Adenosine A(2A) receptor gene expression in the normal striatum and after 6-OH-dopamine lesion

Adenosine A(2A) receptors are present on enkephalinergic medium sized striatal neurons in the rat and have an important function in the modulation of striatal output. In order to establish more accurately whether adenosine transmission is a generalized phenomenon in mammalian striatum we compared the A(2A) R expression in the mouse, rat, cat and human striatum. Secondly we compared the modulation of enkephalin gene expression and A(2A) receptor gene expression in rat striatal neurons after 6-OH-dopamine lesion of the substantia nigra. Hybridization histochemistry was performed with a S-35-labelled radioactive oligonucleotide probe. The results showed high expression of A(2A) adenosine receptor genes only in the medium-sized cells of the striatum in all examined species. In the rat striatum, expression of A(2A) receptors was not significantly altered after lesion of the dopaminergic pathways with 6-OH-dopamine even though enkephalin gene expression was up-regulated. The absence of a change in A(2A) receptor gene expression after 6-OH-dopamine treatment speaks against a dependency on dopaminergic innervation. The maintained inhibitory function of A(2R) R on motor activity in spite of dopamine depletion could be partly responsible for the depression of locomotor activity observed in basal ganglia disorders such as Parkinson's disease.