Metaplasticity is a higher form of synaptic plasticity that is essential for learning and memory, but its molecular mechanisms remain poorly understood. Here, we report that metaplasticity of transmission at CA1 synapses in the hippocampus is mediated by Src family kinase regulation of NMDA receptors (NMDARs). We found that stimulation of G-protein-coupled receptors (GPCRs) regulated the absolute contribution of GluN2A-versus GluN2B-containing NMDARs in CA1 neurons: pituitary adenylate cyclase activating peptide 1 receptors (PAC1Rs) selectively recruited Src kinase, phosphorylated GluN2ARs, and enhanced their functional contribution; dopamine 1 receptors (D1Rs) selectively stimulated Fyn kinase, phosphorylated GluN2BRs, and enhanced these currents. Surprisingly, PAC1R lowered the threshold for long-term potentiation while long-term depression was enhanced by D1R. We conclude that metaplasticity is gated by the activity of GPCRs, which selectively target subtypes of NMDARs via Src kinases.
Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) are two closely related peptides, which can activate protein kinase A (PKA). At least three receptors for PACAP and VIP have been identified. The PACAP-specific receptor, PAC1 receptor, exhibits a higher affinity for PACAP than VIP, whereas VIP receptors, VPAC1-R and VPAC2-R, have similar affinities for PACAP and VIP. Both PACAP/VIP and their cognate receptors are highly expressed in the brain, including the hippocampus. Recently, their roles in the regulation of synaptic transmission have begun to emerge. PACAP/VIP can signal through different pathways to regulate N-methyl-D: -aspartate (NMDA) receptors in CA1 pyramidal cells. The activation of VPAC1/2-Rs increases evoked NMDA currents via the cyclic AMP/PKA pathway. However, the activation of PAC1-R stimulates a PLC/PKC/Pyk2/Src signaling pathway to enhance NMDA receptor function in hippocampal neurons. Furthermore, different concentrations of PACAP induce different effects on the both α-amino-3-hydroxy-5-isoxazole-propionic acid-evoked current and basal synaptic transmission by activating different receptors. Their roles in learning and memory are also demonstrated using transgenic mice and pharmacological methods.
Vasoactive intestinal peptide (VIP) is a 28 amino acid peptide which belongs to a superfamily of structurally related peptide hormones including pituitary adenylate cyclase-activating polypeptide (PACAP). Although several studies have identified the involvement of PACAP in learning and memory, little work has been done to investigate such a role for VIP. At least three receptors for VIP have been identified including the PACAP receptor (PAC1-R) and the two VIP receptors (VPAC receptors). VIP can activate the PAC1-R only if it is used at relatively high concentrations (e.g. 100 nM); however, at lower concentrations (e.g. 1 nM) it is selective for the VPAC receptors. Our lab has showed that PAC1-R activation signals through PKC/CAKβ/Src pathway to regulate NMDA receptors, however there is little known about the potential regulation of NMDA receptors by VPAC receptors. Our studies demonstrated that application of 1nM VIP enhanced NMDA currents by stimulating VPAC receptors as the effect was blocked by VPAC receptor antagonist [Ac-Tyr 1 , D-Phe 2 ]GRF (1-29). This enhancement of NMDA currents was blocked by both RpcAMPS and PKI 14-22 (they are highly specific PKA inhibitors), but not by the specific PKC inhibitor, bisindolylmaleimide I. In addition, the VIP-induced enhancement of NMDA currents was accentuated by inhibition of phosphodiesterase 4, which inhibits the degradation of cAMP. This regulation of NMDA receptors also required the scaffolding protein AKAP. In contrast, the potentiation induced by high concentration of VIP (e.g. 100 nM) was mediated by PAC1-R as well as by Src kinase. Overall, these results show that VIP can regulate NMDA receptors through different receptors and signaling pathways.
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