Of Neurphysiology 1 a Sodium - Activated Potassium Channel Supports High Frequency 2 Firing and Reduces Energetic Costs during Rapid Modulations of 3 Action Potential Amplitude

32 We investigated the ionic mechanisms that allow dynamic regulation of action potential 33 (AP) amplitude as a means of regulating energetic costs of AP signaling. Weakly electric fish 34 generate an electric organ discharge (EOD) by summing the APs of their electric organ cells 35 (electrocytes). Some electric fish increase AP amplitude during active periods or social 36 interactions and decrease AP amplitude when inactive, regulated by melanocortin peptide 37 hormones. This modulates signal amplitude and conserves energy. The gymnotiform 38 Eigenmannia virescens generates EODs at frequencies that can exceed 500 Hz, which is 39 energetically challenging. We examined how E. virescens meets that challenge. E. virescens 40 electrocytes exhibit a voltage-gated Na current with extremely rapid recovery from inactivation 41 (τrecov = 0.3 msec) allowing complete recovery of Na current between APs even in fish with the 42 highest EOD frequencies. Electrocytes also possess an inwardly rectifying K current, and a Na43 activated K current (IKNa) the latter not yet identified in any gymnotiform species. In vitro 44 application of melanocortins increases electrocyte AP amplitude and the magnitudes of all three 45 currents, but increased IKNa is a function of enhanced Na influx. Numerical simulations suggest 46 that changing INa magnitude produces corresponding changes in AP amplitude and that KNa 47 channels increase AP energy efficiency (10-30% less Na influx/AP) over model cells with only 48 voltage-gated K channels. These findings suggest the possibility that E. virescens reduces the 49 energetic demands of high-frequency APs through rapidly recovering Na channels and the novel 50 use of KNa channels to maximize AP amplitude at a given Na conductance. 51