No abstract

1 This study describes the in vitro characterization of two potent and selective 5-HT 6 receptor antagonists at the rat and human recombinant 5-HT 6 receptor. 2 In binding assays with [ 3 H]-LSD, 4-amino-N-(2,6 bis-methylamino-pyrimidin-4-yl)-benzene sulphonamide (Ro 04-6790) and 4-amino-N-(2,6 bis-methylamino-pyridin-4-yl)-benzene sulphonamide (Ro 63-0563) had mean pK i values +s.e.mean at the rat 5-HT 6 receptor of 7.35+0.04 and 7.83+0.01, respectively and pK i values at the human 5-HT 6 receptor of 7.26+0.06 and 7.91+0.02, respectively. 3 Both compounds were found to be over 100 fold selective for the 5-HT 6 receptor compared to 23 (Ro 04-6790) and 69 (Ro 63-0563) other receptor binding sites. 4 In functional studies, neither compound had any signi®cant e ect on basal levels of cyclicAMP accumulation in Hela cells stably expressing the human 5-HT 6 receptor, suggesting that the compounds are neither agonists nor inverse agonists at the 5-HT 6 receptor. However, both Ro 04-6790 and Ro 63-0563 behaved as competitive antagonists with mean +s.e.mean pA 2 values of 6.75+0.07 and 7.10+0.09, respectively. 5 In rats habituated to observation cages, Ro 04-6790 produced a behavioural syndrome similar to that seen following treatment with antisense oligonucleotides designed to reduce the expression of 5-HT 6 receptors. This behavioural syndrome consisted of stretching, yawning and chewing. 6 Ro 04-6790 and Ro 63-0563 represent valuable pharmacological tools for the identi®cation of 5-HT 6 receptors in natural tissues and the study of their physiological function.

The advance of functional genomics revealed the superfamily of G-protein coupled receptors (GPCRs). Hundreds of GPCRs have been cloned but many of them are orphan GPCRs with unidentified ligands. The first identified orphan GPCR is the opioid receptor like orphan receptor, ORL1. It was cloned in 1994 during the identification of opioid receptor subtypes and was de-orphanized in 1995 by the discovery of its endogenous ligand, nociceptin or orphanin FQ (N/OFQ). This receptor was renamed as N/OFQ peptide (NOP) receptor. Several selective ligands acting at NOP receptors or other anti-N/OFQ agents have been reported. These include N/OFQ-derived peptides acting as agonists (cyclo[Cys(10),Cys(14)]N/OFQ, [Arg(14), Lys(15)]N/OFQ, [pX]Phe(4)N/OFQ(1-13)-NH(2), UFP-102, [(pF)Phe(4),Aib(7), Aib(11),Arg(14),Lys(15)]N/OFQ-NH(2)) or antagonists (Phe(1)psi(CH(2)-NH)Gly(2)]N/OFQ(1-13)-NH(2), [Nphe(1)]N/OFQ(1-13)-NH(2), UFP-101, [Nphe(1), (pF)Phe(4),Aib(7),Aib(11),Arg(14),Lys(15)]N/OFQ-NH(2)), hexapeptides, other peptide derivatives (peptide III-BTD, ZP-120, OS-461, OS-462, OS-500), non-peptide agonists (NNC 63-0532, Ro 64-6198, (+)-5a compound, W-212393, 3-(4-piperidinyl)indoles, 3-(4-piperidinyl) pyrrolo[2,3-b]pyridines) and antagonists (TRK-820, J-113397, JTC-801, octahydrobenzimidazol-2-ones, 2-(1,2,4-oxadiazol-5-yl)-1 H-indole, N-benzyl-D-prolines, SB-612111), biostable RNA Spiegelmers specific against N/OFQ, and a functional antagonist, nocistatin. Buprenorphine and naloxone benzoylhydrazone are two opioid receptor ligands showing high affinity for NOP receptors. NOP receptor agonists might be beneficial in the treatment of pain, anxiety, stress-induced anorexia, cough, neurogenic bladder, edema, drug dependence, and, less promising, in cerebral ischemia and epilepsy, while antagonists might be of help in the management of pain, depression, dementia and Parkinsonism. N/OFQ is also involved in cardiovascular, gastrointestinal and immune regulation. Altered plasma levels of N/OFQ have been reported in patients with various pain states, depression and liver diseases. This review summarizes the pharmacological characteristics of, and studies with, the available NOP receptor ligands and their possible clinical implications.

A small series of aryl pyridyl sulfones has been prepared and investigated for its 5-HT(6) receptor binding properties. Thereof, pyrrolidinyl derivative 11 proved to be a very potent (pK(i) 9) and selective 5-HT(6) receptor antagonist. By means of in vivo microdialysis in the frontal cortex and a passive avoidance paradigm, where 11 reversed a scopolamine induced retention deficit, a functional correlation between 5-HT(6) receptors and cholinergic neurotransmission could be shown, supporting the therapeutic potential of 5-HT(6) receptors in the treatment of cognitive deficits.

Although current antipsychotic drugs are effective at treating the psychotic (positive) symptoms of schizophrenia, they have one or more serious side effects, including extrapyramidal symptoms, weight gain, cardiovascular liabilities and type II diabetes. However, recent data from clinical trials of selective neurokinin 3 (NK(3)) receptor antagonists in schizophrenia - osanetant and talnetant - have shown significant improvement in positive symptoms, with no major side-effects reported as yet. Here we discuss the preclinical and clinical evidence that indicates that NK(3) receptor antagonists might represent a new approach to the treatment of schizophrenia and possibly other neuropsychiatric disorders.