Ontogenetic homologous sensitization to the antinociceptive action of quinpirole in rats.

Repeated postnatal treatment of rats with the dopamine receptor agonist, quinpirole results in exaggeration of selected behaviors that are induced by quinpirole in adulthood. To determine whether the antinociceptive response to quinpirole could be similarly enhanced, rats were treated daily from birth with quinpirole HCl (3.0 mg/kg per day i.p. x 28 days) and their response time in the hot plate analgesia test was determined at 4 months. An acute dose of quinpirole HCl (100 or 1000 micrograms/kg i.p.) produced an analgesic response in the neonatally primed rats and in the vehicle controls. More significantly, the effect was substantially greater in the quinpirole-primed group at each of these two doses of quinpirole. This effect of quinpirole was fully attenuated in both groups by treatment with the dopamine receptor antagonist, spiperone HCl (0.30 mg/kg i.p., 1 h before quinpirole). The analgesic effect of morphine sulfate (6.0 mg/kg i.p.) was not greater in the quinpirole-primed group. These findings demonstrate that the ontogenetic sensitization of quinpirole receptors results in enhanced antinociceptive responses to quinpirole in adulthood. This animal model may be useful for studying the involvement of dopamine systems in algesia and analgesia.