Circadian rhythm influences naloxone induced morphine withdrawal and neuronal activity of lateral paragigantocellularis nucleus.

Investigations have shown that the circadian rhythm can affect the mechanisms associated with drug dependence. In this regard, we sought to assess the negative consequence of morphine withdrawal syndrome on conditioned place aversion (CPA) and lateral paragigantocellularis (LPGi) neuronal activity in morphine-dependent rats during light (8:00-12:00) and dark (20:00-24:00) cycles. Male Wistar rats (250-300 g) were received 10 mg/kg morphine or its vehicle (Saline, 2 mL/kg/12 h, s.c.) in 13 consecutive days for behavioral assessment tests. Then, naloxone-induced conditioned place aversion and physical signs of withdrawal syndrome were evaluated during light and dark cycles. In contrast to the behavioral part, we performed in vivo extracellular single-unit recording for investigating the neural response of LPGi to naloxone in morphine-dependent rats on day 10 of morphine/saline exposure. Results showed that naloxone induced conditioned place aversion in both light and dark cycles, but the CPA score during the light cycle was larger. Moreover, the intensity of physical signs of morphine withdrawal syndrome was more severe during the light cycle (rest phase) compare to the dark one. In electrophysiological experiments, results indicated that naloxone evoked both excitatory and inhibitory responses in LPGi neurons and the incremental effect of naloxone on LPGi activity was stronger in the light cycle. Also, the neurons with the excitatory response exhibited higher baseline activity in the dark cycle, but the neurons with the inhibitory response showed higher baseline activity in the light cycle. Interestingly, the baseline firing rate of neurons recorded in the light cycle was significantly different in response (excitatory/inhibitory) -dependent manner. We concluded that naloxone-induced changes in LPGi cellular activity and behaviors of morphine-dependent rats can be affected by circadian rhythm and the internal clock.

Coregulation of sleep-pain physiological interplay by orexin system: An unprecedented review.

Accumulating evidence support the critical role of endogenous orexin system in modulation of various physiological functions. Among these, regulation of pain and wakefulness have extensively been investigated, however, by independent series of studies each focusing a distinct side. It is now well established that orexins induce potent analgesic effect via affecting their receptors within several specific brain structures. These mainly include locus coeruleus (LC), lateral paragigantocellularis (LPGi), ventral tegmental area (VTA), dorsal raphe nucleus (DRN), periaquiductal gray (PAG) and tuberomammillary nuclei (TMN). On the other hand, increased activity of orexinergic neurons enhances general wakefulness. Interestingly, a review of literature reveals that brain regions underlying orexin-mediated analgesia are most probably the site of action for orexin wake-promoting effects as well. The present study first pieces together the existing evidence supporting the rationale for the possibility of sleep-pain coregulation by orexin system and then suggests several shared mechanisms through which orexin can control the two mentioned processes. Furthermore, this study explains how imbalanced orexinergic transmission can cause progressive dysregulation of sleep-pain processing.

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.

The role of orexin type-1 receptors in the development of morphine tolerance in locus coeruleus neurons: An electrophysiological perspective.

Long-term exposure to opioid agonists results in tolerance to their analgesic effects, so the effectiveness of opioid agonists in the management of pain becomes limited. The locus coeruleus (LC) nucleus has been involved in the development of tolerance to opiates. Orexin type-1 receptors (OX1Rs) are highly expressed in LC nucleus. Orexin plays a noteworthy role in the occurrence of morphine tolerance. The purpose of the present study is to investigate the role of orexin type-1 receptors in the development of morphine tolerance in LC neurons. In this study, adult male Wistar rats weighing 250-300g were utilized. Induction of morphine tolerance was obtained by single injection of morphine per day for 6 successive days. An orexin type-1 receptor antagonist (SB-334867) was injected into the lateral ventricle instantly prior to morphine injection. On day 7, the effect of morphine on the electrical activity of LC neurons was studied using in vivo extracellular single unit recording. The results demonstrate that morphine injection for 6 consecutive days led to the development of morphine-induced tolerance in LC neurons. In other words, there was a significant decrease in LC neuronal responsiveness to morphine injection. Inhibitory responses of LC neurons to intraperitoneally applied morphine can be observed with the treatment of the SB-334867 prior to morphine injection. This study showed that OX1R blockade by SB-334867 prevents the development of morphine tolerance in LC neurons. We hope that further studies will lead to considerable progress in understanding the molecular adaptations that contribute to morphine tolerance.

Blockade of orexin type 1 receptors inhibits the development of morphine tolerance in lateral paragigantocellularis nucleus: an electrophysiological approach.

Repetitive administration of opioid agonists is associated with the development of tolerance to the effects of these substances and limits their application. Orexin (also known as hypocretin) is involved in morphine tolerance and dependence. The lateral paragigantocellularis nucleus (LPGi) is a key brain region implicated in the tolerance and dependence to opiates. Orexin type 1 receptor (OXR1) has been detected in LPGi nucleus. In this study the effect of OXR1 blockade was investigated on neural activity of LPGi during the development of morphine tolerance in rats. Male Wistar rats weighing 250-300 g were used in this study. To incite tolerance, morphine sulfate was injected intraperitonealy (10 mg/kg, i.p.) once a day for 6 days. A selective OXR1 antagonist (SB-334867) was microinjected into the right cerebral ventricle (10 µg/10 µl, i.c.v.) immediately before each morphine injection. On day 7, the effect of morphine (10 mg/kg, i.p.) on neural activity of LPGi was investigated using in vivo extracellular single unit recording. In this study morphine injection during 6 days led to the development of morphine tolerance in LPGi neurons which was observed as a significant decrease in responsiveness of LPGi neurons to acute morphine injection. Administration of SB-334867 before each morphine injection could reverse the responses of LPGi neurons to acute morphine injection. This study showed that OXR1 blockade by SB-334867 prevents the development of tolerance to morphine in LPGi neurons. Further studies are required to determine molecular and anatomical mediators which are thought to be involved in this phenomenon.

Orexin type 1 receptor antagonism in Lateral Paragigantocellularis nucleus attenuates naloxone precipitated morphine withdrawal symptoms in rats.

Orexin neuropeptides have been reported to be involved in morphine induced physical dependence and withdrawal. The Lateral Paragigantocellularis (LPGi) is a key brain region implicated in the expression of somatic signs of morphine withdrawal syndrome. Orexin A and orexin type 1 receptor have been found in LPGi neurons but the effect of orexin on the expression of opiate dependence and withdrawal phenomena in this brain structure has not been studied yet. In this study, the effect of intra-LPGi administration of SB 334867 (selective orexin type 1 receptor antagonist) on the behavioral signs of morphine withdrawal syndrome was investigated. Male Wistar rats weighing 250-300 g were rendered dependent by adding morphine sulfate (Temad, Tehran, Iran) to their drinking water in increasing concentrations of 0.1, 0.2, 0.3mg/ml for every 48 h and 0.4 mg/ml during the next 15 days. Behavioral signs of morphine withdrawal were assessed in a transparent cylindrical Plexiglas test chamber (30 cm diameter, 50 cm height) for 25 min. One group of animals received intra-LPGi injection of SB 334867 (0.2 µl, 100 µM) immediately before naloxone. In the control group, SB-334867 vehicle (DMSO 1%, 0.2 µl) was microinjected into LPGi. Our results indicate that intra-LPGi administration of SB 334867 significantly decreases naloxone precipitated morphine withdrawal signs. Thus, it seems that orexin might have a pivotal role in the expression of morphine withdrawal signs through affecting orexin type 1 receptor in LPGi nucleus.