VOLTAGE-GATED POTASSIUM CONDUCTANCES IN GYMNOTUS ELECTROCYTES AB F. SIERRA, a * V. COMAS, b W. BUÑO c AND O. MACADAR b a Unidad Asociada Neurofisiología-IIBCE, Facultad de Ciencias, Uni- versidad de la República, Montevideo, Uruguay b Dpto. de Neurofisiología, Instituto de Investigaciones Biológicas Clemente Estable, Montevideo, Uruguay c Instituto Cajal, CSIC, Madrid, Spain Abstract—Electrocytes are muscle-derived cells that gener- ate the electric organ discharge (EOD) in most gymnotiform fish. We used an in vitro preparation to determine if the complex EOD of Gymnotus carapo was related to the mem- brane properties of electrocytes. We discovered that in addi- tion to the three Na  -mediated conductances described in a recent paper [Sierra F, Comas V, Buño W, Macadar O (2005) Sodium-dependent plateau potentials in electrocytes of the electric fish Gymnotus carapo. J Comp Physiol A 191:1–11] there were four K  -dependent conductances. Membrane de- polarization activated a delayed rectifier (I K ) and an A-type (I A ) current. I A displayed fast voltage-dependent activation- inactivation kinetics, was blocked by 4-aminopyridine (1 mM) and played a major role in action potential (AP) repolariza- tion. Its voltage dependence and kinetics shape the brief AP that typifies Gymnotus electrocytes. The I K activated by de- polarization contributed less to AP repolarization. Membrane hyperpolarization uncovered two inward rectifiers (IR1 and IR2) with voltage dependence and kinetics that correspond to the complex “hyperpolarizing responses” (HRs) de- scribed under current-clamp. IR1 shows “instantaneous” activation, is blocked by Ba 2 and Cs  and displays a voltage and time dependent inactivation that matches the hyperpolarizing phase of the HR. The activation of IR2 is slower and at more negative potentials than IR1 and is resistant to Ba 2 and Cs  . This current fits the depolarizing phase of the HR. The EOD waveform of Gymnotus carapo is more com- plex than that of other gymnotiform fish species, the com- plexity originates in the voltage responses generated through the interactions of three Na  and four K  voltage- and time-dependent conductances although the innerva- tion pattern also contributes [Trujillo-Cenóz O, Echagüe JA (1989) Waveform generation of the electric organ dis- charge in Gymnotus carapo. I. Morphology and innervation of the electric organ. J Comp Physiol A 165:343–351]. © 2006 IBRO. Published by Elsevier Ltd. All rights reserved. Key words: potassium current, potassium channel, A-current, inward rectifier, electric organ discharge. Freshwater weakly electric fish utilize the electric organ discharge (EOD) for electrolocation and communication (Bullock and Heiligenberg, 1986). The EOD results from the activity of an electric organ (EO), composed in most species by electrocytes derived from muscle tissue, and of a neural system of control (Trujillo-Cenóz et al., 1984; Hopkins, 1999). Gymnotus carapo (G. carapo) is a South American pulse-type weakly electric fish with a triphasic EOD which depends on the synchronized firing of action potentials (APs) on the membranes of electrocytes (Trujillo- Cenóz and Echagüe, 1989; Caputi et al., 1989; Caputi, 1999). Different factors (i.e. temperature, steroid hor- mones, and pauses) modify the EOD waveform of G. carapo (Schuster, 2000; Ardanaz et al., 2001; Silva et al., 2002). The underlying cellular mechanisms of these plastic phenomena remain to be investigated in this species. The electrophysiological properties of G. carapo elec- trocytes were previously studied in vivo by Bennett and Grundfest (1959) who showed that these electrocytes fired repetitively when depolarized. The repetitive firing is an indication of repolarizing K + currents that allow the sodium channels to deinactivate (Hille, 2001; Buckingham and Spencer, 2002). In a previous paper we confirmed the repetitive AP discharge of these electrocytes and showed that the repetitive firing relied on the activation of K + - mediated repolarizing conductances (Sierra et al., 2005). In addition, Bennett and Grundfest (1966) described tran- sient responses evoked by hyperpolarization and attrib- uted them to inactivation of K + currents and depolarizing inactivating responses caused by the closure of K + chan- nels. However, our data suggested that the depolarizing “inactivation responses” were Na + -mediated plateau po- tentials caused by the activation of persistent Na + currents (Sierra et al., 2005). However, there is no information on the ionic mechanisms mediating the transient responses induced by membrane hyperpolarization. On the other hand, the electrocytes of two other gym- notiform species, Electrophorus electricus and Sternopy- gus macrurus, discharge a single AP when depolarized. This AP is mediated by a transient Na + current that ini- tiates and terminates the AP, shaping its waveform with minor or no contribution of K + -mediated conductances *Correspondence to: F. Sierra, Dpto. de Neurofisiología, Instituto de Investigaciones Biológicas Clemente Estable, Avda. Italia 3318, CP 11600, Montevideo, Uruguay. Tel: +598-24875532; fax: +598- 2-487-5461. E-mail address: fsierra@iibce.edu.uy (F. Sierra). Abbreviations: AP, action potential; APD, action potential duration; B. pinnicaudatus, Brachyhypopomus pinnicaudatus; EO, electric organ; EOD, electric organ discharge; G. carapo, Gymnotus carapo; Hepes, 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid; HR, hyperpolariz- ing response; I A , A-type potassium current; I K , delayed rectifier potas- sium current; IR1, inward rectifier 1; IR2, inward rectifier 2; I–V, current– voltage; K o , extracellular K + concentration; R in , input resistance; TEA, tetraethylammonium chloride; V–I, voltage– current; V m , membrane po- tential; V 50 , half activation/inactivation voltage; 4-AP, 4-aminopyridine. Neuroscience 145 (2007) 453– 463 0306-4522/07$30.00+0.00 © 2006 IBRO. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.neuroscience.2006.12.002 453