Abstract
1. The intracellular mechanisms activated by the binding of vasopressin to its receptor(s) and which result in the increase of [Ca2+]i were investigated in freshly dissociated supraoptic nucleus neurones. Various pharmacological agents were used to investigate the possible involvement of phospholipase C (PLC) and adenylate cyclase (AC) intracellular pathways in the transduction of the vasopressin action. 2. Both the PLC inhibitor U-73122 and the protein kinase C (PKC) inhibitor calphostin C, reduced the [Ca2+]i rise elicited by vasopressin. The cAMP analogue, 8-Br-cAMP produced an increase in [Ca2+]i and IBMX, a phosphodiesterase inhibitor, potentiated the response to vasopressin. 3. After pre-incubation with the AC inhibitor SQ-22536, 7 out of 18 vasopressin-sensitive neurones showed no inhibition of the vasopressin response, while the response to vasopressin was reduced by greater than 35 % in each of the other 11 neurones. 4. The activation of protein kinase A (PKA) with Sp-cAMPS caused an increase in [Ca2+]i which was additive to the vasopressin-elicited [Ca2+]i increase. After incubation with the PKA inhibitors Rp-cAMPS or H-89, the [Ca2+]i responses triggered by Sp-cAMPS and vasopressin were, respectively, abolished and greatly reduced. 5. A combined administration of SQ-22536 (AC inhibitor) followed by U-73122 (PLC inhibitor), or U-73122 followed by H-89 (PKA inhibitor), virtually abolished the response to vasopressin. 6. In vasopressin-responsive neurones, the pituitary adenylate cyclase-activating polypeptide (PACAP) induced a [Ca2+]i increase similar to the response to vasopressin and in both cases the increase was inhibited to the same extent by a combination of U-73122 and Rp-cAMPS. 7. In conclusion, we suggest that the autoregulation exerted specifically by vasopressin on vasopressin-sensitive neurones involves the activation of both PLC- and AC-linked pathways.