The Ventromedullary Hypotensive Effect of Muscimol in the Anaesthetized Cat

Muscimol, a rigid analogue of GABA, was injected into the CNS of urethane-anaesthetized, normotensive cats. Bilateral microinjections of a small dose of muscimol (100 ng/kg, 0.5 microliter on each side) in the nucleus reticularis lateralis (NRL) induced hypotension. The marked fall in blood pressure obtained by injecting 1 microgram/kg of muscimol unilaterally into the NRL is completely reversed by subsequent local administration of bicuculline (5 microgram/kg, 0.5 microliter), a specific GABA antagonist. These data confirm that a GABAergic inhibitory system modulates the pressor tonic structures localized in the region of the NRL and that the anteroventral part of the medulla oblongata includes a trigger zone for the hypotensive action of muscimol.