Ca2+ dynamics in hippocampal cell cultures. Panel A, Spontaneous synchronized cytoplasmic Ca2+ oscillations in Snap25+/+ hippocampal neurons were abolished by addition of TTX (1 μM). Each trace is from a different neuron within the same culture dish. The inset shows a segment of the recording (prior to TTX, denoted by asterisk) at an expanded time base to show more clearly the synchrony of events. Panel B, Recordings from a culture derived from a homozygote Snap25-/- fetus under identical conditions as illustrated in A. Spontaneous Ca2+ transients in Snap25-/- neurons were very rare and a single event in one neuron is shown at an expanded time base in the inset. Panel C, Depolarization with 55 mM K+ (arrow) led to a sustained Ca2+ elevation in neurons from both. Snap25+/+ (left panel) and Snap25-/- (right panel) cultures. TTX (1 μM) was included in both preparations, prior to K+ application. Panel D, Astrocyte Ca2+ oscillations from the same culture dishes illustrated in Panel C. Under control conditions, spontaneous events were observed in both Snap25+/+ (left panel) and Snap25-/- (right panel) cultures. Following K+ application, an increase in frequency and amplitude of astrocyte events was observed in astrocytes from Snap25+/+ but not in the Snap25-/- preparation. A dashed line is drawn near the initial peak of neuronal Ca2+ increases in the Snap25+/+ preparation, to emphasize the relationship between neuronal and astrocyte signals. A similar relationship was not apparent in the Snap25-/- culture. (See Fig. 3 for group astrocyte data).