Long-term THC shifted brain reward circuit control from rational to emotional regions in rats

What the researchers found

Long-term THC weakened prefrontal cortex glutamate input to the nucleus accumbens shell and strengthened input from the basolateral amygdala and ventral hippocampus. This shifts control of reward processing from cortical (rational) to limbic (emotional) circuits.

Why it matters

This provides a mechanistic explanation for why chronic cannabis use is associated with both addiction vulnerability and psychiatric symptoms: the brain's reward center becomes more responsive to emotional signals and less responsive to rational control.

The numbers in context

THC weakened PFC input, strengthened BLA and vHipp input, and altered properties (but not net strength) of midbrain dopamine neuron input to NAc shell.

How the study worked

In vitro electrophysiology combined with optogenetics in rats to selectively activate and measure specific neural pathways from prefrontal cortex, amygdala, hippocampus, and dopamine neurons to the nucleus accumbens after long-term THC exposure.

What this study cannot tell us

Animal study using in vitro recordings, which may not fully capture in vivo neural dynamics. The THC exposure paradigm may not perfectly model human chronic use. Only the NAc shell subregion was studied.

How to read the evidence

Moderate: sophisticated optogenetic approach providing causal evidence for circuit-level changes, though limited to animal model.

When this study was published

Published in 2020 in Biological Psychiatry.

The bigger picture

The shift from cortical to limbic control of the reward system parallels what is seen in other addictions and psychiatric conditions, suggesting a common neural pathway through which chronic THC exposure increases vulnerability to these disorders.

Questions still open

• Is this circuit remodeling reversible after THC cessation? Does this shift explain individual differences in vulnerability to cannabis-related psychiatric effects? Could interventions targeting prefrontal cortex function prevent these changes?

Common questions

Read the original research

Altered Corticolimbic Control of the Nucleus Accumbens by Long-term Δ9-Tetrahydrocannabinol Exposure.

Biological psychiatry, 87(7), 619-631

Citation

Hwang, Eun-Kyung; Lupica, Carl R. (2020). Altered Corticolimbic Control of the Nucleus Accumbens by Long-term Δ9-Tetrahydrocannabinol Exposure.. Biological psychiatry, 87(7), 619-631. https://doi.org/10.1016/j.biopsych.2019.07.024

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