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

GABA immunoreactivity in the developing rat thalamus and Otx2 homeoprotein expression in migrating neurons

We investigated the expression of gamma-aminobutyric acid (GABA) in the developing rat thalamus by immunohistochemistry, using light, confocal and electron microscopy. We also examined the relationship between the expression of the homeoprotein Otx2, a transcription factor implicated in brain regionalization, and the radial and non-radial migration of early generated thalamic neurons, identified by the neuronal markers calretinin (CR) and GABA. The earliest thalamic neurons generated between embryonic days (E) 13 and 15 include those of the reticular nucleus, entirely composed by GABAergic neurons. GABA immunoreactivity appeared at E14 in immature neurons and processes laterally to the neuroepithelium of the diencephalic vesicle. The embryonic and perinatal periods were characterized by the presence of abundant GABA-immunoreactive fibers, mostly tangentially oriented, and of growth cones. At E15 and E16, GABA was expressed in radially and non-radially oriented neurons in the region of the reticular thalamic migration, between the dorsal and ventral thalamic primordia, and within the dorsal thalamus. At these embryonic stages, some CR- and GABA-immunoreactive migrating-like neurons, located in the migratory stream and in the dorsal thalamus, expressed the homeoprotein Otx2. In the perinatal period, the preponderance of GABAergic neurons was restricted to the reticular nucleus and several GABAergic fibers were still detectable throughout the thalamus. The immunolabeling of fibers progressively decreased and was no longer visible by postnatal day 10, when the adult configuration of GABA immunostaining was achieved. These results reveal the spatio-temporal features of GABA expression in the developing thalamus and suggest a novel role of Otx2 in thalamic cell migration.

Prenatal GABAB1 and GABAB2 receptors: cellular and subcellular organelle localization in early fetal rat cortical neurons

Gamma-aminobutyric acid (GABA)(B) receptors appear to influence developmental events, depending on whether they are found at a synapse or in extrasynaptic areas. Little, if anything, is known as to the cellular and subcellular localization of GABA(B1) and GABA(B2) receptors during early fetal development. We used Western blots, immunohistochemistry, and postembedding immunoelectronmicroscopy to investigate fetal rat brain expression and distribution of these receptor proteins. GABA(B1) is expressed as early as gestational day (GD) 11.5 and 12.5, with immunoreactivity found in the all neuroepithelium, and a high expression in the mantel zone and the cortical area's plate; no immunolabeling for GABA(B2) receptor was observed. Our immunogold studies define a pattern of early GABA(B1) receptor protein in dendrite processes, endoplasmic reticulum, and axon terminals of the cortical neuroepithelium on GD 11.5. On GD 12.5, GABA(B1) receptor immunogold was found in dendrite processes, spines and tree, axon terminals, mitochondria, and intracellular organelles of the cortical neuroepithelium. No synapse formation was apparent as no synaptophysin could be found on either GD 11.5 or 12.5. We suggest that GABA(B1) has a functional role in the early fetal brain during neuronal proliferation and migration, and that it is different from the established functional GABA(B) receptor.

Gabrb3 gene deficient mice exhibit increased risk assessment behavior, hypotonia and expansion of the plexus of locus coeruleus dendrites

Gabrb3 gene deficient (gabrb3(-/-)) mice, control littermates (gabrb3(+/+)) and their progenitor strains C57Bl/6J and 129/SvJ were assessed for changes in the morphology of the main noradrenergic nuclei, the locus coeruleus (LC) and LC-associated behaviors including anxiety and muscle tone. While the area defined by the cell bodies of the LC was found not to differ between gabrb3(-/-) mice and controls, the pericoerulear dendritic zone of the LC was found to be significantly enlarged in gabrb3(-/-) mice. Relative to controls, gabrb3(-/-) mice were also found to be hypotonic, as was indicated by poor performance on the wire hanging task. Gabrb3(-/-) mice also exhibited a significant increase in stretch-attend posturing, a form of risk assessment behavior associated with anxiety. However, in the plus maze, a commonly used behavioral test for assessing anxiety, no significant difference was observed between gabrb3(-/-) and control mice. Lastly, relative to controls, gabrb3(-/-) mice exhibited significantly less marble burying behavior, a method commonly used to assess obsessive-compulsive behavior. However, the poor marble burying performance of the gabrb3(-/-) mice could be associated with the hypotonic condition exhibited by these mice. In conclusion, the results of this study indicate that the gabrb3 gene contributes to LC noradrenergic dendrite development with the disruption of this gene in mice resulting in an enlarged plexus of LC dendrites with a concurrent reduction in muscle tone and marble burying behavior, an increase in risk assessment behavior but no change in the plus maze parameters that are commonly used for assessing anxiety.

Expression of the glycinergic system during the course of embryonic development in the mouse spinal cord and its co-localization with GABA immunoreactivity

To understand better the role of glycine and gamma-aminobutyric acid (GABA) in the mouse spinal cord during development, we previously described the ontogeny of GABA. Now, we present the ontogeny of glycine-immunoreactive (Gly-ir) somata and fibers, at brachial and lumbar levels, from embryonic day 11.5 (E11.5) to postnatal day 0 (P0). Spinal Gly-ir somata appeared at E12.5 in the ventral horn, with a higher density at the brachial level. They were intermingled with numerous Gly-ir fibers reaching the border of the marginal zone. By E13.5, at the brachial level, the number of Gly-ir perikarya sharply increased throughout the whole ventral horn, whereas the density of fibers declined in the marginal zone. In the dorsal horn, the first Gly-ir somata were then detected. From E13.5 to E16.5, at the brachial level, the density of Gly-ir cells remained stable in the ventral horn, and after E16.5 it decreased to reach a plateau. In the dorsal horn, the density of Gly-ir cells increased, and after E16.5 it remained stable. At the lumbar level, maximum expression was reached at E16.5 in both the ventral and dorsal horn. Finally, the co-localization of glycine and GABA was analyzed, in the ventral motor area, at E13.5, E15.5, and E17.5. The results showed that, regardless of developmental stage studied, one-third of the stained somata co-expressed GABA and glycine. Our data show that the glycinergic system matures 1 day later than the GABAergic system and follows a parallel spatiotemporal evolution, leading to a larger population of glycine cells in the ventral horn.

Perinatal subplate neuron injury: implications for cortical development and plasticity

Perinatal brain injury may result in widespread deficits in visual, motor and cognitive systems suggesting disrupted brain development. Neurosensory and cognitive impairment are observed at increasing frequency with decreasing gestational ages, suggesting a unique vulnerability of the developing brain. The peak of human subplate neuron development coincides with the gestational ages of highest vulnerability to perinatal brain injury in the premature infant. At the same time, human thalamocortical connections are forming and being refined by activity-dependent mechanisms during critical periods. Subplate neurons are the first cortical neurons to mature and are selectively vulnerable to early hypoxic-ischemic brain injury in animal models. Timing of subplate neuron death determines the resulting defect in thalamocortical development: very early excitotoxic subplate neuron death results in failure of thalamocortical innervation, while later subplate neuron death interferes with the refinement of thalamocortical connections into mature circuits. We suggest that subplate neuron injury may be a central component of perinatal brain injury resulting in specific neurodevelopmental consequences.

Zac1 is expressed in progenitor/stem cells of the neuroectoderm and mesoderm during embryogenesis: differential phenotype of the Zac1-expressing cells during development

Zac1, a new zinc-finger protein that regulates both apoptosis and cell cycle arrest, is abundantly expressed in many neuroepithelia during early brain development. In the present work, we study the expression of Zac1 during early embryogenesis and we determine the cellular phenotype of the Zac1-expressing cells throughout development. Our results show that Zac1 is expressed in the progenitor/stem cells of several tissues (nervous system, skeleton, and skeletal muscle), because they colocalize with several progenitor/stem markers (Nestin, glial fibrillary acidic protein, FORSE-1, proliferating cell nuclear antigen, and bromodeoxyuridine). In postnatal development, Zac1 is expressed in all phases of the life cycle of the chondrocytes (from proliferation to apoptosis), in some limbic gamma-aminobutyric acid-ergic neuronal subpopulations, and during developmental myofibers. Therefore, the intense expression of Zac1 in the progenitor/stem cells of different cellular lineages during the proliferative cycle, before differentiation into postmitotic cells, suggests that Zac1 plays an important role in the control of cell fate during neurogenesis, chondrogenesis, and myogenesis.

Developmental expression of GABA transporter-1 and 3 during formation of the GABAergic synapses in the mouse cerebellar cortex

In the brain, gamma-amino butyric acid (GABA), released extrasynaptically and synaptically from GABAergic neurons, plays important roles in morphogenesis, expression of higher functions and so on. In the GABAergic transmission system, plasma membrane GABA transporters (GATs) mediate GABA-uptake from the synaptic cleft in the mature brain and are thought to mediate diacrine of cytosolic GABA in the immature brain. In the present study, we focused on two GATs (GAT-1 and GAT-3) in the mouse cerebellar cortex, which are widely localized in neural and glial cells. Firstly, we examined the localization of GATs in the dendrites and cell bodies of developing GABAergic neurons, where GABA is extrasynaptically distributed, to clarify the GABA-diacrine before synaptogenesis. Secondly, we examined the developmental changes in the localization of GATs to reveal the development of the GABA-uptake system. Neither transporter was detected within the dendrites and cell bodies of GABAergic neurons, including Purkinje, stellate, basket and Golgi cells, in the immature cerebellar cortex. GAT-1 was observed within the Golgi cell axon terminals after postnatal day 5 (P5) and presynaptic axons of stellate and basket cells after P7. GAT-3 was localized within the astrocyte processes, sealing the GABAergic synapses in the Purkinje cell and granular layers after P10. These results indicated that GABA-diacrine did not work in the mouse cerebellar cortex. The onset of GAT-1-expression was prior to that of GAT-3. GAT-1 started to be localized within the GABAergic axon terminals during synapse formation. GAT-3 started to be localized within astrocyte processes when they sealed the synapses.

GABAB receptor subunit mRNAs are differentially regulated in pituitary melanotropes during development and detection of functioning receptors coincides with completion of innervation

This study examines the developmental expression of GABAB receptor subunits (GABAB(1a), GABAB(1b), GABAB(2)) in the pituitary intermediate lobe using in situ hybridization, reverse transcriptase-polymerase chain reaction, immunohistochemistry, and Western blots. Receptor functionality was studied by baclofen-stimulated GTPgammaS binding. In the adult rat pituitary all three transcripts were detected in melanotropes, but not in glia, of the intermediate lobe. No transcripts of any subunit were detected in the neural lobe. Transcripts of GABAB(1a) and GABAB(1b), but not of GABAB(2), were detected in specific subpopulations of cells in the anterior lobe. All three transcripts were detected in melanotropes on gestational day 18 using in situ hybridization. Reverse transcriptase-polymerase chain reactions comparing postnatal day 2 and adult transcript levels in the neurointermediate lobe support in situ hybridization data that GABAB(1a) mRNA levels do not change, GABAB(1b) levels increase, and GABAB(2) levels decrease as the rat matures. Thus, GABAB receptor subunit transcripts are differentially regulated in melanotropes during development. In the adult rat both GABAB(1) and GABAB(2) proteins were detected in the neurointermediate lobe using Western blotting and in melanotropes by immunohistochemistry. Developmentally, GABAB(1) protein was not detected until postnatal day 7, but was clearly expressed by postnatal day 15 while GABAB(2) protein could not be detected until postnatal day 15. Functional receptors were found in the intermediate lobe at postnatal day 15 and in the adult. The demonstration of transcripts for GABAB(1a), GABAB(1b) and GABAB(2) subunits at gestational day 18 contrasted with the failure to detect any protein before postnatal day 7, suggesting that the regulation of GABAB subunit isoforms occurs differentially at both the transcriptional and translational level as development progresses. The disparity in the regulation of the receptor subunits may suggest that GABAB(1) could have other functions besides being part of the GABAB receptor heterodimer.

GABAergic signaling in the developing cerebellum

In the adult central nervous system (CNS), GABA is a predominant inhibitory neurotransmitter that regulates glutamatergic activity. Recent studies have revealed that GABA serves as an excitatory transmitter in the immature CNS and acts as a trophic factor for brain development. Furthermore, synaptic transmission by GABA is also involved in the expression of higher brain functions, such as memory, learning and anxiety. These results indicate that GABA plays various roles in the expression of brain functions and GABAergic roles change developmentally in accordance with alterations in GABAergic transmission and signaling. We have investigated morphologically the developmental changes in the GABAergic transmission system and the key factors important for the formation of GABAergic synapses and networks using the mouse cerebellum, which provides an ideal system for the investigation of brain development. Here, we focus on GABA and GABA(A) receptors in the developing cerebellum and address the processes of how GABA exerts its effect on developing neurons and the mechanisms underlying the formation of functional GABAergic synapses.

Reversal of prenatal diazepam-induced deficit in a spatial-object learning task by brief, periodic maternal separation in adult rats

In the rat, prenatal exposure to diazepam (DZ) induces a permanent reduction in GABA/BZ receptor (R) function and behavioural abnormalities. Environmental modifications during early stages of life can influence brain development and induce neurobiological and behavioural changes throughout adulthood. Indeed, a subtle, periodic, postnatal manipulation increases GABA/BZ R activity and produces facilitatory effects on neuroendocrine and behavioural responses. We here investigated the impact of prenatal treatment with DZ on learning performance in adult 3- and 8-month-old male rats and the influence of a brief, periodic maternal separation on the effects exerted by prenatal DZ exposure. Learning performance was examined employing a non-aversive spatial, visual and/or tactile task, the "Can test". Behavioural reactivity, emotional state and fear/anxiety-driven behaviour were also examined using open field (OF), acoustic startle reflex (ASR) and elevated plus-maze (EPM) tests. A single daily injection of DZ (1.5mg/kg, s.c.), over gestational days (GD) 14-20, induced, in an age-independent manner, a severe deficit in learning performance, a decrease in locomotor and explorative activity and an increase in peak amplitude in the ASR. Furthermore, anxiety-driven behaviour in EPM was disrupted. Daily maternal separation for 15 min over postnatal days 2-21 exerted opposite effects in all the paradigms examined. Prenatally DZ-exposed maternal separated rats, in contrast to respective non-separated rats, showed an improvement in learning performance, a decrease in emotionality and a normalization of the exploratory behaviour in EPM. These results suggest that a greater maternal care, induced by separation, can serve as a source for the developing brain to enhance neuronal plasticity and to prevent the behavioural abnormalities induced by prenatal DZ exposure.

Pain reactivity in 2-month-old infants after prenatal and postnatal serotonin reuptake inhibitor medication exposure

OBJECTIVE

In this prospective study, we examined biobehavioral responses to acute procedural pain at 2 months of age in infants with prenatal and postnatal selective serotonin reuptake inhibitor (SSRI) medication exposure. Based on previous findings showing reduced pain responses in newborns after prenatal exposure, we hypothesized that altered pain reactivity would also be found at 2 months of age.

METHODS

Facial action (Neonatal Facial Coding System) and cardiac autonomic reactivity derived from the respiratory activity and heart rate variability (HRV) responses to a painful event (heel-lance) were compared between 3 groups of infants: (1) infants with prenatal SSRI exposure alone (n = 11; fluoxetine, n = 2; paroxetine, n = 9); (2) infants with prenatal and postnatal SSRI (via breast milk) exposure (total n = 30; fluoxetine, n = 6; paroxetine, n = 20; sertraline, n = 4); and (3) control infants (n = 22; nonexposed) during baseline, lance, and recovery periods. Measures of maternal mood and drug levels were also obtained, and Bayley Scales of Infant Development-II were administered at ages 2 and 8 months.

RESULTS

Facial action increased in all groups immediately after the lance but was significantly lower in the pSE group during the lance period. HR among infants in the pSE and ppSE groups was significantly lower during recovery. Using measures of HRV and the transfer relationship between heart rate and respiration, exposed infants had a greater return of parasympathetic cardiac modulation in the recovery period, whereas a sustained sympathetic response continued in control infants. Although postnatal exposure via breast milk was extremely low when infant drug levels could be detected in ppSE infants, changes in HR and HRV from lance to recovery were greater compared among infants with levels too low to be quantified. Neither maternal mood nor the presence of clonazepam influenced pain responses.

CONCLUSIONS

Blunted facial-action responses were observed among infants with prenatal SSRI exposure alone, whereas both prenatal and postnatal exposure was associated with reduced parasympathetic withdrawal and increased parasympathetic cardiac modulation during recovery after an acute noxious event. These findings are consistent with patterns of pain reactivity observed in the newborn period in the same cohort. Given that postnatal exposure via breast milk was extremely low and altered biobehavioral pain reactivity was not associated with levels of maternal reports of depression, these data suggest possible sustained neurobehavioral outcomes beyond the newborn period. This is the first study of pain reactivity in infants with prenatal and postnatal SSRI exposure, and our findings were limited by the lack of a depressed nonmedicated control group, small sample size, and understanding of infant behaviors associated with pain reactivity that could have also have been influenced by prenatal SSRI exposure. The developmental and clinical implications of our findings remain unclear, and the mechanisms that may have altered 5-hydroxytryptamine-mediated pain modulation in infants after SSRI exposure remain to be studied. Treating maternal depression with antidepressants during and after pregnancy and promoting breastfeeding in this setting should remain a key goal for all clinicians. Additional study is needed to understand the long-term effects of prenatal and early postnatal SSRI exposure.

Ethanol modulates the expression of GABAB receptor mRNAs in the prenatal rat brain in an age and area dependent manner

Prenatal ethanol exposure has various deleterious effects on neuronal development. As GABA(B) receptor is known to play an important role during the development of the CNS, we now focused on its mRNA expression pattern in the rat brain during the late gestational days (GD) from 15.5 to GD 21.5. Ethanol's effect was also observed from GD 11.5 to GD 21.5. GABA(B1) receptor mRNA showed a high expression level in GD 15.5 and 19.5, while GABA(B2) receptor mRNA did in GD 15.5 and 21.5. The mRNAs levels depended on age and area during development. Ethanol exposure decreased GABA(B1) receptor from GD 11.5 to GD 19.5 with slight increases in GD 21.5. The decreasing effects were area dependent, with the highest effects in the forebrain including cortex, whereas slight effects were observed in the midbrain and hindbrain. The present results suggest an important role of GABA(B) receptor in the effects of ethanol on prenatal brain developmental processes.

Expression of the sodium-driven chloride bicarbonate exchanger NCBE during prenatal mouse development

In the immature central nervous system (CNS) GABA-mediated excitation is thought to be an important developmental signal. It depends on a high intracellular chloride concentration ([Cl(-)](i)) of the particular neuron. [Cl(-)](i) is a consequence of chloride transport processes across the plasma membrane. The ongoing expression of the KCl-co-transporter KCC2 eventually lowers [Cl(-)](i) in most CNS neurons and thus renders GABA hyperpolarizing. As NCBE, a sodium-dependent chloride-bicarbonate exchanger, also lowers [Cl(-)](i) and may thus modulate the GABA-response, we analyzed its expression during prenatal mouse development before establishment of the mature KCC2 expression. Indeed, NCBE is expressed very early in CNS neurons and precedes the expression of KCC2. Unlike KCC2, NCBE is expressed in the peripheral nervous system and in non-neuronal tissues as the choroid plexus, the dura, and some epithelia including the acid secreting epithelium of the stomach and the duodenal epithelium.

Extrasynaptic localization of GABA in the developing mouse cerebellum

In the adult brain, gamma-amino butyric acid (GABA) is synaptically released and mediates inhibitory transmission. Recent studies have revealed that GABA is a trophic factor for brain development. To reveal the distribution of GABA and its secretion mechanisms during brain development, we investigated the immunohistochemical localization of two molecules, GABA and vesicular GABA transporter (VGAT), which is a GABAergic vesicle protein, in the developing mouse cerebellum by means of newly developed antibodies. Furthermore, we tested the relationship between developmental changes in distribution of above two molecules in the presynapses and ontogeny of GABAergic synapses. GABAergic synapses were detected by immunohistochemistry for the GABA(A) receptor alpha1 subunit, which is an essential subunit for inhibitory synaptic transmission in the mature cerebellar cortex. Until postnatal day 7 (P7), GABA was localized throughout the GABAergic neurons, and VGAT accumulated at axon varicosities and growth cones, where the alpha1 subunit did not accumulate. After P10, both GABA and VGAT became confined to the terminal sites where the alpha1 subunit was localized. These results suggested that GABA was extrasynaptically released from axon varicosities and growth cones by vesicular secretion 'exocytosis' and from all parts of GABAergic neurons during the cerebellar development by non-vesicular secretion 'diacrine'.

Energy metabolism in mammalian brain during development

Production of energy for the maintenance of ionic disequilibria necessary for generation and transmission of nerve impulses is one of the primary functions of the brain. This review attempts to link the plethora of information on the maturation of the central nervous system with the ontogeny of ATP metabolism, placing special emphasis on variations that occur during development in different brain regions and across the mammalian species. It correlates morphological events and markers with biochemical changes in activities of enzymes and pathways that participate in the production of ATP. The paper also evaluates alterations in energy levels as a function of age and, based on the tenet that ATP synthesis and utilization cannot be considered in isolation, investigates maturational profiles of the key processes that utilize energy. Finally, an attempt is made to assess the relevance of currently available animal models to improvement of our understanding of the etiopathology of various disease states in the human infant. This is deemed essential for the development and testing of novel strategies for prevention and treatment of several severe neurological deficits.

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

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

Differential involvement of phosphatidylinositol 3-kinase and p42/p44 mitogen activated protein kinase pathways in brain-derived neurotrophic factor-induced trophic effects on cultured striatal neurons

Brain-derived neurotrophic factor (BDNF) is a potent trophic factor for striatal cells that promotes survival and/or differentiation of GABAergic neurons in vitro. In the present study, we show that the stimulation of cultured striatal cells with BDNF increased the phosphorylation of Akt and p42/p44. This effect was specifically blocked by inhibitors of phosphatidylinositol 3-kinase (PI3-K) pathways (LY294002 and wortmannin) or p42/p44 mitogen-activated protein (MAP) kinase (PD98059 and U0126). BDNF treatment induced an increase in the number of calbindin-positive neurons but not in the number of GABAergic or total cells. Furthermore, BDNF increased the degree of dendritic arborization, soma area and axon length of striatal neurons. However, PD98059 was more effective blocking BDNF effects on calbindin- than on GABA-positive neurons, whereas LY294002 inhibited morphological differentiation in both neuronal populations. Moreover, BDNF induced neuronal survival only through the activation of the PI3-K pathway.

Ontogenic changes of the GABAergic system in the embryonic mouse spinal cord

Numerous studies have demonstrated an excitatory action of GABA early in development, which is likely to play a neurotrophic role. In order to better understand the role of GABA in the mouse spinal cord, we followed the evolution of GABAergic neurons over the course of development. We investigated, in the present study, the ontogeny of GABA immunoreactive (GABA-ir) cell bodies and fibers in the embryonic mouse spinal cord at brachial and lumbar levels. GABA-ir somata were first detected at embryonic day 11.5 (E11.5) exclusively at brachial level in the marginal zone. By E13.5, the number of GABAergic neurons sharply increased throughout the extent of the ventral horn both at brachial and lumbar level. Stained perikarya first appeared in the future dorsal horn at E15.5 and progressively invaded this area while they decreased in number in the presumed ventral gray matter. At E12.5, E13.5 and E15.5, we checked the possibility that ventral GABA-ir cells could belong to the motoneuronal population. Using a GABA/Islet-1/2 double labeling, we did not detect any double-stained neurons indicating that spinal motoneurons do not synthesize GABA during the course of development. GABA-ir fibers also appeared at the E11.5 stage in the presumptive lateral white matter at brachial level. At E12.5 and E13.5, GABA-ir fibers progressively invaded the ventral marginal zone and by E15.5 reached the dorsal marginal zone. At E17.5 and postnatal day 0 (P0), the number of GABA-ir fibers declined in the white matter. Finally, by P0, GABA immunoreactivity that delineated somata was mainly restricted to the dorsal gray matter and declined in intensity and extent. The ventral gray matter exhibited very few GABA-ir cell bodies at this neonatal stage of development. The significance of the migration of somatic GABA immunoreactivity from ventral to the dorsal gray matter is discussed.

Structural effects and neurofunctional sequelae of developmental exposure to psychotherapeutic drugs: experimental and clinical aspects

The advent of psychotherapeutic drugs has enabled management of mental illness and other neurological problems such as epilepsy in the general population, without requiring hospitalization. The success of these drugs in controlling symptoms has led to their widespread use in the vulnerable population of pregnant women as well, where the potential embryotoxicity of the drugs has to be weighed against the potential problems of the maternal neurological state. This review focuses on the developmental toxicity and neurotoxicity of five broad categories of widely available psychotherapeutic drugs: the neuroleptics, the antiepileptics, the antidepressants, the anxiolytics and mood stabilizers, and a newly emerging class of nonprescription drugs, the herbal remedies. A brief review of nervous system development during gestation and following parturition in mammals is provided, with a description of the development of neurochemical pathways that may be involved in the action of the psychotherapeutic agents. A thorough discussion of animal research and human clinical studies is used to determine the risk associated with the use of each drug category. The potential risks to the fetus, as demonstrated in well described neurotoxicity studies in animals, are contrasted with the often negative findings in the still limited human studies. The potential risk fo the human fetus in the continued use of these chemicals without more adequate research is also addressed. The direction of future research using psychotherapeutic drugs should more closely parallel the methodology developed in the animal laboratories, especially since these models have already been used extremely successfully in specific instances in the investigation of neurotoxic agents.

Transient expression of GABAA receptor alpha2 and alpha3 subunits in differentiating cerebellar neurons

In the adult mammalian brain, synaptic transmission mediated by gamma-amino butyric acid (GABA) plays a role in inhibition of excitatory synaptic transmission. During brain development, GABA is involved in brain morphogenesis. To clarify how GABA exerts its effect on immature neurons, we examined the expression of the GABAA receptor alpha2 and alpha3 subunits, which are abundantly expressed before alpha1 and alpha6 subunits appear, in the developing mouse cerebellum using in situ hybridization. Proliferating neuronal precursors in the ventricular zone and external granular layer expressed neither alpha2 nor alpha3 subunits. Hybridization signals for the alpha2 and alpha3 subunit mRNAs first appeared in the differentiating zone at embryonic day 13 (E13). The alpha2 subunit was detected in the migrating and differentiating granule cells and cerebellar nucleus neurons until postnatal day 14 (P14). Hybridization signals for the alpha3 subunit mRNA, on the other hand, were localized in the developing Purkinje cells and cerebellar nucleus neurons, and disappeared from Purkinje cells by the end of first postnatal week. Taken together, this indicated that the alpha2 and alpha3 subunits were abundantly expressed in distinct types of cerebellar neurons after completing cell proliferation while forming the neural network. These results suggest that GABA might extrasynaptically activate the GABAA receptors containing alpha2 and/or alpha3 subunits on the differentiating neurons before finishing the formation of synapses and networks, and could be involved in neuronal differentiation and maturation in the cerebellum.

Effects from prenatal exposure to alprazolam on the social behavior of mice offspring

OBJECTIVE

The purpose of this study was to conduct, in a randomized placebo-controlled manner, social behavior testing on mice offspring that were exposed prenatally to alprazolam.

STUDY DESIGN

A previously described, clinically relevant dose of 0.32 mg/kg of alprazolam (n=8 mice) or a placebo (n=9 mice) was given to gravid C57BL/6 mice by gavage on gestational day 18. Social play, sleep/wake patterns, and male aggression of the exposed offspring were assessed during prejuvenile, juvenile, and adult periods.

RESULTS

Alprazolam did not produce treatment differences in pregnancy outcomes or in dam-pup interactions. Compared with the placebo group, alprazolam-exposed offspring demonstrated less desire to escape (P<.01), more desire to remain alone (P<.02), and shorter periods of being awake (P<.03) on PND 17. Alprazolam-exposed male offspring exhibited more aggression on food restriction (P<.01) and on cage changing (P<.01).

CONCLUSION

Mice offspring that were exposed prenatally to alprazolam demonstrated more individual rather than group activities, avoidance of open areas, and aggression in males. Correlation of these findings in humans is encouraged.

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

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

Nature of inhibitory postsynaptic activity in developing relay cells of the lateral geniculate nucleus

Using intracellular recordings in an isolated (in vitro) brain stem preparation, we examined the inhibitory postsynaptic responses of developing neurons in the dorsal lateral geniculate nucleus (LGN) of the rat. As early as postnatal day (P) 1-2, 31% of all excitatory postsynaptic (EPSP) activity evoked by electrical stimulation of the optic tract was followed by inhibitory postsynaptic potentials (IPSPs). By P5, 98% of all retinally evoked EPSPs were followed by IPSP activity. During the first postnatal week, IPSPs were mediated largely by GABA(A) receptors. Additional GABA(B)-mediated IPSPs emerged at P3-4 but were not prevalent until after the first postnatal week. Experiments involving the separate stimulation of each optic nerve indicated that developing LGN cells were binocularly innervated. At P11-14, it was common to evoke EPSP/IPSP pairs by stimulating either the contralateral or ipsilateral optic nerve. During the third postnatal week, binocular excitatory responses were encountered far less frequently. However, a number of cells still maintained a binocular inhibitory response. These results provide insight about the ontogeny and nature of postsynaptic inhibitory activity in the LGN during the period of retinogeniculate axon segregation.

Substrates and routes of migration of early generated neurons in the developing rat thalamus

We investigated the substrates supporting neuronal migration, and its routes, during early thalamic development in the rat. Neurons and axonal and glial fibres were identified in embryos with single and double immunohistochemistry; dynamic data were obtained with cell tracers in short-term organotypic cultured slices. The earliest thalamic neurons, originating from the ventricular neuroepithelium between embryonic days 13 and 15, include those of the reticular thalamic nucleus. At this developmental stage, calretinin, calbindin or gamma-aminobutyric acid immunostaining revealed both radially and nonradially orientated neurons in the region of reticular thalamic migration, between the dorsal and ventral thalamic primordia. In cultured slices, injections of fluorescent dyes in the neuroepithelium labelled neurons in a migratory stream along radial glia in the same zone. Some labelled fusiform cells departed from this radial trajectory along orthogonal routes within the dorsal thalamus. Confocal microscopy revealed nonradially orientated neurons in close apposition with a fibre system parallel to the lateral thalamic surface. These fibres expressed axonal markers, including the intermediate filament protein alpha-internexin and a polysialylated form of neuronal cell adhesion molecule. Active migration of nonradially orientated neurons along neuronal substrates was confirmed in living cultured slices. In addition, in vitro and ex vivo experiments revealed neurons migrating tangentially in association with glial fibres. These results provide novel evidence that: (i) early generated thalamic neurons follow nonradial routes in addition to glia-linked radial migration; and (ii), nonradially migrating thalamic neurons move along both glial and axonal substrates, which could represent a distinctive feature of thalamic development.

Autocrine/paracrine activation of the GABA(A) receptor inhibits the proliferation of neurogenic polysialylated neural cell adhesion molecule-positive (PSA-NCAM+) precursor cells from postnatal striatum

GABA and its type A receptor (GABA(A)R) are present in the immature CNS and may function as growth-regulatory signals during the development of embryonic neural precursor cells. In the present study, on the basis of their isopycnic properties in a buoyant density gradient, we developed an isolation procedure that allowed us to purify proliferative neural precursor cells from early postnatal rat striatum, which expressed the polysialylated form of the neural cell adhesion molecule (PSA-NCAM). These postnatal striatal PSA-NCAM+ cells were shown to proliferate in the presence of epidermal growth factor (EGF) and formed spheres that preferentially generated neurons in vitro. We demonstrated that PSA-NCAM+ neuronal precursors from postnatal striatum expressed GABA(A)R subunits in vitro and in situ. GABA elicited chloride currents in PSA-NCAM+ cells by activation of functional GABA(A)R that displayed a typical pharmacological profile. GABA(A)R activation in PSA-NCAM+ cells triggered a complex intracellular signaling combining a tonic inhibition of the mitogen-activated protein kinase cascade and an increase of intracellular calcium concentration by opening of voltage-gated calcium channels. We observed that the activation of GABA(A)R in PSA-NCAM+ neuronal precursors from postnatal striatum inhibited cell cycle progression both in neurospheres and in organotypic slices. Furthermore, postnatal PSA-NCAM+ striatal cells synthesized and released GABA, thus creating an autocrine/paracrine mechanism that controls their proliferation. We showed that EGF modulated this autocrine/paracrine loop by decreasing GABA production in PSA-NCAM+ cells. This demonstration of GABA synthesis and GABA(A)R function in striatal PSA-NCAM+ cells may shed new light on the understanding of key extrinsic cues that regulate the developmental potential of postnatal neuronal precursors in the CNS.

Cellular physiology of the neonatal rat cerebral cortex

The early development of the cerebral cortex is characterized by neurogenesis, neuronal migration, cellular differentiation and programmed cell death. Cajal-Retzius cells, developing cortical plate neurons and subplate cells form a transient synaptic circuit which may serve as a template for the formation of cortical layers and columns. These three neuronal cell types show distinct electrophysiological properties and synaptic inputs. Endogenous or exogenous harmful disturbances during this developmental period may lead to the preservation of early cortical circuits, which may act as trigger zones for the initiation of pathophysiological activity.

Long-lasting behavioral changes induced by pre- or neonatal exposure to diazepam in rats

Prenatal treatment with small doses of diazepam may counteract the effect of physical stress in rats, normalizing the time course of neonatal reflexes and the behavioral responses in adulthood. However, prenatal administration of diazepam in greater doses may induce desensitization of gamma-aminobutyric acid A (GABA(A)) receptors and induce hypersensitivity to convulsants. This study was designed to examine in rats the influence of prenatal or neonatal diazepam treatment on development of neonatal reflexes, as index of brain maturation, and susceptibility to pentylenetetrazol-induced convulsions in adulthood. Pregnant Wistar rats were injected with diazepam 2.5 mg/kg/day, intraperitoneally (i.p.) on days 14-21 of pregnancy. Offspring showed a delay in the appearing of neonatal reflexes (cliff aversion, forelimb placing, forelimb grasping and bar holding) except for righting and startle reflexes. At 50 days of age, male rats showed a greater sensitivity to pentylenetetrazol compared to controls. In contrast, females showed a decreased susceptibility to convulsions. The appearance of reflexes in pups exposed to diazepam during neonatal life appeared to be similar to that of controls. Only the appearance of cliff aversion and startle reflexes appeared to be delayed in animals exposed neonatally to diazepam as compared to controls. In adulthood, female rats exposed in early neonatal life to diazepam again showed a resistance to pentylenetetrazol-induced convulsions as compared to male animals. These data suggest that prenatal exposure to diazepam induces long-lasting behavioral changes, which may be influenced by sex-dependent factors.

Postnatal development of the vesicular GABA transporter in rat cerebral cortex

Light and electron microscopic immunocytochemical techniques and Western blotting were used to investigate the postnatal development of the vesicular GABA transporter (VGAT) in the rat somatic sensory cortex. VGAT immunoreactivity was low at birth, it increased gradually through the first and second weeks of life and achieved the adult pattern during the third week. At postnatal day (P)0-P5, VGAT immunoreactivity was associated exclusively to fibers and puncta. Electron microscopic studies performed at P5 showed that all identified synaptic contacts formed by VGAT-positive axonal swellings were of the symmetric type and that a substantial proportion of the boutons appeared not to have formed synapses. From P10 onward, labeled puncta were both scattered in the neuropil and in apposition to unstained cellular profiles; VGAT was also expressed in few GABAergic cell bodies. Western blottings at the same postnatal ages revealed a 55-kDa band whose intensity was weak at P0 (17% of adult), it increased constantly until P15 (P2: 35%; P5: 44%; P10: 68%; P15: 97%), and then leveled off. Overall, the present results show that during neocortical development the expression of VGAT slightly precedes the complete maturation of inhibitory synaptogenesis and suggest that it may contribute to the formation of neocortical GABAergic circuitry.

Morphological development and neurochemical differentiation of cerebellar inhibitory interneurons in microexplant cultures

The cerebellar cortex comprises a rather limited variety of interneurons, prominently among them inhibitory basket and stellate cells and Golgi neurons. To identify mechanisms subserving the positioning, morphogenesis, and neurochemical maturation of these inhibitory interneurons, we analyzed their development in primary microexplant cultures of the early postnatal cerebellar cortex. These provide a well-defined, patterned lattice within which the development of individual cells is readily accessible to experimental manipulation and observation. Pax-2-positive precursors of inhibitory interneurons were found to effectively segregate from granule cell perikarya. They emigrate from the core explant and avoid the vicinity of granule cells, which also emigrate and aggregate into small clusters around the explant proper. This contrasts with the behavior of Purkinje neurons, which remain within the explant proper. During migration, a subset of Pax-2-positive cells gradually acquires a GABAergic phenotype, and subsequently also expresses the type 2 metabotropic receptor for glutamate, or parvalbumin, markers for Golgi neurons and basket or stellate cells, respectively. The latter eventually orient their dendrites such that they take a preferentially perpendicular orientation relative to granule cell axons. Both the neurochemical maturation of basket/stellate cells and the specific orientation of their dendrites are independent of their continuous contact with radially oriented glia or Purkinje cell dendrites projecting from the core explant. Numbers of parvalbumin-positive basket/stellate cells and the prevalence of glutamate-positive neurites, which form a dense network preferentially within cell clusters containing granule cell perikarya and their dendrites, are subject to regulation by chronic depolarization. In contrast, brain-derived neurotrophic factor results in a drastic decrease of numbers of basket/stellate cells. These findings document that granule cell axons (parallel fibers) are the major determinant of basket/stellate cell dendritic orientation. They also show that the neurochemical maturation of cerebellar interneurons is sensitive to regulation by activity and neurotrophic factors.

Metamorphic changes in GABA immunoreactivity in the brainstem of the bullfrog, Rana catesbeiana

We examined immunoreactivity for gamma-aminobutyric acid (GABA) in auditory and vestibular brainstem nuclei of the bullfrog, Rana catesbeiana, across metamorphosis, a developmental period featuring significant anatomical and functional remodeling of the nervous system. In the early larval period, GABA-immunoreactive cell somata were visible in the vestibular nucleus complex and the torus semicircularis, as well as in the spinal cord, cerebellum and optic tectum. Fiber bundles such as the medial longitudinal fasciculus and the lemnsical pathways also exhibited intense label at these early stages. In contrast, only diffuse neuropil label was visible in the dorsolateral nucleus and the superior olivary nucleus at the same stages. This diffuse immunoreactivity became progressively more reduced over larval development, and stained somata were visible in these medullary nuclei by metamorphic climax stages. In the torus semicircularis, the numbers of labeled somata in both the developing laminar and principal nuclei increased over metamorphic development, and became progressively more organized into distinct layers. The adult pattern of GABA-like immunoreactivity in the auditory brainstem was reached by metamorphic climax stages, coincident with the maturation of the opercularis system, and preceding the final development of the external tympanum and the tympanic conduction pathway. The relatively earlier maturation of vestibular, compared to auditory, areas in the medulla might reflect the behavioral importance of vestibular-mediated motor reactions during tadpole life. The distribution of GABA in auditory brainstem nuclei in both developing and adult frogs is comparable to that observed in mammals and birds.

Expression of the brain-specific membrane adapter protein p42IP4/centaurin alpha, a Ins(1,3,4,5)P4/PtdIns(3,4,5)P3 binding protein, in developing rat brain

Inositolphosphates and phosphatidylinositides are important second messengers. Previously p42(IP4), a protein with high affinity for both Ins(1,3,4,5)P(4) and PtdIns(3,4,5)P(3) has been characterized in our laboratory. In the present study mRNA levels of p42(IP4) were quantified during development (ages: 7, 14, 21 days and adult) by means of ribonuclease protection assay in various rat brain regions (cerebellum, cortex, striatum, thalamus, hypothalamus, olfactory bulb, hippocampus and tectum (superior and inferior colliculus)). A high level of p42(IP4) mRNA was detected in the cortex (ca. 1 pg specific RNA per microg of total RNA) which stayed highly independent of the age of the animals. In hippocampus and in the thalamus, p42(IP4) mRNA levels were comparable to those in the cortex in the first and second week postnatally, but decreased to lower levels in the adult brain. In striatum, the mRNA increased, albeit less intensely than in hippocampus and thalamus, until day 21 postnatally, and then decreased in the adult rat brain. Cerebellar p42(IP4) mRNA showed a slow increase within the first 3 weeks postnatally, and remained rather high in the adult brain. The protein expression of p42(IP4), tested within the same samples by Western blot staining, was consistent with mRNA values. For comparison, glutamic acid decarboxylase (isoforms GAD65/GAD67), an enzyme, for which some regional brain specific distribution is already known, was also examined. The mRNA levels of GAD and its developmental regulation clearly differed from that of p42(IP4). In summary, p42(IP4) expressed in several neuronal cell types, did not seem to be restricted to specific developmental stages, but the high absolute expression levels at all developmental stages indicated that p42(IP4) is a protein fundamental for neuronal functioning.

Postnatal development of high-affinity plasma membrane GABA transporters GAT-2 and GAT-3 in the rat cerebral cortex

We investigated the developmental profile of plasma membrane gamma-aminobutyric acid (GABA) transporters (GATs) GAT-2 and GAT-3 expression by immunocytochemistry with affinity-purified polyclonal antibodies in the rat neocortex. At all developmental ages investigated, GAT-2 ir was prominent in the arachnoid and in the trabeculae of the subarachnoid space, whereas it was weak within the cortical parenchyma; the adult pattern was reached during the third week of postnatal life. GAT-3 ir was present at birth and increased rapidly in the first week, when numerous positive cells were present throughout the cortical layers; at P10, GAT-3-positive cells became less numerous and GAT-3 ir switched to the adult pattern, which was expressed at P20. Confocal and electron microscopic investigations showed that GAT-3 positive cells were both neurons and astrocytes. The present evidence indicates that early in development GAT-3 is abundantly expressed in the cerebral cortex, where its expression appears to correlate with developmental variations in GABA levels, and suggests that it accounts for the largest fraction of GABA transport observed in the neonatal cerebral cortex.

The spatial relationship of gamma-aminobutyric acid (GABA) neurons and gonadotropin-releasing hormone (GnRH) neurons in larval and adult sea lamprey, Petromyzon marinus

In this study we examined the spatial relationship of GABA-containing and GnRH-containing neurons by immunocytochemistry and in situ hybridization in larval and adult brains of sea lamprey, Petromyzon marinus. In immunocytochemical studies, GABA-containing neurons were detected early in lamprey development, by day 20 post-fertilization. At this time point, one population of GABA-containing neurons was visualized in the hypothalamus and preoptic area, and another population was located in the olfactory bulb of the telencephalon. By day 30 after fertilization, after the GABA neurons were detected, GnRH-containing neurons were visualized in the preoptic area/rostral hypothalamus region, adjacent to the GABA-containing neurons in the wall of the third ventricle. Similarly, in adult lamprey brains distinct populations of both GABA- and GnRH-containing neurons were located in the hypothalamus adjacent to the third ventricle. To further establish a proximate relationship between GABA and GnRH, the mRNA for glutamate decarboxylase (GAD), the enzyme catalyzing GABA synthesis from glutamate, and GnRH were examined by in situ hybridization in the brains of larval lamprey. These studies also showed that GnRH and GAD are produced in cell populations in and around the third ventricle of the hypothalamus. This close spatial relationship of GABA neurons and GnRH neurons provides a basis for a regulatory role of GABA on GnRH neurons in the sea lamprey.

Unique developmental patterns of GABAergic neurons in rat spinal cord

Gamma-aminobutyric acid (GABA)ergic neurons have been postulated to compose an important component of local circuits in the adult spinal cord, yet their identity and axonal projections have not been well defined. We have found that, during early embryonic ages (E12-E16), both glutamic acid decarboxylase 65 (GAD65) and GABA were expressed in cell bodies and growing axons, whereas at older ages (E17-P28), they were localized primarily in terminal-like structures. To determine whether these developmental changes in GAD65 and GABA were due to an intracellular shift in the distribution pattern of GAD proteins, we used a spinal cord slice model. Initial experiments demonstrated that the pattern of GABAergic neurons within organotypic cultures mimicked the expression pattern seen in embryos. Sixteen-day-old embryonic slices grown 1 day in vitro contained many GAD65- and GAD67-labeled somata, whereas those grown 4 days in vitro contained primarily terminal-like varicosities. When isolated E14-E16 slices were grown for 4 days in vitro, the width of the GAD65-labeled ventral marginal zone decreased by 40-50%, a finding that suggests these GABAergic axons originated from sources both intrinsic and extrinsic to the slices. Finally, when axonal transport was blocked in vitro, the developmental subcellular localization of GAD65 and GAD67 was reversed, so that GABAergic cell bodies were detected at all ages examined. These data indicate that an intracellular redistribution of both forms of GAD underlie the developmental changes observed in GABAergic spinal cord neurons. Taken together, our findings suggest a rapid translocation of GAD proteins from cell bodies to synaptic terminals following axonal outgrowth and synaptogenesis.

Highly differential expression of SN1, a bidirectional glutamine transporter, in astroglia and endothelium in the developing rat brain

The transmitters glutamate and GABA also subserve trophic action and are required for normal development of the brain. They are formed from glutamine, which may be synthesized in glia or extracted from the blood. In the adult, the glutamine transporter SN1 is expressed in the astroglia. SN1 works in both directions, depending on the concentration gradients of its substrates and cotransported ions, and is thought to regulate extracellular glutamine and to supply the neurons with the transmitter precursor. In this article, we have quantified the expression and studied the localization of SN1 at different developmental stages. SN1 is expressed in astroglia throughout the CNS from embryonic stages through adulthood. No indication of SN1 staining in neuronal elements has been obtained at any stage. Quantitative immunoblotting of whole brain extracts demonstrates increasing expression of SN1 from P0, reaching a peak at P14, twice the adult level. A moderate and slower rise and fall of the expression levels of SN1 occurs in the cerebellum. Strong transient SN1-like staining is also found in Bergmann glia and vascular endothelium in the first postnatal weeks. Strong intracellular staining in the same time period suggests a high rate of SN1 synthesis in the early postnatal period. This coincides with the increasing levels of glutamate and GABA in the CNS and with the time course of synaptogenesis. This study suggests that the expression of SN1 is highly regulated, correlating with the demand for glutamine during the critical period of development.

Neurobehavior effects in four strains of mice offspring exposed prenatally to alprazolam

OBJECTIVE

This study was performed to determine whether prenatal exposure to alprazolam affects offspring behavior in different strains of mice.

STUDY DESIGN

Eight to 11 gravid mice of the C3H/He, C57BL/6, A/J, and DBA/2 strains were given either an anxiolytic dose of alprazolam (0.32 mg/kg) or a placebo by gavage on day 18 of an anticipated 19- to 21-day gestation. Neurobehavior tasks were conducted to assess anxiety, learning and memory, and social interaction. Data were analyzed by analysis of variance or a Fisher exact probability test.

RESULTS

Anxiety in alprazolam-exposed offspring was reduced in C3H/He (P <.05) and A/J (P <.05) newborn infants by separation vocalization but may be increased in the C3H/He adult strain on the plus maze task. Learning was slower among C57BL/6 mice exposed to alprazolam (P <.01), whereas memory was reduced in exposed A/J and DBA/2 offspring (P <.05). Alprazolam exposure was associated with more aggression among C3H/He and C57BL/6 male offspring (P <.01) and with less group activity by C57BL/6 offspring (P <.05).

CONCLUSION

Altered behaviors in several mouse strains after prenatal exposure to alprazolam suggests a vulnerability of GABA-benozdiazepine receptor formation in fetal brain development.

Gamma-aminobutyric acidB receptors and the development of the ventromedial nucleus of the hypothalamus

Gamma-aminobutyric acid (GABA) is a highly abundant neurotransmitter in the brain and the ligand for GABA(A), GABA(B), and GABA(C) receptors. Unlike GABA(A) and GABA(C) receptors, which are chloride channels, GABA(B) receptors are G-protein linked and alter cell-signaling pathways. Electrophysiological studies have found GABA(B) receptors in cultured embryonic hypothalamus, but the distribution of these receptors remains unknown. In the present study, we examined the expression of GABA(B) receptors in the ventromedial nucleus of the hypothalamus (VMH) during embryonic mouse development. GABA(B) receptors were present in the VMH at all ages examined, from embryonic day 13 to postnatal day 6. Using a brain slice preparation, we examined the effect of GABA(B) receptor activation on cell movement in the embryonic VMH as the nucleus forms in vitro. The GABA(B) receptor agonist baclofen decreased the rate of cell movement in a dose-dependent manner. Baclofen reduced cell movement by up to 56% compared with vehicle-treated controls. The percentage of cells moving per field and the angles of cell movement were not affected. With our previous findings of GABA(A) receptor activation, it is likely that GABA influences VMH development via multiple mechanisms.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

Relation Between Putative Transmitter Phenotypes and Connectivity of Subplate Neurons During Cerebral Cortical Development

During development, the earliest generated neurons of the mammalian telencephalon reside in a region of the white matter, the subplate, just beneath the cortical plate. Neurons in the subplate are only transiently present in the telencephalon: shortly after birth in the cat the majority have disappeared. During their brief life, however, subplate neurons mature; they extend long-distance and local projections, and express immunoreactivity for GABA and several neuropeptides. In the present study we examined the relation between possible transmitter phenotypes of subplate neurons and their connectivity. To do so, we used a double-label technique in which immunohistochemistry for neuropeptide Y (NPY), somatostatin (SRIF) or calbindin (CaBP) was combined with retrograde tracing. Experiments were performed in neonatal cats and in ferret kits at equivalent postconceptional ages, times when subplate neurons are numerous. Subplate neurons immunoreactive for neuropeptides and CaBP could be double-labelled by an injection of retrograde tracer either into the cortical plate or the white matter, indicating that this particular subset of subplate neurons can make local circuit projections. In contrast, peptide or CaBP immunoreactive subplate neurons could never be retrogradely labelled from a tracer injection into the thalamus. Taken together, these observations indicate that subplate neurons immunoreactive for NPY, SRIF and CaBP are likely to be interneurons exclusively. On the other hand, subplate neurons with long-distance projections to the thalamus or the contralateral hemisphere could be labelled by the retrograde transport of d-[3H]aspartate, suggesting that at least some projection subplate neurons might use an excitatory amino acid as a neurotransmitter. These results indicate that there is a defined relationship between the putative transmitter phenotypes of subplate neurons and their patterns of projection. Interneurons of the subplate express peptidergic properties while projection neurons to the thalamus may use an excitatory amino acid. Thus, these basic organizational features of the transient subplate are reminiscent of those found in the adult cortical layers.

Differential Expression of the GABAA Receptor Complex in the Dorsal Thalamus and Reticular Nucleus: An Immunohistochemical Study in the Adult and Developing Rat

The distribution of the GABAA receptor/benzodiazepine receptor/chloride channel complex was investigated in the thalamus of the rat by means of immunohistochemistry in adulthood, as well as during embryonic and postnatal development, using a monoclonal antibody. In adults, the immunoreactivity for the GABAA receptor complex was intensely expressed by neuronal processes throughout the dorsal thalamus. Neuronal perikaryal membranes were frequently outlined by punctate immunostaining; cell bodies, intrathalamic fibre bundles and the internal capsule did not display immunoreactivity for the GABAA receptor. Regional differences in the expression of the receptor were consistently observed: the immunostaining was much lighter in the thalamic reticular nucleus than in the dorsal thalamic nuclei and, among the latter, the anteroventral nucleus and the ventral nuclear complex displayed the most intense immunopositivity. Immunostaining for the GABAA receptor was already expressed in embryos at E14, and was homogeneously distributed throughout the neuropil of the dorsal and ventral thalamic primordia. During the first two postnatal weeks, a regional differentiation of the immunopositivity was appreciable in the thalamus, with a progressive reduction in the reticular nucleus and a parallel increase in the dorsal thalamic structures. Immunoreactive neuronal perikarya were not observed in the thalamus at any developmental stage. The expression of the GABAA receptor complex appeared to have reached a mature configuration by the end of the third postnatal week. These findings indicate that in adults the GABAA receptor is differentially expressed by thalamic nuclear structures, including the reticular nucleus. Furthermore, the maturation of the receptor in the thalamus undergoes a rearrangement during the first postnatal weeks that results in a considerable regression within the reticular nucleus.

AMPA receptor activation induces GABA release from neurons migrating tangentially in the intermediate zone of embryonic rat neocortex

In the intermediate zone of the embryonic rodent neocortex, neurons migrating tangentially from the basal ganglia express both functional amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors and gamma-aminobutyric acid (GABA). To test the hypothesis of GABA release triggered by AMPA receptor activation, we used whole-hemisphere cultures prepared from rat embryos (day 15). We observed a marked decrease in the number of detectable GABA-positive cells in the intermediate zone after exposure to T-AMPA. This effect was blocked by coapplying GYKI 53655, an AMPA receptor antagonist. The decrease in GABA immunolabelling induced by T-AMPA did not require extracellular calcium. In contrast, it was abolished after sodium substitution by choline, or after coapplication of nipecotic acid, a GABA transporter inhibitor. Exposure to high potassium reduced the number of detectable GABA-positive cells. These results are compatible with carrier-mediated GABA release consecutive to sodium influx. GABA released from neurons migrating tangentially in the intermediate zone after AMPA receptor activation may influence neighbouring elements including radially migrating postmitotic neurons, proliferating progenitors and possibly the tangential cells themselves.

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

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

Prolonged prenatal psychotropic medication exposure alters neonatal acute pain response

Selective serotonin reuptake inhibitors (SSRIs) and benzodiazepines are frequently used to treat maternal depression during pregnancy, however the effect of increased serotonin (5HT) and gamma-amino-butyric acid (GABA) agonists in the fetal human brain remains unknown. 5HT and GABA are active during fetal neurologic growth and play early roles in pain modulation, therefore, if prolonged prenatal exposure alters neurodevelopment this may become evident in altered neonatal pain responses. To examine biologic and behavioral effects of prenatal exposure, neonatal responses to acute pain (phenylketonuria heel lance) in infants with prolonged prenatal exposure were examined. Facial action (Neonatal Facial Coding System) and cardiac autonomic reactivity derived from the relationship between respiratory activity and short term variations of heart rate (HRV) were compared between 22 infants with SSRI exposure (SE) [fluoxetine (n = 7), paroxetine (n = 11), sertraline (n = 4)]; 16 infants exposed to SSRIs and clonazepam (SE+) [paroxetine (n = 14), fluoxetine (n = 2)]; and 23 nonexposed infants during baseline, lance, and recovery periods of a heel lance. Length of maternal SSRI use did not vary significantly between exposure groups-[mean (range)] SE:SE+ 183 (31-281):141 (54-282) d (p > 0.05). Infants exposed to SE and SE+ displayed significantly less facial activity to heel lance than control infants. Mean HR increased with lance, but was significantly lower in SE infants during recovery. Using measures of HRV and the transfer relationship between heart rate and respiration, SSRI infants had a greater return of parasympathetic cardiac modulation in the recovery period, whereas a sustained sympathetic response continued in the control group. Prolonged prenatal SSRI exposure appears to be associated with reduced behavioral pain responses and increased parasympathetic cardiac modulation in recovery following an acute neonatal noxious event. Possible 5HT-mediated pain inhibition, pharmacologic factors and the developmental course remain to be studied.

GAD and GABA transporter (GAT-1) mRNA expression in the developing rat hippocampus

Synaptic inhibition in the mammalian central nervous system is mostly mediated by GABA (gamma-aminobutyric acid). Inhibitory interneurons can be identified by staining for glutamate decarboxylase (GAD), the key enzyme which produces the transmitter. After release, GABA is removed from the extracellular space by specific transporters which are localized at the presynaptic endings of interneurons, in adjacent glial processes and, possibly, also in the postsynaptic target cell membranes. The GABAergic system undergoes profound functional and structural changes during the first 2 weeks of postnatal development, including migration of interneurons and changes in the level of expression and subcellular distribution of GABA transporters. We therefore analyzed the distribution of mRNA coding for GAD and GAT-1 (the main neuronal GABA transporter) in the developing rat hippocampus. Our data show that both transcripts are present in putative interneurons from the first postnatal day and exhibit a largely similar distribution throughout postnatal ontogenesis, with some specific differences in certain hippocampal subfields. Quantification of stained somata confirmed the postnatal redistribution of putative interneurons in the area dentata from dendritic layers towards the hilus. We also found a general staining of principal cell layers for both probes, which differs with postnatal age and between GAD and GAT-1 mRNA. Together, our data reveal a profound reorganization of the GABAergic system in the rat hippocampus during the first weeks of postnatal development.

Development of GABAergic neurons and their transporter in human temporal cortex

The gamma-aminobutyric acid (GABA) system plays an important role in the early development of the hippocampal formation. The immunohistochemical expression of gamma-aminobutyric acid transporter-1, GAT-1, in the human developing temporal cortex was examined, and the distribution of GAT-1 was compared with that of the 67-kDa isoform of glutamic acid decarboxylase as a marker of GABAergic neurons. Four postmortem tissue specimens from young patients with hippocampal sclerosis were also examined. GAT-1 immunoreactivity was present, with a few puncta, in the neuropil of the stratum oriens and in the molecular layer of the dentate gyrus from 21-22 weeks of gestation, and in the stratum lacunosum-moleculare from 26 weeks of gestation. The peak expression of GAT-1 was seen in early infancy and that of glutamic acid decarboxylase in the perinatal period. These findings may reflect the development of GABAergic inhibitory systems, and may be related to the seizure susceptibility in infancy and early childhood. In the temporal lobes with hippocampal sclerosis, GAT-1 immunoreactivity of the neuropil was preserved in the vicinity of the neuronal loss of the hippocampus. This finding may result from the neurotrophic function of GAT-1 and may be related to its ability of neuronal repair and plasticity in childhood.

GABA promotes survival but not proliferation of parvalbumin-immunoreactive interneurons in rodent neostriatum: an in vivo study with stereology

Amino-acid neurotransmitters regulate a wide variety of developmental processes in the mammalian CNS including neurogenesis, cell migration, and apoptosis. In order to investigate the role of GABA in early development of forebrain interneurons, we determined the survival of parvalbumin-immunoreactive GABAergic interneurons in the adult rat striatum following prenatal exposure to either GABA(A) receptor agonist or antagonist. Unbiased stereology was used to quantify parvalbumin-immunoreactive neuron number in the neostriatum of adult rats exposed to the drugs in utero, and the results were compared to pair-fed or vehicle controls. Embryos were exposed to the GABA(A) antagonist (bicuculline) or agonist (muscimol) during previously defined proliferative or post-proliferative periods for parvalbumin-immunoreactive interneurons. Unbiased stereology using the optical fractionator was used to estimate the total number of parvalbumin-immunoreactive neurons in neostriatum of experimental and control rats. No significant alteration in parvalbumin-immunoreactive neuron number was observed in rats treated with either bicuculline (1 or 2mg/kg/day) or muscimol (1mg/kg/day) during the proliferative phase. Administration of bicuculline during the post-proliferative phase significantly reduced parvalbumin-immunoreactive neuron number in the neostriatum. A concomitant decrease in neostriatal volume was also observed, suggesting that the effect is not restricted to parvalbumin-immunoreactive interneurons. Positional analysis revealed loss of normal regional distribution gradients for parvalbumin-immunoreactive neurons in neostriatum of rats exposed to bicuculline in the embryonic post-proliferative phase. This data collectively suggests that GABA promotes survival but not proliferation of parvalbumin-immunoreactive progenitors. GABA may also promote migration of subpopulations of interneurons that ultimately populate the ventral telencephalon.

GABA expression dominates neuronal lineage progression in the embryonic rat neocortex and facilitates neurite outgrowth via GABA(A) autoreceptor/Cl- channels

GABA emerges as a trophic signal during rat neocortical development in which it modulates proliferation of neuronal progenitors in the ventricular/subventricular zone (VZ/SVZ) and mediates radial migration of neurons from the VZ/SVZ to the cortical plate/subplate (CP/SP) region. In this study we investigated the role of GABA in the earliest phases of neuronal differentiation in the CP/SP. GABAergic-signaling components emerging during neuronal lineage progression were comprehensively characterized using flow cytometry and immunophenotyping together with physiological indicator dyes. During migration from the VZ/SVZ to the CP/SP, differentiating cortical neurons became predominantly GABAergic, and their dominant GABA(A) receptor subunit expression pattern changed from alpha4beta1gamma1 to alpha3beta3gamma2gamma3 coincident with an increasing potency of GABA on GABA(A) receptor-mediated depolarization. GABA(A) autoreceptor/Cl(-) channel activity in cultured CP/SP neurons dominated their baseline potential and indirectly their cytosolic Ca(2+) (Ca(2+)c) levels via Ca(2+) entry through L-type Ca(2+) channels. Block of this autocrine circuit at the level of GABA synthesis, GABA(A) receptor activation, intracellular Cl(-) ion homeostasis, or L-type Ca(2+) channels attenuated neurite outgrowth in most GABAergic CP/SP neurons. In the absence of autocrine GABAergic signaling, neuritogenesis could be preserved by depolarizing cells and elevating Ca(2+)c. These results reveal a morphogenic role for GABA during embryonic neocortical neuron development that involves GABA(A) autoreceptors and L-type Ca(2+) channels.