Early activation of Ca2+-permeable AMPA receptors reduces neurite outgrowth in embryonic chick retinal neurons

Calcium entry through Ca2+-permeable AMPA/kainate receptors may activate signaling cascades controlling neuronal development. Using the fluorescent Ca2+-indicator Calcium Green 1-AM we showed that the application of kainate or AMPA produced an increase of intracellular [Ca2+] in embryonic chick retina from day 6 (E6) onwards. This Ca2+ increase is due to entry through AMPA-preferring receptors, because it was blocked by the AMPA receptor antagonist GYKI 52466 but not by the N-methyl-D-aspartic acid (NMDA) receptor antagonist AP5, the voltage-gated Ca2+ channel blockers diltiazem or nifedipine, or by the substitution of Na+ for choline in the extracellular solution to prevent the depolarizing action of kainate and AMPA. In dissociated E8 retinal cultures, application of glutamate, kainate, or AMPA reduced the number of neurites arising from these cells. The effect of kainate was prevented by the AMPA/kainate receptor antagonist CNQX and by GYKI 52466 but not by AP5, indicating that the reduction in neurite outgrowth resulted from the activation of AMPA receptors. Blocking Ca2+ influx through L-type voltage-gated Ca2+ channels with diltiazem and nifedipine prevented the effect of 10-100 muM kainate but not that of 500 muM kainate. In addition, joro spider toxin-3, a blocker of Ca2+-conducting AMPA receptors, prevented the effect of all doses of kainate. Neither GABA, which is depolarizing at this age in the retina, nor the activation of metabotropic glutamate receptors with tACPD mimicked the effects of AMPA receptor activation. Calcium entry via AMPA receptor channels themselves may therefore be important in the regulation of neurite outgrowth in developing chick retinal cells. (C) 2001 John Wiley & Sons. Inc.