- Top of page
- Conflict of interest
Altered colonic sensitivity with increased perception of balloon distension of the colorectal area is one of the manifestations of the functional bowel disorder, irritable bowel syndrome (IBS) (Ritchie, 1973; Stacher and Christensen, 2006; Azpiroz et al., 2007). These observations have led to the consideration that visceral sensory hypersensitivity should be regarded as a significant component of the pathophysiological basis of IBS. Therefore, the modulation of sensory neurotransmission from the intestine could be an effective approach for the therapeutic treatment of IBS. Accordingly, numerous potential targets expressed in sensory afferents from the intestine have been tested in several preclinical models and clinical conditions for their effectiveness to modulate visceral pain (Kirkup et al., 2001; Hicks, 2006).
Despite its relation to GABA, there is little evidence to indicate that pregabalin exerts its pharmacological activity through direct interaction with GABA receptors. Although the mechanism of action is not completely clear, it has been shown that pregabalin binds with high affinity to the α2-δ subunit of voltage-gated calcium channels (Bryans and Wustrow, 1999; Belliotti et al., 2005; Bian et al., 2006). Thus, the compound reduces depolarization-induced calcium influx at nerve terminals, and thereby reduces the release of several excitatory neurotransmitters, such as noradrenaline, glutamate, substance P and calcitonin gene-related peptide, which have been involved in pain mechanisms (Ben-Menachem, 2004; Huckle, 2004; Belliotti et al., 2005; Joshi and Taylor, 2006; Dooley et al., 2007). Consistent with this hypothesis, recent studies in mice with a mutation in the α2-δ1 calcium channel subunit preventing the binding of pregabalin, suggest that the analgesic effects of the compound in mouse pain models are mediated by binding to this subunit (Bian et al., 2006; Field et al., 2006). Pregabalin has been evaluated in multiple clinical trials involving diabetic peripheral neuropathy, post-herpetic neuralgia and epilepsy, and has also been shown to be effective in several animal models of inflammatory and neuropathic pain (Hunter et al., 1997; Dworkin and Kirkpatrick, 2005; Blommel and Blommel, 2007; Field et al., 2007). Furthermore, in animal models of visceral pain, pregabalin has been shown to dose dependently reduce both normal colonic pain responses and colonic hyperalgesia (Eutamene et al., 2000; Diop et al., 2002; Million et al., 2007). A recent report also showed that pregabalin restored sensory thresholds to normal levels in IBS patients with rectal hypersensitivity during rectal balloon distension (Houghton et al., 2007). In the same study, a concomitant increase in rectal compliance was also observed, although its relation to the reduction in sensitivity was unclear.
Several means of assessing anti-nociceptive-like effects of potential visceral analgesics have been developed using in vivo preclinical models. Monitoring the electrical activity of the abdominal muscle in response to colorectal distension (CRD) (the so-called visceromotor response) has been the most common method of choice (Ness and Gebhart, 1988). Indeed, most of the aforementioned effects of pregabalin in animal models were characterized using this parameter. However, we have recently described an alternative readout, termed the mechanical visceral motor response to CRD, which appears to be more sensitive at detecting analgesic-like effects of compounds than electrical recordings and is independent of changes in colonic motility (Tammpere et al., 2005; Arvidsson et al., 2006). In addition, cardiovascular responses have also been demonstrated as valid pseudo-affective responses to noxious visceral stimuli in animals (Ness and Gebhart, 1988). Thus, monitoring these three pseudo-affective reflexes in parallel is expected to provide more confidence in the possible preclinical anti-nociceptive effects of compounds, reinforcing the translational value of results in animals to human conditions.
Although pregabalin has previously been shown to inhibit the visceral motor response using electromyographical (EMG) recordings (Eutamene et al., 2000; Million et al., 2007), plasma levels of pregabalin required for efficacy have not been reported. Thus, if the plasma exposure of pregabalin required for efficacy in preclinical models of visceral pain is relevant to the plasma exposure reached with clinical doses is unknown.
The aim of the present study was therefore to extend previous findings by further assessing the effects of pregabalin on three independent visceral pain-like responses, namely electrical and mechanical visceromotor responses and cardiovascular reflexes, elicited by mechanical stimulation of the colon in conscious rats using distension protocols equivalent to those used in clinical studies. In addition, taking into account the potential effects in colonic compliance observed clinically (Houghton et al., 2007), we assessed, for the first time, the effects of pregabalin in colonic compliance during CRD in animals. Finally, we measured plasma levels of pregabalin to examine how different exposures of pregabalin affected the outcomes with the intention of providing translational relevance to these findings in man.
- Top of page
- Conflict of interest
The present results show that pregabalin reduced the pain-related visceromotor and autonomic responses associated with mechanical stimulation of the colon in rats, confirming its anti-nociceptive properties. In addition, pregabalin also increased the pressure–volume relationship during distension, suggesting that at least part of the analgesic effects of the compound might be associated with the modulation of colonic compliance. The effects of pregabalin on colonic pain and compliance were achieved at doses giving rise to clinically relevant plasma exposures.
Pregabalin significantly increases rectal sensory thresholds to distension in hypersensitive IBS patients and normalizes, rather than desensitizes (that is, make hyposensitive), the perception of rectal distension (Houghton et al., 2007). These observations in humans support findings in animals showing that pregabalin reduced the normal pain response to CRD and also modulated hypersensitivity states (Eutamene et al., 2000; Diop et al., 2002; Million et al., 2007). In the present study, we tested the effects of pregabalin on pain-related visceromotor and autonomic responses to repetitive noxious CRD-induced acute sensitization in normal animals. As previously shown, the CRD at 80 mm Hg induced a viscerosomatic response, indicative of pain, in conscious rats, resulting in augmented activity of the abdominal musculature, as determined measuring its electrical or mechanical activity (Tammpere et al., 2005). Moreover, the magnitude of the response to repetitive distensions tended to increase over time, an effect particularly evident when assessing the mechanical responses to distension (Tammpere et al., 2005), and indicative of acute mechanical sensitization. Oral pregabalin reduced the normal viscerosomatic response to CRD and prevented in a dose-related manner repetitive CRD-induced acute hypersensitivity. The analgesic effects of pregabalin were clearer when assessing the mechanical than the electrical response to CRD of the abdominal musculature. This further confirms our previous results indicating that monitoring the mechanical activity of the abdominal musculature might be more sensitive than the standard electromyographic procedures as a readout for visceral pain-related responses (Tammpere et al., 2005; Arvidsson et al., 2006). Although not determined simultaneously in the same animals, the overall dose-related effects of pregabalin on CRD-induced visceromotor responses show a good correspondence with the mean plasma levels reached after oral dosing. Moreover, the analgesic effects of pregabalin lasted throughout the experimental time (between 21 and 55 min depending on the CRD protocol used, with pregabalin dosed 60 min before), reflecting also the relatively stable plasma levels detected between 60 and 120 min after oral dosing. Effective plasma levels were consistent with those showing efficacy in rodent models of epilepsy and ataxia (Vartanian et al., 2006) or those reported in humans within a therapeutic range (Randinitis et al., 2003; Brodie et al., 2005; Zareba, 2005). In addition, the plasma levels achieved in the current study are likely to be in a similar range to those that can be expected from Houghton et al. (2007) using doses between 50 and 200 mg (Zareba, 2005). Thus, the doses used here and the effects observed might be of therapeutic relevance for IBS patients.
In the present experimental conditions, noxious CRD (80 mm Hg) resulted, in addition to the visceromotor responses, in an autonomic cardiovascular response characterized by an increase in heart rate and arterial blood pressure. Moreover, repetitive CRD resulted also in an increase in these cardiovascular responses, indicative of acute sensitization. These cardiovascular responses are similar to those described previously in rats (Ness and Gebhart, 1988, 2001; Lindström et al., 2008). Pregabalin, at the highest dose tested, also reduced the cardiovascular autonomic responses associated with noxious CRD. Interestingly, pregabalin only prevented the acute sensitization during repetitive CRD, but did not affect the rise in blood pressure or heart rate associated with a normal pain response, as cardiovascular responses at the initiation of the CRD protocol, before the presence of mechanical sensitization, were similar in vehicle- and pregabalin-treated animals. Moreover, at the same dose, pregabalin significantly inhibited the visceromotor responses to phasic ascending CRD and increased the threshold pressure for response during the same CRD protocol or during continuous ramp (0–80 mm Hg) CRD (data not shown).
Altered colonic tone and accommodation to distension have been suggested as contributing mechanisms to the altered colonic sensitivity observed in functional gastrointestinal disorders (Delgado-Aros and Camilleri, 2005). Therefore, an improvement in the pressure–volume relationship during distension, reflected as shift to the left in the pressure–volume relationship curves (that is, greater compliance and improved accommodation), might result in reduced pain sensitivity. Nevertheless, there is no clear correlation between compliance and pain perception. For instance, intrarectal lidocaine reduced compliance in rats but had analgesic effects during CRD (Käll et al., 2007), and the metabotropic glutamate 5 receptor (mGluR5) antagonist MPEP had analgesic effect during CRD without affecting colonic compliance (Lindström et al., 2008). On the other hand, clonidine increased gastric and colonic compliance and also reduced pain perception (Thumshirn et al., 1999; Malcolm et al., 2000), and Wistar Kyoto rats with reduced colonic accommodation responses are also hypersensitive during CRD (Martínez et al., 2007). A recent report showed that pregabalin, in addition to increasing pain thresholds during CRD, also increased colonic compliance in IBS patients with colonic hypersensitivity (Houghton et al., 2007). Similarly, in the present study, pregabalin, at a dose inducing clear analgesia, also modulated colonic compliance, with an increase in the volume response during CRD observed in five out of six animals tested. Altogether, these observations indicate that pregabalin increases colonic compliance in humans and animals. The potential relationship between the analgesic properties of pregabalin and its effects on compliance needs to be further characterized.
In addition to its effects on pain, pregabalin has anxiolytic properties in both animals and humans (Huckle, 2004; Pohl et al., 2005; Rickels et al., 2005; Bandelow et al., 2007). Anxiety might be a contributing factor to the viscerosomatic and autonomic responses elicited by CRD. Therefore, anxiolytic effects of compounds might result in the reduction of pain-related readouts and be misinterpreted as analgesic activity. In this sense, other compounds (such as buspirone or mGluR5 antagonists) with potential anxiolytic activity also had analgesic effects in the CRD model in rats (Sivarao et al., 2004; Lindström et al., 2008). In humans, modulation of anxiety or sedation-like effects was discarded as a contributing factor to the analgesic effects of pregabalin (Houghton et al., 2007). In the present study, anxiety-like behaviours were not directly assessed, although the generation of anxiety and stress during the experimental procedures was minimized as far as possible. In addition, no gross side effects consistent with sedation-like effects, which might interfere with the manifestation of visceromotor responses to pain, were observed at any of the doses tested. This is consistent with data showing that pregabalin caused ataxia and decreased spontaneous locomotor activity at dosages 10–30-fold higher than those active to prevent seizures (ED50 ∼130 μmol kg−1, p.o.) in rodent models of epilepsy (Vartanian et al., 2006). This also agrees with observations with gabapentin in somatic pain models, in which sedation was only observed at a dose of 300 mg kg−1 (approximately 10-fold higher than the maximal dose used in the present study) (Jones and Sorkin, 1998). Thus, although potential central anxiolytic effects of pregabalin cannot be completely ruled out, sedation-like effects can be excluded as a confounding analgesic factor.
In summary, we show that the α2-δ ligand pregabalin has analgesic properties in the CRD model in rats, with consistent effects in several independent visceral pain-related parameters. Pregabalin was effective in attenuating both the viscerosomatic (contractions of the abdominal musculature) and the cardiovascular autonomic responses (hypertension and tachycardia) associated with the noxious mechanical distension of the colon in rats. In addition, pregabalin also modulated colonic tone, resulting in an increase in compliance. These observations support the preliminary clinical data obtained in humans supporting a potential therapeutic use of pregabalin, or other α2-δ ligands, for the treatment of visceral hypersensitivity (Lee et al., 2005; Houghton et al., 2007). Moreover, the CRD protocols used here are closer to those usually applied in clinical conditions than those previously used in similar animal studies (isovolumetric distensions or tonic distensions) (Eutamene et al., 2000; Million et al., 2007), which contributes to the translational value of the present studies. The exact mechanism for the visceral analgesic effects of pregabalin and the potential role of colonic tone warrant further studies. Finally, the present observations, together with the data recently published in humans (Houghton et al., 2007), support the translational value of the CRD model of visceral pain, combined with the simultaneous assessment of multiple surrogate markers of visceral pain, as a predictor for clinical efficacy in humans.