Naunyn-Schmiedeberg's Arch Pharmacol (1982) 319:212-215 Naunyn-Schmiede berg's Archivesof Pharmacology 9 Springer-Verlag 1982 Evidence That the Contractile Response of the Guinea-Pig Ileum to Capsaicin is Due to Substance P Release Loris A. Chahl Faculty of Medicine, Univesity of Newcastle, N.S.W. 2308, Australia Summary. The mechanism of action ofcapsaicin on the guinea- pig isolated ileum was studied. Capsaicin (1.5 x 10 6 tool 1-1) produced a large contractile response of the ileum which ex- hibited marked tachyphylaxis. The response was reduced by treatment of preparations with atropine, 5 • 10-6 mol 1-1, and by substance P-autodesensitization, and was abolished by a combination of these treatments. It was concluded that capsaicin released both substance P and acetylcholine from the guinea-pig ileum. Evidence for a choUnergic component in the action of high concentrations of substance P was also obtained and therefore it is proposed that capsaicin releases substance P from neurones in the ileum, and the released substance P in turn stimulates cholinergic neurones to release acetylcholine, as well as producing direct muscle contraction. Key words: Capsaicin - Substance P - Acetylcholine - Atropine - Guinea-pig ileum Introduction Barth6 and Szolcs/myi (1978) reported that capsaicin pro- duced cholinergic contractions of the guinea-pig isolated ileum by stimulating terminals of extrinsic, non-para- sympathetic neurones. Using the technique of substance P-autodesensitization they found no evidence for the involve- ment of substance P in the response. This finding was unexpected in light of the substance P-releasing action of capsaicin on the spinal cord (Theriault et al, 1979; Gamse et al. 1979; Yaksh et al. 1980; Bucsics and Lembeck, 1981) and sympathetic ganglia (Gamse et al. 1981), and the demon- strated presence of intrinsic and extrinsic neurones contain- ing substance P in the guinea-pig ileum (Costa et al. 1980, 1981 a). However, pretreatment of neonatal or adult rats with capsaicin failed to reduce the concentration of substance P in the rat intestine (Holzer et al. 1980). These results suggested that the substance P-containing neurones in the gastro- intestinal tract are resistant to capsaicin as are certain central substance P-containing neurones (Gamse et al. 1979; Cuello et al. 1981). In order for capsaicin to be a useful tool in the study of the nervous system it will be necessary to identify the substances which it releases in various tissues. Therefore, the present study was undertaken to obtain further information on the substance(s) released in the guinea-pig ileum by capsaicin. Send offprint requests to L. A. Chahl at the above address Methods Ileal segments (1.5- 2 cm) from the distal region of the ileum of adult guinea-pigs of either sex were suspended under 1 g tension in a 2 ml organ bath in oxygenated Tyrode solution at 37~ Isometric contractions were recorded using a Grass force transducer (FT03C) and polygraph. The sensitivity of each preparation to several doses of substance P and usually to acetylcholine was tested at the commencement of each experiment. Capsaicin. Preliminary experiments showed that the con- centration of capsaicin which reliably produced a marked contraction of control preparations was 1.5 x 10- 6 tool 1-1 and therefore this concentration was used in all experiments. The vehicle (10 ~ ethanol and 5 ~o Tween 80 in saline) was also diluted in Tyrode solution and tested in these experi- ments. It was found to produce very slight relaxation of the preparations but no inhibition of responses to acetylcholine or substance P. As has been reported previously (see Barth6 and Szolcs~nyi 1978) rapid tachyphylaxis of guinea-pig ileum preparations to capsaicin occurred and therefore one ad- dition only of capsaicin was made to each preparation. Thus the effects of treatments on the response to capsaicin were determined by comparing responses between different prep- arations by Student's t-tests. The effects of the following treatments on the response to capsaicin were investigated: substance P-autodesensitization (see below) alone and in the presence of atropine 5 x 10 -7 and 5 x 10 6 mol 1 a ; atropine 5 x 10-6tool 1-1; and in another series of experiments, substance P-autodesensitization alone and in the presence of eserine, 3 x 10- s mol 1-1 Substance P-Autodesensitization. The method of Monier and Kitabgi (1980) was used. Six additions, at one minute intervals, of 10 gl of substance P 10 -s mol 1- ~ were added to the bath. This resulted in a final bath concentration of 3 x 10- 7 tool 1-1. After an initial contraction there was gener- al relaxation of the preparations over the 6 min. Prepara- tions desensitized to substance P in the presence of atropine (5 xl0 -v or 5 xl0-6mol 1-1) always relaxed completely so that the tone at the end of the six additions of substance P was similar to that before addition of substance P. However, preparations desensitized in the absence of atropine and particularly in the presence of eserine (3 x 10-Stool 1-1) maintained some degree of tone above that at the commence- ment of desensitization . This was also observed by Holzer and Lembeck (1980). Following desensitization capsaicin (1.5 x 10- 6 mol 1 1) and/or substance P (2.5 or 5 x 10- 7 mol 1-1) were tested without washout of the desensitizing concen- 0028-1298/82/0319/0212/$01.00