Intracoerulear microinjection of orexin-A induces morphine withdrawal-like signs in rats

Orexin (hypocretin) neuropeptides play a pivotal role in expression of opioid withdrawal signs. Hypothalamic orexinergic neurons provide dense afferents for the nucleus locus coeruleus (LC). Somatic signs of opioid withdrawal are associated with the enhanced activity of LC neurons. In addition, intra-LC administration of orexin-A leads to the hyperactivity of LC neurons. The present study was an attempt to investigate whether intra-LC microinjection of orexin-A induces morphine withdrawal-like signs in both morphine dependent and control rats. To end this, adult male Wistar rats weighing 250-300g were used. For induction of morphine dependence, animals received morphine sulfate subcutaneously (10mg/kg, s.c.) at an interval of 12h for 9days. On day 10, intra-LC orexin injection (100μM, 200nl) was carried out two hours after last morphine administration. Morphine withdrawal-like signs were assessed in a transparent Plexiglas test chamber (30cm diameter, 50cm height) for 25min. Orexin-A microinjection induced some morphine withdrawal-like signs in both morphine dependent (chewing, scratching, rearing, teeth chattering, wet-dog shake and paw tremor) and control (chewing, scratching, rearing, sniffing, wet-dog shake and head tremor) rats. Furthermore, microinjection of SB-334867, a selective orexin type-1 receptor antagonist before orexin-A significantly suppressed orexin-induced morphine withdrawal-like signs. It seems that orexin-A, via increasing the activity of LC neurons, mediates the induction of some morphine withdrawal-like signs independent of morphine dependence.