Norio Matsumoto

Somatotopic distribution of trigeminal nociceptive specific neurons within the caudal somatosensory thalamus of cat.

Trigeminal nociceptive specific (NS) neurons were found within the shell region of the caudal part of nucleus ventralis posterior medialis (VPM) of cat. They showed a somatotopic organization. Namely, the mandibular division was represented along the border between the VPM proper and nucleus ventralis posterior medialis parvocellularis (VPMpc). The ophthalmic division was represented dorsolaterally. The maxillary division fell in between. No other types of trigeminal nociceptive neurons were found in the caudal VPM where trigeminal NS neurons were encountered. After cooling the dorsolateral surface of the medulla oblongata caudal to the obex, responses of NS neurons within the shell region of the caudal VPM to noxious stimulation of the peripheral receptive field reversibly disappeared. The responses were also eliminated by the contralateral trigeminal tractotomy at the level of the obex. It was inferred that trigeminal NS neurons within the shell region of the caudal VPM receive nociceptive input relayed via NS neurons located within the marginal layer of the trigeminal subnucleus caudalis.

Location and functional organization of trigeminal wide dynamic range neurons within the nucleus ventralis posteromedials of the cat

Abstract In urethane-chloralose-anesthetized cats, trigeminal wide dynamic range (WDR) neurons were explored in the posterior thalamus. They were found within a narrow zone (about 300 μm wide) of the shell region of the nucleus ventralis posteromedialis (VPM), just rostral to the region where trigeminal nociceptive specific (NS) neurons were located. This narrow zone was somatotopically organized with respect to the center of the receptive field. The mandibular division was represented ventromedially, the ophthalamic division were represented dorsolaterally, and the maxillary division fell in between. Neither NS nor WDR trigeminal units were found within the medial part of the posterior complex of the thalamus (POm).

Facilitation of voluntary swallowing by chemical stimulation of the posterior tongue and pharyngeal region in humans

In this study, we investigated the functional difference between chemical stimulations of the posterior tongue (PT) and pharyngeal region (PR) for facilitation of voluntary swallowing in humans. The PT or PR stimulation consisted of infusion of water (distilled water), 0.3 M NaCl solution or olive oil (non-chemical stimulant) into the PT or the PR through a fine tube at a very slow infusion rate (0.2 ml/min). Water was used as a stimulant of water receptors. A solution of 0.3 M NaCl was used as an inhibitor of the response of water receptors and as a stimulant of salt taste receptors. Excitation of the mucosal receptors would facilitate voluntary swallowing and diminution of sensory inputs from the oral mucosa would induce difficulty in swallowing. Swallowing intervals (SIs) during voluntary swallowing were measured by submental electromyographic activity. Infusion of water into the PR shortened SI (facilitation of swallowing) and infusion of 0.3 M NaCl or olive oil into the same region prolonged it (difficulty in swallowing). On the other hand, infusion of water into the PT prolonged SI and infusion of 0.3 M NaCl into the same region shortened it. The results suggest that water receptors are localized in the PR and that salt taste receptors are almost absent in the PR and present in the PT. With diminution of sensory inputs from the oral mucosa, central inputs would play a dominant role in initiating swallowing voluntarily, and SI would be prolonged. With weak stimulation (infusion of 0.3 M NaCl into the PR or infusion of water into the PT), SI was prolonged and inter-individual variation in SI was pronounced, suggesting that the ability of the central regulation of swallowing to perform repetitive voluntary swallowing varies among subjects. With stimulation of water receptors or salt taste receptors, SI was shortened and inter-individual variation in SI was moderate, suggesting that sensory inputs are important for performing voluntary swallowing smoothly and that the sensory inputs compensate for the difficulty in performing swallowing caused by the central mechanism.

Functional difference of tooth pulp-driven neurons in oral and facial areas of the somatosensory cortex (SI) of the cat.

Single-unit discharges were recorded in the oral and facial areas of the cat somatosensory cortex (SI) while electrical stimuli were individually delivered to eight tooth pulps. The incidence of the tooth pulp-driven (TPD) neurons was 44.7% in the oral area, but only 17.3% in the facial area. Both sets of neurons were also excited by nonnoxious stimulation of the oral structures or of the facial hair, and thus were polymodal. These TPD neurons were confirmed histologically to be in area 3b and were classified into monotooth input type and multitooth input type according to their response to stimulation. Neurons of the monotooth input type appeared three times more frequently in the oral area than in the facial area. The input(s) to the TPD neurons in the former area were slightly stronger from the canine(s) than from the molar(s), but the opposite was the case in the facial area. In the oral area, 83% of the TPD neurons responded with brisk discharges of short latency, whereas 54% of the TPD neurons in the facial area responded with those of a long latency. These findings suggest that the pulpal information to the somatosensory cortex is conveyed by pathways that appear, at least at certain points in the nervous system, to be spatially separated.

Physiological properties of tooth pulp-driven neurons in the first somatosensory cortex (SI) of the cat.

&NA; Tooth pulp‐driven (TPD) neurons are found in the oral area of the first somatosensory cortex (SI) of the cat. They have been classified according to their discharge patterns in response to electrical stimulation of the tooth pulp: the fast (F) type, slow (S) type, and a fast (Fa) type accompanied by afterdischarges. The characteristics of each type of TPD neuron were investigated in cats anesthetized with nitrous oxide and halothane. In surface distribution, there was no biased localization for any of the types. However, F‐type neurons receiving input from the mandibular tooth tended to be found more medially than F‐types receiving maxillary input. These TPD neurons did not change their firing pattern even when the stimulus was intensified. Mean threshold of the F‐type to tooth pulp stimulation was 7.8 ± 1.6V and tended to be lower than that of the S‐type (16.3 ± 3.0V). Graded increases in tooth pulp stimulation produced progressive increases in discharge frequency of both types of neurons. An analysis of the power function in relation to stimulus vs. response demonstrated that the exponent of the S‐type neuron was about 2.0, being significantly larger compared to the 0.8 value for the F‐type. The mean number of pulps afferent to an F‐type was 1.6, compared to 4.8 for an S‐type or 4.3 for an Fa‐type. The results suggest that F‐type TPD neurons may play a more important part in localizing pulpal pain and in recognizing the intensity than the other types.

Tooth pulp stimulation induces c-fos expression in the lateral habenular nucleus of the cat

Detection of Fos protein expression by the peroxidase-antiperoxidase method was used to determine the area in the habenular (Hb) complex responding to electrical stimulation of the tooth pulp in the cat anaesthetized with pentobarbital. In the anaesthetic-injected group, the Fos-positive neurones were found bilaterally in the lateral Hb nucleus (HbL). Tooth pulp stimulation (intensity: 3 times the threshold for jaw-opening reflex) increased the number of positive neurones within the HbL by up to 300%, but did not induce any expression in the medical Hb nucleus. The increase in HbL was inhibited by morphine (2 mg kg-1, i.p.). These findings and the results of previous research suggest that HbL neurones are involved in defensive mechanisms by means of antinociception following noxious stimulation.

Characteristics of the tooth pulp-driven neurons in a functional column of the cat's somatosensory cortex (SI)

SummaryThe columnar arrangement of tooth pulp-driven (TPD) neurons in the first somatosensory cortex (SI) was studied by single unit analysis in the cat anesthetized with nitrous oxide and halo thane. Tactile sensitive neurons in the oral area of SI were arranged functionally in a columnar organization. About 32% of the recorded neurons also responded to tooth pulp stimulation. Thus, the TPD neurons located in area 3b seem to be multireceptive in sensory modality. About 80% of the TPD neurons were F-type which respond with a short latency to tooth pulp stimulation and receive input from a small number of teeth. When a microelectrode penetrated through a single vertical column, most of the F-type TPD neurons encountered received common input from one or two tooth pulps. However, when an electrode was inserted across more than one column, there was no common pulpal input to the TPD neurons encountered in a track. Thus, the TPD neurons in a single column had identical peripheral and pulpal receptive fields, and the same latency of responses to tooth pulp stimulation. These characteristics would support the hypothesis that TPD (presumed nociceptive) neurons are arranged in a vertical column and it may be inferred that the columnar organization of TPD neurons in SI plays an important role in the sensory-discriminative aspects of pulpal pain.

Morphine selectively suppresses the slow response of tooth pulp-driven neurons in first somatosensory cortex (SI) of the cat

Tooth pulp-driven (TPD) neurons are found in the oral area of the first somatosensory cortex (SI) and they have been classified into fast (F-) type, slow (S-) type and fast type accompanied by afterdischarges (Fa). Effects of morphine on single unit responses of the F- and S-type neurons to pulp stimulation were examined by recording the discharges from a single neuron using a microelectrode. Intraperitoneal administration of morphine at 1.5 mg/kg markedly suppressed the response of S-type TPD neurons that fire with a long latency. This effect was completely reversed by naloxone at 0.2 mg/kg. On the other hand, the discharges of F-type neurons that fire with a short latency were not affected by application of morphine at 3 mg/kg. These findings suggest that S-type neurons in SI are concerned with pain perception.

Responses of diencephalic nociceptive neurones to orofacial stimuli and effects of internal capsule stimulation in the rat

The effect of conditioning stimulation of the internal capsule on nociceptive neurones in the rat diencephalon was investigated. The animals were anaesthetised with N(2)O/O(2) (2:1) and 0.5% halothane, and immobilised with pancuronium bromide. Nociceptive neurones responding to noxious stimulation of the face and oral structures were recorded in the ventral posteromedial nucleus, posterior group and zona incerta. These neurones were classified into wide dynamic range and nociceptive-specific types. Functional segregation of these nociceptive neurones was not apparent within the nucleus or between nuclei. A test stimulus with a single rectangular pulse (5-70 V) was applied to the centre of the receptive field; the nociceptive neurones exhibited short- and/or long-latency responses. Both responses in about 45% of the nociceptive neurones were inhibited by conditioning stimuli to the contralateral internal capsule with trains of 33 pulses (300 microA) at 330 Hz. The percent inhibitory effects on the nociceptive neurones of each area were 68.0+/-14.8% (n = 6) in the ventral posteromedial nucleus, 72.8+/-12.4% (n = 4) in the posterior group and 61.5+/-7.5% (n = 4) in the zona incerta. Effective sites for conditioning stimulation were concentrated in the lateral side of the internal capsule, through which the corticofugal fibres from the somatosensory cortex pass. These findings suggest that the transmission of nociceptive information to the diencephalon is modulated by stimulation of the internal capsule at the level of the trigeminal sensory complex in the brainstem. They might provide a novel way to elucidate the neurophysiological basis for antinociception by stimulation of the internal capsule observed in clinical studies.

Electrical stimulation of tooth pulp increases the expression of c-fos in the cat supraoptic nucleus but not in the paraventricular nucleus

Immunoreactivity to Fos protein was detected in the supraoptic (SON) and para-ventricular (PVN) nuclei of the cat using immunohistochemical methods. In the intact animal group, only a few Fos-positive neurons were observed in the PVN, but the SON did not contain any positive neurons. Intraperitoneal injection of pentobarbital sodium (Nembutal: 35 mg/kg) induced c-fos expression in the SON, but not in the PVN. Electrical stimulation of tooth pulp with an intensity that was 3 times the threshold of the jaw-opening reflex (200-600 microA) increased the number of Fos-positive neurons in the SON by up to 388% as compared with those of the Nembutal group, whereas the stimulation did not alter the number in the PVN. The increase was observed throughout the extent of the SON. In addition, morphine treatment (2 mg/kg, i. p.), 5 minutes before tooth pulp stimulation, considerably inhibited the increase in the SON. There were no significant differences among the 3 groups (intact, Nembutal, morphine) in the number of positive neurons in the PVN. These findings suggest that these hypothalamic nuclei have different functional roles and that the SON is involved in nociception and/or the consequent emotional and visceral reactions.