Differential Roles of cAMP and cGMP in Presynaptic Plasticity of Salamander Cone and Rod Photoreceptors in vitro

Abstract

Abstract: : Purpose: Retinal detachment and degeneration cause axonal sprouting and synaptogenesis by cone and rod cells. We have established that photoreceptors require the activity of different Ca²⁺ channels for presynaptic plasticity: rod cells require L-type and cone cells require cGMP-gated channel activity (Zhang and Townes-Anderson, J Neurosci., 2002). In the present experiments, we have investigated the roles of cAMP and cGMP in presynaptic plasticity of cultured photoreceptors. Methods: Isolated adult salamander photoreceptors were incubated for 3 days in a medium containing one of the following reagents: a specific activator of soluble guanylyl cyclase (sGC) YC-1, a cGMP analogue 8Br-cGMP, an inhibitor of sGC ODQ, a cAMP analogue Sp-cAMPS, an inhibitor of adenylyl cyclase SQ22536, an L-type Ca²⁺ antagonist nicardipine (Nc) and a cGMP-gated channel antagonist L-cis-diltiazem (Lcd). The number of newly formed neuritic processes and presynaptic varicosities were examined in cone and rod cells identified by the absence or presence of immunostaining for rod specific opsin. Results: (1) cGMP. In cone cells, 1µM YC-1 and 350µM 8Br-cGMP caused significant increase of process outgrowth and varicosity formation. The stimulatory effect of 350µM 8Br-cGMP could be reduced by 100µM Lcd. In contrast, in rod cells, YC-1 (1, 10 and 100µM) and 8Br-cGMP (35, 350 and 1400µM) caused dose-dependent inhibition of process outgrowth and varicosity formation. Cone growth could be inhibited by ODQ. (2) cAMP. In rod cells, 2-200µM Sp-cAMPS caused significant increase of varicosity formation and process outgrowth. The stimulatory effect of 2uM Sp-cAMPS in rod cells could be reduced by 10µM Nc. SQ22536 (2, 20 and 200µM) caused dose-dependent inhibition of rod process outgrowth and varicosity formation. Conclusion: Presynaptic plasticity of cone cells depends primarily on cGMP-dependent signaling that includes Ca²⁺ influx through cGMP-gated channels; whereas that of rod cells depends primarily on cAMP-dependent signaling that includes Ca²⁺ influx through L-type channels. Thus, the main photoreceptor cell types use distinct mechanisms for axonal plasticity which may be activated during disease.

Keywords: photoreceptors • retinal degenerations: cell biology • plasticity