Morphine enhances and depresses Ca2+-dependent responses in visceral primary afferent neurons.

Morphine at 1 to 100 nM enhances and depresses Ca2+-dependent potentials, the shoulder of the action potential, the hyperpolarizing afterpotential, and the calcium spike in visceral primary afferent neurons of rabbits. These effects could be reversed by naloxone. The enhancement and depression of the Ca2+-dependent potentials are not only concentration-dependent but they are also time-dependent and can be observed with a given concentration of morphine.