Functions of AT1 and AT2 angiotensin receptors in the paraventricular nucleus of the rat, correlating single-unit and cardiovascular responses

The paraventricular hypothalamic nucleus (PVN) is a complex structure with both neuroendocrine and autonomic functions including cardiovascular control. The PVN contains angiotensin II (AngII) immunoreactive cells, fibers, as well as AT1 and AT2 receptors of AngII. We microinjected AngII into the PVN of normotensive anesthetized rats and simultaneously recorded blood pressure, heart rate (HR) and single-unit responses. The roles of AT1 and AT2 receptors in these responses were also evaluated. Microinjection of AngII into the PVN produced a short excitatory single-unit response and two types of pressor responses: short duration with a decrease in HR and long with an increase in HR. Microinjection of losartan, an AT1 antagonist, into the PVN produced two response types, attenuation and augmentation of the pressor and firing rate responses to AngII. Microinjection of PD123319, an AT2 antagonist, into the PVN greatly attenuated pressor and single-unit response to AngII, indicating that the pressor response was mediated through AT2 receptors too. In conclusion, microinjection of AngII into the PVN stimulates neurons resulting in an increase in firing rate and consequently produces a short or long pressor response. These responses were mediated through AT1 and AT2 receptors; however, AT1 receptor may produce inhibition too. The results suggest that AngII of the PVN may be a neurotransmitter playing a role in arterial pressure regulation.