Modulation of gastrointestinal function by MuDelta, a mixed µ opioid receptor agonist/ µ opioid receptor antagonist

BACKGROUND & PURPOSE

Loperamide is a selective µ opioid receptor agonist acting locally in the gastrointestinal (GI) tract as an effective anti-diarrhoeal but can cause constipation. We tested whether modulating µ opioid receptor agonism with δ opioid receptor antagonism, by combining reference compounds or using a novel compound ('MuDelta'), could normalize GI motility without constipation.

EXPERIMENTAL APPROACH

MuDelta was characterized in vitro as a potent µ opioid receptor agonist and high-affinity δ opioid receptor antagonist. Reference compounds, MuDelta and loperamide were assessed in the following ex vivo and in vivo experiments: guinea pig intestinal smooth muscle contractility, mouse intestinal epithelial ion transport and upper GI tract transit, entire GI transit or faecal output in novel environment stressed mice, or four weeks after intracolonic mustard oil (post-inflammatory). Colonic δ opioid receptor immunoreactivity was quantified.

KEY RESULTS

δ Opioid receptor antagonism opposed µ opioid receptor agonist inhibition of intestinal contractility and motility. MuDelta reduced intestinal contractility and inhibited neurogenically-mediated secretion. Very low plasma levels of MuDelta were detected after oral administration. Stress up-regulated δ opioid receptor expression in colonic epithelial cells. In stressed mice, MuDelta normalized GI transit and faecal output to control levels over a wide dose range, whereas loperamide had a narrow dose range. MuDelta and loperamide reduced upper GI transit in the post-inflammatory model.

CONCLUSIONS AND IMPLICATIONS

MuDelta normalizes, but does not prevent, perturbed GI transit over a wide dose-range in mice. These data support the subsequent assessment of MuDelta in a clinical phase II trial in patients with diarrhoea-predominant irritable bowel syndrome.

Chronic treatment with morphine and ethanol, but not cocaine, attenuates IL-1 beta activation of FOS expression in the rat hypothalamic paraventricular nucleus

Interleukin-1 beta (IL-1 beta) is a key mediator of immunological and pathological responses to stress, injury and disease and it has been suggested to have profound effects on neuroendocrine-immune functions. We have shown that central treatment with IL-I beta induces the expression of FOS proto-oncogene protein immunoreactivity (FOS-IR) in several hypothalamic nuclei including the paraventricular nucleus (PVN). Since FOS expression has been used as an anatomical marker of neuronal function, these results suggested that the involvement of IL-1 beta in the neuro-endocrine-immune axis may be mediated through the PVN. Treatment with several substances of abuse has been shown to modify immune function in vivo and in vitro. In this study, we compared the effects of morphine, ethanol and cocaine on IL-1 beta induction of FOS-IR in the rat hypothalamus. Acute treatment with morphine or ethanol induced FOS-IR in several nuclei including the PVN. Cocaine, which induced FOS-IR in the Caudate-Putamen (CPU), nucleus Accumbens (nAcc) and Locus Coeruleus (LC), however, did not induce FOS-IR in the PVN. Chronic treatment with morphine desensitized FOS responsiveness to morphine and IL-1 beta in the PVN since FOS-IR was no longer induced by IL-1 beta or morphine in the PVN after this treatment. Similarly, chronic ethanol treatment desensitized FOS responsiveness to ethanol and to IL-1 beta in the PVN. By contrast, chronic cocaine did not affect FOS responsiveness to IL-1 beta in the PVN even though the treatment was able to desensitize the FOS responsiveness to acute cocaine in the CPU, nAcc, and LC. These results suggest that the PVN may be a site where actions of IL-1 beta converge with those of morphine and ethanol, but not cocaine, to modulate neuro-endocrine-immune functions.

Picrotoxin-induced seizures modified by morphine and opiate antagonists

The effects of naloxone, Tyr-MIF-1, and MIF-1 on morphine-mediated changes in susceptibility to picrotoxin-induced seizures were studied. Rats were pretreated with naloxone, MIF-1, Tyr-MIF-1, or saline. At 15-min intervals, they received a second pretreatment of morphine or saline and then were tested for seizures following a convulsant dose of picrotoxin. Several parameters of specific categories of seizures were scored. Morphine increased the number of focal seizure episodes, duration of postseizure akinesis, and incidence of generalized clonic seizures. Naloxone tended to block the morphine-mediated changes in susceptibility. Tyr-MIF-1 had effects similar to naloxone on duration of postseizure immobility but tended to potentiate the effects of morphine on focal seizure episodes. The effects of morphine and the opiate antagonists on focal seizure episodes and postseizure duration suggest the general involvement of several types of opiate receptors in these picrotoxin-induced behaviors. However, the observation of antagonistic effects for Tyr-MIF-1 on immobility but agonistic effects for focal seizures suggests that the type of effect exerted by opiate agents may depend upon other neuronal variables.

Prolonged analgesia after intracerebroventricular Tyr-W-MIF-1 (Tyr-Pro-Trp-Gly-NH2)

A peptide recently isolated from human and bovine brain, Tyr-W-MIF-1 (Tyr-Pro-Trp-Gly-NH2), was tested for its effects on nociception in the tail-flick test after intracerebroventricular injection in the rat. Tail-flick latencies were significantly increased with a rapid onset and remained significantly elevated for at least 50 min. Naloxone reversed the effect of the peptide, indicating opiate receptor involvement in the response. Met-enkephalin at the same dose produced only slight antinociception. Some animals showed 'barrel-rolling' behavior in addition to the analgesia; this behavior was unusually short-lived, not a prerequisite for the analgesia, and had no apparent persistent effects. The results show that, in addition to previously described opiate-like actions (binding to the mu-receptor and inhibition of electrically induced contractions of the guinea pig ileum), Tyr-W-MIF-1 is capable of inducing significant analgesia.