Effects of synaptic blocking agents on the depolarizing responses of turtle cones evoked by surround illumination

Abstract

The effects of synaptic blocking agents on the antagonistic surround of the receptive field of cone photoreceptors were studied by intracellular recording in the retina of the turtle (Pseudemys scripta elegans). Illumination of a cone's receptive-field surround typically evoked a hybrid depolarizing response composed of two components: (1) the graded synaptic feedback depolarization and (2) the prolonged depolarization, a distinctive, intrinsic response of the cone. The locus of action of synaptic blocking agents was analyzed by comparing their effects on the light-evoked response of horizontal cells, the hybrid cone depolarization evoked by surround illumination, and the pure prolonged depolarization evoked by intracellular current injection.The excitatory amino-acid antagonists, d-O-phosphoserine (DOS) and kynurenic acid (KynA), suppressed the light responses of horizontal cells and eliminated the surround-evoked, hybrid cone depolarization without affecting the prolonged depolarization evoked by current injection. Cobalt at 5-10 mM suppressed horizontal cell responses and thereby eliminated surround-evoked cone depolarizations. Cobalt (5-10 mM) also blocked the current-evoked prolonged depolarization, suggesting that the intrinsic cone mechanisms responsible for the prolonged depolarization are likely to be calcium-dependent.Various GABA agonists and antagonists were found to have no effect on the surround-evoked depolarizations of cones. In contrast, a very low concentration of cobalt (0.5 mM) selectively suppressed the light-evoked feedback depolarization of cones without affecting horizontal cell responses or the current-evoked prolonged depolarization. Cobalt at 0.5 mM thus blocks the light-evoked action of the cone feedback synapse while sparing feedforward synaptic transmission from cones to horizontal cells. The implications of the present work for the possible neurotransmitters used at these synapses is discussed.

The Classical Receptive Field Surround of Primate Parasol Ganglion Cells Is Mediated Primarily by a Non-GABAergic Pathway

“…At higher concentrations, cobalt abolished the entire light response (Fig. 3A, inset, 300 M), presumably by interfering with calcium-dependent photoreceptor synaptic transmission (Thoreson and Burkhardt, 1990). For this cell, 150 M provided a good balance between these two extremes (Fig.…”

“…In outer retina, the antagonistic surround of the cone is blocked by cobalt in monkey (Verweij et al, 2003) and turtle (Thoreson and Burkhardt, 1990) and by carbenoxolone in monkey (Verweij et al, 2003) and teleost fish (Kamermans et al, 2001). The responses of horizontal cells to large flashing spots are also affected by both drugs.…”

Proton-Mediated Feedback Inhibition of Presynaptic Calcium Channels at the Cone Photoreceptor Synapse

“…The effects of kainate and CNQX were assessed by comparing the amplitude of the first signal obtained in the absence of drug with the second signal obtained in its presence. GABA blockers were not used routinely because picrotoxin had no significant effect on the second of the paired stimuli using either kainate (ϳ15% reduction, n ϭ 9; p Ͼ 0.05) or CNQX (ϳ10% reduction; n ϭ 7; p Ͼ 0.05), and it does not appear that GABA mediates feedback (Thoreson and Burkhardt, 1990;Verweij et al, 1996).…”

Synaptic transmission at retinal ribbon synapses

“…O'Bryan (1973), Gerschenfeld and Piccolino (1980) and Burkhardt et al (1988) described Ca 2+ -dependent depolarizations which sometimes led to spikes in turtle cones, responses that were elicited by flashes of annular illumination in the presence of steady central illumination or by current injection into cones. Thoreson and Burkhardt (1990) found that the depolarizing responses in cones elicited by current injection were blocked by submicromolar concentrations of cobalt, suggesting that they depended on calcium. The same low concentrations of cobalt are sufficient to block the surround responses of bipolar cells without attenuating the response to central illumination (Vigh and Witkovsky, 1999).…”

“…Calcium-dependent chloride currents contribute to depolarizing surround responses when the cone membrane potential is below E Cl (Thoreson and Burkhardt, 1990). E Cl in cones appears to be near or slightly negative to the dark resting potential (Kaneko and Tachibana, 1986;Thoreson and Burkhardt, 1991;Kraaij et al, 2000;Thoreson and Bryson, 2005) but in the presence of background illumination, the cone membrane potential often passes below E Cl .…”