OSMOTIC STIMULATION OF VASOPRESSIN MESSENGER-RNA CONTENT IN THE SUPRAOPTIC NUCLEUS REQUIRES SYNAPTIC ACTIVATION

The role of synaptic input to the vasopressin neurons in hypertonicity -induced increase in vasopressin mRNA content was evaluated. Synaptic connection with the anterior hypothalamus is required for hypertonicit y to increase vasopressin release. However, the potential for other me chanisms to induce the increase in vasopressin mRNA content is suggest ed by the fact that hypertonicity induces depolarization of supraoptic neurons independently of synaptic input. Explants of the hypothalamon eurohypophysial system were used to study the effect of depolarization and hypertonicity in the presence and absence of nonspecific synaptic blockade by 15 mM MgSO4 or blockade of excitatory amino acid receptor s with kynurenic acid. Vasopressin release and mRNA content were incre ased by depolarization with 40 mM KCl and by exposure to hypertonicity (P < 0.05). Basal and osmotically stimulated vasopressin release was decreased by MgSO4 and by kynurenic acid. Both agents prevented the hy pertonicity-induced increase in vasopressin mRNA content. Thus either synaptic input or increased VP release is required for hypertonicity t o increase vasopressin mRNA, and excitatory amino acids are implicated in this response.