Conditioned temperature effects using morphine as the unconditioned stimulus

Abstract

The conditioning of body temperature changes using an injection of morphine sulphate as the conditioned stimulus was studied in 30 male Wistar rats. Three groups of animals received daily i.p. injections of either 5, 25, or an increasing dose to 200 mg/kg morphine; a fourth group received saline injections throughout. Rectal temperature was measured in three different environments five times during the day: in a neutral environment, the home cage; in a pre-injection environment, in which animals were placed for a period before the daily injection; and in an injection environment, in which animals remained after the injection. Conditioning trials were followed by a period of abstinence from morphine. Tests for conditioned effects were carried out both during conditioning and after the period of abstinence. During conditioning, animals in the morphine groups, when compared to the saline control animals, showed a conditioned anticipatory hypothermia in the preinjection environment that was opposite in direction to the unconditioned hyperthermia to morphine. In contrast, in the injection environment, animals in the morphine groups showed a conditioned hyperthermia when tested after the period of abstinence. These results suggest a complex interaction between the conditioned and unconditioned temperature responses to morphine.