Abstract
Purpose:
Parasol cells comprise about 15% of the total ganglion cells in the primate, project to the lateral geniculate nucleus and the superior colliculus and display characteristic ON-and OFF-center/surround receptive fields, Y-type non-linear spatial summation, and high achromatic temporal-contrast sensitivity thought to play a role in the cortical coding of object motion. Our purpose was to directly test the hypothesis that the parasol cell’s distinctive spatio-temporal properties are critically dependent on both synaptic inhibition and NMDA receptor-mediated synaptic excitation.
Methods:
We targeted morphologically identified inner, ON-center and outer, OFF-center parasol cells for extracellular and intracellular patch clamp recordings from the intact, light-adapted macaque monkey retina in vitro. A variety of visual stimuli were projected onto the retina to measure spatial, contrast and temporal tuning of both spike discharge and synaptic conductance before and after bath application of GABA, glycine and NMDA receptor antagonists.
Results:
For ON- and OFF-center parasol cells synaptic excitation was mediated by non-NMDA (AMPA/kainate) and NMDA receptors. However the NMDA receptor-mediated nonlinear conductance was low at physiological membrane potentials (chord conductance at -65 mV < 10%). Synaptic inhibition was mediated by glycinergic ‘crossover’ and GABAergic feedforward inhibition. However crossover inhibition was rectified and did not contribute 'push-pull' disinhibition to the light-evoked conductance. Center-surround and Y-type 'frequency-doubling' derived from modulation of an excitatory conductance supporting the hypothesis that bipolar cells with center-surround structure and partially rectified light responses mediate both the linear and non-linear components of the parasol cell receptive field. Contrast sensitivity in the linear range, from 3-20%, was unaffected by selective block of either NMDA, GABA or glycine receptors. Similarly, temporal tuning (peak sensitivity at ~30 Hz) was mediated by modulation of an excitatory conductance and not altered by block of synaptic inhibition.
Conclusions:
The characteristic spatial and temporal-contrast tuning of the parasol-Y cell of primate retina arises primarily by modulation of the excitatory bipolar input, without a significant role for either NMDA-type glutamate receptors or glycinergic crossover inhibition.
Keywords: 531 ganglion cells •
560 inhibitory receptors •
517 excitatory amino acid receptors