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

Effects of season, pinealectomy, and blinding, alone and in combination, on hypothalamic monoaminergic activity in the teleost Channa punctatus (Bloch)

In Channa punctatus, day-night variations in hypothalamic 5-HT (serotonin) and monoamine oxidase (MAO) activity were noticed in preparatory but not other phases (prespawning and postspawning) of the annual reproductive cycle. Hypothalamic MAO, 5-HT, and norepinephrine (NE) activity was found to be high in the prespawning phase and low in the postspawning phase. Dopamine (DA) activity, on the other hand, was high in the postspawning season and low in the prespawning phase. Pinealectomy caused season-dependent effects on hypothalamic monoaminergic activity, with a significant increase in serotonergic activity and a significant reduction in MAO activity at midscotophase during the preparatory phase (March) but not in the prespawning (May-June) or postspawning (September) phases. Hypothalamic catecholaminergic (CA) activity was not influenced by pinealectomy during any of the seasons. To determine whether or not the photoperiodic influences on daily variations of 5-HT and MAO in the preparatory phase are mediated via pineal and/or lateral eyes, fish were pinealectomized and/or blinded in January, when there is no rhythm, and sacrificed in February, when a day-night variation normally sets in. The day-night difference in 5-HT content and activity and MAO activity was not abolished by pinealectomy or blinding alone; but the combination (pinealectomy + blinding) obliterated the daily variation only in 5-HT content and in MAO activity. However, pinealectomy and blinding, alone or in combination, caused a significant elevation of 5-HT activity (not its level) and a significant decrease in MAO activity at midscotophase, with the combination having an additive effect. Hypothalamic CA content or activity was not affected by these regimes. The results show that photoperiodic influence on the daily pattern of 5-HT and MAO activity is mediated through and by the interaction of the pineal and lateral eyes.

Modulation of neuropeptide-stimulated pituitary hormone secretion in hatchling turtles

Neuropeptides that have relatively narrow actions on mammalian pituitary secretion may have divergent effects on pituitary hormone secretion in ectothermal vertebrates. In turtles, secretion of both thyrotropin (TSH) and growth hormone (GH) can be stimulated in vitro by thyrotropin-releasing hormone (TRH) and by members of corticotropin-releasing hormone (CRH) and growth hormone-releasing hormone (GHRH) peptide families. To determine if these neuropeptides share common modes of action, and to study other potential regulators of the turtle pituitary, somatostatin-14 (SRIH) and monoamines were tested for direct effects on in vitro basal and neuropeptide-stimulated TSH and GH secretion. Pituitary glands from young turtles (Pseudemys scripta) were cultured in the presence of 25 nM TRH, ovine CRH, or rat GHRH with or without SRIH. Glands were incubated for several 2-hr periods in medium alone or in medium containing peptides. Preincubation for 4 hr with SRIH (6 or 60 nM) significantly reduced basal and TRH-stimulated TSH and GH output (SRIH present during entire incubation). In another experiment, basal hormone secretion was reduced when SRIH (60 nM) was present only during the 2-hr basal period; however, reduction of TSH and GH responses to TRH required the presence of SRIH (60 nM) during the basal period and the period of stimulation. TSH responses to 25 nM oCRH and rGHRH and GH responses to rGHRH were significantly reduced by preincubation with 60 nM SRIH. The biogenic amines, dopamine (DA), serotonin (5HT), and norepinephrine (NE) (50 or 500 nM) were tested for possible direct actions on basal and neuropeptide-stimulated pituitary TSH and GH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Effects of intratesticular injections of estradiol and gonadotropin-releasing hormone (GnRHA, HOE 766) on plasma androgen levels in intact and hypophysectomized Torpedo marmorata and Torpedo ocellata

The effect of a gonadotropin-releasing hormone analog (GnRHA, HOE766) was studied in hypophysectomized elasmobranch fish Torpedo marmorata and T. ocellata. In addition, estradiol (E2) effects were studied in intact and hypophysectomized (HPX) animals. Plasma androgen concentrations were measured 2 or 6 hr after GnRHA (100 ng or 10 micrograms) or 6 hr after E2 (10(-9) or 10(-6) M) intratesticular injections. Both GnRHA and E2 induced the increase of plasma androgen levels in HPX fish. E2 also enhanced androgen levels in intact animals. It is concluded that GnRH-like substances and E2 may modulate testicular activity in elasmobranch fish.

Dopamine inhibition of gonadotropin and alpha-melanocyte-stimulating hormone release in vitro from the pituitary of the goldfish (Carassius auratus)

The purpose of the present investigation was to examine the receptor specificity of dopamine inhibition of gonadotropin (GtH) and alpha-melanocyte-stimulating hormone (alpha-MSH) release from the goldfish (Carassius auratus) pituitary in vitro. Pars distalis (PD) and neurointermediate lobe (NIL) fragments of the goldfish pituitary were superfused in vitro under various experimental paradigms; eluate from PD and NIL fragments was analyzed for (GtH) and (alpha-MSH), respectively. Spontaneous GtH release from PD fragments was relatively constant over 6 hr; continuous superfusion with dopamine reversibly inhibited spontaneous GtH release with an estimated ED50 of 10(-4.4) M. Domperidone, a specific D-2 receptor antagonist, reversed the inhibitory action of dopamine and increased spontaneous GtH release. Acute treatment of PD fragments with salmon GnRH (sGnRH) stimulated GtH release; dopamine inhibited GtH release from similarly treated fragments with an ED50 of 10(-7.5) M. The spontaneous release of alpha-MSH from NIL fragments was relatively constant over 6 hr; continuous superfusion with dopamine reversibly inhibited this release with an ED50 of 10(-7.2) M. Acute treatment of NIL fragments with thyrotropin-releasing hormone (TRH) caused acute dose-related increases in alpha-MSH release with an ED50 of 10(-8.2) M; dopamine reversibly inhibited alpha-MSH release from similarly treated fragments with an ED50 of 10(-7.7) M. Both stereoisomers of apomorphine, a dopamine agonist, inhibited GtH release from PD fragments treated with sGnRH; in contrast, alpha-MSH release from NIL fragments treated with TRH was stereospecifically inhibited by (-)-apomorphine, but not by (+)-apomorphine. Domperidone reversed (ED50 = 10(-6.6) M) dopamine (10(-6.3) M) inhibition of GtH release from PD fragments treated with sGnRH. In NIL fragments, the inhibitory action of dopamine (10(-6.3) M) was reversed by domperidone (ED50 = 10(-5.5) M), which restored the acute alpha-MSH release response to TRH. These results suggest the involvement of a low-affinity dopamine/neuroleptic receptor in dopamine inhibition of GtH and alpha-MSH release from the pituitary of the goldfish.

Hypothalamic control of prolactin release in the rainbow trout,Salmo gairdneri: in vitro studies

Hypothalamic control of prolactin (PRL) release in immature rainbow troutSalmo gairdneri was investigated using anin vitro perifusion system of the rostral pars distalis. Hypothalamic extract of trout induced a dose-dependent stimulation of PRL release. A similar effect was observed when infusing the medium from a 24h static incubation of the hypothalamus. Extracts from different control tissues (muscle, liver, gut) did not changein vitro release, thus confirming the specificity of this stimulatory effect. Hypothalamic extract from adult male rat, known to contain PRL release inhibiting factors, stimulatedin vitro PRL secretion in rainbow trout. This suggests that PRL cells are predominantly influenced by PRL releasing factors. Measurement of TRH and serotonin content in trout hypothalamus indicated consistent physiological levels of these two factors. HPLC studies of hypothalamic extract showed that immunoreactive - TRH eluted at the same place as labelled TRH standard. Moreover, pizotifen, a serotonin antagonist, partially inhibited the stimulation observed with trout hypothalamic extract. These results suggest that, in immature rainbow trout, PRL release is under stimulatory hypothalamic control and that serotonin and probably TRH play a major role in this control.

Hypothalamic peptides influencing growth hormone secretion in the goldfish,Carassius auratus

In vivo andin vitro techniques were used to examine the influence of various vertebrate peptides on growth hormone (GH) secretion in the goldfish. Tetradecapeptide somatostatin (SRIF-14) was found to inhibit GH secretionin vitro from perifused pituitary fragments, whereas similar concentrations of a salmonid SRIF peptide (sSRIF-25) did not affect GH secretion from the goldfish pituitary fragments. This indicates that SRIF receptors on the goldfish pituitary are very specific for SRIF-14-like peptides. Salmon gonadotropin (GTH)-releasing hormone (sGnRH) was found to elevate serum GH levels in male goldfish. The dopamine antagonist pimozide alone or injected in combination with sGnRH did not influence serum GH levels, although injection of pimozide alone significantly elevated serum GTH levels, in addition to potentiating the effects of sGnRH on GTH secretion. sGnRH stimulated GH secretion from goldfish pituitary fragmentsin vitro, indicating that sGnRH acts directly at the level of the pituitary to stimulate GH secretion in the goldfish. These results suggest that GnRH may also function as a GH-releasing factor in the goldfish, although the release-inhibitory factors for GH and GTH secretion do appear to be separate and distinct. Two human GH-releasing hormone (hGHRH) peptides were found to be ineffective in altering GH secretionin vitro from the perifused pituitary fragments. Consequently, a role for a mammalian GHRH-like peptide in the hypothalamic regulation of GH secretion in the goldfish remains questionable.

Effects of lamprey gonadotropin-releasing hormone and analogs on steroidogenesis and spermiation in male sea lampreys

The biological activities of lamprey GnRH and analogs were determined in the adult male lamprey,Petromyzon marinus. Two successive injections of lamprey GnRH at 0.005, 0.065 or 0.163 μg/g body weight or [D-Ala(6),Pro(9)-NHEt] mammal GnRH at 0.05 μg/g stimulated plasma estradiol and progesterone levels in adult male lampreys undergoing final maturation. In these experiments, a lamprey GnRH putative antagonist, [D-Phe(2,6),Pro(3)] lamprey GnRH, at 0.075, 0.150 or 0.3 μg/g stimulated plasma estradiol but not progesterone levels. Four successive injections of lamprey GnRH at 0.05 or 0.1 μg/g stimulated plasma progesterone and spermiation. [D-Phe(2,6),Pro(3)] lamprey GnRH at 0.05 or 0.1 μg/g depressed plasma progesterone levels and inhibited spermiation. In contrast, lamprey GnRH analog, [D-Ala(6),Pro(9)-OH free carboxylic acid] lamprey GnRH, at 0.05 or 0.1 μg/g stimulated plasma progesterone levels and inhibited spermiation. In summary, lamprey GnRH is biologically active in stimulating the pituitary-gonadal axis in adult male lampreys as determined by steroidogenesis and spermiation.

In vitro binding characteristics of [3H]spiperone to the pituitary of the goldfish (Carassius auratus)

Homogenates of the pituitary of the goldfish (Carassius auratus) were incubated with [3H]spiperone under various experimental paradigms to evaluate the binding characteristics of the goldfish pituitary dopamine receptor. Binding was tissue specific as binding of [3H]spiperone to goldfish pituitary was greater than other tissue types examined, and the magnitude of binding was found to be dependent on pituitary (protein) content; also, specific binding was heat labile. Association was rapid and binding was reversible (dissociable) by addition of excess competing ligand (domperidone, a specific dopamine D2 receptor antagonist); the half-life (t1/2) of dissociation was 9.2 min and the estimated dissociation rate constant (k-1) was 7.56 x 10(-2) min-1); as well, the association rate was temperature dependent. Binding was saturable; saturation analysis using [3H]spiperone indicated a single class of binding sites with an estimated dissociation constant (Kd) and capacity of 7.39 +/- 1.23 x 10(-6) M and 31.56 +/- 2.72 x 10(-9) mol/mg protein, respectively. [3H]Spiperone binding was displaceable; displacement analysis using unlabeled domperidone indicated a single class of binding sites with estimated Kd and capacity of 2.94 +/- 0.54 x 10(-6) M and 19.47 +/- 3.12 x 10(-9) mol/mg protein, respectively. Binding was specifically inhibited by various dopamine antagonists and agonists. The density of binding sites differed significantly between regions of the goldfish pituitary; the number of binding sites in the pars distalis and neurointermediate lobes was estimated as 38.89 +/- 2.07 x 10(-9) mol/mg protein vs 109.45 +/- 25.33 x 10(-9) mol/mg protein, respectively; while the Kd's estimated as 3.73 +/- 0.248 x 10(-6) M vs 4.1 +/- 1.21 x 10(-6) M, respectively, were not significantly different. These data agree with previous in vivo and in vitro findings of the biological actions of dopamine agonists and antagonists in modifying gonadotropic hormone release in the goldfish and represent the first demonstration of the existence and binding characteristics of a dopamine/neuroleptic receptor in the pituitary of a nonmammalian vertebrate.

Relationship between serum growth hormone levels and the brain and pituitary content of immunoreactive somatostatin in the goldfish, Carassius auratus L

In this study, the relationships between endogenous brain and pituitary immunoreactive somatostatin (irSRIF) and circulating growth hormone (GH) levels in the goldfish were examined using two approaches. First, the amount of irSRIF in extracts of the pituitary gland and various brain regions was measured by radioimmunoassay several times throughout the year and was compared to serum GH levels at each time. The amounts of irSRIF in extracts of the pituitary gland, hypothalamus, and telencephalon were found to be inversely related to seasonal changes in serum GH levels, such that irSRIF was highest in these regions when serum GH levels were lowest (November and February). Conversely, irSRIF in these regions was lower in May, June, and July when serum GH levels were highest. These results suggest that endogenous irSRIF in the pituitary and forebrain may participate in the regulation of seasonal changes in serum GH levels in the goldfish. In extracts from other brain regions (thalamus + midbrain and cerebellum + medulla), some changes in the amount of irSRIF were observed among the various sample times, but these variations were not related to changes in serum GH levels. In a second set of experiments, the origin of irSRIF fibers innervating the goldfish pituitary gland was examined by using brain lesioning techniques to destroy regions of the forebrain known to contain irSRIF perikarya and fibers, and subsequently measuring the amount of irSRIF in the pituitary gland. Lesions in the preoptic area of the forebrain resulted in increased serum GH levels concomitant with a decrease in pituitary irSRIF content. This provides direct evidence that the preoptic area is the origin of a somatostatinergic projection inhibiting GH secretion from the goldfish pituitary. Lesions centered in the nucleus lateral tuberis (NLT) pars anterioris did not influence serum GH levels or the pituitary content of irSRIF. In contrast, more posterior lesions centered in the NLT pars posterioris (NLTp) resulted in a dramatic reduction in the amount of irSRIF in the pituitary. This suggests that the majority of irSRIF projections to the goldfish pituitary pass through the area destroyed by the lesion centered in the NLTp; it is also possible that perikarya within this area may be the origin of at least some of the irSRIF-containing fibers in the goldfish pituitary. Together, results from the present study provide evidence of a functional relationship between circulating levels of GH and endogenous brain and pituitary irSRIF in the goldfish.

The effects of TRH on plasma thyroid hormone levels of rainbow trout (Salmo gairdneri) and arctic charr (Salvelinus alpinus)

Plasma levels of L-thyroxine (T4) and 3,5,3'-triiodo-L-thyronine (T3) were measured in 2- to 4-year-old rainbow trout (Salmo gairdneri) and arctic charr (Salvelinus alpinus) intraperitoneally injected with 0.7% NaCl alone (controls) or 0.7% NaCl containing synthetic thyrotropin-releasing hormone (TRH). Blood was obtained from terminally sampled fish or from serially sampled fish cannulated in the dorsal aorta. In trout, TRH (1 microgram/g body wt) significantly raised plasma T4 to maximal values at 2 hr. A dose of 0.1 microgram/g was ineffective in trout but doses below this level elevated plasma T4 in arctic charr. In charr starved for 2 or 3 weeks the plasma T4 response to TRH appeared blunted. There was no consistent influence of TRH on plasma T3 under any condition for either species. It is concluded that TRH influences the salmonid thyroid system at some level to elevate plasma T4, possibly through thyrotroph stimulation.

Several hypothalamic peptides stimulate in vitro thyrotropin secretion by pituitaries of anuran amphibians

The effects of several hypothalamic peptides on hormone secretion by pituitaries of three species of anuran amphibians were investigated using in vitro techniques. Secretion of thyrotropic bioactivity (designated thyrotropin or TSH) was quantified by bioassay of the pituitary incubation medium using thyroxine (T4) production by paired thyroids from the same animals. Pituitaries from adult male Rana pipiens were cultured in medium alone, 10 or 100 ng/ml thyrotropin-releasing hormone (TRH), 1000 ng/ml ovine corticotropin-releasing hormone (oCRH), or 300 ng/ml synthetic mammalian gonadotropin-releasing hormone (mGnRH) (these represent approximately equimolar doses) for two 2-hr incubation periods. TSH secretion by control glands was nondetectable, but glands exposed to TRH increased their secretion of TSH in a dose-dependent manner. Both oCRH and mGnRH also stimulated significant increases in TSH. oCRH produced greater output of TSH than did the other two peptides and mGnRH was less active than TRH. Secretion of immunoreactive gonadotropin (GtH) was increased by mGnRH, but not by the other two peptides. Pituitaries from two other anuran species, Hyla regilla and Xenopus laevis, also responded to 100 ng/ml TRH by releasing TSH. These results provide the first unequivocal evidence that TRH can act directly on the anuran amphibian pituitary to stimulate the secretion of TSH, and suggest that the presence of functional TRH receptors on pituitary thyrotropes may be of greater phylogenetic antiquity than has been assumed previously. Furthermore, these data suggest the potential for multihormonal control of TSH secretion in frogs.

Peptidergic innervation of the adrenocorticotropic hormone (ACTH)- and growth hormone (GH)-producing cells in the pars distalis of the sea bass (Dicentrarchus labrax)

Due to its unique organization, the teleost pituitary is an ideal model in which to investigate the relationship of the nervous system with the pituitary endocrine cells. A light microscope immunocytochemical study of the sea bass pituitary revealed six different neuropeptides in nerve fibers which projected into the pituitary neurohypophysis and bordered the adenohypophysial cells. Double staining showed separate nerve fibers immunoreactive for corticotropin-releasing factor (CRF), vasotocin (VT), somatostatin (SRIF), growth hormone-releasing factor (GRF), and neurotensin (NT) in the vicinity of the adrenocorticotropic hormone-releasing cells (ACTH-cells) in the rostral pars distalis (PD). In the proximal PD cholecystokinin (CCK)-, SRIF-, GRF-, and VT-immunoreactive fibers penetrated between the growth hormone-releasing cells (GH-cells). These results suggest a possible role for CCK, GRF, SRIF, and VT in the modulation of GH-cell activity, while the synthesis and/or secretion of the ACTH-cells might be affected by the release of VT, CRF, SRIF, GRF, and NT.

Control of prolactin secretion in the teleost Oreochromis mossambicus: effects of water acidification

Prolactin secretion is stimulated markedly in fish exposed to water of pH 4. This phenomenon was used to study the control of prolactin secretion. Activation occurs irrespective of changes in plasma osmolarity or plasma sodium and calcium concentrations. After acute acidification of the water, which leads to a substantial fall in plasma osmolarity and plasma electrolyte levels, the activation of the prolactin cells is less marked than after gradual acidification of the water, when plasma osmolarity, plasma sodium, and plasma total and ionic calcium levels are not noticeably affected. When fish bearing an implanted rostral pars distalis of the pituitary gland are exposed to water of pH 4, both the in situ prolactin cells and the prolactin cells of the implant become activated only when the drop in water pH is acute and followed by a reduction in plasma osmolarity and electrolyte levels. When the rate of reduction of the pH is slow and not changing plasma osmolarity or sodium and calcium levels, the in situ prolactin cells are stimulated, but not those of the implants. We conclude that the activation of the prolactin cells in situ in fish in acid water is not mediated by reductions in plasma osmolarity, plasma sodium, total calcium, or ionic calcium, but by hypothalamic mechanisms. The drop in plasma osmolarity and electrolytes probably reflects excessive osmoregulatory stress and this may hamper instead of stimulate the response of the prolactin cells to osmoregulatory disturbance. The physiological significance of the in vitro activation of prolactin cells by reduced ambient osmolarity is unclear.

The intracellular regulation of prolactin cell function in the rainbow trout, Salmo gairdneri

The mechanisms for regulation of prolactin (PRL) secretion in the rainbow trout were investigated. The inhibitory action of dopamine on PRL release in vitro was enhanced by GTP and dopamine also reduced pituitary cAMP content. Forskolin increased both PRL release and cAMP content in vitro, but this effect was prevented by dopamine and did not occur in Ca2+-free medium. The cAMP analogue, dbcAMP increased PRL synthesis in low Ca2+ medium, though release was not significantly affected. The calcium ionophore, A23187, increased PRL release, but this effect was not seen with flunarizine, a voltage-dependent Ca2+-channel blocker. The calmodulin blocker, pimozide, increased PRL synthesis and pituitary PRL content in vivo and a second calmodulin blocker, trifluoperazine, also increased PRL synthesis, though not percentage release, in vitro. Both drugs elevated pituitary cAMP levels. These results indicate an involvement of agonist-dependent Gi proteins, Ca2+, calmodulin, and cAMP in the control of PRL cells in this teleost.

Immunohistochemical investigation of the pituitary of the sturgeon (Acipenser baeri, Chondrostei)

An immunohistochemical study of the sturgeon (Acipenser baeri) pituitary was undertaken using antisera directed against hormones from various classes of vertebrates, including the only pituitary hormone available from sturgeon, gonadotrophin. A positive reaction was obtained after application of antisera towards the following hormones 1-24 synthetic ACTH (1-24 ACTH), melanophore stimulating hormone (MSH), ovine prolactin (oPRL), ovine growth hormone (oGH), salmon growth hormone (sGH), carp gonadotrophin (cGTH) and its beta subunit (βcGTH), sturgeon gonadotrophin (aciGTH), carp thyrotrophin (cTSH) and β subunit of the human thyrotrophin (βhTSH). The results demonstrate that, in general, the sturgeon pituitary resembles that of teleosts as regards the distribution of the different cell types: ACTH and PRL cells in the rostral pars distalis, GTH, TSH and GH cells in the proximal pars distalis and MSH and PAS-cells in pars intermedia. In addition to the topographical organization of the sturgeon pituitary, this study provides data on the immunological relationships between sturgeon pituitary hormones and those of other vertebrates.

Effects of TRH on hormone release from pituitaries of the lizard, Anolis carolinensis

The ability of thyrotropin-releasing hormone (TRH) to stimulate thyrotropin (TSH) from pituitaries of adult male lizards, Anolis carolinensis, was tested in vivo and in vitro. TSH output by pituitaries in vitro was determined by bioassay of the incubation medium using in vitro T4 output by thyroids from the same lizards. Pituitaries incubated without TRH had no detectable TSH secretion during two consecutive (2 or 3 hr) periods of incubation. Incubation in 10 or 100 ng/ml TRH for 2 or 3 hr significantly augmented release of TSH bioactivity in a dose-dependent manner. Pituitaries taken from goitrogen (100 micrograms methimazole/day for 10 days)-treated lizards had elevated basal TSH secretion but did not respond to TRH. TRH injection in vivo (5 micrograms/hr for 10 hr) appeared to stimulate acute release of MSH activity as judged by darkening of skin color after each injection, and plasma T4 was significantly elevated at the end of treatment. These results provide additional evidence that the reptilian thyrotrope has functional TRH receptors and the TSH-stimulating activity of the tripeptide along with its effects on other pituitary cells was present at an early stage of reptilian evolution.

Hypothalamus-hypophysis and testicular GnRH control of gonadal activity in the frog, Rana esculenta: seasonal GnRH profiles and annual variations of in vitro androgen output by pituitary-stimulated testes

The binding of a gonadotrophin-releasing hormone (GnRH) long acting analog (GnRHA), D-Ser (But)6,Pro9-NEt GnRH (HOE 766), to pituitary and testicular extracts and the presence of GnRH-like material in testes and hypothalamuses were measured in the frog, Rana esculenta. Also, the cellular localization of immunoreactive GnRH was investigated in testes by immunohistochemical staining. Furthermore, lyophilized preparations of pituitary crude homogenates from animals caught monthly were tested in vitro for their ability to stimulate androgen production by December testes. Satisfactory results on specific 125I-GnRH binding were difficult to obtain in view of its low binding capacity. Moreover, binding in testicular homogenates was of the same order of magnitude (about 2%) as that found in pituitaries. In a cospecific radioimmunoassay for GnRH nonapeptide, both hypothalamic and testicular extracts gave displacement parallel to the standard curve. Immunoreactive GnRH did not significantly fluctuate in hypothalamuses, while it peaked in testes during December and July. Immunoreactive GnRH was evidenced in June and September testes employing immunohistochemical staining. In particular, the interstitial cells and the Sertoli cells were faintly stained. Testes of December animals stimulated by February pituitaries produced larger quantities of androgens as compared with testes stimulated with hypophyseal preparations from the remaining periods of the year. In conclusion, the present results are consistent with the idea that seasonal changes of the hypothalamus-hypophyseal activity play an important role in regulating the hormonal response in vertebrate testes. Moreover, we report that, in addition to rats, GnRH-like material is present in frog testes and for the first time it has been shown that such putative intratesticular material undergoes seasonal fluctuations in a vertebrate.

Thyroxine and triiodothyronine in plasma and thyroids of the neotenic and metamorphosed axolotl Ambystoma mexicanum: influence of TRH injections

Circulating levels of T3 and T4, as well as T3 and T4 content of the thyroid glands were measured by radioimmunoassay in the neotenic and metamorphosed axolotl Ambystoma mexicanum. In the two experiments which were performed plasma T4 concentrations were more elevated in metamorphosed axolotls, especially in the first experiment (2.12 +/- 0.40 ng/ml vs. 369 +/- 30 pg/ml). T3 plasma values which were only estimated in the second experiment were about five times higher in metamorphosed animals (63.2 +/- 7.4 pg/ml vs. 12.5 +/- 0.8 pg/ml). Also the thyroid hormone content of the glands was higher after metamorphosis. Nevertheless the neotenic gland still contained considerable amounts of T3 (14.7 +/- 1.8 ng and 48.3 +/- 4.8 ng/thyroid, respectively, in the first and second experiment) and T4 (530 +/- 61 ng; 2173 +/- 291 ng/thyroid). Because of the higher T3/T4 ratio found in the plasma compared to the thyroid gland, it was suggested that circulating T3 may be derived partly from peripheral T4 conversion, mainly after metamorphosis. An intravenous injection of 10 micrograms synthetic TRH was able to induce a very significant increase of the plasma T4 concentration (which was maintained during 24 hr) in the metamorphosed axolotls of the first experiment, however, not in those of the second experiment nor in the neotenic animals. Following an injection of 10 mU bovine TSH (first experiment) circulating levels of T4 were raised in both groups. The opposing TRH results could be related with the different control levels of T4 in the two experiments. However, the results indicate that TRH is capable of functioning as a possible thyrotropin-releasing factor in the metamorphosed axolotl.

Effect of osmotic pressure on prolactin release in rainbow trout: in vitro studies

To investigate a possible effect of osmotic pressure on prolactin (PRL) release in rainbow trout, we developed a technique for in vitro perifusion of trout pituitaries. Changes in osmotic pressure similar to those observed in fish plasma during transfer experiments did not induce significant modifications of PRL release. In contrast, high-amplitude variation of osmotic pressure resulted in clear modifications of PRL secretion: hyperosmotic medium caused a reduction in PRL release, while infusion of hyposmotic medium induced a transitory increase in PRL release. By using different concentrations of mannitol, we found that the modifications of prolactin secretion could not be ascribed to alterations of the ionic composition of the medium but actually resulted from variations in the osmotic pressure of the incubation medium. In further experiments osmotic pressure was decreased from 300 to 220 mOsm/kg or from 400 to 300 mOsm/kg; a similar transitory increase in PRL release was observed. Measurement of gonadotropin (GtH) in the perifusion effluent medium showed that PRL and GtH secretion followed similar patterns. Thus, our results suggest a possible mechanical effect of wide changes in osmotic pressure on pituitary cell membranes. These data indicate that the rainbow trout differs notably from nonsalmonid teleost species thus far studied in the lack of sensitivity of its PRL cells to osmotic pressure.

Seasonal and daily variations in hypothalamic monoamine levels and monoamine oxidase activity in the teleost Channa punctatus (Bloch)

In Channa punctatus, a significant daily variation in hypothalamic 5-HT level and monoamine oxidase (MAO) activity was noticed in preparatory phase (February), but not in prespawning (May) or postspawning (November) phases. Hypothalamic dopamine (DA) and noradrenaline (NA), on the other hand, showed marked daily variation in their levels during all the three seasons with peak values in the photophase. The overall activity of MAO (mean +/- SEM on 24-hr period) increased from November to May through February, whereas the 5-HT content which was high in November decreased during February and May. The NA and DA levels were low in November and February and high in May. The catecholamine (CA) content and MAO activity increased with increasing photoperiod and temperature which is indicative of an enhanced CA metabolism.

Hypothalamo-pituitary-interrenal responses to opioid substances in the trout. I. Effects of morphine on the release in vitro of corticotrophin-releasing activity from the hypothalamus and corticotrophin from the pituitary gland

The effects of morphine and naloxone on the secretion in vitro of corticotrophin (ACTH) and and corticotrophin-releasing factor (CRF) by the pars distalis and hypothalamus, respectively, have been studied in the trout. The spontaneous in vitro secretion of corticotrophin by the pars distalis is depressed significantly by the addition of high concentrations of morphine (10(-6)-10(-7) mol/litre) to the incubation medium. The effect is naloxone reversible. Morphine does not influence the response of the pituitary tissue to exogenous CRF41, suggesting that the inhibitory influence of the opiate is exerted primarily on the CRF nerve terminals within the pars distalis and not on the corticotrophs. At considerably lower concentrations (10(-10)-10(-8) mol/litre) morphine stimulates the release of CRF from the isolated trout hypothalamus in vitro. Its effects are dose-dependent and antagonized by naloxone. The results suggest that two anatomically and pharmacologically distinct populations of opioid receptors mediate opposing actions of morphine on the hypothalamo-pituitary-corticotrophic system in the trout.

Prolactin and growth hormone secretion during long-term incubation of the pituitary pars distalis of mature chum salmon, Oncorhynchus keta

The effects of hypotonicity of the medium on prolactin (PRL) and growth hormone (GH) release from the pars distalis (PD) of mature chum salmon pituitary were examined during culture for 4 days. Large amounts of PRL and GH were released during the first 6 hr and secretion decreased rapidly thereafter. The amounts of GH released as well as the residual content in PD were 10 times greater than those of PRL. There was no difference in PRL or GH release between the PD incubated in isotonic medium (325 mOsm) and those in hypotonic medium (250 mOsm) even during the first 6 hr. Female PD secreted more PRL than male PD during the first 6 hr, whereas a sex difference was not observed in GH release. Considerable amounts of both PRL and GH remained in the PD at the end of the culture period. The amount of PRL released during 4 days of culture from female PD (37-39%) was greater than that from male PD (23-26%), and the amount of GH released was less in females (28-32%) than that in males (53-54%). Release of PRL and GH appears to be mainly under stimulatory hypothalamic control in the mature chum salmon.

Central GABAergic innervation of the pituitary in goldfish: a radioautographic and immunocytochemical study at the electron microscope level

The GABAergic innervation of the goldfish pituitary was studied at the light and electron microscope levels by means of radioautography after in vitro incubation in tritiated gamma-aminobutyric acid (GABA) and immunocytochemistry using antibodies against GABA. Following incubation of pituitary fragments in a medium containing tritiated GABA, a selective uptake of the tracer was observed within the digitations of the neurohypophysis. Silver grain clusters were also observed in the adenohypophyseal tissue. At the electron microscope level, this uptake was found to correspond to nerve endings containing small clear and dense-core vesicles. These labeled profiles were located mainly in neurohypophyseal digitations in close apposition with the basement membrane separating the neurohypophysis from the adenohypophysis. However, they were also encountered in direct contact with most adenohypophyseal cell types in the different lobes. These results were confirmed by immunocytochemical data demonstrating the presence of numerous GABA immunoreactive fibers in both anterior and neurointermediate lobes. They were found either in the digitations of the neurohypophysis or in the adenohypophysis in direct contact with the glandular cells with a distribution and an ultrastructural aspect similar to those observed by radioautography. These data demonstrate that the pituitary of teleosts receives a massive GABAergic innervation. Although physiological data providing a functional significance for such an innervation are lacking, the present study suggests that, as already documented in mammals, GABA may be involved in the neuroendocrine regulation of pituitary functions in teleosts.

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

The proximal pars distalis (PPD) of the pituitary of the African catfish, Clarias gariepinus, was studied with immunocytochemical methods at the ultrastructural level. Anti-serum raised against synthetic mammalian luteinizing hormone-releasing hormone (LHRH) was applied on Lowicryl-embedded pituitaries and the antigenic sites were visualized with protein A-gold. In nerve fibers contacting the gonadotropic cells, granulated vesicles with a diameter of 90-120 nm were labeled after this procedure, whereas the glandular cells were not labeled. For the immunocytochemical demonstration of dopaminergic fibers, the preembedding method was performed on Vibratome sections, using highly specific antibodies against dopamine. Immunoreactivity was restricted to fibers containing granulated vesicles with a diameter of approximately 80 nm and terminating on gonadotropic cells. The present data support the results of earlier in vivo and in vitro studies on the catfish pituitary, indicating a dual neuroendocrine regulation of the gonadotropic cells.

Effects of thyrotropin-releasing hormone in vitro on thyrotropin and prolactin release from the turtle pituitary

Effects of synthetic thyrotropin-releasing hormone (TRH) on thyrotropin (TSH) and prolactin (PRL) release by hemipituitaries of adult turtles, Chrysemys picta, were studied in an in vitro superfusion system. Significant increases in the rates of secretion of both immuno-reactive TSH and PRL occurred at doses between 0.01 and 10 ng/ml TRH. TSH secretion increased acutely by two-, to sixfold over nonstimulated secretion levels; responses tended to decline after many hours of continual stimulation, but output remained elevated above baseline in most cases. PRL secretion increased, parallel to TSH secretion during TRH stimulation. No significant difference was found in secretion rates between males and females, and no clear relationship between TRH responsiveness and reproductive stage was evident. These data provide the first direct evidence for the stimulation of TSH secretion by TRH in a reptile and confirm earlier reports that TRH stimulates the release of PRL in the turtle. Although previous in vivo studies indicated that TSH secretion was not affected by TRH in turtles, the present data indicate that the dose sensitivity of the chelonian gland is comparable with that of mammalian and avian pituitaries. Evidence for the role of TRH in endogenous TSH regulation is still lacking in reptiles but the present data provide evidence for functional TRH receptors on the chelonian thyrotrope and, hence, argue against the hypothesis that TSH stimulating activity of TRH evolved relatively recently in association with endothermy.

Increased gonadotropin levels in goldfish do not result in alterations in circulating thyroid hormone levels

To determine whether increases in gonadotropin levels are capable of altering thyroid function in goldfish, plasma thyroid hormone levels were measured following induced changes in endogenous gonadotropin secretion and injection of carp gonadotropin. Radio-frequency lesions placed in the nucleus preopticus periventricularis or monosodium-L-glutamate-induced lesions of the posterior nucleus lateralis tuberis (NLT) of the hypothalamus were capable of stimulating significant increases in plasma gonadotropin levels, but were without effect on plasma triiodothyronine (T3) or thyroxine (T4) at time intervals ranging from 5 hr to 10 days. Likewise, injections of a superactive analog of gonadotropin-releasing hormone resulted in profound increases in gonadotropin levels without associated changes in thyroid hormones. No changes in the circulating levels of T4 or T3 were observed in response to injection of purified carp gonadotropins whereas injection of bovine thyrotropin or carp pituitary extracts stimulated significant increases in T4. Radiofrequency lesions of the pituitary stalk or of the anterior NLT also resulted in significant increases in circulating levels of T4, but not of T3, at 10 and 30 hr postlesion. These results demonstrate that direct acute stimulation of circulating thyroid hormone levels is not an intrinsic action of endogenous goldfish gonadotropin and that activation of the reproductive system, leading to ovulation in some cases, is without effect on blood total thyroid hormone levels. Additionally, these results confirm that hypothalamic inhibition of the pituitary-thyroid axis exists in this teleost fish and demonstrate that interruption of this inhibition results in a time-dependent, high-magnitude increase in circulating thyroxine levels.

Immunocytochemical evidence for alpha-melanocyte-stimulating hormone in the hypothalamus of the frog Rana esculenta

The distribution of immunoreactive alpha-melanocyte-stimulating hormone (alpha-MSH) within the brain of the frog, Rana esculenta, has been studied on adjacent serial sections using an indirect immunofluorescence technique. Immunoreactive cell bodies are found in the anterior part of the preoptic nucleus and in some ventral subependymal cerebrospinal fluid-contacting elements, and in the nucleus infundibularis ventralis. Numerous alpha-MSH-like immunoreactive fibers are present in the preoptic area, in the pars ventralis of the tuber cinereum, and in the outer layer of the median eminence. This staining pattern is completely eliminated after preabsorbing the antiserum with the corresponding antigen, but blocking tests with alpha-MSH-related peptides do not lead to any change in the immunoreaction. From these results it may be inferred that an alpha-MSH-like system is present in the hypothalamic neurosecretory area of R. esculenta, and is probably related to its hypophysiotropic functions. The results are compared to the distribution of alpha-MSH within the hypothalamus of reptiles and mammals.

Morphological evidence for a direct neuroendocrine GABAergic control of the anterior pituitary in teleosts

The anterior pituitary of teleosts is unique among vertebrates in receiving a direct innervation which represents the morphological support of the neuroendocrine control of pars distalis functions. The participation of GABAergic fibers in this innervation was studied by means of immunocytochemistry at the light and electron microscopic levels, using antibodies against GABA. Immunoreactive fibers, characterized by the presence of small clear and dense cored vesicles, were detected in all parts of the gland. Immunopositive terminals were found in close, sometimes synaptic-like, contact with most glandular cell types in the anterior lobe. The data strongly suggest that in teleosts, as in mammals, GABA is involved in the neuroendocrine control of anterior pituitary functions.

Isolation of two forms of growth hormone secreted from eel pituitaries in vitro

Two forms of growth hormone (GH) were purified by chromatofocusing of medium from cultured Japanese eel (Anguilla japonica) pituitaries. The pituitaries were organ-cultured in Eagle's minimum essential medium with Earle's salts. Following polyacrylamide gel electrophoresis of the medium at pH 9.5, two prominent bands were seen with Rf 0.36 and 0.29; they were designated as eGHI and eGHII, respectively. Seven-hundred fifty milliliters of medium, in which 260 pituitaries were cultured for 6-10 weeks, was concentrated by DIAFLO membrane (YM-5) and subjected to gel filtration on a Sephadex G-75 column and to chromatofocusing on a PBE-94 column. eGHI and II were finally purified by gel filtration on a Sephadex G-75 column, yielding 2.0 mg of eGHI and 1.3 mg of eGHII. Both eGHI and eGHII were equipotent to ovine GH in promoting growth of juvenile rainbow trout. The putative GH-producing cells in the proximal pars distalis of the eel pituitary were stained specifically with antisera raised against eGHI or eGHII; no cross-reactivity was seen in the follicular prolactin cells in the rostral pars distalis. As determined by gel isoelectric focusing, eGHI and eGHII have isoelectric points of 6.3 and 6.7, respectively. Identical molecular masses of 23,000 Da were determined by sodium dodecyl sulfate gel electrophoresis. Their amino acid compositions strongly resembled each other; comparison of the partial N-terminal amino acids indicates that sequence 1 to 36 of GHII is exactly the same as 4 to 39 of GHI.

Studies on the regulation of growth hormone release from the proximal pars distalis of male tilapia, Oreochromis mossambicus, in vitro

The in vitro effects of several factors, including cortisol, somatostatin (SRIF), and medium osmotic pressure, on growth hormone (GH) release from the tilapia pituitary were examined in relation to fish size. Spontaneous GH release from the proximal pars distalis (PPD) of approximately 60-g fish was significantly less than that from tissue of fish weighing either approximately 120 or approximately 280 g when incubated in 340 m phi smolal medium. While GH content of the PPD cultures (tissue + medium measured by densitometry) increased consistently with fish size, GH concentration (per microgram of tissue protein) was variable, being highest in 120-g fish and lowest in 280-g fish. Moreover, GH concentration was not related to GH release. Fish size also appeared to be important in the responsiveness of GH cells to stimulation by cortisol (Nishioka et al., 1985) and by increased osmotic pressure. In cultures of PPD from approximately 60-g fish, in which spontaneous release was relatively low, cortisol and increased medium osmotic pressure significantly enhanced release. Cortisol and hyperosmotic medium were without significant effect, however, on GH release from PPD of approximately 120-g fish, which showed high spontaneous release. In contrast, SRIF, a potent inhibitor of GH secretion, was effective in lowering GH release regardless of fish size. Nevertheless, SRIF was apparently more effective in inhibiting GH release from tissue of 60-g fish than from tissue of 120-g fish. Our data suggest that GH secretion may be augmented when smaller tilapia (approximately 60 g) are transferred to seawater, a situation in which blood cortisol and osmotic pressure would presumably be elevated.

Somatostatin and altered medium osmotic pressure elicit rapid changes in prolactin release from the rostral pars distalis of the tilapia, Oreochromis mossambicus, in vitro

Prolactin (PRL) cells in the rostral pars distalis of the tilapia Oreochromis mossambicus respond to somatostatin (SRIF) and reduced medium osmotic pressure within 10-20 min of exposure during perifusion incubation. Pieces of rostral pars distalis tissue were removed from freshwater-adapted tilapia and were preincubated in [3H]leucine in static culture (355 m phi smolal) for 48 hr. Following preincubation, they were placed in the perifusion apparatus and baseline release was established for 3 hr in hyperosmotic medium (355 m phi smolal). Exposure to hyposmotic medium (280 m phi smolal) resulted in a rapid and steep rise in the release of [3H]PRL, which remained elevated for more than 2 hr. When SRIF was added simultaneously with hyposmotic medium, the rise in PRL release normally initiated by reduced osmotic pressure was prevented. Somatostatin also quickly reduced release that had been previously elevated by exposure to hyposmotic medium. The time course of these changes suggests that SRIF and altered osmotic pressure act on PRL secretion in at least partial independence of effects which they may have on PRL synthesis in the tilapia pituitary.

Regulation of androgen production by frog (Rana esculenta) testis: an in vitro study on the effects exerted by estradiol, 5 alpha-dihydrotestosterone, testosterone, melatonin, and serotonin

The possible role of estradiol-17 beta (E2), testosterone (T), 5 alpha-dihydrotestosterone (DHT), melatonin, and serotonin on the regulation of androgen (A) production by the frog, Rana esculenta, testes was studied in vitro. E2 (10(-6) M) inhibited A production whether alone or in combination with oLH (20 micrograms) after 6 hr incubation. After 24 hr incubation. A production was reduced by E2 concentration of around 10(-6) and 10(-9) M. Melatonin and serotonin did not induce any change whichever experimental condition was used. Preincubation for 6 hr with 10(-6) M T or DHT enhanced the oLH-stimulated A production after 18 hr incubation. These data suggest that steroids may regulate their intratesticular levels without passing into the blood stream.

Auditory pathways to the hypothalamus in ranid frogs

Three multisynaptic pathways from the midbrain auditory center (torus semicircularis) of the bullfrog, Rana catesbeiana, to the infundibular hypothalamus were found using the axonal tracer wheat germ agglutinin-horseradish peroxidase. Toral neurons project to the secondary visceral nucleus of the isthmus and to the central and anterior thalamic nuclei of the dorsal thalamus. All 3 of these nuclei project to the infundibular hypothalamus. These findings indicate multiple connections between two centers presumed important for reproductive behavior in frogs, a midbrain sensory region processing acoustic communication signals and a hypothalamic endocrine control area regulating gonadotropin and gonadal steroid secretion.

Relationship between brain gonadotrophin-releasing hormone (GnRH) and seasonal reproductive cycle of "caribe colorado," Pygocentrus notatus

Immunoreactive gonadotrophic hormone-releasing hormone (ir-GnRH) was detected in hypothalamic and telencephalic extracts of the Venezuelan freshwater fish "caribe colorado," Pygocentrus notatus. Hypothalamic ir-GnRH from female fish demonstrated displacement curves parallel to those of synthetic mammalian luteinizing hormone-releasing hormone (LHRH). The content of hypothalamic and telencephalic ir-GnRH from female fish was more than four-fold greater than that of male animals. Also, fluctuations that depended on the reproductive state and environmental conditions (rainfall) occurred in females but not in males. Thus, ir-GnRH levels were higher in hypothalamic and telencephalic extracts from sexually mature females than in those from fish sampled outside the climatically determined breeding season.

beta-Endorphin-like immunoreactivity in the brain of the lizard, Lacerta muralis

Using immunocytochemical methods, a beta-endorphin-like immunoreactive substance was identified in the brain of the lizard Lacerta muralis. beta-Endorphin-like neurons were observed in the dorsal posterior part of the paraventricular nucleus and in the caudal region of the nucleus ventromedialis hypothalami. beta-Endorphin-like immunoreactive fibers were also detected in the median eminence. Another cell group displaying beta-endorphin-like immunoreactivity was found in both subdivisions of the oculomotor nucleus and in the periaqueductal gray of the mesencephalon. In addition, a beta-endorphin-like immunoreaction was observed in the perikarya of the Purkinje cells and in their axonal processes. These patterns of immunoreactivity were completely abolished when a specific antiserum was absorbed with its corresponding antigen or with beta-lipotropin. These control tests suggest that the immunoreaction might correspond to a beta-endorphin- or lipotropin-like reaction. The results are discussed in relation to the possibility that a beta-endorphin-like peptide may be involved in hypophysial regulation or neuromodulator activity in the brain of the lizard L. muralis.

Effects of catecholamines on plasma thyroid hormone levels in arctic charr, Salvelinus alpinus

Plasma levels of L-thyroxine (T4) and 3,5,3'-triiodo-L-thyronine (T3) were measured in arctic charr at 2, 6, or 24 hr after single intraperitoneal injection of epinephrine (E) or norepinephrine (NE). At a dose of approximately 1 microgram/g body wt (sufficient to cause a submaximal dermal melanophore pallor response) plasma T4 was usually elevated at 2 hr, consistently depressed at 6 hr, and unaffected at 24 hr. There was no effect of E on plasma [125I]T4 kinetics or [125I]T4 5'-monodeiodination to [125I]T3. Plasma T3 showed no consistent response to E or NE at any sampling time. At an E dose of 4 ng/g body wt (probably sufficient to cause a physiological elevation in plasma E level), neither plasma T4 nor T3 levels were altered at 6 hr. Acute depression in plasma T4 by the high doses of E and NE may reflect a local neurotransmitter role of catecholamines in inhibiting thyroidal T4 release through action at thyroidal, hypophysial, or hypothalamic levels.

Diverse molecular forms of gonadotropin-releasing hormone in an elasmobranch and a teleost fish

Immunoreactive and biologically active gonadotropin-releasing hormones (GnRHs) in dogfish (Poroderma africanum) and teleost (Coris julis) brain extracts were studied by high-performance liquid chromatography (HPLC), radioimmunoassay with region-specific antisera, and assessment of luteinizing hormone (LH)-releasing activity in a chicken dispersed pituitary cell bioassay. In dogfish brain extract, seven GnRH molecular forms with LH-releasing activity were demonstrated. Three of these forms coeluted with synthetic mammalian GnRH; His5,Trp7,Tyr8-GnRH; and Trp7,Leu8-GnRH on HPLC. The peaks coincident with His5,Trp7,Tyr8-GnRH and Trp7,Leu8-GnRH had immunological and biological properties identical to those of the synthetic peptides. However, the molecular form coeluting with mammalian GnRH had immunological and biological properties different from those of mammalian GnRH and is thus a novel molecular variant of GnRH. The four remaining forms are also novel GnRHs or structurally unrelated peptides with LH-releasing activity. Dogfish systemic blood contained immunoreactive GnRH. In teleost brain extract, three biologically active GnRH forms with LH-releasing activity were present. The major peak of GnRH immunoreactivity coeluted with Trp7,Leu8-GnRH, and a second immunoreactive form coeluted with His5,Trp7,Tyr8-GnRH. The third biologically active peak is a novel, early-eluting molecular variant of GnRH or a structurally unrelated peptide with LH-releasing activity.

Development of a salmon growth hormone radioimmunoassay

This study describes the development of a growth hormone (GH) radioimmunoassay (RIA) using chum salmon (Oncorhynchus keta) GH and an antiserum raised against this preparation. The assay does not cross-react with salmon prolactin and is valid for the genera Salmo and Oncorhynchus. Hypophysectomy of coho salmon (O. kisutch) reduced plasma immunoreactivity to nondetectable levels in seven of eight individuals. Handling stress had no effect upon GH levels in the rainbow trout (Salmo gairdneri) whereas starvation (3 weeks) induced a ninefold increase in plasma immunoreactivity. Plasma GH levels in trout were positively correlated, following a lag phase of 1 week, with the weekly changes in growth rate displayed by this species.

The dopaminergic innervation of the goldfish pituitary. An immunocytochemical study at the electron-microscope level using antibodies against dopamine

The dopaminergic innervation of the goldfish pituitary gland was studied by immunocytochemistry at the electron-microscope level using highly specific antibodies against dopamine coupled to bovine serum albumin with glutaraldehyde. A satisfactory preservation of the tissue was achieved after immersion in 5% glutaraldehyde in phosphate buffer containing sodium metabisulfite to prevent oxidation of the endogenous dopamine. The immunocytochemical procedure was performed on Vibratome sections using the preembedding method. Immunoreactivity was restricted to part of the neurosecretory type-B fibers (diameter of the secretory vesicles lower than 100 nm) in which it was found to occupy the whole cytoplasm. Labeled fibers were observed within the neurohypophysis in the different parts of the gland and in the adenohypophyseal tissue where immunoreactive profiles were detected in close apposition to the different cell types. These data are in agreement with previous results obtained by means of radioautography and further support a role for dopamine in the neuroendocrine regulation of pituitary functions in teleosts.

Estradiol-17 beta and thyrotropin-releasing hormone stimulate prolactin release from the pituitary gland of a teleost fish in vitro

The effects of estradiol-17 beta (E2) and thyrotropin-releasing hormone (TRH) on prolactin (PRL) release were investigated using the organ-cultured rostral pars distalis (RPD) of the tilapia, Oreochromis mossambicus. Spontaneous PRL release into hyperosmotic medium increased in a dose-related manner following E2 pretreatment in vitro. In addition, TRH stimulated a dose-related increase in PRL release from E2-preincubated RPD's, but had no effect on tissues not previously exposed to E2. The maximal PRL response, nearly three times control levels, occurred at 50 nM TRH. Higher doses of TRH were less effective in stimulating PRL release. These findings indicate that TRH may be an important hypothalamic prolactin-releasing factor in the tilapia. Furthermore, the marked potentiation of the action of TRH on PRL release following exposure to E2 suggest that there may be a shift in the control of PRL secretion with changes in the reproductive state of the tilapia.

Direct control of the gonadotroph in a teleost, Poecilia latipinna. II. Neurohormones and neurotransmitters

Pituitaries from male and female mollies were incubated with varying amounts of mammalian LH-RH, arginine vasotocin, dopamine, or serotonin for 18 hr. Ultrastructural differences between control and experimentally treated glands were used to define the direct effects of these neurohormones and neurotransmitters on the gonadotrophic cells of the adenohypophysis. The effects varied in intensity according to the sex and reproductive state of the donor animal. LH-RH stimulated gonadotrophin secretion by the gonadotrophs, as did vasotocin, although to a much lesser extent and with noticeable differences between the sexes. Dopamine inhibited secretion by basally active gonadotrophs and probably from active cells also, although to a lesser extent. Serotonin mildly stimulated secretion at all stages in both sexes. The results of this study indicate the possible involvement of neurohypophysial octapeptides and of monoamines in the direct control of the gonadotroph of Poecilia latipinna.