Substance P acts by releasing 5-hydroxytryptamine from enteric neurons in the stomach of the rainbow trout, Salmo gairdneri.

The effect and mode of action of substance P was studied in a perfused stomach preparation and on isolated strip preparations of the stomach wall from the rainbow trout, Salmo gairdneri. Substance P was excitatory on the stomach muscle wall in a dose-dependent manner. Two other tachykinins, physalaemin and eledoisin, excited the preparations in a similar manner and in the same dose range. The effect of substance P was not antagonized by the substance P analogues [D-Pro2, D-Phe7, D-Trp9]substance P and [D-Pro2, D-Trp7,9]substance P (both 10(-5) M). Tetrodotoxin reduced or abolished the effect of substance P, while no reduction of the response was obtained after atropine, chlorisondamine or phentolamine (all 10(-6) M). 5-Hydroxytryptamine excited the stomach and this effect was not antagonized by tetrodotoxin, suggesting that the action of 5-hydroxytryptamine was direct on the smooth muscle. The 5-hydroxytryptamine antagonist methysergide, in a concentration which selectively blocked the response to 5-hydroxytryptamine, also blocked the response to substance P (10(-9)-10(-8) M). The outflow of 5-[3H]hydroxytryptamine from a preloaded perfused stomach was clearly increased by substance P, and this release was blocked by tetrodotoxin. Immunohistochemistry revealed the presence of nerve fibres and ganglion cells showing 5-hydroxytryptamine-like immunoreactivity in the myenteric plexus and smooth muscle layers of the stomach wall. The immunoreactive cells and nerve fibres were particularly abundant in the pyloric part of the stomach. It is concluded that the main effect of substance P on the stomach wall of the rainbow trout is indirect, via activation of a non-adrenergic, non-cholinergic neuron. The results are compatible with the view that this neuron exerts its action by release of 5-hydroxytryptamine. Supramaximal concentrations (greater than or equal to 10(-7) M) of substance P may in addition have a direct effect on the gastric smooth muscle.