Effects of 5-HT and OA on [Ca²⁺]_i level. Resting [Ca²⁺]_i levels are not changed by 5-HT (A) or OA (B). Activity-dependent calcium influx was activated by a train of 10 Hz action potentials for 30 sec. 5-HT (200 nm) did not create any detectable change in the [Ca²⁺]_iincrease activated by the action potential train (A). OA (2.5 μm) application caused a small but significant decrease in the [Ca²⁺]_i increase activated by the same stimulation as in A (B).