Age‐related decline in striatal dopamine content and motor performance occurs in the absence of nigral cell loss in a genetic mouse model of Parkinson's disease

Abstract

Dopamine cytotoxicity is thought to contribute towards the selective loss of substantia nigra pars compacta dopamine neurons and disease progression in Parkinson's disease. However, the long-term toxicity of dopamine in vivo has not previously been established. The vesicular monoamine transporter 2 (VMAT2) sequesters monoamines into synaptic vesicles, a process that, in addition to being important in normal transmission, may also act to keep intracellular levels of monoamine neurotransmitters below potentially toxic thresholds. The homozygous VMAT2-hypomorphic mouse has an insertion in the VMAT2 gene (Slc18a2). Consequently, VMAT2-deficient mice (VD(-/-)) have an approximately 95% reduction in VMAT2 expression and an equivalent level of dopamine depletion in the striatum which results in moderate motor impairment. Here, we show that L-DOPA induces locomotor hyperactivity in VD(-/-) mice and reverses the deficit in motor coordination and balance as tested with the rotarod. We report that evidence for cytosolic accumulation of dopamine in substantia nigra neurons in these mice is two-fold: firstly, there is reduced phosphorylation of tyrosine hydroxylase at the residue associated with catechol feedback inhibition; and, secondly, there are increased rates of dopamine turnover at 6, 12 and 24 months of age. These animals exhibit a progressive decline in striatal monoamine levels and rotarod performance with increasing age. However, despite these data, there was no loss of nigral dopamine neurons as estimated by quantification of tyrosine hydroxylase-immunoreactive cells in the substantia nigra pars compacta of old VD(-/-) mice (24-month-old), implying that these age-dependent manifestations may be due to senescence alone.

Reduced Vesicular Storage of Dopamine Causes Progressive Nigrostriatal Neurodegeneration

“…Indeed, the interaction between ␣-synuclein and dopamine-derived oxidative species in our VMAT2 LO mice may be the fundamental reason for the neurodegeneration we observed. This is supported by the fact that no loss of THϩ cells was seen in 24-month-old VMAT2 LO mice on a null ␣-synuclein background (Colebrooke et al, 2006).…”

“…3B). Similar dopaminergic defects have been demonstrated in aged VMAT2 LO mice on an ␣-synuclein null background (Colebrooke et al, 2006).…”

“…Striatal dopamine levels remain unchanged in the VMAT2 WT mice up to 12 months of age. In contrast, dopamine levels continue to decline in the aged VMAT2 LO animals, as seen previously in VMAT2 LO mice that are ␣-synuclein null (Colebrooke et al, 2006) (Fig. 1 E).…”

“…Highly chimeric males were bred with C57BL/6 females, and genotype was confirmed by Southern blot analysis. A recent report brought to our attention the C57BL/6 inbred strain of mice originally used to establish the VMAT2 LO mice contained a spontaneous chromosomal deletion spanning the ␣-synuclein gene locus (Specht and Schoepfer, 2001), which was confirmed in the original strain (Mooslehner et al, 2001;Patel et al, 2003;Colebrooke et al, 2006). Through diligent breeding, we eliminated all traces of this mutation from our strain of mice and routinely verify the presence of ␣-synuclein by Southern blot analysis.…”

“…Although the heterozygous VMAT2 mice thrive, the knock-out animals only survive for 1-3 d after birth. We recently generated a transgenic mouse line expressing ϳ5% normal VMAT2 levels [low expressor (VMAT2 LO)] (Mooslehner et al, 2001;Colebrooke et al, 2006). Unlike mice with a complete gene deletion of the gene, VMAT2 LO mice survive into adulthood, providing a unique opportunity to examine the impact of disruption of vesicular function on the aging dopamine system.…”