Immunocytochemical evidence for peptidergic (GnRH) and dopaminergic innervation of the gonadotropic cells in the pituitary of the African catfish, Clarias gariepinus

Thyrotropin-releasing hormone (TRH) in the brain and pituitary of the teleost, Clarias batrachus and its role in regulation of hypophysiotropic dopamine neurons

Thyrotropin-releasing hormone (TRH) regulates the hypothalamic-pituitary-thyroid axis in mammals and also regulates prolactin secretion, directly or indirectly via tuberoinfundibular dopamine neurons. Although TRH is abundantly expressed in teleost brain and believed to mediate neuronal communication, empirical evidence is lacking. We analyzed pro-TRH-mRNA expression, mapped TRH-immunoreactive elements in the brain and pituitary, and explored its role in regulation of hypophysiotropic dopamine (DA) neurons in the catfish, Clarias batrachus. Partial pro-TRH transcript from C. batrachus transcriptome showed six TRH progenitors repeats. Quantitative real-time polymerase chain reaction (qRT-PCR) identified pro-TRH transcript in a number of different brain regions and immunofluorescence showed TRH-immunoreactive cells/fibers in the olfactory bulb, telencephalon, preoptic area (POA), hypothalamus, midbrain, hindbrain, and spinal cord. In the pituitary, TRH-immunoreactive fibers were seen in the neurohypophysis, proximal pars distalis, and pars intermedia but not rostral pars distalis. In POA, distinct TRH-immunoreactive cells/fibers were seen in nucleus preopticus periventricularis anterior (NPPa) that demonstrated a significant increase in TRH-immunoreactivity when collected during preparatory and prespawning phases, reaching a peak in the spawning phase. Although tyrosine hydroxylase (TH)-immunoreactive neurons in NPPa are hypophysiotropic, none of the TRH-immunoreactive neurons in NPPa accumulated neuronal tracer DiI following implants into the pituitary. However, 87 ± 1.6% NPPa TH-immunoreactive neurons were surrounded by TRH-immunoreactive axons that were seen in close proximity to the somata. Superfused POA slices treated with TRH (0.5-2 μM) significantly reduced TH concentration in tissue homogenates and the percent TH-immunoreactive area in the NPPa. We suggest that TRH in the brain of C. batrachus regulates a range of physiological functions but in particular, serves as a potential regulator of hypophysiotropic DA neurons and reproduction.

Connectivity and ultrastructure of dopaminergic innervation of the inner ear and auditory efferent system of a vocal fish

Dopamine (DA) is a conserved modulator of vertebrate neural circuitry, yet our knowledge of its role in peripheral auditory processing is limited to mammals. The present study combines immunohistochemistry, neural tract tracing, and electron microscopy to investigate the origin and synaptic characteristics of DA fibers innervating the inner ear and the hindbrain auditory efferent nucleus in the plainfin midshipman, a vocal fish that relies upon the detection of mate calls for reproductive success. We identify a DA cell group in the diencephalon as a common source for innervation of both the hindbrain auditory efferent nucleus and saccule, the main hearing endorgan of the inner ear. We show that DA terminals in the saccule contain vesicles but transmitter release appears paracrine in nature, due to the apparent lack of synaptic contacts. In contrast, in the hindbrain, DA terminals form traditional synaptic contacts with auditory efferent neuronal cell bodies and dendrites, as well as unlabeled axon terminals, which, in turn, form inhibitory-like synapses on auditory efferent somata. Our results suggest a distinct functional role for brain-derived DA in the direct and indirect modulation of the peripheral auditory system of a vocal nonmammalian vertebrate.

Role of Isotocin in the Regulation of the Hypophysiotropic Dopamine Neurones in the Preoptic Area of the Catfish, Clarias batrachus

Dopamine (DA) has emerged as a potent inhibitory neuromodulator of luteinsing hormone (LH) secretion and reproduction in teleosts. The DA neurones located in the anterior subdivision of nucleus preopticus periventricularis (NPPa) in the preoptic area (POA) innervate the pituitary gland and regulate LH cells. Although a reduction in the inhibitory DAergic tone is crucial for stimulatory action of gonadotrophin-releasing hormone (GnRH) on LH cells, the role of other hypothalamic factors is suggested but not fully understood. Nonapeptide, isotocin (IST) has emerged as a likely candidate that may also influence the LH cell function. IST neurones reside in the nucleus preopticus and innervate LH cells. While IST treatment dramatically elevated LH secretion, the IST levels in brain peaked during spawning. In a pilot study on the catfish, Clarias batrachus, we observed a dense network of IST-immunoreactive (IST-IR) fibres in the NPPa, the region known to harbour hypophysiotropic DA neurones. Application of the double immunofluorescence method showed a dense IST-IR fibre network around the tyrosine hydroxylase-immunoreactive (TH-IR) neurones in the NPPa region. A great majority of the TH-IR neurones in the NPPa were contacted by IST-IR fibres during the spawning phase. The NPPa therefore appears to be a site for the intense interaction of DA and IST. IST-IR fibre innervation in NPPa showed reproduction phase-dependent changes. The percent fluorescent area of IST-IR fibres showed a gradual increase from the resting through prespawning phases (resting: 7.5 ± 1.04; preparatory: 8.6 ± 0.8; prespawning: 15.5 ± 1.4), reaching a peak in the spawning phase (28 ± 2.3; P < 0.001). Compared to the spawning phase, a drastic reduction in IST-IR fibres in the NPPa was observed during the postspawning phase (8.4 ± 0.9; P < 0.001). Superfused slices of the POA of C. batrachus treated with IST peptide resulted in a significant reduction in TH immunoreactivity in the NPPa (Control: 45.3 ± 4.2; IST peptide, 5 μm: 29.4 ± 4.7; P < 0.05). We suggest that the intense interaction between IST and DA in the NPPa, most probably of an inhibitory nature, may be critical for the regulation of LH cells and reproduction in teleosts.

Sexual dimorphism in the hypophysiotropic tyrosine hydroxylase-positive neurons in the preoptic area of the teleost, Clarias batrachus

BACKGROUND

Dopamine (DA) neurons in the anteroventral periventricular nucleus (AVPV) in the preoptic area (POA) of mammals express estrogen receptors, regulate luteinizing hormone (LH) secretion, and show distinct sexual dimorphism. In teleosts, hypophysiotropic DA neurons of the nucleus preopticus periventricularis (NPP), located in the anteroventral POA, express estrogen receptors, innervate LH cells, and emerged as a neuroanatomical substrate for inhibiting LH cells. Interestingly, the NPP and AVPV seem to share several similarities. Whether DAergic neurons in the NPP show sexual dimorphism is, however, not known. Based on the proposed homology to AVPV and previous studies showing greater tyrosine hydroxylase (TH) mRNA and enzyme activity levels in the brain of female catfish, we hypothesize that females have greater number of DAergic neurons in the NPP and correspondingly more TH-immunoreactive fiber innervation of the pituitary.

METHODS

Adult, male and female Clarias batrachus collected during the prespawning phase of their reproductive cycle were used. Fish were anesthetized and perfused transcardially with phosphate-buffered saline (pH 7.4) and 4 % paraformaldehyde in phosphate buffer. Sections through the rostro-caudal extent of the POA and pituitary were processed for TH immunofluorescence. Using double immunofluorescence, the association between TH-immunoreactive fibers and LH cells in the pituitary was explored. Sections were analyzed using semiquantitative analysis.

RESULTS

NPP in POA of C. batrachus has two distinct subdivisions, viz, anterior (NPPa) and posterior (NPPp), and TH neurons were observed in both the subdivisions. Compared to that in the males, a significantly higher (P < 0.05) number of TH neurons was consistently observed in the NPPa of females. TH neurons in NPPp, however, showed no difference in the number or immunoreactivity. Since DA neurons in NPPa are hypophysiotropic, we compared TH-fiber innervation of the pituitary in both sexes. Compared to males, proximal pars distalis and LH cells in this region of the pituitary in females were densely innervated by TH fibers.

CONCLUSIONS

Neurons of NPPa and their innervation to the pituitary seem to be a distinct sexually dimorphic DAergic system in C. batrachus. The DAergic system may serve as a component of the neural mechanisms controlling the sexually dimorphic LH surge in teleosts. Given the similarities shared by NPPa and AVPV, homology between these two nuclei is suggested.

Interaction between dopamine and neuropeptide Y in the telencephalon of the Indian major carp, Cirrhinus cirrhosus

In teleosts, while neuropeptide Y (NPY) has emerged as one of the potent regulators of GnRH-LH axis, entopeduncular nucleus (EN) in the ventral telencephalon serves as major site for NPY synthesis/storage. Neurons of the EN innervate preoptic area and pituitary, respond to gonadal steroids, undergo reproduction phase-related changes, and are believed to convey sex steroid-borne information to GnRH neurons. In spite of the importance of EN, the neural circuitry associated with the nucleus has not been defined. Aim of the present study is to examine the possibility of the dopaminergic regulation of EN. NPY-immunoreactive cells and fibers were extensively distributed in the forebrain and pituitary of Cirrhinus cirrhosus. NPY immunoreactivity was observed in the olfactory receptor neurons, ganglion cells of terminal nerve, and in neurons of area ventralis telencephali/pars lateralis, EN, nucleus preopticus periventricularis (NPP), and nucleus lateralis tuberis. NPY-fibers were observed in the dorsal telencephalon, tuberal area and pituitary. While the area ventralis telencephali/pars intermedialis (Vi) located just above the EN contained a distinct population of tyrosine hydroxylase neurons, their axons seem to innervate NPY neurons in EN. Superfused brain slices containing EN were treated with DA D1- and D2-like receptor agonists. NPY-immunoreactivity in the EN showed significant increase (P<0.001) following DA D1-like receptor agonist, SKF-38393 treatment, but DA D2-like receptor agonist, quinpirole was ineffective. DA may regulate NPY neurons in EN via D1-like receptors. DA-NPY interaction in the EN might be important in the central regulation of reproduction in teleosts.

Tyrosine hydroxylase in the olfactory system, forebrain and pituitary of the Indian major carp, Cirrhinus cirrhosus: organisation and interaction with neuropeptide Y in the preoptic area

Dopamine (DA) inhibits, whereas gonadotrophin-releasing hormone (GnRH) stimulates, luteinisiing (LH) cells in the pituitary of some but not all teleosts. A reduction in the hypophysiotropic dopaminergic tone is necessary for the stimulatory effect of GnRH on LH cells. Neuropeptide Y (NPY) has emerged as one of the potent, endogenous agent that modulates LH secretion directly or indirectly via GnRH. Involvement of NPY in the regulation of hypophysiotropic DA neurones, however, is not known, but there is good evidence suggesting an interaction in the mammalian hypothalamus. DA neurones, identified by tyrosine hydroxylase (TH)-immunoreactivity, were observed widely throughout the brain of the Indian major carp, Cirrhinus cirrhosus. The granule cells and ganglion cells of terminal nerve in the olfactory bulb, and cells in ventral telencephalon and preoptic area (POA) showed conspicuous TH immunoreactivity. In the POA, the nucleus preopticus periventricularis (NPP), divisible into anterior (NPPa) and posterior (NPPp) components, showed prominent TH-immunoreactivity. The majority of TH neurones in NPPa showed axonal extensions to the pituitary and were closely associated with LH cells. The NPPa also appeared to be the site for intense interaction between NPY and DA because it contains a rich network of NPY fibres and few immunoreactive cells. Approximately 89.7 ± 1.5% TH neurones in NPPa were contacted by NPY fibres. Superfused POA slices treated with a NPY Y2 -receptor agonist, NPY 13-36 resulted in a significant (P < 0.001) reduction in TH-immunoreactivity in NPPa. TH neurones in NPPa did not respond to NPY Y1 -receptor agonist, [Leu(31) , Pro(34) ] Neuropeptide Y treatment. We suggest that, by inhibiting DAergic neurones in NPPa via Y2 -receptors, NPY may contribute to the up-regulation of the GnRH-LH cells axis. The microcircuitry of DA and NPY and their interaction in NPPa might be a crucial component in the central regulation of LH secretion in the teleosts.

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.

Gonadotropin releasing hormone (GnRH) neurones of triploid catfish, Heteropneustes fossilis (Bloch): an immunocytochemical study

Gonadotropin-releasing hormone (GnRH), a regulator of gonadal maturation in vertebrates, is primarily secreted by neurosecretory cells of the pre-optic area (POA) in the forebrain of teleosts. GnRH-immunoreactive (GnRH-ir) cells of this area demonstrate positive correlation in number and size of soma with gonadal maturity and directly innervate the pituitary in most teleosts. Gonadal development in triploid fish remains impaired due to genetic sterility. The gonadal immaturity in triploid fish may be due to low levels of gonadotropin and sex steroids during the vitellogenic phase of reproductive cycle. However, the nature of GnRH-ir cells in triploid fish is not yet known. Triploid catfish (H. fossilis) showed significant decrease (P<0.001) in size and number of immunoreactive-GnRH cells of POA and low immunoreactivity in pituitary in comparison to their diploid full-sibs during the late pre-spawning phase of ovarian cycle. This study suggests that low activity of GnRH-cells in triploid may be due to lack of positive feedback stimulation by sex steroids and/or reduced responsiveness of sensory cells to environmental cues required for gonadal maturation in teleosts.

Changes in the pituitary metabolism of monoamines (dopamine, norepinephrine, and serotonin) in female and male rainbow trout (Oncorhynchus mykiss) during gonadal recrudescence

The dynamics of the levels and metabolism of dopamine, norepinephrine, and serotonin were studied in pituitaries of male and female rainbow trout at different stages of gonadal development. In female rainbow trout, the turnover of dopamine (calculated using the inhibitor of tyrosine hydroxylase alpha-methyl-p-tyrosine methyl-ester HCl), serotonin metabolism, and norepinephrine levels decreased in the advanced stage of exogenous vitellogenesis with respect to the initial stage. However, data obtained in males did not show changes in either serotonergic or noradrenergic metabolism during the last stages of gonadal development. However, an increase of dopaminergic turnover was noticed in the male fish at the end of spermiation. Finally, pituitary dopaminergic activity was significantly higher in immature (prepubescent stage) than in adult fish.

Innervation and control of the adenohypophysis by hypothalamic peptidergic neurons in teleost fishes: EM immunohistochemical evidence

Previous light microscopic studies have revealed neuropeptide-immunoreactive neurosecretory fibers in the teleostean neurohypophysis, and ultrastructural work has reported direct innervation of endocrine cells by the terminals of fibers penetrating the adenohypophysis. This paper reviews our recent data from ultrastructural, immunohistochemical, receptor localization, and superfusion studies, which suggest a role for neuropeptides in the control of teleost pituitary secretion. We have used a combination of pre- and post-embedding electron microscopic immunolabeling methods to determine which neuropeptides are present in fibers innervating the pituitaries of three species: Poecilia latipinna, Dicentrarchus labrax, and Clarias gariepinus. Numerous axon profiles with immunoreactivity for the neurosecretory peptides vasotocin and isotocin formed large Herring bodies and terminal-like boutons in contact with corticotropic, growth hormone, thyrotropic, and pars intermedia cells. Numerous melanin-concentrating hormone-immunoreactive fibers and scarcer neurotensin and corticotropin-releasing factor-immunoreactive fibers showed similar distributions, terminating close to pars intermedia and corticotropic cells. Somatostatin, cholecystokinin, galanin, substance P, neuropeptide Y, growth hormone-releasing factor, thyrotropin-releasing hormone, and gonadotropin-releasing hormone-immunoreactivities were found in small calibre fibers penetrating among growth hormone, thyrotropic, and gonadotropic cells. These morphological findings have been supplemented by autoradiographic studies, which showed the distribution of binding sites for vasotocin, isotocin, galanin, and neuropeptide Y ligands over specific groups of pituitary cells, and superfusion studies that showed growth hormone release was stimulated by growth hormone-releasing factor and thyrotropin-releasing hormone, but inhibited by somatostatin. The implications of these results for neuropeptidergic control of teleostean pituitary secretions are discussed.

Central dopaminergic neurons in tilapia: effects of gonadectomy and hypothalamic lesion

The effects of gonadectomy, testosterone and estrogen on the dopamine (DA) neurons were examined by measuring the concentrations of DA and 3,4-dihydroxyphenylacetic acid (DOPAC) in the brain and pituitary of male tilapia. The tuberal area and the pituitary had significantly high levels of DA and low levels of DOPAC, indicating the existence of a rich dopaminergic innervation in these areas. Gonadectomy and sex steroid replacement had no effect on DA and DOPAC levels. Preoptic lesions (14 days survival period) significantly increased DA levels of the pituitary, indicating a possible existence of a preoptico-hypophysial neural system that inhibits pituitary DA synthesis in tilapia. The lack of effect by preoptic (4 days survival period) and posterior hypothalamic lesions on the DA content of the pituitary indicates the absence of dopaminergic innervation of the pituitary by the preoptic and the posterior hypothalamus. Instead, the overall results do suggest the anterior periventricular area as a possible source of pituitary dopaminergic innervation.

Immunolocalization of catecholamine enzymes, serotonin, dopamine and L-dopa in the brain of Dicentrarchus labrax (Teleostei)

Antisera to serotonin (5-HT), dopamine, and L-dopa, and to the catecholamine synthesizing enzymes, tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), and phenylethanolamine N-methyl transferase (PNMT), were used to localize monoamine containing neurones in the brain of Dicentrarchus labrax (sea bass). In the brain stem, 5-HT-immunoreactive (ir) neurones were recognized in the ventrolateral medulla, vagal motor area, medullary, and mesencephalic raphe nuclei and in the dorsolateral isthmal tegmentum. In the hypothalamus, liquor-contacting 5-HT neurones were seen in various regions of the paraventricular organ. Virtually all regions of the brain contained a dense innervation by 5-HT fibres and terminals. DBH-ir neurones were restricted to three brain stem areas: the locus coeruleus, the area postrema, and the reticular formation of the lower medulla. Neurones in these three groups also displayed TH-ir, and in the latter area, PNMT-ir in addition. In the locus coeruleus and area postrema, TH-ir neurones outnumbered DBH-ir neurones, an observation substantiated by the presence of dopamine-ir neurones. In the forebrain, dopamine- and TH-ir neurones were found in the olfactory bulb, ventral/central telencephalon, periventricular preoptic, and suprachiasmatic areas, dorsolateral and ventromedial thalamus, and posterior tuberal nucleus. In the paraventricular organ, the distribution and morphology of dopamine-ir neurones was similar to that observed with anti-5-HT, but the vast majority of cells were not TH-ir, suggesting accumulation of dopamine by uptake from the ventricle, rather than by synthesis. L-dopa-ir neurones were found only in the central telencephalon, preoptic recess, and dorsolateral thalamus. Fibres and terminals immunoreactive for dopamine, TH, and DBH showed a broadly similar distribution. The results are discussed in relation to the monoaminergic systems previously reported in other teleostean species and the mammalian brain.

Gonadotropin-releasing hormone from thai catfish: chromatographic and physiological studies

Two forms of immunoreactive gonadotropin-releasing hormone (GnRH) were extracted from brain-pituitary tissues of Thai catfish, Clarias macrocephalus and C. batrachus. The peptides were detected using high performance liquid chromatography (HPLC) and radioimmunoassay (RIA). In both the HPLC systems, catfish GnRH-I eluted earlier than catfish GnRH-II and also eluted before the synthetic standards of mammalian, lamprey, chicken I, chicken II, and salmon GnRH. Hence, catfish GnRH-I appears to be the most hydrophilic GnRH family member because of this early elution from the HPLC. Catfish GnRH-II eluted in a position similar to that of chicken GnRH-II. This study suggests that catfish GnRH-I is a novel form of GnRH, whereas catfish GnRH-II is the same as chicken GnRH-II. Indirect evidence suggests that the catfish molecule is 10 amino acids in length and has an amide at the C-terminus. Moreover, the novel catfish GnRH appears to be different within the domain of amino acids 5 to 10 compared with mammalian GnRH because it is not recognized by antiserum B-6. An injection of native chicken GnRH-II was more effective than salmon or mammalian GnRH for induced ovulation in C. macrocephalus.

Application of cryosubstitution in neurohormone- and neurotransmitter-immunocytochemistry

Pituitaries of the African catfish, Clarias gariepinus, were prefixed in aldehyde fixatives, frozen in liquid propane and submitted to a cryosubstitution procedure. Ultrathin sections of the Lowicryl HM20-embedded tissue were treated with primary antisera raised in rabbits to gonadotropin releasing hormone (GnRH), vasopressin or gamma amino butyric acid (GABA) respectively. Binding of the primary antisera was visualized with goat anti-rabbit (GAR) labeled with gold. The general morphology of the tissue components in the cryosubstituted pituitaries matches with that obtained after routine embedding procedures. In addition, a strong labeling intensity of the neuropeptides/neurotransmitters investigated in the present study was demonstrated. Due to these qualities cryosubstitution provides optimal conditions for studying co-localization of neurosecretory products, using double-immunostaining procedures. In the pars distalis of the catfish pituitary several types of hypothalamus-derived nerve fibers are present between or synapting on the secretory cells. It is demonstrated that the two known catfish GnRHs are co-localized in the same nerve fiber and within these nerve fibers even co-exist in the same neurosecretory granules. GABA and vasopressin-immunolabeling each occurred in different nerve fibers. The present data demonstrate that cryosubstitution and low temperature-embedding results in an excellent morphological preservation compared to ultracryotomy and a better preserved immunoreactivity of small antigenic molecules in comparison to conventional fixation and embedding techniques.

Serotonin and dopamine turnover in the female rainbow trout (Oncorhynchus mykiss) brain and pituitary: changes during the annual reproductive cycle

Brain serotonin (5HT) and dopamine (DA) turnover were studied at various stages of the reproductive cycle of the female rainbow trout by simultaneous determination by HPLC of neurotransmitters and major related metabolites. An increase of 5HT turnover in telencephalon and hypothalamus and a decrease of DA turnover in pituitary and hypothalamus were observed during the periovulatory period. Some changes also occurred during vitellogenesis: decreased 5HT metabolite in telencephalon and preoptic area and increased DA content in preoptic area. These data suggest that physiological fluctuations of biogenic amines could be involved in both ovarian recrudescence and ovulation, with major effects on the hypothalamo-hypophysial complex during the periovulatory period.

Demonstration of dopamine in electron-dense synaptic vesicles in the pars intermedia of Xenopus laevis, by freeze substitution and postembedding immunogold electron microscopy

The presence of dopamine in the pituitary of the clawed toad Xenopus laevis was studied by light and electron microscope immunocytochemistry, using pre- and postembedding techniques. Light microscopy showed the presence of an intricate, anti-dopamine-positive fibre network throughout the pars intermedia. In preembedded stained material, dopamine appeared to occur in varicosities which make synaptic contacts with both folliculo-stellate cells and melanotrope cells. Post-embedding immunogold staining of freeze-substituted material permitted the localization of anti-dopamine reactivity in electron-dense vesicles in these varicosities. This finding supports the hypothesis that dopamine is involved in the (inhibitory) control of melanotrope cell activity in X. laevis.

Distribution of serotonin- and dopamine-immunoreactivity in the brain of the teleost Clarias gariepinus

The distribution of serotonergic and dopaminergic cell bodies and varicose fibres in the brain of the teleost Clarias gariepinus was studied immunohistochemically using antisera against formaldehyde-conjugated serotonin and dopamine. Many serotonergic and dopaminergic fibres innervated the areas dorsalis telencephali pars medialis and pars lateralis dorsalis, as well as the area ventralis telencephali pars ventralis. In the diencephalon, a large number of serotonergic and some dopaminergic fibres were found in the preoptic nucleus, innervating the cells of this nucleus. In addition, serotonergic and dopaminergic fibres were observed in the pituitary stalk and in all regions of the pituitary gland. Moreover, the diencephalon contained the highest number of serotonin- or dopamine-immunoreactive cell bodies. These cells were confined to the same periventricular nuclei as the nucleus ventromedialis thalami, the nucleus posterior periventricularis, the nucleus lateralis tuberis, the nuclei recessus lateralis and recessus posterioris. Most cells of these nuclei were in contact with the cerebrospinal fluid of the third ventricle. The brainstem contained serotonergic cell bodies in the raphe nuclei and a few serotonergic and dopaminergic fibres. The torus semicircularis was densely innervated by serotonergic fibres and, to a lesser extent, dopaminergic fibres. In the midbrain of Clarias gariepinus, no dopaminergic homologue of the substantia nigra was observed. The results are discussed both in a comparative and a physiological context. In this regard, special attention has been paid to the contribution of hypothalamic monoamines in the regulation of gonadotropin secretion as an essential step in the neuro-endocrine control of reproduction.

Effects of dopamine, norepinephrine and serotonin on secretion of luteinizing hormone, follicle-stimulating hormone and prolactin in ovariectomized, pituitary stalk-transected ewes

Two experiments were conducted in ovariectomized, pituitary stalk-transected ewes to determine if dopamine (DA), norepinephrine (NE) or serotonin (5-HT) alter secretion of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (PRL). In experiment 1, ewes were infused (iv) with saline (control), DA (66 micrograms/kg/min), NE (6.6 micrograms/kg/min) or 5-HT (6.6 micrograms/kg/min). Treatments did not alter pulse frequency, but 5-HT increased (P less than .05) amplitude of pulses of LH and mean concentrations of LH, DA and NE were without effect on basal secretion of LH. DA but not NE or 5-HT decreased (P less than .05) the release of LH in response to gonadotropin hormone-releasing hormone (GnRH, 25 micrograms, im). Concentrations of FSH were not affected by treatments. Secretion of PRL was reduced (P less than .05) by treatment with DA and NE but not 5-HT. Each amine reduced (P less than .05) the release of PRL in response to thyrotropin-releasing hormone (TRH; 3 micrograms, im). In experiment 2, ewes were given DA at doses of 0, 0.66, 6.6 or 66.0 micrograms/kg/min, iv. No dose altered basal LH, but each dose reduced (P less than .05) basal and TRH-induced release of PRL. Key findings from these studies include direct pituitary action for: (1) 5-HT enhanced basal secretion of LH, (2) suppression of GnRH-induced secretion of LH by DA. (3) DA and NE inhibition of PRL secretion, and (4) DA, NE and 5-HT inhibition of release of PRL in response to TRH.

Steroid glucuronides as male pheromones in the reproduction of the African catfish Clarias gariepinus--a brief review

After ovulation, female African catfish are strongly attracted by the odor of male conspecifics. This attraction depends on the presence of the seminal vesicle, a part of the male reproductive organs. Removal of the seminal vesicle illustrates this fact. A low dose of seminal vesicle fluid, added to the water, appears to be highly attractive for catfish which have ovulated. Fractionation of the fluid and testing of the different fractions shows that steroid glucuronides could be responsible for the attraction. These steroid glucuronides can be identified with gas chromatographic-mass spectrometric analysis. A mixture of glucuronides, prepared to resemble the composition of the seminal vesicle fluid, evokes a dose-dependent attraction. The most potent odorant, observed by measuring electrical responses from the olfactory epithelium and from the olfactory tract appears to be 3 alpha,17 alpha-dihydroxy-5 beta-pregnan-20-one-3 alpha-glucuronide.

Changes in hypothalamic catecholamine levels in relation to season, ovariectomy, and 17 beta-estradiol replacement in the catfish, Clarias batrachus (L.)

In female Clarias batrachus, hypothalamic levels of noradrenaline (NA) and dopamine (DA) exhibited seasonal changes during different phases of the annual reproductive cycle. The NA level was high during gonadal recrudescence (prespawning and spawning phases) and low during gonadal quiescence (postspawning and resting phases). A reverse trend was noticed in the DA level. After 3 weeks of ovariectomy (Ovx), the catecholamine (CA) levels were elevated significantly in the preparatory, prespawning, and spawning phases and were unaltered in the postspawning season. Administration of 17 beta-estradiol (E2, 1.0 microgram/g BW) to 3-week Ovx fish resulted in both seasonal and differential effects on the CA levels. The NA level was significantly elevated in the preparatory phase, decreased in the prespawning and spawning phases, and remained unaltered in the postspawning season. The DA level was restored by the steroid treatment in the preparatory phase, but registered a significant elevation in the prespawning and spawning phases. In the prespawning phase, Ovx induced a significant elevation of the hypothalamic NA level after 2, 3, and 4 weeks, but was significantly low after 5 weeks. The DA level registered a significant increase after 2, 3, and 5 weeks of Ovx. Administration of E2 to 3-week Ovx fish elicited dose-dependent responses on CA levels. The administration of a low dose of E2 (0.05 microgram) caused a significant elevation of the NA level, while the 0.1 microgram dose did not alter the amine level in comparison to that of the Ovx vehicle group. The higher doses (0.5 and 1.0 microgram) of E2, however, decreased the NA level significantly below that of the sham vehicle and Ovx vehicle groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypothalamic and telencephalic catecholamine content in the brain of the teleost fish, Pygocentrus notatus, during the annual reproductive cycle

The catecholamines noradrenaline (NA), dopamine (DA), and adrenaline (A) were measured in hypothalamic and telencephalic extracts of the Venezuelan freshwater fish "caribe colorado," Pygocentrus notatus, at different stages of the reproductive cycle. The concentration of NA was found to be significantly higher in the telencephalon than in the hypothalamus, but that of DA was higher in the hypothalamus than in the telencephalon. Fluctuations depending upon the reproductive stage and environmental conditions occurred in both hypothalamus and telencephalon. In the hypothalamus, DA content was highest during the prespawning period (June) as compared to other periods of the cycle. Although the NA concentration was reduced during spawning there was no significant variation during any other period. DA concentrations in both telencephalon and hypothalamus showed a similar pattern of changes. In the telencephalon, NA levels increased between preparatory and prespawning periods but decreased sharply during spawning. No sex differences were observed in either area at any stage of reproduction.

Characterization of dopamine D2 receptors in the pituitary of the African catfish, Clarias gariepinus

Dopamine receptors in the pituitary of the African catfish, Clarias gariepinus, were characterized using [3H]spiperone as radioligand. Specific binding of [3H]spiperone to pituitary membranes reached equilibrium within 60 min of incubation. The binding of the radioligand was tissue specific since the amount of binding was linear with pituitary membrane content in the incubations. In addition, pituitary membranes were observed to bind considerably more [3H]spiperone, compared to membrane preparation of various other tissues. Saturation experiments revealed the presence of a single class of high affinity/low capacity binding sites. The binding characteristics, estimated by Scatchard analysis, were: Kd = 3.2 +/- 0.5 x 10(-9) M and Bmax = 105 +/- 5 fmol/mg protein. Specific binding was displaceable with dopamine and with various specific D2 agonists and antagonists. The nature of displacement curves resembles those observed in studies on mammalian dopamine receptors. Binding experiments with cell fractions, obtained after centrifugation of dispersed pituitary cells over a Percoll density gradient, showed that most [3H]spiperone binding was obtained in an enriched gonadotropic cell fraction. This observation indicates that the receptor characteristics, estimated with the [3H]spiperone assay, are representative for dopamine receptors on the gonadotropic cells.

Differential effects of photoperiod and temperature on hypothalamic monoaminergic activity in the teleost Channa punctatus (Bloch)

In Channa punctatus maintained under ambient photothermal conditions (10.5L:13.5D; 13 ± 2°C) in the postspawning season, hypothalamic dopamine (DA) and norepinephrine (NE) content varied diurnally, but serotonin (5-HT) and monoamine oxidase (MAO) did not. Exposure of the fish to high temperature (25 ± 2°C), with or without altering the photopheriod, increased the NE and DA content, and NE turnover but decreased DA turnover. Increasing the photoperiod (16L:8D) alone had no significant effect on catecholaminergic (CA) content or activity. When the fish were exposed to long photoperiod, with or without altering the temperature, a day-night variation in 5-HT content and activity was induced in the postspawning season. An increase in temperature alone had no significant effect on 5-HT content or activity. Hypothalamic MAO activity was elevated in fish exposed to high temperature alone, or in combination with long photoperiod, but was not affected significantly by long photoperiod treatment. These results indicate that CA activity is influenced largely by ambient high temperature, whereas serotonergic activity is controlled primarily by photoperiod, with high temperature having an additive effect. MAO activity seems to be influenced by both temperature and photoperiod in this species.

Dynamic characteristics of serotonin and dopamine metabolism in the rainbow trout brain: a regional study using liquid chromatography with electrochemical detection

Aminergic metabolism was studied in discrete brain regions of the postovulated female rainbow trout using a liquid chromatography electrochemical detection method. 3 Methoxytyramine (3MT) was the major dopaminergic catabolite, suggesting that catechol-o-methyl transferase is the main dopamine (DA) catabolic enzyme. Two populations of brain regions were found: one with a high DA content and low 3MT/DA ratio (hypothalamus and telencephalon), suggesting that these regions could present a high density of DA perikarya; the other with a high 3MT/DA ratio (pituitary, preoptic area, myelencephalon and optic tectum) suggesting that these regions could present a high density of DA axonal endings. 5 Hydroxytryptamine (5HT) content differed, but an homogeneous distribution of monoamine oxidase was found in different brain regions. High 5HT content was found in the hypothalamus and telencephalon; 5HT was however not detectable in the pituitary.

Pituitary steroids in two teleost species: immunohistological and biochemical studies

Antisera raised against steroid hormones [estradiol-17 beta (E2), testosterone (T), 11 beta-hydroxyandrostenedione (OHA)] were used to localize immunoreactive material in fixed and paraffin-embedded pituitaries of the African catfish and the rainbow trout. Organic extracts of pituitary homogenates were analyzed for steroid hormones by radioimmunoassay and gas chromatography-mass spectrometry (E2 in female catfish only). With the exception of an E2-positive cell type in the catfish neurointermediate lobe, steroid immunoreactivity was found to be restricted to the cytoplasm of adenohypophyseal cells, which were also labeled after incubation with catfish alpha,beta-gonadotropin and salmon gonadotropin antisera, respectively. Steroid levels determined by radioimmunoassay in the catfish ranged between 85 and 628 pg/pituitary, while lower levels (2-8 pg/pituitary) were found in the rainbow trout. E2 was identified by gas chromatography-mass spectrometry at a level of 84 pg/pituitary. The observation that immunolabeling after steroid antiserum incubation is confined mainly to gonadotrops provides morphological evidence for direct steroid effects on this particular cell type.

Changes in hypothalamic monoamine oxidase activity in relation to season, ovariectomy, and 17 beta-estradiol administration in intact and ovariectomized catfish, Clarias batrachus (L.)

Cyclic changes in hypothalamic monoamine oxidase (MAO) activity were noted during the annual ovarian cycle of Clarias batrachus, with a high level in the preparatory phase and a low level in the spawning phase. Administration of 17 beta-estradiol (E2) daily for 3 days induced both season- and dose-dependent responses in enzyme activity. In the preparatory phase, 0.05, 0.1, and 0.5 microgram/g doses of E2 enhanced enzyme activity in a dose-dependent manner, with significant changes being observed in the latter two dose groups. Administration of 1.0 and 10.0 micrograms/g doses decreased enzyme activity, with a significant difference elicited by the latter dose. In the spawning phase, enzyme activity was significantly elevated by 0.05 and 0.1 microgram/g doses. MAO activity decreased insignificantly in the 0.5 and 1.0 microgram/g groups and significantly in the 10.0 micrograms/g E2 group. Enzyme activity decreased significantly 2, 3, and 4 weeks after ovariectomy (prespawning phase), in a time-dependent manner, and increased after 5 weeks (not significantly different from the sham control value) and 6 weeks. Administration of E2 to 3-week ovariectomized fish elicited dose-dependent effects on MAO activity. E2 injections at 0.05 and 0.1 microgram/g doses not only restored the ovariectomy-induced decrease, but also elevated enzyme activity significantly compared with the sham + vehicle control. The higher doses (0.5 and 1.0 microgram/g) were ineffective in restoring enzyme activity, which decreased significantly in the 1.0 microgram/g group compared with ovariectomy and sham control values.(ABSTRACT TRUNCATED AT 250 WORDS)

A new member of the gonadotropin-releasing hormone family in teleosts: catfish gonadotropin-releasing hormone

Two forms of immunoreactive gonadotropin-releasing hormone (GnRH) were detected in extracts of brain-pituitary tissue from the African catfish, Clarias gariepinus. Catfish I GnRH eluted first from reverse-phase HPLC and was present in larger amounts compared with catfish II GnRH. Chromatographic and immunological studies with four antisera provide evidence that catfish I GnRH is unique compared with identified GnRHs from mammal, chicken, salmon, and lamprey. Catfish II GnRH elutes in the same position as chicken II GnRH and the forms cannot yet be distinguished. GnRHs extracted from female and male catfish tissue appear to be similar in terms of the number of peaks eluted, elution position, quantity, and cross-reactivity with the antisera. The results of the HPLC and radioimmunoassay studies suggest that catfish I GnRH is likely to be 10 amino acids in length, and have an amide at the C terminus similar to the other family members. In addition, catfish I GnRH is probably different in the 5 to 10 amino acid region compared with mammalian GnRH. Finally, catfish I GnRH is likely to have a lysine or arginine residue as it is the most hydrophilic family member. The lack of the salmon form of GnRH and the presence of a unique GnRH form constitute another example of the considerable evolutionary variation that has occurred in the catfish family compared with other teleosts.

The dopaminergic regulation of gonadotropin-releasing hormone receptor binding in the pituitary of the African catfish, Clarias gariepinus

In several teleost species, including the African catfish, dopamine acts as an endogenous inhibitor of gonadotropin-releasing hormone (GnRH)-stimulated gonadotropin (GTH) release. The present in vivo study was carried out to investigate whether this inhibitory action of dopamine can be explained by an effect on the pituitary GnRH receptors. To that end, sexually mature female catfish were treated with dopamine and the dopamine antagonist pimozide (PIM), respectively. At different time intervals after injection, the pituitaries were collected, and in a GnRH receptor assay the GnRH-binding parameters were determined. The dopamine treatment affected neither GnRH-binding capacity nor affinity. The PIM treatment resulted in a two-fold increase in pituitary GnRH-binding capacity without affecting binding affinity. The time course of this effect coincided with the potentiating effect of PIM of the GTH-releasing activity of a GnRH analog. It is concluded that the stimulatory effect of PIM on the action of GnRH might, in part, be due to an increased pituitary GnRH-binding capacity. Reversely, these results suggest that the endogenous dopaminergic inhibition of GnRH-stimulated GTH release may be mediated, at least in part, through down-regulation of the pituitary GnRH receptors.

Immunocytochemical localization of LH-RH in the brain and pituitary of the catfish, Clarias batrachus (Linn.)

An elaborate organization of luteinizing hormone-releasing hormone (LH-RH) immunoreactive (ir) cells and fibers was encountered in the olfactory system of Clarias batrachus. In addition to the ir structures in the olfactory nerve, peripheral area of the olfactory bulb, and the medial olfactory tract (MOT), ir cells and fibers were prominently seen in the lamellae of the olfactory organ. Perikarya showing varying degrees of intensity of immunoreaction were observed along the base of the forebrain in the nucleus preopticus basalis lateralis, nucleus preopticus periventricularis, nucleus preopticus, nucleus lateralis tuberis pars posterior, and the pituitary. Some cells were also noticed in the midbrain tegmentum. A well-defined system of ir fibers from the MOT penetrated the telencephalon and curved dorsocaudally into the pars supracommissuralis above the anterior commissure (AC); while some fibers decussate in the AC, others extended posteriorly into the diencephalon. A fairly dense network of beaded ir fibers was seen in the basal forebrain, conspicuous around the organum vasculosum laminae terminalis and caudally traceable as far as the neurohypophysis; some immunostained fibers appear to be directly contacting with the cells of the proximal pars distalis. Fibers were also witnessed in the optic chiasma and in the inner plexiform layer of the retina. Solitary fibers were noticed in certain circumscribed telencephalic areas, caudal hypothalamus, posterior commissure, midbrain tegmentum, cerebellum, and ventral medulla oblongata. The highly organized LH-RH containing system in C. batrachus is indicative of its elaborate role in synchronization of the reproductive processes and the environmental cues.

Characterization of the receptor for gonadotropin-releasing hormone in the pituitary of the African catfish,Clarias gariepinus

Receptors for gonadotropin-releasing hormone (GnRH) were characterized using a radioligand prepared from a superactive analog of salmon GnRH (sGnRH), D-Arg(6)-Pro(9)-sGnRH-NEt (sGnRHa). Binding of(125)I-sGnRHa to catfish pituitary membrane fractions reached equilibrium after 2 h incubation at 4°C. Displacement experiments with several GnRH analogs as well as other peptides, demonstrated the specificity of(125)I-sGnRHa binding. Specific binding was enhanced in the presence of the cation chelator ethylene bis (oxyethylenenitrilo) tetra-acetic acid (EGTA), indicating an inhibitory effect of cations on GnRH-receptor binding. The binding of(125)I-sGnRHa to pituitary membranes was found to be saturable at radioligand concentrations of 5 nM and above. A Scatchard analysis of the saturation data suggested the presence of a single class of high-affinity binding sites (Ka=0.901±0.06×10(9)M(-1), Bmax=1678±150 fmol/mg protein). A comparative study on(125)I-sGnRHa binding to pituitary membrane fractions of male and female catfish, indicated that there were no differences in binding affinity and binding capacity between both sexes. The results demonstrate the presence of specific, saturable GnRH receptors in the African catfish pituitary.