Peripheral nerve injury promotes morphine-seeking behavior in rats during extinction.

Chronic neuropathic pain and prescription opioid abuse represent highly interconnected societal problems. We used a rat model of spared nerve injury (SNI) and an intravenous drug self-administration paradigm to investigate the impact of a neuropathic pain state on morphine-seeking behavior in extinction (i.e. when morphine is withheld). SNI, sham-operated and naive groups exhibited similar levels of active lever presses for morphine infusions on a fixed ratio 1 (FR1) schedule. Self-administration of morphine, but not vehicle, attenuated nerve injury-induced mechanical allodynia in SNI rats. Under these same conditions, mechanical paw withdrawal thresholds in sham-operated and naive groups were largely unaltered. However, SNI rats showed higher levels of morphine-seeking behavior compared to sham-operated or naïve groups in extinction (i.e. when vehicle was substituted for morphine). Interestingly, the perseveration of morphine-seeking behavior observed during extinction was only present in the SNI group despite the fact that all groups had a similar history of morphine self-administration intake. Our results suggest that different motivational states associated with neuropathic pain promote morphine-seeking behavior in extinction. Drug self-administration paradigms may be useful for evaluating analgesic efficacy and motivational properties associated with opioid reinforcers in pathological pain states.