Evaluation of Wistar rats' individual sensitivity to the development of physical dependence on morphine

Behavioral and physiological indices which could be symptoms of individual Wistar rats' sensitivity to the development of physical dependence on morphine, were studied. This state was induced in three different ways: (1) by i.p. injection of increasing doses of the drug; (2) by i.p. injection of morphine as a reward of a rat's visit to a certain compartment of the Y-maze; (3) by voluntary drinking a morphine solution. Animals with high sensitivity to the development of dependence induced by the first way initially demonstrated intensive ambulation and low rearing in the open field and low nociception in the tail flick test. In the second case the 'predisposed' animals had initially higher ambulation also in comparison with the 'resistant' ones, but did not demonstrate significant withdrawal symptoms. The rats imbibing morphine solution exhibited a withdrawal syndrome, low level of initial nociception and received more electrocutaneous stimuli in the Vogel test. Thus, marked differences of sensitivity to the development of morphine dependence are correlated with some initial behavioural and physiological indices and may be genetically and neurochemically determined.