Orexin is involved in morphine-induced physical dependence and withdrawal. The locus coeruleus (LC) nucleus receives dense orexinergic projections, and is shown to express orexin receptors type 1. LC is also a key brain region implicated in morphine tolerance and dependence. However the role of orexinergic transmission at the LC nucleus in morphine dependence and tolerance is unknown. We are studying the effect of orexin neuropeptide at the locus coeruleus nucleus in naloxone-induced morphine withdrawal syndrome and tolerance to the analgesic effect of morphine, using behavioral, extracellular and whole-cell patch clamp recording techniques in rats. The tail flick test using thermal nociceptive stimulation of the tail showed that central administration of orexin receptor type-1 antagonist SB-334867 inhibits the development of tolerance to antinociceptive effect of morphine. Using in vivo extracellular single unit recording, we found that i.c.v. injection of SB-334867 prevents the development of tolerance to morphine in locus coeruleus (LC) neurons. Moreover, our results indicated that intra LC microinjection of SB-334867 prior to each morphine injection or prior to naloxone administration reduces the severity of naloxone-induced morphine withdrawal symptoms. We also used whole-cell patch clamp recording in rat horizontal slices containing the locus coeruleus nucleus to examine the effect of orexin on synaptic transmission. The results showed that in vitro application of orexin-A increases LC spontaneous firing rate and paired-pulse ratio (PPR). It also decreases spontaneous excitatory postsynaptic currents (sEPSCs) frequency of LC neurons, but did not change sEPSCs amplitude. Our electrophysiological data indicate that orexin-A application decreased evoked excitatory postsynaptic currents (eEPSCs) and evoked inhibitory postsynaptic currents (eIPSCs) in LC neurons synapses. It is concluded that orexinergic transmission in the locus coeruleus nucleus might involve in the development of tolerance and physical dependence to morphine. Moreover, our results provide in vitro evidences for a critical role of orexin signaling in LC neurons. It can be deduced that these changes in excitatory synaptic transmission may be elicited by presynaptic rather than presynaptic mechanisms.
School of Cognitive Sciences, Institute for Research in Fundamental Sciences (IPM),
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