Disruption of the CRF2 receptor pathway decreases the somatic expression of opiate withdrawal

Escape from the extremely aversive opiate withdrawal symptoms powerfully motivates compulsive drug-seeking and drug-taking behaviors. The corticotropin-releasing factor (CRF) system is hypothesized to mediate the motivational properties of drug dependence. CRF signaling is transmitted by two receptor pathways, termed CRF1 and CRF2. To investigate the role for the CRF2 receptor pathway in somatic opiate withdrawal, in the present study we used genetically engineered mice deficient in the CRF2 receptor (CRF2-/-).We employed a novel, clinically relevant mouse model of 'spontaneous' opiate withdrawal as well as a classical opioid receptor antagonist (naloxone)-precipitated opiate withdrawal paradigm. To induce opiate dependence, mice were treated with intermittent escalating morphine doses (20-100 mg/kg, i.p.). We found that 8-128 h after the last opiate injection, CRF2-/- mice showed decreased levels of major somatic signs of spontaneous opiate withdrawal, such as paw tremor and wet dog shake, as compared to wild-type mice. Similarly, challenge with naloxone 2 h after the last morphine injection induced lower levels of paw tremor and wet dog shake in CRF2-/- mice as compared to wild-type mice. Despite the differences in somatic signs, wild-type and CRF2-/- mice displayed similar plasma corticosterone responses to opiate dosing and withdrawal, indicating a marginal role for the hypothalamus-pituitary-adrenal axis in the CRF2 receptor mediation of opiate withdrawal. Our results unravel a novel role for the CRF2 receptor pathway in opiate withdrawal. The CRF2 receptor pathway might be a critical target of therapies aimed at alleviating opiate withdrawal symptoms and reducing relapse to drug intake.