Muscarinic M4 Receptors on Cholinergic and Dopamine D1 Receptor-Expressing Neurons Have Opposing Functionality for Positive Reinforcement and Influence Impulsivity.

Anna M Klawonn,Daniel B Wilhelms,David Engblom,Anand Kumar Singh,Jürgen Wess,Sarah H Lindström,Maarit Jaarola,Michael Fritz

doi:10.3389/fnmol.2018.00139

Abstract

The neurotransmitter acetylcholine has been implicated in reward learning and drug addiction. However, the roles of the various cholinergic receptor subtypes on different neuron populations remain elusive. Here we study the function of muscarinic M4 receptors (M4Rs) in dopamine D1 receptor (D1R) expressing neurons and cholinergic neurons (expressing choline acetyltransferase; ChAT), during various reward-enforced behaviors and in a “waiting”-impulsivity test. We applied cell-type-specific gene deletions targeting M4Rs in D1RCre or ChATCre mice. Mice lacking M4Rs in D1R-neurons displayed greater cocaine seeking and drug-primed reinstatement than their littermate controls in a Pavlovian conditioned place preference (CPP) paradigm. Furthermore, the M4R-D1RCre mice initiated significantly more premature responses (PRs) in the 5-choice-serial-reaction-time-task (5CSRTT) than their littermate controls, indicating impaired waiting impulse control. In contrast, mice lacking M4Rs in cholinergic neurons did not acquire cocaine Pavlovian conditioning. The M4R-ChATCre mice were also unable to learn positive reinforcement to either natural reward or cocaine in an operant runway paradigm. Immediate early gene (IEG) expression (cFos and FosB) induced by repeated cocaine injections was significantly increased in the forebrain of M4R-D1RCre mice, whereas it remained normal in the M4R-ChATCre mice. Our study illustrates that muscarinic M4Rs on specific neural populations, either cholinergic or D1R-expressing, are pivotal for learning processes related to both natural reward and drugs of abuse, with opposing functionality. Furthermore, we found that neurons expressing both M4Rs and D1Rs are important for signaling impulse control.

Highlights

Cholinergic neurons form a wide network throughout the central nervous system, including cell populations that can modulate either local or distal neuro-circuitry
To investigate how the auto-receptor population of M4Rs differ from the postsynaptic M4Rs expressed on D1R-neurons in drug-related behaviors, we started out by studying their role in cocaine induced locomotor sensitization
We used a transgenic approach to distinguish the roles of specific neuronal populations of M4Rs in palatable food and cocaine reward behaviors, and in impulsivity

Summary

Introduction

Cholinergic neurons form a wide network throughout the central nervous system, including cell populations that can modulate either local or distal neuro-circuitry. The mesolimbic system is innervated by cholinergic projections arising from two brainstem nuclei: the laterodorsal tegmental nucleus and the pedunculopontine nucleus (Oakman et al, 1995; Dautan et al, 2014). The striatum has its own population of cholinergic interneurons, which constitutes 1%–2% of its neurons. The cholinergic interneurons of the Nucleus Accumbens (NAc) have been studied using modern techniques. Optogenetic activation of cholinergic interneurons in the NAc was shown to enhance local phasic dopamine release (Cachope et al, 2012) and exert control over medium spiny neuron (MSN) activity (Witten et al, 2010). It is not surprising that the neurotransmitter acetylcholine has repeatedly been implicated in learning, decision making and reward functions (Grasing, 2016; Tian et al, 2016)

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