Muscarinic calcium mobilization in the regenerating retina of adult newt

Abstract

We used optical recording with a Ca 2+-sensitive dye, fura2, in living slice preparations from the newt retina at different stages of regeneration. ACh produced the most pronounced [Ca 2+] i rise in progenitor cells and premature ganglion cells of the earlier stage of retinal regeneration, but less pronounced Ca 2+ response in ganglion cells just before, or at the beginning of, synaptogenesis. The [Ca 2+] i rise to ACh was mediated by mAChRs. This was shown by inhibition of the ACh-induced Ca 2+ response with a preincubation of the mAChR antagonist atropine as well as with direct stimulation of the [Ca 2+] i rise by the mAChR agonist muscarine. This muscarine-induced [Ca 2+] i rise was more greatly suppressed by the M1 and/or M3 preferring mAChR antagonists than by the M2 preferring mAChR antagonist. The [Ca 2+] i rise due to muscarine was not suppressed in the absence of extracellular Ca 2+, but suppressed in part in the presence of the L-type voltage-gated Ca 2+ channel blockers, verapamil or nicardipine. Furthermore, thapsigargin (TG), a Ca-ATPase inhibitor, abolished the muscarine-induced [Ca 2+] i rise in the absence of extracellular Ca 2+. These results suggest that the mAChR-mediated [Ca 2+] i rise is mainly a result of a release of Ca 2+ from intracellular stores. TG produced a slow rise in the resting level of [Ca 2+] i. This [Ca 2+] i raise was suppressed as extracellular Ca 2+ was omitted, whereas a rapid rise in [Ca 2+] i occurred when extracellular Ca 2+ was reintroduced, suggesting the occurrence of the capacitative Ca 2+ influx in the progenitor cells and premature ganglion cells of the regenerating newt retina.