Multiplicity of glutamic acid decarboxylases (GAD) in vertebrates: molecular phylogeny and evidence for a new GAD paralog

Single-cell RNA sequencing unravels the transcriptional network underlying zebrafish retina regeneration

In the lesioned zebrafish retina, Müller glia produce multipotent retinal progenitors that generate all retinal neurons, replacing lost cell types. To study the molecular mechanisms linking Müller glia reactivity to progenitor production and neuronal differentiation, we used single-cell RNA sequencing of Müller glia, progenitors and regenerated progeny from uninjured and light-lesioned retinae. We discover an injury-induced Müller glia differentiation trajectory that leads into a cell population with a hybrid identity expressing marker genes of Müller glia and progenitors. A glial self-renewal and a neurogenic trajectory depart from the hybrid cell population. We further observe that neurogenic progenitors progressively differentiate to generate retinal ganglion cells first and bipolar cells last, similar to the events observed during retinal development. Our work provides a comprehensive description of Müller glia and progenitor transcriptional changes and fate decisions in the regenerating retina, which are key to tailor cell differentiation and replacement therapies for retinal dystrophies in humans.

The Brilliance of the Zebrafish Model: Perception on Behavior and Alzheimer’s Disease

Alzheimer’s disease (AD) has become increasingly prevalent in the elderly population across the world. It’s pathophysiological markers such as overproduction along with the accumulation of amyloid beta (Aβ) plaques and neurofibrillary tangles (NFT) are posing a serious challenge to novel drug development processes. A model which simulates the human neurodegenerative mechanism will be beneficial for rapid screening of potential drug candidates. Due to the comparable neurological network with humans, zebrafish has emerged as a promising AD model. This model has been thoroughly validated through research in aspects of neuronal pathways analogous to the human brain. The cholinergic, glutamatergic, and GABAergic pathways, which play a role in the manifested behavior of the zebrafish, are well defined. There are several behavioral models in both adult zebrafish and larvae to establish various aspects of cognitive impairment including spatial memory, associative memory, anxiety, and other such features that are manifested in AD. The zebrafish model eliminates the shortcomings of previously recognized mammalian models, in terms of expense, extensive assessment durations, and the complexity of imaging the brain to test the efficacy of therapeutic interventions. This review highlights the various models that analyze the changes in the normal behavioral patterns of the zebrafish when exposed to AD inducing agents. The mechanistic pathway adopted by drugs and novel therapeutic strategies can be explored via these behavioral models and their efficacy to slow the progression of AD can be evaluated.

Zebrafish as a potential non-traditional model organism in translational bipolar disorder research: Genetic and behavioral insights

Bipolar disorder (BD) is a severe and debilitating illness that affects 1-2% of the population worldwide. BD is characterized by recurrent and extreme mood swings, including mania/hypomania and depression. Animal experimental models have been used to elucidate the mechanisms underlying BD and different strategies have been proposed to assess BD-like symptoms. The zebrafish (Danio rerio) has been considered a suitable vertebrate system for modeling BD-like responses, due to the genetic tractability, molecular/physiological conservation, and well-characterized behavioral responses. In this review, we discuss how zebrafish-based models can be successfully used to understand molecular, biochemical, and behavioral alterations paralleling those found in BD. We also outline some advantages and limitations of this aquatic species to examine BD-like phenotypes in translational neurobehavioral research. Overall, we reinforce the use of zebrafish as a promising tool to investigate the neural basis associated with BD-like behaviors, which may foster the discovery of novel pharmacological therapies.

Neuroendocrine Control of Reproduction in Teleost Fish: Concepts and Controversies

During the teleost radiation, extensive development of the direct innervation mode of hypothalamo-pituitary communication was accompanied by loss of the median eminence typical of mammals. Cells secreting follicle-stimulating hormone and luteinizing hormone cells are directly innervated, distinct populations in the anterior pituitary. So far, ∼20 stimulatory and ∼10 inhibitory neuropeptides, 3 amines, and 3 amino acid neurotransmitters are implicated in the control of reproduction. Positive and negative sex steroid feedback loops operate in both sexes. Gene mutation models in zebrafish and medaka now challenge our general understanding of vertebrate neuropeptidergic control. New reproductive neuropeptides are emerging. These include but are not limited to nesfatin 1, neurokinin B, and the secretoneurins. A generalized model for the neuroendocrine control of reproduction is proposed. Hopefully, this will serve as a research framework on diverse species to help explain the evolution of neuroendocrine control and lead to the discovery of new hormones with novel applications. Expected final online publication date for the Annual Review of Animal Biosciences, Volume 10 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Neuroepithelial cells (NECs) and mucous cells express a variety of neurotransmitters and neurotransmitter receptors in the gill and respiratory air-sac of the catfish Heteropneustes fossilis (Siluriformes, Heteropneustidae): a possible role in local immune defence

Heteropneustes fossilis is an air-breathing teleost inhabiting environments with very poor O₂ conditions, and so it has evolved to cope with hypoxia. In the gills and respiratory air-sac, the sites for O₂ sensing and the response to hypoxia rely on the expression of acetylcholine (Ach) acting via its nicotinic receptor (nAChR). This study examined the expression patterns of neuronal markers and some compounds in the NECs of the gills and respiratory air sac having an immunomodulatory function in mammalian lungs. Mucous cells, epithelial cells and neuroepithelial cells (NECs) were immunopositive to a variety of both neuronal markers (VAChT, nAChR, GABA-B-R1 receptor, GAD679) and the antimicrobial peptide piscidin, an evolutionary conserved humoral component of the mucosal immune system in fish. We speculate that Ach release via nAChR from mucous cells may be modulated by GABA production in the NECs and it is required for the induction of mucus production in both normoxic and hypoxic conditions. The presence of piscidin in mucous cells may act in synergy with the autocrine/paracrine signals of Ach and GABA binding to GABA B R1B receptor that may play a local immunomodulatory function in the mucous epithelia of the gills and the respiratory air sac. The potential role of the NECs in the immunobiological behaviour of the gill/air-sac is at moment a matter of speculation. The extent to which the NECs as such may participate is elusive at this stage and waits investigation.

Early-infantile onset epilepsy and developmental delay caused by bi-allelic GAD1 variants

Gamma-aminobutyric acid (GABA) and glutamate are the most abundant amino acid neurotransmitters in the brain. GABA, an inhibitory neurotransmitter, is synthesized by glutamic acid decarboxylase (GAD). Its predominant isoform GAD67, contributes up to ∼90% of base-level GABA in the CNS, and is encoded by the GAD1 gene. Disruption of GAD1 results in an imbalance of inhibitory and excitatory neurotransmitters, and as Gad1-/- mice die neonatally of severe cleft palate, it has not been possible to determine any potential neurological dysfunction. Furthermore, little is known about the consequence of GAD1 disruption in humans. Here we present six affected individuals from six unrelated families, carrying bi-allelic GAD1 variants, presenting with developmental and epileptic encephalopathy, characterized by early-infantile onset epilepsy and hypotonia with additional variable non-CNS manifestations such as skeletal abnormalities, dysmorphic features and cleft palate. Our findings highlight an important role for GAD1 in seizure induction, neuronal and extraneuronal development, and introduce GAD1 as a new gene associated with developmental and epileptic encephalopathy.

Origin and Evolution of the Neuroendocrine Control of Reproduction in Vertebrates, With Special Focus on Genome and Gene Duplications

In humans, as in the other mammals, the neuroendocrine control of reproduction is ensured by the brain-pituitary gonadotropic axis. Multiple internal and environmental cues are integrated via brain neuronal networks, ultimately leading to the modulation of the activity of gonadotropin-releasing hormone (GnRH) neurons. The decapeptide GnRH is released into the hypothalamic-hypophysial portal blood system and stimulates the production of pituitary glycoprotein hormones, the two gonadotropins luteinizing hormone and follicle-stimulating hormone. A novel actor, the neuropeptide kisspeptin, acting upstream of GnRH, has attracted increasing attention in recent years. Other neuropeptides, such as gonadotropin-inhibiting hormone/RF-amide related peptide, and other members of the RF-amide peptide superfamily, as well as various nonpeptidic neuromediators such as dopamine and serotonin also provide a large panel of stimulatory or inhibitory regulators. This paper addresses the origin and evolution of the vertebrate gonadotropic axis. Brain-pituitary neuroendocrine axes are typical of vertebrates, the pituitary gland, mediator and amplifier of brain control on peripheral organs, being a vertebrate innovation. The paper reviews, from molecular and functional perspectives, the evolution across vertebrate radiation of some key actors of the vertebrate neuroendocrine control of reproduction and traces back their origin along the vertebrate lineage and in other metazoa before the emergence of vertebrates. A focus is given on how gene duplications, resulting from either local events or from whole genome duplication events, and followed by paralogous gene loss or conservation, might have shaped the evolutionary scenarios of current families of key actors of the gonadotropic axis.

A Membrane G-Protein-Coupled Estrogen Receptor Is Necessary but Not Sufficient for Sex Differences in Zebra Finch Auditory Coding

Estradiol acts as a neuromodulator in brain regions important for cognition and sensory processing. Estradiol also shapes brain sex differences but rarely have these concepts been considered simultaneously. In male and female songbirds, estradiol rapidly increases within the auditory forebrain during song exposure and enhances local auditory processing. We tested whether G-protein-coupled estrogen receptor 1 (GPER1), a membrane-bound estrogen receptor, is necessary and sufficient for neuroestrogen regulation of forebrain auditory processing in male and female zebra finches (Taeniopygia guttata). At baseline, we observed that females had elevated single-neuron responses to songs vs males. In males, narrow-spiking (NS) neurons were more responsive to conspecific songs than broad-spiking (BS) neurons, yet cell types were similarly auditory responsive in females. Following acute inactivation of GPER1, auditory responsiveness and coding were suppressed in male NS yet unchanged in female NS and in BS of both sexes. By contrast, GPER1 activation did not mimic previously established estradiol actions in either sex. Lastly, the expression of GPER1 and its coexpression with an inhibitory neuron marker were similarly abundant in males and females, confirming anatomical similarity in the auditory forebrain. In this study, we found: (1) a role for GPER1 in regulating sensory processing and (2) a sex difference in auditory processing of complex vocalizations in a cell type-specific manner. These results reveal sex specificity of a rapid estrogen signaling mechanism in which neuromodulation accounts and/or compensates for brain sex differences, dependent on cell type, in brain regions that are anatomically similar in both sexes.

GABAergic Neurons and Their Modulatory Effects on GnRH3 in Zebrafish

γ-Aminobutyric acid (GABA) is a major amino acid neurotransmitter in the vertebrate brain. To provide detailed information on the distribution of the GABA in zebrafish (Danio rerio), neurons were labeled with mCherry driven by the glutamic acid decarboxylase 67 (gad67) promoter. In the transgenic line Tg(gad67:mCherry), mCherry-positive gad67 cell bodies were predominantly localized to the olfactory bulb, pallial zones, subpallium zones, parvocellular preoptic nucleus, periventricular gray zone of optic tectum, torus semicircularis, posterior tuberculum, medial longitudinal fascicle, caudal zone of periventricular hypothalamus, and oculomotor nucleus. mCherry-positive fibers were widely distributed in the olfactory bulbs, subpallium, thalamus, ventral hypothalamic zone, tectum opticum, mesencephalon, and rhombencephalon. mCherry-positive neurons were also observed in the retina and the spinal cord. The anatomical relationships between GABAergic and gonadotrophin-releasing hormone 3 (GnRH3) neurons were investigated by crossing Tg(gad67:mCherry) fish with the previously established Tg(gnrh3:EGFP) transgenic line. GnRH3 cell bodies and fibers were contacted by GABAergic fibers directly in the ventral telencephalon and anterior tuberal nucleus. A subpopulation of GnRH3 neurons in the ventral telencephalic area was also labeled with mCherry, so some GnRH3 neurons are also GABAergic. GABAB receptor agonist (baclofen) and antagonist (CGP55845) treatments indicated that GABAB receptor signaling inhibited gnrh3 expression in larval fish but was stimulatory in adult fish. The expression of pituitary lhβ and fshβ was stimulated by intraperitoneal injection of baclofen in adult fish. We conclude that GABA via GABAB receptors regulates GnRH3 neurons in a developmentally dependent manner in zebrafish.

Longitudinal assessment of neuronal 3D genomes in mouse prefrontal cortex

Neuronal epigenomes, including chromosomal loopings moving distal cis-regulatory elements into proximity of target genes, could serve as molecular proxy linking present-day-behaviour to past exposures. However, longitudinal assessment of chromatin state is challenging, because conventional chromosome conformation capture assays essentially provide single snapshots at a given time point, thus reflecting genome organization at the time of brain harvest and therefore are non-informative about the past. Here we introduce 'NeuroDam' to assess epigenome status retrospectively. Short-term expression of the bacterial DNA adenine methyltransferase Dam, tethered to the Gad1 gene promoter in mouse prefrontal cortex neurons, results in stable G(methyl)ATC tags at Gad1-bound chromosomal contacts. We show by NeuroDam that mice with defective cognition 4 months after pharmacological NMDA receptor blockade already were affected by disrupted chromosomal conformations shortly after drug exposure. Retrospective profiling of neuronal epigenomes is likely to illuminate epigenetic determinants of normal and diseased brain development in longitudinal context.

Localization of glutamatergic, GABAergic, and cholinergic neurons in the brain of the African cichlid fish, Astatotilapia burtoni

Neural communication depends on release and reception of different neurotransmitters within complex circuits that ultimately mediate basic biological functions. We mapped the distribution of glutamatergic, GABAergic, and cholinergic neurons in the brain of the African cichlid fish Astatotilapia burtoni using in situ hybridization to label vesicular glutamate transporters (vglut1, vglut2.1, vglut3), glutamate decarboxylases (gad1, gad2), and choline acetyltransferase (chat). Cells expressing the glutamatergic markers vgluts 1-3 show primarily nonoverlapping distribution patterns, with the most widespread expression observed for vglut2.1, and more restricted expression of vglut1 and vglut3. vglut1 is prominent in granular layers of the cerebellum, habenula, preglomerular nuclei, and several other diencephalic, mesencephalic, and rhombencephalic regions. vglut2.1 is widely expressed in many nuclei from the olfactory bulbs to the hindbrain, while vglut3 is restricted to the hypothalamus and hindbrain. GABAergic cells show largely overlapping gad1 and gad2 expression in most brain regions. GABAergic expression dominates nuclei of the subpallial ventral telencephalon, while glutamatergic expression dominates nuclei of the pallial dorsal telencephalon. chat-expressing cells are prominent in motor cranial nerve nuclei, and some scattered cells lie in the preoptic area and ventral part of the ventral telencephalon. A localization summary of these markers within regions of the conserved social decision-making network reveals a predominance of either GABAergic or glutamatergic cells within individual nuclei. The neurotransmitter distributions described here in the brain of a single fish species provide an important resource for identification of brain nuclei in other fishes, as well as future comparative studies on circuit organization and function. J. Comp. Neurol. 525:610-638, 2017. © 2016 Wiley Periodicals, Inc.

Zebrafish Get Connected: Investigating Neurotransmission Targets and Alterations in Chemical Toxicity

Neurotransmission is the basis of neuronal communication and is critical for normal brain development, behavior, learning, and memory. Exposure to drugs and chemicals can alter neurotransmission, often through unknown pathways and mechanisms. The zebrafish (Danio rerio) model system is increasingly being used to study the brain and chemical neurotoxicity. In this review, the major neurotransmitter systems, including glutamate, GABA, dopamine, norepinephrine, serotonin, acetylcholine, histamine, and glutamate are surveyed and pathways of synthesis, transport, metabolism, and action are examined. Differences between human and zebrafish neurochemical pathways are highlighted. We also review techniques for evaluating neurological function, including the measurement of neurotransmitter levels, assessment of gene expression through transcriptomic analysis, and the recording of neurobehavior. Finally examples of chemical toxicity studies evaluating alterations in neurotransmitter systems in the zebrafish model are reviewed.

Three Distinct Glutamate Decarboxylase Genes in Vertebrates

Gamma-aminobutyric acid (GABA) is a widely conserved signaling molecule that in animals has been adapted as a neurotransmitter. GABA is synthesized from the amino acid glutamate by the action of glutamate decarboxylases (GADs). Two vertebrate genes, GAD1 and GAD2, encode distinct GAD proteins: GAD67 and GAD65, respectively. We have identified a third vertebrate GAD gene, GAD3. This gene is conserved in fishes as well as tetrapods. We analyzed protein sequence, gene structure, synteny, and phylogenetics to identify GAD3 as a homolog of GAD1 and GAD2. Interestingly, we found that GAD3 was lost in the hominid lineage. Because of the importance of GABA as a neurotransmitter, GAD3 may play important roles in vertebrate nervous systems.

Effects of the insecticide fipronil on reproductive endocrinology in the fathead minnow

Gamma-aminobutyric acid (GABA) and GABA receptors play an important role in neuroendocrine regulation in fish. Disruption of the GABAergic system by environmental contaminants could interfere with normal regulation of the hypothalamic-pituitary-gonadal axis, leading to impaired fish reproduction. The present study used a 21-d fathead minnow (Pimephales promelas) reproduction assay to investigate the reproductive toxicity of fipronil (FIP), a broad-spectrum phenylpyrazole insecticide that acts as a noncompetitive blocker of GABA receptor-gated chloride channels. Continuous exposure up to 5 µg FIP/L had no significant effect on most of the endpoints measured, including fecundity, secondary sexual characteristics, plasma steroid and vitellogenin concentrations, ex vivo steroid production, and targeted gene expression in gonads or brain. The gonad mass, gonadosomatic index, and histological stage of the gonad were all significantly different in females exposed to 0.5 µg FIP/L compared with those exposed to 5.0 µg FIP/L; however, there were no other significant effects on these measurements in the controls or any of the other treatments in either males and females. Overall, the results do not support a hypothesized adverse outcome pathway linking FIP antagonism of the GABA receptor(s) to reproductive impairment in fish.

Postmortem stability of brain GABAergic and glutamatergic receptors and enzymes under ecological conditions

Neurochemical biomarkers have emerged as useful tools for assessing the subclinical neurological impacts of environmental toxicants in birds and other wildlife. Careful consideration of biomarker stability is necessary before implementing their use on tissues from ecological studies, as receptors and enzymes in the brain may be affected by postmortem conditions. The goal of this study was to evaluate the postmortem stability of key GABAergic and glutamatergic receptors (N-methyl-D-aspartate (NMDA), gamma-aminobutyric acid (GABAA-benzodiazepine)) and enzymes (glutamine synthetase (GS), glutamic acid decarboxylase (GAD)) under environmentally relevant field and storage conditions to determine their suitability as biomarkers. We exposed chicken embryo brains to postmortem environmental and storage conditions typical for ecological studies (12, 24, and 48 h at 7 °C or 25 °C; 1, 4, and 8 weeks at -80 °C or -20 °C; 1 or 2 freeze thaw cycles), and measured [3H] MK-801 binding to the NMDA receptor, [3H] flunitrazipam binding to the GABAA-benzodiazepine receptor, GS activity, and GAD activity. We found that [3H] MK-801 binding is stable under all conditions studied. GAD activity was fairly stable under each storage and environmental temperatures for all durations, but was significantly less stable when stored at -20 °C than at -80 °C. [3H] flunitrazipam binding and GS activity were both impacted by environmental and storage temperature and duration, and might best be utilized in studies of samples with similar histories. Our findings here demonstrate that caution is warranted when comparing samples with different collection and storage histories, but that some biomarkers are fairly stable under various conditions.

Molecular characterization and comparative localization of the mRNAs encoding two glutamic acid decarboxylases (GAD65 and GAD67) in the brain of the African lungfish, Protopterus annectens

The existence of two distinct genes encoding two isoforms of glutamic acid decarboxylase (GAD65 and GAD67) has been demonstrated in most vertebrate classes, yet little is known about their differential distributions and functions in the central nervous system in nonmammalian vertebrates. In the present study, we have partially sequenced the cDNAs encoding GAD65 and GAD67 in the lungfish Protopterus annectens and determined their relative distributions in the adult brain by in situ hybridization histochemistry. The expression patterns of the GAD65 and GAD67 mRNAs were globally similar; the highest expression levels being observed in the granular layer of the olfactory bulb, the pallium, the subpallium, the anterior preoptic area, the thalamus, the hindbrain central gray, and the rhombencephalic visceral areas. However, striking differential expression was noticed in several structures. Very high to high concentrations of GAD67 mRNA were seen in the dorsal and ventral aspects of the anterior olfactory nucleus, which is in marked contrast to the very low expression of GAD65 in this region. Similarly, high levels of GAD67 mRNA were observed in the intermediate and ventral parts of the medial pallium that were virtually devoid of GAD65 mRNA. In contrast, GAD65 mRNA was found in the periaqueductal gray that did not express GAD67 mRNA. The differential expression of GAD65 and GAD67 mRNAs in these regions of the lungfish CNS indicates that the two GAD isoforms can be differentially regulated and that they may have distinct physiological roles.

Developmental expression of glutamic acid decarboxylase and of gamma-aminobutyric acid type B receptors in the ascidian Ciona intestinalis

We describe Ciona intestinalis gamma-aminobutyric acid (GABA)-ergic neurons during development, studying the expression pattern of Ci-GAD (glutamic acid decarboxylase: GABA synthesizing enzyme) by in situ hybridization. Moreover, we cloned two GABA(B) receptor subunits (Ci-GABA(B)Rs), and a phylogenetic analysis (neighbor-joining method) suggested that they clustered with their vertebrate counterparts. We compared Ci-GAD and Ci-GABA(B)Rs expression patterns in C. intestinalis embryos and larvae. At the tailbud stage, Ci-GAD expression was widely detected in central and peripheral nervous system (CNS/PNS) precursors, whereas Ci-GABA(B)Rs expression was evident at the level of the precursors of the visceral ganglion. GABA was localized by immunohistochemistry at the same developmental stage. In the larva, Ci-GAD transcripts and GABA immunofluorescence were also detected throughout the CNS and in some neurons of the PNS, whereas transcripts of both GABA(B) receptor subunits were found mainly in the CNS. The expression pattern of Ci-GABA(B)Rs appeared restricted to Ci-GAD-positive territories in the sensory vesicle, whereas, in the visceral ganglion, Ci-GABA(B)Rs transcripts were found in ventral motoneurons that did not express Ci-GAD. Insofar as GABAergic neurons are widely distributed also in the CNS and PNS of vertebrates and other invertebrate chordates, it seems likely that GABA signaling was extensively present in the protochordate nervous system. Results from this work show that GABA is the most widespread inhibitory neurotransmitter in C. intestinalis nervous system and that it can signal through GABA(B) receptors both pre- and postsynaptically to modulate different sensory inputs and subsequent swimming activity.

Role of glutamate decarboxylase (GAD) isoform, GAD65, in GABA synthesis and transport into synaptic vesicles-Evidence from GAD65-knockout mice studies

In GAD65-knockout mice, lack of GAD65 expression was confirmed. The expression level of vesicular GABA transporter (VGAT) was upregulated, and no change in the synaptic vesicles (SV)-associated GAD67 was found. GAD65(-/-) SV transported cytosolic GABA much more efficiently than that of the wild type, further supporting our model that there is a structural and functional coupling between GABA synthesis and packaging into SV. Both full-length and truncated forms of GAD65 could bind to GABAergic SV, indicating the N-terminus is not required for the anchoring of GAD65 to SV. Although both GAD65(-/-) SV reconstituted with either GAD65 or GAD67 could synthesize GABA from [3H] glutamate and transport this newly synthesized GABA into SV, the combined evidence suggests that GAD65 plays a major role in GABA transmission in normal physiological condition. However, GAD67 could serve this role under some pathological conditions.

Expression of gamma-aminobutyric acid and glutamic acid decarboxylases in rat descending colon and their relation to epithelial differentiation

OBJECTIVE

To detect the expression of gamma-aminobutyric acid (GABA) and glutamic acid decarboxylases (GADs; including two isoforms GAD65 and GAD67) in the epithelial growth zones of the descending colon in rats, and to investigate their relation to epithelial differentiation and proliferation.

METHODS

The expression of GABA and GADs in rat descending colon was investigated by immunofluorescent staining and confocal laser scanning techniques, and goblet cells were further investigated by wheat-germ agglutinin histochemistry. In addition, GAD65 and GAD67 mRNAs were also detected by reverse transcription-polymerase chain reaction. Furthermore, evaluation of cell kinetics in colonic epithelia was conducted by ABC immunostaining using a monoclonal antibody against proliferating cell nuclear antigen (PCNA).

RESULTS

Immunoreactive GABA and GADs were distributed in the upper third of the crypts and at the luminal surface in the rat descending colon. Strong staining for GABA and GADs was localized mainly in the cytoplasm of epithelial cells near the neck of the crypts and along the luminal surface. In addition, GABA and GAD65 were also detected at the lamina propria in colonic mucosa. No staining for GABA or GADs was found in goblet cells. GAD65 and GAD67 mRNAs were identified in homogenates of rat descending colon. PCNA labeled nuclei were found in the lower two-thirds of the crypts.

CONCLUSIONS

The expression of GABA and GADs in the maturation and function zones of rat descending colon suggests that GABA may be involved in the differentiation of colonic epithelial cells.

Expression of recombinant goldfish glutamic acid decarboxylase 65 and evidence for differential pH and PLP responsiveness compared to the human enzyme

Glutamic acid decarboxylase (GAD) catalyzes the conversion of glutamate to gamma-aminobutyric acid (GABA) that acts as an important inhibitory neurotransmitter in the vertebrate brain, as well as in the regulation of neuroendocrine function. GAD65 and GAD67 are the two main isoforms that exist in vertebrates. The biochemical properties of recombinant forms of goldfish and human GAD65 were examined. The recombinant goldfish GAD65 (gfGAD65) was expressed at high levels using a maltose binding protein fusion system for biochemical characterization. The human GAD65 (hGAD65) was expressed as a GST fusion and was also purified. The recombinant goldfish GAD65 protein has properties that are different from the human counterpart. In particular, the gfGAD65 is less active at acidic pH compared to hGAD65, which is moderately active over a wider range of acidic and basic pH. Interestingly, however, gfGAD65 is less dependent on a cofactor pyridoxal-5'-L-phosphate (PLP) for activity. In the absence of added PLP, cleaved recombinant gfGAD65 showed approximately 20% of maximal activity whereas hGAD65 showed no detectable activity. The physiological and evolutionary significance of these findings is discussed in light of the conserved function of GAD in two vertebrate species that are separated in evolutionary time by more than 200 million years.

Structural model of human GAD65: prediction and interpretation of biochemical and immunogenic features

The 65 kDa human isoform of glutamate decarboxylase, GAD65, plays a central role in neurotransmission in higher vertebrates and is a typical autoantigen in several human autoimmune diseases, such as insulin-dependent diabetes mellitus (IDDM), Stiff-man syndrome and autoimmune polyendocrine syndrome type I. In autoimmune diabetes, an attack of inflammatory cells to endocrine pancreatic beta-cells leads to their complete destruction, eventually resulting in the inability to produce sufficient insulin for the body's requirements. Even though the etiology of beta-cell destruction is still a matter of debate, the role and antigenic potency of GAD65 are widely recognized. Herein a model of GAD65 is presented, which is based on the recently solved crystal structures of mammalian DOPA decarboxylase and of bacterial glutamate decarboxylase. The model provides for the first time a detailed and accurate structure of the GAD65 subunit (all three domains) and of its dimeric quaternary assembly. It reveals the structural basis for specific antibody recognition to GAD65 as opposed to GAD67, the other human isoform, which shares 81% sequence similarity with GAD65 and is much less antigenic. Literature data on monoclonal antibody binding are perfectly consistent with the detailed features of the model, which allows explanation of several findings on GAD65 immunogenicity. Importantly, by analyzing the active site, we identified the residues most likely involved in catalysis and substrate recognition, paving the way for rational mutagenesis studies of the GAD65 reaction mechanism, specificity and inhibition.

GABAergic synaptic transmission modulates swimming in the ascidian larva

To examine the role of the amino acid GABA in the locomotion of basal chordates, we investigated the pharmacology of swimming and the morphology of GABA-immunopositive neurones in tadpole larvae of the ascidians Ciona intestinalis and Ciona savignyi. We verified that electrical recording from the tail reflects alternating muscle activity during swimming by correlating electrical signals with tail beats using high-speed video recording. GABA reversibly reduced swimming periods to single tail twitches, while picrotoxin increased the frequency and duration of electrical activity associated with spontaneous swimming periods. Immunocytochemistry for GABA revealed extensive labelling throughout the larval central nervous system. Two strongly labelled regions on either side of the sensory vesicle were connected by an arc of labelled fibres, from which fibre tracts extended caudally into the visceral ganglion. Fibre tracts extended ventrally from a third, more medial region in the posterior sensory vesicle. Two rows of immunoreactive cell bodies in the visceral ganglion extended neurites into the nerve cord, where varicosities were seen. Thus, presumed GABAergic neurones form a network that could release GABA during swimming that is involved in modulating the time course and frequency of periods of spontaneous swimming. GABAergic and motor neurones in the visceral ganglion could interact at the level of their cell bodies and/or through the presumed GABAergic fibres that enter the nerve cord. The larval swimming network appears to possess some of the properties of spinal networks in vertebrates, while at the same time possibly showing a type of peripheral innervation resembling that in some protostomes.

Characteristic expression of γ-aminobutyric acid and glutamic acid decarboxylase in epithelial cells of rat descending colon

AIM: To detect the expression of γ-aminobutyric acid (GABA) and glutamate decarboxylases (GADs) in the epithelial growth zones of rat descending colon, and to investigate their relations with cell differentiation and proliferation.

METHODS: Immunocytochemical expression of GABA and GADs in the epithelial cells of rat descending colon was investigated by immunofluorescent staining and confocal laser scanning techniques, and the goblet cells were further revealed by wheat-germ agglutinin (WGA) histochemistry. GAD65 and GAD67 mRNAS were also detected by reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization. Furthermore, evaluation of cell kinetics in colonic epithelia was conducted by [³H]-thymidine autoradiography.

RESULTS: Immunoreactive GABA and GAD65 were distributed in the upper third of the crypts and at the luminal surface in the rat descending colon. Strong staining for GABA and GAD65 was localized mainly in the cytoplasm of epithelial cells near the neck of the crypts and along the luminal surface. Immunoreactivity of GAD67, however, was localized only on the luminal surface. In addition, GABA and GAD65 were detected at lamina propria in clonic mucosa. No staining for GABA or GADs was found in goblet cells. GAD65 and GAD67 mRNAs were both identified in the homogenates of rat descending colon, and the epithelium showed stronger hybridization signal for GAD65 mRNA than that for GAD67. Meanwhile, [³H]-thymidine labeled cells were found in the lower two-thirds of the crypts.

CONCLUSION: The expression of GABA and GADs in the maturation and function zones suggests that GABA might be involved in the differentiation and proliferation of the colonic epithelial cells.

Sex steroid regulation of brain glutamic acid decarboxylase (GAD) mRNA is season-dependent and sexually dimorphic in the goldfish Carassius auratus

GABA, the major inhibitory neurotransmitter of the vertebrate brain, has been shown to play an important role in vertebrate reproduction by regulating LH release and sexual behavior. We have studied the expression of the GABA-synthesizing enzyme, glutamic acid decarboxylase (GAD), in goldfish throughout the reproductive cycle in May (mature), November (early gonadal recrudescence) and February (late gonadal recrudescence) and in response to implanted sex steroids. Levels of GAD67 and GAD65 mRNA levels in the hypothalamus of both males and females were highest in the early stages of gonadal recrudescence. In the telencephalon, a different seasonal pattern of GAD expression was evident. The telencephalic expression GAD67, GAD65 and a novel isoform, GAD3, were highest in sexually mature fish in May. Five-day intraperitoneal implantation of gonad-intact fish with testosterone (T), estradiol (E2) or progesterone (P4) did not affect GAD expression in November and February. This is in contrast to results in May when sex differences in steroid responsiveness were evident. Progesterone decreased hypothalamic GAD67 and GAD65 in females and was without effect in males. All other treatments did not alter GAD67, GAD65 or GAD3 expression in the hypothalamus. Both T and P4 decreased GAD67 and GAD65 levels in the telencephalon of male goldfish but had no effect in females. Serum sex steroid levels in control and implanted mature males and females in May were similar so it is unlikely that sex differences in the GAD responses were a result of differences in serum sex steroid levels. These contrasting effects of sex steroids on males and females suggest important sex differences in the regulation of the GADs in sexually mature goldfish.

GABAergic modulation of the expression of genes involved in GABA synaptic transmission and stress in the hypothalamus and telencephalon of the female goldfish (Carassius auratus)

GABA is one of the most abundant neurotransmitters in the vertebrate central nervous system and is involved in neuroendocrine processes such as development, reproduction, feeding and stress. To examine the effect of GABA on gene expression in the brain, we used a cDNA macroarray containing 26 genes involved in GABA synaptic transmission (GABA receptor subunits, GABA transporters), reproduction (gonadotrophin-releasing hormone isoforms and oestrogen receptor alpha), feeding (neuropeptide Y and cholecystokinin), and stress [corticotrophin-releasing factor (CRF)]. To elevate GABA levels in the brain, we injected female goldfish with gamma-vinyl GABA (300 microg/g of body weight) (24 h), an irreversible inhibitor of the enzyme GABA transaminase (GABA-T). We found that increased levels of GABA in the hypothalamus resulted in a 2.2-fold down-regulation of GABA(A) receptor beta4 subunit mRNA. In the telencephalon, we found that increased GABA levels resulted in a 1.5-fold increase of CRF mRNA and a 1.8-fold decrease of GABA(A) receptor beta2 subunit mRNA. Increasing GABA in the hypothalamus and telencephalon of the goldfish did not significantly affect the mRNA abundance of genes involved in GABA synthesis (glutamic acid decarboxylase isoforms) and degradation (GABA-T), feeding, or reproduction. Our preliminary study suggests that the regulation of GABA receptor subunit mRNA expression by GABA may be a conserved evolutionary mechanism in vertebrates to modulate GABAergic synaptic transmission.

The distribution and cellular localization of glutamic acid decarboxylase-65 (GAD65) mRNA in the forebrain and midbrain of domestic chick

The distribution and cellular localization of GAD65 mRNA in the forebrain and midbrain of domestic chick were examined by in situ hybridization histochemistry with (35)[S]-UTP labeled cRNA probes, using film and emulsion autoradiography. Film autoradiograms showed intense GAD65 labeling in many structures of the basal telencephalon, such as the medial and lateral striatum, the septum, the olfactory tubercle, the lateral bed nucleus of the stria terminalis, and the intrapeduncular nucleus, while the pallial telencephalon showed only a low level of labeling. Emulsion-coated sections revealed that GAD65 mRNA-containing neurons were at least six times more abundant in striatum than pallium, with only a uniformly scattered subpopulation labeled in pallium, and that the vast majority of the large scattered projection neurons of globus pallidus were heavily labeled for GAD65. Prominent labeling was also evident in the nucleus taeniae and subpallial amygdala, but not in the arcopallium in film autoradiograms. Within the diencephalon, the hypothalamus was more GAD65-rich than the thalamus. Additional subtelencephalic cell groups showing prominent labeling included the thalamic reticular nucleus and ventral lateral geniculate nucleus of the diencephalon, the nucleus pretectalis, subpretectalis and spiriformis lateralis of the pretectum, and the magnocellular isthmic nucleus of the optic lobe. Tectal layers 9-10 were also rich in GAD65. These results further clarify GABAergic circuits of the avian forebrain and midbrain, and show them to closely resemble those in mammals.

Characteristic expression of gamma-aminobutyric acid and glutamate decarboxylase in rat jejunum and its relation to differentiation of epithelial cells

AIM

To investigate the expression between gamma-aminobutyric acid (GABA) and glutamate decarboxylase and its relation with differentiation and maturation of jejunal epithelial cells in rat jejunum.

METHODS

Immunohistochemical expression of GABA and glutamate decarboxylase (GAD, including two isoforms, GAD65 and GAD67) was investigated in rat jejunum. Meanwhile, double staining was performed with GAD65 immunohistochemistry, followed by lectin histochemistry of fluorescent wheat germ agglutinin. Furthermore, evaluation of cell kinetics in jejunum was conducted by (3)H-thymidine autoradiography and immunohistochemistry using a monoclonal antibody to proliferating cell nuclear antigen (PCNA).

RESULTS

The cells showing positive immunoreactivity GABA and GAD65 were mainly distributed in the villi in rat jejunum, while jejunal epithelial cells were negative for GAD67. Positive GABA or GAD65 staining was mainly located in the cytoplasm and along the brush border of epithelial cells in the middle and upper portions. In addition, a few GABA and GAD65 strongly positive cells were scattered in the upper two thirds of jejunal villi. Double staining showed that GAD65 immunoreactivity was not found in goblet cells. (3)H-thymidine-labeled nuclei were found in the lower and middle portions of jejunal crypts, which was consistent with PCNA staining. Therefore, GABA and GAD65 were expressed in a maturation or functional zone.

CONCLUSION

The characteristic expression of GABA and GAD suggests that GABA might be involved in regulation of differentiation and maturation of epithelial cells in rat jejunum.

Homozygosity for a missense mutation in the 67 kDa isoform of glutamate decarboxylase in a family with autosomal recessive spastic cerebral palsy: parallels with Stiff-Person Syndrome and other movement disorders

BACKGROUND

Cerebral palsy (CP) is an heterogeneous group of neurological disorders of movement and/or posture, with an estimated incidence of 1 in 1000 live births. Non-progressive forms of symmetrical, spastic CP have been identified, which show a Mendelian autosomal recessive pattern of inheritance. We recently described the mapping of a recessive spastic CP locus to a 5 cM chromosomal region located at 2q24-31.1, in rare consanguineous families.

METHODS

Here we present data that refine this locus to a 0.5 cM region, flanked by the microsatellite markers D2S2345 and D2S326. The minimal region contains the candidate gene GAD1, which encodes a glutamate decarboxylase isoform (GAD67), involved in conversion of the amino acid and excitatory neurotransmitter glutamate to the inhibitory neurotransmitter gamma-aminobutyric acid (GABA).

RESULTS

A novel amino acid mis-sense mutation in GAD67 was detected, which segregated with CP in affected individuals.

CONCLUSIONS

This result is interesting because auto-antibodies to GAD67 and the more widely studied GAD65 homologue encoded by the GAD2 gene, are described in patients with Stiff-Person Syndrome (SPS), epilepsy, cerebellar ataxia and Batten disease. Further investigation seems merited of the possibility that variation in the GAD1 sequence, potentially affecting glutamate/GABA ratios, may underlie this form of spastic CP, given the presence of anti-GAD antibodies in SPS and the recognised excitotoxicity of glutamate in various contexts.

Increased GAD67 mRNA levels are correlated with in vivo GABA synthesis in the MPTP-treated catecholamine-depleted goldfish brain

The role of catecholamine neuronal input on GABAergic activity in the hypothalamus, telencephalon, optic tectum, and cerebellum was investigated in early recrudescent female goldfish (Carassius auratus). A new quantitative technique was developed and validated, permitting concomitant quantification and correlational analysis of glutamic acid decarboxylase 65 (GAD65), GAD67, and GAD3 mRNA levels and in vivo GABA synthesis. Catecholamine depletion was achieved by the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 50 microg/g body weight) and dopamine (DA) depletion verified by HPLC. Endogenous GABA levels were increased by intraperitoneal administration of gamma-vinyl GABA (GVG; 300 microg/g body weight), an inhibitor of the GABA catabolic enzyme GABA transaminase. Treatment with MPTP resulted in a greater than twofold increase in GABA synthesis rate in the optic tectum and telencephalon. The increase in GABA synthesis rate was highly correlated with an increase in GAD67, but not GAD65 or GAD3 mRNA levels. These results suggest that catecholaminergic input exerts inhibitory effects on GABA synthesis rates through the modulation of GAD67 in the optic tectum and telencephalon. Together with previously published observations in rodents and primates, it is suggested that catecholaminergic control of GABA synthesis must have evolved more than 200 million years ago, before the emergence of the teleost fishes.

Kinetic differences between the isoforms of glutamate decarboxylase: implications for the regulation of GABA synthesis

Glutamate decarboxylase (GAD) exists as two isoforms, GAD65 and GAD67. GAD activity is regulated by a cycle of activation and inactivation determined by the binding and release of its co-factor, pyridoxal 5'-phosphate. Holoenzyme (GAD with bound co-factor) decarboxylates glutamate to form GABA, but it also catalyzes a slower transamination reaction that produces inactive apoGAD (without bound co-factor). Apoenzyme can reassociate with pyridoxal phosphate to form holoGAD, thus completing the cycle. Within cells, GAD65 is largely apoenzyme (approximately 93%) while GAD67 is mainly holoenzyme (approximately 72%). We found striking kinetic differences between the GAD isoforms that appear to account for this difference in co-factor saturation. The glutamate dependent conversion of holoGAD65 to apoGAD was about 15 times faster than that of holoGAD67 at saturating glutamate. Aspartate and GABA also converted holoGAD65 to apoGAD at higher rates than they did holoGAD67. Nucleoside triphosphates (such as ATP) are known to affect the activation reactions of the cycle. ATP slowed the activation of GAD65 and markedly reduced its steady-state activity, but had little affect on the activation of GAD67 or its steady-state activity. Inorganic phosphate opposed the effect of ATP; it increased the rate of apoGAD65 activation but had little effect on apoGAD67 activation. We conclude that the apo-/holoenzyme cycle of inactivation and reactivation is more important in regulating the activity of GAD65 than of GAD67.

mRNA analysis in flattened fauna: obtaining gene-sequence information from road-kill and game-hunting samples

Whether gene-sequence information could be obtained using mRNA from road-kill and hunting samples was investigated. Adipose tissue was used to clone cDNA fragments of the hormone leptin and brain tissue was used for the enzyme glutamic acid decarboxylase (GAD). Tissues collected from road-killed animals were used to clone leptin from RNA samples of raccoon (Procyon lotor) and woodchuck (Marmota monax). We were able to extract RNA and clone GAD67 from samples of masked shrew (Sorex cinereus), although the time of death was unknown. We collaborated with hunters who provided tissues from which we cloned leptin and GAD isoforms from beaver (Castor canadensis), red squirrel (Tamiasciurus hudsonicus), black bear (Ursus americanus), and moose (Alces alces americana). Molecular phylogenetic analyses confirmed that the sequences obtained did not result from contamination. A time-course experiment showed that even 24 h after the death of rats, sufficient mRNA remains to amplify leptin from adipose tissue. These results suggest that road-kill and hunting samples could be used as a valuable source of gene-sequence information.

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.

The effect of water temperature on the GABAergic and reproductive systems in female and male goldfish (Carassius auratus)

This study investigated the effect of water temperature on the synthesis of the amino acid neurotransmitter gamma-aminobutyric acid (GABA). In goldfish, GABA stimulates the release of pituitary gonadotropin-II (GTH-II), which regulates gonadal function. Fish were maintained in water of 11, 18, or 24 degrees. In the female and male goldfish, GABA synthesis rates estimated following inhibition of GABA catabolism by gamma-vinyl GABA (GVG) in both the telencephalon (TEL) and the hypothalamus (HYP) were increased in fish held at 24 degrees compared to those at either 11 or 18 degrees (P < 0.05). Additionally, GABA synthesis rates in the pituitary increased in a temperature-dependent manner. Glutamate is the precursor for GABA synthesis; however, no consistent pattern was seen between glutamate and GABA synthesis rates, indicating that glutamate is not a limiting factor in GABA synthesis. Both water temperature and GVG administration increased serum GTH-II levels in female goldfish. However, in male goldfish water temperature had no significant effect on serum GTH-II levels, and GVG injection increased serum GTH-II levels only in fish maintained at 24 degrees. The effects of temperature on the levels of mRNA expression of the GABA-synthesizing enzymes glutamate decarboxylase 65 (GAD(65)) and GAD(67) were measured by semiquantitative PCR. In the TEL and HYP of female goldfish, GAD(65) was not affected, whereas temperature change from 11 to 18 degrees increased (P < 0.05) GAD(67) mRNA levels. These results demonstrate that central GABAergic systems in the goldfish are temperature sensitive.

Sex steroid regulation of glutamate decarboxylase mRNA expression in goldfish brain is sexually dimorphic

Testosterone and oestradiol can modulate GABA synthesis in sexually regressed goldfish. Here we investigated their effects on the mRNA expression of two isoforms of the GABA synthesizing enzyme glutamate decarboxylase (GAD(65) and GAD(67), EC 4.1.1.15). Full-length GAD clones were isolated from a goldfish cDNA library and sequenced. Goldfish GAD(65) encodes a polypeptide of 583 amino acid residues, which is 77% identical to human GAD(65). Goldfish GAD(67) encodes a polypeptide of 587 amino acid residues and is 82% identical to human GAD(67). Goldfish GAD(65) and GAD(67) are 63% identical. Sexually regressed male and female goldfish were implanted with solid silastic pellets containing testosterone, oestradiol or no steroid. Semiquantitative PCR analysis showed that oestradiol significantly increased GAD(65) mRNA expression in female hypothalamus and telencephalon, while testosterone resulted in a significant increase only in telencephalon. GAD(67) mRNA levels were not affected by steroids in females. In contrast, both steroids induced significant decreases of GAD(65) and GAD(67) mRNA levels in male hypothalamus, but had no effect on GAD mRNA expression in male telencephalon. Our results indicate that modulation of GAD mRNA expression is a possible mechanism for steroid action on GABA synthesis, which may have opposite effects in males and females.

Structure of glutamate decarboxylase and related PLP-enzymes: computer-graphical studies

Amino acid sequences of E. coli glutamate decarboxylase (GADa) and those of 36 GAD of different origin were compared by pairwise alignment using computer program CLUSTAL. GADalpha and plant enzymes showed 59.8-67.8% subunit homology, GADalpha and other bacterial GAD--49.8-77.6%, whereas GADalpha and animal enzymes--13.9-58.8%. Two PLP domains exhibited higher homology comparing to that of the whole subunit in the case of GAD67, plant (68.4-73.9%), and bacterial (46.7-83.2%) enzymes. The alignment of PLP-domains of 37 GAD, three group II decarboxylases, and two pyridoxal enzymes with known 3D structures (bacterial ORD and mAAT from chicken heart) allowed us to reveal conserved residues of the active sites. Their functional role is discussed. Modelling of the PLP-binding sites in active centers for GADalpha and human brain GAD67 was done using the Swiss-PdbViewer homology modelling program. Although the homology between GADalpha and GAD67 is rather low, structural similarity of their active sites allows us to consider here a functional convergence. Thus, glutamate decarboxylation by GADalpha may be helpful for understanding general mechanism of this reaction.

The inhibitory effects of (gamma)-aminobutyric acid (GABA) on growth hormone secretion in the goldfish are modulated by sex steroids

Double-labelling studies at the electron microscopic level demonstrated that gamma-aminobutyric acid (GABA)-immunoreactive nerve endings are associated with growth-hormone-secreting cells in the proximal pars distalis of the goldfish pituitary gland, suggesting that GABA may be important for the control of growth hormone release in this species. An in vitro assay for GABA-transaminase activity demonstrated that the pituitary is a site for the metabolism of GABA to succinic acid. In vitro, GABA or the GABA antagonists bicuculline and saclofen did not affect the rate of growth hormone release from dispersed pituitary cells in static incubation. In contrast, intracerebroventricular injection of GABA reduced serum growth hormone levels within 30 min. During the seasonal gonadal cycle, intraperitoneal injection of GABA was without effect in sexually regressed goldfish, but caused a significant decrease in serum growth hormone levels in sexually recrudescent animals. Intraperitoneal implantation of solid silastic pellets containing oestradiol increased serum GH levels fivefold in sexually regressed and recrudescent goldfish; in both groups, GABA suppressed the oestradiol-stimulated increase in circulating growth hormone levels. The effect of oestradiol on basal serum growth hormone levels was specific since progesterone and testosterone were without effect. However, in recrudescent animals treated with progesterone and testosterone, the inhibitory effects of GABA on serum growth hormone levels were absent, indicating a differential role for these steroids in growth hormone release. Taken together, these results demonstrate that GABA has an inhibitory effect on growth hormone release in goldfish.

Distribution of GAD-immunoreactive neurons in the diencephalon of the african lungfish Protopterus annectens: colocalization of GAD and NPY in the preoptic area

The distribution of GABAergic neurons was investigated in the diencephalon of the African lungfish, Protopterus annectens, by using specific antibodies directed against glutamic acid decarboxylase (GAD). A dense population of immunoreactive perikarya was observed in the periventricular preoptic nucleus, whereas the caudal hypothalamus and the dorsal thalamus contained only scattered positive cell bodies. Clusters of GAD-positive cells were found in the intermediate lobe of the pituitary. The diencephalon was richly innervated by GAD-immunoreactive fibers that were particularly abundant in the hypothalamus. In the periventricular nucleus, GAD-positive fibers exhibited a radial orientation, and a few neurons extended processes toward the third ventricle. More caudally, a dense bundle of GAD-immunoreactive fibers coursing along the ventral wall of the hypothalamus terminated into the median eminence and the neural lobe of the pituitary. Double-labeling immunocytochemistry revealed that GAD and neuropeptide tyrosine (NPY)-like immunoreactivity was colocalized in a subpopulation of perikarya in the periventricular preoptic nucleus. The proportion of neurons that coexpressed GAD and NPY was higher in the caudal region of the preoptic nucleus. The distribution of GAD-immunoreactive elements in the diencephalon and pituitary of the African lungfish indicates that GABA may act as a hypophysiotropic neurohormone in Dipnoans. The coexistence of GAD and NPY in a subset of neurons of the periventricular preoptic nucleus suggests that GABA and NPY may interact at the synaptic level.

The role of amino acid neurotransmitters in the regulation of pituitary gonadotropin release in fish

Both glutamate and γ-aminobutyric acid (GABA) are involved in pituitary hormone release in fish. Glutamate serves 2 purposes, both as a neurotransmitter and as a precursor for GABA synthesis. Glutamate can be catabolized to GABA by the actions of 2 distinct but related enzymes, glutamate decarboxylase 65 (GAD65) and GAD67. They derive from 2 different genes that likely arose from an early gene duplication prior to the emergence of teleosts more than 400 million years ago. There is good evidence for the involvement of GABA in luteinizing hormone (LH) release in fish. The mechanism of GABA action to stimulate LH release appears to be a combination of effects on GnRH release, potentiation of gonadotropin hormone-releasing hormone (GnRH) action, and in some cases directly at the LH cell. These actions appear to be dependent on such factors as sex or sex steroid levels, and there may also be species differences. Nevertheless, the stimulatory effects of GABA on LH are present in at least 4 fish species. In contrast, convincing data for the inhibitory effects of GABA on LH release have only been observed in 1 fish species. The sites and mechanisms of action of amino acid neurotransmitters on LH release have yet to be fully characterized. Both N-methyl-D-aspartic acid (NMDA) and S-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type glutamate receptors are likely to have important roles. We suggest that it is a receptor similar to the GABAA type which mediates the effects of GABA on LH release in fish, at least partially acting on the GnRH neuron, but likely directly acting at the gonadotroph as well. GABA may also be involved in regulating the release of other pituitary hormones in fish, namely follicle stimulating hormone (FSH = GTH-I), prolactin, and growth hormone. Based on the findings described in this review, a working model for the involvement of glutamate and GABA in the regulation of LH release in teleost fish is proposed.

Key words: glutamate, GABA, luteinizing hormone, muscimol, patch clamp electrophysiology, reproduction, fish.

Alternative splicing of GAD67 results in the synthesis of a third form of glutamic-acid decarboxylase in human islets and other non-neural tissues

Two forms of glutamic-acid decarboxylase (GAD) have been identified in mammalian tissues: a 65-kDa form (GAD65) and a 67-kDa form (GAD67). Alternate splicing produces one or two smaller variants of GAD67 in the brain of embryonic mice and rats. Additionally, a short, heretofore unidentified transcript homologous to GAD67 has been detected in human testis RNA. Because GAD, the enzyme responsible for gamma-aminobutyric acid production and a key autoantigen in type I diabetes, has unclear function in non-neural tissue, it is important to understand its pattern of expression. Unlike GAD65, GAD67 is not produced in human pancreatic islets. Here, we describe a novel splice variant of GAD67 that is produced in human islets, testis, adrenal cortex, and perhaps other endocrine tissues, but not in brain. This transcript directs the synthesis of a protein without GAD enzymatic activity: GAD25. A unique peptide sequence at the carboxyl terminus of GAD25 is highly conserved between mice, rats, and humans. We conclude that humans produce a third form of GAD in non-neural tissues and that human islets, although they do not synthesize full-length GAD67, do express this shortened variant.