The induction and quantitation of methadone dependence in the rat

Slow-release emulsion (SRE) formulations of methadone were used to induce dependence in rats. Animals were exposed to total methadone doses of either 0, 15, 31.25, 62.5, or 125 mg/kg over 48 h. Withdrawal was induced following intraperitoneal challenge with either naloxone (3 mg/kg) or saline (control), and dependence was assessed in terms of the presence/absence of 13 nominated withdrawal behaviors. Three scoring systems to quantify dose-response relationships for withdrawal are described: (1) using the mean number of withdrawal behaviors per animal within each treatment group; (2) using the sum of the percentage of animals within a treatment group displaying each of the withdrawal behaviors; and (3) a modification of these, to further isolate the naloxone-induced component of the withdrawal score, that is, subtraction of data obtained from saline-challenged animals from those of naloxone-challenged rats. In SRE-treated rats, schemes (1) and (2) gave rise to positive dose-response relationships, while scheme (3) resulted in bell-shaped dose-response curves. To validate the proposed scoring systems, each was applied to data obtained from animals made dependent to methadone via administration in the drinking water. The most appropriate system was that utilizing the mean number of withdrawal behaviors; the method is simple, robust, and amenable to statistical analysis.