DOPAMINE D-2 RECEPTORS REGULATE IN-VITRO MELANOTROPE L-TYPE CA2+ CHANNEL ACTIVITY VIA C-FOS

Dopamine D-2 receptor stimulation of cultured primary melanotropes was found to depress L-type calcium channel activity, whereas D-2 receptor antagonist application increased it. When tested on culture days 10, 16, and 20, control cells displayed increasing rises of intracellular Ca2+ in response to K+ depolarization, indicating an increase in channel activity in the absence of dopaminergic regulation. When treated with 1 mu M bromocriptine from culture day 1, cells showed minimal increase in channel activity. When bromocriptine was added on day 16, intracellular Ca2+ response to high K+ declined by day 20; removal of the agonist on day 16 resulted in the reappearance of increased responsiveness. Thus, in vitro inhibitions could be initiated or reversed with application or withdrawal of dopamine D-2 receptor agonist. Cultured melanotropes were treated with antisense oligodeoxynucleotides directed against the start sequences of the D-2 receptor and c-fos messenger RNA. D-2 receptor antisense nucleotide prevented the depressive effect on channel activity induced by D-2 agonist treatment. c-fos antisense oligodeoxynucleotide blocked the rise in channel activity. The dopamine D-2 receptor antagonist haloperidol, which increased channel activity, could not reverse the c-fos antisense deoxynucleotide block. These results strongly support the idea that the chronic suppression of secretion-related activities by dopaminergic stimulation seen in the intermediate lobe in vivo is effected by chronic suppression of c-fos by D-2 receptors.