Delayed plasticity of the mesolimbic dopamine system following neonatal 6-OHDA lesions.

In this study, we determined the ontogenetic profile (at postnatal days 7, 14, 35, and 90) of tyrodine hydroxylase (TH) mRNA in the ventral mesencephalon, and the levels of TH immunoreactivity (TH-IR) and dopamine (DA) transporter (DAT) sites in the striatum of rats that had received intrastriatal 6-hydroxy dopamine (6-OHDA) or vehicle lesions on day of birth (DO) or postnatal day 1 (P1). TH-IR was significantly decreased in all quadrants of the caudate-putamen at all time points, while TH-IR in the nucleus accumbens was unchanged, as compared to controls. Relative to the earliest time point (P7 lesion group), TH-IR recovered significantly in the medial caudate-putamen (CPu) of the P14, P35 and P90 6-OHDA-lesioned groups. Quantitative autoradiography of [3H]-mazindol binding to DAT sites showed significant, lesion-induced losses throughout the caudate-putamen of the 6-OHDA-lesioned groups at all time points and did not show appreciable recovery. Using in situ hybridization, significant (P < .05) decreases in TH mRNA levels were found at all time points in the lateral and medial substantia nigra pars compacta of 6-OHDA-lesioned animals. TH mRNA levels in the rostral ventral tegmental area (VTA), which were significantly decreased at P7, P14 and P35, returned to control levels at P90. TH mRNA levels in the caudal VTA were unchanged through P35 and became significantly elevated as compared to controls (+22%, P < .05) by P90. Thus, recovery of TH-IR in the medial caudate-putamen occurred prior to the elevation in levels of TH mRNA of the VTA. Our findings suggest that compensation exists in early development in certain subpopulations of mesostriatal DA neurons that differs from that in the adult.