An electron microscope study of glycine-like immunoreactivity in laminae I-III of the spinal dorsal horn of the rat

Lignocaine selectively reduces C fibre-evoked neuronal activity in rat spinal cord in vitro by decreasing N-methyl-D-aspartate and neurokinin receptor-mediated post-synaptic depolarizations; implications for the development of novel centrally acting analgesics

The action of lignocaine on nociceptive transmission in the spinal cord has been studied in vitro using ventral root potential (VRP) recordings from 10-12-day-old rat hemisected spinal cord preparations. Single-shock stimulation of a dorsal root at intensities sufficient to activate high-threshold C-primary afferent fibres elicited VRPs lasting for 15-20 sec in the corresponding ventral root. The VRP consisted of 3 distinct parts: the early, slow and prolonged components, as previously described (Thompson et al. 1992), where the early represents A beta fibre-evoked mono- and polysynaptic responses lasting for tens of milliseconds, the slow is a largely N-methyl-D-aspartic acid (NMDA) receptor-mediated small-calibre afferent-generated component, lasting for about 1.5 sec, and the prolonged is a neurokinin receptor-mediated long-lasting component generated by high-threshold fibres. Lignocaine superfusion (40-60 microM) significantly and reversibly reduced the slow and prolonged components of the C fibre-evoked VRP in a dose-dependent manner without any effect on the early or A beta fibre-mediated component of the VRP. The amplitude of the cumulative VRP generated by repetitive inputs (1 and 10 Hz) was also significantly reduced as was the depolarization produced by bath application of NMDA (100 microM) or substance P (SP, 1 microM) in the presence or absence of tetrodotoxin (TTX) (300 nM). At this dose range lignocaine had no effect on the compound action potential (CAP) elicited by stimulating the sciatic nerve and recorded on the dorsal root. The CAP was only significantly reduced with a 300 microM dose of lignocaine. Application of the opiate, glycine, GABAA and GABAB receptor antagonists, naloxone (1 microM), strychnine (100 microM), bicuculline (100 microM) and phaclofen (100 microM) did not alter the depressant effects of lignocaine on the VRP. Low concentrations of lignocaine have a selective action on nociceptive transmission in the spinal cord which is different and more potent than its local anaesthetic conduction blockade in the periphery. This includes a reduction of direct or synaptically driven NMDA- and NK receptor-mediated post-synaptic depolarizations indicating that this class of sodium channel blockers may be potentially useful as analgesic agents, possibly acting on TTX-resistant sodium ion channels.

Subcellular localization of the GABAA receptor gamma 2 subunit in the rat spinal cord

The fine subcellular organization of the GABAA receptor complex in the adult rat spinal ventral horn was analysed by immunocytochemistry using a specific polyclonal antiserum raised against the gamma 2 subunit. This subunit confers benzodiazepine sensitivity on the chloride channel of the GABAA receptor. With both fluorescent and peroxidase staining, the immunoreactivity was mainly observed in the grey matter and more specifically in the dorsal and ventral horns on medium and large neurons. A high number of immunostained somata were clustered in regions corresponding to motor nuclei. On the neuronal surface, labelling appeared as fluorescent dots over the more diffuse staining that was present on the soma and proximal part of dendrites. At the ultrastructural level, peroxidase end product was in most cases associated with the internal side of postsynaptic differentiations facing terminal boutons enriched with pleiomorphic small clear vesicles. The positively stained synapses were encountered on proximal dendrites of neurons and throughout the neuropil of the ventral horn (layers VII-IX). An immunoreactivity on the postsynaptic membrane was occasionally found to decorate large pieces of membrane not directly apposed to presynaptic active zones. In addition, presynaptic labelling was observed at axoaxonic contacts and at extrasynaptic sites on membranes within boutons, sometimes themselves apposed to gamma 2 immunoreactivity. Finally, we also observed gamma 2 immunoreactivity at the cytosolic face of the plasma membrane of some glial elements. These results give morphological evidence for the involvement of GABAA receptors in both post- and presynaptic inhibition in the rat spinal ventral horn. The presence of gamma 2 subunit immunoreactivity at these different synaptic contacts suggests that the two types of inhibition can be modulated by benzodiazepine drugs. The findings also provide anatomical evidence for the possible regulation of GABA release through an autoreceptor, and for GABAergic communication between neuronal and glial components.

Inhibition by 5-hydroxytryptamine and noradrenaline in substantia gelatinosa of guinea-pig spinal trigeminal nucleus

1. Whole-cell and intracellular recordings were made from neurons in slices of guinea-pig spinal trigeminal nucleus pars caudalis. 2. 5-Hydroxytryptamine (5-HT) hyperpolarized 70% of neurons by activating 5-HT1A receptors. The effect was mimicked by 5-carboxamidotryptamine (5-CT) and (+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronapthalene hydrobromide (8-OH-DPAT) and antagonized by 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)-butyl]-piperazine hydrobromide (NAN 190) and pindobind-5-HT1A. Nine per cent of the neurons were depolarized by 5-HT. 3. In about 20% of recordings, 5-HT also evoked repetitive inhibitory postsynaptic potentials that were mediated by glycine. 4. Noradrenaline (NA) hyperpolarized 71% of neurons. This effect was mediated by activation of alpha 2-adrenoceptors, since 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK14304) also caused a hyperpolarization and idazoxan (1 microM) blocked the hyperpolarization to both NA and UK14304. Phenylephrine depolarized a subset of neurons and this depolarization was blocked by prazosin, suggesting an action mediated by activation of alpha 1-adrenoceptors. 5. NA also evoked repetitive GABAA-mediated inhibitory postsynaptic potentials in about 20% of recordings. The increase in synaptic activity was mimicked by phenylephrine and blocked by prazosin. 6. These results indicate that there are at least two mechanisms through which 5-HT and NA inhibit neurons: (i) in many cells both 5-HT and NA mediate a hyperpolarization through an increase of a potassium conductance; (ii) 5-HT and NA also activated GABA- and glycine-containing interneurons to cause IPSPs in separate groups of cells.

The relationship between glycine and gephyrin in synapses of the rat spinal cord

In order to examine the relationship between gephyrin (the peripheral membrane protein associated with glycine receptors) and glycinergic boutons, we have carried out a post-embedding immunogold study of glycine-like immunoreactivity on sections of rat lumbar spinal cord which had previously been reacted with monoclonal antibody to gephyrin. In all three areas examined (laminae I and II, lamina III and lamina IX) the majority of profiles which were presynaptic at gephyrin-immunoreactive synapses were enriched with glycine-like immunoreactivity. It was estimated that at least 83% of profiles presynaptic to gephyrin-immunoreactive synapses in the superficial dorsal horn (laminae I and II) were glycine-immunoreactive, while for lamina III and the ventral horn (lamina IX) the proportions were at least 91% and 98% respectively. This provides strong evidence that glycine is a transmitter at those synapses where gephyrin- and glycine-like immunoreactivities are both present, but suggests that gephyrin may sometimes be expressed at non-glycinergic synapses and indicates the need for caution in using gephyrin-immunoreactivity as a marker for glycinergic synapses within the spinal cord. By reacting serial sections of dorsal horn with antisera to glycine and GABA, we have shown that many boutons in laminae I-III of the dorsal horn show both types of immunoreactivity and are therefore likely to use both amino acids as inhibitory transmitters. Many of the boutons which were presynaptic at axoaxonic synapses in the ventral part of lamina II and in lamina III were glycine- and GABA-immunoreactive and in many cases the postsynaptic element was the central axon of a type II synaptic glomerulus. Taken together with pharmacological evidence, this suggests that inhibitory interneurons in the dorsal horn which use both GABA and glycine may be important in controlling the flow of information from hair follicle afferents to other spinal neurons.

Primary afferent-evoked glycine- and GABA-mediated IPSPs in substantia gelatinosa neurones in the rat spinal cord in vitro

1. The possible roles of glycine and gamma-aminobutyric acid (GABA) as inhibitory transmitters in the spinal dorsal horn were studied by intracellular recordings from substantia gelatinosa (SG) neurones in transverse slices of the adult rat spinal cord which retained an attached dorsal root. 2. Stimulation of primary afferent A delta fibres evoked an initial excitatory postsynaptic potential (fast EPSP) followed by a short and/or long inhibitory postsynaptic potential (short and long IPSP). The short IPSP, observed in twenty-nine SG neurones (37%) which received inhibitory inputs, had a mean latency of 3.6 ms and a half-decay time of 11 ms, while the long IPSP had a mean latency of 3.7 ms and a half-decay time of 42 ms and was observed in thirty-seven SG neurones (47%). The remaining twelve neurones (16%) exhibited both short and long IPSPs. Both IPSPs reversed polarity at a membrane potential of -70 +/- 4 mV. The short IPSP was reversibly blocked by the glycine receptor antagonist strychnine (0.5-2 microM), while the long IPSP was reversibly blocked by the GABAA receptor antagonist bicuculline (10-20 microM). 3. In the majority of SG neurones, the short and long IPSPs appeared to be disynaptic and were blocked by the non-N-methyl-D-aspartic acid (non-NMDA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 5-10 microM). Both IPSPs were less sensitive (depressed by less than 30%) to the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV; 50-100 microM). 4. In ten SG neurones (13%), bath-applied glutamate (0.5-2 mM) increased the amplitude and frequency of IPSPs, which had a similar time course to that of the short IPSP evoked by afferent A delta fibres. The glutamate-induced short IPSPs were blocked by tetrodotoxin (0.5 microM) or strychnine (0.5-1 microM). In twelve neurones (16%), glutamate hyperpolarized the membrane or increased the amplitude and frequency of IPSPs that had a similar time course to that of the A delta fibre-evoked long IPSPs. The glutamate-induced membrane hyperpolarization and long IPSPs decreased in amplitude with membrane hyperpolarization and reversed polarity at -70 +/- 6 mV. These hyperpolarizing responses were blocked by tetrodotoxin (0.5 microM) or bicuculline (10 microM). 5. These observations suggest that primary afferent A delta fibres activate glycinergic and/or GABAergic interneurones primarily through the non-NMDA receptor subclass and result in inhibition of nearby SG neurones in the dorsal horn of the spinal cord.(ABSTRACT TRUNCATED AT 400 WORDS)

Glycine and GABAA antagonists reduce the inhibition of primate spinothalamic tract neurons produced by stimulation in periaqueductal gray

Amino acids are demonstrated to be important neurotransmitters mediating the inhibitory transmission from nucleus raphe magnus to spinal nociceptive dorsal horn neurons. In this study, the role of glycine and GABA in the inhibitory processes evoked by stimulation in periaqueductal gray (PAG) of responses of primate spinothalamic tract (STT) neurons to cutaneous mechanical and thermal stimuli was investigated by examining the effects of strychnine and bicuculline, antagonists of glycine and GABAA receptors, respectively, introduced into the dorsal horn through a microdialysis fiber. The inhibitory effects of iontophoretic application of glycine and GABAA agonists on STT cell activity evoked by noxious mechanical stimulation of the skin were selectively blocked by their specific antagonist, strychnine or bicuculline, infused into the dorsal horn. Similarly, intra-spinal application of strychnine or bicuculline resulted in a significant reduction in the PAG stimulation-induced inhibition of responses of STT cells to cutaneous stimuli. This reduction was mainly on the PAG-induced inhibition of the responses to noxious mechanical stimuli. Our results suggest that glycinergic and GABAergic inhibitory interneurons in the spinal cord dorsal horn synapsing on STT cells are activated during stimulation in PAG and contribute to descending antinociceptive actions.

Inhibitory neurotransmission in rat spinal cord: co-localization of glycine- and GABAA-receptors at GABAergic synaptic contacts demonstrated by triple immunofluorescence staining

Synaptic inhibition in rat spinal cord is mediated by the amino acids gamma-aminobutyric acid (GABA) and glycine. Most spinal cord neurons respond to both neurotransmitters, suggesting co-expression of GABAA- and strychnine-sensitive glycine-receptors in individual cells. While the distribution of glycine-receptors has been extensively characterized, much less is known about the cellular localization of GABAA-receptors in spinal cord neurons. In the present study, the distribution of GABAA-receptors was analyzed immunohistochemically with a subunit-specific antiserum recognizing the alpha 1-subunit. Their co-localization with glycine-receptors and their apposition to GABAergic axon terminals were assessed by confocal laser microscopy in sections processed for double- and triple-immunofluorescence staining, using a monoclonal antibody against the 93 kDa glycine-receptor-associated protein, gephyrin, and an antiserum to glutamic acid decarboxylase. Staining for the GABAA-receptor alpha 1-subunit decorated the soma and dendrites of numerous neurons in laminae III-VIII and X of the spinal cord, revealing their morphology in clear detail. By contrast, laminae II and IX contained little immunoreactivity for these GABAA-receptors. Double-immunofluorescence staining showed that most GABAA-receptor-positive cells in layers III-VIII and X also exhibited a prominent glycine-receptor immunoreactivity. Both types of receptors had very similar distribution patterns in the cell membrane and were frequently co-localized in sites apposed to GABAergic axon terminals. These results indicate that GABAA- and glycine-receptors may co-exist within single postsynaptic densities, suggesting a possible synergism in the action of GABA and glycine in spinal cord neurons.

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.

Morphine insensitive allodynia is produced by intrathecal strychnine in the lightly anesthetized rat

The acute blockade of spinal glycinergic inhibition with intrathecal strychnine (i.t. STR; a glycine antagonist) in rats induces a change in somatosensory processing which is very similar to the sensory dysesthesia of clinical neural injury pain. In the present study, the effects of i.t. STR were examined in urethane-anesthetized rats. Noxious paw pinch (PP) or tail immersion (TI) in 55 degree C water evoked a pronounced pressor response, increased heart rate (HR) and desynchronized the electroencephalogram; a non-noxious, hair deflection (HD) elicited only minor cardiovascular responses. After i.t. STR (40 micrograms), an identical HD stimulus evoked markedly enhanced cardiovascular responses, resembling those evoked by noxious stimuli, and a HD-evoked motor withdrawal was observed. Consistent STR-dependent responses were only observed if a light plane of anesthesia was maintained for the duration of the experiment. The effects of i.t. STR were dose-dependent and reversible, lasting 15-30 min. Spinal morphine (50 micrograms) completely abolished the cardiovascular responses to PP and TI, but the HD-evoked, STR-dependent cardiovascular and motor withdrawal responses remained unchanged. In contrast, the non-selective excitatory amino acid antagonist, gamma-D-glutamylglycine (DGG; 50 micrograms) was effective in suppressing both the STR-dependent cardiovascular and motor withdrawal responses. These data suggest that STR-dependent responses evoked by non-noxious stimuli are mediated by mechanisms distinct from those of conventional noxious stimuli and that i.t. STR may be useful for investigating the spinal pharmacology of somatosensory processing following the loss of spinal glycinergic inhibition.

Heterogeneous distribution of glycinergic and GABAergic afferents on an identified central neuron

Immunocytochemical methods were used on serial sections to study the glycine- and gamma-amino butyric acid (GABA)ergic innervations of the teleost Mauthner (M) cell. We found different distributions for the boutons containing the two amino acids. Endings filled with GABA predominate on the distal portion of the lateral dendrite (LD) while glycine-positive profiles are more abundant on the soma and within the axon cap (AC), a specialized neuropil surrounding the M-cell initial segment. A few endings containing both transmitters are present on the soma and on the small dendrites issuing ventrally from it. At this level some glutamic acid decarboxylase (GAD)-containing boutons face glycine receptor-93 kD-associated protein, an observation suggesting that the associated glycine functions as a neurotransmitter. Elsewhere on the M-cell, where glycine and GABA are not colocalized, GAD-positive profiles were never observed in front of postsynaptic differentiations with 93 kD labelling. GABA was detected in the small vesicle boutons (SVBs), most of them, following the classification of Tuttle et al., J. Comp. Neurol. 265:254-274, 1987, belonging to the A-type, while glycine was found in the unmyelinated club endings in the AC, and in C- and B-type SVBs, outside this region. All terminals established symmetrical synapses and were filled with a population of pleiomorphic vesicles. Boutons with GABA also contained numerous dense-core vesicles suggesting the presence of an associated peptide(s). A quantitative study of the transmitter content based on the number of the gold particles revealed a variable intensity of the labelling over certain profiles. For GABA, it was maximum at the tip of the LD and it decreased proximally. In contrast, the staining density was constant for glycine along all parts of the cell, except for the ventral dendrite (VD) where it decreased progressively. Taken together, these data suggest that the amino acid content varies, depending upon the location of the synapses on their target neuron.

The use of the optical disector to estimate the number of neurons, glial and endothelial cells in the spinal cord of the mouse--with a comparative note on the rat spinal cord

By means of stereological techniques such as the optical disector, the total number of cells in the spinal cords of five albino mice were estimated. The mean number of cells in the gray and the white matter was 9.0 and 4.1 million, respectively. Of the cells in the gray matter, 4.0 million were classified as neurons, 2.8 million as glial and 1.7 million as endothelial cells whereas 0.4 million remained unclassified. Of the neurons in the gray matter, 1.2 million were located in the cervical region, 1.4 million in the thoracic region, 0.9 million in the lumbar region and 0.4 million in the sacro-coccygeal region. The gray matter in the spinal cord of the mouse contained relatively more neurons and less glial cells than the gray matter in the rat spinal cord. The fraction of cells in the cervical, thoracic, lumbar and sacro-coccygeal part of the cord were, however, quite similar in the two species. In both species the number of neurons in the ventral horns made up about 8-9% of the total neuron population in the gray matter of the cord.

Raphe magnus stimulation-induced antinociception in the cat is associated with release of amino acids as well as serotonin in the lumbar dorsal horn

Stimulation in the nucleus raphe magnus (NRM) inhibits transmission of nociceptive information within the spinal cord through activation of bulbospinal pathways. This study used microdialysis in combination with high pressure liquid chromatography to measure the release of serotonin (5HT) and several amino acids, including glutamate, aspartate and glycine, from the lumbar dorsal horn during electrical stimulation within the NRM in the alpha-chloralose anesthetized cat. Observed release of putative neurotransmitters was correlated with inhibition of nociceptive projection neurons recorded from sites within 800 microns rostral or caudal to the dialysis fiber. NRM stimulus parameters considered to preferentially activate myelinated fibers caused inhibition of nociceptive evoked activity, and increased the release of excitatory amino acids and glycine within the spinal cord, with no detectable release of 5HT. When pulse widths were lengthened and unmyelinated fibers were also activated, increases in 5HT in the spinal dialysate were observed as well. Strychnine administered through the dialysis fiber (0.02-1 mM) antagonized NRM-induced inhibition when 5HT release was not detected. Inhibition produced by stimulation that increased 5HT concentrations was relatively strychnine resistant. These results point to a raphe-spinal inhibitory pathway that is not dependent on 5HT, the activation of which results in the spinal release of glycine.

Inhibitory and excitatory amino acid neurotransmitter binding sites in cynomolgus monkey (Macaca fascicularis) cervical spinal cord

Autoradiography of inhibitory and excitatory amino acid neurotransmitter binding sites in the cervical spinal cord of M. fascicularis spinal cord revealed inhomogeneous distribution of all binding sites in spinal gray matter. Quisqualate-sensitive [3H]glutamate binding, [3H]MK-801 binding, benzodiazepine binding, kainate binding, and GABAB binding had highest levels in the superficial layers of the dorsal horn (laminae 1 and 2) and substantially lower levels in other laminae. [3H]Strychnine binding was more uniformly distributed throughout all laminae with highest levels in the superficial layers of the dorsal horn. These results are similar to those found in other mammals.

Central effects of systemic lidocaine mediated by glycine spinal receptors: an iontophoretic study in the rat spinal cord

The objective of this investigation was to demonstrate the possible interactions of systemic lidocaine (lido) with inhibitory receptors in the spinal cord. In the lumbar dorsal horn of anesthetized and curarized rats, 60 physiologically identified, wide dynamic range (WDR) neurons, were recorded extracellularly. Glutamate, glycine and its selective antagonist, strychnine, were iontophoretically applied onto the neurons either singularly or concurrently. The effects of systemic lido on the drug-induced frequency changes and the interaction with the glycine receptors, using strychnine as a probe, were studied. It was consistently found that: (i) lido (3-4 mg/kg) inhibited the excitatory responses to iontophoretic glutamate, (ii) this inhibition was significantly antagonized by concurrent iontophoretic strychnine, (iii) iontophoretic glycine induced comparable glutamate inhibition that was reversed by strychnine. In contrast, no effect on glutamate-induced excitations was observed when lido was applied by micropressure or a different local anesthetic was systemically administered. The results suggest that central inhibitory effects of lido could be mediated by spinal strychnine-sensitive glycine receptors, activated by lido itself or possibly by its glycine residue-bearing metabolites.

Estimate of the total number of neurons and glial and endothelial cells in the rat spinal cord by means of the optical disector

The total numbers of neurons and glial and endothelial cells in five rat spinal cords were estimated by stereological techniques. Each spinal cord was divided into 12 slabs of equal length. One transverse and one oblique slice was cut from each slab. The volumes of gray and white matter of each cord were then estimated by point-counting techniques on the transverse slices. By means of optical disectors and systematic sampling, the numerical densities of different cell types were estimated on 35 microns-thick plastic sections from the oblique slices. The total cell number was calculated by multiplying the numerical density by the total volume of gray and white matter. On average there were 15.1 and 21.1 million cells in white and gray matter, respectively. Of the cells in gray matter, 6.4 million were judged to be neurons, 4.3 million to be endothelial, and 10.3 million to be glial. Of the neurons, 1.7 million were located in the cervical region, 2.5 million in the thoracic, 1.6 million in the lumbar, and 0.6 million in the sacro-coccygeal region. The methods used are simple to perform, and the counting necessary to obtain a reliable estimate of cell number from one spinal cord can be carried out during the course of 1 day. The only major problem is reliable criteria for unambiguous cell classification.

Immunocytochemical study of somatostatin, neurotensin, GABA, and glycine in rat spinal dorsal horn

In order to determine whether somatostatin coexists with GABA or glycine in neurones in rat spinal dorsal horn, a combined pre- and post-embedding immunocytochemical study was carried out. One hundred six somatostatin-immunoreactive neurones located in lamina II and the dorsal half of lamina III were tested with antiserum or monoclonal antibody to GABA and none of these cells showed GABA-like immunoreactivity. However, 8 out of 13 somatostatin-immunoreactive neurones located deeper in the dorsal horn (ventral lamina III and lamina IV) showed glycine-like immunoreactivity, and 6 of these were also GABA-immunoreactive. We have previously shown that neurotensin-immunoreactive neurones in laminae II and III are also not immunoreactive when tested with GABA antiserum (Todd et al.: Neuroscience 47:685-691, 1992), and a double-labelling fluorescence method was therefore used to compare the distribution of somatostatin and neurotensin within the superficial dorsal horn. The two types of peptide-immunoreactivity were never found in the same profile. These results suggest that somatostatin and neurotensin are present in different populations of non-GABAergic neurones in rat superficial dorsal horn, but that some somatostatin-containing neurones in the deeper part of the dorsal horn contain glycine, with or without GABA.

gamma-Aminobutyric acid and glycine modulate each other's release through heterocarriers sited on the releasing axon terminals of rat CNS

The ability of gamma-aminobutyric acid (GABA) and glycine (Gly) to modulate each other's release was studied in synaptosomes from rat spinal cord, cerebellum, cerebral cortex, or hippocampus, prelabeled with [3H]GABA or [3H]Gly and exposed in superfusion to Gly or to GABA, respectively. GABA increased the spontaneous outflow of [3H]Gly (EC50, 20.8 microM) from spinal cord synaptosomes. Neither muscimol nor (-)-baclofen, up to 300 microM, mimicked the effect of GABA, which was not antagonized by either bicuculline or picrotoxin. However, the effect of GABA was counteracted by the GABA uptake inhibitors nipecotic acid and N-(4,4-diphenyl-3-butenyl)nipecotic acid. Moreover, the GABA-induced [3H]Gly release was Na+ dependent and disappeared when the medium contained 23 mM Na+. The effect of GABA was Ca2+ independent and tetrodotoxin insensitive. Conversely, Gly enhanced the outflow of [3H]GABA from rat spinal cord synaptosomes (EC50, 100.9 microM). This effect was insensitive to both strychnine and 7-chlorokynurenic acid, antagonists at Gly receptors, but it was strongly Na+ dependent. Also, the Gly-evoked [3H]GABA release was Ca2+ independent and tetrodotoxin insensitive. GABA increased the outflow of [3H]Gly (EC50, 11.1 microM) from cerebellar synaptosomes; the effect was not mimicked by either muscimol or (-)-baclofen nor was it prevented by bicuculline or picrotoxin. The GABA effect was, however, blocked by GABA uptake inhibitors and was Na+ dependent. Gly increased [3H]GABA release from cerebellar synaptosomes (EC50, 110.7 microM) in a strychnine- and 7-chlorokynurenic acid-insensitive manner. This effect was Na+ dependent. The effects of GABA on [3H]Gly release seen in spinal cord and cerebellum could be reproduced also with cerebrocortical synaptosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural and immunocytochemical study of lamina II islet cells in rat spinal dorsal horn

In order to compare the ultrastructure of GABA-immunoreactive and nonimmunoreactive islet cells in lamina II of the rat dorsal horn, a combined ultrastructural and immunocytochemical study of nine Golgi-stained neurones was performed. Cell bodies of these neurones were tested with antiserum to GABA, and in most cases with antiserum to glycine, while parts of the cell body and dendritic tree were examined with the electron microscope. Four of the neurones had cell bodies that were immunoreactive with GABA antiserum, and 2 of these were also glycine-immunoreactive, while 2 were not. Cell bodies of the remaining five neurones were not immunoreactive with GABA antiserum, nor, in the 3 cases tested, with glycine antiserum. Three of the GABA-immunoreactive cells possessed vesicle-containing dendrites and were presynaptic at dendrodendritic synapses, whereas no vesicles were observed in the dendrites of any of the neurones that were not GABA-immunoreactive. The axon of one of the nonimmunoreactive cells was found with the electron microscope. It gave rise to boutons that contained round agranular vesicles and a few dense-cored vesicles. Three synapses formed by this axon were identified and all were asymmetric. No obvious differences were detected in the types of profile that were presynaptic to GABA-immunoreactive and nonimmunoreactive cells. These results suggest that GABAergic islet cells are a source of presynaptic dendrites in lamina II of the rat and that some presynaptic dendrites contain GABA and glycine, while others contain GABA without glycine. The nonimmunoreactive islet cells presumably represent a distinct functional class of neurones and some of these may release an excitatory amino acid transmitter, possibly in addition to one or more neuropeptides.

Co-expression of glycine receptor beta subunit and GABAA receptor gamma subunit mRNA in the rat dorsal root ganglion cells

We examined the expression of the beta subunit mRNA of the glycine receptor and the gamma subunit mRNA of the GABAA receptor in the rat dorsal root ganglion (DRG) using in situ hybridization histochemistry with oligonucleotide probes. About 44% and 37% of the all DRG neurons were labeled by the probes for glycine receptor beta subunit and GABAA receptor gamma subunit mRNAs. Labeled neurons were mostly large cells that simultaneously expressed both glycine receptor beta subunit and GABAA receptor gamma subunit mRNA as demonstrated using consecutive sections. Thus, we suggest the possibility that both GABA and glycine presynaptically regulate the activity of neurons involved in low-threshold mechanoreception at axo-axonic synapses in the spinal cord.

Glycine-like immunoreactive input to sympathetic preganglionic neurons

The organization of glycine-like immunoreactive (GLY-LIR) processes was investigated within the sympathetic preganglionic neuropils of male Sprague-Dawley rats and pigeons (Columba livia). Sympathetic preganglionic neurons were retrogradely labeled with horseradish peroxidase following injections into the superior cervical ganglion in rats or into the avian homologue of the mammalian stellate ganglion (paravertebral ganglion 14) in pigeons. Glycine-like immunoreactivity was visualized using postembedding immunoperoxidase and immunogold labeling methods. The neuropils surrounding pigeon sympathetic preganglionic neurons in the principal preganglionic cell column (nucleus of Terni) and in the nucleus intercalatus contained numerous GLY-LIR puncta. Many of these processes appeared to be 'terminal-like' swellings which closely apposed retrogradely labeled preganglionic perikarya and proximal dendrites. GLY-LIR somal and dendritic processes were intermingled among retrogradely labeled preganglionic neurons in the nucleus of Terni. None of these GLY-LIR cells were retrogradely labeled. The neuropils surrounding sympathetic preganglionic neurons in the rat also contained numerous GLY-LIR puncta; there were, however, qualitative differences in the density of such profiles across the preganglionic subnuclei. Within the central autonomic and intercalated regions there were numerous GLY-LIR processes, many of which closely apposed retrogradely labeled sympathetic preganglionic somas and proximal dendrites. Within the principal preganglionic cell column, the nucleus intermediolateralis pars principalis (Ilp), there were very few GLY-LIR 'terminal-like' swellings closely apposed to cell bodies in regions of high somal packing density. In regions were this density diminished, GLY-LIR puncta closely apposed retrogradely labeled perikarya and proximal dendritic processes. GLY-LIR spinal neurons were never observed to be within Ilp proper but were present in areas immediately dorsal (lateral lamina V), medial and ventral (lateral lamina VII). GLY-LIR neurons were never retrogradely labeled. The ultrastructural features of GLY-LIR terminals within the sympathetic preganglionic neuropils of both vertebrates were nearly identical. GLY-LIR terminal boutons formed synaptic contacts with retrogradely labeled preganglionic somas as well as with large and medium-sized proximal dendrites. The majority of identified GLY-LIR terminals, however, contacted non-retrogradely labeled medium and small caliber dendrites within the preganglionic neuropils. Ninety-eight percent of GLY-LIR synapses formed symmetric specializations with the postsynaptic element. Ninety-six percent of the GLY-LIR terminal boutons contained some combination of pleomorphic vesicles. These light and electron microscopic observations support the hypothesis that glycine is localized in terminals presynaptic to sympathetic preganglionic perikarya and dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)

Light and electron microscope study of GABA-immunoreactive neurones in lamina III of rat spinal cord

In order to determine whether different morphological types of neurone in lamina III of rat spinal dorsal horn contain different neurotransmitters, a combined Golgi and immunocytochemical study was performed. Semithin sections through the cell bodies of 52 Golgi-impregnated neurones in this lamina were tested with antisera to GABA and glycine. Thirty of these cells were immunoreactive with anti-GABA antiserum and 25 of these also showed glycine-like immunoreactivity. These cells had dendrites which were oriented along the rostrocaudal axis and occupied lamina III, with some extension into lamina IV and the ventral half of lamina II. Although some of the nonimmunoreactive cells had similar morphology, many of them had dendrites which passed in a dorsal and/or ventral direction and crossed laminar boundaries. Three of the neurones which were immunoreactive with both antisera were examined with the electron microscope. These cells received a variety of synapses including some from axons which resembled low threshold myelinated mechanoreceptive primary afferents. These results indicate that there is a relationship between morphology and function for neurones in lamina III. It is suggested that the inhibitory neurones which contain both GABA and glycine selectively regulate the transmission of information from low threshold mechanoreceptive primary afferents to other dorsal horn neurones.

Relationships between hair-follicle afferent axons and glycine-immunoreactive profiles in cat spinal dorsal horn

In order to identify synapses between hair-follicle afferent axons and glycine-containing structures in cat spinal cord, semithin sections containing physiologically identified primary afferent boutons which had been filled with horseradish peroxidase (HRP) were reacted with anti-glycine antiserum, while adjacent ultrathin sections were examined for synaptic contacts. Four axodendritic synapses between hair-follicle afferent boutons and glycine-immunoreactive dendrites and 4 axoaxonic synapses in which HRP-filled boutons were postsynaptic to immunoreactive axons were identified. These results suggest that glycine is involved in the spinal processing of input from A beta hair-follicle afferent axons.

Immunohistochemical evidence that acetylcholine and glycine exist in different populations of GABAergic neurons in lamina III of rat spinal dorsal horn

Pre-embedding immunohistochemistry with monoclonal antibody to choline acetyltransferase was combined with post-embedding immunohistochemistry with antisera to GABA and glycine in order to study the pattern of coexistence of GABA, glycine and acetylcholine in neurons in lamina III of rat spinal dorsal horn. Of 50 neurons which were choline acetyltransferase immunoreactive, 47 showed GABA-like immunoreactivity and none were immunoreactive with antiserum to glycine, despite the fact that glycine is thought to be present in the majority of GABAergic neurons in lamina III. This suggests that while acetylcholine and glycine can both coexist with GABA in lamina III neurons, they are present in different populations of GABAergic cells. Taken together with recent ultrastructural evidence concerning the synaptic connections of glycinergic and cholinergic structures in the dorsal horn, this suggests that there are functional differences between neurons which contain GABA and glycine and those which contain GABA and acetylcholine.