PECULIAR ALPHA-ADRENOCEPTORS PROMOTE PIGMENT AGGREGATION IN THE TELEOST, SYNBRANCHUS-MARMORATUS MELANOPHORES

1. In S. marmoratus melanophores, the DRC to norepinephrine (NE) was shifted to the right (30-fold) by yohimbine 10(-6) M, but was not affected by labetalol. Methoxamine and phenylephrine were inactive, even in the presence of propranolol and yohimbine. 2. Clonidine was also inactive, but induced a dose-dependent pigment aggregation (ED50 = 1.75 x 10(-6) M), which was diminished (10-fold) by yohimbine 10(-6) M, within Poecilia reticulata melanophores. 3. 4-Bromophenacyl bromide, neomycin sulphate, dibucaine, H-7 and lithium chloride did not affect the DRC to NE. Beta-glycerophosphate 10(-2) M shifted to the right (10-fold) the DRC to NE. 4. The adrenoceptors mediating melanosome aggregation within S. marmoratus melanophores can not be defined by the classical alpha2-agonists, but their activation is unrelated to the DG/IP3 pathway. These peculiar adrenoceptors may be structurally and pharmacologically different from the two, alpha1 and alpha2, subtypes.