Immunocytochemical localization of glutamate decarboxylase in rat spinal cord

Vesicle shape and amino acids in synaptic inputs to phrenic motoneurons: do all inputs contain either glutamate or GABA?

Varicosities that made synapses or direct contacts with retrogradely labelled rat phrenic motoneurons were examined for their content of immunoreactivity for either glutamate or glutamate decarboxylase, the enzyme involved in synthesis of gamma-aminobutyric acid (GABA). Phrenic motoneurons were identified by retrograde tracing from the diaphragm with cholera toxin B subunit conjugated to horseradish peroxidase. Cell bodies and medium-sized to large dendrites were labelled. Preembedding immunocytochemistry identified glutamate decarboxylase-immunoreactive nerve fibres; glutamate-immunoreactive nerve terminals were identified using postembedding immunogold labelling of ultrathin sections. The presence of glutamate- or glutamate decarboxylase immunoreactivity in nerve terminals was correlated with the morphology of the synaptic vesicles. Two major classes of nerve terminals were identified. Nerve terminals with round (presumably spherical) synaptic vesicles (S terminals) comprised 55% of synapses and contacts on phrenic motoneuron somata and 58% of synapses and direct contacts with dendrites. Nerve terminals with flattened synaptic vesicles (F terminals) comprised 42% of synapses direct contacts with somata and 41% of synapses and direct contacts with dendrites. Analysis of immunogold-labelled sections showed that S terminals contained statistically higher levels of glutamate immunoreactivity than F terminals. At the light microscope level, many glutamate decarboxylase-immunoreactive nerve terminals surrounded retrogradely labelled motoneurons. Varicosities with glutamate decarboxylase immunoreactivity made 33% of all synapses and direct contacts on somata, and 33% of synapses and direct contacts with dendrites of the retrogradely labelled phrenic motoneurons. Flattened synaptic vesicles were present in those glutamate decarboxylase-immunoreactive nerve terminals in which synaptic vesicle morphology could be judged. An additional 10% of all nerve terminals were of the F type, but were not glutamate decarboxylase-immunoreactive. Three percent of terminals on somata and 1% of nerve terminals on dendrites could not be classified as S or F types. These findings suggest that more than 90% of all inputs to phrenic motoneuron cell bodies and proximal dendrites could contain either GABA or glutamate. Some of these glutamatergic and GABAergic nerve fibres undoubtedly represent the source of inspiratory drive to, or expiratory inhibition of, phrenic motoneurons.

Effects of GABA receptor antagonists injected spinally on antinociception induced by opioids administered supraspinally in mice

The present study was designed to investigate the modulatory effects of blockade of spinal GABAA and GABAB receptors on antinociception induced by supraspinally administered mu- and epsilon-opioid receptor agonists. The effects of intrathecal (i.t.) injections with GABAA and GABAB receptor antagonists, SR 95531 [2-(3-carboxypropyl)-3-amino-6-(4-mehylphenyl)pyridazinium bromide] and 5-aminovaleric acid, respectively, on the antinociception induced by morphine (a mu-opioid receptor agonist) and beta-endorphin (an epsilon-opioid receptor agonist) injected intracerebroventricularly (i.c.v.) were studied. Antinociception was assayed using the tail-flick test. The i.t. injection of SR 95531 (0.04-0.16 nmol) and 5-aminovaleric acid (32.5-130 nmol), administered alone did not affect the latencies of the tail-flick response, but selectively antagonized the inhibition of the tail-flick response induced by muscimol (a GABAA receptor agonist) and baclofen (a GABAB receptor agonist), respectively. The i.t. injection of SR 95531 attenuated dose-dependently the inhibition of the tail-flick response induced by i.c.v. administered morphine, without affecting the i.c.v. administered beta-endorphin-induced response. 5-Aminovaleric acid attenuated dose-dependently the inhibition of the tail-flick response induced by beta-endorphin, without affecting the response to i.c.v. administered morphine. Our results indicate that GABAA but not GABAB receptors located at the spinal cord appears to be involved in the antinociception induced by morphine administered supraspinally whereas GABAB but not GABAA receptors located at the spinal cord may be involved in the antinociception induced by supraspinally administered beta-endorphin, supporting further the hypothesis that morphine and beta-endorphin administered supraspinally produce their antinociception via the activation of different descending pain inhibitory systems.

Qualitative and quantitative analysis of glycine- and GABA-immunoreactive nerve terminals on motoneuron cell bodies in the cat spinal cord: a postembedding electron microscopic study

The distribution of glycine- and gamma-aminobutyric acid (GABA)-like immunoreactivity (LI) in nerve terminals on the cell soma of motoneurons in the aldehyde-fixed cat L7 spinal cord was examined using postembedding immunogold histochemistry in serial ultrathin sections. Quantitative examination of 405 terminals on eight neurons of alpha-motoneuron size in the L7 motor nuclei from one animal was performed. A majority of the terminals (69%) were immunoreactive to glycine and/or GABA. These terminals contained flat or oval synaptic vesicles, thus classifying them as F type or as C type in one case. In no case was a type-F terminal unlabeled for both glycine and GABA. Most of the immunolabeled terminals were immunoreactive to glycine only (62.5%), whereas 35.4% contained both glycine- and GABA-LI. A very small number of immunolabeled terminals (2%) were immunoreactive to GABA only. In those terminals, where glycine- and GABA-LI coexisted, the gold particle density for each amino acid was only half of that seen in boutons containing only one of the two amino acids. The involvement of glycine and GABA in postsynaptic inhibition of spinal alpha-motoneurons is discussed, with particular reference to the possibility that these two inhibitory amino acids may be coreleased from a significant proportion of the nerve terminals impinging on the cell bodies.

Distribution of immunoreactivity for the beta 2 and beta 3 subunits of the GABAA receptor in the mammalian spinal cord

The localization of GABAA receptors in cat and rat spinal cord was analyzed using two monoclonal antibodies specific for an epitope shared by the beta 2 and beta 3 subunits of the receptor. beta 2/beta 3-subunit immunoreactivity was the most intense in inner lamina II, lamina III, and lamina X, and it was the least intense in lamina IX. In laminae I-III, generally, the staining had a rather diffuse appearance, but the surfaces of small cell bodies in these laminae were outlined clearly by discrete labeling, as were many cell bodies and dendrites in deeper laminae. Rhizotomy experiments and ultrastructural observations indicated that beta 2/beta 3-subunit immunoreactivity in the dorsal horn was largely localized in intrinsic neuropil elements rather than in the terminals of primary afferent fibers, even though labeling overlapped with the terminal fields of different types of primary afferents and was also detected on the membranes of dorsal root ganglion neurons. With few exceptions (most notably, a highly immunoreactive group of dorsolaterally located cells in the cat lumbar ventral horn), motoneurons expressed low levels of beta 2/beta 3-subunit immunoreactivity. Labeling of neuronal membranes was fairly continuous, but focal accumulations of beta 2/beta 3-subunit immunoreactivity were also detected using immunofluorescence. Focal "hot spots" correlated ultrastructurally with the presence of synaptic junctions. Dual-color immunofluorescence revealed that focal accumulations of beta 2/beta 3-subunit immunoreactivity were frequently apposed by glutamic acid decarboxylase (GAD)-immunoreactive terminals. However, the density of continuous-membrane beta 2/beta 3 immunolabeling and GAD terminal density were not correlated in many individual neurons. The results suggest the existence of "classical" (synaptic) and "nonclassical" (paracrine) actions mediated via spinal cord GABAA receptors. The study also revealed the relative paucity of beta 2/beta 3-subunit immunoreactivity postsynaptic to certain GABAergic terminals, particularly those presynaptic to motoneurons or primary afferent terminals.

Gaba-like immunoreactive terminals on lumbar motoneurons of the adult cat. A quantitative ultrastructural study

The aim of this ultrastructural study was to analyse quantitatively the distribution of gamma-aminobutyric acid (GABA)-like immunoreactivity in axon terminals apposed to somatic and proximal dendritic membranes of cat motoneurons in lumbar column 2. Preembedding immunocytochemistry was used to count the GABAergic terminals contacting profiles of eighteen alpha-and six gamma-motoneurons. Of the 1293 terminals counted on the somatic and proximal dendritic compartments of alpha-motoneurons, 197 were GABAergic. In contrast, a total number of only 62 terminals were counted on gamma-motoneurons, of which 8 were GABAergic. These populations of GABAergic terminals were less numerous than the population of glycinergic terminals observed in a previous study. The morphometric characteristics of GABAergic synapses were analyzed using postembedding immunocytochemistry. Most of the GABAergic terminals contained pleomorphic vesicles (F-type boutons, flattened or pleomorphic vesicles). All terminals presynaptic (P boutons) to large terminals containing sphericle vesicles (M-type boutons, characteristic of alpha-motoneurons), were GABA-immunopositive. These results suggest that there are different distributions of the GABAergic control of excitability on gamma- and alpha-motoneurons. GABA appears to be strongly involved in post-synaptic inhibition of alpha-motoneurons, whereas gamma-motoneurons receive very few GABAergic inhibitory inputs. Morphological correlates of GABAergic presynaptic inhibition were seen on alpha- but not on gamma-motoneurons.

Comparative distribution of GAD65 and GAD67 mRNAs and proteins in the rat spinal cord supports a differential regulation of these two glutamate decarboxylases in vivo

Gamma-aminobutyric acid (GABA) synthesis can result from the action of at least two glutamic acid decarboxylase (GAD) isoforms, GAD65 and GAD67, possibly involved in distinct mechanisms. We have made the hypothesis that GAD65 may respond to short-term changes and is present in neurons with a phasic activity, while GAD67 may rather provide GABA for the metabolic pool and for supporting tonic levels of synaptic transmission (Erlander et al.: Neuron 7:91-100, 1991; Feldblum et al.: J Neurosci Res 34:689-706, 1993). In the present work we have tested this hypothesis in the rat spinal cord where both types of activities have been identified. The correlation of GABA immunodetection with the distribution of GAD65 and GAD67 mRNAs and proteins has evinced in the dorsal horn a differential regulation of the two isoforms. In situ hybridization has revealed, in the dorsal horn, relatively higher levels of GAD67 mRNA than of GAD65, while immunodetection of the proteins demonstrated numerous punctate profiles with both GAD antisera. Reverse transcription-polymerase chain reaction (RT-PCR) data confirmed the abundance of the GAD67 transcripts compared to GAD65 in the rat spinal cord. In contrast, within the ventral horn, there was a greter number of GAD67-immunoreactive (IR) profiles mostly located around motoneurons. The paucity of GAD65 immunoreactivity in the ventral horn cannot be related to a different accessibility of the antigens to the epitopes since on the same section a dense GAD65 staining was detected in the dorsal horn. Hence, a number of biochemical and electrophysiological data support the concept of the involvement of glycine as the major inhibitory system within the ventral horn which may explain the low levels of GAD transcription in this region. The paucity of GAD65 in the ventral horn may also reflect a functional difference, suggesting a predominance of GAD67 in neurons under tonic activity. In the dorsal horn, where neurons with phasic and tonic firing patterns have been disclosed, GAD65 may, in addition, provide GABA for responses to short-term changes.

Developmental expression of glycine immunoreactivity and its colocalization with GABA in the embryonic chick lumbosacral spinal cord

The development of immunoreactivity for the putative inhibitory amino acid neurotransmitter glycine was investigated in the embryonic and posthatched chick lumbosacral spinal cord by using postembedding immunocytochemical methods. Glycine immunoreactive perikarya were first observed at embryonic day 8 (E8) both in the dorsal and ventral gray matters. The number of immunostained neurons sharply increased by E10 and was gradually augmented further at later developmental stages. The general pattern of glycine immunoreactivity characteristic of mature animals had been achieved by E12 and was only slightly altered afterward. Most of the immunostained neurons were located in the presumptive deep dorsal horn (laminae IV-VI) and lamina VII, although glycine-immunoreactive neurons were scattered throughout the entire extent of the spinal gray matter. By using some of our previously obtained and published data concerning the development of gamma-aminobutyric acid (GABA)-ergic neurons in the embryonic chick lumbosacral spinal cord, we have compared the numbers, sizes, and distribution of glycine- and GABA-immunoreactive spinal neurons at various developmental stages and found the following marked differences in the developmental characteristics of these two populations of putative inhibitory interneurons. (i) GABA immunoreactivity was expressed very early (E4), whereas immunoreactivity for glycine appeared relatively late (E8) in embryonic development. (ii) In the ventral horn, GABA immunoreactivity declined, whereas immunoreactivity for glycine gradually increased from E8 onward in such a manner that the sum of glycinergic and GABAergic perikarya remained constant during the second half of embryonic development. (iii) Glycinergic and GABAergic neurons showed different distribution patterns in the spinal gray matter throughout the entire course of embryogenesis as well as in the posthatched animal. When investigating the colocalization of glycine and GABA immunoreactivities, perikarya immunostained for both amino acids were revealed at all developmental stages from E8 onward, and the proportions of glycine- and GABA-immunoreactive neurons that were also immunostained for the other amino acid were remarkably constant during development. The characteristic features of the development of the investigated putative inhibitory spinal interneurons are discussed and correlated with previous neuroanatomical and physiological studies.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

Interaction between substance P-immunoreactive central terminals and gamma-aminobutyric acid-immunoreactive elements in synaptic glomeruli in the lamina II of the chicken spinal cord

We investigated the interaction between gamma-aminobutyric acid (GABA)-immunoreactive (IR) elements and substance P (SP)-IR central terminals in synaptic glomeruli in lamina II of the chicken spinal cord in order to ascertain how pain information is modulated in the spinal dorsal horn. We combined the peroxidase-antiperoxidase (PAP) technique and the protein A-gold (PAG) technique to observe the synaptic relationship between these two components. At the light microscopic level, we observed both GABA-IR and SP-IR elements in the lamina II. GABA-IR elements were also observed in the lamina III. At the electron microscopic level, the following three GABA-IR elements formed synapses with the SP-IR central terminals in synaptic glomeruli: (1) elements which appeared to be axon terminals containing tightly-packed pleomorphic clear vesicles; (2) elements which appeared to be vesicle-containing dendrites with loosely-packed clear and dense-cored vesicles (DCVs); and (3) dendrites without synaptic vesicles. The first type of element was always presynaptic to the SP-IR central terminal. The second type was postsynaptic, presynaptic or in some cases reciprocal to the SP-IR central terminals. The third type was postsynaptic to the SP-IR central terminal. These results suggest that the SP-containing primary afferents activate GABA-containing dendrites and that the SP-containing primary afferents are inhibited presynaptically by GABA-containing neurons through axo-axonic and dendro-axonic synapses.

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.

The distribution of NADPH-d in the central grey region (lamina X) of rat upper thoracic spinal cord

The distribution of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) in the central grey region (lamina X of Rexed) of the rat upper thoracic cord was examined by LM and EM. Numerous NADPH-d positive neuronal somata and fibres were present in the subependymal areas of the central grey region at levels T1-T3. Most of the neurons were located dorsal to the central canal in horizontal sections through this region. Many medially-directed NADPH-d positive fibres arising from neurons in n. intermediolateralis pars principalis, n. intercalatus spinalis and longitudinally-directed NADPH-d positive fibres arising from neurons in n. intercalatus pars paraependymalis formed a subependymal plexus. In horizontal sections through the central canal, some NADPH-d positive nerve fibres appeared to traverse the ependyma to enter and run along the central canal. By EM, NADPH-d reaction products were localized on the nuclear membrane, outer mitochondrial membrane and Golgi apparatus of both neurons and ependymal cells and in some axon terminals containing pleomorphic and round agranular synaptic vesicles. Present results suggest that besides the traditional monoamine-, amino acid- and peptide-containing axon terminals, the central grey region also contains fibres in which nitric oxide is utilized as a neurotransmitter or neuromodulator. The finding of NADPH-d positive fibres in the central canal suggests that nitric oxide may be released into DPH-cerebrospinal fluid. Since some of the ependymal cells were NADPH-d positive, it is suggested that they may be involved in the modulation of nitric oxide levels in the cerebrospinal fluid.

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.

The isolated mammalian spinal cord

This review considers: spinal cord slices; isolated spinal cord sagitally or transversely hemisected; whole spinal cord; respiration control--[brain-stem spinal cord; brain-stem spinal cord with attached lungs]; nociception--[spinal cord with tail]; fictive locomotion--[spinal cord with one hind limb; spinal cord with two hind limbs]. Much of the functional circuitry of the CNS can be studied in the isolated spinal cord with the additional advantage that the isolated spinal cord can be perfused with known concentrations of ions, neurotransmitters, agonists, antagonists, and anaesthetics. These can be washed away, the circuitry allowed to recover and other drugs or different concentrations applied. Future preparations including the complete spinal cord, the two hind limbs, and a sagittal section of the complete brain will allow greater understanding of the multiple sensory and motor pathways and their interactions in the CNS.

A radiometric microassay for choline acetyltransferase. Some observations on the spinal cord of the chicken embryo

This paper describes cation-exchange methods for separating acetyl[3H] coenzyme A from [acetyl-3H]choline. Blanks for the routine method were approximately 0.05% of the substrate radioactivity; product recoveries were approximately 97%. The cation-exchange method was more efficient than the standard methods using either anion-exchange chromatography or periodide precipitation. The cation-exchange method was also more specific than either of the other two standard methods for estimating choline acetyltransferase (ChAT) activity. ChAT activity was detected in the chicken lumbar spinal cord on embryonic day (E) 2 1/4 with the cation-exchange method. This developmental stage is about 6 hours before the final mitosis of any neuroblast in the ventral horn. Total ChAT activity per lumbar spinal cord increased more than 10,000-fold between E 3 and E 18. Changes in ChAT activity in the lumbar spinal cord following limb-bud extirpation appeared to mirror (with a phase lag) the changes in the number of motoneurons in the lateral motor column.

[D-Pen2-D-Pen5]enkephalin, a delta opioid agonist, given intracerebroventricularly in the mouse produces antinociception through medication of spinal GABA receptors

Intracerebroventricular (ICV) administration of [D-Pen2-D-Pen5]enkephalin (DPDPE), a delta opioid receptor agonist, activates a descending antinociceptive pathway that inhibits the tail-flick response in mice. Involvement of spinal GABA receptors in this response was studied by giving GABA antagonist intrathecally. First, antinociception produced by intrathecally administered isoguvacine, a GABAA agonist, was inhibited by intrathecal bicuculline (GABA receptor antagonist) or picrotoxin (chloride channel antagonist). Then, antinociception induced by ICV DPDPE was antagonized by intrathecal picrotoxin and bicuculline in a dose-and time-dependent manner. Second, intrathecal administration of 2-hydroxysaclofen, a GABAB antagonist (which inhibited antinociception induced by a GABAB agonist, baclofen, given IT), produced a shift of the dose-response curve for ICV DPDPE to the right. GABAA agonist, baclofen, given IT), produced a shift of the dose-response curve for ICV DPDPE to the right. GABAA and B antagonists given together intrathecally produced a greater than additive antagonistic effect against ICV DPDPE-induced antinociception. Thus, the delta agonist action of DPDPE in the brain leads to activation of descending spinal pathways which involve mediation by spinal GABAA and GABAB receptors in the antinociceptive response.

GABAergic and pallidal terminals in the thalamic reticular nucleus of squirrel monkeys

The ultrastructure of synaptic terminals from the external segment of the globus pallidus and of other synaptic terminals positive for gamma-aminobutyric acid (GABA) was examined in the thalamic reticular nucleus (TRN) of squirrel monkeys. Two GABA-positive terminals types were commonly encountered within the TRN neuropil. The most common type of GABAergic terminals (F terminals) are filled with dispersed pleomorphic synaptic vesicles and clusters of mitochondria. These terminals establish multiple symmetric synapses upon the somata and dendrites of TRN neurons. The external pallidal terminals, labeled with WGA-HRP, arise from thinly myelinated axons and correspond to the medium to large F terminals. A less prevalent population of smaller GABAergic synaptic profiles was also identified. The synaptic profiles in this second group contain considerably fewer pleomorphic synaptic vesicles in small irregular clusters and fewer mitochondria, establish symmetric synapses, are postsynaptic to other axonal terminals, are presynaptic to dendrites and soma, and are unlabeled following pallidal injections of WGA-HRP.

Increased sensitivity to the antinociceptive activity of (+/-)-baclofen in an animal model of chronic neuropathic, but not chronic inflammatory hyperalgesia

Modulation of sensory afferent inputs to the spinal cord by GABA appears to be an important physiological mechanism and may provide an antinociceptive control system. In the present study we have evaluated the antinociceptive activity of the GABAB receptor agonist, (+/-)-baclofen, in rats with unilateral chronic inflammatory or neuropathic hyperalgesia. (+/-)-Baclofen was antinociceptive in untreated control animals and both animal models. In the neuropathic model the sensitivity to (+/-)-baclofen was significantly increased by 3-fold in the ipsilateral limb. By contrast, in animals with chronic inflammation no difference in sensitivity between ipsilateral and contralateral limbs to (+/-)-baclofen was observed. Receptor autoradiographic analysis in spinal cord sections revealed no increase in the density of GABAB receptor binding sites and no change in receptor affinity in the neuropathic model.

Immunohistochemical evidence for GABA and glycine-containing trigeminal premotoneurons in the guinea pig

Electrophysiological studies have suggested that inhibition of trigeminal motoneurons during mastication and the jaw-opening reflex are mediated by last-order interneurons (premotoneurons) utilizing GABA and glycine [Chandler et al. (1985), Brain Res., 325:181-186; Enomoto et al. (1987), Neurosci. Res., 4:396-412; Goldberg and Nakamura (1968), Experientia, 24:371-373; Kidokoro et al. (1968), J. Neurophysiol., 31:695-708; Nakamura et al. (1978), Exp. Neurol., 61:1-14]. In the present study we performed a series of double-labeling experiments in guinea pigs to determine the location of neurons which contain GABA (gamma aminobutyric acid) or glycine that project to the trigeminal motor nucleus (Mo5). This was accomplished by performing immunohistochemical staining in combination with a retrograde tract tracing technique using colloidal gold bound to inactivated WGA-HRP (wheat germ agglutin-horseradish peroxidase) (gWGA-HRP) as our retrograde tracer. Neurons which had a positive immunoreactivity to GABA or GAD (glutamic acid decarboxylase) and contained the retrograde marker were located in regions adjacent to the Mo5 such as the intertrigeminal, supratrigeminal, peritrigeminal and rostral portions of the parvocellular reticular formation alpha. Neurons which had a positive immunoreactivity to glycine and contained the retrograde marker were identified in the parvocellular reticular formation, the spinal trigeminal nucleus oralis, supratrigeminal and intertrigeminal regions. These data provide anatomical evidence for GABAergic and glycinergic projections to Mo5.

Transient increase in expression of GAD65 and GAD67 mRNAs during postnatal development of rat spinal cord

Gamma-aminobutyric acid (GABA) is thought to be one of the classic neurotransmitters acting as a developmental signal. To understand the role for GABA in development, we investigated the expression of transcripts encoding two forms of the GABA-synthesizing enzyme glutamate decarboxylase (GAD65 and GAD67) in the cervical enlargement of the rat spinal cord at successive postnatal days--P0, P7, P14, P21, and P90 (adult)--by using in situ hybridization histochemistry. Cells hybridized with two oligonucleotide probes designed to detect GAD65 and GAD67 mRNAs were widely distributed in all laminae, except in motoneurons of the spinal cord. The integrated densities of hybridization signals were measured across all layers of the gray matter. The relative number of GAD mRNA-labeled cells was determined within each of four regions: laminae I-III, laminae IV-VI, laminae VII and VIII, and lamina X. There was a transient increase in both the integrated density and the relative number of hybridized cells between P7 and P14, after which there was a marked decline to adult levels (lowest). An overall decrease in the number of GAD mRNA-labeled cells was evident in all layers, but a dramatic drop occurred in a subpopulation of cells within ventral portions of the spinal cord. The distribution patterns and postnatal changes in expression of the mRNAs encoding GAD65 and GAD67 were similar and closely paralleled reported changes in the abundance of GAD65 and GAD67 proteins and their product, GABA. Transient increases in GAD mRNA expression during the early postnatal period coincide with, and may be linked to, synapse formation and synapse elimination of the developing spinal cord.

Synaptic terminal coverage of primate triceps surae motoneurons

This study examined the synaptic terminal coverage of primate triceps surae (TS) motoneurons at the electron microscopic level. In three male pigtail macaques, motoneurons were labeled by retrograde transport of cholera toxin-horseradish peroxidase that was injected into TS muscles bilaterally and visualized with tetramethylbenzidine stabilized with diaminobenzidine. Somatic, proximal dendritic, and distal dendritic synaptic terminals were classified by standard criteria and measured. Overall and type-specific synaptic terminal coverages and frequencies were determined. Labeled cells were located in caudal L5 to rostral S1 ventral horn and ranged from 40 to 74 microns in diameter (average, 54 microns). The range and unimodal distribution of diameters, the label used, and the presence of C terminals on almost all cells indicated that the 15 cell bodies and associated proximal dendrites analyzed here probably belonged to alpha-motoneurons. Synaptic terminals covered 39% of the cell body membrane, 60% of the proximal dendritic membrane, and 40% of the distal dendritic membrane. At each of these three sites, F terminals (flattened or pleomorphic vesicles, usually symmetric active zones, average contact length 1.6 microns) were most common, averaging 52%, 56%, and 58% of total coverage and 56%, 57%, and 58% of total number of cell bodies, proximal dendrites, and distal dendrites respectively. S terminals (round vesicles, usually asymmetric active zones, average contact length 1.3 microns) averaged 24%, 29%, and 33% of coverage and 33%, 35%, and 36% of number at these three sites, respectively. Thus, S terminals were slightly more prominent relative to F terminals on distal dendrites than on cell bodies. C terminals (spherical vesicles, subsynaptic cisterns associated with rough endoplasmic reticulum, average contact length 3.5 microns) constituted 24% and 11% of total terminal coverage on cell bodies and proximal dendrites, respectively, and averaged 11% and 6% of terminal number at these two locations. M terminals (spherical vesicles, postsynaptic Taxi bodies, some with presynaptic terminals, average contact length 2.7 microns) were absent on cell bodies and averaged 3% and 7% of total coverage and 2% and 5% of terminals on proximal and distal dendrites, respectively. Except for M terminals, which tended to be smaller distally, terminal contact length was not correlated with location. Total and type-specific coverages and frequencies were not correlated with cell body diameter. Primate TS motoneurons are similar to cat TS motoneurons in synaptic terminal morphology, frequency, and distribution. However, primate terminals appear to be smaller, so that the fraction of membrane covered by them is lower.

Light- and electron-microscopic analysis of neuropeptide Y-immunoreactive profiles in the cat spinal dorsal horn

The organization of neuropeptide Y-containing profiles in the dorsal horn of cat lumbosacral spinal cord was examined in an immunocytochemical study employing a specific antiserum against neuropeptide Y. Light-microscopic inspection revealed heavy concentrations of immunoreactive axons and varicosities within the superficial layers of the dorsal horn (laminae I and II) and only low to moderate numbers of positive terminals in the deeper layers (laminae III-VI). Neuropeptide-Y immunoreactivity in the superficial laminae occurred primarily as single punctate terminals, although in sagittal sections long rostrocaudally orientated fibres were also found. Immunoreactive fibres in the deeper layers were usually long and beaded. Two-hundred and eight neuropeptide Y-immunoreactive profiles throughout laminae I-VI were examined through serial sections with the electron microscope, and the overwhelming majority (n = 194) was confirmed to be axon terminals, most of which (95%) formed synaptic junctions. These terminals were packed with small irregularly shaped agranular vesicles, together with a number of large dense-core vesicles. Immunoreactivity was homogeneously scattered throughout the cytoplasm, and was also associated with the dense-core vesicles. A few neuropeptide Y-containing profiles (n = 14) were difficult to classify but they could have been vesicle-containing dendrites. The postsynaptic targets of neuropeptide Y-positive terminals were similar throughout each dorsal horn lamina. Most frequently, neuropeptide Y-positive boutons formed axodendritic and axosomatic synaptic junctions (range = 64% of synapses in laminae V/VI to 83% in lamina III). A smaller proportion of synapses were found upon other axon terminals and in laminae I-III the postsynaptic axon terminals were sometimes the central boutons of glomeruli. A number of terminals, especially those in lamina II, formed multiple synapses which often comprised a triadic arrangement. These findings suggest that neuropeptide Y regulates spinal sensory transmission through both a postsynaptic action upon dorsal horn neurons and a presynaptic action upon primary afferent terminals.

Intergeniculate leaflet: an anatomically and functionally distinct subdivision of the lateral geniculate complex

The intergeniculate leaflet (IGL) in the rat is a distinctive subdivision of the lateral geniculate complex that participates in the regulation of circadian function through its projections to the circadian pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. The present investigation was undertaken to provide a precise definition of the IGL and a characterization of its neuronal organization including neuronal morphology, chemical phenotype, connections, and synaptic organization. The IGL extends the entire rostrocaudal length of the geniculate complex and contains a distinct population of small to medium neurons. In Golgi preparations, the neurons are multipolar with dendrites largely confined to the IGL. The neurons can be subdivided into three groups on the basis of neurotransmitter content and projections: (1) neurons that contain GABA and neuropeptide Y and project to the SCN; (2) neurons that contain GABA and enkephalin and project to the contralateral IGL; and (3) a small group of neurons that projects to the SCN but not characterized as yet by neurotransmitter content. The IGL receives dense, bilateral input from retinal ganglion cells and dense substance P input of unknown origin. A number of neurons in the anterior hypothalamic area and, particularly, the retrochiasmatic area project to the IGL, and there are sparse projections from brainstem monoamine and cholinergic neurons. The synaptic organization of the IGL is complex with afferents terminating in glomerular complexes that include axoaxonic synaptic interactions. Virtually all IGL afferents synapse upon dendrites and spines, with the densest synaptic input occurring on the distal portions of the dendritic arbor. The organization of the IGL and its connections as revealed in this analysis is in accord with its role in the integration of visual input with other information to provide feedback regulation of the SCN pacemaker.

Developmental changes in the distribution of gamma-aminobutyric acid-immunoreactive neurons in the embryonic chick lumbosacral spinal cord

The development of gamma-aminobutyric acid (GABA)-immunoreactive neurons was investigated in the embryonic and posthatch chick lumbosacral spinal cord by using pre- and postembedding immunostaining with an anti-GABA antiserum. The first GABA-immunoreactive cells were detected in the ventral one-half of the spinal cord dorsal to the lateral motor column at E4. GABAergic neurons in this location sharply increased in number and, with the exception of the lateral motor column, appeared throughout the entire extent of the ventral one-half of the spinal gray matter by E6. Thereafter, GABA-immunoreactive neurons extended from ventral to dorsal regions. Stained perikarya first appeared at E8 and then progressively accumulated in the dorsal horn, while immunoreactive neurons gradually declined in the ventral horn. The general pattern of GABA immunoreactivity characteristic of mature animals had been achieved by E12 and was only slightly altered afterwards. In the dorsal horn, most of the stained neurons were observed in laminae I-III, both at the upper (LS 1-3) and at the lower (LS 5-7) segments of the lumbosacral spinal cord. In the ventral horn, the upper and lower lumbosacral segments showed marked differences in the distribution of stained perikarya. GABAergic neurons were scattered in a relatively large region dorsomedial to the lateral motor column at the level of the upper lumbosacral segments, whereas they were confined to the dorsalmost region of lamina VII at the lower segments. The early expression of GABA immunoreactivity may indicate a trophic and synaptogenetic role for GABA in early phases of spinal cord development.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct immunocytochemical localization of 5-hydroxytryptamine receptors in the adult rat spinal cord: a light and electron microscopic study using an anti-idiotypic antiserum

In the present study, we performed the immunodetection of serotonergic (5-HT) receptor subtypes in the spinal cord by using an anti-idiotypic antiserum (TH8) at light and electron microscopic levels. This antibody has been shown to recognize 5-HT1B, 5-HT1C, and 5-HT2 receptor subtypes (Tamir et al.: J Neurochem 57:930-942, 1991). The TH8 immunoreactivity was observed in the dorsal and ventral horns of the gray matter. Light microscopy revealed that small cell bodies located in laminae I-III of the dorsal horn were intensely immunolabeled. A more homogenous and discrete staining was also observed throughout the entire dorsal horn. In the ventral horn, motoneurons were also immunoreactive (IR). Peroxidase deposits were observed as numerous patches covering the motoneuronal surface. Numerous interneurons were moderately and homogeneously immunostained. With the electron microscope, most of the labeled structures were identified as neurons (dendrites and perikarya) in both the dorsal and ventral horns. In the dorsal horn, immunoreactivity was present in dendrites and neuronal perikarya. A large majority of the immunoreactivity found in dendrites was not associated with synaptic differentiations. Indeed, the dendrites, in which peroxidase deposit was seen, were not locally involved in synapses. Very scarce synaptic varicosities were observed in close apposition with IR dendrites. In the ventral horn, TH8 immunoreactivity was present in dendrites, with an accumulation of peroxidase deposit on the active zone of synapses, facing presynaptic membranes. Both the postsynaptic membrane and the submembrane area were IR. In addition, a few astroglial fine processes were immunostained; most of them were observed in the dorsal horn. Scarce IR astroglial profiles were observed in the ventral horn. These observations show that such an antiserum constitutes a useful tool for the ultrastructural analysis of 5-HT receptor distribution. Finally, correlation between the immunocytochemical localization of 5-HT receptor subtypes and the modes of 5-HT transmission in the spinal cord (wiring and volume transmissions) is discussed in the present report.

Gamma-aminobutyric acid is released in the dorsal horn by electrical spinal cord stimulation: an in vivo microdialysis study in the rat

The mechanism underlying the beneficial effect of electrical stimulation of the posterior surface of the spinal cord in chronic pain states are unknown. The prolonged pain relief following a short stimulation period is believed to imply the activation of long-lasting neurochemical processes, mainly in the spinal cord, but possibly also involving other parts of the central nervous system. Previous studies have demonstrated that substance P and serotonin are released in the cat dorsal horn during spinal cord stimulation (SCS) with electrical parameters similar to those used in the clinic. However, gamma-aminobutyric acid (GABA) has also been hypothesized to play a role in the effect of SCS, but there have been no studies of the possible effects of SCS on GABA release. The authors applied SCS to anesthetized rats and monitored the extracellular concentration of GABA in the lumbar dorsal horns by microdialysis and a sensitive reverse-phase high-performance liquid chromatography technique. After 30 minutes of SCS, the GABA level increased significantly (by almost 270%) in comparison with the basal level recorded before stimulation, from 3.6 +/- 1.0 nmol/L to 13.1 +/- 2.2 nmol/L (mean +/- the standard error of the mean; P < 0.05). The peak release was delayed and appeared in the 30-minute fraction collected after stimulation. Also, perfusion of the dialysis probes with potassium (100 mmol/L) induced an increase of the GABA level. In control experiments without electrical stimulation, slowly decreasing GABA levels were observed throughout the experiments. The present results may suggest an involvement of GABA in the mechanism of SCS-induced pain relief.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a GABAergic interface between cortical afferents and brainstem projection neurons in the rat central extended amygdala

The synaptic circuitry of the intrinsic GABAergic system of the central extended amygdala (CEA) in relation to efferent neurons and cortical afferents was examined in the present study. Neurons in the CEA projecting to the dorsal vagal complex and the parabrachial complex were identified by the retrograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP). Postembedding GABA-immunocytochemistry revealed that GABA-immunoreactive (GABA-IR) terminals formed largely symmetrical synaptic contacts with the perikarya and proximal dendritic processes of almost all WGA-HRP-labeled neurons in the CEA. To determine the relationship between cortical afferents and CEA GABAergic neurons, WGA-HRP was used to anterogradely label afferents from the insular cortex in combination with postembedding immunogold detection of GABA. Cortical afferents formed asymmetrical synaptic contacts predominantly on small dendrites and dendritic spines. Many of the dendrites postsynaptic to cortical terminals in the central nucleus were immunoreactive for GABA although only relatively few spines were GABA-IR. Combining pre-embedding GAD-immunocytochemistry with cortical lesions resulted in approximately 40% of degenerating terminals of insular cortical origin in the central nucleus in contact with small, GAD-IR dendrites and spines. The present results demonstrate that the neurons providing the major CEA outputs to the brainstem receive an extensive GABAergic innervation, strongly supporting our proposal that CEA efferent neurons are under strong tonic inhibition by intrinsic GABAergic neurons. Further, our finding that the major cortical input to the central nucleus preferentially innervates intrinsic GABAergic neurons suggests that these neurons in the CEA may serve as an interface between the principal inputs and outputs of this forebrain region.

Perispinal progestins enhance the antinociceptive effects of muscimol in the rat

The intrathecal (IT) injection of progesterone (PROG) or three of its ring A-reduced metabolites (5 beta,3 alpha-pregnanolone, 5 alpha,3 alpha-pregnanolone, or 5 beta,3 beta-pregnanolone) did not significantly alter any of two pain thresholds (vocalization threshold to tail shock, VTTS, or tail flick latency, TFL) in ovariectomized rats when tested in a wide range of doses (2.5-250 micrograms). When combined with a subanalgesic dose of muscimol (MUSC; 1 microgram IT), PROG and its two 3 alpha-hydroxy derivatives, but not the 3 beta, caused significant analgesia in the VTTS but not in the TFL test. No clear dose-response relationships were noted in the analgesic response to the combination of the progestins and MUSC. The present results indicate that PROG, either directly or through its ring A-reduction, can modulate nociceptive information by enhancing the action of GABA agonists on GABAA receptors.

Gamma-aminobutyric acid (GABA) immunoreactivity in the mouse adrenal gland

Gamma-aminobutyric acid (GABA) immunoreactivity was revealed by immunocytochemistry in the mouse adrenal gland at the light and electron microscopic levels. Groups of weakly or faintly GABA immunoreactive chromaffin cells were often seen in the adrenal medulla. By means of immunohistochemistry combined with fluorescent microscopy, these GABA immunoreactive chromaffin cells showed noradrenaline fluorescence. The immunoreaction product was seen mainly in the granular cores of these noradrenaline cells. These results suggest the co-existence of GABA and noradrenaline within the chromaffin granules. Sometimes thick or thin bundles of GABA immunoreactive nerve fibers with or without varicosities were found running through the cortex directly into the medulla. In the medulla, GABA immunoreactive varicose nerve fibers were numerous and were often in close contact with small adrenaline cells and large ganglion cells; a few, however, surrounded clusters of the noradrenaline cells, where membrane specializations were formed. Single GABA immunoreactive nerve fibers, and thin or thick bundles of the immunoreactive varicose nerve fibers ran along the blood vessels in the medulla. The immunoreaction deposits were observed diffusely in the axoplasm and in small agranular vesicles of the GABA immunoreactive nerve fibers. Since no ganglion cells with GABA immunoreactivity were found in the adrenal gland, the GABA immunoreactive nerve fibers are regarded as extrinsic in origin.

GABAergic neurons in human spinal cord cultures: a computer-aided analysis of normal and thienyl-phencyclidine-treated cells

gamma-Aminobutyric acid (GABA)-containing neurons were studied in dissociated cell cultures of human spinal cords from 6-10-week-old fetuses using immunohistochemistry with anti-GABA antibodies. Light microscopy showed two types of immunoreactive (IR) neurons: (1) IR neurons with short neuritic processes remaining near the cell body (small neuritic tree neurons); and (2) IR neurons with long neuritic processes extending far from the cell body (large neuritic tree neurons). Both types were studied at different ages in vitro, in control and in thienyl phencyclidine (TCP)-treated cultures by means of computer reconstructions and morphometric parameters. A discriminant analysis permitted the recognition of three populations: whatever the age, the control and TCP-treated neurons with small neuritic trees were not discriminated from each other and were considered to be one population whereas the 98 DIV control and both 21 DIV and 98 DIV TCP-treated cells with large neuritic trees were clearly separated from each other and from the small cell population. In all models, an astrocytic labeling, weaker than that of the neurons, was observed. The nature of these neurons (probably interneurons) intrinsic to the spinal cord is discussed in view of previous findings concerning the anatomical distribution and organization of the GABAergic system in the spinal cord.

Evidence that neuropeptide Y is present in GABAergic neurons in the superficial dorsal horn of the rat spinal cord

In order to determine whether or not neuropeptide Y coexists with GABA or glycine in rat dorsal horn, we have examined 84 neuropeptide Y-immunoreactive neurons in laminae I-III with a combined pre- and postembedding immunocytochemical method. All of the neuropeptide Y-immuno-reactive neurons were also GABA-immunoreactive, but they were either non-immunoreactive or weakly immunoreactive with the glycine antiserum. In addition, a double-label immunofluorescence method was used to search for co-localization of neuropeptide Y and [Met]enkephalin in spinal cord. Although the two types of peptide immunoreactivity often coexisted in varicosities around the central canal and in the ventral horn, such coexistence was not seen in the superficial dorsal horn. These results suggest that neuropeptide Y is present in GABAergic neurons in laminae I-III of rat dorsal horn, but that it is largely or completely restricted to those neurons which do not contain glycine. In addition, the cells that contain GABA and neuropeptide Y appear to form a different population from those that contain GABA and [Met]enkephalin. Neuropeptide Y administered by intrathecal injection causes analgesia, and there is evidence that this may involve a presynaptic mechanism. The results of the present study suggest that neuropeptide Y may act in conjunction with GABA to produce presynaptic inhibition of nociceptive primary afferents.

Neuropeptide Y-immunoreactive terminals form axo-axonic synaptic arrangements in the substantia gelatinosa (lamina II) of the cat spinal dorsal horn

The ultrastructural organization of nerve terminals containing neuropeptide Y-immunoreactivity was studied in the substantia gelatinosa of the cat spinal dorsal horn. Seventy immunoreactive boutons were examined through serial sections and 67 of them were found to form between one and five synaptic junctions with dendrites (59.5% of synapses), somata (3% of synapses) and other axon terminals (37.5% of synapses). The postsynaptic axon terminals were often the central boutons of glomeruli. These findings suggest that neuropeptide Y regulates spinal sensory transmission through both a postsynaptic action upon dorsal horn neurons and a presynaptic action upon primary afferent terminals.

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.

Neuroepithelial cells in the rat spinal cord express glutamate decarboxylase immunoreactivity in vivo and in vitro

It is unknown whether neuroepithelial cells in the mammalian central nervous system express neurotransmitter-synthesizing enzymes. In this study, expression of glutamate decarboxylase (GAD), the gamma-aminobutyric acid (GABA)-synthesizing enzyme, was examined in proliferative cells and postmitotic neuroblasts in embryonic rat spinal cord. Immunostaining coronal sections of the embryonic spinal cord with K2 antiserum, which recognizes GAD proteins encoded by the GAD67 gene, revealed intensely stained neuroepithelial cells in the basal plate at embryonic day (E) 13, in the intermediate plate between E 13-16, and last seen in the alar plate at E 16. Nissl counterstaining demonstrated that a small number of these GAD-immunoreactive cells adjacent to the neural tube lumen were mitotic. The ventral-to-dorsal gradient of GAD expression in precursor cells and postmitotic neuroblasts correlates anatomically and temporally with the sequential generation of motoneurons, commissural neurons, and interneurons in the dorsal horn. Some of these GAD-immunoreactive neuroepithelial cells may re-enter the mitotic cycle, while others are postmitotic neuroblasts presumably migrating to the intermediate zone to differentiate into young neurons. Double-immunostaining cells acutely dissociated from E 11-18 spinal cords with K2 and anti-bromodeoxyuridine antisera, following a bromodeoxyuridine pulse in vivo, revealed considerable numbers of DNA-synthesizing cells immunoreactive for GAD. The absolute number of double-stained cells peaked during E 12-15, coinciding with terminal cell division in most spinal neurons. These observations suggest that spinal neuronal precursors can synthesize GAD-related proteins prior to, or during, the terminal cell cycle. Although GAD immunoreactivity revealed by K2 antiserum was detected in proliferative cells and in migrating postmitotic neuroblasts, GABA immunoreactivity was never detectable in these cells. These early embryonic GAD-immunoreactive neuroepithelial cells may either synthesize levels of GABA that cannot be detected immunocytochemically, and/or express enzymatically inactive GAD-related proteins.

Transient expression of GABA immunoreactivity in the developing rat spinal cord

The development of GABAergic neurons in the spinal cord of the rat has been investigated by immunocytochemical staining of frozen sections with anti-gamma-aminobutyric acid (GABA) antiserum. In the cervical cord, GABA-immunoreactive fibers first appeared at embryonic day (E) 13 in the presumptive white matter within the ventral commissure, ventral funiculus, and dorsal root entrance zone, and in the ventral roots. There were no GABA-immunoreactive cell bodies detected at this age. By E14, motoneurons, the earliest generated spinal cells, were the first cell population to become GABA-immunoreactive at the cell body level. Thereafter, GABA-immunoreactive neurons increased progressively in number and extended from ventral to dorsal regions. GABA-immunoreactive relay neurons within lamina I of the dorsal horn were initially detected at E17. Interneurons in the substantia gelatinosa, the latest generated cells in the spinal cord, were also the last to express the GABA immunoreactivity at E18. Immunoreactive neurons peaked in intensity and extent at E18 and 19. GABA immunoreactivity was only detectable in neurons within the intermediate and marginal zones 1-3 days after they withdrew from the cell cycle. This contrasts to glutamate decarboxylase immunoreactivity, which is detected in precursor cells in the ventricular zone prior to, or during, withdrawal from the cell cycle. Toward the end of gestation, GABA immunoreactivity declined in intensity and extent. This regression began in the ventral horn of the cervical region and ended in the dorsal horn of the lumbosacral region. During the first week after birth, immunoreactivity in motoneurons and in many other neurons within the ventral horn, intermediate gray, and deeper layers of the dorsal horn disappeared, and only in those neurons predominantly within the superficial layers of the dorsal horn did it persist into adulthood. Thus, the expression and regression of GABA immunoreactivity in the spinal cord followed ventral-to-dorsal, rostral-to-caudal, and medial-to-lateral gradients. These observations indicate that the majority of embryonic spinal neurons pass through a stage of transient expression of GABA immunoreactivity. The functional significance of this transient expression is unknown, but it coincides with the period of intense neurite growth of motoneurons, sensory neurons, and interneurons, and of neuromuscular junction formation, suggesting that the transient presence of GABA may play an important role in the differentiation of sensorimotor neuronal circuits.

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.

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.

Immunohistochemical evidence that Met-enkephalin and GABA coexist in some neurones in rat dorsal horn

A pre-embedding immunohistochemical method to detect Met-enkephalin was combined with postembedding immunohistochemistry with GABA and glycine antisera, in order to determine whether or not Met-enkephalin coexisted with either of these inhibitory transmitters in neuronal cell bodies within the superficial dorsal horn of the rat. The distribution of immunostaining with the three antisera was similar to that which has been described previously. Of 74 enkephalin-immunoreactive neurones in laminae II and III, 51 were immunoreactive with the GABA antiserum and 23 were not. All of the neurones which were not GABA-immunoreactive were located in lamina II. None of the enkephalin-immunoreactive cells showed glycine-like immunoreactivity. These results suggest that enkephalin is present both in GABAergic neurones and in neurones which do not contain GABA within the rat superficial dorsal horn. It is likely that the population of neurones immunoreactive with both enkephalin and GABA antisera includes lamina II islet cells and that the population which were enkephalin-immunoreactive but not GABA-immunoreactive includes stalked cells. In addition, this latter group may correspond to those cells which possess both enkephalin- and substance P-like immunoreactivity and which have been described previously in this area.

Primary afferent interactions: analysis of calcitonin gene-related peptide-immunoreactive terminals in contact with unlabeled and GABA-immunoreactive profiles in the monkey dorsal horn

The present study analyses the relationship of calcitonin gene-related peptide (CGRP)-immunoreactive primary afferent terminals with unlabeled and GABA-immunoreactive profiles in the primate (Macaca fascicularis) dorsal horn. One-hundred CGRP-immunoreactive terminals located in the superficial dorsal horn were quantitatively analysed and all profiles in apposition or in synaptic contact with these terminals were categorized as either axon terminals or dendrites with or without vesicles. These profiles were then further classified as to whether they were GABA-immunoreactive. All of the CGRP-immunoreactive terminals demonstrated axodendritic interactions; in addition to dendrites without vesicles, approximately half of the CGRP-immunoreactive terminals had dendrites with vesicles as postsynaptic elements. Of the dendrites with vesicles, 25/53 were GABAergic but only 3/67 of the postsynaptic dendrites without vesicles were GABAergic. GABAergic vesicle-containing dendrites were the most prominent CGRP-GABAergic interaction. Axoaxonic and dendroaxonic interactions were a rare occurrence, thus the classical anatomical substrate for primary afferent depolarization involving GABA- and CGRP-immunoreactive terminals could not be substantiated. CGRP-GABAergic interactions often involved diadic and triadic arrangements. These findings are discussed in relation to previously described primary afferent synaptology, primary afferent-GABAergic interactions and spinal cord mechanisms for modulation of noxious input.

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.

A glycinergic projection from the ventromedial lower brainstem to spinal motoneurons. An ultrastructural double labeling study in rat

In the present study it was determined whether glycine was present in the descending brainstem projections to spinal motoneurons in the rat. For this purpose injections of wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) were made in the ventromedial part of the lower brainstem at the levels of the rostral inferior olive and the caudal facial nucleus. After perfusion, WGA-HRP histochemistry was performed, followed by the postembedding immunogold technique with an antibody against glycine. Electron microscopical examination of the lumbar motoneuronal cell groups showed that 15% of the WGA-HRP labeled terminals, derived from the ventromedial reticular formation, were also labeled for glycine. The majority (91%) of these double labeled terminals were of the F-type (containing many flattened vesicles), while the remaining 9% were of the S-type (containing mostly spherical vesicles). Many of the double labeled terminals established a synapse, mostly with proximal and distal dendrites. The present data, combined with our previous findings that 40% of the projections from the same ventromedial brainstem area to lumbar motoneurons contained gamma-aminobutyric acid (GABA), indicate that over 50% of these brainstem projections contain GABA and/or glycine, exerting a direct inhibitory effect on spinal motoneurons. The possibility that the glycinergic fibers within these projections play an important role in producing muscle atonia during rapid eye movement (REM) sleep is discussed.

GABA-synthesizing neurons in the medulla: their relationship to serotonin-containing and spinally projecting neurons in the rat

GABA-synthesizing neurons were identified in the medulla of the rat by peroxidase-antiperoxidase (PAP) immunohistochemistry for glutamic acid decarboxylase (GAD). Using diaminobenzidine (DAB) either alone or intensified with silver, a relatively large number of GAD-immunoreactive neurons were evident within the reticular formation, raphe nuclei and vestibular nuclei. In all these areas, profuse GAD-immunoreactive varicosities appeared to contact the soma and dendrites of both non-GABA and GABA neurons. These observations suggest that GABA neurons may act as interneurons or local projection neurons within the medulla and accordingly exert a potent inhibitory and/or disinhibitory control on bulbar projection neurons. Within the ventral reticular formation (pars alpha and ventralis of the gigantocellular reticular field) and raphe magnus, large numbers of prominent GAD-immunoreactive neurons resembled in size and morphology and overlapped in distribution the serotonin-immunoreactive neurons of the same regions. However, by sequential double immunostaining utilizing DAB as a chromogen for serotonin (5-HT) and benzidine dihydrochloride (BDHC) for GAD, it was found that GAD-containing neurons were distinct from 5-HT-containing neurons. Following injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the upper cervical spinal cord and combined processing for WGA-HRP (using tetramethylbenzidine [TMB] with cobalt) and immunohistochemistry (with DAB), a contingent of spinally projecting neurons were found to contain GAD. The GAD-immunoreactive reticulo- and raphe-spinal neurons were most frequent within the pars alpha and ventralis of the gigantocellular reticular fields and the raphe magnus, where they were approximately equal in number to the coexistent, but distinct 5-HT spinally projecting neurons. GABA neurons of the medulla may thus contribute directly to the bulbar inhibitory influence upon spinal sensory and motor systems.

Immunohistochemical evidence for the coexistence of substance P, thyrotropin-releasing hormone, GABA, methionine-enkephalin, and leucin-enkephalin in the serotonergic neurons of the caudal raphe nuclei: a dual labeling in the rat

By means of dual immunohistochemical labeling on the same brain section examined with a light microscope, the present study reports the presence with serotonin (5-hydroxytryptamine; 5-HT) of gamma-aminobutyric acid (GABA), substance P (SP), thyrotropin-releasing hormone (TRH), leucin-enkephalin (LEU-enk), or methionine-enkephalin (MET-enk), within the same neuron in the nuclei raphe magnus, raphe obscurus, and raphe pallidus of the rat. On the one hand, peptides or GABA are detected with specific rabbit antibodies by indirect peroxidase labeling using peroxidase-conjugated Fab fragments, and on the other, 5-HT is detected with a rabbit antibody against the BSA-serotonin conjugate by radio-immunocytochemistry using [125I]-labeled protein A. The possible coexistence of TRH and SP in these neurons is also investigated by using peroxidase labeling and radio-immunocytochemical detection, respectively. In the whole caudal raphe nuclei the proportion of each coexisting peptide with 5-HT appears in decreasing order as: TRH greater than SP greater than MET-enk # LEU-enk greater than GABA. In all instances the level of coexistence differs considerably in B1-B2 vs. B3 cell groups. No SP/TRH dually labeled cells have ever been found in any of the serotonergic nuclei of the caudal raphe. Given the evidence that these raphe nuclei project possibly to the spinal cord, these data constitute an anatomical substrate for the several distinct physiological functions presumably subserved by 5-HT in the cord, namely the modulation of nociception, motor, and autonomic functions.

Actions of (-)-baclofen on rat dorsal horn neurons

The actions of a gamma-aminobutyric acid B (GABAB) agonist, (-)-baclofen, on the electrophysiological properties of neurons and synaptic transmission in the spinal dorsal horn (laminae I-IV) were examined by using intracellular recordings in spinal cord slice from young rats. In addition, the effects of baclofen on the dorsal root stimulation-evoked outflow of glutamate and aspartate from the spinal dorsal horn were examined by using high performance liquid chromatography (HPLC) with flourimetric detection. Superfusion of baclofen (5 nM to 10 microM) hyperpolarized, in a stereoselective and bicuculline-insensitive manner, the majority (86%) of tested neurons. The hyperpolarization was associated with a decrease in membrane resistance and persisted in a nominally zero-Ca2+, 10 mM Mg(2+)- or a TTX-containing solution. Our findings indicate that the hyperpolarizing effect of baclofen is probably due to an increase in conductance to potassium ions. Baclofen decreased the direct excitability of dorsal horn neurons, enhanced accommodation of spike discharge, and reduced the duration of Ca(2+)-dependent action potentials. Baclofen depressed, or blocked, excitatory postsynaptic potentials evoked by electrical stimulation of the dorsal roots. Spontaneously occurring synaptic potentials were also reversibly depressed by baclofen. Whereas baclofen did not produce any consistent change in the rate of the basal outflow of glutamate and aspartate, the stimulation-evoked release of the amino acids was blocked. The present results suggest that baclofen, by activating GABAB receptors, may modulate spinal afferent processing in the superficial dorsal horn by at least two mechanisms: (1) baclofen depresses excitatory synaptic transmission primarily by a presynaptic mechanism involving a decrease in the release of excitatory amino acids, and (2) at higher concentrations, the hyperpolarization and increased membrane conductance may contribute to the depressant effect of baclofen on excitatory synaptic transmission in the rat spinal dorsal horn.

Quantitative analysis and postsynaptic targets of GABA-immunoreactive boutons within the rat trigeminal motor nucleus

We have used the post-embedding immunogold labelling method using antibodies to gamma-aminobutyric acid (GABA) to obtain quantitative data on the distribution, frequency, postsynaptic targets and ultrastructural characteristics of GABA-immunoreactive (GABA-IR) boutons in the trigeminal motor nucleus of rats. We have also combined this method with horseradish peroxidase tracing to obtain specific evidence for termination of some GABA-IR boutons onto identified jaw-elevator motoneurones. Twenty-eight percent of all synapses in the motor nucleus involved GABA-IR boutons. Seventy-three percent of the GABA-IR boutons formed axo-dendritic synapses, 13% axo-somatic synapses and 14% axo-axonic synapses. Ninety-three percent of GABA-IR boutons formed symmetrical synapses. Overall, 58% of all boutons contained only flattened vesicles, while 26% contained round vesicles and 16% a mixture of vesicle types. Measurements of bouton cross sectional area, apposition length, and active zone length were obtained from serial reconstructions of 15 GABA-IR boutons and 30 unlabelled boutons. In each case mean values for GABA-IR boutons were significantly smaller than those for nonlabelled boutons.

Ovarian steroid modulation of [3H]muscimol binding in the spinal cord of the rat

[3H]Muscimol binding was measured in the lumbar spinal cord of female rats by in vitro quantitative autoradiography. Ovariectomized rats were treated subcutaneously with either oil, estradiol benzoate (EB) or EB plus progesterone (P) in a regime known to reliably induce sexual receptivity. The level of [3H]muscimol binding was highest in laminae I-III and in the region around the central canal. Binding was lower in laminae IV-VI and was frequently undetectable in the ventral horn. There was a significant increase in the level of binding in laminae I-III after EB treatment. There was also a significant increase after treatment with EB+P in comparison to both the ovariectomized and EB-treated groups in this same region of the spinal cord.