Chemogenetic Inhibition and Optogenetic Depotentiation of the Prelimbic Cortex to Paraventricular Thalamus Pathway Attenuate Abstinence-Induced Plasticity and Heroin Seeking in Rats.

Opioid use disorder (OUD) is characterized by high relapse rates, underscoring an urgent need to identify neurobiological mechanisms of opioid seeking. The paraventricular thalamus (PVT) is a key brain region implicated in drug seeking in humans and rodents. While the role of its downstream targets is beginning to be clarified, the upstream inputs that regulate PVT activity during opioid seeking remain poorly understood. The prelimbic cortex (PL) provides substantial excitatory input to the PVT and has a well-established role in cue-guided behaviors and drug seeking. However, the contribution of the PL → PVT pathway to heroin seeking is yet to be elucidated. To address this knowledge gap, we trained male and female rats to self-administer heroin for 12†d and then used a pathway-specific chemogenetic approach to transiently inhibit PL → PVT projections during a cued heroin seeking test following 14†d of abstinence. We found that chemogenetic inhibition of this pathway significantly reduced heroin seeking. We next examined synaptic plasticity at PL → PVT synapses ex vivo on withdrawal day 14 and found that abstinence from heroin self-administration induced an increase in AMPA/NMDA ratio compared with saline controls. Importantly, the increase in synaptic strength was normalized by applying an ex vivo optogenetic long-term depression (LTD) protocol. Finally, we applied the same optogenetic LTD protocol in vivo immediately before the cued seeking test and found that depotentiation of PL → PVT synaptic activity significantly reduced heroin seeking. These findings identify PL → PVT inputs as key drivers of heroin seeking and demonstrate that weakening this pathway reduces opioid seeking.