Actions of midazolam in the spinal cord of the cat.

Midazolam, which induces anaesthesia in humans at intravenous doses of 0.3 mg/kg, did not anaesthetize cats at doses of 20 mg/kg. Nevertheless, intravenous doses as small as 0.3 mg/kg enhanced spinal primary afferent depolarization and presynaptic inhibition of spinal monosynaptic reflexes, and both intravenous and microelectrophoretic administration of midazolam enhanced the inhibitory effect of GABA on spinal neurones and the depolarization of Ia afferent terminations by GABA and piperidine-4-sulphonate. Some degree of specificity was demonstrated for the inhibitory effect of GABA in relation to those of glycine and noradrenaline, and the enhancement by midazolam of inhibition by GABA was blocked by R015-1788, which alone was inactive. Although these results are consistent with proposals that depressant benzodiazepines enhance the effectiveness of GABA as a central transmitter, such an effect alone may not fully account for the anaesthesia produced by midazolam in humans.

4,5,6,7-Tetrahydroisothiazolo[5,4-c]pyridin-3-ol and related analogues of THIP. Synthesis and biological activity.

The thio analogues of the GABA (gamma-aminobutyric acid) agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol), the GABA uptake inhibitor THPO (4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol), and the glycine antagonist THAZ (5,6,7,8-tetrahydro-4H-isoxazolo[4,5-d]azepin-3-ol) have been synthesized and tested biologically on single neurons in the cat spinal cord and in vitro by using synaptic membrane preparations obtained from rat brains. In contrast to THIP, thio-THIP (4,5,6,7-tetrahydroisothiazolo[5,4-c]pyridin-3-ol, 5) was only a weak GABA agonist. Thio-THPO (4,5,6,7-tetrahydroisothiazolo[4,5-c]pyridin-3-ol, 10) was slightly weaker than THPO as an inhibitor of GABA uptake in vitro, and these two compounds were approximately equipotent in enhancing the inhibition of the firing of cat spinal neurons by GABA. Like THAZ and structurally related bicyclic isoxazole zwitterions, thio-THAZ (5,6,7,8-tetrahydro-4H-isothiazolo[4,5-d]azepin-3-ol, 15) was an antagonist at glycine receptors on cat spinal neurons. The I/U ratios, which reflect the ability of neutral amino acids to penetrate the blood-brain barrier (BBB), were calculated for 5 (I/U = 16), 10 (63), and 15 (200). These low I/U ratios, compared with the findings that THIP (I/U = 500 or 1500) and THPO (I/U = 2500) enter the brain after systemic administration, suggest that the thio analogues may penetrate the BBB very easily.

Jun, Fos and Krox in the thalamus after C-fiber stimulation: coincident-input-dependent expression, expression across somatotopic boundaries, and nucleolar translocation.

Expression of the inducible transcription factors Jun, Fos and Krox is commonly used to map neurons in the brain that are activated by sensory inputs. However, some neurons known to be electrically excited by such inputs do not always express these factors. In particular, stimulation of hindlimb sensory nerve C-fibers induces expression of c-Fos in the medial thalamus (the mediodorsal, intermediodorsal, centrolateral and centromedial), but not in the lateral thalamus (the ventroposterolateral, ventroposteromedial and posterior group). We hypothesized that c-Fos expression might only occur in these lateral areas after more complex stimulation patterns, or that only other transcription factors can be induced in these areas by such stimuli. Thus we examined the effects of single, repeated and coincident C-fiber inputs on expression of six inducible transcription factors in the medial, lateral and reticular thalamus of the rat. A weak C-fiber input caused by noxious mechanical stimulation of the skin of one hindpaw did not induce expression of c-Fos, FosB, Krox-20 or Krox-24; but it did reduce the basal expressions of c-Jun and JunD in both the medial and lateral areas. An intense input produced by electrical stimulation of all the C-fibers in one sciatic nerve also failed to induce expression of c-Fos, FosB, Krox-20 or Krox-24 in the medial or lateral areas. However, in the medial thalamus it increased c-Jun and reduced the basal expression of JunD, whereas in the lateral thalamus it had no effect on c-Jun but again reduced the basal expression of JunD. With repeated stimulation, i.e. when the noxious stimulus was applied to the contralateral hindpaw 6 h after the sciatic stimulation, there was again no induction of c-Fos, FosB or Krox-20 in the medial thalamus; but there was an increase in c-Jun and Krox-24, and a decrease in JunD levels. In the lateral thalamus the repeated stimulation again failed to induce c-Fos, but the expressions of FosB, c-Jun and Krox-24 were increased, and that of JunD was again reduced. With coincident stimulation, i.e. when a stimulus was applied to each hindpaw simultaneously, c-Fos and Krox-24 remained absent; but there was a marked induction of FosB and Krox-20, a strong repression of c-Jun, and no effect or a reduction of the basal levels of JunD. This coincident stimulation also caused FosB to appear in the nucleolus of many thalamic neurons. MK-801, but not L-NAME, blocked all these changes. In summary, noxious stimulation affects the expression of all transcription factors in the medial, lateral and reticular thalamus in a complex manner depending upon the inducible transcription factor considered, the thalamic nucleus, and the stimulation paradigm. The expression of some transcription factors uniquely after simultaneous inputs suggests they act as coincidence detectors at the gene level.

Expression of immediate early gene proteins following axotomy and inhibition of axonal transport in the rat central nervous system.

The expression of the immediate early gene-encoded proteins c-Jun, Jun B, Jun D, c-Fos, Fos B and Krox-24 in central neurons following transection of, or inhibition of, axonal transport in their axons was investigated in the rat using immunocytochemistry. Transection of the medial forebrain bundle, which produces an essentially complete axotomy of neurons in the ipsilateral mammillary nucleus, substantia nigra pars compacta, ventral tegmental area and parafascicularis, induced the expression of c-Jun, Jun D and, to a lesser extent, Krox-24, in these nuclei. Microinjection of colchicine into the medial forebrain bundle to chemically inhibit axonal transport similarly induced the expression of these proteins in these areas. The expression of the proteins was first evident 24 h after transection, reached a maximum at 48 h and was still present after 10 days. However, after 30 days the proteins were absent from the substantia nigra, ventral tegmentum and parafascicularis, and were still present only in the mammillary nuclei. The other immediate early genes, Jun B, c-Fos and Fos B, were never expressed above the basal levels seen in untreated rats. Transection of the corpus callosum and the hippocampal commissure, which produces only a partial axotomy of neurons in the cerebral cortex and hippocampus, respectively, did not induce the expression of any of the genes in these neurons. Microinjection of colchicine or vinblastine to produce a localized inhibition of axonal transport in the cerebral cortex, hippocampus, thalamus and cerebellum also induced the expression of c-Jun, Jun D and, again to a lesser extent, Krox-24, in neurons surrounding the injection site. In contrast to this selective expression, administration of the neuronal excitant metrazole induced the expression of all six immediate early gene proteins in central nervous system neurons. These results demonstrate that transection of, or inhibition of, transport in the axons of central neurons induces a particular pattern of expression of transcriptionally operating immediate early genes that may be related to the regenerative competency of the neurons.

c-FOS-like immunoreactivity in rat brainstem neurons following noxious chemical stimulation of the nasal mucosa.

It has previously been shown that noxious and non-noxious peripheral stimuli induce c-fos expression in spinal dorsal horn neurons. In the present study we have examined the expression of c-fos in brainstem neurons following noxious chemical stimulation of the respiratory region of the nasal mucosa. In urethane-anaesthetized rats we injected mustard oil or applied CO2 pulses to the right nasal cavity. In control animals we applied paraffin oil or a continuous flow of air. A further group of control animals was anaesthetized and not subjected to any experimental treatment. Two hours after the first stimulus the rats were perfused with 4% phosphate-buffered paraformaldehyde. Brainstem sections were incubated with primary antiserum against the FOS protein and processed according to the ABC method. Only the mustard oil-treated rats had obvious signs of rhinitis and displayed FOS-positive cells in laminae I and II of the subnucleus caudalis and in the subnucleus interpolaris of the trigeminal brainstem nuclear complex as well as in the medullary lateral reticular nucleus. These areas are known to be involved in the processing of nociceptive information. Although CO2 pulses applied to the nasal mucosa are known to evoke pain sensations in man we did not observe any FOS-positive neurons in trigeminal and reticular brainstem areas of CO2-treated rats. This lack of c-fos expression probably results from the fact that unlike mustard oil, CO2 did not induce any apparent inflammatory reactions. In all animals c-fos expression was found in the nucleus of the solitary tract and in the area postrema. Staining in these areas might partly result from factors related to anaesthesia, changed respiration parameters and stress. Since the mustard oil-treated rats displayed the highest levels of immunoreactivity in the nucleus of the solitary tract and in the area postrema, additional effects specifically related to nociceptive input are very likely.

c-JUN-like immunoreactivity in the CNS of the adult rat: basal and transynaptically induced expression of an immediate-early gene.

An immunocytochemical study of dorsal root ganglia, spinal cord and medulla oblongata was performed with antisera against the c-jun proto-oncogene encoded protein. The c-JUN-like immunoreactivity was restricted to the cell nucleus. In the CNS of untreated rats a basal c-JUN-like immunoreactivity was present in the nuclei of two types of neurons: motor and autonomic. Labelled nuclei could be seen in many motoneurons of the ventral horn of the entire length of spinal cord and the lower medulla oblongata, as well as in the area of the nucleus hypoglossus, the dorsal motor nucleus of nucleus vagus, nucleus ambiguus, nucleus facialis, nucleus abducens and motor nucleus of nucleus trigeminus. Additionally, labelled nuclei were found in the preganglionic sympathetic and preganglionic parasympathetic cells of the nucleus intermediolateralis and nucleus intercalatus in the spinal cord. In the medulla oblongata we found a cluster of cells with c-JUN-like immunoreactivity in an area between the dorsomedial part of the oral nucleus spinalis trigeminalis and the lateral border of the knee of facial nerve. Additionally, a second cluster of c-JUN-like immunoreactivity cells was visible between the ventromedial part of the oral nucleus spinalis trigeminalis and the lateral border of the rostral nucleus facialis. Examination of the characteristics of all cell groups with a basal c-JUN-like immunoreactivity in the spinal cord and lower brainstem revealed an overlapping distribution with cholinergic cell groups. Basal c-JUN-like immunoreactivity was also seen in the dorsal root ganglion cells. We examined the factors which can effect the expression of the c-JUN protein. Maximal expression of c-JUN-like immunoreactivity was observed after electrical stimulation of primary afferents. Stimulation of sciatic nerve at a strength sufficient to recruit A delta- and C-fibres produced c-JUN-like immunoreactivity in many nuclei of the ipsilateral dorsal horn of the lumbar spinal cord. c-JUN-like immunoreactivity was first detectable at 30 min following the end of stimulation, reached a maximum after 1 h, remained unchanged for another 1 h and declined to the basal level after 16 h. The distribution of c-JUN-like immunoreactivity in the lumbar cord coincided with the region of termination of sciatic nociceptive afferents. Contralateral c-JUN-like immunoreactivity appeared after 4 h. After noxious mechanical stimulation of the plantar hindpaw c-JUN-like immunoreactivity occurred in the spinal area of termination of nociceptive afferents of the tibial nerve. Noxious stimulation did not provoke additional c-JUN-like immunoreactivity in dorsal root ganglia.(ABSTRACT TRUNCATED AT 400 WORDS)

The coexistence of neuropeptides in feline sensory neurons.

The coexistence of immunoreactivity to the peptides substance P, bombesin, calcitonin gene-related peptide and somatostatin has been determined in single, lumbar and sacral dorsal root ganglion cells in the cat. Colchicine pretreated L7 and S1 dorsal root ganglia were embedded in wax and cut into 5 microns sections. Groups of four, serially adjacent sections were reacted with antisera to one of four peptides using avidin-biotin immunocytochemistry. It was thus possible to determine the coincidence of the four peptides in single cell bodies by examining the immunoreactivity in a ganglion cell in one section and then locating the same cell in three adjacent sections. As a comparison, this procedure was repeated on a different population of ganglion cells using antiserum to substance P, bombesin and calcitonin gene-related peptide only. The results indicate that different combinations of three or four peptides may occur in single, small diameter sensory neurons in the cat. It would appear that immunoreactivity to bombesin and/or calcitonin gene-related peptide coexists with immunoreactivity to substance P in some dorsal root ganglion cells. However, immunoreactivity to each of these peptides was also found to occur alone in single cells. Immunoreactivity to calcitonin gene-related peptide but not to the other three peptides was found to occur in some medium-sized cell bodies (up to 70 microns). Somatostatin-like immunoreactivity was found to have a high level of coexistence with substance P-like immunoreactivity in cells which contained immunoreactivity to these two peptides only. Immunoreactivity to all the four peptides tested was found to occur in 18-26% of ganglion cells which contained at least one peptide.

Potentiated expression of FOS protein in the rat spinal cord following bilateral noxious cutaneous stimulation.

A noxious mechanical or chemical stimulus to the ventral skin of one hindpaw induced the expression of FOS proteins ipsilaterally in the spinal dorsal horn neurons in the rat. The number of FOS-labelled cells reached a maximum at 2-3 h, and decayed to basal levels within 6 h after the stimulus. When a first noxious stimulus was applied to the contralateral hindpaw 1-1.5 h prior to this stimulus, the number of FOS-labelled cells increased, over all laminae, to 153% (mechanical) and 164% (chemical) compared to the number produced by a single stimulus. This effect of a prior stimulus in increasing the number of FOS-labelled cells produced by a contralateral stimulus persisted for several hours after the first stimulus. The results are interpreted as a sensitization of dorsal horn neurons induced by peripheral noxious stimuli, which is manifest at the molecular biological level.

Coexistence of peptide immunoreactivity in sensory neurons of the cat.

The coexistence of the neuropeptides substance P, cholecystokinin, somatostatin and vasoactive intestinal polypeptide in cat sensory neurons has been examined using peroxidase-anti-peroxidase immunocytochemistry. Attempts were also made to locate cells containing bombesin, neurotensin, [Met]enkephalin and [Leu]enkephalin but no immunoreactivity was found when antisera to these peptides was used. Cells in the dorsal root ganglia were studied by cutting 5 microns serial wax sections or 15 microns cryostat sections. Coexistence was established by applying the antiserum to each peptide to serially adjacent 5 microns sections and establishing the presence of peptide-like immunoreactivity in each of 4 different sections through a single cell. Results showed that the distribution and combinations of coexistence of these neuropeptides in the cat is extremely complex; three and sometimes all four antisera showing immunoreactivity with a single cell. About 21% of all ganglion cells contained some immunoreactivity but there were certainly some small cells which did not contain any immunoreactivity. The coexistence of these peptides differed markedly from that previously reported in the rat suggesting that interspecific differences in the neuropeptide content of cells might be much greater than they are for classical neurotransmitters. The results are discussed in relation to the possible role of neuropeptides and the regulation of their production by sensory neurons.

Spinal interneurone depression by DS103-282.

The depressant effect of 5-chloro-4-(2-imidazolin-2-yl-amino)-2,1,3-benzothiodiazole (DS103-282) on the polysynaptic excitation of interneurones in the cat spinal cord appears to be related to a postsynaptic reduction in the effectiveness of excitatory transmitters than to interference with their presynaptic release.

Prolonged and selective induction of Fos-related antigen(s) in striatal neurons after 6-hydroxydopamine lesions of the rat substantia nigra pars compacta.

Unilateral 6-hydroxydopamine (6-OHDA) lesions of the rat substantia nigra lead to a large widespread and long-lasting (greater than 3 months) increased expression of Fos-related antigen(s) (FRAs) in striatal neurons ipsilateral to the side of the lesion. However, Fos and Jun expression were only very slightly increased in a few scattered neurons in the dopamine-denervated striatum. These results demonstrate that FRAs are induced long-term in striatal neurons following dopamine-depletion. This increased production of FRAs may be related to neuropeptide and/or D2 dopamine receptor upregulation that also occurs in the dopamine-denervated striatum.

Differential expression of immediate early genes after hippocampal long-term potentiation in awake rats.

The pattern of expression of fos and jun family immediate early genes following the induction of long-term potentiation (LTP) was investigated in the dentate gyrus of awake rats. Rapid, transient increases in the levels of c-jun and jun-B mRNA and protein, and in the levels of Fos-related proteins (FRAs), occurred in the dentate gyrus after LTP-inducing tetanization of the perforant path. A delayed, and more prolonged induction occurred for jun-D mRNA and protein. The induction of c-Jun, Jun-B, Jun-D and Fos-related proteins was prevented by administration of an N-methyl-D-aspartate receptor antagonist, which also blocked LTP induction, and by pentobarbital, which reduced but did not block LTP. These findings show that differential expression of fos and jun gene family members occurs in a distinct pattern following LTP in awake rats. The responsive genes may participate in the biochemical cascade leading to the long-term stabilization of synaptic modifications.

The induction and potentiation of c-Jun and c-Fos expression in spinal neurons mediated by NK1 and NK2 receptors.

Specific antagonists were used to determine NK1 and NK2 receptor mediation of the expression of c-Jun and c-Fos inducible transcription factors. (ITFs) in dorsal horn neurons. The induction of c-Jun by C-fiber stimulation was strongly reduced in the superficial laminae by NK1 and NK2 antagonists, but only weakly in the deep laminae by NK2 antagonism. c-Fos induction was reduced in all laminae by both NK1 and NK2 antagonism but less than with c-Jun. The potentiation of c-Jun expression, caused by a preceding stimulus, was abolished in all laminae by NK1 and NK2 antagonists, whereas that of c-Fos was reduced in all laminae but again less than that of c-Jun. Thus, the expressions of ITFs mediated by NK1 and NK2 receptors are complex, depending upon the neurons and stimulus considered, and they are not a measure of the neurons' electrophysiological responsiveness.

Selective expression of Jun proteins following axotomy and axonal transport block in peripheral nerves in the rat: evidence for a role in the regeneration process.

Expression of the protein products of the immediate-early genes (IEGs), members of the fos, jun and krox families (Jun, Fos, and Krox, resp.) was investigated in the spinal cord and sensory ganglia (DRG) of normal rats; and following transection of, block of axonal transport in, or electrical stimulation of their peripheral axons. The nuclei of many moto- and DRG neurons showed a faint basal immunoreactivity (IR) for Jun proteins, but not for Fos or Krox proteins. There was a strong and selective induction of Jun-IR in moto- and DRG neurons after peripheral nerve transection or crush, or colchicine- or vinblastine-induced block of axonal transport. The Jun-IR induced by nerve transection disappeared after nerve regeneration. In contrast, Jun, Fos and Krox proteins were all induced transynaptically in spinal dorsal horn neurons following electrical stimulation of the C-fibers in the afferent nerves. Thus in differentiated neurons in vivo these IEG proteins can be expressed either independently or concomitantly depending on the type of stimulus.

The localization and release of substance P and calcitonin gene-related peptide at nerve fibre endings in rat cutaneous nerve neuroma.

This study uses radiological and immunocytochemical techniques to investigate the localization, content, transport and release of substance P- and calcitonin gene-related peptide-like immunoreactivity (SP-LI and CGRP-LI, respectively) in nerve fibre endings in 1-, 3- and 5-week-old cutaneous nerve neuromas. Neuromas were induced by ligating and transecting the saphenous nerve in anaesthetized Sprague-Dawley rats. The content of both neuropeptides in 3-week-old saphenous nerve neuroma was significantly reduced compared to that in normal saphenous nerve. At 5 weeks the levels of the peptides in the neuromas had returned to normal but remained reduced in the nerve just proximal to the neuroma. Following a 24-h ligation of the nerve proximal to 3-week-old neuromas there was a diminished immunocytochemical staining for SP-LI and CGRP-LI both proximal and distal to the ligature when compared to that seen at ligations of normal nerves. This indicates a decreased transport of the neuropeptides both to and from the 3-week-old neuromas. The density of neuropeptide staining at ligatures of nerves with 5 week or older neuromas had increased, but still remained less than that seen at ligations of normal nerve. Both a basal and a bradykinin-induced release of SP-LI and CGRP-LI from nerve fibre endings in the neuroma was demonstrated. The basal release was demonstrated by exposing the neuromas, in situ, to solutions containing 50 microM morphine plus 2 mM CoCl2 for 24 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Brainstem peptidergic neurons projecting to the medial and lateral thalamus and zona incerta in the rat.

The presence of neuropeptides in brainstem neurons that project to the medial and lateral thalamus and zona incerta has been studied in the rat. Brainstem neurons were retrogradely labeled from the medial and lateral thalamus and the zona incerta by colloidal gold-WGA-HRP and, after silver intensification of the retrograde label, their content of immunoreactivity for nine different neuropeptides was determined after colchicine administration. The medial thalamus and zona incerta both received a large peptidergic input and the lateral thalamus a smaller input from neurons in several brainstem nuclei. These were principally from the locus coeruleus, parabrachial nucleus, the dorsal raphe and the dorsal tegmentum. The principal input to the medial thalamus arose from neurotensin, neuropeptide Y and galanin neurons in the locus coeruleus, neurotensin neurons in the dorsal tegmentum, dynorphin neurons in the parabrachial nucleus and dorsal tegmentum, galanin neurons in the dorsal raphe, substance P neurons in the lateral and dorsal periaqueductal grey and calcitonin gene-related peptide neurons in the nucleus paragigantocellularis. The principal peptidergic input to the zona incerta was from dynorphin neurons in the nucleus of the solitary tract, bombesin neurons in the lateral reticular nucleus, calcitonin gene-related peptide and cholecystokinin neurons in the dorsal tegmentum, substance P, bombesin and galanin neurons in the locus coeruleus, dynorphin and substance P neurons in the lateral periaqueductal grey and cholecystokinin neurons in the substantia nigra, ventral tegmental nucleus and raphe linearis. The principal peptidergic input to the lateral thalamus came from calcitonin gene-related peptide and cholecystokinin neurons in the dorsal tegmentum, calcitonin gene-related peptide and galanin neurons in the locus coeruleus; substance P, neuropeptide Y, galanin and calcitonin gene-related peptide neurons in the dorsal raphe, substance P neurons in the lateral periaqueductal gray, galanin neurons in the nucleus interpedunculus and cholecystokinin neurons in the raphe linearis. In all these cases, from 25% to virtually all of the projection neurons in the brainstem nucleus could contain immunoreactivity to the neuropeptide. A lesser, but significant peptidergic input to the thalamus and zona incerta also arose from the trigeminal nucleus, the substantia nigra, the nucleus of the solitary tract, the lateral reticular nucleus, the interpeduncular nucleus, the raphe linearis, the paragigantocellularis, the inferior olive and ventral tegmental area. Overall, the neuropeptides most frequently present in the projection neurons were substance P, calcitonin gene-related peptide, galanin and cholecystokinin. Bombesin, neuropeptide Y, neurotensin and dynorphin were less common; and enkephalin was present in only a small percentage of projection neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of naloxone, morphine and methionine enkephalinamide on Ia afferent terminations in the cat spinal cord.

Naloxone, morphine, Met5-enkephalinamide (MENKA) and procaine were administered microelectrophoretically near extracellularly stimulated extensor muscle group Ia afferent fibres and terminations in the lumbar spinal cord of cats anaesthetized with pentobarbitone sodium. Observations were made of effects on the electrical threshold, on the depolarizing action of GABA or piperidine-4-sulphonate (P4S), and on bicuculline-sensitive primary afferent depolarization (PAD) generated by tetanic stimulation of flexor muscle low threshold afferents. All 4 agents reversibly elevated the threshold of Ia fibres in the dorsal column and Ia terminations in the ventral horn. The depolarizations of terminations by GABA or P4S were also reduced, an effect, which for all except MENKA, probably accounted for a concomitant reduction in PAD. In the absence of a consistent effect on either threshold or depolarization by GABAmimetics, MENKA reversibly diminished PAD, an action blocked by naloxone. Intravenously administered naloxone, in doses known to enhance spinal monosynaptic excitation in the cat, had no effect on GABAergic PAD and little or no effect on Ia termination threshold. The results are discussed in relation to a naloxone-sensitive effect of MENKA which reduces transmitter release from GABAergic axo-axonic synapses on Ia terminals, but which does not account for the enhancement of spinal reflexes by naloxone.

Antagonists of synaptic and amino acid excitation of neurones in the cat spinal cord.

In the spinal cord of the anaesthetized cat microelectrophoretically administered (+/-)-cis-2,3-piperidine dicarboxylate (2,3-PDA), (+/-)-cis-2,5-piperidine dicarboxylate (2,5-PDA), gamma-D-glutamylglycine (gamma DGG), beta-D-aspartyl-beta-alanine (beta DAA), (+/-)-2-amino-4-phosphonobutyrate (2-APB), (+/-)-2-amino-5-phosphonovalerate (2-APV) and (+/-)-2-amino-7-phosphonoheptanoate (2-APH) were assessed as antagonists of chemical excitation of dorsal horn interneurones and Renshaw cells by N-methyl-D-aspartate (NMDA), L-aspartate, quisqualate (QUIS), kainate and L-glutamate, and of monosynaptic and polysynaptic excitation by impulses in primary afferent fibres of muscle and cutaneous origin. Whereas polysynaptic excitation of interneurones was readily and reversibly depressed by 2-APV, 2-APH, beta DAA, gamma DGG and 2,3-PDA, all of which also reduced excitation by NMDA (and L-aspartate) more than that by QUIS (and L-glutamate), no selective antagonism of monosynaptic excitation could be demonstrated. In particular, 2,3-PDA, which depressed excitation by kainate to a greater extent than that by either QUIS or NMDA, appeared to have no effect on monosynaptic excitation. The results support the involvement of L-aspartate as the transmitter of some spinal excitatory interneurones, but none of the antagonists tested were considered suitable for assessing the role of L-glutamate as the transmitter of some spinal primary afferent fibres.

Effects of noradrenaline and 5-hydroxytryptamine on spinal Ia afferent terminations.

When administered microelectrophoretically, noradrenaline (NA) and 5-hydroxytryptamine (5-HT) increased the thresholds of the terminal portions of extensor muscle Ia afferents stimulated extracellularly near lumbar motoneurons of anesthetized cats. This effect, and the concomitant increase in the electrical resistance of the extracellular medium near the orifices of multibarrel micropipettes, could be reversibly altered by ouabain. The results suggest further evidence is required of a direct effect of these amines at transmitter-related receptors on Ia terminations since the observed increase in threshold may be indirect, resulting from the sodium-dependent uptake of the administered amines by neurons and glia.

The electrophysiological and morphological characteristics of feline dorsal root ganglion cells.

The electrophysiological characteristics of physiologically typed L7 and S1 dorsal root ganglion (DRG) cells have been studied in the cat by intracellular recording. The injection of the fluorescent dye Lucifer Yellow has enabled us to study the morphology of these neurons. Our results show that over the entire range of primary sensory afferents there is a linear relationship between the peak rate of rise of somatic action potentials (dv/dt) and axonal conduction velocity. There is a prominent inflexion on the repolarizing phase of the somatic action potentials of group III and group IV afferents. This is not seen in the action potentials of group II or group I afferents. These results correlate with the observation that the total action potential duration (APD) is inversely related to conduction velocity. Primary afferent somata were observed to have an ellipsoidal shape with the long axis in the rostrocaudal dimension. It was observed that for all afferents studied the volume of a dorsal root ganglion cell was linearly related to its peripheral axonal conduction velocity. We were able to show further that group IV somata, some of whose axons supplied nociceptors, were among the smallest in the ganglion.