Abstract
Purpose::
The diversity of mammalian amacrine cell morphology is well-studied: there are about 30 different forms. However, there appear to be a limited number of relationships between excitation and inhibition impinging upon amacrine cells. Here we explore the physiology and pharmacology of these interactions. This study elucidates the recursive GABAergic and glycinergic pathways that shape the responses of different amacrine cells.
Methods::
We patch clamped amacrine cells in the rabbit retinal slice, and measured excitation, inhibition, and voltage responses to bright vs. dark flashes and sinusoidal modulation of intensity. We used the inhibitory receptor blockers picrotoxin, strychnine, SR95531, APB, and TPMPA to determine the neurotransmitters that mediate the inhibition. Alexa Fluor 488 was added to the intracellular pipette so that we could correlate the physiology and pharmacology with morphology.
Results::
We measured from over 200 amacrine cells, and found four major forms of interaction between excitation and inhibition. Different inhibitory inputs were distinguished pharmacologically: 1. ON amacrine cells (those receiving ON excitation), are inhibited by OFF signals. This inhibition is attenuated by strychnine. The application of APB obliterated the ON excitation. 2. OFF amacrine cells, (those receiving OFF excitation), are inhibited by ON signals. This inhibition is attenuated by strychnine. The application of APB obliterated the ON inhibition. 3. ON-OFF amacrine cells do not appear to receive inhibition, and their response is not affected by any blockers of inhibition. 4. ON-OFF inhibition measured in amacrine cells (that receive either ON or OFF excitation) can be pharmacologically parsed into two components: the ON inhibition is strychnine-sensitive, and the OFF inhibition is picrotoxin/SR95531-sensitive.
Conclusions::
The major form of interaction, encompassing 80% of the ON cells and 40% of the OFF cells, is glycinergic "crossover inhibition", where ON amacrine cells receive OFF inhibition, and OFF amacrine cells receive ON inhibition. The majority of ON-OFF amacrine cells do not receive any inhibitory inputs. The ON and OFF phases of ON-OFF inhibition to amacrine cells are mediated by separate transmitters, suggesting that the ON and OFF phases of inhibition are mediated by different classes of amacrine cells, and not by a single ON-OFF amacrine cell. These relatively few relationships between excitation and inhibition encompass the behavior of all amacrine cells studied.
Keywords: amacrine cells • electrophysiology: non-clinical • retina: proximal (bipolar, amacrine, and ganglion cells)