Buprenorphine Antinociception is Abolished, but Naloxone-Sensitive Reward is Retained, in μ-Opioid Receptor Knockout Mice

Abstract

Buprenorphine is a relatively nonselective opioid receptor partial agonist that is used in the management of both pain and addiction. To improve understanding of the opioid receptor subtypes important for buprenorphine effects, we now report the results of our investigation on the roles of m-, d-, and k-opioid receptors in antinociceptive responses and place preferences induced by buprenorphine. Buprenorphine antinociception, assessed by hot-plate and tail-flick tests, was significantly reduced in heterozygous m-opioid receptor knockout (MOR-KO) mice and abolished in homozygous MOR-KO mice. In contrast, buprenorphine retained its ability to establish a conditioned place preference (CPP) in homozygous MOR-KO, although the magnitude of place preference was reduced as the number of copies of wild-type m-opioid receptor genes was reduced. The remaining CPP of buprenorphine was abolished by pretreatment with the nonselective opioid antagonist naloxone, but only partially blocked by pretreatment with either the d-selective opioid antagonist naltrindole or the k-selective opioid antagonist norbinaltorphimine. These data, and biochemical confirmation of buprenorphine actions as a partial d-, m-, and k-agonist, support the ideas that m-opioid receptors mediate most of analgesic properties of buprenorphine, but that m-and d-and/or k-opioid receptors are each involved in the rewarding effects of this drug.

Abolished thermal and mechanical antinociception but retained visceral chemical antinociception induced by butorphanol in μ-opioid receptor knockout mice

“…Both the analgesic effects of morphine in the tail-flick and hotplate tests and the rewarding effects of morphine in self-administration tests are abolished in MOP knockout (KO) mice (Loh et al, 1998;Sora et al, 1997bSora et al, , 2001). Buprenorphine, a nonselective opioid receptor partial agonist, has no analgesic effect in the tail-flick and hotplate tests but a significant rewarding effect in the conditioned place preference tests in homozygous MOP-KO mice (Ide et al, 2004). These observations are especially interesting because the distributions of DOP and KOP are not apparently altered in MOP-KO mice (Loh et al, 1998;Matthes et al, 1996;Sora et al, 1997b).…”

“…Butorphanol bound with higher affinity than morphine to membranes prepared from MOP/CHO, DOP/CHO, and KOP/CHO cells. The morphine results were obtained from previous data (Ide et al, 2004) that were analyzed according to the present methods. The affinities of butorphanol for MOP and KOP were equivalent and higher than for DOP.…”

“…IC 50 values of butorphanol were lower than those of morphine for MOP/CHO, DOP/CHO, and KOP/CHO cells (Table 1). The morphine results were obtained from previous data (Ide et al, 2004) that were analyzed according to the present methods.…”

“…Mice were placed on a 52 ± 0.2°C hot-plate, and latencies to hind-paw-lick or jump were recorded. We selected a relatively lower temperature (52°C) to examine the mild thermal antinociceptive effects of opioid partial agonists (Ide et al, 2004). The cut-off time was 60 s. Tail-flick testing was carried out according to the method of D'Amour and Smith (1941) with slight modifications using a commercially available apparatus consisting of an irradiator for heat stimulation and a photosensor for detection of tail-flick behavior (Model MK-330A, Muromachi Kikai Co., Tokyo, Japan).…”

“…Chinese hamster ovary (CHO) cell lines stabling expressing human -, -, and -opioid receptors (MOP/CHO, DOP/CHO, and KOP/CHO, respectively) were established as previously described (Ide et al, 2004 H]U69593 to KOP were 1.7 ± 0.3 nM (n = 4), 2.2 ± 0.2 nM (n = 4), and 2.5 ± 0.2 nM (n = 3), respectively. B max estimates of receptor densities in these cell lines were 2300 ± 160, 3000 ± 270, and 5000 ± 450 fmol/mg protein, respectively.…”