Immunocytochemical localization of glutamate decarboxylase in rat spinal cord

Are baclofen-sensitive GABAB receptors present on primary afferent terminals of the spinal cord?

The site of action of the antispastic drug baclofen has long been considered to reside in the spinal cord although supraspinal effects have also been reported. This beta-chlorophenyl derivative of the neurotransmitter gamma-aminobutyric acid (GABA) depresses both monosynaptic and polysynaptic transmission in the cord possibly through a decrease in transmitter release rather than by any antagonism at postsynaptic receptors. Recently, baclofen has been shown to be a selective ligand for a bicuculline-insensitive GABA receptor (GABAB) site that occurs widely in the mammalian central nervous system including the spinal cord. The apparent importance of the cord in the therapeutic effects of this drug prompted us to ask whether they involve GABAB site activation. As an initial step we have located these receptors by autoradiography, comparing them with classical GABAA sites. We report here that GABAB sites, unlike GABAA sites, are present in high concentrations in laminae I, II, III and IV of the dorsal horn and that after the neonatal administration of capsaicin this binding is reduced by 40-50%.

A Golgi Type II neuron in trigeminal nucleus oralis: a Golgi study in the rat

This Golgi study identifies a class of small neurons in trigeminal nucleus oralis (Vo) that satisfies all the morphological criteria for a Golgi Type II neuron. The cylindrical-shaped dendritic arbor extends up to 500 micron in the rostrocaudal axis and is confined to Vo. The unmyelinated axon generates a highly branched collateral axonal plexus within or near the dendritic tree and it does not project out of Vo. This Golgi Type II neuron is considered to be an inhibitory interneuron and probably participates in a variety of inhibitory phenomena known to occur in Vo.

Ultrastructure of chemically defined neuron systems in the dorsal horn of the monkey. II. Methionine-enkephalin immunoreactivity

Enkephalinergic axons and terminals were identified by the PAP immunohistochemical method in lamina I (marginal zone) and lamina IIO (outer substantia gelatinosa) in the dorsal horn of the monkey spinal cord. Synaptic profiles with enkephalin-like immunoreactivity (MELI) contained clear, round, vesicles, sometimes a few large granular vesicles, and usually formed asymmetrical contacts. MELI terminals forming synaptic contacts with various sizes of dendrites and with dendritic spines were the most common type of relationship found; axosomatic contacts were few. Additionally, two types of complexes were observed in which an MELI terminal formed a specialized apposition with an unlabelled terminal. The contact often resembled a synapse and in most cases the MELI terminal was suspected to be presynaptic. One complex consisted of a MELI terminal apposing the LGV type terminal (containing large granular vesicles), which in turn was presynaptic to a dendrite. (The identity of the LGV terminal could not be determined, but it had some characteristics similar to those described for substance P terminals and for a class of primary afferents in the monkey dorsal horn). The other type of complex consisted of a MELI terminal apposing an R-type terminal (containing small, round, clear vesicles) which was in turn presynaptic to a dendrite. Often, the MELI terminal also formed a synapse onto the same dendrite. The axodendritic, axospinous and axosomatic contacts of MELI terminals in the superficial dorsal horn may produce some of the depressive postsynaptic-like effects of enkephalin iontophoresis onto dorsal horn neurons. In these cases the responses of dorsal horn neurons to both low threshold and nociceptive primary afferents is suppressed. However, the opiate receptor-dependent PAD of C-fibers observed in the dorsal horn may be mediated by the MELI complexes formed with LGV and R terminals found in lamina I.

Actions of opiates, substance P, and serotonin on the excitability of primary afferent terminals and observations on interneuronal activity in the neonatal rat's dorsal horn in vitro

Approximately 5 segments of lumbo-thoracic spinal cord together with connected dorsal root ganglia were removed from 1-11-day-old rats and maintained in vitro. Dorsal root afferents, recorded from the ganglion and stimulated at the root entry zone, had conduction velocities typical of unmyelinated fibers (less than 2 m/s). The spinal terminals of individual afferents showed increased excitability with bath application of substance P and serotonin and decreased excitability with morphine sulfate, [D-ala2]methionine-enkephalinamide, manganese ions and magnesium ions. Naloxone by itself elicited no change in excitability, although it appeared to reduce the ongoing effect of opiates. Neurons recorded extracellularly in the dorsal horn responded to afferent volleys with one or more of 3 distinct phases: an excitation roughly coincident with the volley's arrival, a 50-300 ms period of inhibition, and a late excitation of 150-300 ms latency. The excitability results are accounted for by a model in which substance P, gamma-aminobutyric acid and possibly other depolarizing agents are contained in interneurons which synapse on afferent terminals. These interneurons could receive inhibitory enkephalinergic input, and, in the neonate but not the adult, excitatory serotoninergic input. An alternate scheme would have enkephalin and serotonin acting directly on afferent terminals, although perhaps by non-synaptic diffusion since the appropriate synapses have not been seen in histochemical studies. Such an action for enkephalin might explain the existence of opiate receptor on afferent terminals. The interneuronal responses to afferent volleys are parallel in most aspects to those found in the dorsal horns of adult mammals in vivo.

Localization of cholecystokininlike immunoreactivity in the rat spinal cord, with particular reference to the autonomic innervation of the pelvic organs

The distribution of cholecystokinin in the spinal cord was investigated by immunohistochemistry. Throughout the length of the spinal cord cholecystokinin immunoreactivity was found in laminae I and II, in the spinal reticular nucleus, and in the surroundings of the central canal. On the basis of the cholecystokinin pattern lamina II could be divided into a dorsal and ventral part. In the lumbar and sacral spinal cord additional terminal fields of cholecystokinin immunoreactive boutons unique to these levels were found. They corresponded to the intermediolateral nucleus and to the medial lumbar sympathetic nucleus dorsal to the central canal in the first and second lumbar segment. Also the intermediolateral nucleus in L6-S1 received a dense cholecystokinin positive input. Moreover, the area surrounding the central canal in L6-S1 contained many cholecystokinin immunoreactive structures. Combined retrograde tracing and immunocytochemistry revealed that the two cholecystokinin terminal fields characteristic for L1-L2 and that surrounding the intermediolateral nucleus in L6-S1 were situated corresponding to preganglionic neurons innervating pelvic organs through the hypogastric nerve or the pelvic nerves. It thus appears that the unique lumbosacral cholecystokinin is related to nuclei influencing pelvic structures, pointing to a special need for regulation of the organs involved in evacuation and sexual functions. Moreover, it is demonstrated that the caudal part of the spinal sympathetic system differs from the more cranial part with respect to type of afferent connections. The origin of the spinal cholecystokinin was investigated and it was found that neither complete transection of the spinal cord nor ipsilateral sectioning of three or four dorsal roots induced visible changes in the cholecystokinin staining pattern. Treatment of the caudal spinal cord with colchicine revealed the presence of cholecystokinin immunoreactive neurons in the intermediate gray, at the lateral border of the dorsal horn, in the dorsal horn proper, and in the substantia gelatinosa. These findings indicate that the majority of spinal cholecystokinin has a spinal origin.

Single unit analysis of the posteroventral cochlear nucleus of the decerebrate cat

Single unit recordings were obtained in the cochlear nuclear complex of the unanesthetized, decerebrate cat. Sixty-six of 282 units were localized to the posteroventral cochlear nucleus, 17 from the multipolar cell area and 49 from the octopus cell area. Spontaneous rates ranged from less than 1 to 75 spikes per second in the multipolar cell area and from less than 1 to 135 spikes per second in the octopus cell area. Poststimulus time histograms revealed four response types, at the best frequency, in the posteroventral cochlear nucleus. These responses were: (1) primary-like (maximum response shortly after the stimulus onset, followed by a reduction in activity to a steady state); (2) chopper (similar to primary-like but with multiple peaks in the first 10-15 milliseconds); (3) onset-ex (onset response followed by a low level of excitation); and (4) onset-in (onset response followed by inhibition). The onset-in responses represented the first observations of inhibition, at best frequency, for onset units in the mammalian cochlear nuclear complex. Analysis of interspike interval distributions showed that both spontaneous and driven activity consisted of irregular intervals for all four response types. Activity-intensity functions for primary-like, chopper and onset-ex units showed monotonic increases with increases in stimulus intensity. Activity-intensity functions for onset-in units were non-monotonic. Latency-intensity functions for primary-like, chopper and onset-ex units exhibited monotonic decreases with increases in intensity. Latency-intensity functions for onset-in units were non-monotonic. In contrast to primary-like, chopper and onset-ex units, onset-in units do not retain the intensity and temporal information coded in the eighth nerve, as least for stimuli above 2 kilohertz. It is hypothesized that a depolarization block, caused by the massive eighth nerve input to octopus cells, is responsible for the inhibition observed from onset-in units.

Light microscopic and ultrastructural localization of immunoreactive substance P in the dorsal horn of monkey spinal cord

Light- and electron-microscopic localization of substance P in the monkey spinal cord was studied by the peroxidase anti-peroxidase technique with the particular aim of examining types of interactions made by substance P-positive boutons with other neuronal elements in the dorsal horn. By light-microscopy dense labeling for immunoreactive substance P was found in laminae I, II (outer zone) and V (lateral region), consistent with findings in other mammalian species. By electron-microscopy, substance P-positive staining was mostly in unmyelinated and in some thinly myelinated small diameter fibers. Substance P-positive terminals contained both large granular vesicles (80-120 nm diameter), which were filled with reaction product, and clear round vesicles (40-60 nm). Substance P-positive large granular vesicles were sometimes observed near presynaptic sites and in contact with dense projection there. Immunoreactive substance P boutons were small to large in size (1-4 micron), formed synapses with somata and large dendrites and were the central axons of synaptic glomeruli where they were in synaptic contact with numerous small dendrites and spines. Substance P-labeled axons frequently formed synapses with dorsal horn neurons which were also postsynaptic to other types of axons. Substance P-positive profiles participated in numerous puncta adhaerentia with unlabeled cell bodies, dendrites and axons. Only rarely, some suggestive evidence was obtained indicating that axons might synapse onto substance P-containing boutons. Biochemical analysis of monkey spinal cord tissue extracts, undertaken to characterize more precisely the immunoreactive substances, indicated that only substance P and its oxide derivative were detected with the antiserum used in the immunocytochemistry. These morphological findings show that substance P is contained within a class of axon terminals, many of which have been shown previously in the monkey to originate from the dorsal root. The results suggest that modulation of substance P primary afferents terminating in the outer dorsal laminae of the monkey spinal cord occurs in part via axonal inputs onto dorsal horn neurons postsynaptic to the primary afferent.

The depolarization of feline ventral horn group Ia spinal afferent terminations by GABA

The unmyelinated terminal regions of extensor muscle Ia afferent fibres were stimulated electrically near lumbar motoneurones in anaesthetised cats using 300 microseconds pulses of less than 1 microA passed through the central NaCl barrel of seven barrel micropipettes. Such terminations were identified by anodal blocking factors of less than four and the latency of the antidromic impulse recorded in the appropriate peripheral muscle nerve. Although the effects of microelectrophoretically administered GABA were occasionally complex, the most consistent finding was a reduction in termination threshold followed by an increase. Both this reduction in threshold by GABA, and that produced by tetanic stimulation of low threshold flexor afferents (PAD) were diminished by microelectrophoretic bicuculline methochloride. This GABA antagonist alone elevated the threshold of some terminations but did not reduce the depolarizing action of either potassium or L-glutamate. Furthermore, since reductions in threshold by GABA, but not by either potassium or L-glutamate, were associated with a decrease in PAD, GABA appears to increase terminal membrane conductance. Since neither GABA nor bicuculline methochloride influenced the threshold or afferent depolarization of non-terminal regions of Ia fibres, there results are consistent with the function of GABA as a depolarizing transmitter at gabergic axoaxonic synapses upon the terminals of Ia afferent fibres synapsing with motoneurones.

The cytoarchitecture of GABAergic neurons in rat spinal cord

Glutamic acid decarboxylase (GAD), the enzyme that synthesizes the transmitter gamma-aminobutyric acid (GABA), has previously been localized within synaptic terminals in rat spinal cord by immunocytochemistry. In the present study, GAD was localized within the somata and dendrites of GABA neurons following colchicine injections into rat lumbar spinal cord. All regions of the spinal gray matter contained GAD-positive somata except the motoneuron pools (lamina IX). GAD-positive somata also were observed in the ependymal layer and in the dorsolateral funiculus. Small GAD-positive somata, averaging 9 X 13 micrometer in size, were located in laminae I-III, and the size of GAD-positive somata increased for cells located in progressively more ventral laminae, reaching a maximum in lamina VII where somal size averaged 12 X 19 micrometer. Lamina I contained two classes of GAD-positive cell bodies; lenticular shaped, intermediate size neurons that were reminiscent of stalked cells, and a smaller cell type that was elongated in the sagittal plane. GAD-positive somata in laminae II and III had the size and position of islet cells. In laminae IV-VI, GAD-positive somal profiles averaged 12 X 17 micrometer in size. Lamina IV neurons were concentrated along laminar edges, while those in laminae V and VI were distributed more homogeneously. In lamina VIII, GAD-positive cell bodies appeared in groups of 3 or 4 and were smaller than those in lamina VII. Lamina X contained GAD-positive somal profiles averaging 12 X 16 micrometer in size. In the ependymal layer, there were two types of cerebrospinal fluid (CSF)-contacting neurons that contained GAD; one spherical and the other elongated. Both types sent extensions into the central canal where these processes expanded into 4-5 micrometer-wide end bulbs. CSF-contacting cells with sizes and shapes similar to the GAD-positive ones were seen to receive synapses in electron micrographs. The widespread distribution of GABA neurons in spinal cord was suggestive of diverse functions for these cells, encompassing conventional synaptic roles and, perhaps, an involvement in hormonally modulated communication via GABAergic, CSF-contacting neurons.

Electrophysiological studies with the spastic mutant mouse

Electromyographic (EMG) studies were carried out with the genetically spastic mouse (spa, autosomal recessive), obtained from matings of B6C3a/a, spa/+ heterozygotes. Spastic homozygotes exhibited high amplitude repetitive EMG bursts during spontaneous activity. Following an electrical stimulus to hindlimb or forelimb, high amplitude stereotyped EMG bursts were recorded from contralateral limbs in spastic mice, but were not observed in phenotypically unaffected littermates or normal C57BL/6J mice. The timing and latency of this stereotyped response to an electrical stimulus was consistent with the participation of spinal cord neuronal pathways. In normal C57BL/6J mice the administration of strychnine (0.65 mg/kg), but not picrotoxinin (up to convulsant doses), reproduced all of the behavioral and EMG features observed in spastic homozygotes. We hypothesize that the symptoms in the spastic mutant may result from a deficiency of strychnine-sensitive (presumably glycinergic) inhibition in the spinal cord.

Immunocytochemical evidence for the existence of GABAergic neurons in the nucleus raphe dorsalis. Possible existence of neurons containing serotonin and GABA

It has been established that nerve cell bodies of the nucleus raphe dorsalis (NRD) belong to ascending 5-hydroxytryptamine systems. These neurons could be modulated by GABAergic interneurons or interposed GABA neurons. A high glutamate decarboxylase (GAD) activity in the NRD and a specific high-affinity uptake mechanism for GABA suggest the presence of GABA synthesizing elements in the NRD. Anti-GAD antibodies were used by an immunocytochemical procedure to demonstrate the presence of GABAergic elements. Anti-GAD antibodies were previously tested in the cerebellum and substantia nigra. Large amounts of GAD-positive reaction product were observed in the cytoplasm of some neurons (fusiform, ovoid or multipolar) or appeared as punctate deposits apposed to dendrites, soma and dispersed in the neuropil of the NRD. At the electron microscopic level, GAD-positive reaction product was observed within the cytoplasm of numerous somata in sections from colchicine-treated rats. GAD-positive staining was observed in numerous fibers or axonal terminals and two types of morphologically different fibers could be distinguished. The first displays small clear vesicles and few large granular vesicles (LGV) (80-100 nm), the second displays only clear round vesicles (40-60 nm). After 5,7-dihydroxytryptamine treatment (a neurotoxic for 5-HT terminals), the immunocytochemical labeling is much decreased. Some reactive neurons are still dispersed in the nucleus but the fibers containing LGV are no longer observed. These results strongly suggest that some neuronal elements in the NRD are morphologically, pharmacologically and anatomically similar to 5-HT neurons described at this level. Such cell elements could possess a double GABA and 5-HT potentiality. If this is not the case, a population of GABA neurons could be sensitive to 5,7-DHT and so have the capacity to take up 5-HT. The other reactive elements, insensitive to 5,7-DHT, could represent the GABAergic interneurons postulated at this level. Numerous GAD positive fibers or axon terminals were observed in synaptic contact with dendrites, axons or soma of other neurons. The chemical nature of the neuronal postsynaptic elements remains unknown. These findings strongly support the hypothesis for GABA-mediated inhibition in the NRD.

High affinity GABA receptors-autoradiographic localization

The distribution of the high affinity gamma-aminobutyric acid (GABA) receptor labeled by [3H]muscimol, has been studied in the rat brain by light microscopic autoradiography. Receptors in slide-mounted tissue sections were labeled in vitro with [3H]muscimol. Most of the gray matter areas presented grain densities significantly higher than background or white matter areas. Wide variations in receptor densities were found between different brain areas and nuclei. Areas with very high grain densities are the granule cell layer of the cerebellum, external plexiform layer of the olfactory bulb and nuclei of the thalamus, such as the ventral nucleus, lateral nucleus and dorsal geniculate body. The molecular layer of the hippocampus and the external (I-IV) layers of the cortex are also rich in GABA receptors. The basal ganglia have moderate concentrations of receptors, while the pons, medulla and brainstem have only low concentrations of autoradiographic grains. These distributions are discussed in correlation with the known distribution of GABAergic terminals and the presence of inhibitory GABAergic mechanisms.

Pharmacology of GABA-mediated inhibition of spinal cord neurons in vivo and in primary dissociated cell culture

In this paper it is shown that the postsynaptic GABA-receptor chloride ion channel complex is composed of several functional subunits. There are probably at least two stereospecific locations on the receptor for GABA-binding and both must be occupied to obtain an increase in chloride conductance. The interaction between these sites is uncertain but there could be either positive cooperativity between the sites or only a requirement that both sites are occupied without occupation of either site affecting the affinity for GABA of the other site. There is a chloride conductance channel coupled to the GABA receptor which opens for an average of 20 msec and has an average conductance of 18 pS. The GABA-coupled chloride channel may or may not have the same composition as the glycine coupled chloride channel. In addition to the GABA-recognition site and the chloride ion channel, GABA-receptors must have additional binding sites or modulator sites where drugs can bind to modify GABA activation of the GABA receptor. The convulsant PICRO binds to a site which is independent of the GABA site and PICRO reduces GABA responses. Barbiturates and benzodiazepines augment GABA-responses without reducing GABA-binding and thus they must bind to a modulator site independent of the GABA recognition site. Whether or not this is the same site as the PICRO binding site is uncertain. Thus, the GABA-receptor-chloride ion channel complex is composed of at least: 1) two GABA-binding sites; 2) a chloride ion channel; 3) a convulsant binding site (PICRO-binding site) and 4) an anticonvulsant binding site. This organization serves several obvious purposes. First, since two GABA-molecules are required to activate GABA-coupled chloride ion channels, the dose-response relationship for GABA is sigmoidal and steep. Thus minor shifts in GABA affinity will produce large alterations in GABA-responses and the GABA receptor can be easily modulated. Second, since the receptors has binding sites for convulsant and anticonvulsant compounds which decrease and increase GABA-responses, GABAergic inhibition can easily be modulated.

A re-evaluation of the neurochemical and antinociceptive effects of intrathecal capsaicin in the rat

The effect of intrathecal administration of capsaicin in the rat on thermal nociceptive thresholds and on the content of substance P, somatostatin and glutamic acid decarboxylase in the dorsal horn of the spinal cord was determined. The results suggest that the depletion of spinal cord substance P induced by capsaicin may not by itself be sufficient to explain the observed changes in noxious thermal thresholds, which may be related instead to non-specific damage to the spinal cord.

Supraspinal inhibition of the excitation of dorsal horn neurones by impulses in unmyelinated primary afferents: lack of effect by strychnine and bicuculline

In barbiturate anaesthetized cats, tonic descending inhibition of the excitation of lumbar dorsal horn neurones by impulses in unmyelinated primary afferents was measured by reversibly cooling a more cephalic segment. Administered electrophoretically in the substantia gelatinosa and at sites progressively closer to cell bodies, amounts of bicuculline and strychnine adequate to reduce segmental inhibition failed to reduce supraspinal inhibition. Intravenous bicuculline and strychnine also failed to reduce supraspinal inhibition.

Selective effects of (-)-baclofen on spinal synaptic transmission in the cat

When ejected microelectrophoretically near spinal interneurones of cats anaesthetised with pentobarbitone and under conditions where postsynaptic excitability was maintained artificially at a constant level, (-), but not (+), -baclofen selectively reduced monosynaptic excitation by impulses in low threshold muscle (Ia and Ib) and cutaneous (Aalpha) afferents. Polysynaptic excitation of interneurones and Renshaw cells by impulses in higher threshold afferents was less affected, and baclofen had little or no effect on the cholinergic monosynaptic excitation of Renshaw cells. Glycinergic and gabergic inhibitions of spinal neurones were relatively insensitive to baclofen. These stereospecific actions of baclofen, produced by either a reduction in the release of excitatory transmitter or postsynaptic antagonism, suggest that Ia, Ib, and Aalpha afferents may release the same excitatory transmitter which differs from that of spinal excitatory interneurones. Microelectrophoretic (-), but not (+), -baclofen also reduced primary afferent depolarization of ventral horn Ia extensor afferent terminations produced by impulses in low threshold flexor afferents, without altering either the electrical excitability of the terminations or their depolarization by electrophoretic GABA or L-glutamate. This stereospecific action of baclofen is interpreted as a reduction in the release of GABA at depolarizing axo-axonic synapses on Ia terminals.

Physiology and morphology of substantia gelatinosa neurons intracellularly stained with horseradish peroxidase

Neurons in Rexed's layer II were physiologically characterized with natural and electrical stimuli applied to their cutaneous receptive fields. The neurons were then intracellularly stained with horseradish peroxidase. Three general patterns of physiological responses were found. Nociceptive specific neurons did not respond to gentle mechanical stimulation. Most responded exclusively to tissue-damaging stimuli. Some also responded to moderately heavy pressure, but these responded to noxious stimuli with an increased discharge frequency. Wide dynamic range neurons responded to both gentle mechanical stimulation and to tissue-damaging stimulation. Low-threshold mechanoreceptive neurons responded only to gentle mechanical stimulation. Some of the low-threshold mechanoreceptive neurons were innervated by primary afferents with unmyelinated axons. Excepting those low-threshold mechanoreceptive neurons with input from unmyelinated afferents, the patterns of primary afferents innervation of layer II neurons were similar to the patterns of innervation that have been found for neurons in layers I and IV-V. All but 2 of the 22 neurons that we found were recognized as being of two general morphological types. Stalked cells had their perikarya situated along the superficial border of layer II. Most of their dendrites traveled ventrally while spreading out rostrocaudally. This gave their dendritic arbors a fan-like shape. Stalked cell axons arborized largely in layer I. Islet cell perikarya were found throughout layer II. Most of their dendrites traveled rostrocaudally. Their dendritic arbors were shaped like cylinders with their long axes parallel to the long axis of the spinal cord. Islet cell axons arborized in the immediate vicinity of their dendritic territories, within layer II. Stalked cells and those islet cells whose dendritic arbors were largely contained within the superficial one-third of layer II (layer IIa) were either nociceptive specific or wide dynamic range neurons. The islet cells whose dendritic arbors were largely within the deeper two-thirds of layer II (layer IIb) were all low-threshold mechanoreceptive neurons. These observations suggest that layers IIa and IIb have different functional roles and that stalked cells and islet cells are separate and distinct components of the neural circuitry of the superficial dorsal horn.

Reduction of [3H]muscimol binding sites in rat dorsal spinal cord after neonatal capsaicin treatment

Two-day-old rats were pretreated with 50 mg/kg of capsaicin. After 3--4 months, specific binding of [3H]muscimol and [3H]strychnine was measured in membrane preparations from dorsal spinal cord. A 20-30% decrease of the number of [3H]muscimol binding sites was observed after capsaicin treatment. In contrast, [3H]strychnine binding was unchanged. The results provide indirect evidence for a presynaptic location of GABA receptors on capsaicin-sensitive primary afferent neurons.

GABA neurons are the major cell type of the nucleus reticularis thalami

Glutamic acid decarboxylase (GAD), the synthesizing enzyme for the neurotransmitter gamma-aminobutyric acid (GABA), has been localized in a large number of neuronal somata within the nucleus reticularis thalami (NR) of rat brain by light microscopic immunocytochemical methods. GAD-positive staining of neuronal somata and proximal dendrites is observed in the NR of normal (untreated) rats, and this staining is substantially enhanced following colchicine injection into the lateral cerebral ventricle. GAD-positive neuronal cell bodies are prominent throughout the dorsoventral and rostrocaudal extents of the NR and, thus, form a band around the entire lateral aspect of the thalamus. In the lateral part of the NR, oval-shaped neurons with elongated GAD-positive dendritic processes are oriented parallel to the narrow axis of the NR and lie perpendicular to the penetrating fascicles of unstained thalamocortical and corticothalamic fibers. Semithin (2 micrometers) sections confirm that GAD-positive reaction product is contain within the cytoplasm of cell bodies and proximal dendrites. In addition, GAD-positive punctate structures, representing axon terminals, are present in the neuropil and, occasionally, are observed in close proximity to positively-stained neuronal somata. This finding suggests that GABA-mediated inhibition of GABA neurons may occur in the NR. The large number of GAD-positive cell bodies within the NR contrasts with a paucity of positively-stained somata in the more internally located thalamic nuclei. Within these nuclei, GAD-positive punctate structures that represent GABAergic synaptic sites are a characteristic feature. Since previous anatomical studies have demonstrated that a large proportion or reticularis neurons project into the thalamus, it is suggested that many of these GAD-positive punctate structures are the axon terminals of reticularis neurons. Through these projections, reticularis neurons may contribute to GABA-mediated inhibition within many of the thalamic nuclei.

Neurochemistry of the auditory system

Two areas of auditory biochemistry are reviewed: the identification and characterization of neurotransmitters in the auditory system and the biochemical approach to the study of genetic hearing disorders. Studies to identify neurotransmitters at major auditory synapses are outlined. Evidence supporting glutamate or aspartate as the neurotransmitter for the auditory nerve is presented. The application of biochemistry to the study of genetic hearing disorders is discussed.

Neuronal uptake of [3H]GABA and [3H]glycine in laminae I-III (substantia gelatinosa Rolandi) of the rat spinal cord. An autoradiographic study

[3H]GABA or [3H]glycine were injected into the subarachnoidal space of adult rats at C4-C5 level. After 10-60 min, the animals were perfused with 4% paraformaldehyde-0.5% glutaraldehyde and thick sections of the cervical spinal cord were postosmicated and Epon embedded. Light microscope autoradiographs of transverse cord sections showed numerous silver grains over the dorsal column and laminae I-III, higher grain densities occurring over lamina I for GABA and lamina III for glycine. In [3H]GABA-injected animals nerve cell bodies in lamina I or at the transition to lamina II appeared strongly labeled in light and electron microscope autoradiographs. These cells were smaller and less rich in RER than marginal cells and poor in axosomatic synaptic contacts. High grain densities appeared over axon terminals synapsing with dendrites in laminae I-II and over the light peripheral axon endings of synaptic glomeruli of laminae II-III. After [3H]glycine treatment, a number of nerve cell bodies were labeled in lamina III. It is suggested that two types of inhibitory interneurons occur in the rat gelatinosa, one GABAergic with cell body in lamina I, and another glycinergic in lamina III.

Abnormal protein patterns in multiple sclerosis

Sera from patients with multiple sclerosis (MS) and those obtained from normal subjects are indistinguishable by regular 5% or 7% polyacrylamide gel electrophoresis. However, 11 out of 15 MS sera examined by gradient polyacrylamide gel electrophoresis showed three distinct protein bands. None of the sera obtained from 10 normal subjects showed the characteristic protein patterns when they were examined by gradient gel electrophoresis. Similar results were obtained with de-albumin serum samples or with serum proteins precipitable at 50% ammonium sulfate saturation. These three proteins have now been purified to homogeneity by preparative gradient gel electrophoresis. Molecular weights of these proteins were estimated from gradient gel electrophoresis as 398,000, 363,000 and 302,000 daltons, respectively.

The GABA neurons and their axon terminals in rat corpus striatum as demonstrated by GAD immunocytochemistry

Glutamic acid decarboxylase (GAD, EC 4.1.1.15), the enzyme which catalyzes the alpha-decarboxylation of L-glutamate to form the neurotransmitter gamma-aminobutyric acid (GABA), was localized immunocytochemically in rat neostriatum, pallidum and entopeduncular nucleus. A large amount of GAD-positive reaction product was observed in both the pallidum and entopeduncular nucleus in light microscopic preparations and was localized ultrastructurally to axon terminalis that surrounded dendrites and large somata. In the neostriatum the relative numbers of GAD-positive axons terminals per unit area were substantially less than in the pallidum. GAD-positive terminals predominantly formed symmetric synapses with somata, dendrites and spines, but a small number of them formed asymmetric synapses with either dendrites or spines. The presence of GAD within these terminals is consistent with results of other investigations which have indicated that the striatopallidal and striatoentopeduncular pathways as well as neostriatal local circuit neurons and/or collaterals from neostriatal projection neurons, use GABA as a neurotransmitter. GAD-positive reaction product was also localized within the somata and dendrites of neostriatal and pallidal neurons in colchicine-injected preparations. The GAD-positive somata in the pallidum were medium-sized neurons and since such cells project to the substantia nigra, our results are in agreement with those from other studies which demonstrate a GABAergic, pallidonigral pathway. In the neostriatum, GAD-positive somata were identified light microscopically as medium-sized neurons with either round or fusiform shapes. Electron microscopic examinations also showed GAD-positive reaction product within the perikaryal and dendritic cytoplasm of these neurons, as well as in dendritic spines. These findings are in accord with the results of studies which have indicated that medium-sized, spinous neurons of the neostriatum give rise to a GABAergic, striatonigral pathway. The significance of GAD localization within these neostriatal neurons is discussed in relation to recent findings which show that substance P is contained within this same class of striatonigral projection neuron.

Behavioral and biochemical analysis of GABA-mediated inhibition in the early chick embryo

Exogenous gamma-aminobutyric acid (GABA) decreased spontaneous motility in 4-, 6-, 7-, 9-, and 13-day chick embryos; the younger embryos were more sensitive. Neither the positional isomers of GABA, alpha-aminobutyric acid (AABA) and beta-aminobutyric acid, nor the principle GABA catabolite, succinic acid, decreased motility in 4-day embryos. Several semi-rigid GABA analogues decreased motility in 4-day embryos with a potency that paralleled their effectiveness in displacing [3H]GABA in ligand-binding studies. The effects of AABA and GABA on hind-limb motility were quantitatively similar in thoracic spinal and sham-operated 7-day embryos. Bicuculline and picrotoxin elicited absolute motility increases at 6, 7 and 9 days of incubation. Picrotoxin and two bicyclophosphate GABA antagnoists elicited relative motility increases while bicuculline elicited an absolute motility increase at 4 days. The two bicyclophosphates increased motility with a potency that paralleled their electrophysiological effectiveness. L-Glutamic acid decarboxylase (GAD) activity was detected in the embryonic lumbar spinal cord at all ages examined (3--7 days) using a new radiometric cation-exchange method. Gamma-Aminobutyric acid transaminase (GABA-T) activity was detected in the lumbar spinal cord at the earliest age examined (day 5). Both GAD and GABA-T activity were detected at earlier ages than previously reported. GABA receptors, and the enzymes necessary for the synthesis and degradation of GABA, all appear to be present at (or before) the onset of spontaneous motility. GABA-mediated transmission appears to be present at 6 days and perhaps as early as 4 days.

Inhibitory, GABAergic nerve terminals decrease at sites of focal epilepsy

Using an immunocytochemical method for the localization of the gamma-aminobutyric acid (GABA) synthesizing enzyme, glutamic acid decarboxylase (GAD), we have observed GABAergic nerve terminals distributed throughout all layers of normal monkey sensorimotor cortex. These terminals displayed ultrastructural characteristics that suggested that they arose from aspinous and sparsely spinous stellate neurons. In monkeys (Macaca mulatta and M. fascicularis) made epileptic by cortical application of alumina gel, a highly significant numerical decrease of GAD-positive nerve terminals occurred at sites of seizure foci indicating a functional loss of GABAergic inhibitory synapses. A loss of such inhibition at seizure foci could lead to epileptic activity of cortical pyramidal neurons.

Tissue and regional distribution of cysteic acid decarboxylase. A new assay method

A sensitive and rapid assay method method for cysteic acid decarboxylase was develped which combined the selectivity of ion exchange resin (a complete retention of the substrate, cysteic acid, and exclusion of the product, taurine) with the speed of a vacuum filtration. The synthesis and purification of 35S-labeled cysteic acid were described. The validity of the assay was established by the identification of the reaction product as taurine. With this new method, the decarboxylase activity was measured in discrete regions of bovine brain. Putamen had the highest activity, 172 pmol taurine formed/min/mg protein (100%), followed by caudate nucleus, 90%; cerebral cortex, 82%; hypothalamus, 81%; cerebellar cortex, 79%; cerebellar peduncle, 59%; thalamus, 42%; brain stem, 25%; pons, 10%; and corpus callosum, 3%. The decarboxylase activity in various mouse tissues was also determined as follows: liver, 403; brain, 145; kidney, 143; spinal cord, 59; lung, 21; and spleen, 10 pmol taurine formed/min/mg. No activity could be detected in skeleton muscle and heart, suggesting a different biosynthetic pathway for taurine synthesis in these tissues. The advantages and disadvantages of the new assay method are also discussed.

Immunocytochemical localization of dopamine-beta-hydroxylase in rat locus coeruleus and hypothalamus

Dopamine-beta-hydroxylase (DBH), the enzyme that converts dopamine to norepinephrine, has been localized in light and electron microscopic preparations of rat brain by an immunocytochemical method using a peroxidase--anti-peroxidase Fab complex. In light microscopic preparations, DBH-specific reaction product was observed in somata and proximal processes of neurons in the locus coeruleus and subcoeruleus as well as within distal axons of the principal adrenergic fiber system. DBH-specific reaction product was also observed within small (1--2 micrometer), punctate structures in the interstitial nucleus of the stria terminalis and the para- and periventricular nuclei of the hypothalamus. Electron microscopic results demonstrated on association of DBH-specific reaction product with the Golgi apparatus of neuronal somata in the locus coeruleus and subcoeruleus. DBH-positive reaction product was also seen in association with small (35-55 nm) agranular synaptic vesicles and large (80--100 nm), probable granular vesicles within axonal varicosities and terminals in the interstitial nucleus of the stria terminalis. Occasionally, DBH-containing axonal varicosities and terminals were observed to form synapse-like junctions with dendritic profiles, but most of the observed DBH-positive axonal structures did not establish identifiable synaptic relationships.

Correlation of changes in the GABA-ergic system with the development of spasticity in paraplegic cats

Following spinal cord transection there occurred decreases in Km and Vmax of glutamate decarboxylase (GAD) both above and below the lesion, and an initial decrease in the concentration of GABA. Concomitantly, there was a gradual decrease in presynaptic inhibition. Eight to 12 weeks after spinal cord transection, Km and Vmax for GAD returned to control values, but the GABA content of the spinal cord below the lesion increased significantly and presynaptic inhibition became maximally depressed. These results suggested that during the chronic phase of spinal cord injury there is a decrease in release of GABA, the interneuronal inhibitory neurotransmitter which mediates presynaptic inhibition. Diazepam, a GABA enhancer, increased presynaptic inhibition in acute and chronic spinal cats, this being accompanied by a reduction in somatic muscular spasticity. The degree of this enhancement by diazepam, however, is attenuated with gradual loss of presynaptic inhibition. In the acute cat, a conditioning volley applied to cutaneous afferents blocked the inhibition of the monosynaptic response to extensor motoneurones. In contrast, in chronic spinal cats (eight to 12 weeks), the duration of complete blockade was markedly reduced and was followed by a prolonged period which cutaneous nerve stimulation potentiated the monosynaptic discharge. Similar to GABA, there also occurred an increase of substance P below the level of the lesion. Other neurotransmitters (e.g., norepinephrine, serotonin) accumulated above and disappeared below the transection level. Although somatic msucular spasticity appears to be, to some extent, due to GABA dysfunction in the spinal cord, alterations in "normal" functioning of other neurotransmitters and the loss of supraspinal control also contribute to this state.

Aspinous and sparsely-spinous stellate neurons in the visual cortex of rats contain glutamic acid decarboxylase

Glutamic acid decarboxylase (GAD), the enzyme that synthesizes the neurotransmitter gamma-amino-butyric acid (GABA), has been localized in the rat visual cortex by immunocytochemical methods with both light and electron microscopy. In both colchicine-injected and non-injected preparations of the visual cortex, GAD-positive reaction product was observed in somata, proximal dendrites and axon terminals of non-pyramidal neurons. The GAD-positive terminals were observed to form symmetric synaptic junctions most commonly with dendritic shafts and somata of pyramidal and stellate neurons and less frequently with initial axon segments of pyramidal neurons and dendritic spines. In colchicine-injected preparations, GAD-positive somata were located in all cortical layers including the immediately subjacent white matter. In contrast, sections from non-injected rats displayed GAD-positive somata within a superficial and a deep cortical band. The GAD-positive somata observed in both types of preparations received both symmetric and asymmetric synaptic junctions, lacked apical dendrites, and had radially oriented dendrites of small diameter. These characteristics of GAD-positive neurons indicate that they are aspinous and sparsely-spinous stellate neurons. The localization of GAD within these neurons in combination with physiological and pharmacological data indicate that these local circuit neurons mediate GABA-ergic inhibition in the neocortex.

Golgi studies of the neurons in layer II of the dorsal horn of the medulla (trigeminal nucleus caudalis)

The translucent band which lies just beneath the spinal V tract at the lower end of the spinal trigeminal nucleus (nucleus caudalis) can be divided into three layers. These three layers are distinguished by textural differences in their neuropil and by the morphology and laminar distribution of the axons and dendrites of their neurons. Layer II contains four different kinds of interneurons. Stalked cells are named after their short, stalk-like branches. Their cell bodies are found in greatest numbers in the outer half of layer II. Their coneshaped dendritic arbors extend medially across layers II and III and sometimes extend into layer IV. Their axons form extensive, canopy-like arborizations in layer I. Stalked cells are considered to be excitatory interneurons receiving input on their dendritic spines from primary axonal endings in the layers II and III glomeruli and transferring it to the dendrites of the layer I projection neurons. Layer II contains three kinds of Golgi type II inteneurons, i.e, neurons whose axons branch repeatedly within the confimes of their dendritic arbors. Islet cells similar to those found in layer III (Gobel), '75a), are found in small clusters in layer II. Their dendrites and axons are largely confined in layer II. The dendrites of the arboreal cell burst, in tree-like fashion, into highly focal dendritic arbors confined largely in layer II while the extensive rostral and caudal dendritic arbors of the II-III border cell lie largely in layers II and III with a few branches extending into layer I. The axons of both of these interneurons arborize in layers II and III with a few collaterals extending into layer I. Islet cells, arboreal cells and II-III border cells are considered to be inhibitory interneurons. They are strategically situated to interrupt transmission between primary axonal endings in layers II and III and the layer I projection neurons by altering the output of the stalked cells.