Relationship between cyclic AMP-stimulated and native gonadotropin-releasing hormone-stimulated gonadotropin release in the goldfish

Comparative aspects of GnRH-Stimulated signal transduction in the vertebrate pituitary - Contributions from teleost model systems

Gonadotropin-releasing hormone (GnRH) is a major regulator of reproduction through actions on pituitary gonadotropin release and synthesis. Although it is often thought that pituitary cells are exposed to only one GnRH, multiple GnRH forms are delivered to the pituitary of teleost fishes; interestingly this can include the cGnRH-II form usually thought to be non-hypophysiotropic. GnRHs can regulate other pituitary cell-types, both directly as well as indirectly, and multiple GnRH receptors (GnRHRs) may also be expressed in the pituitary, and even within a single pituitary cell-type. Literature on the differential actions of native GnRH isoforms in primary pituitary cells is largely derived from teleost fishes. This review will outline the diversity and complexity of GnRH-GnRHR signal transduction found within vertebrate gonadotropes as well as extra-gonadotropic sites with special emphasis on comparative studies from fish models. The implications that GnRHR transduction mechanisms are GnRH isoform-, function-, and cell-specific are also discussed.

Nitric oxide and guanylate cyclase signalling are differentially involved in gonadotrophin (LH) release responses to two endogenous GnRHs from goldfish pituitary cells

Nitric oxide synthase (NOS) immunoreactivity is present in goldfish gonadotrophs. The present study investigated whether two native goldfish gonadotrophin-releasing hormones (GnRHs), salmon (s)GnRH and chicken (c)GnRH-II, use NOS/nitric oxide (NO) and soluble guanylate cyclase (sGC)/cyclic (c)GMP/protein kinase G (PKG) signalling to stimulate maturational gonadotrophin [teleost gonadotrophin-II, luteinising hormone (LH)] release. In cell column perifusion experiments with dispersed goldfish pituitary cells, the application of three NOS inhibitors (aminoguanidine hemisulphate, 1400W and 7-nitroindazole) and two NO scavengers [2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) and rutin hydrate] reduced sGnRH-elicited, but not cGnRH-II-induced, LH increases. The NO donor sodium nitroprusside (SNP) increased NO production in goldfish pituitary cells in static incubation. SNP-stimulated LH release in column perifusion was attenuated by PTIO and the sGC inhibitor 1H-(1,2,4)oxadiazolo[4,3-a]quinoxalin-1-oneon (ODQ), and additive to responses elicited by cGnRH-II, but not sGnRH. ODQ and the PKG inhibitor KT5823 decreased sGnRH- and cGnRH-II-stimulated LH release. Similarly, the LH response to dibutyryl cGMP was reduced by KT5823. These results indicate that, although only sGnRH uses the NOS/NO pathway to stimulate LH release, both GnRHs utilise sGC/PKG to increase LH secretion.

Differential involvement of protein kinase C and protein kinase A in ghrelin-induced growth hormone and gonadotrophin release from goldfish (Carassius auratus) pituitary cells

Ghrelin (GRLN) and its receptor have been identified and characterised in goldfish brain and the pituitary, and recent evidence shows that goldfish (g)GRLN(19) induces both growth hormone (GH) and maturational gonadotrophin (LH) release through an extracellular Ca(2+) -dependent mechanism in goldfish. To further understand the role of GRLN in hormone release, the present study examined the involvement of protein kinase C (PKC) and protein kinase A (PKA) in gGRLN(19) -induced GH and LH release and corresponding Ca(2+) signals in primary cultures of goldfish pituitary cells. Treatments with PKC inhibitors, Bis-II and Gö 6976, significantly reduced gGRLN(19) -induced GH and LH release and their corresponding intracellular Ca(2+) signals in identified somatotrophs and gonadotrophs, respectively. gGRLN(19) was unable to further stimulate hormone release or Ca(2+) signals when cells were pretreated with the PKC agonist, DiC8. PKA inhibitors, H-89 and KT 5720, inhibited gGRLN(19) -induced LH release and Ca(2+) signals in gonadotrophs but not GH release or Ca(2+) signals in somatotrophs. Interestingly, pretreatment of pituitary cells with the adenylate cyclase activator forskolin potentiated gGRLN(19) -induced GH, but not LH, release, although it had no effect on intracellular Ca(2+) signals in either cell type. Taken together, the results suggest that PKC is an important intracellular component in gGRLN(19) -induced GH and LH release, whereas PKA is involved in gGRLN(19) -elicited LH release. Furthermore, the PKA pathway potentiates gGRLN(19) -induced GH release via a Ca(2+) -independent mechanism. Overall, the present study provides insight into the neuroendocrine regulation of GH and LH release by elucidating the mechanistic aspects of GRLN, a hormone involved in many critical physiological processes, including pituitary functions.

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.

Extracellular signal-regulated kinase mediates gonadotropin subunit gene expression and LH release responses to endogenous gonadotropin-releasing hormones in goldfish

The possible involvement of extracellular signal-regulated kinase (ERK) in mediating the stimulatory actions of two endogenous goldfish gonadotropin-releasing hormones (salmon (s)GnRH and chicken (c)GnRH-II) on gonadotropin synthesis and secretion was examined. Western blot analysis revealed the presence of ERK and phosphorylated (p)ERK in goldfish brain, pituitary, liver, ovary, testis and muscle tissue extracts, as well as extracts of dispersed goldfish pituitary cells and HeLa cells. Interestingly, a third ERK-like immunoreactive band of higher molecular mass was detected in goldfish tissue and pituitary cell extracts in addition to the ERK1-p44- and ERK2-p42-like immunoreactive bands. Incubation of primary cultures of goldfish pituitary cells with either a PKC-activating 4beta-phorbol ester (TPA) or a synthetic diacylglycerol, but not a 4alpha-phorbol ester, elevated the ratio of pERK/total (t)ERK for all three ERK isoforms. The stimulatory effects of TPA were attenuated by the PKC inhibitor GF109203X and the MEK inhibitor PD98059. sGnRH and cGnRH-II also elevated the ratio of pERK/tERK for all three ERK isoforms, in a time-, dose- and PD98059-dependent manner. In addition, treatment with PD98059 reduced the sGnRH-, cGnRH-II- and TPA-induced increases in gonadotropin subunit mRNA levels in Northern blot studies and sGnRH- and cGnRH-II-elicited LH release in cell column perifusion studies with goldfish pituitary cells. These results indicate that GnRH and PKC can activate ERK through MEK in goldfish pituitary cells. More importantly, the present study suggests that GnRH-induced gonadotropin subunit gene expression and LH release involve MEK/ERK signaling in goldfish.

Mechanisms for gonadotropin-releasing hormone potentiation of growth hormone rebound following norepinephrine inhibition in goldfish pituitary cells

In the goldfish, norepinephrine (NE) inhibits growth hormone (GH) secretion through activation of pituitary alpha(2)-adrenergic receptors. Interestingly, a GH rebound is observed after NE withdrawal, which can be markedly enhanced by prior exposure to gonadotropin-releasing hormone (GnRH). Here we examined the mechanisms responsible for GnRH potentiation of this "postinhibition" GH rebound. In goldfish pituitary cells, alpha(2)-adrenergic stimulation suppressed both basal and GnRH-induced GH mRNA expression, suggesting that a rise in GH synthesis induced by GnRH did not contribute to its potentiating effect. Using a column perifusion approach, GnRH given during NE treatment consistently enhanced the GH rebound following NE withdrawal. This potentiating effect was mimicked by activation of PKC and adenylate cyclase (AC) but not by induction of Ca(2+) entry through voltage-sensitive Ca(2+) channels (VSCC). Furthermore, GnRH-potentiated GH rebound could be alleviated by inactivation of PKC, removal of extracellular Ca(2+), blockade of VSCC, and inhibition of Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII). Inactivation of AC and PKA, however, was not effective in this regard. These results, as a whole, suggest that GnRH potentiation of GH rebound following NE inhibition is mediated by PKC coupled to Ca(2+) entry through VSCC and subsequent activation of CaMKII. Apparently, the Ca(2+)-dependent cascades are involved in GH secretion during the rebound phase but are not essential for the initiation of GnRH potentiation. Since GnRH has been previously shown to have no effects on cAMP synthesis in goldfish pituitary cells, the involvement of cAMP-dependent mechanisms in GnRH potentiation is rather unlikely.

PKC and ERK are differentially involved in gonadotropin-releasing hormone-induced growth hormone gene expression in the goldfish pituitary

Gonadotropin-releasing hormone (GnRH) is produced by the hypothalamus and stimulates the synthesis and secretion of gonadotropin hormones. In addition, GnRH also stimulates the production and secretion of growth hormone (GH) in some fish species and in humans with certain clinical disorders. In the goldfish pituitary, GH secretion and gene expression are regulated by two endogenous forms of GnRH known as salmon GnRH and chicken GnRH-II. It is well established that PKC mediates GnRH-stimulated GH secretion in the goldfish pituitary. In contrast, the signal transduction of GnRH-induced GH gene expression has not been elucidated in any model system. In this study, we demonstrate, for the first time, the presence of novel and atypical PKC isoforms in the pituitary of a fish. Moreover, our results indicate that conventional PKCα is present selectively in GH-producing cells. Treatment of primary cultures of dispersed goldfish pituitary cells with PKC activators (phorbol ester or diacylglycerol analog) did not affect basal or GnRH-induced GH mRNA levels, and two different inhibitors of PKC (calphostin C and GF109203X) did not reduce the effects of GnRH on GH gene expression. Together, these results suggest that, in contrast to secretion, conventional and novel PKCs are not involved in GnRH-stimulated increases in GH mRNA levels in the goldfish pituitary. Instead, PD98059 inhibited GnRH-induced GH gene expression, suggesting that the ERK signaling pathway is involved. The results presented here provide novel insights into the functional specificity of GnRH-induced signaling and the regulation of GH gene expression.

Regulation of fish gonadotropins

Neurohormones similar to those of mammals are carried in fish by hypothalamic nerve fibers to regulate directly follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Gonadotropin-releasing hormone (GnRH) stimulates the secretion of FSH and LH and the expression of the glycoprotein hormone alpha (GPalpha), FSHbeta, and LHbeta, as well as their secretion. Its signal transduction leading to LH release is similar to that in mammals although the involvement of cyclic AMP-protein kinase A (cAMP-PKA) cannot be ruled out. Dopamine (DA) acting through DA D2 type receptors may inhibit LH release, but not that of FSH, at sites distal to activation of protein kinase C (PKC) and PKA. GnRH increases the steady-state levels of GPalpha, LHbeta, and FSHbeta mRNAs. Pituitary adenylate cyclase-activating polypeptide (PACAP) 38 and neuropeptide Y (NPY) potentiate GnRH effect on gonadotropic cells, and also act directly on the pituitary cells. Whereas PACAP increases all three subunit mRNAs, NPY has no effect on that of FSHbeta. The effect of these peptides on the expression of the gonadotropin subunit genes is transduced differentially; GnRH regulates GPalpha and LHbeta via PKC-ERK and PKA-ERK cascades, while affecting the FSHbeta transcript through a PKA-dependent but ERK-independent cascade. The signals of both NPY and PACAP are transduced via PKC and PKA, each converging at the ERK level. NPY regulates only GPalpha- and LHbeta-subunit genes whereas PACAP regulates the FSHbeta subunit as well. Like those of the mammalian counterparts, the coho salmon LHbeta gene promoter is driven by a strong proximal tripartite element to which three different transcription factors bind. These include Sf-1 and Pitx-1 as in mammals, but the function of the Egr-1 appears to have been replaced by the estrogen receptor (ER). The GnRH responsive region in tilapia FSHbeta 5' flanking region spans the canonical AP1 and CRE motifs implicating both elements in conferring GnRH responsiveness. Generally, high levels of gonadal steroids are associated with high LHbeta transcript levels whereas those of FSHbeta are reduced when pituitary cells are exposed to high steroid levels. Gonadal or hypophyseal activin also participate in the regulation of FSHbeta and LHbeta mRNA levels. However, gonadal effects are dependent on the gender and stage of maturity of the fish.

Multiplicity of gonadotropin-releasing hormone signaling: a comparative perspective

GnRH regulation of GtH synthesis and release involves PKC- and Ca(2+)-dependent pathways. There are differential signaling mechanisms in different cells, tissues and species. Signaling mechanisms involved in GnRH-mediated GtH release appear to be more conserved compared to that of GnRH-induced GtH gene expression. This may in part be due to different 5' regulatory regions on the GtH-subunit genes. Cell type specific expression of various signaling and/or exocytotic components may also be responsible for the observed differences in signaling between gonadotropes and somatotropes in the goldfish and tilapia pituitaries. However, this can not explain the observed differences in post receptor mechanisms for sGnRH and cGnRH-II in gonadotropes which is more likely to result from the existence of GnRH receptor subtypes. Support for this hypothesis is also provided by observations on mechanisms of autocrine/paracrine regulation of ovarian function by sGnRH and cGnRH-II in the goldfish ovary in which GnRH antagonists only block GnRH stimulation of oocyte meiosis and do not affect inhibitory effects of sGnRH. It should be easier to explain observed variations concerning GnRH-induced responses as more information becomes available on different types of GnRH receptors, and their distribution and function in mammals and non-mammalian vertebrates.

Caffeine-stimulated GTH-II release involves Ca(2+) stores with novel properties

Modulation of Ca(2+) stores with 10 mM caffeine stimulates robust secretion of gonadotropin (GTH-II) from goldfish gonadotropes. Although both endogenous forms of gonadotropin-releasing hormone (GnRH) utilize a common intracellular Ca(2+) store, sGnRH, but not cGnRH-II, uses an additional caffeine-sensitive mechanism. We examined caffeine signaling by using Ca(2+) imaging, electrophysiology, and cell-column perifusion. Although caffeine inhibited K+ channels, this action appeared to be unrelated to caffeine-induced GTH-II release, because the latter was insensitive to tetraethylammonium. The effects of caffeine also were not mediated by the cAMP/protein kinase A pathway. Instead, caffeine-evoked GTH-II responses were Ca(2+) signal dependent because they were abolished by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid loading. Caffeine generated localized Ca(2+) signals that began near secretory granules. Surprisingly, caffeine-stimulated GTH-II release was insensitive to 100 microM ryanodine and, unlike GnRH action, was unaffected by inhibitors of voltage-gated Ca(2+) channels or sarco(endo)plasmic reticulum Ca(2+)-ATPases. Collectively, these data indicate that caffeine-stimulated GTH-II release is not mediated by typical agonist-sensitive Ca(2+) stores found in endoplasmic reticulum.

Pacap stimulation of gonadotropin-II secretion in goldfish pituitary cells: mechanisms of action and interaction with gonadotropin releasing hormone signalling

Pituitary adenylate cyclase-activating polypeptide (PACAP) has recently been shown to be a hypophysiotropic factor in the goldfish. In this study, we examined the mechanisms of PACAP action on goldfish maturational gonadotropin (GTH-II) release using primary cultures of pituitary cells. The GTH-II response to mammalian PACAP1-38 (mPACAP) was inhibited by a PACAP receptor antagonist suggesting a receptor-mediated action. Addition of either an adenylate cyclase inhibitor or a protein kinase A (PKA) inhibitor reduced the mPACAP-induced GTH-II release. In addition, when GTH-II release was already stimulated by either forskolin or 8-bromo-cAMP (8Br-cAMP), mPACAP did not further increase GTH-II secretion. These results strongly implicated the involvement of an adenylate cyclase/cAMP/PKA pathway in PACAP-stimulated GTH-II release. Although mPACAP induced a rise in intracellular Ca2+ level in identified gonadotropes, results with voltage-sensitive Ca2+ channel inhibitors indicated that the GTH-II responses to mPACAP, forskolin and 8Br-cAMP did not depend upon Ca2+ entry through these channels. Two protein kinase C (PKC) inhibitors did not affect mPACAP-elicited GTH-II release, and mPACAP further increased GTH-II secretion in the presence of PKC activators. These results indicate that PKC-dependent elements are not essential for the stimulatory action of mPACAP in gonadotropes. Interestingly, while GTH-II responses to a stimulatory concentration of mPACAP were additive to responses elicited by maximal effective concentrations of two endogenous gonadotropin releasing hormones (GnRHs), a subthreshold concentration of mPACAP potentiated GnRH and PKC activator stimulation of GTH-II secretion. Similarly, submaximal concentrations of forskolin potentiated the GTH-II response to the PKC activator, tetradecanoyl phorbol acetate. These data suggest that PACAP and its cAMP-dependent signalling mechanisms provide an alternate stimulatory input to goldfish gonadotropes and may influence the effectiveness of the major neuroendocrine control exerted by the PKC-dependent GnRH signalling pathway.

GnRH stimulates LH release directly via inositol phosphate and indirectly via cAMP in African catfish

In African catfish, two gonadotropin-releasing hormone (GnRH) peptides have been identified: chicken GnRH (cGnRH)-II and catfish GnRH (cfGnRH). The GnRH receptors on pituitary cells producing gonadotropic hormone signal through inositol phosphate (IP) elevation followed by increases in intracellular calcium concentration (¿Ca(2+)(i)). In primary pituitary cell cultures of male African catfish, both cGnRH-II and cfGnRH dose dependently elevated IP accumulation, ¿Ca(2+)(i), and the release of the luteinizing hormone (LH)-like gonadotropin. In all cases, cGnRH-II was more potent than cfGnRH. The GnRH-stimulated LH release was not associated with elevated cAMP levels, and forskolin-induced cAMP elevation had no effect on LH release. With the use of pituitary tissue fragments, however, cAMP was elevated by GnRH, and forskolin was able to stimulate LH secretion. Incubating these fragments with antibodies against cfGnRH abolished the forskolin-induced LH release but did not compromise the forskolin-induced cAMP elevation. This suggests that cfGnRH-containing nerve terminals are present in pituitary tissue fragments and release cfGnRH via cAMP signaling on GnRH stimulation, whereas the GnRH receptors on gonadotrophs use IP/¿Ca(2+)(i) to stimulate the release of LH.

Signal transduction mechanisms mediating secretion in goldfish gonadotropes and somatotropes

The intracellular signal transduction mechanisms mediating maturational gonadotropin and somatotropin secretion in goldfish are reviewed. Several major signaling mechanisms, including changes in intracellular [Ca2+], arachidonic acid cascades, protein kinase C, cyclic AMP/protein kinase A, calmodulin, nitric oxide, and Na+/H+ antiport, are functional in both cell types. However, their relative importance in mediating basal secretion and neuroendocrine-factor-regulated hormone release differs according to cell type. Similarly, agonist- and cell-type-specificity are also present in the transduction pathways leading to neuroendocrine factor-modulated maturational gonadotropin and somatotropin release. Specificity is present not only in the actions of different regulators within the same cell type and with the same ligand in the two cell types, but this also exists between isoforms of the same neuroendocrine factor within a single cell type. Other evidence suggests that function-selectivity of signaling may also result from differential modulation of Ca2+ fluxes from different sources. The interaction of different second messenger systems provide the basis by which regulation of maturational gonadotropin and somatotropin release by multiple neuroendocrine factors can be integrated at the target cell level.

Key words: Ca2+ signaling, cAMP, PKC, arachidonic acid, NO.

Norepinephrine regulation of growth hormone release from goldfish pituitary cells. II. Intracellular sites of action

Previous results suggest that norepinephrine decreases growth hormone (GH) release in goldfish by means of alpha-2 adrenoceptor activation. The intracellular mechanisms by which norepinephrine inhibits GH release were examined in the present study using dispersed goldfish pituitary cells. In 2-h static incubation experiments, norepinephrine and the alpha-2 agonist clonidine decreased basal GH release and the GH responses to stimulation by the dopamine D1 agonist SKF38393 and two native gonadotropin-releasing hormones (GnRH). Norepinephrine also reduced GH responses to the adenylate cyclase activator forskolin, two protein kinase C (PKC) activators (phorbol ester and synthetic diacylglycerol), and two Ca2+ ionophores (ionomycin and A23187). Similarly, norepinephrine applied as a 1-h pulse in cell column perifusion experiments reduced basal GH release and abolished the GH response to a 5-min pulse of arachidonic acid. In goldfish, D1-stimulated GH release is mediated by AC-, arachidonic acid-and Ca2+-dependent pathways, whereas GnRH action is coupled to PKC-and Ca2+-dependent mechanisms. These results suggest that norepinephrine activation of alpha-2 receptors inhibits ligand-induced GH secretion by actions subsequent to activation of these second messenger cascades. To further characterize norepinephrine mechanisms of action on unstimulated hormone release, the ability of norepinephrine and an alpha-2 agonist to affect activation of two second messenger cascades under basal conditions was also investigated. Static incubation with clonidine reduced cAMP production in a time-and dose-dependent manner, suggesting that norepinephrine inhibitory action can also be expressed at the level of cAMP production. Resting intracellular free calcium levels in single, identified goldfish somatotropes was unaffected by norepinephrine. However, the inhibitory effects of norepinephrine on basal GH secretion was not observed in the presence of a voltage-sensitive Ca2+ channel agonist. Whether these channels are targets for norepinephrine action on unstimulated GH release requires further investigation.

Differences in mGnRH and cGnRH-II contents in pituitaries and discrete brain areas of Rana rugulosa W. according to age and stage of maturity

(1) In tadpoles, chicken-II gonadotropin-releasing hormone (cGnRH-II) could be measured in the brains before metamorphosis, but mammalian gonadotropin-releasing hormone (mGnRH) did not appear until the stage of metamorphosis, i.e. cGnRH-II appeared earlier than mGnRH during ontogenesis. (2) During the metamorphic climax, mGnRH content increased more rapidly than cGnRH-II; the content of mGnRH was about two times of that of cGnRH-II. (3) In juveniles and adults, the content of mGnRH and cGnRH-II, and the distribution pattern of mGnRH (but not cGnRH-II) in the brains and pituitaries changed with age and stages of gonadal development. mGnRH mainly distributed in the rostral brain areas, whereas cGnRH-II had a widespread distribution in the brain. (4) Both mGnRH and cGnRH-II were present in the pituitaries at each stage of maturity. The gonadotropin-releasing hormone (GnRH) content at sexually maturity was significantly higher than that at other stages of gonadal development, and the content of mGnRH was about 15-18 times of that of cGnRH-II. (5) These results suggest that both mGnRH and cGnRH-II are potentially involved in the direct regulation of pituitary gonadotropes, and mGnRH may be the major active form, cGnRH-II may also serve as a neurotransmitter or neuromodulator in the brain.

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)

Involvement of protein kinase C in the modulation of gonadotropin and growth hormone secretion from dispersed goldfish pituitary cells

It has been established that secretion of gonadotropin (GtH) and growth hormone (GH) release in goldfish are both stimulated by GtH-releasing hormone (GnRH); in addition GtH secretion is inhibited by dopamine D2 mechanisms. In the present study, depletion of protein kinase C (PKC) in goldfish pituitary cells reduced the GtH and GH responses to GnRH and an activator of PKC in static culture. In perifusion studies, GtH released in response to sGnRH analog was greatly attenuated in PKC-depleted cells, however, hormone responses to forskolin were enhanced. Stimulation of dopamine D2 receptors reduced the GtH, but not the GH, responses elicited by PKC activators. These results indicate that PKC participates in the GtH and GH responses to natural neuroendocrine regulators in the goldfish.

Intracellular mechanisms mediating gonadotropin and growth hormone release in the goldfish, Carassius auratus

Evidence for the involvement of Ca(2+), protein kinase C, cAMP, and arachidonic acid metabolism in mediating gonadotropin (GTH) and growth hormone (GH) release in the goldfish is reviewed. Models for the signal transduction pathways mediating GTH-releasing hormone (GnRH) and dopamine actions on GTH and GH secretion are postulated. A novel hypothesis that two GnRHs which bind to the same receptor type activate different transduction cascade in two different cell types (GTH vs. GH) as well as within the same cell type (GTH) is presented.