Activation of the cGMP/nitric oxide signal transduction system by nicotine in the retina

Abstract

Acetylcholine is one of the primary excitatory neurotransmitters/neuromodulators in the retina, but little is known about the downstream signaling pathways it can activate. The present study immunocytochemically examines the potential sources of acetylcholine and the location of the nicotinic cholinergic receptors in the turtle retina. It also examines how activation of these receptors can influence the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) signal-transduction pathways. Photoreceptors, amacrine cells, and potentially ganglion cells contain choline acetyltransferase-like immunoreactivity (LI). Nicotinic acetylcholine receptors are immunocytochemically localized on photoreceptors, horizontal, bipolar, and ganglion cells. Nitric oxide imaging indicates that stimulation with nicotine increases NO production primarily in photoreceptors, horizontal, Müller, bipolar, and ganglion cells. In turn, very select populations of amacrine cells respond to this NO with increased levels of cGMP-LI. Selective inhibitors reveal that nitric oxide synthase is involved in most, but not all, of these increases in cGMP-LI. These results show that acetylcholine can activate the NO/cGMP signal-transduction pathways in both the inner and outer retina. This indicates that both of the major excitatory retinal transmitters, glutamate and acetylcholine, can stimulate NO production that increases levels of cGMP-LI in overlapping populations of retinal cells.