Role of presynaptic and postsynaptic IP3-dependent intracellular calcium release in long-term potentiation in sympathetic ganglion of the rat.

Abstract

In the rat superior cervical ganglion, a form of long term potentiation (LTP) can be elicited by a brief high frequency stimuli applied to the preganglionic nerve. Cumulative evidence shows that a transient increase in cytoplasmic Ca²+ concentration is essential for the generation of the ganglionic LTP. Calcium influx and calcium release from intracellular calcium stores contribute to LTP. However, the differential role of presynaptic and postsynaptic calcium signaling has not been established. Herein, by using heparin, a membrane-impermeant inositol trisphosphate receptor (IP3R) blocker, we explored the contribution of presynaptic and postsynaptic IP3-sensitive calcium stores to the ganglionic LTP. The LTP was produced by a conditioning train of 40 Hz for 3 s. We analyzed the effects of heparin on the posttetanic potentiation: PTP magnitude and PTP time constant, and on two parameters that describe the LTP: LTP decay time (elapsed time required by the potentiated response to fall to 20% above the basal value) and LTP extent (the integral of the potentiated response). Heparin (100 and 200 μg/ml) was loaded in the preganglionic, the postganglionic, or in both nerves. We found that in all tested conditions heparin significantly decreased LTP but practically did not affect PTP. The preganglionic and postganglionic inhibitory effects of heparin were not additive. De-N-sulfated heparin, an ineffective IP3R blocker, had no effect on LTP, but abolished the heparin blocking effect. Data suggest that presynaptic and postsynaptic IP3-dependent intracellular calcium release equally contribute to ganglionic LTP, supporting our proposal of a trans-synaptic mechanism for LTP.