Modeling the Oscillating Dipole Properties of Electric Organ Discharge in the Weakly Electric Fish, Eigenmannia

Abstract

One of the most studied weakly electric fish, Eigenmannia produces a continuous high frequency electric organ discharge (EOD) used to sense nearby objects and communicate with conspecifics. The EOD of an individual fish has a characteristic frequency (within the species range, 250-600 Hz) which it shifts when necessary to avoid jamming. The nearly dipolar oscillating electric field yields zero net current and is generated by parallel columns of identical, synchronously discharging electrocyte cells. Recent findings from whole fish respirometry (during high-frequency signaling over a range of frequencies) have renewed interest in the frequency-dependent energetics of the EODs (Lewis et al 2014 J Neurosci 34:197) but the modeling based on past analyses is missing some key features of the in-vivo electrolyte operation. We have constructed a model for the neurally-driven electrocyte action potentials (APs) that underlie the EOD. APs are initiated by brief post-synaptic cation influxes through AChR channels at each electrocyte's innervated posterior end. This yields a head positive current while the anterior end serves mainly as a capacitor whose discharge yields the head negative current of the oscillating dipole cycle. To maintain the appropriate [Na]in and [K]out levels, Na/K-ATPase pumps run continuously. Modeling the activity of this ATP consuming protein gives us access to electrocyte energetics and frequency-dependent EOD efficiency. Modeling the posterior-anterior current flows gives us access to the electric field patterns produced by the electrocytes.