Developmental Brain Research, 24 (1986) 261 - 270 261 Elsevier

BRD 50330

The Functional Development of Descending Inhibitory Pathways in the Dorsolateral Funiculus of the Newborn Rat Spinal Cord

MARIA FITZGERALD and MARTIN KOLTZENBURG

Cerebral Functions Group, Department of Anatomy, University College London, London WC1E 6BT ( U. K.)

(Accepted July 16th, 1985)

Key words: spinal cord - - development - - descending inhibition - - dorsal horn - - brainstem

The postnatal development of descending inhibition in the spinal cord has been studied in the rat. Electrophysiological recordings were made in neonatal rat pups of the activity in single lumbar dorsal horn cells evoked by stimulation of the skin of the hindlimb. De- scending inhibition was tested by observing the effect of stimulation of the dorsolateral funiculus (DLF) at thoracic level on the dorsal horn cell responses. In adults the DLF is known to contain descending axons from the brainstem which inhibit dorsal horn cell activity. Such inhibition was always observed in days 22-24 rat pups. At 18 days of age it was present but required higher-intensity stimulation to produce an effect. On day 12 only half the dorsal horn cells tested were inhibited by DLF stimulation and then only weakly. On day 9 no cells were inhibited. Application of horseradish peroxidase to DLF axons in the lumbar cord resulted in retrograde labelling of cells in the medulla, pons and midbrain. The labelling on day 6 was comparable to the adult. The results show that despite the early anatom- ical existence of a descending DLF pathway, there is no functional descending inhibition until days 10-12 of life. It is suggested that this is due to delayed maturation of crucial interneurones in the dorsal horn or to insufficient levels of 5-hydroxytryptamine or other neurochemicals in the descending DLF axon terminals.

INTRODUCTION

The adult spinal cord is under considerable inhibi-

tory control from supraspinal centres 6,8,19.20,42.45. A

particularly well-documented descending system is

the spinal projection from the brainstem travelling in

the dorsolateral funiculus (DLF) 1,11.22. A major

source of these axons is the rostral ventromedial me-

dulla which includes the midline nucleus raphe mag-

nus and the adjacent reticular formation J,27,29,33.

These descending fibres mediate a strong inhibitory

influence on spinal cord neurons, in particular on

their responses to nociceptive or C-fibre afferent in-

put 13,1s,48. The pathway has been proposed to be part

of an endogenous analgesic system capable of con-

trolling the afferent pain input at segmental level and

thus the information that arrives at the higher centres

of the brain ~2. Descending inhibitory controls are

also likely to have more general importance for sen-

sory transmission, controlling the i npu t -ou tpu t rela-

tions at the spinal levelg, 44.

The aspect of supraspinal control studied here is its

development and maturat ion in the neonate. Pre-

vious work has led to the proposal that the spinal cord

is relatively free from descending control in newborn

animals and that the motor behaviour of neonatal

cats and rats is a result of a series of segmental re-

flexes uninfluenced by higher centres in the brain2,5,

10,17,38,46. This idea has come from reflex studies and

from behavioural experiments following lesions to

the CNS. Anatomical studies have shown that de-

scending pathways mature both before and after

birth 7,28,32.41 but until now, there has been no parallel

neurophysiological studies necessary to form any

conclusions about the functional development of

these pathways.

The experiments reported here are the first study

of the physiological development of a descending

pathway; that from the brainstem to the spinal cord

via the DLF. In vivo electrophysiological recording

Correspondence: M. Fitzgerald, Cerebral Functions Group, Department of Anatomy, University College London, Gower Street, London WCIE 6BT, U.K.

0165-3806/86/$03.50 © 1986 Elsevier Science Publishers B.V. (Biomedical Division)

7 7 _ { } - -

of dorsal horn cells in newborn rats has recently been

achieved in our laboratory and the postnatal matura-

tion of peripheral afferent input anti receptive field

properties in the dorsal horn have been described I5.

It was therefore possible to investigate the devel-

opmenl ot descendintz influences from the brain on

these cells by applying controlled electrical stimula-

tion to the DLF and observing changes in dorsal horn

cell responses. As a parallel stud~ we have also invcs-

titzated the state of anatomical developmenl of the

descending DLF axons and their cell bodie~ in the

brainstem.

If the spinal cord is free of major descending influ-

ences in the immediate postnatal period it will be pos-

sible io assess the functional role of these pathways as

they become active. This will lead to a better under-

standing of both the adult function of descending

pathways and their control of sensory inputs and of

the developmental function where arrival or new in-

puts from the brain may be a crucial step in the matu-

ration of the spinal cord.

MATERIAI,S AND METtIODS

Physiological in vestigauon Wistar rat pups of both sexes aged 9-26 days were

used. Initial experiments were carried out on adult

animals. The rats were anaesthetized with an i.p. in-

jection of urethane (100 mg/kg), given a subcuta-

neous injection of 20 ug atropine sulphate and the

trachea was cannulated. Two laminectomies were

performed, one at upper thoracic level and one to ex-

pose the lumbar enlargement. The cord was sec-

tioned rostrally at the thoracic level to isolate it from

higner centres. The dura was removed and the cord

covered with warm mineral oil. The animal was then

paralysed with gallamine trlethiodide fFlaxedil: 20

mg/kg) and artificially ventilated. The heart rate was

monitored throughout the exper iment and the rectal

temperature kept constant at 36-37 °C with a heat-

ing pad.

When the surgery was complete a tungsten-m-

glass stimulating electrode {211-30/~m bare tip) was

inserted into the dorsolateral fasciculus in the upper

thoracic cord below the level of section. Caudal to

the stimulating electrode at lower thoracic level, the

dorsal columns were crushed with watchmakers for-

ceps. The position of the stimulation electrode was

marked with a lesion at the end of the experiment by

applying a current of 100 uA for 10 ~ This lesion and

the dorsal column lesion were verified histologically

by removing the cord at the end of the exper iment

and fixing it in 10C4 formol saline, berial 50-urn-thick

frozen sections were cut and staincd with cresvl fasl

violet, The lesions were then cleartv visible as is illus-

trated in Fig. 1B

A

Iool" ov I

!

t , i

lOms

\

( '! / I J 1 o

\\\ ..__i II \ i \ I

J

i i

250 um

Fig. 1, A: the evoked volley recorded in the lumbar dorsal horn by stimulation of the dorsal columns in the upper thoracic cord at ltJ0 uA. 2(X)/~s. Upper trace: a stimulus applied caudal to a lesion of the dorsal columns. Lower trace: stimulus applied ros- tral to the lesion, sa, stimulus artefact. B: camera tucida draw- ing of a 5t)-~m section through the thoracic cord of a day-17 rat pup at the end of an experiment. Nissl staining showed up the dorsal column lesion (cross-hatched) and the site of DLF stimu- lation (doubly cross-hatched} clearly.

Recordings of dorsal horn cells were made in the

medial L4 cord with a tungsten microelectrode. Sin- gle units were isolated while the skin of the foot and lower limb was manually stimulated. The receptive

field (RF) of units encountered during the search were tested for their response to brush, touch and pinch. Only wide dynamic range neurons responding to both light brush and intense pinch were used in this investigation and all the cells were recorded in the mid dorsal horn, laminae I I I - V as measured from the recording depth. After mapping the RF, pin elec- trodes were placed in the centre of the cells' cuta- neous RF in the skin and single pulses of 0.5 Hz were applied, slowly increasing from 0 to 10 mA, 500/~s

until a clear maximal response was elicited. The re- sponses were recorded as raster dot displays on an os-

cilloscope. After 30 s of control peripheral stimula- tion, the effect of DLF stimulation was tested. Trains (200-1800 ms) were applied to the ipsilateral DLF at 5-100 Hz, 200/~s triggered in synchrony with the pe- ripheral stimulation. The intensity was raised from 0 to 300 ~A and the effect on the peripherally evoked responses recorded. Judged by the data of Ranck39 myelinated axons can be activated up to 1 mm away with 250/~A; unmyelinated ones at about half that distance 39. At our normal stimulus intensity, 50/~A, it

is reasonable to assume that the stimulation was re- stricted to the dorsolateral quadrant. The height of the action potentials evoked by skin stimulation was

monitored during DLF stimulation. Inhibitions re- ported here were true disappearances of spikes and

not a decrease of spike height. The effectiveness of the dorsal column lesions was also checked in each experiment. A complete lesion resulted in a total ab- sence of the evoked potential in the lumbar cord when the dorsal columns were stimulated as illus- trated in Fig. 1A, where a comparison can be made between the stimulus applied rostral (bottom trace) and caudal (top trace) to the dorsal column lesion.

Anatomical investigation Two rat pups aged 5 days, two aged 24 days and

one adult rat were anaesthetized with i.p. pentobar- bitone (20 mg/kg for the pups, 40 mg/kg for the adult). Under sterile conditions, a small length of the lumbar cord was exposed by removal of only one or two bony vertebral arches (L2 and L3). A slit was made in the dura and a small crystal of horseradish

263

peroxidase (HRP) (Type VI, Sigma) was crushed

into the dorsolateral quadrant on one side with watchmakers forceps. The wound was then closed in two layers, muscle and skin, and the rats were allow- ed to regain consciousness. The pups were returned to their litters and recovered uneventfully. After 48 h

(72 for the adult) the rats were sacrificed and per- fused transcardially with 1.25% glutaraldehyde and

1.0% paraformaldehyde. The cord and brain were then removed and transverse frozen 50-/~m sections were cut on a cryostat. The sections were reacted with tetramethylbenzidine and lightly counterstained with neutral red 35. The site of HRP application in the

cord was identified and then the labelling of cells in the brainstem carefully studied. Serial sections from medullary to midbrain level were examined under the microscope and stained cells marked on brain-

stem maps drawn from the atlas of Paxinos and Wat- son 37 using the camera lucida attachment.

RESULTS

Physiological study Adults. The results on adult rats were in agreement

with previous studies~a,as,4L Neurons in the deep lam-

inae of the dorsal horn are readily inhibited by DLF

stimulation. The long latency response evoked by

100

~5C

\ \ \

\

J~ adult 24 20 16 12

~r

8 days

Fig. 2. A graph showing the percentage of dorsal horn cells whose C fibre-evoked responses were inhibited by DLF stimu- lation at different ages. The value on day 23 is an average of days 22-24, on day 20 an average of days 19-21 and day 16 an average of days 15-18. The values for days 12, 10 and 9 were all recorded at those particular ages. Numbers of cells are given in the text.

264

stimulation of peripheral C fibres was particularly af-

fected, although in some cases the A fibre-evoked re-

sponse was also reduced. Fifteen units were exam-

ined. All had an early A-evoked response at 3-10 ms

latency and a late C-evoked response at 95-18(I ms

latency. The C-evoked response was inhibited by

DLF stimulation in 13 out of the 15 units (Figs. 2 and

3AI and in 3 of these the A response was also re-

duced, but the inhibitory influences on A responses

were not systematically studied. The threshold for in-

hibition of C-evoked responses was often as low as

2(t uA and raising the intensity to 50-100 uA totally

eradicated the C-evoked response

The efficiency of the inhibitory effect depended on

the frequency of the DLF stimulation and on the

length of the stimulus train, again confirming pre-

vious reports~. The aim here was Io find stimulus

parameters that produced opumum inhibition in

adults while being suitable for use iu immature CNS.

so that comparisons could be made between age

groups. Very-high-frequency stimulation as has been

used in previous adult studies~3.4~ cannot be followed

in neonatal CNS. Frequencies or 50 Hz and 20 Hz

were found here to be more than adequate in adults.

short trains of 200 ms at 100 u A producing inhibitions

of 50-500 ms and 20-100 ms respectively. Increasing

A adult C day 12

I

B day 17 D 10

50 m s e c

Fig. 3. Raster dot displays of dorsal horn cell responses to peripheral skin st imulation and the effect of D L F st imulat ion on those re- sponses Peripheral skin s t imulat ion (5 m A . 500/Ls) is applied regularly once every 2 s f rom the bot tom of thedisp lay to the top and each ensuing sweep is represented horizontally Each dot is a spike. A clear early and late band of do ts occurs with each s t imulus reprc- senting A- and C-evoked activity. At various intervals DL F stimulation is applied, marked with arrows (~---) and by thc very regular bands of dots which are due to the s t imulus artifacts. A a dorsal horn unit from an adult rat. The C-evoked response is inhibited more effectively by 50 Hz (top and bot tom trains~ than by 20 Hz (middle train l DLF stimulation (50/~A-) and even more effectively by a longer 50-Hz train. B: a unit from a day-17 rat pup. Again 50-Hz (bottom train; DLF s t imulat ioninhibi ts the ( \ e v o k e d response more effectively than 20 Hz [top train) C: a unit from a day-12 pup. 20-Hz (top) DLF s t imulat ion (100 p A ) is ineIfect iveand 5fl Hz (bot tom) produces slight inhibition. D: a unit from a day-10 pup. 20-Hz. 200 uA DLF stimulation (top) has no effect, nor does a 50-Hz. 200-~A train (bottom).

the length of the train produced more effective inhi-

bition (Fig. 3A). Optimum effects were produced by

continuous stimulation for a period of up to 10 s as.

These parameters were therefore used for age com-

parisons and, as described in the next section, were

well suited to the rather weak low-frequency evoked

responses observed in immature cord. The classic ap-

proach to inhibition of condition-test interval meas-

urement was not applicable here. In two units using

the above regime, the inhibition outlasted the stimu-

lation period for several seconds but usually the nor-

mal response returned soon after the end of DLF stimulation. In one cell there were a few seconds of

after-excitation when the DLF stimulation was

turned off. Two cells were not inhibited at all by DLF

stimulation and in one of these raising the intensity to

200 HA produced excitation in the form of an in- creased evoked C response and background dis-

charge.

Immature rats. From postnatal day 19 onwards,

the effects of DLF stimulation closely resemble those

in the adult. Fig. 2 shows that the percentage of inhib-

ited cells on days 22-24 is the same as in the adult

(n = 7). On days 19-21, the percentage is rather less,

but at this stage, the inhibitory effects were compara-

ble to the adult in that 10 s trains of 50 Hz, 100 HA,

200~s totally eradicated the C-evoked response. Ob-

vious differences in the response to DLF stimulation

appeared at and below the age of 18 days.

Firstly, the peripheral afferent input is not fully

mature at this time. This has been described pre- viously L~ and was confirmed here. Whereas in adult

spinalized preparation the cord is excitable and cells

with long-latency C fibre inputs are easy to isolate, in

the young animals, particularly those aged 9-12 days, such responses are much harder to find. The C

fibre-mediated responses, when present, are often

scattered and unreliable as in Fig. 3C, D. It was also noted that receptive fields were larger than in the

adultlL On days 14-18, 12 units were studied. All showed a clear early A response at 3 .5-8.0 ms laten-

cy and a late C response at 60-95 ms. The C-evoked responses in 10 of the units (84%) were inhibited. The effects were somewhat weaker, however, than in

older groups. In only 6 of these 10 units was the inhi- bition complete: in 4 some C-evoked response re- mained on DLF stimulation (Fig. 3B). Furthermore, the intensities of stimulation required for inhibition

265

were greater at 80-200 HA, 50 Hz. On day 10-12,

the inhibitory effect of DLF stimulation was poor.

Ten cells were recorded on day 12. The A response

occurred at 5-15 ms and the late C response at

85-110 ms (conduction velocities being slower in im-

mature rats). Only 5 out of these 10 cells (50%)

showed a reduction of C response on DLF stimula-

tion. The effect was only partial as illustrated in Fig.

3C. On days 9 and 10, 11 cells were recorded. A fi- bre-evoked activity occurred at 4 - 8 ms and C-

evoked activity at 70-120 ms. On day 10 only 1 out of

6 cells (17%) was inhibited by DLF stimulation and

on day 9 none were affected (n = 5). The stimulation

A

sec°L L

1 2 s ~ '

B

' 5 0 msec J

Fig. 4. A: a rate-meter record of the ongoing activity of a dor- sal horn unit recorded in a 9 day old pup. Pinching the skin (connected arrows) of the limb contralateral to the receptive field resulted in inhibition of the activity. B: a raster dot display of the same units' response to electrical skin stimulation (5 mA, 500 fls, 0.5 Hz) of its receptive field. DLF stimulation (100- 200HA; top and bottom) at 20 Hz and 50 Hz (middle) has no in- hibitory effect.

intensities required to evoke inhibition on days

10-12 were higher still than at days 14-18. at

100-200 uA. 50 Hz. Fig. 3C shows the most impres-

sive inhibitory effect seen on da~ 12. illustrating the

fact that it ts not strong at this age and Fig. 3D shows

ineffective DLF stimulation observed on day l(I. The

immaturity of the CNS was taken into account. Low-

er frequencies of 5 and 1~) Hz were tested as well as

20. 50 and 100 Hz in case of frequency following

problems All were tried al different intensities. At

this young age the evoked C responses are scattered

and weak and yet the DLF stimulation is less effec-

tive than in the adult where responses are powerful

and time-locked.

Despite little or no inhibition from DLF stimula-

tion on days 9-10. it would not be true to say that the

dorsal horn cells cannot be subjected to inhibitory in-

fluences at all Fig. 4A illustrates the inhibition of

background firing of a dorsal horn cell in a day-9 pup

caused by pinching the skin of the foot contralateral

to the receptive field The early development of this

local segmental inhibitory interaction has been de-

scribed previously15 The same cell (Fig. 4B) is. how-

ever. totally unaffected by DLF stimulation.

Anatornical study

The lack of physiological responses al days 9 -10

from DLF stimulation led to an anatomical investiga-

tion of the projections from the brainstem to the dor-

sal horn via the DLF in the neonate. Two rat pups

aged 6 days. two aged 22 days and one adult were

used for this study Axons in the DLF were labelled

by crushing HRP into the dorsolateral quadrant on

one side of the lumbar cord and the retrogradely la-

belled projection neurons in the bramstem were

stained in serial sections 24-48 h later. The site of

HRP uptake was later found to include the whole

dorsolateral quadrant .

The results showed little difference between label-

ling in the 3 groups of animals. Fig. 5 shows examples

of typical sections through medulla, pons and mid-

brain in a day-6 and a day-22 pup. Labelled cells were

found throughout these areas. They were clustered

around the midline raphe nuclei and the medial retic-

ular formation in the medulla, ventrolateral reticular

nuclei in the pons and in and around the red nucleus

in the midbrain. No attempt was made to look for

cells further rostrally. The pattern and distribution of

A B

f • . .

' ~ , \ .) / " :,':.: /

'5 (, , /

2 x ~

Fig, 5. Camera tueida drawings ol 4 t}p~cal S0-~tm sections through medulla, pons and midbrain ol a da~-6 IA) and a day- 22 (B~ rat pup The drawings are based on the atla~ of Paxinos and Watson 3~_ ['he black dots represent HRP-labetIcd cell bod- ies as seen oll the sections. The dorsolaterai quadranl ol the lumbar cord on the left side was labelled with HRP 48 h pre- viously

labelling was the same in the day 6 and adult brain

and although detailed counts were not undertaken.

the number of labelled cells also did not seem greatl)

different in the two groups.

DISCUSSION

The postnatal development of supraspinal control

has been studied by investigating the functional and

anatomical maturat ion of descending inhibitory

pathways in the DLF. The efferent projection from

the raphe nuclei and adjacent reticular formation is a

major inhibitory pathway to the spinal cord and trav-

els in the DLFt.~I ..... 8.,3 Earlier investigations re-

ported that 80 -90% of wide dynamic range dorsal

horn neurons were inhibited by raphe stimula-

tion 13.1~,4s. The inhibitory effects were particularly

powerful with high frequencies of s t imulat ion and the

longer the stimulus train the more effective the inhi-

bition. The present findings in adult rats using DLF

stimulation agree well with these previous results

Nevertheless, the effects of DLF stimulation cannot be attributed solely to raphe efferents. There are oth- er fibre tracts of diencephalic and mesencephalic ori- gin in the DLF (see ref. 47) as confirmed here from the HRP labelling. Furthermore, the DLF contains ascending axons from dorsal horn neurons projecting to the brain 34. The spinal cord was sectioned in these

experiments so secondary central effects of stimula- tion can be ruled out, but such dorsal horn cells (mainly located in lamina 134 , and so not observed in these experiments) would be antidromically acti- vated by excitation of their axons. DLF stimulation is not, therefore, highly specific but the aim here was not to scrutinize individual pathways, but rather to investigate descending inhibitory controls in the im- mature cord.

The results show that inhibitory pathways in the DLF only become functional at 10 days after birth and only become well-established by the 3rd week. At the onset, the inhibition is slight and high stimulus intensities are needed to produce an effect. Even at day 18, although the majority of cells are inhibited, stimulus intensities which are capable of eradicating the response in adult rats have only a weak effect. There is evidence that at least part of the descending inhibition is mediated by fast-conducting myelinated fibres which project from the nucleus raphe magnus to the spinal cord 47. Myelination of the DLF is slow and not complete until postnatal day 24 (Koltzen- berg, unpublished observations) and this may ac- count for the higher stimulus intensities required to evoke an effect. It seems reasonable to conclude from these experiments that on day 18 descending DLF inhibitory connections are fully established.

In contrast to the slow physiological development, fibres projecting from the brainstem are apparently present at an early stage. Labelling of the descending axons in the DLF with HRP demonstrated the pres- ence, on day 6, of projection cells in the medulla and other parts of the brainstem, comparable to the adult. The anatomical development of descending pathways in the spinal cord has been studied pre- viously in some detail 7,17,32.41. Leong et al. have dem- onstrated the presence of neurons projecting the spi- nal cord throughout the brainstem at birth zs. Most appeared fully developed; only the trigeminospinal, solitariospinal, tectospinal and cerebellospinal groups needed 10-20 days to reach maturity. Corti-

267

cospinal projections do not develop until after birth, connections being established in the lumbar cord over days 6-9, well after brainstem projections 41.

The slow maturation of descending inhibitory con- trol agrees well with previous observations. Spinal transection has little effect on evoked reflexes and motor behaviour in rats before the age of 21 days 3s.46.

In kittens few differences in flexor or extensor re- flexes were observed after spinalization up to an age of 2-3 weeks 10. Spinal shock does not follow spinali- zation until 3 weeks after birth 46. It was observed in the present experiments that the increase in excita- bility of dorsal horn neurons that follows spinaliza- tion in adult rats, due to removal of descending inhi- bition, did not occur in the immature rat pups.

The lack of descending inhibition in the first 10 days of life must be seen against a background of im- maturity within the dorsal horn itself. Although the termination of peripheral afferents in the dorsal horn is established prenatally and immediately postnatal- ly 16.43 the properties of dorsal horn cells have still not fully matured at day 10. The cells in deeper laminae studied here do not respond to C fibre afferent input until days 7-8 and by the second postnatal week, al- though present, the C fibre-evoked responses are weak and scattered ~5. The cells in neonatal rat cord also have larger receptive fields, higher thresholds and show more tendency to habituate or have af- terdischarges. These properties are, however, far more pronounced immediately after birth and have greatly subsided by the second week of life 15. Never- theless, afferent synaptic connections are still appar- ently relatively weak and the final balance of inhibi- tory and excitatory connections and receptive field organization is not established. One might suppose such responses to be easier to inhibit by activation of descending pathways than the tightly coupled adult input/output relationships, but this is not the case.

What might be the reason for the slow functional development of the descending DLF pathway in con- trast to its early anatomical maturation? There are several possibilities. The projecting fibres may arrive in the lumbar cord from the brainstem at birth but not develop synaptic connections for some time. Such a 'waiting' period has been described for other fibre systems such as the corticospinai tract TM but is un- likely to be the full explanation here since the time lag is usually only 2-3 days and here we are looking

at d iscrepancies of 4 - 1 0 days at least. A n o t h e r possi-

bility is that the descendin~ fibre te rmina ls lack

neuro t ransmi t te r . A b o u t half the raphe neurons pro-

iecting to the adult cord conta in 5 -hydroxyt ryp ta -

mine ( 5 - H T p and there is good ev idence for 5 - H T

being the t ransmi t t e r invo lved in descend ing inhibi-

tion 12.a". Interes t ingly , the m o n o a m i n e d is t r ibut ion

in the cord has been shown to be sparse in the perma-

tal pe r iod and not ma tu re until the 2nd or 3rd pos tna-

tal w e e k 3~, a l though b iochemica l assays indicate that

adult levels of 5 - H T are ach ieved within the first

postnata l week >. Appl ica t ion of 5 - H T to isolated

neona ta l cord 'in vitro" ef fec t ive ly lowered the

th reshold of C fibre te rminals showing that 5 - H T re-

cep tors are funct ional in this period:~. A fur ther pos-

s i n e reason for the fai lure to p roduce descend ing in-

hibi t ion before day 10 may be that the process re-

quires the ac t iva t ion of local i n t e r n e u r o n e s which arc

slow to ma tu re . Bicknel l and B e a P have d e m o n -

s t ra ted that i n t e rneu rons m substant ia ge la t inosa do

not begin to ma tu re until the pos tnata l pe r iod in con-

trast to the dorsal horn p ro j ec t i on neurons which ma-

ture before birth. Perhaps these m t e r n e u r o n s are

crucial for descend ing inhibi t ion. It ~s cer ta inly not

t rue that all inhibi tory in t e rneu rons are react ive at

this t ime. Con t ra l a t e ra l inhibi t ion ~4 was clearly ob-

se rved in the dorsal horn and has been shown to be

present f rom birth t5. It has been obse rved by several

au thors that some local segmenta l inhibi tory net-

works funct ion early in d e v e l o p m e n t well be fo re the

appea rance of descend ing inhibi t ion lt~.-~y,~<4'~ In the

present study pro jec t ing neu rons and in t e rneu rons

were not dis t inguished, but this x~ould p rov ide useful

in format ion .

O n e may specula te on the unpor tance of descend-

ing cont ro l in the deve lop ing animal and the possible

reasons for its la te ma tu ra t ion . The dorsal horn ot the

spinal cord unde rgoes cons ide rab le physiological

change in the i m m e d i a t e pos tnata l pe r iod LS. T h e pe-

r ipheral a f ferent input is a crucial factor in estab-

lishing the final o rganiza t ion . T h e system may be de-

s igned such that the afferent input can exer t its influ-

ences on the cord and establ ish basic ref lex pat terns

be fo re new signals descend f rom tl~c bra in to p roduce

more sophis t ica ted m o t o r behav lours The arrival of

descend ing cont ro l in the third week of life co inc ides

with the appea rance of m o r e c o o r d i n a t e d behav tou r

pa t te rns m rat pups 4~. The descend ing f ibres may no1

only br ing electr ical signals but also chemica l mes-

sengers. A b o u t two- th i rds of 5 -HT-con ta in ing neu-

rons in the nucleus raphe magnus also conta in cn-

kephal in > and o the r raphe neurons also conta in sub-

s tance P and thy ro t rop in - re l eas ing h o r m o n e 24. Noth-

ing is known about the funct ional s ignif icance of this

coex is tence or i ndeed of its o n t o g e n y but these pep-

t ides may well be impor t an t d e v e l o p m e n t a l signals.

REFERENCES

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2 Bekoff, A., Embryonic development of the neural circuitry underlying motor co-ordination. In W. Maxwell Cowan (Ed.), Studies in Developmental Neurobiotogy. Oxford University Press, Oxford, 198l. pp. 134-170.

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