Hyperinnervation of the striatum by dorsal raphe afferents after dopamine-depleting brain lesions in neonatal rats.

Dopamine-depleting brain lesions produced in neonatal rats by intraventricular injection of 6-hydroxydopamine (6-OHDA) are known to increase striatal levels of serotonin by adulthood. We now report that such lesions lead to an increased innervation of the striatum by serotonin-containing neurons from the dorsal raphe nucleus. This hyperinnervation was revealed as a 300% increase in the number of retrogradely labeled neurons in the dorsal raphe after horseradish peroxidase was injected into the adult striatum. The degree of hyperinnervation was reduced sharply when serotonin-depleting brain lesions were given to adult animals that had received 6-OHDA as neonates.