Contribution of zona incerta to osmotically induced drinking in rats

Spontaneous extracellular activity was recorded from single neurons in the rostral zona incerta (ZI) of urethan-anesthetized rats. Ventricular injections of hyperosmotic saline (1 or 2 microliter of NaCl solutions with osmolarities of 0.6 or 1.2 osmol/l) and distilled water (1 or 2 microliter) at the level of the anteroventral third ventricle (AV3V) changed the firing rate of ZI neurons. By comparison, ventricular injections of osmotic solutions at the level of the dorsal third ventricle were relatively ineffective. Injections of osmotic solutions (0.2 or 0.5 microliter of 1.2 osmol/l NaCl) into the medial preoptic area (MPO) also changed the firing rate of ZI neurons, whereas control injections into the caudate putamen were ineffective. In another series of rats, injections of procaine into the region of the rostral ZI significantly reduced drinking to injections of hyperosmotic saline into the ipsi- but not the contralateral MPO. The ZI, AV3V, and MPO have previously been reported to contribute to the neural regulation of fluid intake. These findings provide additional evidence for a role of the ZI in drinking and suggest that part of the central pathway for osmotic thirst involves a projection to neurons in the ZI from osmoreceptors in the MPO.