Glutamate-immunoreactive terminals synapse on primate spinothalamic tract cells

Somatotopy in the Medullary Dorsal Horn As a Basis for Orofacial Reflex Behavior

The somatotopy of the trigeminocervical complex of the rat was defined as a basis for describing circuitry for reflex behaviors directed through the facial motor nucleus. Thus, transganglionic transport of horseradish peroxidase conjugates applied to individual nerves/peripheral receptive fields showed that nerves innervating oropharyngeal structures projected most rostrally, followed by nerves innervating snout, periocular, and then periauricular receptive fields most caudally. Nerves innervating mucosae or glabrous receptive fields terminated densely in laminae I, II, and V of the trigeminocervical complex, while those innervating hairy skin terminated in laminae I-V. Projections to lamina II exhibited the most focused somatotopy when individual cases were compared. Retrograde transport of FluoroGold (FG) deposited into the facial motor nucleus resulted in labeled neurons almost solely in lamina V of the trigeminocervical complex. The distribution of these labeled neurons paralleled the somatotopy of primary afferent fibers, e.g., those labeled after FG injections into a functional group of motoneurons innervating lip musculature were found most rostrally while those labeled after injections into motoneurons innervating snout, periocular and preauricular muscles, respectively, were found at progressively more caudal levels. Anterograde transport of injections of biotinylated dextran amine into lamina V at different rostrocaudal levels of the trigeminocervical complex confirmed the notion that the somatotopy of orofacial sensory fields parallels the musculotopy of facial motor neurons. These data suggest that neurons in lamina V are important interneurons in a simple orofacial reflex circuit consisting of a sensory neuron, interneuron and motor neuron. Moreover, the somatotopy of primary afferent fibers from the head and neck confirms the "onion skin hypothesis" and suggests rostral cervical dermatomes blend seamlessly with "cranial dermatomes." The transition area between subnucleus interpolaris and subnucleus caudalis is addressed while the paratrigeminal nucleus is discussed as an interface between the somatic and visceral nervous systems.

Inflammation-induced shift in the valence of spinal GABA-A receptor-mediated modulation of nociception in the adult rat

The objective of this study was to assess the impact of persistent inflammation on spinal gamma-aminobutyric acid-A (GABA-A) receptor-mediated modulation of evoked nociceptive behavior in the adult rat. Nocifensive threshold was assessed with von Frey filaments applied to the dorsal surface of the hind paw. The GABA-A receptor agonist muscimol, the antagonist gabazine, the benzodiazepine receptor agonist midazolam, and antagonists PK11195 and flumazenil were administered spinally in the presence and absence of complete Freund's adjuvant (CFA)-induced inflammation. In naive rats, muscimol increased and gabazine decreased nociceptive threshold. After CFA, the effects of these compounds were reversed: Low doses of muscimol exacerbated the inflammation-induced decrease in nociceptive threshold and gabazine increased nociceptive threshold. Midazolam increased nociceptive threshold both in the presence and absence of inflammation. Flumazenil but not PK11195 blocked the analgesic effects of midazolam. These findings indicate that inflammation-induced changes in GABA-A signaling are complex and are likely to involve several distinct mechanisms. Rectifying the changes in GABA-A signaling may provide effective relief from hypersensitivity observed in the presence of inflammation.

PERSPECTIVE

An inflammation-induced shift in spinal GABA-A receptor signaling from inhibition to excitation appears to underlie inflammatory pain and hypersensitivity. Use of GABA-A receptor selective general anesthetics in association with therapeutic interventions may be contraindicated. More importantly, rectifying the changes in GABA-A signaling may provide effective relief from inflammatory hypersensitivity.

The role of calcitonin gene-related peptide (CGRP) in the generation and maintenance of mechanical allodynia and hyperalgesia in rats after intradermal injection of capsaicin

This study was designed to assess the role of calcitonin gene-related peptide (CGRP) and its receptor in the generation and maintenance of secondary mechanical allodynia and hyperalgesia induced by intradermal injection of capsaicin in rats. Paw withdrawal responses (PWRs) to von Frey hairs with different bending forces applied on the rat paw were tested in this study. CGRP(8-37), a specific antagonist of CGRP 1 receptors, was delivered through a microdialysis fiber inserted across the dorsal horn. Post- and pretreatment paradigms were followed. When CGRP(8-37) was administered 1h after capsaicin injection, the mechanical allodynia and hyperalgesia were partially reversed in a dose-dependent manner. On the other hand, when rats were treated with CGRP(8-37) prior to capsaicin injection, the PWRs to von Frey applications were significantly reduced as compared to control animals. Collectively, these results suggest that CGRP receptors present in the dorsal horn are involved in the generation and maintenance of nociceptive behaviors associated with cutaneous inflammation.

State-related inhibition by GABA and glycine of transmission in Clarke's column

During the state of active sleep (AS), Clarke's column dorsal spinocerebellar tract (DSCT) neurons undergo a marked reduction in their spontaneous and excitatory amino acid (EAA)-evoked responses. The present study was performed to examine the magnitude, consistency of AS-specific suppression, and potential role of classical inhibitory amino acids GABA and glycine (GLY) in mediating this phenomenon. AS-specific suppression of DSCT neurons, expressed as the reduction in mean spontaneous firing rate during AS versus the preceding episode of wakefulness, was compared across three consecutive sleep cycles (SC), each consisting of wakefulness (W), AS, and awakening from AS (RW). Spontaneous spike rate did not differ during W or RW between SC1, SC2, and SC3. AS-specific suppression of spontaneous firing rate was found to be consistent and measured 40.3, 31.5, and 41.6% in SC1, SC2, and SC3, respectively, indicating that such inhibition is marked and stable for pharmacological analyses. Microiontophoretic experiments were performed in which the magnitude of AS-specific suppression of spontaneous spike activity was measured over three consecutive SCs: SC1-control (no drug), SC2-test (drug), and SC3-recovery (no drug). The magnitude of AS-specific suppression during SC2-test measured only 11.7 or 14.6% in the presence of GABA(A) antagonist bicuculline (BIC) or GLY antagonist strychnine (STY), respectively. Coadministration of BIC and STY abolished AS-specific suppression. AS-specific suppression of EAA-evoked DSCT spike activity was also abolished in SC2-test after BIC or STY, respectively. We conclude that GABA and GLY mediate behavioral state-specific inhibition of ascending sensory transmission via Clarke's column DSCT neurons.

Differential effects of NMDA and group I mGluR antagonists on both nociception and spinal cord protein kinase C translocation in the formalin test and a model of neuropathic pain in rats

Coincident with nociception, both noxious chemical stimulation of the hind paw and chronic constriction injury (CCI) of the sciatic nerve produce an increase in protein kinase C (PKC) translocation in the spinal cord of rats. Noxious stimulus-induced PKC translocation likely depends on glutamate activity at either N-methyl-D-aspartate (NMDA) receptors or group I metabotropic glutamate receptors (mGluR1/5) in the spinal cord dorsal horn. This study compares nociceptive responses to, and the alterations in membrane-associated PKC, induced by noxious chemical stimulation of the hindpaw and CCI of the sciatic nerve, as well as their modulation by both NMDA and mGluR1/5 receptor antagonists. Three groups of rats were given a single intrathecal (i.t.) injection of either vehicle, dizocilpine maleate (MK-801, 60 nmol), an NMDA receptor antagonist, or (S)-4-carboxyphenylglycine (S)-4CPG, (150 nmol), an mGluR1/5 antagonist, 10 min prior to a 50 microl of 2.5% formalin injection into the ventral surface of one hind paw. Another three groups of rats were given twice daily injections of either vehicle, MK-801 (30 nmol) or (S)-4CPG (90 nmol) i.t. for 5 days starting 30 min before CCI or sham injury of the sciatic nerve. Nociceptive responses were assessed for a 60 min period after the formalin injection in the first three groups, and tests of mechanical and cold allodynia were performed on days 4, 8, 12 and 16 after CCI for the latter three groups. Furthermore, changes in the levels of membrane-associated PKC, as assayed by quantitative autoradiography of the specific binding of [3H]-phorbol 12,13-dibutyrate ([3H]-PDBu) in the dorsal horn of the lumbar spinal cord sections, were assessed in formalin-injected rats (at 5, 25 and 60 min) and in neuropathic rats 5 days after CCI, treated (as above) with vehicle, MK-801 or (S)-4CPG. The results indicate that i.t. treatment with MK-801 significantly reduced nociceptive scores in the formalin test and also produced a significant suppression of formalin-induced increases in [3H]-PDBu binding in laminae I-II, III-VI and X of the lumbar spinal cord. In contrast, i.t. treatment with (S)-4CPG failed to significantly affect either nociceptive behaviours in the formalin test or formalin-induced increases in [3H]-PDBu binding in laminae I-II and III-VI of the lumbar spinal cord. On the other hand, i.t. treatment with either MK-801 or (S)-4CPG produced a significant reduction in mechanical and cold hypersensitivity, as well as [3H]-PDBu binding in laminae I-II and III-VI of the lumbar spinal cord, after CCI. These results suggest that while NMDA, but not mGluR1/5, receptors are involved in translocation of PKC and nociception in a model of persistent acute pain, both types of receptors influence the translocation of PKC in dorsal horn and mechanical and cold allodynia in a model of chronic neuropathic pain.

Role of neurotransmitters in sensitization of pain responses

Injection of capsaicin into the skin results in pain, primary heat and mechanical hyperalgesia, and secondary mechanical allodynia and hyperalgesia. Sensory receptors in the area of secondary mechanical allodynia and hyperalgesia are unaffected, and so the sensory changes must be due to central actions of the initial intense nociceptive discharge that follows the capsaicin injection. Central sensitization of the responses of spinothalamic tract neurons lasts several hours, but can be prevented by spinal cord administration of non-NMDA and NMDA glutamate receptor antagonists or NK1 substance P receptor antagonists. The long-lasting increase in excitability of spinothalamic tract cells depends on the activation of several second messenger cascades (PKC, PKA, and NO/PKG signal transduction pathways). The excitability change also depends on activation of calcium/calmodulin-dependent kinase II, which is consistent with the proposal that this central sensitization response is a form of long-term potentiation.

Stimulus-function, wind-up and modulation by diffuse noxious inhibitory controls of responses of convergent neurons of the spinal trigeminal nucleus oralis

Extracellular unitary recordings were made from 53 spinal trigeminal nucleus oralis (Sp5O) convergent neurons in halothane-anaesthetized rats. The neurons had an ipsilateral receptive field including mainly oral or perioral regions. They responded to percutaneous electrical stimulation with two peaks of activation. The first had a short latency (4.3 +/- 0.3 ms) and low threshold (0.35 +/- 0.04 mA), whereas the second had a longer latency (68.1 +/- 3.4 ms) and higher threshold (7.3 +/- 0.5 mA). Intracutaneous injection of capsaicin (0.1%) produced a strong and rapid reduction of the long-latency responses of Sp5O convergent neurons with little effect on the short-latency responses. In most cases (73%), the long-latency responses exhibited a wind-up phenomenon during repetitive (0.66 Hz) suprathreshold electrical stimulation. These results suggest that C-fibres mediate the long-latency response of Sp5O convergent neurons. Regarding the C-fibre-evoked responses, a linear relationship between the intensity of the applied current and the magnitude of the response was found within the one to three times threshold range. The Sp5O convergent neurons also encoded the intensity of mechanical stimuli applied to the skin or mucosa in the 5-50 g ranges. The evoked activity of Sp5O convergent neurons could be suppressed by noxious heat applied to the tail (52 degrees C) and long-lasting poststimulus effects followed this. These findings show that convergent neurons in the Sp5O resemble those in the deep laminae of the spinal dorsal horn and spinal trigeminal nucleus caudalis, and further support that the Sp5O plays a part in the processing of nociceptive information from the orofacial region.

Branching and/or collateral projections of spinal dorsal horn neurons

Branching and/or collateral projections of spinal dorsal horn neurons is a common phenomenon. Evidence is presented for the existence of STTm/STTl, STTc/STTi, STT/SMT, STT/SRT, SCT/DCPS, SST/DCPS, SCT/SST, STT/SHT, STeT/SHT, STeTs and other doubly or multiply projecting spinal neurons that have been anatomically and physiologically identified and named based on the locations of the cells of origin and their terminations in the brain. These newly discovered spinal projection neurons are characterized by a single cell body and branched axons and/or collaterals that project to two or more target areas in the brain. These novel populations of neurons seem to be a fuzzy set of spinal projection neurons that function as an intersection set of the corresponding single projection spinal neurons and to be at an intermediate stage phylogenetically. Identification strategies are discussed, and general concluding remarks are made in this review.

Synaptic organization of substance P, glutamate and GABA-immunoreactive boutons on functionally identified neurons in cat spinal deeper dorsal horn

In order to determine how nociceptive input conveyed by the C-fibers terminating in superficial laminae of the spinal cord reaches the wide dynamic range (WDR) cells in deeper dorsal horn, which functions as ascending projection pathway, the morphological features of some WDR cells in the deeper dorsal horn of the cat lumbar spinal cord were studied by intracellular injection of horseradish peroxidase and physiological characterization. One of the fully stained neurons with somata in lamina V and dendrites that entered lamina II were examined by electron microscopy. Immunogold staining of ultrathin sections through the labeled proximal dendrites in lamina II revealed that these dendrites received numerous synapses from substance P and glutamate immunoreactive (IR) axons, which were considered originating from C-fibers. In addition, many GABA-IR terminals were found presynaptic to the labeled dendrites. The results, therefore, suggest that the information carried by primary afferent can be sent from the superficial dorsal horn to the deeper laminae through monosynaptic contacts between C-fiber terminals and the long dorsal dendrites of some WDR cells in the deeper laminae, and that GABAergic system is involved in postsynaptic control to modulate the transmission of nociceptive sensory information.

Organization of the zona incerta in the macaque: an electron microscopic study

BACKGROUND

The zona incerta (ZI) receives projections from many telencephalic and brainstem structures. On the basis of its connectivity and physiology, this nucleus has been implicated in the control of saccadic eye movements. Because of the complexity of its afferent signals and its simple efferent signal, there must be much local interaction within the ZI to integrate these various afferents. The purpose of this study was to investigate, at the ultrastructural level, whether the ZI contains the anatomical substrata which could subserve the control of eye movements.

METHODS

Blocks of tissue from the ZI of macaque monkeys were prepared for electron microscopy using standard techniques. Some of these animals were taken specifically for electron microscopy. Others had received injections of tracer substances and were prepared for electron microscopy subsequent to tracer visualization.

RESULTS

Cell bodies of medium-large neurons were found in our preparations. They have large nucleoli and relatively small volumes of karyoplasm. Cell bodies and dendrites of all sizes have many synaptic contacts. Three types of synaptic profiles were found, designated Types 1, 2, and 3. Type 1 profiles are symmetrical and contact cell bodies and small dendrites. Type 2 profiles are thought to be presynaptic dendrites and may have symmetrical or asymmetrical contacts. Type 3 profiles are asymmetrical and primarily contact small dendrites. Many synapses contacted vesicle-containing profiles. In some cases, it was clear that these profiles participated in serial synapses on presumptive presynaptic dendrites. Other profiles appeared to be axoaxonic contacts.

CONCLUSIONS

Afferent and efferent signals are likely to be modulated extensively within the ZI. Therefore, there needs to be complex interactions between neuronal elements of the ZI and its afferents. This study demonstrates that this nucleus possesses the structural substrata to subserve diverse roles, such as the gating of saccadic eye movements.

Neuroanatomy of the pain system and of the pathways that modulate pain

We review many of the recent findings concerning mechanisms and pathways for pain and its modulation, emphasizing sensitization and the modulation of nociceptors and of dorsal horn nociceptive neurons. We describe the organization of several ascending nociceptive pathways, including the spinothalamic, spinomesencephalic, spinoreticular, spinolimbic, spinocervical, and postsynaptic dorsal column pathways in some detail and discuss nociceptive processing in the thalamus and cerebral cortex. Structures involved in the descending analgesia systems, including the periaqueductal gray, locus ceruleus, and parabrachial area, nucleus raphe magnus, reticular formation, anterior pretectal nucleus, thalamus and cerebral cortex, and several components of the limbic system are described and the pathways and neurotransmitters utilized are mentioned. Finally, we speculate on possible fruitful lines of research that might lead to improvements in therapy for pain.

Electron microscopy of immunoreactivity patterns for glutamate and gamma-aminobutyric acid in synaptic glomeruli of the feline spinal trigeminal nucleus (Subnucleus Caudalis)

We studied the ultrastructure of the synaptic organization in the feline spinal trigeminal nucleus, emphasizing specific neurotransmitter patterns within lamina II of the pars caudalis/medullary dorsal horn. Normal adults were perfused, and Vibratome sections from pars caudalis were processed for electron microscopy. Ultrathin sections were reacted with antibodies for the excitatory neurotransmitter glutamate (Glu) and for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) by using postembedding immunogold techniques. Both single- and double-labeled preparations were examined. Results with single labeling show that Glu-immunoreactive terminals have round synaptic vesicles and form asymmetric synaptic contacts onto dendrites. GABA-immunoreactive axon terminals and vesicle-containing dendrites have pleomorphic vesicles, and the axon terminals form symmetric contacts onto dendrites and other axons. Double labeling on a single section shows glomeruli with central Glu-immunoreactive terminals that are presynaptic to dendrites, including GABA+ vesicle-containing dendrites. These Glu+ terminals are also postsynaptic to GABA+ axon terminals, and these GABA-immunoreactive terminals may also be presynaptic to the GABA+ vesicle-containing dendrites. Quantitative analyses confirm the specificity of the Glu and GABA immunoreactivities seen in the various glomerular profiles. The results suggest that a subpopulation of Glu-immunoreactive primary afferents (excitatory) may be under the direct synaptic influence of a GABA-immunoreactive intrinsic pathway (inhibitory) by both presynaptic and postsynaptic mechanisms.

Glutamatergic and GABAergic input to rat spinothalamic tract cells in the superficial dorsal horn

The distribution of synaptic terminals onto spinothalamic tract cells (types I and II) of the superficial dorsal horn was determined with special reference to the amino acid transmitters glutamate and gamma-aminobutyric acid. Fifteen spinothalamic cells retrogradely labeled from the thalamus with the neuroanatomical tracer wheatgerm agglutinin conjugated to horseradish peroxidase were sectioned for electron microscopy. Serial sections from several levels through each cell were immunostained for glutamate and gamma-aminobutyric acid using a postembedding immunogold technique. Perimeter measurements of spinothalamic cell somata and dendrites and the lengths of apposition for all terminal profiles in contact with the spinothalamic cells were obtained from electron micrographs using a digitizing tablet. These data were used to determine the density of terminals on the soma and dendrites. In addition, the terminal population on these cells was categorized by transmitter content (glutamate, gamma-aminobutyric acid, or unlabeled). The results demonstrate that terminal density increased on dendrites relative to their distance from the soma. Glutamatergic and GABAergic input composed 37% and 20% of the terminal population, respectively, and these percentages remained uniform for the soma and dendrites. There were no significant differences among the 15 cells analyzed for this study. The results, therefore, suggest that both type I and type II STT cells of the superficial DH have similar synaptic organizations.

Infusion of substance P or neurokinin A by microdialysis alters responses of primate spinothalamic tract neurons to cutaneous stimuli and to iontophoretically released excitatory amino acids

The responses of 25 spinothalamic tract (STT) neurons to mechanical and thermal stimulation of the skin, as well as to a battery of iontophoretically applied excitatory amino acids (EAAs), were tested before and then during microdialysis of substance P (SP) or neurokinin A (NKA) into the dorsal horn of anesthetized monkeys. Neither peptide had significant effects on the background activity or the responses to mechanical or thermal stimulation of the skin. However, each peptide produced significant increases in the responses to iontophoretic application of one or more EAAs. In addition, following combined application of the EAAs and either SP or NKA, the responses of the cells to mechanical stimulation of the skin increased. Combined application of SP and NKA failed to produce an increase in responses to either the EAAs or to cutaneous stimuli that was greater than that produced by either peptide alone. It is concluded that SP and NKA produce an increase in the responses of STT cells to iontophoretic applications of EAAs and the combined effects of these compounds produce sustained increases in responses to mechanical stimulation of the skin. These changes mimic those observed when STT cells are sensitized by peripheral noxious stimuli, suggesting that the mechanism of induction and expression of sensitization involves the facilitation of dorsal horn neuron responses to EAAs by tachykinins.

Synaptic organization of excitatory and inhibitory boutons associated with spinal neurons which project through the dorsal columns of the cat

The cell bodies and proximal dendrites of postsynaptic dorsal column neurons were examined for synaptic boutons which displayed immunoreactivity for the principal excitatory and inhibitory neurotransmitters, glutamate and GABA. The neurons were labelled by retrograde transport of horseradish peroxidase and GABA or glutamate-containing boutons were revealed by performing postembedding immunogold reactions on electron microscope sections. Five neurons were examined and all of them were postsynaptic to boutons which contained either GABA or glutamate. Quantitative analysis of two of the cells revealed that more than 90% of the synaptic profiles associated with them displayed immunogold reactions for these transmitters. Analysis of series of alternate sections, which were reacted for either GABA or glutamate, showed that there was no overlap in the populations of immunoreactive boutons. Furthermore, GABA and glutamate immunoreactions were associated with boutons which had different morphological characteristics. In addition, some large glutamate-enriched boutons were postsynaptic to small boutons which displayed immunogold reactions for GABA. This study demonstrates morphological bases for direct excitation, postsynaptic inhibition and presynaptic inhibition of postsynaptic dorsal column cells.

Dorsal column inhibition of nociceptive thalamic cells mediated by gamma-aminobutyric acid mechanisms in the cat

Cells in posterior parts of the cat thalamus were investigated. Responses in single units excited by electrical stimulation in the lateral funiculus (LF), the dorsal column nucleus (DCN) or the canine tooth pulp (TP) were analysed. All cells had a spontaneous resting activity which could be increased by extracellular iontophoretic application of DL-homocysteic acid (DLH) and decreased by gamma-aminobutyric acid (GABA). No effect on the spontaneous firing rate was observed following iontophoresis of the selective GABA-antagonists, picrotoxin (GABA-A receptor antagonist) or saclofen (GABA-B receptor antagonist). However, the decreased firing following GABA application was partially blocked by picrotoxin but not by saclofen. A phasic inhibition induced by DCN stimulation in nociceptive thalamic cells is indicated since simultaneous administration of picrotoxin increased the evoked response. This type of inhibitory mechanism could not be detected following LF or TP stimulation. The extracellular activity evoked by electrical stimulation of LF or TP was significantly depressed by preceding electrical stimulation in the DCN. This inhibition was reversed by simultaneous administration of picrotoxin, indicating an involvement of GABA-A receptors. The reversal of the DCN-induced depression of the late responses following LF stimulation occurred after application of saclofen. It is suggested that this effect is partly mediated via GABA-B receptors. Results from the present study indicate an interaction in the thalamus between presumed low-threshold (DCN) and presumed nociceptive afferents (LF and TP) similar to that previously described in the spinal cord.

Amino acid immunocytochemistry of primary afferent terminals in the rat dorsal horn

We combined transganglionic tracing methods with postembedding electron microscopic immunocytochemistry to determine whether identified primary afferent fibers terminating in spinal laminae I-IV may use glutamate and aspartate as neurotransmitters. Sciatic injections of wheat-germ agglutinin conjugated to horseradish peroxidase labeled fine afferent fibers with terminals in laminae I-II of the lumbar spinal cord, whereas injections of the B subunit of cholera toxin conjugated to horseradish peroxidase labeled primary afferent terminals in deeper laminae. Many labeled primary afferent terminals in superficial laminae were involved in glomerular synaptic arrangements; others established nonglomerular contacts. Most glomerular arrangements were clearly immunopositive for glutamate, compared with dendrites, astrocytes, or terminals immunopositive for gamma-aminobutyric acid (GABA). The degree of enrichment varied in labeled terminals of different morphological types. Aspartate was enriched, though to a lesser degree than glutamate, in labeled central terminals of glomeruli in superficial laminae. Labeled primary afferent terminals in laminae III-IV were immunopositive for glutamate, though at lower levels than glomerular terminals in superficial laminae. Aspartate was not enriched in these terminals compared with dendrites, glia, and GABA-positive terminals. These results support a neurotransmitter role for glutamate in primary afferents to the dorsal horn. Quantitative differences in the content of glutamate in identified primary afferent terminals may be related to functional differences. Enrichment of aspartate in terminals in superficial but not deep laminae is compatible with a role for this amino acid in sustained, NMDA-mediated phenomena characteristic of activity in fine caliber afferents.

Evidence of a role for N-methyl-D-aspartate (NMDA) receptors in the facilitation of tail withdrawal after spinal transection

Peripheral injury produces a characteristic excitation of spinal cord dorsal horn cells (wind-up) which is associated with a facilitation of spinal nociceptive reflexes (hyperalgesia). These phenomena are believed to be mediated by a trauma-induced increase in the release of excitatory amino acids (EAAs). A similar increase in the activity of dorsal horn neurons and spinal reflexes occurs after spinal transection. Therefore, the present studies examined the possibility that EAAs, acting through the NMDA receptor, might also be involved in behavioral hyperalgesia produced by central injury. The first experiment assessed the effect of pretreatment with the NMDA antagonist, ketamine, on the facilitated tail flick (TF) response of spinally transected rats. Separate groups of animals were spinalized under isoflurane anesthesia alone, intramuscular ketamine anesthesia alone, or a combination of isoflurane and intrathecal ketamine. The TF was examined 24 h later, before and 30 min after an intrathecal injection of morphine. In the second experiment, the effect of intraperitoneal or intrathecal ketamine on the TF was assessed to separate groups of rats that underwent spinal transection or sham surgery under isoflurane anesthesia. Pretreatment with either systemic or intrathecal ketamine did not alter TF facilitation or morphine-induced antinociception in spinal rats. However, both systemic and intrathecal ketamine significantly increased TF latencies in spinal, relative to intact rats. These results indicate that ketamine did not prevent the development of spinal reflex facilitation, but it selectively reduced this reaction once it was established in spinal rats. The data support an involvement of EAAs in reflex facilitation produced by spinal transection.

Intrathecal co-administration of morphine and excitatory amino acid agonists produce differential effects on the tail-flick of intact and spinal rats

Previous reports, that intrathecal morphine is more potent on the tail-flick test of acute spinal rats than intact rats, suggested that spinal opiate analgesia was attenuated by neurotransmitter release from descending pathways. To determine if this phenomenon involved excitatory amino acids (EAAs), 0.25 nm of N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) were i.t. co-administered with morphine to Intact and Spinal rats. NMDA potentiated morphine antinociception in Intact but not Spinal rats; AMPA had no effect in Intact rats, but significantly reduced morphine antinociception in Spinal rats. The data suggest a reciprocal descending, modulatory influence on the spinal interaction between EAAs and morphine.

Neurotransmitters in subcortical somatosensory pathways

Investigations during recent years indicate that many different neuroactive substances are involved in the transmission and modulation of somesthetic information in the central nervous system. This review surveys recent developments within the field of somatosensory neurotransmission, emphasizing immunocytochemical findings. Increasing evidence indicates a widespread role for glutamate as a fast-acting excitatory neurotransmitter at different levels in somatosensory pathways. Several studies have substantiated a role for glutamate as a neurotransmitter in primary afferent neurons and in corticofugal projections, and also indicate a neurotransmitter role for glutamate in ascending somatosensory pathways. Other substances likely to be involved in somatosensory neurotransmission include the neuropeptides. Many different peptides have been detected in primary afferent neurons with unmyelinated or thinly myelinated axons, and are thus likely to be directly involved in primary afferent neurotransmission. Some neurons giving rise to ascending somatosensory pathways, primarily those with cell bodies in the dorsal horn, are also immunoreactive for peptides. Recent investigations have shown that the expression of neuropeptides, both in primary afferent and ascending tract neurons, may change as a result of various kinds of peripheral manipulation. The occurrence of neurotransmitters in intrinsic neurons and neurons providing modulating inputs to somatosensory relay nuclei (the dorsal horn, the lateral cervical nucleus, the dorsal column nuclei and the ventrobasal thalamus) is also reviewed. Neurotransmitters and modulators in such neurons include acetylcholine, monoamines, GABA, glycine, glutamate, and various neuropeptides.

GABA-immunoreactive terminals synapse on primate spinothalamic tract cells

Gamma-aminobutyric acid (GABA) is a putative inhibitory neurotransmitter in the vertebrate nervous system. Several lines of evidence suggest that GABA plays an important role in the processing and modulation of sensory input in the spinal cord dorsal horn. In the present study, the relationship between GABA-immunoreactive (GABA-IR) terminals and spinothalamic tract (STT) cells in the monkey lumbar cord was investigated. Physiologically characterized STT cells, one located in lamina V and two located in lateral lamina IV, were intracellularly injected with horseradish peroxidase (HRP). A fourth STT cell, located in lamina I, was retrogradely labeled following injection of HRP into the contralateral thalamus. Immunogold labeling of ultrathin sections through the cell bodies and proximal dendrites of the STT neurons demonstrated that the percentage of the GABA-IR terminals in contact with these profiles was 24.7% and 36%, respectively. The average STT surface length contacted by GABA-IR terminals for cell bodies and proximal dendrites was 18.2% and 26.7%, respectively. For the lamina I cell, 7 out of 35 (20%) of the terminals were GABA-IR and they covered 9.6% of the surface analyzed. These data demonstrate that GABA-IR terminals synapse directly on STT cells, constituting a substantial proportion of the terminal population on these cells. Furthermore, compared to the cell bodies, a greater percentage of the input on the proximal dendrites is GABAergic. These anatomical data are consistent with the findings of a previously published iontophoretic study that demonstrated that GABA can exert a strong inhibitory influence on STT cells. These findings are discussed in relation to GABAergic involvement in tonic and phasic inhibition of STT neurons.