R. Sumino

Morphological and electrophysiological properties of ACCx nociceptive neurons in rats.

A total of 33 neurons with cutaneous receptive fields were recorded from the anterior cingulate cortex (ACCx) and successfully injected with neurobiotin. All neurons were in area 24 of the ACCx. Neurons from the ACCx had large receptive fields (RFs), usually bilateral, and some had RFs covering the whole body surface. Most of the neurons were in the deep laminae and had a pyramidal soma with thick apical dendrites and many spines. Thirteen of 33 neurons were classified as pyramidal nociceptive specific (NS) neurons and 12 as noxious-tap neurons, 3 neurons received inhibitory input and were in lamina V. Two non-pyramidal noxious-tap neurons were located in lamina V and 1 pyramidal noxious-tap neuron was located in lamina VI, and 2 pyramidal NS neurons were in lamina III. Axon collaterals of NS neurons were mainly distributed around the soma, whereas those of noxious-tap neurons were also distributed far from the soma. A large number of varicosities were observed on the axon collaterals of both NS and noxious-tap neurons. Our results suggest that NS neurons in the ACCx send information locally to the vicinity of the soma, while noxious-tap neurons send information to a wider area of the ACCx.

Effects of intrastriatal injections of selective dopamine D-1 and D-2 agonists and antagonists on jaw movements of rats

The effects of bilateral intrastriatal injections of the selective D-1 and D-2 antagonists, SCH23390 and sulpiride on apomorphine-induced jaw movements were studied in ketamine-anaesthetized rats after C1 spinal transection. A photo-transducer attached to the lower mandible automatically detected jaw movements. Apomorphine (0.2, 0.5 and 1.0 mg/kg i.v.) dose dependently increased jaw movements, an effect prevented by prior administration into the ventral striatum of either SCH23390 (0.1, 0.5 and 1 microgram) or sulpiride (125 ng). To be effective, SCH23390 had to be given less than 30 min before apomorphine whereas sulpiride had to be given earlier. Sulpiride injected into the dorsal striatum potentiated the effects of apomorphine, an action prevented by administering the sulpiride with SCH23390. Local application of the selective D-1 and D-2 agonists, SKF38393 (5 micrograms) and quinpirole (10 micrograms) into sites within the ventral striatum from which repeated jaw movements could be obtained by electrical stimulation, also evoked jaw movements; the effects of combining the two drugs were much greater than the effects of either drug alone.

Fos protein induction in the medullary dorsal horn and first segment of the spinal cord by tooth-pulp stimulation in cats

&NA; Electrophysiological studies using the single neuron recording technique have led to the hypothesis that nociceptive neurons in the medullary dorsal horn (MDH) and the first segment of the spinal cord (C1) encode the stimulus intensity of noxious stimuli applied to the tooth pulp. The present study utilized the Fos protein technique in combination with electrical and chemical stimulation of the tooth pulp to test this hypothesis. Upper canine tooth‐pulp stimulation with intensities just above the threshold stimulus intensity for evoking the jaw‐opening reflex (JOR) did not produce a clear expression of Fos protein‐like immunoreactive (LI) cells in the MDH and C1 of cats. Fos protein‐LI cells were mainly found in the superficial laminae (laminae I–II) of the MDH and C1 after tooth‐pulp stimulation of 200% of the JOR threshold intensity. When higher intensities (400–600% of the JOR threshold intensity) or mustard oil were applied, Fos protein‐LI cells were also found in laminae III–IV as well as in laminae I–II. The number of Fos protein‐LI cells significantly increased when 600% of the JOR threshold intensity or mustard oil was applied. Furthermore, the rostro‐caudal distribution of Fos protein‐LI cells was greater following increases in stimulus intensities and the greatest after mustard oil application. These data suggest that the change in number and spatial arrangement of nociceptive neurons in the MDH and C1 reflect changes in the encoding of the stimulus intensity applied to the tooth pulp.

Fos induction in the medullary dorsal horn and C1 segment of the spinal cord by acute inflammation in aged rats

In order to elucidate the effect of aging on nociceptive neurons in the central nervous system, c-fos was used as a marker of excitability of neurons in the medullary dorsal horn (MDH) and the first spinal segment (C1) following noxious stimulation of the lateral face of young and aged rats. The distribution of c-fos-positive cells was dense in the superficial laminae and sparse in the deep laminae of the MDH and C1 in both young and aged animals following subcutaneous injection of formalin into the lateral face, whereas few c-fos-positive cells were labeled after saline injection. The distribution of c-fos-positive cells in the superficial laminae of the aged rats was found to be denser and more rostro-caudally expanded compared to that in the young rats. C-fos-positive cells were distributed more rostro-caudally in aged than in young rats. There was no difference between young and aged rats in the distribution of c-fos-positive cells in the deep laminae. Substance P (SP), 5-HT and calcitonin gene-related peptide-like immunoreactive (CGRP-LI) fibers and varicosities showed similar distribution density in the MDH and C1. Furthermore, many 5-HT-LI aberrant fibers and varicosities were observed in the MDH and C1 of the aged rats. The SP-LI and CGRP-LI cells in the trigeminal ganglion of aged rats were larger than those of young rats. These findings suggest that a deficit of the descending 5-HT inhibitory system produces the increment of c-fos-positive cells in the MDH and C1 of aged rats, resulting in the recruitment of a larger number of neurons in the superficial laminae of the MDH and C1 for conveying nociceptive sensory information to the central nervous system.

Responses of bradykinin sensitive tooth-pulp driven neurons in cat cerebral cortex

SummaryThe properties of single cortical neurons responding to electrical stimulation of the tooth-pulp and to intrapulpal application of bradykinin were studied in the cat. The activities of tooth-pulp driven neurons (TPNs) were recorded from the middle and anterior parts of the coronal gyrus of the cerebral cortex. Bradykinin-sensitive tooth-pulp driven neurons (BK-TPNs) were located in layer IV of area 3b of the anterior part of the coronal gyrus. These neurons had a large cutaneous oro-facial receptive field and received a nociceptive input from the facial skin as well as from the tooth-pulp. The BK-TPNs had a higher threshold and longer latency to electrical stimulation than TPNs insensitive to bradykinin (non BK-TPNs). These findings suggest that BK-TPNs in this cortical area may be involved in sensory processing of noxious information from trigeminal regions.

Cortical potentials associated with voluntary biting movement in humans

We compared the distribution of Bereitschaftspotentials (BPs) on both sides of the scalp preceding jaw biting movements in order to identify the relationship between the cortical regions and the activation of the masseter muscle in 10 healthy subjects. The BPs were recorded from the midline-central, central and temporal areas of the scalp according to the international 10-20 system, preceding self-paced biting on one side. The cortical negative potentials began 1.0 approximately 1.5 s before the EMG onset of the masseter muscle. All of these negative potentials could be considered to be BPs, and the additional negative slope component (NS) occurred 70 approximately 80 ms before the EMG onset of the masseter muscle. The BPs were detected from all the recorded regions of the scalp, while the NS was observed only from the bilateral temporal area. The amplitudes of BPs and NSs were largest in the temporal areas (T3 and T4) that were ipsilateral to the biting. The rates of occurrence of NS at T3 and T4 ipsilateral temporal areas were 80% and 60%, respectively. These results suggest that unilateral biting movements may be controlled mainly from the ipsilateral hemisphere.

Comparisons of the sensation perceived and intradental nerve activity following temperature changes in human teeth

SummaryThe relationship between the intradental nerve responses and subjective sensory ratings evoked by thermal stimulation of the teeth was studied in man. Recordings were taken from a total of 12 thermally sensitive units from the inferior dental nerve following thermal tooth stimulation, of which seven responded to both heating and cooling, two were exclusively cold-sensitive and three exclusively heat-sensitive. The early and late sensory responses following both cold and heat stimulation of the tooth were observed. The late sensory responses were more unstable than the early sensory responses. The mean threshold of the early sensory responses to tooth cooling was 13.6±1.9° C (n = 9) and that to heat stimulation was 48.4±4.8° C (n=10). The firing frequency of the heat-sensitive, but not the cold sensitive, units increased linearly in proportion to the increase in magnitude of the early sensory ratings.

Response properties of primary somatosensory cortical neurons responsive to cold stimulation of the facial skin and oral mucous membrane

The distribution and response characteristics of the primary somatosensory cortical (SI) neurons activated by cold stimulation of the facial skin and oral mucous membrane were studied in cats. The discharge activities of 53 cold-sensitive SI neurons that responded to a decrease in temperature of the facial skin and/or oral mucous membrane were recorded. Each of these neurons was classified according to its responsiveness to mechanical stimulation as follows: LTM (low-threshold mechanoreceptive, 14/53), WDR (wide dynamic range, 39/53) and NS (nociceptive-specific, none identified). Encoding and non-encoding SI cold-sensitive neurons were identified, according to their responsiveness to decremental thermal stimulation. Only 14 cold-sensitive SI neurons demonstrated increased firing frequencies when subjected to incremental stimulus intensity increases and were classified as the encoding-type, whereas 39 non-encoding-type neurons did not.

Response properties of nociceptive neurons in the spinal dorsal horn of aged rat

In a previous study using a rat model of inflammatory hyperalgesia. we suggested that peripheral intlammation activated the descending modulation system from the nucleus locus coeruleus (LC) The aim of the present study was to demonstrate this notion neurochemicall) One week before the experiment, male Sprague-Dawle) rats recei\,ed bilateral electrollzic lesions of the LC A coaxial microdialysis probe was inserted into the dorsal horn, and the dialysates sere collected sequentially e\er?; 20 min The noradrenaline (N.4) concentration in the dialqsate Has measured bk high performance liquid chromatography with electrochemical detection For induction of peripheral inflammation. carrageenan nas injected subcutaneousl) into the plantar surface of the left hindpaw. In the non-operated control group (n=7). 3 and 4 h after induction of inflammation. the 5.4 level increased significant& compared with the basal level ofNA In the LC-lesioned group (n=7). following induction of inflammation. the NA concentration did not change significantly compared with the basal level of Nr\ Statistical analysis of \A le\,els for the LC-lesioned and the non-operated control rats revealed a significant difference bet\\een groups at _. 1 3 and -1 h This result suggests that peripheral inflammation produces acti\.ation of descending NA-containing neurons projected to the dorsal horn from the LC

1802 Effects of aging on escape behavior and c-fos expression following noxious heat stimulation to the hind paw in rats

SHIKO CHICHIBU, ATSUSHI CHIBA, HIROMOTO NAKANISHI Several different types of mechanoreceptive hairs were found on the carapace of the American crayfish (Procambarus clarkii). Innervating sensory neurons are the detectors of physical components of the surrounding water current. As the base of the hairs is similar to that of the joystick, hair shaft displaces with the water current. The firing pattern of sensory neurons is classified into Acceleration type, Velocity type and Displacement type according to correlation to the stimulus pattern. Within the displacement angle of 1 degree from the resting position, the shape of the receptive field became close to the resting spontaneous firing level, though only the longer axis of the receptive field was recognizable. The degree of the ratio, longer axis/shorter axis, was largest in the A-type neuron, and then the V-type, and the D-type decreasing to l,O. With minute stimuli, neural informations sent through the A-type neurons are estimated more important than those of other two types.