Depressed excitability and ion currents linked to slow exocytotic fusion pore in chromaffin cells of the SOD1^G93A mouse model of amyotrophic lateral sclerosis

Abstract

Altered synaptic transmission with excess glutamate release has been implicated in the loss of motoneurons occurring in amyotrophic lateral sclerosis (ALS). Hyperexcitability or hypoexcitability of motoneurons from mice carrying the ALS mutation SOD1(G93A) (mSOD1) has also been reported. Here we have investigated the excitability, the ion currents, and the kinetics of the exocytotic fusion pore in chromaffin cells from postnatal day 90 to postnatal day 130 mSOD1 mice, when motor deficits are already established. With respect to wild-type (WT), mSOD1 chromaffin cells had a decrease in the following parameters: 95% in spontaneous action potentials, 70% in nicotinic current for acetylcholine (ACh), 35% in Na(+) current, 40% in Ca(2+)-dependent K(+) current, and 53% in voltage-dependent K(+) current. Ca(2+) current was increased by 37%, but the ACh-evoked elevation of cytosolic Ca(2+) was unchanged. Single exocytotic spike events triggered by ACh had the following differences (mSOD1 vs. WT): 36% lower rise rate, 60% higher decay time, 51% higher half-width, 13% lower amplitude, and 61% higher quantal size. The expression of the α3-subtype of nicotinic receptors and proteins of the exocytotic machinery was unchanged in the brain and adrenal medulla of mSOD1, with respect to WT mice. A slower fusion pore opening, expansion, and closure are likely linked to the pronounced reduction in cell excitability and in the ion currents driving action potentials in mSOD1, compared with WT chromaffin cells.

Faster kinetics of quantal catecholamine release in mouse chromaffin cells stimulated with acetylcholine, compared with other secretagogues

“…In a recent study performed to analyse the alterations undergone by the fusion pore in MCCs from wild‐type and in SOD1 G93A transgenic mouse models of ALS, we also found that control CCs stimulated with ACh had a fusion pore kinetics faster than CCs stimulated with K + (Calvo‐Gallardo et al . ).…”

“…), and the SOD 1 G93A mouse model of amyotrophic lateral sclerosis carrying the mutation G93A in the enzyme superoxide dismutase 1 (Calvo‐Gallardo et al . ), are also displayed at the bottom.…”

“…), in the ALS mouse model SOD1 G93A (Calvo‐Gallardo et al . ), and in the Huntington's disease mouse model carrying a knocked out huntingtin‐associated protein 1 (HAP1) (Mackenzie et al . ).…”

“…; Calvo‐Gallardo et al . ). The electrodes were calibrated by perfusing 50 μM norepinephrine dissolved in standard Tyrode and measuring the current elicited; only electrodes that yielded a current between 200 and 400 pA were used.…”

“…; Calvo‐Gallardo et al . ) is a reflection of a more general dysfunction of the exocytotic release of neurotransmitters in the brain. Hence, the exploration of this hypothesis may have interest from a double point of view namely, (i) search for alterations of the exocytotic machinery that operates both centrally and peripherally, and (ii) inquire on the altered CC catecholamine secretory patterns linked to changes in the activity of the sympathoadrenal axis occurring during those stressful conflicts arising as secondary outcomes of neurodegenerative diseases.…”