Enkephalin but not morphine modulates the motor activity in the frog spinal cord in vitro.

In the isolated frog spinal cord, methionine-enkephalin (ME, up to 3 X 10(-5) M) hyperpolarized the resting ventral and dorsal root potentials. These hyperpolarizations remained even under Ca2+-free conditions. ME depressed the fast component of the electrically stimulated spinal reflex and enlarged the following depolarizing component. Morphine (10(-4) M) had no apparent ME-like effects. ME may directly and indirectly affect the motoneuron and modulate the spinal motor activity in the frog spinal cord.