Differential contribution of Ih to the integration of excitatory synaptic inputs in substantia nigra pars compacta and ventral tegmental area dopaminergic neurons

The selective vulnerability of substantia nigra pars compacta (SNc) dopaminergic (DA) neurons is an enigmatic trait of Parkinson's disease (PD), especially if compared to the remarkable resistance of closely related DA neurons in the neighboring ventral tegmental area (VTA). Overall evidence indicates that specific electrophysiological, metabolic and molecular factors underlie SNc vulnerability, although many pieces of the puzzle are still missing. In this respect, we recently demonstrated that 1‐methyl‐4‐phenylpyridinium (MPP+), the active metabolite of the parkinsonizing toxin 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP), alters the electrophysiological properties of SNc DA neurons in vitro by inhibiting the hyperpolarization‐activated current (Ih). Here, we present an electrophysiological investigation of the functional role of Ih in the integration of synaptic inputs in identified SNc and VTA DA neurons, comparatively, in acute midbrain slices from TH‐GFP mice. We show that pharmacological suppression of Ih increases the amplitude and decay time of excitatory postsynaptic potentials, leading to temporal summation of multiple excitatory potentials at somatic level. Importantly, these effects are quantitatively more evident in SNc DA neurons. We conclude that Ih regulates the responsiveness to excitatory synaptic transmission in SNc and VTA DA neurons differentially. Finally, we present the hypothesis that Ih loss of function may be linked to PD trigger mechanisms, such as mitochondrial failure and ATP depletion, and act in concert with SNc‐specific synaptic connectivity to promote selective vulnerability. We show that pharmacological Ih blockade alters the excitability of midbrain dopamine neurons in acute slices by increasing the amplitude and decay time of EPSPs, thus leading to temporal summation of excitatory inputs at somatic level. Quantitatively, these effects appear more evident in SNc compared to VTA DA neurons, correlating to differential post‐synaptic expression of Ih. Ih loss of function may be linked to mitochondrial failure and ATP depletion, and act in concert with SNc‐specific synaptic connectivity to promote selective vulnerability in Parkinson's Disease.