Preferences for opioids by the weight pulling method in rats.

The preference for morphine and codeine was studied by means of the antagonistic conflict behavior between the positive drive of drug intake and the negative drive of weight pulling in rats. An apparatus was developed in which rats were compelled to pull the weight for the intake of drug-admixed food. The experiments began with the preadministration of the drug through the repetition of CFF schedule. The schedule consisted of one choice trial between the intake of normal food and drug-admixed food followed by two consecutive forced trials, in which the rats were forced to take the drug-admixed food only. In the test trial, the findings were that the rats which had already shown a drug seeking behavior toward morphine or codeine pulled weight to take each drug and that the reinforcing effects of these drugs on the drug seeking behavior depended on the treatment period of these drugs. The reinforcing effect of codeine was weaker than one of morphine. It is suggested that the reinforcing effects of these opioids can be evaluated quantitatively by the weight pulling method in rats.