Naunyn-Schmiedeberg's Arch Pharmacol (1994) 350:356-360 Naunyn-Schmiedeberg's Archivesof Pharmacoloffy © Springer-Verlag 1994 5-Hydroxytryptamine receptors with a 5-HT 6 receptor-like profile stimulating adenylyl cyclase activity in pig caudate membranes Philippe Schoeffter, Christian Waeber* Preclinical Research, 386/744, Sandoz Pharma Ltd., CH-4002 Basel, Switzerland Received: 5 February 1994/Accepted: 31 May 1994 Abstract. This study deals with the characterization of 5-hydroxytryptamine (5-HT, serotonin) receptors posi- tively linked to adenylyl cyclase in membranes from pig brain caudate. 5-HT and related agonists induced a con- centration-dependent stimulation of adenylyl cyclase ac- tivity in pig caudate membranes, with the following rank order of potency (mean pECs0 values): 5-HT (7.1)_>5- methoxytryptamine (6.9) > 5-carboxamidotryptamine (5.6) > sumatriptan (< 5). Maximal stimulation by 5-HT averaged 35 pmol cyclic AMP/min/mg protein over a bas- al activity of 159pmol cyclic AMP/min/mg protein. 5-Methoxytryptamine and 5-carboxamidotryptamine had similar efficacies to that of 5-HT, whereas sumatriptan was about half efficacious. Other compounds known as agonists at some 5-HT receptors were weakly potent (mean pECs0 values < 5). They include the 5-HT1A recep- tor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin hy- drobromide (8-OH-DPAT), the 5-HT 4 receptor agonist, renzapride and the 5-HT 2 receptor agonist, (1-(2,5-di- methoxy-4-iodophenyl)-2 aminopropane) (DOI). In an- tagonist studies, methiothepin (0.1 and i ~tmol/1) shifted the 5-HT curve to the right with no depression of the Emax, yielding pK B values of 7.4-8.0. Clozapine (1 ~tmol/1) also produced surmountable antagonism of 5-HT-induced effects (pKB 6.9). Ketanserin (10 Ixmol/1) weakly antagonized 5-HT (pKB 5.0). The 5-HT 4 receptor antagonists, tropisetron (ICS 205-930) and SDZ 205-557 (2-methoxy-4-amino-5-chloro-benzoic acid 2-(diethyl- amino) ethyl ester), each at I ~tmol/1, did not significant- ly alter the concentration-response curve of 5-HT. The present receptor shares some characteristics of the recent- ly cloned 5-HT 6 receptor (Monsma et al. (1993) Mol Pharmacol 43:320-327): similar pharmacological pro- file, location (striatum) and ability to stimulate adenylyl cyclase. It may thus represent the functional 5-HT 6 re- ceptor in its natural environment. * Present address: Stroke Research Laboratory, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA Correspondence to: P. Schoeffter at the above address Key words: 5-Hydroxytryptamine (5-HT) - 5-HT recep- tors - 5-HT6 receptors - Adenylyl cyclase - Pig caudate Introduction According to a recent review (Humphrey et al. 1993), the classification of receptors for 5-hydroxytryptamine (5-HT, serotonin) now distinguishes four main receptor types. The 5-HT 1 receptor class itself consists of 5 sub- types (5-HT1A, 5-HT1B, 5-HT~D, 5-HT1E and 5-HT~F), all negatively coupled to adenylyl cyclase. 5-HT2 recep- tors (with three subtypes including the former 5-HTlc receptor, now called 5-HT2c and the 'rat fundus recep- tor', now referred to as 5-HTzB) stimulate phospholipase C. 5-HT 3 receptors are ligand-gated ion channels and 5-HT 4 receptors, whose structure is unknown, stimulate adenylyl cyclase activity. Additional 5-HT receptor genes have been cloned, with little structural similarities to each other and to those previously cloned. Thus, another three putative 5-HT receptors (5-HT 5, 5-HT 6 and 5-HT7) are in the process of being acknowledged (Hoyer et al. 1994), but they remain to be indentified in functional models. The present work deals with the characterization of 5-HT receptors positively coupled to adenylyl cyc!ase in membranes from pig brain caudate. It has long been known that 5-HT carl stimulate adenylyl cyclase activity in striatum, of which caudate is a part (Fillion et al. 1979, 1980; Chneiweiss et al. 1984). However, in these earlier studies, the pharmacological characterization of the re- ceptors mediating this effect was hampered by the lack of selective compounds and/or the limited knowledge of 5-HT receptors. We now show that these receptors have a unique profile and resemble the recently cloned 5-HT 6 receptors (Monsma et al. 1993) in many aspects: similar pharmacological profile, location (striatum) and ability to stimulate adenylyl cyclase. A preliminary account to this work has been commu- nicated to the German Pharmacological Society (1993 Spring Meeting, Mainz; Schoeffter and Waeber 1993).