Methamphetamine-induced behavioral sensitization in mice: alterations in mu-opioid receptor

Abstract

We had previously demonstrated that opioid receptors contribute to the induction and expression of behavioral sensitization induced by repeated daily injection with 2.5 mg/kg of methamphetamine for 7 days. Using the same regimen, the present study investigated the alterations in mu-opioid receptor during the induction (on days 2, 5, and 8) and expression (on days 11 and 21) periods of behavioral sensitization. Radioligand binding revealed that the maximal binding of mu-opioid receptor was not changed on days 2 and 5, but down-regulated on day 8. After cessation of drug treatment, the maximal binding of mu-opioid receptor gradually and time-dependently returned to normal level on day 11 and up-regulated on day 21. In contrast, no changes in delta- and kappa-opioid receptors were detectable on any given day examined. The potency of DAMGO for [(35)S]-GTPgammaS coupling was enhanced on days 2, 5, 11, and 21. Moreover, 1 muM of naltrexone or beta-chlornaltrexamine significantly suppressed the basal [(35)S]-GTPgammaS coupling on days 2, 11, and 21. These findings indicate enhanced responsiveness and elevated constitutive activity of mu-opioid receptor. In summary, our data clearly demonstrate that alterations in mu-opioid receptor are involved in and may contribute to the sensitization to locomotor stimulating effect of methamphetamine.

Figure 1

Effects of METH on receptor binding in brain membranes during the induction period. Saturation analyses of radioligand binding using [³H]-DAMGO, [³H]-DPDPE, or [³H]-U69,593 were performed in the brain from mice 24 h after the first, fourth, and last daily injections of saline or METH on days 2 (a), 5 (b), and 8 (c), respectively. Data are mean values (mean ± S.E.M.) of four independent experiments.

Figure 2

Effects of METH on receptor binding in brain membranes during the expression period. Saturation analyses of radioligand binding using [³H]-DAMGO, [³H]-DPDPE, or [³H]-U69,593 were performed in the brain from saline- or METH-pretreated mice after short-term and long-term periods of abstinence on days 11 (a) and 21 (b), respectively. Data are mean values (mean ± S.E.M.) of four independent experiments.

Figure 3

Effects of METH on DAMGO-stimulated [³⁵S]-GTPγS coupling in brain membranes during the induction period. [³⁵S]-GTPγS coupling concentration–response curves for DAMGO were obtained in the brain from mice 24 h after the first, fourth, and last daily injections of saline or METH on days 2 (a), 5 (b), and 8 (c), respectively. Data are expressed as % of basal level (mean ± S.E.M.) under identical conditions of six independent experiments.

Figure 4

DAMGO-stimulated [³⁵S]-GTPγS coupling in wild-type and µ-opioid receptor knockout mice. [³⁵S]-GTPγS coupling concentration–response curves for DAMGO were obtained in the brain from wild-type and µ-opioid receptor knockout mice without any treatment. Data are expressed as % of basal level (mean ± S.E.M.) under identical conditions of three independent experiments.

Figure 5

Effects of METH on DAMGO-stimulated [³⁵S]-GTPγS coupling in brain membranes during the expression period. [³⁵S]-GTPγS coupling concentration-response curves for DAMGO were obtained in the brain from saline- or METH-pretreated mice after short-term and long-term periods of abstinence on days 11(a) and 21 (b), respectively. Data are expressed as % of basal level (mean ± S.E.M.) under identical conditions of six independent experiments.

Figure 6

Effects of antagonists on basal [³⁵S]-GTPγS coupling in brain membranes during the induction period. Effects of NAT (a) and β-CNA (b) were determined in the brain from mice 24 h after the first, fourth, and last daily injections of saline or METH on days 2, 5, and 8, respectively. Data are expressed as % of basal level (mean ± S.E.M.) under identical conditions (n = 6). *p < 0.05, **p < 0.01, compared with response of corresponding saline-pretreated group, according to Bonferroni post-tests after a two-way ANOVA (METH treatment × days of treatment).

Figure 7

Effects of antagonists on basal [³⁵S]-GTPγS coupling in brain membranes during the expression period. Effects of NAT (a) and β-CNA (b) were determined in the brain from saline- or METH-pretreated mice after short-term and long-term periods of abstinence on days 11 and 21, respectively. Data are expressed as % of basal level (mean ± S.E.M.) under identical conditions (n = 5). *p < 0.05, **p < 0.01, compared with response of corresponding saline-pretreated group, according to Bonferroni post-tests after a two-way ANOVA (METH treatment × days of abstinence).