Substance P facilitates the release of acetylcholine from motor nerve terminals of frogs.

Neurotransmitters can modulate the release of other neurotransmitters from presynaptic nerve terminals, as has been described for "presynaptic inhibition" in the spinal cord (Eccles, 1964). Catecholamine and 5-hydroxytryptamine can alter the release of acetylcholine (ACh) from pre ganglionic nerve terminals in sympathetic ganglia (Christ and Nishi, 1971; Kuba et al. 1981; Hirai and Koketsu, 1980). Substance P, an endogenous polypeptide, has been considered to be a neurotransmitter candidate for the non-cholinergic slow postsynaptic potential in the mammalian myenteric plexus (Katayama and North, 1978; Johnson et al. 1981), inferior mesenteric ganglia (Konishi et al. 1979; Dun and Jiang, 1982) and bullfrog sympathetic ganglia (Nishi et al. 1980). The present experiments demonstrate that sub stance P in low concentrations (0.5-4.2ƒÊ M) increases the amount of ACh released from motor nerve terminals of the frog neuromuscular junction. It is suggested that substance P might have an important role as a presynaptic modulator of nicotinic cholinergic transmission. Sartorius muscles with intact sciatic neryes were isolated from frogs (Rang nigromaculata). The preparations were continuously superfused with Ringer solution. Conventional microelectrode techniques were employed for recording the end-plate potential (EPP) using microelectrodes filled with 3 M KCl and having a tip resistance of 20 M fl . The ionic composition of the Ringer solution was (mM) : NaCl 112, KCl 2, CaCl 2 1.8 and NaHCO3 2.4. To obtain an appropriate amplitude of EPPs, the sciatic nerves were stimulated at a rate of 0.2 Hz in low Ca 2 + -high Mg 2 + Ringer solutions (CaCl2 0.6-1.1 mM, MgCl2 4.3-7.0 mM) and in Ringer solution containing d-tubocurarine (d-TC) (2ƒÊM). Quantal content was calculated by the variance method (del Castillo and Katz, 1954) from the mean amplitude and standard deviation of 60 EPPs (5min) induced before, during and after the application of substance P. Changes in the quantal content during the application of substance P were expressed as percent of the control quantal content. To obtain the ACh-induced end plate potentials (ACh potentials) and ACh-induced currents (ACh currents), ACh was directly applied to the end-plate by iontophoresis from a microelectrode filled with 1 M ACh with a tip resistance of 70 Mƒ¶. The voltage-clamp methods were similar to