OPIOID, CALCIUM, AND ADRENERGIC-RECEPTOR INVOLVEMENT IN PROTOPINE ANALGESIA

The analgesic effect of protopine (Pro), an alkaloid isolated from Pap averaceae, was confirmed by tail-pinch and hot-plate tests when given sc 10-40 mg . kg-1, and 20 - 40 mg . kg-1 inhibited the spontaneous mo vements of mice. Prp 40 mg . kg-1 increased the sleeping rate, prolong ed the sleeping duration, and shortened the sleeping latency in mice h ypnotized by ip pentobarbital sodium 30 mg . kg-1. Pro 10 - 40 mg . kg -1 did not affect the inflammatory reaction induced by xylene and egg white. An icv injection of Pro 20 - 200 mug/mouse showed a remarkable analgesic effect in mice. The icv pretreatment of naloxone 2 mug block ed the analgesic effect completely. CaCl2 40 mug/mouse (icv) or methot rexate 10 mg . kg-1(ip), an agonist of Ca2+ channel, showed a complete blockade of the analgesia, while nifedipine 100 mg-kg-1 (po), a block er of Ca2+ channel, enhanced the analgesic effect. The ip pretreatment of reserpine 4 mg . kg-1 reduced the Pro analgesia. Phentolamine 10 m g . kg-1(ip), an alpha-adrenergic blocker, tended to weaken the analge sia, but propranolol 10 mg . kg-1(ip), a beta-blocker, did not affect it. These results suggest that Pro displays its analgesic effect mainl y through the opioid and calcium systems and partly through the adrene rgic mechanism.