Transient GABA immunoreactivity in cranial nerves of the chick embryo

Acetylcholine Neurons Become Cholinergic during Three Time Windows in the Developing Mouse Brain

Acetylcholine (ACh) neurons in the central nervous system are required for the coordination of neural network activity during higher brain functions, such as attention, learning, and memory, as well as locomotion. Disturbed cholinergic signaling has been described in many neurodevelopmental and neurodegenerative disorders. Furthermore, cotransmission of other signaling molecules, such as glutamate and GABA, with ACh has been associated with essential roles in brain function or disease. However, it is unknown when ACh neurons become cholinergic during development. Thus, understanding the timeline of how the cholinergic system develops and becomes active in the healthy brain is a crucial part of understanding brain development. To study this, we used transgenic mice to selectively label ACh neurons with tdTomato. We imaged serial sectioned brains and generated whole-brain reconstructions at different time points during pre- and postnatal development. We found three crucial time windows-two in the prenatal and one in the postnatal brain-during which most ACh neuron populations become cholinergic in the brain. We also found that cholinergic gene expression is initiated in cortical ACh interneurons, while the cerebral cortex is innervated by cholinergic projection neurons from the basal forebrain. Taken together, we show that ACh neuron populations are present and become cholinergic before postnatal day 12, which is the onset of major sensory processes, such as hearing and vision. We conclude that the birth of ACh neurons and initiation of cholinergic gene expression are temporally separated during development but highly coordinated by brain anatomical structure.

Developmental exposure to methylmercury alters GAD67 immunoreactivity and morphology of endothelial cells and capillaries of midbrain and hindbrain regions of adult rat offspring

INTRODUCTION

Methylmercury (MeHg) is an environmental contaminant that is of particular concern in Northern Arctic Canadian populations. Specifically, organic mercury compounds such as MeHg are potent toxicants that affect multiple bodily systems including the nervous system. Developmental exposure to MeHg is a major concern, as the developing fetus and neonate are thought to be especially vulnerable to the toxic effects of MeHg. The objective of this study was to examine developmental exposure to low doses of MeHg and effects upon the adult central nervous system (CNS). The doses of MeHg chosen were scaled to be proportional to the concentrations of MeHg that have been reported in human maternal blood samples in Northern Arctic Canadian populations.

METHOD

Offspring were exposed to MeHg maternally where pregnant Sprague Dawley rats were fed cookies that contained MeHg or vehicle (vehicle corn oil; MeHg 0.02 mg/kg/body weight or 2.0 mg/kg/body weight) daily, throughout gestation (21 days) and lactation (21 days). Offspring were not exposed to MeHg after the lactation period and were euthanized on postnatal day 450. Brains were extracted, fixed, frozen, and sectioned for immunohistochemical analysis. A battery of markers of brain structure and function were selected including neuronal GABAergic enzymatic marker glutamic acid decarboxylase-67 (GAD67), apoptotic/necrotic marker cleaved caspase-3 (CC3), catecholamine marker tyrosine hydroxylase (TH), immune inflammatory marker microglia (Cd11b), endothelial cell marker rat endothelial cell antigen-1 (RECA-1), doublecortin (DCX), Bergmann glia (glial fibrillary acidic protein (GFAP)), and general nucleic acid and cellular stains Hoechst, and cresyl violet, respectively. Oxidative stress marker lipofuscin (autofluorescence) was also assessed. Both male and female offspring were included in analysis. Two-way analysis of variance (ANOVA) was utilized where sex and treatment were considered as between-subject factors (p* <0.05). ImageJ was used to assess immunohistochemical results.

RESULTS

In comparison with controls, adult rat offspring exposed to both doses of MeHg were observed to have (1) increased GAD67 in the cerebellum; (2) decreased lipofuscin in the locus coeruleus; and (3) decreased GAD67 in the anterior CA1 region. Furthermore, in the substantia nigra and periaqueductal gray, adult male offspring consistently had a larger endothelial cell and capillary perimeter in comparison to females. The maternal high dose of MeHg influenced RECA-1 immunoreactivity in both the substantia nigra and periaqueductal gray of adult rat offspring, where the latter neuronal region also showed statistically significant decreases in RECA-1 immunoreactivity at the maternal low dose exposure level. Lastly, males exposed to high doses of MeHg during development exhibited a statistically significant increase in the perimeter of endothelial cells and capillaries (RECA-1) in the cerebellum, in comparison to male controls.

CONCLUSION

Findings suggest that in utero and early postnatal exposure to MeHg at environmentally relevant doses leads to long-lasting and selective changes in the CNS. Exposure to MeHg at low doses may affect GABAergic homeostasis and vascular integrity of the CNS. Such changes may contribute to neurological disturbances in learning, cognition, and memory that have been reported in epidemiological studies.

Development of the trigeminal motor neurons in parrots: implications for the role of nervous tissue in the evolution of jaw muscle morphology

Vertebrates have succeeded to inhabit almost every ecological niche due in large part to the anatomical diversification of their jaw complex. As a component of the feeding apparatus, jaw muscles carry a vital role for determining the mode of feeding. Early patterning of the jaw muscles has been attributed to cranial neural crest-derived mesenchyme, however, much remains to be understood about the role of nonneural crest tissues in the evolution and diversification of jaw muscle morphology. In this study, we describe the development of trigeminal motor neurons in a parrot species with the uniquely shaped jaw muscles and compare its developmental pattern to that in the quail with the standard jaw muscles to uncover potential roles of nervous tissue in the evolution of vertebrate jaw muscles. In parrot embryogenesis, the motor axon bundles are detectable within the muscular tissue only after the basic shape of the muscular tissue has been established. This supports the view that nervous tissue does not primarily determine the spatial pattern of jaw muscles. In contrast, the trigeminal motor nucleus, which is composed of somata of neurons that innervate major jaw muscles, of parrot is more developed compared to quail, even in embryonic stage where no remarkable interspecific difference in both jaw muscle morphology and motor nerve branching pattern is recognized. Our data suggest that although nervous tissue may not have a large influence on initial patterning of jaw muscles, it may play an important role in subsequent growth and maintenance of muscular tissue and alterations in cranial nervous tissue development may underlie diversification of jaw muscle morphology.

Excitatory actions of GABA in developing chick vestibular afferents: effects on resting electrical activity

The aim of this study was to characterize the effect of γ-aminobutyric acid (GABA) in the resting multiunit activity of the vestibular afferents during development using the isolated inner ear of embryonic and postnatal chickens (E15-E21 and P5). GABA (10(-3) to 10(-5) M; n = 133) and muscimol (10(-3) M) elicited an increase in the frequency of the basal discharge of the vestibular afferents. We found that GABA action was dose-dependent and inversely related to animal age. Thus, the largest effect was observed in embryonic ages such as E15 and E17 and decreases in E21 and P5. The GABAA receptor antagonists, bicuculline (10(-5) M; n = 10) and picrotoxin (10(-4) M; n = 10), significantly decreased the excitatory action of GABA and muscimol (10(-3) M). Additionally, CNQX 10(-6) M, MCPG 10(-5) M and 7ClKyn 10(-5) M (n = 5) were co-applied by bath substitution (n = 5). Both the basal discharge and the GABA action significantly decreased in these experimental conditions. The chloride channel blocker 9-AC 0.5 mM produced an important reduction in the effect of GABA 10(-3) (n = 5) and 10(-4) M (n = 5). Thus, our results suggest an excitatory role of GABA in the resting activity of the vestibular afferents that can be explained by changes in the gradient of concentration of Cl(-) during development. We show for the first time that the magnitude of this GABA effect decreases at later stages of embryonic and early postnatal development. Taking into account the results with glutamatergic antagonists, we conclude that GABA has a presynaptic action but is not the neurotransmitter in the vestibular afferent synapses, although it could act as a facilitator of the spontaneous activity and may regulate glutamate release.

Neuroendocrine cells are present in the domestic fowl ovary

Neuroendocrine cells are present in virtually all organs of the vertebrate body; however, it is yet uncertain whether they exist in the ovaries. Previous reports of ovarian neurons and neuron-like cells in mammals and birds might have resulted from misidentification. The aim of the present work was to determine the identity of neuron-like cells in immature ovaries of the domestic fowl. Cells immunoreactive to neurofilaments, synaptophysin, and chromogranin-A, with small, dense-core secretory granules, were consistently observed throughout the sub-cortical ovarian medulla and cortical interfollicular stroma. These cells also displayed immunoreactivity for tyrosine, tryptophan and dopamine β-hydroxylases, as well as to aromatic L-DOPA decarboxylase, implying their ability to synthesize both catecholamines and indolamines. Our results support the argument that the ovarian cells previously reported as neuron-like in birds, are neuroendocrine cells.

GABAergic neurons in the lateral superior olive of the hamster are distinguished by differential expression of gad isoforms during development

Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter that is synthesized by two isoforms of glutamic acid decarboxylase (GAD), GAD65 and GAD67. Using in situ hybridization and immunocytochemical techniques in hamsters, we investigated the postnatal development of GAD isoforms within the lateral superior olive (LSO) where GABAergic neurons form part of a descending efferent projection to the cochlea. In the neonatal hamster LSO, GAD67 immunoreactivity, GAD67 transcript labeling, and intense GABA immunostaining are at low levels. However, robust GAD65 mRNA expression is found throughout the LSO during the early postnatal period. The neonatal GABAergic expression patterns are in stark contrast to the adult where the LSO has robust GAD67 mRNA expression and weak GAD65 mRNA expression. Cells exhibiting intense GABA immunolabeling were also found in the same LSO locations as robust GAD67 mRNA expression and intense GAD67 immunoreactivity. Additionally, GAD67-positive cells in the LSO were retrogradely labeled from the cochlea confirming that these cells are a part of the lateral olivocochlear system. The late onset of GAD67 expression and intense GABA immunoreactivity in LSO neurons are consistent with the relatively late maturation of the lateral olivocochlear neurons inferred from previous studies. During development, these data lead us to conclude that the GABAergic portion of the lateral olivocochlear system is distinguished by preferential GAD67 expression, intense GABA immunoreactivity, and relatively late postnatal onset.

Avian superior olivary nucleus provides divergent inhibitory input to parallel auditory pathways

The avian auditory brainstem displays parallel processing, a fundamental feature of vertebrate sensory systems. Nuclei specialized for temporal processing are largely separate from those processing other aspects of sound. One possible exception to this parallel organization is the inhibitory input provided by the superior olivary nucleus (SON) to nucleus angularis (NA), nucleus magnocellularis (NM), and nucleus laminaris (NL) and contralateral SON (SONc). We sought to determine whether single SON neurons project to multiple targets or separate neuronal populations project independently to individual target nuclei. We introduced two different fluorescent tracer molecules into pairs of target nuclei and quantified the extent to which retrogradely labeled SON neurons were double labeled. A large proportion of double-labeled SON somata were observed in all cases in which injections were made into any pair of ipsilateral targets (NA and NM, NA and NL, or NM and NL), suggesting that many individual SON neurons project to multiple targets. In contrast, when injections involved the SONc and any or all of the ipsilateral targets, double labeling was rare, suggesting that contralateral and ipsilateral targets are innervated by distinct populations of SON neurons arising largely from regionally segregated areas of SON. Therefore, at the earliest stages of auditory processing, there is interaction between pathways specialized to process temporal cues and those that process other acoustic features. We present a conceptual model that incorporates these results and suggest that SON circuitry, in part, functions to offset interaural intensity differences in interaural time difference processing.

Domain-restricted expression of two glutamic acid decarboxylase genes in midgestation mouse embryos

Glutamic acid decarboxylase (GAD) is the biosynthetic enzyme for gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system (CNS) of vertebrates. In addition to the adult CNS, GABA and GAD also have been detected in embryos, although their precise localization and specific functions in embryonic development have not been elucidated. In this paper, the authors studied the cellular distribution of two GAD isoforms, GAD65 and GAD67, in midgestation mouse embryos by in situ hybridization histochemistry. With few exceptions, it was found that GAD65 and GAD67 mRNAs are localized in overlapping cellular domains of the embryonic CNS that later develop into regions with a strong GABAergic contribution. The GAD-expressing cells are situated in the differentiating zone of the embryonic day 10.5 (E10.5) through E11.5 CNS and in the subventricular zone and the mantle zone of the E12.5 CNS, which suggests that they are committed neuronal precursors. By using a specific serum for GABA, a similar pattern of distribution was obtained, indicating that GAD mRNAs are translated efficiently into enzymatically active GAD, which produces embryonic GABA. The expression domains of GAD overlap with those of genes that are known to be involved in the patterning of the embryonic CNS. The two GAD mRNAs also are detected outside of the embryonic CNS in various cell types, mainly those of placodal and neural crest origin. This pattern of expression is consistent with the notion that GAD and its product, GABA, play a signaling role during development.

Rhombomere origin plays a role in the specificity of cranial motor axon projections in the chick

Guidance of cranial motor axons to their targets conforms to a segmental plan in the chick embryo. Trigeminal motor neurons lie within rhombomeres 2 and 3 and project via an exit point in rhombomere 2 to innervate the first branchial arch. Facial motor neurons lie within rhombomeres 4 and 5 and grow out via an exit point in rhombomere 4 to innervate the second branchial arch. We have investigated the axial level-specific matching of motor neurons and branchial arches using donor to host transplantation in avian embryos. Previous work has shown that rostrocaudal reversal of a single hindbrain segment (rhombomere 3) leads to misprojection of a contingent of trigeminal axons via the facial nerve exit point. Using the same experimental manipulation in chick embryos and quail-chick chimaeras, we have analysed the pathways of these aberrant projections. We have found that in the majority of embryos analysed from stage 19 to 31, trigeminal axons from the transplanted rhombomere projected towards second branchial arch muscles, in addition to their normal first arch muscle targets. However, from stage 32 to 36, aberrant projections to second arch-derived muscles were detected only in a small minority of embryos. These experiments show that trigeminal motor neurons show a lack of specificity in their early projection into the periphery but that inappropriate projections may be later eliminated. This suggests that segmental mechanisms intrinsic to the hindbrain specify motor neurons with respect to their eventual innervation pattern.

Regulation of cell-type specific expression of lacZ by the 5'-flanking region of mouse GAD67 gene in the central nervous system of transgenic mice

The transcriptional regulation of the murine gene encoding the 67-kDa form of glutamic acid decarboxylase (GAD67) was studied by beta-galactosidase histochemistry in transgenic mice carrying fusion genes between progressively longer portions of the 5'-upstream regulatory region of GAD67 and E. coli lacZ. No expression was detected in brains of mice carrying 1.3 kb of upstream sequences including a housekeeping and two conventional promoters, and two negative regulatory elements with homology to known silencers. In mice carrying the same portion of the promoter region plus the first intron, lacZ expression in the adult central nervous system was found in few, exclusively neuronal sites. The number of correctly stained GABAergic centres increased dramatically with increasing the length of the 5'-upstream region included in the construct which suggests that multiple putative spatial enhancers are located in this region. Their action is influenced by epigenetic mechanisms that may be due to site-of-integration and transgene copy-number effects. Additional cis-acting elements are needed to obtain fully correct expression in all GABAergic neurons of the adult central nervous system.

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.

Two glutamate decarboxylase forms corresponding to the mammalian GAD65 and GAD67 are expressed during development of the chick telencephalon

The gamma-aminobutyric acid (GABA)-synthesizing enzyme glutamate decarboxylase (GAD) was studied during development of the chick telencephalon. By means of reverse-phase HPLC analysis, we showed that GABA indeed accumulates during embryogenesis, whereas the levels of glutamate, the substrate for GAD, are more or less unchanged up to later developmental stages. The enzyme activity increased approximately 25-fold from embryonic day 3 to embryonic day 17. Immunoblotting data revealed that two GAD proteins, of approximately 65 and 67 kDa, were present during the period investigated. Furthermore, Northern blot analysis with probes obtained from rat cDNA sequences, as well as a chicken-specific probe for GAD65 generated by means of reverse transcriptase-polymerase chain reaction (RT-PCR), strengthened the interpretation that the chick embryo expresses genes corresponding to GAD65 and GAD67. The rat probes recognized transcript sizes of 3.9 kb (GAD65) and 5.6 kb (GAD67), sizes which are different from those of the rat brain (Erlander et al., Neuron, 7, 91-100, 1991). Sequencing of the RT-PCR products revealed a high level of homology (82% at the nucleotide level) between the mammalian and chick GAD65 genes. Taken together, these findings suggest that the chick embryo expresses two GAD genes during embryogenesis. The functional properties of each gene product remain to be investigated.

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.

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.

The development of GABA immunoreactivity in the retina of the zebrafish (Brachydanio rerio)

The goal of this study was to determine the pattern of gamma-aminobutyric acid (GABA) expression in the retina and optic nerve of the zebrafish (Brachydanio rerio) during embryonic development. Zebrafish embryos were fixed at intervals between 1 and 4 days postfertilization, and semithin plastic sections were prepared for postembedding immunocytochemistry with antisera against GABA. Sections were also prepared from several adult zebrafish eyes for comparison. GABA immunoreactivity first appeared in the optic nerve at 2 days postfertilization, and by 2.5 days the inner nuclear layer (INL), inner plexiform layer (IPL), retinal ganglion cell layer, and optic nerve were all positive for GABA. The GABA expression in the retinal ganglion cell layer and optic nerve was transient, however, and these structures were largely unlabeled by 4 days postfertilization. The pattern of GABA immunoreactivity at 4 days resembled that seen in the adult zebrafish: A large population of presumptive amacrine cells was labeled at the base of the INL, and the IPL was positive for GABA, as were occasional cells in the ganglion cell layer. Horizontal cells, particularly at the retinal margins, were also GABA positive beginning at about 3 days postfertilization. The transient expression of GABA in retinal ganglion cells and their axons during the period when synaptic contacts are being established both within the retina and between the retina and central targets suggests that GABA may have a role in the development of this system, in addition to serving as a classical neurotransmitter.

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)

Calcium dependence of differentiation of GABA immunoreactivity in spinal neurons

The developmental regulation of neurotransmitter synthesis has been extensively studied and appears in many cases to depend on electrical activity. The central nervous system of the Xenopus embryo and young larva is an attractive subject for such studies, since action potentials first elicited from Xenopus spinal neurons at the time of closure of the neural tube are long in duration and calcium-dependent. Moreover, cells exhibit spontaneous elevations of intracellular calcium during this early period as a consequence of calcium influx through voltage-dependent channels, which induces calcium release from intracellular stores. Since the early differentiation of Xenopus spinal neurons in dissociated cell culture parallels development in vivo, we have examined the maturation of gamma-aminobutyric acid (GABA) immunoreactivity in cultured neurons and explored its dependence on spontaneous calcium influx at early stages of development. We find that specific GABA immunoreactivity develops in spinal neurons in dissociated cell culture with the same time course previously defined in vivo. Additionally, this process requires calcium influx that occurs spontaneously through voltage-dependent channels. The appearance of GABA immunoreactivity is blocked by transcriptional inhibitors. The early appearance of GABA raises the possibility that it may play additional roles at early stages of development.

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.

Expression of gamma-aminobutyric acid and calcium binding protein-parvalbumin by chick motoneurons

The expression of calcium binding protein parvalbumin (PV) and gamma-aminobutyric acid (GABA) was studied in the chick motoneurons by using pre- and postembedding immunocytochemistry. Our data reveal that PV and GABA are colocalized in the majority, but not all, of chick lumbo-sacral spinal motoneurons innervating the somatic muscles. It is suggested that, in this neuromuscular system, GABA does not act as a classical inhibitory neurotransmitter but, combined with calcium, could be involved, at least in part, in the maintenance of neurons and the prevention of cell death as in certain neurodegenerative disorders.

Acetyl-L-carnitine has a neuromodulatory influence on neuronal phenotypes during early embryogenesis in the chick embryo

Studies from this laboratory and others have demonstrated that neuroblasts in early embryogenesis exhibit a high degree of plasticity with respect to neurotransmitter phenotype. The critical period within which these neuroblasts are sensitive to the effects of endogenous neurotrophins has been defined as 1-3 days of development in the chick embryo. In this study, we examined the influence of acetyl-L-carnitine (ALCAR) administered in ovo during embryonic days 1-3 (E1-E3) and sacrificed at embryonic day 8 (E8) on cholinergic and GABAergic neuronal phenotypes using as neuronal markers the activities of choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD), respectively. Phenotypic expression was assessed in 3 distinct anatomical regions of the embryonic brain: cerebral hemispheres (CH), optic lobes (OL), and diencephalon-midbrain-brainstem (DMBS). A single administration of ALCAR at embryonic day 1 resulted in a dose-dependent increase in ChAT activity and decrease in GAD activity in CH. ChAT activity was again increased and GAD activity decreased in CH from embryos that were administered ALCAR (100 micrograms/50 microliters/day) daily from embryonic day E1 to E3. No change was observed in either ChAT or GAD activity in OL in response to ALCAR administration during the critical period (E1-E3) at doses ranging from 10 to 500 micrograms/day. In the DMBS, the activity of ChAT exhibited a marked increase at lower doses (10 micrograms) followed by a marked decrease at higher doses (500 micrograms) of ALCAR. The decrease in ChAT activity in DMBS was again observed at an ALCAR dose of 100 micrograms/day when administered from E1 to E3.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient expression of GABA immunoreactivity in the developing rat spinal cord

The development of GABAergic neurons in the spinal cord of the rat has been investigated by immunocytochemical staining of frozen sections with anti-gamma-aminobutyric acid (GABA) antiserum. In the cervical cord, GABA-immunoreactive fibers first appeared at embryonic day (E) 13 in the presumptive white matter within the ventral commissure, ventral funiculus, and dorsal root entrance zone, and in the ventral roots. There were no GABA-immunoreactive cell bodies detected at this age. By E14, motoneurons, the earliest generated spinal cells, were the first cell population to become GABA-immunoreactive at the cell body level. Thereafter, GABA-immunoreactive neurons increased progressively in number and extended from ventral to dorsal regions. GABA-immunoreactive relay neurons within lamina I of the dorsal horn were initially detected at E17. Interneurons in the substantia gelatinosa, the latest generated cells in the spinal cord, were also the last to express the GABA immunoreactivity at E18. Immunoreactive neurons peaked in intensity and extent at E18 and 19. GABA immunoreactivity was only detectable in neurons within the intermediate and marginal zones 1-3 days after they withdrew from the cell cycle. This contrasts to glutamate decarboxylase immunoreactivity, which is detected in precursor cells in the ventricular zone prior to, or during, withdrawal from the cell cycle. Toward the end of gestation, GABA immunoreactivity declined in intensity and extent. This regression began in the ventral horn of the cervical region and ended in the dorsal horn of the lumbosacral region. During the first week after birth, immunoreactivity in motoneurons and in many other neurons within the ventral horn, intermediate gray, and deeper layers of the dorsal horn disappeared, and only in those neurons predominantly within the superficial layers of the dorsal horn did it persist into adulthood. Thus, the expression and regression of GABA immunoreactivity in the spinal cord followed ventral-to-dorsal, rostral-to-caudal, and medial-to-lateral gradients. These observations indicate that the majority of embryonic spinal neurons pass through a stage of transient expression of GABA immunoreactivity. The functional significance of this transient expression is unknown, but it coincides with the period of intense neurite growth of motoneurons, sensory neurons, and interneurons, and of neuromuscular junction formation, suggesting that the transient presence of GABA may play an important role in the differentiation of sensorimotor neuronal circuits.

GABA immunoreactive axons and growth cones in the developing chicken optic nerve and tract

Immunohistochemical studies of the chicken embryo optic tract using an antibody to gamma-aminobutyric acid (GABA) reveal that the tract is initially free of GABA immunoreactive axons. During the second week of incubation, GABA+ axons appear in the tract, chiasm, and optic nerve. The number of GABA+ axons in the optic nerve increases through E18, although few are recognizable after hatching. Detailed staining of GABA+ growth cones confirmed that virtually all the GABA+ axons in the optic nerve were growing toward the retina. Taken together, the findings suggest that the GABA+ axons in the chiasm and nerve are largely a transient extension of the GABA+ optic tract cells, the tectogeniculate projection, or both.

Age-dependent extrasynaptic modulation of axonal conduction by exogenous and endogenous GABA in the rat optic nerve

To ascertain whether endogenous gamma-aminobutyric acid (GABA) exists and exerts physiological effects in the optic nerve, we compared the effects of GABA and related drugs on the neonatal (1 to 22 days of age) and adult (greater than 6 months) rat optic nerve in vitro. GABA (10(-4)-10(-3) M) reversibly depressed the amplitude and increased the latency of compound action potentials in the neonatal optic nerve. In the adult optic nerve, GABA (10(-4)-10(-3) M) had no significant effect on the compound action potential. The GABA-A receptor agonist, isoguvacine (10(-4)-10(-3) M), mimicked these GABA effects on the neonate and adult optic nerve. Lower concentrations (10(-5) M) of GABA increased excitability of the neonatal optic nerve but produced no discernible effects on the adult optic nerve. The GABA-uptake inhibitor, nipecotic acid (10(-3) M), mimicked the effects of GABA (10(-5) M) on the neonatal optic nerve. The GABA-A receptor blockers, picrotoxin and bicuculline (10(-6)-10(-3) M), decreased the latency of compound action potentials in the neonatal optic nerve. Membrane potential recordings indicate that while GABA (10(-5)-10(-3) M) depolarized the neonatal optic nerve dose-dependently, picrotoxin hyperpolarized the axons. In the adult optic nerve, neither GABA-uptake inhibitors nor GABA-A receptor blockers had significant effects on the compound action potential. These results suggest that functional GABA-A receptors and GABA are present in the neonatal rat optic nerve and depolarize axons under physiological conditions. However, this does not appear to be the case in the adult optic nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone-releasing hormone and somatostatin influence neuronal expression in developing chick brain. III. GABAergic neurons

We have shown that the endogenous neuropeptides, growth hormone-releasing hormone (GHRH) and somatostatin (SRIF) influence expression of both cholinergic and catecholaminergic neuronal phenotypes in developing chick brain as assessed by the activities of choline acetyltransferase and tyrosine hydroxylase, respectively (Dev. Brain Res., 49 (1989) 275-280; Brain Research, 512 (1990) 297-303). In this study we examined the effects of GHRH and SRIF on GABAergic neuronal expression in ovo using activity of glutamate decarboxylase (GAD) as a neuronal marker. Chick embryos were administered GHRH or SRIF in ovo via the air sac on embryonic days 1, 3, 5 and 7, sacrificed at day 8 and the activity of GAD assayed in whole brain homogenates. GAD activity was significantly reduced in peptide-treated embryos as compared to controls. Similar results were obtained when GHRH was administered in a single dose at days 1 or 3 or when SRIF was administered in a single dose at day 3; GAD activity was significantly reduced as compared with control embryos. In contrast, embryos treated with either GHRH or SRIF on day 5 of development showed no difference in GAD activity as compared to controls. These data support our previous findings that endogenous neuropeptides such as GHRH and SRIF possess important properties with respect to neuronal phenotypic expression. They further define the critical period of sensitivity to these neuropeptides as 1-3 days of embryonic development.(ABSTRACT TRUNCATED AT 250 WORDS)

The formation of axonal pathways in developing cranial nerves

The roles of a variety of molecules including cell adhesion molecules and growth factors in the development of cranial nerves have begun to be understood in detail. In the course of embryonic development, cranial nerves are differentiated in concordance with the development of the metameric facial structure called 'ectomeres'. Each ectomere parallels the segmentation of the hindbrain called the 'rhombomere', in which pairs of metameric units cooperate to generate the repeating sequence of cranial branchiomotor nerves. A number of genes, including homeobox genes, are expressed in a rhombomere-specific pattern. For the formation of the olfactory nerve, it is suggested that several carbohydrate residues play important roles in receptor-target specificity. In the optic nerve, a combination of multiple cell adhesion molecules contributes to neurite growth in a developmental stage-specific manner. The development of the trigeminal nerve is under the control of both cell adhesion molecules and several growth factors. There is evidence that some of the adhesion molecules are expressed in a modality-specific way. There are also several molecules, such as 11p15 or TAG1/SNAP which are expressed only in selected cranial nerves. The growth rate of neurites also varies according to the individual nerves. Thus each cranial nerve has its own intrinsic properties and their outgrowth is the outcome of these properties and their interactions with surrounding non-neuronal tissues.

Expression of nerve growth factor (NGF) receptors in the brain and retina of chick embryos: comparison with cholinergic development

The expression of nerve growth factor receptor (NGFR) transcripts was investigated with in situ hybridization techniques in the CNS of chick embryos from 3 days of incubation (E3) to 14 days posthatch (P14). The time course and distribution of NGFR expression was compared with the development of the cholinergic phenotype. Cholinergic properties were assessed by immunolabeling for choline acetyltransferase (ChAT) and histochemistry for acetylcholinesterase (AchE) activity. NGFR transcripts are expressed transiently in the inner plexiform layer and ganglion cell layer of the retina (E4-P1), neostriatum and hippocampus (E18), infundibular hypothalamus (E7-18), spiriform complex (E9-15), layers 2, 3 (E9-18), and 10 (E11-18) of the optic tectum, nucleus mesencephalicus profundus, pars ventralis (E9-18), parvicellular isthmic nucleus (E7-P1), magnocellular isthmic nucleus (E9-E18), nucleus semilunaris (E7-18), isthmo-optic nucleus (E7-P14), rostral motor nuclei (E5-18), developing cerebellum (E7-15), internal granule cell layer (E11-18) and Purkinje cell layer (E15-P14) of the cerebellar cortex, and the inferior olivary nucleus (E9-15). A small number of neuronal populations with embryonic expression of NGFR remain strongly NGFR-positive in the posthatch animal:habenular nuclei (labeled after E5), nucleus subrotundus (after E9), mesencephalic trigeminal nucleus (after E5), caudal parts of locus ceruleus and nucleus subceruleus (after E7), medullar reticular nuclei (after E11), and motor nuclei IX, X, and XII (after E9). The majority of neuronal populations with NGFR expression show cholinergic properties in development, and NGFR expression always precedes the onset of ChAT immunoreactivity. Postnatal expression of growth factor receptors is largely confined to neurons of the reticular type. NGFR expression in avian CNS nuclei differs from that in mammals. Early loss of NGFR expression in the cholinergic basal forebrain (which remains strongly NGFR positive in mammals) and persistent NGFR expression in parts of the avian locus ceruleus indicate changes of growth factor receptor expression and growth factor requirements in phylogeny. Knowledge of the time and distribution of NGFR expression in the chick embryo will facilitate the assessment of specific functions of NGF and NGF-like molecules in an embryonic model with easy access for experimental manipulations.(ABSTRACT TRUNCATED AT 400 WORDS)

Ontogeny of the calcium binding protein parvalbumin in the rat nervous system

In the adult rat brain, the calcium-binding protein parvalbumin is preferentially associated with spontaneously fast-firing, metabolically active neurons and coexists with gamma-amino-butyric acid (GABA) in cortical inhibitory interneurons. Whether this is so in developing neurons has not been explored. To this end, we have used parvalbumin immunohistochemistry to study expression of this protein in the rat nervous system during pre- and postnatal life. Our results indicate that parvalbumin first appears at embryonic day 13 in sensory system of the spinal cord, in the vestibular (VIII), the trigeminal (V) and the visuomotor (III, IV, VI) systems, and develops rapidly during the following days. In these locations the expression of parvalbumin coincides with the beginning of physiological activity in nerve cells. In the gamma-aminobutyric acid (GABA)-containing interneurons of the cerebral cortex and the hippocampus, as well as in the Purkinje cells of the cerebellum, parvalbumin only appears postnatally. It lags behind the development of GABA-immunoreactivity by 1 to 2 weeks. The beginning of its expression, in the cerebellum at least, coincides with the arrival of excitatory synaptic input and the onset of spontaneous activity. Thus, during the development of the nervous system, the expression of parvalbumin is subordinate to the establishment of physiological activity.

Expression of GABA-immunoreactivity by spinal motoneurons of some vertebrates

Electrophysiological and biochemical investigations have shown that gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the vertebrate central nervous system. However, the present study shows that some motoneurons located in the spinal cord of young chickens and adult monkeys display a GABA-like immunoreactivity. The expression of GABA immunoreactivity in vertebrate motoneurons suggests that this inhibitory amino acid is colocalized with acetylcholine and could play a role in the neuromuscular transmission.