Abstract
Purpose: :
GABA is important in mediating inhibition in the retina and shaping its visual output, the summation of which can be assessed at the level of the retinal ganglion cell (RGC). We evaluated the role of GABACR–mediated inhibition in shaping spatial and temporal aspects of RGC responses to standing contrasts under light adapted conditions.
Methods: :
C57B6/J (WT) and GABACR null (Null) mice were anesthetized and RGC action potentials were recorded in vivo, from the optic nerve using a tungsten electrode. Optimal receptive field (RF) center size was determined using a series of spot stimuli of varying diameter. Responses to an optimal RF surround annulus also were examined. Spots and annuli (bright for ON– (100 cd/m2) and dark for OFF–center (3 cd/m2) RGCs) were presented on a light adapted background (20 cd/m2).
Results: :
Consistent with our previous findings only changes in ON–center RGCs were noted when RF center excitatory responses were probed with an optimal spot. When the sustained nature of ON–center RGCs was assessed, we observed that Null cells were more transient than in WT mice, in that Null cell responses were less frequently maintained over the two second stimulus presentation. In WT cells, a decrease in firing rate (inhibition) was evoked when the center stimulus was removed. All aspects of the post–stimulus response were similar between ON–center Null and WT RGCs. In contrast, both the magnitude and the duration of this inhibition was reduced in OFF–center Null RGCs compared to WT. RF surround responses, probed with an annulus, revealed two types of Null ON–center RGCs. In one class, the annulus elicited a response similar to WT RGCs, where its onset evoked an inhibition of spiking and its removal evoked a transient increase in spiking (excitation). In the other class, onset of the annulus elicited excitation with no post–stimulus response. In contrast, the annulus–evoked response profiles in OFF–center Null and WT RGCs were similar.
Conclusions: :
GABACR–mediated inhibition is vital for shaping the inhibitory surround of one type of ON–center RGCs but not of the second type or any of the OFF–center RGCs. Within the RF center of OFF RGCs, however, the absence of GABACR–mediated inhibition alters post stimulus inhibition. These findings suggest a differential role of GABACR–mediated inhibition both across the RF center and surround and across the ON and OFF pathways.
Keywords: retina: proximal (bipolar, amacrine, and ganglion cells) • electrophysiology: non-clinical • inhibitory receptors