Unilateral dopamine denervation blocks corticostriatal LTP

The nigrostriatal dopaminergic projection is crucial for the striatal processing of motor information received from the cortex. Lesion of this pathway in rats causes locomotor alterations that resemble some of the symptoms of Parkinson's disease and significantly alters the excitatory transmission in the striatum. We performed in vitro electrophysiological recordings to study the effects of unilateral striatal dopamine (DA) denervation obtained by omolateral nigral injection of 6-hydroxydopamine (6-OHDA) in the formation of corticostriatal long-term potentiation (LTP). Unilateral nigral lesion did not affect the intrinsic membrane properties of striatal spiny neurons. In fact, these cells showed similar pattern of firing discharge and current-voltage relationship in denervated striata and in naive controlateral striata. Moreover, excitatory postsynaptic potentials (EPSPs) evoked by stimulating corticostriatal fibers and recorded from DA-denervated slices showed a pharmacology similar to that observed in slices obtained from controlateral intact striata. Conversely, in magnesium-free medium, high-frequency stimulation (HFS) of cortico-striatal fibers produced LTP in slices from nondenervated striata but not in slices from 6-OHDA-denervated rats. After denervation, in fact, no significant changes in the amplitude of extra- and intracellular synaptic potentials were recorded after the conditioning HFS. The absence of corticostriatal LTP in DA-denervated striata might represent the cellular substrate for some of the movement disorders observed in Parkinson's disease.