Journal of Neurophysiology, Vol 74, Issue 2 506-518, Copyright © 1995 by APS
Diverse current and voltage responses to baclofen in an identified molluscan photoreceptor
L. D. Matzel, I. A. Muzzio and R. F. Rogers
Department of Psychology, Rutgers University, New Brunswick, New Jersey 08902, USA.
1. gamma-Aminobuturic acid-B (GABAB) 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 GABAB
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 microM) to the
terminal branches of the B photoreceptors induced a slow,
concentration-dependent hyperpolarization (approximately 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 microM) 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 (IL),
and the increase was blocked by TEA. The baclofen-induced increase of IL
was accompanied by an increase in amplitude and a negative shift in the
voltage dependence of a slow, steeply voltage-dependent K+ current (IK),
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
(ICa) were reduced by baclofen. The reduction of ICa resulted in a
concomitant suppression of a Ca(2+)-dependent K+ current (IK-Ca) that was
sufficient to account for the reduction of the AHP after evoked action
potentials.(ABSTRACT TRUNCATED AT 400 WORDS)
