Glutamatergic transmission in the trigeminal nucleus assessed with local blood flow

Stimulation of the superior sagittal sinus in humans is pain-producing and in experimental animals leads to excitation of neurons in the caudal trigeminal nucleus and dorsal horns of the C(1/)C(2) cervical spinal cord: the trigeminocervical complex. Neuronal excitation is generally associated with an increase in local blood flow due to flow/metabolism coupling and we have used local blood flow in the trigeminocervical complex to examine the role of N-methyl-D-aspartate (NMDA)-mediated transmission in these neurons. Cats were anaesthetised with alpha-chloralose (60 mg/kg, ip; supplements 20 mg/kg iv) after surgical preparation under halothane (0.5-3%). Animals were paralysed with gallamine triethiodide to prevent possible movement artefact distorting the laser Doppler signals. The superior sagittal sinus was isolated for electrical stimulation (150 V; 250 microsec duration; 0.5, 1, 2, 5, 10 and 20 Hz) and the dorsal surface of the spinal cord exposed at the C(2) level. Blood flow was recorded from the region over the trigeminocervical complex by careful placement of a laser Doppler flow probe. Flow was recorded continuously by an online collection programme and NMDA-mediated transmission modulated by intravenous administration of MK-801 (0.4, 1 and 4 mg/kg, iv) at the stimulation frequency of 5 Hz. Stimulation of the superior sagittal sinus produced a stimulus-locked, frequency-dependent increase in blood flow in the region of the trigeminocervical complex. The mean maximum response was 39+/-4% at 20 Hz. MK-801 had no effect on the resting flow signal but markedly attenuated the SSS-evoked response in a dose-dependent manner. The mean maximum response after 4 mg/kg MK-801 was 13+/-2%. NMDA-mediated transmission is likely to be involved in nociceptive trigeminovascular transmission within the trigeminocervical complex and offers a possible target for both acute and preventative treatment of migraine.

Parabrachial area and nucleus raphe magnus inhibition of corneal units in rostral and caudal portions of trigeminal subnucleus caudalis in the rat

The cornea has been used extensively as a means to selectively stimulate trigeminal nociceptive neurons. The aim of this study was to determine the effects of descending modulatory control pathways on corneal unit activity by comparing the effects of conditioning stimulation of the pontine parabrachial area (PBA CS) and nucleus raphe magnus (NRM CS). Electrical stimulation of the cornea at A- and C-fiber intensities was used to activate neurons in two regions of the trigeminal spinal nucleus, the subnucleus interpolaris/caudalis transition (Vi/Vc, 'rostral units') and laminae I-II at the subnucleus caudalis/cervical cord transition (Vc/C1, 'caudal units'), in chloralose-anesthetized rats. Corneal units were further classified according to convergent cutaneous receptive field properties and PBA projection status. None of 48 rostral and 23/28 caudal units projected to the ipsilateral or contralateral PBA. PBA CS inhibited the cornea-evoked responses (<75% change from control) of approximately 65% of rostral and caudal units regardless of neuronal class. For rostral corneal units, PBA CS inhibited A- and C-fiber input equally (15+/-3 and 18+/-14% of control, respectively), whereas among caudal units, A-fiber input was inhibited more than C-fiber input (26+/-5 and 64+/-12% of control, respectively, P<0.01). The magnitude of NRM CS inhibition on cornea-evoked activity of both rostral and caudal units was not different from that seen after PBA CS. Glutamate microinjections into PBA also inhibited rostral and caudal corneal units (6/9 tested). These results indicate that corneal input to rostral and caudal units is modified by activation of descending controls from the PBA and NRM. The significance for processing corneal sensory information is discussed in terms of functional differences between rostral and caudal neurons.

Experimental study of medullary trigeminal evoked potentials: development of a new method of intraoperative monitoring of the medulla oblongata

OBJECT

The goal of this study was to develop a new method of intraoperative monitoring of functions located in the lateral portion of the medulla oblongata. Based on the fact that the spinal trigeminal nucleus and tract are located in the lateral portion of the medulla oblongata, the authors intended to investigate the efficacy of trigeminal evoked potentials (TEPs) in intraoperative monitoring for assessing functions of the medulla oblongata.

METHODS

Trigeminal evoked potentials induced by electrical stimulation of the infraorbital nerve were recorded from the dorsolateral portion of the medulla oblongata (M-TEP) and the cerebral sensory cortex (C-TEP) in dogs. When the lateral one-sixth portion of the medulla was cut, the amplitude of the M-TEP decreased markedly, but the amplitude of the C-TEP and the somatosensory evoked potential (SSEP) did not decrease. When the lateral one-third portion of the medulla was cut, the amplitude of the SSEP decreased, but that of the C-TEP showed no change. When the medulla was retracted, the amplitude of the M-TEP was more sensitive than that of SSEP. Pathological examinations revealed that retraction force less than 10 g and a reduction in the amplitude of the M-TEP less than 50% were safe.

CONCLUSIONS

These results suggest that M-TEPs obtained from the dorsolateral portion of the medulla oblongata by electrical stimulation of the trigeminal nerve are clinically applicable as a new means of intraoperative monitoring of the functions of the medulla oblongata.

Upper cervical afferents to the motor trigeminal nucleus and the subnucleus oralis of the spinal trigeminal nucleus in the rat: an anterograde and retrograde tracing study

Upper cervical afferents to the motor trigeminal nucleus (Vmo) and the subnucleus oralis (Vo) neurons projecting contralaterally to the cervical cord were demonstrated in the rat. Axon-terminals were labeled with biotinylated dextran and neurons with cholera toxin subunit B. Axons from the C2 and C3 segments terminated ipsilaterally on the somata and proximal dendrites of Vmo neurons. In the Vo, terminals of axons from the C2 and C3 segments were densely distributed on the somata, and proximal to distal dendrites of neurons projecting contralaterally to the cervical cord. The ipsilateral cervical input to the Vmo would modulate the activity of motoneurons of masticatory muscles while that to the Vo neurons subserves the feedback control of the trigemino-spinal reflex.

The difference in temporal distribution of c-Fos immunoreactive neurons between the medullary dorsal horn and the trigeminal subnucleus oralis in the rat following experimental tooth movement

The difference in temporal distribution of c-Fos-immunoreactivity (Fos-IR) was assessed in the medullary dorsal horn (MDH) and in the dorsomedial part of the trigeminal subnucleus oralis (Vodm) following experimental tooth movement of the rat maxillary molars. The number of MDH c-Fos-immunoreactive neurons increased bilaterally at 2 h and decreased markedly by 12 h, and then increased again with a small peak at 48 h. In contrast, Vodm c-Fos expression was not up-regulated until 12 h, but increased in number after 24 h, which increase lasted until 72 h. These findings indicate that experimental tooth movement induced nociceptive c-Fos response in a biphasic manner. Furthermore, the later response appeared after 24 h, and lasted for a few days, mainly manifested in the Vodm during experimental tooth movement.

Responses of medullary dorsal horn neurons to corneal stimulation by CO(2) pulses in the rat

Corneal-responsive neurons were recorded extracellularly in two regions of the spinal trigeminal nucleus, subnucleus interpolaris/caudalis (Vi/Vc) and subnucleus caudalis/upper cervical cord (Vc/C1) transition regions, from methohexital-anesthetized male rats. Thirty-nine Vi/Vc and 26 Vc/C1 neurons that responded to mechanical and electrical stimulation of the cornea were examined for convergent cutaneous receptive fields, responses to natural stimulation of the corneal surface by CO(2) pulses (0, 30, 60, 80, and 95%), effects of morphine, and projections to the contralateral thalamus. Forty-six percent of mechanically sensitive Vi/Vc neurons and 58% of Vc/C1 neurons were excited by CO(2) stimulation. The evoked activity of most cells occurred at 60% CO(2) after a delay of 7-22 s. At the Vi/Vc transition three response patterns were seen. Type I cells (n = 11) displayed an increase in activity with increasing CO(2) concentration. Type II cells (n = 7) displayed a biphasic response, an initial inhibition followed by excitation in which the magnitude of the excitatory phase was dependent on CO(2) concentration. A third category of Vi/Vc cells (type III, n = 3) responded to CO(2) pulses only after morphine administration (>1.0 mg/kg). At the Vc/C1 transition, all CO(2)-responsive cells (n = 15) displayed an increase in firing rates with greater CO(2) concentration, similar to the pattern of type I Vi/Vc cells. Comparisons of the effects of CO(2) pulses on Vi/Vc type I units, Vi/Vc type II units, and Vc/C1 corneal units revealed no significant differences in threshold intensity, stimulus encoding, or latency to sustained firing. Morphine (0.5-3.5 mg/kg iv) enhanced the CO(2)-evoked activity of 50% of Vi/Vc neurons tested, whereas all Vc/C1 cells were inhibited in a dose-dependent, naloxone-reversible manner. Stimulation of the contralateral posterior thalamic nucleus antidromically activated 37% of Vc/C1 corneal units; however, no effective sites were found within the ventral posteromedial thalamic nucleus or nucleus submedius. None of the Vi/Vc corneal units tested were antidromically activated from sites within these thalamic regions. Corneal-responsive neurons in the Vi/Vc and Vc/C1 regions likely serve different functions in ocular nociception, a conclusion reflected more by the difference in sensitivity to analgesic drugs and efferent projection targets than by the CO(2) stimulus intensity encoding functions. Collectively, the properties of Vc/C1 corneal neurons were consistent with a role in the sensory-discriminative aspects of ocular pain due to chemical irritation. The unique and heterogeneous properties of Vi/Vc corneal neurons suggested involvement in more specialized ocular functions such as reflex control of tear formation or eye blinks or recruitment of antinociceptive control pathways.

Substance P receptor-like immunoreactive neurons in the caudal spinal trigeminal nucleus send axons to the gelatinosus thalamic nucleus in the rat

By means of substance P receptor (SPR) immunofluorescence histochemistry combined with Fluoro-Gold (FG) fluorescent retrograde labeling, SPR-like immunoreactive neurons in the caudal spinal trigeminal nucleus of the rat were observed to send their axons to the gelatinosus thalamic nucleus with a clear ipsilateral dominance. FG/SPR double-labeled neurons were distributed mainly in the ventral part of lamina I at the rostral level of the caudal spinal trigeminal nucleus. The percentages of FG/SPR-LI neurons in the total number of SPR-LI neurons and FG-labeled neurons are 10.5% and 31.1%, respectively. The present results suggest that trigemino-gelatinosus thalamic projection neurons with SPR-LI in the caudal spinal trigeminal nucleus might receive SP-containing, nociceptive primary afferent fibers from the orofacial region and transmit nociception to the gelatinosus thalamic nucleus.

Projections from subnucleus oralis of the spinal trigeminal nucleus to contralateral thalamus via the relay of juxtatrigeminal nucleus and dorsomedial part of the principal sensory trigeminal nucleus in the rat

Following injection of HRP into contralateral thalamus, retrogradely labeled cells were observed in principal sensory trigeminal nucleus (Vp) and an area of juxtatrigeminal nucleus (JX) formerly described by John and Tracey (1987). When PHA-L was delivered to dorsomedial part of the subnucleus oralis (Vodm), PHA-L labeled terminals were seen in dorsomedial part of the Vp (Vpdm) and in the JX region. Comparing the distribution of PHA-L labeled terminal field with that of HRP labeled JX neurons showed that the labeled terminals and neurons were overlapped closely in the JX. The distribution patterns of the labeled terminals and JX neurons were also the same: viewed on the coronal planes caudal-rostrally, both of the labelings began to appear at the levels where the facial nerve root was just broken. Rostrally, at middle levels of the motor trigeminal nucleus (Vmo), the labelings showed their typical view covering dorsal and ventral JX (dJX, vJX). The labelings disappeared at rostral poles of the Vmo and Vp. When injections of PHA-L into the Vodm and HRP into the contralateral thalamus was made in one rat, the contacts between Vodm projecting terminals labeled with PHA-L and HRP labeled trigemino-thalamic neurons were seen in the JX and also in the Vpdm. Then, electron microscopic (EM) study was done, injections of kainic acid into the Vodm and HRP into the contralateral thalamus was performed simultaneously. After EM embedding, the JX and Vpdm regions were selected, ultrathin sections were cut and observed with EM. In both areas, axo-somatic and axo-dendritic synapses were seen between degenerated boutons and HRP labeled somata or dendrites. Namely, the Vodm projecting terminals synapsed on trigemino-thalamic neurons in the JX and Vpdm. Anyway, axo-dendritic synapses was the main type of observed synapses. Thus, the present work demonstrated 1. the JX containing a group of trigemno-thalamic neurons was a target of special projections froin the Vodm; 2. The Vodm neurons projected to the contralateral thalamus through the relay of JX and Vpdm neurons.

Jaw-muscle spindle afferent feedback to the cervical spinal cord in the rat

Putative synaptic contacts between masticatory-muscle spindle afferents and brainstem neurons which project to the cervical spinal cord were studied in rats by combining retrograde and intracellular neuronal labeling. Spinal cord projecting neurons were retrogradely labeled via injection of horseradish peroxidase unilaterally or bilaterally into cervical spinal cord segments C2 through C5. Twenty-four hours after the injection of horseradish peroxidase, one to five jaw-muscle spindle afferent axons were physiologically identified and intracellularly stained with biotinamide on each side of the brainstem. Horseradish-peroxidase-labeled neurons were found bilaterally in the supratrigeminal region, trigeminal principal sensory nucleus, parvicellular reticular nucleus including its alpha division, spinal trigeminal subnuclei oralis and interpolaris and the medullary reticular formation. Retrogradely labeled neurons were most numerous in the spinal trigeminal subnucleus oralis, parvicellular reticular formation and the ventral part of the spinal trigeminal subnucleus interpolaris. A small number of horseradish-peroxidase-labeled neurons were also present in the trigeminal mesencephalic nucleus and spinal trigeminal subnucleus caudalis. Appositions between jaw-muscle spindle afferent boutons and spinal projecting neurons were found in the supratrigeminal region, dorsomedial portions of the trigeminal principal sensory nucleus and spinal trigeminal subnuclei oralis and interpolaris, and the parvicellular reticular formation including its alpha division. Putative synaptic contacts were most frequent in the parvicellular reticular formation and the dorsomedial portion of the trigeminal subnucleus oralis. These results indicate that some orofacial proprioceptive feedback transmitted via the mesencephalic trigeminal nucleus reaches the cervical spinal cord directly and suggests that jaw-muscle spindle afferent feedback reaches the cervical spinal cord predominately via relays in the dorsomedial part of the spinal trigeminal subnucleus oralis and the parvicellular reticular formation. It is hypothesized that these pathways are primarily involved in the coordination of jaw and neck movement during mastication and biting.

Distribution of trigeminal fibers in the primary facial gustatory center of channel catfish, Ictalurus punctatus

Previous studies in several fishes including catfish, have shown that primary trigeminal nerve (NV) axons terminate not only in the principal and spinal trigeminal nuclei, but in the facial (gustatory) lobes. The present study was undertaken to determine the extent and distribution of trigeminal terminations within the facial lobe (FL) and principal trigeminal nucleus (nVpr) in the channel catfish, Ictalurus punctatus. In order to reveal the distribution of trigeminal fibers, the carbocyanine dye, diI, was applied to the central cut stump of the trigeminal root in isolated, paraformaldehyde-fixed brains. After a diffusion period of 10-90 days, the brains were serially sectioned on a vibratome and examined with epifluorescence. The trigeminal motor nucleus (nVm) and principal sensory nucleus lie near the level of entrance of NV. The majority of primary trigeminal fibers, however, sweep caudally after entering into the brain to form the descending root. At the level of the caudal third of the FL, collaterals emitted by the descending root fibers turn medially and dorsally to terminate in the FL. The trigeminal fibers are coarser than the facial nerve (NVII) fibers which terminate within the same structure. The trigeminal fibers terminate throughout the FL except for the lateral-most lobule which contains the representation of taste buds innervated by the recurrent branch of NVII, i.e., those over the trunk and tail of the animal. These results show that in catfish, the trigeminal input to the primary gustatory complex is restricted to those portions of the nucleus receiving chemosensory inputs from the face and barbels, i.e., the trigeminally innervated sensory fields.

Convergence of meningeal and facial afferents onto trigeminal brainstem neurons: an electrophysiological study in rat and man

Headache is often accompanied by referred pain in the face. This phenomenon is probably due to a convergence of afferent inputs from the meninges and the face onto central trigeminal neurons within the medullary dorsal horn (MDH). The possible existence and extent of this convergence was examined in rat and man. MDH neurons activated by stimulation of the parietal meninges were tested for convergent tactile and noxious mechanical input from all three facial branches of the trigeminal nerve. All 21 units with meningeal input could also be activated by facial stimuli. Brush stimuli applied to the supraorbital nerve area activated 86%, to the infraorbital nerve area 29%, and to the mental nerve area none of the units. Pinch stimuli applied to the supraorbital nerve area activated 95%, to the infraorbital nerve area 86%, and to the mental nerve area 52% of the units. The results suggest convergence of meningeal and facial inputs concentrated on the supraorbital nerve in rat. In man convergence was examined by probing neuronal excitability of MDH applying the blink reflex (BR) during Valsalva maneuver which probably increases intracranial pressure. The BR evoked by supraorbital nerve stimulation remained unchanged, while the BR evoked by mental nerve stimulation was significantly facilitated. This facilitation may be due to convergence of meningeal and facial inputs onto trigeminal neurons in man.

Early appearance of acetylcholinergic, serotoninergic, and peptidergic neurons and fibers in the developing human central nervous system

Animal experiments have already shown that neurotransmitters and neuropeptides are not only important for normal functioning of the adult central nervous system (CNS) but are also crucial to its development. However, information on the spatio-temporal distribution of these endogenous substances in the developing human CNS is still scarce. With the use of immunocytochemical staining and a constant supply of properly fixed human abortuses from southern China, an early appearance of acetylcholinesterase, enkephalin, and substance P immunoreactivities was detected first in the spinal cord (weeks 5 to 7 of gestation), then in the brainstem nuclei (weeks 11 to 12). Their overlapping localizations in many regions of the CNS suggest possible interactions among neurons containing these substances, which are in turn important for the proper establishment of the neuronal circuitry. Immunoreactivity for neuropeptide Y appeared initially in the lateral region of upper segments of the spinal cord at week 12 of gestation, then spread latero-medially and cranio-caudally to the sacral region. In the hippocampus, neuropeptide Y neurons appeared from week 15 onwards. Serotoninergic neurons were found in the dorsal raphe nucleus at week 10 and then decreased in number as the fetus grew older. Somatostatin releasing inhibitory factor, vasopressin, and oxytocin were detected in the hypothalamus from weeks 12 to 14 onwards, and monoamine oxidase, succinic dehydrogenase, parvalbumin, calbindin D28K, and vasoactive intestinal peptide were found in the visual cortex at midgestation. The early appearance and the abundance of the neurotransmitters and neuropeptides in the developing CNS indicate that they may play a key role in neuronal differentiation.

Response properties of trigeminal brain stem neurons with input from dura mater encephali in the rat

The responsiveness of trigeminal brain stem neurons to selective local mechanical and chemical stimulation of the cranial dura mater was examined in a preparation in the rat. The dura mater encephali was exposed and its surface stimulated with electrical pulses through bipolar electrodes. Extracellular recordings were made from neurons in the subnucleus caudalis of the spinal trigeminal nucleus. Single neurons driven by meningeal input were identified by their responses to electrical stimulation and to probing their receptive fields on the dura. Facial receptive fields were defined mechanically. Chemical stimuli (a combination of inflammatory mediators, bradykinin, prostaglandin E2, serotonin, capsaicin and acidic Tyrode's solution) were applied topically to the dura and by injection through a catheter into the superior sagittal sinus. All neurons with input from the parietal dura mater had convergent input from the facial skin, with preponderance of the periorbital region. Proportions of units were activated by the combination of inflammatory mediators (55%), bradykinin (64.5%), acidic Tyrode's solution (64.1%) and capsaicin (78.6%). We conclude that, among the chemical mediators of inflammation, bradykinin and low pH are the most effective chemical stimuli in activating meningeal nociceptors. These stimuli may be important during meningeal inflammatory processes that lead to the generation of headaches.

Stimulus-function, wind-up and modulation by diffuse noxious inhibitory controls of responses of convergent neurons of the spinal trigeminal nucleus oralis

Extracellular unitary recordings were made from 53 spinal trigeminal nucleus oralis (Sp5O) convergent neurons in halothane-anaesthetized rats. The neurons had an ipsilateral receptive field including mainly oral or perioral regions. They responded to percutaneous electrical stimulation with two peaks of activation. The first had a short latency (4.3 +/- 0.3 ms) and low threshold (0.35 +/- 0.04 mA), whereas the second had a longer latency (68.1 +/- 3.4 ms) and higher threshold (7.3 +/- 0.5 mA). Intracutaneous injection of capsaicin (0.1%) produced a strong and rapid reduction of the long-latency responses of Sp5O convergent neurons with little effect on the short-latency responses. In most cases (73%), the long-latency responses exhibited a wind-up phenomenon during repetitive (0.66 Hz) suprathreshold electrical stimulation. These results suggest that C-fibres mediate the long-latency response of Sp5O convergent neurons. Regarding the C-fibre-evoked responses, a linear relationship between the intensity of the applied current and the magnitude of the response was found within the one to three times threshold range. The Sp5O convergent neurons also encoded the intensity of mechanical stimuli applied to the skin or mucosa in the 5-50 g ranges. The evoked activity of Sp5O convergent neurons could be suppressed by noxious heat applied to the tail (52 degrees C) and long-lasting poststimulus effects followed this. These findings show that convergent neurons in the Sp5O resemble those in the deep laminae of the spinal dorsal horn and spinal trigeminal nucleus caudalis, and further support that the Sp5O plays a part in the processing of nociceptive information from the orofacial region.

Convergence of nociceptive and non-nociceptive input onto the medullary dorsal horn in man

Referred pain arising in orofacial pain states is probably due to convergence of different somatosensory input onto the medullary dorsal horn (MDH). To examine convergence between nociceptive and non-nociceptive input onto the MDH, the blink reflex (BR) was applied. R1- and R2-components can be evoked by innocuous stimuli, but only the R2 is elicited by painful heat. The BR was elicited by innocuous electrical stimuli applied to the supraorbital nerve. A conditioning painful heat pulse which did not evoke any BR was homotopically applied to the left forehead preceding the electrical stimulus by 75 ms. While R1 remained unchanged, the R2 was facilitated by about 30%. This study demonstrates a convergence of low-threshold mechanoreceptive and nociceptive inputs onto interneurons of the MDH in man.

Morphine microinjected into the nucleus raphe magnus does not block the activity of spinal trigeminal nucleus oralis convergent neurons in the rat

This study investigated the effects of morphine microinjection into the nucleus raphe magnus (RMg) on electrically evoked C-fiber activities of convergent neurons in the spinal trigeminal nucleus oralis (Sp5O), in halothane-anesthetized rats. Although the neurons could be depressed by systemic morphine (6 mg/kg, i.v.) in a naloxone-reversible fashion, morphine microinjected into the RMg (2. 5 microgram or 5 microgram) neither depressed their C-fiber-evoked responses, nor the diffuse noxious inhibitory controls acting on them. It is concluded that the RMg is not involved in reinforcing descending inhibitory controls that are tonic or triggered by noxious stimuli acting on Sp5O convergent neurons.

Characterization of blink reflex interneurons by activation of diffuse noxious inhibitory controls in man

The blink reflex consists of an early, pontine R1-component and a late, medullary R2-component. R1 and R2 can be evoked by innocuous stimuli, but only the R2 also by painful heat, suggesting that the R2 is mediated by wide dynamic range neurons (WDR) of the spinal trigeminal nucleus. Remote noxious stimuli suppress the activity in WDR neurons via activation of diffuse noxious inhibitory controls (DNIC), whereas low-threshold mechanoreceptive neurons (LTM) are unaffected. In order to characterize the trigeminal interneurons of R1 and R2 we investigated the modulation of the blink reflex by remote painful heat. The blink reflex was elicited in 11 healthy subjects by innocuous electrical pulses applied to the left supraorbital nerve. The remote, painful heat stimuli were applied by a Peltier type thermode to the left volar forearm. Remote painful heat of 44 to 46 degreesC significantly suppressed the R2 by 15% (p<0.01), while the R1 remained unchanged. These results provide further evidence that the R2 is mediated by medullary WDR neurons and the R1 by pontine LTM neurons.

Serotonin inhibits trigeminal nucleus activity evoked by craniovascular stimulation through a 5HT1B/1D receptor: a central action in migraine?

The development of serotonin (5HT1B/1D) agonists as treatments for the acute attack of migraine has resulted in considerable interest in their mechanism of action and, to some extent, renewed interest in the role of serotonin (5-hydroxytryptamine; 5HT) in the disorder. The initial synthesis of this class of compounds was predicated on the clinical observation that intravenous 5HT terminated acute attacks of migraine. In this study the superior sagittal sinus was isolated in the alpha-chloralose (60 mg/kg i.p. and 20 mg/kg i.v. injection supplementary 2 hourly) anesthetized cat. The sinus was stimulated electrically (120V, 250 microsec duration, 0.3 Hz), and neurons of the trigeminocervical complex in the dorsal C2 spinal cord were monitored using electrophysiological methods. After baseline recordings in each animal, 5HT (15 microg/kg/min) was infused for 5 minutes in the presence of either vehicle (group A) or the 5HT1B/1D antagonist GR127935 (100 microg/kg i.v. injection; group B). The baseline probability of cell firing after sagittal sinus stimulation was 0.61 +/- 0.1 at a latency to the fastest peak of 11.1 +/- 0.4 msec. In group A, 5HT infusion alone had a small effect of increasing mean blood pressure (12 +/- 3 mm Hg), which in itself did not alter cell firing. In group A, 5HT alone had an inhibitory effect on evoked trigeminal activity, which developed 15 to 20 minutes after commencement of the infusion. The inhibition of cell firing lasted for 20 minutes, after which the activity returned to baseline. In group B, the combination of 5HT and GR127935 had no effect on trigeminal cell firing, although the small hypertensive effect was still present. These data indicate that 5HT inhibits evoked trigeminal nucleus firing via the 5HT1B/1D receptor at which GR127935 is an antagonist. It is likely that some part of the effect of 5HT in migraine relates to inhibition of trigeminal nucleus activity, just as it is likely that some part of the effect of the triptans is also mediated at this central site and may be complementary to their nonneuronal actions. Moreover, the data highlight the case for describing this class of headache as neurovascular headaches rather than vascular headaches, to recognize the implicit contribution of the trigeminovascular system to their pathophysiology.

Morphine administered in the substantia gelatinosa of the spinal trigeminal nucleus caudalis inhibits nociceptive activities in the spinal trigeminal nucleus oralis

The present study investigates the effects of morphine microinjection into the spinal trigeminal nucleus caudalis (Sp5C) or the spinal trigeminal nucleus oralis (Sp5O) on C-fiber-evoked activities of Sp5O convergent neurons, after supramaximal percutaneous electrical stimulation in halothane-anesthetized rats. When it was microinjected into the Sp5O, morphine (2.5 microg in 0. 25 microl) never depressed the C-fiber-evoked responses of Sp5O convergent neurons (n = 13), whereas these neurons were responsive to the inhibitory effects of systemic morphine (6 mg/kg, i.v.) in a naloxone-reversible manner. On the contrary, morphine microinjected into the Sp5C produced a naloxone-reversible inhibition of the C-fiber-evoked responses of Sp5O neurons (n = 14). The magnitude and the time course of this effect varied according to the location of the injection sites. After microinjection into the superficial laminae (n = 7), a strong depressive effect of morphine (7 +/- 5% of control) on the C-fiber-evoked responses was apparent as soon as 5 min after the injection and could always be reversed by naloxone, administered either intravenously (0.4 mg/kg) or locally (2.5 microg in 0.6 microl) at the same site as morphine. After microinjection into deeper laminae (V-VI), a significant depressive effect (34 +/- 5% of control) of morphine could be detected only 20 min after the injection and was reversed only by intravenous administration of naloxone. These results suggest that morphine exerts its antinociceptive action on Sp5O convergent neurons by blocking the C-fiber inputs that relay in the Sp5C substantia gelatinosa. The mechanisms that underlie the activation of Sp5O convergent neurons by C-fibers and the inhibition of C-fiber-evoked responses of Sp5O convergent neurons by morphine microinjected into the Sp5C are discussed.

Selective blockade of substance P or neurokinin A receptors reduces the expression of c-fos in trigeminal subnucleus caudalis after corneal stimulation in the rat

Stimulation of the cornea activates neurons in two distinct regions of the spinal trigeminal nucleus: at the transition between trigeminal subnucleus interpolaris and subnucleus caudalis and at the transition between trigeminal subnucleus caudalis and the upper cervical spinal cord as estimated by expression of the immediate early gene, c-fos. To determine if receptors for substance P or neurokinin A, neurokinin 1 and neurokinin 2 receptors, respectively, contribute to the production of Fos-positive neurons in these brainstem regions, receptor-selective antagonists were given intracerebroventricularly 15 min prior to stimulation of the cornea in anesthetized rats. The number of Fos-positive neurons produced in superficial laminae at the trigeminal subnucleus caudalis/cervical cord transition by application of the selective small fiber excitant, mustard oil, to the corneal surface was reduced by the neurokinin 1 receptor antagonist, CP99,994 (5-100 nmol, i.c.v.) and the neurokinin 2 receptor antagonist, MEN10,376 (0.01-1.0 nmol, i.c.v.). Combined pretreatment with CP99,994 and the competitive N-methyl-D-aspartate receptor antagonist, CPP, caused a greater reduction in c-fos expression at the subnucleus caudalis/cervical cord transition than after either drug alone suggesting interaction between receptors for glutamate and substance P. Tachykinin receptor antagonists did not reduce the number of Fos-positive neurons produced at the subnucleus interpolaris/subnucleus caudalis transition. The elevation in plasma concentration of adrenocorticotropin, but not the increases in arterial pressure or heart rate, evoked by corneal stimulation was prevented by pretreatment with CP99,994 or MEN10,376 at doses lower than those needed to reduce c-fos expression. The results indicate that receptors for substance P and neurokinin A contribute to the transmission of sensory input from corneal nociceptors to brainstem neurons in trigeminal subnucleus caudalis and to increased activity of the hypothalamo-pituitary axis that accompanies acute stimulation of the cornea.

Trigeminal-reticular connections: possible pathways for nociception-induced cardiovascular reflex responses in the rat

Cardiovascular regulatory neurons of the ventral medulla and pons are thought to have an important role in the mediation of trigeminal nociception-induced reflex cardiovascular responses. However, the neural pathways that link the spinal trigeminal nucleus with ventral medullary and pontine autonomic cell groups are poorly understood. The present study utilized injections of the highly sensitive anterograde tracer substance biotinylated dextran combined with immunocytochemistry for tyrosine hydroxylase, the synthesizing enzyme for catecholamines, to investigate the distribution and morphology of projections from the spinal trigeminal subnucleus caudalis to ventral medullary and pontine catecholaminergic cell groups. Injection of biotylinated dextran into the dorsal subnucleus caudalis produced dense anterograde labeling in dorsal regions of the medullary and pontine reticular formation including the dorsal medullary reticular field, the parvicellular reticular field, and the parvicellular reticular field pars anterior. In the ventral medullary and pontine reticular formation, light anterograde labeling tended to be distributed in close proximity to the distal dendrites of catecholaminergic neurons located in the C1, A1, and A5 regions. Injections of anterograde tracer into the dorsal medullary reticular field produced dense anterograde labeling in the ventral medullary and pontine reticular formation. Numerous terminal-like varicosities were observed in close proximity to catecholaminergic neurons located in the C1, A1, and A5 regions. These data suggest that trigeminal pain-induced reflex cardiovascular responses involve indirect projections that terminate in the dorsal medullary and pontine reticular formation before reaching ventral medullary and pontine catecholaminergic cell groups known to be involved in cardiovascular regulation.

Central distribution of synaptic contacts of primary and secondary jaw muscle spindle afferents in the trigeminal motor nucleus of the cat

Little is known about the differences of the terminations of group Ia and group II afferents within the brainstem or spinal cord. The present study was performed to classify cat jaw muscle spindle afferents by the use of succinylcholine (SCh) and to examine the morphological characteristics of the physiologically classified afferents at the light and electron microscopic levels through the use of the intra-axonal horseradish peroxidase (HRP) injection technique. The effects of SCh on stretch responses of 119 jaw muscle spindle afferents from the masseter were examined. The SCh converted the single skew distribution of the values for dynamic index (DI) into a bimodal one. Fifty-eight and 61 afferents were classified as group Ia and group II afferents, respectively. The central projections of 17 intra-axonally stained afferents (10 group Ia and 7 group II afferents) were examined. The spindle afferents terminated mainly in the supratrigeminal nucleus (Vsup), region h, and the dorsolateral subdivision of trigeminal motor nucleus (Vmo.dl) but differed in the pattern of projections of group Ia and group II afferents. The proportion of group Ia afferent terminals was higher in Vmo.dl but lower in Vsup than that of group II afferents. In Vmo.dl, the proportion of group Ia afferent terminals was higher in the central region but lower in the more outer regions than that of group II afferents. The ultrastructure of serially sectioned afferent boutons (63 group Ia and 72 group II boutons) also was examined. The boutons from the two groups were distributed widely from the soma to small-diameter dendrites, but the frequency of synaptic contacts on proximal dendrites was higher in group Ia than group II afferents. The present study provides evidence that the two groups of jaw muscle spindle afferents differ in their central projection and the spatial distribution of their synaptic contacts on Vmo.dl neurons.

Cortical somatosensory and trigeminal inputs to the cat superior colliculus: light and electron microscopic analyses

Two different axonal transport tracers were used in single animals to test the hypothesis that the expansive intermediate gray layer of the cat superior colliculus (stratum griseum intermediale, SGI) is composed of sensorimotor domains. The results show that two sensory pathways, the trigeminotectal and the corticotectal arising from the fourth somatosensory area, commingle in patches across the middle tier of the SGI. Furthermore, the data reveal that tectospinal cells are distributed within these patches. Taken together, these results show a commingling of functionally related afferents and a consistent spatial relationship between these afferents and tectospinal neurons. These relationships indicate that the SGI consists of domains that can be distinguished by their unique combinations of afferent and efferent connections. The ultrastructural characteristics and synaptic relationships of these somatosensory afferent pathways suggest that they have distinct roles within the sensorimotor domain of the SGI. The trigeminotectal terminals are relatively small, contain round vesicles and make asymmetrical synapses on small, presumably distal, dendrites. We submit that these trigeminal terminals bestow the basic receptive field properties upon SGI neurons. In contrast, the somatosensory corticotectal terminals are relatively large, contain round vesicles, make asymmetrical synapses, participate in triads, and are presynaptic to proximal dendrites. We suggest that these cortical terminals bestow integrative abilities on SGI neurons.

Nigral axon terminals are in contact with parvicellular reticular neurons which project to the motor trigeminal nucleus in the rat

We examined whether in the rat the descending fibers from the substantia nigra pars reticulata (SNr) were in contact with premotor neurons projecting to the motor trigeminal nucleus (Vm), using a combined anterograde and retrograde tracing technique. After ipsilateral injections of cholera toxin B subunit (CTb) into the Vm and biotinylated dextranamine (BDA) into the SNr, numerous CTb-labeled neurons were distributed bilaterally with slightly ipsilateral dominance in the parvicellular reticular formation (RFp), where many BDA-labeled axons with bouton-like varicosities were found bilaterally with a clear-cut ipsilateral dominance. The overlapping distribution of these labeled axons and neurons was more prominent in the rostral RFp than in the caudal RFp. Within the neuropil of the RFp, some of the BDA-labeled axons made synapses with the somata and proximal dendrites of CTb-labeled neurons. Thus, the present study demonstrated the existence of an indirect pathway from the SNr to the Vm, relayed by the RFp.

Single neurons in the spinal trigeminal and dorsal column nuclei project to both the cochlear nucleus and the inferior colliculus by way of axon collaterals: a fluorescent retrograde double-labeling study in the rat

A number of single neurons in the spinal trigeminal nucleus (STN) and the dorsal column nucleus (DCN) were found to project simultaneously to the cochlear nucleus (CoN) and the external cortex of the inferior colliculus (ICe) by way of axon collaterals. Each rat was injected with Fluoro-Gold (FG) into CoN on one side and with tetramethylrhodamine-dextran amine (TMR-DA) into ICe on the side ipsilateral or contralateral to the FG injection. In these rats, a number of neuronal cell bodies in DCN and the interpolar and caudal subnuclei of STN were double-labeled retrogradely with both FG and TMR-DA, mainly on the side ipsilateral to the FG injection into CoN. These neurons in STN and DCN might mediate somatosensory inputs simultaneously to the two lower brainstem nuclei, CoN and ICe, which constitute the relays of the auditory pathway.

Elicitation, modification, and conditioning of the rabbit nictitating membrane response by electrical stimulation in the spinal trigeminal nucleus, inferior olive, interpositus nucleus, and red nucleus

Elicitation of responses by electrical brain stimulation (EBS) was related to the synaptic distance of the target nucleus from the accessory abducens. Specifically, responses to EBS in the spinal trigeminal nucleus (TRIG) and red nucleus (RN) increased as a positive function of stimulation parameters. Responding to EBS in the interpositus nucleus (IP) was lower, and responding to EBS in the inferior olive (IO) was negligible. EBS in the TRIG, IP, and RN nuclei was then paired with a tone conditioned stimulus (CS). The CS modified responses for EBS in RN and TRIG but not IP. CS-EBS pairings yielded conditioned response (CR) acquisition, in which Groups TRIG, IP, and RN reached asymptotes of 90%, 70%, and 43% CRs, respectively. Thus, contrary to previous findings, EBS in the efferent pathway can support CR acquisition. The results are discussed with respect to the role of projections from the RN to the cerebellar cortex and the TRIG nucleus.

Granule cell activation of complex-spiking neurons in dorsal cochlear nucleus

Dorsal cochlear nucleus (DCN) principal cells receive, in addition to their well known auditory inputs, various nonauditory inputs via a cerebellar-like granule cell circuit located in the superficial layers of the DCN. Activation of this circuit (granule cell axons make excitatory synapses on the principal cells but also contact inhibitory interneurons that project to the principal cells) produces strong inhibition of the principal cells. Here we investigate the role of cartwheel cells, homologs of cerebellar Purkinje cells, in producing this inhibition. The responses of type IV units (one type of principal cells) and of cartwheel cells were recorded to ortho- and antidromic activation of the granule cells (i. e., by stimulation of their inputs from the somatosensory cuneate and spinal trigeminal nuclei and by direct stimulation of their parallel fiber axons). Cartwheel cells were identified on the basis of recording depth and complex action potential shape. A four-pulse facilitation paradigm (four pulses at 50 msec intervals) was used; this stimulus allows separation of the apparently simple inhibitory somatosensory response of type IV units into a three-component (inhibition-excitation-inhibition) response. As expected, cartwheel cells are excited by granule cell activation; the latencies and four-pulse amplitudes of these responses correspond to the properties of the second, long-latency inhibitory component of type IV responses. The source of the first, short-latency inhibitory response is still unknown. Nevertheless, these results show that cartwheel cells convey inhibitory polysensory information to DCN principal cells.

Trigeminal and chemoreceptor contributions to bradycardia during voluntary dives in rats

This study investigates the importance of chemoreceptive and trigeminal information during voluntarily initiated diving in rats. The heart rate responses to simulated diving are unaffected by chemoreceptor drive [McCulloch, P.F., and N. H. West. Am. J. Physiol. 263 (Regulatory Integrative Comp. Physiol. 32): R1049-R1056, 1992] but are reversibly eliminated by infusion of glutamate receptor antagonists into the spinal trigeminal nuclei [McCulloch, P. F., I. A. Paterson, and N. H. West. Am. J. Physiol. 269 (Regulatory Integrative Comp. Physiol. 38): R669-R677, 1995]. To investigate the role of chemoreceptor drive in conscious dives, rats were made hypercapnic, hyperoxic, or hypoxic predive. The role of trigeminal input was explored by infusing the glutamatergic antagonists D-2-amino-7-phosphoheptanoic acid and 6,7-dinitroquinoxaline-2,3-dione into the region of the trigeminal nuclei. The alteration of arterial blood gases predive had no effect on diving bradycardia. Trigeminal blockade reduced the intensity of the bradycardia but did not abolish it. Chemoreceptor input does not play a significant role in determining heart rate during conscious diving in rats. The attenuation, rather than abolition, of bradycardia on trigeminal blockade suggests either that we achieved incomplete blockade or that an additional spectrum of sensory inputs not present in simulated diving is important in determining the underwater heart rate during conscious diving in rats.

Primary- and secondary-like jaw-muscle spindle afferents have characteristic topographic distributions

Single jaw-muscle spindle afferent axons were characterized physiologically and intracellularly stained to determine whether particular physiological types of spindle afferent show distinctive morphologies. Microelectrodes filled with either horseradish peroxidase (HRP) or biotinamide (Neurobiotin) were advanced into the mesencephalic trigeminal nucleus (Vme) in anesthetized rats. Intracellular recordings then were characterized by their response: to palpation of the jaw muscles; when pressure was applied to the teeth and during passive ramp and hold and sinusoidal jaw movement. Seventy-one afferents were characterized physiologically and injected with HRP; an additional 61 afferents were typed and injected with biotinamide. The response of 43 stained neurons was recorded in the presence of suxamethonium. The major projection areas of these afferents were the: trigeminal motor nucleus (Vmo); region dorsal to Vmo; reticular formation, spinal trigeminal nucleus, superior cerebellar peduncle and Vme. One afferent type was modulated strongly during stretching of the jaw-elevator muscles. Based on their high sensitivity during stretching of the jaw muscles and/or their silencing during the release phase of muscle stretch, these afferents were classified as primary-like spindle afferents. These afferents projected most strongly to Vmo. A second type of afferent was modulated only modestly during stretching of the jaw-elevator muscles. These tonic afferents were classified as secondary-like spindle afferents because of their low dynamic sensitivity during ramp muscle stretch and their continued discharge during the release phase of muscle stretch. Secondary-like afferents projected most strongly to the region dorsal to Vmo. Boutons (n = 3,834) from 11 afferents were studied in detail. Secondary-like afferents had statistically larger boutons within Vmo. In both secondary- and primary-like spindle afferents, only a small number of boutons were associated closely with the somata and proximal dendrites of trigeminal motoneurons. In these cases, however, two to five boutons appeared to contact individual motoneurons, implying multiple monosynaptic inputs to a selective subset of jaw-elevator motoneurons. Some "giant" boutons were present dorsal to Vmo and in Vme. These results demonstrate that dynamically sensitive and nondynamically sensitive jaw-elevator muscle spindle afferents project preferentially to different regions. Primary-like spindle afferents are capable of providing feedback related to the dynamic phases of muscle stretch and project most heavily to Vmo. Secondary-like spindle afferents can transmit a feedback signal associated with muscle length and project most strongly to the supratrigeminal region. Both types of afferent have projections caudal to Vmo that may serve longer latency jaw-muscle stretch reflexes and/or the projection of proprioceptive information to the thalamus and cerebellum.

Trigeminal autonomic pathways involved in nociception-induced reflex cardiovascular responses

Reflex cardiovascular responses elicited by noxious oro-facial stimulation are well known but the neural pathways that underlie trigeminal cardiovascular reflex reactions remain to be elucidated. In previous studies, we have shown that noxious electrical stimulation of the mandibular incisor in the anesthetized rat elicits increases in mean arterial blood pressure and heart rate (Allen, G.V., Barbrick, B. and Esser, M.J., Trigeminal parabrachial connections: possible pathway for nociception-induced cardiovascular reflex responses, Brain Res., 715 (1996) 125-135). In this study, microinjections of the presynaptic blocker, cobalt chloride, or the anesthetic agent, lidocaine, were made into selected brainstem sites to identify neural pathways that are involved in mediation of the reflex pressor responses. Ipsilateral and bilateral injections of chemical blocker into the dorsomedial spinal trigeminal nucleus, pars caudalis, lateral parabrachial nucleus and the rostral ventral lateral medulla/caudal A5 region attenuated the reflex cardiovascular response. Bilateral injections of cobalt chloride into the dorsomedial subnucleus caudalis resulted in 70-100% attenuation of the reflex pressor response. Bilateral injections of cobalt chloride and/or lidocaine into the lateral parabrachial nucleus or the rostral ventral lateral medulla/A5 region resulted in 43-57% and 44-100% attenuation of the reflex pressor response, respectively. There were no significant differences in the degree or duration of attenuation of the reflex pressor responses produced by cobalt chloride compared to that produced by lidocaine injections. The reflex pressor responses usually returned to baseline levels approximately 60 min following injection of the chemical blocker substance. The results indicate that noxious electrical stimulation of the mandibular incisor elicits a reflex increase in mean arterial blood pressure which is initially mediated in the dorsomedial spinal trigeminal nucleus, pars caudalis and is subsequently mediated in the lateral parabrachial nucleus and the rostral ventral lateral medulla/caudal A5 region.

Morphine and somatostatin analogue reduce c-fos expression in trigeminal subnucleus caudalis produced by corneal stimulation in the rat

The influence of morphine and somatostatin on nociceptor-evoked activation of central trigeminal neurons and cardiovascular reflex responses was assessed in barbiturate-anaesthetized rats. Morphine or the somatostatin analogue, octreotide, was given intracerebroventricularly 20 min prior to application of mustard oil to the corneal surface. The expression of the immediate early gene, c-fos, was used to estimate neuronal activation within the spinal trigeminal nucleus. Morphine reduced the number of Fos-positive neurons produced at the transition region between trigeminal subnucleus caudalis and the upper cervical spinal cord, whereas c-fos expression at the subnucleus interpolaris/caudalis transition was not affected significantly. Morphine also reduced the arterial pressure and heart rate responses to corneal stimulation in proportion to the dose of morphine and required a threshold dose similar to that which reduced c-fos expression. Naloxone prevented the morphine-induced inhibition of c-fos expression and cardiovascular reflex responses to corneal stimulation. Somatostatin analogue reduced the number of Fos-positive neurons at the subnucleus caudalis/cervical cord transition, but not at the subnucleus interpolaris/caudalis transition, an effect that was not prevented by naloxone. Somatostatin analogue did not blunt the cardiovascular responses evoked by corneal stimulation. A subthreshold dose of morphine plus a threshold dose of somatostatin analogue caused a greater inhibition of Fos-positive neurons at the subnucleus caudalis/cervical cord transition, but not in reflex-evoked autonomic responses, than the same dose of either drug alone. Intracerebroventricular administration of morphine and somatostatin analogue inhibit corneal activation of neurons within the superficial laminae at the subnucleus caudalis/cervical cord transition through opioid and non-opioid-dependent neural pathways, respectively. By contrast, the low sensitivity of corneal-responsive neurons at the subnucleus interpolaris/caudalis transition to analgesics suggests that these neurons are not simply a rostral extension of the medullary dorsal horn. Correlation analyses suggest that morphine-induced inhibition of cardiovascular responses to corneal stimulation depend on the activity of neurons at the subnucleus caudalis/cervical cord transition and not on those at the subnucleus interpolaris/caudalis transition region.

Central projection of calcitonin gene-related peptide (CGRP)- and substance P (SP)-immunoreactive trigeminal primary neurons in the rat

Substance P (SP) is implicated in transmission of primary afferent nociceptive signals. In primary neurons, SP is colocalized with calcitonin gene-related peptide (CGRP), which is another neuropeptide marker for small to medium primary neurons. CGRP coreleased with SP augments the postsynaptic effect of SP and thereby modulates the nociceptive transmission. This study demonstrates the distribution of CGRP-like immunoreactivity (-ir) and SP-ir in the lower brainstem of normal rats and after trigeminal rhizotomy or tractotomy at the level of subnucleus interpolaris (Vi). By comparing the results obtained from normal and deafferented rats, we analyzed the central projection of trigeminal primary nociceptors. The CGRP-immunoreactive (-ir) trigeminal primaries projected to the entire rostrocaudal extent of the spinal trigeminal nucleus, the principal nucleus (PrV), the paratrigeminal nucleus (paraV), and the lateral subnucleus of solitary tract nucleus (STN) on the ipsilateral side. The trigeminal primaries projecting to the spinal trigeminal nucleus, paraV and STN also contained SP-ir. The ipsilateral trigeminal primaries were the exclusive source of CGRP-ir terminals in the PrV, the Vi and the dorsomedial nucleus within the subnucleus oralis (Vo). The medullary dorsal horn (MDH) and the lateral edge of Vo received convergent CGRP-ir projection from the ipsilateral trigeminal primaries and other neurons. The glossopharyngeal and vagal primaries are candidates for the source of CGRP-ir projection to the Vo and the MDH, while the dorsal root axons supply the MDH with CGRP-ir terminals. In addition, contralateral primary neurons crossing the midline appear to contain CGRP and to terminate in the MDH.

Cortical and medullary somatosensory projections to the cochlear nuclear complex in the hedgehog tenrec

Various tracer substances were injected into the spinal cord, the dorsal column nuclei, the trigeminal nuclear complex and the somatosensory cortex in Madagascan hedgehog tenrecs. With the exception of the cases injected exclusively into the spinal cord all injections gave rise to sparse, but distinct anterograde projections to the cochlear nuclear complex, particularly the granular cell domain within and outside of the dorsal cochlear nucleus. Among these cochlear afferents the projection from the primary somatosensory cortex is the most remarkable because the hedgehog tenrec has one of the lowest encephalisation indices among mammals and a similar cortico-cochlear connection has not been demonstrated so far in other species.

Encoding of corneal input in two distinct regions of the spinal trigeminal nucleus in the rat: cutaneous receptive field properties, responses to thermal and chemical stimulation, modulation by diffuse noxious inhibitory controls, and projections to the parabrachial area

To determine whether corneal input is processed similarly at rostral and caudal levels of the spinal trigeminal nucleus, the response properties of second-order neurons at the transition between trigeminal subnucleus interpolaris and subnucleus caudalis (Vi/Vc) and at the transition between subnucleus caudalis and the cervical spinal cord (Vc/C1) were compared. Extracellular single units were recorded in 68 Sprague-Dawley rats under chloralose or urethan/chloralose anesthesia. Neurons that responded to electrical stimulation of the cornea at the Vi/Vc transition region (n = 61) and at laminae I/II of the Vc/C1 transition region (n = 33) were classified regarding 1) corneal mechanical threshold; 2) cutaneous mechanoreceptive field, if present; 3) electrical input characteristics (A and/or C fiber); 4) response to thermal stimulation; 5) response to the small-fiber excitant, mustard oil (MO), applied to the cornea; 6) diffuse noxious inhibitory controls (DNIC); and 7) projection status to the contralateral parabrachial area (PBA). On the basis of cutaneous receptive field properties, neurons were classified as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR), nociceptive specific (NS), or deep nociceptive (D). All neurons recorded at the Vc/C1 transition region were either WDR (n = 19) or NS (n = 14). In contrast, 54% of the Vi/Vc neurons had no cutaneous receptive field. Of those Vi/Vc neurons that had a cutaneous receptive field, 57% were LTM, 25% were WDR, and 18% were D. All Vc/ C1 neurons responded to noxious thermal and MO stimulation. Only 22 of 47 and 13 of 19 Vi/Vc corneal units responded to thermal or MO stimulation, respectively. At the Vc/C1 transition region, 12 of 17 neurons demonstrated DNIC, whereas at the Vi/Vc transition region, DNIC was present in only 4 of 26 neurons. Of 15 Vc/C1 corneal units, 12 could be antidromically activated from the contralateral PBA (average latency 6.29 ms, range 1.8-26 ms). None of 22 Vi/Vc corneal units tested could be antidromically activated from the PBA. These findings suggest that neurons in laminae I/II at the Vc/C1 transition and at the Vi/Vc transition process corneal input differently. Neurons in laminae I/II at the Vc/C1 transition process corneal afferent input consistent with that from other orofacial regions. Corneal-responsive neurons at the Vi/Vc transition region may be important in motor reflexes or in recruitment of descending antinociceptive controls.

Short- and long-term transganglionic changes in the central terminations of transected vibrissal afferents in the rat

Previous light and electron microscopic studies in rat and cat have shown that transection of peripheral sensory nerve branches leads to alterations in the central branches of primary sensory neurons, so-called transganglionic changes. In this study the changes in choleragenoid (B)-horseradish peroxidase B-HRP-labeled primary sensory terminals and axons in the trigeminal nuclear complex 3-90 days following transection of vibrissae nerves in the rat have been studied. Since regeneration of the transected vibrissa nerve was not prevented, these experiments allowed the examination of degenerative changes in the earlier stage after nerve injury as well as those present during nerve regeneration and target reinnervation. Two different experimental approaches were used, depending on the postlesion survival time. For short-term experiments the deep vibrissa nerve was injected with a solution of B-HRP. Forty-eight hours later the nerve was transected at its entry in the follicle, and after survival times ranging from 3 to 15 days sections from the subnucleus caudalis and spinal trigeminal nucleus, were prepared for electron microscopic examination. For long-term experiments involving a 16- to 90-day posttransection survival time, the deep vibrissa nerve was cut first. Then B-HRP was injected into the reinnervated follicle 2 days before killing the rats. Atypical HRP-labeled terminals were seen from 4 to 90 days survival time. The changes observed included atypical swollen vesicles or lack of vesicles in parts of the terminals apposed to the synaptic cleft. Other terminals displayed dense clusters of vesicles, flocculent cytoplasm, and/or neurofilamentous hyperplasia. No evidence of complete disintegration or phagocytosis by glial cells was observed. From 4 to 12 days survival time the changes were most commonly seen in the larger terminals, from 19-90 days in smaller terminals. From 10 days survival time and onward, changes in axons were observed. The most commonly seen alterations were axons with expanded myelin sheaths. Normal-labeled terminals were seen at all survival times examined. Compared with earlier studies of transganglionic changes in the vibrissa system occurring after infraorbital nerve or vibrissa row nerve injury, the changes seen in this study are less pronounced. These observations indicate (1) that the initial changes in the central processes of peripherally injured vibrissae nerves are less extensive than those occurring after infraorbital nerve transection, possibly because of the distally located lesion, and (2) that transganglionic changes occur also after the injured nerve has regenerated.

A comparative and ultrastructural study of synaptic contacts established by primary sensory fibers in the spinal trigeminal nucleus of the rat

A quantitative evaluation of the types of afferent synaptic contacts in the pars oralis, using transganglionic degeneration and a comparison of previous data obtained from the pars interpolaris (Lapa & Bauer, 1992), of the rat was performed. Following left inferior alveolar nerve transection or partial pulpectomy of the first and second left lower molar teeth well-defined degenerating terminals appeared bilaterally. In both experiments, the majority of these afferent synapses formed single asymmetric contacts with intermediate and distal dendritic segments in the pars oralis. Fewer contacts were observed with dendritic spines, proximal dendritic segments, perikarya, and other terminals. Double and multiple synaptic contacts, preferentially with small dendritic profiles, were also found. Pars oralis showed higher density of degenerating terminals and higher proportion of the contralateral contacts than pars interpolaris suggesting that it is a prime input area and that it may play a role in the bilateral management of sensory information. Pars oralis showed a higher density of contacts with intermediate and distal dendritic segment and a lower density of double contacts in comparison to the pars interpolaris. Partial pulpectomy revealed a distribution in synaptic types similar to that following IAN transection suggesting that sensory fibers conveying pain-related stimuli are not distinguished from fibers of other sensory modalities as to preference of synaptic contacts. The overall pattern demonstrates a structural organization of the sensory inputs to the spinal trigeminal nucleus regarding the bilateral handling of sensory information.

Trigeminocerebellar and trigemino-olivary projections in rats

Retrograde and anterograde neuronal tracers (HRP, biocytin, biotinylated dextran-amine) were used to study the organisation of trigeminocerebellar and trigemino-olivary connections, focusing on the connectivity between trigeminal nuclear regions and the sagittal zones of the rat cerebellar cortex. Trigeminocerebellar projections were bilateral, but mostly ipsilateral. Direct trigeminocerebellar fibres originated mostly in the principal trigeminal nucleus (VP) and pars oralis (Vo), pars interpolaris (Vi), and to a lesser extent in pars caudalis (Vc) of the spinal trigeminal nucleus. Consistent projections were found from the Vc to cerebellar lobules IX and X. The trigeminal fibres terminated in the cerebellum in an organised fashion. The ventral part of the VP, Vo and Vi projected to the medial A zone and the C3 and D2 subzones, whereas the dorsal part of the nuclei projected to the lateral A zone and the C2, D0 and D1 subzones. In lobules IX and X, the organisation was different. The medial half of the VP, Vo, Vi and Vc projected to the lateral aspects of these lobules whereas their lateral part projected to their medial aspects. Trigeminal projections to the deep cerebellar nuclei were also present. Projections to a given sagittal zone concomitantly reached its corresponding nuclear target. Trigemino-olivary projections were principally contralateral. The Vo, Vi and Vc projected to the rostromedial dorsal accessory olive, the adjacent dorsal leaf and the dorsomedial part of the ventral leaf of the principal olive, which are known to project subzones C3, D0 and D1 of the rat cerebellar cortex.

N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor antagonism reduces Fos-like immunoreactivity in central trigeminal neurons after corneal stimulation in the rat

The role of glutamate receptors in processing noxious sensory input from the cornea was assessed in barbiturate-anesthetized rats. Animals were treated with selective antagonists for N-methyl-D-aspartate or non-N-methyl-D-aspartate receptor subtypes prior to application of mustard oil to the corneal surface. Neural activation was estimated from the number of neurons that produced Fos, the protein product of the immediate early gene, c-fos, as detected by immunocytochemistry. Fos-positive neurons were found at two distinct regions of the spinal trigeminal nucleus: the subnucleus interpolaris/caudalis transition and the subnucleus caudalis/upper cervical cord transition. The number of Fos-positive neurons was reduced dose-dependently by the competitive N-methyl-D-aspartate receptor antagonist, 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (0.08-8 nmol, i.c.v.), or by the non-N-methyl-D-aspartate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (2.5-250 nmol, i.c.v.). The greatest reduction in Fos-positive cells was seen at the subnucleus caudalis/upper cervical cord transition after blockade of either receptor subtype. Combined blockade of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors did not cause a further reduction in the number of Fos-positive neurons than was seen after the highest dose of either antagonist alone. Peripheral or central administration of the nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester, had no effect on the number of Fos-positive neurons after corneal stimulation. These results suggest that corneal input to neurons at the subnucleus caudalis/upper cervical cord transition, and to a lesser extent, at the subnucleus interpolaris/subnucleus caudalis transition depends on excitatory amino acid transmission. Both N-methyl-D-aspartate and non-N-methyl-D-aspartate glutamate receptor subtypes, but not the formation of nitric oxide, contribute to the processing of acute corneal stimuli by central trigeminal neurons.

The Kölliker-Fuse nucleus mediates the trigeminally induced apnoea in the rat

The present study examined whether the Kölliker-Fuse nucleus (KF) plays a role in mediating the trigeminally induced apnoea which occurs after noxious perturbation of the nasal mucosa. We stimulated the ethmoidal nerve (EN5) electrically and recorded respiratory responses before and after injections of the calcium channel blocker CoCl2 into the KF. Unilateral EN5 stimulations resulted in an apnoea or in a reduction of respiratory frequency and tidal volume. EN5 stimulations immediately after ipsilateral CoCl2 injections into the caudal KF caused only minor respiratory suppression, indicating a blockade of synaptic transmission. Recovery of the respiratory responses was observed 15-120 min after the CoCl2 injection. Our data strongly suggest that the caudal KF is an obligatory relay site for trigeminally induced apnoea.

Impairment of suckling response, trigeminal neuronal pattern formation, and hippocampal LTD in NMDA receptor epsilon 2 subunit mutant mice

Multiple epsilon subunits are major determinants of the NMDA receptor channel diversity. Based on their functional properties in vitro and distributions, we have proposed that the epsilon 1 and epsilon 2 subunits play a role in synaptic plasticity. To investigate the physiological significance of the NMDA receptor channel diversity, we generated mutant mice defective in the epsilon 2 subunit. These mice showed no suckling response and died shortly after birth but could survive by hand feeding. The mutation hindered the formation of the whisker-related neuronal barrelette structure and the clustering of primary sensory afferent terminals in the brainstem trigeminal nucleus. In the hippocampus of the mutant mice, synaptic NMDA responses and longterm depression were abolished. These results suggest that the epsilon 2 subunit plays an essential role in both neuronal pattern formation and synaptic plasticity.

The trigeminally evoked blink reflex. II. Mechanisms of paired-stimulus suppression

The paired-stimulus paradigm, presentation of a pair of identical reflex-eliciting stimuli to the supraorbital nerve (SO) with an interstimulus interval of less than 2 s, evokes a response to the second, test, stimulus which is less than that elicited by the first, conditioning, stimulus. In this study, we investigated the site of this suppression and its pharmacology in the alert guinea pig. Both the early (R1) and the late (R2) component of the SO-evoked blink reflex exhibited suppression in the paired-stimulus paradigm. Initiation of suppression appeared to be specific to the afferent limb of the reflex rather than the result of motor activity generated by the conditioning stimulus. Neither acoustic conditioning stimuli nor air puffs that elicited blinks via another branch of the trigeminal nerve suppressed the test response. Extremely weak SO shocks, however, that did not directly elicit a reflex, caused suppression of the response to subsequent SO stimuli of normal intensity. Paired stimulus suppression of the R1 component appeared to involve activation of GABAB receptors within the spinal trigeminal nucleus. Both systemic injections and microinjections of baclofen into the spinal trigeminal nucleus enhanced R1 suppression, whereas identical injections of CGP35348, a GABAB antagonist, diminished R1 suppression. Furthermore, single-unit recordings in alert animals revealed that spinal trigeminal neurons exhibited suppression in the paired-stimulus paradigm that resembled that of the R1 component of the blink reflex. These findings showed that sensory gating underlies paired-stimulus suppression of the SO-evoked blink reflex and that activation of GABAB receptors plays an important role in this process.

Connections to cerebellar cortex (Larsell's HVI) in the rabbit: a WGA-HRP study with implications for classical eyeblink conditioning

Conditioned eyeblink responses are presumably learned in the cerebellum and relayed to motoneurons by way of the red nucleus. Projections from the red nucleus to cerebellar cortex (Larsell's lobule HVI) could be important for shaping temporally adaptive features of the conditioned response. Rabbits that had pipettes containing wheat germ agglutinated horseradish peroxidase (WGA-HRP) implanted unilaterally into HVI showed retrograde labeling of neurons within subregions of the contralateral red nucleus implicated in eyeblink conditioning by lesioning and recording studies. Retrogradely labeled neurons were also observed in the pontine nuclei, inferior olive, and spinal trigeminal nucleus pars oralis. Projections to HVI provide a possible neural substrate for implementing time-derivative computational models of learning in the cerebellum. Time-derivative models are capable of describing the timing and topography of conditioned responses.

The trigeminally evoked blink reflex. I. Neuronal circuits

In this study, we characterized the pathways that generate the trigeminal blink reflex in the guinea pig. Blinks were evoked by stimulation of the supraorbital branch of the trigeminal nerve and measured by recording electromyographic activity in the lid-closing orbicularis oculi muscle (OOemg) and, in one case, lid position. Blinks evoked by stimulation of the supraorbital nerve consisted of two bursts of muscle activity ipsilateral to the side of stimulation. The first, R1, had a latency of 6.9 ms and the second, R2, had a latency of 17.25 ms. Increasing stimulus intensity to 3 times threshold for evoking an ipsilateral blink elicited an R1 and R2 response contralaterally, with latencies of 9.2 ms and 19.25 ms, respectively. We investigated the causes for this bipartite response that is seen in the guinea pig, as well as other mammals including humans. The two-component response could arise from different populations of afferents, or from different central circuits, or a combination of these two causes. Multiunit recording in the trigeminal ganglion and simultaneous measurement of the OOemg showed that activation of A beta afferents alone was sufficient to elicit both the R1 and the R2 responses, but that activation of A delta afferents could enhance both responses. Different neural circuits, however, produce the R1 and R2 responses. Transganglionic tracing with wheatgerm agglutin or choleragenoid subunit of cholera toxin bound to HRP revealed that primary afferents from the supraorbital branch of the trigeminal nerve terminated densely in the dorsal horn of spinal cord segment C1 and in the caudalis-interpolaris border region of the spinal trigeminal nucleus. Injections of HRP into the orbicularis oculi motoneuron region of the facial nucleus showed that both of these regions projected to the facial nucleus. Hemisections at the level of C1 eliminated the R2 blink response, but not the R1 response, evoked by stimulation of the supraorbital branch of the trigeminal nerve. Subsequent hemisections at the level of the obex eliminated the R1 response. Microinjections of the GABAB agonist baclofen into the spinal trigeminal nucleus at the level of the obex abolished the R1 but not the R2 response. Thus, the spinal trigeminal nucleus produces the R1 component, whereas the R2 component originates in the C1 region of the spinal cord.

A medullary dorsal horn relay for the cardiorespiratory responses evoked by stimulation of the nasal mucosa in the muskrat Ondatra zibethicus: evidence for excitatory amino acid transmission

Stimulation of the upper respiratory tract, including the nasal mucosa, with water, vaporous irritants, or gases, induces a collation of several cardiorespiratory responses including an apnea and bradycardia and often some change in arterial blood pressure. Since the nasal mucosa is innervated by branches of the trigeminal nerve, it implies that some part of the trigeminal system within the central nervous system mediates the autonomic responses induced by nasal stimulation. In the present study, respirations, heart rate and arterial pressure were monitored in muskrats anesthetized with a mixture of chloralose-urethane. We induced a bradycardia and apnea by stimulating the nasal mucosa of muskrats with brief (5 s) transnasal application of vapors of ammonia hydroxide. In an effort to determine the central site where the trigeminal mediation of the cardiorespiratory responses occurs, small nanoliter injections of 2% lidocaine were made bilaterally into the subnucleus caudalis of the spinal trigeminal nucleus (referred to as the medullary dorsal horn) to determine if the responses could be blocked. The responses could be blocked when the lidocaine injections on both sides were placed in the rostral, ventral parts of the medullary dorsal horn, but persisted when the injections were placed elsewhere. Since lidocaine blocks both neurons and fibers of passage, nanoliter injections of kynurenate, a general excitatory amino acid antagonist, were used in a similar paradigm to circumvent the problem of blocking only fibers of passage.(ABSTRACT TRUNCATED AT 250 WORDS)

An intact glutamatergic trigeminal pathway is essential for the cardiac response to simulated diving

Nasal water flow plus concomitant expiratory apnea in anesthetized (Innovar-Vet), paralyzed, and artificially ventilated rats produces immediate bradycardia. To investigate the origin of this response, four procedures were used to block the trigeminal pathway. 1) Trigeminal receptors within the nasal passages were anesthetized by infusing local anesthetic through the external nares. 2) Trigeminal nerves that innervate the nasal passages were sectioned bilaterally as they passed through the orbit. 3) The trigeminal neural pathway was blocked within the brain stem by either electrolytically lesioning or infusing local anesthetic into the spinal trigeminal nucleus interpolaris (Sp5I). 4) Synaptic transmission within Sp5I was prevented by infusing glutamate receptor antagonists D-2-amino-7-phosphonoheptanoic acid and 6,7-dinitroquinoxaline-2,3-dione. After each of the procedures was completed, the cardiovascular responses to nasal water flow plus apnea were either attenuated or eliminated. The major conclusion of this study is that an intact glutamatergic trigeminal pathway is required for manifestation of the cardiovascular responses to nasal stimulation. Evidence also suggests that N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors are both required for synaptic neurotransmission within Sp5I.

Topographic organization of spinal and trigeminal somatosensory pathways to the rat parabrachial and Kölliker-Fuse nuclei

We examined the organization of somatosensory projections to the parabrachial (PB) and Kölliker-Fuse (KF) nuclei by employing the retrograde and anterograde axonal transport of Fluorogold and Phaseolus vulgaris-leucoagglutinin (PHA-L), respectively. Small PHA-L injections were made into different parts of the spinal trigeminal complex, including the paratrigeminal nucleus, and into different segments and laminae of the spinal dorsal horn. The subnuclear distribution of axonal labeling in the PB and KF was mapped with a camera lucida. Our results show that the somatosensory input to the PB and KF is highly organized. Neurons in the spinal trigeminal nuclei project predominantly to the KF and to the ventral portion of the external lateral PB. Neurons in the paratrigeminal nucleus project to the ventral lateral PB, the external medial PB, and to caudal aspects of the medial PB. These findings were supported by retrograde tracing experiments with Fluorogold. Spinal cord neurons located in the superficial dorsal horn (laminae I-II) of upper cervical segments project specifically to the ventral portion of the external lateral PB and, although more sparsely, to various other lateral PB nuclei. In contrast, neurons in the superficial dorsal horn of thoracic and lumbar spinal segments project mainly to the dorsal lateral and the central lateral PB. Finally, neurons in the lateral reticulated area and the lateral spinal nucleus of all spinal segments project almost exclusively to the internal lateral PB, whereas neurons in the respective nuclei of upper cervical segments also project to the KF. From our data we conclude that the somatosensory projections to the PB and KF are topographically organized. It is assumed that these pathways, which run from trigeminal and spinal neurons through the PB and KF to various forebrain, medullary, and spinal nuclei, form functionally different neural circuits that are involved in somatoautonomic processing.

Somatosensory effects on neurons in dorsal cochlear nucleus

1. Single units and evoked potentials were recorded in dorsal cochlear nucleus (DCN) in response to electrical stimulation of the somatosensory dorsal column and spinal trigeminal nuclei (together called MSN for medullary somatosensory nuclei) and for tactile somatosensory stimuli. Recordings were from paralyzed decerebrate cats. 2. DCN principal cells (type IV units) were strongly inhibited by electrical stimulation (single 50-microA bipolar pulse) in MSN or by somatosensory stimulation. Units recorded in the fusiform cell and deep layers of DCN were inhibited, suggesting that the inhibition affects both types of principal cells (i.e., both fusiform and giant cells). 3. Interneurons (type II units) that inhibit principal cells were only weakly inhibited by electrical stimulation and were never excited, demonstrating that the inhibitory effect on principal cells does not pass through the type II circuit. In the vicinity of the DCN/PVCN (posteroventral cochlear nucleus) boundary, units were encountered that were excited by electrical stimulation in MSN; some of these neurons responded to sound, and some did not. Their response properties are consistent with the hypothesis that they are deep-layer inhibitory interneurons conveying somatosensory information to the DCN. 4. Analysis of the evoked potentials produced by electrical stimulation in MSN suggests that the somatosensory inputs activate the granule cell system of the DCN molecular layer. A model based on previous work by Klee and Rall was used to show that the distribution of evoked potentials in DCN can be explained as resulting from radial currents produced in the DCN molecular and fusiform-cell layers by synchronous activation of granule cells inputs to fusiform and cartwheel cells. Current-source density analysis of the evoked potentials is consistent with this model. Thus molecular layer interneurons (cartwheel and stellate cells) are a second possible source of inhibition to principal cells. 5. With lower stimulus levels (20 microA) and pulse-pair stimuli (50- to 100-ms interstimulus interval), three components of the inhibitory response can be recognized in both fusiform cell layer and deep layer type IV units: a short-latency inhibition that begins before the start of the evoked potential; a longer-latency inhibition whose timing corresponds to the evoked potential; and an excitatory component that occurs on the rising phase of the evoked potential. The excitatory component is usually overwhelmed by the inhibitory components and could be derived from granule cell inputs; the long-latency inhibitory component could be derived from cartwheel cells or the hypothesized deep-layer inhibitory interneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanisms of oral sensation

Sensory nerves that supply mechanoreceptors in the mucosal lining of the oral cavity, pharynx, and larynx provide the substrate for a variety of sensations. They are essential for the perception of complex or composite sensory experiences including oral kinesthesia and oral stereognosis. Relevant to the concerns of the oral health care delivery specialist they also contribute to initiation of reflexes and coordination and timing of patterned motor behaviors. The response of oral mechanoreceptors to natural stimuli is determined to a large degree by morphological factors such as the nature of the relationship between nerve ending and certain cellular specializations, their distribution in the mucosa, the diameter of their primary afferent nerve fibers, and the central distribution of these fibers in the brainstem. Because of morphological similarities to certain cutaneous mechanoreceptors, the mucosal lining may be considered as an internal continuation of the large "receptor sheet" for localization and detection of mechanical stimuli. In some regions of the oral, pharyngeal, and laryngeal mucosa, this analogy is appropriate whereas in others, existing data suggest a different role consistent with regionally specific demands (i.e., initiation of protective reflexes).

Functional properties of cells in rat trigeminal subnucleus interpolaris following local serotonergic deafferentation

We have previously demonstrated increases in serotonin (5-HT) content and immunoreactivity within spinal trigeminal subnucleus interpolaris (SpVi) that are correlated with the functional changes observed in this subnucleus following adult infraorbital nerve (ION) transection. To assess the possible functional significance of this change, we have examined the influence of 5-HT afference upon the normal response properties of cells in SpVi. We employed local depletion of the transmitter, using 5,7-dihydroxtryptamine (5,7-DHT), in combination with extracellular single-cell recording. Chromatographic methods revealed a 97.6% depletion of 5-HT 24 hr after neurotoxin injection. Immunocytochemical procedures revealed depletion of 5-HT throughout SpVi. Physiological recordings were made from 403 SpVi cells in 5,7-DHT-injected rats and 387 cells in vehicle-injected rats. All recordings were made 19-27 hr after injection. Horseradish peroxidase (HRP) deposits from the recording electrode were used to mark recording tracks. 5-HT depletion did not influence receptive field (RF) location, size, or continuity, or the dynamic response characteristics of SpVi cells. It did, however, (1) alter the probability that certain types of somatosensory receptor surfaces would activate local-circuit neurons, and (2) influence the rate of firing of spontaneously active SpVi cells. There was a significant increase in the proportion of vibrissa-sensitive cells with infraorbital RF components, and a concurrent decrease in the proportion of guard-hair-sensitive cells. It therefore appears that 5-HT input to SpVi is necessary for some mechanoreceptive features of the normal functional organization of this area. These functional changes were interesting in that they were opposite to those found following adult ION transection, which increases 5-HT within SpVi. Thus, changes in 5-HT central afference to SpVi that follow ION damage may be responsible for at least one type of functional change observed following this peripheral lesion.