Repeated injections of orexin-A developed behavioral tolerance to its analgesic effects in rats

Objective(s):Reduction of pharmacological effectiveness or tolerance appears following repeated administration of many analgesic drugs. We investigated tolerance to anti-nociceptive effects of orexin-A, an endogenous potent analgesic peptide using the hot-plate test.Materials and Methods:Orexin-A was microinjected ICV (intracerebroventricular) with an interval of 12 hr for 7 continuous days and its anti-nociceptive responses were measured on days 1, 4 and 7 using the hot-plate test following the first day of administration. Orexin-A was used at a dose of 100 pmol to induce analgesic effects. Results:ICV administration of orexin-A produced an effective anti-nociception on the first day of experiment as measured by hot-plate 5, 15, and 30 min after the injection, in comparison with both baselines (hot-plate test one day before the beginning of orexin-A administration and control, saline-administrated group). However, repeated administration of orexin-A on the following days revealed a significant reduction in this analgesic effect during day 4 to day 7. However, to rule out any associative tolerance resulting from learning related to experimental procedures and/or environmental cues, a single injection of orexin-A was administrated to animals of control group (which were receiving saline during 7 days of experiments) and the analgesic effect was observed. Conclusion:These results, for the first time, indicated the appearance of tolerance to anti-nociceptive effects of orexin-A, following repeated administrations of this agent

Involvement of Endogenous Opioid System in Swim Stress-Induced Pain Modulation During the Interphase of the Formalin Test

Introduction: Some evidence demonstrates endogenous inhibitory pathways of pain involved in the interphase (phase between early and later phase) of the formalin test. We previously showed that swimming stress modulates the pain-related behaviors during the interphase of the formalin test. In this study, we evaluated the role of the endogenous opioid system in modulating nociceptive responses of the formalin test. Methods: Swim stress was performed in different heights of water (5, 25, 50 cm) in a swimming tank. The mean nociceptive scores were measured during phase 1 (1-7 min), interphase (8-14 min), and phase 2 (15-90 min) of the formalin test. Opioid receptor antagonist, naloxone (3 mg/kg; IP) was injected immediately before swim stress. Results: Swim stress attenuated nociceptive behaviors in the first phase and increased the duration of interphase in the formalin test in a water-height-dependent manner, compared to the control group. Naloxone significantly increased nociceptive behaviors in the first phase, interphase, and the second phase of the formalin test, compared to the control group. Conclusion: Stress could affect the nociceptive response. Swim stress in different heights of water could have different effects on the nociception in different phases of the formalin test. In addition, the involvement of the endogenous opioid system is further demonstrated in the swim stress-induced modulation of pain behaviors in phase 1, phase 2, as well as interphase of formalin test in rats

Effect of Food Deprivation on Formalin-Induced Nociceptive Behaviors and Beta-Endorphin and Sex Hormones Concentration in Rats

Background: The present study examined the possible role of endogenous opioidergic system in effect of food deprivation on formalin-induced nociceptive behaviors in male and female rats. Also, we investigated the effect of food deprivation on the plasma level of beta-endorphin and sex hormones. Methods: Food was withdrawn 48 h prior to performing the formalin test, but water continued to be available ad libitum. The formalin was injected into hind plantar paw. Results: There is significant difference between male and female control rats during phase 2B. Following 48-h food deprivation, both male and female rats exhibited enhanced nociceptive behavior in response to formalin. Food deprivation for 12 and 24 h increased and for 48 h decreased beta-endorphin level in male and female rats. Food deprivation for 24 h decreased testosterone level in male, while it had no significant effect on female rats and food deprivation for 48 h decreased testosterone level in both sexes. Food deprivation for 24 h increased estradiol level in female and that for 48 h had no significant effect on male and female rats. Conclusions: The present study demonstrates the existence of food deprivation for 48 h causes enhancement of nociception in the formalin test in male and female rats that has correlation with decrease in plasma beta-endorphin and testosterone levels

Orexin Receptor-1 (OX1R)

Orexin or hypocretin; a peptide that resembles the molecular structure of the gastric peptide secretin, was discovered in 1996 by two independent groups of researchers. One group named it orexin after the Greek word orexis which means “appetite,” while the other group named it hypocretin since it was synthesized in the hypothalamus and resembles secretin (Sakurai et al. 1998). Both the nomenclatures for orexins are currently in use. We would adhere to “orexin” for references to the peptides in this article. These hypothalamic peptides are produced by orexin neurons in the dorsomedial (DMH), lateral (LH), and perifornical hypothalamic areas (PFA) (Sakurai et al. 1998). Since these areas of the hypothalamus are involved in arousal, sleep-wake cycle, thermoregulation, feeding and appetite, the function of orexins is implicated in the regulation of these behaviors. Furthermore, studies have shown that orexins are also involved in emotion, reward and drug seeking. Since these neuropeptides control a wide range of functions important for survival, they are implicated in a variety of neuropsychiatric conditions like obesity, drug addiction and sleep disorders (Sakurai et al. 1998). Further research revealed the identification of two types of orexinergic peptides, namely, orexin A and orexin B. The common precursor, prepro-orexin mRNA, is differentially cleaved to yield either orexin A, a 33-amino acid peptide of 3562 Da with two sets of intrachain disulfide bonds (Fig. 1), or orexin B, a 28-amino acid linear peptide of 2937 Da (Ebrahim et al. 2002). Interestingly, the molecular structure of orexin A is conserved among numerous mammalian species. Both the orexinergic subtypes are ligands of the G protein-coupled cell-surface receptor (GPCR), HFGAN72. Later in 1998, this orphan receptor was recognized as orexin receptor-1 (OX1R) or hypocretin receptor-1 because of the pioneering work by Sakurai T. et al. (1998) (Fig. 1). We would adhere to “OX1R” for references to the orexin receptor-1 in this chapter

Involvement of the orexin/hypocretin system in the pharmacological effects induced by Δ(9)-tetrahydrocannabinol

BACKGROUND AND PURPOSE: Anatomical, biochemical and pharmacological evidence suggest the existence of a crosstalk between the orexinergic and endocannabinoid systems. While the orexin receptor 1 (OX1 receptor) modulates the reinforcing properties of cannabinoids, the participation of orexins in the acute pharmacological effects of Δ(9) -tetrahydrocannabinol (THC) remains unexplored. EXPERIMENTAL APPROACH: We assessed the possible role of orexins in THC-induced hypolocomotion, hypothermia, antinociception, anxiolytic- and anxiogenic-like effects and memory impairment. Selective OX1 and OX2 receptor antagonists and OX1 knockout (KO) mice as well as prepro-orexin (PPO) KO mice were used as pharmacological and genetic approaches. CB1 receptor levels in control and PPO KO mice were evaluated by immunoblot analysis. The expression of c-Fos after THC treatment was analysed in several brain areas in wild-type mice and in mice lacking the PPO gene. KEY RESULTS: The hypothermia, supraspinal antinociception and anxiolytic-like effects induced by THC were modulated by orexins through OX2 receptor signalling. OX1 receptors did not seem to be involved in these THC responses. No differences in CB1 receptor levels were found between wild-type and PPO KO mice. THC-induced increase in c-Fos expression was reduced in the central amygdala, medial preoptic area and lateral septum in these mutant mice. CONCLUSIONS AND IMPLICATIONS: Our results provide new findings to further clarify the interaction between orexins and cannabinoids. OX1 and OX2 receptors are differently implicated in the pharmacological effects of cannabinoids.This work was supported by the Instituto de Salud Carlos III grants [#PI13/00042 and #RD12/0028/0023 (RTA-RETICS)], by the Spanish Ministry of Science (#SAF2011-29864 and #SAF2014-59648-P), National Plan on Drugs (#2014I019) and the Catalan Government (SGR2014-1547