The effects of morphine administered into the vertebral artery on the somatosympathetic A- and C-reflexes in anesthetized cats.

The effects of morphine (0.2-200 micrograms/kg) administered into the brain stem via a vertebral artery on the somatosympathetic A- and C-reflexes evoked in a cardiac sympathetic nerve by electrical stimulation of somatic myelinated (A) and unmyelinated (C) afferent nerve fibers in anesthetized cats were examined. The intravertebral arterial administration of 20-200 micrograms/kg morphine depressed dose-dependently both the somatosympathetic A- and C-reflexes. Following the administration of 200 micrograms/kg morphine, the amplitudes of the sympathetic A- and C-reflexes were depressed to 71 +/- 8% and 80 +/- 8% of the control magnitude, respectively. Injection of 2-200 micrograms/kg morphine caused hypotension in a dose-dependent manner. It is concluded that morphine administered in the brain stem via the vertebral artery produced a nonselective and slight depression of both the somatosympathetic A- and C-reflexes by acting at the level of brain stem.