Modulation of pressor response to muscle contraction via monoamines following AMPA-receptor blockade in the ventrolateral medulla

Abstract We hypothesized that cardiovascular responses to static muscle contraction are mediated via changes in extracellular concentrations of monoamines (norepinephrine, dopamine and serotonin) following the administration of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, an AMPA-receptor antagonist) into the rostral (RVLM) or caudal (CVLM) ventrolateral medulla. For the RVLM experiments ( n = 8), a 2-min static muscle contraction increased the mean arterial pressure (MAP) and heart rate (HR) by 23 ± 2 mmHg and 28 ± 8 bpm, respectively. During this contraction, the concentrations of norepinephrine, dopamine, and serotonin within the RVLM increased by 278 ± 52%, 213 ± 23%, and 232 ± 24%, respectively. Microdialysis of CNQX (1.0 μ M) for 30 min into the RVLM attenuated the increases in MAP and HR ( 11 ± 2 mmHg and 14 ± 5 bpm) without a change in developed muscle tension. The levels of norepinephrine, dopamine, and serotonin within the RVLM were also attenuated. In contrast, microdialysis of CNQX into the CVLM ( n = 8) potentiated the contraction-evoked responses in MAP ( 21 ± 2 vs 33 ± 5 mmHg) and HR ( 25 ± 5 vs 46 ± 8 bpm) without any effect on the monoamine levels within the CVLM region. These results suggest that AMPA-receptor blockade within the RVLM and CVLM has opposing effects on cardiovascular responses during static muscle contraction. In addition, such receptor blockade modulates extracellular concentrations of monoamines within the RVLM but not in the CVLM. These results provide evidence that AMPA receptors within the ventrolateral medulla play a role in exercise pressor reflex.