LACK OF EVIDENCE FOR AXONAL TRANSPORT OF D1 AND D2 RECEPTORS IN THE NIGRO-STRIATAL PATHWAY OF THE RAT

The possibility that D1 and D2 dopamine receptors are axonally transported in the nigro-striatal pathway has been investigated in the rat by placing a coronal knife cut (sparing the striato-nigral pathway) through the medial forebrain bundle (MFB) and autoradiographically examining the density of D2 (as labeled by [3H]spiperone in the presence of ketanserin) and D1 (as labeled by [3H]SCH 23390) receptors. The efficacy of MFB transection has been assessed by measuring in parallel the binding of [3]ketanserin, a ligand that has been reported to be axonally transported in this bundle. At 12 h post-transection, there was a minor accumulation of [3H]spiperone binding on both sides of the transection. However, (+)butaclamol (1 microM) failed to displace the ligand build-up at the knife cut, thus demonstrating the nonspecific nature of [3H]spiperone accumulation. Similar results were observed at 24, 48, and 72 h after severing the MFB. MFB transection also failed to cause changes in specific [3H]SCH 23390 binding at the knife cut at 12-72 h post-surgery. In contrast, a dramatic accumulation of [3H]ketanserin binding sites was observed rostral and caudal to the cut at 12 h post-transection, attesting to the efficacy of the lesion. These results confirm the existence of both anterograde and retrograde transport of [3H]ketanserin binding sites and suggest that D1 and D2 receptors are not axonally transported in fibers of the nigro-striatal pathway.