Involvement of NMDA Receptor and L-type Calcium Channel in the Excitatory Action of Morphine

Involvement of NMDA Receptor and L-type Calcium Channel in the Excitatory Action of Morphine

<P> We studied the excitatory action of morphine on the responses of dorsal horn neuron to iontophoretic application of excitatory amino acid and C-fiber stimulation by using the <I>in vivo</I> electrophysiological technique in the rat. In 137 of the 232 wide dynamic range (WDR) neurons tested, iontophoretic application of morphine enhanced the WDR neuron responses to N-methyl-D-aspartate (NMDA), kainate, and graded electrical stimulation of C-fibers. Morphine did not have any excitatory effects on the responses of low threshold cells. Morphine-induced excitatory effect at low ejection current was naloxone-reversible and reversed to an inhibitory action at high ejection current. NMDA receptor, calcium channel and intracellular Ca<SUP>2&#8290;</SUP> antagonists strongly antagonized the morphine-induced excitatory effect. These results suggest that changes in intracellular ionic concentration, especially Ca<SUP>2&#8290;</SUP>, play an important role in the induction of excitatory effect of morphine in the rat dorsal horn neurons.