ENDOGENOUS ANGIOTENSIN WITHIN THE ROSTRAL VENTROLATERAL MEDULLA FACILITATES THE SOMATOSYMPATHETIC REFLEX

Objective: To determine whether endogenous angiotensin modifies the sy naptic excitation of sympatho-excitatory neurons in the rostral part o f the ventrolateral medulla. Design and methods: Experiments were perf ormed on anaesthetized rabbits with denervated arterial and cardiopulm onary baroreceptors. Arterial pressure, heart rate and renal sympathet ic nerve activity were measured. The average sympatho-excitatory refle x response evoked by short-train stimulation of the sciatic nerve was measured before and at various times after micro-injection of the non- specific angiotensin receptor antagonist [Sar(1),Thr(8)]-angiotensin I I (80 pmol) into the contralateral rostral ventrolateral medulla. Beca use the central pathway mediating this somatosympathetic reflex includ es a synapse within the contralateral (but not ipsilateral) rostral ve ntrolateral medulla, a change in the evoked response after blockade of angiotensin receptors in the contralateral rostral ventrolateral medu lla indicates a role of these receptors in synaptic transmission withi n this region. As a control, the effects on this reflex of micro-injec tion of [Sar(1),Thr(8)]-angiotensin II into the ipsilateral rostral ve ntrolateral medulla were also measured. Results: After injection of [S ar(1),Thr(8)]-angiotensin II into the contralateral rostral ventrolate ral medulla, the evoked sympatho-excitatory refer response was signifi cantly reduced compared with the response before injection. In contras t, injection of [Sar(1),Thr(8)]-angiotensin II into the ipsilateral ro stral ventrolateral medulla had no significant effect on the evoked sy mpatho-excitatory reflex response, although contralateral and ipsilate ral injections of [Sar(1),Thr(8)]-angiotensin II had very similar effe cts on baseline arterial pressure and renal sympathetic nerve activity . Conclusions: The results indicate that, in the anaesthetized rabbit with denervated arterial and cardiopulmonary baroreceptors, endogenous angiotensin is tonically released within the rostral ventrolateral me dulla and facilitates the synaptic excitation of sympatho-excitatory n eurons reflexly evoked by stimulation of afferent fibres in the sciati c nerve.