We have exposed primary dispersed hypothalamic cultures from 14-day-old fetal Sprague-Dawley rats to substances known to either elevate adenosine 3',5'-cyclic monophosphate (cAMP) levels or increase vasopressin (VP) secretion. The levels of VP in the medium collected from the cultures were determined by radioimmunoassay, and the number of neurophysin (NP)-positive cells after immunohistochemistry was counted. cAMP-elevating agents, 3-isobutyl-1-methylxanthine (200 microM) and forskolin (25 microM), in combination (I-F) maintained NP synthesis and VP secretion in 19-day cultures. I-F replacement by K+ (28 mM), isoproterenol (10 microM), glutamate (10 microM), or bicuculline (10 microM) during the last week of culture resulted in maintenance of NP expression and transient stimulation of VP secretion, but these agents did not induce NP expression independently of I-F treatment. In contrast, exposure to the dopamine D1 agonist SKF-38393 (10 microM) significantly increased NP expression independently and after replacement of I-F. Dopamine D1A receptors were detected by immunofluorescence on NP-expressing cells, providing a morphological basis for this response. These results suggest a role for D1A receptors in the regulation of VP gene expression.