A distinct thermoreceptive subregion of lamina I in nucleus caudalis of the owl monkey

A Distinct Thermoreceptive Subregionof Lamina I in Nucleus Caudalis

of the Owl Monkey

A.D. (BUD) CRAIG,1* EN-TAN ZHANG,1 AND ANDERS BLOMQVIST2

1Division of Neurosurgery, Barrow Neurological Institute, Phoenix, Arizona 850132Division of Cell Biology, Faculty of Health Sciences, University of Linkoping,

Linkoping, Sweden

ABSTRACTAn immunohistochemically distinct zone was identified in the superficial aspect of

trigeminal nucleus caudalis of the New World owl monkey that is not immunoreactive forsubstance P or serotonin, in stark contrast to the dense staining present in the surroundinglaminae I and II. Thionin-stained sections in different planes showed that this is a subregionof lamina I containing clusters of neurons that appear to have pyramidal or polygonal somata.Extracellular microelectrode recordings in this region revealed clusters of thermoreceptive-specific (COLD) cells with nasal or labial receptive fields, whereas nociceptive neurons werefound in the adjacent portions of lamina I. Anterograde tracer injections in this regionproduced trigeminothalamic terminal labeling in the site homologous to the lamina Ispino-thalamo-cortical relay nucleus identified previously in the Old World macaque monkeyand in humans. Retrograde tracer injections involving this thalamic site, where recordings oftrigeminal COLD-like neurons were obtained, produced clusters of retrogradely labeledtrigeminothalamic neurons in this immunohistochemically distinct subregion of lamina I,nearly all of which are pyramidal neurons. We conclude that the nocturnal owl monkey has aspecialized perinasal thermoreceptive trigeminothalamic sensory pathway that is probably ofbehavioral significance during olfactory sniffing. In addition, these observations corroborateother findings that have indicated that lamina I COLD cells are pyramidal neurons and arenot physiologically modulated by substance P or serotonin, in contrast to nociceptive neurons.J. Comp. Neurol. 404:221–234, 1999. r 1999 Wiley-Liss, Inc.

Indexing terms: dorsal horn; trigeminal; thalamus; temperature; anterograde labeling

It has been recognized since the inception of ex-perimental neurology that the sense of temperature is adiscrete form of somatosensation that has two aspects(Weber, 1846; Sherrington, 1900; Hensel, 1981; Hellon,1983). First, afferent sensory information regarding thetemperature of the skin and deep tissues of the body isimportant for autonomic activity, homeostasis, and thermo-regulation. Thermoregulation is a defining feature ofmammalian physiology and must be a primary role ofthermoreception (Schonbaum and Lomax, 1990). This isthe enteroceptive aspect of thermal sensibility. Second,discriminative sensory information regarding externaltemperatures is important for environmental explorationand for object recognition (i.e., composition and nature),helping to determine the appropriate actions for behav-ioral movement within and manipulation of the externalenvironment (Darian-Smith, 1984). This is the (extero-and) teloreceptive aspect of thermal sensibility. There is

psychophysical evidence that differentiates these two as-pects in humans (Cabanac, 1972; Mower, 1976).

The correlates of these two aspects in the centralrepresentation of thermoreceptive sensibility have notbeen described. Specific thermoreceptive primary afferentfibers have been well studied, but knowledge of theircentral representation is still quite limited (Hensel, 1981;Hellon, 1983; Darian-Smith, 1984). Within the spinal ortrigeminal dorsal horn, there are thermoreceptive-specificneurons in lamina I, where nociceptive-specific neuronsare also found (Christensen and Perl, 1970; Dostrovsky

Grant sponsor: Swedish Medical Research Council; Grant number: 7879;Grant sponsor: National Institutes of Health; Grant number: NS25616;Grant sponsor: James R. Atkinson Memorial Pain Research Fund adminis-tered by the Barrow Neurological Foundation.

*Correspondence to: Dr. A. D. Craig, Division of Neurosurgery, BNI, 350West Thomas Road, Phoenix, AZ 85013. E-mail: [email protected]

Received 5 June 1998; Revised 27 August 1998; Accepted 8 September1998

THE JOURNAL OF COMPARATIVE NEUROLOGY 404:221–234 (1999)

r 1999 WILEY-LISS, INC.

and Hellon, 1978; Craig, 1996b). In addition, there areneurons sensitive to both thermal and mechanical cutane-ous stimuli (T1M cells) in laminae III–V, where primarilylow-threshold mechanoreceptive neurons are found (Bur-ton, 1975; Iggo and Ramsey, 1976; Poulos, 1981). Thethermoreceptive-specific lamina I neurons are almost allcool-sensitive (COLD) neurons that show graded excita-tory responses to cooling and inhibitory responses towarming, with thresholds near normal skin temperature(specific warm-sensitive neurons seem to be quite rare)(Dostrovsky and Hellon, 1978; Poulos, 1981; Craig andHunsley, 1991; Craig and Bushnell, 1994; Dostrovsky andCraig, 1996a). In contrast, the activity of the T1M cellsseems to reflect the nonlinear, weak cooling sensitivity ofslowly adapting mechanoreceptors (Iggo and Ramsey, 1976;Darian-Smith, 1984). Nonetheless, several studies sug-gested that the T1M cells might be important for discrimi-native (i.e., teloreceptive) thermosensation, because, ex-cept for a small locus of specific cool-sensitive neuronsresponsive to the tongue (Landgren, 1960; Poulos andBenjamin, 1968; Auen et al., 1980), they were the only typeof thermoreceptive cells that could be found in the somato-sensory thalamus of cats and monkeys (Burton et al., 1970;Martin and Manning, 1971; Poulos, 1981; Bushnell et al.,1993).

Recent studies have provided evidence that thermorecep-tive-specific lamina I neurons may be involved in both theenteroceptive and the teloreceptive aspects of thermalsensibility. Lamina I neurons project to a variety ofautonomic and preautonomic sites in the spinal cord andbrainstem (Craig, 1993, 1995, 1996b), and such pathwaysare likely involved in the enteroceptive aspect of thermore-ception, as well as of nociception. In addition, both thermo-receptive and nociceptive lamina I neurons project directlyto the thalamus (Dostrovsky and Hellon, 1978; Craig andKniffki, 1985; Ferrington et al., 1987; Dostrovsky andCraig, 1996a; Craig, 1996b). Their axons ascend in thelateral spinothalamic tract, which is critical for thermaland nociceptive sensibility (Norrsell, 1989; Craig, 1991;Ralston and Ralston, 1992). In the Old World macaquemonkey and in humans, a dedicated lamina I spino- andtrigemino-thalamic terminus has been identified (the pos-terior part of the ventral medial nucleus, or VMpo; Craig etal., 1994), and the neurons in this thalamic nucleusrespond specifically to thermal or to noxious stimuli inmonkey and apparently human (Lenz et al., 1993b; Craiget al., 1994; Dostrovsky et al., 1996). Furthermore, stimu-lation in this region in awake humans can produce discretesensations of cold or of pain (Dostrovsky et al., 1992; Lenzet al., 1993a), consistent with a direct relationship be-tween the activity of spinothalamic lamina I COLD neu-rons and the perceptual discrimination of cool stimuli(Davies et al., 1983; Craig and Bushnell, 1994). However,this spino-thalamo-cortical pathway seems to relay spe-cific thermoreceptive and nociceptive activity to the insu-lar cortex (Craig, 1996a,b), which has been associated withlimbic and autonomic (i.e., enteroceptive) function ratherthan with discriminative somatosensation (Mufson andMesulam, 1984; Augustine, 1996). Nonetheless, recentPET functional imaging data indicate that innocuous coolstimuli activate the insula but not the primary somatosen-sory cortex in humans (Craig et al., 1996).

The present study was initiated by the observation of aregion with an anomalous pattern of staining in thetrigeminal dorsal horn during an immunohistochemical

examination of the neighboring dorsal column nuclei ofowl monkeys (Blomqvist and Broman, 1993), the speciesthat serendipitously happened to be available. Our pre-sent characterization of this region, using several func-tional anatomical approaches, indicates that it can beconsidered the substrate for a specialized thermoreceptivetrigemino-thalamic sensory pathway that may not beinvolved with thermoregulation but that may insteadprovide an enhanced discriminative thermosensory capa-bility. We infer that this finding signifies a pathway for theteloreceptive aspect of thermoreception that must be ofbehavioral significance to this nocturnal primate. Thisinference supports the intriguing implications that thediscriminative aspect of thermal sensibility may be sub-served by insular cortex rather than by the main somato-sensory cortex, but conjointly, that the enteroceptive andthe teloreceptive aspects of thermal sensibility may not beentirely separate yet even at this level. A preliminaryreport was made (Blomqvist et al., 1995).

MATERIALS AND METHODS

Data were obtained from eight owl monkeys (Aotusnancymai or trivirgatus) of either sex. All procedures werein accordance with the guiding principles of the NationalInstitutes of Health and were approved by the localinstitutional animal care and use committees.

Immunohistochemical examination

Material was examined for this study from three ani-mals used in a prior study of the immunohistochemicalorganization of the dorsal column nuclei (Blomqvist andBroman, 1993). Briefly, they were overdosed with pentobar-bital (60 mg/kg i.p.) and perfused transcardially with 3–4liters of 4% paraformaldehyde in warm 0.1 M PB (pH 7.4)after a short rinse with warm saline (0.9% sodium chlo-ride). The tissue was stored in fresh fixative for 3 hoursand then in cold phosphate buffered saline overnight. TheC1 segment and the caudal medulla were cut with aVibratome into 40-µm sections in the transverse plane inone case. In the other two cases, the tissue was separatedat the midline and then sectioned in the transverse planeon one side and the horizontal or sagittal plane on theother. One-in-ten series from the transverse sections andone-in-three series from the longitudinal sections wereprocessed as detailed in Blomqvist and Broman (1993) forthionin, substance P (SP), and serotonin (5HT) by using5-day incubations at 4°C in low concentrations of primaryrabbit antibodies (5HT, 1:12,000 or SP, 1:6,000; Incstar,Stillwater, MN), detection with peroxidase antiperoxidase(PAP), and visualization with 3,3’-diaminobenzidine tetra-hydrochloride (DAB) followed by osmication (0.1% OsO4,3–4 minutes). The vehicle solution contained 0.2% bovineserum albumin, 0.2% Triton X-100, and 3.3% normalserum. Controls included comparison with prior studiesand preadsorption of the primary antibody with SP or a5HT–formaldehyde–bovine serum albumin conjugate. Thesections were dehydrated in alcohol and cover-slipped withEukitt. Photographs were made with Kodak Technical Panfilm and 43 and 103 planapochromatic objectives.

Physiologic recordings and tracer injections

Recordings were made in five animals that were tranquil-ized with ketamine (15 mg i.m.) then anesthetized withpentobarbital (20–30 mg/kg i.v.). A bolus injection of dexa-

222 A.D. CRAIG ET AL.

methasone (10 mg i.v.) was given. The neck, head, and facewere shaved. A rectal thermistor was inserted, and aninfrared lamp was used to maintain core temperature at36.5°C. A digital oximeter probe was placed on the fingers.Local anesthetics were used at pressure and incisionpoints. The animals were mounted in a stereotaxic head-holder. Surgical exposures were made with sterile proce-dures.

Trigeminal recordings. In three animals, the C1 spi-nal cord and medulla were exposed by laminectomy. Smallholes were made in the pia to insert a platinum-platedtungsten-in-glass microelectrode (tip ca. 15-µm width),with which extracellular single- and multi-unit recordingswere made from the left superficial trigeminal dorsal horn(nucleus caudalis). Lamina I neurons were identified as inprior work by their responses to natural noxious andthermal stimuli (Craig and Kniffki, 1985; Craig and Hun-sley, 1991; Craig and Bushnell, 1994; Dostrovsky andCraig, 1996a). (These electrodes generally produce onlyfield potentials and background ‘‘hash’’ in lamina II, andmulti-unit responses to low threshold [innocuous] stimuliin laminae III-IV.) Lamina I cells (or clusters) were classi-fied as follows: cooling-sensitive thermoreceptive-specific(COLD) neurons if they responded to a cold beaker or a wetice cube, were inhibited by warming with a radiant heatlamp, and were unresponsive to other stimuli; nociceptive-specific (NS) neurons if they responded only to noxiouspinch (handheld anatomical forceps) or noxious radiantheat (at a latency of 5 to 8 seconds from a fixed distance); orpolymodal nociceptive (HPC) neurons if they respondedonly to noxious heat, pinch, and cold. Receptive fields (RFs)were determined by manual mapping with adequatestimuli and recorded by verbal descriptions or on draw-ings. In one animal, a thermoelectric Peltier stimulator(2 3 2 cm) was used to apply controlled thermal stimuli(Craig and Hunsley, 1991; Craig and Bushnell, 1994;Dostrovsky and Craig, 1996a). Magnetic tape was used forbackup, and documentation was obtained with oscillo-scopic photographs or thermographic recordings. Lesionswere made at selected locations by passing 7–10 µA for20–60 seconds and verified by recording.

Trigeminal anterograde tracer injections. In two ofthe three animals in which trigeminal recordings weremade, an injection of PHA-L (2.5% in 0.01 M PB, pH 7.4)was made at a selected site by replacing the microelectrodewith a micropipette inserted at the same location andpassing 8 µA of positive current (5 seconds on, 5 secondsoff, 6 minutes in one case, 12 minutes in the other). Aftersurvival (one for 2 weeks, one for 3 weeks), the animalswere overdosed with pentobarbital and perfused transcar-dially with 1 liter of warm, heparinized, phosphate buff-ered saline, pH 7.5, followed by 1 liter of 4% paraformalde-hyde and 0.2% picric acid in acetate buffer (pH 6.3), andfinally 1.5 liter of cold 4% paraformaldehyde and 0.05%glutaraldehyde with 10% sucrose in PB (pH 8.0). The C1segment, brainstem, and thalamus were stored in coldfixative with 30% sucrose for 4 hours and then overnight incold 30% sucrose. Transverse 50-µm frozen sections werecut from each block. Two one-in-three series from C1 weredouble-labeled immunohistochemically for PHA-L and SP,or for PHA-L and 5HT, and the remaining one-in-threeseries was stained with thionin. For the brainstem, one-in-four series were processed for PHA-L or stained withthionin; for the thalamus, a one-in-three series was stained

with thionin, and the remaining sections were double-labeled for PHA-L and calbindin 28 kD.

Double-labeling immunohistochemistry. Immunore-active PHA-L was visualized with an avidin/Texas Redprocedure (in Tris buffer containing 0.1% Triton-X), asdetailed previously (Craig, 1991; a modification of themethod of Gerfen and Sawchenko, 1984). Double-labelingwas accomplished with the indirect fluorescein isothiocya-nate (FITC) method, or with sequential avidin/biotin com-plex (ABC) reactions, by incubating simultaneously inboth primary antibodies and by using sera of differentorigins (see Craig et al., 1992; Craig, 1995). Goat anti–PHA-L primary antiserum (Vector Laboratories, Burlin-game, CA, 1:1,350) was mixed with rabbit anti-SP (Incstar,1:4,000 to 1:10,000) or rabbit anti-5HT (Incstar, 1:2,000 to1:8,000) or monoclonal mouse anti-calbindin (Sigma, St.Louis, MO, 1:1,000) in 2% normal horse serum (2 days),followed by biotinylated donkey anti-goat secondary anti-serum (4 hours; Accurate, Westbury, NY), avidin/TexasRed (4 hours; Vector Laboratories, 1:500), and 0.01%avidin and 0.01% biotin rinses (30 minutes), then eitherFITC-labeled donkey anti-rabbit serum (4 hours; Accu-rate, 1:200) or biotinylated donkey anti-rabbit serum (4hours; Accurate or Vector Laboratories; 1:200) followed byavidin/FITC (4 hours; 1:500, DCS grade, Vector Laborato-ries). This resulted in red-fluorescent, PHA-L-labeledlamina I terminals and green-fluorescent SP or 5HT orcalbindin labeling. Controls included the omission of theprimary antibody or, when available, preadsorption of theprimary with the appropriate antigen. The sections weremounted, dried, rinsed in xylene, cover-slipped with DPX,and examined with rhodamine and fluorescein (NikonG-2A, B-2A) filters. The fluorescent labeling was localizedwith respect to the pseudo-myeloarchitectonic backgroundimage of the tissue, and then vessels were used to comparewith the adjacent thionin sections. Photographs weremade with Fujichrome 400 film (push-processed one stop)and digitized with a Polaroid film scanner at 2,200 dpi.Digital images of thionin-stained sections were made witha Leaf Microlumina scanner (at 3,380 3 2,253 pixels) and a23, 43, or 103 planapochromatic objective, and AdobePhotoshop was used to enhance contrast and apply labels.

Thalamic retrograde tracer injections. In two ani-mals, a craniectomy was made to expose the right thala-mus. Microelectrode recordings during repeated stereo-taxic penetrations were used to identify somatosensorythalamus. A small region at the posterior aspect of somato-sensory thalamus containing nociceptive and thermorecep-tive neurons was located; in the Old World macaquemonkey, this region has been recognized physiologicallyand anatomically as the lamina I spino-thalamo-corticalrelay nucleus, where it has been termed the posteriorventral medial nucleus, or VMpo (Craig et al., 1994;Dostrovsky and Craig, 1996). Single- and multi-unit re-sponses to adequate stimulation were characterized,mapped, and recorded as before. An injection was made ata selected site in each animal through a micropipetteattached to a Hamilton 10-µl syringe of 1.0–1.5 µl of 1%cholera toxin subunit B (CTb, Sigma). A second injectionwas made in each case at a neighboring site (1.0 mmrostral, lateral, or both). In one animal, the same proce-dure was also followed on the contralateral side, butFITC-labeled dextran (3,000 molecular weight, lysine fix-able; Molecular Probes) was injected. After survival (2weeks), the animals were perfused and the tissue removed

COLD SUBREGION OF LAMINA I 223

as above. Frozen 50-µm sections were cut from the thala-mus in the frontal plane and from the C1/medulla in eitherthe sagittal plane or an oblique tangential plane betweentransverse and sagittal. Alternate sections were stainedwith thionin or were processed by using standard immuno-fluorescence for SP (avidin/Texas Red) and FITC-labeleddextran, or were immunohistochemically double-labeledfor SP and CTb with two serial ABC reactions (as inWestlund and Craig, 1996). In this process, the firstreaction (CTb) was stained brown with DAB and thesecond (SP) was stained purple with VIP (Vector Laborato-ries). The sequence (with rinsing) was as follows: mixedmonoclonal primary anti-CTb (1:1000, supplied by Dr.Marianne Wikstrom, as used previously [Zhang et al.,1996)]) and rabbit anti-SP (Incstar, 1:10,000; 2 days, 4°C),then biotinylated horse anti-mouse (Vector Laboratories,1:400, 4 hours), ABC (Vector Laboratories; 1 hour), 0.025%DAB and 0.0003% H2O2 (5 to 20 minutes), 0.01% avidin (30minutes), 0.01% biotin (30 minutes), biotinylated donkeyanti-rabbit (Accurate, 1:400, 4 hours), ABC (1 hour), andVIP (as prescribed by Vector Laboratories). The materialwas documented as above. The spinal labeling in one ofthese animals was used in a prior study (Zhang and Craig,1997).

RESULTS

Several different experiments were performed to charac-terize the thermoreceptive subregion of lamina I. Theseincluded immunohistochemical examination, physiologicrecordings, anterograde and retrograde labeling, and com-bined immunohistochemical and retrograde labeling. Thesestudies are described in sequence.

Immunohistochemical examination

Blocks of material from both sides of three owl monkeyswere separated and cut so that the trigeminal n. caudaliswas examined in four series of transverse sections, in oneseries of horizontal sections, and in one series of sagittalsections. Dense labeling for SP or for 5HT was obtained inthe superficial dorsal horn, or laminae I-II. In each case, aregion was observed within lamina I in which few or no SP-and 5HT-labeled fibers are present. Figure 1 shows acompact portion of this region in adjacent thionin andimmunohistochemically labeled transverse sections at lowand high power, and Figure 2 shows this in adjacenthorizontal sections from a different case. The SP- and5HT-labeled sections demonstrate the stark contrast be-tween this region and the dense labeling in the adjacentlaminae I-II, and they show that where this SP-freesubregion is thickest, it displaces or indents the SPlabeling in the superficial dorsal horn. The thionin sectionsshow that this subregion contains many large, darklystained neurons that have generally pyramidal or polygo-nal shapes, and that it extends into the spinal tract of thetrigeminal nerve (Fig. 1D).

The SP-free subregion of lamina I is centered between 1and 2 mm caudal to the obex, and it is clearly recognizableover an extent of 1 to 2 mm rostrocaudally and 500 to 800µm mediolaterally (or tangentially). The caudal portion ofthe SP-free region consists of pockets of darkly stainedcells in the most superficial aspect of lamina I that aresurrounded by areas where lamina I is a thin marginallayer that contains immunohistochemical staining for SP(Fig. 2). Progressing rostrally, these pockets of cells en-

large and expand into the trigeminal tract as interstitialclusters of lamina I cells (see Fig. 3). In its rostral portion(Fig. 1), where it is thickest, the SP-free region appears tooccupy all of lamina I dorsoventrally, and the zone of SPand 5HT labeling (in lamina II) is relatively thinner. At themost rostral end of the nucleus caudalis, the SP-free regiondiminishes, but a different large cluster of neurons ap-pears within the dorsal portion of the trigeminal tract thatextends rostrally over the nucleus interpolaris; this is theinterstitial nucleus of Cajal (see Light, 1992). In contrastto the SP-free subregion of lamina I, this region containsdense SP immunoreactivity.

Physiologic recordings

Recordings were obtained from lamina I neurons during12 microelectrode penetrations into nucleus caudalis inthree animals. In two of these cases, anterograde tracerinjections were made (see below). A total of 16 single unitswere isolated, of which 7 were NS cells, 7 were COLD cells,and 2 were HPC cells. In addition, recordings were ob-tained from 16 multi-unit clusters of COLD cells. Nearlyall of these cluster recordings contained only COLD cells,which had a common receptive field. In only one such locusdid a single neuronal response to a noxious stimulus occur(pinch). In two penetrations in which one or more clustersof COLD cells were recorded, nociceptive cells were re-corded at a deeper site, just before lamina II was encoun-tered.

As in prior recordings (Craig and Hunsley, 1991;Dostrovsky and Craig, 1996a), the COLD units and theclusters of COLD cells all displayed ongoing activity thatwas inhibited by warming. They were not excited bynoxious pinch or heat, nor by innocuous brushing normechanical stimulation (if the probe used was warmed toskin temperature). These COLD neurons were very sensi-tive, readily excited by ice held even several centimetersfrom the skin or inhibited by the experimenter’s warmbreath.

The RFs of the solitary nociceptive or COLD neuronsthat were isolated were distributed over the face, includingall three trigeminal divisions, and the general pattern ofRF locations matched the overall topographic organizationof trigeminal lamina I in other species (Dostrovsky andHellon, 1978). In contrast, the RFs of the large clusters ofCOLD cells were nearly all in the maxillary division (Fig.3A); most occurred near, on, or even inside the nares,nearby in the mystacial portion of the infraorbital region,or on the labial or gingival aspect of the upper lip. Withinthis limited distribution, the sequence of RFs within thecluster zone also matched the overall topographic organiza-tion (Fig. 3A). The RFs of COLD cell clusters were gener-ally quite small, on the order of 20–50 mm2. The cells withRFs at the nares or within the nostril were most obvious,because their ongoing activity was clearly enhanced witheach inspiration of room air (e.g., see Fig. 4, thalamicrecordings).

Figure 3 shows the reconstruction of one microelectrodepenetration from an animal in which seven penetrationswere made. This particular track passed through almostthe full tangential extent of the SP-free subregion oflamina I near its thickest portion. The photomicrographsin Figure 3B,C show the lesion made at the bottom of thepenetration, marked by the large arrowhead. Although theplane of section is not exactly the same as the angle ofpenetration, and the thionin section suffered some shrink-


age distortion, it is clear that the microelectrode passedthrough the approximately 450-µm span of cell clusters inthe SP-free subregion of lamina I, delimited in Figure 3B,Cby the small arrowheads. Within this region, dense clus-ters of COLD cells were recorded at depths between 200and 650 µm. The physiologic records in Figure 3A docu-ment that these COLD neurons showed brisk responses to

cooling and were inhibited by warming. The quantitativestimulus-response function obtained for the isolated singleunit denoted with the bold numeral 1 in Figure 3A (whoseRF was accessible with the Peltier stimulator) shows thatthis COLD cell had a steeply increasing (but negativelyaccelerating) discharge rate with decreasing temperature.This finding is similar to the response functions of COLD

Fig. 1. Immunohistochemical identification of the distinct subre-gion of lamina I in the owl monkey. Low (A), middle (B), and high (D)power photomicrographs of a thionin-stained transverse sectionthrough nucleus caudalis. The boxes indicate the locations of thesuccessively higher magnification images that focus on the cluster ofpyramidal shaped lamina I somata. The adjacent section, stainedimmunohistochemically for substance P (SP), is shown in C at low

power and in E at higher power, and the next adjacent section, stainedfor serotonin (5HT), is shown in F at high power. The arrowheads inthese photomicrographs show that the region of the cluster of neuronshighlighted in D does not contain immunoreactive fibers, in contrast tothe remainder of laminae I–II. Dorsal is up, and lateral is to the left.Scale bar in E 5 520 µm in A,C, 125 µm in B,E,F, and 56 µm in D.


lamina I spinothalamic neurons in the cat (Craig andHunsley, 1991) and the macaque monkey (Dostrovsky andCraig, 1996a).

Anterograde tracer injections

To determine the projections of the COLD neurons inthis distinct SP-free subregion of trigeminal lamina I, wemade a PHA-L injection in two of the three monkeys inwhich these neurons were recorded. In the first case, asingle large injection (8 µA, 12 minutes) was made thatspread throughout the SP-free region and into the sur-rounding lamina I. In the second case, two small injections(8 µA, 6 minutes) were made spaced 0.4 mm apartrostrocaudally, both of which were centered in the cellclusters of the SP-free region and spread very little intothe adjacent lamina I. This is documented in Figure 4,which shows one of the PHA-L injections in this case (Fig.4B1) and the SP labeling in the same section (Fig. 4B2; thebright spot in the SP-free cell cluster is the intensecross-fluorescence at the center of the injection site). Theoscillographic trace in Figure 4A shows the thermal re-sponses of the cluster of COLD units that was recorded atthis injection site.

In both cases, terminal labeling is located bilaterally inthe brainstem at several sites that are consistent withprior examinations of the projections of lamina I cells

(Burton et al., 1979; Craig, 1995; Westlund and Craig,1996), i.e., in the nucleus of the solitary tract (A2 region),the caudal and rostral ventrolateral medulla (A1 and C1regions), and the ventrolateral (A5) and dorsolateral (A7)regions of the caudal pons. In the case with the largeinjection, there is bilateral labeling in the medial parabra-chial nucleus; but in the case with PHA-L injections nearlyrestricted to the SP-free subregion of lamina I, there is nolabeling in the parabrachial nucleus. There is sparse or nolabeling in the periaqueductal gray in either case.

In the thalamus of both cases, there is fairly denseterminal labeling with large clusters of boutons located inan area of small, moderately thionin-stained neurons inthe posterior region, that is, medial and dorsal to thesuprageniculate nucleus, ventral to the anterior pulvinar,and lateral to nucleus limitans. This region containsmoderately dense calbindin immunoreactive fiber stainingand it is immediately posterior to and, more rostrally,immediately lateral and dorsal to the compact region ofdarkly stained neurons and very dense calbindin labelingthat is the basal part of the ventral medial (VMb) nucleus(also called the parvocellular part of the ventral posteriormedial nucleus). Thus, this terminal labeling appears tooccur within a region that is anatomically equivalent tothe posterior part of the ventral medial nucleus (VMpo),

Fig. 2. Horizontal sections through nucleus caudalis of the owlmonkey, showing at low (A) and higher (B) power the cluster of laminaI neurons in the distinct subregion with little or no immunohistochemi-cal labeling for substance P (SP) (C) and serotonin (5HT) (D). Thearrowheads in C and D outline the cluster of lamina I neurons visible

in (B) and show that this subregion does not contain immunoreactivestaining for SP and 5HT. In A, the rostral (main) cuneate is visible atthe lower right, and lamina III/IV cells are visible in the medial aspectof the nucleus caudalis. Scale bar in D 5 520 µm in A, 125 µm in B–D.Lateral is up, and rostral is to the right.


Fig. 3. Physiologic characterization of the thermoreceptive sub-stance P (SP)-free subregion of lamina I. A: Sequence of recordingsfrom thermoreceptive-specific (COLD) neurons along the microelec-trode penetration that passed through the clusters of neurons in theSP-free subregion shown in B, delimited by arrowheads. The microelec-trode entry point and the more superficial recordings were located in aneighboring section. C: The adjacent thionin section (lower power inD), which suffered some shrinkage during dehydration. (The magnifi-cation in C has been increased slightly to match that of the section

shown in B.) The lesion made at the bottom of the electrode track isindicated by the large filled circle in A and the large arrowheads in Band C. The RF locations numbered 1, 2, and 3 are indicated on thefigurine of a (macaque) monkey’s face. Sample responses and thethermal stimulus-response function of the unit recorded at location 1are shown in lower A, and a record of the multi-unit cluster responseobtained at location 3 is also shown. Dorsal is up, and lateral is to theleft. Scale bar in D 5 75 µm in B,C, 250 µm in D.

identified in prior work in the macaque thalamus as adedicated, topographic lamina I spino-thalamo-corticalrelay (Craig et al., 1994). The terminal PHA-L labeling inboth cases occupies only the ventromedial portion ofanterior VMpo; our prior observations in the owl monkeyhave shown that spinal lamina I terminals correspond-ingly occur in the more posterior and lateral portion of thisregion (shown in Fig. 1 of Blomqvist et al., 1996). In thecase with the injections that were nearly restricted to the

SP-free subregion of lamina I, the terminal labeling inVMpo is the only labeling in the thalamus (Fig. 4C). It iscentered on a portion of VMpo that contains clusters ofsmall, polygonal neurons that are coherent and differen-tiable (slightly more densely packed) from the remainderof this region, which are denoted in Figure 4E. In the casewith the larger PHA-L injection, dense labeling is alsocentered on this region, yet the labeling extends over alarger portion of VMpo, and in addition, is present in the

Fig. 4. Documentation of the thalamic projection of cells in thethermoreceptive SP-free subregion of lamina I. A: The recordedactivity of a small cluster of thermoreceptive-specific (COLD) unitsthat had an RF on the middle upper lip. (Note that the amplitude ofthe large spike diminished when the unit firing frequency increased.)The iontophoretic injection of PHA-L made at this site is shown in B1,and the adjacent section in B2 shows that this injection is within,and nearly restricted to, the interstitial SP-free subregion of lamina I.

C: The focus of the field of anterograde terminal labeling found in thiscase, which is located at the site indicated by the arrows in thethionin-stained section shown in E. D: The low power photomicro-graph shows the region of the thalamus from which the image in E wastaken. CM, centre median; Pla, anterior pulvinar; Pll, lateral pulvinar;VMb, basal part of the ventral medial n.; VMpo, posterior part of theventral medial n.; VPM, ventral posterior medial n. Scale bar in E 5480 µm in B1,B2, 100 µm in C, 2.27 mm in D, and 1.0 mm in E.


ventral posterior medial (VPM) nucleus more rostrally,where three separate, small bursts of terminations arelocated in its medial aspect.

Retrograde tracer injections

Injections of the retrograde tracer CTb (Ericson andBlomqvist, 1988) were made in the right thalamus of twoanimals, and injections of a different retrograde tracer,FITC-labeled 3,000 molecular weight dextran, were alsomade in the left thalamus of one of these. The injectionswere guided by recordings. As in the macaque monkey(Craig et al., 1994), VMpo was identified by the presence ofnociceptive- and thermoreceptive-specific (cooling sensi-tive) neurons posterior and medial to the intraoral repre-sentation of the main somatosensory thalamus. In allthree thalami, a small region was found in which clusters

of COLD-like neurons with RFs on the nose, lips, tongue,or hand were recorded. In each case, one injection wasmade at this site and another was made 1 mm anterior, 1mm lateral, or both, to this site.

Figure 5B shows the injection made at the site at whichCOLD-like neurons were recorded in one case in a thionin-stained section; the cytoarchitectonic location of this sitecorresponds to that of the locus of anterograde labelingshown in Figure 4. The cluster of COLD-like neuronsrecorded at this site (Fig. 5B, lower trace) had RFs on thenares, they responded to cooling and were inhibited bywarming, and they had ongoing activity that was drivenrhythmically by each inspiration of (cool) room air.

The two cases in which CTb injections were made haveretrogradely labeled neurons throughout the contralateraltrigeminal lamina I. These neurons are well labeled, and

Fig. 5. Documentation of the retrograde labeling of pyramidallamina I neurons in the substance P (SP)-free subregion. B: Thelocation of a CTb injection site. Clusters of thermoreceptive-specific(COLD)-like neurons recorded at this site had RFs on the nares andresponded with inspiration or to thermal stimuli as shown in the lowertrace. A: The photomicrograph of a sagittal section from the contralat-eral nucleus caudalis in this case shows retrogradely CTb-labeledlamina I neurons both apart from and within the region of dense SP

fiber labeling. C: The drawing shows the labeled neurons and theregion of SP labeling (shading); the location of the photomicrograph inA is also marked. D: The shapes of the symbols identifies the types ofcells found. The same cell is marked with an arrow in A, C, and D. CM,centre median; Pla, anterior pulvinar; Pll, lateral pulvinar; VPM,ventral posterior medial n. Dorsal is up, and rostral is to the left for A,C, and D; dorsal is up, and lateral is to the right for B. Scale bar in A 5100 µm in A, 1.2 mm in B, and 220 µm in C,D.


their somata and primary dendrites are clearly visible.Figure 5A shows a photomicrograph from a sagittal sectionin which CTb-labeled lamina I trigeminothalamic cells arevisible both within the region of dense SP fiber labeling aswell as in the adjacent clusters of the SP-free subregion.The drawing in Figure 5C represents, for a broader area ofthis section, the distribution and the apparent somatalshapes of the labeled cells, and the shading indicates theregion of SP immunoreactivity. As in our prior studies ofretrogradely CTb-labeled lamina I spinothalamic neurons(Zhang et al., 1996; Zhang and Craig, 1997), these lamina Ineurons were classified as fusiform, pyramidal, and multi-polar, based on somatodendritic shape. The identificationof cell shape in nucleus caudalis is more difficult than inspinal horizontal sections, because lamina I curves ventro-laterally, so a sagittal, horizontal, or oblique section is notcompletely tangential and because the arborizations oflamina I cells in nucleus caudalis are not strictly tangen-tial (Gobel, 1978). Nonetheless, the distribution of celltypes illustrated in Figure 5D with different symbolsreveals that almost all of the retrogradely labeled neuronsin the SP-free subregion of lamina I could be classified aspyramidal cells. A few were multipolar cells.

The injection of FITC-labeled dextran, which spreadover a smaller region of the thalamus, produced denselabeling of trigeminal lamina I cells in the SP-free subre-gion, and less labeling of cells in the remainder of lamina I.Again, the labeled cells in the SP-free zone are nearly allpyramidal in shape, as shown in the pair of color photomi-crographs in Figure 6 from an obliquely cut section. Thelarge cell near the vessel on the left, for example, appearsnearly identical to the pyramidal cell labeled P1 in Figure4 of Zhang and Craig (1997), and it is similar to theintracellularly labeled pyramidal COLD cell 27–176 shownin Figure 4 of Han et al. (1998).

DISCUSSION

By using several, complementary functional anatomicaltechniques, we have obtained data indicating that there isa distinct subregion of lamina I in the trigeminal dorsalhorn of the owl monkey. This subregion differs from thenormal lamina I because it does not contain immunoreac-tivity for SP or 5HT. The clusters of pyramidal-shapedlamina I neurons in this subregion project to a smallportion of VMpo in the contralateral thalamus. Physiologi-cally, the neurons in this subregion of lamina I are COLDcells with small receptive fields focused on the nose andupper lip. Similar COLD-like neurons can be found attheir projection terminus in VMpo. In contrast to otherlamina I cells, these neurons appear to provide little or noinput to the parabrachial nucleus, a major brainstemvisceral integration site, or to the periaqueductal gray, themesencephalic substrate for homeostasis. Together, thesedata indicate that the owl monkey has a specializedthermoreceptive trigeminothalamic sensory pathway.

The striking lack of SP and 5HT inputs to this subre-gion, which differentiates it from the surrounding laminaI, is consistent with the absence of nociceptive neurons andthe presence of thermoreceptive-specific neurons in thissubregion, because of the physiologic association of SP and5HT with nociception. Substance P is contained in nocicep-tive primary afferent fibers (Knyihar-Csillik et al., 1990;Cuello et al., 1993), and superficial dorsal horn neuronsthat respond to SP are nociceptive (De Koninck et al.,

1992). Furthermore, 5HT and SP are associated withdescending brainstem inputs that modulate nociceptiveneurons in the superficial dorsal horn (Marson, 1989;Reddy et al., 1990; Bernau et al., 1993), whereas thermore-ceptive-specific neurons in spinal and trigeminal lamina Iare little affected by such descending modulation (Dawsonet al., 1981; Dostrovsky et al., 1983; Mokha et al., 1987;Craig and Hunsley, 1991). Thus, the present findingsprovide anatomic corroboration for the physiologic observa-tions of differential descending effects on thermoreceptiveand nociceptive lamina I neurons. Examinations of theimmunoreactivity of this subregion for calcitonin generelated peptide or enkephalin (Chung et al., 1988; Arvids-son et al., 1995) or other neurochemicals and receptorsthat are selectively associated with nociception in thesuperficial dorsal horn might also reveal differential stain-ing. Furthermore, the immunohistochemical labeling dif-ferentiates this thermoreceptive subregion of lamina Ifrom the interstitial nucleus (of Cajal) in the trigeminaltract, which is basically an extension of lamina I thatcontinues rostrally from n. caudalis above n. interpolaris.The interstitial nucleus stains for SP and 5HT fibers andcontains both nociceptive and thermoreceptive neuronswith intra-oral input (see Light, 1992), in contrast to thisspecialized thermoreceptive subregion.

Fig. 6. Color photomicrographs of a double-labeled section showingpyramidal lamina I neurons retrogradely labeled with fluoresceinisothiocyanate–dextran (A) within the substance P (SP)-free region (B,Texas Red). The SP-free subregion shows background autofluores-cence, but few labeled fibers. The same vessel is marked with an x inboth micrographs. Oblique transverse/sagittal section, dorsal/rostralis up, lateral is to the left. Scale bar 5 100 µm in B (applies to A,B).


This thermoreceptive-specific subregion contains pre-dominantly pyramidal neurons, whereas fusiform andmultipolar cells are present in the adjacent portions oflamina I, where nociceptive cells were recorded. Thisfinding suggests that lamina I pyramidal neurons arethermoreceptive-specific COLD cells in the owl monkey.This role is consistent with prior findings on structural/functional relationships in lamina I. Three major morpho-logic types of lamina I neurons have been identified in rat,cat, and monkey: fusiform, pyramidal, and multipolar(Gobel, 1978; Lima and Coimbra, 1986; Zhang et al., 1996;Zhang and Craig, 1997). Intracellular labeling evidenceobtained in the cat clearly indicates that these morpho-logic types correspond directly to the three physiologicclasses of nociceptive-specific (NS), thermoreceptive-specific (COLD), and HPC cells, respectively (Han et al.,1998). The same physiologic classes of lamina I spinotha-lamic neurons have been identified in the monkey(Dostrovsky and Craig, 1996a). Thus, the present observa-tions provide strong support for the notion that the samestructure/function correlation exists for monkey lamina Ineurons as in the cat. These observations are also consis-tent with the recent finding in the monkey that fusiformand multipolar cells express the SP (NK-1) receptor,whereas pyramidal cells generally do not (Yu et al., 1998).

The COLD cells recorded in this subregion had smallreceptive fields focused on the nasal and labial regions.They were remarkably sensitive, compared with the spino-thalamic and trigeminothalamic lamina I COLD neuronswe have recorded before in cat and macaque monkey(Craig and Hunsley, 1991; Craig and Bushnell, 1994;Dostrovsky and Craig, 1996a; Craig, 1996b). They hadongoing discharge at normal skin temperature, thus en-abling small temperature changes in either the warmingor cooling direction to be readily detected, and they werecapable of encoding static temperatures with a steepresponse function. Particularly striking were neurons withreceptive fields on the nares or within the nostril that wereactivated with each inspiration of room temperature air.These COLD cells occurred predominantly in clusters ofmany units with the same receptive fields, indicating ahigh density of representation. In addition, there was adistinctly topographic order within this cluster region.These characteristics together provide evidence that thissubstrate forms the basis for a high resolution, finelydiscriminative thermosensory map, reminiscent of thewell-developed mechanosensory maps of the fingertips(Merzenich and Kaas, 1980; Romo et al., 1998) or therecently described map of the star-nosed mole’s snout inthe somatosensory system (Catania and Kaas, 1997). Weinfer that this subregion is specialized for the discrimina-tion of small temperature differences at or in front of theowl monkey’s nose, that is, for teloreceptive thermalsensation.

The ascending projection of this subregion is consistentwith this interpretation. Our data indicate that the neu-rons in this thermoreceptive trigeminal subregion of laminaI project to the thalamus, but apparently not to two majorhomeostatic integration sites in the brainstem. In thethalamus, they terminate in a small portion of VMpo,which is a topographically organized, nociceptive andthermoreceptive lamina I relay nucleus that we haveidentified previously in the macaque monkey and in hu-mans (Craig et al., 1994). Evidence in the macaque sug-gests that the thermoreceptive neurons in VMpo form a

topographic map at its most rostral and medial aspect thatis segregated from its nociceptive component (Bushnelland Craig, 1993; unpublished observations), and this isconsistent with the present observations. The lamina Iterminations in VMpo consist of dense clusters of largeboutons, and we have shown in prior work that ascendingspinal terminals in VMpo in the owl monkey are glutama-tergic and form triadic arrangements with postsynapticrelay cell and presynaptic GABAergic dendrites (Blom-qvist et al., 1996), all of which are lemniscal featuresindicative of high synaptic and temporal fidelity. Accord-ingly, we found in the present experiments that neurons inthe appropriate portion of VMpo were thermoreceptive-specific, COLD-like neurons that had small receptive fieldsfocused on the nose, and that some were sensitive to thetemperature of inspired air, just like the COLD neurons inthe trigeminal lamina I subregion. These characteristicsall support the inference that this is a specialized trigemi-nothalamic pathway for discriminative thermal sensation.

The observation that this specialized thermosensitivityis focused at the nose may signify its behavioral impor-tance in relation to the ethology of the owl monkey (Baer etal., 1994). The owl monkey is the only nocturnal monkeyknown, and accordingly, it has several beneficial adapta-tions. For example, its visual system is highly adapted formovement detection in the dark over a broad field of view,appropriate for its appetite for moths and other flyinginsects that are active at night. The owl monkey is alsohighly osmotic and has a well-developed olfactory system,as do all New World monkeys and also nocturnal prosim-ians; yet, the olfactory system of the owl monkey is largerthan that of any other New World monkey, and it has agreatly enhanced ability to use olfactory cues for orienta-tion and food localization (Bolen and Green, 1997). Theseanimals regularly smell and taste foods (such as figs,flowers, and leaves) before eating (perhaps to confirmripeness and palatability). They also perform frequentscent marking with a pelvic gland and with urine, buttheir reproductive behavior does not depend on such cues(Wright, 1989). Ethologists emphasize that such olfactorycues can be very efficient markers in the rain forest atnight, when the humid air is cold and still, and that the useof olfactory cues for navigation and trail marking by theowl monkey may be a significant aid to its nocturnalforaging (Wright, 1989; Bolen and Green, 1997). Notably,the cold night air also provides the appropriate conditionfor enhanced thermal discrimination, and a specializedthermosensitivity focused at the nose would inherentlyprovide a valuable accessory signal when sampling olfac-tory cues during nocturnal foraging, or perhaps whenanother animal is near in the dark.

In particular, this suggestion stems from the observa-tions made by Wright (1989) in a careful field comparisonof the owl monkey’s behavior with that of a comparablediurnal New World monkey (Callicebus molloch) in thesame ecologic niche. Wright identified a significant differ-ence in foraging behavior. Whereas the diurnal monkeyforages widely in the rain forest canopy and rarely returnsto sleep at the same site, an owl monkey group (actually, asmall nuclear family) generally follows the same shortcircular foraging route each moonless night and returns tothe same tree hole to sleep. (The tree hole providesnecessary refuge from diurnal predators such as eaglesand harpies; the nocturnal predators are snakes andfelids. On moonlit nights, the owl monkeys behave more


like the diurnal monkeys; they are much more active andvocal, they travel widely, they interact with neighboringgroups, and they sleep in different sites.) Wright (1989)observed that the owl monkeys use olfactory signals tomark their foraging route and their tree hole, and sheinferred that they use these trail markings for navigationin the pitch darkness. We note that sniffing these mark-ings naturally provides an adequate stimulus for the owlmonkey’s specialized thermosensory pathway. The cold airinspired while sniffing the scent markings will activate orinhibit the thermosensory neurons with receptive fields onthe nares and in the nostril (as it did during our record-ings), and this evoked activity reliably indicates the differ-ence in temperature between the scent marking and theambient air. Because the temperature of such a trailmarking, at or near body temperature when first made butsteadily declining in the cold night air, provides a fairlyaccurate measure of the time elapsed since the markingwas made, this thermosensory information signals thefreshness of the trail marking. In other words, it revealshow ‘‘warm’’ the trail is. Thus, the sampled olfactory inputprovides the ‘‘who’’ and ‘‘where’’ aspects of the trail mark-ings, and the thermosensory input signals the ‘‘when.’’ Itseems likely that the discriminative thermosensory infor-mation carried by this specialized pathway might haveconsiderable survival value for a family of small owlmonkeys trying to follow one another, in pitch darkness,along an arboreal foraging trail through the rain forest.

These considerations suggest that this specialized path-way may be unique to the nocturnal owl monkey amongmonkeys, yet it might be present in other nocturnalmammals. In the course of prior work, we have examinedthe trigeminal complex in rat, rabbit, cat, dog, raccoon,macaque, and human brains, by using thionin and immu-nohistochemical stains. Although some of these species arenocturnal (e.g., the raccoon) and thermoreceptors areknown to cluster around the nose, eyes, and mouth, in allof these species, we have seen no evidence of a similarsubregion in trigeminal lamina I and none have beendescribed by others. Previous descriptions of the trigemi-nal complex in another New World monkey, the squirrelmonkey (Saimiri sciureus), also do not mention such aregion (Tiwari and King, 1974; Ganchrow, 1978). Neverthe-less, as a supplement, we examined nucleus caudalis intwo squirrel monkeys with similar histologic methods. Weobserved that lamina I is expanded into the trigeminaltract in this species, relative to that of macaques or cats,yet there is not a compact SP-free subregion like that inthe owl monkey. This supports the view that this thermo-sensory specialization is unique to the nocturnal owlmonkey among monkeys. In addition, we examined themedulla of one nocturnal prosimian (Galago senegalensis).The laminar organization of nucleus caudalis is particu-larly well differentiated in this primate, and it has twolarge, compact clusters of lamina I neurons in the mostrostral aspect of nucleus caudalis. There is a dorsomedialcluster that has neurons of all shapes and more sparse SPand 5HT staining than the remainder of lamina I, and aventrolateral cluster that has neurons that appear to begenerally pyramidal or polygonal and virtually no SP or5HT staining. Based on the topographic organization ofnucleus caudalis, the dorsolateral cluster might be associ-ated with oral receptive fields and, thus, is likely homolo-gous to the interstitial nucleus of Cajal, whereas theventrolateral cluster might be associated with nasal recep-

tive fields. We speculate that this latter cluster may behomologous to the specialized, thermoreceptive, SP-freesubregion of the owl monkey. If this is so, it would indicatethat an enhanced thermosensory capability focused at thenose, whereas unique to the nocturnal owl monkey amongmonkeys, could be a significant, convergent developmentamong nocturnal, arboreal primates.

Discriminative somatosensation in primates is usuallyassociated with the primary somatosensory cortex (Mer-zenich and Kaas, 1980; Romo et al., 1998), but the probableterminus of this thermosensory trigeminothalamic path-way by way of VMpo lies along the dorsal margin of theinsular cortex (Craig, 1996a,b), just posterior to the corti-cal projection of the gustatory pathway (Pritchard et al.,1986). The present considerations, therefore, suggest thatthis portion of insular cortex subserves discriminativethermal sensibility. Only few thermoreceptive neuronshave yet been recorded in this region in primates(Dostrovsky and Craig, 1996b), but there is supportiveevidence for a role of insular cortex in specific thermalsensibility in rats (Kosar et al., 1986). This suggestion isconsistent also with the activation of this region, but notprimary somatosensory cortex, by innocuous cool stimula-tion in human PET functional imaging (Craig et al., 1996),as well as with the clinical defect in thermal sensation thatcan result from lesions in this region (Berthier et al., 1988;Boivie and Leijon, 1991; Schmahmann and Leifer, 1992).This suggestion could have significant meaning for thecortical representation of pain as well, because this sug-gests the possibility that the VMpo terminus in insularcortex may similarly be involved in discriminative aspectsof nociception. Yet, the insula has been viewed as a limbicsensory cortex, based on its visceral afferent inputs and itsinterconnections with amygdala, hypothalamus, cingu-late, and brainstem homeostatic regions (Allen et al., 1991;Yasui et al., 1991; Oppenheimer et al., 1992; Augustine,1996; Clasca et al., 1997). Thus, it seems at this pointprospective to consider that the insula may actually beinvolved with both teloreceptive and enteroceptive aspectsof thermoreception. Activity from both the thermosensoryand the gustatory insular pathways may converge andbecome integrated with olfactory information at the nextstation in area 13 of the orbital cortex (Carmichael andPrice, 1995), in accordance with the suggestion of Carmi-chael and Price (1996) that this area is involved in feedingand foraging behavior, and this site may be more involvedin contextual evaluation of the thermosensory informa-tion.

In summary, we have identified a specialized trigemino-thalamic thermosensory pathway in the owl monkey. Thispathway could provide the basis for an enhanced, discrimi-native sensitivity to the temperature of items at or nearthe nose, such as the scent markings used by this noctur-nal primate for navigation while foraging in pitch dark-ness. This specialized substrate offers corroborative sup-port for prior evidence that thermoreceptive neurons inlamina I are anatomically and physiologically distinct. Theprobable cortical terminus of this pathway in the insulamay be involved in both enteroceptive (homeostatic) andteloreceptive (discriminative) aspects of thermoreception.

ACKNOWLEDGMENTS

We thank Ludmila Mackerlova, Maribeth Tatum, Eliza-beth O’Campo, and Kark Krout for technical assistance.


We are grateful to Dr. Vania Apkarian (Syracuse) and Dr.Jon Kaas (Nashville) for supplying material from squirrelmonkey and bushbaby for comparative histologic analyses.

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A distinct thermoreceptive subregion of lamina I in nucleus caudalis of the owl monkey

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