Clearance of neurotransmitter from the cone synaptic cleft in goldfish retina

Dendrites of bipolar and horizontal cells protrude deeply into the synaptic terminals of cones in goldfish retina. This arrangement gives the impression that the cone synaptic terminal surrounds a morphologically shielded compartment, the cone synaptic cleft, from which clearance of neurotransmitter by diffusion is limited. In this study the time constant of this clearance has been approached in two ways: (1) the morphological parameters determining the clearance (extracellular synaptic volume and leak area), were estimated using morphometric methods. These data were introduced into a diffusion model of the cone pedicle, yielding a time constant for the clearance of < 1 msec; (2) the time constant of the light onset response when the glutamate transporter in the cone was blocked with DL-threo-beta-hydroxyaspartate or dihydrokainate, was interpreted as the time constant of the clearance, yielding values of almost 650 msec compared to around 90 msec in control conditions. 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 decay time constant was about 800 msec. This suggests that the intracellular Ca-concentration in the synaptic terminal and hence the glutamate release by the cones drops with a similar large time constant. These results indicate that the cone pedicle in goldfish does not limit the clearance of neurotransmitters from the synaptic cleft and that the fast light onset response of horizontal cells under control conditions is due to activation of glutamate transporters by hyperpolarization of the cone membrane potential while the glutamate release drops slowly. The slow horizontal cell light onset response in beta-hydroxyaspartate or dihydrokainate may be due to a slow reduction of the glutamate release by the cones at light onset.