Involvement of glutamatergic mechanisms in the median preoptic nucleus in the dipsogenic response induced by angiotensinergic activation of the subfornical organ in rats.

Experiments were done to investigate the role of glutamatergic systems in the median preoptic nucleus (MnPO) in the water ingestion induced by administration of angiotensin II (ANG II) in the subfornical organ (SFO) in the awake rat. Microdialysis methods were utilized to quantify the extracellular content of glutamate (Glu) in the region of MnPO. Microinjection of ANG II (10-10 M) into the SFO significantly increased the release of Glu in the MnPO in the rats under the condition that water is available for drinking and the rats under the condition that water is not available for drinking. The amount of initial maximal increases in the Glu levels elicited by the ANG II injection was quite similar in drinking and non-drinking rats, whereas the duration of the response was much longer in non-drinking than in drinking rats. The amount of water ingestion in 20 min immediately after the ANG II injection was significantly enhanced by previous injections of N-methyl-D-aspartate (NMDA, 10 μM) into the MnPO, while the ANG II-induced water ingestion was attenuated by pretreatment with the NMDA antagonist dizocilpine (MK-801, 10 μM). The amount of water intake elicited by the ANG II injection into the SFO was enhanced by previous injections of either the non-NMDA agonist kainic acid (KA, 50 μM) or quisqualic acid (QA, 50 μM) into the MnPO. On the contrary, the ANG II-induced drinking response was diminished by pretreatment with the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 μM) in the MnPO. Each injection of NMDA, KA, and QA into the MnPO produced drinking behavior. These results imply that the glutamatergic neural pathways to the MnPO may transmit the information for eliciting drinking in response to ANG II acting at the SFO. Our data further provide evidence that the ANG II-induced dipsogenic response may be mediated through both NMDA and non-NMDA glutamatergic receptor mechanisms in the MnPO.