Receptor adaptive responsiveness in disease models: 6-OHDA lesioned and spontaneously hypertensive rats

Dopamine receptors status after unilateral nigral 6-OHDA lesion. Autoradiographic and in situ hybridization study in the rat brain

The physiological effects of dopamine (DA) are mediated by several distinct receptor subtypes. The effects of unilateral nigral 6-hydroxydopamine (6-OHDA) lesions on DA receptors were investigated by receptor autoradiography using the D1 selective ligand [3H]SCH 23390 as well as the D2 ligand [3H]spiroperidol. mRNA distribution was studied by in situ hybridization. Lesioned rats were sacrificed at different time intervals. Receptor binding studies were performed on tissue sections using selective ligands. [35S]UTP labeled RNA probes were prepared from the different cDNA (D1, D2, D3) and used for in situ hybridization. A specific loss of receptor binding sites and mRNA hybridization was found in the lesioned substantia nigra pars compacta (SNc) at all times examined. Receptor binding studies revealed a different time-dependent increase in both D1 and D2 receptors. In situ hybridization showed that only D2 receptor mRNA increased in the caudate-putamen (CPu) of the lesioned side 15 d after 6-OHDA. No changes were observed in D1 and D3 receptor mRNA during the entire time-course.

Autoradiographic study of striatal D1 and D2 dopamine receptors in 6-OHDA-lesioned rats receiving foetal ventral mesencephalic grafts and chronic treatment with L-dopa and carbidopa

Foetal dopamine cell suspensions or sham preparations were implanted into the denervated striatum of rats with a unilateral 6-hydroxy-dopamine (6-OHDA) lesion of the medial forebrain bundle. Some animals were also treated with L-DOPA (200 mg/kg/24 h) and carbidopa (25 mg/kg/24 h) in the drinking water for 5 weeks, followed by a 3-week drug-free period. Rotational responses to apomorphine and (+)-amphetamine were assessed, and the density of D1 and D2 dopamine receptors was evaluated autoradiographically in striatal slices exposed to [3H]SCH 23390 or [3H]spiperone. Foetal grafts reduces apomorphine-induced contralateral rotation and prevented the development of apomorphine-induced stereotypy. Foetal grafts abolished (+)-amphetamine-induced ipsilateral rotation. These effects of the grafts were not altered by treatment with L-DOPA. A unilateral 6-OHDA lesion of the nigrostriatal pathway resulted in an ipsilateral increase in D2 receptor density most marked in the lateral and dorsomedial quadrants of the striatum compared with the contralateral side. Foetal ventral mesencephalic grafts implanted into the lesioned striatum decreased D2 receptor density to levels found in the contralateral intact striatum. Chronic L-DOPA and carbidopa treatment did not alter the effect of the grafts. A 6-OHDA lesion resulted in a reduction of D1 receptor density in the lateral areas of the lesioned striatum at Level 2. The presence of a foetal ventral mesencephalic graft either alone or together with L-DOPA treatment did not alter the lesion-induced changes in D1 binding density.

Injections of 6-hydroxydopamine in the substantia nigra of the rat brain: morphological and biochemical effects

Rats with unilateral injections of 6-hydroxydopamine (6-OHDA) into the substantia nigra pars compacta were classified as active and inactive according to the intensity of their spontaneous and/or apomorphine-induced turning behavior (TB), and sacrificed at different survival times for morphological and biochemical analysis. In active rats, at any survival time, dopaminergic fluorescence in the nigrostriatal system as well as dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) content of the nucleus caudatus-putamen drastically decreased on the brain side ipsilateral to the injection. Dopaminergic fluorescence as well as DA and DOPAC content of the mesolimbic system ipsilateral to the injection also decreased. In inactive rats, at any survival time, 6-OHDA-induced lesions only partially involved both nigrostriatal and mesolimbic systems. Our results are indicative of a good correlation between the intensity of TB and the extent of 6-OHDA-induced lesions, as assessed by morphological and biochemical analysis.