Noxious stimuli and Met-enkephalin release from nucleus reticularis gigantocellularis

The nucleus reticularis gigantocellularis of the rat medulla oblongata was perfused in situ, and effects of noxious stimuli on the release of immunoreactive Met-enkephalin were examined. Formalin-induced and thermal but not mechanical stimuli increased the "tonic" release of immunoreactive Met-enkephalin from this nucleus. No "phasic" increase was observed following the three forms of stimulation. A topical application of dibucaine abolished the formalin-induced increase in the release of immunoreactive Met-enkephalin. Therefore, the possibility that persistent noxious stimuli may activate the Met-enkephalin-containing fibers in the nucleus reticularis gigantocellularis has to be given consideration.     

Age-related alterations in responses of nucleus basalis magnocellularis neurons to peripheral nociceptive stimuli

The present study investigated the effects of peripheral noxious stimuli on the spontaneous activity of the nucleus basalis magnocellularis (nbM) neurons in young, adult and old rats. Single unit extracellular recordings from the nbM neurons were obtained with glass micropipettes in urethane-anesthetized rats. A total of 104 units were antidromically identified as nbM-cortical neurons. Noxious but not innocuous mechanical stimulation elicited responses in 72% of the 104 neurons. Most of them were excited. The receptive fields were usually very large and bilateral. Most of the neurons also responded to noxious thermal, chemical and electrical stimuli. No marked differences were observed in the incidence of neurons having different spontaneous firing rates, firing patterns and response type among the three age groups. However, the latency of responses evoked by noxious thermal or electrical stimulation and the threshold of excitatory responses evoked by electrical stimulation were increased with aging. The duration and peak-firing rate of excitatory responses evoked by noxious thermal, chemical or electrical stimulation were decreased in old rats. These findings indicate that there might be some functional changes in the nbM neurons and its projection pathway with aging, which impair their responsive ability to peripheral nociceptive stimuli.     

Effects of cerebellar nuclei stimulation on the electrical activity of the neurons in the centrum medianum nucleus in cats

The responses evoked in the neurons of the centrum medianum nucleus of the thalamus by stimulation of the deep cerebellar nuclei were studied. A marked predominance of activation effects during single shocks and lowfrequency stimulation was revealed. This was most striking with stimulation of the dentate nucleus. Parallel with increases in stimulation repetition rates, there was a noticeable rise both in unit responsiveness and in depressive responses. In general the percentage of unidirectional responses (either activation or depression) was considerably greater than that of responses which changed in direction with increases in stimulus rate. A predominance of tonic post-stimulation effects was also revealed. This was more marked after dentate or fastigial stimulation. In some centrum medianum neurons single shocks to the cerebellar nuclei evoked phasic responses. One type of these responses had a constant latency of 2- to 12-ms duration. A second type showed somewhat longer, although inconstant, latencies (≤20 ms). Single shocks to the dentate nucleus produced two temporal classes of latency (2 to 6 and 13 to 17 ms). In another type of centrum medianum neuron single shocks to the cerebellar nuclei, especially dentate and interpose, elicited depression of spontaneous firing for periods of 50 to 100 ms. A clear synchronization of spontaneous neuronal activity was evoked by low-frequency stimulation as well. Nearly 50% of the responsive neurons could be driven by excitation of the cerebellar nuclei. Convergent effects occurred more often with higher frequencies than with single-shock stimulation.     

Neuronal receptive field properties in feline nucleus reticularis gigantocellularis

Nucleus reticularis gigantocellularis (NGC) has been shown, using both behavioral and physiological techniques, to be involved in the processing of nociceptive information. However, previous studies of the receptive fields of NGC neurons have utilized only innocuous stimuli. Thus, while neurons in NGC may play an important role in nociception, the receptive field properties of these cells remain to be defined. This investigation was designed to determine the receptive field properties of neurons in NGC using nociceptive and innocuous stimuli. Receptive fields were determined for 127 neurons in NGC. Eighty-seven percent of the NGC neurons studied responded exclusively to noxious stimuli, while 13% also responded to innocuous stimuli. None of the neurons studied responded exclusively to innocuous stimuli. The receptive fields of most NGC neurons (63%) were large, discontinuous, and bilaterally symmetrical. Eighty-one percent of NGC neurons received convergent inputs from both spinal and trigeminal systems. These receptive field properties differ from those previously reported using only innocuous stimulation     

Ascending inhibition of nociceptive neurons in the nucleus ventralis posterolateralis following conditioning stimulation of the nucleus raphe magnus

Recordings were made from neurons in the nucleus ventralis posterolateralis (VPL) of urethane-chloralose-anesthetized cats, following both noxious mechanical stimulation of the integument and electrical stimulation of the greater splanchnic nerve (SPL). The effects of stimulating the nucleus raphe magnus (NRM) on responses obtained from these units were investigated. Units responding to noxious mechanical stimulation of the integument with SPL input were found in the posterior shell region of the VPL. Responses elicited from these units by electrical stimulation of the SPL were inhibited following conditioning stimulation in or near the NRM. Inhibition could still be demonstrated after bilateral section of the dorsolateral funiculi at the level of C₃−C₄. Responses of these units to electrical stimulation of the ventrolateral funiculus (VLF) of the cervical cord were also inhibited following conditioning stimulation in or near the NRM. These results suggest that inhibition of these units produced by conditioning NRM stimulation may be partially mediated by an ascending pathway, in addition to the well-known descending spinal pathways. Glutamate stimulation of the NRM inhibited responses of nociceptive VPL units to SPL stimulation, but responses of the same units to VLF stimulation were little affected by the glutamate stimulation of the NRM. Inhibition of responses of nociceptive VPL units to SPL stimulation may be due to anti-dromic excitation of brainstem neurons having efferent connection with the NRM.     

Responses of caudate nucleus neurons to acoustic stimulation during functional blockade of the centrum medianum of the thalamus in the cat

Responses of caudate nucleus to sound clicks (0.5/s) were recorded extracellularly in chronic experiments on cats under conditions of functional blockade of centrum medianum. Analysis of average poststimulus histograms has shown that functional blockade of centrum medianum in 30% of the caudate nucleus neurons induced a decrease in phasic activating responses to sound signals and in 52% of neurons their disappearance. A decrease in the level of spontaneous caudate nucleus neurons activity was also found. Direct pathways from centrum medianum to caudate nucleus are discussed for their significance in transmission of sound signals to caudate nucleus neurons.     

Feedback loop between locus coeruleus and spinal trigeminal nucleus neurons responding to tooth pulp stimulation in the rat

IGARASHI, S., M. SASA AND S. TAKAORI. Feedback loop between locus coeruleus and spinal trigeminal nucleusneurons responding to tooth pulp stimulation in the rat. BRAIN RES. BULL. 4(1) 75–83, 1979.—Studies were performed to elucidate reciprocal relationships between locus coeruleus (LC) and spinal trigeminal nucleus (STN) neurons responding to tooth pulp (TP) stimulation using rats anesthetized with α-chloralose. LC conditioning stimulation inhibited STN field potential as well as orthodromic spike generation of STN neurons produced by ipsilateral TP stimulation, confirming the previous findings in cats that LC neurons played an inhibitory role in the orthodromic transmission in STN neurons. Forty-one out of 56 LC neurons were activated by ipsilateral TP stimulation and 12 neurons by stimulation of both ipsi- and contralateral TP. STN stimulation usually excited LC neurons with a significantly shorter latency than did TP stimulation, including three LC neurons with a latency of less than 2.0 msec. These results indicate the existence of input from TP to LC neurons via multisynapses. In addition, neurons antidromically activated by STN stimulation were found in LC. It is highly probable, therefore, that there is a feedback loop between LC and STN, which might control input from TP to STN.     

Cholecystokinin-containing and nociceptive neurons in rat edinger-westphal nucleus

The putative peptide neurotransmitter cholecystokinin (CCK) is co-localized witsubstance P (SP) in a dense cluster of neurons located in the rat Edinger-Westphal (EW) nucleus. In an attempt to record electrophysiologically from these CCK-containing neurons, we have identified a group of nociceptive neurons located within the confines of the EW nucleus. The firing pattern of these nociceptive neurons is erratic, sometimes wita bursting pattern and at other times witfairly regular rates whicvary generally from 1 to 7 Hz. These neurons respond to noxious stimuli, sucas toe pinch, witan increase in rate and sometimes enter an apparent depolarization blockade (preceded by an increase in the duration and a decrease in the amplitude of the action potential). Systemically administered morphine suppresses botthe spontaneous firing rate and the toe pinch-induced increase in firing rate. Naloxone is able to reverse the effects of morphine. Althougwe have identified in the EW area a moderate density of terminals containing enkephalin-like immunoreactivity, morphine locally applied via microiontophoresis is largely without effect on the firing of these neurons. We hypothesize that the opiate-induced suppression of these nociceptive neurons is not mediated directly on the EW cells, but rather indirectly througafferent systems.     

Noxious stimuli excite neurons in nucleus submedius of the normal and arthritic rat

Anatomical studies have revealed the existence of an ascending pathway originating in the spinal cord and medullary dorsal horn, relaying in nucleus submedius (Sm) in medial thalamus and terminating in ventrolateral orbital cortex. It has been suggested that this pathway may be involved in the transmission of nociceptive information. In the present study extracellular recordings were obtained from neurons in Sm of anesthetized arthritic and normal rats. Mechanical and thermal stimuli were delivered to various regions of the body to determine the types of somatic stimuli which could activate Sm neurons. Over 40% of the 146 neurons studied responded to somatic stimuli. In the normal rats only high intensity mechanical and thermal stimuli were effective in inducing responses. In the arthritic rats lower intensity mechanical stimuli, joint movements and high intensity thermal stimuli were effective. Such stimuli produce nociceptive reactions in the freely moving arthritic rat. Almost all the responses were excitatory and generally lasted the entire duration of the 15-s stimuli employed. In some cases after-discharges were present. The receptive fields of the neurons were in almost all cases large and bilateral. These findings support the hypothesis that Sm may be involved in mediating the affective-motivational aspects of pain.     

Reactions of neurons of the reticular and ventral anterior thalamic nuclei to electrical stimulation of the centrum medianum of the thalamus

Responses of 145 reticular (R) and 158 ventral anterior (VA) thalamic neurons to electrical stimulation of centrum medianum (CM) were studied in cats anaesthetized with thiopental sodium (30-40 mg/kg intraperitoneally) and immobilized with d-tubocurarine (1 mg/kg). 4.1% of R and 4.4% of VA neurons under study responded to CM stimulation by antidromic spike (latency 0.3-2.0 ms). The conduction velocity of antidromic excitation in axons of those neurons was found to be 1.7-7.6 m/s. There were neurons which responded by antidromic spike to the other thalamic nuclei stimulation as well as to CM. This fact is the electrophysiological proof of the axonal branching in these neurons. 53.8% of R and 46.9% of VA neurons responded to CM stimulation with orthodromic excitation. Two groups of cells were separated among neurons excited orthodromically. The first group neurons responded to CM stimulation by discharges composed of 6-12 spikes with frequency of 130-640 per second. The neurons of the second group generated a single spike. Inhibitory reactions were noticed only in 0.7% of R and in 4.4% of VA neurons. It is shown that afferent impulses from relay nuclei, lateral posterior nucleus and motor cortex converged to some R and VA neurons responding to CM.     

Thalamic paraventricular nucleus neurons collateralize to innervate the prefrontal cortex and nucleus accumbens

The prefrontal cortex and nucleus accumbens are primary recipients of medial thalamic inputs, prominently including projections from the thalamic paraventricular nucleus. It is not known if paraventricular neurons collateralize to innervate both the prefrontal cortex and nucleus accumbens. We used dual retrograde tract tracing methods to examine this question. A small population of paraventricular neurons was found to innervate the prefrontal cortex and medial nucleus accumbens. These data suggest that the thalamic paraventricular nucleus may coordinately influence activity in the prefrontal cortex and ventral striatum.     

Distribution of thalamic nociceptive neurons activated from the tail of the rat

The purpose of the present study was to map systematically in the thalamus the distribution of neurons processing nociceptive information from the tail of the rat. Pentobarbital-anesthetized and gallamine-paralyzed rats were used. Glass microelectrodes were used to record extracellularly from thalamic neurons. Noxious radiant heat stimuli were applied to the tail with a tail-flick apparatus, and the recorded neurons were localized with horseradish peroxidase deposits or by marking electrodes left in situ. A number of 121 neurons were tested of which 45 responded. Of these, 13 were located in the ventrobasal complex (VB), 17 were located in the central lateral nucleus and the parafascicular nucleus of the intralaminar nuclei (ILN). The rest of the responding neurons were located in the posterior group, the reticular thalamic nucleus, and the zona incerta. The nucleus submedius was not examined specifically. It is concluded that the VB and the ILN are two of the most important thalamic nuclei for processing nociceptive information from the tail of the rat.     

Interactions between superficial and deep dorsal horn spinal cord neurons in the processing of nociceptive information

In acute rat spinal cord slices, the application of capsaicin (5 μm , 90 s), an agonist of transient receptor potential vanilloid 1 receptors expressed by a subset of nociceptors that project to laminae I–II of the spinal cord dorsal horn, induced an increase in the frequency of spontaneous excitatory and spontaneous inhibitory postsynaptic currents in about half of the neurons in laminae II, III–IV and V. In the presence of tetrodotoxin, which blocks action potential generation and polysynaptic transmission, capsaicin increased the frequency of miniature excitatory postsynaptic currents in only 30% of lamina II neurons and had no effect on the frequency of miniature excitatory postsynaptic currents in laminae III–V or on the frequency of miniature inhibitory postsynaptic currents in laminae II–V. When the communication between lamina V and more superficial laminae was interrupted by performing a mechanical section between laminae IV and V, capsaicin induced an increase in spontaneous excitatory postsynaptic current frequency in laminae II–IV and an increase in spontaneous inhibitory postsynaptic current frequency in lamina II that were similar to those observed in intact slices. However, in laminae III–IV of transected slices, the increase in spontaneous inhibitory postsynaptic current frequency was virtually abolished. Our results indicate that nociceptive information conveyed by transient receptor potential vanilloid 1‐expressing nociceptors is transmitted from lamina II to deeper laminae essentially by an excitatory pathway and that deep laminae exert a ‘feedback’ control over neurons in laminae III–IV by increasing inhibitory synaptic transmission in these laminae. Moreover, we provide evidence that laminae III–IV might play an important role in the processing of nociceptive information in the dorsal horn.     

Differential effects of trigeminal tractotomy on Aδ- and C-fiber-mediated nociceptive responses

In this study we have tested in the rat, whether trigeminal tractotomy, which deprives the spinal trigeminal nucleus caudalis (Sp5C) of its trigeminal inputs, affected differentially nociceptive responses mediated by C- vs. Aδ-nociceptors from oral and perioral regions. Tractotomy had no effect on the threshold of the jaw opening reflex, induced by incisive pulp stimulation (Aδ-fiber-mediated), but blocked the formalin response (mainly C-fiber-mediated). These results suggest that nociceptive responses mediated by trigeminal C-fibers completely depend on the integrity of the Sp5C, while intraoral sensations triggered Aδ-fibers (especially of dental origin) are primarily processed in the rostral part of the spinal trigeminal nucleus.     

Extracellular glutamate, aspartate and arginine increase in the ventral posterolateral thalamic nucleus during nociceptive stimulation

Although there is evidence that the thalamus plays a remarkable role in pain processing few in vivo studies on the thalamic neurochemical correlates of pain have been done. In the present experiments a combination of capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIF) and microdialysis in freely moving rats was used to measure extracellular arginine, glutamate and aspartate in the thalamus during the formalin test. Microdialysis probes were implanted in the left ventral posterolateral (VPL) nucleus of the thalamus in rats. Samples were collected every 30 s, derivatized with fluorescein isothyocyanate and injected into a CZE-LIF instrument. After nine baseline samples, a subcutaneous formalin (5%, 50 microl) injection in the right hind paw caused an increase of arginine, glutamate and aspartate that lasted for about 3 min. These increases were calcium and nerve impulse dependent. These results indicate that the release of arginine, glutamate and aspartate may mediate rapid pain neural transmission in the VPL nucleus of the thalamus.     

Effect of different concentrations of iontophoretic nociceptin on distinct classes of nociceptive neurons in rat spinal cord

Iontophoretically applied nociceptin (NC) was tested at different concentrations on the activity of spinal nociceptive specific (NS) and wide dynamic range (WDR) neurons. Low NC dosages inhibited the noxious response of NS neurons, higher dosages inhibited the noxious responses of the WDR neurons but had little effect on the non-noxious response. Naloxone did not antagonize the NC effect. Thus, appropriate dosages of NC may be selective, both for neuronal classes and for sensory modalities.     

Parvalbumin- and calretinin-immunoreactive trigeminal neurons innervating the rat molar tooth pulp

Calcium-binding proteins and neuropeptides were examined in trigeminal neuronal cell bodies retrogradely labeled with Fast blue (FB) from the maxillary molar tooth pulp of the rat. FB-labeled cells were located in the maxillary division of the trigeminal ganglion. 30 and 50% of the labeled cells were immunoreactive for parvalbumin and calcitonin gene-related peptide (CGRP), respectively. The coexpression of these substances was observed in 9.5% of FB-labeled cells. On the other hand, 2.4% of FB-labeled cells exhibited calretinin-immunoreactivity (CR-ir) and 20% tachykinin (TK)-ir. The coexpression of CR and TK was observed in 1.9% of FB-labeled cells, i.e., most of CR-ir FB-labeled neurons coexpressed TK-ir. An immuno-EM method revealed that all parvalbumin-ir nerve fibers in the root pulp were myelinated and that CGRP-ir nerve fibers were both myelinated (15%) and unmyelinated (85%). The present study indicated that primary nociceptors innervating the rat molar both pulp contained parvalbumin and CR and coexpressed these calcium-binding proteins and neuropeptides. It was suggested that peripheral axons of parvalbumin-ir tooth pulp primary neurons are all myelinated. Most peripheral CR-ir axons are probably unmyelinated because TK-ir myelinated axons have never been demonstrated in any peripheral organ.