Increased firing frequency of intracardiac neurons from Scn5a-1798insD mice: functional relevance beyond the cardiomyocyte

Background Mutations in SCN5A, which encodes the cardiac sodium channel Nav1.5, are associated with an increased risk for arrhythmias and sudden cardiac death. Carriers of the SCN5A-1795insD overlap mutation display features of Brugada syndrome and Long QT syndrome type 3, and develop severe bradycardia leading to excessive QT-prolongation and sudden death. The latter occurs predominantly during states of high parasympathetic tone (at rest or during the night). Since Nav1.5 is also expressed in intracardiac neurons (ICNs), SCN5A mutations may have functional implications beyond the cardiomyocyte. Purpose To explore whether ICNs from mice carrying the Scn5a-1798insD mutation (the murine equivalent of SCN5A-1795insD) display functional alterations. Methods and Results ICNs were isolated from fat pads located in the pulmonary veins and sinoatrial node regions of the hearts of wild-type (WT) and Scn5a-1798insD mice. Immunocytochemistry demonstrated that the majority of ICNs were cholinergic in nature (as indicated by ChAT staining) and confirmed the presence of Nav1.5 in ICNs. To evoke repetitive action potential (AP) firing in single ICNs, 500 ms-sustained depolarizing current pulses of increasing amplitude (0-150 pA) were applied. Two types of neurons were identified based on their firing patterns: accommodating and tonic. AP firing frequency did not differ between WT and Scn5a-1798insD accommodating ICNs, while it was significantly increased in Scn5a-1798insD tonic ICNs as compared to WT (at 100 pA, AP count/500 ms, WT: 30.3±1.7; Scn5a-1798insD: 36.7±2.1, p=0.02). AP properties were unchanged in Scn5a-1798insD accommodating ICNs, whereas a significant increase in maximal upstroke velocity (Vmax) and a decrease in resting membrane potential (RMP) were observed in Scn5a-1798insD tonic ICNs as compared to WT (Vmax, WT: 147.9±14.5 V/s, Scn5a-1798insD: 199.9±22.3 V/s, p=0.04; RMP, WT: -55.2±1.6 mV, Scn5a-1798insD: -48.4±1.6 mV, p=0.004). When incorporated into a multi-layered computational model of the autonomic nervous system, these observed AP changes were predicted to increase acetylcholine release and, consequently, reduce heart rate. Conclusions We demonstrate for the first time that SCN5A mutations can exert functional effects in other cell types besides cardiomyocytes. In mouse ICNs, the Scn5a-1798insD mutation alters AP properties and increases firing frequency, which is predicted to increase acetylcholine release and reduce heart rate. Hence, mutation-induced effects in ICNs may underlie, at least in part, the observed bradycardia and as such contribute to arrhythmogenesis.