Neurotransmitter‐induced currents in retinal bipolar cells of the axolotl, Ambystoma mexicanum.

Abstract

1. Whole-cell patch clamping was used to study the membrane properties of isolated bipolar cells and the currents evoked in them by putative retinal neurotranmitters. 2. Isolated bipolar cells show an approximately ohmic response to voltage steps over most of the physiological response range, with an average input resistance of 1.3 G.omega. and resting potential of -35 mV. These values are underestimates because of the shunting effect of the seal between the patch electrode and the cell membrane. Depolarization beyond -30 mV produces rapid activation (10-100 ms) of an outward current (carried largely by potassium ions), which then inactivates slowly (0.5-2 s). 3. Of five candidates for the photoreceptor transmitter, four (aspartate, N-acetylhistidine, cadaverine, putrescine) had no effect on bipolar cells. The fifth substance, L-glutamate, opened ionic channels with a mean reversal potential of -12 mV in some cells (presumed hyperpolarizing biplar cells), and closed channels with a mean reversal potential of -13 mV in other cells (presumed depolarizing bipolar cells). 4. The conductance increase induced by glutamate in presumed hyperpolarizing bipolar cells was associated with an increase in membrane current noise. Noise analysis suggested a single-channel conductance for the glutamate-gated channel of 5.4 pS. The power spectrum of the noise increase required the sum of two Lorentzian curves to fit it, suggesting that the channel can exist in three states. 5. The conductance decrease induced by glutamate in presumed depolarizing bipolar cells was associated with a decrease in membrane current noise that could be described as the sum of two Lorentzian spectra, and which suggested a single-channel conductance of 11 pS. The noise decrease implies that the channels closed by glutamate are not all open in the absence of the transmitter. 6 GABA (.gamma.-aminobutyric acid) and glycine, transmitters believed to mediate lateral inhibition in the retina, open chloride channels in isolated bipolar cells, and increase the membrane current noise. Noise analysis suggested that the channels gated by GABA and glycine have conductances of 4.4 and 7.5 pS respectively. The noise spectra required the sum of two Lorentzian curves to fit them. 7. By whole-cell patch clamping cells in retinal slices, the synaptic transmitter released by photoreceptors was shown to close channels with an extrapolated reversal potential around -3 mV in depolarizing bipolar cells. The membrane properties of depolarizing bipolar cells in the retina were similar to those of isolated bipolar cells, except that their membrane capacitance was twice as large and their input resistance was one-tenth as large as for isolated cells. 8. We conclude that the time-dependent potassium current in axolotl bipolar cells plays little role in shaping the bipolar cells'' light response because it is not significantly activated in the physiological potential range. The conductances modulated by the putative photoreceptor and lateral inhibitory transmitters are comparable to the cell conductance in the absence of transmitter. The voltage noise produced by the ionic channels in the bipolar cell membrane may hinder the detection of dim stimuli by these cells.