Peptidergic innervation of caudal neurosecretory neurons

[The caudal neurosecretory system, the other "neurohypophysial" system in fish]

The caudal neurosecretory system (CNSS) is a neuroendocrine complex whose existence is specific to fishes. Structurally, it has many similarities with the hypothalamic-neurohypophyseal complex of other vertebrates. However, it differs regarding its position at the caudal end of the spinal cord and the nature of the hormones it secretes, the most important being urotensins. The CNSS was first described more than 60 years ago, but its embryological origin is totally unknown and its role is still poorly understood. Paradoxically, it is almost no longer studied today. Recent developments in imaging and genome editing could make it possible to resume investigations on CNSS in order to solve the mysteries that still surround it.

Neuromodulatory effects of GnRH on the caudal neurosecretory Dahlgren cells in female olive flounder

Gonadotropin-releasing hormone (GnRH) is considered a key player in reproduction. The caudal neurosecretory system (CNSS) is a unique neurosecretory structure of fish that may be involved in osmoregulation, nutrition, reproduction, and stress-related responses. However, a direct effect of GnRH on Dahlgren cells remains underexplored. Here, we examined the electrophysiological response of Dahlgren cell population of the CNSS to GnRH analog LHRH-A₂ and the transcription of related key genes of CNSS. We found that GnRH increased overall firing frequency and may be changed the firing pattern from silent to burst or phasic firing in a subpopulation of Dahlgren cells. The effect of GnRH on a subpopulation of Dahlgren cells firing activity was blocked by the GnRH receptor (GnRH-R) antagonist cetrorelix. A positive correlation was observed between the UII and GnRH-R mRNA levels in CNSS or gonadosomatic index (GSI) during the breeding season. These findings are the first demonstration of the ability of GnRH acts as a modulator within the CNSS and add to our understanding of the physiological role of the CNSS in reproduction and seasonal adaptation.

Multiple functions of non-hypophysiotropic gonadotropin releasing hormone neurons in vertebrates

Gonadotropin releasing hormone (GnRH) is a hypophysiotropic hormone that is generally thought to be important for reproduction. This hormone is produced by hypothalamic GnRH neurons and stimulates the secretion of gonadotropins. On the other hand, vertebrates also have non-hypophysiotropic GnRH peptides, which are produced by extrahypothalamic GnRH neurons. They are mainly located in the terminal nerve, midbrain tegmentum, trigeminal nerve, and spinal cord (sympathetic preganglionic nerves). In vertebrates, there are typically three gnrh paralogues (gnrh1, gnrh2, gnrh3). GnRH-expression in the non-hypophysiotropic neurons (gnrh1 or gnrh3 in the terminal nerve and the trigeminal nerve, gnrh2 in the midbrain tegmentum) occurs from the early developmental stages. Recent studies have suggested that non-hypophysiotropic GnRH neurons play various functional roles. Here, we summarize their anatomical/physiological properties and discuss their possible functions, focusing on studies in vertebrates. GnRH neurons in the terminal nerve show different spontaneous firing properties during the developmental stages. These neurons in adulthood show regular pacemaker firing, and it has been suggested that these neurons show neuromodulatory function related to the regulation of behavioral motivation, etc. In addition to their recognized role in neuromodulation in adult, in juvenile fish, these neurons, which show more frequent burst firing than in adults, are suggested to have novel functions. GnRH neurons in the midbrain tegmentum show regular pacemaker firing similar to that of the adult terminal nerve and are suggested to be involved in modulations of feeding (teleosts) or nutrition-related sexual behaviors (musk shrew). GnRH neurons in the trigeminal nerve are suggested to be involved in nociception and chemosensory avoidance, although the literature on their electrophysiological properties is limited. Sympathetic preganglionic cells in the spinal cord were first reported as peptidergic modulatory neurons releasing GnRH with a putative function in coordinating interaction between vasomotor and exocrine outflow in the sympathetic nervous system. The functional role of non-hypophysiotropic GnRH neurons may thus be in the global modulation of neural circuits in a manner dependent on internal conditions or the external environment.

Neuropeptides isotocin and arginine vasotocin in urophysis of three fish species

In this study, for the first time, both neuropeptides isotocin (IT) and arginine vasotocin (AVT) have been identified and measured in urophysis, the neurohaemal organ of the caudal neurosecretory system of teleost fish. So far, AVT, but not IT, was quantified by radioimmunoassay (RIA) in urophysis of several fish species. We have used high-performance liquid chromatographic assay with fluorescence detection (HPLC-FL) preceded by solid-phase extraction (SPE) and liquid chromatography-electrospray ionization triple-quadrupole tandem mass spectrometry (LC-ESI MS/MS) technique to determine both neuropeptides in urophysis of three fish species. The efficiency of peptide's SPE extraction was 79-85%. In HPLC-FL method, the limits of detection (LOD) and quantification (LOQ) were estimated as 1.0 and 3.4 pmol/mL for IT and 0.25 and 2.20 pmol/mL for AVT. In LC-MS/MS method, LOD and LOQ were estimated as 0.4 and 1.2 pmol/mL for IT and 0.06 and 0.2 pmol/mL for AVT. The chromatographic methods are good alternative for RIA, because enable to measure both nonapeptides simultaneously in one sample. In round goby (Neogobius melanostomus), three-spined stickleback (Gasterosteus aculeatus) and sea bream (Sparus aurata), urophysial IT concentrations ranged between 0.056 and 0.678 pmol/mg tissue and AVT concentrations ranged between 0.0008 (or even below detection threshold) and 0.084 pmol/mg tissue.

The contribution of lower vertebrate animal models in human reproduction research

Many advances have been carried out on the estrogens, GnRH and endocannabinoid system that have impact in the reproductive field. Indeed, estrogens, the generally accepted female hormones, have performed an unsuspected role in male sexual functions thanks to studies on non-mammalian vertebrates. Similarly, these animal models have provided important contributions to the identification of several GnRH ligand and receptor variants and their possible involvement in sexual behavior and gonadal function regulation. Moreover, the use of non-mammalian animal models has contributed to a better comprehension about the endocannabinoid system action in several mammalian reproductive events. We wish to highlight here how non-mammalian vertebrate animal model research contributes to advancements with implications on human health as well as providing a phylogenetic perspective on the evolution of reproductive systems in vertebrates.

Three gonadotropin-releasing hormone neuronal groups with special reference to teleosts

Gonadotropin-releasing hormone (GnRH) is a decapeptide, which has been isolated from the hypothalamus as a releasing hormone of gonadotropins from the pituitary. However, subsequent morphological studies have demonstrated the presence of multiple GnRH neuronal groups outside the hypothalamus and preoptic area. In most vertebrate lineages studied to date, GnRH neuronal groups are present along the terminal nerve and in the midbrain tegmentum, in addition to a population in the preoptico-hypothalamic areas. The presence of GnRH fibers in extrahypothalamic areas has also been demonstrated, indicating a significance for GnRH neurons in functions other than those that are purely hypophysiotropic. Among vertebrate lineages, GnRH neurons have been most extensively studied in teleost fish through morphological, electrophysiological, behavioral and molecular approaches. To date, studies on differential roles of GnRH neuronal groups have been mostly restricted to teleosts. In the present review, the anatomy and functions of each GnRH neuronal group are reconsidered, based mainly on knowledge from teleosts. Recent findings in teleosts indicate that the preoptico-hypothalamic GnRH neurons are hypophysiotropic and that GnRH neurons of the terminal nerve and midbrain tegmentum regulate neural activities in various regions, including extrahypothalamic areas. The latter populations presumably serve as neuromodulatory systems to control aspects of neural functions such as reproductive behavior. Similar functional differentiation may be generalized to other vertebrate lineages as well.

Electrical activity of caudal neurosecretory neurons in seawater- and freshwater-adapted flounder: responses to cholinergic agonists

The caudal neurosecretory system (CNSS) of the euryhaline flounder is involved in osmoregulatory responses underlying adaptation to seawater and freshwater. This study compared electrophysiological activity and responses to cholinergic agonists in the neuroendocrine Dahlgren cells in an in vitro preparation taken from fully seawater- (SWA) or freshwater-adapted(FWA) fish. Resting membrane and action potential parameters showed few differences between SWA and FWA cells. The hyperpolarisation-activated sag potential and depolarising afterpotential were present under both conditions;however, amplitude of the latter was significantly greater in SWA cells. The proportions of cells within the population exhibiting different firing patterns were similar in both adaptation states. However, bursting parameters were more variable in FWA cells, suggesting that bursting activity was less robust. The muscarinic agonist, oxotremorine, was largely inhibitory in Dahlgren cells, but increased activity in a non-Dahlgren cell population,α neurons. Nicotine promoted bursting activity in SWA Dahlgren cells,whereas it inhibited over half of FWA cells.

Evolutionary aspects of cellular communication in the vertebrate hypothalamo-hypophysio-gonadal axis

This review emphasizes the comparative approach for developing insight into knowledge related to cellular communications occurring in the hypothalamus-pituitary-gonadal axis. Indeed, research on adaptive phenomena leads to evolutionary tracks. Thus, going through recent results, we suggest that pheromonal communication precedes local communication which, in turn, precedes communication via the blood stream. Furthermore, the use of different routes of communication by a certain mediator leads to a conceptual change related to what hormones are. Nevertheless, endocrine communication should leave out of consideration the source (glandular or not) of mediator. Finally, we point out that the use of lower vertebrate animal models is fundamental to understanding general physiological mechanisms. In fact, different anatomical organization permits access to tissues not readily approachable in mammals.

Type II gonadotropin-releasing hormone receptor transcripts in human sperm

GnRH regulates reproduction via the well-characterized mammalian pituitary GnRH receptor (type I). In addition, two homologous genes for a second form of the GnRH receptor (type II) are present in the human genome, one on chromosome 14 and the second on chromosome 1. The chromosome 14 gene is ubiquitously transcribed at high levels in the antisense orientation but lacks exon 1, required to encode a full-length receptor. In comparison, the chromosome 1 gene contains all three exons. The issue of whether this gene is transcribed in any human tissue(s), and whether these transcripts encode a functional receptor protein, remains unresolved. We have directly addressed this by screening a panel of human RNAs by hybridization and RT-PCR. These analyses showed that, unlike the chromosome 14 gene, chromosome 1 gene expression is limited and of low abundance. Exon 1-containing transcripts were detected by in situ hybridization in mature sperm and in human postmeiotic testicular cells. Further sequence analysis revealed that although all the potential coding segments were present, the human transcripts, like the gene, contain a stop codon within the coding region and a frame-shift relative to other mammalian GnRH receptors. Although this suggests that the human gene may be a transcribed pseudogene, a functional type II GnRH receptor cDNA has recently been cloned from monkeys. Given the well-established role of GnRH in spermatogenesis and reported evidence of type II GnRH receptor immunoreactivity in human tissues, it is possible that the chromosome 1 gene is functional.

Nitric oxide and neuromodulation in the caudal neurosecretory system of teleosts

Although evidence exists that nitric oxide (NO) mediates neuroendocrine secretion in mammals, the involvement of NO in the neuroendocrine regulation of non-mammalian vertebrates has yet to be investigated in detail. The present review conveys several recent data, suggesting that NO plays a modulatory role in the caudal neurosecretory system (CNSS) of teleosts. The presence and distribution of neuronal NO synthase (nNOS) was demonstrated in the CNSS of the Nile tilapia Oreochromis niloticus by means of NADPHd histochemistry, NOS immunohistochemistry, NOS immunogold electron microscopy, the citrulline assay for NOS activity and Western blot analysis. NO production by the caudal spinal cord homogenates was also evaluated by the oxyhemoglobin assay. On the whole, these findings indicate that caudal neurosecretory cells express NOS enzymes and presumably produce NO as a cotransmitter. Moreover, the comparison of the nNOS distribution with that of urotensins I and II (UI and UII) suggests that neurosecretory Dahlgren cells belong to two different functional subpopulations: a population of UI/UII secreting nitrergic neurons and a population of non-nitrergic neurons, which principally secrete UII. These results implicate NO as a putative modulator of the release of urotensins from the neurosecretory axon terminals. Therefore, like in mammals, NO appears to influence neuroendocrine secretion in teleosts.

Immunohistochemical localization of three different prepro-GnRHs in the brain and pituitary of the European sea bass (Dicentrarchus labrax) using antibodies to the corresponding GnRH-associated peptides

The distribution of the cells expressing three prepro-gonadotrophin-releasing hormones (GnRH), corresponding to salmon GnRH (sGnRH), seabream GnRH (sbGnRH), and chicken GnRH-II (cGnRH-II) forms, was studied in the brain and pituitary of the sea bass (Dicentrarchus labrax) by using immunohistochemistry. To circumvent the cross-reactivity problems of antibodies raised to GnRH decapeptides, we used specific antibodies generated against the different sea bass GnRH-associated peptides (GAP): salmon GAP (sGAP), seabream GAP (sbGAP), and chicken-II GAP (cIIGAP). The salmon GAP immunostaining was mostly detected in terminal nerve neurons but also in ventral telencephalic and preoptic perikarya. Salmon GAP-immunoreactive (ir) fibers were observed mainly in the forebrain, although sGAP-ir projections were also evident in the optic tectum, mesencephalic tegmentum, and ventral rhombencephalon. The pituitary only receives a few sGAP-ir fibers. The seabream GAP-ir cells were mainly detected in the preoptic area. Nevertheless, sbGAP-ir neurons were also found in olfactory bulbs, ventral telencephalon, and ventrolateral hypothalamus. The sbGAP-ir fibers were only observed in the ventral forebrain, innervating strongly the pituitary gland. Finally, chicken-II GAP immunoreactivity was only detected in large synencephalic cells, which are the origin of a profuse innervation reaching the telencephalon, preoptic area, hypothalamus, thalamus, pretectum, posterior tuberculum, mesencephalic tectum and tegmentum, cerebellum, and rhombencephalon. However, no cIIGAP-ir fibers were detected in the hypophysis. These results corroborate the overlapping of sGAP- and sbGAP-expressing cells in the forebrain of the sea bass, and provide, for the first time, unambiguous information on the distribution of projections of the three different GnRH forms expressed in the brain of a single species.

Bursting properties of caudal neurosecretory cells in the flounder Platichthys flesus, in vitro

Bursting activity in type 1 Dahlgren cells was studied using intra- and extracellular recording from an in vitro preparation of the caudal neurosecretory system of the euryhaline flounder. 45% of cells showed spontaneous bursts of approximately 120s duration and 380s cycle period. Similar bursts were triggered by short duration (<5s) depolarising or hyperpolarising pulses. Cells displayed a characteristic depolarising after potential, following either an action potential with associated afterhyperpolarisation, or a hyperpolarising current pulse. This depolarising after potential was related to a ‘sag’ potential, which developed during the hyperpolarising pulse. Both the depolarising after potential and the sag potential occurred only in cells at more depolarised (<60mV) holding potentials. In addition, the amplitude of the depolarising after potential was dependent on the amplitude and the duration of the hyperpolarising pulse. The depolarising after potential following action potentials may provide a mechanism for facilitating repetitive firing during a burst. Extracellular recording revealed similar bursting in individual units which was not, however, synchronised between units. Spontaneous bursting activity recorded both intra- and extracellularly was inhibited by application of a known neuromodulator of the system, 5-hydroxytryptamine. This study provides a basis for investigating the relationship between physiological status, Dahlgren cell activity and neuropeptide secretion.

The caudal neurosecretory system: control and function of a novel neuroendocrine system in fish

The caudal neurosecretory system (CNSS) of fish was first defined over 70 years ago yet despite much investigation, a clear physiological role has yet to be elucidated. Although the CNSS structure is as yet thought to be confined to piscine species, the secreted peptides, urotensins I and II (UI and UII), have been detected in a number of vertebrate species, most recently illustrated by the isolation of UII in humans. The apparent importance of these peptides, suggested by their relative phylogenetic conservation, is further supported by the complex control mechanisms associated with their secretion. The CNSS in teleosts is known to receive extensive and diverse innervation from the higher central nervous system, with evidence for the presence of cholinergic, noradrenergic, serotonergic, and peptidergic descending inputs. Recent observations also suggest the presence of glucocorticoid receptors in the flounder CNSS, supporting previous evidence for a possible role as a pituitary-independent mechanism controlling cortisol secretion. The most convincing evidence as to a physiological role for the CNSS in fish has stemmed from the direct and indirect influence of the urotensins on osmoregulatory function. Recent advances allowing the measurement of circulating levels of UII in the flounder have supported this. In addition, there is evidence to suggest some seasonal variation in peptide levels supporting the notion that the CNSS may have an integrative role in the control of coordinated changes in the reproductive, osmoregulatory and nutritional systems of migratory euryhaline species.

Development of the caudal neurosecretory system of the nile tilapia Oreochromis niloticus: an immunohistochemical and electron microscopic study

The development of the caudal neurosecretory system (CNSS) of the Nile tilapia, Oreochromis niloticus, has been investigated by means of UI/oCRF (urotensin I/ovine corticotropin-releasing factor) immunohistochemistry and transmission electron microscopy. UI-like immunoreactive perikarya and fibers are first detected in the caudal spinal cord of larval fish about 4 days after hatching (stage 21). In the region of the future urophysis two bundles of strongly immunoreactive neurosecretory fibers are observed. At this stage, neurosecretory axons terminate on the meninx sheath of the spinal cord with immature neurosecretory terminals. The histogenesis of the urophysis begins at stage 24. The future neurohemal organ consists of a small ventral swelling of the spinal cord, which is associated with dilated vessels. At this stage, neurosecretory axons terminate on the basal lamina of the ingrowing blood vessels. Further development occurs by means of progressive branching of vessels and the concomitant increase in the number of neurosecretory terminals. In the caudal spinal cord, immunoreactive neurons also increase in number and progressively differentiate morphologically. Typical features of the mature CNSS are recognizable in 4-month-old juveniles. Data suggest that in tilapia both the synthesis and the release of urophysial hormones begin before morphogenesis of the neurohemal organ takes place.

Gonadotropin-releasing hormone genes: phylogeny, structure, and functions

Gonadotropin-releasing hormone (GnRH, previously called leutinizing hormone-releasing hormone, LHRH) is the final common signaling molecule used by the brain to regulate reproduction in all vertebrates. Recently, genes encoding two other GnRH forms have been discovered. Here we present a phylogenetic analysis that shows that the GnRH genes fall naturally into three distinct branches, each of which shares not only a molecular signature but also characteristic expression sites in the brain. The GnRH genes appear to have arisen through gene duplication from a single ancestral GnRH whose origin predates vertebrates. Several lines of data support this suggestion, including the fact that all three genes share an identical exonic structure. The existence of three distinct GnRH families suggests a new, natural nomenclature for the genes, and in addition, we present a logical proposal for naming the peptide sequences. The two recently discovered GnRH genes are unusual because they encode decapeptides that are identical in all the species in which they have been found. The control of gene expression also differs among the three gene families as might be expected since they have had separate evolutionary trajectories for perhaps 500 million years.

Testosterone differentially regulates expression of GnRH messenger RNAs in the terminal nerve, preoptic and midbrain of male tilapia

The purpose of the present study was to examine the regulation of three molecular variants of gonadotropin-releasing hormone (GnRH)-encoding mRNAs by testosterone in the male tilapia Oreochromis niloticus. Tilapias castrated for two weeks were injected intraperitoneally with sesame oil or 5 microgram/g testosterone for 7 days. In situ hybridization histochemistry was performed using 35S-labelled 30-mer antisense oligonucleotide probes complementary to exon two (bases 1-30) of salmon-, seabream-, and chicken II-GnRH. Computerized image analysis was performed to quantify GnRH mRNA expression in the terminal nerve ganglia (nucleus olfactoretinalis) and in individual cells of the preoptic area and the midbrain tegmentum. Testosterone treatment significantly elevated terminal nerve salmon-GnRH mRNA, reduced preoptic seabream-GnRH mRNA but had no effect on midbrain chicken II-GnRH mRNA levels. The total number and size of preoptic and midbrain GnRH mRNA-containing neurons or the total volume of the terminal nerve ganglia in testosterone-treated animals did not differ significantly from oil-treated animals. The midbrain chicken II-GnRH neurons are not targets of testosterone. These results demonstrate for the first time differential regulation of subpopulations of GnRH neurons with molecular diversity and different topography.

Second gene for gonadotropin-releasing hormone in humans

Gonadotropin-releasing hormone (GnRH) is a decapeptide widely known for its role in regulating reproduction by serving as a signal from the hypothalamus to pituitary gonadotropes. In addition to hypothalamic GnRH (GnRH-I), a second GnRH form (pGln-His-Trp-Ser-His-Gly-Trp-Tyr-Pro-Gly; GnRH-II) with unknown function has been localized to the midbrain of many vertebrates. We show here that a gene encoding GnRH-II is expressed in humans and is located on chromosome 20p13, distinct from the GnRH-I gene that is on 8p21-p11.2. The GnRH-II genomic and mRNA structures parallel those of GnRH-I. However, in contrast to GnRH-I, GnRH-II is expressed at significantly higher levels outside the brain (up to 30x), particularly in the kidney, bone marrow, and prostate. The widespread expression of GnRH-II suggests it may have multiple functions. Molecular phylogenetic analysis shows that this second gene is likely the result of a duplication before the appearance of vertebrates, and predicts the existence of a third GnRH form in humans and other vertebrates.

Gonadotropin-releasing hormone neurons in the gourami midbrain: a double labeling study by immunocytochemistry and tracer injection

There are three groups of gonadotropin-releasing hormone (GnRH) neurons in the teleost brain. Midbrain GnRH neurons in the dwarf gourami send axons to various areas of the central nervous system. However, it is not clear whether midbrain GnRH neurons form a cell cluster separate from the nucleus of the medial longitudinal fasciculus (nMLF), which has been reported to project to the spinal cord. Thus, we performed a double labeling study. GnRH neurons were immunostained but were very faintly labeled with biocytin injected into the spinal cord. In contrast, nMLF neurons were strongly labeled with biocytin but were GnRH-immunonegative. GnRH neurons are distributed at almost the same rostrocaudal levels as nMLF neurons, but they constitute a separate cell group dorsocaudal to nMLF neurons.

The caudal neurosecretory system and its afferent synapses in the goldfish, Carassius auratus: morphology, immunohistochemistry, and fine structure

Morphological features of the goldfish caudal neurosecretory system were investigated by means of immunohistochemical localization of urotensins I and II (UI and UII) and electron microscopic examination of the caudal neurosecretory neurons, the urophysis, and the synaptic neuropil. The aim of the work is to provide a detailed morphological description of the afferent synapses to the caudal neurons and to analyze their distribution through the rostrocaudal extension of the caudal neurosecretory system. Three morphologically different types of neurosecretory cells have been identified according to size and shape: large, medium, and small Dahlgren cells. The three different-sized cells share similar patterns of immunoreactivity with the UI (or oCRF) and the UII antisera. Electron microscopic examination of the synaptic neuropil throughout the caudal system revealed the presence of four types of terminals: dense-cored-vesicle end bulbs (DC), spherical-vesicle end bulbs (S), flattened-vesicle end bulbs (F), and granular-vesicle end bulbs (G). The present study demonstrates that the small Dahlgren cells receive different synaptic inputs from the large and the medium neurosecretory cells. Indeed, G terminals are only found on the small Dahlgren cells, whereas DC, S, and F terminals are distributed on the large, medium, and small Dahlgren cell bodies and proximal processes.

Adrenergic receptor activation hyperpolarizes the caudal neurosecretory cells of the flounder, Platichthys flesus

The physiological factors that govern activity of the caudal neurosecretory system in teleost fish are poorly understood. Immunocytochemical evidence indicates that the neurosecretory Dahlgren cells are innervated by descending monoaminergic fibres. Using intracellular recording techniques in an isolated preparation of the posterior spinal cord of the flounder (Platichthys flesus) we have demonstrated that superfusion of adrenaline or noradrenaline (10(-7) - 10(-3) M) causes hyperpolarization of Dahlgren cells (up to -30 mV). This hyperpolarization is likely to reflect an inhibitory effect of noradrenergic nerves on the neurosecretory system in vivo, reducing the rate of hormone release. Fluctuations in the input resistance and membrane time constant suggest involvement of a multiplicity of cellular mechanisms, including the opening and closing of populations of ion-selective channels. Superfusion with dopamine (10(-7) - 10(-3) M) had no effect. Superfusion with the beta-adrenoreceptor agonist, isoprenaline, caused hyperpolarization but to a markedly lesser extent than the maximum effect of adrenaline or noradrenaline, suggesting that their effects are mediated, only in part, by a beta-adrenoreceptor subtype. Superfusion of the preparation with a membrane permeable, non-hydrolysable cyclic AMP analogue (8-[4-chlorophenylthio]-cAMP) resulted in a slight hyperpolarization which was accompanied by a small, but significant, increase in input resistance. These data are consistent with at least part of the beta-adrenoreceptor mediated effect involving closure of cAMP-sensitive ion channels. Superfusion with the alpha 1-adrenoreceptor agonist, phenylephrine, had no effect on any electrophysiological parameter studied. However, the alpha 2-adrenoreceptor agonist, clonidine, caused hyperpolarization which again failed to reach the maximum level produced by adrenaline or noradrenaline. Together, these data suggest that the adrenergic inhibition of Dahlgren cell activity is mediated by both alpha 2- and beta-adrenoreceptor subtypes.

Three gonadotropin-releasing hormone genes in one organism suggest novel roles for an ancient peptide

Gonadotropin-releasing hormone (GnRH) is known and named for its essential role in vertebrate reproduction. Release of this decapeptide from neurons in the hypothalamus controls pituitary gonadotropin levels which, in turn, regulate gonadal state. The importance of GnRH is underscored by its widespread expression and conservation across vertebrate taxa: five amino acids are invariant in all nine known forms, whereas two others show only conservative changes. In most eutherian mammals, only one form, expressed in the hypothalamus, is thought to exist, although in a recent report, antibody staining in developing primates suggests an additional form. In contrast, multiple GnRH forms and expression loci have been reported in many non-mammalian vertebrates. However, evidence based on immunological discrimination does not always agree with analysis of gene expression, since GnRH forms encoded by different genes may not be reliably distinguished by antibodies. Here we report the expression of three distinct GnRH genes in a teleost fish brain, including the sequence encoding a novel GnRH preprohormone. Using in situ hybridization, we show that this form is found only in neurons that project to the pituitary and exhibit changes in soma size depending on social and reproductive state. The other two GnRH genes are expressed in other, distinct cell populations. All three genes share the motif of encoding a polypeptide consisting of GnRH and a GnRH-associated peptide. Whereas the GnRH moiety is highly conserved, the GnRH-associated peptides are not, reflecting differential selective pressure on different parts of the gene. GnRH forms expressed in nonhypothalamic regions may serve to coordinate reproductive activities of the animal.

Multiple gonadotropin-releasing hormone (GnRH)-immunoreactive systems in the brain of the dwarf gourami, Colisa lalia: immunohistochemistry and radioimmunoassay

The present study characterizes gonadotropin-releasing hormone (GnRH) neuronal groups that are located in several different brain regions by investigating GnRH molecular species and projection patterns in an anabantid fish, Colisa lalia. First, we examined the molecular species of GnRHs in extracts of the brain and the pituitary by reverse-phase high-performance liquid chromatography followed by radioimmunoassays. We found salmon GnRH (sGnRH), chicken GnRH-II (cGnRH-II), and an unfamiliar GnRH-like substance. Next, to examine the distribution of each GnRH molecule in different GnRH neuronal groups, we performed immunohistochemistry using four kinds of antisera and an antibody. Furthermore, we performed brain lesioning experiments of terminal nerve (TN) cells, the most conspicuous GnRH-immunoreactive cells in Colisa lalia. Comparisons of immunoreactive structures between TN-lesioned fish and untreated fish elucidated the projection area of each neuronal group. Three major neuronal groups were observed. TN-GnRH cells, which are located in the transitional area between the olfactory bulb and the telencephalon, showed strong sGnRH and weaker cGnRH-II immunoreactivity. TN-GnRH cells projected to wide areas of the central nervous system from the olfactory bulb to the spinal cord. The second group, located in the preoptic area, showed only sGnRH immunoreactivity and projected only to the pituitary. The third one, located in the midbrain tegmentum, exhibited strong cGnRH-II and weaker sGnRH immunoreactivity. This cell group projected mainly to brain regions posterior to the hypothalamus and the spinal cord. These different projection patterns suggest functional differentiation of each GnRH neuronal group.

Gonadotropin-releasing hormones in microdissected brain regions of an amphibian: concentration and anatomical distribution of immunoreactive mammalian GnRH and chicken GnRH II

Mammalian and chicken II gonadotropin-releasing hormones (mGnRH, cGnRH II) were extracted from 350 microns diameter punches from brains of a urodele amphibian, Taricha granulosa, and measured by means of radioimmunoassay (RIA) with specific antisera. Measurable quantities of both peptides were found in the lateral pallium, the subpallium (along the course of the nervus terminalis), the preoptic area, habenula, optic tectum, infundibulum, paraventricular organ/posterior tubercle of the caudal diencephalon, medulla, and cerebrospinal fluid. Highest concentrations of both peptides were in the preoptic area and infundibulum, suggesting a role in gonadotropin release. In most extrahypothalamic regions, cGnRH II concentrations exceeded those of mGnRH, suggesting that cGnRH II may function as a neurotransmitter in many sites, perhaps to control reproductive behaviors. Results are largely consistent with immunocytochemical (ICC) analyses, except that RIA revealed small amounts of both peptides not found by ICC in some areas of the brain. Results from this microdissection/RIA study and prior ICC studies in amphibians support the conclusions that GnRH cell bodies in the terminal nerve and preoptic area, which project mainly to the median eminence and habenula, express mGnRH, and that GnRH cell bodies in the caudal diencephalon, which project widely throughout the CNS, express cGnRH II. Comparative data support the view that cGnRH II, and the neural systems in which it is expressed, evolved early in vertebrate phylogeny and have been highly conserved.

Immunocytochemical localization of mammalian GnRH (gonadotropin-releasing hormone) and chicken GnRH-II in the brain of the European silver eel (Anguilla anguilla L.)

Using specific antibodies for the two molecular forms of gonadotropin-releasing hormone (GnRH) present in the European eel, Anguilla anguilla, (mammalian GnRH, mGnRH, and chicken GnRH II, cGnRH-II), we employed immunocytochemistry to determine the distribution of these two peptides in the brain and in the pituitary. The results indicate that mGnRH and cGnRH-II are localized in different neurons: mGnRH-immunoreactive (ir) perikaria were observed in the olfactory bulbs, the junction between olfactory bulbs and telencephalon (nucleus olfactoretinalis), the telencephalon, the preoptic region and the mediobasal hypothalamus. These cell bodies are located along a continuum of ir-fibers that could be traced from the olfactory nerve to the pituitary. Mammalian GnRH-ir fibers were detected in many parts of the brain (olfactory bulbs, ventral telencephalon, hypothalamus, optic tectum, mesencephalon) and in the pituitary. Chicken GnRH-II-ir cell bodies were detected in the nucleus of the medial longitudinal fasciculus of the midbrain tegmentum, but only scattered fibers could be detected in different parts of the brain. The pituitary exhibited very few cGnRH-II-ir fibers, contrasting with an extensive mGnRH innervation. These results are in agreement with our previous data obtained in the same species using specific radioimmunoassays for mGnRH and cGnRH-II. They demonstrate a differential distribution of the two forms of GnRH in the brain of the eel, as in the brain of some other vertebrate species, and suggest differential physiological roles for the two GnRH forms in the eel. They also provide information concerning the evolution of the GnRH systems in vertebrates.

Antibodies against different forms of GnRH distinguish different populations of cells and axonal pathways in a urodele amphibian, Taricha granulosa

Neurons immunoreactive to the peptide hormone gonadotropin-releasing hormone (GnRH) have been identified in the posterior diencephalon or anterior midbrain of diverse vertebrates. These cells are located caudal to the more well-characterized GnRH neurons in the nervus terminalis and septo-preoptic area, and are believed to express one or more of the nonmammalian forms of the GnRH. The present study utilized immunocytochemical techniques to determine whether the posterior GnRH group is present in a urodele amphibian, the newt Taricha granulosa. Antibodies directed against different molecular forms of GnRH were used to evaluate the immunological properties of GnRH-containing neurons in amphibians. An antibody selective for mammalian GnRH labeled perikarya in the nervus terminalis (terminal nerve) and septo-preoptic region, as described previously. Thick fibers that arise from terminal nerve and septo-preoptic neurons project mainly to the median eminence, medial pallium and habenula. An antibody selective for chicken GnRH II labeled cell bodies in the paraventricular organ and posterior tubercle of the caudal diencephalon, and thin fibers that project widely throughout the central nervous system. Region-specific staining with different GnRH antibodies supports the interpretation that different molecular forms of GnRH are expressed by neuroanatomically distinguishable systems.

Multiple embryonic origins of gonadotropin-releasing hormone (GnRH) immunoreactive neurons

Experiments were conducted to test the hypothesis that gonadotropin-releasing hormone immunoreactive (GnRH-ir) and FMRFamide-ir neurons present in the brain and nervus terminalis originate in the embryonic olfactory placode. The olfactory placodes were bilaterally extirpated in stage 26 or stage 29 embryos of the axolotl, Ambystoma mexicanum, which were then reared for 4-8 months before they were examined immunohistochemically. In experimental subjects with bilateral loss of olfactory epithelia, nerves and bulbs, there was complete absence of GnRH- and FMRFamide-ir neurons in the terminal nerve, and in septal and preoptic areas, and complete absence of large diameter peptidergic fibers associated with the TN-septo-preoptic system. However, GnRH-ir perikarya in the posterior tubercle, and FMRFamide-ir perikarya in the ventral hypothalamus, and small diameter peptidergic fibers were not affected by placodal ablation. These results support the hypothesis that contrary to recent reports, GnRH-ir neurons have more than one embryonic origin. Region-specific patterns of staining with antisera directed against different molecular forms of GnRH support the interpretation that GnRH-ir neurons of placodal origin express mammalian GnRH, whereas GnRH-ir neurons of non-placodal origin, in the posterior tubercle, express chicken GnRH II.

A second gene for gonadotropin-releasing hormone: cDNA and expression pattern in the brain

In vertebrates, the gonadotropin-releasing hormone (GnRH) decapeptide is secreted from hypothalamic nerve terminals to regulate reproduction via control of synthesis and release of pituitary gonadotropins. Only one GnRH peptide has been found in mammals, with one exception, although numerous other vertebrate species express more than one of the eight known decapeptide forms as shown by immunocytochemical labeling of distinct cell groups in the brain. However, neither the functional nor the evolutionary relationships among these GnRH forms are clear, because only one preprohormone gene sequence from any species has been reported. The most ubiquitous alternative form of GnRH is [His5,Trp7,Tyr8]GnRH (also referred to as chicken-II), which differs from the mammalian sequence at amino acids 5, 7, and 8. This peptide has been shown to have the most potent releasing-hormone activity, although immunocytochemical staining has suggested it is synthesized only in the mesencephalon. Here we report the cloning and expression pattern of the gene for the precursor of this form from the teleost fish Haplochromis burtoni. This is the second GnRH-encoding gene to be characterized in this species. The newly discovered preprohormone gene differs from that previously reported in two ways. First, whereas the original gene predicts only a single associated peptide, this one predicts two associated peptides, both of which appear to be unique. Second, the gene for [His5,Trp7,Tyr8]GnRH is expressed in only one cell group in the mesencephalon. In contrast, the previously reported gene is expressed only in the terminal nerve. The striking differences between the preprohormone structure and localization suggest that the genes coding for the two known GnRH forms in H. burtoni did not arise from a recent duplication event. Interestingly, neither of the two genes found to date in this species is expressed in cells which project from the hypothalamus to the pituitary, suggesting that yet a third gene coding for GnRH may exist.

Comparative distribution of mammalian GnRH (gonadotrophin-releasing hormone) and chicken GnRH-II in the brain of the immature Siberian sturgeon (Acipenser baeri)

The brain of the sturgeon has recently been shown to contain at least two forms of GnRH (gonadotropin-releasing hormone), mammalian GnRH (mGnRH) and chicken GnRH-II (cGnRH-II). In this study, we compared the distribution of immunoreactive (ir) mGnRH and cGnRH-II in the brain of immature Siberian sturgeons (Acipenser baeri). The overall distribution of mGnRH was very similar to the distribution of sGnRH in teleosts such as salmonids or cyprinids. mGnRH-ir perikarya were observed in the olfactory nerves and bulbs the telencephalon, the preoptic region, and the mediobasal hypothalamus. All these cell bodies are located along a continuum of ir-fibers that could be traced from the olfactory nerve to the hypothalamopituitary interface. No ir-fibers were observed in the anterior lobe of the pituitary, but a few were seen to enter the neurointermediate lobe. mGnRH-ir fibers were detected in many parts of the brain, particularly in the forebrain. mGnRH-ir cerebrospinal fluid-contacting cells were observed in the telencephalon, the preoptic region, and the mediobasal hypothalamus. In contrast, cGnRH-II was present mainly in the posterior brain, although a few ir axons were seen in the above-mentioned territories. In particular, cGnRH-II-ir cells bodies, negative for mGnRH, were consistently observed in the nucleus of the medial longitudinal fasciculus of the midbrain tegmentum. The cGnRH-II innervation in the optic tectum, cerebellum, vagal lobe, and medulla oblongata was more abundant than the mGnRH innervation in the same areas. This study provides evidence that the organization of the GnRH systems in a primitive bony fish is highly similar to that reported in teleosts and further documents the differential distribution of two forms of GnRH in the brain of vertebrates.

Immunohistochemical localization of chicken gonadotropin-releasing hormones I and II (cGnRH I and II) in turkey hen brain

The distribution of cells and fibers immunoreactive (ir) for either chicken gonadotropin-releasing hormone I (cGnRH I; [Gln8]GnRH) or II ([His5,Trp7,Tyr8]GnRH) was determined in brains of turkey hens to reveal whether these peptides occur in separate neuronal systems. ir-cGnRH I cells were located: along the medial aspect of the ventriculus lateralis, nucleus accumbens, and bed nucleus of the stria terminalis; ventral to the tractus septomesencephalicus and extending medially to the third ventricle, and caudally into the lateral hypothalamic area; and in a diffuse band extending from the nucleus preopticus medialis to the nucleus dorsomedialis anterior thalami. cGnRH I fibers were evident in these areas in addition to the hippocampus, nucleus subhabenularis medialis, nucleus ventromedialis hypothalami, and median eminence. Two groups of ir-cGnRH II cells were observed: a magnocellular group lying between the substantia grisea centralis and the nucleus ruber; and a parvicellular group lying medial to the nucleus of the basal optic root and extending into the lateral hypothalamic area. ir-cGnRH II fibers were prominent in limbic structures (cortex piriformis, lateral to nucleus taeniae, hippocampus); olfactory areas (tuberculum olfactorium, nucleus subhabenularis lateralis, nucleus septalis lateralis); areas that in other avian species have steroid-concentrating cells or receptors (medial edge of lobus parolfactorius, nucleus septalis medialis, nucleus periventricularis magnocellularis, nucleus dorsomedialis posterior thalami); and areas containing ir-GnRH I cells or fibers but not in median eminence. These results suggest that cGnRH I and II occur in separate neuronal systems and that cGnRH II does not directly promote pituitary gonadotropin secretion.

Immunocytochemical demonstration of salmon GnRH and chicken GnRH-II in the brain of masu salmon, Oncorhynchus masou

We have recently developed sensitive and specific radioimmunoassays (RIAs) for salmon gonadotropin-releasing hormone (sGnRH) and chicken GnRH-II (cGnRH-II) and have measured the contents of both GnRHs in the rainbow trout brain. Our results showed that contents of the two GnRHs are variable among different brain regions. Therefore, in order to confirm the differential distribution of the two GnRHs by a different technique, we examined the distribution of immunoreactive sGnRH and cGnRH-II in the brain of masu salmon by using immunocytochemical techniques. sGnRH immunoreactive (ir) cell bodies were scattered in the transitional areas between the olfactory nerve and the olfactory bulb, the ventral olfactory bulb, between the olfactory bulb and the telencephalon, the ventral telencephalon, and the preoptic area. These sGnRH-ir cell bodies were dispersed in a strip-like region running rostrocaudally in the most ventral part of the ventral telencephalon. sGnRH-ir fibers were distributed in the various brain regions from the olfactory bulb to the spinal cord. They were especially abundant in the olfactory bulb, ventral telencephalon, preoptic area, hypothalamus, deep layers of the optic tectum, and thalamus. sGnRH-ir fibers also innervated the pituitary directly. cGnRH-II-ir cell bodies were found in the nucleus of the medial longitudinal fasciculus (nMLF). The distribution of cGnRH-II-ir fibers was similar to that of sGnRH-ir fibers, except that cGnRH-II-ir fibers were absent in the pituitary. The number of cGnRH-II-ir fibers was much fewer than that of sGnRH-ir fibers. The results of the present immunocytochemical study are in basic agreement with those of our previous RIA study. Thus, we suggest that in masu salmon, sGnRH not only regulates gonadotropin (GTH) release from the pituitary but also functions as a neuromodulator in the brain, whereas cGnRH-II functions only as a neuromodulator.

Gonadotropin hormone-releasing hormone (GnRH) immunoreactivity in the mesencephalon of sharks and rays

Other than association with the terminal nerve (TN), little is known concerning the distribution of gonadotropin hormone-releasing hormone (GnRH) in elasmobranchs. The purpose of this study was to identify GnRH immunoreactivity in the brains of three elasmobranch species with special regard to the mesencephalon. The round stingray (Urolophus halleri), thornback guitarfish (Platyrhinoidis triseriata), and leopard shark (Triakis semifasciatus) were used and immunocytochemistry was performed with antisera to both salmon and mammalian GnRH. A large GnRH-immunoreactive (ir) nucleus extends rostrocaudally for approximately 1.5 mm along and adjacent to the midline of the midbrain near the area of the oculomotor nerve. GnRH-ir fibers surround the nucleus and are found diffusely throughout the mesencephalon; some of the fibers may contact the ventricle. The medulla and spinal cord contain ir fibers that most likely originate from the midbrain nucleus. Mesencephalic GnRH-ir cell groups have been reported in representatives of all vertebrate classes with the exception of agnathans and mammals. Such a well-developed cell group in elasmobranchs may aid in understanding the evolution of GnRH systems with regard to the mesencephalon as well as providing insight to the functional significance of this cell group. Possible homologies to mesencephalic GnRH systems reported in other vertebrates is discussed as well.

Ontogeny of immunoreactive gonadotropin-releasing hormone neuronal systems in amphibians

The ontogeny of gonadotropin-releasing hormone (GnRH) systems was investigated in 3 anuran amphibians (genus Rana) by means of immunocytochemical (ICC) techniques and antibodies generated against 3 different forms of GnRH. Antisera that recognize primarily chicken II and mammalian GnRHs revealed two anatomically and developmentally distinct GnRH systems. One system, referred to here as the forebrain-spinal cord system, contained GnRH immunoreactive (ir) fibers extending from the rostral diencephalon through the ventromedial brainstem to the spinal cord. Intensity of labeling was robust in the youngest, premetamorphic tadpoles, but decreased with age. GnRH immunolabeling in the hypothalamic-pituitary tract was not detected until late prometamorphosis and increased with age. Development of GnRHir in the hypothalamic-pituitary tract coincided with first appearance of GnRHir in the terminal nerve in R. catesbeiana, but not in R. cascadae or R. aurora, suggesting species differences. Comparisons of results obtained with antisera to different forms of GnRH support the interpretation that the forebrain-spinal cord system, hitherto undescribed in amphibians, develops first and synthesizes a non-mammalian, chicken II-like GnRH, and that the hypothalamic-pituitary system develops later and synthesizes primarily mammalian GnRH. We speculate that the forebrain-spinal cord system may represent a GnRH innervation of frog sympathetic ganglia, and that the two GnRH systems are chemically and embryonically distinct.

Distribution of serotonin in the caudal neurosecretory complex. A light and electron microscopic study

The caudal neurosecretory complex (CNc) of poecilids has previously been shown to receive serotonergic inputs. In the present study, immunohistochemical techniques were applied at the light and electron microscopic levels to characterize serotonergic terminals in the neuroendocrine nucleus. A dense plexus of varicose fibers observed in the rostral CNc neuropil was absent in the spinal cords of deafferented fish, indicating that the origin of this input was extranuclear. Ultrastructural study revealed no direct contacts between labeled structures and neuroendocrine cells. Non-synaptic terminals (varicosities) were the predominantly labeled structures in the neuropil. Synaptic terminals were observed on cellular and axonal targets in the CNc. Small cells containing 70 nm dense-core vesicles received serotonergic input on their perikarya. Labeled synapses were also found on unlabeled axon terminals which made axo-axonal synapses on neuroendocrine processes. Non-synaptic terminals may be responsible for a variety of serotonin-mediated effects in the CNc. Synaptic interactions with local catecholaminergic and afferent cholinergic inputs to the CNc are likely.

Phylogeny and ontogeny of gonadotropin-releasing hormone: comparison of guinea pig, rat, and a protochordate

Immunoreactive gonadotropin-releasing hormone (ir-GnRH) was detected in brain extracts of newborn and 10-day-old rats and in adult guinea pigs; it was also present in extracts of the neural ganglion and gland of a protochordate. Radioimmunoassay (RIA) using different GnRH antisera after high-performance liquid chromatography (HPLC) revealed that the dominant form of GnRH is the mammalian form (pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2) both during ontogenesis in the rat and in the adult guinea pig known to have variant forms of other peptide hormones. None of the other forms of GnRH identified in nonmammalian species to date appear to be present in the rat or guinea pig. A small amount of an unidentified HPLC early eluting form of GnRH is present, but detection by antiserum B-6 implies that it is also mammalian GnRH, with the possibility of changes in positions 2-4. The molecular form of GnRH in a protochordate, the sea squirt Chelyosoma productum, is distinct from salmon and mammalian GnRHs. Cross-reactivity with the sea squirt GnRH-like molecule was highest with an antiserum made against lamprey GnRH; the same antiserum was used to stain nerve fibers in the neural ganglion and some of its roots. This is the first report using RIA, HPLC, and immunocytochemistry to show that protochordates have GnRH-like material. The results suggest that GnRH may have been present at the transition between the invertebrates and vertebrates.

Existence of a GnRH immunoreactive nucleus in the dorsal midbrain tegmentum of the chameleon

The GnRH system of the chameleon brain was studied at light microscopic and ultrastructural levels by use of an immunohistochemical technique with antibodies directed against salmon gonadotrophin-releasing hormone. Immunoreactive (IR) perikarya were found in the anterior midbrain tegmentum. At this level numerous IR cell bodies were detected around the fasciculus longitudinalis medialis (FLM). The more rostral neurons were observed dorsal to the FLM and progressively tended to be lateral to it along the midline. More caudally, they were found ventral to the FLM. At the electron microscope level, these cells were seen to contain large granular vesicles and to receive numerous synaptic inputs. A prominent pathway was traced from these cell bodies along the medulla oblongata to the spinal cord. A second IR pathway ascended rostrally to the habenular complex. No IR perikarya were located in the anterior brain including the olfactory bulbs.

Brainstem location of serotonin neurons projecting to the caudal neurosecretory complex

Serotonergic fibers in the caudal neurosecretory complex (CNc) of poeciliids originate from neurons within, and extrinsic to this spinal cord nucleus. In the present study, retrograde tracing and immunofluorescence techniques were combined to localize extrinsic serotonergic projection neurons. The entire spinal cord and brain were sectioned after Fast Blue (FB) or horseradish peroxidase (HRP) was implanted in the CNc. No HRP or FB filled neurons were found in the spinal cord. Retrogradely filled neurons were found bilaterally in dorsolateral and ventromedial reticular nuclei, and the dorsal midbrain tegmentum. Fusiform cells in the medullary fasciculus longitudinalis medialis filled with FB but not HRP. Serotonin immunopositive neurons were found surrounding the third ventricle, in the raphe and in medullary reticular nuclei. Double labelled neurons in the medial reticular nucleus were determined to be the source of serotonergic projections to the CNc. Reticular projection nuclei are strategically situated to receive visceral sensory input from rhombencephalic cranial nerves. These putative pathways may provide an anatomical substrate by which visceral sensory information is transmitted to the CNc.

Monoamines in the caudal neurosecretory complex: biochemistry and immunohistochemistry

Monoaminergic inputs to the caudal neurosecretory complex (CNc) of Poecilia latipinna have been identified using histofluorescence and immunohistochemical techniques. The present study was undertaken to identify specific monoamines and determine the relative contribution of indolamines and catecholamines in supraspinal and intrinsic innervation of the nucleus. The CNc was deafferented by transecting the spinal cord rostral to the CNc. Ten days subsequently, CNc of spinal-transected and control fish were processed for either biochemical or immunohistochemical analysis. Norepinephrine and serotonin were detected in pooled samples of control CNc. Following deafferentation, the content of both monoamines was diminished. Using immunohistochemical labeling for serotonin or for the catecholamine-synthesizing enzymes, tyrosine hydroxylase (TH) or dopamine-beta-hydroxylase (DBH), the number of monoamine fibers was decreased in deafferented CNc compared to control. A substantial serotonergic innervation remains after deafferentation, as evidenced by serotonin-positive neurons and heavy, varicose fibers. Occasional TH/DBH-positive cells and fibers remain after deafferentation. These data suggest that both norepinephrine and serotonin are associated with descending supraspinal projections, while serotonin predominates as the intrinsic monoamine.

Differential distribution of two molecular forms of gonadotropin-releasing hormone in discrete brain areas of goldfish (Carassius auratus)

Two molecular forms of gonadotropin-releasing hormone (GnRH) were identified in the extracts of various brain areas, spinal cord and pituitary in female and male goldfish and had chromatographic and immunological properties similar to [His5, Trp7, Tyr8]-GnRH (cGnRH-II) and [Trp7,Leu8]-GnRH (sGnRH). Radioimmunoassay using different GnRH antisera after high pressure liquid chromatography did not reveal significant peaks of mammalian GnRH, [Gln8]-GnRH and [Tyr3,Leu5,Glu6,Trp7,Lys8]-GnRH in the brain extracts. The proportion of cGnRH-II-like immunoactivity to sGnRH-like immunoactivity was higher in the caudal brain areas compared to the rostral areas. The differential distribution of two GnRH forms suggest that the different GnRH forms may have different physiological functions.