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

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.

Neuroendocrine control by dopamine of teleost reproduction

While gonadotropin-releasing hormone (GnRH) is considered as the major hypothalamic factor controlling pituitary gonadotrophins in mammals and most other vertebrates, its stimulatory actions may be opposed by the potent inhibitory actions of dopamine (DA) in teleosts. This dual neuroendocrine control of reproduction by GnRH and DA has been demonstrated in various, but not all, adult teleosts, where DA participates in an inhibitory role in the neuroendocrine regulation of the last steps of gametogenesis (final oocyte maturation and ovulation in females and spermiation in males). This has major implications for inducing spawning in aquaculture. In addition, DA may also play an inhibitory role during the early steps of gametogenesis in some teleost species, and thus interact with GnRH in the control of puberty. Various neuroanatomical investigations have shown that DA neurones responsible for the inhibitory control of reproduction originate in a specific nucleus of the preoptic area (NPOav) and project directly to the region of the pituitary where gonadotrophic cells are located. Pharmacological studies showed that the inhibitory effects of DA on pituitary gonadotrophin production are mediated by DA-D2 type receptors. DA-D2 receptors have now been sequenced in several teleosts, and the coexistence of several DA-D2 subtypes has been demonstrated in a few species. Hypophysiotropic DA activity varies with development and reproductive cycle and probably is controlled by environmental cues as well as endogenous signals. Sex steroids have been shown to regulate dopaminergic systems in several teleost species, affecting both DA synthesis and DA-D2 receptor expression. This demonstrates that sex steroid feedbacks target DA hypophysiotropic system, as well as the other components of the brain-pituitary gonadotrophic axis, GnRH and gonadotrophins. Recent studies have revealed that melatonin modulates the activity of DA systems in some teleosts, making the melatonin-DA pathway a prominent relay between environmental cues and control of reproduction. The recruitment of DA neurons for the neuroendocrine control of reproduction provides an additional brain pathway for the integration of various internal and environmental cues. The plasticity of the DA neuroendocrine role observed in teleosts may have contributed to their large diversity of reproductive cycles.

Specific [3H]spiperone binding sites in the pituitary of rainbow trout (Oncorhynchus mykiss) and goldfish (Carassius auratus)

Dopamine, a catecholamine neurohormone, modulates pituitary hormone release in teleost fishes and other vertebrates. The existence and binding parameters of a pituitary dopamine-neuroleptic receptor from trout were examined and compared with those from goldfish. Pituitary homogenate was incubated with [3H]spiperone (D2 antagonist) under several experimental paradigms; incubations were terminated by filtration and bound 3H radioactivity was assessed by liquid scintillation spectroscopy. Specific binding of [3H]spiperone was tissue dependent. Equilibrium displacement analyses using domperidone (D2 antagonist) indicated a single class of binding site (LIGAND) with Kd = 2.49 +/- 0.89 microM and a capacity of 3.10 +/- 0.45 nmol/mg protein; the goldfish Kd and capacity were both significantly (p < 0.05) larger: Kd = 4.63 +/- 0.30 microM and capacity = 20.66 +/- 2.03 nmol/mg protein. The Kd and capacity for the trout pars distalis (2.45 +/- 0.33 microM and 3.27 +/- 0.24 nmol/mg protein, respectively) did not differ significantly (p < 0.05) from that of the neurointermediate lobe (2.50 +/- 0.08 microM and 3.58 +/- 0.56 nmol/mg protein, respectively). Dopamine D2 receptor ligands differentially displaced [3H]spiperone from the trout pituitary, while D1 ligands, a D4 ligand, and a 5-hydroxytryptamine (5HT2) receptor antagonist had only small nonspecific effects. Comparison of the trout and goldfish pituitary dopamine-neuroleptic receptor indicates conservation of receptor affinity (Kd); however, differences in receptor numbers and in the distribution of receptors between the pars distalis and neurointermediate lobe in the two species may be due in part to species or developmental differences, and may reflect differences in the role(s) and degrees of influence of dopamine in these fishes.

Possible sites of dopaminergic inhibition of gonadotropin release from the pituitary of a teleost fish, tilapia

The present study is an attempt to find sites of dopaminergic inhibition along the transduction cascades culminating in gonadotropin (GtH) release in a teleost fish, tilapia. Experiments were carried out on perifused pituitary fragments and in primary culture of trypsinized pituitary cells. Salmon GnRH, chicken GnRH I and II stimulated GtH release in culture with estimated ED50 values of 15.56 pM, 2.55 nM and 8.65 pM, respectively. Apomorphine (APO; 1 microM) totally abolished this stimulation. Dopamine (DA; 1 microM) reduced both basal and GnRHa-stimulated GtH release from perifused pituitary fragments but did not alter the formation of cAMP. In a similar perifusion experiment DA abolished GtH release in response to forskolin (10 microM) with no reduction in cAMP formation. This indicates that one site of the dopaminergic inhibition is distal to cAMP formation, an indication not compatible with the classic characteristic of DA D2 type mode of action. The inhibition of GtH release in culture, caused by 1 microM APO, the specific DA D2 agonists LY 171555 (LY) or bromocryptine (BRCR) could not be reversed by activating protein kinase C (PKC) by DiC8 or the phorbol ester TPA. This would indicate a site for DA action distal to PKC. However, the stimulatory effect of arachidonic acid (AA; 50 microM) in perifusion was not reduced by DA (1 microM) or by APO, LY or BRCR in culture, which suggests a site for DA action proximal to AA formation. APO, LY and BRCR reduced GtH release in response to the Ca2+ ionophore A23187, however, their inhibitory effect was reversed by 10 microM ionomycin.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Topographical relationships between catecholamine- and neuropeptide-containing fibers in the median eminence of the newt, Triturus alpestris. An ultrastructural immunocytochemical study

Dopaminergic and peptidergic nerve fibers were simultaneously demonstrated with a double-labeling technique at the ultrastructural level. The first antibody, raised against tyrosine hydroxylase, was applied during the preembedding phase and visualized with the peroxidase method. The second antibody, raised against one of the peptides met-enkephalin, somatostatin or gonadotropin-releasing hormone (GnRH), was applied to the ultrathin sections and visualized with gold-labeled goat anti-rabbit IgG. The fibers of both categories were present in the zona externa of the median eminence, frequently contacting the basal lamina of the portal vessels. In addition, topographical relationships between different types of nerve fibers were observed in the perivascular areas, although there were no morphological signs of synaptic specializations. Using serial sections, it could be established that one GnRH-fiber contacted both a dopaminergic fiber and a fiber immunoreactive for met-enkephalin. The observations support earlier physiological data concerning the regulation of the hypothalamo-hypophyseal axis, with special emphasis on the release of neurohormones in the median eminence of the newt.

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.

Alterations in pituitary GnRH and dopamine receptors associated with the seasonal variation and regulation of gonadotropin release in the goldfish (Carassius auratus)

Seasonal variations in the serum concentrations of gonadotropin (GtH) and the serum GtH response to intraperitoneal injection of domperidone, a specific dopamine receptor antagonist, were examined in goldfish. In addition, the effects of in vivo treatment of goldfish with a superactive analog of salmon gonadotropin-releasing hormone (sGnRH-A) and domperidone on the binding parameters of pituitary GnRH and dopamine receptors were investigated in goldfish. Serum concentrations of GtH and the maximum GtH response to domperidone increased in correlation with advancing gonadal maturation; values increased from those in sexually regressed fish in January to maximal levels observed in fish in late stages of gonadal recrudescence in March, followed by a decrease with gonadal regression. At all stages, injection of domperidone increased serum concentrations of GtH in a dose-related manner; however, the ED50 of domperidone did not vary significantly over the course of the reproductive cycle. Multiple injections of sGnRH-A caused a progressively increasing and more prolonged serum GtH response; as well, multiple sGnRH-A treatment significantly potentiated the serum GtH response to domperidone without altering the ED50 of domperidone. sGnRH-A treatment caused a significant increase in the number of dopamine/neuroleptic receptors in the goldfish pars distalis, accompanied by a nonsignificant increase in dopamine/neuroleptic receptors in the neurointermediate lobe, and significantly increased the number of high-affinity GnRH receptors in the goldfish pituitary. Treatment with domperidone also significantly increased pituitary high-affinity GnRH receptor numbers. Receptor affinities were not significantly altered by either sGnRH-A or domperidone treatment.(ABSTRACT TRUNCATED AT 250 WORDS)