Figure 6. AMPA increases axoplasmic [Ca²⁺] in spinal cord dorsal column axons measured using confocal microscopy.

A and B, example time courses from one experiment of axoplasmic Ca²⁺ levels as reported by fluorescence change of Oregon Green-488 BAPTA-1 in normal, non-ischaemic dorsal column axons. AMPA (100 μm; plus 30 μm cyclothiazide to reduce desensitization) induced a substantial Ca²⁺ rise which was significantly blunted by ryanodine. A, shows Ca²⁺-dependent (Oregon Green-488 BAPTA-1) and Ca²⁺-independent (Alexa Fluor 594) fluorescence separately. B, illustrates the ratio of Ca²⁺-dependent to Ca²⁺-independent fluorescence to correct for the small downward drift in fluorescence over time. C, bar graph of mean axoplasmic Ca²⁺ fluorescence changes at 30 min. Fluorescence increased by a mean of 86 ± 53% above baseline after 30 min of AMPA exposure. Addition of ryanodine (50 μm) reduced the AMPA-evoked Ca²⁺ increase to 16 ± 34% (P < 10⁻⁷). These data suggest that AMPA receptors increase axonal [Ca²⁺] partly by controlling release of Ca²⁺ from ryanodine receptor-dependent axonal Ca²⁺ stores (see text). n = 48 and 26 axons for AMPA and AMPA + ryanodine, respectively.