Electrophysiological evidence for a presynaptic mechanism of morphine withdrawal in the neonatal rat spinal cord.

Dorsal and ventral root depolarizing responses to capsaicin (1 microM) and substance P (SP; 1 microM) were measured from the isolated, hemisected spinal cord of the neonatal rat. Capsaicin depolarized the dorsal and ventral roots. The mechanism of ventral root depolarization was presynaptic; since dorsal root depolarization preceded the ventral, and the ventral depolarization was eliminated when synaptic transmission was blocked in the absence of calcium. SP depolarized the ventral root without affecting the dorsal root. The SP-induced depolarization of the ventral root was reduced but not abolished by blocking synaptic transmission with low calcium, suggesting that SP acted postsynaptically on motoneurons and excitatory interneurons to depolarize the ventral root. Morphine (10 microM) abolished the capsaicin-induced ventral root depolarization, but only slightly suppressed the SP response (30%). The capsaicin-induced depolarization of the ventral root was enhanced greatly (238%) when morphine, which had been in the superfusion for 1 h, was removed and naloxone (1 microM) was added to the superfusion solution, whereas the SP response was not augmented during withdrawal from acute morphine. Furthermore, a putative SP antagonist ([D-Arg1, D-Pro2, D-Tryp7,9, Leu11]-SP) prevented the augmented capsaicin ventral root response during precipitated withdrawal. These data provide electrophysiological evidence for a presynaptic mechanism of acute morphine withdrawal in the neonatal rat spinal cord.