Photoreceptors and CSF-Contacting Neurons in the Pineal Organ of a Teleost Fish Have Direct Axonal...

8/10/2019 Photoreceptors and CSF-Contacting Neurons in the Pineal Organ of a Teleost Fish Have Direct Axonal Connections

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The Journal of Neuroscience, April 1987, 7(4): 987-995

Photoreceptors and CSF-Contacting Neurons in the Pineal Organ of aTeleost Fish Have Direct Axonal Connections with the Brain: AnHRP-Electron-Microscopic Study

Peter EkstrGm

Department of Zoology, University of Lund, Lund, Sweden, and Department of Anatomy and Cytobiology, University ofGiessen, Giessen, Federal Republic of Germany

Neural signals transmitted from the pineal organ to the brainin cold-blooded vertebrates provide information about am-bient illumination, information of importance for the syn-chronization of act ivi ty rhythms with the light-dark cycle.

The ultrastructure of intrapineal projection neurons (pinealganglion cells) was studied after retrograde filling withHRP through their cut axons. The dominating neuronal typeis a small bipolar cell. I t is present in largest numbers in thepineal stalk. This cell type displays several morphologicalfeatures characteristic of cerebrospinal fluid (CSF)-con-tatting neurons. An apical dendritelike process extends to-ward the central lumen of the pineal organ. This dendriticprocess contains numerous mitochondria, it may have sev-eral fine branches, and it may possess a ciliumlike struc-ture. An axon emerges from the basal pole of the neuronand joins the pineal tract. This CSF-contacting neuron ispostsynaptic to photoreceptor basal pedicles with ribbon-type synapses. Such synapses may occur on the neuronalsoma but are mostly observed on small, basally located pro-cesses in the vicinity of the axon. There is a significantsimilarity between this cell type and the bipolar cells bear-ing a Landolts club in the retina. In the rostra1 part of thepineal end-vesicle , several large photoreceptors were la-beled. These photoreceptors may, consequently, have ax-ons more than 1 mm long. An intriguing possibility is thatthis previously unknown vertebrate photoreceptor type con-veys graded potentials over long distances.

The pineal organ of poikilothermic vertebrates s a photosensorystructure (Meissl and Dodt, 198 1). The pineal photoreceptors,which for comparative anatomical reasons have been termed

conelike Eakin and Westfall, 196 1; Oksche, 197 ), respond witha graded hyperpolarization to light stimulations of all wave-lengths (Tabata et al., 1975; Pu and Dowling, 1981; Morita etal., 1985; EkstrGm and Meissl, 1986; Meissl and EkstrGm, 1986).The so-called ganglion cells, i.e., the intrapineal neurons thatproject into the brain via the pineal tract (references n Ekstrom,1984; Ekstriim and van Veen, 1984) are spontaneously active

Received Mar. 22, 1986; revised June 25, 1986; accepted Aug. 8, 1986.This study was supported by the Alexander van Humboldt-foundation, Bonn,

F.R.G., and by the Swedish N atural Science Research Council (Grant B8554-100). The support of Prof. A. Oksche and Dr. H.-W. Korf, Giessen, during theexperiments is gratefully acknowledged. I would like to thank Ms. R. Wall& forher exuert technical assistance. and Ms. I. Norline for ohotoeraohic aid.

Co&espondence should be ahdressed to Peter E&r&, Dep%nent ofZoology,University of Lund, H elgonavsgen 3, S-223 62 Lund, Sw eden.Copyright 0 1987 Society for Neuro science 0270-6474/87/04098 7-09 02.00/O

in the dark. Light stimulation of all wavelengths causes nhi-bition followed by Off responses t stimulus offset. These arethe achromatic or luminance units (Meissl and Dodt, 198 1). Aminor proportion of the ganglion cells, the chromatic units, are

inhibited by short-wavelength exposure and are excited by sub-sequent medium- or long-wavelength exposure Meissl and Dodt,1981).

In some teleosts, the area of the skull covering the pinealorgan is specialized, the pineal window. This window seemsto facilitate light penetration to the organ (Meissl and Dodt,198 1). In any case, t does not provide any means or focusingof the incident light, and, in view of the irregular organizationof photoreceptors n the pineal epithelium, which is often strong-ly convoluted (Vollrath, 198 ), it seems nlikely that anythingbut diffuse ight reaches he pineal organ. Thus, the pineal organmay transmit information about ambient light intensity and,possibly, also about spectral composition. This information isimportant for the regulation of cyclic physiological parameters,which are coordinated by the daily light-dark changes. Theneural signals ransmitting this information to the brain areprobably unobscured by signals elated to contrast and move-ment.

In the brain, however, neural information from the pinealorgan s obviously directly integrated with signals rom the ret-ina. Experimental studies of the central projections of pinealneurons have revealed axonal connections with the habenularnuclei, the periventricular hypothalamus and tegmentum, thedorsal thalamus, and the pretectal area (EkstrBm, 1984, 1985;Ekstrijm and van Veen, 1984). The latter areas coincide withthose receiving retinofugal innervation (Ekstrom, 1984).

In spite of the simple light responses f the pineal organ,

the underlying neural circuitry has never been analyzed. Thepineal organ of the rainbow trout is a favorable model to studythe underlying interactions of photoreceptors and neurons. Itencompasses chromatic units and a small number of chromaticunits (Dodt, 1963; Morita, 1966). This is the first part of a seriesof investigations attempting to analyze the synaptic circuitry ofa simple vertebrate photoreceptor organ, by means of ultra-structural analysis of neural elements hat have been dentifiedby their central connections (present study), and of electro-physiologically characterized single units (P. Ekstrijm and H.Meissl, unpublished observations).

Materials and Methods

Twenty ainbow rout,Salmo gairdneri

Richardson, 0-20 cm n length,were obtained rom a local hatchery and kept as specified n an earlier


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988 Ek strCim - Pineal Photoreceptors

study (Ekstrom and Korf, 1985). For the experiments, the fishes weredecapitated and the pineal area was exposed. A crystal of HRP (Sigma,grade VI) was dissolved in the cerebrospinal fluid (CSF) surroundingthe exposed pineal s talk, which was then cut with microscissors. ExcessHRP was rinsed away with tissue culture medium (Dulbeccos modifiedEagle Medium, DMEM; Ekstrom, 1985) after 1 min. The pineal organwas excised and placed in fresh DMEM. Care was taken not to touchthe pineal organ, which was lifted by the surrounding dorsal sac tissue.

The excised pineal organs were incubated in DMEM overnight at 4C(for details, see Ekstriim, 1985; Ekstrdm and Korf, 1985).As controls, pineal organs were excised and incubated overnight with-

out application of HRP to the lesioned stalk (2 specimens) or incubatedovernight in DMEM to which HRP had been added-ca. 10 crystals to4 specimens. (One of these was mechanically injured ca. 300 pm fromthe lesion site.) Further, the handling and incubation of the pineal organsat 4C ensure a minimal pinocytotic uptake from the pineal lumen(references in Ekstriim, 1985).

After incubation, the pineal organs were fixed in 2.5% glutaraldehydein 0.1 M Sorensen phosphate buffer (pH 7.2) for 1 hr, subsequentlyrinsed in several changes of phosphate buffer, and transferred to 0.1MTris-HCl buffer (pH 7.2). The HRP was visualized as follows: (1) 60min preincubation in 0.025% 3,3-diaminobenzidine tetrahydrochloride(DAB) in 0.1M Tris-HCl bufferCDH 7.2) containine. 0.01 M imidazole(Ekstrom, 1985); (2) 45 min incubation in freshly made preincubationmedium, containing 0.015% hydrogen peroxide; (3) 60 min incubationin fresh medium as described in (2). All incubations were performed atroom temperature, in the dark. After a thorough rinse in Tris-HCl buffer,the specimens were transferred to 0.1M Sorensen phosphate buffer andpostfixed in 2% 0~0, in 0.1M Sorensen buffer fo r 1 hr at 4C. Followinga thorough rinse in phosphate buffer, the specimens were dehydratedin graded alcohol series, and transferred via propylene oxide to sof tAraldite.

Serial 5 pm sections of the pineal organs were mounted on precleanedglass slides, sprayed with a thin silicon film (Baysilon, Bayer) to facilita tereembedding, and stained with toluidine blue/Azure II (Ekstrom et al.,1983). After light-microscopical analysis , the 5 pm sections werereembedded in sof t Araldite. Thin to ultrathin (gray to gold) serialsections were collected on single slot or barred copper grids, poststainedwith uranyl acetate and lead citrate (EkstrBm et al., 1983), and examinedin a Zeiss EM 10 electron microscope.

ResultsAfter application of HRP to the severed pineal tract and over-night incubation of the explanted pineal organs, neural elementsmassively abeled with HRP-reaction product were observed nthe pineal stalk and end-vesicle (for a detailed light-microscop-ical analysis, see Ekstrom and Korf, 1985). The control exper-iments support the interpretation that these neural elementshave been abeled by retrograde diffusion of HRP through theiraxons. Pineal organs hat were incubated in DMEM overnightin the absence of HRP never contained any labeled elements,ruling out the possibility of the presence of cells with a highendogenous eroxidase activity. Pineal organs hat were incu-bated in the presence of HRP likewise never contained mas-

sively labeled elements, except in the stalk in the immediatevicinity of the lesion site (these being etrogradely filled via theiraxons) or in the immediate vicinity of the injured basal amina(1 specimen). After incubation with HRP in the incubation me-dium, the pineal lumen always contained HRP-reaction prod-uct. This was never the case after application of HRP to thepineal stalk and was never observed n the specimens nalyzedbelow.

In the pineal stalk, most labeled neurons were oriented withtheir long axes more or less adially. Somatal shapes aried fromround to elongate (Figs. l-4). In most neurons, only a thincytoplasmic sheet surrounded the nucleus, which almost filledthe soma Fig. 1). From the apical pole of the soma, a constrictedmyoidlike process Figs. l-3) extended oward the central lumenof the pineal stalk. The shape of this apical process was variedfrom cell to cell: In some cases, t was relatively long and thin;in others, it was bulbous (Figs. 1, 2). It often possessed cil-iumlike structure (Fig. 4). Because f the heavy HRP filling ofthese neurons, the cytoplasm was too dense or ciliary basalrootlets or microtubuli to be discerned.

The apical process, with or without ciliumlike appendages,seemed o lie in contact with widenings of the extracellular space(Fig. 5) which sometimes could be observed to be continuouswith the pineal lumen (Fig. 6). It may be assumed hat they aregenerally in contact with the pineal lumen for the followingreasons. The pineal epithelium seems o be divided into anapical and a basal portion by a junctional complex, similar tothe external unctional complex (external limiting membrane)

of the retina. This junctional complex connects photoreceptorcells and supporting cells, so that the apical portion-in the caseof photoreceptors, the inner and outer segments-is in contactwith the pineal lumen, while the basal portion-the soma andbasal pedicles-is in contact with the perivascular space Omuraet al., 1985). In the present study, the apical processes f theHRP-labeled cells were oriented toward the pineal lumen andwere always observed n the vicinity of photoreceptor outer orinner segments, r nearby 9 x 2 + 2 cilia arising from the apicalpoles of supporting cells.

The apical process nvariably contained numerous mito-chondria and, in a few cases, ynaptic ribbons and presynapticdensities (Fig. 7). Some neurons had an apical process withseveral thin branches. Such branches were always too thin tocontain any mitochondria (Figs. 5, 8). They were never post-synaptic to any receptor or neural structure.

Apical processes, ith numerous mitochondria, were n a fewcases associated with irregular whorls of membrane lamellae.The cytoplasmic side of these amellae contained HRP-reactionproduct.

The basal pole of these abeled neurons gave rise to a processdirected more or less oward the peripheral basal amina of thepineal stalk. This basal process, which could give rise to severalshorter branches and evaginations, seemed o represent a smalldendrite formation. From its proximal portion, the axon emergedand joined other centrifugally coursing axons, forming part ofthe pineal tract. The shorter branches and evaginations were he

most commonly observed postsynaptic sites or the basal ped-icles of the photoreceptor cells Figs. 9, 10). Photoreceptors werealso observed to terminate directly on neural somata, often inassociation with small cytoplasmic evaginations (Fig. 11).

In the pineal end-vesicle, HRP-labeled elements were essen-tially similar to those observed in the pineal stalk [except forthe large neurons n the rostra1 pineal ganglion-see Ekstromand Korf ( 198 5) which will be analyzed in a following paper].

Figure I, Two HRP-filled neurons in the pineal stalk. Note the long apical process (asterisk) and its myoidlike part (M). N, nucleus. Scale ba1.0 pm. Figure 2, HRP-filled neuron with a bulbous apical process (asterisk) with a short myoidlike portion (M). Note the basal process (opearrow) from which the axon emerges. N, nucleus. Scale bar, 1.0 Wm. Figure 3, HRP-filled neuron with a large apical process (asterisk) filled witmitochondxia. M, myoidlike structure; N, nucleus. Scale bar, 1.0 pm. Figure 4, The long and slender apical process (asterisk) of an HRP-filledneuron. Note the ciliumlike structure (C). Scale bar, 1 O pm.


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The Journal of Neuroscience, April 1987, i(4) 989


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990 Ekst rijm Pineal Photoreceptors

Figure 5, Apical process with microvillous protrusions (curved arrows). Note the cilium (C) with basal rootlet (open arrow) from an unstaincell. Scale bar, 0.5 pm. Figure 6, Apical process (asterisk) o f an HRP-labeled neuron lies in direct contact with the pineal lumen (star) . The pinealumen is an extension of the third ventricle. Note the lamellar stacks of photoreceptor outer segments (05) near the labeled neuron. Scale bar, 0.5pm. Figure 7, Apical process with a synaptic ribbon (arrowhead) and a possible presynaptic density(arrow). Scale ar, 0.5 pm.

However, in addition, very large photoreceptor cells were ret-rogradely labeled with HRP. A photoreceptor soma typicallycontained a large nucleus and was connected, via a well-devel-oped myoid zone, with a arge ellipsoid. Each ellipsoid containedlarge numbers of mitochondria and gave rise to a modifiedcilium with membrane lamellae-photoreceptor outer seg-ment-with surrounding microvilli (Figs. 12, 13). Up to 40lamellae were observed o originate in 1 modified cilium. Thesephotoreceptors were not observed o possess ither pre- or post-synaptic specializations adjacent to other photoreceptors orneural elements.

The results are summarized n Figure 14.

DiscussionThe present study challenges revailing views of the pineal organon the following points. First, the second-order neurons n thepineal stalk, which receive synaptic input from photoreceptors,are CSF-contacting neurons. This provides us with another typeof CSF-contacting neuron (other than the photoreceptors) hatmay have evolved into the pinealocytes of mammals. Second,CSF-contacting neurons n the pineal organ might indicate thepresence of sensory or secretory elements hat are postsynapticto photoreceptor cells. Third, CSF-contacting neurons in thepineal organ may also be presynaptic to other neural elements.


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The Journal of Neuroscience, April 1987, 7(4) 99

Figure 8, Apical process ith thin, filamentous, rotrusions curved arrows). M, myoidlike part. Scale ar, 1 O pm. Figure 9, Photoreceptor asalpedicle asterisk) with synaptic ibbons arrowheads) adjacent o an HRP-filled neuron. Note he nterdigitation f the neuronal asal rocess star)and he photoreceptor asal edicle small asterisk). Scale ar, 0.5 pm.

Fourth, the presence of CSF-contacting neurons n the pinealstalk that have axonal projections nto the brain is not in accordwith the view of pinealocytes/photoreceptors s ntrapineal CSF-contacting neurons with only local connections. Fifth, the pres-ence of pineal photoreceptors hat have long axonal connectionsindicates the possibility that they may directly influence theactivity in various brain centers by means of graded potentials.

The observation that a large proportion of the second-orderneurons-in fact, the majority-may be a previously unrecog-nized type of CSF-contacting neuron prompts the followingquestion: What is the phylogenetic relationship between theseCSF-contacting neurons and the pinealocytes of mammals? heconcept that the pineal photoreceptors of poikilothermic ver-tebrates have evolved into secretory pinealocytes that are in-directly influenced by light has been supported by a vast numberof comparative histological, histochemical, and ultrastructuralstudies reviewed by Collin and Oksche, 1981). Similarly, theconcept that pinealocytes (including the photosensory phylo-genetic predecessors) re special types of CSF-contacting neu-rons is supported by numerous comparative studies reviewedby Vigh-Teichmann and Vigh, 1983, 1985). Accordingly, thetypical photoreceptors and all transitory stages oward secretorypinealocytes have received much more attention than other cel-

lular elements of the poikilotherm pineal organ. While pineal-ocytes typically make up for about 80 of the cellular elementsof the mammalian pineal (Vollrath, 198 ), photoreceptors con-stitute a much smaller part of the total cell number in, at least,the pineal organ of the rainbow trout. Thus, the CSF-contactingneurons n the pineal stalk might also be considered orerunnersof mammalian pinealocytes. In this context, it is interesting tonote the strong similarity between he CSF-contacting neuronswith bulbous apical processes nd pinealocyte type I cells ofPtvet and Collin (1976), which were considered a rudimentaryphotoreceptor.

Two major objections might be raised. First, it is well estab-lished that there are several histochemical similarities betweenmammalian pinealocytes and pineal photoreceptors of poikilo-thermic vertebrates. They contain indolamines review by Moll-er and van Veen, 198 ), and they contain proteins that shareimmunological properties with the retinal photoreceptor-spe-cific proteins cu-transducin van Veen et al., 1986b), S-antigen(Mirshahiet al., 1984; Korfetal., 1986b; vanveenetal., 1986a),and even opsin (Korf et al., 1985). Second, they have beencharacterized by axonal processes erminating within the pinealorgan (Collin and Oksche, 198 1). However, we also have in-direct evidence that the CSF-contacting neurons n the pineal


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The Journal of Neuroscience, April 1987, i(4) 9

Figure 14. Schematic representation of synaptic connections of neural elements that were retrogradely labeled with HRP (stippled cells). I,Photoreceptor in the rostra1 part of the pineal organ, with direct axonal connections with the brain.This type of photoreceptor as not observedto be either pre- or postsynaptic to other neural elements. 2, Neuron in the rostra1 part of the pineal organ. In this part, an accumulation ofprojection neurons is found (EkstrGm and Korf, 1985). The synaptic connections of these neurons are still under s tudy (P. Ekstrbm, unpublishedobservations), but &is assumed that they receive mainly direct input from photoreceptor cells (open arrowheads). 3, CSF-contacting neuron in thepineal stalk, eceiving xodendritic arrowhead) or axosomatic open arrow) inputs rom photoreceptor ells. , CSF-contacting euron n the pinealstalk, receiving axodendritic nput (arrowhead) from photoreceptor ells. This cell type was sometimes bserved o be presynaptic o unknownelements (. 5, Photoreceptors in the rostral part of the pineal organ, with basal synapses on projection neurons. 6 and 7, Photoreceptors in thepineal stalk, where they may synapse on basal dendrites (6) or on neuronal somata (7). Large arrows denote xonal connections ith the brain.

stalk may share histochemical properties with the mammalianpinealocyte, since he majority of the cells n the pineal stalk of

the rainbow trout are immunoreactive with antibodies against5-HT, S-antigen, or-transducin, or opsin (P. Ekstrom, R. G.Foster, H. W. Korf, and J. J. Schalken, unpublished observa-tions). Furthermore, we have recently been able to demonstratethat S-antigen-immunoreactive pinealocytes n the golden ham-ster pineal organ display axonal processes hat can be tracedinto the habenular nuclei and the dorsal thalamus (Korf et al.,1986a). We observed similar projections of S-antigen-immu-noreactive processes rom the pineal organ into the habenularnuclei in the rainbow trout, and in a cyprinid fish, Phoxinusphoxinus (P. Ekstriim, R. G. Foster, H. W. Korf, and J. J.Schalken, unpublished observations).

It has ong been a matter of debate whether the dendritelike,ciliated, intraventricular protrusions of CSF-contacting neurons

represent sensory or secretory specializations, although mostauthors favor the sensory functions (Leonhardt, 1980; Vigh-Teichmann and Vigh, 1983). The principal similarity betweenthe CSF-contacting neurons n the pineal stalk and the bipolarcells of the retina that possess Landolts club should be stressed.Vigh et al. (1983) were the first to point out the general similarity

t

between these retinal neurons and CSF-contacting neurons. Itis possible hat Landolt clubs, as well as pineal CSF-contacting

neurons, might have a sensory unction, analyzing some aspectof the cerebrospinal luid. The CSF-contacting neurons n thepineal stalk are, as he bipolars of the retina, second-order neu-rons to photoreceptor cells. We have, unfortunately, no infor-mation regarding their mode of signal transmission, whethergraded potentials or action potentials.

The presence of synaptic ribbons and presynaptic densitiesin the apical processes f some CSF-contacting neurons s in-triguing. So far, I have been unable to make a conclusive iden-tification of the postsynaptic elements, but these presynapticspecializations could be an indication of feedback mechanismsdirectly influencing the pineal photoreceptors. Putative feedbacksynapses nto photoreceptor cells have been described n thetrout pineal organ by Omura and Ali (1980).

According to the classical view, the mammalian pinealocytepossesses nly short axonlikc processes hat display terminalformations, either on other pinealocytes (by synaptic ribboncontacts), on the basal amina of the perivascular space, or inthe perivascular space e.g., Wartenberg, 1968; Vigh et al., 1975).Similarly, the pineal photoreceptor of poikilotherms has been

.

Figure 10, Photoreceptor basal pedicle with synaptic ribbons (arrowheads) in contact with a neuronal basal process (star). Scale bar, 0.5 pm.Figure 11, Cross-sectioned synaptic ribbon (arrowhead) adjacent to an HRP-labeled neuronal soma. N, nucleus. Scale bar, 0.1 pm. Figure 12, Theellipsoid of an HRP-filled photoreceptor cell in the rostra1 part of the pineal organ. A modified cilium (C) gives rise to numerous membranelamellae, forming the photoreceptor outer segment (open arrow). Note the microvillous protrusions (curved arrows). m, Mitochondria. Scale bar,1.0 pm. Figure 13, Same HRP-labeled photoreceptor as shown in Figure 12. M, myoid region; m, mitochondria; N, nucleus; curved arrows,microvillous protrusions; open arrow, photoreceptor outer segment. Note that an adjacent photoreceptor (05) is unstained, showing that the HRP-labeling of certain photoreceptor cells is not due to an unspecific , general, uptake of HRP by the pineal tissue. Scale bar, 1.0 hrn.


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994 Eks trtim * Pineal Photoreceptors

described as possessing short basal pedicles, with terminals eitheron dendrites of intrapineal neurons (with synaptic ribbons) orin the perivascular space (without synaptic specializations). Theformer type should then represent the true sensory photorecep-tor, and the latter a rudimentary photoreceptor type with in-dolaminergic and/or peptidergic secretory functions (review inCollin and Oksche, 198 1). In all cases, pinealocytes and pinealphotoreceptors were regarded as possessing only local, intra-pineal, connections. The presence of HRP-labeled, CSF-con-tatting neurons and photoreceptors with direct axonal projec-tions to the brain indicates the presence of a previouslyunrecognized cell lineage in the pinea l pedigree.

The significance of pineal photoreceptors with long-range pro-jections is enigmatic. Vertebrate photoreceptors are typicallyendowed with short axons and regulate their synaptic outputwith graded hyperpolarizing potentials in response to light(Dowling and Dubin, 1984). The signal decrement of these grad-ed potentials is no problem, since most photoreceptor axons(i.e., the distance the signal must be propagated) are short, inthe range of 10 pm. Propagation of neural signals by action

potentials, which would overcome the difficulties presented bythe long distances covered by the pineal photoreceptor axonsunder consideration, is not known from vertebrate photorecep-tors. However, evidence is accumulating that graded potentialsare utilized in neural signaling over relatively long distances.

Nondecremental propagation of graded potentials seems tooccur over distances of 400-500 Frn in the thin (0.5 pm di-ameter), passively conducting axon of goldfish horizontal cells(Weiler and Zettler, 1979; Sakuranaga and Naka, 1985). Thisis not compatible with cable theory. Rather, Nelson et al. (1975)calculated that this thin interconnecting axon should functionas an isolation of the dendritic from the axonal arborizations.Fovea1 cones in the primate retina have axons up to 600 pmlong (Polyak, 1941), and amacrine cells may have dendriticarbors where graded potentials convey signals over distances inthe millimeter range (Vallerga and Deplano, 1984). Thus, thereis no general evidence against the proposal that pineal photo-receptors, which have been retrogradely labe led by HRP via thepineal tract, could be able to transmit photic information to thebrain by means of passively conducted graded potentials.

Photoreceptors in invertebrates may possess very long axons(ca. 1O-80 mm), along which graded potentials are propagatedwith litt le, if any, decrement (Shaw, 1972; Hudspeth et al., 1977;Behrens and Fahy, 198 1). Similarly , second-order visual inter-neurons may transmit graded potentials over relatively longdistances (Wilson, 1978). These axons are relatively thick, withdiameters ranging between 10 and 25 pm. On the contrary,

pineal photoreceptor axons are only ca. 1 pm thick, and cabletheory would predict that a passively conducted graded potentialshould attenuate to microvolt levels very rapidly.

However, graded potentials in the micro- and millivolt rangeshave been reported to be sufficient to elicit postsynaptic re-sponses (Fain et al., 1977; Stuart and Oertel, 1978). Such smallpotentials may be discriminated from membrane noise by elec-trical coupl ing of photoreceptors (Shaw, 1972; Lamb and Simon,1976). There are indications that electrical coupling exists be-tween pineal photoreceptors in the rainbow trout. Intracellularinjections of Lucifer yellow revealed extensive dye-coupling ofpineal photoreceptors (Ekstrom and Meissl, 1986).

Propagation of graded potentials over long distances could bemade possible by a higher specific membrane resistivity in the

outer membrane of the axon than in the soma (Hudspeth et al.,1977; Wilson, 1978). Unfortunately, it is not possible to measurethe specific membrane resistivity for the ca. 1 Km-thick pinealphotoreceptor axons. However, assuming membrane resistivi-ties in the range of those calculated by Hudspeth et al. (1977)or Wilson (1978), the attenuation of signals along the pinealtract should not be so great that postsynaptic responses couldnot still be elicited, at least theoretically.

Before we can speculate about what type of information isconveyed by the directly projecting photoreceptors and CSF-contacting neurons, we need to know their exact axonal ter-mina tion sites and what type(s) of terminals they make. Theymay transmit neural signals as retinal photoreceptors, i.e., byrelease of glutamate or a related substance in the dark (Brandonand Lam, 1983) but they may also act by release of indolamines,neuropeptides, or other substances. Unfortunately, our attemptsto identify the neurotransmitters used by the centrally projectingelements of the pineal organ have been fruitless (Ekstrom andKorf, 1986a, b). Identification of the transmitter substances andultrastructural characterization of the terminal formations of

the pinealofugal axons are urgently needed to resolve this issue.

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Ekstriim, P., and H.-W. Korf (1986b) Substance P-like immunoreac-tive neurons in the photosensory pineal organ of the rainbow trout,Salmo gairdneri Richardson (Teleostei). Cell Tissue Res. 246: 359-364.

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Photoreceptors and CSF-Contacting Neurons in the Pineal Organ of a Teleost Fish Have Direct Axonal...

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