RECONSTRUCTION OF IONIC CURRENTS IN A MOLLUSCAN PHOTORECEPTOR

Two-microelectrode voltage-clamp measurements were made to determine t he kinetics and voltage dependence of ionic currents across the soma m embrane of the Hermissenda type B photoreceptor. The voltage-dependent outward potassium currents, I(A) and I(Ca2+-K+), the inward voltage-d ependent calcium current, I(Ca2+) and the light-induced current, I(Igt ), were then described with Hodgkin-Huxley-type equations. The fast-ac tivating and inactivating potassium current, I(A), was described by th e equation; I(A)(t) = gA(max){m(a)infinity[1 - exp(-t/tau(ma)]}3 x {h( a)infinity [1 - exp(-t/tau(ha))] + exp(-t/tau(ha))}(V(m) - E(K)), wher e the parameters m(a)infinity, h(a)infinity, tau(ma), and tau(ha) are functions of membrane potential, V(m), and m(a)infinity and h(a)infini ty are steady-state activation and inactivation parameters. Similarly, the calcium-dependent outward potassium current, I(Ca2+-K+), was desc ribed by the equation, I(Ca2+-K+) (t) = g(c)(max)(m(c)infinity)(V(C)){ 1 - exp[-t/tau(mc)(V(C))]})pc{h(c)infinity(V(C))[1 - exp(-t/tau(hc))] + exp(-t/tau(hc)(V(C))]}pc(V(C) - E(K)). In high external potassium, I (Ca2+-K+) could be measured in approximate isolation from other curren ts as a voltage-dependent inward tail current following a depolarizing command pulse from a holding potential of -60 mV. A voltage-dependent inward calcium current across the type B soma membrane, I(Ca2+), acti vated rapidly, showed little inactivation, and was described by the eq uation: I(Ca2+) = g(Ca)(max)[1 + exp[(-V(m) - 5)/7]-1 (V(m) - E(Ca)), where g(Ca)(max) was 0.5 muS. The light-induced current, with both fas t and slow phases was described by: I(Igt)(t) = I(Igt1) + I(Igt2) + I( Igt3), I(Igti) = g(Igti) [1 - exp(-t(on)/tau(mi))]3 exp(-t(on)/tau(hi) )(V(m) - E(Igti)) (i = 1, 2). For i = 3, I(Igt)(t) = g(Igt3)m3(3)h3(V( m) - E(Igt3))exp(-t(on)/tau(on)) x exp(-t(off)/tau(off)). Based on the se reconstructions of ionic currents, learning-induced enhancement of the long lasting depolarization (LLD) of the photoreceptor's light res ponse was shown to arise from progressive inactivation of I(A), I(Ca2-K+), and I(Ca2+).