Effects of noradrenergic denervation by anti-DBH-saporin on behavioral responsivity to l-DOPA in the hemi-parkinsonian rat

Dopamine (DA) replacement with l-DOPA remains the most effective pharmacotherapy for motor symptoms of Parkinson's disease (PD) including tremor, postural instability, akinesia, and bradykinesia. Prolonged l-DOPA use frequently leads to deleterious side effects including involuntary choreic and dystonic movements known as l-DOPA induced dyskinesias (LID). DA loss in PD is frequently accompanied by concomitant noradrenergic (NE) denervation of the locus coeruleus (LC); however, the effects of NE loss on l-DOPA efficacy and LID remain controversial and are often overlooked in traditional animal models of PD. The current investigation examined the role of NE loss in l-DOPA therapy by employing the NE specific neurotoxin anti-DA-beta hydroxylase saporin (αDBH) in a rat model of PD. Rats received unilateral 6-hydroxydopamine lesions of the medial forebrain bundle to deplete nigral DA and intraventricular injection of vehicle (DA lesioned rats) or αDBH (DANE lesioned rats) to destroy NE neurons bilaterally. Results indicated that αDBH infusion drastically reduced NE neuron markers within the LC compared to rats that received vehicle treatment. Behaviorally, this loss did not alter the development or expression of l-DOPA- or DA agonist-induced dyskinesia. However, rats with additional NE lesions were less responsive to l-DOPA's pro-motor effects. Indeed, DANE lesioned animals rotated less and showed less attenuation of parkinsonian stepping deficits following high doses of l-DOPA than DA lesioned animals. These findings suggest that severe NE loss may reduce l-DOPA treatment efficacy and demonstrate that degradation of the NE system is an important consideration when evaluating l-DOPA effects in later stage PD.