Comparing tonic and phasic dendritic calcium in cholinergic pedunculopontine neurons and dopaminergic substantia nigra neurons

Abstract

Several brainstem nuclei degenerate in Parkinson's disease (PD). In addition to the well‐characterized dopaminergic neurons of the substantia nigra pars compacta (SNc), the cholinergic neurons of the pedunculopontine nucleus (PPN) also degenerate in PD. One leading hypothesis of selective vulnerability is that pacemaking activity and the activation of low‐threshold L‐type calcium current are major contributors to tonic calcium load and cellular stress in SNc dopaminergic neurons. However, it is not yet clear whether the vulnerable PPN cholinergic neurons share this property. Therefore, we used two‐photon dendritic calcium imaging and whole‐cell electrophysiology to evaluate the role of L‐type calcium channels in tonic and phasic dendritic calcium signals in PPN and SNc neurons. In addition, we investigated N‐ and P/Q‐type calcium channel regulation of firing properties and dendritic calcium in PPN neurons. We found that blocking L‐type channels reduces tonic firing rate and dendritic calcium levels in SNc neurons. By contrast, the tonic calcium load in PPN neurons did not depend on L‐, N‐ or P/Q‐type channels. However, we found that blocking either L‐type (with nifedipine) or N‐ and P/Q‐type (with omega‐conotoxin MVIIC) channels reduces phasic calcium influx in PPN dendrites. Together, these findings show that L‐type calcium channels play different roles in the activity of SNc and PPN neurons, and suggest that low‐threshold L‐type channels are not responsible for tonic calcium levels in PPN cholinergic neurons and are therefore not likely to be a source of selective vulnerability in these cells.

Inhibitory basal ganglia nuclei differentially innervate pedunculopontine nucleus subpopulations and evoke opposite motor and valence behaviors.

“…Although oIPSC amplitude is indicative of the strength of a synapse, it does not always correlate with the functional impact on a neuron's action potential output 20,43,44 . Because cholinergic PPN neurons display spontaneous firing [45][46][47][48][49] , we characterized the impact of SNr-mediated inhibition on action potential firing rate. While optically stimulating SNr axons with the same 20 Hz blue light protocol as above, we observed a decrease in firing rate in the recorded cholinergic PPN neuron (Figure 2J).…”

“…The internal solution contained neurobiotin (0.1-0.3%) for post-hoc staining. These reagents enable reliable electrophysiology recordings in adult brainstem neurons 45,84 .…”