Prenatal ontogeny of the GABAergic system in the rat brain: an immunocytochemical study

GABA, GAD, and GABA(A) receptor alpha4, beta1, and gamma1 subunits are expressed in the late embryonic and early postnatal neocortical germinal matrix and coincide with gliogenesis

Increasing evidence indicates that the classical, fast-acting neurotransmitter gamma-amino butyric acid (GABA) may initially act as morphogen in cell proliferation and differentiation via specific receptors. In view of the potential roles for GABA in central nervous system development, we examined the expression of GABA, GABA(A) receptor beta1 and gamma1 subunits by immunocytochemistry and the expression of transcripts for two GABA-synthesizing enzymes, glutamate decarboxylase (GAD65, GAD67 mRNAs), and for alpha4, beta1, and gamma1 subunits of GABA(A) receptor by in situ hybridization in the developing neocortex. Tissue sections were taken from embryonic days (E) 17 and E20 embryos and newborn rats (P0). The embryos' mothers and newborn rats had been injected with 5-bromo-2'-deoxyuridine (BrdU) and had survived for 2 hours. At E17, BrdU-positive cells were largely restricted in the synthetic zone at the ventricular margin when cortical neurogenesis was still active. GAD mRNAs and GABA immunoreactivity were detected in the subventricular zone, while alpha4, beta1, and gamma1 subunits were abundant in the ventricular zone. At E20 and P0, when neurogenesis had largely ceased and gliogenesis had commenced, BrdU-positive cells were found throughout the ventricular zone with GABA, GAD mRNAs, and alpha4, beta1, and gamma1 subunits. GABA, GAD mRNAs and alpha4, beta1, and gamma1 subunit signals intensified in the ventricular zone from E17 to P0 as gliogenesis proceeded. Thus, specific components of a putative GABAergic circuit are expressed in cells of the ventricular zone during the late embryonic/early postnatal period coincident with gliogenesis, suggesting a role for GABA in glial cell proliferation.

Afferent arrival and onset of functional activity in the trigeminothalamic pathway of the rat

In this study, a novel in vitro slice preparation has been used to study the anatomical and physiological development of the trigeminothalamic pathway in the prenatal and neonatal rat. Anterograde tracing studies showed that the most rostral trigeminal fibres had reached the cephalic flexure by embryonic day (E)15, and entered the diencephalon by E16. By E17 the first few fibres had reached the ventroposteromedial thalamic nucleus (VPM) where they terminated in growth cones. The projection was more substantial and fibres had begun branching by E18, and arbors were more elaborate by E19. The fibres densely filled the nucleus by the day of birth (PO). The physiological studies showed that postsynaptic responses to stimulation of the trigeminal nerve or principal sensory nucleus (Pr5) could first be recorded at E17. Reliable responses to stimulation of either the nerve or Pr5 were recorded from E18 on. Stimulation of Pr5 enabled both axonal and synaptic signals to recorded in VPM. A GABAergic influence was acting to decrease the overall level of excitability in the thalamus from E18. In prenatal animals, the excitatory response was primarily mediated by NMDA receptors, and by P1 a non-NMDA mediated component was beginning to appear. These results demonstrate that the capacity for axonal conduction in the trigeminothalamic fibres and synaptic transmission in the thalamus are present from the time that anatomical connections are first established.

Increased expression of GABA transporters, GAT-1 and GAT-3, in the deafferented superior colliculus of the rat

GABA transporters (GATs) play a critical role in the translemmal transport of GABA in neurons and glial cells. Two major brain GATs, GAT-1 and GAT-3, are found in astrocytes in the adult brain. Astroglia demonstrate morphological and molecular changes in response to brain injury and deafferentation. The present study was designed to determine whether the expression of GATs changes after nerve deafferentation using the rat superior colliculus (SC) as a model. The immunoreactivity for GAT-1 and GAT-3, as well as GABA and glutamic acid decarboxylase (GAD)-65 and GAD-67, was studied in the SC of control rats and rats with unilateral optic nerve transections. Immunolabeling for both GAT-1 and GAT-3 was increased in the neuropil of the denervated SC as compared to that for the SC of control rats or for the unaffected SC of experimental rats. In contrast, immunoreactivity for GABA, GAD-65 and GAD-67 was not altered. The change in the immunolabeling of GAT-1 and GAT-3 was detectable at 1 day postlesion and became more evident between 3-30 days postlesion. At the electron microscopic level, immunoreactivity for both GAT-1 and GAT-3 in the unaffected SC was localized to astrocytic processes, whereas GAT-1 immunolabeling was also present in synaptic terminals. In the deafferented SC, immunolabeling for both GATs was elevated in the somata and processes of hypertrophied astrocytes as compared to that in the unaffected SC, whereas GAT-1 labeling in neuronal profiles was largely unchanged. A substantial increase of GAT-1 and GAT-3 in astrocytes following optic nerve transection suggests that these cells play a role in modulating GABA's action in the deafferented SC.

Functional GABA(A) receptors on human glioma cells

Glioma cells in acute slices and in primary culture, and glioma-derived human cell lines were screened for the presence of functional GABA(A) receptors. Currents were measured in whole-cell voltage clamp in response to gamma-aminobutyric acid (GABA). While cells from the most malignant glioma, the glioblastoma multiforme, did not respond to GABA, an inward current (under our experimental conditions with high Cl- concentration in the pipette) was induced in gliomas of lower grades, namely in 71% of oligodendroglioma cells and in 62% of the astrocytoma cells. Glioma cell lines did not express functional GABA(A) receptors, irrespective of the malignancy of the tumour they originate from. The currents elicited by application of GABA were due to activation of GABA(A) receptors; the specific agonist muscimol mimicked the response, the antagonists bicuculline and picrotoxin blocked the GABA-activated current and the benzodiazepine receptor agonist flunitrazepam augmented the GABA-induced current and the benzodiazepine inverse agonist DMCM decreased the GABA current. Cells were heterogeneous with respect to the direction of the current flow as tested in gramicidin perforated patches: in some cells GABA hyperpolarized the membrane, while in the majority it triggered a depolarization. Moreover, GABA triggered an increase in [Ca2+]i in the majority of the tumour cells due to the activation of Ca2+ channels. Our results suggest a link between the expression of GABA receptors and the growth of glioma cells as the disappearance of functional GABA(A) receptors parallels unlimited growth typical for malignant tumours and immortal cell lines.

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.

Ontogeny of sex differences in the mammalian hypothalamus and preoptic area

1. There are numerous sites in the nervous system where steroid hormones dramatically influence development. Increasing interest in mechanisms in neural development is providing avenues for understanding how gonadal steroids alter the ontogeny of these regions during sexual differentiation. 2. An increasing number of researchers are examining effects of gonadal steroids on neurite outgrowth, cell differentiation, cell death, cell migration, and synaptogenesis. The interrelated timing of these events may be a key aspect influenced by gonadal steroids throughout development. 3. The preoptic area and hypothalamus are characteristically heterogeneous in terms of cell type (e.g., different neuropeptides) and cell derivation. Perhaps a major reason for the ontogeny of sexual differences in the preoptic area and hypothalamus lies in the convergence of many different cell types from diverse sources (i.e., proliferative zones surrounding the lateral and third ventricles, and the olfactory placodes) that can be influenced in an interactive manner by gonadal steroid mechanisms. 4. The characterization of multiple mechanisms (e.g., trophic, migratory, apoptotic, fate, etc.,) that contribute to permanent changes in brain structure and ultimately function is essential for unraveling the process of sexual differentiation.

Excitatory neurotransmission and sexual differentiation of the brain

During normal development there is a perinatal sensitive period during which the male brain is exposed to high levels of gonadal steroids, resulting in permanent differentiation of neural substrates. The cellular mechanisms mediating hormonally induced sexual differentiation remain largely unknown. In the adult brain, steroids exert profound influences on the amino acid transmitters, GABA, and glutamate. We have found steroid regulation of amino acid neurotransmission during the perinatal sensitive period and propose this may be functionally related to sexual differentiation of the brain. Specifically, the mRNA coding for the rate-limiting enzyme in GABA synthesis, glutamic acid decarboxylase (GAD), is up to twice as high in some steroid-concentrating regions of the neonatal male brain compared to females, including the arcuate nucleus, dorsomedial nucleus, and the CA1 region of hippocampus. Sex differences in GABA tissue concentrations positively correlate with GAD mRNA differences in several brain regions. There are also sex differences in protein levels of GABA(A) receptor subunits. In parallel with these findings are significantly higher levels of binding to the non-NMDA glutamate receptor in steroid-concentrating regions of male brain. Given that GABA is an inhibitory transmitter and glutamate is an excitatory amino acid, these results initially appear paradoxical. However, in contrast to its inhibitory action in the adult brain, early in development GABA is actually excitatory and acts in a manner analogous to glutamate. Therefore, the combination of increased excitatory GABAergic and glutamatergic activity should result in substantially higher levels of neuronal excitation in the male brain. We speculate that an increased level of neuronal excitation is a potential mechanism mediating the permanent masculinization of the brain.

Origin and route of tangentially migrating neurons in the developing neocortical intermediate zone

Neuroblasts produced in the ventricular zone of the neocortex migrate radially and form the cortical plate, settling in an inside-out order. It is also well known that the tangential cell migration is not negligible in the embryonic neocortex. To have a better understanding of the tangential cell migration in the cortex, we disturbed the migration by making a cut in the neocortex, and we labeled the migrating cells with 1,1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) in vivo and in vitro. We also determined the birth dates of the cells. Disturbance of tangential cell migration caused an accumulation and disappearance of microtubule-associated protein 2 immunoreactive (MAP2-IR) cells on the ventral and dorsal side of the cut, respectively, which indicated that most of the MAP2-IR cells in the intermediate zone (IZ) were migrating toward the dorsal cortex. The DiI injection study in vivo confirmed the tendency of the direction of cell migration and suggested the origin of the cells to be in the lateral ganglionic eminence (LGE). DiI injection into the LGE in vitro confirmed that the LGE cells cross the corticostriatal boundary and enter the IZ of the neocortex. The migrating cells acquired multipolar shape in the IZ of the dorsal cortex and seemed to reside there. A 5-bromo-deoxyuridine incorporation study revealed that the migrating MAP2-IR cells in the IZ were early-generated neurons. We concluded that the majority of tangentially migrating cells were generated in the LGE and identified as a distinct population that was assumed not to have joined the cortical plate.

GABA(A) receptor subunit expression in intrastriatal striatal grafts comparison between normal developing striatum and developing striatal grafts

Expression of the alpha1, alpha2 and beta2/3 GABA(A) receptor subunits in maturing cell-suspension striatal grafts and in normal developing striatum was studied by immunocytochemistry. During normal postnatal development, the alpha1 subunit was present in the striatum only at very low density, while the alpha2 and beta2/3 subunits were present with a patchy distribution, in some patches at high density. Double-staining techniques indicated that DARPP-32 (a marker of striatal projection neurons) was not colocalized with alpha1, but was present in some beta2/3-positive areas and all alpha2-positive areas. In striatal grafts, alpha1 immunoreactivity was first detected 2 weeks post-grafting (p.g.), and by 3-10 weeks p.g. the pattern was similar to that observed in mature grafts (1 year p.g.), in which alpha1-immunopositive patches surrounding DARPP-32-positive (i.e. striatum-like) areas are observed. Alpha2 and beta2/3 immunoreactivity was observed within the first week p.g., and by 3-10 weeks p.g. was similar to that observed in mature grafts (i.e. immunoreactivity throughout the graft but with patches of different intensity). During graft maturation there was a marked decline in alpha2 immunoreactivity in DARPP-32-negative areas, as is observed during normal development of the globus pallidus and ventral pallidum. Interestingly, alpha1- and beta2/3-positive fibers (perhaps mostly dendrites) entered DARPP-32-positive patches from DARPP-32-negative areas. This study indicates that the time course of expression of GABA(A) receptor subunits in grafted striatal neurons, closely matches that of morphological maturation of the transplant, that of the development of functional synaptic activity and that of GABA(A) receptor subunit immunoreactivity in normal developing striatum. Our results also suggest that there are significant interactions between DARPP-32-positive and DARPP-32-negative areas with respect to the expression of GABA(A) receptors, and support the suggestion that miniature 'striatopallidal systems' may develop within grafts; such interactions may be important for the functional integration of striatal grafts with the host brain.

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.

Expression of a glutamate transporter subtype, EAAT4, in the developing human cerebellum

A glutamate transporter subtype, EAAT4, is closely related to removal of glutamate from the synaptic cleft. Immunohistochemistry for EAAT4 demonstrated the specific distribution and localization of its expression in the developing human cerebellum. Purkinje cells showed faint EAAT4 immunostaining at 17 gestational weeks (GW), which became increasingly intense from 23 GW to the infantile period. In the late fetal to early infantile periods, Purkinje cells showed marked immunoreactivity. After the late infantile period, EAAT4 immunoreactivity was the same in extent as in the adult pattern. Its intracellular localization also changed with development. EAAT4 immunoreactivity was demonstrated in the short processes of Purkinje cells in the early embryonic period, in the cell bodies and dendrites in the late fetal to early infantile periods, and then in the spines after the late infantile period. In the adult cerebellum, immunoreactivity was detected strongly in the spines of Purkinje cells and weakly in the cell bodies. No immunoreactivity was found in the axons or axon terminals of the cells. Thus, the glutamate transporter exhibits developmental changes in its distribution in the cerebellum and its localization in Purkinje cells. EAAT4 immunoreactivity may be related to the dendritic arborization of cells in the molecular layer.

Regulation of GABA(A) receptor subunit mRNA expression by the pesticide dieldrin in embryonic brainstem cultures: a quantitative, competitive reverse transcription-polymerase chain reaction study

Cyclodiene organochlorine pesticides, such as dieldrin, inhibit gamma-aminobutyric acid (GABA)ergic neurotransmission by blocking the Cl- channel of GABA(A) receptors. This action may make the developing nervous system especially vulnerable to these neurotoxins, which could interfere with the trophic actions of GABA on developing neurons and alter expression of GABA(A) receptors. We have used an in vitro model to determine whether exposure to dieldrin alters developmental expression of GABA(A) receptor subunit mRNA transcripts. Dissociated cell cultures were prepared from embryonic day 14 (E14) brainstem and cultured in serum-containing medium for 1 day in vitro (DIV), then treated for 2 DIV with 10 microM dieldrin in serum-free medium. This dose was based on preliminary experiments and previous studies (Nagata et al.: Brain Res 645:19-26, 1994; Pomes et al.: J Pharmacol Exp Ther 271:1616-1623, 1994). Absolute amounts of alpha1, beta3, gamma1, gamma2S and gamma2L mRNA transcripts were quantified in these cultures by quantitative, competitive reverse transcription-polymerase chain reaction (RT-PCR) using subunit-selective internal standards. The most abundant GABA(A) subunit transcript was beta3, which was much more highly expressed than gamma2S, gamma1, gamma2L, or alpha1 subunit mRNAs. Dieldrin differentially regulated expression of these transcripts. Levels of beta3 subunit transcripts were significantly increased (by 300%) by dieldrin, whereas expression of gamma2S and gamma2L transcripts were decreased (by 50% and 40%, respectively). However, dieldrin did not alter the ratio of gamma2S to gamma2L transcripts, indicating that it did not affect alternative splicing of gamma2 transcripts. Dieldrin appeared to increase expression of alpha1 subunit transcripts, but this effect was not statistically significant. Dieldrin did not significantly alter expression of gamma1 subunit transcripts. These results support the hypothesis that in utero exposure to cyclodiene pesticides could pose a risk to the developing brain by virtue of their ability to alter gene expression of GABA(A) receptor subunits, which could produce GABA(A) receptors with altered functional properties.

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.

Postnatal development of membrane properties and delta oscillations in thalamocortical neurons of the cat dorsal lateral geniculate nucleus

The development of membrane properties, firing patterns, and delta oscillations in neurons of the cat dorsal lateral geniculate nucleus (dLGN) was investigated in vitro during the first 7 postnatal weeks. Compared with adult neurons, the resting membrane potential was more depolarized at postnatal days 1-9 (P1-P9), the input resistance was higher at P1-P7, and action potentials had a higher threshold and a smaller amplitude at P1-P3 and a longer duration at P1-P9. At P1-P3 trains longer than 200 msec were rarely observed, and trains with more than three action potentials were only present in 41% of the neurons, whereas at P1-P7 the normalized slope of the instantaneous frequencies at the first five interspike intervals was smaller than in the adult. A long-lasting (up to 6 sec) afterhyperpolarization followed a short train of action potentials in 88 and 30% of neurons at P1-P3 and P30-P32, respectively, but it was rarely observed in the adult. The low-threshold Ca2+ potential could evoke a burst of action potentials since P1. However, at P1-P7 the number of action potentials per burst was smaller (range, one to five), and at P1-P9 their maximum instantaneous frequency was lower (<190 Hz) than in the adult (range, six to eight, and 344 Hz, respectively). No delta oscillations were observed until P17, and their frequency (0.36 Hz) was lower than that in the adult (1.8 Hz). The percentage of neurons displaying delta oscillations and their frequency reached adult values by the end of the seventh postnatal week, i.e., well after the maturation of the membrane properties and firing patterns (second postnatal week). In conclusion, the maturation of the electrophysiological properties of thalamocortical neurons in the cat dLGN is completed later than the retinogeniculate axon segregation (Shatz CJ, 1983), and the immaturity of the oscillatory, and not of the burst-firing, activity is a limiting factor in the development of delta waves.

GABAergic transcallosal neurons in developing rat neocortex

In the mature cerebral cortex the interhemispheric connections across the corpus callosum appear to be essentially completely excitatory on the basis of both immunocytochemical and electrophysiological studies. During late embryonic development, however, immunocytochemical staining reveals numerous GABA-positive fibres in the callosum, which later largely disappear. The origin of these fibres and whether they represent functional GABAergic neurons has not been established. In the present study we used a combination of retrograde labelling in vivo with electrophysiology and immunocytochemistry in cell culture to show that transiently at birth in rat pups a substantial number of transcallosal cortical cells are functional GABAergic neurons. Possible roles and fates for these neurons are discussed.

Initial development of gamma-aminobutyric acid immunoreactivity in the human cerebral cortex

The development of cortical cells immunoreactive for gamma-aminobutyric acid (GABA) was studied in human cerebral cortex in the first trimester of gestation (from 4 to 13 gestational weeks; g.w.). The first GABA-immunoreactive (IR) cells were observed at 6.5 g.w., i.e., before the appearance of the cortical plate, which gives rise to a majority of the adult cortical layers. GABA-IR cells were found initially in the telencephalic wall, where a lateromedial gradient in the density of GABA-positive cells was observed at this early developmental time point, but not at later stages. At 7 g.w., as the cortical plate emerged in the ventrolateral region of the cerebral vesicle, GABA-immunoreactive cells were found dorsal and ventral to the developing cortical plate. At this stage, immunoreactivity was also observed in the other transient developmental zones of the cortical anlage: in the subplate layer and in the intermediate, subventricular and ventricular zones. From 8 to 9 g.w. and continuing throughout the end of the studied period (13 g.w.), GABA-IR cells were distributed throughout the full width of the telencephalic wall, and, at 13 g.w., the newly formed subpial granular layer contained GABA-immunoreactive cells, as well. However, the predominant sites for GABA immunoreactivity remained the prospective layer I and the subplate. The population of GABA-positive cells described here was not immunoreactive for glial fibrillary acidic protein (GFAP) at any gestational age examined and, therefore, probably represents GABA-containing neurons. The observation that GABA-IR neurons appear in human developing cortex slightly before the cortical plate formation and beginning of synaptogenesis (6.5 g.w.) suggests that GABA plays an important role in the initial organization of the developing human cerebral cortex.

Localisation of amino acid neurotransmitters during postnatal development of the rat retina

We used postembedding immunocytochemistry to determine the localisation of the amino acid neurotransmitters glutamate, gamma-aminobutyrate (GABA), and glycine, potential neurotransmitter precursors (aspartate and glutamine), and taurine in the rat retina during postnatal development. All amino acids investigated were present at birth; however, only the inhibitory neurotransmitters GABA and glycine displayed neuronal localisation. GABA was localised in a sparse population of amacrine cells, and glycine immunoreactivity was found in cells within the ventricular zone that appeared to migrate through the neuroblastic layer. Glutamate labelling was diffuse across the retina until postnatal day (PND) 8. Localisation of glutamine was evident within Müller's cells by PND 6, in agreement with the known age of onset of glutamine synthetase activity. Based on the findings of uptake of radiolabelled glutamate and GABA by PND 8 and changes in immunoreactivity, we propose that Müller's cells evolve at PND 6-8 from their precursor cells, the radial glial cells. Evidence for differences in glutamate turnover in the infant retina was seen on examination of aspartate and glutamine immunoreactivity. Aspartate labelling was weak until PND 11, when ganglion cells and some amacrine cells were labelled. Unlike the mature retina, a large number of amacrine cells were glutamine immunoreactive in the PND 6 retina. One reason for the observed differences in precursor pooling may be a lack of neuronal neurotransmitter release and overall low metabolic activity. We also investigated the response of the developing retina to ischaemic insult to test the physiological hypoxia model of vascular development. Our findings are consistent with the hypothesis that the developing retina has increased tolerance to ischaemic insult. Our findings suggest that, although the retina is morphologically adult like by PND 8, there are differences in neurotransmitter turnover in the immature rat retina.

Immunoreactivity for GABA plasma membrane transporter, GAT-1, in the developing rat cerebral cortex: transient presence in the somata of neocortical and hippocampal neurons

The immunoreactivity for a gamma-aminobutyric acid (GABA) membrane transporter, GAT-1, was examined in the neocortex and hippocampal formation of developing rats from the day of birth (postnatal day 0, P0) to the adult stage. The immunolabeling was mainly localized to the neuropil, but was also in a select population of cell bodies during a limited time period. Layers I and VIb of neocortex exhibited relatively high reactivity at birth, but diminished their staining with development. In contrast, GAT-1 immunoreactivity in the neuropil in the cortical plate and its derivatives was light at birth, but increased rapidly during the first 2-3 postnatal weeks in an inside-out order. An adult pattern with immunoreactive puncta more densely distributed in layers II to IV than the deeper layers was completed by P30-45. The neuropil reactivity in the hippocampal formation at P0 was greater than that in the neocortex, densely localized in a supragranular band, and less densely in the hilus of the dentate gyrus and the strata radiatum and oriens of the hippocampus. This pattern was basically maintained at later stages except that the immunoreactivity in the supragranular band diminished, whereas that in the subgranular zone was enhanced. A population of cell bodies morphologically characteristic of cortical and hippocampal interneurons was substantially immunolabeled for GAT-1 by P5 and remained until P30. At the electron microscopic level, GAT-1 immunoreactivity was localized mainly to axon terminals and astrocytes between P5 and P45, but was also found in neuronal somata and their dendrites between P5 and P30. Our data show a differential postnatal development of GAT-1 immunoreactivity in the rat cerebral cortex, including a transient presence of immunoreactivity in the somata of a subpopulation of cerebral interneurons and a developmental downregulation of GAT-1 expression in the earliest generated cortical elements (layers 1 and VIb). The findings in the present study suggest that GAT-1 expression in the neocortex and hippocampus may relate to the functional maturation of the GABAergic system.

GABAA receptor stimulation promotes survival of embryonic rat striatal neurons in culture

In order to clarify the functional role of gamma-aminobutyric acid (GABA) in developing brain, we investigated the effect of GABA on the survival of embryonic rat striatal neurons in dissociated cell culture. Chronic exposure of striatal cultures to GABA resulted in a significant increase in the number of surviving neurons. The effect of GABA was concentration-dependent (1-1000 microM) and was blocked by a GABAA receptor antagonist, bicuculline (100 microM), or a GABAA chloride channel blocker, picrotoxin (100 microM), but not by a GABAB receptor antagonist, 2-hydroxysaclofen (100 microM). In addition, the GABAA receptor agonist muscimol mimicked the effect of GABA, promoting cell survival in a concentration-dependent manner (0.01-100 microM), while the GABAB receptor agonist baclofen (up to 100 microM) had no significant effect. The GABA-induced enhancement of neuronal survival was suppressed by the L-type voltage-dependent Ca2+ channel blockers nifedipine (1-3 microM) and nicardipine (1-5 microM). Protein kinase inhibitors, H-7 (10-30 microM) or genistein (3 microM), also suppressed GABA-induced enhancement of neuronal survival. These results suggest that stimulation of GABAA receptors enhances survival of embryonic striatal neurons, and that the effect is mediated by Ca2+ influx through L-type voltage-dependent Ca2+ channels, initiating intracellular signaling cascades that involve activation of H-7- and genistein-sensitive protein kinases.

Cryopreserved GABAergic neurons in cultures of rat cerebral cortex and mesencephalon: a comparative morphometric study with anti-GABA antibodies

Blocks of embryonic rat cerebral cortex and mesencephalon were cryopreserved and stored for up to 1 year in liquid nitrogen at -196 degrees C with 7.5% dimethylsulfoxide (DMSO) as cryoprotectant. After thawing, these tissues were only mechanically dissociated and the cells were cultured for 2-7 weeks before immunocytochemical staining with anti-GABA (gamma-aminobutyric acid) antibodies. The freeze-stored GABA-immunoreactive (IR) mesencephalic neurons were compared, with computerized morphometry, to fresh mesencephalic cells and to their fresh and frozen cerebral cortical counterparts. A part of the cortical cells was treated with thienyl-phencyclidine (TCP) in order to assess the potential morphological effects of this neuroprotective agent upon these cortical neurons. Two types of GABA-IR neurons (small and large neuritic field cells) could be evidenced in both structures without any difference between fresh and frozen materials, but with significant quantitative morphological differences linked to their anatomical source. GABAergic phenotype is expressed similarly in fresh and frozen cultured neurons with intrinsically programmed morphological features and only minor influences of epigenetic factors. Small and large neuritic field GABA-IR neurons represent, respectively, local and long-range circuits of inhibition, strongly reminiscent of those described in vivo and which remain unchanged in culture even after freeze-storage.

Expression pattern of glutamate decarboxylase (GAD) in the developing cortex of the embryonic chick brain

The development of the GABAergic system in the chick embryo telencephalon has been studied. Special emphasis was placed on the development of glutamate decarboxylase (GAD) between embryonic day 8 (E8) and E17. The GABA immunoreactivity and neuron-specific enolase expression was detected simultaneously in glutardialdehyde fixed sections, which confirmed that GABAergic cells exhibit neuronal phenotype. The GAD expression was studied by means of immunohistochemistry on cryo-sectioned material both at the light and electron microscopic levels. Furthermore, the presence and localization of GAD65 and GAD67 mRNAs were studied with an in situ hybridization technique with digoxigenin-labeled RNA probes. Protein expression as well as mRNA appearance mostly coincided both temporally and spatially. In the parahippocampal area, as well as in other regions of the developing cortex, GAD staining was seen from E8 onwards. The number of positive cells increased as did the intensity of staining up to E14. As observed in the electron microscope, the GAD protein was co-localized with GABA in most cases, although some GAD-positive cells devoid of GABA-staining also were observed. The pattern of GAD mRNA expression was in general similar to that of GAD immunostaining. Both GAD65 and GAD67 mRNA were detected during the entire period. Furthermore, GAD67 mRNA localization spatially was more correlated with GAD protein expression. The study provides evidence for the notion that development of the GABAergic system occurs rapidly during embryogenesis and, as suggested from mRNA data, that two forms of GAD with slight difference in distribution can contribute to this.

Electron microscopic evidence for coexistence of leucine5-enkephalin and gamma-aminobutyric acid in a subpopulation of axon terminals in the rat locus coeruleus region

We recently described ultrastructural evidence for morphologically heterogeneous axon terminals containing the endogenous opioid peptide, methionine5-enkephalin (ENK), that formed synapses with neurons containing the catecholamine synthesizing enzyme, tyrosine hydroxylase, in the locus coeruleus (LC) of the rat brain. The morphological characteristics of these terminals suggested that ENK may be co-localized with either an excitatory or inhibitory amino acid. To further test this hypothesis, we combined immunogold-silver localization of gamma-aminobutyric acid (GABA) and immunoperoxidase labeling for ENK in single sections through the LC, in the present study, to determine whether ENK and GABA were contained within single axon terminals. Light microscopic analysis of ENK and GABA immunoreactivities in the LC indicated that both transmitters were enriched in the dorsal pons. Although electron microscopy revealed that ENK and GABA were located primarily in axon terminals, some dendrites also contained immunolabeling for GABA. The dense core vesicles were consistently the most immunoreactive in ENK containing axon terminals and were identified toward the periphery of the axon terminal distal to the synaptic specialization. Axon terminals containing either ENK or GABA immunoreactivities contained pleomorphic vesicles as well as large dense core vesicles, varied in size and formed heterogeneous types of synaptic specializations (i.e. asymmetric vs. symmetric). Approximately 38% (n = 76) of the axon terminals containing ENK immunoreactivity (n = 200) also contained GABA. Some axon terminals containing peroxidase labeling for ENK (22%; n = 44) converged on common targets with GABA-labeled axon terminals. Finally, a few ENK-labeled axon terminals (14%; n = 28) formed asymmetric (excitatory-type) synapses with dendrites containing gold-silver labeling for GABA. The results, therefore, indicate that the opioid peptide, ENK, and the inhibitory amino acid, GABA, may influence LC neurons by concerted actions via (1) release from a common axon terminal, and (2) via separate sets of afferents converging on similar portions of the plasmalemma of target neurons. Furthermore, these studies also suggest a cellular substrate for opioid inhibition of LC neurons via activation (i.e. asymmetric synapses) of inhibitory GABAergic neurons. Future studies are required to determine whether the receptive sites for ENK and GABA are located at similar sites on the plasma membranes of LC neurons pre- or postsynaptically and whether there is differential release of either transmitter from single terminals in the LC.

GABA metabolism during status epilepticus in the developing rat brain

The rate of synthesis of GABA, the major inhibitory neurotransmitter, was determined in parietal cortex and hippocampus during SE induced by systemic administration of lithium (3 mEq/kg) followed 20 h later by pilocarpine (100 mg/kg) in 1-4-week-old rats. Our results show that the immature hippocampus is better capable of maintaining GABA synthesis in the face of SE at the earliest stages of development studied (74.1% of basal in 1-week-old) and that development results in a progressive decline in the ability to maintain GABA synthesis in the face of SE (44.1% of basal by 4 weeks) that may parallel the ontogeny of self-sustaining seizures. Our data describe an aspect of developmental GABA neurochemistry which may in part explain the relative resistance of the immature hippocampus to seizure spread and of certain types of seizure-induced damage.

Postnatal development of GABA-immunoreactive terminals in the reticular and ventrobasal nuclei of the rat thalamus: a light and electron microscopic study

The postnatal development of inhibitory GABAergic circuits in the thalamic reticular and ventrobasal nuclei was studied in rats ranging from the day of birth to the end of the third postnatal week by means of a postembedding immunogold staining procedure to visualize GABA. In the reticular nucleus, GABA labeling was present from birth in cell bodies, dendrites, growth cones and a few synaptic terminals, whereas in the ventrobasal nucleus it was exclusively in axonal processes identifiable as growth cones, vesicle-rich profiles and synaptic terminals. In both nuclei, GABA-labeled synaptic terminals were, however, very scarce and immature in neonatal animals and they became numerous and morphologically mature only after the end of the second postnatal week. These findings suggest that inhibitory synaptic responses in the somatosensory thalamus are not yet fully mature throughout the first two postnatal weeks and support the hypothesis that GABA may initially play trophic roles. The relatively late maturation of the thalamic GABAergic system may have important functional consequences, as the reticulothalamic circuits are responsible for the generation of spindle wave oscillations whose cellular mechanisms are also involved in the generation of spike-and-wave (absence) seizures in humans and in animal models.

Embryonic and postnatal expression of four gamma-aminobutyric acid transporter mRNAs in the mouse brain and leptomeninges

The distribution of gamma-aminobutyric acid (GABA) transporter mRNAs (mGATs) was studied in mouse brain during embryonic and postnatal development using in situ hybridization with radiolabeled oligonucleotide probes. Mouse GATs 1 and 4 were present in the ventricular and subventricular zones of the lateral ventricle from gestational day 13. During postnatal development, mGAT1 mRNA was distributed diffusely throughout the brain and spinal cord, with the highest expression present in the olfactory bulbs, hippocampus, and cerebellar cortex. The mGAT4 message was densely distributed throughout the central nervous system during postnatal week 1; however, the hybridization signal in the cerebral cortex and hippocampus decreased during postnatal weeks 2 and 3, and in adults, mGAT4 labeling was restricted largely to the olfactory bulbs, midbrain, deep cerebellar nuclei, medulla, and spinal cord. Mouse GAT2 mRNA was expressed only in proliferating and migrating cerebellar granule cells, whereas mGAT3 mRNA was absent from the brain and spinal cord throughout development. Each of the four mGATs was present to some degree in the leptomeninges. The expression of mGATs 2 and 3 was almost entirely restricted to the pia-arachnoid, whereas mGATs 1 and 4 were present only in specific regions of the membrane. Although mGATs 1 and 4 may subserve the classical purpose of terminating inhibitory GABAergic transmission through neuronal and glial uptake mechanisms, GABA transporters in the pia-arachnoid may help to regulate the amount of GABA available to proliferating and migrating neurons at the sub-pial surface during perinatal development.

Guidance of circumferentially growing axons by netrin-dependent and -independent floor plate chemotropism in the vertebrate brain

Netrin-1, a diffusible signal secreted by floor plate cells at the ventral midline of the vertebrate CNS, can attract ventrally migrating axons and repel a subset of dorsally migrating axons in the spinal cord and rostral hindbrain in vitro. Whether netrin-1 can act as a global cue to guide all circumferentially migrating axons is, however, unknown. Here, we show that netrin-1 can attract alar plate axons that cross the floor plate along its entire rostrocaudal axis. Dorsally directed axons forming the posterior commissure are, however, repelled by the floor plate by a netrin-independent mechanism. These results suggest that netrin-1 functions as a global guidance cue for attraction to the midline. Moreover, floor plate-mediated chemorepulsion may also operate generally to direct dorsal migrations, but its molecular basis may involve both netrin-dependent and -independent mechanisms.

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.

Cellular and subcellular localization of delta opioid receptor immunoreactivity in the rat dentate gyrus

To study a potential locus of action of opioids in the rat dentate gyrus, we examined the localization of the delta opioid receptor (DOR) by immunocytochemistry. Two antisera raised to unique, non-overlapping peptide sequences located within the extracellular N-terminal sequence of DOR were tested. By light microscopy, numerous neurons in the central hilar region were intensely labeled for DOR, while the granule cell layer contained light DOR immunoreactivity. To further characterize hilar neuron cell types which contained DOR, sections through the dentate gyrus were double labeled using immunofluorescence with antisera to DOR and either gamma-aminobutyric acid (GABA), neuropeptide Y (NPY), or somatostatin-28 antisera. Most DOR-labeled perikarya also contained GABA and NPY, while a subpopulation contained somatostatin. Electron microscopic examination of sections labeled for DOR revealed that the immunoreactivity was common in profiles which exhibited the morphological characteristics of granule cells, as well as those of non-granule cells. DOR immunoreactivity was located at postsynaptic sites within neuronal perikarya (2%), dendrites (27%), and dendritic spines (22%); as well as in presynaptic axon terminals (25%) and glia (23%) (n = 279). In dendrites and dendritic spines, DOR immunoreactivity was most often associated with the plasmalemmal surface near asymmetric synapses. In axon terminals, DOR immunoreactivity primarily surrounded small, clear vesicles, and was less consistently found on the plasmalemmal surface. The distribution of DOR-labeled profiles overlapped with, but was not restricted to regions known to contain enkephalin. These data suggest that opiates acting at the DOR can modulate both hilar neurons and granule cells both pre- and postsynaptically.

Beta-adrenergic receptor activation promotes process outgrowth in an embryonic rat basal forebrain cell line and in primary neurons

A clonal cell line, AS583-8.E4.22, from the embryonic day 15 rat basal forebrain was established using retrovirus-mediated transduction of a temperature-sensitive mutant of the simian virus 40 (SV40) large tumour antigen. The cell line expresses cytoskeletal and neurotransmitter features indicative of neuronal commitment. In response to agents that increase intracellular cAMP, including forskolin and catecholamines, the cell line exhibits rapid process outgrowth and growth cone formation that does not require new gene expression or protein synthesis. The neurite outgrowth induced by catecholamines is mediated by beta 2-adrenergic receptors and is characterized by a rapid, reversible redistribution of filamentous actin. Neurons from primary cultures of embryonic day 15 basal forebrain were also found to respond to beta-adrenergic receptor agonists by enhancing growth cone formation. These results suggest that catecholamines provide cues that induce cytoskeletal rearrangements leading to neuronal process outgrowth and growth cone formation in the developing basal forebrain and possibly other neuronal progenitor cell populations. The neuronal basal forebrain cell line provides an ideal model to study the signalling mechanisms underlying the catecholamine-induced process outgrowth.

Development of cholinergic and GABAergic neurons in the rat medial septum: different onset of choline acetyltransferase and glutamate decarboxylase mRNA expression

In the present study, we have investigated the developmental expression of the transmitter-synthesizing enzymes choline acetyltransferase (ChAT) and glutamate decarboxylase (GAD) in rat medial septal neurons by using in situ hybridization histochemistry. In addition, we have employed immunostaining for ChAT and the calcium-binding protein parvalbumin, known to be contained in septohippocampal GABAergic neurons. A large number of GAD67 mRNA-expressing neurons were already observed in the septal complex on embryonic day (E) 17, the earliest time point studied. During later developmental stages, there was mainly an increase in the intensity of labeling. Neurons expressing ChAT mRNA were first recognized at E 20, and their number slowly increased during postnatal development of the septal region. The adult pattern of ChAT mRNA-expressing neurons was observed around postnatal day (P) 16. By using a monoclonal ChAT antibody, the first immunoreactive cells were not seen before P 8. Similarly, the first weakly parvalbumin-immunoreactive neurons were seen in the septal complex by the end of the 1st postnatal week. These results indicate that in situ hybridization histochemistry may be an adequate method to monitor the different development of transmitter biosynthesis in cholinergic and GABAergic septal neurons. Moreover, the late onset of ChAT mRNA expression would be compatible with a role of target-derived factors for the differentiation of the cholinergic phenotype.

Development of GABA and calcium binding proteins immunoreactivity in the rat hippocampus following neonatal anoxia

The consequences of neonatal anoxia (N2 100% for 25 min at 30 h after birth) on the rat hippocampus were studied 7-60 days postnatally with immunocytochemistry for gamma-aminobutyric acid (GABA), parvalbumin (PV) and calbindin-D28k (CB). In both sham-treated and anoxic rats, GABA immunoreactivity presented a mature expression since early stages, while PV and CB immunoreactivity showed a major postnatal development. In anoxic animals, a significant reduction in the number of hippocampal GABA-immunoreactive neurons was observed at all time-points analysed, a transitory effect on PV immunoreactivity was seen at P7 and P21, while no modifications in the number of CB-immunoreactive neurons could be found. Thus, selective vulnerability of GABA-containing neurons and relative resistance of neurons in which PV or CB immunoreactivity is present or is expressed later, occur in the hippocampus after neonatal anoxia. The role of calcium binding proteins (CBP) in nerve cell protection is discussed.

Ultrastructural relationships between leu-enkephalin- and GABA-containing neurons differ within the hippocampal formation

Electrophysiological studies have suggested that the excitatory actions of opioids in the hippocampal formation are mediated by inhibition of interneurons containing GABA; however, an anatomical basis for this interaction has never been established. Thus, we sought to determine the relationship between leu-enkephalin (LE)-containing axon terminals and GABAergic neurons using dual labeling immunohistochemistry and electron microscopy. In the CA1 region of the hippocampus, LE-labeled terminals (n = 99) were in direct contact with GABA-labeled perikarya and dendrites (18%), and directly apposed to GABA-labeled axon terminals (14%). In the molecular layer of the dentate gyrus, LE-containing terminals (n = 125) occasionally apposed GABA-containing terminals (8%). In the hilus of the dentate gyrus, LE-containing terminals (n = 165) often contacted GABA-containing perikarya and dendrites (39%), but rarely apposed GABA-containing terminals (3%). In the CA3 region of the hippocampus, only a few LE-labeled mossy fiber boutons (n = 102) contacted the shafts of GABA-labeled dendrites (4%). The results demonstrate that leu-enkephalin-containing terminals have a different anatomical relationship with GABA-containing profiles in each subregion of the hippocampal formation. In the CA1 region of the hippocampus, the data support the numerous electrophysiological studies indicating that LE functions in modulating inhibitory GABAergic neurons by both pre- and postsynaptic mechanisms. In the outer molecular layer of the dentate gyrus the localization suggests some presynaptic regulation of GABAergic terminals. In the hilus of the dentate gyrus, the study also supports the contention that LE may have an important role in regulating inhibition of GABA-containing neurons. In comparison, in the CA3 region of the hippocampus, LE may have a more limited role in regulating GABAergic inhibition by direct association.

Immunohistochemical investigation of gamma-aminobutyric acid ontogeny and transient expression in the central nervous system of Xenopus laevis tadpoles

The ontogeny of the gamma-aminobutyric acid (GABA)-positive neurons in the brain of Xenopus laevis tadpoles was investigated by means of immunohistochemistry, using specific antibodies both against GABA and its biosynthetic enzyme, glutamate decarboxylase (GAD). The results obtained with the two antisera were comparable. The GABA system differentiates very early during development. At stages 35/36, numerous GABA-positive neurons were seen throughout the prosencephalon and formed two main bilateral clusters within the lateral walls of the forebrain that ran caudally toward the hindbrain. Other GABA-immunolabeled cell bodies, together with a conspicuous network of GABAergic fibers, were seen in the posterior hypothalamus. In the spinal cord, the lateral marginal zone was GABA-positive, as were Rohon-Beard neurons, interneurons, and Kolmer-Agdhur cells. A very rich GABA innervation was observed in the pars intermedia of the pituitary. At stage 50, plentiful immunopositive neurons and fibers were found in the telencephalic hemispheres, the diencephalon, and the mesencephalon (optic tectum and tegmentum). By stage 54, the number of GABA-immunoreactive neurons in the posterior hypothalamus had decreased, so that, at stage 58, there were very few GABA-labeled cell bodies in the dorsolateral walls of the infundibulum, despite a strong GABAergic innervation within the median eminence and the pars intermedia. From stage 58 to stage 66, the distribution pattern was very similar to that described in the adult X. laevis and in other amphibian species. These results point to transient GABA expression within the hypothalamus, possibly related to either 1) a naturally occurring cell death or 2) a phenotypic switch.

Developmental profile of GABAA-receptors in the marmoset monkey: expression of distinct subtypes in pre- and postnatal brain

Gamma aminobutyric acid (GABA)A-receptors are expressed in fetal mammalian brain before the onset of synaptic inhibition, suggesting their involvement in brain development. In this study, we have analyzed the maturation of the GABAA-receptor in the marmoset monkey forebrain to determine whether distinct receptor subtypes are expressed at particular stages of pre- and postnatal ontogeny. The distribution of the subunits alpha 1, alpha 2, and beta 2,3 was investigated immunohistochemically between embryonic day 100 (6 weeks before birth) and adulthood. Prenatally, the alpha 2- and beta 2,3-subunit-immunoreactivity (-IR) was prominent throughout the forebrain, whereas the alpha 1-subunit-IR appeared in selected regions shortly before birth. The alpha 2-subunit-IR disappeared gradually to become restricted to a few regions in adult forebrain. By contrast, the alpha 1-subunit-IR increased dramatically after birth and replaced the alpha 2-subunit in the basal forebrain, pallidum, thalamus, and most of the cerebral cortex. Staining for the beta 2,3-subunits was ubiquitous at every age examined, indicating their association with either the alpha 1- or the alpha 2-subunit in distinct receptor subtypes. In neocortex, the alpha 1 -subunit-IR was first located selectively to layers IV and VI of primary somatosensory and visual areas. Postnatally, it increased throughout the cortex, with the adult pattern being established only during the second year. The switch in expression of the alpha 1- and alpha 2- subunits indicates that the subunit composition of major GABAA-receptor subtypes changes during ontogeny. This change coincides with synaptogenesis, suggesting that the emergence of alpha 1- GABAA-receptors parallels the formation of inhibitory circuits. A similar pattern has been reported in rat, indicating that the developmental regulation of GABAA-receptors is conserved across species, possibly including man. However, the marmoset brain is more mature than the rat brain at the onset of alpha 1-subunit expression, suggesting that alpha 1-GABAA-receptors are largely dispensable in utero, but may be required for information processing after birth.

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.

Pre- and postnatal expression of amino acid neurotransmitters, calcium binding proteins, and nitric oxide synthase in the developing superior colliculus

Neurons within the superior colliculus (SC) contain a variety of neurochemicals, including the amino acid neurotransmitters GABA and glutamate, the calcium binding proteins calbindin and parvalbumin, and the neuromodulator nitric oxide. We have examined the development of expression of these substances using antibody immunocytochemistry. These results are summarized in Fig. 10. GABA and calbindin are expressed very early in development, at a time when cells are still dividing and migrating from the subventricular zone. The expression of both GABA and CB is maximal at around E40-46, the age at which these cells have just established their adult lamination and extrinsic afferents have begun to grow into the tectum. GABA and CB likely play diverse roles during this stage of development, including the regulation of intracellular calcium during cell migration and neurite outgrowth. Glutamate is expressed somewhat later in development while parvalbumin immunoreactivity does not appear until shortly after birth. These two substances continue to increase in density throughout the period of postnatal growth, at a time when synapse formation and evoked electrical activity are beginning to develop. Both PV and glutamate may be involved in one or both of these activity-dependent processes. Nitric oxide synthase (NOS) is expressed at different times in different cell groups. NOS appears very early in prenatal development in cells within the SVZ and in the deep gray layer of SC. On the other hands, cells within the intermediate gray layer of SC do not express NOS until shortly before birth. The igl cells that express NOS at this age are clustered neurons similar to those that project to the CFR in the adult. NOS expression occurs in these cells at precisely the time when axons begin to form patches that innervate these clusters. Based upon this temporal correlation, we hypothesize that nitric oxide may regulate synapse formation in this cell group.

Axon terminals immunolabeled for dopamine or tyrosine hydroxylase synapse on GABA-immunoreactive dendrites in rat and monkey cortex

Dopamine afferents to the cortex regulate the excitability of pyramidal neurons via a direct synaptic input. However, it has not been established whether dopamine also modulates pyramidal cell activity indirectly through synapses on gamma-aminobutyric acid (GABA) interneurons, and whether such inputs differ across cortical regions and species. We sought to address these issues by an immunocytochemical electron microscopic approach that combined peroxidase staining for dopamine or tyrosine hydroxylase (TH) with a pre-embedding gold-silver marker for GABA. In the deep layers of the rat prefrontal cortex and in the superficial layers of the monkey prefrontal and primary motor cortices, terminal varicosities immunoreactive for dopamine or TH formed primarily thin, symmetric synapses on distal dendrites. Both GABA-immunoreactive dendrites as well as unlabeled spines and dendrites were contacted by dopamine- or TH-immunoreactive terminals. Synaptic specializations were detected at some, but not all of these contacts. The relative frequency of these appositional and synaptic contacts did not appear to differ between the rat and monkey prefrontal cortex, or between the monkey prefrontal and motor cortices. Across regions and species, labeled and unlabeled targets of dopamine- or TH-positive terminals received additional synaptic input from unlabeled, and occasionally GABA-immunoreactive terminals. Close appositions between dopamine- or TH-immunoreactive and GABA-positive terminals were observed only rarely. These findings indicate that dopamine afferents provide direct synaptic inputs to GABA local circuit neurons in a consistent fashion across cortical regions and species. Thus, dopamine's cellular actions involve direct as well as modulatory effects on both GABA interneurons and pyramidal projection neurons.

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.

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.

Fate of GABAergic septohippocampal neurons after fimbria-fornix transection as revealed by in situ hybridization for glutamate decarboxylase mRNA and parvalbumin immunocytochemistry

Many septohippocampal neurons are GABAergic and are affected by transection of the fimbria-fornix, like the septohippocampal cholinergic cells. Here we have studied the changes that occur in GABAergic septohippocampal neurons following fimbria-fornix transection. For labeling of septohippocampal projection neurons, adult Sprague-Dawley rats received injections of the fluorescent tracer Fluoro-Gold into the hippocampus 1 week prior to bilateral transection of the fimbria-fornix. After axotomy, rats were allowed to survive for varying periods ranging from 3 weeks to 18 months. Following fixation of the animals, sections through the septal region were either stained by in situ hybridization for glutamate decarboxylase (GAD) mRNA or immunostained for parvalbumin (PARV), which is known to be present in GABAergic septohippocampal neurons. In situ hybridization for GAD mRNA revealed no statistically significant changes in cell number 3 weeks and 6 months postlesion. In contrast, PARV-immunoreactive neurons were reduced to 35% of control 3 weeks postlesion. This value increased to 66% after 6 months of survival. As seen in the electron microscope, axotomized PARV-positive neurons exhibited characteristics of vital cells. Most neurons contained lysosomes associated with Fluoro-Gold, resulting from retrograde labeling prior to fimbria-fornix transection. We conclude that mainly PARV-containing GABAergic neurons in the medial septal nucleus (MS) project to the hippocampus and are thus heavily affected by the lesion but are able to survive and restore the synthesis of PARV. The lack of significant changes in the number of GAD mRNA-expressing cells is explained by the presence of numerous GABAergic MS neurons not projecting to the hippocampus.

Differentiation of pluripotent embryonic stem cells into the neuronal lineage in vitro gives rise to mature inhibitory and excitatory neurons

Embryonic stem (ES) cells represent a suitable model to analyze cell differentiation processes in vitro. Here, we report that pluripotent ES cells of the line BLC 6 differentiate in vitro into neuronal cells possessing the complex electrophysiological and immunocytochemical properties of postmitotic nerve cells. In the course of differentiation BLC 6-derived neurons differentially express voltage-dependent (K+, Na+, Ca2+) and receptor-operated (GABAA, glycine, AMPA, NMDA receptors) ionic channels. They generate fast Na(+)-driven action potentials and are functionally coupled by inhibitory (GABAergic) and excitatory (glutamatergic) synapses as revealed by measurements of postsynaptic currents. Moreover, BLC 6-derived neurons express neuron-specific cytoskeletal, cell adhesion and synaptic vesicle proteins and exhibit a Ca(2+)-dependent GABA secretion. Thus, the ES cell model enables the investigation of cell lineage determination and signaling mechanisms in the developing nervous system from a pluripotential stem cell to a differentiated postmitotic neuron. The in vitro differentiation of neurons from ES cells may be an excellent approach to study by targeted gene disruption a variety of neuronal functions.

GABA-induced motility of spinal neuroblasts develops along a ventrodorsal gradient and can be mimicked by agonists of GABAA and GABAB receptors

During embryogenesis, neuroblasts proliferate within germinal zones, then migrate to their final positions. Although many neurons migrate along radial glial fibers, evidence suggests that environmental factors, as yet unidentified, also influence neuroblast movement. In vivo, nerve growth factor (NGF) and gamma-aminobutyric acid (GABA) colocalize near target destinations of migratory neuroblasts. In vitro, embryonic spinal neurons migrate towards NGF and GABA (Behar et al.: J Neurosci 14:29-38, 1994), implying that the molecules may act as chemoattractants in vivo. Here, we have used an in vitro assay of migration to show that migratory responses to these attractants develop along a ventrodorsal gradient that parallels terminal mitosis during cord development, and that GABA stimulates chemokinesis (motility without a gradient) via heterogeneous receptors involving separate signalling pathways. Both GABAA (muscimol) and GABAB (baclofen) agonists mimicked the effects of GABA in stimulating chemokinesis. Muscimol-induced motility was only blocked by GABAA antagonists (bicuculline or picrotoxin), whereas migration to baclofen was blocked by antagonists of both GABAA and GABAB (2-hydroxysaclofen) receptors. Migration to baclofen, but not muscimol, was abolished in the presence of 8-bromo cAMP or pertussis toxin, indicating that the former, but not the latter, attractant may stimulate motility via Gi/Go GTP binding proteins, and that PKA may modulate migratory responses to baclofen. Migration to GABA was partially attenuated by each of the GABA receptor antagonists. These results lead us to conclude that the natural ligand stimulates neuroblast motility via heterogeneous receptors coupled to different signalling mechanisms.

Benzodiazepine-insensitive mice generated by targeted disruption of the gamma 2 subunit gene of gamma-aminobutyric acid type A receptors

Vigilance, anxiety, epileptic activity, and muscle tone can be modulated by drugs acting at the benzodiazepine (BZ) site of gamma-aminobutyric acid type A (GABAA) receptors. In vivo, BZ sites are potential targets for endogenous ligands regulating the corresponding central nervous system states. To assess the physiological relevance of BZ sites, mice were generated containing GABAA receptors devoid of BZ sites. Following targeted disruption of the gamma 2 subunit gene, 94% of the BZ sites were absent in brain of neonatal mice, while the number of GABA sites was only slightly reduced. Except for the gamma 2 subunit, the level of expression and the regional and cellular distribution of the major GABAA receptor subunits were unaltered. The single channel main conductance level and the Hill coefficient were reduced to values consistent with recombinant GABAA receptors composed of alpha and beta subunits. The GABA response was potentiated by pentobarbital but not by flunitrazepam. Diazepam was inactive behaviorally. Thus, the gamma 2 subunit is dispensable for the assembly of functional GABAA receptors but is required for normal channel conductance and the formation of BZ sites in vivo. BZ sites are not essential for embryonic development, as suggested by the normal body weight and histology of newborn mice. Postnatally, however, the reduced GABAA receptor function is associated with retarded growth, sensorimotor dysfunction, and drastically reduced life-span. The lack of postnatal GABAA receptor regulation by endogenous ligands of BZ sites might contribute to this phenotype.

Intrauterine cocaine exposure of rabbits: persistent elevation of GABA-immunoreactive neurons in anterior cingulate cortex but not visual cortex

The effects of prenatal cocaine exposure on the development of the rabbit cerebral cortex were studied. Two cortical areas were compared: primary visual cortex (VC) and anterior cingulate cortex (ACC). ACC was selected because behavioral deficits observed in cocaine-exposed infants suggest the involvement of ACC. In addition, ACC receives dense dopaminergic innervation and cocaine's action in inhibiting the re-uptake of dopamine is believed to underly the rewarding properties of cocaine. VC was selected as a control area because there is no evidence of behavioral deficits associated with visual perception in cocaine-exposed infants, and because VC receives minimal dopaminergic innervation. Two aspects of cortical development were studied: (i) cortical morphology, growth and cytoarchitectonic organization; and (ii) the development of the GABAergic neurotransmitter system. Measures of postnatal cortical growth, including cortical lamination, cell number and soma size, were compared in cocaine-exposed or control (saline) rabbits aged P5-P60. There was no difference between cocaine and saline animals in any of these parameters, and cortical cytoarchitecture appeared normal. However, despite the absence of major abnormalities in cortical development, we found that the number of GABA-immunoreactive neurons in cocaine-exposed animals was significantly higher than normal in ACC. This effect was highly consistent, was present in all laminae and at all ages studied, and persisted into maturity (P60). In contrast, in VC, the number of GABA-immunoreactive neurons in cocaine-exposed animals did not differ from normal. We suggest that increased GABA immunoreactivity may reflect a compensatory response to excessive excitatory input to ACC. A change in the balance of excitation and inhibition in ACC, reflecting 'noisy' or dysfunctional intracortical circuitry, may underly the emotional lability and attentional deficits characteristically described in infants exposed in utero to cocaine.

Developmental expression of parvalbumin mRNA in the cerebral cortex and hippocampus of the rat

Parvalbumin (PARV) belongs to the family of calcium-binding proteins bearing the EF hand domain. Immunocytochemical studies in the cerebral cortex have demonstrated that neurons containing PARV include two types of GABAergic interneurons, namely, basket and axo-axonic chandelier cells. The present study examines the onset and pattern of PARV mRNA expression during the development of rat neocortex and hippocampus by means of 'in situ' hybridization with an oligonucleotide probe corresponding to rat PARV cDNA. In animals aged P0-P6 no signal was detected above background in neocortex or hippocampus. At P8, a few cortical cells displayed a number of silver grains just above background levels. By P10 PARV mRNA-expressing cells in the neocortex were detected almost exclusively in layer V of somatosensory, frontal and cingulate cortices. At P12 PARV mRNA was mainly detected in layers IV, V and VIa. By P14 there was a marked overall increase in the entire neocortex, including layer II-III, both in the number of cells and in their intensity of labelling. Further maturation in the pattern of PARV mRNA concentration was observed between P16 and P21. In the hippocampus low hybridization was observed at P10-P12. In subsequent stages both the number of positive cells and the intensity of labelling increased steadily. No clear-cut radial gradients for the expression of PARV mRNA were observed in the hippocampal region. Our results show that the developmental radial gradient followed by PARV mRNA expression in the neocortex does not follow an 'inside-out' gradient, consistent with previous immunocytochemical findings. Taken together, these data indicate that the developmental sequence followed by the PARV protein directly reflects mRNA abundance and suggest that PARV mRNA expression correlates with the functional maturation of cortical interneurons.

Morphological and GABA-immunoreactive development of the embryonic chick telencephalon

The development of neurons utilizing gamma-aminobutyric acid (GABAergic neurons) in prosencephalon and telencephalon from chicken embryonic days 4-14 (E4-E14) was studied by means of immunohistochemistry. Furthermore, routine histology and transmission electron microscopy. respectively, were performed in order to study the morphological development in the designated area. The main finding is that development of GABAergic neurons in the chick telencephalon is rapid; the GABA neurons are appearing in bulk at day 8, being "overexpressed" at days 10-11, decreasing in numbers thereafter and achieving mature morphology on day 14, which is considerably faster than in the rodent. Morphological analysis revealed that the prosencephalon mainly consisted of a thin layer of undifferentiated neuroblasts in the E4 embryo. By E6, the prosencephalon had increased in thickness and occasional cells outside the neuroepithelium showed a more mature morphology with a few cells weakly staining positive for GABA. At E8, the prospective granular and subventricular layers had developed. At E14, the appearance of the telencephalon is approximating that of the adult since both ependymal cells and morphologically mature neurons can be seen.

Postnatal maturation of GABA-immunoreactive neurons of rat medial prefrontal cortex

A light microscopic immunocytochemical approach has been used to examine the distribution and maturation of gamma-aminobutyric acid- (GABA) containing cells in rat medial prefrontal cortex (mPFC) at progressive postnatal stages. Between P1 and P5, labeled cells in the cortical plate show less differentiated morphological characteristics when compared to cells in the deeper laminae. By P10, however, most labeled cells in superficial laminae show more differentiated characteristics with some having a distinctive multipolar appearance. Between P1 and P5, there is a significant increase (50%) in the density of GABA-containing cells in the superficial laminae, while concurrently there is an overall decreases in the subjacent deeper laminae. As the cortex continues to expand, there is a corresponding decrease in the density of GABA-immunoreactive cells in the outer two-thirds of the cortical mantle until approximately P15, stabilizing at 20-25 cells/100,000 microns2 for all laminae. Between P1 and P15, there is also a significant increase (133%) in the average size of labeled cells, followed by a gradual decrease of 30% between P15 and P41. During P1-7, there is a marked increase in the density of labeled axosomatic terminals in both the superficial (200%) and deep laminae (116%). In the superficial layers, however, the density of labeled terminals again increases by 86% between P12 and P18. In general, the present findings are consistent with the idea that there is a progressive maturation of the intrinsic GABAergic system in rat mPFC in a classic "inside-out" pattern, and this involves extensive postnatal changes occurring during the first 3 postnatal weeks.