Effects of verapamil on morphine-induced euphoria, analgesia and respiratory depression in humans.

Organic calcium (Ca++) channel antagonists enhance opiate-induced analgesia and antagonize respiratory depression produced by morphine in rodents. Our preliminary data indicated that verapamil reduces the subjective effects of morphine in humans. We therefore assessed morphine-verapamil interactions in 12 experienced, male polydrug users with histories of heroin abuse by using a double-blind, cross-over study design. Treatments consisted of two drug infusions. Either verapamil, 2.5 or 10 mg, or saline was infused, 30 ml i.v. over 2 min; half way through this infusion either 10 mg of morphine or saline was infused, 3 ml i.v. over 10 sec, via a second catheter. Autonomic parameters, responsiveness to pain and subjective self-reports of mood and feeling state were measured over 4 hr. Analgesia was measured using a finger pressure test and hand immersion in ice water. Respiration was measured by using respiratory inductive plethysmography and transcutaneous CO2 levels. The Addiction Research Center Inventory (ARCI) was used to measure the subjective effects. Morphine had a liminal effect on pain threshold, but verapamil potentiated this effect to elevate pain threshold significantly. Verapamil did not affect the ability of morphine to increase pain endurance or to produce respiratory depression. Morphine produced positive affective responses, as demonstrated by elevated scores on the Morphine-Benzedrine Group subscale of the ARCI. Verapamil alone produced no effects on any ARCI subscales; however, 10 mg of verapamil significantly reduced morphine-elevated MBG scores over a 3-hr period. The results suggest the euphorigenic and analgesic effects of opioids may be differentiated by using Ca++ channel blockers.