Our recent studies have identified a group of neurons in the basal forebrain (BF) region that encodes motivational salience using robust bursting responses (Lin &Nicolelis, 2008). Such bursting responses lead to faster decision speed (Avila &Lin, 2014) and generate an event-related potential response in the frontal cortex (Nguyen &Lin, 2014). During the current reporting period, our research effort focused on two main directions: (1) how does BF encoding of motivational salience emerge during learning? (2) establish the platform to conduct optogenetic experiments in awaking behaving mice, with the goal of determining the neurochemical identity of salience-encoding BF neurons. (1) While our previous studies have observed basal forebrain bursting in response to over-trained conditioned stimuli, it has remained unclear how basal forebrain neurons acquire the bursting response to conditioned stimuli during the learning process. We investigated this question by recording basal forebrain neuronal activity in rats that have been trained in a two-alternative auditory discrimination task and subsequently encountered a new learning context in which one of the auditory cues was replaced by a light. We found that basal forebrain neurons showing bursting responses to the learned auditory cue gradually developed a bursting response to the novel light over several sessions, closely mirroring two parameters of learning in this task: the discrimination of the new stimulus (light) from the no-stimulus condition (catch trial), and the session-average reaction time. These results support that the basal forebrain bursting response emerges in parallel with, but not before, the acquisition of a new stimulus-outcome contingency during learning. The basal forebrain bursting response to conditioned stimuli does not reflect an attention signal required for new learning. Rather, basal forebrain bursting continues to determine decision speed even during the learning process and therefore reflects the outcome of the learning process. (2) We have previously suggested that salience-encoding basal forebrain neurons are non-cholinergic neurons based on the lack of firing rate modulation across wake-sleep cycles. However, the neurochemical identity of salience-encoding neurons remains unclear. Determining their neurochemical identity represents a key step in further understanding their functions especially because BF is a neuroanatomically complex region comprised of multiple macrosystems and multiple populations of projection neurons. A powerful approach to determine the neurochemical identity of salience-encoding BF neurons is to optogenetically label specific neuronal populations in the BF, and simultaneously record single unit activity and photo-stimulate this region in awake behaving mice. As a first step toward this goal, we established an experimental platform in transgenic Cre mice combining operant behavior, electrophysiology and optogenetics. We established operant training paradigms in mice similar to those previously used to identify salience-encoding BF neurons in rats, which require mice to maintain fixation in nosepoke ports and then respond quickly to reward-predicting sound or light stimuli and collect reward in a custom-built photobeam lickometer. Mice were able to achieve high hit rates (>90%) and high trial numbers in a session (>150 rewarded trials), with fast reaction times (<300ms). To adapt optogenetic experiment for mice in operant chambers, we miniaturized existing microdrive designs that record EEG, LFP and single unit activity while also incorporating optic fibers for photo-stimulation. This miniaturized optrode design allowed mice to move freely in operant chambers without hindrance and achieve the pre-surgery levels of hit rate and trial number. Taken together, these results establish an experimental platform for optogenetic experiments in freely behaving mice using operant chambers, and serve as the basis for further studies to elucidate the neurochemical identity and functions of salience-encoding BF neurons through photo-tagging and perturbation experiments.
