GABAergic modulation of the ANG II-induced drinking response in the rat medial preoptic nucleus

The present study was designed to examine the participation of gamma-aminobutyric acid (GABA) receptor mechanisms in the medial preoptic nucleus (MPO) in the drinking response caused by angiotensin II (ANG II) activation of the subfornical organ (SFO) in the awake rat. Local administration of ANG II (5 pmol, 50 nl) into the SFO elicited drinking. The water intake induced was significantly attenuated by previous injections (50 nl) into the MPO of the GABA(A) agonist muscimol (0.5, 5 and 50 pmol), but not by the GABA(B) agonist baclofen (0.5, 5 and 50 pmol) or vehicle, into the MPO. On the other hand, the ANG II-induced water intake was significantly enhanced by previous injections (50 nl) into the MPO of the GABA(A) antagonist bicuculline (0.5 and 5 pmol), but not the GABA(B) antagonist phaclofen (0.05, 0.5 and 5 pmol). Muscimol (50 nmol) injected into the MPO significantly reduced the water intake elicited by intracellular fluid depletion (i.e., hypertonic saline: 2 M, 2 ml/kg bw ip), whereas bicuculline (5 pmol) was without effect. These results show the involvement of the GABAergic system within the MPO in the dipsogenic responses induced by ANG II acting at the SFO and intracellular fluid depletion, and suggest that the system may serve to attenuate the ANG II-induced dipsogenic response through GABA(A) receptors.