Opposite responses to aversive stimuli in lateral habenula neurons.

Appropriate behavioural strategies to cope with unexpected salient stimuli require synergistic neuronal responses in diverse brain regions. Among them, the epithalamic lateral habenula (LHb) plays a pivotal role in processing salient stimuli of aversive valence. Integrated in the complex motivational circuit, LHb neurons are indeed excited by aversive stimuli, including footshock (Fs). However, whether such excitation is a common feature represented throughout the LHb remains unclear. Here, we combined single-unit extracellular recordings in anaesthetized mice with juxtacellular labelling to describe the nature, location and pharmacological properties of Fs-driven responses within the LHb. We find that, along with Fs-excited cells, about 10% of LHb neurons display Fs-mediated inhibitory responses. Such inhibited neuronal population, in contrast to Fs-excited neurons, display regular and high frequency activity at baseline and is clustered in the medial portion of the LHb. Juxtacellular labelling of Fs-excited and inhibited neurons unravels that both populations are of glutamatergic type, as they co-localized with the EAAC1 glutamatergic transporter but not with the GAD67 GABAergic marker. Moreover, while the excitatory responses to Fs require both AMPA and NMDA receptors, the inhibitory responses rely instead on GABA <sub>A</sub> channels. Taken together, our results indicate that two functionally and partly segregated LHb neuronal ensembles encode Fs in an opposite fashion. This highlights the neuronal complexity in the LHb for processing aversive external stimuli