Functional relationship between subfornical organ cholinergic stimulation and cellular activation in the hypothalamus and AV3V region.

The subfornical organ (SFO) has been suggested to be important for water intake and secretion of vasopressin (AVP). However, the role of the SFO cholinergic mechanism in the control of body fluid regulation is not clear. This study determined the effects of local cholinergic stimulation in the SFO produced by administration of physostigmine on drinking and cellular excitation in the anterior third ventricle (AV3V) region and in the supraoptic and paraventricular nuclei (SON and PVN). The results showed that injection of physostigmine into the SFO induced water intake and c-fos expression in the AV3V area as well as in the AVP containing neurons in the hypothalamus. Pretreatment of the SFO with mecamylamine, a nicotinic receptor antagonist, had no effect on physostigmine induced behavioral and c-fos responses. The muscarinic receptor blocker atropine, however, abolished both drinking and cellular activation after injection of physostigmine into the SFO. Immunostaining experiments demonstrated positive acetyltransferase (ChAT) in the SFO. Intensive ChAT immunoreactivity was located in the cholinergic fibers in the SFO. Together, the results indicate that SFO cholinergic mechanisms are important in co-operation with the AV3V and hypothalamic neurons in the control of thirst and AVP-mediated body fluid homeostasis.