Homers at the Interface between Reward and Pain

Area of Science:

• Neuroscience
• Molecular Biology
• Pharmacology

Background:

• Pain can change how the brain responds to opioids, likely through neuroadaptations in pain pathways and the limbic system.
• Nerve injury increases expression of glutamate receptors and Homer scaffolding proteins, which influence drug reward sensitivity.

Purpose of the Study:

• To investigate the role of Homer proteins in the interaction between pain and drug reward.
• To explore how nerve injury affects the brain's response to heroin reward.

Main Methods:

• Mice underwent chronic constriction injury (CCI) of the sciatic nerve to induce neuropathic pain.
• Expression of Homer proteins, glutamate receptors (mGluR5, GluN2A/B), and kinase activity were measured in brain regions.
• Behavioral responses to heroin (conditioned place preference/aversion) were assessed in wild-type, Homer knockout, and genetically modified mice.

Main Results:

• CCI mice exhibited pain hypersensitivity and a conditioned place aversion to low-dose heroin, unlike controls.
• Disrupting Homer proteins or mGluR5-Homer interactions blocked heroin's rewarding effects (conditioned place preference) in CCI mice.
• While Homer1c in the nucleus accumbens was not essential for heroin preference, its increase in other limbic areas may alter heroin's motivational properties after nerve injury.

Conclusions:

• Increased glutamate receptor/Homer/kinase activity in limbic structures, potentially outside the nucleus accumbens, is critical for switching heroin's incentive properties following nerve injury.
• These findings highlight the role of Homer proteins in neuropathic pain's impact on opioid reward and have implications for opioid pharmacotherapy.