Effect of longitudinal muscle-myenteric plexus removal and indomethacin on the response to tachykinin NK-2 and NK-3 receptor agonists in the circular muscle of the guinea-pig ileum

1 The effect of removal of the longitudinal muscle-myenteric plexus (LM-MP) and/ or indomethacin (10 -μM) on the response to the tachykinin NK-2 receptor selective agonist, [βAla8]NKA(4–10), or to the NK-3 receptor selective agonist, senktide, was investigated by measuring mechanical activity (isotonic recording) of circular muscle (ring preparation) of the guinea-pig ileum. 2 Indomethacin (10 μM) increased the percentage of ileal rings displaying spontaneous activity, either intact or LM-MP-free. The response to senktide (10nM and 1 μM) was lower in LM-MP-free than in intact ileal rings, either in the absence or presence of indomethacin. The response to a low concentration (10 nM) of [βAla8] NKA (4–10) was enhanced in LM-MP-free rings and by indomethacin. 3 In intact ileal rings, the response to senktide was unaffected by atropine (3 μM) alone or by the tachykinin NK-2 receptor antagonist MEN 10,376 (10 μM) alone while it was reduced by the combined administration of the two antagonists. The response to senktide was greatly reduced by tetrodotoxin (TTX, 1 μM). Senktide-induced contractions (10 nM) were also reduced by the blocker of N-type voltage-sensitive calcium channels, ω-conotoxin (CTX, 0.1 μM). 4 In about 30% of preparations tested, an inhibitory response (decrease in spontaneous activity) to 10 nM senktide, was disclosed in CTX-treated intact ileal rings. This inhibitory effect was TTX-sensitive. 5 In LM-MP-free ileal rings, the response to senktide was abolished or reduced by atropine and MEN 10,376, alone or in combination, and was also reduced or abolished by TTX and CTX. 6 The response to [βAla8]NKA (4–10) was inhibited by MEN 10,376, in both intact and LM-MP-free ileal rings while it was unaffected by atropine, TTX or CTX. 7 These results indicate that indomethacin pretreatment induces a regular background activity for studying the motor response to tachykinins in the circular muscle of the ileum, probably by blocking the formation of relaxant prostanoids. A further increase in sensitivity to direct smooth muscle stimulation (NK-2 receptor agonist) can be obtained by removal of the LM-MP. The response to NK-3 receptor stimulation is diminished but not abolished by removal of the LM-MP, suggesting that NK-3 receptors are located on neuronal bodies of myenteric neurons, but possibly also at other sites (possibly, nerve terminals). Furthermore, the presence of NK-3 receptors on myenteric plexus neurons, which inhibit circular muscle activity, is suggested.