Differential expression of voltage-activated calcium channels in functionally distinct brainstem motoneurons controlling airway and extraocular muscles

Abstract

Voltage-activated calcium channels are important in controlling motoneuron (MN) excitability and repetitive firing behaviour through their effects on calcium-dependent potassium channels, and the after-hyperpolarization (AHP), in particular the medium AHP (mAHP). Differential expression of voltage-activated calcium channels may therefore be important in establishing the task-specific firing properties of functionally distinct motoneuron pools. Differential expression of these channels may also be involved in the variable susceptibility of MNs to disruptions in calcium homeostasis, which are implicated in the pathology of neurodegenerative diseases such as Amyotrophic Lateral Sclerosis (ALS). To further explore the expression of calcium channels in brainstem MNs and the possibility that differential expression of calcium channels predisposes some MNs to degeneration, we compared voltage-activated calcium channels in hypoglossal (XII) and oculomotor (III) MNs, that differ not only in their firing behaviour, but in their vulnerability to degeneration in ALS. XII MNs, which innervate the genioglossus muscle of the tongue and degenerate in ALS, produce rhythmic bursts of action potentials during inspiration that last ~200–400 ms. Discharge frequency peaks between 25–50 Hz [1]. Oculomotor motoneurons, which innervate extraocular muscles and are resistant to degeneration in ALS, can produce sustained firing rates of 300 Hz during fixation and peak discharge rates of 600 Hz for up to 25 ms during saccades [2]. Voltage-activated calcium channel expression was compared between XII and III MNs electrophysiologically in neonatal rat brain slices using whole-cell patch-clamp recording techniques, and immunohistochemically using antibodies to different channel subtypes. Current densities of low voltage-activated (LVA) calcium currents were similar in XII (-5.9 ± 1.1 pA/pF, n = 9) and III (-4.6 ± 0.8 pA/pF, n = 13) MNs. Immunolabelling for the α1 G subunit of the LVA calcium channel was also similar between MN pools. In contrast, high voltage-activated (HVA) calcium current density was two-fold greater in XII (-38.2 ± 3.0 pA/pF, n = 33) compared with III (-19.5 ± 1.4 pA/pF, n = 40) MNs. Use of HVA calcium channel antagonists (nimodipine and nifedipine for L-type; ω-agatoxin-TK for P/Q-type; ω-conotoxin-GVIA for N-type) revealed that the majority of this difference reflected greater P/Q-type currents in XII MNs (XII: -15.4 ± 0.9 pA/pF, n = 6; III: -4.4 ± 1.2 pA/pF, n = 12). Immunohistochemical analysis of P/Q channel expression supported greater expression in XII MNs. N-type currents were not significantly different in XII (-4.3 ± 1.2 pA/pF, n = 9) and III MNs (-8.0 ± 1.7 pA/pF; n = 11). L-type currents, defined by nimodipine or nifedipine sensitivity, were not detected. These data suggest that differential expression of voltage-activated calcium channels may not contribute to the distinct firing properties of XII and III MNs since activation of the mAHP is primarily associated with calcium flux through N-type channels [3] and these channels do not differ between XII and III MNs. However, our data do support the possibility that greater expression of HVA calcium channels may predispose XII MNs to degeneration during periods of metabolic stress as seen in ALS. The greater density of P/Q-type HVA calcium channels in XII MNs would allow more calcium to enter MNs upon depolarisation and increase the likelihood of interaction with calcium channel antibodies found in some ALS patients.