Induction of physical dependence on codeine in the rat by drug-admixed food ingestion.

The developmental process of physical dependence on codeine has been explored in rats treated with codeine-admixed food (0.5 mg/g food) during 1 to 7 days. In rats treated with codeine for more than 2 days, body weight loss was markedly observed after the abrupt codeine withdrawal. The intensity and time course of body weight loss increased according to the duration of codeine treatment. After the codeine withdrawal, behavioral signs such as diarrhea, ptosis and vocalization were observed. In the naloxone-precipitated withdrawal test, rats treated with codeine for 1 day manifested a loss of body weight after naloxone challenge, and the intensity of body weight loss increased according to the duration of codeine treatment. After naloxone injection, the codeine-treated rats showed abnormal behaviors such as diarrhea, ptosis, teeth chattering , salivation, body shakes, vocalization, nose bleed, irritability, lacrimation and writhing. The total score, evaluated by the ranking system for precipitated withdrawal behaviors, was correlated with the duration of codeine treatment. These results suggest that naloxone-precipitated withdrawal signs are powerful in comparison with that after codeine withdrawal, and the weight loss is a common index for quantitative assessment of physical dependence on narcotics in the natural and naloxone-precipitated withdrawal tests. It is concluded that the drug-admixed food ingestion method has the advantage of rapidly inducing a high degree of physical dependence on codeine.