Hypothalamic control of the pars distalis in fishes, amphibians, and reptiles

Direct control of the gonadotroph in a teleost, Poecilia latipinna: gonadal steroids

Pituitaries from male and female fish were incubated with 1, 5, and 10 micrograms/ml of oestradiol-17 beta, testosterone, 17 alpha OH-progesterone, and 17 alpha,20 beta-dihydroxy-4-pregnan-3-one for 18 hr. Ultrastructural differences between control and experimentally treated glands were used to define the direct effects of these steroids on the gonadotropic cells of the adenohypophysis. The effects of the steroids differed according to sex and reproductive state of the donor animal. Oestradiol and testosterone stimulated gonadotropin secretion by active gonadotrophs, but inhibited it in inactive cells. Both the progesterones generally inhibited gonadotropin secretion although 17 alpha,20 beta-dihydroxy-4-pregnan-3-one had no effect on active gonadotrophs. The four steroids investigated all show potential for direct control of gonadotropin secretion in Poecilia latipinna although factors affecting the balance of these actions, and their relative importance in vivo, remain to be elucidated.

Multiple forms of gonadotropin-releasing hormone in amphibian brains

Several forms of gonadotropin-releasing hormone (GnRH)-like molecules were found in brains of both anurans (frogs) and urodeles (salamanders). The presence of the mammalian-like GnRH molecule was confirmed by HPLC and cross-reactivity studies. Small amounts of salmonid-like GnRH molecules in the brains of frogs (Rana pipiens, Hyla regilla) and salamanders (Taricha granulosa, Ambystoma gracile) were detected by comparing the HPLC chromatographic pattern and immunological reactivity of the brain extracts with native trout and synthetic salmon GnRH. This nonmammalian form of GnRH in the amphibian brain is similar and perhaps identical, at least by indirect evidence, to a form of GnRH reported earlier to be in sympathetic ganglion, retina, chromaffin tissue, and tadpole brain. If two of the amphibian GnRH molecules prove to be mammalian and salmon GnRH, then it is likely that two separate genes in amphibians code for the distinct primary structures of the molecules. The most parsimonious interpretation of the presence of both mammalian- and salmon-like GnRH in anurans and urodeles is that a common phylogenetic ancestor also possessed the two forms of GnRH. Thus the mammalian form of GnRH may well have been present in labyrinthodont amphibians. Independent of evolutionary origin, the functions of the different GnRH molecules in amphibians are unknown.

Somatostatin stimulates ACTH release in brown trout (Salmo trutta L.)

Injections of somatostatin (SRIF) produced small but significant increases in the serum cortisol concentration of brown trout in which endogenous ACTH secretion was suppressed with dexamethasone. SRIF did not stimulate in vitro secretion of cortisol by trout interrenal tissue. These results suggest that SRIF may play a minor role in ACTH secretion in salmonids.

Cloning and sequence analysis of cDNA encoding urotensin I precursor

The primary structure of the precursor of urotensin I, a neuropeptide hormone from the caudal neurosecretory system of the carp Cyprinus carpio, has been determined by analyzing the nucleotide sequence of cloned DNA complementary to the mRNA encoding it. The precursor consists of 145 amino acid residues and the carboxyl terminus represents the 41-amino acid sequence of urotensin I, preceded by Lys-Arg and followed by Gly-Lys. Sequence homology as well as similar organization of the precursors of urotensin I and mammalian corticotropin-releasing factors suggest that they are evolutionarily related. RNA transfer blot analysis indicates that mRNA encoding the precursor of urotensin I is present only in the spinal cord and not in the brain, intestine, liver, or kidney of the carp.

Responses to salinity and the effect of prolactin on whole animal transepithelial potential in the gobiid teleost, Gillichthys mirabilis

Whole-animal transepithelial potential (TEP) of the goby, Gillichthys mirabilis, was measured. TEP was the same whether the electrode was implanted in the dorsal muscle or in the peritoneal cavity. TEP response to different salinities was linear from 0 to 100% seawater, and stable from 100 to 200% seawater. TEP decreased significantly in 225 and 250% seawater, and the fish became dehydrated. Plasma Na and Cl concentrations showed a linear relationship with increase in salinity from 0 to 200% seawater. In hypophysectomized Gillichthys, TEP was significantly lower than in sham-operated fish, but hypophysectomized fish with a rostral pars distalis autotransplant showed TEP similar to that of intact fish. Plasma Na and Cl in hypophysectomized fish were lower than in intact and RPD-autotransplanted fish, suggesting that prolactin is involved in maintaining normal whole-animal TEP.

In vitro release of growth hormone from the pituitary gland of tilapia, Oreochromis mossambicus

The release of growth hormone from the proximal pars distalis of the tilapia, Oreochromis mossambicus, was significantly stimulated by cortisol (1 microgram/ml) in an in vitro system. Growth hormone released into the medium and remaining in the tissue was measured by densitometry after gel electrophoresis. Neither triiodothyronine (6.7 ng/ml) nor equimolar concentrations of thyroxin altered the release of growth hormone. In combination with cortisol, triiodothyronine did not alter the effect of cortisol alone.

Occurrence of beta-endorphin-like immunoreactivity in the brain of the teleost, Boops boops

By immunocytochemical methods the present study describes beta-endorphin-like immunoreactive substance in the brain of Boops boops. Beta-Endorphin-like neurons and fibers were detected in both dorsal and ventral components of the preoptic nucleus and in the nucleus lateralis tuberis. This localization has been discussed in relation to the presence, in the same area, of a well-defined neurosecretory system involved in hypophysial regulation. A beta-endorphin-like immunoreaction was also detected in the Purkinje cells and in processes within the cerebellum molecular layer. Although this last finding remains enigmatic it may suggest a neuromodulatory activity for cerebellum beta-endorphin-like substance. No immunoreaction was observed when the specific antiserum was absorbed with corresponding antigen and with beta-LPH. These tests led to the conclusion that the immunostaining reaction might correspond to a beta-endorphin- or lipotropin-like reaction. Further, the present results show the phylogenetic antiquity of a beta-endorphinergic or lipotropinergic system in the brain, with a stable evolutionary history as a hypophysial regulatory factor or neuromodulatory agent.

Immunocytochemistry of luteinizing hormone-releasing hormone and sexual maturation of the frog brain: comparisons of juvenile and adult bullfrogs (Rana catesbeiana)

An unlabeled antibody enzyme immunocytochemical procedure has been combined with an adjacent serial section approach to localize mammalian-like immunoreactive luteinizing hormone-releasing hormone (ir-LH-RH) in brains of juvenile and adult bullfrogs (Rana catesbeiana). In juvenile animals modest specific immunocytochemical staining was found over cell bodies in the anterior preoptic area (aPOA) and in scattered neurosecretory endings in the outer layer of the median eminence (ME). In brains of adults, a comparatively robust staining was observed for perikarya in the medial septal nucleus, as well as in the aPOA; intense immunostaining of a well-differentiated ME also was noted for adults. Specific immunocytochemical staining was absent from other brain regions and in control preparations. Accordingly, changes both in the number and distribution of cells stained and in the quality of immunostaining were coincident with sexual maturation of the brain. This initial study of juvenile and adult bullfrogs suggests that ir-LH-RH in septal and preoptic areas may exert distinctly differing activities.

Somatostatin-like immunoreactivity in the pituitary and brain of three teleost fish species: somatostatin as a potential regulator of prolactin cell function

Somatostatin-like immunofluorescence occurs in the hypothalamus and neurohypophysis of three euryhaline teleosts: tilapia, killifish, and mudsucker. This immunofluorescence was eliminated by incubating the primary antibody with excess somatostatin or somatostatin-28 but not with urotensin II, a partial analogue of somatostatin. In all three fishes, the strongest reaction was seen in the proximal pars distalis and parts of the pars intermedia. Strongly fluorescing processes from cells of the preoptic nucleus extend toward the pituitary. Distinct fluorescence was also associated with the neurohypophysis penetrating into the rostral pars distalis in the tilapia but not in the killifish or mudsucker. In the tilapia, an extensive network of immunofluorescent fibers and small cells were present in the anterior dorsolateral telencephalon, in addition to a moderately fluorescing group of cells anterolateral to the preoptic nucleus. A small area of diffuse fluorescence was also seen in the anterior dorsolateral midbrain tegmentum. Previous physiological studies have implicated somatostatin as a regulator of prolactin cell activity in tilapia. The present study demonstrates the route by which somatostatin may be delivered to the rostral pars distalis to inhibit prolactin secretion.

Elevation of plasma glucose, alanine, and urea levels by mammalian ACTH in the American bullfrog (Rana catesbeiana)

The effects of a single infusion of mammalian ACTH on plasma glucose, alanine, urea, and lactate were determined in the American bullfrog (Rana catesbeiana). The ACTH (10 U/250 g body wt) was administered, and serial blood samples were collected via a nonocclusive cannula chronically placed in the right truncus arteriosus. Plasma metabolite levels were estimated by standard enzymatic techniques. The plasma metabolites declined following the surgery to levels that were relatively stable by postoperative Day 2. The levels did not vary significantly for the remainder of the 3- or 4-day pretreatment period and in the control bullfrogs during the 48-hr experiments. Plasma glucose levels were essentially unchanged from the time-zero levels at 6 hr following ACTH infusion. Plasma glucose levels subsequently increased to levels that were approximately 24% greater than the control levels by 24 hr and then declined to near control levels by 48 hr. Plasma alanine increased to levels that were approximately 60% greater than the control levels by 12 hr after ACTH treatment and returned to essentially the time-zero levels by 24 hr. Plasma urea rose to levels that were approximately 110% greater than the control levels by 45 min after ACTH infusion, but urea returned to essentially the time-zero levels by 1.5 through 3 hr. Plasma urea increased again to levels that were approximately 90% greater than the control levels by 6 hr and returned to essentially the initial levels by 24 hr. Plasma lactate levels were not significantly influenced by ACTH treatment. The results suggest that a function of the bullfrog hypothalamic-pituitary-adrenocortical axis is to regulate gluconeogenesis from alanine, and probably other glucogenic amino acids.

Multiple molecular forms of gonadotropin-releasing hormone in teleost fish brain

Gonadotropin-releasing hormone (GnRH) immunoreactive peptides in extracts of hake (Merluccius capensis) and tilapia (Tilapia sparrmanii) brain were investigated by high performance liquid chromatography (HPLC) and radioimmunoassay with region-specific antisera. In hake brain, content and concentration of GnRH was higher in the pituitary gland than in the hypothalamic lobes or extrahypothalamic brain. Hake pituitary gland GnRH was purified by six consecutive HPLC systems. The major GnRH molecular form co-eluted with salmon brain GnRH (Trp7, Leu8-GnRH) in four different HPLC systems which were specifically designed to separate the four natural vertebrate GnRHs (mammalian, salmon, chicken I and II). The immunoreactive peak in the final purification step had a retention time identical to that of Trp7, Leu8-GnRH and an UV absorbance (280 nm) peak appropriate for two tryptophan residues in the peptide, as in Trp7, Leu8-GnRH. Six additional less hydrophobic forms of GnRH were detected. Tilapia brain extract contained two major GnRH molecular forms which had identical retention times to chicken GnRH I (Gln8-GnRH) and Trp7, Leu8-GnRH in an HPLC system which separates the natural vertebrate GnRHs. The immunological properties of these two immunoreactive peaks, determined by relative interaction with four region-specific GnRH antisera raised against vertebrate GnRHs, were identical to those of Gln8-GnRH and Trp7, Leu8-GnRH. Additional GnRH molecular forms were also detected. In summary, these findings indicate that a major GnRH molecule in hake pituitary gland is Trp7, Leu8-GnRH, while tilapia brain contains both Trp7, Leu8-GnRH and Gln8-GnRH. Additional GnRH molecular forms were detected in both species.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of temperature and darkness on testosterone concentration in the testes of intact frogs (Rana esculenta) treated with gonadotrophin-releasing hormone analog (HOE 766)

Testicular testosterone was determined by radioimmunoassay in the frog (Rana esculenta) kept in total darkness, at a high or a low temperature (24 or 4 degrees C), and treated with a gonadotrophin-releasing hormone analog (GnRHa, HOE 766). Prolonged exposure to dark conditions seemed to inhibit hypotalamic functions. Moreover, it is shown that high temperature interacts positively with GnRHa treatment on testicular testosterone concentration.

Environmental control of prolactin synthesis in the teleost fish Oreochromis (formerly Sarotherodon) mossambicus

In the cichlid fish Oreochromis mossambicus prolactin cell activity is inversely related to the osmolarity and the Ca2+ concentration of the ambient water. Prolactin cell activity was estimated, at the end of a 3-week experimental period, by determination of the rate of prolactin synthesis during incubation of the rostral parts of the pituitary gland, in the presence of [3H]lysine. Since the secretory activity of isolated prolactin cells is known to be inversely related to the osmolarity of the incubation medium, the possibility was investigated that the effects of changes in ionic composition of the ambient water on prolactin secretion in vivo are mediated by changes in osmolarity of the blood plasma. No support was found for this hypothesis. In fish exposed to high water osmolarities prolactin cell activity was reduced, while plasma osmolarity increased. In contrast, at high Ca2+ concentrations of the water, when prolactin secretion was inhibited to a similar extent, plasma osmolarity was significantly reduced. Although direct effects of plasma osmolarity on prolactin cells cannot be excluded completely, it is unlikely that plasma osmolarity is the predominant factor in the control of prolactin cell activity in situ. The physiological significance of the capacity of isolated prolactin cells to respond to changes in osmolarity of the ambient medium--a capacity shared with some other endocrine cell types--is therefore unclear.

The development of ACTH-like substance during tadpole metamorphosis

The interrenals of tadpoles of Rana catesbeiana at various developmental TK stages were frozen, sectioned, and prepared histochemically for assessment of delta 5-3 beta-hydroxysteroid dehydrogenase (delta 5-3 beta-HSD) activity. The interrenals of hypophysectomized (HX) tadpoles from the same batch of eggs as control tadpoles were prepared similarly for delta 5-3 beta-HSD activity following a series of intraperitoneal injections of different concentrations of mammalian ACTH. The histochemically stained sections of control and HX tadpoles were scanned with a computerized microscope spectrophotometer for diformazan absorbance. Two regression equations were obtained: Y = 0.1460 + 0.0015X and Y' = 0.1494 + 0.0893X', where Y or Y' = delta 5-3 beta-HSD activity absorbance (for the control and HX tadpoles, respectively), X = developmental stages of control, and X' = exogenous ACTH dosage administered to the HX tadpoles. Taking absorbance of delta 5-3 beta-HSD activity as the common factor, a curve of concentrations of ACTH-like substance against TK stages was made. The developing concentration of ACTH-like substance was very low at first and then rose during metamorphic climax. The trend of the development of ACTH-like substance was discussed from the view point of thyroid and delta 5-3 beta-HSD activities.

The brain-pituitary-gonadal axis in the rainbow trout, Salmo gairdneri. III. Absence of an inhibiting action of testosterone on gonadotrophin release in juveniles

In juvenile rainbow trout the effects of exogenous testosterone and of a synthetic gonadotrophin-releasing hormone (GnRH) on the secretion of gonadotrophin (GTH) were investigated. Treatment with implanted testosterone resulted in an accumulation of GTH in the pituitary, but did not affect the concentration of GTH in the plasma. After the testosterone implants were removed, the levels of testosterone in the circulation dropped to undetectable or control values, but the concentration of GTH in the plasma did not increase. These results indicate that testosterone stimulated the synthesis and storage of GTH, and did not prevent the release of this hormone. The synthetic GnRH, des-Gly10[D-Ala6]LH-RH ethylamide (LH-RHa) stimulated the release of GTH when injected into testosterone-pretreated fish, indicating that accumulated GTH is present in a releasable pool.

Effects of dopamine and norepinephrine on in vitro spontaneous and gonadotropin-releasing hormone-induced gonadotropin release by dispersed cells or fragments of the goldfish pituitary

Enzymatically dispersed goldfish pituitary cells or freshly prepared goldfish pituitary fragments continue to secrete gonadotropin spontaneously in a column perifusion system. After the establishment of basal secretion rates, treatment of dispersed pituitary cells with 5 and 500 nM dopamine, or pituitary fragments with 50 and 100 nM dopamine, decreased the amount of gonadotropin released into the perifusate. Perifusion with 500 nM dopamine also abolished the gonadotropin-release response to a 10 nM solution of a luteinizing hormone-releasing hormone analogue in both perifusion systems. Perifusion of pituitary dispersed cells or fragment preparations obtained from sexually regressed goldfish with 50 nM norepinephrine consistently increased the amount of gonadotropin released into the perifusate. These results provide in vitro evidence for direct dopamine inhibition of spontaneous gonadotropin release, blockade by dopamine of gonadotropin-releasing hormone actions, and norepinephrine stimulation of gonadotropin secretion in goldfish.

Evidence for dopaminergic and serotonergic regulation of prolactin cell activity in the trout Salmo gairdneri

The control of prolactin (PRL) cell activity in Salmo gairdneri was investigated in vivo and in vitro. In some in vivo experiments treatment was followed by estimation of pituitary PRL content by gel electrophoresis or of PRL cell nuclear area by light microscopy. In the remainder, treatment was followed by incubation of the pituitary glands in drug-free medium for estimation of PRL synthesis and release. The dopamine precursor, L-dopa (20 mg/kg), reduced pituitary PRL content. Conversely, the dopamine-receptor blocker, domperidone (10 mg/kg), increased total PRL content and amount released in the subsequent incubation. The initial serotonin precursor, L-tryptophan (75 mg/kg), increased pituitary PRL content and PRL cell nuclear area. 5-HTP (20 mg/kg), the immediate serotonin precursor, increased both percentage PRL release and total PRL levels during subsequent incubation. Pargyline (25 mg/kg) treatment to inhibit serotonin catabolism elevated PRL levels in pituitary and medium during subsequent incubation. The serotonin synthesis blocker, parachlorophenylalanine (pCPA; 100 mg/kg), nonsignificantly reduced PRL cell nuclear area. When this was followed by incubation, percentage PRL release and total PRL fell significantly. During in vitro incubation, dopamine (2 micrograms/ml) reduced the release of PRL into the medium, while serotonin (10(-5) M) increased PRL release. These results suggest that both an inhibitory dopaminergic and a stimulatory serotonergic system may be involved in PRL cell regulation in S. gairdneri. The lack of any significant effect of cortisol (1 microgram/ml), somatostatin (300 ng/ml). GABA (100 mg/ml) and TRH 100 ng/ml) on PRL release in vitro suggested little or no involvement of these putative regulatory factors in PRL cell regulation.

Immunohistochemical localization of substance P-like immunoreactivity in the hypothalamus of the lizard, Podarcis sicula, R

The distribution of substance P (SP) immunoreactivity was investigated in the hypothalamus (preoptic area included) of the lizard by single and double immunocytochemical procedures, SP-immunopositive cell bodies were seen in the paraventricular nucleus and periventricular hypothalamic gray (including the paraventricular organ) together with some more lateral elements. Extensive nerve fibers were seen in the white matter and surrounding the paraventricular and supraoptic neurons, and more caudally reaching the hypothalamic periventricular gray, suggesting a massive involvement of SP-like substance in the control of hypothalamic neuroendocrine areas.

In vitro effects of thyrotropin-releasing hormone and somatostatin on prolactin and growth hormone release by the pituitary of Poecilia latipinna. I. An electrophoretic study

Pituitary glands were removed from Poecilia latipinna which had been maintained in one-third seawater and were incubated for 18 hr in media of either 300 mosmol/kg (OP300) or 340 mosmol/kg (OP340) osmotic pressure for measurement of both total and newly synthesised prolactin (PRL) and growth hormone (GH) release. Thyrotropin-releasing hormone (TRH) at 100 ng/ml increased release of total and newly synthesised PRL into OP340, but not into OP300, medium. Conversely, 300 ng/ml of somatotropin-release-inhibiting factor (SRIF) inhibited total and newly synthesised PRL release into OP300, but not OP340, medium. At 1000 ng/ml, SRIF inhibited total PRL release into both media, but newly synthesised PRL release was reduced significantly only in OP300 medium. The release of GH was unaffected by 100 ng/ml TRH in OP300 medium, but both total and newly synthesised GH release were enhanced by this dose in OP340 medium. SRIF at 300 ng/ml reduced total GH release into OP300 medium, whereas the release of newly synthesised GH was inhibited in OP340 medium. At 1000 ng/ml, SRIF inhibited total GH release into both media, but release of the newly synthesised hormone was not significantly altered. These results suggest that TRH can stimulate and SRIF inhibit both PRL and GH release by Poecilia pituitaries, but that these effects may be modulated by plasma osmotic pressure.

Pituitary gonadotropin release by graded electrical stimulation of the preoptic area in the male bullfrog, Rana catesbeiana

Acute (5 min) graded electrical stimulation (biphasic pulses: amplitude, 50, 200, and 500 microA; frequency, 10 Hz; duration, 0.5 msec) of the medial preoptic area (including the medial and lateral septal nuclei and amygdala, pars medialis) induced significant elevations in circulating gonadotropins (more luteinizing hormone, LH, than follicle-stimulating hormone, FSH) in male bullfrogs, Rana catesbeiana. Pituitary response to all currents was rapid and of brief duration; levels of plasma gonadotropins (LH and FSH) peaked at 5 min (the end of stimulation) and decreased rapidly thereafter (LH by at least 50% after 30 min). A stimulus-response relationship was apparent; the magnitude of LH maxima was directly dependent on current intensity between 50 and 500 microA. In general, pituitary responses characteristic of acute electrical stimulation were similar to those induced by injection of mammalian gonadotropin-releasing hormone (GnRH). This is the first direct physiological demonstration of GnRH-like activity in the brain of the bullfrog.

Effects of dopamine and apomorphine on gonadotropin release from the transplanted pars distalis in goldfish

Male goldfish bearing pars distalis transplants from other male goldfish have increased serum gonadotropin (GtH) levels, due to the high spontaneous release rate of GtH by the transplants. Intraperitoneal injection of dopamine or its agonist, apomorphine, each reduced the elevated serum GtH levels caused by the release from the transplanted pars distalis. These results suggest that dopamine has GtH-release-inhibitory activity and acts directly on gonadotrophs to inhibit spontaneous secretion of GtH.

Effects of catecholaminergic agonists and antagonists on serum gonadotropin concentrations and ovulation in goldfish: evidence for specificity of dopamine inhibition of gonadotropin secretion

The elevated serum gonadotropin (GtH) levels in goldfish receiving two injections of des Gly10, [D-Ala6] LH-RH ethylamide (LH-RH-A), given 12-hr apart, were reduced by apomorphine, a dopamine agonist, injected at either the first or the second LH-RH-A injection. Serum GtH concentrations in goldfish given two injections of LH-RH-A at a 3-hr interval were also depressed by bromocriptine, a specific D-2 dopamine receptor agonist, administered simultaneously with both LH-RH-A injections. Injections of dopamine antagonists, pimozide or metoclopramide (a specific D-2 antagonist) caused increased serum GtH concentrations in normal goldfish, but no changes were found following injections of the alpha-adrenergic antagonist phentolamine, the beta-adrenergic antagonist propranolol, or the sympathomimetic agent octopamine. Injection of pimozide or metoclopramide at the time of the second of two LH-RH-A injections given at an interval of 12 hr potentiated the LH-RH-A-induced increase in serum GtH concentrations; injections of phentolamine, propranolol, or octopamine did not alter the response to LH-RH-A. Injections of pimozide or metoclopramide also increased the frequency of ovulation in LH-RH-A-injected gravid female goldfish. These results suggest that stimulation of dopamine receptors can block the potentiating effect of multiple injections of GtH-releasing hormone, as well as ongoing LH-RH-A-stimulated release. The results also indicate that the dopamine inhibition of GtH secretion is specific and may be mediated by receptors resembling the D-2 type receptors in mammals.

Immunohistochemical localization of corticotropin-releasing factor (CRF)-like immunoreactivity in the hypothalamus of the newt, Triturus cristatus

The distribution of corticotropin-releasing factor (CRF) immunoreactivity was investigated in the hypothalamus and preoptic area of the newt by single and double immunocytochemical procedures. CRF immunopositive cell bodies were seen in the preoptic area (from the anterior wall of the preoptic recess to the dorsal parts of the preoptic nucleus) and in the tuberal portions of the posterior hypothalamus. Abundant nerve fibres are seen in the outer zone of the median eminence, while the pars nervosa lacks CRF-immunoreactivity. CRF immunoreactive material is seemingly separated from neurophysins- and somatostatin immunoreactive cell bodies and fibres.

Gonadotropin-releasing hormone (Gn-RH) in striped mullet (Mugil cephalus), milkfish (Chanos chanos), and rainbow trout (Salmo gairdneri): comparison with salmon Gn-RH

Immunoreactive gonadotropin-releasing hormone (Gn-RH) was extracted from brains of striped mullet, milkfish, rainbow trout, and chum salmon with acetone/HCl and petroleum ether. High pressure liquid chromatography and cross-reactivity studies show mullet, milkfish, and trout brains to contain a peptide chromatographically and immunologically identical to synthetic salmon Gn-RH, while the mammalian form of Gn-RH is detectable in none of these fishes. Gn-RH is present in immature 7-month-old and 4-year-old milkfish. A second immunoreactive peptide is separable by HPLC in all the fish studied. This "early-eluting" form of Gn-RH is unlikely to be a precursor; its cross-reactivity with antisera R-42 and #185 suggests that any modification is in the C-terminal region. Several possible roles for this peptide are advanced.

Influences of norepinephrine and alpha-adrenergic mechanisms on gonadotropin secretion in female goldfish, Carassius auratus

Intraperitoneal or intraventricular (3rd cranial ventricle) injection of norepinephrine (NE) caused an increase in serum gonadotropin (GtH) concentrations in female goldfish at times of the year when the ovaries were regressed or at early stages of recrudescence, but had no influence at other stages of the ovarian cycle. In a previous report it was shown that clonidine (alpha-agonist) increased serum GtH levels in fish at early or mid-ovarian recrudescence [J. P. Chang, A. F. Cook, and R. E. Peter (1983) Gen. Comp. Endocrinol. 49, 22-31]; however, intraperitoneal injection of clonidine had no effect on circulating GtH levels when tested in fish at late ovarian recrudescence. In fish at early stages of ovarian recrudescence, intraperitoneal injection of phentolamine (alpha-blocker) did not alter GtH levels, but it blocked the NE-induced increase in serum GtH levels when injected simultaneously with NE. These results suggest that NE stimulates GtH release by alpha-adrenergic mechanisms at the brain and/or pituitary level in female goldfish in a sexually regressed or early ovarian recrudescence condition.

Distribution of 5-hydroxytryptamine (serotonin) in the brain of the teleost Gasterosteus aculeatus L

The distributions of serotoninergic neurons in the brain of the three-spined stickleback was demonstrated with the indirect peroxidase-antiperoxidase (PAP) immunohistochemical method with antibodies against serotonin. Serotoninergic perikarya were demonstrated in the brainstem reticular formation (nucleus raphe dorsalis, nucleus raphe medialis, and nucleus tegmenti dorsalis lateralis) and in the periventricular ventral thalamus and hypothalamus (nucleus ventromedialis thalami, nucleus posterioris periventricularis, nucleus recessus lateralis, and nucleus recessus posterioris). After pharmacological pretreatment of the animals with a monoamine oxidase inhibitor, serotoninergic perikarya were also visualized in area praetectalis and in the medial brainstem, caudal to nucleus raphe medialis. Whereas the cell groups of the brainstem give rise to both ascending and descending pathways, it was not possible to analyze the distribution of efferent projections from the diencephalic cell groups. Distribution of serotoninergic axons showed marked regional differences. Only scattered varicose fibers were demonstrated in the cerebellum, the facial lobes, and the lateral line lobes. In the mesencephalon, the dorsal periventricular tegmentum and the central gray receive only small numbers of serotoninergic axons, while torus semicircularis and the visual layers of tectum opticum are profusely innervated. In the diencephalon, the hypothalamus and ventral thalamus generally display the highest density of serotoninergic axons. Exceptions are found in nucleus glomerulosus and the ventromedial portion of lobus inferioris, where densities are low. In the telencephalon, the density of serotoninergic axons is very high in area dorsalis pars medialis and pars lateralis dorsalis, but low in area dorsalis pars dorsalis and pars lateralis ventralis, and intermediate in area ventralis.

Human pancreatic growth hormone-releasing factor (hpGRF) stimulates growth hormone release in goldfish

Synthetic human pancreatic growth hormone-releasing factor (hpGRF) was injected intraperitoneally in different dosages, either as one injection or two injections 3 hours apart, into goldfish. Serum GH levels were increased by certain dosages of hpGRF under both treatments. This is the first demonstration of GH-releasing activity of hpGRF in a teleost fish.

Identification of the second gonadotropin-releasing hormone in chicken hypothalamus: evidence that gonadotropin secretion is probably controlled by two distinct gonadotropin-releasing hormones in avian species

A new peptide having gonadotropin-releasing activity distinct from the known luteinizing hormone-releasing hormones ( LHRHs ) has been identified in a chicken hypothalamic extract. The existence of [Gln8]LHRH in avian hypothalamus has been reported previously. The new molecular species of gonadotropin-releasing activity, named chicken gonadotropin-releasing hormone II (chicken GnRH-II), has been isolated recently in a yield of 7 micrograms, starting from 10,000 chicken hypothalami. Structural analyses have been performed on the peptide fragments derived from chymotryptic and thermolytic digests of chicken GnRH-II by amino acid analyses and terminal analyses. The full sequence of chicken GnRH-II has been determined to be: pGlu-His-Trp-Ser-His-Gly-Trp-Tyr-Pro-Gly-NH2. A synthetic decapeptide with the above sequence was verified to be chromatographically identical to natural chicken GnRH-II. For further structural confirmation, chymotryptic and thermolytic peptides from synthetic and natural chicken GnRH-II also were identified. Thus, the structure of chicken GnRH-II has been definitely established. The gonadotropin-releasing potency of chicken GnRH-II was about 32% of that of mammalian LHRH and 8 times more potent than chicken LHRH, as estimated from the bioassay with rat anterior pituitary cells. Our results indicate that gonadotropin secretion is probably controlled by two distinct GnRHs , at least in avian species.

Environmental salinity and the thyroidal response to thyrotropin in juvenile coho salmon (Oncorhynchus kisutch)

The effect of environmental salinity on the thyroidal response, measured by plasma concentrations of thyroxine (T4), to exogenous bovine thyrotropin (bTSH) was assessed early and late in the smoltification of coho salmon (Oncorhynchus kisutch). A single injection of 100 mIU bTSH significantly increased plasma T4 concentrations in fish in both fresh water and seawater, and both early (March 19) and late (May 24) in smoltification. In fish transferred to seawater, the increase in T4 titers was somewhat earlier and its duration shorter, suggesting an environmentally induced change in the kinetics of T4 entry and exit from the blood stream. The overall response to TSH was greater near the beginning (March 19) of the smoltification-associated T4 increase than near the end (May 24), suggesting a development-related change in thyroid function.

Effects of chicken and mammalian gonadotropin-releasing hormones (GnRH) on in vivo pituitary gonadotropin release in amphibians and reptiles

The ability of mammalian and chicken gonadotropin-releasing hormones (GnRH) and their agonistic analogs to stimulate in vivo gonadotropin release were tested in a frog (Rana catesbeiana), snake (Naja naja), and turtle (Sternotherus odoratus). In the frog, chicken and mammalian GnRH were equipotent in stimulating the release of FSH and LH. Attendant increases in plasma androgen and the occurrence of spermiation confirmed the release of biologically active gonadotropin. Neither of the GnRH preparations or their agonists produced significant changes in plasma hormones in either of the reptiles. In light of comparable data for the actions of these GnRH preparations in mammals and birds, it appears that species specificity in the response to different GnRHs does not correlate well with the nature of the homologous hypothalamic GnRH molecule.

Temporal effects of infused corticosterone and aldosterone on plasma glucose levels in the American bullfrog (Rana catesbeiana)

The effects of a single infusion of corticosterone or aldosterone on plasma glucose levels were compared in the American bullfrog (Rana catesbeiana). The corticoids were administered, and serial blood samples were collected, via a cannula placed in the common iliac artery. Plasma glucose was estimated by the glucose oxidase method. Plasma glucose levels were essentially unchanged from the time-zero levels at 3 hr after the infusion of 1.0 mg/100 g body wt of corticosterone. The levels subsequently increased to maxima that were approximately 45% greater than the time-zero levels at 9 through 24 hr and then declined to approximately the initial levels by 48 hr after treatment. Infusion of 0.24 mg/100 g body wt of aldosterone did not significantly alter plasma glucose levels. The results suggest that elevated circulating corticosterone is not involved in the primary hyperglycemic response to a stress, but may function synergistically and sequentially with elevated circulating catecholamines in subsequent compensatory adjustments.

Immunocytochemistry of luteinizing hormone-releasing hormone in brains of breeding eastern narrow-mouthed toads (Gastrophryne carolinensis)

Unlabeled antibody-enzyme immunocytochemistry was used to localize mammalian-like immunoreactive luteinizing hormone-releasing hormone in brains of breeding eastern narrow-mouthed toads (Gastrophryne carolinensis). Intense immunostaining occurred over the infundibular floor and the median eminence, yet staining of few perikarya in the anterior preoptic area was minimal. This pattern of markedly regional immunostaining differs from that reported for other anurans.

Immunocytochemical localization of luteinizing hormone-releasing hormone in the brain of the goldfish Carassius auratus

Using an indirect immunofluorescence technique, LH-RH was localized in the brain of the goldfish (Carassius auratus). Immunoreactive cell bodies were inconsistantly found in two hypothalamic nuclei: the anteroventral nucleus preopticus periventricularis (npp) and the posterior nucleus lateralis tuberis (nltp). Numerous immunoreactive fibers were observed in hypothalamic as well as in the extrahypothalamic parts of the brain and in the proximal pars distalis of the pituitary gland. Our results are discussed in relation to the neuroendocrine control of the pituitary gonadotropic function in teleosts.

TRH receptors in fish

The brains of goldfish and other teleosts contain high-affinity binding sites for [3H][3-methyl-His2]thyrotropin-releasing hormone [( 3H]MeTRH) which closely resemble TRH receptors in mammalian brain and pituitary gland in apparent dissociation constant (KD = 3-4 nM), in pharmacology for eight TRH analogs, and in exhibiting marked regional differences in the density of binding sites, with highest binding in the cerebrum and lowest in the cerebellum. Fish brain differs from mammalian brain in containing a prominent additional class of much lower affinity [3H]MeTRH binding sites (KD about 15 microM), of unknown nature, which also exhibit regional differences. Fish pituitary glands contain high-affinity [3H]MeTRH binding sites, but insufficient tissue has prevented full characterization. Little or no saturable binding of [3H]MeTRH was detected in several goldfish peripheral tissues.

Dopaminergic regulation of hatching in fish embryos

Enveloped medaka embryos and denuded zebrafish embryos were exposed to agents that are known to modify the activity of dopaminergic systems. Precocious emergence of medaka embryos was found in the presence of pimozide, salsolinol, and alpha-methyl-rho-tyrosine, whereas delayed hatching occurred with bromocriptine and apomorphine. Moreover, the hatching rate in the light period of medaka eggs, exposed to a 12-hr light/12-hr dark cycle, is significantly higher than in the dark period. Precocious hatching enzyme secretion from denuded zebrafish embryos is caused by salsolinol, whereas dopamine has an opposite effect. At the same time it turned out that in controls hatching enzyme release from denuded zebrafish embryos is well correlated with hatching of enveloped zebrafish embryos. These results do not support the hypothesis proposed by several authors that hatching enzyme is solely mechanically released, but suggest a controlling influence of dopamine receptors, probably located in the developing central nervous system. Assuming a stimulating effect of prolactin on teleostean hatching enzyme secretion, the present data indicate that hypothalamic-hypophyseal tracts are functional at the time of hatching.

Effects of monosodium L-glutamate on pituitary innervation in goldfish, Carassius auratus

Monosodium L-glutamate was injected intraperitoneally into goldfish at a dosage of 2.5 mg/g body wt. The first noticeable effect in the pituitary, 30 min postinjection, was marked swelling of pituicytes in the neurohypophysis (NH). Eighteen hours postinjection a large number of degenerating type B fibers were found in the NH of the proximal pars distalis (PPD), while all type A fibers, and type B fibers in other parts of the pituitary, remained unaffected. Because glutamate causes a lesion in the nucleus lateral tuberis (NLT), from the anterior margin of the pituitary stalk through to the posterior end of the nucleus, and a minor lesion in the nucleus preopticus periventricularis, it is likely that the NLT is the origin of the degenerative type B fibers in the proximal NH. The significance of these results is discussed in relation to the neuroendocrine regulation of the pituitary functions in goldfish.

Effects of hypothalamic lesions on serum growth hormone levels and growth rates in goldfish, Carassius auratus

Serum growth hormone (GH) levels and growth rates of goldfish were measured concurrently following discrete electrothermic lesions of a number of hypothalamic nuclei. Following lesions of the nucleus preopticus periventricularis (NPP), significant increases in serum GH levels were evident at 4 weeks postlesioning in comparison to both sham-operated and normal control groups. In addition, the NPP-lesioned fish showed significant increases in both total body weight and standard length increments compared to control groups. Lesions in other hypothalamic areas including the nucleus anterior tuberis, nucleus lateralis tuberis and the nucleus recessus lateralis had no consistent effect on growth rates or serum GH levels. The present results suggest that the NPP is concerned with the inhibition of GH secretory function in the goldfish.

Secretory activity of Poecilia latipinna (Teleostei) pituitary in vitro: rostral pars distalis and proximal pars distalis

The ultrastructure of each adenohypophysial secretory cell type was examined in pituitaries of adult female mollies (Poecilia latipinna) after various periods in vitro, and with varied medium osmotic pressure (OP), Na+, and Ca2+ concentrations. Prolactin (PRL) cells were markedly activated by 18 hr, and after 7 or 14 days were almost totally degranulated, with massive arrays of Golgi and RER. Reduction in OP, but not Na+ or Ca2+, caused an additional activation of PRL cells after periods of 18 hr or longer. Corticotroph (ACTH) cells became noticeably activated by 4 hr, and were possibly affected by OP, but not Na+ or Ca2+. Growth hormone (GH) cells were activated by 6 hr, and after 18 hr were quite degranulated with extensive arrays of RER. OP had no effect on GH cells before 3 days, when reduced OP appeared to cause an additional activation, with the appearance of large irregular secretory granule (SG)-like inclusions. Na+ and Ca2+ again had no effect. Gonadotrophic (GtH) cells appeared to be little affected by in vitro incubation; however, the very active cells from vitellogenic fish underwent a reduction in dilated RER after prolonged culture. Thyrotrophic (TSH) cells gradually became activated in vitro, but the response again varied with the sexual condition of the fish. Neither GtH nor TSH cells were affected by OP, Na+, or Ca2+. The findings are discussed in relation to hypothalamic control, via releasing/inhibiting factors, of adenohypophysial cell activity.

Presence and distribution of radioimmunoassayable LHRH in the European eel, Anguilla anguilla

Eel brain contains a factor which exhibited a complete cross-reaction in a radioimmunoassay (RIA) for mammalian luteinizing hormone-releasing hormone (LHRH) and which exhibited a behavior similar to that of synthetic mammalian LHRH on Sephadex G-25. Eel brains contained 920 +/- 135 pg of this LHRH-like immunoreactive factor (IR-LHRH); 75% of immunoreactivity was present in the di- and mes-encephalon, 21% in the telencephalon and 4% in the hind-brain. Thus, IR-LHRH in eel brain exhibited a much wider distribution than in mammalian brain. Pituitary also contained a large amount of IR-LHRH (304 +/- 69 pg) at a concentration even higher than in median hypothalamic lobe; this result is probably related to the fact that, in teleost, median eminence is included into the pituitary. An immunoreactive substance was also present in the blood. In conclusion, the lack of sexual development and the low gonadotropic function in the eel, are apparently not related to an absence of LHRH.