The enkephalinase inhibitor, GB 52, does not affect nociceptive flexion reflexes nor pain sensation in humans

The effects of intravenous administration of 2.5 mg/kg of GB 52, a highly potent derivative of the enkephalinase inhibitor, thiorphan, were studied on the threshold of both the nociceptive reflex (Tr) and sensation of pain (Tp) as well as on the thresholds of both recruitment of the maximal nociceptive reflex response (Tmr) and tolerable pain (Tip), elicited by electrical stimulation of the sural nerve in normal and relaxed volunteers. It was found that neither the nociceptive motor responses (Tr and Tmr) nor the subjective reports of pain (Tp and Tip), were significantly affected by GB 52. It is concluded that, in the experimental conditions used, the transmission of nociceptive messages at the spinal level is not tonically modulated by any enkephalinergic system.