Genetically targeted connectivity tracing excludes dopaminergic inputs to the interpeduncular nucleus from the ventral tegmentum and substantia nigra

Abstract

The “habenulopeduncular system” consists of the medial habenula (MHb) and its principal target of innervation, the interpeduncular nucleus (IP). Neurons in the ventral MHb express acetylcholine along with glutamate, and both the MHb and IP are rich in nicotinic acetylcholine receptors. Much of the work on this system has focused on nicotinic mechanisms and their clinical implications for nicotine use, particularly because the IP expresses the α5 nicotinic receptor subunit, encoded by the CHRNA5 gene, which is genetically linked to smoking risk. A working model has emerged in which nicotine use may be determined by the balance of reinforcement mediated in part by nicotine effects on dopamine reward pathways, and an aversive “brake” on nicotine consumption encoded in the MHb-IP pathway. However, recent work has proposed that the IP also receives direct dopaminergic input from the ventral tegmental area (VTA). If correct, this would significantly impact the prevailing model of IP function. Here we have used Chrna5^Cre mice to perform rabies virus-mediated retrograde tracing of global inputs to the IP. We have also used Cre-dependent adeno-associated virus (AAV) anterograde tracing using Slc6a3^Cre (DAT^Cre) mice to map VTA dopaminergic efferents, and we have examined tract-tracing data using other transgenic models for dopaminergic neurons available in a public database. Consistent with the existing literature using non-genetic tracing methods, none of these experiments show a significant anatomical connection from the VTA or substantia nigra to the IP, and thus do not support a model of direct dopaminergic input to the habenulopeduncular system.

SIGNFICANCE STATEMENT

The interpeduncular nucleus is the central node in a descending brain pathway linking the medial habenula of the epithalamus to the mesopontine tegmentum. Neurons in this “habenulopeduncular pathway” express unusual nicotinic acetylcholine receptors and are thought to play a role in behavioral responses to nicotine. In particular, the IP expresses the α5 nicotinic receptor, human variants of which confer increased risk of smoking. Recently it has been reported that inputs from midbrain dopaminergic neurons to the IP also influence behavior. Here we have used a mouse genetic model to map specific inputs to the α5-expressing neurons in the IP. Contrary to the published reports, we do not identify dopaminergic inputs to the IP.