Enhanced quinpirole response in rats lesioned neonatally with 5,7-dihydroxytryptamine

The ontogenic destruction of dopamine (DA) neurons in rat brain is associated with supersensitization of DA D₁ receptors. This effect is attenuated when rats are cotreated in ontogeny with the serotonin (5-HT) neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT). In an attempt to determine whether 5-HT fibers might have a similar modulatory role on the sensitivity of the DA D₂ receptor complex, we pretreated rats with desipramine HCl (20 mg/kg IP, base), 1 h before the DA neurotoxin, 6-hydroxydopamine (6-OHDA; 134 μg ICV, base) and/or 5,7-DHT (75 μg ICV) and/or vehicle. At about 3 months after birth dose-effect curves for quinpirole-induced oral activity were constructed for each group of rats. We found that quinpirole, an agonist for the DA D₂ receptor complex, produced a dose-related increase in oral activity in all groups of rats. After a 200 μg/kg dose of quinpirole HCl, however, neonatal 5,7-DHT-lesioned rats had a peak oral response of 54.4 ± 5.1 (mean and SEM) vs. 22.6 ± 4.8 for control rats (p < 0.01). In neonatal 6-OHDA-lesioned rats this dose of quinpirole increased oral activity to 36.8 ± 5.8 oral movements (p < 0.05 vs. control). In rats lesioned with both 5,7-DHT and 6-OHDA, the oral response was not different from control. The enhanced oral response to quinpirole in 5,7-DHT-lesioned rats was attenuated by spiperone, an antagonist for the DA D₂ receptor complex. These findings are believed to be the first to demonstrate that receptors of the DA D₂ complex become sensitized after ontogenic injury to 5-HT fibers. This effect is opposite to the attenuated sensitivity of DA D₁ receptors in rats with a similar 5-HT lesion.