RVLM glycine receptors mediate GABAA and GABAB independent sympathoinhibition from CVLM in rats

The caudal ventrolateral medulla (CVLM) provides tonic inhibitory and also excitatory inputs to the rostral ventrolateral medulla (RVLM). These experiments evaluated the role of RVLM γ-amino butyric acid (GABA) receptor subtypes and glycine receptors in mediating CVLM sympathoinhibition. In Inactin anesthetized female rats, the CVLM and RVLM were functionally defined by pressor and depressor responses to microinjected GABA (500 pmol, 50 nl). Although reduced, pressor and sympathoexcitatory responses due to inhibition of the CVLM with GABA persisted following ipsilateral RVLM GABAA receptor blockade (bicuculline, BIC, 400 pmol, 100 nl; n = 12) in rats with contralateral nucleus tractus solitarius (NTS) lesion. In the presence of either ipsilateral (+ contralateral NTS lesion; n = 8) or bilateral (n = 6) GABAA and GABAB receptor blockade of the RVLM (400 pmol BIC + 400 pmol CGP35348, 100 nl), inhibition of the CVLM still increased MAP and renal sympathetic nerve activity (RSNA). Thus neither GABAB receptors nor a contralateral CVLM to RVLM GABAergic pathway explains residual responses to CVLM blockade. The addition of strychnine (300 pmol, 100 nl) to the RVLM eliminated responses to CVLM inhibition, suggesting that a GABAA and GABAB independent sympathoinhibitory influence from CVLM to RVLM is mediated by glycine receptors. Decreases in MAP and RSNA due to activation of the CVLM with glutamate (500 pmol, 50 nl) were reversed to increases in the presence of RVLM GABAA receptor blockade (n = 7). Thus, a sympathoexcitatory pathway from the CVLM can be activated in the presence of RVLM GABA receptor blockade, but sympathoinhibitory influences from the CVLM predominate.