Optogenetic Manipulation of Postsynaptic cAMP Using a Novel Transgenic Mouse Line Enables Synaptic Plasticity and Enhances Depolarization Following Tetanic Stimulation in the Hippocampal Dentate Gyrus.

Thomas T. Luyben,Kenichi Okamoto,Jayant Rai,Hang Li,John Georgiou,Ariel Avila,Graham L. Collingridge,Takashi Tominaga,Mei Zhen

doi:10.3389/fncir.2020.00024

Abstract

cAMP is a positive regulator tightly involved in certain types of synaptic plasticity and related memory functions. However, its spatiotemporal roles at the synaptic and neural circuit levels remain elusive. Using a combination of a cAMP optogenetics approach and voltage-sensitive dye (VSD) imaging with electrophysiological recording, we define a novel capacity of postsynaptic cAMP in enabling dentate gyrus long-term potentiation (LTP) and depolarization in acutely prepared murine hippocampal slices. To manipulate cAMP levels at medial perforant path to granule neuron (MPP-DG) synapses by light, we generated transgenic (Tg) mice expressing photoactivatable adenylyl cyclase (PAC) in DG granule neurons. Using these Tg(CMV-Camk2a-RFP/bPAC)3Koka mice, we recorded field excitatory postsynaptic potentials (fEPSPs) from MPP-DG synapses and found that photoactivation of PAC during tetanic stimulation enabled synaptic potentiation that persisted for at least 30 min. This form of LTP was induced without the need for GABA receptor blockade that is typically required for inducing DG plasticity. The paired-pulse ratio (PPR) remained unchanged, indicating the cAMP-dependent LTP was likely postsynaptic. By employing fast fluorescent voltage-sensitive dye (VSD: di-4-ANEPPS) and fluorescence imaging, we found that photoactivation of the PAC actuator enhanced the intensity and extent of dentate gyrus depolarization triggered following tetanic stimulation. These results demonstrate that the elevation of cAMP in granule neurons is capable of rapidly enhancing synaptic strength and neuronal depolarization. The powerful actions of cAMP are consistent with this second messenger having a critical role in the regulation of synaptic function.

Highlights

Hippocampal synapses play a critical role in learning and memory and exhibit post-synaptic NMDA receptor-dependent long-term potentiation (LTP) of synaptic transmission (Collingridge et al, 1983; Moser et al, 1998; Gilbert et al, 2001)
To address the role of cAMP in synaptic plasticity at medial perforant path (MPP) fibers synapsing onto dentate gyrus (DG) granule neurons (MPP-DG synapses), we designed transgenic (Tg) mice expressing photoactivatable adenylyl cyclase (PAC)
We constructed a transgene in which the CaMKIIα promoter drives the expression of a red fluorescence protein (RFP)-tagged PAC (Figure 1A)

Summary

Introduction

Hippocampal synapses play a critical role in learning and memory and exhibit post-synaptic NMDA receptor-dependent long-term potentiation (LTP) of synaptic transmission (Collingridge et al, 1983; Moser et al, 1998; Gilbert et al, 2001). CAMP function in synaptic plasticity at excitatory synapses has conventionally been studied using pharmacological and genetic approaches (Abel et al, 1997; Barad et al, 1998; Wong et al, 1999; Navakkode et al, 2004; Govindarajan et al, 2011; Park et al, 2016). Recent advances in optogenetic techniques have made it possible to control spatiotemporal signaling functions within living neurons (Kim et al, 2015; Murakoshi et al, 2017), providing powerful tools for addressing dynamic molecular processes at synapses. In this regard, optogenetic manipulation of cAMP production has begun to show promise. Methods to control cAMP with spatiotemporal precision within intact brain tissue remain limited

Methods

Results

Conclusion