Angiotensin II responsiveness of rat paraventricular and subfornical organ neurons in vitro.

The responsiveness of neurons in the hypothalamic paraventricular nucleus to angiotensin II was investigated using extracellular single unit recording techniques in rat brain slices. Bath application of angiotensin II at a concentration of 3 x 10(-7) M for 2-5 min resulted in excitatory responses in 50.4% of 141 paraventricular cells tested. The mean increase in firing rate was 2.12 +/- 0.20 (mean +/- S.E.M.) spikes/s, which represents a mean increase in activity of 149.8 +/- 16.5%. Angiotensin II-sensitive neurons usually displayed irregular, phasic, or very slow spontaneous activity, with the majority of these neurons located in the magnocellular region. Under physiological blockade of synaptic transmission with low Ca2+/high Mg2+ medium, neuronal responses to this peptide remained in 12 (92.3%) of 13 cells tested. Application of three successive doses of angiotensin II ranging from 3 x 10(-9)-3 x 10(-7) M showed that neuronal responses were dose-dependent with an estimated threshold of 10(-8) M. In comparison with angiotensin III, angiotensin II not only stimulated more paraventricular cells, but usually induced larger excitatory responses. Angiotensin II subtype 1 receptor antagonist losartan completely blocked angiotensin II responsiveness in each of 14 paraventricular cells tested whereas PD 123319, an angiotensin II subtype 2 receptor antagonist, exhibited a partial inhibitory effect in about one half of another 13 cells. In addition, single unit in vitro subfornical organ recordings demonstrate that angiotensin II evokes greater excitatory responses than in the paraventricular nucleus and that these effects are abolished by losartan application. These results support the hypothesis that within both the paraventricular nucleus and subfornical organ angiotensin II is a bioactive peptide which modulates neuronal activity and thus may exert significant control over neuroendocrine and autonomic functions.