Striatal norepinephrine efflux in L-DOPA-induced dyskinesia

L-DOPA remains the primary treatment for Parkinson's disease (PD). Unfortunately, its therapeutic benefits are compromised by the development of abnormal involuntary movements (AIMs) known as L-DOPA-induced dyskinesia (LID). The norepinephrine (NE) system originating in the locus coeruleus is profoundly affected in PD and known to influence dopamine (DA) signaling. However, the effect of noradrenergic loss on L-DOPA-induced striatal monoamine efflux and Parkinsonian motor behavior remains controversial and is frequently overlooked in traditional animal models of LID. Thus, the current study sought to determine whether degeneration of the DA and/or NE system(s) altered L-DOPA-induced striatal monoamine efflux in hemiparkinsonian rats with additional NE loss induced by the potent NE-toxin α DA beta hydroxylase (DBH)-saporin. Sham-, DA-, NE-, and dual DA + NE-lesioned rats were treated with L-DOPA (6 mg/kg, s.c.) for 2 weeks. Thereafter, L-DOPA-mediated striatal monoamine efflux was measured with in vivo microdialysis, and concurrent AIMs testing occurred to determine responsiveness to L-DOPA. Noradrenergic lesions exacerbated parkinsonian motor deficits but did not significantly alter LID expression or corresponding L-DOPA-induced striatal monoamine efflux. Interestingly, L-DOPA-induced striatal NE efflux rather than DA efflux, corresponded more closely with dyskinesia severity. Moreover, marked reductions in striatal NE tissue concentration did not appear to impact L-DOPA-induced striatal NE efflux. The current study implicates L-DOPA-induced striatal NE as an important factor in LID expression and demonstrates the importance of developing treatment strategies that co-modulate the NE and DA systems.