Activity-dependent enhancement of presynaptic Ca²⁺ signaling by ryanodine receptors at the hippocampal mossy fiber synapse

Abstract

Presynaptic ryanodine-sensitive Ca²⁺ store has been suggested to be involved in long-term potentiation (LTP) at the hippocampal mossy fiber synapses. However, the mode of activation of presynaptic ryanodine receptors remains mostly unknown. In this study, the conditions to activate presynaptic Ca²⁺ store at the mossy fiber synapse were examined using presynaptic Ca²⁺ measurements in acute hippocampal slice preparations. Presynaptic Ca²⁺ transients evoked by single shocks to the mossy fibers were not affected significantly by application of the ryanodine receptor blocker TMB-8 (100 μM). In contrast, presynaptic Ca²⁺ transients elicited by train stimuli (20 Hz, 10 times) were partially suppressed by TMB-8, although the responses to the first stimulus were almost unaffected. TMB-8 also suppressed the latter phase, but not the initial phase, of presynaptic Ca²⁺ transients elicited by tetanic stimulation (100 Hz, 100 times) of the mossy fibers. These findings suggested that the presynaptic ryanodine receptors release Ca²⁺ from intracellular stores in a use-dependent manner, and thereby amplify unique presynaptic long-term synaptic plasticity at the hippocampal mossy fiber synapses. [J Physiol Sci. 2007;57 Suppl:S84]