Abstract
Abstract: :
Purpose: Voltage clamp recordings from identified Off bipolar cells in the ground squirrel show that each anatomical type (West's b2, b3, and b7) has a distinct quantal response shape. The goal is to test whether these shape differences arise via transmitter release from distinct presynaptic vesicle pools. Methods: Simultaneous spontaneous events were recorded in pairs of postsynaptic neurons that shared a common presynaptic cone. Pairs included combinations of Off bipolar cells (b2, b3, and b7) and horizontal cells (HCs). Slices were obtained from ground squirrel retina according to published procedures, mounted in a chamber on a Zeiss Axioscop 2FS microscope, and superfused continuously with an AMES based saline containing picrotoxin and strychnine. Recording noise was minimized by using Cs as the main intracellular cation, coating pipettes with dental wax, and in some cases by severing axon terminals. cAMP was included in the pipette solution to minimize HC coupling. Membrane voltages were maintained at –70 mV. B2 cells could be distinguished from b3/7 cells based on soma shape alone. b3 and b7 cells could not easily be distinguished without an axon. The occurrence of joint spontaneous events was evaluated by superimposing traces and by computing cross–correlation functions. Results: Spontaneous and evoked quantal currents had comparable shapes. HC and b2 cell events were 10–20 pA in amplitude, had rise times (20–80%) of 0.15–0.2 ms, and decayed with a time constant of about 0.5 ms. B3 and b7 responses were smaller (1.5–4 pA) and had slower time courses (0.5–1.0 ms rise, 2–5 ms decay). Cross–correlation analysis showed joint events as a central peak in the following pairs (n): b2 & b3 (4), b3/7 & b3/7 (1), b2 & b7 (1), HC & b2 (2), HC & b3/7 (3), and HC & b3 (1). Shuffling traces abolished the peaks. A b2 & b3 and a HC & b2 pair were analyzed in detail. Individual events from each cell were isolated, superimposed, and averaged. The shape of the cross–correlation function of averaged events closely matched that obtained from the simultaneous traces. The area under the average event cross–correlation function allows an estimate of the joint event rate. Joint event rates were 64 Hz for the HC & b2 pair (12% of events on average) and 22 Hz in the b2 & b3 pair (35% of events on average). Conclusions: Cell–type specific differences in quantal response shape do not arise from differences in presynaptic release pool or mechanism. Since an individual vesicle can activate both horizontal and all types of Off bipolar cells, and since HC contacts are juxtaposed with ribbon release sites, the inference is that the characteristic bipolar cell quantal response shapes are produced after ribbon–mediated release.
Keywords: retina: distal (photoreceptors, horizontal cells, bipolar cells) • synapse • excitatory neurotransmitters