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 μM) totally abolished this stimulation. Dopamine (DA; 1 μM) 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 μM) 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 μM 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 μM) in perifusion was not reduced by DA (1 μM) 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 μM ionomycin. The stimulatory effect of ionophore A23187 differed from that of ionomycin in that A23187 was able to stimulate GtH release only in the presence of Ca²⁺ in the medium, whereas ionomycin stimulated the release also in the absence of the ion, and even in the presence of EGTA. It is assumed that ionomycin at 10 μM promotes the mobilization of Ca²⁺ from intracellular stores. As neither ionomycin nor AA caused any leakage of lactic dehydrogenase from cultured pituitary cells, the GtH released in response to this agonist is specific and cannot be attributed to damage of the cells' membrane. It is proposed that, in addition to sites distal to cAMP formation and proximal to AA formation, DA may inhibit GtH release from the pituitary of tilapia at sites distal to Ca²⁺ influx and at a step proximal to Ca²⁺ mobilization from intracellular sources.