Multiple alpha 2-adrenoceptor signalling pathways mediate pigment aggregation within melanophores

It has previously been shown that alpha 2-adrenoceptors (alpha 2-ARs) mediate pigment granule (melanosome) aggregation in melanophores of the teleost fish Labrus ossifagus. The present investigation scrutinized the signalling mechanisms of melanosome aggregation induced by sympathetic nerve stimulation or by exogenous addition of alpha-AR agonists and cAMP analogues. The following was observed: i) nerve-induced melanosome aggregation was associated with a rapid decrease in the cAMP level; ii) noradrenaline or medetomidine (an alpha 2-AR agonist) caused melanosome aggregation and reduced the cAMP content; iii) RP-8-Cl-cAMP, a membrane-permeating inhibitor of protein kinase A induced melanosome aggregation; and iv) B-HT 920 (an alpha 2-AR agonist) and methoxamine (an alpha 1-AR agonist) induced melanosome aggregation, although they did not reduce cAMP. It has been suggested that in some teleost species alpha 1-ARs mediate melanosome aggregation by increasing the level of intracellular calcium. However, we found that the effect of methoxamine in melanophores from Labrus ossifagus could be blocked by yohimbine (an alpha 2-AR antagonist) but not by equimolar concentration of prazosin (an alpha 1-AR antagonist). Furthermore, 1 microM ionomycin (a calcium ionophore) did not induce melanosome aggregation. Our findings therefore do not indicate that alpha 1-ARs and/or an increase in intracellular calcium mediate melanosome aggregation in Labrus ossifagus. Our results suggest that alpha 2-AR-mediated melanosome aggregation is induced by multiple signalling pathways. One of these involves a reduction in cAMP, but none involves an increase in intracellular calcium.