Abstract
Purpose: :
Network adaptation involves numerous mechanisms but the role of nitric oxide (NO) in these processes is poorly understood. This paper demonstrates that NO can regulate network adaptive processes.
Methods: :
Whole–cell currents were recorded from ganglion cells in light–adapted retinal slice of tiger salamander. EPSCs and IPSCs were evoked by light flashes (dur 500 ms; 9–10 log Q) and by focal electrical stimulation (zaps; dur 1msec .2–1.2 µA) of bipolar cells. Desensitization index (Di) was quantified as 1 – average steady state response / first response, for peak response or charge, so no desensitization = 0 and complete desensitization = 1. Drugs were applied through a gravity–fed microperfusion system. Glycinergic inhibition was blocked with strychnine (10µM); D1 dopamine receptors were blocked with SCH23390 (10µM); NO synthesis was blocked by LNAME (250 µM).
Results: :
Desensitization was activated with brief, repetitive stimulation at short inter–stimulus intervals (12 stimuli; 5 sec ISI). Responses evoked by light or zaps were largest for the first stimulus and were reduced in subsequent stimuli to a new steady state response typically after 3–7 repetitions. Di was slightly greater for light–evoked IPSCs (.37) than for EPSCs (.26) but EPSCs were quite variable. LNAME had no significant effect on light–evoked EPSC magnitude but did reduce the first and steady state IPSC response by 86%. LNAME increased Di for light–evoked IPSCs but not EPSCs. Zap–evoked responses were less variable and exhibited greater desensitization of EPSCs (.70) and IPSCs (.64). LNAME reduced the EPSC peak (by 12%) but did not significantly alter the EPSC Di. In contrast, LNAME reduced the first IPSC by 72% and decreased the steady state response by 86%; IPSC Di was significantly increased (from .64 to .89; p<.01). The effects of LNAME were rapidly reversible following washout, or with the addition of the NO donor DETA (10µM).
Conclusions: :
In the absence of NO, GABAergic amacrine cell output is reduced significantly. Increasing NO, which occurs in response to illumination, would upregulate the output of GABAergic amacrine cells, but would not dramatically alter the output of bipolar cells. This is probably due to the differences in transmitter release from conventional amacrine cell synapses and bipolar ribbon synapses.
Keywords: synapse • amacrine cells • nitric oxide