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 80 mmHg with 10 mmHg intervals induced a clear stimulus-response relationship with a strong correlation (r2=0.93) between the electrical and mechanical responses. Twelve repeated phasic distensions at 80 mmHg 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 (1 mg/kg, i.v.) did not affect the mechanical response to distension at 80 mmHg, suggesting that colonic activity per se, does not contribute to the balloon pressure variations during CRD in the current experimental set-up. The μ-opioid receptor agonist fentanyl at a dose of 1.5 μg/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.