Morphological evidence for a direct neuroendocrine GABAergic control of the anterior pituitary in teleosts

Distribution of gamma-aminobutyric acid immunoreactivity in the brain of the Siberian sturgeon (Acipenser baeri): Comparison with other fishes

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system (CNS) of vertebrates. Immunohistochemical techniques with specific antibodies against GABA or against its synthesizing enzyme, glutamic acid decarboxylase (GAD) allowed characterizing GABAergic neurons and fibers in the CNS. However, studies on the CNS distribution of GABAergic neurons and fibers of bony fishes are scant and were done in teleost species. With the aim of understanding the early evolution of this system in bony vertebrates, we analyzed the distribution of GABA-immunoreactive (-ir) and GAD-ir neurons and fibers in the CNS of a basal ray-finned fish, the Siberian sturgeon (Chondrostei, Acipenseriformes), using immunohistochemical techniques. Our results revealed the presence and distribution of GABA/GAD-ir cells in different regions of the CNS such as olfactory bulbs, pallium and subpallium, hypothalamus, thalamus, pretectum, optic tectum, tegmentum, cerebellum, central grey, octavolateralis area, vagal lobe, rhombencephalic reticular areas, and the spinal cord. Abundant GABAergic innervation was observed in most brain regions, and GABAergic fibers were very abundant in the hypothalamic floor along the hypothalamo-hypophyseal tract and neurohypophysis. In addition, GABA-ir cerebrospinal fluid-contacting cells were observed in the alar and basal hypothalamus, saccus vasculosus, and spinal cord central canal. The distribution of GABAergic systems in the sturgeon brain shows numerous similarities to that observed in lampreys, but also to those of teleosts and tetrapods.

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.

Distribution of GABA-immunoreactive neurons in the forebrain of the goldfish, Carassius auratus

The distribution of gamma-aminobutyric acid (GABA) immunoreactivity was studied in the forebrain (tel- and diencephalon) of the goldfish by means of immunocytochemistry on Vibratome sections using antibodies against GABA. Positive perikarya were detected in the olfactory bulbs and in all divisions of the telencephalon, the highest density being found along the midline. In the diencephalon, GABA-containing cell bodies were found in the hypothalamus, in particular in the preoptic and tuberal regions. The inferior lobes, the nucleus recessus lateralis, and more laterodorsal regions, such as the nucleus glomerulosus and surrounding structures, also exhibited numerous GABA-positive perikarya. Cell bodies were also noted in the thalamus, in particular in the dorsomedial, dorsolateral and ventromedial nuclei. The relative density of immunoreactive fibers was evaluated for each brain nucleus and classified into five categories. This ubiquitous distribution indicates that, as in higher vertebrates, GABA most probably represents one of the major neurotransmitters in the brain of teleosts.

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.

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.

Distribution of GABA-Like Immunoreactive Cell Bodies in the Brains of Two Amphibians, Rana catesbeiana and Xenopus laevis

The distribution of the neurotransmitter gamma-aminobutyric acid (GABA) is not well understood for non-mammalian vertebrates. We thus used immunocytochemistry to locate putative GABAergic cells in the brains of male bullfrogs (Rana catesbeiana) and South African clawed frogs (Xenopus laevis). GABA-immunoreactive cell bodies were broadly distributed throughout the brains of both species with similar general patterns. In both, the greatest numbers of GABA-positive cells were found in the olfactory bulb, thalamus, and optic tectum, but virtually no major brain region lacked GABAergic cells. Species differences were also apparent. The density of GABA-immunoreactive cells was substantially higher in many areas of the bullfrog brain, compared to Xenopus. Bullfrogs possessed extensive cell populations in the medial pallium, preoptic area, optic tectum, torus semicircularis and tegmentum but cells were fewer in these locations in Xenopus. In the bullfrog hindbrain, GABA-immunoreactive cell bodies were restricted to very narrow and distinct populations. In Xenopus, however, cells in a similar position were fewer and spread more extensively. The distribution of GABA cells in the brain of these two species supports the hypotheses that GABA is involved in control of olfaction, audition, vision and vocalization. However, differences in the distribution of GABA between the bullfrog and Xenopus suggest that the extent of the GABAergic influence might vary between species.

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 distribution of GABA-immunoreactive neurons in the brain of the silver eel (Anguilla anguilla L.)

The distribution of GABA-immunoreactivity was studied in the brain of the silver eel (Anguilla anguilla) by means of antibodies directed against GABA. Immunoreactive neuronal somata were distributed throughout the brain. Positive perikarya were detected in the internal cellular layer of the olfactory bulb, and in all divisions of the telencephalon, the highest density being observed along the midline. Numerous GABA-reactive cell bodies were found in the diencephalon, particularly in the preoptic and tuberal regions of the hypothalamus, and the dorsolateral, dorsomedial and ventromedial thalamic nuclei. In the optic tectum, the majority of GABA-positive cell bodies were located in the periventricular layer. A number of immunolabeled cell bodies were observed in different tegmental structures, notably the torus semicircularis. In the cerebellum, the Purkinje cells were either very intensely or very weakly immunoreactive. In the rhombencephalon, reactive cell bodies were observed in the eminentia granularis, the valvula cerebellaris, the octavolateral nucleus, the lobus vagus and in the vagal and glossopharyngeal motor nuclei. Intensely immunoreactive axons and terminals were observed in the external granular layer and internal cellular layer of the olfactory bulb. In the telencephalon, the highest density of reactive fibres and boutons was found in the fields of the medial wall. Many immunolabeled fibres were seen in the medial and lateral forebrain bundles. In the diencephalon, intense labelling of fibres and terminals were observed in the nuclei situated close to the midline. In the optic tectum the highest density of reactive fibres was seen in the sfgs, the layer to which the retina projects massively. Finally, in the rhombencephalon the strongest labelling of neurites was observed in the nuclei of the raphé, the nucleus octavocellularis magnocellularis and the nuclei of the IXth and Xth cranial nerves. The GABAergic system of the eel, which is well developed, appears to be generally comparable to that described in tetrapod vertebrates.

GABA-ergic control of prolactin release in rainbow trout (Oncorhynchus mykiss) pituitaries in vitro

The involvement of γ-aminobutyric acid (GABA) in the control of prolactin (PRL) release was investigated in rainbow trout using both perifused pituitary fragments and pituitary cells in primary culture. In our perifusion system, infusion of GABA (10(-6) to 10(-4) M) caused an inhibition of PRL release (between 20 and 40%). Administration on perifused pituitary fragments of 3APS, a GABAa agonist, mimicked this inhibitory effect. Moreover, bicuculline, a specific antagonist of GABAa receptors, totally abolished GABA effect. When tested on cultured pituitary cells during 40h exposure, GABA (10(-5) M) caused a significant decrease in PRL release (24.5%). Baclofen, a specific agonist for GABAb receptor tested at 10(-6) and 10(-5) M, also inhibited PRL released from cultured pituitary cells. These results demonstrate that GABA inhibits PRL release by acting directly on pituitary cells and that probably both types of GABA receptor (a and b) are involved in this regulation.

Localization and characterization of diazepam-binding inhibitor (DBI)-like peptides in the brain and pituitary of the trout (Salmo gairdneri)

The distribution of diazepam-binding inhibitor (DBI)-like peptide(s) in the brain and pituitary of the trout was determined by the indirect immunofluorescence technique using an antiserum raised against synthetic rat octadecaneuropeptide (ODN). Numerous immunoreactive perikarya and processes were observed in the basal hypothalamus, within the pars lateralis of the nucleus lateralis tuberis. In the pituitary, ODN-immunoreactive processes were visualized in the neurohypophysial tract, projecting into the pars intermedia, and the pars distalis. Reverse-phase high-performance liquid chromatography combined to radioimmunoassay quantification was used to characterize the DBI-related material in tissue extracts. In both pituitary and hypothalamic extracts, the major immunoreactive form eluted with a retention time higher than that of rat ODN. In the hypothalamus, a minor peak co-eluting with the synthetic ODN standard was also resolved. The existence of peptides related to mammalian DBI in the hypothalamo-hypophysial complex of the trout suggests these neuropeptides may participate in the control of pituitary hormone release.

[The intermediate lobe of the pituitary, model of neuroendocrine communication]

The intermediate lobe of the pituitary is composed of a homogeneous population of endocrine cells, the melanotrophs, which secrete several bioactive peptides including alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin. In contrast to most endocrine glands which are richly vascularized, the intermediate lobe of the pituitary contains very few blood vessels; in some species, the pars intermedia is virtually totally avascular. In contrast, pituitary melanotrophs are richly supplied by nerve fibers originating from the hypothalamus. The pars intermedia thus appears as a pure model of neuroendocrine communication, i.e. it is an archetype of the mode of transducing interface between the central nervous system and endocrine effectors. In mammalian species, different types of nerve terminals containing dopamine, norepinephrine, gamma-aminobutyric acid (GABA) and serotonin have been identified. In lower vertebrates, particularly in fish and amphibians, the pars intermedia is also innervated by peptidergic fibers which are though to take part in regulation of the secretory activity of the melanotroph. In these animals, the pars intermedia is regarded as a major center of neuroendocrine integration and an exceptional model to investigate the process of communication between the brain and the endocrine glands. The purpose of the present review is to summarize our current knowledge on the synthesis, processing and release of peptide hormones from pars intermedia cells and to survey the multiple regulatory mechanisms which are involved in the control of the activity of pituitary melanotrophs. Proopiomelanocortin, a multifunctional precursor. Pituitary melanotrophs synthetise a major precursor protein called proopiomelanocortin (POMC) which generates through proteolytic cleavage several biologically active peptides including adrenocorticotropic hormone (ACTH), endorphins and MSHs. In lower vertebrates, alpha-MSH is generally considered as the major hormone secreted by melanotrophs, in that it is involved in the process of skin colour adaptation. The post-translational processing of POMC, which yields to the mature hormones released by melanotrophs, includes a number of steps: glycosylation, phosphorylation, tissue-specific proteolytic cleavage, amidation and acetylation. Some of these posttranslational modifications can be regulated by neuroendocrine factors. For instance, in frogs, it has been shown that dopamine inhibits acetylation of alpha-MSH and thus reduces the secretion of the biologically active form of the peptide. The intermediate lobe of the pituitary: a model of neuroendocrine integration. In most vertebrate species, the intermediate lobe of the pituitary is innervated by catecholamine-containing fibers. In particular, the presence of dopaminergic nerve fibers has been observed in the pars intermedia of mammals and poikilotherms.(ABSTRACT TRUNCATED AT 400 WORDS)

A presumptive role for GABA in the stimulatory effects of Des-Gly10, [D-Ala6]-LHRH-ethylamide and pimozide on the gonadotropin release in carp

To investigate the effect of endogenous gamma-aminobutyric acid (GABA) on the blood maturating gonadotropin (GtH) levels, or to study its interaction with pimozide (dopamine antagonist) and a luteinizing hormone-releasing hormone analog (LHRH-a), sexually mature male and female carps were treated with drugs that may either inhibit GABA biosynthesis or GABA degradation. In females the irreversible inhibitor of GABA-transaminase, gamma-vinyl GABA (GVG), which was to increase the endogenous GABA-ergic tone, had no influence on GtH release. On the other hand, the increased GtH response to the combination of pimozide (PIM) and LHRH-a was clearly enhanced by the administration of 3-mercaptopropionic acid (MPA), an inhibitor of the rate limiting enzyme of GABA-biosynthesis. In males the GABA-ergic compound, valproic acid (DPA) decreased LHRH-a stimulated GtH levels. In male carps that received PIM to diminish the dopaminergic inhibition of GtH release, the spermiating response to LHRH-a was increased by administration of MPA. These data suggest that GABA interacts with the action of dopamine and the gonadotropin releasing hormone (GnRH) on the release of GtH.

Localization of CRF-like immunoreactivity in the brain and pituitary of teleost fish

Immunocytochemical techniques were applied to brain and pituitary sections of eleven teleost species. A corticotropin-releasing factor (CRF)-antiserum allowed the identification of a CRF-like system in these species. Perikarya were labeled in the preoptic nucleus. Labeled fibers were traced laterally, then ventrally close to the optic chiasma, forming two symmetrical tracts running through the basal hypothalamus. These ended in the rostral neurohypophysis (NH) close to ACTH cells as shown by double immunostaining. Other fibers, often more variquous, ended in the caudal NH close to melanocorticotropic cells. In Salmo fario, small perikarya also stained in the nucleus lateralis tuberis. The CRF-like system appears distinct from that of somatostatin. In Anguilla, adjacent sections stained with CRF- and vasotocin (AVT)-antisera respectively showed that these two peptides coexist in some perikarya. As few fibers containing only AVT end in the rostral NH, they probably do not control ACTH cells directly. AVT fibers terminate mostly in the caudal NH close to melanocorticotropic cells. Some extra-hypothalamic fibers suggest that CRF may also act as a neurotransmitter. The plurality of hormones showing a CRF-like activity in teleosts is considered.