Peripheral μ-opioid receptors attenuate the augmented exercise pressor reflex in rats with chronic femoral artery occlusioin

Recently, opioid receptors have been shown to be expressed on group III and IV afferents, which comprise the sensory arm of the exercise pressor reflex. Although the stimulation of opioid receptors in the central nervous system has been shown to attenuate the exercise pressor reflex, the effect on the reflex of their stimulation in the periphery is unknown. We therefore tested the hypothesis that the activation of peripheral μ-opioid receptors attenuates the exercise pressor reflex. The pressor responses to static contraction were compared before and after the injection of the μ-opioid receptor agonist [D-Ala²,N-MePhe⁴,Glyol⁵] enkephalin (DAMGO; 1 μg) into the abdominal aorta of decerebrated rats in which one femoral artery had been occluded 72 h previously (n = 10) and in control rats whose femoral arteries were freely perfused (n = 8). DAMGO attenuated the peak pressor response to contraction in rats whose femoral arteries had been occluded (before: increase of 34 ± 3 mmHg and after: increase of 22 ± 2 mmHg, P = 0.008); the inhibitory effect of DAMGO was prevented by the injection of naloxone (100 μg) into the abdominal aorta (before: increase of 29 ± 5 mmHg and after: increase of 29 ± 5 mmHg, P = 0.646, n = 7). An intravenous injection of DAMGO (1 μg, n = 6) had no effect on the peak pressor response to contraction in both groups of rats. DAMGO had no effect on the peak pressor response to contraction in rats whose femoral arteries were freely perfused (before: Δ 23 ± 4 mmHg, after: Δ 23 ± 3 mmHg, n = 6) but appeared to have a small effect on topography of the response. DAMGO had no effect on the peak pressor response to tendon stretch in both groups of rats (both P > 0.05). We conclude that the stimulation of peripheral μ-opioid receptors attenuates the exercise pressor reflex in rats whose femoral arteries have been ligated for 72 h.