PET scan studies of histamine receptors in the human brain

S.16 Histamine

in the bmin: focus on dementia and schizophrenia

this preparation, because HA neurons and their ascending projections are intact, while their descending projections and the ascending projections from the lower brainstem are largely (if not totally) interrupted, these data thus strongly support the importance of HArgic ascending projections in cortical arousal by demonstrating their ability to ensure cortical activation in the case of a deficit of the more caudal brainstem ascending activating systems. Among the widespread ascending outputs of HA cells, those projecting massively to the cholinergic basal forebrain could be of great importance since we have identified Ht-receptor mRNA in a large number of cholinergic neurons in this region (unpublished data). Furthermore, it has been shown that HA induces direct excitation of these corticalpetal cholinergic cells and increases significantly their discharge rate5. With respect to the descending projections of HA neurons and their role in cortical activation, on the other hand, we have found that these neurons send out also heavy inputs to the brainstem. Particularly, very dense axons are seen to reach the mesopontine tegmentum, which contains cholinergic neurons playing a key role in tonic cortical activation by their diencephalic projections2. Our double immunohistochemical studies further show that in this structure, HA axons form dense networks of very fine and vericose fibers and terminal-like dots in close proximity to a large number of cholinergic neurons and seem to make contact with them. Moreover, we have observed, by in situ hybridization coupled with immunocytochemistry of choline acethyhransferase in the guinea-pig, a strong expression of HI-receptor mRNA within cholinergic neurons in the mesopontine tegmentum (unpublished data). All these anatomical data suggest that the activity of mesopontine cholinergic neurons may be under a descending control of HA neurons. To verify this hypothesis and to explore the functional role of these HArgic descending inputs, we have further performed in situ pharmacological studies coupled with polygraphic recordings and spectral analysis of the cortical EEG in the cat. It is in the cholinergic mesopontine tegmentum that application of HA or a HI-receptor agonist by microdialysis disrupts cortical spindles and slow waves and enhances cortical fast rhythms, resulting in a long-lasting quiet waking state. The effects of HA are attenuated by systemic or in situ pretreatment with mepyramine, which, when injected alone, produces an increase in SWS’. Since in the mesopontine tegmentum, presumed cholinergic ascending neurons discharge tonically during cortical activation of W2, and since HA causes excitation of mesopontine cholinetgic neurons via HI-recepto&, we suggest that the HArgic descending afferents in the mesopontine tegmentum could promote cortical activation and W via, at least partially, activation of Ht -receptors situated on cholinergic neurons’. Taking together, these results indicate that 1) HA neurons constitute one of the major excitatory sources for cortical activation during waking and thus play an important role in brain arousal; that 2) the mechanisms involved include both their ascending and descending projections and implicate their interactions with other brain activating systems, particularly, choline& neurons and that 3) the close interactions between HArgic and cholinergic systems and the increase in cortical fast rhythms seen with ciproxifan suggest, in addition, a cognitive function for HArgic neurons. Refemnces [I] Lin et al, 1 Nemxci. 16, 1523-1537,1996 [Z] Sakai et al, In: The Diphencephalon & Sleep (Man& & Marini eds), p171198. Raven Press, 1990 [3] Haas, In: The histamine receptors (Schwartz & Haas eds), ~161-178, WileyLiss, 1992 [4] Ligneau et al, JPET, 287,658-666, 1998 [5] Khateb et al, Meurosci. 69, 495-506, 1995

@YKGl

iioz;J;istamine

in attention

and learning

J.L. Muir. School of Psychology. Card@ University, Wales, UK The role of the basal forebrain cholinergic system in cognitive and attentional functions has been the subject of considerable investigation

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following the suggestion that degeneration of this system may be responsible for many of the cognitive deficits associated with neurodegenerative diseases such as Alzheimer’s disease. Animal studies using a variety of behavioural tasks have investigated the role of the basal forebrain (BF) cholinergic system in cognitive and attentional function. Such studies (lesion and pharmacological) have also been conducted with a mind to investigating the feasibility of chohnergic replacement strategies. However, the effectiveness of such treatment for AD patients is limited by the fact that their cholinergic system is substantially compromised, thus reducing the availability of cholinergic neurons upon which to target such treatments. More recently, interest has developed in the potential of a novel therapeutic strategy for the treatment of cognitive dysfunction in the elderly and in Alzheimer’s disease patients, involving the use of histamine compounds. Unlike cholinergic neurons which are severely damaged in these patients, histaminergic neurons may be relatively spared and thus provide a means of cognitive enhancement via facilitation of histamine release from relatively intact neurons. Previous studies performed in mice using first generation H3-receptor antagonists, such as thioperamide, suggested enhancement of learning ability, e.g. in the Morris water maze. We have now assessed the H3-antagonist strategy in rata using Ciproxifan, a compound enhancing histamine release in brain with increased potency and have conducted initial studies using a second compound, which may be even more potent than Ciproxifan. The aim of the present series of studies was therefore to investigate the effect of administration of these novel H3-receptor antagonists on several behavioural paradigms used previously to assess the role of the cholinergic system or the effects of normal ageing on cognitive function. In these studies, impairments in the ability of rats to detect brief visual targets following cholinergic lesions were significantly improved following administration of the histaminergic compounds. In contrast, such compounds were ineffective in improving the short term memory deficit observed in normal animals on the delayed nomnatching-to-position task and had a limited effect in improving spatial learning in aged animals in the Morris water maze. Therefore, the results at the present time suggest that these histaminergic antagonists may have a role in enhancing attentional function.

-1

PET scan studies of histamlne human brain

receptors

in the

K. Yanai, T. Watanabe, M. Itoh’. Tohoku University School of Medicine, Department of Pharmacology, Sendai 980-85 75; I Cyclotron and Radioisotope Center, Tohoku University, Sendai 980-8578, Japan Histaminergic neurons are demonstrated in the rat brain. The cell bodies are located at posterior hypothalamus, and are shown to be involved in various physiological functions through H 1, H2, and H3 receptors. A role for brain histamine has long been thought to exist in arousal, sleep-wake cycle, appetite control, seizures, learning and memory, aggressive behaviors and emotion. These data were mainly obtained in rodents through classical pharmacological experiments and, recently, by the study of knockout mice. Positron emission tomography (PET) is a unique method to examine brain chemistry non-invasively in humans. We have been developing and utilizing the PET techniques to understand the functions of histaminergic neuronal system in the human brain. For histamine receptors, several radiotracers for selective labeling of Hl, H2, and H3 receptors have been synthesized. We have been utilizing two carbon11 labeled Hl-receptor ligands, [“C]pyrilamine and [“Cldoxepin for human PET studies (1). In this paper, we mainly describe the human PET studies of [“Cldoxepin. Three topics of our studies on Hl receptor imaging are as follows: the effects of normal and abnormal aging on Hi receptors, epilepsy and HI receptors, and Hl receptor occupancy by sedating and non-sedating antihistaminics. In control young subjects, a high density of Hl-receptors was observed in the cingulate gyms, frontal cortex, temporal cortex, and hippocampus. On the contrary, the cerebellum and pons have few Hl receptors. The Hl receptor binding in the cortex was significantly decreased with age (2). The frontal and temporal cortex showed age-dependent decrease in binding of approximately 12 percent per decade. Statistical parametric

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S. I6 Histamine in the brain: focus on dementia and schizophrenia

mapping (SPM) analysis is often used to visualize the differences in PET imaging. The kinetic parameter, k3, was obtained on a pixel by pixel analysis by a 3-compartment model analysis. Data were then spatially normalized to the Talairach atlas and smoothed. A significant decrease in Hl receptor binding with normal aging was similarly found in the fronto-temporal cortex, cingulate gyrus, and hippocampus. We also examined the brains of Alzheimer disease using [“C]doxepin. A significant decrease in Hi receptor binding was observed in the cortex of Alzheimer’s disease when compared to age- and sex-matched controls. We previously reported that histamine is an endogenous anticonvulsant. When the brain level of histamine is high, the duration of convulsion is shortened. In accordance with this finding, Hl receptor binding was increased in the focus of complex partial seizure. By contrast, glucose metabolism was decreased in these loci (3). Glucose metabolism was inversely correlated with the binding of [ 1’Cldoxepin in the foci. Our tiding of increased histamine Hl receptor binding in the focus of epilepsy is consistent with the data on the mu opiod receptor obtained with [“C]carfentanil. The third topic is the penetration of antihistamine through the bloodbrain barrier. It is well known that many first-generation antihistamines cause drowsiness and impaired performance. Several second-generation antihistamines were developed as drugs with few side effects of sedation. Terfenadine and epinastine are often used in the treatment of allergies as the second-generation antihistamines. After taking 2 mg dchlorpheniramine, a first-generation antihistamine, the specific binding to Hl-receptors in the brain was markedly decreased. On the other hand, the second-generation antihistamines such as terfenadine and epinastine did not have any significant effects on the distribution of Hl receptor binding (4). The PET images of Hl receptors obtained after taking non-sedating antihistamines are almost identical to those of controls. This result clearly demonstrated that the second-generation antihistamines do not penetrate the blood-brain barrier. We also examined the relationship between the CNS effects and HI receptor occupancy in humans (5). An attentiondemanding reaction time task was given to normal young volunteers before and after intravenous administration of d-chlorpheniramine, dChlorpheniramine could not cause any significant increase of reaction time at a dose of 1 mg. However, the intravenous administartion of the 2 mg prolonged the reaction time. In this dose (2 mg), the Hl receptor occupancy was approximately over 70%. The administration of 1 mg of d-chlorpheniramine occupied 56% of total Hl receptors. These data indicate that at least 60% of HI receptors had to be blocked for subjects to impair their performance. This paper demonstrated that PET imaging using [“Cldoxepin is useful for measuring the distribution of Hl receptors in human brain and their alterations in diseases. Other useful applications of this PET method are for direct evaluation of the sedative characteristics of antihistamines, antidepressants and neuroleptics. References [l] Yanai, K., Watanabe, T., Hatazawa, J., Iwata, R., Ishiwata, K., Yokoyama, H., Megufo K., Itoh M., Takahashi, T., Ido, T. Matswawa, T., 1992. Mapping of histamine Ht receptors in human brain by positron emission tomography with [“C]pyrilamine. J. Newochem. 59, 128-136. [2] Yanai, K., Watanabe, T., Megwo, K., Yokoyama, H., Sate, I., Sasano, K., Itoh, M., Iwata, R., Takahashi, T., Ido, T., 1992. Age-dependent-decrease in Histamine Hl receptor in human brains revealed by PET. NeuroReport 3,433436. [3] Iinoma, K., Yokoyama, H., Otsuki, T., Yanai, K., Watanabe, T., Ido, T., Itoh, M., 1993. Histamine HI receptors in complex partial seizure. Lancet 341, 238 [4] K. Yanai, J. H. Ryu, T. Watanabe, R. Iwata, T. Ido, Y. Sawai, K. Ito, and M. Itoh. Histamine HI receptor occupancy in human brains at?er single oral doses of histamine Hl antagonists measured by positron emission tomography. Br. J. Pharmacol. 116: 1649-1655 (1995) [5] Yanai, K., Okamwa, N., Tagawa, M., Itoh, M., Watanabe, T., 1999. New findings in pharmacological effects induced by antihistamines: From PET studies to knockout mice. Clinical and Experimental Allergy, 1999 Supplement 2 in press

Is.16.051

Potential role of histamine in schizophrenia

J.-M. Arrang’ , S. Morisset’ , C. Ito’, J. Tardivel-Lacombe’, M.-O. Krebs2, J.-P Olie2, H. Loo2, M.-F. PoirieZ, E. J6nsson3, G. Sedval13, M.-A. Crocq4, P Sokolod , J.-C. Schwartz’. ‘INSEM, UlO9, Paris; ‘Service Hospitalo-Universitaire de Sant4 Mentale et Thlmpeutique, C.H. Sainte-Anne, Paris; 4FORENM Centre Hospitaliec Rouffach, Fmnce 3Department of Psychiatty and Psychology, Karolinska Hospital, Stockholm, Sweden The relationships between histamine and neuropsychiatric diseases are not well understood (1) but several recent pieces of evidence suggest a role of histaminergic neurons, and/or a modification of their activity, in schizophrenia. We have first evaluated the changes in histamine neuton activity induced in mouse brain by administration of various ‘typical’ and ‘atypical’ neuroleptics by measuring the levels of te&methylhistamine (t-MeHA), a major histamine metabolite in brain. “Qpical’ antipsychotics such as haloperidol or sulpiride, decreased by 20% t-MeHA levels in brain, revealing a tonic activation of histaminergic neurons by endogenous dopamine. This enhancing effect of dopamine was also observed after administration of metamphetamine that induced 3 h later, a strong increase (by 100%) in cortical and striatal t-MeHA levels. This increase was completely blocked by haloperidol, a D2/D3-receptor antagonist, but was not modified after administration of nafadotride used at a dose inducing a selective blockade of D3 receptors. Moreover, the enhancing effect of metamphetamine remained unchanged in the brain of mice lacking functional D3 receptors. It is therefore concluded that the stimulation by endogenous dopamine of histaminergic neurons occurs via selective activation of D2 receptors. The role attributed to histaminergic neurons in wakefulness and cognition suggests that some sedative effects of ‘typical’ neuroleptics may result from this interaction between dopamine and histamine. In contrast to ‘typical’ neuroleptics, various ‘atypical’ antipsychotics such as clozapine or olanzapine, dramatically increase histamine neuron activity. This effect which is observed at low dosage, is comparable to the effect of the most potent H3-receptor antagonists but is due to 5-HTZA-receptor blockade, being reproduced by ketanserin, a ~-HT~Areceptor antagonist and reversed by DOI, a 5-HTzA-receptor agonist. This effect of ‘atypical’ neuroleptics on histaminergic neurons does not seem to account for their reduced extrapyramidal side-effects since H3receptor ligands did not modify haloperidol-induced catalepsy. However, the pro-cognitive properties of histaminergic neurons suggest that it may underlie the significant efficacy of these compounds against the negative symptomatology of schizophrenia (2). Recently, several polymorphisms were detected in a british population in the coding region of the HZ-receptor gene (3), among which a A649G transition leading to one amino acid substitution in the third intracellular loop of the receptor, was associated with schizophrenia (4). However, by direct sequencing and restriction analysis of DNA from 53 Swedish controls, 52 French controls and 88 French schizophrenic patients, we could not confirm the existence of these polymorphisms. Moreover, a G543A transition that was not reported in the british study, was detected in the coding region of the gene but did not change the ammo acid sequence of the receptor. We also found in the promoter region two additional polymorphisms that may regulate the expression of the H2receptor gene. The first one was a A-592G transition located in the minimal promoter and the second one, a G-1018A transition located in an enhancer element. Nevertheless, in 88 schizophrenic subjects, the incidence of the three polymorphisms was not significantly different from controls. These data may suggest an unexpected variability of the HZ-receptor gene polymorphisms in different geographical areas but do not support an association of these allelic variations with schizophrenia (5). By applying a similar methodology, a conservative transition was found in the coding region of the Hl -receptor gene. After DNA analysis of 138 controls, it was shown to occur with a very low incidence (l2%) with no homozygotes. A metaanalysis from two french independent