Neurokinin-3 (NK 3 ) receptors are concentrated in forebrain and basal ganglia structures within the mammalian CNS. This distribution, together with the modulatory influence of NK 3 receptors on monoaminergic neurotransmission, has led to the hypothesis that NK 3 receptor antagonists may have therapeutic efficacy in the treatment of psychiatric disorders. Here we describe the in vitro and in vivo characterization of the highly selective NK 3 receptor antagonist talnetant (SB-223412). Talnetant has high affinity for recombinant human NK 3 receptors (pK i 8.7) and demonstrates selectivity over other neurokinin receptors (pK i NK 2 ¼ 6.6 and NK 1 o4). In native tissuebinding studies, talnetant displayed high affinity for the guinea pig NK 3 receptor (pK i 8.5). Functionally, talnetant competitively antagonized neurokinin B (NKB)-induced responses at the human recombinant receptor in both calcium and phosphoinositol second messenger assay systems (pA 2 of 8.1 and 7.7, respectively). In guinea pig brain slices, talnetant antagonized NKB-induced increases in neuronal firing in the medial habenula (pK B ¼ 7.9) and senktide-induced increases in neuronal firing in the substantia nigra pars compacta (pK B ¼ 7.7) with no diminution of maximal agonist efficacy, suggesting competitive antagonism at native NK 3 receptors. Talnetant (3-30 mg/kg i.p.) significantly attenuated senktide-induced 'wet dog shake' behaviors in the guinea pig in a dose-dependent manner. Microdialysis studies demonstrated that acute administration of talnetant (30 mg/kg i.p.) produced significant increases in extracellular dopamine and norepinephrine in the medial prefrontal cortex and attenuated haloperidol-induced increases in nucleus accumbens dopamine levels in the freely moving guinea pigs. Taken together, these data demonstrate that talnetant is a selective, competitive, brain-penetrant NK 3 receptor antagonist with the ability to modulate mesolimbic and mesocortical dopaminergic neurotransmission and hence support its potential therapeutic utility in the treatment of schizophrenia.

Antipsychotic drugs (APD) are widely prescribed for the treatment of schizophrenia. The APD are differentiated into typical and atypical based on the lower incidence of extra-pyramidal side-effects associated with the newer atypical APD. It was suggested that atypicality may arise from an interaction with the 5-hydroxytryptamine (5-HT)(2) receptor and specifically on the 5-HT(2):dopamine D(2) affinity ratio. It is now realised that multiple subtypes of these receptors exist and that in addition, atypical APD interact with many monoamine receptors. The aim of the present study was to characterise the interaction of APD with a variety of monoamine receptors in terms of both affinity and efficacy. The data produced has highlighted that the atypical profile of APD such as olanzapine and clozapine may reflect antagonism of the 5-HT(2A) and 5-HT(2C) receptors, whilst that of, ziprasidone and quetiapine may reflect partial agonist activity at the 5-HT(1A) receptor, and that of aripiprazole may reflect partial agonist activity at the 5-HT(1A) receptor as well as is its claimed partial agonist activity at the dopamine D(2) receptor.