Figure 6. Regulation of somatodendritic DA release by RyR-gated intracellular Ca2+ stores.
a. Representative record of Fluo-5F fluorescence during RyR-activation by caffeine (5 mM), showing the time course of the resultant increase in intracellular Ca2+ concentration [Ca2+]i in an SNc DAergic neuron. Recordings were made in nominally zero [Ca2+]o and in the presence of TTX (1 μM). b. Average [DA]o versus time profiles in the SNc evoked by local stimulation (30 pulses, 10 Hz) in the absence and presence of caffeine (Caff, 5 mM, n = 6). c. Summary of the effect of caffeine on peak [DA]o; control peak [DA]o was taken as 100%. Activation of RyRs by caffeine increased evoked [DA]o (n = 6, ***p < 0.001 vs. control), implicating RyR-gated Ca2+ stores in somatodendritic DA release. d. Average [DA]o versus time profiles in SNc during caffeine (3 mM) after pretreatment with dantrolene (10 μM), a RyR blocker (left panel, n = 8) or CPA (30 μM), a SERCA inhibitor (right panel, 30 μM, n = 6). e. Summary of the effect of caffeine in dantrolene or CPA on peak [DA]o; control peak [DA]o in either dantrolene or CPA alone was taken as 100%. Enhancement of evoked [DA]o by caffeine is prevented by pretreatment with dantrolene (n = 8, p > 0.05, Caff + dant vs. dant alone) or CPA (n = 6, p > 0.05, Caff + CPA vs. CPA alone), confirming the involvement of release of Ca2+ from RyR-gated stores in caffeine-mediated somatodendritic DA release. f. Average [DA]o versus time profiles in SNc in the absence and presence of dantrolene in 2.4 mM [Ca2+]o (left panel, 10 μM, n = 8) and 1.2 mM [Ca2+]o (right panel, 10 μM, n = 8). g. Summary of the effect of dantrolene on peak [DA]o; control peak [DA]o was taken as 100%. Although dantrolene had little effect on evoked [DA]o in 2.4 mM [Ca2+]o (n = 8, p > 0.05 vs. control), a significant decrease in evoked [DA]o was revealed when dantrolene was applied in 1.2 mM [Ca2+]o (n = 8, **p < 0.01 vs. control), implicating RyR activation in somatodendritic DA release.