Immunohistochemical and Ultrastructural characterization of the terminal nerve ganglion cells of the ayu, Plecoglossus altivelis (Salmoniformes, Teleostei)

Expression of tachykinin3 and related reproductive markers in the brain of the African cichlid fish Astatotilapia burtoni

Neurokinin B, encoded by the tachykinin3 gene, plays a crucial role in regulating reproduction in mammals via KNDy neurons and interaction with GnRH. Previous work in teleost fishes has focused on hypothalamic tac3 expression for its role in reproduction, but detailed studies on extra-hypothalamic tac3 expression are limited. Here, we identified two tac3 genes in the social African cichlid fish Astatotilapia burtoni, only one of which produces a functional protein containing the signature tachykinin motif. In situ hybridization for tac3a mRNA identified cell populations throughout the brain. Numerous tac3a cells lie in several thalamic and hypothalamic nuclei, including periventricular nucleus of posterior tuberculum, lateral tuberal nucleus (NLT), and nucleus of the lateral recess (NRL). Scattered tac3-expressing cells are also present in telencephalic parts, such as ventral (Vv) and supracomissural (Vs) part of ventral telencephalon. In contrast to other teleosts, tac3 expression was absent from the pituitary. Using double-fluorescent staining, we localized tac3a-expressing cells in relation to GnRH and kisspeptin cells. Although no GnRH-tac3a colabeled cells were observed, dense GnRH fibers surround and potentially synapse with tac3a cells in the preoptic area. Only minimal (<5%) colabeling of tac3a was observed in kiss2 cells. Despite tac3a expression in many nodes of the mesolimbic reward system, it was absent from tyrosine hydroxylase (TH)-expressing cells, but tac3a cells were located in areas with dense TH fibers. The presence of tac3a-expressing cells throughout the brain, including in socially relevant brain regions, suggest more diverse functions beyond regulation of reproductive physiology that may be conserved across vertebrates.

Evolutionary Viewpoint on GnRH (gonadotropin-releasing hormone) in Chordata - Amino Acid and Nucleic Acid Sequences

GnRH (gonadotropin-releasing hormone) is a supreme hormone regulating reproductive activity in most animals. The sequences of amino acid and nucleic acid of GnRH reported up to now are examined from the evolutionary framework of Chordata. All identified GnRH are classified into GnRH1, GnRH2, or GnRH3. In all three forms of GnRH both N-terminal and C-terminal are conserved, which allows for effective binding to their receptors. The three amino acids in the middle of GnRH1 sequence have altered diversely from the primitive Chordata, which is indicative of the adaptation process to the ambient environment. GnRH2 and GnRH3 sequences are well conserved. There are more diverse modifications in the nucleic acids than in amino acid sequence of GnRH1. These variations can result from meiosis, mutation, or epigenetics and indicate that GnRH is the product of natural selection.

The role of the terminal nerve and GnRH in olfactory system neuromodulation

Animals must regulate their sensory responsiveness appropriately with respect to their internal and external environments, which is accomplished in part via centrifugal modulatory pathways. In the olfactory sensory system, responsiveness is regulated by neuromodulators released from centrifugal fibers into the olfactory epithelium and bulb. Among the modulators known to modulate neural activity of the olfactory system, one of the best understood is gonadotropin-releasing hormone (GnRH). This is because GnRH derives mainly from the terminal nerve (TN), and the TN-GnRH system has been suggested to function as a neuromodulator in wide areas of the brain, including the olfactory bulb. In the present article we examine the modulatory roles of the TN and GnRH in the olfactory epithelium and bulb as a model for understanding the ways in which olfactory responses can be tuned to the internal and external environments.

Association of cocaine- and amphetamine-regulated transcript and neuropeptide Y in the forebrain and pituitary of the catfish, Clarias batrachus: a double immunofluorescent labeling study

Cocaine- and amphetamine-regulated transcript (CART) and neuropeptide Y (NPY) are involved in the regulation of food intake, body weight, pituitary hormones, and reproduction. While CART and NPY occupy overlapping fields in the brain of mammals, little is known about the interaction between these peptide-containing systems in other vertebrates. We explored neuroanatomical associations between CART and NPY in the olfactory system, forebrain and pituitary of the catfish, Clarias batrachus, using double immunofluorescence method. NPY-containing fascicles from olfactory receptor neurons innervated the olfactory glomeruli and mitral cell layer in close association with CART-containing terminal fields. Distinct CART- or NPY-containing fibers were seen in the medial olfactory tract. In the dorsal telencephalon, CART- and NPY-immunoreactive axons were closely associated in area dorsalis telencephali/pars lateralis dorsalis (Dld), and posterioris (Dlp). In the ventral telencephalon, while most of the cells of nucleus entopeduncularis (NE) showed the presence of CART as well as NPY, a few cells with only NPY-immunoreactivity were observed. Similarly, a CART and NPY colocalized cell population was prominent in the preoptic area (POA); and a small population of cells with NPY-immunoreactivity was also evident. Other areas where CART and NPY were colocalized included fibers in the tuberal area, inferior lobe, neurohypophysis, proximal pars distalis and pars intermedia of the pituitary. No association between CART and NPY was observed in the thalamus and habenular ganglion. These results suggest that CART- and NPY-peptidergic systems may interact in NE, POA, tuberal area, certain telencephalic areas and pituitary and jointly process information relating to reproduction, feeding and neuroendocrine regulation.

Structural and functional evolution of gonadotropin-releasing hormone in vertebrates

The neuropeptide gonadotropin-releasing hormone (GnRH) has a central role in the neural control of vertebrate reproduction. This review describes an overview of what is currently known about GnRH in vertebrates in the context of its structural and functional evolution. A large body of evidence has demonstrated the existence of three paralogous genes for GnRH (GnRH1, GnRH2 and GnRH3) in the vertebrate lineage. They are most probably the products of whole-genome duplications that occurred early in vertebrate evolution. Although GnRH3 has been identified only in teleosts, comparative genomic analyses indicated that GnRH3 has not arisen from a teleost-specific genome duplication, but has been derived from an earlier genome duplication in an ancestral vertebrate, followed by its loss in the tetrapod lineage. A loss of other paralogous genes has also occurred independently in different vertebrate lineages, leading to species-specific differences in the organization of the GnRH system. In addition to the GnRH3 gene, the GnRH2 gene has been deleted or silenced in certain mammalian species, while some teleosts seem to have lost the GnRH1 or GnRH3 gene. The duplicated GnRH genes have undergone subfunctionalization during the evolution of vertebrates; GnRH1 has become the major stimulator of gonadotropins and probably other pituitary hormones as well, whereas GnRH2 and GnRH3 would have functioned as neuromodulators, affecting reproductive behaviour. Conversely, in cases where a paralogous gene for GnRH has been lost, one of the remaining paralogues appears to have adopted its role.

Terminal nerve-derived neuropeptide y modulates physiological responses in the olfactory epithelium of hungry axolotls (Ambystoma mexicanum)

The vertebrate brain actively regulates incoming sensory information, effectively filtering input and focusing attention toward environmental stimuli that are most relevant to the animal's behavioral context or physiological state. Such centrifugal modulation has been shown to play an important role in processing in the retina and cochlea, but has received relatively little attention in olfaction. The terminal nerve, a cranial nerve that extends underneath the lamina propria surrounding the olfactory epithelium, displays anatomical and neurochemical characteristics that suggest that it modulates activity in the olfactory epithelium. Using immunocytochemical techniques, we demonstrate that neuropeptide Y (NPY) is abundantly present in the terminal nerve in the axolotl (Ambystoma mexicanum), an aquatic salamander. Because NPY plays an important role in regulating appetite and hunger in many vertebrates, we investigated the possibility that NPY modulates activity in the olfactory epithelium in relation to the animal's hunger level. We therefore characterized the full-length NPY gene from axolotls to enable synthesis of authentic axolotl NPY for use in electrophysiological experiments. We find that axolotl NPY modulates olfactory epithelial responses evoked by l-glutamic acid, a food-related odorant, but only in hungry animals. Similarly, whole-cell patch-clamp recordings demonstrate that bath application of axolotl NPY enhances the magnitude of a tetrodotoxin-sensitive inward current, but only in hungry animals. These results suggest that expression or activity of NPY receptors in the olfactory epithelium may change with hunger level, and that terminal nerve-derived peptides modulate activity in the olfactory epithelium in response to an animal's changing behavioral and physiological circumstances.

The centrifugal visual system of vertebrates: a comparative analysis of its functional anatomical organization

The present review is a detailed survey of our present knowledge of the centrifugal visual system (CVS) of vertebrates. Over the last 20 years, the use of experimental hodological and immunocytochemical techniques has led to a considerable augmentation of this knowledge. Contrary to long-held belief, the CVS is not a unique property of birds but a constant component of the central nervous system which appears to exist in all vertebrate groups. However, it does not form a single homogeneous entity but shows a high degree of variation from one group to the next. Thus, depending on the group in question, the somata of retinopetal neurons can be located in the septo-preoptic terminal nerve complex, the ventral or dorsal thalamus, the pretectum, the optic tectum, the mesencephalic tegmentum, the dorsal isthmus, the raphé, or other rhombencephalic areas. The centrifugal visual fibers are unmyelinated or myelinated, and their number varies by a factor of 1000 (10 or fewer in man, 10,000 or more in the chicken). They generally form divergent terminals in the retina and rarely convergent ones. Their retinal targets also vary, being primarily amacrine cells with various morphological and neurochemical properties, occasionally interplexiform cells and displaced retinal ganglion cells, and more rarely orthotopic ganglion cells and bipolar cells. The neurochemical signature of the centrifugal visual neurons also varies both between and within groups: thus, several neuroactive substances used by these neurons have been identified; GABA, glutamate, aspartate, acetylcholine, serotonin, dopamine, histamine, nitric oxide, GnRH, FMRF-amide-like peptides, Substance P, NPY and met-enkephalin. In some cases, the retinopetal neurons form part of a feedback loop, relaying information from a primary visual center back to the retina, while in other, cases they do not. The evolutionary significance of this variation remains to be elucidated, and, while many attempts have been made to explain the functional role of the CVS, opinions vary as to the manner in which retinal activity is modified by this system.

Glomerular territories in the olfactory bulb from the larval stage of the sea lamprey Petromyzon marinus

The goal of this study was to investigate the spatial organization of olfactory glomeruli and of substances relevant to olfactory sensory neuron activity in the developing agnathan, the sea lamprey Petromyzon marinus. A 45-kD protein immunoreactive to G(olf), a cAMP-dependent olfactory G protein, was present in the ciliary fraction of sea lamprey olfactory epithelium and in olfactory sensory neurons of larval and adult sea lampreys. This result implies that G(olf) expression was present during early vertebrate evolution or evolved in parallel in gnathostome and agnathostome vertebrates. Serial sectioning of the olfactory bulb revealed a consistent pattern of olfactory glomeruli stained by GS1B(4) lectin and by anterograde labeling with fluorescent dextran. These glomerular territories included the dorsal cluster, dorsal ring, anterior plexus, lateral chain, medial glomeruli, ventral ring, and ventral cluster. The dorsal, anterior, lateral, and ventral glomeruli contained olfactory sensory axon terminals that were G(olf)-immunoreactive. However, a specific subset, the medial glomeruli, did not display this immunoreactivity. Olfactory glomeruli in the dorsal hemisphere of the olfactory bulb, the dorsal cluster, dorsal ring, anterior plexus, lateral chain, and medial glomeruli, were seen adjacent to 5HT-immunoreactive fibers. However, glomeruli in the ventral hemisphere, the ventral ring, and ventral cluster did not display this association. The presence of specific glomerular territories and discrete glomerular subsets with substances relevant to olfactory sensory neuron activity suggest a spatial organization of information flow in the lamprey olfactory pathway.

Distribution and development of FMRFamide-like immunoreactive neuronal systems in the brain of the brown trout, Salmo trutta fario

The distribution of Phe-Met-Arg-Phe-amide (FMRFamide) peptide-immunoreactive (FMRF-ir) cells and fibers in the terminal nerve and central nervous system was investigated in developing stages and adults of the brown trout, Salmo trutta fario. The first FMRF-ir neurons appeared in the terminal nerve system of 8-mm embryos in and below the olfactory placode. In the brain, FMRF-ir neurons were first observed in the rostral hypothalamus, primordial hypothalamic lobe, mesencephalic laminar nucleus, and locus coeruleus of 12- to 13 -m embryos. After hatching, FMRF-ir cells appeared in the lateral part of the ventral telencephalic area and the anterior tuberal nucleus. In adult trout, FMRF-ir cells were observed in all these areas. The number of FMRF-ir neurons increased markedly in some of these populations during development. Dense innervation by FMRF-ir fibers was observed in the dorsal and lateral parts of the dorsal telencephalic area, and in the ventral telencephalic area, the lateral preoptic area, the medial hypothalamic and posterior tubercle regions, midbrain tegmentum and rhombencephalic reticular areas, the central gray, the superior raphe nucleus, the secondary visceral nucleus, the vagal nuclei, and the area postrema. Fairly rich FMRF-ir innervation was also observed in the optic tectum and some parts of the torus semicircularis. The saccus vasculosus and hypophysis received a moderate amount of FMRF-ir fibers. Innervation of most of these regions appeared either in late alevins or fry, although FMRF-ir fibers in the preoptic area, hypothalamus, and reticular areas appeared in embryos. Comparative analysis of the complex innervation pattern observed in the brain of trout suggests that FMRF is involved in a variety of functions, like the FMRF family of peptides in mammals.

Development of three distinct GnRH neuron populations expressing two different GnRH forms in the brain of the African catfish (Clarias gariepinus)

The early development of both the catfish gonadotropin-releasing hormone (cfGnRH)- and the chicken GnRH-II (cGnRH-II) system was investigated in African catfish by immunocytochemistry by using antibodies against the GnRH-associated peptide (GAP) of the respective preprohormones. Weakly cfGnRH-immunoreactive (ir) neurons and fibers were present at 2 weeks after hatching (ph) but only in the ventral telencephalon and pituitary. Two weeks later, cfGnRH fibers and neurons were also observed in more rostral and in more caudal brain areas, mainly in the preoptic area and hypothalamus. Based on differences in temporal, spatial, and morphologic appearance, two distinct cfGnRH populations were identified in the ventral forebrain: a population innervating the pituitary (ventral forebrain system) and a so-called terminal nerve (TN) population. DiI tracing studies revealed that the TN population has no neuronal connections with the pituitary. The cGnRH-II system is present from 2 weeks ph onward in the midbrain tegmentum and only their size and staining intensity increased during development. Based on the comparison of GnRH systems amongst vertebrates, we hypothesize that during fish evolution, three different GnRH systems evolved, each expressing their own molecular form: the cGnRH-II system in the midbrain, a hypophysiotropic GnRH system in the hypothalamus with a species-specific GnRH form, and a salmon GnRH-expressing TN population. This hypothesis is supported by phylogenetic analysis of known GnRH precursor amino acid sequences. We hypothesize, because the African catfish is a less advanced teleost species, that it contains the cfGnRH form both in the ventral forebrain system and in the TN population.

Molecular cloning and tissue-specific expression of a gonadotropin-releasing hormone receptor in the Japanese eel

Gonadotropin-releasing hormone (GnRH) is a key regulatory neuropeptide involved in the control of reproduction in vertebrates. In the Japanese eel, one of the most primitive teleost species, two molecular forms of GnRH, mammalian-type GnRH and chicken-II-type GnRH (cGnRH-II), have been identified. This study has isolated a full-length cDNA for a GnRH receptor from the pituitary of the eel. The 3233-bp cDNA encodes a 380-amino acid protein which contains seven hydrophobic transmembrane domains and N- and C-terminal regions. The exon/intron organization of the open reading frame of the eel GnRH receptor gene was also determined. The open reading frame consists of three exons and two introns. The exon-intron splice site is similar to that of the GnRH receptor genes of mammals reported so far. Expression of the eel GnRH receptor was detected in various parts of the brain, pituitary, eye, olfactory epithelium, and testis. This result suggests that GnRH has local functions in these tissues in addition to its actions on gonadotropin synthesis and release in the pituitary. This tissue-specific expression pattern is similar to that of the eel cGnRH-II. Furthermore, the present eel receptor shows very high amino acid identity with the catfish and goldfish GnRH receptors, which are highly selective for the cGnRH-II. These results suggest that the cGnRH-II acts through binding to the present receptor in the eel.

Immunohistochemical cell types in the terminal nerve ganglion of the cloudy dogfish, Scyliorhinus torazame, with special regard to neuropeptide Y/FMRFamide-immunoreactive cells

Previous studies showed immunoreactivities for neuropeptide Y (NPY), molluscan cardioexcitatory tetrapeptide (FMRFamide), and gonadotropin-releasing hormone (GnRH) in the terminal nerve of elasmobranchs. The present immunohistochemical study demonstrated two types of cells, i.e. GnRH- and NPY/FMRFamide-positive cells, in the terminal nerve ganglion of the elasmobranch Scyliorhinus torazame. The second cell type (non-GnRH element) contained a substance with a common structure or epitope recognized by anti-NPY and anti-FMRFamide antibodies. The NPY/FMRFamide-like immunoreactivity was associated with granules 70-130 nm in diameter, found in the cell bodies, axons, and axon endings.

FMRFamide-like immunoreactivity in the olfactory system responds to morphine treatment in the teleost Clarias batrachus: involvement of opiate receptors

In view of the close relationship between the FMRF-related peptides and the central opiate-sensitive system, we investigated the effects of morphine, alone and in combination with naloxone, on the FMRFamide-like immunoreactivity in the olfactory system of the teleost, Clarias batrachus. In the olfactory system of normal and untreated fish, FMRFamide-like immunoreactivity was confined to the ganglion cells and fibers of the terminal nerve; the cells in the olfactory epithelium per se or the olfactory nerve were not immunoreactive. Intensely immunoreactive cells appeared in the olfactory epithelium following 2 h of intracranial morphine administration. FMRFamide-like immunoreactivity also appeared in the olfactory nerve fibers as they ran caudally and arborized in the glomerular layer of the bulb. However, immunoreactivity in the ganglion cells of the terminal nerve and the ensuing fibers was abolished, suggesting the transport/release of the immunoreactive material. Pretreatment with naloxone, a potent opiate receptor antagonist, reversed the effects of morphine, suggesting the involvement of opiate receptors in the regulation of the ganglion cells of the terminal nerve. The results provide initial immunocytochemical evidence in favor of a relationship between the opiates and FMRFamide-containing systems within the framework of the olfactory system.