Immunohistochemical investigation of gamma-aminobutyric acid ontogeny and transient expression in the central nervous system of Xenopus laevis tadpoles

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.

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.

From nerve net to nerve ring, nerve cord and brain--evolution of the nervous system

The puzzle of how complex nervous systems emerged remains unsolved. Comparative studies of neurodevelopment in cnidarians and bilaterians suggest that this process began with distinct integration centres that evolved on opposite ends of an initial nerve net. The 'apical nervous system' controlled general body physiology, and the 'blastoporal nervous system' coordinated feeding movements and locomotion. We propose that expansion, integration and fusion of these centres gave rise to the bilaterian nerve cord and brain.

Prosomeric organization of the hypothalamus in an elasmobranch, the catshark Scyliorhinus canicula

The hypothalamus has been a central topic in neuroanatomy because of its important physiological functions, but its mature organization remains elusive. Deciphering its embryonic and adult organization is crucial in an evolutionary approach of the organization of the vertebrate forebrain. Here we studied the molecular organization of the hypothalamus and neighboring telencephalic domains in a cartilaginous fish, the catshark, Scyliorhinus canicula, focusing on ScFoxg1a, ScShh, ScNkx2.1, ScDlx2/5, ScOtp, and ScTbr1 expression profiles and on the identification α-acetylated-tubulin-immunoreactive (ir), TH-ir, 5-HT-ir, and GFAP-ir structures by means of immunohistochemistry. Analysis of the results within the updated prosomeric model framework support the existence of alar and basal histogenetic compartments in the hypothalamus similar to those described in the mouse, suggesting the ancestrality of these subdivisions in jawed vertebrates. These data provide new insights into hypothalamic organization in cartilaginous fishes and highlight the generality of key features of the prosomeric model in jawed vertebrates.

Regional expression of Pax7 in the brain of Xenopus laevis during embryonic and larval development

Pax7 is a member of the highly conserved Pax gene family that is expressed in restricted zones of the central nervous system (CNS) during development, being involved in early brain regionalization and the maintenance of the regional identity. Using sensitive immunohistochemical techniques we have analyzed the spatiotemporal pattern of Pax7 expression in the brain of the anuran amphibian Xenopus laevis, during development. Pax7 expression was first detected in early embryos in the basal plate of prosomere 3, roof and alar plates of prosomere 1 and mesencephalon, and the alar plate of rhombomere 1. As development proceeded, Pax7 cells were observed in the hypothalamus close to the catecholaminergic population of the mammillary region. In the diencephalon, Pax7 was intensely expressed in a portion of the basal plate of prosomere 3, in the roof plate and in scattered cells of the thalamus in prosomere 2, throughout the roof of prosomere 1, and in the commissural and juxtacommissural domains of the pretectum. In the mesencephalon, Pax7 cells were localized in the optic tectum and, to a lesser extent, in the torus semicircularis. The rostral portion of the alar part of rhombomere 1, including the ventricular layer of the cerebellum, expressed Pax7 and, gradually, some of these dorsal cells were observed to populate ventrally the interpeduncular nucleus and the isthmus (rhombomere 0). Additionally, Pax7 positive cells were found in the ventricular zone of the ventral part of the alar plate along the rhombencephalon and the spinal cord. The findings show that the strongly conserved features of Pax7 expression through development shared by amniote vertebrates are also present in the anamniote amphibians as a common characteristic of the brain organization of tetrapods.

GABA expression and regulation by sensory experience in the developing visual system

The developing retinotectal system of the Xenopus laevis tadpole is a model of choice for studying visual experience-dependent circuit maturation in the intact animal. The neurotransmitter gamma-aminobutyric acid (GABA) has been shown to play a critical role in the formation of sensory circuits in this preparation, however a comprehensive neuroanatomical study of GABAergic cell distribution in the developing tadpole has not been conducted. We report a detailed description of the spatial expression of GABA immunoreactivity in the Xenopus laevis tadpole brain at two key developmental stages: stage 40/42 around the onset of retinotectal innervation and stage 47 when the retinotectal circuit supports visually-guided behavior. During this period, GABAergic neurons within specific brain structures appeared to redistribute from clusters of neuronal somata to a sparser, more uniform distribution. Furthermore, we found that GABA levels were regulated by recent sensory experience. Both ELISA measurements of GABA concentration and quantitative analysis of GABA immunoreactivity in tissue sections from the optic tectum show that GABA increased in response to a 4 hr period of enhanced visual stimulation in stage 47 tadpoles. These observations reveal a remarkable degree of adaptability of GABAergic neurons in the developing brain, consistent with their key contributions to circuit development and function.

Phylotypic expression of the bHLH genes Neurogenin2, Neurod, and Mash1 in the mouse embryonic forebrain

In the anamniote model animals, zebrafish and Xenopus laevis, highly comparable early forebrain expression patterns of proneural basic helix-loop-helix (bHLH) genes relevant for neurogenesis (atonal homologs, i.e., neurogenins/NeuroD and achaete-scute homologs, i.e., Ascl/ash) were previously revealed during a particular period of development (zebrafish: 3 days; frog: stage 48). Neurogenins/NeuroD on the one hand and Ascl1/ash1 on the other hand exhibit essentially mutually exclusive spatial patterns, probably reflecting different positional information received within the neural tube, and appear to underlie glutamatergic versus GABAergic neuronal differentiation, respectively. Significant data suggest that similar complementary localizations of these proneural genes and corresponding differentiation pathways also exist in the mouse, the prominent mammalian model. The present article reports on detailed mouse brain bHLH gene expression patterns to fill existing gaps in the identification of expression domains, especially outside the telencephalon. Clearly, there are strong similarities in the complementarity of territories expressing Ascl1/Mash 1 versus neurogenins/NeuroD in the entire mouse forebrain, except for the pretectal alar plate and basal plate of prosomeres 1-3. The analysis substantiates localization of neurogenins/NeuroD in the pallium, eminentia thalami, and dorsal thalamus, and expression of Ascl1/Mash 1 in the striatal and septal subpallium, preoptic region, ventral thalamus, and hypothalamus, which is highly similar to the situation described in Xenopus and zebrafish. Thus, all three vertebrate model species display a "phylotypic stage or period" corresponding to a temporally and spatially defined control of neurogenesis during forebrain development, ultimately resulting in the differentiation of distinct populations of glutamatergic versus GABAergic neurons.

The distribution of GAD67-mRNA in the adult zebrafish (teleost) forebrain reveals a prosomeric pattern and suggests previously unidentified homologies to tetrapods

We used in situ hybridization on sections to examine the distribution of GAD67-expressing cell populations in the entire forebrain of the adult zebrafish. GAD67 is predominantly expressed in the olfactory bulb (OB), all regions of the subpallium (including the dorsal, ventral, central, and lateral nucleus of the area ventralis [Vd, Vv, Vc, and Vl, respectively]), as well as preoptic (PPa, PPp, and PM), pretectal (PPd, PPv, PCN, PSp, and PSm), ventral (= pre-) thalamic (I, VM, and VL), hypothalamic (Hr, Hi, and Hc), preglomerular (P, PGa, PGl, PGm, and RT), and posterior tubercular (TPp and TPm) nuclei. Only scattered GAD67-expressing cells are seen in all pallial zones (Dm, Dd, Dc, Dl, and Dp) and in the previously unidentified bed nucleus of the stria medullaris (BNSM). The BNSM appears to be the adult teleostean derivative of the larval eminentia thalami (EmT). We identify the GAD67-positive entopeduncular nucleus proper (EN) as being homologous to the entopeduncular nucleus of nonprimate mammals. GAD67 is strongly expressed in the anterior thalamic nucleus (A). The anterior thalamic nucleus is laterally bordered by a distinct GAD67-expressing cell population, which we interpret as the previously unidentified reticular thalamic nucleus (RTN) of teleosts. Furthermore, we identified a GAD67-positive thalamic nucleus, the intercalated nucleus (IC), which is sandwiched between the GAD67-negative dorsal (DP) and central posterior (CP) thalamic nuclei. Overall, the distribution of GAD67-expressing cells highly resembles the distribution of gamma-aminobutyric acid (GABA)/GAD67-expressing cells found in the early zebrafish (teleost) forebrain and thus allows us to propose a prosomeric fate map of GABAergic cell populations.

GABA distribution in lamprey is phylogenetically conserved

The localization of gamma-aminobutyric acid (GABA) has been well described in most classes of vertebrates but not in adult lampreys. The question if the GABA distribution is similar throughout the vertebrate subphylum is therefore still to be addressed. We here investigate two lamprey species, the sea lamprey, Petromyzon marinus, and the river lamprey, Lampetra fluviatilis, and compare the GABA pattern with that of other vertebrates. The present immunohistochemical study provides an anatomical basis for the general distribution and precise localization of GABAergic neurons in the adult lamprey forebrain and brainstem. GABA-immunoreactive cells were organized in a virtually identical manner in the two species. They were found throughout the brain, with the following regions being of particular interest: the granular cell layer of the olfactory bulb, the nucleus of the anterior commissure, the septum, the lateral and medial pallia, the striatum, the nucleus of the postoptic commissure, the thalamus, the hypothalamus, and pretectal areas, the optic tectum, the torus semicircularis, the mesencephalic tegmentum, restricted regions of the rhombencephalic tegmentum, the octavolateral area, and the dorsal column nucleus. The GABA distribution found in cyclostomes is very similar to that of other classes of vertebrates, including mammals. Since the lamprey diverged from the main vertebrate line around 450 million years ago, this implies that already at that time the basic vertebrate plan for the GABA innervation in different parts of the brain had been developed.

Thyroid hormone and γ-aminobutyric acid (GABA) interactions in neuroendocrine systems

Thyroid hormones (THs) have critical roles in brain development and normal brain function in vertebrates. Clinical evidence suggests that some human nervous disorders involving GABA(gamma-aminobutyric acid)-ergic systems are related to thyroid dysfunction (i.e. hyperthyroidism or hypothyroidism). There is experimental evidence from in vivo and in vitro studies on rats and mice indicating that THs have effects on multiple components of the GABA system. These include effects on enzyme activities responsible for synthesis and degradation of GABA, levels of glutamate and GABA, GABA release and reuptake, and GABA(A) receptor expression and function. In developing brain, hypothyroidism generally decreases enzyme activities and GABA levels whereas in adult brain, hypothyroidism generally increases enzyme activities and GABA levels. Hyperthyroidism does not always have the opposite effect. In vitro studies on adult brain have shown that THs enhance GABA release and inhibit GABA-reuptake by rapid, extranuclear actions, suggesting that presence of THs in the synapse could prolong the action of GABA after release. There are conflicting results on effects of long term changes in TH levels on GABA reuptake. Increasing and decreasing circulating TH levels experimentally in vivo alter density of GABA(A) receptor-binding sites for GABA and benzodiazepines in brain, but results vary from study to study, which may reflect important regional differences in the brain. There is substantial evidence that THs also have an extranuclear effect to inhibit GABA-stimulated Cl(-) currents by a non-competitive mechanism in vitro. The thyroid gland exhibits GABA transport mechanisms as well as enzyme activities for GABA synthesis and degradation, all of which are sensitive to thyroidal state. In rats and humans, GABA inhibits thyroid stimulating hormone (TSH) release from the pituitary, possibly by action directly on the pituitary or on hypothalamic thyrotropin-releasing hormone neurons. In mice, GABA inhibits TSH-stimulated TH release from the thyroid gland. Taken together, these studies provide strong support for the hypothesis that there is reciprocal regulation of the thyroid and GABA systems in vertebrates.

Characterization of the binding of [3H]CGP54626 to GABAB receptors in the male bullfrog (Rana catesbeiana)

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the vertebrate brain. GABA activates both ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors in mammals. Whether non-mammalian vertebrates possess receptors with similar characteristics is not well understood. We used a mammalian GABA(B)-specific antagonist to determine the pharmacology of putative receptors in the brain of an anuran amphibian, the male bullfrog (Rana catesbeiana). Receptor binding assays with the antagonist [(3)H]CGP54626 revealed a single class of high affinity binding sites (with a K(D) of 2.97 nM and a B(max) of 2619 fmol/mg protein). Binding was time- and temperature-dependent, saturable and specific. Specific binding of [(3)H]CGP54626 was inhibited by several mammalian GABA(B) receptor agonists and antagonists. The rank order potency of agonists was: GABA = SKF97541 > (R)-Baclofen > 3-APPA. The rank order for antagonists was: CGP54626 = CGP55845 > CGP52432 > CGP35348. The GABA(A) receptor ligands muscimol and SR95531 had very low affinity for [(3)H]CGP54626 binding sites, while bicuculline compounds had no affinity. Binding of GABA was positively modulated by CGP7930. Taurine did not allosterically modulate GABA binding but did inhibit [(3)H]CGP54626 binding in a linear fashion. Bullfrog brain thus possesses binding sites with significant similarity to mammalian GABA(B) receptors. These receptors differ from mammalian receptors, however, in dissociation kinetics, ligand specificity and allosteric modulation.

A phylotypic stage in vertebrate brain development: GABA cell patterns in zebrafish compared with mouse

A recent comparison of early forebrain gene expression in mouse and zebrafish revealed highly comparable expression patterns of developmentally relevant genes, for example, of proneural (Neurogenin1, NeuroD, Mash1/Zash1a) genes involved in neurogenesis at a particular time window (mouse: embryonic day 12.5/13.5; zebrafish: 3 days). Here we extend this analysis to the description of gamma-aminobutyric acid (GABA) cell patterns in the early postembryonic zebrafish brain (i.e., during early secondary neurogenesis). We find again an astonishing degree of correspondences of GABA cell patterns between zebrafish and mouse during this previously established critical time window, for example, regarding absence of GABA cells in certain forebrain regions (pallium, dorsal thalamus, eminentia thalami) or with respect to the spatiotemporal occurrence of GABA cells (e.g., late cerebellar GABA cells). Furthermore, there is perfect correlation with previously established proneural gene expression patterns (i.e., absence of Mash1/Zash1a gene expression in GABA-cell-free forebrain regions) between mouse and zebrafish. The available information in additional vertebrate species, especially in Xenopus, is also highly consistent with our analysis here and suggests that a "phylotypic stage" of neurogenesis during vertebrate brain development may be present.

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 and development of glutamic acid decarboxylase immunoreactivity in the spinal cord of the dogfishScyliorhinus canicula(elasmobranchs)

The adult distribution and development of gamma-aminobutyric acid (GABA)-synthesizing cells and fibers in the spinal cord of the lesser spotted dogfish (Scyliorhinus canicula L.) was studied by means of immunohistochemistry using antibodies against glutamic acid decarboxylase (GAD). Complementary immunostaining with antibodies against GABA, tyrosine hydroxylase (TH), and HuC/HuD (members of the Hu/Elav family of RNA-associated proteins) and staining with a reduced silver procedure ("en bloc" Bielschowski method), Nissl, and hematoxylin were also used. In adults, GAD-immunoreactive (GAD-ir) cells were observed in the ventral horns, in the spinal nucleus of the dorsal horn, at the base of the dorsal horns, and around the central canal, where some GAD-ir cells were cerebrospinal fluid-contacting (CSF-c). In addition, a few GAD-ir cells were observed in the lateral funiculus between the ventral horn and the marginal nucleus. The adult spinal cord was richly innervated by GAD-ir fibers. Large numbers of GAD-ir fibers and boutons were observed in the dorsal and ventral horns and also interstitially in the dorsal, lateral, and ventral funiculi. In addition, a rich GAD-ir innervation was observed in the marginal nucleus of the spinal cord. In the embryonic spinal cord, GAD-ir cells develop very early: The earliest cells were observed in the very thin mantle/marginal layer of stage 22 embryos in a short length of the spinal cord. At stages 25 and 26, several types of GAD-ir cells (commissural and noncommissural) were distinguished, and two of these cells were of CSF-c type. At stages 28, 30, and 31, the GAD-ir populations exhibited a marked longitudinal columnar organization. Double-immunolabeling experiments in embryos showed the presence of two different GAD-ir CSF-c cell populations, one ventral that is simultaneously TH-ir and other more dorsal that is TH-negative. By stage 33 (prehatching), GAD-expressing cells are present in virtually all loci, as in adults, especially in the ventral horn and base of the dorsal horn. The present results for the lesser spotted dogfish suggest an important role for gamma-aminobutyric acid in sensory and motor circuits of the spinal cord.

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

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

Dopamine and histamine in the developing stomatogastric system of the lobster Homarus americanus

Dopamine and histamine are neuromodulators found in the adult stomatogastric nervous system (STNS) of several crustacean species. We used antibodies against tyrosine hydroxylase (TH) and histamine to map the distribution and developmental acquisition of the dopamine and histamine neurons in the STNS of the lobster, Homarus americanus. Embryos, larvae, juvenile and adult animals were studied. TH labeling was present in the STNS as early as E80-85 (80-85% of embryonic development). A subset of preparations in embryos, larvae, juveniles, and adults contained 1-5 labeled somata in the stomatogastric ganglion. Histamine staining appeared in the STNS as early as E50. The distribution of both TH and histamine staining remained relatively constant through development. Electrophysiological recordings demonstrated that receptors for both amines are present in the embryo. Bath application of dopamine increased the frequency of the pyloric rhythm in embryos, and evidence for dopaminergic activation of peripherally initiated spiking in motor axons was seen. In embryos and adults, histamine inhibited the motor patterns produced by the stomatogastric ganglion (STG). These data suggest that the dopaminergic and histaminergic systems in H. americanus appear relatively early in development and that the effects of each are largely maintained through development.

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

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

CB1 cannabinoid receptors in amphibian spinal cord: relationships with some nociception markers

The role of cannabinoids in spinal analgesia has so far been investigated in mammals and the interactions between cannabinoid receptors and markers involved in nociception have been described in the rat spinal cord. An endocannabinoid system is well developed also in the amphibian brain. However, the anatomical substrates of pain modulation have been scarcely investigated in anamniotes, neither is there reference to such a role for cannabinoids in lower vertebrates. In the present paper we employed multiple cytochemical approaches to study the distribution of CB1 cannabinoid receptors and their morphofunctional relationships with some nociception markers (i.e. Substance P, nitric oxide synthase, GABA and mu opioid receptors) in the spinal cord of the anuran amphibian Xenopus laevis. We found a co-distribution of CB1 receptors with the aforementioned signaling molecules, as well as a more limited cellular co-localization, in the dorsal and central fields of the spinal cord. These regions correspond to the mammalian laminae I-IV and X, respectively, areas strongly involved in spinal analgesia. Comparison of these results with those previously obtained in the mammalian spinal cord, reveals a number of similarities between the two systems and suggests that cannabinoids might participate in the control of pain sensitivity also in the amphibian spinal cord.

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

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

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.