Title:Repeated Administration of Orexin into the Thalamic Paraventricular
Nucleus Inhibits the Development of Morphine-Induced Analgesia
Volume: 29
Issue: 1
Author(s): Fatemeh Samani and Masoumeh Kourosh Arami*
Affiliation:
- Department of Neuroscience, School of Advanced Technologies in Medicine, Iran University of Medical Sciences,
Tehran, Iran
- Neuroscience Research Center, Iran University of Medical Sciences, Tehran, Iran
Keywords:
Paraventricular nucleus, Orexin, Morphine, Nociceptive behaviors, Formalin test, Analgesic effect.
Abstract: Background: Hypothalamic neuropeptides, orexins, play pivotal roles in nociception
and pain modulation.
Objective: In this study, we investigated the effect of the administration of orexin into the paraventricular
nucleus (PVT) on the development of morphine-induced analgesia in rats.
Method: Male Wistar rats weighing 250-300g received subcutaneous (s.c.) chronic morphine (6,
16, 26, 36, 46, 56 and 66 mg/kg, 2 ml/kg) at an interval of 24 hours for 7 days. Animals were divided
into two experimental groups in which the orexin (100 μM, 200 nl) and its vehicle were microinjected
into the PVT nucleus for 7 days before each morphine injection. Then, the formalin test was
performed for the assessment of pain-related behaviors.
Results: The results demonstrated that the rats pretreated by intra-PVT orexin exhibited higher
pain-related behaviors than the morphine-treated group. The analgesic effects of morphine were significantly
lower in orexin plus morphine-treated rats than the vehicle plus morphine-treated ones.
Conclusion: Our findings suggested that the animals receiving the prolonged intra-PVT application
of orexin before morphine injection demonstrated a significant increase in the development of
nociceptive behaviors in all phases. There fore, the present study highlighted a new area of the
brain involved in the effect of orexin on analgesia induced by morphine.