European Journal of Pharmacology, 155 (1988) 297-299 297 Elsevier EJP 20226 Short communication Pertussis toxin inactivates the presynaptic serotonin autoreceptor in the hippocampus Francesca Passarelli 1, Tomaso Costa 1,. and Osborne F.X. Almeida 1,2 J Department of Neuropharmacology, Max-Planck-lnstitut fur Psychiatrie, Am Klopferspitz 18a, D-8033 Planegg-Martinsried, and 2 Institute of Pharmacology, Toxicology and Pharmacy, Faculty of Veterinary Medicine, Ludwig-Maximilians-University, Ki~niginstrasse 18, D-8000 Mi~nchen 22, F.R.G. Received 9 August 1988, accepted 23 August 1988 The serotonin (5-HT) agonists, D-lysergic acid diethylamide (LSD) and 5-methoxytryptamine (5-Me-O-T), dose dependently inhibited the K+-evoked outflow of [3H]5-HT from preloaded rat hippocampal slices in vitro, indicating activation of the 5-HT autoreceptor. However, this effect was abolished in slices pretreated with pertussis toxin. It is thus concluded that the 5-HT autoreceptor in the hippocampus is coupled to G proteins. 5-HT autoreceptors; G proteins; Pertussis toxin; Hippocampus; D-Lysergic acid diethylamide 1. Introduction 2. Materials and methods There is evidence that presynaptic inhibitory serotonin autoreceptors modulate the release of serotonin (5-HT) in various brain areas (Chesselet, 1984). It was recently shown that the somato- dendritic 5-HT autoreceptor (5-HTIA subtype) of the dorsal raphe mediates its inhibitory effects by interacting with a GTP-binding regulatory protein (G protein) sensitive to pertussis toxin (Innis and Aghajanian, 1987). Moreover, studies on the auto- receptor regulation of the a2-adrenoceptor in the rabbit hippocampus have shown that pertussis toxin treatment diminished the inhibitory and facilitatory effects of adrenoceptor agonists and antagonists, respectively (Allgaier et al., 1985). We studied the effect of pertussis toxin treatment on the K+-evoked release of 5-HT in rat hippocampal slices to examine whether the 5-HT autoreceptor which controls [3H]5-HT-stimulated release in the hippocampus might also be coupled to G-proteins. * To whom all correspondence should be addressed. The methods used have been described in detail elsewhere (Passarelli et al., 1987). Briefly, slices of rat hippocampus (0.4 mm thickness) were first incubated for 4 h at 37°C in the presence or absence of 100 ng/ml pertussis toxin (obtained from LIST Biological LABS, Campbell, CA, U.S.A.) in Krebs medium which was bubbled with a mixture of 02 and CO 2 (95:5) and had the following composition (in mM): NaC1 118, KC1 4.7, CaC12 1.3, MgC12 1.2, Na2HPO 4 1, NaHCO 3 25, glucose 11.1, Na2EDTA 0.04 and ascorbic acid 0.11 (hereafter referred to as Krebs medium). The slices were washed then preloaded with [3H]5- HT (0.1 #M, 30 Ci/mmol) in Krebs medium for 30 rain at 37 °C and finally transferred to superfu- sion chambers (3 slices/each). Superfusion was carried out with Krebs medium at 37 °C at a rate of 0.5 ml/min. Two stimulation pulses (4 min each) were applied at 60 and 104 min, respec- tively, from the beginning of the superfusion with Krebs medium that contained 20 mM KC1. Sam- ples were collected at 4-min intervals and the 0014-2999/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)