GABA as a trophic factor during development

Sex-specific behavioural deficits in adulthood following acute activation of the GABAA receptor in the neonatal mouse

INTRODUCTION

Sex differences exist in the prevalence of neurodevelopmental disorders (NDDs). Part of the aetiology of NDDs has been proposed to be alterations in the balance between excitatory and inhibitory neurotransmission, leading to the question of whether males and females respond differently to altered neurotransmitter balance. We investigated whether pharmacological alteration of GABAA signalling in early development results in sex-dependent changes in adult behaviours associated with NDDs.

METHODS

Male and female C57BL/6J mice received intraperitoneal injections of 0.5mg/kg muscimol or saline on postnatal days (P) 3-5 and were subjected to behavioural testing, specifically open field, light dark box, marble burying, sucralose preference, social interaction and olfactory habituation/dishabituation tests between P60-90.

RESULTS

Early postnatal administration of muscimol resulted in reduced anxiety in the light dark box test in both male and female adult mice. Muscimol reduced sucralose preference in males, but not females, whereas female mice showed reduced social behaviours. Regional alterations in cortical thickness were observed in the weeks following GABAA receptor activation, pointing to an evolving structural difference in the brain underlying adult behaviour.

CONCLUSIONS

We conclude that activation of the GABAA receptor in the first week of life resulted in long-lasting changes in a range of behaviours in adulthood following altered neurodevelopment. Sex of the individual affected the nature and severity of these abnormalities, explaining part of the varied pathophysiology and neurodevelopmental diagnosis that derive from excitatory/inhibitory imbalance.

Neurogenesis of GABAergic cells in the retina of malnourished rats

The present study investigated how prenatal protein malnutrition affects the neurogenesis of GABAergic cells in the retina. Rats were treated with a multi-deficient diet, with only 8% of protein that was administered during the gestational and suckling periods. Pregnant mothers and pups from malnourished and control (fed with 22% protein) groups received a single intra-peritoneal injection of [3H]-thymidine at six developmental ages, from E14 to PN4, and the pups were sacrificed at PN18. Eyes were enucleated and cryosections of the retina were double labeled for GABA-immunocytochemistry and for autoradiography. The percentage of double labeled cells, in the retinal inner nuclear and ganglion cell layers, was determined for both groups. Qualitative and quantitative results showed that double labeled cells [GABA+/thymidine+] were present since E14, when mitotic activity for GABAergic cells starts, in both GCL and INL layers. The peak rate of GABAergic cell generation was reached in control animals injected with [3H]-thymidine at E18 in both central and peripheral sectors of the retina, but only at E20 in the malnourished group. The generation of cells of GABA phenotype showed a significant delay in both layers of the retina in the malnourished group. At PN4, close to the age that GABAergic mitotic activity ends in the control group, double labeled cells were significantly higher in the malnourished group. Our data showed a delay in GABAergic cell generation in the malnourished group when compared to the control group that might result in significant functional consequences in the developing retina.

GABA-induced intracellular Ca2+ elevation in the embryonic chick brainstem slice

We examined the intracellular Ca2+ ([Ca2+]i) elevation evoked by GABA in an 8-day embryonic chick brainstem slice using a Ca imaging technique with Ca green-1 AM. When small quantities of GABA were pressure-ejected on the surface of the slice, the [Ca2+]i elevation was clearly detected. The GABA-induced [Ca2+]i elevation was eliminated in a Ca2+-free solution, whereas the previously reported GABA-induced light-scattering change was independent of extracellular Ca2+. Although, micro-application of glycine or glutamate also induced [Ca2+]i elevation, these changes were smaller than that by GABA. These results suggest that the GABA-induced [Ca2+]i elevation is due to Ca2+ entry resulting from membrane depolarization and may play an important role in the development of the central nervous system (CNS).

Ontogeny of gamma-aminobutyric acid-immunoreactive neurons in the rhombencephalon and spinal cord of the sea lamprey

The development of neurons expressing gamma-aminobutyric acid (GABA) in the rhombencephalon and spinal cord of the sea lamprey (Petromyzon marinus) was studied for the first time with an anti-GABA antibody. The earliest GABA-immunoreactive (GABAir) neurons appear in late embryos in the basal plate of the isthmus, caudal rhombencephalon, and rostral spinal cord. In prolarvae, the GABAir neurons of the rhombencephalon appear to be distributed in spatially restricted cellular domains that, at the end of the prolarval period, form four longitudinal GABAir bands (alar dorsal, alar ventral, dorsal basal, and ventral basal). In the spinal cord, we observed only three GABAir longitudinal bands (dorsal, intermediate, and ventral). The larval pattern of GABAir neuronal populations was established by the 30-mm stage, and the same populations were observed in premetamorphic and adult lampreys. The ontogeny of GABAergic populations in the lamprey rhombencephalon and spinal cord is, in general, similar to that previously described in mouse and Xenopus.

IL-4 increases GABAergic phenotype in rat retinal cell cultures: involvement of muscarinic receptors and protein kinase C

Interleukin-4 (IL-4) is an anti-inflammatory cytokine. During injuries, infections and neurodegenerative diseases, high levels of this molecule are expressed in the brain. In the present work, we investigated the effect of IL-4 on GABAergic differentiation of retinal cells kept in vitro. We analyzed either the uptake of [3H]-gamma-aminobutyric acid (GABA) or the expression of glutamic acid decarboxylase (GAD-67) following IL-4 treatment. We have also investigated the pharmacological modulation of the [3H]-GABA uptake by cholinergic activation. Our results demonstrate that IL-4 increases the uptake of [3H]-GABA after 48 h in culture in a dose-dependent manner (0.5-100 U/ml). The maximal effect was obtained with 5 U/ml (75% increase). This effect was blocked by 1 mM of nipecotic acid, demonstrating the involvement of the GAT-1 subtype of GABA transporter. The IL-4 effect depends on M1 muscarinic activity, an increase in intracellular calcium levels, tyrosine kinase activity and protein kinase C (PKC) activity. Treatment with IL-4 for 48 h induced an increase of 90% in the number of GAD- and GABA-immunoreactive cells when compared with control cultures. Our results indicate that IL-4 modulates the GABAergic phenotype of retinal cells in culture. This result can suggest an important role for this cytokine either during the normal development of retinal circuitry or during neuroprotection after injuries.

Neonatal toluene exposure selectively alters sensitivity to different chemoconvulsant drugs in juvenile rats

Toluene is an abused solvent widely used in several commercial products. Recent evidence indicates that this solvent is a noncompetitive inhibitor of N-methyl-D-aspartate (NMDA) receptors and enhances gamma-aminobutyric acid(A) (GABA(A)) receptor-mediated synaptic currents. Since NMDA and GABA(A) receptors have been implicated in seizures, this study investigated whether toluene exposure during synaptogenesis period alters the NMDA and GABA(A) receptor-mediated seizure susceptibility in juvenile rats. Neonatal rats were administered toluene (1 g/kg ip) daily over postnatal days (PN) 4-9. Rats were administered NMDA (10 mg/ml), picrotoxin (2 mg/ml), pentylenetetrazol, (5 mg/ml) and 4-aminopyridine (4-AP; 2 mg/ml) via timed tail vein infusion on PN 34-36. Toluene exposure increased sensitivity to NMDA, picrotoxin and pentylenetetrazol, but did not affect 4-aminoyridine-induced seizures in both male and female rats. These results suggest that toluene may possess a risk to the developing brain by inducing a long-term alteration in the function of NMDA and GABA(A) receptors.

Ontogeny of gamma-aminobutyric acid-immunoreactive neuronal populations in the forebrain and midbrain of the sea lamprey

Although brain organization in lampreys is of great interest for understanding evolution in vertebrates, knowledge of early development is very scarce. Here, the development of the forebrain and midbrain gamma-aminobutyric acid (GABA)-ergic systems was studied in embryos, prolarvae, and small larvae of the sea lamprey using an anti-GABA antibody. Ancillary immunochemical markers, such as proliferating cell nuclear antigen (PCNA), calretinin, and serotonin, as well as general staining methods and semithin sections were used to characterize the territories containing GABA-immunoreactive (GABAir) neurons. Differentiation of GABAir neurons in the diencephalon begins in late embryos, whereas differentiation in the telencephalon and midbrain was delayed to posthatching stages. In lamprey prolarvae, the GABAir populations appear either as compact GABAir cell groups or as neurons interspersed among GABA-negative cells. In the telencephalon of prolarvae, a band of cerebrospinal fluid-contacting (CSF-c) GABAir neurons (septum) was separated from the major GABAir telencephalic band, the striatum (ganglionic eminence) primordium. The striatal primordium appears to give rise to most GABAir neurons observed in the olfactory bulb and striatum of early larval stages. GABAir populations in the dorsal telencephalon appear later, in 15-30-mm-long larvae. In the diencephalon, GABAir neurons appear in embryos, and the larval pattern of GABAir populations is recognizable in prolarvae. A small GABAir cluster consisting mainly of CSF-c neurons was observed in the caudal preoptic area, and a wide band of scattered CSF-c GABAir neurons extended from the preoptic region to the caudal infundibular recess. A mammillary GABAir population was also distinguished. Two compact GABAir clusters, one consisting of CSF-c neurons, were observed in the rostral (ventral) thalamus. In the caudal (dorsal) thalamus, a long band extended throughout the ventral tier. The nucleus of the medial longitudinal fascicle contained an early-appearing GABAir population. The paracommissural pretectum of prolarvae and larvae contained a large group of non-CSF-c GABAir neurons, although it was less compact than those of the thalamus, and a further group was found in the dorsal pretectum. In the midbrain of larvae, several groups of GABAir neurons were observed in the dorsal and ventral tegmentum and in the torus semicircularis. The development of GABAergic populations in the lamprey forebrain was similar to that observed in teleosts and in mouse, suggesting that GABA is a very useful marker for understanding evolution of forebrain regions. The possible relation between early GABAergic cell groups and the regions of the prosomeric map of the lamprey forebrain (Pombal and Puelles [ 1999] J. Comp. Neurol. 414:391-422) is discussed in view of these results and information obtained with ancillary markers.

Optical approaches to embryonic development of neural functions in the brainstem

The ontogenetic approach to physiological events is a useful strategy for understanding the functional organization/architecture of the vertebrate brainstem. However, conventional electrophysiological techniques are difficult or impossible to employ in the early embryonic central nervous system. Optical techniques using voltage-sensitive dyes have made it possible to monitor neural activities from multiple regions of living systems, and have proven to be a useful tool for analyzing the embryogenetic expression of brainstem neural function. This review describes recent progress in optical studies made on embryonic chick and rat brainstems. Several technical issues concerning optical recording from the embryonic brainstem preparations are discussed, and characteristics of the optical signals evoked by cranial nerve stimulation or occurring spontaneously are described. Special attention is paid to the chronological analyses of embryogenetic expression of brainstem function and to the spatial patterning of the functional organization/architecture of the brainstem nuclei. In addition, optical analyses of glutamate, GABA, and glycine receptor functions during embryogenesis are described in detail for the chick nucleus tractus solitarius. This review also discusses intrinsic optical signals associated with neuronal depolarization. Some emphases are also placed on the physiological properties of embryonic brainstem neurons, which may be of interest from the viewpoint of developmental neurobiology.

Neurogenesis of GABAergic cells in the chick retina

Two classes of retinal neurons in the chick retina, the horizontal and the amacrine cells, are GABAergic. This study evaluates the neurogenesis of glutamic acid decarboxylase immunoreactive cells in the chick retina. Twenty-five microCi [3H]thymidine was injected into eggs of 2-10 days and the embryos were sacrificed at embryonic day 18 (E18). Glutamic acid decarboxylase immunohistochemistry was revealed by avidin-biotin complex method followed by autoradiography of thymidine. We used the cumulative method for counting autoradiographic grains. At E3, 10% of the amacrine cells were thymidine negative/glutamic acid decarboxylase positive and this rate remained constant until E6. From E6 to E8 about 80% of the amacrine cells were thymidine negative/glutamic acid decarboxylase positive. At E9, 100% of these neurons had been generated. On the other hand, at E3 only 1.5% of the horizontal cells had been generated (thymidine negative/glutamic acid decarboxylase positive) while at E6 this number increased to 10%. From E6 to E9 the neurogenesis pattern was similar to that found for amacrine cells. Our data show that the great majority (80%) of glutamic acid decarboxylase positive amacrine and horizontal cells proliferate between E6 and E9, i.e. the last 3 days of the neurogenesis period. From E3 to E6 only 20% of the glutamic acid decarboxylase positive amacrine and horizontal cells are generated, which suggests that glutamic acid decarboxylase positive cells may require a specific signal at about E6, which triggers their withdrawal from the cell cycle.

Molecular signals for development of neuronal circuitry in the retina

In this review article, we summarize recently accumulated knowledge regarding the molecular mechanisms, which control retinal development. Retinal neurons are born in two waves of cytogenesis. In the first wave, neurons of cone circuitry are generated, whereas in the second wave, rod circuitry is added. Neurons generated in these two waves of cytogenesis differ in many respects, including the molecular cues used for migrational guidance. The neurons generated in the second phase of proliferation are arranged in radial columns associated with Müller cells, whereas those of the first phase are often found outside the radial columns. Certain early born cone photoreceptors may form templates for the arrangement of additional mosaics of other cell types. These mosaic arrangements of cell bodies are subsequently refined by lateral displacement of cells and apoptosis. Müller cells may play an important role in directing migration of second phase neurons within groups of radial columns and also in guiding the projections of these neurons so that specific connections are formed. The Müller cell's ability to exert these influences perhaps resides in a variety of cell adhesion molecules such as L1/NgCAM, F11, and 5A11, which are expressed on the surface of Müller cells and retinal neurons. CAMs also promote neurite outgrowth through second messenger pathways.

Developmental expression of GABA(A) receptors in retinal transplants

gamma-Aminobutyric acid (GABA) has transiently been found in certain retinal cells during development, and thus it has been suggested that besides its role as an inhibitory neurotransmitter, it also plays a role during the development of the retina. Further it has been suggested that this developmental role of GABA is mediated through GABA(A) receptors. Retinal cell transplants are being tried for the treatment of degenerative retinal disorders. Even though the donor tissue continues to proliferate, to develop and to differentiate after transplantation, its development is not entirely normal. Various neurotransmitters have been found in retinal transplants, but the receptors, which are needed for their action, have not been demonstrated. It was therefore of interest to see the expression of GABA(A) receptors during the development of the transplants. Embryonic day (E) 15 rabbit retinas were transplanted into the eyes of adult rabbits. Transplants were allowed to survive for various times so that the grafts attained the equivalent ages of (donor age + survival time) E 19, 21, 26, 29 and postnatal (PN) day 2, 5, 9, 19 and 95. On formaldehyde-fixed cryostat sections of the transplant tissue immunohistochemistry was performed. Antibodies against the alpha(1) and beta(2/3) subunits of the GABA(A) receptors were used to demonstrate these receptors. No immunoreactivity was detected in transplants of ages E 19 and 22. The GABA(A) receptor beta(2/3) subunit first appeared in E 29 transplants, whereas the GABA(A) receptor alpha(1) subunit was first detected at PN 2. At these ages faint immunoreactivity was detected in certain plexiform layers in the transplants. In older ages the immunoreactivity increased and also appeared in certain cells lying in between the rosettes along with the plexiform layers that are equivalent to the inner and the outer plexiform layers of the normal retina. The development of the GABA(A) receptor alpha(1) and beta(2/3) subunit immunoreactivity compared well with that during the development of the normal retina. The results suggest that GABA present in the retinal transplants can exert its action through its receptors.

Postnatal development of GABAA receptor beta1, beta2/3, and gamma2 immunoreactivity in the rat retina

GABA (gamma-aminobutyric acid) is the major inhibitory neurotransmitter in the mammalian central nervous system and plays an important role in neuronal physiology during ontogenesis. The distribution of the beta1-, beta2/3-, and gamma2-subunit of the GABAA receptor in the rat retina was studied during postnatal development using immunohistochemical methods. All subunits were found at birth. However, each subunit showed a unique staining pattern with a different local distribution. The immunoreactivity pattern changed during the time course of postnatal development for each of the proteins investigated. A clustered distribution at presumptive synaptic sites as indicated by a punctate staining pattern of the inner plexiform layer was detected as early as the second day of postnatal development. However, diffuse staining of presumptive extrasynaptic sites was found throughout development. The typical adult layering of immunoreactivity into distinctive bands appeared later in development, characteristically in the second postnatal week. The results of the present study suggest that GABAA receptor expression precedes the formation of functional synapses and changes along with cellular differentiation of the rat retina. Developmentally regulated changes in GABAA receptor composition and distribution indicate possible functions for this receptor during retinal ontogeny.

Pharmacologic and therapeutic aspects of the developmentally regulated expression of GABA(A) and GABA(B) receptors: cerebellar granule cells as a model system

Cerebellar granule neurons can be conveniently kept in culture. They constitute a useful model to study regulation of glutamatergic activity, in particular the inhibitory action of GABA (7-aminobutyrate). GABA exerts an inhibitory action on evoked transmitter release acting on both GABA(A) and GABA(B) receptors. The functional properties of these receptors are dependent upon the environment of the neurons during early development in culture as the expression of both receptor subtypes is enhanced by exposure of the neurons to GABA(A) receptor agonists. Thus, the inducible GABA(A) receptors are of low affinity and lack benzodiazepine sensitivity, and the G-protein coupling differs among the native and the inducible GABA(B) receptors. Moreover, the GABA(A) and the GABA(B) receptors are functionally coupled, leading to a disinhibitory action of GABA. Therefore drugs exhibiting selective agonist or antagonist action on subclasses of GABA(A) and GABA(B) may be of potential use as regulators of glutamatergic excitatory activity.

Pharmacological and functional implications of developmentally-regulated changes in GABA(A) receptor subunit expression in the cerebellum

The cerebellum undergoes many morphological, pharmacological, and electrophysiological changes during the first 3 weeks of postnatal development. The purpose of this review is to present the most up to date synopsis of the pharmacological and functional changes in, gamma-aminobutyric acid (GABA) type A receptors during this time of cerebellar maturation. Since most of the diversity in cerebellar, GABA(A) receptor pharmacology lies within the granule cell layer, research groups have focused on this area of the cerebellum to study the developmental changes in GABA(A) receptor subunit expression and the neurodifferentiating factors involved in regulating this expression. Thus, it is important to note that developmental changes in GABA(A) receptor composition and its corresponding pharmacology will be essential for determining the type of GABA-mediated transmission that occurs between neuronal contacts in the neonatal and subsequently in the mature cerebellum.

Dopamine D2 receptor effects on GABA(A) receptor expression may modify melanotrope peptide release

Stimulation of melanotrope dopamine D2 receptors decreases mitotic rate, calcium channel activity, and the biosynthesis of several proteins. This study demonstrates that D2 receptor activation also affects GABA(A) receptor beta2/beta3 subunit immunoreactivity. Following chronic treatment with haloperidol, a D2 receptor antagonist, GABA(A) receptor immunoreactivity increased, whereas it decreased after chronic treatment with bromocriptine, a dopamine D2 receptor agonist. Thus, these data indicate that D2 function regulates GABA(A) receptor expression in melanotropes, a mechanism by which peptide release may be modified.

Differences in the retinal GABA system among control, spastic mutant and retinal degeneration mutant mice

Immunocytochemical methods were used to compare the GABA system in control mice and two mutant strains: spastic which has reduced glycine receptors and retinal degeneration mutant in which the photoreceptors degenerate and reportedly have increased GABA and GAD levels. We found that the spastic mutant retina had reduced GABA-immunoreactivity (IR) in the proximal retina, reduced staining for GAD-1440 in the OPL, and reduced GABAA receptor staining in the OPL, compared to control. The retinal degeneration mutant retinas had enhanced GABA-IR throughout the retina, particularly in Müller cells, bipolar cells and IPL, and enhancement of GABAA receptor staining in the OPL, compared to control. The distributions of GABA-IR, GAD-1440-IR and GABAA receptor-IR in retinas of spastic mutant mice that also expressed the retinal degeneration phenotype resembled those found in retinas of mice that expressed only the retinal degeneration phenotype rather than those that expressed only the spastic mutation. No differences were observed among the conditions for GAD-65, GAD-67 or GABA-T. Our results with the spastic and retinal degeneration mutant mice demonstrate that attenuation in the glycinergic system and photoreceptor degeneration, respectively, is accompanied by alterations in different aspects of the GABA system, giving impetus for caution in the interpretation of experiments involving genetic manipulation of complex phenotypes.

Expression of two forms of glutamic acid decarboxylase (GAD67 and GAD65) during postnatal development of the cat visual cortex

The postnatal development of GAD67 and GAD65 protein expression and of GAD67 positive neurons and GAD65 containing axon terminals in cat visual cortex was studied. Western blot analysis showed that the expression of both GAD67 and GAD65 increased to approximately two-thirds of the adult level during the first 5 postnatal weeks and gradually increased thereafter. In adult cats, immunohistochemistry showed that GABA and GAD67 containing neurons were found in all cortical layers. Faint cell body staining was seen with the antibody to GAD65, but it densely labeled puncta. In neonates, GABA and GAD67 immunoreactivity was most intense in two distinct bands, one superficial (Layer 1/Marginal zone), another deep (Layer VI/Subplate). Unlike in adults, GAD65 positive cell bodies were clearly evident in neonates and distributed similarly to, but less frequently than, GABA and GAD67. These GAD65 positive cells frequently had morphologies suggestive of embryonic cells and largely disappeared in older animals. During postnatal development, the neurochemical differentiation of GAD67 positive neurons and GAD65 positive axon terminals across visual cortical laminae followed an inside-outside developmental pattern, which reached adult levels after 10 weeks of age. These results suggest that postnatal development of the visual cortical GABA system involves three distinct processes: (A) a dying off of embryonic GABA cells which could play a role in formation of the cortical plate; (B) a period of relative quiescence of the VC GABA system in the first 5 postnatal weeks which could maximize excitatory NMDA effects during the rising phase of the critical period; (C) the prolonged postnatal maturation of the adult GABA system which could be involved in the crystallization of adult physiological properties and the disappearance of neural plasticity.

Analysis of the distribution of glycine and GABA in amacrine cells of the developing rabbit retina: a comparison with the ontogeny of a functional GABA transport system in retinal neurons

The objectives of this study were to (1) determine whether the glycinergic and GABAergic amacrine cells in the developing rabbit retina were neurochemically distinct at birth, (2) determine if the ratio of GABAergic to glycinergic amacrine cells was constant during development, (3) determine whether the capacity to take up a GABA analogue was restricted to GABAergic neurons, and (4) whether initiation of GABA transport into GABAergic neurons preceded the presence of a content of GABA in these neurons. We have used a novel strategy to immunolocalize a non-endogenous GABA analogue, gamma-vinyl GABA, which is taken up into neurons by a GABA transporter. Examination of serial semithin resin-embedded sections of neonatal rabbit retinae that had been immunolabelled for glycine, GABA or gamma-vinyl GABA revealed that at 1 day postnatum, 60% of amacrine cells contain glycine but not GABA and did not accumulate gamma-vinyl GABA, which is similar to the percentage of glycinergic amacrine cells in the adult retina. The vast majority of the remaining amacrine cells contained GABA and many also transported gamma-vinyl GABA; however, a significant number of GABA-containing cells failed to accumulate gamma-vinyl GABA suggesting that possession of a content of GABA did not have to be preceded by, or be concomitant with, the presence of a GABA transport system. By 10 days postnatum, over 99% of GABA-containing amacrine cells also transported gamma-vinyl GABA indicating their functional maturity. Analysis of the horizontal cells revealed no evidence for uptake of gamma-vinyl GABA, but another GABA analogue, diaminobutyric acid, which is a substrate both for the neuron-associated GABA transporter and the glial GABA transporter, was accumulated into some horizontal cells at 21 days postnatum, a time point when these cells also contain endogenous GABA. We conclude that amacrine cells are committed to being GABAergic or glycinergic at, or prior to birth, and that in some amacrine cells, expression of a content of GABA may occur prior to the capacity to transport GABA. Conversely, in some ganglion cells transport of gamma-vinyl GABA may precede a content of GABA.

Morphologic and neurochemical target selectivity of regenerating adult photoreceptors in vitro

Regenerating adult central nervous system (CNS) neurons must re-establish synaptic circuits in an environment very different from that present during development. However, the complexity of CNS circuitry has made it extremely difficult to assess the selectivity and mechanisms of synaptic regeneration at the cellular level in vivo. The synaptic preferences of adult photoreceptors were examined by using a defined cell culture system known to support regenerative process growth, presynaptic varicosity formation, and establishment of functional synapses. Immunolabeling for synaptic vesicle protein 2 and ultrastructural analysis demonstrated that cell-cell contacts made by photoreceptors were synaptic in nature. Target selectivity was determined by quantitative analysis of contacts onto normal and novel target cell types in cultures in which opportunities to contact all retinal cell types were present. Target cells were identified by morphology and immunolabeling for the amino acid neurotransmitters glutamate, aspartate, gamma-aminobutyric acid (GABA), and glycine. Regenerating photoreceptors showed a strong preference for novel multipolar cell targets (amacrine and ganglion cells) over normal photoreceptor, horizontal, and bipolar cell targets. Additionally, photoreceptors were selective for targets containing the transmitter GABA. These results indicate first, that the normal synaptic partners for photoreceptors are not intrinsically the optimal targets for regenerative synapse formation, and second, that GABA may modulate synaptic targeting by adult photoreceptors.

Prominent expression of two forms of glutamate decarboxylase in the embryonic and early postnatal rat hippocampal formation

Immunohistochemical methods were used to determine the earliest times of detection for two forms of glutamate decarboxylase (GAD67 and GAD65) in the embryonic and early postnatal rat hippocampal formation and to determine whether their distribution patterns differed from each other and from those of the adult. Both GAD67- and GAD65-containing neurons were observed as early as embryonic day 17 (E17)-E18 in the hippocampus and E19 in the dentate gyrus, and this was substantially earlier than GAD had been detected previously in the hippocampal formation. The two GAD isoforms displayed very similar distribution patterns, but these patterns were distinctly different from those of the adult. From E17 to E20, GAD67 and GAD65 were expressed in neuronal cell bodies throughout the hippocampal and dentate marginal zones (future dendritic layers), and relatively few existed within the principal cell body layers, where GAD-positive neurons are frequently concentrated in the adult. At E21 to postnatal day 1 (P1), there was a sudden shift from a predominance of GAD-containing cell bodies within the developing dendritic regions to a meshwork of GAD-positive processes with terminal-like varicosities in these same regions. This pattern also contrasted with that of the adult, in which GAD-labeled terminals are highly concentrated in the principal cell layers. Electron microscopic observations of the GAD-labeled processes at P1 confirmed their axon-like appearance and demonstrated that the immunoreactivity was consistently localized in vesicle-filled regions that were often closely apposed to and, in some instances, established synaptic contacts with dendritic profiles. The present identification of an early abundance of GAD-containing structures in the hippocampal formation and the marked change in their distribution during development complement recent observations of developmental changes in the functioning of the GABA system and provide additional support for the early involvement of this neurotransmitter system in hippocampal development.

Presence of mRNA for glutamic acid decarboxylase in both excitatory and inhibitory neurons

Neurons in very low density hippocampal cultures that are physiologically identified as either GABAergic inhibitory or glutamatergic excitatory all contain mRNA for the gamma-aminobutyric acid (GABA) synthetic enzyme, glutamic acid decarboxylase (GAD), as detected by single cell mRNA amplification and PCR. However, consistent with the physiology, immunocytochemistry revealed that only a subset of the neurons stain for either GAD protein or GABA. A similar fraction hybridize with RNA probes for GAD65 and GAD67. Hippocampal CA1 pyramidal neurons in slice preparations, which are traditionally thought to be excitatory, also contain mRNA for GAD65 and GAD67. Hippocampal neurons in culture did not contain mRNA for two other neurotransmitter synthesizing enzymes, tyrosine hydroxylase, and choline acetyl transferase. These data suggest that in some neurons, presumably the excitatory neurons, GAD mRNA is selectively regulated at the level of translation. We propose that neurotransmitter phenotype may be posttranscriptionally regulated and neurons may exhibit transient phenotypic plasticity in response to environmental influences.

GABA and GABA-A receptors are maximally expressed in association with cone synaptogenesis in neonatal rabbit retina

Previous studies have shown the cone photoreceptors form reciprocal synapses with horizontal cells during the first week after birth in rabbits. These synapses constitute pioneering elements of the developing outer plexiform layer. We now report that antibodies against the alpha-1 and against the beta-2/3 subunits of the GABA-A receptor label a highly restricted sublamina in the developing outer plexiform layer known to contain nascent cone photoreceptor terminals. Staining is relatively weak at birth, increases to maximal levels between postnatal days 5 and 7, and is significantly reduced in the adult. These results support recent calcium imaging studies which have shown that the activation of GABA-A receptors causes an increase in intracellular free calcium in cones, an effect which is observed only at 3-9 days after birth. The transient expression of GABA-A receptors in this region coincides with the period of peak expression of GABA immunoreactivity in horizontal cells. A direct functional link between GABAergic transmission and cone synaptogenesis is suggested by previous reports that GABA-A receptor antagonists cause disruption of cone synaptogenesis. Together these findings support the notion that GABA functions as a developmental neurotransmitter which is produced by horizontal cells and interacts with developing cone axons in order to facilitate synaptic linkage between these two cells types.

Use-dependent regulation of GABAA receptors

Prolonged occupancy of GABAA receptors by ligands, including GABA and benzodiazepine agonists, sets in motion a series of mechanisms that can be termed use-dependent regulation. These mechanisms can be subdivided into two distinct pathways, one for GABAA receptor downregulation and another for upregulation. Treatment of cortical neurons with GABA or benzodiazepines in cultures opens the pathway for GABAA receptor downregulation, which includes (in putative temporal order): (1) desensitization (tachyphylaxis), (2) sequestration (endocytosis) of subunit polypeptides and uncoupling of allosteric interactions between GABA and benzodiazepine binding sites, (3) subunit polypeptide degradation, and (4) repression of subunit gene expression. The end-point of GABAA receptor downregulation, a reduction in receptor number, is postulated to be established initially by degradation of the receptor protein and then maintained by a diminished level of de novo synthesis. Benzodiazepine treatment of many preparations, including cells expressing recombinant GABAA receptors, may elicit only desensitization, sequestration, or uncoupling, without a decline in receptor number. Components of the GABAA receptor downregulation pathway are also evoked by chronic administration of GABAmimetics, benzodiazepines, barbiturates, and neurosteroids in animals. This downregulation correlates with the establishment of tolerance to and physical dependence on the pharmacological effects of these drugs, suggesting a cellular model for this behavior. The upregulation of GABAA receptors is observed as one of the neurotrophic actions of GABA, primarily in cultured cerebellar granule cells. Upregulation in culture is caused by enhanced expression of genes for GABAA receptor subunits and correlates with the establishment of GABAergic circuitry in the developing cerebellum. Thus, both the upregulation and downregulation of GABAA receptors appear to represent use-dependent pathways for guiding synaptic plasticity in the vertebrate central nervous system.

Ontogeny of GABA-immunoreactive neurons in the central nervous system in a teleost, gasterosteus aculeatus L

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is known to exert various neurotrophic actions in the developing nervous system, but little is known about its distribution in the central nervous system during early development. We have studied the development of GABA-immunoreactive (GABAir) neurons during embryogenesis of a teleost fish, the three-spined stickleback. As early as 51 h postfertilization (PF; hatching occurs 144-168 h PF, and the first monoaminergic neurons appear around 72 h PF) GABAir neurons appear in the ventral prosencephalon caudal to the optic recess, in the ventral mesencephalon, and in the spinal cord. Then, there is a gradual addition of GABAir cell groups in the rostral prosencephalon and ventral rhombencephalon (66 h PF), dorsal and caudal hypothalamus and pretectum (72 h PF), ventral hypothalamus (78 h PF), preoptic region, thalamus, and in the mesencephalon and rhombencephalon (96 h PF). GABAir axons appear in the spinal cord already at 51 h PF, and then gradually appear in the various tracts of the early axonal scaffold of pathfinding fibers, so that by 96 h PF the entire axonal scaffold contains GABAir fibers. It appears likely that GABAergic axons contribute a major population to the formation of the axonal scaffold. Moreover, in the prosencephalon GABAir neurons are arranged in clusters that may reflect a neuromeric organization with six prosencephalic neuromeres.

Differentiation of photoreceptors, glia, and neurons in the retina of the cichlid fish Aequidens pulcher; an immunocytochemical study

Light-microscopic immunocytochemistry was carried out to investigate the developmental dynamics of several neurochemical markers in the retina of blue acara (Aequidens pulcher). As a rule, double-label experiments were performed in order to determine the absolute and relative timing of the appearance of these markers. The diameter of eye-ball (from 0.6 to 1.2 mm) and the body length (from 4.6 to 9.4 mm) enlarged in parallel during the observation period of 2 to 9 days after spawning (day 2-9); hatching took place usually on day 2. Immunoreactive proliferating cell nuclear antigen (ir-PCNA) was present in all neuroblasts (the embryonic homogeneous cell stage; day 1.0-2.0), but was lost progressively in a center-to-periphery and apparent proximal-to-distal sequence as the cells and layers differentiated. In late larvae and juveniles, ir-PCNA was confined to a ring of dividing neuroblasts at the retinal margin and to a population of scattered rod precursors in the outer nuclear layer. Immunoreactive structures of representative antigens progressively appeared after ir-PCNA had decayed. Around hatching, at the synaptic separation stage (day 2.0-2.5), luteinizing hormone-releasing hormone-ir centrifugal fibers, visinin-ir cones, glial fibrillary acidic protein-ir structures and gamma-aminobutyric acid-ir cell bodies appeared, which were followed by the emergence of rhodopsin-ir rods and tyrosine hydroxylase-ir interplexiform cells (on day 2.5-3.0) and serotonin-, neuropeptide Y- and substance P-ir amacrine cells (on day 3.0-4.0). The results indicate that photoreceptor cells, and especially rods start to differentiate at an earlier stage of retinogenesis than has previously been proposed. In addition, an extraretinal tissue in the brain identified as the prospective pineal organ was found to be visinin- and rhodopsin-immunoreactive on day 1.5-2.0 before these photoreceptor-specific antigens became positive in the retina.

Differential distribution of GABAA receptor subunits in soma and processes of cerebellar granule cells: effects of maturation and a GABA agonist

Quantitative analysis of the density of alpha 1 and beta 2/3 GABAA receptor subunits was performed at the electron microscope level after indirect pre-embedding immunogold labeling with subunit-specific antibodies of rat cerebellar granule cell cultures grown for 4 or 8 days and in the presence or absence of the GABAA receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4c]pyridin-3-ol (THIP). THIP (150 microM) induced a 2-fold increase in the number of alpha 1 and beta 2/3 subunits in both cell bodies and processes in 4-day-old cultures. Extending the culture period to 8 days led to a polarization of the receptor expression, since the increase in the number of subunits selectively was observed in the processes. Moreover, a general subcellular differentiation of the receptor population was observed in all culture conditions, since the ratio between the two subunits (beta 2/3; alpha 1) was four times higher in cell bodies compared to processes. A detailed analysis of the less mature (4-day-old) cultures revealed the existence of two populations of neurons exhibiting differences in the average number of receptors. During maturation neurons with few receptors developed into cells with a higher density of receptors resulting in a single population of the latter neurons, a process enhanced by exposure to THIP. This may indicate that receptor development is a discontinuous process with individual neurons following different temporal patterns. In double-labeling experiments, a spatially close association of the alpha 1 and beta 2/3 subunits could be seen, but the subunits were more frequently found separated from each other. In spite of the fact that exposure of the neurons to THIP increased the total number of receptor subunits, its presence apparently prevented formation of receptors with this subunit composition. Interestingly, receptor subunit clusters, consisting of alpha 1 alone, were more frequently observed than composite (alpha 1; beta 2/3) clusters. This substantiates the view that receptors not having alpha 1 and beta 2/3 subunits in the same complex may exist.

Modulatory actions of gamma aminobutyric acid (GABA) on GABA type A receptor subunit expression and function

Gamma aminobutyric acid (GABA) is present in the central nervous system (CNS) during very early embryogenesis. It is therefore likely to play a role not only as a neurotransmitter but also as a signal molecule for neuronal differentiation, growth, and development. It has been firmly established that formation of synapses is strengthened by GABA, and the expression of certain subunits of the GABA type A (GABAA) receptor complex is clearly promoted by GABA. This latter effect of GABA may have profound implications for the functional activity of GABAergic synapses since the pharmacological properties of GABAA receptors are governed by the subunit composition of the receptor complex. Dynamic changes in GABAA receptor expression and diversity during development and differentiation may therefore play important roles for the inhibitory potential of the CNS during mature stages.

Early development of GABA-like immunoreactive cells in the retina of turtle embryos

Gamma aminobutyric acid (GABA) is one of the earliest neuroactive substances appearing in the developing central nervous system. The distribution and the time course of the appearance of GABA-like immunoreactivity in the retina of the turtle Emys orbicularis were investigated from embryonic stage 13 to hatching. The first GABA-like immunoreactive cells were observed at stage 14. These cells were located in both the scleral third of the neuroblastic layer and the inner layers of the retina. They were identified as presumptive immature horizontal cells and amacrine cells, respectively. The observation of numerous labelled fibers in the nerve fiber layer suggests that some of the GABA-like immunoreactive cells in the layers were ganglion cells. The development of GABA-like immunoreactive cells followed a gradient of maturation from central to peripheral retina. At hatching, the central retina appeared nearly morphologically mature. In conclusion, GABA is present before the morphofunctional maturation of the retina and this precocious existence supports the idea of its involvement in a neurotrophic role preceding the establishment of synaptic connections and neurotransmitter function.

Agonist administration in ovo down-regulates cerebellar GABAA receptors in the chick embryo

Chick embryos with an undeveloped blood-brain barrier were used to examine the down-regulation of GABAA receptors in vivo. The GABAA receptor agonist isoguvacine (5 mumol) was applied to the vascularized chorioallantoic membrane of 8 day embryos. This treatment was repeated on embryonic days 11, 14, and 17, and the embryos were sacrificed on day 18 (stage 42). Isoguvacine administration reduced the clonazepam-displaceable binding of [3H]flunitrazepam to washed cerebellar membranes by 34.0 +/- 3.0% compared to vehicle-treated controls. Binding reductions of lower magnitude were found in membranes from the cerebrum and optic lobes. Administration of isoguvacine had no significant effect on the wet weights of whole embryos or cerebella, the yield of cerebellar membranes, or the binding of [3H]N-methylscopolamine. The reduction of [3H]flunitrazepam binding to cerebellar membranes was dose-dependent, allowing a half saturation value of 8 microM isoguvacine to be estimated. Scatchard analysis showed that the Bmax for [3H]flunitrazepam binding was reduced by 28.3 +/- 6.7% compared to controls, without a change in the Kd. Embryonic exposure to isoguvacine also caused a reduction of 43.6 +/- 6.0% in the binding of the GABAA receptor channel ligand [35S]t-butylbicyclophosphorothionate to washed cerebellar membranes. Taken together, these results indicate that isoguvacine induces a down-regulation of the receptor subunits in vivo. However, measurements of cerebellar GABAA receptor mRNAs for the alpha 1, beta 2L, beta 2S, beta 4, gamma 1, gamma 2L, and gamma 2S subunits by reverse transcriptase-polymerase chain reaction (RT-PCR) revealed no significant alterations by isoguvacine administration. The data suggest that translational or post-translational mechanisms, rather than those modulating the synthesis or stability of subunit mRNAs, take precedence in establishing GABAA receptor down-regulation.

Postnatal ontogeny of GABAB binding in rat brain

The postnatal development of GABAB binding sites in rat brain was studied by quantitative receptor autoradiography using [3H]GABA under selective conditions. Binding levels peak at regionally specific times during the first three weeks of life and then decline to adult levels. GABAB binding peaked in the globus pallidus, vestibular and spinal trigeminal nuclei, and the CA3 region of the hippocampus at postnatal day 3; in the striatum, nucleus accumbens, inferior olive, septum, dentate gyrus and CA1 region of the hippocampus at postnatal day 7; in the neocortex and thalamus at postnatal day 14; and in the medial geniculate at postnatal day 21. Following these regionally specific peaks, binding decreased to postnatal day 28 levels. Further significant decreases in binding were observed in all regions examined between postnatal day 28 and adulthood. Comparisons of binding site pharmacology reveal equipotent displacement of GABAB binding by several competitive agonists and antagonists in postnatal day 7 and adult rat brain, indicating that immature and adult binding sites have similar pharmacological properties with regard to these compounds. The GABAB receptor antagonist CGP 54626A, however, inhibited binding more potently in the postnatal day 7 thalamus and neocortex than in these areas in the adult brain. The guanyl nucleotide analogue guanosine 5'-O-(3-thiotriphasphate) inhibited GABAB binding extensively in both postnatal day 7 and adult brain. The non-competitive antagonist zinc also inhibited GABAB binding at both ages and was more potent in postnatal day 7 brain than in adult brain. Saturation analyses reveal two binding sites with similar affinities in both immature and adult rat brain, indicating that postnatal modulation of GABAB binding reflects changes in binding site density rather than modulation of binding site affinity. While immature GABAB binding sites share most pharmacological characteristics with adult binding sites and appear to be coupled to G-proteins at an early age, their interactions with zinc and CGP 54626A suggest that GABAB binding sites in immature brain may have a distinct pharmacological profile. Our data suggest significant regional and pharmacological changes in GABAB binding during development. The implications of these findings are discussed with regards to a possible role of GABAB receptors in the development of the central nervous system.

Developmental loss of GABA- and glycine-induced depolarization and Ca2+ transients in embryonic rat dorsal horn neurons in culture

More than 90% of dorsal horn neurons from embryonic day 15-16 rats responded to the inhibitory amino acids GABA and glycine by a transient elevation of intracellular Ca2+ concentration ([Ca2+]i) when maintained in culture for < 1 week. This [Ca2+]i response has previously been shown to be due to depolarization and subsequent Ca2+ entry through voltage-gated Ca2+ channels following activation of bicuculline-sensitive GABAA receptors and strychnine-sensitive glycine receptors. Both the number of cells responding to GABA and glycine and the amplitude of the [Ca2+]i response diminished over time in culture. By 30 days in culture, none of the cells responded to GABA, muscimol or glycine by elevation of [Ca2+]i. The loss of the [Ca2+]i response was not due to a change in the abundance or the properties of voltage-gated Ca2+ channels, since over the same period of time dorsal horn neurons showed a large increase in the amplitude of the [Ca2+]i transient in response to 30 mM K+. Nor was the loss of the [Ca2+]i response due to a loss of GABA and glycine receptors. Instead, the decrease in the [Ca2+]i response over time paralleled a similar change in the electrophysiological responses. More than 90% of the neurons tested were depolarized in response to inhibitory amino acids during the first week in culture. After 30 days, all neurons tested responded to GABA and glycine with a hyperpolarization. These observations add support to the suggestion that GABA and glycine may excite dorsal horn neurons early in development and play a role in postmitotic differentiation.

Equivalent cell density in three areas of neonatal rat cerebral cortex

Using the disector method, equivalent cell densities (number of cells per unit volume) were found in the frontal (motor), parietal (somatosensory) and occipital (visual) cortex of neonatal rat (1.2 x 10(6) cells/mm3). As determined with GABA post-embedding immunocytochemistry on semithin sections, at least 11% of these cells expressed GABA. Because neonate frontal cortex is thicker than the parietal or the occipital cortex, the number of cells under an unit area of pial surface was greater in this cortex. This clearly differed from the adult pattern, where the greatest number of cells beneath an unit pial area is that of the somatosensory versus the frontal and the occipital cortex. It suggests that the attainment of the adult number of cells in a cortical column is achieved through differential cell death in the different areas and that the amount of thalamic input could be a crucial determinant of the adult cytoarchitecture.

Quantitative analysis of transient GABA expression in embryonic and early postnatal rat spinal cord neurons

GABA expression was investigated using biochemical analysis of spinal cord homogenates and immunocytochemical analysis of cells acutely dissociated from the embryonic and postnatal rat spinal cord. gamma-Aminobutyric acid (GABA) was detected by both methods as early as embryonic day 13 (E13). At E13, the percentage of neurons that were GABA+ was 0.5%. This value increased during embryogenesis, peaked during the first two postnatal weeks to just over 50%, and declined to approximately 20% by the third postnatal week emphasizing the transient nature of GABA expression. At E17 there was a pronounced, positive ventro-dorsal and rostro-caudal gradient of GABA+ cells that persisted until just before birth. At this time the gradients reversed in cervical and lumbosacral regions indicating that GABA immunoreactivity in discrete anatomical regions is also a transient phenomenon. During the embryonic period GABA immunoreactivity was diffusely distributed throughout cell bodies and proximal processes. At E21, both GABA and synaptophysin were present in the same cells. However the two antigens did not co-localize point for point. By postnatal day 21 GABA immunoreactivity appeared in puncta that co-localized entirely with puncta of synaptophysin immunoreactivity. The sizable percentage of neurons that transiently express GABA during development, and the fact that it can be detected prior to the synaptic form of glutamic acid decarboxylase (GAD65), suggest that the amino acid may play a significant role during differentiation before it functions as an inhibitory neurotransmitter.

The role of GABA during development of the outer retina in the rabbit

Horizontal cells are among the first to mature in the neonatal mammalian retina and they are the first to establish the position of the outer synaptic layer which is subsequently formed by invading terminals of both rod and cone photoreceptors. During the period of cone synaptogenesis, horizontal cells transiently express the full complement of GABAergic properties (uptake, release, synthesis and storage of GABA); later during development of rod terminals, these properties are down-regulated. Given the reports of GABA's role in other developing neuronal systems (for review: 10), we have examined the effect that GABA, produced from horizontal cells, might have on photoreceptor maturation in rabbit retina. Results from our previous studies show that lesioning the horizontal cell with kainic acid in vivo leads to a displacement of cone photoreceptor cells and a disappearance of their synaptic terminals, while rod cells maintain their normal position and produce an overabundance of terminals. Similar effects are seen with the GABA-A receptor antagonists, picrotoxin and bicuculline. New evidence from 3H-thymidine studies suggests that the effects of kainic acid are specific and that cell division, migration and differentiation in other cell types do not appear to be affected. This body of work is summarized and possible mechanisms of action are suggested which could account for the apparent ability of GABA to help maintain the normal position of cone cell bodies and regulate cone synaptogenesis.

Modulation by GABA of neuroplasticity in the central and peripheral nervous system

Apart from being a prominent (inhibitory) neurotransmitter that is widely distributed in the central and peripheral nervous system, gamma-aminobutyric acid (GABA) has turned out to exert trophic actions. In this manner GABA may modulate the neuroplastic capacity of neurons and neuron-like cells under various conditions in situ and in vitro. In the superior cervical ganglion (SCG) of adult rat, GABA induces the formation of free postsynaptic-like densities on the dendrites of principal neurons and enables implanted foreign (cholinergic) nerves to establish functional synaptic contacts, even while preexisting connections of the preganglionic axons persist. Apart from postsynaptic effects, GABA inhibits acetylcholine release from preganglionic nerve terminals and changes, at least transiently, the neurochemical markers of cholinergic innervation (acetylcholinesterase and nicotinic receptors). In murine neuroblastoma cells in vitro, GABA induces electron microscopic changes, which are similar in principle to those seen in the SCG. Both neuroplastic effects of GABA, in situ and in vitro, could be mimicked by sodium bromide, a hyperpolarizing agent. In addition, evidence is available that GABA via A- and/or B-receptors may exert direct trophic actions. The regulation of both types of trophic actions (direct, receptor-mediated vs. indirect, bioelectric activity dependent) is discussed.

Postnatal ontogenesis of neurons containing GABAA alpha 1 subunit mRNA in the rat forebrain

The expression of GABAA receptor alpha 1 subunit mRNA in the postnatal rat forebrain was examined by in situ hybridization histochemistry. In most regions, including the isocortex, olfactory bulb, amygdala, septum, nucleus of the diagonal band, bed nucleus of the stria terminalis, basal ganglia, thalamus, and hypothalamus, the expression of alpha 1 subunit mRNA was low at birth but showed a dramatic increase during the early postnatal period. Adult levels of expression were reached at around the second or third week of life in these regions. However, in the caudate-putamen, and the nucleus accumbens, the expression of this subunit was only transient.

GABAA receptor subunit messenger RNAs show differential expression during cortical development in the rat brain

Developmental changes of the expression of various GABAA receptor subunits (alpha 1, alpha 3, alpha 4, beta 1-3, and gamma 2) were examined in the fetal rat cerebral cortex using in situ hybridization histochemistry. The subunits showed three main patterns of development. The alpha 1 subunit showed the first pattern, in which no expression was observed during embryonic development. The alpha 4 and beta 1 subunits showed the second pattern, in which expression was observed in both the undifferentiated neuroepithelium and the developing cortical layers. The alpha 3, beta 2, beta 3, and gamma 2 subunits showed the third pattern, in which expression was only seen in the developing cortical layers. These findings strongly suggest the following: (i) the alpha 1 subunit is involved in GABAergic transmission in the mature cerebral cortex; (ii) the alpha 4 and beta 1 subunits are involved in both the differentiation of the neuroepithelium and the development of the cortical plate, and (iii) the alpha 3, beta 2, beta 3, and gamma 2 subunits are involved in the development of the cortical plate. Subunits already expressed on embryonic day 13 (beta 1, beta 3, and gamma 2) appear especially likely to have a special role in neuronal development.

gamma-Aminobutyric acid immunoreactivity in multiple cell types of the developing rabbit retina

We have previously demonstrated that the neonatal rabbit retina contains a larger complement of cells that accumulate [3H]-GABA than does the adult. In order for these neurons to be classified as GABAergic, they must also contain endogenous GABA. We now report that these same neonatal cell populations are also immunoreactive to GABA antisera. In frozen sections from rabbit retina, treated with GABA antisera, immunoreactive processes in both synaptic layers were observed at postnatal day 1. The appearance of immunofluorescent fibers precedes that of photoreceptor and bipolar cell terminals in the outer plexiform layer and is diminished by postnatal day 5. Also noted, was a 50% decrease in the density of GABA-immunoreactive cell bodies in the inner nuclear and ganglion cell layers, accompanied by an increase in cell volume and a shift toward a more spherical cell shape of the remaining cells. At postnatal day 1 and 3, we also observed immunoreactive cells having the characteristic morphology of interplexiform cells. This cell type sends branches to both the outer and inner plexiform layers, thus a morphological basis for communication between the two developing plexiform layers is present as early as postnatal day 1. Thus, retinas from neonatal rabbits have a larger complement of cells that stain for endogenous GABA than does the adult. These results coupled with our previous studies suggest that GABAergic properties are expressed by a larger number of cell types in the neonate than in the adult and are consistent with the general hypothesis that GABA functions as a trophic agent during development.

Region-specific expression of GABAA receptor alpha 3 and alpha 4 subunits mRNAs in the rat brain

The expression of mRNAs encoding the alpha 3 and alpha 4 subunits of the gamma-aminobutyric acid A (GABAA) receptor in the rat brain was investigated by in situ hybridization histochemistry. Both subunits showed a wide but uneven distribution, which did not coincide with the distribution of any other subunit so far reported. The cerebral cortex, anterior olfactory nucleus, lateral septum, subiculum, lateral and medial nuclei of the amygdaloid complex, anterior nuclei of the thalamus, pars compacta of the substantia nigra, trigeminal sensory nuclei, and cochlear nucleus were some of the areas where strong expression of mRNA for both the alpha 3 and alpha 4 subunits was detected. In the mitral cell layer of the olfactory bulb, the preoptic area and locus coeruleus, strong expression of only the alpha 3 subunit was detected. In the granular cell layer of the olfactory bulb, caudate-putamen, tenia tecta, pyramidal cell layer of the CA region and granular cell layer of the dentate gyrus in the hippocampal formation, dorsomedial and ventrolateral nuclei of the thalamus, dorsal part of the lateral geniculate body, preolivary nuclei and pontine nuclei, only the alpha 4 subunit showed strong expression. The diverse distribution of these two subunits is considered to indicate that each has a different role in the central nervous system.

The postnatal development of benzodiazepine receptor sites in the chick optic lobe is modulated by environmental lighting

The present paper describes the ability of benzodiazepine receptor sites to undergo light mediated-plastic changes during the early postnatal development of the chick optic lobe. The postnatal development pattern of these receptors was studied under different levels of light stimulation, i.e. normal-, light-and dark-rearing. At hatching the specific binding of [3H]Flunitrazepam was 0.23 +/- 0.01 pmol/mg protein. The developmental profile shows a sharp and transient peak of receptor overexpression between the 1st and the 2nd postnatal day in three experimental groups. Between the 2nd and the 6th day significant differences were found between the three groups, being this difference maximal during the peak of overexpression. In fact, on the 2nd day the specific [3H]Flunitrazepam binding showed an increase of 17% (P < 0.0005) and a decrease of 34% (P < 0.0005) for light- and dark-reared animals as compared with normally-reared ones. The changes in receptor density were transient since from the 6th day onward they gradually disappeared, being almost identical in the three groups by the day 15. At this moment the number of benzodiazepine receptor sites stabilized at the adult level. Scatchard analysis at the 2nd postnatal day revealed that the differences observed in the high affinity benzodiazepine binding sites between the three groups were due to modifications in the total number of binding sites while the affinity remained unchanged. The maximal number of binding sites were: 2.76 +/- 0.03, 3.40 +/- 0.01 and 1.46 +/- 0.11 pmol/mg protein in normally-, light- and dark-reared chicks, respectively; while the apparent dissociation constants were unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

Different postnatal development profiles of neurons containing distinct GABAA receptor beta subunit mRNAs (beta 1, beta 2, and beta 3) in the rat forebrain

The expression of three beta subunit (beta 1, beta 2, and beta 3) mRNAs for gamma-aminobutyric acidA receptor in the postnatal rat forebrain was examined by in situ hybridization histochemistry with probes synthesized for the respective subunit mRNAs. The developmental expression of these subunit mRNAs conformed to one of three patterns. Pattern I was high expression of the mRNA at birth and a constant or increasing expression thereafter. In contrast, pattern II was no or very low expression of the mRNA at birth, with expression quickly increasing to reach the adult level in the early postnatal period. Pattern III was the transient expression of the subunit mRNA or else a marked decrease of its expression after a peak in the early postnatal period. On the basis of this classification, the expression of beta 3 subunit mRNA followed pattern I in most regions of the forebrain, such as the isocortex, the olfactory bulb and some of its related areas, the hippocampal formation, the amygdala, the septum, the bed nucleus of the stria terminalis, the caudate-putamen, the nucleus accumbens, the globus pallidus, the ventral pallidum, and the hypothalamus. In some areas, such as the magnocellular preoptic nucleus, the thalamus, and the subthalamic nucleus, pattern III was seen for this subunit. However, none of the regions of the brain showed pattern II expression of beta 3 subunit mRNA. In contrast, the expression of beta 1 and beta 2 subunit mRNAs followed pattern II in most regions of the forebrain. These included the expression of beta 1 subunit mRNA in the isocortex, the olfactory bulb, the hippocampal formation, the amygdala, the septum, the bed nucleus of the stria terminalis, the thalamus, and the hypothalamus, and the expression of beta 2 subunit mRNA in the isocortex, the olfactory bulb and some of its related areas, the amygdala, the nucleus of the diagonal band, the caudate-putamen, the thalamus, and the hypothalamus. Pattern I was not found for beta 1 subunit mRNA, although it was seen in some areas for beta 2 subunit mRNA, such as the ventral pallidum, the globus pallidus, and the magnocellular preoptic nucleus. On the other hand, pattern III was followed by beta 1 subunit mRNA in the anterior olfactory nucleus, the olfactory tubercle, and the piriform cortex, and the same pattern for the beta 2 subunit was also found in the olfactory tubercle, the hippocampal formation, the septum, the bed nucleus of the stria terminalis, and the nucleus accumbens.(ABSTRACT TRUNCATED AT 400 WORDS)

Different postnatal ontogenic profiles of neurons containing beta (beta 1, beta 2 and beta 3) subunit mRNAs of GABAA receptor in the rat thalamus

The postnatal ontogeny of neurons containing different GABAA receptor beta (beta 1, beta 2 and beta 3) subunit mRNAs were examined in the rat thalamus using in situ hybridization histochemistry. Neurons containing beta 1 or beta 2 subunit mRNA developed remarkably postnatally, while most neurons were already strongly labeled with beta 3 probe at birth. However, beta 3 subunit mRNA decreased rapidly after birth, few cells being labeled with this probe at day 35 and thereafter.