Comparative localization of mRNAs encoding two forms of glutamic acid decarboxylase with nonradioactive in situ hybridization methods

Alterations in GABAergic biomarkers in the autism brain: research findings and clinical implications

Autism is a pervasive developmental disorder characterized by repetitive stereotyped behavior, social-emotional deficits, and delayed or absent language abilities. There are known neuropathologies in the autism brain affecting limbic, cerebellar, and cortical structures but the neurochemical profile of affected individuals, revealed in postmortem tissue studies, is only recently emerging. One major component that appears highly impacted in autism is the GABAergic system. It is now apparent that there are widespread significant effects in many distributed regions in the autism brain revealed by histochemical, autoradiographic, and biochemical studies. The key synthesizing enzymes for GABA, glutamic acid decarboxylase type 65 and 67 (GAD65 and GAD67), are decreased in the cerebellum and closer examination of mRNA levels revealed that it is largely due to decreases in Purkinje cells and a subpopulation of larger dentate neurons as measured by in situ hybridization studies. Other cell types had either normal GAD levels (Golgi cells, smaller dentate interneurons, and stellate cells) or increased levels (basket cells). GABA receptor density, number, and protein expression are all decreased in the cerebellum and in select cortical areas. GABA(A) and GABA(B) subunit protein expression was significantly reduced in cerebellum, BA 9 and BA 40. Benzodiazepine binding sites were significantly reduced in the hippocampus and anterior cingulate cortex (BA 24). Taken together, data from these studies suggest that there is a marked dysregulation of the inhibitory GABA system in the autism brain affecting particular biomarkers localized to specific cell types and lamina likely influencing circuitry and behavior.

Specific regulation of NRG1 isoform expression by neuronal activity

Neuregulin 1 (NRG1) is a trophic factor that has been implicated in neural development, neurotransmission, and synaptic plasticity. NRG1 has multiple isoforms that are generated by usage of different promoters and alternative splicing of a single gene. However, little is known about NRG1 isoform composition profile, whether it changes during development, or the underlying mechanisms. We found that each of the six types of NRG1 has a distinct expression pattern in the brain at different ages, resulting in a change in NRG1 isoform composition. In both human and rat, the most dominant are types III and II, followed by either type I or type V, while types IV and VI are the least abundant. The expression of NRG1 isoforms is higher in rat brains at ages of E13 and P5 (in particular type V), suggesting roles in early neural development and in the neonatal critical period. At the cellular level, the majority of NRG1 isoforms (types I, II, and III) are expressed in excitatory neurons, although they are also present in GABAergic neurons and astrocytes. Finally, the expression of each NRG1 isoform is distinctly regulated by neuronal activity, which causes significant increase in type I and IV NRG1 levels. Neuronal activity regulation of type IV expression requires a CRE cis-element in the 5' untranslated region (UTR) that binds to CREB. These results indicate that expression of NRG1 isoforms is regulated by distinct mechanisms, which may contribute to versatile functions of NRG1 and pathologic mechanisms of brain disorders such as schizophrenia.

Brain expression and song regulation of the cholecystokinin gene in the zebra finch (Taeniopygia guttata)

The gene encoding cholecystokinin (Cck) is abundantly expressed in the mammalian brain and has been associated with such functions as feeding termination and satiety, locomotion and self-stimulation, the modulation of anxiety-like behaviors, and learning and memory. Here we describe the brain expression and song regulation of Cck in the brain of the adult male zebra finch (Taeniopygia guttata), a songbird species. Using in situ hybridization we demonstrate that Cck is highly expressed in several discrete brain regions, most prominently the caudalmost portion of the hippocampal formation, the caudodorsal nidopallial shelf and the caudomedial nidopallium (NCM), the core or shell regions of dorsal thalamic nuclei, dopaminergic cell groups in the mesencephalon and pons, the principal nucleus of the trigeminal nerve, and the dorsal raphe. Cck was largely absent in song control system, a group of nuclei required for vocal learning and song production in songbirds, although sparse labeling was detected throughout the striatum, including song nucleus area X. We also show that levels of Cck mRNA and the number of labeled cells increase in the NCM of males and females following auditory stimulation with conspecific song. Double labeling further reveals that the majority of Cck cells, excluding those in the reticular nucleus of the thalamus, are non-GABAergic. Together, these data provide the first comprehensive characterization of Cck expression in a songbird, and suggest a possible involvement of Cck regulation in important aspects of birdsong biology, such as perceptual processing, auditory memorization, and/or vocal-motor control of song production.

Identification of prolactin-sensitive GABA and kisspeptin neurons in regions of the rat hypothalamus involved in the control of fertility

High levels of circulating prolactin are known to cause infertility, but the precise mechanisms by which prolactin influences the neuroendocrine axis are yet to be determined. We used dual-label in situ hybridization to investigate whether prolactin-receptor (PRLR) mRNA is expressed in GnRH neurons. In addition, because γ-aminobutyric acidergic and kisspeptin neurons in the rostral hypothalamus are known to regulate GnRH neurons and, hence, might mediate the actions of prolactin, we investigated whether these neurons coexpress PRLR mRNA. (35)S-labeled RNA probes to detect PRLR mRNA were hybridized together with digoxigenin-labeled probes to detect either GnRH, Gad1/Gad2, or Kiss1 mRNA in the rostral hypothalamus of ovariectomized (OVX), estradiol-treated rats. Additional sets of serial sections were cut through the arcuate nucleus of OVX rats, without estradiol replacement, to examine coexpression of PRLR mRNA in the arcuate population of kisspeptin neurons. PRLR mRNA was highly expressed throughout the rostral preoptic area, particularly in periventricular regions surrounding the third ventricle, and there was a high degree of colocalization of PRLR mRNA in both Gad1/Gad2 and Kiss1 mRNA-containing cells (86 and 85.5%, respectively). In contrast, only a small number of GnRH neurons (<5%) was found to coexpress PRLR mRNA. In the arcuate nucleus of OVX rats, the majority of Kiss1 mRNA-containing cells also coexpressed PRLR mRNA. These data are consistent with the hypothesis that, in addition to a direct action on a small subpopulation of GnRH neurons, prolactin actions on GnRH neurons are predominantly mediated indirectly, through known afferent pathways.

Characterization of Kiss1 neurons using transgenic mouse models

Humans and mice with loss-of-function mutations of the genes encoding kisspeptins (Kiss1) or kisspeptin receptor (Kiss1r) are infertile due to hypogonadotropic hypogonadism. Within the hypothalamus, Kiss1 mRNA is expressed in the anteroventral periventricular nucleus (AVPV) and the arcuate nucleus (Arc). In order to better study the different populations of kisspeptin cells we generated Kiss1-Cre transgenic mice. We obtained one line with Cre activity specifically within Kiss1 neurons (line J2-4), as assessed by generating mice with Cre-dependent expression of green fluorescent protein or β-galactosidase. Also, we demonstrated Kiss1 expression in the cerebral cortex and confirmed previous data showing Kiss1 mRNA in the medial nucleus of amygdala and anterodorsal preoptic nucleus. Kiss1 neurons were more concentrated towards the caudal levels of the Arc and higher leptin-responsivity was observed in the most caudal population of Arc Kiss1 neurons. No evidence for direct action of leptin in AVPV Kiss1 neurons was observed. Melanocortin fibers innervated subsets of Kiss1 neurons of the preoptic area and Arc, and both populations expressed melanocortin receptors type 4 (MC4R). Specifically in the preoptic area, 18-28% of Kiss1 neurons expressed MC4R. In the Arc, 90% of Kiss1 neurons were glutamatergic, 50% of which also were GABAergic. In the AVPV, 20% of Kiss1 neurons were glutamatergic whereas 75% were GABAergic. The differences observed between the Kiss1 neurons in the preoptic area and the Arc likely represent neuronal evidence for their differential roles in metabolism and reproduction.

Heterogeneity of the supramammillary-hippocampal pathways: evidence for a unique GABAergic neurotransmitter phenotype and regional differences

The supramammillary nucleus (SuM) provides substantial projections to the hippocampal formation. This hypothalamic structure is involved in the regulation of hippocampal theta rhythm and therefore the control of hippocampal-dependent cognitive functions as well as emotional behavior. A major goal of this study was to characterize the neurotransmitter identity of the SuM-hippocampal pathways. Our findings demonstrate two distinct neurochemical pathways in rat. The first pathway originates from neurons in the lateral region of the SuM and innervates the supragranular layer of the dorsal dentate gyrus and, to a much lesser extent, the ventral dentate gyrus. This pathway displays a unique dual phenotype for GABAergic and glutamatergic neurotransmission. Axon terminals contain markers of GABAergic neurotransmission, including the synthesizing enzyme of GABA, glutamate decarboxylase 65, and the vesicular GABA transporter and also a marker of glutamatergic neurotransmission, the vesicular glutamate transporter 2. The second pathway originates from neurons in the most posterior and medial part of the SuM and innervates exclusively the inner molecular layer of the ventral dentate gyrus and the CA2/CA3a pyramidal cell layer of the hippocampus. The axon terminals from the medial part of the SuM contain the vesicular glutamate transporter 2 only. These data demonstrate for the first time the heterogeneity of the SuM-hippocampal pathways, not only from an anatomical but also a neurochemical point of view. These pathways, implicated in different neuronal networks, could modulate different hippocampal activities. They are likely to be involved differently in the regulation of hippocampal theta rhythm and associated cognitive functions as well as emotional behavior.

Differential distribution of activated spinal neurons containing glycine and/or GABA and expressing c-fos in acute and chronic pain models

The inhibitory transmitters GABA and glycine play an important role in modulating pain transmission, both in normal and in pathological situations. In the present study we have combined in situ hybridization for identifying spinal neurons that use the transmitter(s) glycine and/or GABA (Gly/GABA neurons) with immunohistochemistry for c-fos, a marker for neuronal activation. This procedure was used with acute pain models induced by the injection of capsaicin or formalin; and chronic pain models using Complete Freund's Adjuvant (CFA, chronic inflammation), and the spared nerve injury (SNI) model (neuropathic pain). In all models Gly/GABA neurons were activated as indicated by their expression of c-fos. The pattern of Gly/GABA neuronal activation was different for every model, both anatomically and quantitatively. However, the averaged percentage of activated neurons that were Gly/GABA in the chronic phase (≥20h survival, 46%) was significantly higher than in the acute phase (≤2h survival, 34%). In addition, the total numbers of activated Gly/GABA neurons were similar in both phases, showing that the activation of non-Gly/GABA (presumed excitatory) neurons in the chronic phase decreased. Finally, morphine application equally decreased the total number of activated neurons and activated Gly/GABA neurons. This showed that morphine did not specifically activate Gly/GABA neurons to achieve nociceptive inhibition. The present study shows an increased activity of Gly/GABA neurons in acute and chronic models. This mechanism, together with mechanisms that antagonize the effects of GABA and glycine at the receptor level, may determine the sensitivity of our pain system during health and disease.

Biodistribution and safety assessment of AAV2-GAD following intrasubthalamic injection in the rat

BACKGROUND

The steps necessary to translate promising new biological therapies to the clinic are poorly documented. For gene therapy, there are unique aspects that need to be addressed in biodistribution studies. Notably, the spread of the vector beyond the intended target cells or tissue may result in persistent unwanted biological activity or unpredictable biological events; thus, it is critical to evaluate the risks associated with viral vector-mediated gene transfer prior to embarking on human clinical trials.

METHODS

In the present study, we conducted a comprehensive assessment of vector biodistribution throughout the brain, blood and major organs of rats that had been injected via the subthalamic nucleus with recombinant adeno-associated virus (AAV) expressing glutamic acid decarboxylase (GAD). In addition, behavioral and histological analyses were also performed.

RESULTS

AAV genomes were not detected in blood or cerebrospinal fluid, and did not disseminate to organs outside of the brain in the majority of animals. In the brain, an average of 97.3% of AAV2-GAD genomes were restricted to the area of the ipsilateral subthalamic nucleus (STN). There were no discernable effects of AAV2-GAD on general health, and a behavioral assessment of the animals did not reveal any alteration in general behavior, exploration, locomotion or motor symmetry.

CONCLUSIONS

The present study met Food and Drug Administration requirements, in addition to efficacy and toxicity studies in rodents and nonhuman primates, to support and supplement a Phase II clinical trial invloving the gene transfer of AAV2-GAD to the human STN for the potential therapy of Parkinson's disease.

Male and female odors induce Fos expression in chemically defined neuronal population

Olfactory information modulates innate and social behaviors in rodents and other species. Studies have shown that the medial nucleus of the amygdala (MEA) and the ventral premammillary nucleus (PMV) are recruited by conspecific odor stimulation. However, the chemical identity of these neurons is not determined. We exposed sexually inexperienced male rats to female or male odors and assessed Fos immunoreactivity (Fos-ir) in neurons expressing NADPH diaphorase activity (NADPHd, a nitric oxide synthase), neuropeptide urocortin 3, or glutamic acid decarboxylase mRNA (GAD-67, a GABA-synthesizing enzyme) in the MEA and PMV. Male and female odors elicited Fos-ir in the MEA and PMV neurons, but the number of Fos-immunoreactive neurons was higher following female odor exposure, in both nuclei. We found no difference in odor induced Fos-ir in the MEA and PMV comparing fed and fasted animals. In the MEA, NADPHd neurons colocalized Fos-ir only in response to female odors. In addition, urocortin 3 neurons comprise a distinct population and they do not express Fos-ir after conspecific odor stimulation. We found that 80% of neurons activated by male odors coexpressed GAD-67 mRNA. Following female odor, 50% of Fos neurons coexpressed GAD-67 mRNA. The PMV expresses very little GAD-67, and virtually no colocalization with Fos was observed. We found intense NADPHd activity in PMV neurons, some of which coexpressed Fos-ir after exposure to both odors. The majority of the PMV neurons expressing NADPHd colocalized cocaine- and amphetamine-regulated transcript (CART). Our findings suggest that female and male odors engage distinct neuronal populations in the MEA, thereby inducing contextualized behavioral responses according to olfactory cues. In the PMV, NADPHd/CART neurons respond to male and female odors, suggesting a role in neuroendocrine regulation in response to olfactory cues.

Neuroanatomical characterization of endogenous opioids in the bed nucleus of the stria terminalis

Numerous neuroanatomical data indicate that the bed nucleus of the stria terminalis (BST) provides an interface between cortical and amygdaloid neurons, and effector neurons modulating motor, autonomic and neuroendocrine responses. Distinct divisions of the BST may be involved in stress response, homeostatic regulation, nociception, and motivated behaviors. Endogenous opioid peptides and receptors are expressed in the BST, but their exact distribution is poorly characterized. The present study used in situ hybridization in order to characterize the endogenous opioid system of the BST, focusing on both enkephalin and dynorphin neuropeptides, as well as their respective receptors (mu, delta, and kappa opioid receptors). We report that preprodynorphin mRNA is observed in distinct nuclei of the BST, namely the fusiform, oval and anterior lateral nuclei. In contrast, there is a widespread expression of preproenkephalin mRNA in both anterior and posterior divisions of the BST. Similarly, mu and kappa opioid receptors are broadly expressed in the BST, whereas delta opioid receptor mRNA was observed only in the principal nucleus. For further characterization of enkephalin-expressing neurons of the BST, we performed a double fluorescent in situ hybridization in order to reveal the coexpression of enkephalin peptides and markers of GABAergic and glutamatergic neurons. Although most neurons of the BST are GABAergic, there is also a modest population of glutamatergic cells expressing vesicular glutamate transporter 2 (VGLUT2) in specific nuclei of the BST. Finally, we identified a previously unreported population of enkephalinergic neurons expressing VGLUT2, which is principally located in the posterior BST.

GABAergic transmission to gonadotropin-releasing hormone (GnRH) neurons is regulated by GnRH in a concentration-dependent manner engaging multiple signaling pathways

Gonadotropin-releasing hormone (GnRH) neurons are the central regulators of fertility. GnRH stimulates or inhibits GnRH neuronal activity depending on dose. The mechanisms for these actions remain unknown. We hypothesized GnRH acts in part by altering fast synaptic transmission to GnRH neurons. GABAergic and glutamatergic postsynaptic currents (PSCs), both of which can excite these neurons, were recorded from GnRH neurons in brain slices from adult intact and orchidectomized (ORX) males. ORX enhanced the frequency of GABA transmission to GnRH neurons, but had no effect on glutamatergic transmission. Effects of ORX on GABAergic transmission were reversed by estradiol replacement, suggesting GABA is a mediator of steroid feedback in males. GABAergic neurons express type-1 GnRH receptor (GnRHR-1). Low GnRH (20 nm) reduced GABAergic PSC frequency in GnRH neurons from both ORX and intact mice. High GnRH (2 microm) had no effect on either GABAergic or glutamatergic transmission to GnRH neurons. To investigate mechanisms mediating low-dose GnRH suppression of GABAergic transmission, GABAergic PSCs were recorded after arresting G(alphai) activity with pertussis toxin (PTX). PTX abolished the suppressive effect of low GnRH. Moreover, PTX uncovered a stimulatory effect of high GnRH on GABAergic transmission. These data suggest low-dose GnRH suppresses GnRH firing rate in part by decreasing GABAergic transmission to the GnRH neurons, independent of gonadal hormone milieu. Low-dose GnRH appears to exert the suppressive effect by activating GnRHR-I coupled to G(alphai). The concentration-dependent effects of GnRH may be mediated in part by changes in affinity of GnRH to GnRHR-I coupled to different G(alpha) proteins.

Specific knock-down of GAD67 in the striatum using naked small interfering RNAs

Small interfering RNAs (siRNA) are double-stranded RNAs of 9-29 nucleotides designed to reduce the expression of homologous genes by a process known as RNA interference (RNAi). Most studies using siRNA in neurons have been performed in mammalian cell cultures. Only few reports have reported the effects of in vivo infusion of siRNA into the brain. In the present study, we performed local intracerebral infusions of naked siRNA against glutamic acid decarboxylase 67 (GAD67) mRNA to engineer specific knock-down of GAD67 protein in the striatum of adult rats. Directly injecting a mix of GAD67 siRNAs into the striatum decreased the levels of corresponding mRNA, evaluated by quantitative real-time PCR. In particular, we show that GAD67 mRNA expression is reduced in the striatum for 3, 6, and 24h following intrastriatal injection of GAD67 siRNA and is restored at 72h. Relative to controls, the levels of GAD67 protein were also lower in the striatum for 6 and 24h after injection. No changes in GAD65 expression, one of the two isoforms of GAD, were detected in the striatum, which further validates the specificity of the siRNA. We demonstrate the efficiency of the RNAi strategy for producing a specific and selective down-regulation of GAD67 in the adult rat brain. This suggests that siRNA-mediated gene knock-down constitute a valid methodological approach for studying the functional consequences of a transient decrease of a gene expression in a brain structure.

Identification of neurotransmitters and co-localization of transmitters in brainstem respiratory neurons

Identifying the major ionotropic neurotransmitter in a respiratory neuron is of critical importance in determining how the neuron fits into the respiratory system, whether in producing or modifying respiratory drive and rhythm. There are now several groups of respiratory neurons whose major neurotransmitters have been identified and in some of these cases, more than one transmitter has been identified in particular neurons. This review will describe the physiologically identified neurons in major respiratory areas that have been phenotyped for major ionotropic transmitters as well as those where more than one transmitter has been identified. Although the purpose of the additional transmitter has not been elucidated for any of the respiratory neurons, some examples from other systems will be discussed.

Signal intensities of radiolabeled cRNA probes used alone or in combination with non-isotopic in situ hybridization histochemistry

This study addressed the question of whether radioactive hybridization signal intensities are reduced in combined isotopic and non-isotopic double in situ hybridization (DISH) compared with those in single in situ hybridization (ISH). Non-isotopic digoxigenin (Dig)-labeled hybrids were detected using an alkaline phosphatase (AP) enzymatic reaction which results in nitroblue tetrazolium chloride (NBT)/5-bromo-4-chloro-3-indolyl phosphate (BCIP)-salt precipitation that could shield S35-radiation from penetrating to the surface. Sections were plastic coated of with 2% parlodion to prevent a chemical reaction between AP and developer during processing of the photosensitive emulsion, which could further reduce radioactive hybridization signal detection by autoradiography. We used DISH with a hybridization cocktail of radioactive S35- and Dig-labeled GAD67 cRNA probes. In order to avoid competition for the same complementary sequence, the probes were directed towards different sequences of the glutamic acid decarboxylase (GAD67) mRNA, resulting in co-detection of isotopic and non-isotopic hybrids in close to 100% of GAD67 positive cells. Quantitation of autoradiograms showed that there was no reduction of autoradiographic signal intensity from S35-labeled hybrids in the presence of Dig-labeled hybrids. Plastic coating of single or dual hybridized sections did not reduce the radioactive signal intensity. When mRNAs for nicotinic acetylcholine receptor (nAChR) subunits were detected with subunit specific S35-labeled cRNA probes in GAD67 hippocampal interneurons the total numbers of nAChR subunit expressing cells remained the same in single or double hybridized sections even for low abundant mRNAs. Together, these results indicate that combined radioactive and non-radioactive DISH does not interfere with the detection of the radiation signal from the S35-labeled hybrids, and neither specificity nor sensitivity is compromised.

Adult and embryonic GAD transcripts are spatiotemporally regulated during postnatal development in the rat brain

Background

GABA (gamma-aminobutyric acid), the main inhibitory neurotransmitter in the brain, is synthesized by glutamic acid decarboxylase (GAD). GAD exists in two adult isoforms, GAD65 and GAD67. During embryonic brain development at least two additional transcripts exist, I-80 and I-86, which are distinguished by insertions of 80 or 86 bp into GAD67 mRNA, respectively. Though it was described that embryonic GAD67 transcripts are not detectable during adulthood there are evidences suggesting re-expression under certain pathological conditions in the adult brain. In the present study we systematically analyzed for the first time the spatiotemporal distribution of different GADs with emphasis on embryonic GAD67 mRNAs in the postnatal brain using highly sensitive methods.

Methodology/Principal Findings

QPCR was used to precisely investigate the postnatal expression level of GAD related mRNAs in cortex, hippocampus, cerebellum, and olfactory bulb of rats from P1 throughout adulthood. Within the first three postnatal weeks the expression of both GAD65 and GAD67 mRNAs reached adult levels in hippocampus, cortex, and cerebellum. The olfactory bulb showed by far the highest expression of GAD65 as well as GAD67 transcripts. Embryonic GAD67 splice variants were still detectable at birth. They continuously declined to barely detectable levels during postnatal development in all investigated regions with exception of a comparatively high expression in the olfactory bulb. Radioactive in situ hybridizations confirmed the occurrence of embryonic GAD67 transcripts in the olfactory bulb and furthermore detected their localization mainly in the subventricular zone and the rostral migratory stream.

Conclusions/Significance

Embryonic GAD67 transcripts can hardly be detected in the adult brain, except for specific regions associated with neurogenesis and high synaptic plasticity. Therefore a functional role in processes like proliferation, migration or synaptogenesis is suggested.

Distribution and progression of amyloid-beta deposits in the amygdala of the aged macaque monkey, and parallels with zinc distribution

In this study, we have mapped amyloid beta (Abeta) deposition in the amygdala of five aged Japanese monkeys (from 23 to 30 years old). In brief, the aged monkey amygdala shows a topographic distribution of Abeta deposits that is subnucleus specific and exhibits a distinct temporal progression. The pattern is similar to the distribution of Abeta deposits in the human amygdala of Alzheimer's patients and of high plaque nondemented cases. The spatial distribution and temporal progression were correlated with the distribution of free zinc (Zn), which is known to mediate Abeta aggregation. For the basolateral group of subnuclei in particular, there is a clear dorsoventral gradient in the progressive distribution of Abeta. Abeta depositions first appear in the ventral division of the lateral nucleus and parvicellular division of the accessory basal nucleus, and then extend into the ventral part of the basal and paralaminar nuclei. All these nuclei are also Zn-dense. Conversely, Zn-weak nuclei, which are more dorsally situated (i.e. dorsal division of lateral nucleus and magnocellular division of basal nucleus) showed only a low level of Abeta deposits, even in brains with the greatest Abeta burden. In contrast to the basolateral group, the central and medial nuclei and cortical group had Abeta deposits only at later stages. In the central and medial nuclei, we identified a lateromedial gradient of Abeta deposits, again similar to the gradient of Zn-distribution. In the cortical group, Abeta deposits are densest in the deep layer, where Zn is also densest. Thus, we suggest the macaque amygdala, with its clear topographic distribution of Abeta deposits, may be an effective model for examining the complex mechanisms of vulnerability to Abeta deposits. A primate model would be advantageous for experimental interventions geared toward therapeutic protection from Alzheimer's disease, including by microarray analysis and genetic manipulation.

Decreased GAD65 mRNA levels in select subpopulations of neurons in the cerebellar dentate nuclei in autism: an in situ hybridization study

The laterally positioned dentate nuclei lie in a key position in the cerebellum to receive input from Purkinje cells in the lateral cerebellar hemisphere participating in both motor and cognitive functions. Although neuropathology of the four cerebellar nuclei using Nissl staining has been qualitatively reported in children and adults with autism, surprisingly the dentate nuclei appeared less affected despite reported reductions in Purkinje cells in the posterolateral cerebellar hemisphere. To determine any underlying abnormalities in the critically important GABAergic system, the rate-limiting GABA synthesizing enzyme, glutamic acid decarboxylase (GAD) type 65 was measured via in situ hybridization histochemistry in dentate somata. GAD65 mRNA labeling revealed two distinct subpopulations of neurons in adult control and autism postmortem brains: small-sized cells (about 10-12 microm in diameter, presumed interneurons) and larger-sized neurons (about 18-20 microm in diameter, likely feedback to inferior olivary neurons). A mean 51% reduction in GAD65 mRNA levels was found in the larger labeled cells in the autistic group compared with the control group (P=0.009; independent t-test) but not in the smaller cell subpopulation. This suggests a disturbance in the intrinsic cerebellar circuitry in the autism group potentially interfering with the synchronous firing of inferior olivary neurons, and the timing of Purkinje cell firing and inputs to the dentate nuclei. Disturbances in critical neural substrates within these key circuits could disrupt afferents to motor and/or cognitive cerebral association areas in the autistic brain likely contributing to the marked behavioral consequences characteristic of autism.

Origins of cortical GABAergic neurons in the cynomolgus monkey

In human most cortical gamma-aminobutyric acidergic (GABAergic) neurons are produced in the proliferative zones of the dorsal telencephalon in contrast to rodents. We report that in cynomolgus monkey fetuses cortical GABAergic neurons are generated in the proliferative zones of the dorsal telencephalon, in addition to the proliferative region of the ventral telencephalon, the ganglionic eminence (GE), however, with a temporal delay. GABAergic neuron progenitors labeled for Mash1 and GAD65 were present mainly in the GE at embryonic days (E) 47-55, and in the entire dorsal telencephalon at E64-75. These progenitors within the dorsal telencephalon are generated locally rather than in the GE. The ventral and dorsal lineages of cortical GABAergic neurons display different laminar distribution. Early generated GABAergic neurons from the GE mostly populate the marginal zone and subplate, whereas cortical plate GABAergic neurons originate from both ventral and dorsal telencephalon. A differential regulation of the two GABA synthesizing enzymes (GAD65 and GAD67) parallels GABAergic neuron differentiation. GAD65 is preferentially expressed in GABAergic progenitors and migrating neurons, GAD67 in morphologically differentiated neurons. Therefore, the dorsal telencephalic origin of cortical GABAergic neurons is not human-specific but appears as a former event in the ascent of evolution that could provide GABAergic neurons to an expending neocortex.

Phenotype, compartmental organization and differential vulnerability of nigral dopaminergic neurons

The degeneration of nigral dopaminergic (DA-) neurons is the histopathologic hallmark of Parkinson's disease (PD), but not all nigral DA-cells show the same susceptibility to degeneration. This starts in DA-cells in the ventrolateral and caudal regions of the susbtantia nigra (SN) and progresses to DA-cells in the dorsomedial and rostral regions of the SN and the ventral tegmental area, where many neurons remain intact until the final stages of the disease. This fact indicates a relationship between the topographic distribution of midbrain DA-cells and their differential vulnerability, and the possibility that this differential vulnerability is associated with phenotypic differences between different subpopulations of nigral DA-cells. Studies carried out during the last two decades have contributed to establishing the existence of different compartments of nigral DA-cells according to their neurochemical profile, and a possible relationship between the expression of some factors and the relative vulnerability or resistance of DA-cell subpopulations to degeneration. These aspects are reviewed and discussed here.

Expression and adhesion profiles of SynCAM molecules indicate distinct neuronal functions

Cell-cell interactions through adhesion molecules play key roles in the development of the nervous system. Synaptic cell adhesion molecules (SynCAMs) comprise a group of four immunoglobulin (Ig) superfamily members that mediate adhesion and are prominently expressed in the brain. Although SynCAMs have been implicated in the differentiation of neurons, there has been no comprehensive analysis of their expression patterns. Here we examine the spatiotemporal expression patterns of SynCAMs by using reverse transcriptase-polymerase chain reaction, in situ hybridization, and immunohistological techniques. SynCAMs 1-4 are widely expressed throughout the developing and adult central nervous system. They are prominently expressed in neurons throughout the brain and are present in both excitatory and inhibitory neurons. Investigation of different brain regions in the developing and mature mouse brain indicates that each SynCAM exhibits a distinct spatiotemporal expression pattern. This is observed in all regions analyzed and is particularly notable in the cerebellum, where SynCAMs display highly distinct expression in cerebellar granule and Purkinje cells. These unique expression profiles are complemented by specific heterophilic adhesion patterns of SynCAM family members, as shown by cell overlay experiments. Three prominent interactions are observed, mediated by the extracellular domains of SynCAMs 1/2, 2/4, and 3/4. These expression and adhesion profiles of SynCAMs together with their previously reported functions in synapse organization indicate that SynCAM proteins contribute importantly to the synaptic circuitry of the central nervous system.

Functional role of local GABAergic influences on the HPA axis

Neuronatomical and pharmacological studies have established GABA-mediated inhibition of the HPA axis at the level of the PVN. The origin of this innervation is a series of local hypothalamic and adjacent forebrain regions that project to stress-integrative hypophysiotropic CRH neurons. While a role in tonic inhibition of the stress axis is likely, this system of inhibitory loci is also capable of producing a dynamic braking capacity in the context of the neuroendocrine stress response. The latter function is mediated in large part by glutamatergic forebrain afferents that increase GABA release at the level of the PVN. In addition, this local GABA system can be inhibited by upstream GABAergic projection neurons, producing activation of the HPA axis via removal of GABAergic tone. This PVN projecting GABA network interfaces with a wide range of homeostatic mechanisms, and is capable of biochemical plasticity in response to chronic stress. Collectively, the elements of this system provide for exquisite control of neuroendocrine activation in the face of stressful stimuli, and loss of this regulatory capacity may underlie many stress-related disorders.

Loss of GABAergic neurons in the subiculum and its functional implications in temporal lobe epilepsy

Clinical and experimental evidence suggest that the subiculum plays an important role in the maintenance of temporal lobe seizures. Using the pilocarpine-model of temporal lobe epilepsy (TLE), the present study examines the vulnerability of GABAergic subicular interneurons to recurrent seizures and determines its functional implications. In the subiculum of pilocarpine-treated animals, the density of glutamic acid decarboxylase (GAD) mRNA-positive cells was reduced in all layers. Our data indicate a substantial loss of parvalbumin-immunoreactive neurons in the pyramidal cell and molecular layer whereas calretinin-immunoreactive cells were predominantly reduced in the molecular layer. Though the subiculum of pilocarpine-treated rats showed an increased intensity of GAD65 immunoreactivity, the density of GAD65 containing synaptic terminals in the pyramidal cell layer was decreased indicating an increase in the GAD65 intensity of surviving synaptic terminals. We observed a decrease in evoked inhibitory post-synaptic currents that mediate dendritic inhibition as well as a decline in the frequency of miniature inhibitory post-synaptic currents (mIPSCs) that are restricted to the perisomatic region. The decrease in mIPSC frequency (-30%) matched with the reduced number of perisomatic GAD-positive terminals (-28%) suggesting a decrease of pre-synaptic GABAergic input onto pyramidal cells in epileptic animals. Though cell loss in the subiculum has not been considered as a pathogenic factor in human and experimental TLE, our data suggest that the vulnerability of subicular GABAergic interneurons causes an input-specific disturbance of the subicular inhibitory system.

Characterization of GABAergic neurons in rapid-eye-movement sleep controlling regions of the brainstem reticular formation in GAD67-green fluorescent protein knock-in mice

Recent experiments suggest that brainstem GABAergic neurons may control rapid-eye-movement (REM) sleep. However, understanding their pharmacology/physiology has been hindered by difficulty in identification. Here we report that mice expressing green fluorescent protein (GFP) under the control of the GAD67 promoter (GAD67-GFP knock-in mice) exhibit numerous GFP-positive neurons in the central gray and reticular formation, allowing on-line identification in vitro. Small (10-15 microm) or medium-sized (15-25 microm) GFP-positive perikarya surrounded larger serotonergic, noradrenergic, cholinergic and reticular neurons, and > 96% of neurons were double-labeled for GFP and GABA, confirming that GFP-positive neurons are GABAergic. Whole-cell recordings in brainstem regions important for promoting REM sleep [subcoeruleus (SubC) or pontine nucleus oralis (PnO) regions] revealed that GFP-positive neurons were spontaneously active at 3-12 Hz, fired tonically, and possessed a medium-sized depolarizing sag during hyperpolarizing steps. Many neurons also exhibited a small, low-threshold calcium spike. GFP-positive neurons were tested with pharmacological agents known to promote (carbachol) or inhibit (orexin A) REM sleep. SubC GFP-positive neurons were excited by the cholinergic agonist carbachol, whereas those in the PnO were either inhibited or excited. GFP-positive neurons in both areas were excited by orexins/hypocretins. These data are congruent with the hypothesis that carbachol-inhibited GABAergic PnO neurons project to, and inhibit, REM-on SubC reticular neurons during waking, whereas carbachol-excited SubC and PnO GABAergic neurons are involved in silencing locus coeruleus and dorsal raphe aminergic neurons during REM sleep. Orexinergic suppression of REM during waking is probably mediated in part via excitation of acetylcholine-inhibited GABAergic neurons.

Novel neurons in ventral tegmental area fire selectively during the active phase of the diurnal cycle

The ventral tegmental area (VTA) contains dopamine (DA) and gamma-aminobutyric acid (GABA) neurons involved in motivation and behavioral state. These phenomena are also influenced by circadian factors. The goal of our studies was to examine the impulse activity of neurochemically identified VTA neurons during dark (active) vs light (rest) phases of the circadian cycle. Using extracellular single-unit recordings with juxtacellular labeling in anesthetized rats, we found multiple neuronal subpopulations including 'novel neurons' that selectively fired during the dark phase. These novel neurons were electrophysiologically categorized into two groups, 'novel wide-spike' and 'novel thin-spike' neurons. Characterization of novel wide-spike neurons found they were consistently non-dopaminergic and non-GABAergic [tyrosine hydroxylase (TH)(-), glutamic acid decarboxylase (GAD)(-)]. However, they were inhibited by the D2 agonist quinpirole, an effect that could be reversed by the D2 antagonist eticlopride. Physiologically, they were fast firing (mean = 18.9 +/- 1.2 spikes/s), low bursting neurons (median = 6.2 +/- 3.0% of spikes in bursts) with spike durations > or = 2.0 ms, but slightly shorter than TH(+) neurons. They were also consistently non-responsive to footpad stimulation. The novel thin-spike neurons were neurochemically heterogeneous, and were located more ventrally than thin-spike neurons found during the light phase. These findings reveal previously unknown populations of VTA neurons whose activities are sensitive to diurnal phase, and whose functions may be in the temporal regulation of arousal and motivational processes.

Acute ethanol induces Fos in GABAergic and non-GABAergic forebrain neurons: a double-labeling study in the medial prefrontal cortex and extended amygdala

The purpose of this study was to further address the hypothesis that ethanol activates GABAergic neurons in specific brain neurocircuits that mediate motivated behavior and control of action, such as the central extended amygdala and medial prefrontal cortex. Male Sprague-Dawley rats received habituation to 7 days of daily intragastric administration of water (5 ml/kg) followed by a single acute intragastric dose of ethanol (2.5 g/kg) or water then, 2 h later, by paraformaldehyde perfusion. Rats left undisturbed in the animal room throughout the experiment were also perfused (naive group). Brain sections were processed for single Fos immunohistochemistry or dual Fos immunohistochemistry/glutamic acid decarboxylase (GAD) mRNA in situ hybridization. Intragastric water administration increased the number of Fos-immunoreactive cells in the infralimbic cortex and lateral part of the central nucleus of the amygdala compared with the naive group. Ethanol administration increased the number of Fos-immunoreactive cells in the infralimbic (+57.5%) and prelimbic (+105.3%) cortices, nucleus accumbens shell region (+88.2%), medial part of the central nucleus of the amygdala (+160%), and lateral part of the bed nucleus of the stria terminalis (+198.8%) compared with the water-treated group. In the nucleus accumbens shell region, central nucleus of the amygdala, and bed nucleus of the stria terminalis, more than 80% of Fos-immunoreactive neurons were GABAergic after ethanol administration. In contrast, in the prelimbic cortex, 75% of Fos-immunoreactive neurons were not GABAergic. These results constitute new evidence for region-specific functional interactions between ethanol and GABAergic neurons.

Cortical-like functional organization of the pheromone-processing circuits in the medial amygdala

The medial amygdala (MeA) is a critical center for processing pheromonal signals that regulate social and reproductive behaviors, but the fundamental cellular mechanisms underlying signal processing in the MeA have remained largely unknown. Some studies suggest that the MeA belongs to the striatum and provides inhibitory output to hypothalamic areas including the ventromedial hypothalamus (VMH). By combining tract tracing, genetic labeling of GABAergic neurons, and immunostaining against markers for glutamatergic synapses, we found that a majority of MeA neurons projecting to the VMH are glutamatergic. Whole cell patch-clamp recordings revealed that VMH-projecting neurons form a homogeneous population in terms of morphological and intrinsic properties. Nearly all cells possess I(h) and I(T) and in some cases they can give rise to postinhibitory rebound spikes. Morphological analysis of neurobiotin-filled cells revealed neurons with long dendritic arbors that extend to the MeA external layer and within the amygdala. Thus the VMH-projecting neurons in the MeA differ from the medium spiny neurons, the principal neurons of striatum, in terms of intrinsic physiological properties and morphology. In contrast, they resemble a subset of pyramidal cells in deep piriform cortex. Similar to pyramidal cells in piriform cortex, the VMH-projecting neurons in the MeA received direct excitatory input from their upstream sensory areas and inhibitory input from local GABAergic neurons. We conclude that pheromonal signals relayed to the VMH are processed by unique cortical, but not striatal, circuitry in the MeA.

The neurochemically diverse intermedius nucleus of the medulla as a source of excitatory and inhibitory synaptic input to the nucleus tractus solitarii

Sensory afferent signals from neck muscles have been postulated to influence central cardiorespiratory control as components of postural reflexes, but neuronal pathways for this action have not been identified. The intermedius nucleus of the medulla (InM) is a target of neck muscle spindle afferents and is ideally located to influence such reflexes but is poorly investigated. To aid identification of the nucleus, we initially produced three-dimensional reconstructions of the InM in both mouse and rat. Neurochemical analysis including transgenic reporter mice expressing green fluorescent protein in GABA-synthesizing neurons, immunohistochemistry, and in situ hybridization revealed that the InM is neurochemically diverse, containing GABAegric and glutamatergic neurons with some degree of colocalization with parvalbumin, neuronal nitric oxide synthase, and calretinin. Projections from the InM to the nucleus tractus solitarius (NTS) were studied electrophysiologically in rat brainstem slices. Electrical stimulation of the NTS resulted in antidromically activated action potentials within InM neurons. In addition, electrical stimulation of the InM resulted in EPSPs that were mediated by excitatory amino acids and IPSPs mediated solely by GABA(A) receptors or by GABA(A) and glycine receptors. Chemical stimulation of the InM resulted in (1) a depolarization of NTS neurons that were blocked by NBQX (2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonoamide) or kynurenic acid and (2) a hyperpolarization of NTS neurons that were blocked by bicuculline. Thus, the InM contains neurochemically diverse neurons and sends both excitatory and inhibitory projections to the NTS. These data provide a novel pathway that may underlie possible reflex changes in autonomic variables after neck muscle spindle afferent activation.

Changes in vesicular transporters for gamma-aminobutyric acid and glutamate reveal vulnerability and reorganization of hippocampal neurons following pilocarpine-induced seizures

The reorganizations of the overall intrinsic glutamatergic and gamma-aminobutyric acid (GABA)-ergic hippocampal networks as well as the time course of these reorganizations during development of pilocarpine-induced temporal lobe epilepsy were studied with in situ hybridization and immunohistochemistry experiments for the vesicular glutamate transporter 1 (VGLUT1) and the vesicular GABA transporter (VGAT). These transporters are particularly interesting as specific markers for glutamatergic and GABAergic neurons, respectively, whose expression levels could reflect the demand for synaptic transmission and their average activity. We report that 1) concomitantly with the loss of some subpopulations of VGAT-containing neurons, there was an up-regulation of VGAT synthesis in all remaining GABA neurons as early as 1 week after pilocarpine injection. This enhanced synthesis is characterized by marked increases in the relative amount of VGAT mRNAs in interneurons associated with increased intensity of axon terminal labeling for VGAT in all hippocampal layers. 2) There was a striking loss of mossy cells during the latent period, demonstrated by a long-term decrease of VGLUT1 mRNA-containing hilar neurons and associated loss of VGLUT1-containing terminals in the dentate gyrus inner molecular layer. 3) There were aberrant VGLUT1-containing terminals at the chronic stage resulting from axonal sprouting of granule and pyramidal cells. This is illustrated by a recovery of VGLUT1 immunoreactivity in the inner molecular layer and an increased VGLUT1 immunolabeling in the CA1-CA3 dendritic layers. These data indicate that an increased activity of remaining GABAergic interneurons occurs during the latent period, in parallel with the loss of vulnerable glutamatergic and GABAergic neurons preceding the reorganization of glutamatergic networks.

Quantitative analysis of glutamatergic and GABAergic neurons expressing 5-HT(2A) receptors in human and monkey prefrontal cortex

5-hydroxytryptamine (5-HT) or serotonin 2A receptors play an important role in modulation of prefrontal cortex (PFC) activity and have been implicated in the physiopathology of psychiatric disorders. There is no quantitative information on the percentage of glutamatergic and GABAergic cells that express 5-HT(2A) receptors in human and monkey PFC. We have used double in situ hybridization to quantify the mRNA co-localization of 5-HT(2A) receptor with the glutamatergic transporter vesicular glutamate transporter 1, and with the GABAergic marker glutamic acid decarboxylase 65/67 and in parvalbumin and calbindin GABAergic cell populations. Our results show that nearly every glutamatergic cell (86-100%) in layers II-V expressed 5-HT(2A) receptor mRNA in both species. This percentage was lower in layer VI (13-31%). In contrast, not all the GABAergic interneurons (13-46%) expressed 5-HT(2A) receptor mRNA. This receptor was expressed in 45-69% of parvalbumin and in 61-87% of calbindin positive cells. These results indicate that, while the majority of glutamatergic neurons can be sensitive to 5-HT action via 5-HT(2A) receptors, this modulation occurs only in a limited population of GABAergic interneurons and provides new neuroanatomical information about the role played by serotonin through 5-HT(2A) receptors in the PFC and on the sites of action for drugs such as antipsychotics and antidepressants used in treatment of psychiatric disorders.

Gamma-aminobutyric acid-containing sympathetic preganglionic neurons in rat thoracic spinal cord send their axons to the superior cervical ganglion

Gamma-aminobutyric acid (GABA)-containing fibers have been observed in the rat superior cervical ganglion (SCG) and, to a lesser extent, in the stellate ganglion (STG). The aim of present study is to clarify the source of these fibers. No cell body showed mRNAs for glutamic acid decarboxylases (GADs) or immunoreactivity for GAD of 67 kDa (GAD67) in the cervical sympathetic chain. Thus, GABA-containing fibers in the ganglia are suggested to be of extraganglionic origin. GAD67-immunoreactive fibers were found not in the dorsal roots or ganglia, but in the ventral roots, so GABA-containing fibers in the sympathetic ganglia were considered to originate from the spinal cord. Furthermore, almost all GAD67-immunoreactive fibers in the sympathetic ganglia showed immunoreactivity for vesicular acetylcholine transporter, suggesting that GABA was utilized by some cholinergic preganglionic neurons. This was confirmed by the following results. 1) After injection of Sindbis/palGFP virus into the intermediolateral nucleus, some anterogradely labeled fibers in the SCG were immunopositive for GAD67. 2) After injection of fluorogold into the SCG, some retrogradely labeled neurons in the thoracic spinal cord were positive for GAD67 mRNA. 3) When the ventral roots of the eighth cervical to the fourth thoracic segments were cut, almost all GAD67- and GABA-immunoreactive fibers disappeared from the ipsilateral SCG and STG, suggesting that the vast majority of GABA-containing fibers in those ganglia were of spinal origin. Thus, the present findings strongly indicate that some sympathetic preganglionic neurons are not only cholinergic but also GABAegic.

Decreased GAD67 mRNA levels in cerebellar Purkinje cells in autism: pathophysiological implications

The recent identification of decreased protein levels of glutamate decarboxylase (GAD) 65 and 67 isoforms in the autistic cerebellar tissue raises the possibility that abnormal regulation of GABA production in individual neurons may contribute to the clinical features of autism. Reductions in Purkinje cell number have been widely reported in autism. It is not known whether the GAD changes also occur in Purkinje cells at the level of transcription. Using a novel approach, the present study quantified GAD67 mRNA, the most abundant isoform in Purkinje cells, using in situ hybridization in adult autistic and control cases. The results indicate that GAD67 mRNA level was reduced by 40% in the autistic group (P < 0.0001; two-tailed t test), suggesting that reduced Purkinje cell GABA input to the cerebellar nuclei potentially disrupts cerebellar output to higher association cortices affecting motor and/or cognitive function. These findings may also contribute to the understanding of previous reports of alterations in the GABAergic system in limbic and cerebro-cortical areas contributing to a more widespread pathophysiology in autistic brains.

Glutamic acid decarboxylase 65, 67, and GABA-transaminase mRNA expression and total enzyme activity in the goldfish (Carassius auratus) brain

GAD65 and GAD67 are the two major isoforms of the enzyme that converts glutamate into GABA in a single step reaction. Despite studies describing GAD65 and GAD67 mRNA expression in the mammalian brain, both GAD65 and GAD67 mRNA expression has not yet been fully described for a non-mammalian vertebrate model. Similarly, the expression patterns of GABA-T mRNA, the major enzyme involved in metabolizing GABA, have not been described for any vertebrate. In the present study, we utilized non-radioactive in situ hybridization to localize GAD65, GAD67, and GABA-T in the adult goldfish brain and complimented this with an in vitro assessment of total GAD and GABA-T enzyme activities. A partial fragment of goldfish GABA-T was cloned for a riboprobe that showed approximately 92% deduced amino acid identity to zebrafish GABA-T and 78% identity to human GABA-T. Transcripts for GAD65, GAD67, and GABA-T were detected throughout the brain and were detected largely in the medial and ventral regions of the telencephalon, nucleus preopticus, nucleus recessus lateralis of the hypothalamus, and Purkinje cell layer of the cerebellum. GAD65 mRNA was significantly more abundant in the nucleus recessus posterioris of the hypothalamus than GAD67 and GABA-T mRNA. Total GAD and GABA-T specific enzyme activity was highest in the hypothalamus and optic tectum and GABA-T activity was significantly higher than total GAD enzyme activity. Our results show that GAD65, GAD67, and GABA-T mRNAs are generally correlated with total GAD and GABA-T activity and all three transcripts have a largely overlapping mRNA distribution in the goldfish forebrain.

Classification of projection neurons and interneurons in the rat lateral amygdala based upon cluster analysis

Neurons in the rat lateral amygdala in situ were classified based upon electrophysiological and molecular parameters, as studied by patch-clamp, single-cell RT-PCR and unsupervised cluster analyses. Projection neurons (class I) were characterized by low firing rates, frequency adaptation and expression of the vesicular glutamate transporter (VGLUT1). Two classes were distinguished based upon electrotonic properties and the presence (IB) or absence (IA) of vasointestinal peptide (VIP). Four classes of glutamate decarboxylase (GAD67) containing interneurons were encountered. Class III reflected "classical" interneurons, generating fast spikes with no frequency adaptation. Class II neurons generated fast spikes with early frequency adaptation and differed from class III by the presence of VIP and the relatively rare presence of neuropeptide Y (NPY) and somatostatin (SOM). Class IV and V were not clearly separated by molecular markers, but by membrane potential values and spike patterns. Morphologically, projection neurons were large, spiny cells, whereas the other neuronal classes displayed smaller somata and spine-sparse dendrites.

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.

Acute changes in the neuronal expression of GABA and glutamate decarboxylase isoforms in the rat piriform cortex following status epilepticus

The piriform cortex (PC) is the largest region of the mammalian olfactory cortex with strong connections to other limbic structures, including the amygdala, hippocampus, and entorhinal cortex. In addition to its functional importance in the classification of olfactory stimuli, the PC has been implicated in the study of memory processing, spread of excitatory information, and the facilitation and propagation of seizures within the limbic system. Previous data from the kindling model of epilepsy indicated that alterations in GABAergic inhibition in the transition zone between the anterior and posterior PC, termed here central PC, are particularly involved in the processes underlying seizure propagation. In the present study we studied alterations in GABAergic neurons in different parts of the PC following seizures induced by kainate or pilocarpine in rats. GABA neurons were labeled either immunohistochemically for GABA or its synthesizing enzyme glutamate decarboxylase (GAD) or by in situ hybridization using antisense probes for GAD65 and GAD67 mRNAs. For comparison with the PC, labeled neurons were examined in the basolateral amygdala, substantia nigra pars reticulata, and the hippocampal formation. In the PC of controls, immunohistochemical labeling for GABA and GAD yielded consistently higher neuronal densities in most cell layers than labeling for GAD65 or GAD67 mRNAs, indicating a low basal activity of these neurons. Eight hours following kainate- or pilocarpine-induced seizures, severe neuronal damage was observed in the PC. Counting of GABA neurons in the PC demonstrated significant decreases in densities of neurons labeled for GABA or GAD proteins. However, a significantly increased density of neurons labeled for GAD65 and GAD67 mRNAs was determined in layer II of the central PC, indicating that a subpopulation of remaining neurons up-regulated the mRNAs for the GAD isoenzymes. One likely explanation for this finding is that remaining GABA neurons in layer II of the central PC maintain high levels of activity to control the increased excitability of the region. In line with previous studies, an up-regulation of GAD67 mRNA, but not GAD65 mRNA, was observed in dentate granule cells following seizures, whereas no indication of such up-regulation was determined for the other brain regions examined. The data substantiate the particular susceptibility of the central PC to seizure-induced plasticity and indicate that this brain region provides an interesting tool to study the regulation of GAD isoenzymes.

Stable expression of the vesicular GABA transporter following photothrombotic infarct in rat brain

Before exocytotic release of the inhibitory neurotransmitter GABA, this amino acid has to be stored in synaptic vesicles. Accumulation of GABA in vesicles is achieved by a specific membrane-integrated transporter termed vesicular GABA transporter. This vesicular protein is mainly located at presynaptic terminals of GABAergic interneurons. In the present study we investigated the effects of focal ischemia on the expression of the vesicular GABA transporter. Vesicular GABA transporter mRNA and protein expression was examined after photothrombosis in different cortical and hippocampal brain regions of Wistar rats. In situ hybridization and quantitative real-time RT-PCR were performed to analyze vesicular GABA transporter mRNA. Both vesicular GABA transporter mRNA-stained perikarya and mRNA expression levels remained unaffected. Vesicular GABA transporter protein-containing synaptic terminals and somata were visualized by immunohistochemistry. The pattern of vesicular GABA transporter immunoreactivity as well as the protein expression level revealed by semiquantitative image analysis and by Western blot remained stable after stroke. The steady expression of vesicular GABA transporter mRNA and protein after photothrombosis indicates that the exocytotic release mechanism of GABA is not affected by ischemia.

Distribution of GABAergic neurons in the striatum of amygdala-kindled rats: An immunohistochemical and in situ hybridization study

A large body of experimental evidence suggests that the basal ganglia circuitry may be part of a remote control system modulating the spread of epileptic seizures. In the kindling model of temporal lobe epilepsy, this endogenous inhibitory control mechanism seems to be impaired. Neurochemical and neurophysiological studies have indicated that the activity of the GABAergic projection from the striatum to the substantia nigra pars reticulata is reduced in kindled rats, but the exact mechanisms involved in this observation are not known. Possible explanations include a kindling-induced loss of striatal GABAergic projection neurons to the substantia nigra or enhanced inhibition of these neurons by GABAergic interneurons. In the present experiments, the GABAergic system of the striatum (caudate-putamen) of amygdala-kindled rats and controls was studied immunohistochemically with a monoclonal antibody to GABA and with nonisotopic in situ hybridization with cRNA probes selective for glutamic acid decarboxylase 65 (GAD65) and GAD67, respectively. Compared to sham controls, an increased density of neurons heavily labeled for GAD67 mRNA was observed in the anterior striatum of kindled rats when cells were counted 6 weeks after the last kindled seizure. This subgroup of striatal GABAergic neurons has been suggested previously to correspond to the medium-sized aspiny interneurons in the striatum, indicating that kindling is associated with an increased activity of these neurons. Our previous finding of reduced GAD and GABA levels in synaptosomes isolated from the substantia nigra of kindled rats together with the present observation of increased density of GABAergic striatal interneurons in such rats suggest that kindling affects the regulation of the GABAergic projections from the striatum to the substantia nigra rather than directly damaging GABAergic neurons in the striatum.

Histological and cytological study of the bed nuclei of the stria terminalis in adult rat. II. Oval nucleus: Extrinsic inputs, cell types, neuropil, and neuronal modules

The oval nucleus (Ov) of the bed nuclei of the stria terminalis was studied in adult rats. The Ov is composed of 11 neuron types distributed into a shell and a core domain. The stria terminalis, internal capsule, ventral amygdaloid pathway, and medial forebrain bundle are the main sources of afferents to the neuropil of the Ov. The nucleus shell contains abundant intrinsic neurons possibly connected among themselves and with the core by centripetal axon collaterals. Series of intrinsic neurons in the shell, linked with both short-axon and projecting neurons in the core, suggest a centripetal control of projecting neurons. In situ hybidization for vesicular glutamate transporter (VGlu) and glutamic acid decarboxylase (GAD) show numerous GAD-synthesizing neurons and an absence of VGlu-synthesizing neurons. In the electron microscope, the neuropil of the Ov contains axospinous, axoshaft, axosomatic, mixed (i.e., chemical-electrical), and axoaxonic synapses, in order of frequency. Synaptic boutons in apposition with the initial segment, represent an additional axoaxonic interaction. Further neural synchronization of the Ov occurs via gap junctions between somata, soma-dendrite, and possibly by apposition between axon terminals. The putative inputs from the major tracts of the forebrain coupled with the cytological organization of the Ov make it one of the most complex structures of the mammalian brain.

Molecular taxonomy of major neuronal classes in the adult mouse forebrain

Identifying the neuronal cell types that comprise the mammalian forebrain is a central unsolved problem in neuroscience. Global gene expression profiles offer a potentially unbiased way to assess functional relationships between neurons. Here, we carried out microarray analysis of 12 populations of neurons in the adult mouse forebrain. Five of these populations were chosen from cingulate cortex and included several subtypes of GABAergic interneurons and pyramidal neurons. The remaining seven were derived from the somatosensory cortex, hippocampus, amygdala and thalamus. Using these expression profiles, we were able to construct a taxonomic tree that reflected the expected major relationships between these populations, such as the distinction between cortical interneurons and projection neurons. The taxonomic tree indicated highly heterogeneous gene expression even within a single region. This dataset should be useful for the classification of unknown neuronal subtypes, the investigation of specifically expressed genes and the genetic manipulation of specific neuronal circuit elements.

Localization of GABA (gamma-aminobutyric acid) markers in the turtle's basal optic nucleus

Recent physiological data have demonstrated that retinal slip, the sensory code of global visual pattern motion, results from complex interactions of excitatory and inhibitory visual inputs to neurons in the turtle's accessory optic system (the basal optic nucleus, BON). In the present study, the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), its synthetic enzyme, glutamic acid decarboxylase (GAD-67) and its receptor subtypes GABA(A) and GABA(B) receptors were localized within the BON. GABA antibodies revealed cell bodies and processes, whereas antibodies against GAD revealed a moderate density of immunoreactive puncta throughout the BON. GAD in situ hybridization labeled BON cell bodies, indicating a possible source of inhibition intrinsic to the nucleus. Ultrastructural analysis revealed terminals positive for GAD that exhibit symmetric synaptic specializations, mainly at neuronal processes having small diameters. Neurons exhibiting immunoreactivity for GABA(A) receptors were diffusely labeled throughout the BON, with neuronal processes exhibiting more labeling than cell bodies. In contrast, GABA(B)-receptor-immunoreactive neurons exhibited strong labeling at the cell body and proximal neuronal processes. Both these receptor subtypes are functional, as evidenced by changes of visual responses of BON neurons during application to the brainstem of selective receptor agonists and antagonists. Therefore, GABA may be synthesized by BON neurons, released by terminals within its neuropil and stimulate both receptor subtypes, supporting its role in mediating visually evoked inhibition contributing to modulation of the retinal slip signals in the turtle accessory optic system.

Enhanced Fos expression in glutamic acid decarboxylase immunoreactive neurons of the mouse periaqueductal grey during opioid withdrawal

Previous studies using c-Fos immunohistochemistry suggest that a sub-population of neurons in the midbrain periaqueductal gray region is activated during opioid withdrawal. The neurochemical identity of these cells is unknown but cellular physiological studies have implicated GABAergic neurons. The present study investigated whether GABAergic neurons are activated in the mouse periaqueductal gray during opioid withdrawal using dual-antibody immunohistochemistry for Fos and glutamic acid decarboxylase. Both chronic opioid treatment and naloxone-precipitated opioid withdrawal increased Fos expression in the periaqueductal gray, with the greatest increase being four-fold in the caudal ventrolateral subdivision following withdrawal. Neurons stained for both Fos and glutamic acid decarboxylase were greatly enhanced in all subdivisions of the periaqueductal gray following withdrawal, particularly in the lateral and ventrolateral divisions where the increase was up to 70-fold. These results suggest that activation of a subpopulation of GABAergic interneurons in the periaqueductal gray plays a role in opioid withdrawal.

GABAergic neurons in the central region of the spinal cord: a novel substrate for sympathetic inhibition

Homeostatic maintenance of widespread functions is critically dependent on the activity of the sympathetic nervous system. This activity is generated by the CNS acting on the sole output cells in the spinal cord, sympathetic preganglionic neurons (SPNs). SPNs are subject to control from both supraspinal and spinal inputs that exert effects through activation of direct or indirect pathways. A high proportion of indirect control is attributable to activation of spinal interneurons in a number of locations. However, little is known about the different groups of interneurons with respect to their neurochemistry or function. In this study, we report on a novel group of GABAergic interneurons located in the spinal central autonomic area (CAA) that directly inhibit SPN activity. In situ hybridization studies demonstrated a group of neurons that contained mRNA for glutamic acid decarboxylase (GAD)65 and GAD67 within the CAA. Combining in situ hybridization with trans-synaptic labeling from the adrenal gland using pseudorabies virus identified presympathetic GABAergic neurons in the CAA. Electrical stimulation of the CAA elicited monosynaptic IPSPs in SPNs located laterally in the intermediolateral cell column. IPSPs were GABAergic, because they reversed at the chloride reversal potential and were blocked by bicuculline. Chemical activation of neurons in the CAA hyperpolarized SPNs, an effect that was also bicuculline sensitive. We conclude that the CAA contains GABAergic interneurons that impinge directly onto SPNs to inhibit their activity and suggest that these newly identified interneurons may play an essential role in the regulation of sympathetic activity and thus homeostasis.

Neurochemical characterization of hypothalamic neurons involved in attack behavior: glutamatergic dominance and co-expression of thyrotropin-releasing hormone in a subset of glutamatergic neurons

The electrical stimulation of a specific hypothalamic area rapidly evokes attacks in rats. Noteworthy, attack-related hypothalamic structures were identified in all species studied so far. The area has been extensively mapped in rats, and its anatomical connections have been studied in detail. However, technical difficulties precluded earlier the precise identification of the neural elements mediating the aggressive effects of stimulation. It now appears that a dense and distinct group of glutamatergic cells expressing vesicular glutamate transporter 2 mRNA extends over the entire hypothalamic attack area. Rostral parts overwhelmingly contained glutamatergic neurons. In more caudal parts, glutamatergic and fewer GABAergic neurons were found. The remarkable similarity in the distribution of hypothalamic attack area and glutamatergic cell groups suggests that these cells mediate the aggressive effects of stimulation. Surprisingly, thyrotropin releasing hormone mRNA was co-localized in a subset of glutamatergic neurons. Such neurons were present at all rostro-caudal levels of the hypothalamic attack area, except for that part of the hypothalamic attack area extending into the ventro-lateral part of the ventromedial hypothalamic nucleus. Earlier data on the projections of hypothalamic thyrotropin releasing hormone neurons suggest that this subpopulation plays a specific role in attack behavior. Thus, we identified three neuronal phenotypes in the hypothalamic structure that is involved in the induction of attacks: glutamatergic neurons co-expressing thyrotropin releasing hormone, glutamatergic neurons without thyrotropin releasing hormone, and GABAergic neurons dispersed among the glutamatergic cells. Assessing the specific roles and connections of these neuron subpopulations would contribute to our understanding of the mechanisms underlying attack behavior and aggression.