Brain Research, 539 (1991) 191-195 191 Elsevier BRES 16251 Cholinergic nerve terminals of human cerebral cortex possess a GABA transporter whose activation induces release of acetylcholine Giambattista Bonanno 1, Antonio Ruelle 2, Gian Carlo Andrioli 2 and Maurizio Raiteri 1 Ilstituto di Farmacologia e Farmacognosia, Universitd degli Studi di Genova, Genova (Italy) and 2Divisione di Neurochirurgia, Ospedali Galliera, Genova (Italy) (Accepted 14 August 1990) Key words: ~-Aminobutyric acid uptake; Acetylcholine release; Human neocortex; Release regulation; Transmitter coexistence The effect of ~,-aminobutyric acid (GABA) on the release of [3H]acetylcholine ([3H]ACh) from human cerebral cortex nerve terminals was investigated using synaptosomes prepared from neurosurgical specimens (which had to be removed to reach deeply located tumors) prelabeled with [3H]choline and exposed in superfusion to varying concentrations of GABA. The amino acid (3-100 /~M) increased in a concentration-dependent manner (maximal effect: 40%; ECso = 14.7 gM) the release of [3H]ACh but not that of [3H]choline. The GABA A receptor agonist muscimol (up to 100 gM) did not increase significantly the release of [3H]ACh. Accordingly, the effect of GABA was insensitive to the GABA A receptor antagonist bicuculline. The release of [3H]ACh was not affected by the GABA B receptor agonist (-)-baclofen (100-300 gM). The GABA-induced [3H]ACh release was counteracted by two inhibitors of GABA uptake, N-(4, 4-diphenyl-3-butenyl)nipecotic acid (SKF 89976A) and nipecotic acid. Moreover, the enhancing effect of GABA on [3H]ACh release was clearly Na+-dependent and was reduced by almost 90% in presence of 23 mM NaCI. The data indicate that, similarly to what had been observed in the rat, cholinergic nerve terminals in the human cerebral cortex possess a GABA transporter. Activation of this carrier brings about release of newly synthesized ACh. GABA and ACh might co-exist in some cerebrocortical nerve endings in the vertebrate brain, including man. INTRODUCTION During a recent study of the effects of y-aminobutyric acid (GABA) on the release of acetylcholine (ACh) in the rat brain an unexpected observation was made. The amino acid augmented the release of [3H]ACh from rat hippocampus synaptosomes prelabeled with [3H]choline; however, this effect was not mediated by a GABAergic presynaptic heteroreceptor. The GABA-induced [3H]- ACh release was instead selectively blocked by inhibitors of GABA uptake. This finding led to the conclusion that, in the rat brain, nerve terminals exist which not only possess a choline transporter linked to ACh synthesis, but also a GABA carrier, activation of which brings about release of ACh 2. Reciprocally, the release of endogenous GABA from rat hippocampus synaptosomes was found to be enhanced by choline, an effect which was prevented by hemicholinium-3, a blocker of the high affinity choline uptake 13. These novel findings may be variously interpreted. One possibility is that some neurons express a hetero- carrier instead of a heteroreceptor to permit release modulation. These neurons might even co-store GABA and ACh and therefore possess co-carriers having not only the well-known reuptake function but also that of permitting reciprocal regulation of release between the two co-transmitters. Although the mechanisms underlying this phenome- non are now being actively investigated in our laboratory using experimental animals, any further experimentation would obviously be most justified if the process also occurs in the human brain. We have therefore investi- gated the effects of GABA on the release of ACh in fresh specimens of human cerebral cortex. MATERIALS AND METHODS Characteristic of human specimens Samples of human cerebral cortex were obtained from patients undergoing neurosurgery. The tissues used were removed by the surgeon to reach deeply located tumors. The samples represented parts of frontal (2), temporal (2), parietal (1) and occipital (2) lobes and were obtained from 4 male and 3 female patients (aged 49-61 years). The tissues were obtained and processed separately on different days. After premedication with atropine and meperidine, anesthesia was induced with Penthotal and maintained with 70% nitrous oxide in 30% oxygen and 0.5-1% isoflurane. Pancuronium was employed to obtain muscular relaxation. Preparation of synaptosomes Immediately after removal, the tissue was placed in a physiolog- Correspondence: M. Raiteri, Istituto di Farmacologia e Farmacognosia, Universit~t degli Studi di Genova, Viale Cembrano 4, 16148 Genova, Italy. 0006-8993/91/$03.50 © 1991 Elsevier Science Publishers B.V. (Biomedical Di~,ision)