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Research Article Open Access
Epinephrine is an important neuroendocrine regulator to control growth hormone (GH) secretion in vertebrates. Somatolactin (SL), the latest member of the GH family, is a novel pituitary hormone with diverse functions in fish. In a previous report it was shown that epinephrine had a potent inhibitory effect on SL release in fish. However, very little is known about the mechanisms responsible for epinephrine inhibition of SL gene expression. In primary cultures of tilapia neurointermediate lobe (NIL) cells, epinephrine not only reduced SL mRNA levels, but could also abolish pituitary adenylate cyclase-activating polypeptide (PACAP)-stimulated SL gene expression. The inhibitory effects of epinephrine on SL gene expression were mimicked by additions of α2-adrenergic agonists clonidine and UK14304, whereas similar treatments with the α1-agonist cirazoline or the β-agonist isoproterenol had no effects in this regard. In parallel experiments, the SL response to epinephrine was significantly abolished by co-incubations with the α2- antagonist yohimbine, but the α1- or β-antagonist was not effective in this regard. In tilapia NIL cells, the α2-adrenergic agonist clonidine suppressed cAMP production and blocked forskolin and PACAP induction of total cAMP production. By using a pharmacological approach, the adenylate cyclase (AC) activator- and cAMP analog-stimulated SL responses were blocked treatment with clonidine. Furthermore, adrenergic inhibition of SL gene expression was also mimicked by inhibiting AC and blocking protein kinase A (PKA). These results, as a whole, suggest that α2-adrenergic stimulation can downregulate SL gene transcription by inhibiting the AC/cAMP-dependent mechanism at the tilapia pituitary level.
Epinephrine, Somatolactin, Tilapia, Pituitary cells, Signaling pathways, Cancer Pharmacology