Involvement of DHP voltage-sensitive calcium channels and protein kinase C in thyroliberin (TRH) release by...

The intracellular mechanisms regulating the process of thyroliberin (TRH) release were studied using fetal hypothalamic neurons grown in serum-free medium. In particular, we compared the effects of dihydropyridine (DHP) derivatives, omega-conotoxin and phorbol esters on basal and K+-evoked TRH release from 12 days in vitro (DIV) neurons. BAY K 8644, a DHP calcium channel agonist increased in a dose-related manner basal and K+-evoked TRH release. PN 200-110, an antagonist of DHP-sensitive calcium channels, completely suppressed the effect of BAY K 8644, whatever the extracellular K+ concentration, but did not modify basal or K+-evoked TRH release. In contrast, omega-conotoxin partially inhibited the two latter processes. The active phorbol ester 12-O-tetradecanoyl-phorbol-beta-acetate (TPA), and to a lesser extent Sn-1,2-dioctanoylglycerol (DAG), triggered TRH release. This effect was specific, time and dose dependent and only partly dependent on extracellular calcium. Simultaneous addition of BAY K 8644 and TPA to the cells displayed a synergistic effect. The same compounds were studied on younger neurons (6-DIV cultures): BAY K 8644 stimulated TRH release whereas neither 60 mM K+ nor TPA did. These results suggest that TRH release can be mediated at least by two intracellular routes: (i) increase of intracellular calcium mediated by the opening of different types of voltage sensitive calcium channels, and (ii) activation of protein kinase C (PKC). The asynchrony in the maturation of the intracellular mechanisms underlying TRH release may be explained by different subcellular localizations of these mechanisms in neurons and is discussed in relation to synapse differentiation.