Coregulation of voltage-dependent kinetics of Na(+) and K(+) currents in electric organ.

The electric organ cells of Sternopygus generate action potentials whose durations vary over a fourfold range. This variation in action potential duration is the basis for individual variation in a communication signal. Thus, action potential duration must be precisely regulated in these cells. We had observed previously that the inactivation kinetics of the electrocyte Na(+) current show systematic individual variation. In this study, using a two-electrode voltage clamp, we found that the voltage-dependent activation and deactivation kinetics of the delayed rectifying K(+) current in these cells covary in a graded and predictable manner across fish. Furthermore, when Na(+) and K(+) currents were recorded in the same cell, their voltage-dependent kinetics were highly correlated. This finding illustrates an unprecedented degree of coregulation of voltage-dependent properties in two molecularly distinct ionic channels. Such a coregulation of ionic channels is uniquely observable in a cell specialized to generate individual differences in electrical activity and in which the results of biophysical control mechanisms are evident in behaving animals. We propose that the precise coregulation of the voltage-dependent kinetics of multiple ionic currents may be a general mechanism for regulation of membrane excitability.

Estrogen modifies an electrocommunication signal by altering the electrocyte sodium current in an electric fish, Sternopygus.

Many species of electric fish emit sexually dimorphic electrical signals that are used in gender recognition. In Sternopygus, mature females produce an electric organ discharge (EOD) that is higher in frequency and shorter in pulse duration than that of mature males. EOD pulse duration is determined by ion currents in the electrocytes, and androgens influence EOD pulse duration by altering the inactivation kinetics of the electrocyte sodium current. We examined whether estrogen modulates the female-specific EOD and, if so, whether it regulates EOD pulse duration by acting on the same androgen-sensitive ion current in the electrocytes. We implanted gonadectomized Sternopygus with either empty SILASTIC capsules (control), one capsule filled with estradiol-17beta (E2; low dose), or three capsules of E2 (high dose). Twelve days after implantation, E2-treated fish had plasma E2 levels approximately 3.3-fold (low dose) or approximately 7.1-fold (high dose) higher than controls. After implantation, both E2-treated groups had higher EOD frequency and shorter EOD pulse duration than controls and their own preimplantation values. Through immunocytochemistry, we identified immunoreactive estrogen receptors in the nuclei of electrocytes, indicating that these cells are directly responsive to estrogen. In addition, voltage-clamp studies showed that E2 affected the electrocyte ion currents kinetics: the sodium inactivation time constant was significantly lower in E2-treated fish than in controls. Thus, sexual dimorphism in the electrocommunication signal results, at least in part, from estrogens and androgens acting in opposite directions on the same ion current in the electrocytes.

Individual variation in and androgen-modulation of the sodium current in electric organ.

Electric fish of the genus Sternopygus produce a sinusoidal electric organ discharge (EOD) of low frequencies in males, high frequencies in females, and overlapping and intermediate frequencies in juveniles. Correspondingly, the cells of the electric organ, the electrocytes, generate action potentials which are of long duration in mature males, short duration in females, and intermediate duration in immatures. The androgen dihydrotestosterone (DHT) lowers EOD frequency and increases electrocyte action potential duration. We examined the electrocytes under voltage clamp to determine whether variations in the kinetic properties of the Na+ current might underlie these phenomena. We found that the fast inactivation time constants of the peak Na+ current (0 mV) ranged from 0.5 to 4.7 msec and varied systematically with EOD frequency and action potential duration. Voltage dependence of steady-state inactivation also varied with EOD frequency with the midpoint of inactivation being more positive in fish with low EOD frequencies. There was no correlation between the voltage at which the Na+ current activates, voltage at peak current, reversal potential, rate of recovery from inactivation, or TTX sensitivity and EOD frequency. We tested whether DHT influenced Na+ current inactivation by recording from electrocytes before and after juvenile fish of both sexes were implanted with a DHT-containing or empty capsule. We found that inactivation time constants were significantly slower in DHT implanted, but not control, fish. This is the first observation of functionally relevant individual variation in the kinetics of a Na+ current and the first demonstration that the kinetics of a Na+ current may be modulated by an androgen.

Abdominal positioning interneurons in crayfish: participation in behavioral acts.

Premotor interneurons involved in the abdominal positioning behaviors of the crayfish, Procambarus clarkii, were studied intracellularly, along with motoneuron activity, in semi-intact preparations during episodes of fictive behavior. Each impaled cell was tested by injecting depolarizing current and examining the motor output. If a response was evoked then the cell was classified as a flexion-producing interneuron (FPI), extension-producing interneuron (EPI) or mixed output interneuron (MOI). A platform drop/rise procedure was then used to elicit abdominal extension-like and flexion-like responses. Interneurons that were active during positioning behavior were silenced by hyperpolarization to determine their contribution in generating the underlying motor program. The data were used to assess the degree of participation of these interneurons in abdominal positioning behavior. Fewer than half of the FPIs, EPIs and MOIs became active during the behavioral episodes. Strength of response to depolarizing current was not correlated with the probability that a cell would fire during behavior. Hyperpolarization tests showed that typical FPIs, EPIs and MOIs were only responsible for a small part of the overall motor output. Also, interneurons, regardless of their FPI or EPI classification, were often observed to fire during both flexion-like and extension-like behaviors. Responses of FPIs, EPIs and MOIs to repeated platform movements suggest that these cells may fire according to a probability distribution depending on: (1) strength of the stimulus; (2) location of the stimulus; (3) location of the interneuron. Most identified cells could not readily be assigned to a specific behavior except for the 'T' cell type, which seems intimately involved in most flexion behaviors. The results of this study support the hypothesis that there are few if any 'command neurons', as defined by Kupfermann and Weiss (1978), in the crayfish abdominal positioning system. Abdominal positioning behavior, therefore, is probably under the control of a large network of cells each contributing a small part to the overall motor output.