Dependence of sympathetic vasomotor tone on bilateral inputs from the rostral ventrolateral medulla in the rabbit: role of baroreceptor reflexes.

A unilateral microinjection of muscimol into the pressor region in the rostral ventrolateral medulla (RVLM) of anaesthetised baroreceptor-denervated rabbits resulted in large and sustained decreases in mean arterial pressure, renal sympathetic nerve activity and heart rate (maximal decreases of 41 +/- 4 mmHg, 64 +/- 3%, and 59 +/- 8 beats/min, respectively). Subsequently, muscimol microinjection into the contralateral RVLM pressor region resulted in further but much smaller decreases in these variables. In contrast, it is well established that in baro-intact animals unilateral inactivation of the RVLM pressor region has little effect on resting sympathetic activity or arterial pressure - bilateral inactivation is required to produce large and sustained decreases. The results of the present study indicate that the baroreceptor reflex plays a crucial role in maintaining resting sympathetic vasomotor activity under circumstances in which the activity of RVLM presympathetic neurons is partially impaired.