Opposing effects of intracerebroventricularly injected norepinephrine on oxytocin and vasopressin neurons in the paraventricular nucleus of the rat.

Our previous study shows that intracerebroventricularly (i.c.v.) injected norepinephrine (NE) had different effects on the discharge of different firing patterns of magnocellular neurons in the paraventricular nucleus (PVN). In the present study we further classified antidromically identified magnocellular neurons into two groups: vasopressin (VP) and oxytocin (OT) secreting neurons, and found that of all 48 cases of magnocellular neurons, NE had mainly excitatory effects on 36 cases of putatively OT-secreting neurons, and inhibitory effects on 12 cases of VP-secreting neurons. The third ventricular injected NE had almost the same effect on two types of neurons as that of lateral ventricular injection, partly ruling out the possibility that the lateral ventricularly injected NE may have acted indirectly on the magnocellular neurons in PVN. The results show that different mechanisms may be involved in mediating the effect of i.c.v. injected NE on VP- and OT-secreting neurons in the PVN.