Presynaptic modulation by L‐glutamate and GABA of sympathetic co‐transmission in rat isolated vas deferens

1 The modulatory effects of L‐glutamate and its structural analogues, and of γ‐aminobutyric acid (GABA), on sympathetic co‐transmission were studied in the rat isolated vas deferens exposed to electrical field stimulation (EFS). 2 Application of exogenous L‐glutamate caused a concentration‐dependent (1 μ m ‐3 mM) inhibition of the rapid twitch component of the biphasic EFS contraction. However, L‐glutamate (1 μ m ‐3 mM) had a minimal effect on the phasic contraction induced by exogenous adenosine 5′‐triphosphate (ATP, 150 μ m ) and noradrenaline (50 μ m ). Unlike L‐glutamate, D‐glutamate had no effect on the EFS contraction. 3 The L‐glutamate‐induced inhibition of the EFS contractions was significantly attenuated by the glutamate decarboxylase (GAD) inhibitor 3‐mercapto‐propionic acid (150 μ m ) and was abolished in the presence of the GABA transaminase (GABA‐T) inhibitor, 2‐aminoethyl hydrogen sulphate (500 μ m ). 4 The L‐glutamate‐induced inhibition of the electrically evoked contraction was not affected by the adenosine A 1 ‐receptor antagonist, 8‐cyclopentyl‐1, 3‐dipropylxanthine (DPCPX)(30 nM), reactive blue 2 (30 μ m ) or the GABA A receptor antagonist bicuculline (50 μ m ). However, the GABA B receptor antagonist 2‐hydroxysaclofen (50 μ m ) significantly inhibited the L‐glutamate effect. 5 Similar to L‐glutamate, GABA also caused a concentration‐dependent (0.1 – 100 μ m ) inhibition of the EFS contractions. This GABA‐induced inhibition was not affected by either the GABA A receptor antagonist bicuculline (50 μ m ) or reactive blue 2 (30 μ m ). However, a significant attenuation of the GABA‐mediated effect was recorded with the GABA B receptor antagonist 2‐hydroxysaclofen (50 μ m ). Contractions of the vas deferens induced by exogenous ATP and noradrenaline were not affected by GABA (0.1 – 100 μ m ). 6 The L‐glutamate analogues, N‐methyl‐D‐aspartate (NMDA) (1 μ m ‐1 mM) and quisqualate (Quis 0.1 μ m ‐0.3 mM) had no effect, whilst kainate (Kain, 1 μ m ‐1 mM) caused an inhibition of the EFS‐induced contractions. Effects of Kain could be abolished by the non‐NMDA receptor antagonist 6‐cyano‐7‐nitroquinoxaline‐2, 3‐dioxine (CNQX, 10 μ m ). NMDA, Quis and Kain had no effect on the exogenous ATP‐ or noradrenaline‐induced contractions. 7 It is concluded that the excitatory amino acid L‐glutamate modulates the electrically evoked vas deferens contraction through conversion to the inhibitory amino acid GABA by a specific GABA transaminase. The GABA formed may then act on GABA B receptors and cause inhibition of the contraction through a presynaptic mechanism.