The expression of neuropeptide-induced excessive grooming behavior in dopamine D1 and D2 receptor-deficient mice

Grooming behavior in rodents has long been related to dopamine receptors in the brain. However, the relative contribution of dopamine D-1-like receptors (D-1 and D-5) and D-2-like receptors (D-2, D-3 and D-4) in this behavior has not been established yet. Spontaneous novelty-induced grooming (as assessed with a 30-min sampling test) was reduced in knockout mice lacking the dopamine D-1 receptor. Furthermore, the intracerebroventricular (i.c.v.) injection of small quantities of oxytocin, prolactin or the adrenocorticotrophic hormone 1-24 fragment, ACTH-(1-24) was followed by a diminished level of novelty-induced excessive grooming. These neuropeptides caused a sustained increase in grooming level of control animals (wild type). Interestingly, the i.c.v, injection of beta-endorphin enhanced novelty-induced grooming to a level similar in control and knockout mice. The systemic administration of the dopamine D-2 receptor antagonist,sulpiride did not suppress the residual grooming activity shown by animals injected with oxytocin, prolactin or ACTH-(1-24), and did not change the behavioral expression of those injected with beta-endorphin. In contrast, the systemic administration of the opioid receptor antagonist, naloxone, totally suppressed the residual grooming activity of oxytocin-, prolactin- or ACTH-(1-24)-injected mice and of those treated with beta-endorphin. In contrast with the behavioral deficit observed in dopamine D-1 receptor-deficient mice, dopamine D-2 receptor-null animals showed a normal expression of spontaneous novelty-induced grooming and a high level of grooming activity induced by i.c.v. injection of oxytocin, prolactin, ACTH-(1-24) or beta-endorphin. Again, the peripheral injection of naloxone was followed by a suppression of neuropeptide-induced excessive grooming in these animals. These data suggest that dopamine D-1 receptors are involved in the expression of novelty-induced grooming in mice. In contrast, dopamine D-2 receptors seem not to be important for the expression of this behavior. Furthermore, neuropeptide-enhanced grooming involves dopamine D-1, but not dopamine D-2 receptors. However, neurotransmitters other than dopamine (e.g., endorphins) may play a supplementary role in neuropeptide-enhanced grooming in mice. (C) 1999 Elsevier Science B.V. All rights reserved.