A54 DELTA OPIOID RECEPTOR SIGNALING MEDIATES OPIOID INDUCED TOLERANCE AND HYPERALGESIA IN COLONIC SENSORY NERVES

Background

While opioid drugs are widely used for treating abdominal pain, prolonged exposure to opioids can induce tolerance and paradoxically increase pain. We previously showed in colonic afferent nerves that chronic morphine treatment causes tolerance and paradoxical hyperalgesia, however the mechanisms are unknown.

Aims

To evaluate the role of delta opioid receptor (DOR) signaling in opioid induced tolerance and hyperalgesia in colonic nerves during chronic morphine exposure.

Methods

C57BL/6 mice were injected (i.p.) twice daily for 7 days with escalating doses of morphine (10, 20, 30 mg/kg for the first 3 days, then 40mg/kg on 4th to 7th day); a subset of mice were also injected with 2.5 mg/kg naltrindole (NTI), a DOR antagonist. The analgesic response in vivo was monitored daily using the tail-flick test. To assess the effect of chronic morphine exposure, mice were euthanized on day 8, and the dorsal root ganglia (DRGs) and colons were harvested. Isolated DRG neurons were incubated with 1μM morphine (30 min) and neuronal excitability was determined by measuring the rheobase (amount of current required to elicit an action potential) using perforated patch clamp. To assess changes in colonic afferent nerve excitability, ex vivo afferent nerve recordings were obtained from flat-sheet colon preparations to determine the effects of 1μM morphine (10 min perfusion) on mechanosensitivity by probing with 1g von Frey hair.

Results

The tail flick test showed that the DOR antagonist NTI inhibited (30%) the maximal antinociceptive effect of morphine and reduced morphine tolerance. In patch clamp recordings, 1μM morphine paradoxically increased the excitability of small DRG neurons from morphine mice (rheobase decreased 28%; p<0.05, 2-way ANOVA), whereas this excitatory effect was absent on neurons from NTI+morphine mice. Acute application of 1μM morphine had no effect on afferent nerve responses to probing in morphine mice (14.9 vs 16.7 Hz; p=ns, paired t test, n=15) whereas it inhibited the afferent nerve response to probing in NTI+morphine mice (15.2 vs 9.9 Hz; p<0.001, paired t test, n=16). Furthermore, most single units from morphine mice had an increased response to probing following acute morphine application (7/15 vs 2/16 units excited), whereas most units form NTI-morphine mice were inhibited (14/16 vs. 3/15 units inhibited, P<0.001, Chi-square).

Conclusions

These findings suggest that DOR signaling plays a significant role in the development of opioid tolerance and hyperalgesia in colonic afferent nerves induced by chronic morphine treatment. Thus, antagonists of DOR may help to mitigate these side effects induced by opioids.

Funding Agencies

CCC

A213 A NOVEL PH-SENSITIVE OPIOID ANALGESIC THAT IS SELECTIVELY ACTIVATED IN ACIDIC INFLAMMATORY ENVIRONMENTS

Background

Opioids drugs are effective analgesics for inflammatory disorders such as inflammatory bowel disease (IBD) but their effects at non-inflammed sites can cause serious morbidity and even death. Exploiting the knowledge that tissue pH in inflamed tissues is acidic (e.g. 6.5–7.0), a novel opioid analgesic, ±)-N-(3-fluoro-1-phenethylpiperidine-4-yl)-N-phenylpropionamide (NFEPP), with a low acid dissociation constant, was developed that selectively activates peripheral µ-opioid receptor (MOPr) at acidic pH. Thus, pH-sensitive binding of NFEPP could selectively inhibit nociceptive nerves in the inflamed colon and have no effect on non-inflamed tissues outside the GI tract.

Aims

Evaluate whether NFEPP causes inhibition of colonic nociceptors at acidic pH’s, which mimic the inflamed colon.

Methods

To evaluate pH sensitive property of NFEPP to activate MOPr, dorsal root ganglia (DRG) neurons from C57BL/6 mice were exposed to the MOPr agonists NFEPP (300nM, 15 min) or DAMGO (100nM, 15 min) or vehicle at pH 6.5 or 6.8 or 7.4. Neuronal excitability was measured by recording the rheobase (minimum current to fire an action potential) using patch clamp recordings of isolated dorsal root ganglia neurons. In parallel ex vivo studies of mouse colon, extracellular recordings were obtained from afferent nerves innervating the distal colon. Afferent responses to probing with von Frey hair (1 gm) were examined before and after exposure to NFEPP (300nM, 5 min superfusion) at pH 6.5 and 7.4 respectively. Oneway ANOVA and post hoc Dunnett and Bonferroni tests were used to analyze the data.

Results

In patch clamp studies, NFEPP caused a decrease in DRG excitability at pH 6.5 and 6.8 (increased rheobase 21.3%, p<0.05 and 28.9%, p<0.05 respectively compared to vehicle) but had no effect at physiological pH 7.4. DAMGO, a MOPr agonist, caused inhibition of nociceptor excitability at pH 7.4 (increased rheobase 25.2%, p<0.05 compared to vehicle) as shown in previous experiments, but had no effect at pH 6.5 and 6.8. Vehicle had no effect at the different pH’s. In colonic afferent nerve recordings, NFEPP significantly attenuated afferent response (28.9% P<0.01) to probing at pH 6.5 and this effect was reversed after a 15 min washout. At pH 7.4 NFEPP had no effect on afferent nerve firing.

Conclusions

NFEPP activated MOPr at acidic pH causing inhibition of colonic nociceptors. This pH-selective agonist provides a new strategy to relieve pain at the site of inflammation while being devoid of any of unwanted activity in non-inflamed organs.

Funding Agencies

CCC

A127 THE ROLE OF ENDOGENOUS OPIOIDS IN MITIGATING OPIOID TOLERANCE IN A PRE-CLINICAL MODEL OF IBD

Background

While opioid drugs are used to treat pain in IBD patients, escalating doses can induce tolerance and paradoxical increased pain signaling. Endogenous opioids are released by immune cells during chronic colitis and have an analgesic action but it is unknown whether they can mitigate the development of tolerance to opioid drugs in IBD.

Aims

To evaluate whether endogenous opioids released during chronic inflammation prevent the development of tolerance and/or increased pain signaling following chronic morphine treatment.

Methods

Three groups of C57BL/6 mice were studied: 1) control, 2) chronic colitis induced with 3 cycles of 2% DSS (cDSS mice) and 3) chronic DSS mice injected with escalating doses of morphine on the last 7 days of DSS (10, 20, 30 mg/kg for the first 3 days, then 40mg/kg on 4 to 7 day) (cDSS-morphine mice). Ex vivo afferent nerve recordings were obtained from flat-sheet distal colon preparations (day 31) to determine their mechanosensitivity by probing with a 1g von Frey hair before and after acute morphine (1μM, 10 min perfusion). In parallel experiments, supernatants were obtained from the proximal colon and incubated overnight with isolated dorsal root ganglia (DRG) neurons from control mice. Neuronal excitability was then examined by measuring the rheobase using perforated patch clamp.

Results

In afferent nerve recordings, acute application of 1μM morphine inhibited the colonic afferent response to probing in control mice (17.86 vs 12.31 Hz; p<0.05), whereas no effect of morphine was seen in cDSS mice. In cDSS-morphine mice acute morphine perfusion paradoxically increased the mean afferent response to probing (13.99 vs 17.28 Hz, p<0.05). Single units were analyzed to identify how many mechanosensitive responses were increased, decreased or not affected by acute morphine perfusion. Compared to control mice, cDSS mice exhibited increased numbers of unaffected units (6/18 vs 1/14) whereas cDSS-morphine mice had decreased number of inhibited units (1/17 vs 11/14) and increased number of excited units (8/17 vs. 2/14) (Figure 1). In patch clamp recordings, colonic supernatants from cDSS mice reduced the excitability of DRG neurons (rheobase increased 28%; p<0.05), however, this effect was lost in neurons incubated with cDSS-morphine supernatants.

Conclusions

Endogenous opioids released during cDSS colitis do not mitigate tolerance to chronic morphine exposure and may diminish the response to acute morphine. Moreover, the paradoxical hyperexcitable response to acute morphine induced by chronic morphine was not blocked by endogenous opioids. Our patch clamp data suggest that endogenous opioid actions are lost following chronic morphine exposure and could result from inhibition of their release from immune cells.

Funding Agencies

CCC

A304 FOOD ANTIGEN-STRESS INTERACTION INCREASES PERIPHERAL PAIN SIGNALING IN A MOUSE MODEL OF IBS

Background

Food and stress are common triggers of symptoms in irritable bowel syndrome (IBS). However, it is unknown whether a specific food antigen alone can induce symptoms such as abdominal pain or if a second trigger, such as stress, is required for a food antigen to increase pain signaling.

Aims

To determine if a food antigen (ovalbumin) can induce hyperexcitabiity in nociceptive neurons alone and/or in combination with stress.

Methods

Mice were exposed to water avoidance stress (WAS; 1 hr for 6 days). On day 2, mice were gavaged ovalbumin (20mg) daily after completing WAS (WAS/OVA). These were compared to 3 groups: no WAS and no ovalbumin (control); WAS but no ovalbumin (WAS); and ovalbumin (OVA) alone. On day 8, WAS, OVA and WAS/OVA mice were given ovalbumin (50 μg) subcutaneously; 3 hours later euthanized. Colons were removed and incubated with ovalbumin (100 μg/ml) for 4 hrs. Supernatants were collected and DRG neurons from control mice were incubated overnight with supernatants from each of the four groups. Changes in neuronal excitability were examined by measuring the rheobase (minimal current to evoke an action potential) using perforated patch clamp recording techniques. Stress effects on intestinal permeability in the ileum and colon were assessed in Ussing chambers. One way ANOVA and Bonferroni post hoc test or unpaired t test were used to analyze the data.

Results

Incubation with supernatants obtained from WAS/OVA mice evoked hyperexcitability in DRG neurons compared to incubation with control supernatant (rheobase: control = 82 ± 10 pA vs WAS/OVA = 56 ± 4 pA, p < 0.05). Similarly, WAS/OVA supernatant decreased the rheobase compared to both OVA mice (OVA = 80 ± 8 pA, p<0.05) and WAS mice (WAS = 81 ± 9 pA, p<0.05). The effect of supernatants from OVA mice and WAS mice did not differ from controls. In a separate series of experiments, the PAR2 antagonist GB83 inhibited the effect of WAS/OVA supernatant (p < 0.01). There was no effect of GB83 on WAS or OVA supernatants. Stress decreased tissue resistance in ileum (p<0.05) but not in colon.

Conclusions

The food antigen ovalbumin induced hyperexcitability in DRG nociceptive neurons only when combined with stress. This action was PAR2 dependent suggesting a role of tissue proteases. Stress may cause a loss of oral tolerance to ovalbumin by increasing mucosal permeability in the small intestine. The interaction of food antigens and stress may be a mechanism of meal induced increase in abdominal pain in IBS.

Funding Agencies

CIHR