| Summary: | 碩士 === 國立臺灣大學 === 藥理學研究所 === 91 === Abstrat
Orexins, also known as hypocretins, is a novel hypothalamic peptide family with two members: orexin A/ hypocretin- 1 and orexin B/ hypocretin- 2. Orexin A and orexin B consist of 33 and 28 amino acids, respectively, and share 46% sequence homology. Two receptors, OX1R and OX2R, were identified from G- protein coupled orphan receptors to be the specific receptors of orexins. OX1R has lower affinity to orexin B, compared to orexin A but OX2R displays similar affinity to orexin A and orexin B. Orexin- containing neurons were found to localize mainly at the hypothalamus and thalamus. Thus, orexins have been implicated in several hypothalamus- regulated physiological functions including arousal, feeding, locomotor stimulation and autonomic stimulation.
Given that orexin fibers extending to the central gray and the spinal cord and that orexin- containing neurons were also found in the spinal cord, orexins were suggested to be involved in pain regulation. Indeed, orexin A was recently shown have an antinociceptive effect when given systemically or intracerebroventricularly. In the present study, we further investigated if intrathecal injection of orexins induces antiallodynic effect in a postoperative pain animal model.
Intrathecal injection with orexin A (0.03- 1 nmol) and orexin B (0.1- 3 nmol) dose- dependently reduced incision- induced allodynia with the ED50s at 0.059 and 2.5 nmol, respectively. The antiallodynic effect of 0.3 nmol of orexin A was abolished by co- injection with specific orexin A antibody (1: 500/ 5 mL). The effect of 0.3 nmol of orexin B was abolished by orexin B antibody (0.05- 0.1 mg). Co- injection with 30 nmol of SB- 334897, a selective OX1R blocker, completely antagonized the antiallodynic effect of 0.3 nmol of orexin A but partially reversed the effect of 0.3 nmol of orexin B. Increasing the dose of SB- 334897 to 100 nmol still did not completely antagonize the effect of orexin B. Co- injection with 50 nmol naloxne affected neither the effect of orexin A nor that of orexin B. Co- injection with suramin, 30- 90 nmol, a P2x ATP receptor antagonist， attenuated the effects of orexin A and orexin B. Strychnine, at doses without convulsive effects (1 mg) antagonized the effect of orexin B.
In animals with higher basal withdrawal threshold, intrathecal injection with orexin A antibody (1:500/ 5 mL), orexin B antibody (0.05 mg), SB- 334867 (30- 100 nmol), suramine (30- 90 nmol) or strychnine (1 mg) alone, increased the allodynic response induced by incision injury. It is suggested that endogenous orexins are elicited by paw incision and through activation of spinal OX1 receptors, tonically attenuate the allodynia induced by incision injury，which may serve as an endogenous self- protection mechanism.
In addition to the antiallodynic effect, intrathecal injection of orexin A，but lseldom orexin B, caused profound exophthalmos less than 10 min after injection. The exophthalmos induced by 0.3 nmol of orexin A was abolished by intrathecal injection of orexin A antibody (1:500/ 5 mL) but not by naloxone (50 mM) or 30 nmol of SB- 334867. Increasing the dose of SB- 334867 to 100 nmol delayed the onset and shortened the duration of exophthalmos but did not abolished it. Intrathecal co- injection with suramin 30- 90 nmol also only slightly delayed its onset and shortened the duration. Pretreatment with guanethidine by intraperitoneal injection for 5 days completely prevented the exophthalmos induced by intrathecal orexin A. It is suggested the exophthalmos might be attributed to an activation of preganglionic neurons within the spinal cord induced by intrathecal orexin A，and then exciting the cervical ganglia to cause the superior and inferior palpebral smooth muscle of Müller muscles that are innervated by cervical postganglionic adrenergic terminals.
It is concluded that intrathecal injection of orexins attenuate the allodynic response induced by incision injury. The antiallodynic effect of orexin A is mediated by OX1 receptors but not by opioid receptors. In addition to OX1 receptors, OX2 receptors may also be invoved in the antiallodynic effect of orexin B. The anitallodynic effect of intrathecal orexins are suggested to be attributed to an activation of glycinergic inhibitory neurons through purinergic P2x receptors in the spinal cord. Upon paw incision, endogenous orexins might be released to activate the spinal OX1 receptors as an endogenous analgesic protection mechanism.
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