Anna Ravnefjord

Effects of pregabalin on visceral pain responses and colonic compliance in rats

Pregabalin, which binds to the α2‐δ subunit of voltage‐gated calcium channels, increased the threshold for pain during colorectal distension (CRD) in irritable bowel syndrome (IBS) patients. We tested the effects of oral pregabalin on the visceral pain‐related viscerosomatic and autonomic cardiovascular responses to CRD and colonic compliance in rats.

The GABAB receptor agonist, baclofen, and the positive allosteric modulator, CGP7930, inhibit visceral pain-related responses to colorectal distension in rats

Activation of GABA(B) receptors by the selective agonist baclofen produces anti-nociceptive effects in animal models of somatic pain. The aim of the present study was to evaluate the effect of baclofen and the GABA(B) receptor positive allosteric modulator CGP7930 on pseudo-affective responses to colorectal distension in rats. Female Sprague-Dawley rats were subjected to repeated, noxious colorectal distension (CRD) (12 distensions at 80 mmHg, for 30 s with 5 min intervals). The visceromotor response (VMR) and cardiovascular responses (mean arterial blood pressure (ABP) and heart rate (HR)) to CRD were monitored in conscious, telemetrized animals. Baclofen (0.3-3 micromol/kg, i.v.) reduced the VMR to CRD dose-dependently, reaching a 61% maximal inhibition (p < 0.001). The highest doses of baclofen attenuated CRD-evoked increases in ABP by 17% (p > 0.05) and reduced the change in HR by 48% (p < 0.01). CGP7930 (3-30 micromol/kg, i.v.) reduced the VMR to CRD in a dose-dependent fashion with a maximal inhibition of 31% (p < 0.05). The highest dose of CGP7930 also attenuated the increase in ABP by 18% (p > 0.05) and inhibited the increase in HR by 24% (p < 0.05) associated with CRD. Neither baclofen nor CGP7930 affected colorectal compliance. The results suggest that activation of GABA(B) receptors produces anti-nociceptive effects in a rat model of mechanically induced visceral pain. While CGP7930 was less efficacious than baclofen overall, positive allosteric modulation of GABA(B) receptors may represent a valid approach in the treatment of visceral pain conditions, with the possibility of an improved safety profile compared to full agonism.

Involvement of the transient receptor potential vanilloid 1 (TRPV1) in the development of acute visceral hyperalgesia during colorectal distension in rats.

Transient receptor potential vanilloid 1 (TRPV1) channels have been implicated in pain mechanisms and, particularly, in the development of hyperalgesia. We used selective TRPV1 antagonists (NGV-1, SB-750364 and JYL 1421) to assess the role of TRPV1 channels in repetitive noxious colorectal distension (CRD)-induced visceral pain responses in rats. Isobaric CRD (80 mmHg) induced a viscerosomatic response, indicative of visceral pain associated to the distension procedure. Repetition (12 consecutive distensions) of the CRD resulted in an increase in the response over time (119+/-23% increase at distension 12, P<0.05 vs response during the 1st distension) indicative of acute mechanical sensitization. NGV-1 (0.1, 0.3, 1 or 3 micromol/kg, i.v.) prevented in a dose-related manner the development of sensitization, without inducing hypoalgesic responses. SB-750364 (30 micromol/kg, i.v.) had a transitory effect, partially reducing the sensitization response, while JYL 1421 (4.7 micromol/kg, i.v.) was without effect. In the same conditions, the cannabinoid receptor 1 (CB(1)) agonist, WIN55,212-2 (0.1 micromol/kg) reduced pain responses leading to a hypoalgesic state. At 3 micromol/kg, NGV-1, did not affect the pressure-volume relationship during CRD, indicating that TRPV1 channels do not modulate colonic compliance. These observations suggest that TRPV1 channels are involved in the development of acute mechanical colonic hyperalgesia during repetitive noxious CRD in rats. Antagonism of TRPV1 channels might result in antihyperalgesic effects without hypoalgesic activity and might be beneficial in the treatment of visceral pain disorders, such as irritable bowel syndrome. These observations warrant the clinical assessment of TRPV1 antagonists for the treatment of visceral pain.

Evaluation of ticagrelor pharmacodynamic interactions with reversibly binding or non-reversibly binding P2Y12 antagonists in an ex-vivo canine model

INTRODUCTIONnAs ticagrelor, clopidogrel and cangrelor therapies may be used in the same clinical setting, their potential pharmacodynamic interactions are of interest. Hence, we investigated possible interactions between these agents in dogs using a variety of switching protocols.nnnMETHODSnSix male dogs all received 7 different dosing regimens separated by 1-5week washout periods: cangrelor (1μg/kg/min, intravenous infusion); ticagrelor (0.8mg/kg, oral); clopidogrel (3mg/kg, intravenous injection); cangrelor together with ticagrelor initiated 10minutes after cangrelor infusion start or clopidogrel given 30minutes after cangrelor infusion start; ticagrelor followed by clopidogrel given 3 or 7hours after ticagrelor dosing. ADP-induced whole blood platelet aggregation was measured by impedance aggregometry.nnnRESULTSnMean maximum inhibition of platelet aggregation (IPA) was 81-87% at 6minutes (cangrelor), 3hours (ticagrelor) and 4hours (clopidogrel) postdosing and platelet function recovered after 1.5hours, 12hours, and 9days, respectively. IPA at 2hours post clopidogrel was reduced to 39% when clopidogrel was given during cangrelor infusion versus 69% for clopidogrel alone. With clopidogrel dosed 3hours after ticagrelor, IPA was reduced after washout of ticagrelor to 38% at 24hrs vs. 68% for clopidogrel alone, but an interaction was not seen when clopidogrel was dosed 7hours after ticagrelor. No pharmacodynamic interaction occurred between ticagrelor and cangrelor.nnnCONCLUSIONSnThe extent of the pharmacodynamic drug-drug interactions observed between clopidogrel and cangrelor or ticagrelor apparently depends on the level of receptor occupancy when clopidogrel is administered. Importantly, no significant pharmacodynamic interaction occurred between ticagrelor/clopidogrel when clopidogrel was given at clinical trough IPA levels with ticagrelor. No significant pharmacodynamic interaction occurred with cangrelor and ticagrelor.

The selective 5-hydroxytryptamine 1A antagonist, AZD7371 [3(R)-(N,N-dicyclobutylamino)-8-fluoro-3,4-dihydro-2H-1-benzopyran-5-carboxamide (R,R)-tartrate monohydrate] (robalzotan tartrate monohydrate), inhibits visceral pain-related visceromotor, but not autonomic cardiovascular, responses to colorectal distension in rats.

5-Hydroxytryptamine 1A (5-HT1A) receptors have been suggested as a target for the treatment of irritable bowel syndrome (IBS). A recent clinical trial investigating the efficacy of the selective 5-HT1A antagonist AZD7371 [3(R)-(N,N-dicyclobutylamino)-8-fluoro-3,4-dihydro-2H-1-benzopyran-5-carboxamide (R,R)-tartrate monohydrate] showed no symptomatic improvement in IBS patients. We characterized the mechanisms mediating potential analgesic effects of AZD7371 in a model of colorectal distension (CRD)-induced visceral pain in rats to understand its mechanism of action and the lack of clinical efficacy. Visceromotor and cardiovascular responses (telemetry) were assessed in conscious rats during noxious CRD (80 mm Hg). Effects of AZD7371 (3–300 nmol/kg i.v.; 1–30 μmol/kg p.o.) and a reference 5-HT1A antagonist, WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridyl)cyclohexanecarboxamide maleate salt; 3–300 nmol/kg i.v.), were assessed. Effects of intracerebroventricular AZD7371 were also evaluated. Intravenous AZD7371 or WAY-100635 and oral AZD7371 dose-dependently inhibited visceromotor responses to CRD (ED50, 203, 231, and 14 μmol/kg, respectively). In telemetrized rats, oral AZD7371 inhibited visceromotor responses to CRD without affecting the concomitant hypertensive and tachycardic responses. Intracerebroventricular AZD7371 did not affect visceromotor responses, whereas it inhibited micturition. None of the doses tested induced visible gross side effects. AZD7371, likely acting at a spinal site, inhibited the visceromotor but not the cardiovascular responses to visceral pain in the CRD model in rats. Although agents effective on multiple pain-related readouts in the CRD model (e.g., pregabalin or clonidine) alleviate IBS symptoms, AZD7371, which is effective on only one pain-related pseudoaffective readout, does not. Data from preclinical CRD models of visceral pain need to be interpreted cautiously as it relates to their clinical translational value.

Oral baclofen reduces visceral pain-related pseudo-affective responses to colorectal distension in rats: Relation between plasma exposure and efficacy

Abstract Objective. We previously showed that activation of GABAB receptors by intravenous baclofen reduces pseudo-affective responses to colorectal distension in rats. Here we evaluate the potential clinical significance of these observations. Material and Methods. Clinically relevant colorectal distension protocols were used to assess the effects of oral baclofen on visceromotor and autonomic cardiovascular responses in conscious rats. Plasma levels of baclofen were monitored to provide clinical relevance to the doses used. Conscious female Sprague–Dawley rats were subjected to repeated noxious colorectal distension (12 × 80 mmHg), ascending-phasic colorectal distension (10–80 mmHg, 10 mmHg increments) or ramp colorectal distension (10 min ramp at 8 mmHg/min). Visceromotor and cardiovascular responses (mean arterial blood pressure and heart rate) were monitored. Pain-related response thresholds were assessed using ascending-phasic and ramp colorectal distension. Results. Baclofen (1–10 μmol/kg, p.o.) reduced the visceromotor response to colorectal distension, reaching a 40% maximal inhibition (p < 0.05). The highest dose (10 μmol/kg, p.o.) also inhibited pain-related cardiovascular responses in telemetrized rats (50–55% reduction in colorectal distension-evoked hypertensive and tachycardic responses; p < 0.05). Similar thresholds for pain-related visceromotor responses were determined during ramp or ascending-phasic colorectal distension (34.1 ± 1.9 and 31.7 ± 3.2 mmHg, respectively). Baclofen (10 μmol/kg, p.o.) increased thresholds to 71.1 ± 3.7 and 77.5 ± 1.8 mmHg during ramp and ascending-phasic colorectal distension, respectively (p < 0.001). Plasma levels of baclofen were 3.3 ± 0.2 μmol/l at 90 min post-dosing, corresponding to the end of the colorectal distension procedure. Conclusions. Oral baclofen, at plasma levels similar to those reported safe and within a therapeutic range in humans, produced significant visceral anti-nociceptive effects in rats.

Oral clonidine inhibits visceral pain-related viscerosomatic and cardiovascular responses to colorectal distension in rats.

The alpha(2)-adrenoceptor agonist, clonidine, modulates colorectal sensorimotor functions in humans and, given intrathecally, has analgesic effects in the colorectal distension (CRD) model in rats. We tested the effects of systemic clonidine on the visceral pain-related viscerosomatic and autonomic cardiovascular responses to CRD and colonic compliance in rats using clinically relevant CRD protocols. The activity of the abdominal musculature (viscerosomatic response), monitored by electromyography and intracolonic manometry, and changes in arterial blood pressure and heart rate, monitored by telemetry, were assessed simultaneously in conscious rats during CRD. Pressure-volume relationships during CRD served as a measure of colonic compliance. Clonidine (50-200 nmol/kg, p.o.) dose-dependently inhibited the viscerosomatic response to phasic, noxious CRD (12 distension at 80 mm Hg). At 200 nmol/kg clonidine also attenuated the increase in blood pressure (70+/-7% inhibition, P<0.05) and heart rate (67+/-16% inhibition, P<0.05) associated to noxious CRD. Similar effects were observed after i.v. administration. Likewise, clonidine (200 nmol/kg, p.o.) reduced the response to ascending phasic CRD (10-80 mm Hg) and significantly increased the threshold pressure for pain-related responses. Clonidine (50 or 150 nmol/kg, i.p.) did not affect the pressure-volume relationship during phasic CRD (2-20 mm Hg). These results show that systemic clonidine, at doses devoid of visible side effects, has analgesic effects in the CRD model of visceral pain in rats without affecting colonic compliance. These observations confirm the analgesic activity of systemic clonidine on visceral pain, support the translational value of the rat CRD model to humans and show that manometry is more sensitive than electromyography detecting pain-related responses.

Acute colonic ischaemia in rats results in long-term structural changes without alterations of colonic sensitivity

Colonic ischaemia and mast cells have been involved in the pathophysiology of the functional gastrointestinal disorder irritable bowel syndrome, although the cause–effect relationships remain unknown. We assessed long‐term histopathological and functional changes associated to an acute ischaemic episode (1u2003h) of the colon, followed by 8‐week recovery, in rats. Functional colonic alterations [sensitivity during colorectal distension (CRD), compliance and propulsive motility] were assessed regularly during the recovery. Colonic histopathology (presence of inflammation, morphometric alterations and variations in neuronal density in the enteric nervous system) 8‐week postischaemia was assessed. Following ischaemia, none of the functional parameters tested (motility, sensitivity and compliance) were affected. At necropsy, the colon presented an overall normal appearance with an increase in weight of the ischaemic area (mg/cm: 99u2003±u20036; Pu2003<u20030.05 vs. control: 81u2003±u20034 or sham ischaemia: 81u2003±u20033). Histopathological evaluations revealed the presence of a local infiltrate of mast cells in the area of ischaemia (nb of mast cells: 142u2003±u200350; Pu2003<u20030.05 vs. control, 31u2003±u200314 or sham ischaemia: 40u2003±u200316), without other significant alterations. Animals subjected to colonic ischaemia and treated 8u2003weeks later with the mast cell degranulator, compound 48/80, showed no changes in CRD‐related pain responses. These studies show that acute colonic ischaemia is associated with the presence of a long‐term local infiltration of mast cells, located within the serosa and muscle layers, despite the absence of functional changes, including colonic sensitivity. Considering the important pathophysiological functions of mast cells, the observed mast cell infiltration may be involved in ischaemia‐induced functional changes yet to be characterized.