Substance P provoked gamma-aminobutyric acid release from the myenteric plexus of the guinea-pig small intestine.

The release of [3H]gamma-aminobutyric acid (GABA) from the isolated small intestine of the guinea-pig pre-loaded with [3H]GABA was measured in the presence of substance P and vasoactive intestinal polypeptide (VIP). Substance P (10(-10)-10(-7) M) produced a dose-dependent increase in the fractional rate of [3H]GABA release. VIP, even at 10(-7) M, did not affect the spontaneous [3H]GABA release nor the release of [3H]GABA evoked by electrical transmural stimulation (0.5 ms, 15 V, 10 Hz for 30 s). The release of endogenous GABA from the isolated small intestine was measured in the presence of substance P (10(-9) M). After 60 min superfusion, the spontaneous release of GABA was 4.61 +/- 0.14 pmol min-1 g-1 wet wt. (n = 20). Substance P (10(-9) M) produced an approximate 2-fold spontaneous release of endogeneous GABA (8.74 +/- 0.21 pmol min-1 g-1 wet wt. (n = 10)). Perfusion with Ca-free medium containing 1 mM-EGTA and tetrodotoxin (3 X 10(-7) M) inhibited the release of endogenous GABA evoked by substance P (10(-9) M). (D-Pro2, D-Trp7,9) substance P (10(-6) M) antagonized the release of endogenous GABA evoked by substance P (10(-9) M). These results indicate that substance P induces a neuronal release of GABA through its receptor located in the guinea-pig small intestine. Substance P (10(-11)-10(-7) M) produced a dose-dependent increase in the fractional rate of [3H]acetylcholine (ACh) release from the isolated small intestine pre-loaded with [3H]choline. The release of [3H]ACh evoked by substance P (10(-9) M) was inhibited by perfusion with Ca-free medium containing 1 mM-EGTA, tetrodotoxin (3 X 10(-7) M) and (D-Pro2, D-Trp7,9)substance P (10(-6) M). Bicuculline (10(-6) M) inhibited the release of [3H]ACh evoked by substance P (10(-9) M) by 68.1 +/- 4.6% (n = 5), thereby suggesting that the substance P-evoked ACh release is partly mediated through the endogenous GABA released by substance P. These results provide evidence for the neurotransmitter role of GABA and a possible excitatory role of substance P on the GABAergic neurones in the myenteric plexus of the guinea-pig small intestine.