Goldfish brain somatostatin-28 differentially affects dopamine- and pituitary adenylate cyclase-activating polypeptide-induced GH release and Ca(2+) and cAMP signals.

Dopamine (DA) and pituitary adenylate cyclase-activating polypeptide (PACAP) stimulate goldfish growth hormone (GH) release via cAMP- and Ca(2+)-dependent pathways while DA also utilizes NO. In this study, identified goldfish somatotropes responded to sequential applications of PACAP and the DA D1 agonist SKF38393 with increased intracellular Ca(2+) levels (Ca(2+)), indicating that PACAP and DA D1 receptors were present on the same cell. A native goldfish brain somatostatin (gbSS-28) reduced SKF38393-stimulated cAMP production and PACAP- and NO donor-elicited GH and Ca(2+) increases, but not PACAP-induced cAMP production nor the GH and Ca(2+) responses to forskolin, 8-bromo-cAMP and SKF38393. gbSS-28 might inhibit PACAP-induced GH release by interfering with PACAP's ability to increase Ca(2+) in a non-cAMP-dependent manner. However, DA D1 receptor activation bypassed gbSS-28 inhibitory effects on cAMP production and NO actions via unknown mechanisms to maintain a normal Ca(2+) response leading to unhampered GH release.