The stimulation of vasopressin gene expression in cultured hypothalamic neurons by cyclic adenosine 3',5'-monophosphate is reversible.

Abstract

Previous studies have demonstrated that the number of vasopressin (VP) neurons present in primary diencephalic cultures can be markedly augmented by treatment with drugs that elevate intracellular cAMP. To evaluate the effect of this drug treatment on VP secretion by hypothalamic cultures and to determine if this represents a developmental phenomenon or a mechanism involved in the continuing dynamic regulation of the VP gene, we have exposed primary dispersed hypothalamic cultures derived from 14-day-old fetal Sprague-Dawley rats to forskolin (25 microM) and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX; 500 microM), either continually or intermittently, for up to 39 days. Culture medium was collected throughout the culture period for VP RIA, and at the end of the culture period, cultures were stained immunocytochemically for neurophysin (NP). As reported by previous investigators, exposure to the drugs for 11 days resulted in an increase in the number of NP-positive neurons. The increase was sustained with longer periods of exposure up to 39 days. IBMX and forskolin treatment also resulted in detectable release of VP into the culture medium, which increased from 1.4 +/- 0.15 pg/ml at 11 days to 8.4 +/- 0.6 pg/ml after 32 days of drug treatment. The VP concentration remained undetectable (< 1.25 pg/ml) in nontreated cultures throughout this period. The effect on VP expression did not require immediate exposure to the drugs in culture, but did require the continuous presence of the drugs. Removal of the drugs from days 11-18 of culture resulted in an almost complete loss of NP-positive cells; however, reexposure to the drugs reinstated NP expression in a time-dependent fashion. The effect of IBMX/forskolin treatment on the expression of other neuronal markers was also evaluated. The treatment did not alter the total number of neurons, and there was no evidence of stimulation of oxytocin expression. There was a marked increase in the number and size of neurons stained immunocytochemically for tyrosine hydroxylase and a small increase in the number of cells staining for somatostatin. These results demonstrate that treatment with cAMP-elevating drugs markedly and selectively elevates VP secretion from dispersed hypothalamic cultures, but continuous exposure to the drugs is necessary to sustain the effect. These findings suggest that although cAMP is required in hypothalamic cultures for VP gene expression, it may also participate in the dynamic regulation of VP gene transcription in response to physiological challenges.