SYMPATHOINHIBITION EVOKED FROM CAUDAL MIDLINE MEDULLA IS MEDIATED BY GABA RECEPTORS IN ROSTRAL VLM

The present study was performed to determine whether the powerful depr essor and sympathoinhibitory response that can be evolved from neurons in the caudal midline medulla is mediated by gamma-aminobutyric acide rgic (GABAergic) inhibition of sympathoexcitatory neurons in the rostr al part of the ventrolateral medulla (VLM). In anesthetized barointact and barodenervated rabbits, bilateral microinjections of bicuculline into sympathoexcitatory sites in the rostral VLM resulted in a sustain ed increase in renal sympathetic nerve activity and abolished or rever sed the depressor and sympathoinhibitory response evoked by glutamate microinjection into the caudal midline medulla. By contrast, the sympa thoinhibitory response evoked from the caudal midline medulla persiste d when the background level of renal sympathetic nerve activity was re flexly raised by baroreceptor unloading. The results indicate that 1) the depressor and sympathoinhibitory response evoked by stimulation of neurons in the caudal midline medulla is mediated by a GABAErgic syna pse in the rostral VLM and 2) there are also sympathoexcitatory neuron s in the caudal midline medulla whose presence is revealed by blockade of the more powerful sympathoinhibitory response.