Oxymorphone-induced analgesia and colonic motility measured in colorectal distension

Toward Directing Opioid Receptor Signaling to Refine Opioid Therapeutics

The mu opioid receptor (MOR) is a diversely regulated target for the alleviation of pain in the clinical setting. However, untoward side effects such as tolerance, dependence, respiratory suppression, constipation, and abuse liability detract from the general activation of these receptors. Studies in genetically modified rodent models suggest that activating G protein signaling pathways while avoiding phosphorylation of the receptor or recruitment of β-arrestin scaffolding proteins could preserve the analgesic properties of MOR agonists while avoiding certain side effects. With the development of novel MOR "biased" agonists, which lead to preferential activation of G protein pathways over receptor phosphorylation, internalization, or interaction with other effectors, this hypothesis can be tested in a native, physiological setting. Overall, it is clear that the MOR is not a simple on-off switch and that the diverse means by which the receptor can be regulated may present an opportunity to refine therapeutics for the treatment of pain.

Assessment of visceral pain-related pseudo-affective responses to colorectal distension in mice by intracolonic manometric recordings

Recently, a new manometric method has been proposed to quantify visceromotor responses (VMR) to colorectal distension (CRD) in rats. This method is based on monitoring pressure changes within the distending balloon during CRD. This study assesses the applicability of such a technique to the quantification of VMRs to CRD in mice. Electrical activity of the abdominal muscles and pressure changes within the distending balloon (mechanical response) were simultaneously recorded in conscious mice during CRD (phasic ascending, 10-80 mm Hg, or repetitive, 55 mm Hg). There was a clear stimulus-response relationship with a strong correlation between electrical and mechanical responses during the ascending (r(2) = 0.899, n = 7) or repetitive phasic CRD (r(2) = 0.926, n = 8). Repetitive phasic distensions (55 mm Hg) increased the mechanical and electrical responses by 71 +/- 20% and 42 +/- 16%, respectively (pulses 10-12 vs. 1-3; n = 8, both P < .01). Atropine (0.5 or 1 mg/kg, subcutaneously) did not affect the mechanical response to CRD. The mu-opioid agonist, fentanyl (0.05 mg/kg, subcutaneously), completely prevented the sensitizing response associated to repetitive distensions. These results show that noninvasive, surgery-free manometry of intracolonic pressure is a reliable method to assess VMRs to CRD in mice. The analgesic effect of compounds could be determined, indicating that the method can be used in pharmacologic studies.

PERSPECTIVE

The model presented to assess visceral pain in mice allows a broad use of this species in pharmacological studies and will be of use in the characterization of potential targets and new drugs for the treatment of human pathologies with visceral pain arising from the gut as a significant component.

Evaluation of pseudo-affective responses to noxious colorectal distension in rats by manometric recordings

Recordings of electromyographic (EMG) activity in the abdominal musculature are generally used to quantify the pseudo-affective visceromotor response induced by colorectal distension (CRD) in rodents. The present study describes a non-invasive, manometric method to quantify the magnitude of the abdominal contractions evoked by CRD. CRD-induced increases in EMG activity in female rats (electrical response) were compared to phasic changes in balloon pressure (mechanical response). A phasic increasing CRD paradigm from 10 to 80mmHg with 10mmHg intervals induced a clear stimulus-response relationship with a strong correlation (r(2)=0.93) between the electrical and mechanical responses. Twelve repeated phasic distensions at 80mmHg increased the mechanical response by 133+/-53% (P<0.01), while the electrical response only increased by 20+/-19% (P>0.05), when comparing the last distension to the first. Atropine methyl bromide (1mg/kg, i.v.) did not affect the mechanical response to distension at 80mmHg, suggesting that colonic activity per se, does not contribute to the balloon pressure variations during CRD in the current experimental set-up. The mu-opioid receptor agonist fentanyl at a dose of 1.5microg/kg (i.v.) significantly reduced the mechanical response to CRD (P<0.01) while the electrical response was not affected. The present study shows that phasic bursts in EMG activity from the abdominal musculature occur simultaneously with balloon pressure variations, which may represent a non-invasive alternative to EMG recordings. Furthermore, the mechanical response is a more sensitive parameter for detecting both hyperalgesic and analgesic responses.

Epidural analgesia with a combination of bupivacaine and buprenorphine in rats

We determined the analgesic effects of epidural administration of either bupivacaine at 62.5, 125, 250, and 500 micrograms/kg; buprenorphine at 5 and 10 micrograms/kg; and the combination of bupivacaine at 125 micrograms/kg and buprenorphine at 5 or 10 micrograms/kg, using the tail flick (TF) and colorectal distension (CD) tests in rats and compared the results with those obtained using morphine at 100 micrograms/kg. In both the TF and CD tests, all doses of bupivacaine alone produced potent anti-nociceptive effects, although the effect rapidly diminished after 20-30 min of administration. The administration of buprenorphine at 10 and 5 micrograms/kg produced mild to moderate anti-nociceptive effects in both the TF and CD tests, and the effects were relatively constant throughout the 2-h experimental period. Combinations of 5 or 10 micrograms/kg of buprenorphine with 125 mg/kg of bupivacaine produced a significantly higher percentage of maximum possible analgesic effect (%MPE) than that of the calculated additive effect of each drug alone in the TF and CD tests. The analgesic effect of this combination was similar to that of morphine. Minimal ataxia was observed in rats administered this combination.

Models of Visceral Nociception

Pains arising from the viscera constitute a large portion of clinically treated pains. They are characterized by poor localization; immobility with tonic increases in muscle tone; and vigorous but nonspecific changes in autonomic function, such as changes in respiration, heart rate, and blood pressure. Tissue-damaging stimuli do not reliably produce visceral pain, so the study of visceral nociception in nonhuman animals requires identification of appropriate stimuli and responses. This article defines "noxious" visceral stimuli as those that produce pain in humans, result in aversive behaviors in animals, and evoke responses that are inhibited by manipulations known to be analgesic in humans. To be valid, the measured responses must be reliable, inhibited by known analgesics, and not inhibited by nonanalgesics. Using these criteria as measures of validity, the author examined several visceral pain models. The writhing test (application of intraperitoneal irritants) failed to meet these criteria; however, responses to small bowel distension, colonic-rectal distension, artificial ureteral calculosis, urinary tract distension, and the intravesical application of irritants met most, if not all, of the criteria. Other models, such as responses to biliary system distension, to reproductive organ stimulation, to the focal application of algesic agents onto various viscera, and to ischemic stimuli, met some of these criteria. This information should assist readers in decisions related to the use of visceral pain models.

Antinociceptive effects of oxymorphone-butorphanol-acepromazine combination in cats

OBJECTIVE

To determine the antinociceptive effects of oxymorphone, butorphanol, and acepromazine individually and in combination to a noxious visceral stimulus in cats.

STUDY DESIGN

Randomized, blinded controlled study.

ANIMALS

Eight healthy mixed-breed cats (four male, four female) weighing 4.4 +/- 1.2 kg and aged 1 to 2 years old.

METHODS

A silastic balloon catheter was inserted per rectum and inflated at various pressures. Physiological parameters (respiratory rate, pulse rate, and blood pressure) were also recorded. Subjects were administered individual and combined intravenous (i.v.) doses of 0.025, 0.05, 0.10, and 0.20 mg/kg oxymorphone and 0.025, 0.05, 0.10, and 0.20 mg/kg butorphanol. A further study of various ratios of butorphanol and oxymorphone (3:1, 2:1, 1:1, 1:2, and 1:3), at a combined equivalent dose of 0.1 mg/kg, was performed in four cats per dose combination. In a separate study, four cats were administered combined i.v. doses of 0.05 mg/kg each of oxymorphone and butorphanol or 0.05 mg/kg each of oxymorphone, butorphanol, and acepromazine.

RESULTS

Combined doses of 0.05 and 0.10 mg/kg of oxymorphone and butorphanol showed mainly additive with some synergistic antinociceptive interactions and the combined dose of 0.2 mg/kg of each agent demonstrated additional antinociceptive effects, P < .05. Additional studies showed that various ratios of the two agents at a total combined dose of 0.10 mg/kg i.v. did not produce levels of antinociception that were significantly different from each other, P > .05. Acepromazine (ACE) significantly increased the magnitude of antinociception at 15 minutes when administered in combination with oxymorphone and butorphanol, P < .05. Also, physiological variables were unaffected by these drug combinations.

CONCLUSIONS

Low doses of oxymorphone and butorphanol in combination can produce greater levels of antinociception than when used individually. ACE, in conjunction with oxymorphone and butorphanol, produced even greater levels of antinociception than the two-opioid drug combination.

CLINICAL RELEVANCE

Oxymorphone, butorphanol, and ACE can be used in combination to produce additive or synergistic effects without adverse effects in cats. These data suggest that ACE and butorphanol at low doses given as preanesthetic medication followed by a mu opioid (eg, oxymorphone) after surgery at low doses may provide an effective method of pain management in the cat.

Endogenous opiates: 1995

This article is the eighteenth installment of our annual review of research concerning the opiate system. It includes articles published during 1995 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects. The specific topics covered this year include stress: tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.