Simultaneous glutamate and γ-aminobutyric acid release within ventrolateral medulla during skeletal muscle contraction in intact and barodenervated rats

The purpose of this study was to determine if baroreflex modulates cardiovascular responses and neurotransmitter release within rostral (RVLM) and caudal (CVLM) ventrolateral medulla during static contraction of skeletal muscle using anesthetized rats. We evoked cardiovascular responses by a static muscle contraction and measured simultaneous release of glutamate and γ-aminobutyric acid (GABA) in both the RVLM and CVLM using microdialysis probes, two inserted bilaterally into the RVLM and two into the CVLM. In intact anesthetized rats, a muscle contraction increased release of glutamate concomitantly in both the RVLM and CVLM along with significant increases in heart rate and arterial blood pressure. In contrast, concentrations of GABA increased within the RVLM, but decreased significantly within the CVLM during the pressor response. These changes were due to contraction-evoked activation of muscle afferents since tibial nerve stimulation following muscle paralysis failed to evoke glutamate, GABA, or any cardiovascular changes. On the other hand, static muscle contractions in baroreceptor denervated rats augmented the increases in heart rate and blood pressure. Furthermore, muscle contraction significantly enhanced the release of glutamate in the RVLM but attenuated its release in the CVLM. In addition, concentrations of GABA within the RVLM were attenuated following a muscle contraction in denervated rats without any changes in GABA within the CVLM. These results demonstrate that the baroreceptors influence cardiovascular responses to static muscle contraction associated with dynamic changes in glutamate and GABA release within the RVLM and CVLM.