Changes in core temperature and feeding in rats by levorphanol and dextrorphan

Abstract

Male Sprague–Dawley rats received saline for 5 days before and 5 days after daily subcutaneous injections of levorphanol or dextrorphan tartrate (8 mg base/kg) for 10 days. Core temperatures, measured by telemetry, and acquisition of food pellets on a continuous reinforcement schedule were monitored simultaneously and recorded every 30 min for each rat throughout the experiment. After the first levorphanol injections signs of acute intoxication were apparent, and a mild but delayed hyperthermia was observed and food intake declined. With repeated injections of levorphanol peak hyperthermia increased and occurred with a shorter latency after administration, as did a phase of 'stimulated' feeding activity. This phase of vigorous feeding during light hours markedly disrupted the characteristic diurnal pattern of daily food intake. During withdrawal temperatures decreased, feeding became more intermittent, and signs of increased irritability were evident in the levorphanol group. Patterns during daily injections of dextrorphan, and after its withdrawal, were similar to those in the saline control period. These results indicate that the patterns of change in temperature and feeding responses to levorphanol are similar to those found earlier with heroin, codeine, and morphine, and that these changes involve stereospecific receptors.