1 RT ± PCR technique was used to clone the human 5-HT 4(e) receptor (h5-HT 4(e) ) from heart atrium. We showed that this h5-HT 4(e) receptor splice variant is restricted to brain and heart atrium. 2 Recombinant h5-HT 4(e) receptor was stably expressed in CHO and C6-glial cell lines at 347 and 88 fmol mg 71 protein, respectively. Expression of h5-HT 4(e) receptors at the cell membrane was con®rmed by immunoblotting. 3 The receptor binding pro®le, determined by competition with [3 H]-GR113808 of a number of 5-HT 4 ligands, was consistent with that previously reported for other 5-HT 4 receptor isoforms. Surprisingly, we found that the rank order of potencies (EC 50 ) of 5-HT 4 agonists obtained from adenylyl cyclase functional assays was inversely correlated to their rank order of anities (K i ) obtained from binding assays. Furthermore, EC 50 values for 5-HT, renzapride and cisapride were 2 fold lower in C6-glial cells than in CHO cells. 4 ML10302 and renzapride behaved like partial agonists on the h5-HT 4(e) receptor. These results are in agreement with the reported low ecacy of the these two compounds on L-type Ca 2+ currents and myocyte contractility in human atrium. 5 A constitutive activity of the h5-HT 4(e) receptor was observed in CHO cells in the absence of any 5-HT 4 ligand and two 5-HT 4 antagonists, GR113808 and ML10375, behaved as inverse agonists. 6 These data show that the h5-HT 4(e) receptor has a pharmacological pro®le which is close to the native h5-HT 4 receptor in human atrium with a functional potency which is dependent on the cellular context in which the receptor is expressed.

SL65.0155 [5-(8-amino-7-chloro-2,3-dihydro-1,4-benzodioxin-5-yl)-3-[1-(2-phenyl ethyl)-4-piperidinyl]-1,3,4-oxadiazol-2(3H)-one monohydrochloride] is a novel benzodioxanoxadiazolone compound with high affinity for human 5-hydroxytryptamine (5-HT) 4 receptors (K i of 0.6 nM) and good selectivity (greater than 100-fold for all other receptors tested). In cells expressing the 5-HT 4(b) and 5-HT 4(e) splice variants, SL65.0155 acted as a partial agonist, stimulating cAMP production with a maximal effect of 40 to 50% of serotonin. However, in the rat esophagus preparation, SL65.0155 acted as a 5-HT 4 antagonist with a pK b of 8.81. In addition, SL65.0155 potently improved performance in several tests of learning and memory. In the object recognition task, it improved retention at 24 h when administered i.p. or p.o. (0.001-0.1 mg/kg). This effect was antagonized by the 5-HT 4 antagonist SDZ 205,557, itself without effect, demonstrating that the promnesic effects of SL65.0155 are mediated by 5-HT 4 agonism. SL65.0155 also reversed the cognitive deficits of aged rats in the linear maze task and the scopolamineinduced deficit of mice in the water maze task. Furthermore, the combined administration of an inactive dose of SL65.0155 with the cholinesterase inhibitor rivastigmine resulted in a significant promnesic effect, suggesting a synergistic interaction. SL65.0155 was devoid of unwanted cardiovascular, gastrointestinal, or central nervous system effects with doses up to more than 100-fold higher than those active in the cognitive tests. These results characterize SL65.0155 as a novel promnesic agent acting via 5-HT 4 receptors, with an excellent preclinical profile. Its broad range of activity in cognitive tests and synergism with cholinesterase inhibitors suggest that SL65.0155 represents a promising new agent for the treatment of dementia.Alzheimer's disease (AD) is a chronic neurodegenerative disorder characterized by progressive cognitive decline, debilitating behavioral impairment, and ultimately death. Neurofibrillary tangles and amyloid plaques are the pathological hallmarks of this disease, which is also characterized by a dramatic loss of cholinergic neurons in brain regions involved in higher cognitive functions. It is widely accepted that the loss of cholinergic markers is one of the major neurochemical deficits in AD and that this deficit correlates well with loss of cognitive abilities (Bierer et al., 1995;Palmer, 1996). Despite this, cholinergic therapies, such as inhibitors of acetylcholinesterase, have had only partial success (e.g., Volger, 1991), possibly due to the multifactorial nature of the deficits and poorly understood etiology of AD. Thus, there is a major need for novel therapeutic approaches that can alleviate the symptoms of the disease.In addition to the cholinergic deficit, loss of several other neurotransmitter systems has been reported in AD, including norepinephrine, dopamine (see Gottfries, 1990 for a review), and 5-hydroxytryptamine (5-HT) (Palmer, 1996; MeltArticle, publicat...

1 Among the ®ve human 5-HT 4 (h5-HT 4 ) receptor isoforms, the h5-HT 4(a) receptor was studied with a particular emphasis on the molecular interactions involved in ligand binding. For this purpose, we used site-directed mutagenesis of the transmembrane domain. Twelve mutants were constructed with a special focus on the residue P4.53 of helix IV which substitutes in h5-HT 4 receptors the highly conserved S residue among the rhodopsin family receptors. The mutated receptors were transiently expressed in COS-7 cells.