CLEARANCE OF NEUROTRANSMITTER FROM THE CONE SYNAPTIC CLEFT IN GOLDFISH RETINA

Dendrites of bipolar and horizontal cells protrude deeply into the syn aptic terminals of cones in goldfish retina, This arrangement gives th e impression that the cone synaptic terminal surrounds a morphological ly shielded compartment, the cone synaptic cleft, from which clearance of neurotransmitter by diffusion is limited, In this study the time c onstant of this clearance has been approached in two ways: (1) the mor phological parameters determining the clearance (extracellular synapti c volume and leak area), were estimated using morphometric methods, Th ese data were introduced into a diffusion model of the cone pedicle, y ielding a time constant for the clearance of <1 msec; (2) the time con stant of the light onset response when the glutamate transporter in th e cone was blocked with DL-threo-beta-hydroxyaspartate or dihydrokaina te, was interpreted as the time constant of the clearance, yielding va lues of almost 650 msec compared to around 90 msec in control conditio ns, The decay time-constant of the Ca-dependent tail-currents in cones was used, as an approximation of the dynamics of the intracellular Ca -concentration and thus of the glutamate release by the cones, The dec ay time constant was about 800 msec, This suggests that the intracellu lar Ca-concentration in the synaptic terminal and hence the glutamate release by the cones drops with a similar large time constant, These r esults indicate that the cone pedicle in goldfish does not limit the c learance of neurotransmitters from the synaptic cleft and that the fas t light onset response of horizontal cells under control conditions is due to activation of glutamate transporters by hyperpolarization of t he cone membrane potential while the glutamate release drops slowly, T he slow horizontal cell light onset response in beta-hydroxyaspartate or dihydrokainate may be due to a slow reduction of the glutamate rele ase by the cones at light onset. Copyright (C) 1996 Elsevier Science L td