Microinjection of orexin-A into the locus coeruleus area induces morphine withdrawal-like behaviors in morphine dependent rats

Introduction: It has been shown that orexin peptides have a role in opioid withdrawal behaviors. Orexin-expressing neurons that are present in the hypothalamic nuclei send dense projections to the Locus Coeruleus (LC). Withdrawal syndrome is temporally associated with hyperactivity of LC neurons. However, LC neurons do not show withdrawalinduced hyperactivity in the brain slices from morphine-dependent rats. Thus, it has been suggested that the increase in LC neuronal activity seen in vivo is mediated by extrinsic factors. Therefore, this study was carried out to find whether LC microinjection of orexin-A can induce withdrawal behaviors. Methods: Adult male Wistar rats weighing 250-300 grams were rendered morphine dependent by subcutaneous injection of morphine sulfate (10 mg/kg) at an interval of 12 h for 9 days. On day 10, intra-LC microinjection of orexinA (100 μM, 200 nl) was performed two hours after morphine administration. Thereafter, somatic signs of withdrawal were evaluated in a Plexiglas chamber (30 cm diameter, 50 cm height) during a period of 25 min. Results: Orexin-A induced several signs of morphine withdrawal including chewing, scratching, rearing, teeth chattering, wet-dog shake and paw tremor. Acute LC microinjection of an orexin type 1 receptor antagonist, SB334867-A, prior to orexin-A prevents the expression of these signs. Conclusion: It may be concluded that orexin, via orexin type 1 receptor at LC acts as an extrinsic factor in the expression of morphine withdrawal syndrome.