Interactive reportAccelerated release and production of orphanin FQ in brain of chronic morphine tolerant rats1
Introduction
The successful cloning of δ 1, 2, μ 3, 4and κ opioid receptors 5, 6quickly led to the discovery of a novel member of the opioid receptor gene family designated orphan opioid like receptor (ORL1) [7]or LC132 [8], Oprl [9], ROR-C [10], MOR-C [11]. The receptor structurally displays approximately 65% identity with μ, κ and δ opioid receptor and is negatively coupled with adenylyl cyclase, but no opioid ligands have been found to interact with this receptor 7, 9, 10, 12, 13. Orphanin FQ (OFQ), the putative endogenous ligand of ORL1, was a newly discovered heptadecapeptide first isolated from porcine hypothalamus in 1995 [14]. While OFQ structurally resembles the existing endogenous opioid peptides, especially dynorphin A (1–17), it does not bind to the classical opioid receptors with high affinity [14]. Much to our surprise, OFQ exhibited a role different from traditional opioids in vivo pharmacological study. The initial behavioral study reported that i.c.v. administration this new neuropeptide in mice produced hyperalgesia in the hot plate test, hence it was named nociceptin according to its apparent pronociceptive properties [15]. Recent studies, however, demonstrated that i.c.v. injection of OFQ could dose-dependently reversed systemic morphine-induced antinociception 16, 17, electroacupuncture-induced antinociception [18], opioid-mediated stress-induced antinociception [16]and functionally antagonized the analgesic effect induced by selective opioid receptor agonists DAMGO (μ-selective), DPDPE (δ-selective) and U-50, 488H (κ-selective) in mice [19]. In other words, OFQ acts as an anti-opioid peptide at least at supraspinal level [20].
In situ hybridization and Northern hybridization analysis 21, 22revealed that preproOFQ mRNA is widely distributed and highly expressed in central gray and central tegmental area of the midbrain, amygdala and hypothalamic nuclei. Tian et al. in our laboratory found that OFQ IgG administrated cerebroventricularly (i.c.v.) to rats reversed chronic morphine tolerance [23], suggesting that OFQ may play an important role in the development of morphine tolerance. To test the hypothesis that an increased production and release of endogenous OFQ is the possible mechanism of morphine tolerance, we used radioimmunoassay (RIA), a specific and sensitive method for measuring minute amount of neuropeptide, to assess whether there is a change in the amount of OFQ-ir in cerebroventricular perfusate and in the brain tissue of periaqueductal gray (PAG) and amygdala, which contain abundant OFQ [21]and ORL1 transcripts [24].
Section snippets
Chemicals and animals
Morphine HCl was purchased from Qinhai Drug Company (China). OFQ and OFQ RIA kits were products of Phoenix Pharmaceuticals (USA). Morphine HCl and OFQ were dissolved in normal saline and buffer, respectively. Adult female Wistar rats weighing 200–250 g were provided by the Animal Center in Beijing Medical University. They were housed six in a cage with food pellets and water available ad libitum.
Nociceptive testing
Experiments were performed in a temperature-controlled room (20±1°C). Nociceptive sensitivity was
The development of chronic morphine tolerance in rats
In rats that were given an increasing dose of morphine (10–60 mg/kg, s.c. t.i.d.), the analgesic effect induced by a testing dose of morphine (10 mg/kg) was decreased almost linearly from 113±18% on the 1st day to 78±17% on the 3rd day and 16±7% on the 5th day (Fig. 1). The basal TFL was not significantly changed during this period (4.5±0.1 s and 4.8±0.2 s at the 1st and 5th days of injection, respectively).
Changes of OFQ-ir in the brain perfusate (BP), PAG and amygdala of chronic morphine tolerance rats
Rats were randomly divided into six groups (n=11-13/group). Three groups respectively
Discussion
There is a battery of evidence pointing to the conclusion that brain OFQ may play an important role in the mechanism underlying morphine tolerance. This conclusion has been strengthened by the fact that in homozygous mutant mice whose OFQ receptor gene was knocked out, tolerance was not readily induced by systemic morphine injection with a dose of 10 mg/kg, once a day for 5 days [28]. In addition, chronic morphine tolerance induced by multiple injections of morphine (3 times a day for 6 days,
Conclusion
Our results suggests that administration morphine of high dose and long time course may trigger the OFQ system in the central nervous system to exert a delayed negative feedback control, which may constitute one of the mechanisms of morphine tolerance, especially in the later stage of the development of tolerance.
Acknowledgements
The project was supported by the National Natural Science Foundation of China and a grant to JSH from NIDA, USA.
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Published on the World Wide Web on 18 March 1999.