GABA influences the ultrastructure composition of cerebellar granule cells during development in culture

Extracellular molecular signals shaping dendrite architecture during brain development

Proper growth and branching of dendrites are crucial for adequate central nervous system (CNS) functioning. The neuronal dendritic geometry determines the mode and quality of information processing. Any defects in dendrite development will disrupt neuronal circuit formation, affecting brain function. Besides cell-intrinsic programmes, extrinsic factors regulate various aspects of dendritic development. Among these extrinsic factors are extracellular molecular signals which can shape the dendrite architecture during early development. This review will focus on extrinsic factors regulating dendritic growth during early neuronal development, including neurotransmitters, neurotrophins, extracellular matrix proteins, contact-mediated ligands, and secreted and diffusible cues. How these extracellular molecular signals contribute to dendritic growth has been investigated in developing nervous systems using different species, different areas within the CNS, and different neuronal types. The response of the dendritic tree to these extracellular molecular signals can result in growth-promoting or growth-limiting effects, and it depends on the receptor subtype, receptor quantity, receptor efficiency, the animal model used, the developmental time windows, and finally, the targeted signal cascade. This article reviews our current understanding of the role of various extracellular signals in the establishment of the architecture of the dendrites.

GABAergic control of neurite outgrowth and remodeling during development and adult neurogenesis: general rules and differences in diverse systems

During development, Gamma-aminobutyric acidergic (GABAergic) neurons mature at early stages, long before excitatory neurons. Conversely, GABA reuptake transporters become operative later than glutamate transporters. GABA is therefore not removed efficiently from the extracellular domain and it can exert significant paracrine effects. Hence, GABA-mediated activity is a prominent source of overall neural activity in developing CNS networks, while neurons extend dendrites and axons, and establish synaptic connections. One of the unique features of GABAergic functional plasticity is that in early development, activation of GABA_A receptors results in depolarizing (mainly excitatory) responses and Ca²⁺ influx. Although there is strong evidence from several areas of the CNS that GABA plays a significant role in neurite growth not only during development but also during adult neurogenesis, surprisingly little effort has been made into putting all these observations into a common framework in an attempt to understand the general rules that regulate these basic and evolutionary well-conserved processes. In this review, we discuss the current knowledge in this important field. In order to decipher common, universal features and highlight differences between systems throughout development, we compare findings about dendritic proliferation and remodeling in different areas of the nervous system and species, and we also review recent evidence for a role in axonal elongation. In addition to early developmental aspects, we also consider the GABAergic role in dendritic growth during adult neurogenesis, extending our discussion to the roles played by GABA during dendritic proliferation in early developing networks versus adult, well established networks.

Caveolae in fibroblast-like synoviocytes: static structures associated with vimentin-based intermediate filaments

The fibroblast-like synoviocyte is a CD13-positive cell-type containing numerous caveolae, both single and interconnected clusters. In unstimulated cells, all single caveolae at the cell surface and the majority of those localized deeper into the cytoplasm were freely accessible from the medium, as judged from electron microscopy of synoviocytes exposed to the membrane impermeable marker Ruthenium Red. Caveolar internalization could be induced by a CD13 antibody or by cholera toxin B subunit (CTB). Thus, in experiments using sequential labeling with Alexa 488- and 594-conjugated CTB, about 50% of CTB-positive caveolae were internalized by 5 min of chase, and these remained inaccessible from the cell surface for periods up to 24 h. No colocalization with an endosomal marker, EEA1, or Lysotracker was observed, indicating that internalized caveolae clusters represent a static compartment. Vimentin was identified as the most abundant protein in detergent resistant membranes (DRM's), and by immunogold electron microscopy caveolae were seen in intimate contact with intermediate-size filaments. These observations indicate that vimentin-based filaments are responsible for the spatio-temporal fixation of caveolae clusters. RECK, a glycosylphosphatidylinositol-anchored protein acting as a negative regulator of cell surface metalloproteinases, was also localized to the caveolae clusters. We propose that these clusters function as static reservoirs of specialized lipid raft domains where proteins involved in cell-cell interactions, such as CD13, can be sequestered by binding to RECK in a regulatory manner.

Demonstration of extensive GABA synthesis in the small population of GAD positive neurons in cerebellar cultures by the use of pharmacological tools

Cultures of dissociated cerebella from 7-day-old mice were maintained in vitro for 1-13 days. GABA biosynthesis and degradation were studied during development in culture and pharmacological agents were used to identify the enzymes involved. The amount of GABA increased, whereas that of glutamate was unchanged during the first 5 days and both decreased thereafter. The presence of aminooxyacetic acid (AOAA, 10 microM) which inhibits transaminases and other pyridoxal phosphate dependent enzymes including GABA-transaminase (GABA-T), in the culture medium caused an increase in the intracellular amount of GABA and a decrease in glutamate. The GABA content was also increased following exposure to the specific GABA-T inhibitor gamma-vinyl GABA. From day 6 in culture (day 4 when cultured in the presence of AOAA) GABA levels in the medium were increased compared to that in medium from 1-day-old cultures. Synthesis of GABA during the first 3 days was demonstrated by the finding that incubation with either [1-(13)C]glucose or [U-(13)C]glutamine led to formation of labeled GABA. Synthesis of GABA after 1 week in culture, when the enzymatic machinery is considered to be at a more differentiated level, was shown by labeling from [U-(13)C]glutamine added on day 7. Altogether the findings show continuous GABA synthesis and degradation throughout the culture period in the cerebellar neurons. At 10 microM AOAA, GABA synthesis from [U-(13)C]glutamine was not affected, indicating that transaminases are not involved in GABA synthesis and thus excluding the putrescine pathway. At a concentration of 5 mM AOAA GABA labeling was, however, abolished, showing that glutamate decarboxylase, which is inhibited at this level of AOAA, is responsible for GABA synthesis in the cerebellar cultures. In conclusion, the present study shows that GABA synthesis is taking place via GAD in a subpopulation of the cerebellar neurons, throughout the culture period.

Different chloride electrochemical gradients across the plasma membrane in subcellular compartments of rat cerebellum granules

The effects of GABA on intracellular Ca2+ have been studied in neonatal rat cerebellum granule cells (CGC) in culture by Oregon Green and two-photon excitation fluorescence microscopy. This technique allowed the study of [Ca2+]i both in cell bodies and neurites. Working with a perfusion chloride concentration corresponding to the average extracellular level, we found that GABA induced an increase in [Ca2+]i in the cell bodies in many of the cells studied with a maximum at day 4 in vitro. This effect disappeared after day 6. However, no increase in [Ca2+]i was ever found in neurites at standard [Cl-]e. On the other hand, an increase of [Ca2+]i was found also in neurites when [Cl-]e was close to zero. The [Ca2+]i increases were blocked by both bicuculline methiodide and nimodipine. The results indicate the presence of an outward directed electrochemical gradient for chloride in the cell bodies which results in depolarization by GABA via GABA(A) receptor activation. Calcium ion influx ensues due to activation of voltage-gated calcium channels (VGCC). This phenomenon may mediate the well-known trophic effect of GABA on these cells at this developmental stage, via an action of [Ca2+]i on the transcriptional activity of the nucleus. No calcium accumulation takes place in neurites due to either no or a reverse (hyperpolarizing) electrochemical gradient for chloride ions. Such a circumstance in later developmental stages may be of importance for the phasic component of GABA-mediated inhibition.

Role of calcium and kinases on the neurotrophic effect induced by gamma-aminobutyric acid

An increasing body of evidence supports a trophic action of gamma-aminobutyric acid (GABA) during nervous system development. The purported mediator of these trophic effects is a depolarizing response triggered by GABA, which elicits a calcium influx in immature CNS cells. This Mini-Review focuses on the neurotrophic role of neural activity and GABA and some of the most common intracellular cascades activated by depolarization and trophic factors. Several biological effects induced by GABA in the developing nervous system are reviewed, with particular emphasis on what is known about calcium-dependent neurotrophic effects induced by GABA and its intracellular mechanisms.

GABA-induced neurite outgrowth of cerebellar granule cells is mediated by GABA(A) receptor activation, calcium influx and CaMKII and erk1/2 pathways

During neuronal development, GABAA-mediated responses are depolarizing and induce an increase in the intracellular calcium concentration. Since calcium oscillations can modulate neurite outgrowth, we explored the capability of GABA to induce changes in cerebellar granule cell morphology. We find that treatment with GABA (1-1000 microm) induces an increase in the intracellular calcium concentration through the activation of GABA(A) receptors and voltage-gated calcium channels of the L-subtype. Perforated patch-clamp recordings reveal that this depolarizing response is due to a chloride reversal potential close to - 35 mV. When cells are grown in depolarizing potassium chloride concentrations, a shift in reversal potential (Erev) for GABA is observed, and only 20% of the cells are depolarized by the neurotransmitter at day 5 in vitro. On the contrary, cells grown under resting conditions are depolarized after GABA application even at day 8. GABA increases the complexity of the dendritic arbors of cerebellar granule neurons via a calcium-dependent mechanism triggered by voltage-gated calcium channel activation. Specific blockers of calcium-calmodulin kinase II and mitogen-activated protein kinase kinase (KN93 and PD098059) implicate these kinases in the intracellular pathways involved in the neuritogenic effect of GABA. These data demonstrate that GABA exerts a stimulatory role on cerebellar granule cell neuritogenesis through calcium influx and activation of calcium-dependent kinases.

Neurotransmitters, neuropeptides and calcium binding proteins in developing human cerebellum: a review

Many endogenous neurochemicals that are known to have important functions in the mature central nervous system have also been found in the developing human cerebellum. Cholinergic neurons, as revealed by immunoreactivities towards choline acetyltransferase or acetylcholinesterase, appear early at 23 weeks of gestation in the cerebellar cortex and deep nuclei. Immunoreactivities gradually increase until the first postnatal month. Enkephalin is localized in the developing cerebellum, initially in the fibers of the cortex and deep nuclei at 16-20 weeks and then also in the Purkinje cells, granule cells, basket cells and Golgi cells at 23 weeks onward. Another neuropeptide, substance P, is localized mainly in the fibers of the dentate nucleus from 9 to 24 weeks but substance P immunoreactivity declines thereafter. GABA, an inhibitory neurotransmitter of the central nervous system, starts to appear at 16 weeks in the Purkinje cells, stellate cells, basket cells, mossy fibers and neurons of deep nuclei. GABA expression is gradually upregulated toward term forming networks of GABA-positive fibers and neurons. Catecholaminergic fibers and neurons are also detected in the cortex and deep nuclei at as early as 16 weeks. Calcium binding proteins, calbindin D28K and parvalbumin, make their first appearance in the cortex and deep nuclei at 14 weeks and then their expression decreases toward term, while calretinin appears later at 21 weeks but its expression increases with fetal age. The above findings suggest that many neurotransmitters, neuropeptides and calcium binding proteins (1) appear early during development of the cerebellum; (2) have specific temporal and spatial expression patterns; (3) may have functions other than those found in the mature neural systems; and (4) may be able to interact with each other during early development.

Development of GABA, glycine, and their receptors in the auditory brainstem of gerbil: a light and electron microscopic study

Inhibitory synaptic transmission is known to play an important role during the maturation of central auditory pathways. While there is a lot of information on the modulatory role of glycine (Gly) on the postsynaptic target nuclei in the developing auditory brain stem, such a role for gamma-aminobutyric acid (GABA) in the lateral superior olive (LSO) of neonatal gerbil has been only recently reported (Kotak and Sanes [1997] Soc Neurosci Abst 23:1549; Kotak et al. [1998] J Neurosci 18:4646-4655). Here we present further immunohistochemical findings and the first ultrastructural evidence documenting a significant decrease in the postsynaptic localization of the beta2,3 subunit of the GABA(A) receptor from postnatal day (P)4 to P14 in the LSO of gerbil and the shift in the location of most of the staining from dendritic to astroglial over the same time course. There was a concomitant increase in staining for the Gly receptor (GlyR) anchoring protein, gephyrin. At the same time, GABA and Gly did not show a significant change in their staining pattern, suggesting that the transmitter levels are not particularly indicative of the inhibitory function in the neonatal gerbil LSO, but their receptors on the postsynaptic cells are. The observations of the present study suggest that the early GABAergic inhibition may be important in establishing appropriate synaptic contacts in the LSO of gerbil.

GABAA receptors mediate trophic effects of GABA on embryonic brainstem monoamine neurons in vitro

The inhibitory neurotransmitter GABA may act as a trophic signal for developing monoamine neurons in embryonic rat brain, because GABA neurons and their receptors appear in brainstem during generation of monoamine neurons. To test this hypothesis, we used dissociated cell cultures from embryonic day 14 rat brainstem, which contains developing serotonin (5-HT), noradrenaline (tyrosine hydroxylase; TH), and GABA neurons. Immunocytochemistry and reverse transcription-PCR (RT-PCR) revealed the presence of multiple alpha, beta, gamma, and delta subunits in these cultures. Competitive RT-PCR demonstrated high levels of beta3 subunit transcripts. Expression of functional GABAA receptors was demonstrated using 36Cl- flux assays. To investigate GABAergic regulation of neuronal survival and growth, cultures were treated for 1-3 d in vitro with 10 microM GABA and/or GABAA antagonist (bicuculline or the pesticide dieldrin). The effects of treatments were quantified by analysis of immunoreactive 5-HT, TH, and GABA neurons. GABAA receptor ligands differentially regulated neuronal survival and growth depending on neurotransmitter phenotype. GABA exerted positive effects on monoamine neurons, which were countered by bicuculline (and dieldrin, 5-HT neurons only). By itself, bicuculline produced inhibitory effects on both 5-HT and TH neurons, whereas dieldrin potently inhibited 5-HT neurons only. GABA neurons responded positively to both antagonists, but more strongly to bicuculline. Taken together, these results demonstrate that the activation/inhibition of GABAA receptors produces opposite effects on the development of embryonic monoamine and GABA neurons. This suggests that these neurotransmitter phenotypes may express GABAA receptors that differ in fundamental ways, and these differences determine the developmental responses of these cells to GABAergic stimuli.

Temporal patterns and depolarizing actions of spontaneous GABAA receptor activation in granule cells of the early postnatal dentate gyrus

Whole cell patch-clamp recordings were used to investigate the properties of the gamma-aminobutyric acid type A (GABAA) receptor-mediated spontaneous synaptic events in immature granule cells of the developing, early postnatal day (P0-P6) rat dentate gyrus. With Cs-gluconate-filled whole cell patch pipettes at 0 mV in control medium, spontaneous inhibitory postsynaptic currents (sIPSCs) occurred in prominent bursts (peak amplitude of the bursts 406.9 +/- 58.4 pA; intraburst IPSC frequency 71.0 +/- 12.4 Hz) at 0.05 +/- 0.02 Hz in every immature granule cell younger than P7. Between the bursts of IPSCs, lower frequency (1.7 +/- 0.7 Hz), interburst IPSCs could be observed. Bicuculline and picrotoxin as well as the intracellularly applied chloride-channel blockers CsF- and 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid (DIDS) abolished the intraburst as well as the interburst IPSCs, indicating that the IPSCs were mediated by GABAA receptor channels. The bursts of IPSCs, but not the interburst IPSCs, were blocked by the simultaneous application of the glutamate receptor antagonists 2-amino-5-phosphovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione, indicating the importance of the glutamatergic excitatory drive onto the interneurons in the early postnatal dentate gyrus. The spontaneously occurring excitatory postsynaptic currents in immature granule cells, observable after the intracellular blockade of GABAA receptor channels with CsF- and DIDS, appeared exclusively as single events at low frequencies, i.e., they did not occur in prominent bursts. Gramicidin-based perforated patch-clamp recordings determined that the reversal potential for the burst of IPSCs (-46.6 +/- 3.1 mV) was more depolarized than the resting membrane potential (-54.2 +/- 4.2 mV) but more hyperpolarized than the action potential threshold (-41. 8 +/- 1.7 mV). The depolarizing action of the bursts of synaptic events most often evoked only a single action potential per burst. Simultaneous whole cell patch recordings, with KCl-filled patch pipettes at -60 mV in current clamp from pairs of immature granule cells of the developing dentate gyrus, determined that the bursts of IPSPs took place in a similar temporal pattern but with imperfect synchrony in neighboring granule cells (average lag between the onsets of the bursts between granule cell pairs 77.7 +/- 8.6 ms). These results show that the spontaneous activation of GABAA receptors in immature dentate granule cells displays unique properties that are distinct from the temporal patterns and biophysical features of spontaneous GABAA receptor activation taking place in the developing Ammon's horn and in the adult dentate gyrus.

The ontogenic appearance of tyrosine hydroxylase-, serotonin-, gamma-aminobutyric acid-, calcitonin gene-related peptide-, substance P-, and synaptophysin-immunoreactivity in rat pituitary gland

The initial appearance of tyrosine hydroxylase (TH)-, serotonin (5-HT)-, gamma-aminobutyric acid (GABA)-, calcitonin gene-related peptide- (CGRP), substance P-, and synaptophysin-immunoreactivity in the rat pituitary gland, and in the related brain regions was investigated. Several groups of TH-immunoreactive neurons were first detected in the brain stem on day E17, and in the hypothalamus on day E18, followed by TH-immunoreactivity in the median eminence and infundibulum on E19-E20. TH-positive fibers appeared in the posterior lobe on day E20 and in the intermediate lobe on day P0. 5-HT-immunoreactivity was first detected on day E17 in neurons and nerve fibers in the brain stem and in the median eminence, respectively. On day E18, a few 5-HT-immunoreactive fibers were detected in the posterior lobe of the pituitary, although they were consistently seen in the infundibulum from day E19. In newborn rats, some 5-HT-immunoreactive fibers, but no neurons, were seen in the hypothalamus. GABA immunoreactivity appeared on day E17 in several nerve fibers of the infundibulum and the posterior lobe. Some neurons in the cortex and ventral hypothalamus transiently expressed GABA-immunoreactivity on day E17. In newborn rats, a plexus of GABA-immunoreactive fibers was detected for the first time in the intermediate lobe. No CGRP-immunoreactive fibers could be detected in the prenatal pituitary. On day P10, CGRP-immunoreactive fibers were first observed in the anterior lobe. Later their number considerably increased, while only sporadic fibers could be found in the intermediate or posterior lobes. No substance P-immunoreactivity could be detected in any of the lobes in the embryonic or developing postnatal rat pituitary, instead the adult anterior lobe occasionally showed some substance P-immunoreactive fibers. Synaptophysin-immunoreactivity was first detected in the posterior lobe on day E20, followed shortly by its expression in the intermediate lobe in newborn rats. The time course of GABA and 5-HT expression revealed in the present study suggests that these transmitters, which are initially expressed in the developing pituitary clearly before synaptic maturation, may act as trophic molecules during the prenatal period.

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.

Modulation of GAP-43 mRNA by GABA and glutamate in cultured cerebellar granule cells

Expression of GAP-43 in the cerebellum and selected regions of the brain has been shown to be developmentally regulated. Localization of GAP-43 mRNA within granule cells of the immature and mature rat cerebellum has been demonstrated by in situ hybridization. Higher levels are detected in the neonate compared to the adult. To determine if the cerebellar neurotransmitters, GABA (gamma-amino-butyric acid) and glutamate are involved in the modulation of GAP-43 expression, cultured cerebellar granule cells were exposed to these transmitters. Cultures were treated with glutamate, GABA, or the agonists/antagonists to their receptors in serum-free media for 5-7 days. Analysis of the levels of GAP-43 mRNA by in situ hybridization indicated that a 7-day exposure to GABA (25 and 50 microM) significantly lowered levels of granule cell GAP-43 mRNA. Specific agonists to the GABAA (muscimol) and GABAB (baclofen) receptors produced a decrease similar to that observed for GABA. Results from these studies also indicated that exposure to non-NMDA (CNQX) and NMDA (CPP, MK-801) glutamate receptor antagonists, and a metabotropic receptor glutamate agonist (ACPD), decreased the level of GAP-43 mRNA. The involvement of GABA and glutamate in the modulation of GAP-43 expression was corroborated by Northern hybridization. These studies revealed that a 5-day exposure to GABA decreased the cellular content of GAP-43 mRNA by 21% whereas exposure to glutamate resulted in a 37% increase. Findings from the studies reported here, using an in vitro cerebellar granule cell model, suggest that levels of GAP-43 mRNA, in vivo, are modulated by input from both excitatory glutamatergic mossy fibers and inhibitory GABAergic Golgi interneurons. Thus, modulation of GAP-43 mRNA by these neurotransmitters may influence granule cell maturation during development in the neonate and neuroplasticity in the adult, possibly at the parallel fiber-Purkinje cell synapse.

A ventrodorsal GABA gradient in the embryonic retina prior to expression of glutamate decarboxylase

GABA is known to function as a neurotransmitter in the mature nervous system, and in immature neurons it has been linked to neurotrophic actions. While most GABA is generated by glutamate decarboxylase (GAD), an alternative synthetic pathway is known to originate from putrescine, which is converted via gamma-aminobutyraldehyde in an aldehyde-dehydrogenase-requiring step to GABA. In a search for the role of two aldehyde dehydrogenases expressed in segregated compartments along the dorsoventral axis of the developing retina, we assayed dorsal and ventral retina fractions of the mouse for GABA by high performance liquid chromatography. We found GABA to be present in the embryonic retina, long before expression of GAD, and ventral GABA levels exceeded dorsal levels by more than three-fold. Postnatally, when GAD became detectable, overall GABA levels increased, and the ventrodorsal concentration difference disappeared. Our observations indicate that prior to the formation of synapses the embryonic retina contains a ventrodorsal GABA gradient generated by an alternate synthetic pathway.

Slow kinetics of miniature IPSCs during early postnatal development in granule cells of the dentate gyrus

Whole-cell patch-clamp recordings were used to investigate the properties of GABAA receptor-mediated postsynaptic currents during development in dentate gyrus granule cells from neonatal [postnatal day 0 (P0)] to adult rats in brain slices. The frequency of miniature IPSCs (mIPSCs) was low at birth and increased progressively with age. The mIPSCs of all ages could be satisfactorily fitted with the sum of a single exponential rise and single exponential decay. From P0 to P14, both the rise time and the decay time constants were significantly longer than in the adult. The mIPSC rise and decay kinetics did not change during the first 2 postnatal weeks, but during the third week the kinetics sped up and by P21 attained adult values. In contrast, the amplitude of the mIPSCs did not change during development. The synaptic GABAA receptors in immature and adult cells showed differential sensitivity to modulators. The subunit-specific benzodiazepine agonist zolpidem increased the decay time constant of the IPSCs of immature granule cells with a reduced potency compared with the adult. Furthermore, zinc decreased the amplitude and decay time constant of mIPSCs from developing granule cells, whereas it had no effect on mIPSCs in adult neurons. The results reveal for the first time that until the end of the second postnatal week the synaptic GABAA receptor-mediated currents in dentate granule cells display slower rise and decay kinetics but similar amplitudes compared with adult, resulting in a net decrease in synaptic charge transfer during development.

Temporal and spatial expression of glutamic acid decarboxylases in human fetal brain

The expression of two isoforms of glutamic acid decarboxylase, GAD67 and GAD65, was analyzed in central nervous system (CNS) tissues obtained from normal second trimester human fetuses after elective termination of pregnancy. After RT-PCR amplification of sequences contained in total RNA extracts, Southern blotting indicated that GAD67 and GAD65 mRNAs can be detected in frontal pole tissue as early as the 12th week of gestation (12 GW). GAD67 message is strongly expressed during early second trimester and decreases slightly thereafter but remains abundant. In contrast, GAD65 message decreases rapidly and becomes undetectable by the 19 GW. However, GAD67 and GAD65 are similar in their spatial expression in the CNS at 22 GW. GAD67 and GAD65 messages are highly expressed in the cerebellum but expressed in low levels, if at all, in the spinal cord during this gestational period. These results suggest that GAD67 may have a greater role in neuron differentiation than GAD65 during human brain development.

Effects of nefiracetam on amnesia animal models with neuronal dysfunctions

The effects of N-(2,6-dimethylphenyl)-2-(2-oxo-1-pyrrolidinyl) acetamide (nefiracetam; DM-9384), on learning and memory in several amnesia animal models with neuronal dysfunctions were investigated. Nefiracetam improved scopolamine-, bicuculline-, picrotoxin-, ethanol-, chlordiazepoxide- and cycloheximide-induced amnesia. Anti-amnesic action of nefiracetam on scopolamine model was antagonized by nifedipine and flunarizine, but not by diltiazem. Repeated administration of nefiracetam to AF64A-treated animals improved impairment of learning and memory as well as the alterations in cholinergic and monoaminergic neurotransmitters in the hippocampus. Basal forebrain (BF) lesioned rats induced by excitotoxin or by thermal coagulation showed impairment of learning accompanied by a marked reduction in choline acetyltransferase (ChAT) and acetylcholine esterase activities. Nefiracetam improved the learning deficit of the BF-lesioned rats. Nefiracetam also improved the carbon monoxide-induced delayed and acute amnesia. Nefiracetam stimulated acetylcholine release in the frontal cortex. Repeated administration of nefiracetam increased ChAT activity, gamma-aminobutyric acid (GABA) turnover and glutamic acid decarboxylase activity, and facilitated the Na(+)-dependent high-affinity GABA uptake. Nefiracetam activated the high voltage-activated (N/L-type) Ca2+ channel. The dose-response curves of nefiracetam were bell-shaped in both behavioral and biochemical studies. Therefore, it is suggested that nefiracetam improves the dysfunction of cholinergic, GABAergic and/or monoaminergic neuronal function by acting at Ca2+ channel and enhancing the release of neurotransmitters, and modifies impairment of memory processes induced by drugs and hypoxia.

Biphasic response of spinal GABAergic neurons after a lumbar rhizotomy in the adult rat

The expression of gamma-aminobutyric acid (GABA) and of the isoforms of the enzyme involved in its synthesis, glutamic acid decarboxylase (GAD), is modified in several rat brain structures in different injury models. The aim of the present work was to determine whether such plasticity of the GABAergic system also occurred in the deafferented adult rat spinal cord, a model where a major reorganization of neural circuits takes place. GABAergic expression following unilateral dorsal rhizotomy was studied by means of non-radioactive in situ hybridization to detect GAD67 mRNA and by immunohistochemistry to detect GAD67 protein and GABA. Three days following rhizotomy the number of GAD67 mRNA-expressing neurons was decreased in the superficial layers of the deafferented horn, while GABA immunostaining of axonal fibres located in this region was highly increased. Seven days after lesion, on the other hand, many GAD67 mRNA-expression neurons were bilaterally detected in deep dorsal and ventral layers, this expression being correlated with the increased detection of GAD67 immunostained somata and with the reduction of GABA immunostaining of axons. GABA immunostaining was frequently found to be associated with reactive astrocytes that exhibited intense immunostaining for glial fibrillary acidic protein (GFAP) but remained GAD67 negative. These results indicate that degeneration of afferent terminals induces a biphasic response of GABAergic spinal neurons located in the dorsal horn and show that many spinal neurons located in deeper regions re-express GAD67, suggesting a possible participation of the local GABAergic system in the reorganization of disturbed spinal networks.

Prenatal binge-like alcohol exposure alters neurochemical profiles in fetal rat brain

The majority of studies examining the effects of prenatal exposure to alcohol on neurotransmitter levels have furnished results that are divergent (increase, decrease or no change). The present study assessed six neurochemical compounds [norepinephrine (NE), dopamine (DA), dihydroxyphenylacetic acid (DOPAC), serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA), gamma-aminobutyric acid (GABA)] from the same brain tissue. Pregnant Sprague-Dawley rats were given 5.1 g/kg alcohol (by gavage) either daily from embryonic day 1 (E1) through E20 or E20 only. In addition, pairfed/intubated (PF/INT) and ad lib chow (Chow) groups were included as controls. The dams were sacrificed and the fetuses were removed on E20. Binge-like alcohol exposure throughout gestation (E1-E20) produced significantly higher brain to body weight ratios compared with all other groups. Alcohol exposure did not produce changes in NE levels, although the E1-E20 exposure to alcohol reduced the contents of DA and 5-HT compared with the PF/INT and Chow controls. In addition, the E20 alcohol treatment reduced both DA and 5-HT levels compared with the E1-E20 alcohol treatment. DOPAC and 5-HIAA contents were affected by the prenatal treatments insofar as the 5-HIAA levels were decreased in E/1-20 and E20 animals relative to both controls, while the DOPAC levels were decreased in E/1-20, E20 and PF/INT groups compared to the Chow group; however, both metabolites were unaffected by the difference in alcohol treatment duration. Moreover, GABA levels were increased in fetuses exposed to alcohol from E1-E20 compared with all other groups. Collectively, these findings suggest that binge-like alcohol exposure prior to and during neurotransmitter development affects the baseline content of several neurotransmitters.

Activity-independent segregation of excitatory and inhibitory synaptic terminals in cultured hippocampal neurons

Cultured hippocampal neurons were used as a model system to address experimentally the spatial and temporal sequence leading to the appropriate sorting of excitatory and inhibitory synaptic terminals to different cellular target domains and the role of neural activity in this process. By using antibodies against glutamic acid decarboxylase 65 (GAD65) and synaptophysin, we examined the development and segregation of GABAergic and non-GABAergic synaptic terminals on single neurons. Electron microscopy confirmed that GAD65-labeled swellings observed using light microscopy corresponded to synaptic boutons. From the time at which GABAergic terminals first appeared, they developed at a more rapid rate on neuronal somata than non-GABAergic terminals did, such that by 18 d in culture, 60% of the total boutons on somata were GABAergic. By contrast, the majority (70%) of boutons on dendrites were non-GABAergic. These data suggest that inhibitory synaptic terminals are targeted preferentially to or maintained on cell somata at the expense of excitatory terminals. Interestingly, non-GABAergic terminals were not inhibited from forming synapses on cell somata, because in the absence of GABAergic terminals they attained the same total somatic terminal density seen in the presence of GABAergic terminals. Chronic blockade of neuronal activity did not affect the differential targeting of GABAergic and non-GABAergic axons; however, it did reduce the extent of dendritic arborization. Our findings support a two-step model for synaptic segregation whereby the majority of terminals is initially targeted in an activity-independent manner to the appropriate cellular domains, but an additional developmental mechanism serves to further restrict and refine the original synaptic distribution.

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.

Basal expression and induction of glutamate decarboxylase and GABA in excitatory granule cells of the rat and monkey hippocampal dentate gyrus

The excitatory, glutamatergic granule cells of the hippocampal dentate gyrus are presumed to play central roles in normal learning and memory, and in the genesis of spontaneous seizure discharges that originate within the temporal lobe. In localizing the two GABA-producing forms of glutamate decarboxylase (GAD65 and GAD67) in the normal hippocampus as a prelude to experimental epilepsy studies, we unexpectedly discovered that, in addition to its presence in hippocampal nonprincipal cells, GAD67-like immunoreactivity (LI) was present in the excitatory axons (the mossy fibers) of normal dentate granule cells of rats, mice, and the monkey Macaca nemestrina. Using improved immunocytochemical methods, we were also able to detect GABA-LI in normal granule cell somata and processes. Conversely, GAD65-LI was undetectable in normal granule cells. Perforant pathway stimulation for 24 hours, which evoked population spikes and epileptiform discharges in both dentate granule cells and hippocampal pyramidal neurons, induced GAD65-, GAD67-, and GABA-LI only in granule cells. Despite prolonged excitation, normally GAD- and GABA-negative dentate hilar neurons and hippocampal pyramidal cells remained immunonegative. Induced granule cell GAD65-, GAD67-, and GABA-LI remained elevated above control immunoreactivity for at least 4 days after the end of stimulation. Pre-embedding immunocytochemical electron microscopy confirmed that GAD67- and GABA-LI were induced selectively within granule cells; granule cell layer glia and endothelial cells were GAD- and GABA-immunonegative. In situ hybridization after stimulation revealed a similarly selective induction of GAD65 and GAD67 mRNA in dentate granule cells. Neurochemical analysis of the microdissected dentate gyrus and area CA1 determined whether changes in GAD- and GABA-LI reflect changes in the concentrations of chemically identified GAD and GABA. Stimulation for 24 hours increased GAD67 and GABA concentrations sixfold in the dentate gyrus, and decreased the concentrations of the GABA precursors glutamate and glutamine. No significant change in GAD65 concentration was detected in the microdissected dentate gyrus despite the induction of GAD65-LI. The concentrations of GAD65, GAD67, GABA, glutamate and glutamine in area CA1 were not significantly different from control concentrations. These results indicate that dentate granule cells normally contain two "fast-acting" amino acid neurotransmitters, one excitatory and one inhibitory, and may therefore produce both excitatory and inhibitory effects. Although the physiological role of granule cell GABA is unknown, the discovery of both basal and activity-dependent GAD and GABA expression in glutamatergic dentate granule cells may have fundamental implications for physiological plasticity presumed to underlie normal learning and memory. Furthermore, the induction of granule cell GAD and GABA by afferent excitation may constitute a mechanism by which epileptic seizures trigger compensatory interictal network inhibition or GABA-mediated neurotrophic effects.

Role of GABAA receptors in the GABA attenuation of ethanol neurotoxicity

We have previously reported that GABA reverses the neuronotoxic effects of ethanol in neuroblast-enriched cultures derived from 3-day-old whole chick embryo (E3WE). In the present study, we examined the effects of GABA agonists and antagonists on morphological growth patterns and on cholinergic neuronal phenotypic expression, using choline acetyltransferase (ChAT) activity as a marker. E3WE neuroblast-enriched cultures showed positive immunoreactivity for neurofilament and as previously reported, control cultures exhibited the characteristic pattern of outgrowth of neurites of varying thickness radiating from the aggregates. In contrast, cultures grown in ethanol consisted of neuronal aggregates lacking fasciculation but having a complex network of individual thin neurites. Both GABA and GABAA agonist muscimol enhanced neuritic fasciculation and arborization in control and ethanol-treated cultures, and this growth enhancement was inhibited by GABAA antagonist bicuculline. No effects were noted with GABAB agonist baclofen. GABA increased ChAT activity in E3WE control cultures, as previously reported. A similar effect was seen with GABAA agonist muscimol, but not with GABAB agonist baclofen. However, the GABA effect was not apparent in the presence of GABAB antagonist phaclofen. Thus, it appears that the cholinotrophic effects of GABA are mediated by both GABAA and GABAB receptors. In ethanol-treated cultures the already-reported ChAT decline was reversed by GABA and muscimol, but not by baclofen. Moreover, the GABA effect in ethanol-treated cultures was not antagonized by GABAB antagonist phaclofen, suggesting that the GABA effect was mediated by a GABAA receptor. We conclude from these findings that the cholinotrophic effects of GABA are mediated by GABAA and GABAB receptors, while the rescuing effects of GABA in the ethanol-treated cultures are mediated via GABAA receptors.

GABA-induced increases in [Ca2+]i in retinal neurons of postnatal rabbits

Previous studies have indicated that gamma-aminobutyric acid (GABA) plays an important trophic role in the synapse formation between horizontal cells and photoreceptors in postnatal rabbit retina. However, the mechanism of the GABA effect has not been identified. Using fluo-3 Ca2+ imaging and confocal laser scanning microscopy we examined the effect of GABA on [Ca2+]i during postnatal retinal development. GABA (100 microM) evoked a fast and transient increase of [Ca2+]i in selected populations of freshly dissociated retinal cells from postnatal rabbits. This increase was apparent on postnatal day 1 and reached a maximum on day 5. Little increase in [Ca2+]i was observed in retinal cells isolated from adult rabbits. GABA receptor antagonists, picrotoxin and bicuculline, significantly reduced the response. The GABAB agonist, baclofen, did not evoke any [Ca2+]i changes. The GABA-induced increase in [Ca2+]i was observed in all retinal layers in neonatal retinal whole-mount explants. In the outer retina, the increase was seen in cone photoreceptors which were specifically labeled with peanut agglutinin (PNA). The GABA-induced increase in [Ca2+]i may provide an important mechanism for regulating cone synaptogenesis in the outer plexiform layer of the postnatal retina.

Effects of the nootropic drug nefiracetam on the GABAA receptor-channel complex in dorsal root ganglion neurons

The effects of nefiracetam on GABA-induced chloride currents were studied with rat dorsal root ganglion neurons in primary culture using the whole-cell patch-clamp technique. The dose-response curve for GABA-induced currents was shifted by 16 microM to lower concentrations by 10 microM nefiracetam while the maximal response was reduced by 22.84 +/- 0.68%. Thus at a low concentration (10 microM) of GABA, the chloride currents were potentiated by nefiracetam in a concentration-dependent manner. With 10 microM nefiracetam, the potentiation occurred slowly and the recovery after washout was also slow. The desensitization of the GABAA receptor at high concentration (100 microM) of GABA was accelerated by nefiracetam. The recovery process of chloride currents from desensitization was not affected by nefiracetam. KT 5720 (0.56 microm), a specific protein kinase A (PKA) inhibitor, blocked the transient potentiation of GABA-activated currents by nefiracetam, but did not affect the acceleration of desensitization. Nefiracetam suppression of GABA-induced currents was also abolished by KT 5720 or the pertussis toxin. Thus, nefiracetam may inhibit Gi/G(o) proteins leading to a cascade of events that increase the intracellular cAMP level, activate the PKA system, and suppress GABA-induced currents. Nefiracetam-induced transient potentiation and acceleration of desensitization of GABA-induced currents may involve other pathways. The nefiracetam modulation of the GABAA receptor function will result in a nootropic effect on the central nervous system through modification of synaptic transmission.

Immunohistochemical studies of GABA and parvalbumin in the developing human cerebellum

The localization of GABA and parvalbumin was studied in the developing cerebellum of human fetuses from 16 to 28 weeks of gestation. The avidin-biotin complex immunohistochemical method combined with silver staining were used to reveal the presence of GABA- and parvalbumin-positive neurons and nerve fibres. As early as the 16th week of gestation, GABA immunopositivity was observed in the cerebellar cortex and the deep nuclei. GABA-positive neurons included Purkinje cells, stellate and basket cells of the cerebellar cortex and neurons in the deep nuclei. The gradient of immunoreactivity increased with the maturing cells, being weak at 16 weeks and becoming markedly pronounced at 28 weeks of gestation. GABA-immunopositive mossy fibres were observed in the granular cell layer at 16 weeks, and by 28 weeks, a robust fibre network was present in the cortex and deep nuclei. Immunohistochemical localization for parvalbumin indicates that weak immunoreactivity was observed in Purkinje cells, stellate and basket cells at 16 weeks of gestation, increasing in intensity with advancing age, notably in the Purkinje cells which had acquired an elaborate arbor of neurites at 28 weeks of gestation. In the deep nuclei, parvalbumin-positive cells and nerve fibres were observed throughout the 16 to 28 week period. These results indicate that GABA- and parvalbumin-positive neurons and fibres appeared as early as 16 weeks of gestation, expressing a high degree of immunoreactivity by the 28 week of fetal age.

GABA and glutamate depolarize cortical progenitor cells and inhibit DNA synthesis

We have found that, during the early stages of cortical neurogenesis, both GABA and glutamate depolarize cells in the ventricular zone of rat embryonic neocortex. In the ventricular zone, glutamate acts on AMPA/kainate receptors, while GABA acts on GABAA receptors. GABA induces an inward current at resting membrane potentials, presumably owing to a high intracellular Cl- concentration maintained by furosemide-sensitive Cl- transport. GABA and glutamate also produce increases in intracellular Ca2+ in ventricular zone cells, in part through activation of voltage-gated Ca2+ channels. Furthermore, GABA and glutamate decrease the number of embryonic cortical cells synthesizing DNA. Depolarization with K+ similarly decreases DNA synthesis, suggesting that the neurotransmitters act via membrane depolarization. Applied alone, GABAA and AMPA/kainate receptor antagonists increase DNA synthesis, indicating that endogenously released amino acids influence neocortical progenitors in the cell cycle. These results demonstrate a novel role for amino acid neurotransmitters in regulating neocortical neurogenesis.

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

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

Polyamines and ripening of photoreceptor outer segments in chicken embryos

Polyamines and their related monoacetyl derivatives were studied in rod outer segment (ROS) and cone outer segment (COS) of photoreceptor cells from chick embryo retina during eye development (7th-18th days). Putrescine was found to be necessary, in the second phase of retinogenesis, to sustain both ROS and COS differentiation and, after acetylation, gamma-aminobutyric acid synthesis. On the other hand, spermidine and even more spermine intervene in the third phase of development when photoreceptors mature. Moreover, the presence of N1-acetylspermidine already at the 7th day indicates that in the outer segment of photoreceptor cells too, as in the whole retina, putrescine synthesis comes about by two pathways. One pathway involves ornithine decarboxylase; the other, spermidine/spermine N1-acetyltransferase and FAD-dependent polyamine oxidase activities that convert spermidine to putrescine via N1-acetylspermidine. These different biosynthetic pathways are probably also decisive in permitting gamma-aminobutyric acid synthesis, which is very important in the ripening process of neural retina.

Ethanol neurotoxicity on neuroblast-enriched cultures from three-day-old chick embryo is attenuated by the neuronotrophic action of GABA

In the present study, using neuroblast-enriched cultures derived from three-day-old chick embryos (E3WE), we examined the morphological effects of ethanol and/or GABA, as well as the developmental profile of the cholinergic and GABAergic neuronal phenotypes, as assessed by the activities of choline acetyltranferase (ChAT) and glutamate decarboxylase (GAD). Cultures exposed to ethanol (50 mM) exhibited smaller and fewer aggregates than controls with a neuritic network that lacked fasciculation. In cultures treated with GABA (10(-5) M) alone or ethanol+GABA the size and number of the neuronal aggregates was increased and also neuritic arborization and fasciculation was enhanced. Thus, addition of GABA restored the normal growth pattern in the ethanol-treated cultures. As previously shown, E3WE culture treated with ethanol alone showed a decrease in both ChAT and GAD activities compared to controls. Both cholinergic and GABAergic neuronal phenotypes were enhanced in cultures treated with GABA as assessed by increases in ChAT and GAD activities, respectively, compared to controls. Moreover, in cultures treated concomitantly with ethanol and GABA both ChAT and GAD activities were higher than in ethanol-alone-treated cultures. Thus, the presence of GABA in the ethanol-treated cultures counteracted the decline in ChAT and GAD activities observed in the ethanol-alone-treated cultures. We conclude that GABA through its neuronotrophic actions can rescue neuroblasts from ethanol insult and restore neuronal phenotypes.

NGF mRNA is expressed by GABAergic but not cholinergic neurons in rat basal forebrain

Nerve growth factor (NGF) supports the survival and biosynthetic activities of basal forebrain cholinergic neurons and is expressed by neurons within lateral aspects of this system including the horizontal limb of the diagonal bands and magnocellular preoptic areas. In the present study, colormetric and isotopic in situ hybridization techniques were combined to identify the neurotransmitter phenotype of the NGF-producing cells in these two areas. Adult rat forebrain tissue was processed for the colocalization of mRNA for NGF with mRNA for either choline acetyltransferase, a cholinergic cell marker, or glutamic acid decarboxylase, a GABAergic cell marker. In both regions, many neurons were single-labeled for choline acetyltransferase mRNA, but cells containing both choline acetyltransferase and NGF mRNA were not detected. In these fields, virtually all NGF mRNA-positive neurons contained glutamic acid decarboxylase mRNA. The double-labeled cells comprised a subpopulation of GABAergic neurons; numerous cells labeled with glutamic acid decarboxylase cRNA alone were codistributed with the double-labeled neurons. These data demonstrate that in basal forebrain GABAergic neurons are the principal source of locally produced NGF.

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.

GABA attenuates the neurotoxic effects of ethanol in neuron-enriched cultures from 8-day-old chick embryo cerebral hemispheres

Neuron-enriched cultures were prepared from 8-day-old chick embryo cerebral hemispheres and exposed to ethanol (50 mM), GABA (10(-5) M) and ethanol (50 mM) + GABA (10(-5) M) from day 4 to 8 in culture. At day 8, control, ethanol, GABA and ethanol + GABA-treated cultures were examined morphologically and biochemically. Choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD) activities were used as markers for cholinergic and GABAergic neuronal phenotypic expression, respectively. Control cultures showed more numerous and large neuronal aggregates as well as prominent neuritic bundles. Moreover, cultures treated with GABA depicted even more numerous neuronal aggregates with interconnecting neurites as compared to control. In contrast, ethanol-treated cultures exhibited smaller neuronal aggregates with less prominent neuritic bundles than control. However, cultures treated concomitantly with ethanol + GABA exhibited numerous and larger aggregates than cultures treated with ethanol alone. Neuritic bundles which were highly reduced in ethanol-treated cultures became prominent in the presence of GABA. As previously reported, ethanol alone enhanced ChAT and reduced GAD activities. GABA given alone enhanced the expression of both neuronal phenotypes. When GABA was given concomitantly with ethanol the decline in GAD and the rise in ChAT observed in ethanol-treated cultures was restored by GABA to almost control levels. Thus, ethanol-induced alterations in morphology and neuronal phenotypes were counteracted by the neurontrophic effect of GABA.

Induction of low-affinity GABAA receptors by the GABA-agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) in cultured rat cerebellar granule cells is prevented by inhibition of polyamine biosynthesis

GABAA agonist-induced formation of low-affinity GABAA receptors in cultured cerebellar granule cells was studied in the presence or absence of alpha-difluoromethylornithine (DFMO), a blocker of polyamine formation. High- and low-affinity GABAA receptors were monitored by Scatchard analysis of [3H]GABA binding to membranes from cells cultured for either 4 or 10 days in the presence or absence of the GABA agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP). Cultures grown for 4 days were exposed to THIP and DFMO for an additional period of 6 hr (acute exposure), whereas cultures grown for 10 days were exposed to the same agents during the entire culture period (chronic exposure). Regardless of the culture period or drug exposure protocol, control cells expressed only a high-affinity (KD 7 nM) binding site for GABA, whereas the cultures treated with THIP for either 6 hr or 10 days exhibited an additional low-affinity binding site (KD approximately 500 nM). Chronic exposure to DFMO prevented the THIP induction of low-affinity GABAA receptors, whereas acute exposure to DFMO had no effect on the ability of THIP to induce low-affinity GABAA receptors. Measurements of the intracellular polyamine concentration demonstrated a slight decrease in the putrescine level in the granule cells exposed to DFMO or THIP + DFMO for 6 hr. In contrast, granule cells chronically (10 days) exposed to DFMO or THIP + DFMO were depleted of putrescine and spermidine. Hence, the ability of THIP to induce low-affinity GABAA receptors was prevented by the simultaneous depletion of the cellular content of putrescine and spermidine, whereas inhibition of ornithine decarboxylase and of putrescine formation was not sufficient to prevent THIP-induced receptor formation.

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.

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.

Identification and function of glycine receptors in cultured cerebellar granule cells

Poly(A)+ mRNA was isolated from cultured mouse cerebellar granule cells and injected into Xenopus oocytes. This led to the expression of receptors that evoked large membrane currents in response to glycine. Current-responses were also obtained after application of beta-alanine and taurine, but these were very low relative to that of glycine (maximal beta-alanine and taurine responses were 8 and 3% of that of glycine, respectively). The role of glycine receptors on K(+)-evoked transmitter release in cultured cerebellar granule cells was also assayed. Release of preloaded D-[3H]aspartate evoked by 40 mM K+ was dose dependently inhibited by glycine, and the concentration producing half-maximal inhibition was 50 microM. Taurine, beta-alanine, and the specific GABAA receptor agonist isoguvacine also inhibited K(+)-evoked release, and the maximal inhibition was similar for all agonists (approximately 40%). The EC50 value was 200 microM for taurine, 70 microM for beta-alanine, and 4 microM for isoguvacine. Bicuculline (150 microM) antagonized the inhibitory effect of isoguvacine (150 microM) but not that of glycine (1 mM). In contrast, strychnine (20 microM) antagonized the inhibitory effect of glycine (1 mM) but not that of isoguvacine (150 microM). The pharmacology of the responses to beta-alanine and taurine showed that these agonists activate both glycine and GABAA receptors. The results indicate that cultured cerebellar granule cells translate the gene for the glycine receptor and that activation of glycine receptors produces neuronal inhibition.

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

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

Analysis of the anatomical distribution of GAD67 mRNA encoding truncated glutamic acid decarboxylase proteins in the embryonic rat brain

During development of the central nervous system (CNS) the gene that encodes the 67 kDa form of glutamic acid decarboxylase (GAD) undergoes alternative splicing. The alternatively spliced variants include an exon (referred to as ES, for embryonic stop) that contains a premature stop codon. The detection of mRNA containing the ES exon in embryonic rat brain has been previously reported (Proc. Natl. Acad. Sci., 87 (1990) 8771-8775). We have used in situ hybridization to identify the anatomical distribution of ES mRNA in the embryonic rat brain during two stages of development, embryonic day 17 (E17) and E20. At E17, GAD67 mRNA was expressed in several CNS regions that were destined to contain GABAergic neurons when mature. ES transcripts were predominantly localized to ventricular zones and other regions associated with populations of proliferative cells at E17 and E20. At both ages, however, the alternatively spliced variants were also detected in regions of brain associated with migratory or post-mitotic neurons. GAD67 transcripts that did not include the ES exon were localized to anatomical areas that contained post-mitotic, and often post-migratory neurons. The temporal and spatial disappearance of mRNA containing the ES exon generally followed a caudal-to-rostral gradient which paralleled neuronal terminal mitosis and differentiation.

Transient presence of GABA in astrocytes of the developing optic nerve

Immunostaining and high-pressure liquid chromatography (HPLC) were used to study the developmental time course of astrocytic gamma-aminobutyric acid (GABA) expression in rat optic nerve. GABA immunostaining was carried out on cultured astrocytes, and on whole optic nerve. Confocal scanning laser microscopy was used to obtain optical sections in excised whole tissue in order to localize the cellular origins of GABA within the relatively intact optic nerve. GABA immunoreactivity was localized in astrocytes identified by GFAP staining; GABA staining was most intense in early neonatal optic nerve and attenuated over 3 weeks of postnatal development. The staining was pronounced in the astrocyte cell bodies and processes but not in the nucleus. There was a paucity of GABA immunoreactivity by postnatal day 20, both in culture and in whole optic nerve. A biochemical assay for optic nerve GABA using HPLC indicated a relatively high concentration of GABA in the neonate, which rapidly attenuated over the first 3 postnatal weeks. Immunoreactivity for the GABA synthesis enzyme glutamic acid decarboxylase (GAD) was pronounced in neonates but also attenuated with development. These results indicate that GABA and the GABA synthesis enzyme GAD are localized in astrocytes of optic nerve, and that their expression is transient during postnatal development.

Neurotrophic effects of NMDA receptor activation on developing cerebellar granule cells

Glutamate acting on N-methyl-D-aspartate (NMDA) receptors controls a variety of aspects of neuronal plasticity in the adult and developing brain. This review summarizes its effects on developing cerebellar granule cells. The glutamatergic mossy fibre input to cerebellar granule cells exerts a neurotrophic effect on these cells during development. The investigation of potential neurotrophic agents can be carried out using enriched granule cell cultures. Considerable evidence now indicates that glutamate acting on N-methyl-D-aspartate receptors is an important neurotrophic factor that regulates granule cell development. In culture, neurite growth, differentiation and cell survival are all stimulated by N-methyl-D-aspartate receptor activation. The intracellular pathways involved following Ca2+ entry through the N-methyl-D-aspartate receptor channel are beginning to be elucidated. The cerebellar granule cell culture system may provide an ideal model to investigate the molecular mechanisms involved in long term N-methyl-D-aspartate receptor-mediated changes in neuronal function.

Nefiracetam (DM-9384) preserves hippocampal neural cell adhesion molecule-mediated memory consolidation processes during scopolamine disruption of passive avoidance training in the rat

Scopolamine (0.15 mg/kg), a muscarinic antagonist, when administered during training or at a discrete 6-h posttraining time point, is demonstrated to inhibit the recall of a step-down passive avoidance response when tested at 24 and 48 h after task acquisition. Nefiracetam (3 mg/kg), a piracetam-related nootropic, when given with scopolamine during training tended to improve task recall, and this effect was more pronounced when given at the 6-h posttraining time. Co-administration of nefiracetam with scopolamine was not necessary to achieve the antiamnesic action, as nefiracetam given during training significantly improved the memory deficits produced by scopolamine at the 6-h posttraining time. The paradigm-specific increase in hippocampal neural cell adhesion molecule sialylation, which is observed during consolidation of a passive avoidance response, was attenuated by the presence of scopolamine during training and at the 6-h posttraining time, and this effect was reversed by co-administration of nefiracetam, albeit in a paradigm-independent manner. These results suggest nefiracetam exerts a neurotrophic action that protects memory consolidation from drug interventive insults.

Depolarization by K+ and glutamate activates different neurotransmitter release mechanisms in GABAergic neurons: vesicular versus non-vesicular release of GABA

Neurotransmitter release and changes in the concentration of intracellular free calcium ([Ca++]i) were studied in cultured GABAergic cerebral cortical neurons, from mice, upon depolarization with either an unphysiologically high potassium concentration (55 mM) or the physiological excitatory neurotransmitter glutamate (100 microM). Both depolarizing stimuli exerted prompt increases in the release of preloaded [3H]GABA as well as in [Ca++]i. However, the basic properties of transmitter release and the increase in [Ca++]i under a variety of conditions were different during stimulation with K+ or glutamate. Potassium-evoked release of [3H]GABA consisted of two phases, a rapid, large and transient phase followed by a smaller, more persistent second phase. The rapid phase was inhibited (60%) by nocodazole which reduced the number of vesicles in the neurites by 80%. This rapid phase of the GABA release was also reduced by organic (verapamil) and inorganic (Co++) Ca++ channel blockers but was insensitive to the GABA transport inhibitor SKF 89976A. In contrast, the second phase was less sensitive to nocodazole and Ca++ channel antagonists but could be inhibited by SKF 89976A. The glutamate-induced [3H]GABA release, which was mainly mediated by N-methyl-D-aspartate receptors, consisted of a single, sustained phase. This was insensitive to nocodazole, partly inhibited by verapamil and could be blocked by Co++ as well as SKF 89976A. The action of Co++ could be attributed to a block of N-methyl-D-aspartate-associated ion channels. These findings strongly suggest that the majority of the K(+)-stimulated GABA release is dependent upon vesicles whereas the glutamate induced release is non-vesicular and mediated by a depolarization-dependent reversal of the direction of high-affinity GABA transport. The basic differences in the mode of action of the two depolarizing stimuli were reflected in the properties of the increase in [Ca++]i elicited by 55 mM K+ and 100 microM glutamate, respectively. The K(+)-induced increase in [Ca++]i was reduced by both verapamil and Ca(++)-free media whereas the corresponding glutamate response was only sensitive to Ca(++)-free conditions. Exposure of the cells to nocodazole or SKF 89976A had no effect on the ability of K+ or glutamate to increase [Ca++]i. Altogether, the results clearly demonstrate that K(+)-induced transmitter release from these GABAergic neurons is vesicular in nature whereas that induced by the neurotransmitter glutamate is not.

Effect of insulin on GABAergic development in the embryonic chick retina

We investigated the role of insulin in GABAergic differentiation in the embryonic chick retina at different embryonic ages using glutamate decarboxylase (GAD) and high-affinity GABA uptake as developmental markers. Both these GABAergic markers exhibit developmentally programmed increases in activity during retinogenesis that also occur in culture. Insulin stimulated GABA uptake in retina neurons at all embryonic ages in a dose-dependent manner and GAD activity by 30% in embryonic retina neurons after 11 days of development. The stimulation of GABA uptake by insulin was blocked by addition of ouabain suggesting a role for the Na+,K+ ATPase. The same concentration of insulin caused a 76% stimulation of protein synthesis in these retinal cells, and previous work demonstrated that insulin also stimulates cholinergic differentiation in the chick retina (Hausman et al., Dev. Brain. Res. 59, (1991) 31-37). Thus, there was no selective stimulation of GABAergic differentiation by insulin but likely a neurotrophic effect. The increase in GAD activity in neurons from post-11-day embryonic neurons contrasts with our previous findings at embryonic days 6-7 where there is little change in GAD activity after addition of insulin. It is possible that the failure of insulin to stimulate GAD activity during early retina development is due to the increased accumulation of GABA in the presence of insulin. GABA levels were increased more than two-fold by 100 ng/ml insulin.

Many spinal cord cells transiently express low molecular weight forms of glutamic acid decarboxylase during embryonic development

At early developmental stages in the rat spinal cord (embryonic day 13), when neuronal progenitors are still proliferating, most differentiating neurons express truncated forms of glutamic acid decarboxylase (GAD) (approximately 25 kDa) which are the products of alternative splicing of the GAD67 gene. These truncated proteins do not appear to synthesize gamma-aminobutyric acid (GABA). The amino acid is detected in cells only after alternative splicing of the GAD67 gene generates a full-length, 67 kDa enzymatically active form of GAD. Both the 67 kDa GAD and GABA colocalize and appear diffusely distributed in the cytoplasm of embryonic neurons. GABA does not appear associated with synaptic vesicles until after birth, when its intracellular distribution becomes punctate and it colocalizes with synaptophysin. At this time, it also colocalizes with an immunologically distinct 65 kDa GAD protein encoded by a second GAD gene (GAD65). Expression of different GAD-related proteins with distinct intracellular distributions during development suggests that GABA, the product of these enzymes, may have trophic or metabolic roles during spinal cord differentiation.

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.