Angiotensin II as a factor inhibiting the fear response.

Angiotensin II, if injected into the lateral ventricles of rabbits in doses of 0.015-0.15 microgram, has an inhibitory action on the fear response evoked by electrical stimulation of the ventromedial hypothalamic nucleus, but in doses of 1-10 ng it blocks the inborn behavioral fear responses in rats. On microionophoretic application of angiotensin II to single neurons in the cerebral cortex and parafascicular complex of the thalamus, predominantly activation responses were observed. Predominance of inhibitory neuronal responses were noted in structures of the hypothalamus and mesencephalic reticular formation to angiotensin II. Responses of cortical and subcortical neurons to angiotensin II are potentiated after stimulation of the "fear center" in the ventromedial hypothalamus. The hypothesis was put forward that depression of the fear response after administration of angiotensin II is connected with changes in cortico-subcortical relations, during which ascending activating influences of the mesencephalic reticular formation on the cerebral cortex are abolished due to descending influences of cortical and thalamic neurons.