Immunocytochemical study of GABAA receptors in the cat visual cortex

Neocortical inhibitory activities and long-range afferents contribute to the synchronous onset of silent states of the neocortical slow oscillation

During slow-wave sleep, neurons of the thalamocortical network are engaged in a slow oscillation (<1 Hz), which consists of an alternation between the active and the silent states. Several studies have provided insights on the transition from the silent, which are essentially periods of disfacilitation, to the active states. However, the conditions leading to the synchronous onset of the silent state remain elusive. We hypothesized that a synchronous input to local inhibitory neurons could contribute to the transition to the silent state in the cat suprasylvian gyrus during natural sleep and under ketamine-xylazine anesthesia. After partial and complete deafferentation of the cortex, we found that the silent state onset was more variable among remote sites. We found that the transition to the silent state was preceded by a reduction in excitatory postsynaptic potentials and firing probability in cortical neurons. We tested the impact of chloride-mediated inhibition in the silent-state onset. We uncovered a long-duration (100-300 ms) inhibitory barrage occurring about 250 ms before the silent state onset in 3-6% of neurons during anesthesia and in 12-15% of cases during natural sleep. These inhibitory activities caused a decrease in cortical firing that reduced the excitatory drive in the neocortical network. That chain reaction of disfacilitation ends up on the silent state. Electrical stimuli could trigger a network silent state with a maximal efficacy in deep cortical layers. We conclude that long-range afferents to the neocortex and chloride-mediated inhibition play a role in the initiation of the silent state.

Fast recruitment of recurrent inhibition in the cat visual cortex

Neurons of the same column in L4 of the cat visual cortex are likely to share the same sensory input from the same region of the visual field. Using visually-guided patch clamp recordings we investigated the biophysical properties of the synapses of neighboring layer 4 neurons. We recorded synaptic connections between all types of excitatory and inhibitory neurons in L4. The E–E, E–I, and I–E connections had moderate CVs and failure rates. However, E–I connections had larger amplitudes, faster rise-times, and shorter latencies. Identification of the sites of putative synaptic contacts together with compartmental simulations on 3D reconstructed cells, suggested that E–I synapses tended to be located on proximal dendritic branches, which would explain their larger EPSP amplitudes and faster kinetics. Excitatory and inhibitory synapses were located at the same distance on distal dendrites of excitatory neurons. We hypothesize that this co-localization and the fast recruitment of local inhibition provides an efficient means of modulating excitation in a precisely timed way.

High-affinity choline uptake and acetylcholine-metabolizing enzymes in CNS white matter. A quantitative study

The presence of nicotinic and muscarinic receptors suggests the occurrence of cholinergic neurotransmission in white matter; however no quantitative information exists on acetylcholine formation and breakdown in white matter. We compared white structures of pig brain (fimbria, corpus callosum, pyramidal tracts, and occipital white matter) to gray structures (temporal, parietal and cerebellar cortices, hippocampus, and caudate) and found that sodium-dependent, high-affinity choline uptake in white structures was 25-31% of that in hippocampus. White matter choline acetyltransferase activity was 10-50% of the hippocampal value; the highest activity was found in fimbria. Acetylcholine esterase activity in white structures was 20-25% of that in hippocampus. The caudate, which is rich in cholinergic interneurons, gave values for all three parameters that were 2.8-4 times higher than in hippocampus. The results suggest a certain capacity for cholinergic neurotransmission in central nervous white matter. The white matter activity of pyruvate dehydrogenase, which provides acetyl-CoA for acetylcholine synthesis, ranged between 33 and 50% of the hippocampal activity; the activity in the caudate was similar to that in hippocampus and the other gray structures, which was true also for other enzymes of glucose metabolism: hexokinase, phosphoglucomutase, and glucose-6-phosphate dehydrogenase. Acetylcholine esterase activity in white matter was inhibited by the nerve agent soman, which may help explain the reported deleterious effect of soman on white matter. Further, this finding suggests that acetylcholine esterase inhibitors used in Alzheimer's disease may have an effect in white matter.

Immunohistochemical analysis of subcortical white matter astroglia of infant and adult primate brains, with a note on resident neurons

An immunohistochemical analysis of brain subcortical white matter astroglia from human (infant, adult) and adult monkey (Cebus apella, Macaca nemestrina) cases without any known neurological disease, is described. Expression of synaptic vesicle-associated proteins, excitatory amino acid transporters (EAAT1 and EAAT2) and GABAA Ralpha2 receptor produced coarse punctate labeling in human adult white matter astrocytes. A finer, generalized, punctate labeling was observed in human infants and adult C. apella monkeys. Labeling of neuronal somata and processes with microtubule-associated proteins (MAP2a-c) and neuron nuclear (NeuN) antibodies, was also observed in subcortical white matter of humans and monkeys. Results suggest competence of subcortical white matter astroglia of the primate brain to participate in various transmitter regulatory pathways. It is also proposed that, collectively with resident neurons, they may exert some role in affecting the transfer of information that takes place through the various associational and projecting fiber systems coursing through this brain compartment.

Muscimol and baclofen differentially suppress retinotopic and nonretinotopic responses in visual cortex

This study relates to local field potentials and single-unit responses in cat visual cortex elicited by contrast reversal of bar gratings that were presented in single, double, or multiple discrete patch (es) of the visual field. Concurrent stimulation of many patches by means of the pseudorandom, binary m-sequence technique revealed interactions between their respective responses. An analysis identified two distinct components of local field potentials: a fast local component (FLC) and a slow distributed component (SDC). The FLC is thought to be a primarily postsynaptic response, as judged by its relatively short latency. It is directly generated by thalamocortical volleys following retinotopic stimulation of receptive fields of a small cluster of single cells, combined with responses to recurrent excitation and inhibition derived from the cells under study and immediately neighboring cells. In contrast, the SDC is thought to be an aggregate of dendritic potentials related to the long-range lateral connections (i.e. long-range coupling). We compared the suppressive effects of a GABAA-receptor agonist, muscimol, on the FLC and SDC with those of a GABAB-receptor agonist, baclofen, and found that muscimol more strongly suppressed the FLC than the SDC, and that the reverse was the case for baclofen. The differential suppression of the FLC and SDC found in the present study is consistent with the notion that intracortical electrical signals related to the FLC terminate on the somata and proximal/basal dendrites, while those related to the SDC terminate on distal dendrites.

Postnatal expression profile of OBCAM implies its involvement in visual cortex development and plasticity

This study examined the expression of a neuron-specific cell adhesion molecule, OBCAM (opioid-binding cell adhesion molecule), at both the mRNA and protein levels in the cat primary visual cortex at various postnatal ages, using cDNA array analysis and immunocytochemistry. Results obtained using both methods showed that the expression level of OBCAM was high in young and low in older and adult visual cortex. OBCAM-immunoreactivities were associated predominantly with perikarya and dendrites of pyramidal neurons, and OBCAM-immunopositive neurons were present in all cortical layers. Immunostaining of OBCAM in adult visual cortex showed a reduced number of immunopositive neurons and neurites and relatively lower staining intensities as compared with younger animals. In addition, the number of OBCAM-immunopositive neurons was significantly higher in the visual cortex of 4-month-old animals dark-reared from birth than those in age-matched normally reared animals. These results suggest that OBCAM may play an important role in visual cortex development and plasticity.

Squid p196, a new member of the myosin-V class of motor proteins, is associated with motile axoplasmic organelles

Axoplasmic organelles obtained from the squid giant axon move on actin filaments at an average velocity of 1 microm/s [Nature 356 (1992) 722]. The unconventional myosins, in particular, the myosin-V class of motor proteins, represent the most likely candidates to have a role in this motility. Experiments were performed to determine whether a member of the myosin-V class of unconventional myosins is present in axoplasm and optic lobes. Western blots of axoplasm probed with an affinity purified antibody to chicken brain myosin-V (CBM-V) showed cross-reactivity with a protein of Mr 196 kD (p196) which was subsequently purified from squid optic lobes using a modification of a protocol for the purification of CBM-V [Methods Enzymol. 298 (1998) 3; Cell 75 (1993) 215]. Western blots of CBM-V probed with an alpha-p196 polyclonal IgG showed cross-reactivity with CBM-V. Purified p196 has been found to be a calmodulin (CaM) binding protein that possesses calcium-stimulated actin-activated ATPase activity. Equilibrium density fractionation of motile axoplasmic organelle preparations has revealed that p196 cosedimented with the peak organelle fraction into Percoll gradients in the presence of cytochalasin B and ATP. Based on this evidence, we conclude that the p196 present in axoplasm and purified from optic lobes is a squid homolog of CBM-V and functions as a motor for fast transport of membranous organelles on actin filaments in neurons.

A deficit of functional GABA(A) receptors in neurons of beta 3 subunit knockout mice

Mice whose gamma-aminobutyric acid type A (GABA(A)) beta3 subunit gene is inactivated ('beta3 knockout mice') have been previously shown to have epilepsy, hypersensitive behavior, cleft palate, and a high incidence of neonatal mortality. In this study, we analyze whole-cell responses to GABA in neurons from beta3+/+, beta3+/- and beta3-/- mice. We demonstrate markedly decreased responses to GABA in both hippocampal and dorsal root ganglion neurons isolated from beta3-/- mice without major differences in the GABA concentration-response curves. We also utilize the subunit selective pharmacology of Zn2+ and the anticonvulsant drug loreclezole to help infer the presence of beta2 and gamma subunits in the GABA(A) receptors remaining in neurons from beta3-/- mice.

Localization of CAM II kinase-alpha, GAD, GluR2 and GABA(A) receptor subunit mRNAs in the human entorhinal cortex

The human entorhinal cortex (ERC) is an important relay between neocortical association areas and the hippocampus. Pathology in this area, including disturbances in its unique cytoarchitecture and alterations in neurotransmitter receptor binding, has been implicated in several neuropsychiatric disorders but details of the patterns of gene expression for molecules involved in the major neurotransmitter systems in this cortex have been lacking. We used in situ hybridization histochemistry to localize the mRNAs for several proteins which are involved in excitatory and inhibitory neurotransmission in the human ERC. Labelling of mRNA for a glutamate receptor subunit (GluR2) and for a marker of glutamatergic cortical neurons (alpha type II calcium/calmodulin-dependent protein kinase) were distributed in a laminar manner which matched the cellular packing seen on the Nissl sections, with particularly high levels of labelling in the layer II (pre-alpha) cell clusters characteristic of this cortex. Cells labelled for the mRNA of 67 kDa glutamic acid decarboxylase, the synthesizing enzyme of GABA, were distributed diffusely throughout all layers, not concentrated in the cell clusters, and were present in higher numbers in layer III. The labelling of mRNAs for the alpha1, beta2 and gamma2 subunits of the GABA(A) receptor, however, was distributed in a laminar pattern similar to that for GluR2 and CAM II kinase mRNAs, implying a high concentration of inhibitory synapses on the excitatory cells which express these mRNAs.

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.

Brain GABAA receptors studied with subunit-specific antibodies

Brain GABAA/benzodiazepine receptors are highly heterogeneous. This heterogeneity is largely derived from the existence of many pentameric combinations of at least 16 different subunits that are differentially expressed in various brain regions and cell types. This molecular heterogeneity leads to binding differences for various ligands, such as GABA agonists and antagonists, benzodiazepine agonists, antagonists, and inverse agonists, steroids, barbiturates, ethanol, and Cl- channel blockers. Different subunit composition also leads to heterogeneity in the properties of the Cl- channel (such as conductance and open time); the allosteric interactions among subunits; and signal transduction efficacy between ligand binding and Cl- channel opening. The study of recombinant receptors expressed in heterologous systems has been very useful for understanding the functional roles of the different GABAA receptor subunits and the relationships between subunit composition, ligand binding, and Cl- channel properties. Nevertheless, little is known about the complete subunit composition of the native GABAA receptors expressed in various brain regions and cell types. Several laboratories, including ours, are using subunit-specific antibodies for dissecting the heterogeneity and subunit composition of native (no reconstituted) brain GABAA receptors and for revealing the cellular and subcellular distribution of these subunits in the nervous system. These studies are also aimed at understanding the ligand-binding, transduction mechanisms, and channel properties of the various brain GABAA receptors in relation to synaptic mechanisms and brain function. These studies could be relevant for the discovery and design of new drugs that are selective for some GABAA receptors and that have fewer side effects.

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.

Expression patterns and deprivation effects on GABAA receptor subunit and GAD mRNAs in monkey lateral geniculate nucleus

The expression and regulation of seven GABAA receptor subunit gene transcripts were examined in the adult monkey lateral geniculate nucleus. In situ hybridization histochemistry was used to localize mRNAs of the genes encoding monkey-specific alpha 1, alpha 2, alpha 4, alpha 5, beta 1, beta 2, and gamma 2 subunits. The highest levels of expression in the nucleus were for alpha 1, beta 2, and gamma 2 subunit transcripts. The levels were substantially higher in the magnocellular than in the parvocellular layers. Alpha-2, alpha 4, alpha 5, and beta 1 subunit mRNAs were expressed at much lower levels, and magno- and parvocellular layers had approximately equal levels of expression. Following 4- or 21-day periods of monocular deprivation induced by intravitreal injections of tetrodotoxin, levels of the alpha 1, alpha 2, alpha 4, alpha 5, beta 1, and beta 2 mRNAs were decreased in the deprived geniculate laminae. Adjacent sections hybridized with probes specific for 67-kDa glutamic acid decarboxylase (GAD) mRNA also showed decreased levels of expression in deprived laminae after the 21-day deprivation period. Levels of gamma 2 receptor subunit mRNA were unaffected by monocular deprivation. In our previous studies, a clearly significant downregulation is observed for this subunit transcript in the visual cortex of monkeys deprived for equivalent times. The differential expression and responses to deprivation in the lateral geniculate nucleus suggest region-specific regulation of GABAA receptor subunit genes.

Antibodies to the rat beta 3 subunit of the gamma-aminobutyric acid A receptors

Polymerase chain reaction was used to amplify the cDNA region that codes for the large intracellular loop of the beta 3 subunit of the gamma-aminobutyric acidA/benzodiazepine receptors (GABAAR/BZDR) from rat brain. The amplified cDNA was inserted into the prokaryotic expression vector pGEX-3X and a fusion protein containing glutathione-S-transferase and beta 3 intracellular loop moieties was expressed in bacteria. The fusion protein was affinity-purified and it was used to raise a rabbit anti-beta 3 antiserum. The anti-beta 3 antiserum immunoprecipitated the gamma-aminobutyric acidA receptor from rat and bovine brain. Immunoblots of the affinity-purified GABAAR/BZDR from bovine brain revealed that the anti-beta 3 antiserum reacted with a 57 kDa peptide, whereas the monoclonal antibody 62-3G1 that recognized both beta 2 and beta 3 reacted with 55 and 57 kDa peptides. The anti-beta 3 antiserum showed specificity for the beta 3 subunit vs beta 2 and beta 1.

Immunocytochemical localization of the beta 2 subunit of the gamma-aminobutyric acidA receptor in the rat brain

An antiserum to the beta 2 subunit of the rat gamma-aminobutyric acid (GABAA) receptor was prepared by immunizing a rabbit with a fusion protein expressed in bacteria. The fusion protein had the large, intracellular loop expanding between the putative M3 and M4 transmembrane domains of the beta 2 subunit fused to staphylococcal protein A (SPA). The antiserum immunoprecipitated both the solubilized and the affinity-purified GABAA receptors. The anti-beta 2 antibodies were affinity purified on immobilized beta 2 intracellular loop peptide. The antibodies recognized a 55-57 kDa peptide in immunoblots of either crude membranes from rat cerebral cortex or affinity-purified GABAA receptors from bovine cerebral cortex. Immunocytochemistry with the affinity-purified antibody has revealed for the first time the localization of the beta 2 subunit in the rat brain. A comparative study of the regional and cellular immunoreactivities of the affinity-purified anti-beta 2 antibody and the monoclonal antibody 62-3G1 (which recognizes both beta 2 and beta 3 subunits) is presented. The procedure described for generating and preparing specific anti-beta 2 subunit antibodies that are valuable for immunocytochemistry could be extended to other GABAA receptor subunits.

Noise, neural codes and cortical organization

Cortical circuitry must facilitate information transfer in accordance with a neural code. In this article we examine two candidate neural codes: information is represented in the spike rate of neurons, or information is represented in the precise timing of individual spikes. These codes can be distinguished by examining the physiological basis of the highly irregular interspike intervals typically observed in cerebral cortex. Recent advances in our understanding of cortical microcircuitry suggest that the timing of neuronal spikes conveys little, if any, information. The cortex is likely to propagate a noisy rate code through redundant, patchy interconnections.

Astrocytic GABA receptors

GABA receptors are distributed widely throughout the central nervous system on a variety of cell types. It has become increasingly clear that astrocytes, both in cell culture and tissue slices, express abundant GABAA receptors. In astrocytes, GABA activates Cl(-)-specific channels that are modulated by barbiturates and benzodiazepines; however, the neuronal inverse agonist methyl-4-ethyl-6, 7-dimethoxy-beta-carboline-3-carboxylate enhances the current in a subpopulation of astrocytes. The properties of astrocytic GABAA receptors, therefore, are remarkably similar to their neuronal counterparts, with only a few pharmacological exceptions. In stellate glial cells of the pituitary pars intermedia, GABA released from neuronal terminals activates postsynaptic potentials directly. The physiological significance of astrocytic GABAA-receptor activation remains unknown, but it may be involved in extracellular ion homeostasis and pH regulation. At present, there is considerably less evidence for the presence of GABAB receptors on astrocytes. The data that have emerged, however, indicate a prominent role for second-messenger regulation by this receptor.