NAV, AChR and Na/K PUMP Densities as a Function of EOD Frequency: Predictions for and Observations from the Weakly Electric Fish Eigenmannia

Abstract

Eigenmannia virescens individuals, to sense and communicate, continually generate an electric organ discharge (EOD) at some frequency within the range 200-600Hz. The skeletal muscle-derived EOs are comprised of syncytial electrocytes, each of whose innervated excitable posterior membranes continually fires action potentials in response to cholinergic stimuli. EOD-related whole animal O2-consumption reveals a nonlinear increase in the cost of EODs with increasing frequency (Lewis et al 2014 J Neurosci 34:197). We modeled the excitable membrane firing at 200-600Hz. Na+ entry through Nav and acetylcholine channels (AChRs) was tallied. A dynamic steady-state of continual firing implies near-perfect ion homeostasis achieved by Na,K-ATPase (pump) activity. Modeling showed that Nav channel density, but not AChR density, would need to increase as firing frequency increased. To counter the influx through higher density Nav channels, pump density would also need to increase. Modeling, which assumes synaptic transmission operating at very high speed (200 Hz is already extraordinary), predicts frequency-independent AChR density for electrocytes; this will need to be tested. Testing is underway, however, for Nav channel and the pump densities. E.virescens have two Nav1.4 isoforms (Nav1.4a, Nav1.4b); they are expressed in electrocytes only on the innervated posterior membrane. Electrocytes have one Na,K-ATPase alpha subunit isoform, expressed both anteriorly and posteriorly. Nav1.4a is expressed exclusively in the EO, while Nav1.4b is expressed in both skeletal muscle and EO. We used quantitative RT-PCR to measure transcription levels of genes encoding Nav1.4a, Nav1.4b and the Na,K-ATPase. We found that transcription levels of Nav1.4a, and Na+/K+ ATPase have a strong positive correlation with EOD frequency, while the transcription levels of Nav1.4b showed no correlation with EOD frequency.