DIFFERENTIAL MODULATION OF VOLTAGE-DEPENDENT CURRENTS IN HERMISSENDA TYPE-B PHOTORECEPTORS BY SEROTONIN

1. In identified photoreceptors in the eyes of Hermissenda, serotonin (5-HT) enhances the peak and plateau phases of light-evoked generator potentials and modulates light-dependent and voltage-dependent currents. In addition, electrophysiological and morphological studies indicate that 5-HT may contribute to cellular plasticity detected in the visual system of Hermissenda produced by classical conditioning procedures. With the use of conventional two-electrode voltage-clamp techniques, we examined the effects of 5-HT on three distinct currents recorded across the soma membranes of identified lateral and medial type B photoreceptors in the isolated circumesophageal nervous system. 2. The inward rectifier (I(ir)), a putative K+-dependent conductance, activates in 0-Na artificial seawater at membrane potentials greater than -60 mV, is voltage dependent, noninactivating, and reaches steady-state within 800 ms to 3 s at -100 mV. Bath application of 10(-4) M 5-HT consistently enhanced the magnitude of I(ir) at all potentials tested (-60 to -100 mV) and, in some cases, allowed expression of I(ir), which was not initially detectable before the application of 5-HT. 3. The major component of outward K+ current in type B photoreceptors with I(A) and I(K(Ca)) blocked is the delayed rectifier [I(K(v))].5-HT (10(-4) M) produced both an increase as well as a decrease in the peak amplitude of I(K(v)) and consistently slowed its inactivation rate and reduced twin-pulse inactivation. 4. A previously identified outward K+ current in type B photoreceptors has properties that are similar to the A current (I(A)). Voltage separation of I(A) from I(K(v)) was achieved by activating I(A) in the range -50 to -20 mV, from a V(h) of -80 mV. Here we report that, consistent with previous studies, 5-HT (10(-4) M) reduces the peak amplitude of I(A) in type B photoreceptors.