Developing a novel animal model for the study of Parkinson's disease : focusing on the early loss of noradrenergic neurons

Parkinson’s disease (PD) is most commonly associated with dopaminergic neuronal loss inthe substantia nigra (SN), but recent evidence suggests that concomitant loss of noradrenergic neurons in the locus coeruleus (LC) also heavily contributes to PD pathology and symptomology. The LC more specifically appears to influence PD progression and severity because neurodegeneration in the LC consistently precedes nigral loss in PD patients and because LC damage exacerbates neurotoxic effects of MPTP (a neurotoxin that preferentially targets and destroys dopaminergicneurons) at the SN. Establishing a viable PD animal model thus requires inclusion of both SN and LC brain regions. The primary goal of this study was to determine whether focal injection of 6-hydroxydopamine (6-OHDA), a neurotoxin to which noradrenergic cells are particularly susceptible, induces sufficient LC damage in mice to be comparable to the Parkinsonian brain (~ 50% loss). Extent of neuronal damage was determined by quantifying the LC density of two synthetic enzymes in the catecholamine synthesis pathway. Coupling this LC model with MPTP further allowed us to simulate PD disease progression and to observe the neuroprotective effects of the LC on the SN. We assayed for the norepinephrine transporter (NET), a synaptic signaling protein, in an attempt to characterize the mechanism of LC protective action (direct innervation versus diffuse neuromodulation). Among the 6-OHDA dosages (10, 12, and14 μg) and drug incubation times (2 and 3 weeks) tested, we chose the 10 μg-3 week condition to serve as the experimental condition used with MPTP injections given its ability to induce significant LC damage without behavioral complications. Prior 6-OHDA injection exacerbated MPTP toxicity at the SN and thereby confirmed LC neuroprotective action at the SN, though NET data suggest an indirect mechanism. Given its effectiveness in mirroring Parkinsonian neurodegeneration and LC-SN interaction, a combined 6-OHDA/MPTP protocol may be considered an adequate model for recreating PD pathology and progression in mice.