Twelve- to sixteen-month-old (G2019S) LRRK2 transgenic mice prepared by us displayed progressive neuronal death of substantia nigra pars compacta (SNpc) dopaminergic cells. In the present study, we hypothesized that prior to a late-phase death of SNpc dopaminergic neurons, (G2019S) LRRK2 also causes an early-phase neuronal dysfunction of SNpc dopaminergic cells in the (G2019S) LRRK2 mouse. Eight to nine-month-old (G2019S) LRRK2 transgenic mice exhibited the symptom of hypoactivity in the absence of the degeneration of SNpc dopaminergic neurons or nigrostriatal dopaminergic terminals. Whole-cell current-clamp recordings of SNpc dopaminergic cells in brain slices demonstrated a significant decrease in spontaneous firing frequency of SNpc dopaminergic neurons of 8-month-old (G2019S) LRRK2 mice. Carbon fiber electrode amperometry recording using striatal slices showed that (G2019S) LRRK2 transgenic mice at the age of 8 to 9. months display an impaired evoked dopamine release in the dorsolateral striatum. Normal nigrostriatal dopaminergic transmission is required for the induction of long-term synaptic plasticity expressed at corticostriatal glutamatergic synapses of striatal medium spiny neurons. Whole-cell voltage-clamp recordings showed that in contrast to medium spiny neurons of 8 to 9-month-old wild-type mice, high-frequency stimulation of corticostriatal afferents failed to induce long-term depression (LTD) of corticostriatal EPSCs in medium spiny neurons of (G2019S) LRRK2 mice at the same age. Our study provides the evidence that mutant (G2019S) LRRK2 causes early-phase dysfunctions of SNpc dopaminergic neurons, including a decrease in spontaneous firing rate and a reduction in evoked dopamine release, and impairment of corticostriatal LTD in the (G2019S) LRRK2 transgenic mouse.
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