INTRACEREBROVENTRICULAR INJECTION OF CHOLINE INCREASES PLASMA OXYTOCIN LEVELS IN CONSCIOUS RATS

In the present study, we examined the effect of intracerebroventricula rly (i.c.v.) injected choline on both basal and stimulated oxytocin re lease in conscious rats. I.c.v. injection of choline (50-150 mu g) cau sed time- and dose-dependent increases in plasma oxytocin levels under normal conditions. The increase in plasma oxytocin levels in response to i.c.v. choline (150 mu g) was greatly attenuated by the pretreatme nt of rats with atropine (10 mu g; i.c.v.), muscarinic receptor antago nist. Mecamylamine (50 mu g; i.c.v.), a nicotinic receptor antagonist, failed to suppress the effect of 150 mu g choline on oxytocin levels. Pretreatment of rats with 20 mu g of hemicholinium-3 (HC-3), a specif ic inhibitor of choline uptake into nerve terminals, greatly attenuate d the increase in plasma oxytocin levels in response to i.c.v. choline injection. Osmotic stimuli induced by either oral administration of 1 mi hypertonic saline (3 M) following 24-h dehydration of rats (type 1 ) or an i.c.v. injection of hypertonic saline (1 M) (type 2) increased plasma oxytocin levels significantly, but hemorrhage did not alter ba sal oxytocin concentrations. The i.c.v. injection of choline (50, 150 mu g) under these conditions caused an additional and significant incr ease in plasma oxytocin concentrations beyond that produced by choline in normal conditions. These data show that choline can increase plasm a oxytocin concentrations through the stimulation of central cholinerg ic muscarinic receptors by presynaptic mechanisms and enhance the stim ulated oxytocin release.