Effects of peripheral and spinal κ-opioid receptor stimulation on the exercise pressor reflex in decerebrate rats

The exercise pressor reflex is greater in rats with ligated femoral arteries than it is in rats with freely perfused femoral arteries. The exaggerated reflex in rats with ligated arteries is attenuated by stimulation of μ-opioid and δ-opioid receptors on the peripheral endings of thin-fiber muscle afferents. The effect of stimulation of κ-opioid receptors on the exercise pressor reflex is unknown. We tested the hypothesis that stimulation of κ-opioid receptors attenuates the exercise pressor reflex in rats with ligated, but not freely perfused, femoral arteries. The pressor responses to static contraction were compared before and after femoral arterial or intrathecal injection of the κ-opioid receptor agonist U62066 (1, 10, and 100 μg). Femoral arterial injection of U62066 did not attenuate the pressor responses to contraction in either group of rats. Likewise, intrathecal injection of U62066 did not attenuate the pressor response to contraction in rats with freely perfused femoral arteries. In contrast, intrathecal injection of 10 and 100 μg of U62066 attenuated the pressor response to contraction in rats with ligated femoral arteries, an effect that was blocked by prior intrathecal injection of the κ-opioid receptor antagonist nor-binaltorphimine. In rats with ligated femoral arteries, the pressor response to stimulation of peripheral chemoreceptors by sodium cyanide was not changed by intrathecal U62066 injections, indicating that these injections had no direct effect on the sympathetic outflow. We conclude that stimulation of spinal, but not peripheral, κ-opioid receptors attenuates the exaggerated exercise pressor reflex in rats with ligated femoral arteries.