Captopril and capsaicin modify opioid withdrawal in the morphine-dependent rat

The involvement of neurokinins, especially substance P, in the opiate withdrawal syndrome was studied by treating rats with drugs that have been reported to increase (captopril) or decrease (capsaicin) tissue levels of substance P. Preliminary experiments with captopril (0.1, 0.3, 1 or 3 mg/kg, SC) showed that the 0.3 mg/kg dose enhanced some of the naloxone-precipitated withdrawal signs. Captopril alone had no effect in the morphine-dependent rat. On experimental days, either saline or captopril (0.3 mg/kg) was injected (SC) immediately before naloxone in morphine-dependent rats that were pretreated (4 to 10 days before the morphine pellet implantation) with either capsaicin (125 mg/kg, SC) or the capsaicin vehicle (N = 8 for each of 4 groups). Capsaicin treatment inhibited the following withdrawal signs: rhinorrhea, lacrimation and salivation. Captopril increased the occurrence of these secretory responses in vehicle-treated but not in capsaicin-treated animals. Other withdrawal signs were not altered by either captopril or capsaicin treatment. The results support the conclusion that substance P and related neurokinins may be involved in the expression of some signs of opioid withdrawal.