Linking the CHRNA5 SNP to drug abuse liability: From circuitry to cellular mechanisms

Understanding the habenula: A major node in circuits regulating emotion and motivation

Over the last decade, the understanding of the habenula has rapidly advanced from being an understudied brain area with the Latin name 'habena" meaning "little rein", to being considered a "major rein" in the control of key monoaminergic brain centers. This ancient brain structure is a strategic node in the information flow from fronto-limbic brain areas to brainstem nuclei. As such, it plays a crucial role in regulating emotional, motivational, and cognitive behaviors and has been implicated in several neuropsychiatric disorders, including depression and addiction. This review will summarize recent findings on the medial (MHb) and lateral (LHb) habenula, their topographical projections, cell types, and functions. Additionally, we will discuss contemporary efforts that have uncovered novel molecular pathways and synaptic mechanisms with a focus on MHb-Interpeduncular nucleus (IPN) synapses. Finally, we will explore the potential interplay between the habenula's cholinergic and non-cholinergic components in coordinating related emotional and motivational behaviors, raising the possibility that these two pathways work together to provide balanced roles in reward prediction and aversion, rather than functioning independently.

A common SNP in Chrna5 enhances morphine reward in female mice

The single nucleotide polymorphism (SNP) D398N (rs16969968) in CHRNA5, the gene encoding the α5 subunit of the nicotinic acetylcholine receptors (nAChR), has been associated with both nicotine and opiate dependence in human populations. Expression of this SNP on presynaptic VTA dopaminergic (DA) neurons is known to cause a reduction in calcium signaling, leading to alterations in transmitter signaling and altered responses to drugs of abuse. To examine the impact of the Chrna5 SNP on opiate reward and underlying dopaminergic mechanisms, mice harboring two copies of the risk-associated allele (Chrna5 A/A) at a location equivalent to human rs16969968 were generated via CRISPR/cas9 genome editing. We sought to determine whether Chrna5 A/A mice show differences in sensitivity to rewarding properties of morphine using the conditioned place preference paradigm. When mice were tested two weeks after conditioning, female Chrna5 A/A mice showed significantly enhanced preference for the morphine-paired chamber relative to WT females, suggesting that this genotype may enhance opioid reward specifically in females. In contrast, Chrna5 genotype had no effect on locomotor sensitization in male or female mice. Relative to WT females, peak amplitude of ACh-gated currents recorded from VTA DA neurons in Chrna5 A/A females was potentiated 1 day after conditioning with morphine. Increased FOS expression was also observed in Chrna5 A/A mice relative to WT mice following exposure to the morphine CPP chamber. We propose that impaired α5 nAChR subunit function alters DA neuron response following repeated morphine exposures, and that this early cellular response could contribute to enhanced opiate reward two weeks after conditioning.

Nicotine and opioid co-dependence: Findings from bench research to clinical trials

Concomitant use of tobacco and opioids represents a growing public health concern. In fact, the mortality rate due to smoking-related illness approaches 50% among SUD patients. Cumulative evidence demonstrates that the vulnerability to drugs of abuse is influenced by behavioral, environmental, and genetic factors. This review explores the contribution of genetics and neural mechanisms influencing nicotine and opioid reward, respiration, and antinociception, emphasizing the interaction of cholinergic and opioid receptor systems. Despite the substantial evidence demonstrating nicotine-opioid interactions within the brain and on behavior, the currently available pharmacotherapies targeting these systems have shown limited efficacy for smoking cessation on opioid-maintained smokers. Thus, further studies designed to identify novel targets modulating both nicotinic and opioid receptor systems may lead to more efficacious approaches for co-morbid nicotine dependence and opioid use disorder.