Inhibition of isotonic sodium chloride intake in the rat by selective tachykinin agonists

The present study investigated the effect of the central injection of selective tachykinin (TK) agonists on the need-free intake of 0.9% NaCl in rats. Isotonic NaCl was offered for 60 min (between 1800 and 1900 h); water was offered for 4 h (between 1800 and 2200 h). The TK agonists were injected into the third ventricle just before access to fluids. The NK3-selective agonists [Asp5,6, MePhe8]substance P(5–11) and Succ[Asp6,MePhe8]substance P(6–11), as well as the NK1-selective agonist [Sar9, Met(O2)11]substance P, markedly reduced salt intake, the threshold dose for their effects being 5 ng/rat. The NK2-selective agonist GR64349 reduced salt intake only at 500 ng/rat. At the dose of 31.2 ng/rat, neither the NK1 nor the NK3 agonists inhibited water intake, when water was the only fluid offered (between 1800 and 2200 h), or modified food intake, but also the need-free food-deprived rats. The present study shows that a) TKs inhibit not only the need-induced salt intake, but also the need-free intake of isotonic saline, b) this effect is behaviorally selective, and c) the effect is apparently mediated by NK1 and NK3 receptors. The finding that TKs suppress salt intake in a large variety of experimental conditions supports the idea that the antinatriorexic effect of TKs is independent of the physiological and hormonal status of the animal. It is hypothesized that TKs might modify taste sensitivity or the hedonic evaluation of the salty taste.