Optogenetic Dissection of Interneuron Microcircuits Driving Cholinergically-Activated Rhythms in CA1 Hippocampus

Abstract

Perisomatic GABAergic inhibition is thought to play a prominent role in hippocampal oscillations associated with the release of acetylcholine (ACh) from septal cholinergic afferents. A prominent hypothesis states that parvalbumin (PV)-containing basket cells, activated concurrently by ACh and fast glutamatergic input, drive these rhythms. However, this model - generated solely from in vitro studies using bath application of cholinergic agonists - has never been tested under conditions of endogenous ACh release. To investigate the selective effects of endogenous ACh on inhibitory microcircuits in slices, we have taken an optogenetic approach by injecting choline acetyltransferase (ChAT)-Cre mice with a Cre-dependent AAV vector carrying the light-activated cation channel Channelrhodopsin2 (ChR2). In acute hippocampal slices taken from these animals, brief trains of blue light generate sustained low-frequency rhythmic IPSCs in CA1 pyramidal cells, and inhibitory local field potentials (LFPs), even in the presence of ionotropic glutamate receptor antagonists. These rhythms are almost entirely blocked by the muscarinic ACh receptor (mAChR) antagonist atropine, and are potently suppressed by the endocannabinoid-mediated process of depolarization-induced suppression of inhibition (DSI). This suggests that the IPSCs are driven type 1 cannabinoid receptor (CB1R)-expressing interneurons, which in the hippocampus comprise the cholecystokinin (CCK)-positive basket cells, not PV+ basket cells. Surprisingly, the oscillations are also greatly inhibited by activation of mu-opioid receptors, which are indeed highly concentrated on the terminals of PV+ basket cells, but not CCK+ cells. However, selective pharmacological inhibition as well as optogenetic silencing of PV+ cells using the light-driven chloride pump Halorhodopsin (NpHR), or the proton pump Archaerhodopsin (Arch), had no significant effect on the rhythms. Surprisingly, IPSCs evoked through direct optogenetic stimulation of CCK+ cells were sensitive to a mu-opioid receptor agonist. This finding is inconsistent with previous anatomical studies, and suggests the presence of an inhibitory oscillator in CA1 which is activated by endogenous ACh, driven solely by CCK+ basket cells, and regulated by both endogenous cannabinoids and opioids.