Improved Version of ChETA Promotes Aggression in the Medial Amygdala

The development of optogenetic tools has significantly advanced our understanding of neural circuits and behavior. The medial amygdala, posterior dorsal subdivision (MeApd) is part of a distributed network controlling social behaviors such as mating and aggression. Previous work showed that activation of GABAergic neurons in mouse MeApd using channelrodopsin-2 (ChR2 H134R ) promoted aggression. In a recent study, Baleisyte et al. (2022) confirmed these findings using the same reagents (i.e. ChR2 H134R ), but also reported that a different ChR2 variant with faster kinetics—ChETA—inhibited rather than promoted aggression when high laser power, long duration photostimulation conditions were used. As ChETA is known to have a substantially lower photocurrent than ChR2 and other opsins, an improved version of ChETA (i.e. ChR2 E123T/T159C ; ChETA TC ) was subsequently developed. ChETA TC has larger photocurrents than the original ChETA while maintaining fast kinetics and low plateau depolarization. Here we show that activating MeApd GABAergic neurons using the improved ChETA TC promotes aggression, similar to ChR2 H134R , suggesting that the results obtained using the original ChETA are not due to a difference in channel kinetics. Furthermore, we found that ChETA TC is capable of driving a rapid onset of aggression within 200-300 milliseconds of stimulation, suggesting that this effect reflects direct activation of MeApd GABAergic neurons. We conclude that the different behavioral phenotypes observed using the original ChETA vs. ChETA TC and ChR2 likely reflects the weaker photocurrents in ChETA vs. other opsins, and/or the long duration/high power photostimulation conditions used with ChETA. Consistent with this conclusion, the results obtained using ChR2 or ChETA TC are complementary to findings from loss-of-functions experiments using optogenetic inhibition, chemogenetic inhibition, and neuronal ablation. These data support a positive-acting role of MeApd Vgat + neurons in aggression. Our findings, in conjunction with studies of Berndt et al. (2011), suggest that the improved ChETA TC should be used when faster kinetics than ChR2 offers are required.