DIVERSE CURRENT AND VOLTAGE RESPONSES TO BACLOFEN IN AN IDENTIFIED MOLLUSCAN PHOTORECEPTOR

1. gamma-Aminobuturic acid-B (GABA(B)) receptors play a role in the mediation of slow inhibitory postsynaptic potentials in mammalian as well as some nonmammalian species. In identified photoreceptors from the marine mollusc Hermissenda, recent evidence has suggested that GABA, as well as the GABA(B) receptor agonist baclofen, might simultaneously modulate multiple conductances on the postsynaptic membrane. Here, using intracellular current-clamp and single-electrode voltage-clamp techniques, we have characterized responses to baclofen in the B photoreceptors of the Hermissenda eye. 2. Microapplication of baclofen (12.5-62.5 mu M) to the terminal branches of the B photoreceptors induced a slow, concentration-dependent hyperpolarization (approximate to 3-8 mV) that was accompanied by a cessation of spontaneous action potentials and a positive shift in firing threshold. Both the hyperpolarization and the shift in spike threshold in response to baclofen were attenuated largely by the K+ channel blocker tetraethylammonium chloride (TEA; 50 mM). 3. Bath application of baclofen (100 mu M) decreased the amplitude, duration, and the afterhyperpolarization (AHP) of evoked action potentials. Although baclofen's effect on spike duration and amplitude persisted in the absence of extracellular Ca2+, the reduction of the AHP by baclofen was eliminated, suggesting that multiple conductances mediated the baclofen-induced modification of the action potential. 4. Using a single-electrode voltage-clamp technique, microapplication of baclofen to the terminal branches of the B photoreceptor produced a slow, net outward current (<0.5 nA) that reversed near the equilibrium potential for K+ and shifted to more positive potentials when extracellular K+ was increased, in approximate agreement with the Nernst equation for K+. 5. Baclofen induced an increase in amplitude of the nonvoltage dependent leak conductance (I-L), and the increase was blocked by TEA. The baclofen-induced increase of I-L was accompanied by an increase in amplitude and a negative shift in the voltage dependence of a slow, steeply voltage-dependent K+ current (I-K), which displays selective sensitivity to TEA but does not normally contribute to leak conductance. The amplitude and steady-state inactivation of a fast, transient K+ current, as well as the amplitude of an inwardly rectifying K+ current were unaffected by baclofen. 6. Both the rate of activation as well as the amplitude of a voltage-dependent Ca2+ current (I-ca) were reduced by baclofen. The reduction of I-ca resulted in a concomitant suppression of a Ca2+-dependent K+ current (I-k-ca) that was sufficient to account for the reduction of the AHP after evoked action potentials. 7. In total, these results suggest that the baclofen-induced hyperpolarization and spike narrowing are attributable to a postsynaptic increase in K+ conductance, possibly through the I-K class channel, whereas a reduction of the AHP results from a decrease in I-K-ca that is attributable to a reduction in voltage-dependent Ca2+ influx. These responses, as well as the direct reduction of I-ca by baclofen, are likely to result in a functional decrease in intracellular Ca2+ concentration during an action potential, suppressing secretion onto postsynaptic targets. Thus GABA(B) receptors on these neurons may contribute to both pre- and postsynaptic forms of inhibition.