Preclinical investigations into the antipsychotic potential of the novel histamine H3 receptor antagonist GSK207040.

OBJECTIVES: To test the novel nonimidazole histamine H3 receptor antagonist 5-[(3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazapin-7-yl)oxy]-N-methyl-2-pyrazinecarboxamide (GSK207040) in a series of behavioral and neurochemical paradigms designed to evaluate its antipsychotic potential. MATERIALS AND METHODS: Acute orally administered GSK207040 was investigated for its capacity to reverse a 24-h-induced deficit in novel object recognition memory, deficits in prepulse inhibition (PPI) induced by isolation rearing, and hyperlocomotor activity induced by amphetamine. The acute neurochemical effects of GSK207040 were explored by analyzing rat anterior cingulate cortex microdialysates for levels of dopamine, noradrenaline, and acetylcholine and by c-fos immunohistochemistry. The potential for interaction with the antipsychotic dopamine D2 receptor antagonist haloperidol was explored behaviorally (spontaneous locomotor activity and catalepsy), biochemically (plasma prolactin), and via ex vivo receptor occupancy determinations. RESULTS: GSK207040 significantly enhanced object recognition memory (3 mg/kg) and attenuated isolation rearing-induced deficits in PPI (1.0 and 3.2 mg/kg) but did not reverse amphetamine-induced increases in locomotor activity. There was no evidence of an interaction of GSK207040 with haloperidol. GSK207040 (3.2 mg/kg) raised extracellular concentrations of dopamine, noradrenaline, and acetylcholine in the anterior cingulate cortex and c-fos expression in the core of the nucleus accumbens was increased at doses of 3.2 and 10.0 mg/kg. CONCLUSIONS: The behavioral and neurochemical profile of GSK207040 supports the potential of histamine H3 receptor antagonism to treat the cognitive and sensory gating deficits of schizophrenia. However, the failure of GSK207040 to reverse amphetamine-induced locomotor hyperactivity suggests that the therapeutic utility of histamine H(3) receptor antagonism versus positive symptoms is less likely, at least following acute administration.

Increased expression of the NR2A NMDA receptor subunit in the prefrontal cortex of rats reared in isolation.

A hypofunction of the N-methyl-D-aspartate (NMDA) receptor has been implicated in the pathophysiology of schizophrenia. Compelling evidence of altered NMDA receptor subunit expression in the schizophrenic brain has not, however, so far emerged. Rats reared in isolation exhibit several characteristics, including disturbed sensory gating, which resemble those seen in schizophrenia. To explore the possibility that NMDA receptor dysfunction may contribute to the behavioral and neurochemical consequences of rearing rats in isolation, we compared NMDA receptor subunit expression in brains of rats which were housed in isolation and which displayed a deficit in prepulse inhibition of the acoustic startle response with that of socially housed controls. An initial microarray analysis revealed a 1.26-fold increase in NR2A transcript in the prefrontal cortex, but not in the nucleus accumbens, of rats reared in isolation compared with those housed socially. In contrast, NR1, NR2B, NR2C, NR2D, NR3A, and NR3B subunit expression was unchanged in either brain area. In a second cohort of animals, in situ hybridization revealed increased NR2A mRNA expression in the medial prefrontal cortex, an observation that was substantiated by increased [(3)H]CGP39653 binding suggesting that NR2A receptor subunit protein expression was also elevated in the medial prefrontal cortex of the same animals. No changes in expression of NR1 or NR2B subunits were observed at both mRNA and protein level. Altered NR2A subunit expression in the medial prefrontal cortex of rats reared in isolation suggests that NMDA receptor dysfunction may contribute to the underlying pathophysiology of this preclinical model of aspects of schizophrenia.

Effect of the selective dopamine D3 receptor antagonist SB-277011-A on regional c-Fos-like expression in rat forebrain.

SB-277011-A is a dopamine D(3) receptor antagonist that exhibits over 100-fold selectivity over dopamine D(2) receptors and a broad spectrum of other receptor, ion channels, and enzymes. We employed c-Fos immunohistochemistry to characterise the functional neuroanatomical effects of acute administration of SB-277011-A and observed a time-dependent increase in the density of c-Fos-like positive nuclei in rat forebrain with maximal effects observed 2 h post-dose. The relative influence of the different brain regions on the overall effect of SB-277011-A was ranked by partial least squares discriminant analysis loadings plot which indicated that sites within the nucleus accumbens exerted the greatest influence on the separation of the vehicle and SB-277011-A treatment groups. At the 2 h time-point, c-Fos-like expression was shown to be significantly elevated (p<0.05) in the core and shell of the nucleus accumbens, at both rostral and caudal levels, and in the lateral septum. No significant changes were detected in the caudate nucleus (lateral or medial) or in the cingulate, infralimbic prefrontal, or somatosensory cortices. The capacity of SB-277011-A to trigger immediate early gene expression in these limbic regions of rat brain adds to a growing consensus of the potential utility of dopamine D(3) receptor antagonism in psychiatric disorders including schizophrenia and drug dependency.

Pharmacological and immunohistochemical characterization of the APJ receptor and its endogenous ligand apelin.

Apelin peptides have recently been identified to be the endogenous ligands for the G protein-coupled receptor APJ. However, little is known about the physiological roles of this ligand-receptor pairing. In the present study we investigated the pharmacology of several apelin analogues at the human recombinant APJ receptor using radioligand binding and functional assays. This has led to the identification of key residues in the apelin peptide required for functional potency and binding affinity through structure-activity studies. In particular, we have identified that replacement of leucine in position 5, or arginine in position 2 and 4 of the C-terminal apelin peptide, apelin-13, resulted in significant changes in pharmacology. We also investigated the detailed localization of pre-proapelin and APJ receptor mRNA in a wide range of human, rat and mouse tissues using quantitative RT-PCR, and carried out a detailed immunohistochemical study of the distribution of the APJ receptor in rat brain and spinal cord. Interestingly, the APJ receptor was not only co-localized in white matter with GFAP in the spinal cord, but was also clearly localized on neurones in the brain, suggesting that this receptor and its peptide may be involved in a wide range of biological process yet to be determined.