Regulation of dihydropyridine-sensitive Ca(2+) channels during naloxone-induced opioid supersensitivity in rats.
The Ca(2+) channel blocker, nimodipine, increases the chronic naltrexone-induced supersensitivity to morphine analgesia in mice without affecting the density of up-regulated mu-opioid receptors. In the present study, the change in dihydropyridine-sensitive Ca(2+) channels associated with chronic naloxone-induced supersensitivity to morphine analgesia was studied in rat whole-brain membranes using a radiolabeled Ca(2+) channel blocker, [3H]PN200-110 [isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5-methoxycarbonylpyridine-3-carboxylate] (0.02-1.0 nmol/l). Rats were chronically treated with naloxone (1 mg/kg, i.p.), nimodipine (1 mg/kg, i.p.) or their respective vehicles twice daily for 10 days. On the 11th day, 16 h after the last injection of either nimodipine or naloxone, morphine (2 mg/kg, i.p.)-induced tail-flick analgesia was determined or rats were killed for the binding study. Chronic naloxone significantly potentiated (+84%) the morphine-induced analgesia. Chronic nimodipine also potentiated (+76%) morphine analgesia. In rats treated with nimodipine and naloxone, there was an additive increase (206%) in morphine analgesia. In binding studies, chronic naloxone resulted in a significant decrease (-39%) in the density (B(max)) of [3H]PN200-110 binding with no change in K(d) value when compared to the effect of chronic vehicle. Chronic nimodipine resulted in a slight but significant (+14.5%) increase in the B(max) of [3H]PN200-110. However, when nimodipine was co-administered with naloxone, it not only reversed the down-regulation but actually up-regulated (+25%) [3H]PN200-110 binding with no change in K(d) value. Our results show significant down-regulation of [3H]PN200-110 binding in association with naloxone-induced analgesic supersensitivity to morphine. The supersensitivity was also observed following chronic blockade of up-regulated Ca(2+) channels by nimodipine. These results indicate an important role of L-type Ca(2+) channel regulation in naloxone-induced analgesic supersensitivity to morphine.
Gullapalli
Srinivas
S
Department of Pharmacology and Toxicology, National Institute of Pharmaceutical Education and Research, Phase-X, Sector 67, SAS Nagar (Mohali), Punjab 160 062, India.
Ramarao
Poduri
P
D000698:Analgesia; D000818:Animals; D001921:Brain / Q000187:drug effects / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D004347:Drug Interactions; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000378:metabolism; D017275:Isradipine / Q000378:metabolism*; D008297:Male; D009020:Morphine; D009270:Naloxone / Q000494:pharmacology*; D009553:Nimodipine / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000378:metabolism
D002121:Calcium Channel Blockers; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D009270:Naloxone; D009553:Nimodipine; D009020:Morphine; D017275:Isradipine