Diversity of lateral habenula neuron roles in negative valence events

Abstract

The lateral habenula (LHb) integrates critical information regarding aversive stimuli that shapes decision making and behavioral responses. Three major LHb outputs innervate dorsal raphe nucleus (DRN), ventral tegmental area (VTA), and the rostromedial tegmental nucleus (RMTg). LHb neurons that project to these targets are segregated and nonoverlapping, and this led us to consider whether they have distinct molecular phenotypes and adaptations to stress exposure in chapter 2. In order to capture a time-locked profile of gene expression after repeated forced swim stress, we used intersectional expression of RiboTag in rat LHb neurons and next-gen RNA sequencing to interrogate the RNAs actively undergoing translation from each of these pathways. The “translatome” in the neurons comprising these pathways was similar at baseline, but diverged after stress, especially in the neurons projecting to the RMTg. Using weighted gene co-expression network analysis, we found one module, which had an overrepresentation of genes associated with phosphoinositide 3 kinase (PI3K) signaling, comprising genes downregulated after stress in the RMTg-projecting LHb neurons. Reduced PI3K signaling in RMTg-projecting LHb neurons may be a compensatory adaptation that alters the functional balance of LHb outputs to GABAergic vs. monoaminergic neurons following repeated stress exposure.Additionally, while primarily glutamatergic, LHb neurons express numerous neuropeptides, such as pituitary adenylate cyclase-activating polypeptide (PACAP), which itself has been associated with anxiety and stress disorders. In chapter 3, we explored these neurons. Using Cre-dependent viral vector expression, we characterized these neurons based on their anatomical projections and found that they projected to raphe and rostromedial tegmentum but not ventral tegmental area. Using RiboTag to capture ribosomal-associated RNA from these neurons and reanalysis of existing single cell RNA sequencing data, we did not identify a unique molecular phenotype that characterized these PACAP-expressing neurons in LHb. In order to understand the function of these neurons, we chemogenetically excited PACAP-expressing neurons using virally-mediated gene transfer of DREADDs in PACAP-Cre mice and tested the mice using open field test, contextual fear conditioning, sucrose preference, novelty suppressed feeding, and conditioned place preference. We found that activating these neurons produce behaviors opposite to what is expected from the LHb as a whole – they decreased anxiety-like and fear behavior and produced a conditioned place preference. In conclusion, PACAP-expressing neurons in LHb represents a unique population of cells that oppose the actions of the remainder of LHb neurons by being rewarding or diminishing the negative consequences of aversive events.