Excessive grooming induced by the administration of codeine and morphine

Previous studies have shown that peripheral injections of the codeinone RX 336-M can induce excessive grooming in rats in an age-dependent fashion. The present experiment demonstrates that codeine, itself, also induces excessive grooming but with apparent equal effectiveness at all ages tested. Because of the structural similarity between codeine and morphine, the effects of intraperitoneally administered morphine were examined as well. Morphine was found to produce a temporal course of excessive grooming quite different from that produced by codeine. Morphine did not affect grooming in the first half (30 min) of the observation period, but accentuated it, briefly, for the next 30-45 min. Intraventricular administration of codeine at doses of 0.3 or 1.0 microgram/3 microliter had no effect on grooming in animals previously shown to demonstrate excessive grooming in response to either dose.