THE OXYTOCIN-INDUCED INWARD CURRENT IN VAGAL NEURONS OF THE RAT IS MEDIATED BY G-PROTEIN ACTIVATION BUT NOT BY AN INCREASE IN THE INTRACELLULAR CALCIUM-CONCENTRATION

The neuropeptide oxytocin can depolarize parasympathetic preganglionic neurons in the dorsal motor nucleus of the vagus nerve of the rat by generating a sustained inward current, which is sodium-dependent and t etrodotoxin-insensitive. The second messenger activated by oxytocin re ceptor binding is, however, not yet known. In the present study, we at tempted to characterize it by using the whole-cell recording technique and brainstem slices. When loaded with GTP-gamma-S, a non-hydrolysabl e analogue of GTP, vagal neurons generated a persistent inward current in the absence of agonist and the oxytocin effect was suppressed, sug gesting that the peptide-evoked current was mediated by G-protein acti vation. Loading vagal neurons with the calcium chelator s(2-aminopheno xy)ethane-N,N,N',N',tetraacetic-acid (BAPTA) suppressed a calcium-depe ndent, slowly decaying potassium after-current but did not affect the oxytocin response, suggesting that the latter was not mediated by an a gonist-induced increase in the intracellular calcium concentration. Pr otein kinase C (PKC) activation was probably not involved, since the p eptide-evoked current was not modified by loading neurons with the PKC inhibitor H7. Thus, the oxytocin-evoked current in vagal neurons was probably not mediated by phospholipase C-beta (PLC-beta) activation. L oading neurons with 8-Br-cAMP or with an adenylyl cyclase activator (f orskolin) reduced the oxytocin-evoked current by about half. SQ 22536, an adenylyl cyclase inhibitor, reduced this current by a similar amou nt. However, the peptide-evoked current was unaffected by Rp-cAMPS and SpcAMPS, an inhibitor and an activator, respectively, of cAMP-depende nt protein kinase (PKA). We suggest that oxytocin activates two distin ct signalling pathways in vagal neurons: one which is cAMP-dependent, but PKAindependent, and one, unidentified, which is PLC-P-and cAMP-ind ependent. Each pathway accounts for about half of the peptide effect a nd both appear to involve G-protein activation.