Dopamine D2 receptors regulate in vitro melanotrope L-type Ca2+ channel activity via c-fos

Dopamine D2 receptor stimulation of cultured primary melanotropes was found to depress L-type calcium channel activity, whereas D2 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 microM 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 D2 receptor agonist. Cultured melanotropes were treated with antisense oligodeoxynucleotides directed against the start sequences of the D2 receptor and c-fos messenger RNA. D2 receptor antisense nucleotide prevented the depressive effect on channel activity induced by D2 agonist treatment. c-fos antisense oligodeoxynucleotide blocked the rise in channel activity. The dopamine D2 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 D2 receptors.