Prolonged depolarization in turtle cones evoked by current injection and stimulation of the receptive field surround.

Abstract

2. A distinctive depolarizing response was evoked by flashing an annulus of light while steadily illuminating the centre of the receptive field. The response, here called 'the prolonged depolarization', was found in 67 % of a sample of 125 cones and could reach some 20 mV in amplitude.3. The prolonged depolarization is characterized by a set of properties which include: the capacity to persist up to 17 s after the flash, a stereotypical waveform, a long period of temporal facilitation, a very narrow dynamic range, and a long refractory period (30-45 s).4. Depolarizing current pulses (0-01-0X1 nA) evoke a prolonged depolarization which is similar to and functionally interchangeable with that evoked by light. The prolonged depolarization is thus apparently generated by a voltage-sensitive mechanism intrinsic to the cone.5. Brief depolarizing spikes were recorded in a small fraction of cones. The spikes appear to be dissociable from the prolonged depolarization although both might arise from similar regenerative mechanisms.

Synaptic transmission at retinal ribbon synapses

“…But what prevents the cell from depolarizing beyond −40 mV in darkness? As discussed previously, I Ca is partially active at −40 mV and when stimulated by extrinsic depolarization or by a strong depolarizing input from the receptive field surround, I Ca can become regeneratively active allowing photoreceptors to produce large, and often long-lasting, calcium-dependent transients (Burkhardt et al, 1988;Gerschenfeld and Piccolino, 1980;Akopian et al, 1997). One way that photoreceptors minimize these depolarizing overshoots is by the activation, above −30 mV, of large outward currents (I Kx , I KCa , I Cl(Ca) ;Owen, 1987;Moriondo et al, 2001).…”

“…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.…”

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

“…Turtle cones also occasionally exhibit brief, calcium-dependent spikes . However, spikes are only rarely observed in untreated retinas (Fuortes et al, 1973;Burkhardt et al, 1988) and were not examined in the present study.…”

Reciprocal Interactions Between Calcium and Chloride in Rod Photoreceptors

“…Generation of a prolonged depolarization produces a substantial shunt reduction of photoreceptor light responses (Burkhardt et al 1988), and the Ca 2ϩ influx accompanying such a sustained depolarization can be damaging to neurons. A number of mechanisms help to keep I Ca activity limited to a tonic low level and prevent more frequent activation of Ca 2ϩ action potentials in photoreceptors.…”

“…Rods produce both spike-like and prolonged regenerative Ca 2ϩ action potentials (Burkhardt et al 1988;Fain et al 1977). Generation of a prolonged depolarization produces a substantial shunt reduction of photoreceptor light responses (Burkhardt et al 1988), and the Ca 2ϩ influx accompanying such a sustained depolarization can be damaging to neurons.…”