Cloning and sequence analysis of two cDNAs encoding cyclin A and cyclin B in the zebra mussel Dreissena polymorpha.

Cyclins are key components in the progression of both mitotic and meiotic cell cycle control. Full-length cDNA clones encoding cyclin A and cyclin B were isolated from a zebra mussel testis cDNA library. The clones contained open reading frames of 419 and 434 amino acids, had similarity to cyclins A and B from other species, but also some unique features in their sequences. Cyclin A and B mRNA was expressed in testis, ovary, gill, mantle, muscle, and eggs, as shown by specific polymerase chain reaction.

Cloning and sequence analysis of a hypothalamic cDNA encoding a D1c dopamine receptor in tilapia.

Physiological and pharmacological studies have indicated that during acid stress a D1-like dopamine receptor becomes functional on intermediate pituitary melanocyte-stimulating hormone cells of tilapia (Oreochromis mossambicus). As a first step towards physiological expression studies we isolated a D1-like dopamine receptor from a tilapia hypothalamus cDNA library. Construction of a phylogenetic tree of most of the D1-like receptors known in human, rat, Xenopus, goldfish and Drosophila revealed that the here presented clone is most likely the tilapia equivalent of the Xenopus D1c dopamine receptor.

A role for di-acetyl alpha-melanocyte-stimulating hormone in the control of cortisol release in the teleost Oreochromis mossambicus.

In stressed tilapia, Oreochromis mossambicus, total alpha-melanocyte-stimulating hormone (alpha-MSH) levels and di-acetyl alpha-MSH/mono-acetyl alpha-MSH (di:mono) ratios are elevated. We therefore investigated the role of alpha-MSH in the regulation of the pituitary-interrenal axis. The corticotrophic activities of des-acetyl alpha-MSH, mono-acetyl alpha-MSH and di-acetyl alpha-MSH were compared. These forms of alpha-MSH were isolated from neurointermediate lobes and tested in a superfusion experiment with homologous interrenal tissue. The corticotrophic activity of di-acetyl alpha-MSH was the highest, followed by that of des-acetyl alpha-MSH and mono-acetyl alpha-MSH. Apparently, acetylation of alpha-MSH is of functional significance for corticotrophic action. Di-acetyl alpha-MSH proved to be about 100 times less potent than ACTH(1-39): the half-maximal stimulating concentrations for ACTH and di-acetyl alpha-MSH were 0.89 nmol/l and 110 nmol/l respectively. Surprisingly, a superfusate from neurointermediate lobes proved to be only about three times less active than a superfusate from the pituitary pars distalis, in which the corticotrophic activity is attributable to its ACTH content. When selectively stripped of all forms of alpha-MSH by passage through a Sepharose column coated with an antiserum against alpha-MSH, the neuro-intermediate lobe superfusate was devoid of corticotrophic activity. Thus alpha-MSH appears to be the corticotrophic factor in the superfusate of the neurointermediate lobe. After the same treatment, the corticotrophic activity of the pars distalis superfusate was not affected.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of differential release of alpha-melanocyte-stimulating hormone forms from the pituitary of a teleost fish, Oreochromis mossambicus.

Using high-performance liquid chromatography (HPLC) in combination with radioimmunoassay, three forms of alpha-MSH (des-acetyl, mono-acetyl and di-acetyl alpha-MSH) were separated and identified in tilapia neurointermediate lobes and plasma, and in medium from lobes superfused in vitro. The presence of acetylated forms in lobe extracts indicated that the peptides are acetylated intracellularly. Di-acetyl alpha-MSH was, especially in comparison with monoacetyl alpha-MSH, relatively more abundant in lobe extracts than in plasma. This suggests that the three forms of alpha-MSH are not released according to their relative intracellular abundances. The possibility of regulation of this differential release by dopamine and TRH was investigated, using a microsuperfusion system. Dopamine was a potent inhibitor of alpha-MSH release, but did not modulate the relative abundance of the different forms of alpha-MSH released from the MSH cells. TRH was a potent stimulator of alpha-MSH release. It enhanced in vitro the release of di-acetyl alpha-MSH more than the release of mono-acetyl alpha-MSH. Thus tilapia may be able to modulate not only the quantitative but also the qualitative signal from the MSH cells. This might enhance the flexibility of the animals to respond to environmental challenges.

A specific role for TRH in release of diacetyl alpha-MSH in tilapia stressed by acid water.

After exposure of tilapia for 7 days to low-pH water, plasma thyrotropin-releasing hormone (TRH) levels were elevated, and the melanocyte-stimulating hormone (MSH) cells in the pituitary pars intermedia had increased in size and showed enhanced synthetic and secretory activity. The MSH cells became more sensitive to TRH but not to corticotropin-releasing hormone (CRH). Stimulation by TRH (but not by CRH) of the MSH cells of tilapia exposed to low pH specifically enhanced the release of diacetyl alpha-MSH, the most potent corticotropic form of alpha-MSH. Acute stress imposed by handling and confinement for 1 h elevated the plasma cortisol level but did not affect blood plasma alpha-MSH levels. We conclude that stimulation by TRH is pivotal for an enhanced release of diacetyl alpha-MSH during low-pH adaptation. These results are further evidence of a role for TRH and alpha-MSH in the activation of the hypothalamopituitary-interrenal axis during adaptation to low pH.

Acid stress induces a D1-like dopamine receptor in pituitary MSH cells of Oreochromis mossambicus.

A 7-day exposure of tilapia (Oreochromis mossambicus) to water with a pH of 4.5 activates their pituitary melanophore-stimulating hormone (MSH) cells to preferentially release diacetyl alpha-MSH as an important corticotrope (13). We here focus on the control of alpha-MSH release by dopamine in tilapia exposed to water with low pH ("low-pH tilapia"). The MSH cells of low-pH tilapia showed a decreased sensitivity to inhibitory concentrations (10(-7)-10(-5) M) of dopamine compared with controls. Low concentrations (10(-14)-10(-8) M) of dopamine stimulated the release of alpha-MSH in low-pH tilapia but not in controls. Strong pharmacological evidence for a stimulatory dopamine receptor (D1-like) was obtained: the D1-agonists SKF-38393 and 6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazep ine hydrobromide (6-chloro APB) had a stimulatory effect on the release of alpha-MSH in low-pH tilapia MSH cells but not in controls. The selective D2-agonists quinpirole and 2-hydroxy apomorphin inhibited the release of alpha-MSH in controls as well as in low-pH tilapia, and there was no difference in the sensitivity of the cells to these agonists. We conclude that only MSH cells of low-pH exposed tilapia exhibit a D1-like receptor activity. A comparable D2-like receptor activity, as demonstrated by specific D2-receptor agonists, is present in both controls and low-pH-adapted fish. The apparent loss of sensitivity of the MSH cells to inhibitory concentrations of dopamine, therefore, must be caused by the activation of the D1-like receptors and not by changes in the activity of the D2-like receptor proper. Stimulatory concentrations of dopamine not only quantitatively but also qualitatively enhanced the corticotropic activity of the released alpha-MSH, as indicated by the elevated ratio of diacetyl and monoacetyl alpha-MSH. This effect was mimicked by the D1-like agonists SKF-38393 and 6-chloro APB, indicating that the D1-like receptor activity is responsible for the enhancement of the di/mono ratio.