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Effects of kappa opioids in the inflamed rat colon
1The University of Iowa, College of Medicine, Department of Pharmacology, Iowa City 52242, USA.
Abstract:
The objective of this study was to examine the antinociceptive effects of peripherally restricted kappa-opioid receptor agonists (ORAs) in a rat model of inflammatory bowel disease produced by intracolonic instillation of trinitrobenzine sulfonic acid (TNBS). Antinociceptive effects of mu-(morphine) and kappa-ORAs (EMD 61,753 and ICI 204,488) were evaluated in a behavioral model of visceral nociception. The effects of these agonists and a delta-ORA (SNC 80) on responses of pelvic nerve afferent fibers innervating the colon were also tested. In the behavioral study, systemic injections of morphine and both kappa-ORAs dose-dependently inhibited the visceromotor response to colorectal distension in rats with uninflamed or inflamed colons. The inhibitory effects of kappa-ORAs, but not morphine, were significantly greater in rats with colons inflamed 4 days previously by TNBS. A mu-receptor-selective dose (30 microg/kg) of naloxone methiodide (NLXM) blocked the inhibitory effect of morphine, but not of EMD 61,753. In the single-fiber study, neither morphine nor the delta-ORA SNC 80 attenuated the responses of pelvic nerve afferent fibers, whereas kappa-ORAs dose-dependently inhibited responses of pelvic nerve afferent fibers with significantly greater potency in the inflamed colon. Pretreatment with a non-opioid receptor-selective dose (2 mg/kg) of NLXM produced a rightward shift in the dose-response function of EMD 61,753. The greater potency of kappa-ORAs in the TNBS-inflamed condition suggests a peripheral upregulation of kappa-opioid receptors in colonic inflammation.
Insights
Peripherally restricted kappa-opioid receptor agonists show enhanced antinociceptive effects in inflammatory bowel disease models. This suggests a potential peripheral upregulation of kappa-opioid receptors during colonic inflammation.
Area of Science:
- Pharmacology
- Gastroenterology
- Neuroscience
Background:
- Inflammatory bowel disease (IBD) is associated with visceral pain.
- Opioid receptors are key targets for pain management.
- Peripherally acting analgesics are desirable to minimize central side effects.
Purpose of the Study:
- To investigate the antinociceptive effects of peripherally restricted kappa-opioid receptor agonists (ORAs) in a rat model of inflammatory bowel disease.
- To compare the efficacy of kappa-ORAs with mu-ORAs (morphine) and delta-ORAs in visceral nociception.
- To explore the role of peripheral opioid receptor expression in inflammation-induced hyperalgesia.
Main Methods:
- A rat model of inflammatory bowel disease was induced using trinitrobenzine sulfonic acid (TNBS).
- Visceromotor responses to colorectal distension were measured behaviorally.
- Single-fiber electrophysiology was used to record pelvic nerve afferent fiber activity.
- Pharmacological blockade with naloxone methiodide (NLXM) was employed to assess receptor specificity.
Main Results:
- Both kappa-ORAs and morphine demonstrated dose-dependent inhibition of visceromotor responses.
- Kappa-ORAs exhibited significantly greater antinociceptive effects in TNBS-inflamed colons compared to uninflamed colons.
- Morphine's effects were blocked by a mu-selective dose of NLXM, while kappa-ORA effects were not.
- Kappa-ORAs, but not morphine or a delta-ORA (SNC 80), inhibited pelvic nerve afferent fiber responses, with greater potency in inflamed conditions.
Conclusions:
- Peripherally restricted kappa-ORAs possess potent antinociceptive properties in inflammatory bowel disease.
- The enhanced efficacy of kappa-ORAs in inflamed colons suggests peripheral kappa-opioid receptor upregulation.
- Kappa-opioid receptor agonists represent a promising therapeutic strategy for managing visceral pain in inflammatory bowel disease.