The effect of some putative neurotransmitters on the release of 5-hydroxytryptamine and gamma-aminobutyric acid from slices of the rat midbrain raphe area.

The effect of various putative transmitters has been studied on the efflux of ³H]5-hydroxytryptamine and ³H]γ-aminobutyric acid from small superfused slices of the midbrain raphe area of the rat. The slices apparently contained functionally intact terminals for these transmitters since a depolarizing stimulus (50 mM KCl) stimulated the rate of efflux of ³H]5-hydroxytryptamine and ³H]γ-aminobutyric acid in a calcium-dependent fashion. Furthermore, slices accumulated radioactivity with apparent high affinity mechanisms. γ-Aminobutyric acid (50 and 100 μ m) and substance P (100 and 500μ m) stimulated the efflux of ³H]5-hydroxytryptamine. The effect of γ-aminobutyrate was blocked by picrotoxin (50 μ m) but not by strychnine (1 μ m). Other inhibitory amino acids (β-alanine, taurine and glycine) were without effect on the release of ³H]5-hydroxytryptamine. l-Noradrenaline (0.2–1 mM) stimulated the efflux of ³H]γ-aminobutyric acid but not that of 3H]5-hydroxytryptamine. Phentolamine (10μ m) but not (±)-propranolol (10 μ m) abolished the effect of 1 mM noradrenaline. Neither dopamine nor 5-hydroxytryptamine influenced the efflux of 3H]γ-aminobutyric acid.It is possible that interactions in the midbrain raphe area proposed from other studies can be mediated through presynaptic influences on transmitter release.