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

Dopamine inhibits somatolactin gene expression in tilapia pituitary cells through the dopamine D2 receptors

Dopamine (DA) is an important neurotransmitter in the central nervous system of vertebrates and possesses key hypophysiotropic functions. Early studies have shown that DA has a potent inhibitory effect on somatolactin (SL) release in fish. However, the mechanisms responsible for DA inhibition of SL gene expression are largely unknown. To this end, tilapia DA type-1 (D1) and type-2 (D2) receptor transcripts were examined in the neurointermediate lobe (NIL) of the tilapia pituitary by real-time PCR. In tilapia, DA not only was effective in inhibiting SL mRNA levels in vivo and in vitro, but also could abolish pituitary adenylate cyclase-activating polypeptide (PACAP)- and salmon gonadotropin-releasing hormone (sGnRH)-stimulated SL gene expression at the pituitary level. In parallel studies, the specific D2 receptor agonists quinpirole and bromocriptine could mimic the DA-inhibited SL gene expression. Furthermore, the D2 receptor antagonists domperidone and (-)-sulpiride could abolish the SL response to DA or the D2 agonist quinpirole, whereas D1 receptor antagonists SCH23390 and SKF83566 were not effective in this respect. In primary cultures of tilapia NIL cells, D2 agonist quinpirole-inhibited cAMP production could be blocked by co-treatment with the D2 antagonist domperidone and the ability of forskolin to increase cAMP production was also inhibited by quinpirole. Using a pharmacological approach, the AC/cAMP pathway was shown to be involved in quinpirole-inhibited SL mRNA expression. These results provide evidence that DA can directly inhibit SL gene expression at the tilapia pituitary level via D2 receptor through the AC/cAMP-dependent mechanism.

Endocrine regulation of prolactin cell function and modulation of osmoreception in the Mozambique tilapia

Prolactin (PRL) cells of the Mozambique tilapia, Oreochromis mossambicus, are osmoreceptors by virtue of their intrinsic osmosensitivity coupled with their ability to directly regulate hydromineral homeostasis through the actions of PRL. Layered upon this fundamental osmotic reflex is an array of endocrine control of PRL synthesis and secretion. Consistent with its role in fresh water (FW) osmoregulation, PRL release in tilapia increases as extracellular osmolality decreases. The hyposmotically-induced release of PRL can be enhanced or attenuated by a variety of hormones. Prolactin release has been shown to be stimulated by gonadotropin-releasing hormone (GnRH), 17-β-estradiol (E2), testosterone (T), thyrotropin-releasing hormone (TRH), atrial natriuretic peptide (ANP), brain-natriuretic peptide (BNP), C-type natriuretic peptide (CNP), ventricular natriuretic peptide (VNP), PRL-releasing peptide (PrRP), angiotensin II (ANG II), leptin, insulin-like growth factors (IGFs), ghrelin, and inhibited by somatostatin (SS), urotensin-II (U-II), dopamine, cortisol, ouabain and vasoactive intestinal peptide (VIP). This review is aimed at providing an overview of the hypothalamic and extra-hypothalamic hormones that regulate PRL release in euryhaline Mozambique tilapia, particularly in the context on how they may modulate osmoreception, and mediate the multifunctional actions of PRL. Also considered are the signal transduction pathways through which these secretagogues regulate PRL cell function.

Effects of salmon GnRH and sex steroid hormones on expression of genes encoding growth hormone/prolactin/somatolactin family hormones and a pituitary-specific transcription factor in masu salmon pituitary cells in vitro

Expression of genes encoding growth hormone (GH), prolactin (PRL), and somatolactin (SL) in growing and maturing salmon was stimulated by gonadotropin-releasing hormone (GnRH) analog during particular periods of the life cycle. GnRH therefore appears to directly and/or indirectly regulate gene expression for GH, PRL, and SL in combination with the pituitary-gonadal axis, such as sex steroid hormones. Direct effects of salmon GnRH (sGnRH), estradiol-17beta (E2), testosterone, and 11-ketotestosterone (11KT) on the amounts of GH, PRL, and SL mRNAs were thus examined using primary pituitary cell cultures of masu salmon at the four reproductive stages. We also determined the amounts of mRNA encoding pituitary specific POU homeodomain transcription factor (Pit-1) by real-time polymerase chain reactions. The amounts of GH, PRL, and SL mRNAs in the control cells elevated with gonadal maturation, coincidently with those of Pit-1 mRNA. sGnRH at 1.0 nM elevated the amounts of all mRNAs examined in the pre-spawning females, whereas significant effects were not observed with 100 nM sGnRH at any reproductive stages. Sex steroid hormones had no significant effects before initiation of gonadal maturation and at the maturing stage. In the males, E2 tended to decrease the amounts of SL mRNA in the pre-spawning stage. In the females, E2 and 11KT increased the amounts of PRL and SL mRNAs in the pre-spawning stage, but halved those of PRL mRNA in the spawning stage. The amounts of Pit-1 mRNA changed coincidently with those of PRL and SL mRNAs at all examined stages. The effects of E2 alone were abolished by 100 nM sGnRH. The present results indicated that both sGnRH and steroid hormones directly modulate synthesis of Pit-1, and further expression of PRL and SL genes. sGnRH may indirectly regulate GH/PRL/SL family hormone genes through the pituitary-gonadal axis, particularly in the late stage of gametogenesis.

Melatonin modulates secretion of growth hormone and prolactin by trout pituitary glands and cells in culture

In Teleost fish, development, growth, and reproduction are influenced by the daily and seasonal variations of photoperiod and temperature. Early in vivo studies indicated the pineal gland mediates the effects of these external factors, most probably through the rhythmic production of melatonin. The present investigation was aimed at determining whether melatonin acts directly on the pituitary to control GH and prolactin (PRL) secretion in rainbow trout. We show that 2-[125I]-iodomelatonin, a melatonin analog, binds selectively to membrane preparations and tissue sections from trout pituitaries. The affinity was within the range of that found for the binding to brain microsomal preparations, but the number of binding sites was 20-fold less than in the brain. In culture, melatonin inhibited pituitary cAMP accumulation induced by forskolin, the adenyl cyclase stimulator. Forskolin also induced an increase in GH release, which was reduced in the presence of picomolar concentrations of melatonin. At higher concentrations, the effects of melatonin became stimulatory. In the absence of forskolin, melatonin induced a dose-dependent increase in GH release, and a dose-dependent decrease in PRL release. Melatonin effects were abolished upon addition of luzindole, a melatonin antagonist. Our results provide the first evidence that melatonin modulates GH and PRL secretion in Teleost fish pituitary. Melatonin effects on GH have never been reported in any vertebrate before. The effects result from a direct action of melatonin on pituitary cells. The complexity of the observed responses suggests several types of melatonin receptors might be involved.

Localization of tyrosine hydroxylase and its messenger RNA in the brain of rainbow trout by immunocytochemistry and in situ hybridization

This report describes the distribution of tyrosine hydroxylase (TH)-expressing structures in the brain of rainbow trout (Oncorhynchus mykiss). TH neurons have been localized by the use of two complementary techniques, immunocytochemistry and in situ hybridization of TH messenger RNA. Results obtained from in situ hybridization and immunocytochemistry were in agreement. TH cells were observed in many areas of the brain, with a higher density at the level of the olfactory bulbs where TH-positive neurons are abundant in the internal cell layer. In the telencephalon, two populations of TH neurons can be distinguished: one group is located in the area ventralis telencephali pars dorsalis, and the other group is located in the area ventralis telencephali pars ventralis and extends laterally in the area ventralis telencephali pars lateralis. Many labeled neurons are also seen in the preoptic area as well as in the hypothalamus, where several clusters of TH-positive cells are observed. Some of these neurons located in the paraventricular organ grow a short cytoplasmic extension directed to the ventricular wall and are known to be cerebrospinal fluid-contacting cells. The most caudal TH neurons are observed at the level of the locus caeruleus. At the level of the pituitary, TH-positive fibers are observed in the neurohypophysis. The TH-immunoreactive innervation at the level of the pituitary provides a neuroanatomic basis for the effects of dopamine and/or norepinephrine on the release of pituitary hormones in fish.

The effects of ions and hypothalamic factors on the in vitro activity of rainbow trout prolactin cells

The influence of various ions and of dopamine and somatostatin on the in vitro activity of rainbow trout prolactin (PRL) cells was investigated. There was a positive correlation between medium Ca2+ concentration and both PRL synthesis and release up to 1.8 mM Ca2+, above which no further increase occurred. Even with no Ca2+ in the medium, there was still PRL secretion during the incubation. Replacement of Ca2+ with Ba2+ in the medium did not elevate either total PRL levels or PRL release above that in Ca2 +)-free medium. Neither elevated Mg2+ nor increased medium K+ had any effect on PRL synthesis or release. Dopamine inhibited PRL release but not synthesis, as did the D2 receptor agonist, apomorphine. However, the D2 receptor antagonist, (+)-butaclamol was unable to prevent the action of dopamine on PRL release. Somatostatin inhibited both PRL synthesis and release in normal Ca2+ medium, but release only in reduced Ca2+ medium. Thus, Ca2+, dopamine, and somatostatin may all have roles in regulating prolactin secretion in this fish. In addition, oPRL reduced trout PRL release, indicating a possible negative feedback mechanism for trout PRL secretion.

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

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