Background and purpose: Histamine H3 receptor antagonists are currently being evaluated for their potential use in a number of central nervous system disorders including Alzheimer's Disease (AD). To date, little is known about the state of H3 receptors in AD. Experimental approach: In the present study we used the radiolabelled H3 receptor antagonist [ 3 H]GSK189254 to investigate H3 receptor binding in the amyloid over-expressing double mutant APPswe ¥ PSI.MI46V (TASTPM) transgenic mouse model of AD and in post-mortem human AD brain samples. Key results: No significant differences in specific H3 receptor binding were observed between wild type and TASTPM mice in the cortex, hippocampus or hypothalamus. Specific [ 3 H]GSK189254 binding was detected in sections of human medial frontal cortex from AD brains of varying disease severity (Braak stages I-VI). With more quantitative analysis in a larger cohort, we observed that H3 receptor densities were not significantly different between AD and age-matched control brains in both frontal and temporal cortical regions. However, within the AD group, [ 3 H]GSK189254 binding density in frontal cortex was higher in individuals with more severe dementia prior to death. Conclusions and implications:The maintenance of H3 receptor integrity observed in the various stages of AD in this study is important, given the potential use of H3 antagonists as a novel therapeutic approach for the symptomatic treatment of AD.

Background and purpose: Histamine H3 receptor antagonists are currently being evaluated in clinical trials for a number of central nervous system disorders including narcolepsy. These agents can increase wakefulness (W) in cats and rodents following acute administration, but their effects after repeat dosing have not been reported previously. Experimental approach: EEG and EMG recordings were used to investigate the effects of acute and repeat administration of the novel H3 antagonist GSK189254 on the sleep-wake cycle in wild-type (Ox+/+) and orexin knockout (Ox-/-) mice, the latter being genetically susceptible to narcoleptic episodes. In addition, we investigated H3 and H1 receptor expression in this model using radioligand binding and autoradiography. Key results: In Ox+/+ and Ox-/-mice, acute administration of GSK189254 (3 and 10 mg·kg -1 p.o.) increased W and decreased slow wave and paradoxical sleep to a similar degree to modafinil (64 mg·kg -1 ), while it reduced narcoleptic episodes in Ox-/-mice. After twice daily dosing for 8 days, the effect of GSK189254 (10 mg·kg -1 ) on W in both Ox+/+ and Ox-/-mice was significantly reduced, while the effect on narcoleptic episodes in Ox-/-mice was significantly increased. Binding studies revealed no significant differences in H3 or H1 receptor expression between Ox+/+ and Ox-/-mice. Conclusions and implications:These studies provide further evidence to support the potential use of H3 antagonists in the treatment of narcolepsy and excessive daytime sleepiness. Moreover, the differential effects observed on W and narcoleptic episodes following repeat dosing could have important implications in clinical studies.

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Using a tissue bath system which allowed the simultaneous measurement of electrically-induced [3H]noradrenaline release and neurogenic contraction under identical conditions, we investigated the prejunctional inhibitory activity of the selective 5-HT(1D/1B) receptor agonists BRL 56905 ((+/-)-3-amino-6-carboxamido-1,2,3,4-tetrahydrocarbazole) and SKF 99101H (3-(2-dimethylaminoethyl)-4-chloro-5-propoxyindole hemifumarate), compared to sumatriptan and 5-HT. Transmural electrical stimulation (2 Hz) of dog saphenous vein induced consistent increases in [3H]noradrenaline release as well as reproducible contractile responses (<10% decrease over four stimulation periods). BRL 56905, SKF 99101H, sumatriptan and 5-HT (60 nM-6 microM) inhibited electrically-evoked [3H]noradrenaline release and neurogenic contractile responses in dog saphenous vein. However, despite being measured under identical conditions, the inhibition of [3H]noradrenaline release was consistently greater than the inhibition of neurogenic contraction induced by a particular concentration of agonist, suggesting that neurogenic contractile responses in dog saphenous vein result from the combined release of noradrenaline and other non-noradrenergic neurotransmitters. Under the present assay conditions, since the agonists produced only small (BRL 56905, sumatriptan and 5-HT) or marginal (SKF 99101H) contractile responses, it is unlikely that this is the cause of the discrepancy observed between inhibition of release and inhibition of contraction. The inhibitory effects of BRL 56905, sumatriptan and 5-HT were blocked by the 5-HT(1D/1B) receptor antagonist methiothepin, consistent with the involvement of canine ca-5-HT(1D/1B) receptors in inhibiting neurotransmitter release and subsequent smooth muscle contraction in dog saphenous vein. The present results show that the novel 5-HT(1D/1B) receptor agonists BRL 56905 and SKF 99101H are at least as potent as sumatriptan and 5-HT, at activating prejunctional inhibitory ca-5-HT(1D/1B) heteroreceptors on sympathetic axon terminals in dog saphenous vein. In addition, when measured simultaneously in the same tissue preparation, [3H]noradrenaline release was inhibited to a much greater extent than neurogenic contraction by any particular agonist.