Avoidance conditioning to tooth pulp stimulation in the cat after bulbar transection

Effects of lesions in the trigeminal oralis and caudalis subnuclei on different orofacial nociceptive responses in the rat

Behavioural responses to two different orofacial noxious stimulations were analysed following lesion of spinal trigeminal subnucleus oralis (Sp5O) in the rat. Lesions were obtained by intranuclear microinjections of quinolinic acid (0.4 microliter of 60 nmol/microliter solution). The control groups received microinjection of saline. Noxious stimulation was a subcutaneous injection of formalin into the upper lip or electrical stimulation of the tooth pulp. The measured behavioural responses were duration of rubbing induced by the formalin injection and thresholds of the jaw-opening reflex (JOR), head rotation (HR) and face rubbing (FR) evoked by the pulp stimulation. In addition, formalin injection was also performed in two groups of rats that had received intranuclear injection of quinolinic acid or saline into rostral subnucleus caudalis (Sp5C). Rubbing duration was not significantly modified by the lesion of Sp5O, whereas a significant decrease occurred after the lesion of rostral Sp5C. After the lesion of Sp5O, an increase in the threshold of JOR was observed whereas the thresholds of HR and FR were not significantly modified. These results suggest that Sp5O is not necessary for the processing and relay of nociceptive inputs triggered by intense stimulations of oral and perioral areas. However further experiments are needed to reconcile these results with the relevant data obtained from cell recording experiments which indicate the existence, in Sp5O, of neuronal activities related to the sensory discriminative aspect of intense nociception.

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.

Effects of tractotomy on nociceptive reactions induced by tooth pulp stimulation in the rat

The effects of a trigeminal tractotomy on nociceptive reactions induced by electrical stimulation of an inferior incisor were studied in freely moving rats. Behavioral tests were based on the observation of three reactions: jaw opening reflex, face scratching with forepaws, and head rotation toward the stimulated side. These reactions appear successively when the stimulation intensity increases. The results were evaluated by comparing a test group with a sham-operated group. Both groups were prepared surgically in the same way but only the former had undergone tractotomy. The results were as follows: The threshold of the jaw opening reflex triggered by stimulation of the tooth pulp ipsilateral to the tractotomy was not modified; however, the threshold of the face-scratching and the head rotation reactions rose (P less than or equal to 0.05) after tractotomy. These data suggest that the rostral part of the trigeminal sensory complex is involved in oral nociception. However, the modification of the face-scratching and head rotation thresholds reveals that the subnucleus caudalis also participates in this function when a certain level of pain is reached. The thresholds of the three nociceptive reactions evoked by stimulation of the contralateral tooth pulp were significantly lower in the test group than those in the sham-operated group (P less than or equal to 0.001), which is interpreted as being indicative of an modulating effect of the subnucleus caudalis on the contralateral trigeminal sensory complex. This effect was probably revealed by the use of a sham-operated group as a control.

The rostral part of the trigeminal sensory complex is involved in orofacial nociception

Single units responsive to noxious mechanical stimulation of orofacial receptive fields were recorded within the ventrobasal complex of the rat thalamus. The induced activities were compared before and after deafferentation of the subnucleus caudalis by a trigeminal tractotomy performed at the obex level. The receptive fields activated by noxious stimulation were classified as 'oral' when included in the oral, perioral or paranasal areas, and as 'facial' when included in facial regions distant from the oral cavity. After tractotomy, the unit responses to noxious stimulation of an oral field remained unchanged in 8 cases, decreased in 3 cases, and were suppressed in 4 cases. For units responding to noxious stimulation of a facial field, the responses were suppressed in 8 cases, decreased in two cases and remained unchanged in two other cases. So it appears that the rostral part of the trigeminal sensory complex (1) receives nociceptive afferents mainly from the oral and perioral areas and (2) is a relay in ascending pathways which convey painful sensations.

Cutting rostral trigeminal nuclear complex projections preferentially affects perioral nociception in the rat

Sagittal knife cuts were made medial to rostral trigeminal brainstem nuclei in 11 rats. These cuts resulted in significant group deficits in facial thermal nociception at only two of the 5 ipsilateral sites tested. The affected sites were close to the vibrissa and oral cavity, suggesting that rostral trigeminal projections signal anterior rather than more caudal facial nociception. Three rats were also tested for EEG arousal responsiveness to innocuous air puffs before and after the cuts at the same facial sites. Only one of the rats demonstrated a significant deficit at one of the 5 sites tested, suggesting that the rostral projections interrupted in this study are not necessary for innocuous facial sensation.

Neuronal responses in rostral trigeminal brain-stem nuclei of macaque monkeys after chronic trigeminal tractotomy

Unilateral trigeminal tractotomy was carried out at the level of the obex, just rostral to the subnucleus caudalis, in five young adult Macaca fascicularis monkeys. The animals had been trained previously to perform a behavioral shock avoidance task in response to electrical stimulation of dental pulp and facial skin. Tractotomy produced an elevation in the stimulus strength which elicited escape behavior when facial skin was stimulated but not when the tooth pulp was stimulated. Unit activity, evoked by electrical stimulation of the tooth pulp and facial skin as well as innocuous and noxious mechanical stimulation of orofacial regions, was recorded from neurons in the trigeminal main sensory nucleus and the subnuclei oralis and interpolaris of the spinal nucleus 8 to 12 weeks after tractotomy. Primary afferent input to these nuclei is unaffected by the tractotomy which is located more caudally. The tractotomy interrupts primary afferent input into the trigeminal nucleus caudalis and also intranuclear connections between caudalis and the more rostral nuclei. Forty-one units contralateral and 47 ipsilateral to the tractotomy were studied. Thirty-six of the units responded only to low-threshold mechanical or electrical stimulation of orofacial zones, 46 were responsive to innocuous mechanical and electrical stimulation of orofacial zones and also to electrical stimulation of the dental pulp. Six units responded only to dental pulp stimulation. No statistically significant differences between the populations of neurons ipsilateral and contralateral to the tractotomies were found relating to the size or location of the peripheral receptive fields, latencies, thresholds, mean firing densities, or responsiveness to the various forms of stimulation. The behavioral results suggest that trigeminal relay neurons rostral to the obex are able to signal dental pain sensation, and the physiological studies confirm that the firing of such neurons is unaffected by tractotomy. The physiological studies demonstrate that the firing patterns of relay neurons activated by natural and electrical cutaneous facial stimuli and which are located in trigeminal brain-stem nuclei rostral to the obex are also not affected by tractotomy. The cutaneous facial analgesia observed after tractotomy thus appears to be due to deafferentation of relay neurons in trigeminal nucleus caudalis rather than to alterations in coding patterns in rostrally located trigeminal neurons due to interruption of the intratrigeminal pathway between the caudal and rostral nuclear groups.

Lesions of the mid-spinal trigeminal complex are effective in producing perioral thermal hypoalgesia

The hypoalgesic effects of lesions in the rat spinal trigeminal complex at the levels of subnucleus interpolaris and caudal subnucleus oralis were investigated. Lesions of the trigeminal tract, nucleus, or adjacent reticular formation resulted in significant elevations in escape latencies to noxious thermal stimulation of the ipsilateral perioral area. The nuclear lesions were significantly more effective in producing latency elevations than were the reticular formation lesions. Behavioral and anatomical evidence is presented suggesting that the mid-spinal trigeminal complex, including the ascending trigeminal intranuclear pathways, participate in perioral thermal nociception.

Effects of trigeminal tractotomy on facial thermal nociception in the rat

Wire knife-cuts lesioning the descending trigeminal tract and trigeminal sensory nuclear complex at the level of subnucleus interpolaris significantly elevated response latencies to noxious thermal stimuli applied to one perioral locus of facial skin. Group analyses of four other ipsilateral sites did not reveal significant effects. Medial cuts, extensively damaging the sensory nuclear complex as well as the tract, were more likely to produce significant elevations than were lateral cuts. These results suggest that tractotomy does not result in complete facial analgesia in the rat, and that the effectiveness of tractotomy may in part be due to interruption of intranuclear trigeminal projections.

Pathways for orofacial pain sensation in the trigeminal brain-stem nuclear complex of the Macaque monkey

Eleven Macaque monkeys underwent a variety of lesions in the trigeminal afferent system; namely, tractotomy, rhizotomy, and radiofrequency destruction of various components of the bulbospinal trigeminal nuclear complex. Behavioral responses were evaluated before and after the lesions using a quantitative paradigm which measured lever-pressing in response to electrical stimulation of the dental pulp or facial skin, and by assessing adversive responses to facial cutaneous and intraoral pin-scratch. Thresholds for lever-pressing in response to cutaneous facial stimulation were elevated by tractotomy, elevated further by a combination of tractotomy and rhizotomy of the seventh, ninth, and 10th cranial nerves and cervical dorsal roots C1-3, and maximally elevated by complete radiofrequency destruction of the trigeminal nucleus caudalis. These lesions also abolished adversive responses to cutaneous facial pin-scratch. None of these lesion combinations, however, altered lever-pressing responses to dental pulp stimulation. Radiofrequency destruction of the trigeminal nuclei principalis, oralis, and interpolaris caused elevations of lever-pressing thresholds in response to dental pulp stimulation, and also smaller but statistically significant elevations on cutaneous electrical stimulation. Mild reductions in adversive responses to cutaneous pin-scratch were also produced by these rostral nuclear lesions, suggesting analgesia. The experiments suggest that primary afferent fibers for dental pain perception travel only in the trigeminal nerve and that these fibers relay via the trigeminal brain-stem nuclei principalis, oralis, and interpolaris. Primary afferent fibers for cutaneous facial pain perception travel in the trigeminal, facial, glossopharyngeal, and vagus nerves, and the upper cervical dorsal and ventral roots, and these afferents relay mainly in the trigeminal nucleus caudalis.

The central projections of tooth pulp afferent neurons in the rat as determined by the transganglionic transport of horseradish peroxidase

Transganglionic transport of horseradish peroxidase (HRP) or horseradish peroxidase-wheat germ agglutinin conjugate (HRP-WGA) was used to map in detail the central projections of trigeminal primary afferent neurons that innervate the dental pulp organ of the rat. In each of ten animals, 0.5-2.0 microliters of enzyme solution was injected into the pulp chamber of the first maxillary molar tooth. Postmortem examination of the decalcified teeth in all cases showed that the HRP/HRP-WGA remained confined to the pulp chamber and pulp roots, with no spread of enzyme into periapical tissues. HRP-labeled tooth pulp afferent fibers projected to all four rostrocaudal subdivisions of the ipsilateral trigeminal brainstem nuclear complex (TBNC) and to the upper cervical spinal cord. The labeled terminal fields formed a column that stretched relatively uninterrupted from just caudal to the rostromedial tip of the trigeminal principal sensory nucleus to at least the C2 segment of the spinal cord. The density of the afferent projection varied markedly from one rostrocaudal level of the TBNC to the next but was heaviest in an area encompassing the caudal one-half of the principal sensory nucleus and the rostral two-thirds of pars oralis. Fibers projected only lightly to pars caudalis, where they terminated preferentially in laminae I, IIa, and the junctional zone between laminae IV and V. HRP-labeled terminals in C1 and C2 were located almost exclusively in laminae I. In the dorsoventral axis, the terminal fields in the TBNC were located in a surprisingly dorsal part of the complex, well within what has been shown by others to be largely an area of termination for mandibular division fibers. Most fibers ended in medial parts of the TBNC, with the exception of two modestly labeled terminal fields located in the lateral aspects of rostral pars oralis and rostral pars caudalis. No labeled fibers terminated in the contralateral TBNC or contralateral cervical spinal cord.

Inhibition of intracranial self-stimulation in brain stem-transected cats--a proposed mechanism of aversive effects produced by brain stimulation

Effects of intracranial self-stimulation of central 'punishment areas' were studied on an operant conditioning of vertical eye movements in the midpontine pretrigeminal cats as well as in the encéphale isolé cats. In 36 pretrigeminal cats, the ventromedial hypothalamus (VMH), basal amygdaloid nuclei (AMY), dorsal central gray (CG) of the midbrain and the thalamic nuclei such as the ventralis posteromedialis (VPM) and ventralis posterolateralis (VPL) were tested. No suppression of eye movements indicating a passive avoidance conditioning from stimulation of these 'punishment areas' was obtained in 92 electrode tip sites. In 49 encéphale isolé cats, stimulation of the VPM associated with contraction of the facial muscles, demonstrated a marked passive avoidance effect on the eye movements. After blocking both the trigeminal (5N) and facial nerves (7N), VPM stimulation no longer produced an increase of facial EMG activity and the suppressive effect on eye movements was abolished. Extracranial blockade of 7N alone, which induced facial muscle paralysis also showed similar effects. Bilateral blockade of cranial nerves from acoustic (8N) to hypoglossal (12N) nerves had no significant effect on the avoidance conditioning. The mass neural activity recorded from the 5N showed a marked increase of discharge by VPM stimulation which was reduced significantly after 7N blockade. These results may suggest a possibility that punishing effects of brain stimulation depend on feedback from the periphery (muscles, blood vessels and visceral organs), whereas reward effects essentially depend on neural circuitry confined within the forebrain above the rostral pons.