Role of tachykinins in castor oil diarrhoea in rats

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Abstract
We set out to ascertain the role of tachykinins, neurokinin A and substance P, in castor oil-induced diarrhoea in rats as disclosed by the inhibitory effect of the non-peptide NK1- and NK2-receptor antagonists, SR 140333 and SR 48968, respectively. SR 48968 (0.02 to 20 μg kg−1, s.c. or p.o.), and the opioid receptor agonist loperamide (1–10 mg kg−1, p.o.), dose-dependently prevented castor oil effects: % inhibition vs castor oil, diarrhoea 0 to 100, increase in faecal mass 7 to 90 and water content 16 to 90. SR 140333 (0.02 to 20 μg kg−1, s.c.) and the platelet activating factor antagonist SR 27417 (5 to 500 μg kg−1, p.o.) did not prevent the increase in faecal water content, but reduced faecal mass (35 to 66%) and diarrhoea (0 to 57%). The R-enantiomers of tachykinin NK1 and NK2 receptor antagonists, SR 140603 and SR 48605 (both at 2 or 20 μg kg−1, s.c.) had no effect other than reducing faecal mass at the highest dose tested. SR 48968 (20 μg kg−1, p.o.) but not loperamide (10 mg kg−1, p.o.) given 24 h before castor oil, still slightly but significantly reduced by 30% the increase of faecal mass output; both treatments significantly reduced (30 to 70%) the effect of castor oil on faecal water content, although the incidence of diarrhoea was only slightly less than in controls. In castor oil-treated rats, naloxone (2 mg kg−1, s.c.) completely blocked the antidiarrhoeal action of loperamide (10 mg kg−1, p.o.) but not of SR 48968 (20 μg kg−1, p.o.); a similar result was obtained on faecal mass and water content. Castor oil strongly increased the occurrence of manometrically recorded propulsive giant contractions (500 to 1000% over control values) of transverse and distal colon, this effect being significantly prevented (80 to 100%) by SR 48968 and loperamide and partially by SR 140333 (35% distal colon, 70% transverse colon). In castor oil free rats, loperamide but not SR 48968 or SR 140333 significantly reduced by 50% the gastrointestinal transit of a charcoal test meal, as well as 24 h faecal mass output. Consistently, loperamide, unlike the tachykinin receptor antagonists, had a dramatic effect on manometric recordings of intestinal motility, reducing all kinds of colonic contractions. Our findings suggest that castor oil diarrhoea in rats entails activation of NK1 and NK2 receptors by endogenous tachykinins, whose antagonists may have a potential as antidiarrhoeal agents free from the constipating action of opioids.