CaMKIIα Promoter-Controlled Circuit Manipulations Target Both Pyramidal Cells and Inhibitory Interneurons in Cortical Networks

Abstract

A key assumption in studies of cortical functions is that excitatory principal neurons, but not inhibitory cells express calcium/calmodulin-dependent protein kinase II subunit α (CaMKIIα) resulting in a widespread use of CaMKIIα promoter-driven protein expression for principal cell manipulation and monitoring their activities. Using neuroanatomical and electrophysiological methods we demonstrate that in addition to pyramidal neu-rons, multiple types of cortical GABAegic cells are targeted by adeno-associated viral vectors (AAV) driven by the CaMKIIα promoter in both male and female mice. We tested the AAV5 and AAV9 serotype of viruses with either Channelrhodopsin 2 (ChR2)-mCherry or Archaerhodopsin-T-green fluorescent protein (GFP) constructs, with different dilutions. We show that in all cases, the reporter proteins can visualize a large fraction of different interneuron types, including parvalbumin (PV), somatostatin (SST), neuronal nitric oxide synthase (nNOS), neuropeptide Y (NPY), and cholecystokinin (CCK)-containing GABAergic cells, which altogether cover around 60% of the whole inhibitory cell population in cortical structures. Importantly, the expression of the excitatory opsin Channelrhodopsin 2 in the interneurons effectively drive spiking of infected GABAergic cells even if the immunodetectability of reporter proteins is ambiguous. Thus, our results challenge the use of CaMKIIα pro-moter-driven protein expression as a selective tool in targeting cortical glutamatergic neurons using viral vectors.