A study on the central osmoreceptor mechanism controlling drinking

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We have demonstrated that there is an osmosensitive neural circuit connecting the organum vasculosum of the lamina terminalis(OVLT),median preoptic nucleus(MnPO)and supraoptic nucleus and suggested that the circuit controls the vasopressin and oxytocin release. We examined whether the osmoreceptor complex plays a role in control of drinking. Cannulae were implanted into the OVLT,MnPO or SON of male rats and the effects of injections(0.2mu1)of isotonic or hypertonic CSF into each of the regions on the volume of water drunk in 10 min were measured. Hypertonic CSF injected into the OVLT,MnPO or SON was found to induce drinking. The rats were then deprived of water 15 hours and tested for the effect of lidocaine injection into each of the brain regions. The lidocain block of the activity of the OVLT,MnPO or SON reduced the volume of water drunk during the first 10 min after resuming free access to water. These results suggest that the osmosensitive neural circuit connecting the OVLT,MnPO and SON plays a role in the osmotic control of drinking as well as vasopressin release.The significance of the paraventricular nucleus for the dipsogenic effect of angiotensin II(AII) has been shown(Gutman et al, 1988). So we made attempt to investigate whether or not the SON is also involved in the central AII control of thirst. The microinjection(0.2mu1)of 10^<-6>M AII into the SON significantly increased water-intake. However,the microinjection of 10^<-4>M saralasin(AII antagonist)into the SON did not influence water intake in 15 hour water-deprived rats. Thus it is concluded that the angiotensinergic mechanism in the SON does not play a role in drinking in water-deprived dehydrated rat.