State-Dependent cAMP Sensitivity of Presynaptic Function Underlies Metaplasticity in a Hippocampal Feedforward Inhibitory Circuit

SummaryAt hippocampal mossy fiber (MF)-st. lucidum interneuron (SLIN) synapses, mGluR7 serves as a metaplastic switch controlling bidirectional plasticity. mGluR7 activation during high-frequency stimulation (HFS) triggers presynaptic LTD due to persistent P/Q-type Ca2+ channel inhibition. However, following mGluR7 internalization HFS produces presynaptic LTP. Surprisingly, LTP is not a simple molecular reversal of Ca2+ channel depression. Rather, mGluR7 activation/internalization controls plasticity polarity by gating cAMP sensitivity of release. While naive surface mGluR7 expressing MF-SLIN synapses are insensitive to cAMP elevation, synapses that have internalized mGluR7 robustly potentiate following cAMP increases. Moreover, MF-SLIN LTP requires adenylate cyclase (AC) and protein kinase A (PKA) activities. We also discovered an association between mGluR7 and RIM1α, an active zone molecule required for AC/PKA-dependent presynaptic LTP. Importantly, the mGluR7-RIM1α interaction is regulated by mGluR7 activation, and mice lacking RIM1α are deficient in MF-SLIN LTP. We conclude that state-dependent cAMP sensitivity controlled by mGluR7-RIM1α interactions underlies MF-SLIN metaplasticity.