The effect of zinc on maturational gonadotropin (GtH2) secretion modulated by GABAergic system in common carp (Cyprinus carpio L.)

Abstract

Studies were carried out on the effects of GABA and agonists or antagonists of its receptors on in vivo or in vitro spontaneous secretion as well as LHRH analog (D-Ala6 LHRH) stimulated maturational gonadotropic hormone (GtH2) secretion in carp fed the diet containing zinc. A possible modulatory action of dopamine in these processes was examined by simultaneous administration of pimozide, a dopamine (D2) receptor antagonist. Injections of isonipecotic acid - an antagonist of GABAA receptors (INIP), or bicuculline, an antagonist of GABAA receptors (BIC), increased the GnRH stimulated GtH2 secretion only in the control fish, but not in fish fed the diet containing zinc, suggesting that zinc inhibited this stimulatory influence. By contrast baclofen, an agonist of GABAB receptors (BAC), increased the GtH2 response to GnRH-A only in zinc treated fish. In vitro experiments demonstrated that GABA did not modify the GnRH-A stimulated GtH2 secretion from the pituitary of normal or zinc treated fish. These results suggest that although zinc does not affect the in vivo GtH2 release modulated by GABAergic drugs, it can block the tonic GABAergic inhibition of GtH2 secretion in vitro Therefore, zinc may temporarily eliminate the GABAA type of receptors from the GABAergic regulation of GtH2 secretion.