RESPONSE SENSITIVITY AND VOLTAGE GAIN OF THE ROD- AND CONE-BIPOLAR CELL SYNAPSES IN DARK-ADAPTED TIGER SALAMANDER RETINA. Xiong-Li Yang and Samuel M. Wu. Cullen Eye Institute, Baylor College of Medicine, Houston, Texas 77030.
APStracts 4:126N, 1997.
ABSTRACT
Rods, cones and bipolar cells were recorded in superfused, flat-mounted isolated retinas of the larval tiger salamander,Ambystoma tigrinum, under dark- adapted conditions. Voltage responses of 24 rods, 15 cones and 41 bipolar cells in dark-adapted retinas to 500 nm light steps of various intensities were listed and fitted with hyperbolic functions, and their step sensitivities_ and relative sensitivities (Log å) were estimated. In the linear response-intensity ranges, the step sensitivity of rods, SS(rod), is about 1.0 mV photon -1æm 2 sec., or 0.034 mV Rh*-1 sec rod; and that of the cones, SS (cone), is about 0.00146 mV photon-1 æm2 sec., or 0.000048 mV Rh*-1 sec rod. _The rod and cone responses were relatively homogenous with little variations in response amplitude and sensitivity. In contrast, bipolar cell responses were heterogenous with large variations in response amplitue and sensitivity. The maximum response amplitude bipolar cells varied from 5 mV to 25 mV, and the relative response sensitivity (Log å) varied more than six log units (-8.11 to -2.32). The step sensitivity of bipolar cells in the linear response-intensity range varied from 0.0000438 to 51.82 mV photon-1 æm2 sec. Bipolar cells in dark-adapted tiger salamander retinas fell into two groups according to their relative sensitivities, with very few cells falling in the intermediate light intensity region. The mixed bipolar cells _(DBCM and HBCM) exhibited relative response sensitivity ranged from -8.11 to -5.54, and step sensitivity ranged from 1.22 to 51.82 mV photon-1 æm2 sec._ The cone- driven bipolar cells (DBCC and HBCC) exhibited relative response sensitivity ranged from -3.45 to -2.32, and step sensitivity ranged from 0.0000438 to 0.00201 mV photon -1æm2 sec. The chord voltage gain of the rod-DBCM or rod - HBCM synapses near the rod dark membrane potential ranged from 1.14 to 48.43, and that of the cone-DBCC or cone-HBCC synaptic gain near the cone dark membrane potential ranged from 0.03 to 1.38. The highest voltage gains were found near the rod or cone dark membrane potentials. By the use of linear subtraction method, we studied the synaptic inputs from cones to 5 mixed bipolar cells, and th_e voltage gains of the cone synapses in each of the bipolar cells _was very close to the voltage gain of the rod synapses. This result suggests that although the responses of mixed bipolar cells are mainly mediated by rods when lights of short and medium wavelengths are used, their responses to long wavelength lights (>650 nm) are mediated by both rods and cones with comparable synaptic gains. Functional roles of the mixed and cone- driven bipolar cells in information processing in dark-adapted retinas are discussed. 

Received 15 November 1996; accepted in final form 1 July 1997.
APS Manuscript Number J905-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 24 July 1997