Effects of ventrolateral medullary AMPA-receptor antagonism on pressor response during muscle contraction.

Effects of administering 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) at a concentration that preferentially blocks alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors into rostral ventrolateral medulla (rVLM) or caudal ventrolateral medulla (cVLM) on cardiovascular responses elicited during static muscle contraction were investigated using anesthetized rats. Two microdialysis probes were inserted bilaterally into either the rVLM or the cVLM using stereotaxic guides. A tibial nerve stimulation-evoked static muscle contraction for 30 s increased mean arterial pressure (MAP) and heart rate (HR) by 27 +/- 3 mmHg and 28 +/- 4 beats/min, respectively. Microdialysis of CNQX into the rVLM for 30 min attenuated the contraction-evoked increases in MAP and HR (10 +/- 2 mmHg and 12 +/- 2 beats/min). Developed tensions were similar during the contractions before and after microdialyzing CNQX. In contrast, administration of CNQX into the cVLM potentiated the muscle contraction-evoked cardiovascular responses (MAP, 25 +/- 4 vs. 39 +/- 6 mmHg; HR, 27 +/- 3 vs. 42 +/- 3 beats/min), with no change in developed tensions. Results demonstrate that AMPA receptors within the rVLM and the cVLM appear to play opposite modulatory roles in the central integration of cardiovascular responses elicited during static muscle contraction.