neurosciencestuff:

Recalling traumatic memories enhances the rewarding effects of morphine in male rats, finds new research published in JNeurosci. These findings may help to explain the co-occurrence of post-traumatic stress disorder (PTSD) and addiction.

More than half of PTSD patients also struggle with substance abuse,
yet the underlying neural mechanisms of their addiction are not clear.
Dopamine receptors in the prefrontal cortex may play a role, as they are
involved in the processing of both fear- and reward-related memories.

Steven Laviolette and colleagues examined the involvement of two
dopamine receptors in the recall of a traumatic experience – a
footshock – and subsequent preference for morphine. Rats that were
reminded of the troubling experience by an associated scent showed a
greater freezing fear response and spent more time in an environment
where they previously received a dose of morphine that ordinarily does
not produce a preference for a morphine-paired environment. This effect
was blocked by activation of the dopamine receptor D1R. A different
dopamine receptor, D4R, increased freezing behavior and reward
sensitivity after the recall of a minor footshock that does not produce a
traumatic memory under normal conditions. The results suggest that
abnormal dopamine signals in the prefrontal cortex may underlie the
ability of traumatic memories to predispose individuals to addiction by
increasing their sensitivity to the rewarding effects of drugs such as
opioids.

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