The histaminergic system in human thalamus: correlation of innervation to receptor expression

The role of heterodimers formed by histamine H3 receptors and dopamine D1 receptors on the methamphetamine-induced conditioned place preference

RATIONALE

The rewarding effect of Methamphetamine (METH) is commonly believed to play an important role in METH use disorder. The altered expression of dopamine D1 receptor (D1R) has been suggested to be essential to the rewarding effect of METH. Notably, D1R could interact with histamine H3 receptors (H3R) by forming a H3R-D1R heteromer (H3R-D1R).

OBJECTIVES

This study was designed to specifically investigate the involvement of H3R-D1R in the rewarding effect of METH.

METHODS

C57BL/6 mice were treated with intraperitoneal injections of a selective H3R antagonist (Thioperamide, THIO; 20 mg/kg), an H1R antagonist (Pyrilamine, PYRI; 10 mg/kg), or microinjections of cytomegalovirus (CMV)-transmembrane domain 5 (TM5) into the nucleus accumbens (NAc). The animal model of Conditioned Place Preference (CPP) was applied to determine the impact of H3R-D1R on the rewarding effect of METH.

RESULTS

METH resulted in a significant preference for the drug-associated chamber, in conjunction with increased H3R and decreased D1R expression in both NAc and the ventral tegmental area (VTA). THIO significantly attenuated the rewarding effect of METH, accompanied by decreased H3R and increased D1R expression. In contrast, pyrilamine failed to produce the similar effects. Moreover, the inhibitory effect of THIO on METH-induced CPP was reversed by SKF38393, a D1R agonist. Furthermore, SCH23390, a D1R antagonist, counteracted the ameliorative effect of SKF38393 on THIO. Co-immunoprecipitation (CO-IP) experiments further demonstrated the specific interaction between H3R and D1R in METH CPP mice. The rewarding effect of METH was also significantly blocked by the interruption of CMV-transmembrane domain 5 (TM5), but not CMV-transmembrane domain 7 (TM7) in NAc.

CONCLUSION

These results suggest that modulating the activity of H3R-D1R complex holds promise for regulating METH use disorder and serves as a potential drug target for its treatment.

Mapping the primate thalamus: systematic approach to analyze the distribution of subcortical neuromodulatory afferents

Neuromodulatory afferents to thalamic nuclei are key for information transmission and thus play critical roles in sensory, motor, and limbic processes. Over the course of the last decades, diverse attempts have been made to map and describe subcortical neuromodulatory afferents to the primate thalamus, including axons using acetylcholine, serotonin, dopamine, noradrenaline, adrenaline, and histamine. Our group has been actively involved in this endeavor. The published descriptions on neuromodulatory afferents to the primate thalamus have been made in different laboratories and are not fully comparable due to methodological divergences (for example, fixation procedures, planes of cutting, techniques used to detect the afferents, different criteria for identification of thalamic nuclei…). Such variation affects the results obtained. Therefore, systematic methodological and analytical approaches are much needed. The present article proposes reproducible methodological and terminological frameworks for primate thalamic mapping. We suggest the use of standard stereotaxic planes to produce and present maps of the primate thalamus, as well as the use of the Anglo-American school terminology (vs. the German school terminology) for identification of thalamic nuclei. Finally, a public repository of the data collected under agreed-on frameworks would be a useful tool for looking up and comparing data on the structure and connections of primate thalamic nuclei. Important and agreed-on efforts are required to create, manage, and fund a unified and homogeneous resource of data on the primate thalamus. Likewise, a firm commitment of the institutions to preserve experimental brain material is much needed because neuroscience work with non-human primates is becoming increasingly rare, making earlier material still more valuable.

Structural connectivity of the human massa intermedia: A probabilistic tractography study

The role of massa intermedia (MI) is poorly understood in humans. Recent studies suggest its presence may play a role in normal human neurocognitive function while prior studies have shown the absence of MI correlated with psychiatric disorders. There is growing evidence that MI is likely a midline white matter conduit, responsible for interhemispheric connectivity, similar to other midline commissures. MI presence was identified in an unrelated sample using the Human Connectome Project database. MI structural connectivity maps were created and gray matter target regions were identified using probabilistic tractography of the whole brain. Probabilistic tractography revealed an extensive network of connections between MI and limbic, frontal and temporal lobes as well as insula and pericalcarine cortices. Women compared to men had stronger connectivity via their MI. The presented results support the role of MI as a midline commissure with strong connectivity to the amygdala, hippocampus, and entorhinal cortex.

Histamine receptors, agonists, and antagonists in health and disease

Histamine in the brain is produced by a group of tuberomamillary neurons in the posterior hypothalamus and a limited number of mast cells in different parts of the brain. Four G-protein-coupled receptors mediate the effects of histamine. Two of these receptors, H3 and H4 receptors, are high-affinity receptors in the brain and immune system, respectively. The two classic histamine receptors, H1 receptor and H2 receptor, are well known as drug targets for allergy and gastric ulcer, respectively. These receptors have lower affinity for histamine than the more recently discovered H3 and H4 receptors. The H1 and H2 receptors are important postsynaptic receptors in the brain, and they mediate many of the central effects of histamine on, e.g., alertness and wakefulness. H3 receptor is a pre- and postsynaptic receptor, which regulates release of histamine and several other neurotransmitters, including serotonin, GABA, and glutamate. H4 receptor is found in cerebral blood vessels and microglia, but its expression in neurons is not yet well established. Pitolisant, a H3 receptor antagonist, is used to treat narcolepsy and hypersomnia. H1 receptor antagonists have been used to treat insomnia, but its use requires precautions due to potential side effects. H2 receptor antagonists have shown efficacy in treatment of schizophrenia, but they are not in widespread clinical use. H4 receptor ligands may in the future be tested for neuroimmunological disorders and potentially neurodegenerative disorders in which inflammation plays a role, but clinical tests have not yet been initiated.

Histamine, histamine H3 receptor, and alcohol use disorder

Alcohol use disorder is associated with several mental, physical, and social problems. Its treatment is difficult and often requires a combination of pharmacological and behavioural therapy. The brain histaminergic system, one of the wake-active systems that controls whole-brain activity, operates through three neuronal GPCRs. The histamine H₃ receptor (Hrh3), which is expressed in many brain areas involved in alcohol drinking and alcohol reward, can be targeted with a number of drugs developed initially for cognitive disorders and/or disorders related to sleep, wakefulness, and alertness. In all rodent alcohol drinking models tested so far, H₃ receptor antagonists have reduced alcohol drinking and alcohol-induced place preference and cue-induced alcohol reinstatement. Several H₃ receptor antagonists tested and found to be safe for humans could be subjected to clinical tests to treat alcohol use disorder. Preference should be given to short-acting drugs to avoid the sleep problems associated with the wake-maintaining effects of the drugs. LINKED ARTICLES: This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc.

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein-coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.

Role of the Histamine H3 Receptor in the Central Nervous System

The G_i/o protein-coupled histamine H₃ receptor is distributed throughout the central nervous system including areas like cerebral cortex, hippocampus and striatum with the density being highest in the posterior hypothalamus, i.e. the area in which the histaminergic cell bodies are located. In contrast to the other histamine receptor subtypes (H₁, H₂ and H₄), the H₃ receptor is located presynaptically and shows a constitutive activity. In detail, H₃ receptors are involved in the inhibition of histamine release (presynaptic autoreceptor), impulse flow along the histaminergic neurones (somadendritic autoreceptor) and histamine synthesis. Moreover, they occur as inhibitory presynaptic heteroreceptors on serotoninergic, noradrenergic, dopaminergic, glutamatergic, GABAergic and perhaps cholinergic neurones. This review shows for four functions of the brain that the H₃ receptor represents a brake against the wake-promoting, anticonvulsant and anorectic effect of histamine (via postsynaptic H₁ receptors) and its procognitive activity (via postsynaptic H₁ and H₂ receptors). Indeed, H₁ agonists and H₃ inverse agonists elicit essentially the same effects, at least in rodents; these effects are opposite in direction to those elicited by brain-penetrating H₁ receptor antagonists in humans. Although the benefit for H₃ inverse agonists for the symptomatic treatment of dementias is inconclusive, several members of this group have shown a marked potential for the treatment of disorders associated with excessive daytime sleepiness. In March 2016, the European Commission granted a marketing authorisation for pitolisant (Wakix^R) (as the first representative of the H₃ inverse agonists) for the treatment of narcolepsy.

Mutations in the histamine N-methyltransferase gene, HNMT, are associated with nonsyndromic autosomal recessive intellectual disability

Histamine (HA) acts as a neurotransmitter in the brain, which participates in the regulation of many biological processes including inflammation, gastric acid secretion and neuromodulation. The enzyme histamine N-methyltransferase (HNMT) inactivates HA by transferring a methyl group from S-adenosyl-l-methionine to HA, and is the only well-known pathway for termination of neurotransmission actions of HA in mammalian central nervous system. We performed autozygosity mapping followed by targeted exome sequencing and identified two homozygous HNMT alterations, p.Gly60Asp and p.Leu208Pro, in patients affected with nonsyndromic autosomal recessive intellectual disability from two unrelated consanguineous families of Turkish and Kurdish ancestry, respectively. We verified the complete absence of a functional HNMT in patients using in vitro toxicology assay. Using mutant and wild-type DNA constructs as well as in silico protein modeling, we confirmed that p.Gly60Asp disrupts the enzymatic activity of the protein, and that p.Leu208Pro results in reduced protein stability, resulting in decreased HA inactivation. Our results highlight the importance of inclusion of HNMT for genetic testing of individuals presenting with intellectual disability.

The human histaminergic system in neuropsychiatric disorders

Histaminergic neurons are exclusively located in the hypothalamic tuberomamillary nucleus, from where they project to many brain areas. The histaminergic system is involved in basic physiological functions, such as the sleep-wake cycle, energy and endocrine homeostasis, sensory and motor functions, cognition, and attention, which are all severely affected in neuropsychiatric disorders. Here, we present recent postmortem findings on the alterations in this system in neuropsychiatric disorders, including Parkinson's disease (PD), Alzheimer's disease (AD), Huntington's disease (HD), depression, and narcolepsy. In addition, we highlight the need to validate animal models for these diseases and also for Tourette's syndrome (TS) in relation to alterations in the histaminergic system. Moreover, we discuss the potential for, and concerns over, the use of novel histamine 3 receptor (H3R) antagonists/inverse agonists as treatment for such disorders.

Intrinsic properties and neuropharmacology of midline paraventricular thalamic nucleus neurons

Neurons in the midline and intralaminar thalamic nuclei are components of an interconnected brainstem, limbic and prefrontal cortex neural network that is engaged during arousal, vigilance, motivated and addictive behaviors, and stress. To better understand the cellular mechanisms underlying these functions, here we review some of the recently characterized electrophysiological and neuropharmacological properties of neurons in the paraventricular thalamic nucleus (PVT), derived from whole cell patch clamp recordings in acute rat brain slice preparations. PVT neurons display firing patterns and ionic conductances (I_T and I_H) that exhibit significant diurnal change. Their resting membrane potential (RMP) is maintained by various ionic conductances that include inward rectifier (Kir), hyperpolarization-activated nonselective cation (HCN) and TWIK-related acid sensitive (TASK) K⁺ channels. Firing patterns are regulated by high voltage-activated (HVA) and low voltage-activated (LVA) Ca²⁺ conductances. Moreover, transient receptor potential (TRP)-like nonselective cation channels together with Ca²⁺- and Na⁺-activated K⁺ conductances (K_Ca; K_Na) contribute to unique slow afterhyperpolarizing potentials (sAHPs) that are generally not detectable in lateral thalamic or reticular thalamic nucleus neurons. The excitability of PVT neurons is also modulated by activation of neurotransmitter receptors associated with afferent pathways to PVT and other thalamic midline nuclei. We report on receptor-mediated actions of GABA, glutamate, monoamines and several neuropeptides: arginine vasopressin, gastrin-releasing peptide, thyrotropin releasing hormone and the orexins (hypocretins). This review represents an initial survey of intrinsic and transmitter-sensitive ionic conductances that are deemed to be unique to this population of midline thalamic neurons, information that is fundamental to an appreciation of the role these thalamic neurons may play in normal central nervous system (CNS) physiology and in CNS disorders that involve the dorsomedial thalamus.

Glutamatergic neurons induce expression of functional glutamatergic synapses in primary myotubes

Background

The functioning of the nervous system depends upon the specificity of its synaptic contacts. The mechanisms triggering the expression of the appropriate receptors on postsynaptic membrane and the role of the presynaptic partner in the differentiation of postsynaptic structures are little known.

Methods and Findings

To address these questions we cocultured murine primary muscle cells with several glutamatergic neurons, either cortical, cerebellar or hippocampal. Immunofluorescence and electrophysiology analyses revealed that functional excitatory synaptic contacts were formed between glutamatergic neurons and muscle cells. Moreover, immunoprecipitation and immunofluorescence experiments showed that typical anchoring proteins of central excitatory synapses coimmunoprecipitate and colocalize with rapsyn, the acetylcholine receptor anchoring protein at the neuromuscular junction.

Conclusions

These results support an important role of the presynaptic partner in the induction and differentiation of the postsynaptic structures.

Histamine in neurotransmission and brain diseases

Apart from its central role in the mediation of allergic reactions, gastric acid secretion and inflammation in the periphery, histamine serves an important function as a neurotransitter in the central nervous system. The histaminergic neurons originate from the tuberomamillary nucleus of the posterior hypothalamus and send projections to most parts of the brain. The central histamine system is involved in many brain functions such as arousal, control of pituitary hormone secretion, suppression ofeating and cognitive functions. The effects of neuronal histamine are mediated via G-protein-coupled H1-H4 receptors. The prominent role of histamine as a wake-promoting substance has drawn interest to treat sleep-wake disorders, especially narcolepsy, via modulation of H3 receptor function. Post mortem studies have revealed alterations in histaminergic system in neurological and psychiatric diseases. Brain histamine levels are decreased in Alzheimer's disease patients whereas abnormally high histamine concentrations are found in the brains of Parkinson's disease and schizophrenic patients. Low histamine levels are associated with convulsions and seizures. The release of histamine is altered in response to different types of brain injury: e.g. increased release of histamine in an ischemic brain trauma might have a role in the recovery from neuronal damage. Neuronal histamine is also involved in the pain perception. Drugs that increase brain and spinal histamine concentrations have antinociceptive properties. Histaminergic drugs, most importantly histamine H3 receptors ligands, have shown efficacy in many animal models of the above-mentioned disorders. Ongoing clinical trials will reveal the efficacy and safety of these drugs in the treatment of human patients.

Altered histamine H3 receptor radioligand binding in post-mortem brain samples from subjects with psychiatric diseases

BACKGROUND AND PURPOSE

Histamine is a modulatory neurotransmitter in the brain. Auto- and hetero-histamine H3 receptors are present in human brain and are potential targets of antipsychotics. These receptors may also display disease-related abnormalities in psychiatric disorders. Here we have assessed how histamine H3 receptors in human brain may be affected in schizophrenia, bipolar disorder, major depression.

EXPERIMENTAL APPROACH

Histamine H3 receptor radioligand binding assays were applied to frozen post-mortem prefrontal and temporal cortical sections and anterior hippocampal sections from subjects with schizophrenia, bipolar disorder, major depression and matched controls.

KEY RESULTS

Compared with the controls, increased H3 receptor radioligand binding was found in dorsolateral prefrontal cortex of schizophrenic subjects (especially the ones who were treated with atypical antipsychotics), and bipolar subjects with psychotic symptoms. No differences in H3 receptor radioligand binding were found in the temporal cortex. In hippocampal formation of control subjects, H3 receptor radioligand binding was prominent in dentate gyrus, subiculum, entorhinal cortex and parasubiculum. Decreased H3 binding was found in the CA4 area of bipolar subjects. Decreased H3 binding in CA2 and presubiculum of medication-free bipolar subjects was also seen.

CONCLUSIONS AND IMPLICATIONS

The results suggest that histamine H3 receptors in the prefrontal cortex take part in the modulation of cognition, which is impaired in schizophrenic subjects and bipolar subjects with psychotic symptoms. Histamine H3 receptors probably regulate connections between hippocampus and various cortical and subcortical regions and could also be involved in the neuropathology of schizophrenia and bipolar disorder.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.

Implications of activity-dependent neurotransmitter-receptor matching

Electrical activity has numerous roles in early neuronal development. Calcium transients generated at low frequencies regulate neural induction and neuronal proliferation, migration and differentiation. Recent work demonstrates that these signals participate in specification of the transmitters expressed in different classes of neurons. Matching of postsynaptic receptor expression with the novel expression of transmitters ensues. These findings have intriguing implications for development, mature function and evolution of the nervous system.

Histamine and Schizophrenia

With the availability of an increased number of experimental tools, for example potent and brain-penetrating H1-, H2-, and H3-receptor ligands and mutant mice lacking the histamine synthesis enzyme or the histamine receptors, the functional roles of histaminergic neurons in the brain have been considerably clarified during the recent years, particularly their major role in the control of arousal, cognition, and energy balance. Various approaches tend to establish the implication of histaminergic neurons in schizophrenia. A strong hyperactivity of histamine neurons is induced in rodent brain by administration of methamphetamine or NMDA-receptor antagonists. Histamine neuron activity is modulated by typical and atypical neuroleptics. H3-receptor antagonists/inverse agonists display antipsychotic-like properties in animal models of the disease. Because of the limited predictability value of most animal models and the paucity of drugs affecting histaminergic transmission that were tried so far in human, the evidence remains therefore largely indirect, but supports a role of histamine neurons in schizophrenia.

Pre-synaptic histamine H3 receptors regulate glutamate, but not GABA release in rat thalamus

We have investigated the presence of histamine H(3) receptors (H(3)Rs) on rat thalamic isolated nerve terminals (synaptosomes) and the effect of their activation on glutamate and GABA release. N-alpha-[methyl-(3)H]histamine ([(3)H]-NMHA) bound specifically to synaptosomal membranes with dissociation constant (K(d)) 0.78+/-0.20 nM and maximum binding (B(max)) 141+/-12fmol/mg protein. Inhibition of [(3)H]-NMHA binding by histamine and the H(3)R agonist immepip fit better to a two-site model, whereas for the H(3)R antagonist clobenpropit the best fit was to the one-site model. GTPgammaS (30 microM) decreased [(3)H]-NMHA binding by 55+/-4% and made the histamine inhibition fit better to the one-site model. Immepip (30 nM) induced a modest, but significant increase (113+/-2% of basal) in [(35)S]-GTPgammaS binding to synaptosomal membranes, an effect prevented by clobenpropit (1 microM) and by pre-treatment with pertussis toxin. In thalamus synaptosomes depolarisation-induced, Ca(2+)-dependent glutamate release was inhibited by histamine (1 microM, 25+/-4% inhibition) and immepip (30 nM, 38+/-5% reduction). These effects were reversed by clobenpropit (1microM). Conversely, immepip (up to 1 microM) had no effect on depolarisation-evoked [(3)H]-GABA release. Extracellular synaptic responses were recorded in the thalamus ventrobasal complex by stimulating corticothalamic afferents. H(3)R activation reduced by 38+/-7% the glutamate receptor-mediated field potentials (FPs), but increased the FP2/FP1 ratio (from 0.86+/-0.03 to 1.38+/-0.05) in a paired-pulse paradigm. Taken together, our results confirm the presence of H(3)Rs on thalamic nerve terminals and show that their activation modulates pre-synaptically glutamatergic, but not GABAergic neurotransmission.

Histamine H(1) and H(3) receptors in the rat thalamus and their modulation after systemic kainic acid administration

In rat thalamus, histamine H(1) receptor and isoforms of H(3) receptor were expressed predominantly in the midline and intralaminar areas. Correspondingly, higher H(1) and H(3) receptor binding was also detected in these areas. All isoforms of H(3) receptor were expressed in several thalamic nuclei, but there were minor differences between their expression patterns. H(1) mRNA expression was high in the ventral thalamus, but the H(1) binding level was low in these areas. Since increased brain histamine appears to have an antiepileptic effect through the H(1) receptor activity, kainic acid (KA)-induced status epilepticus in rat was used to study modulation of H(1) and H(3) receptors in the thalamus following seizures. After systemic KA administration, transient decreases in mRNA expression of H(1) receptor and H(3) receptor isoforms with full-length third intracellular loops were seen in the midline areas and the H(1) receptor mRNA expression also decreased in the ventral thalamus. After 1 week, a robust increase in mRNA expression of H(3) receptor isoforms with a full-length third intracellular loop was found in the ventral posterior, posterior, and geniculate nuclei. The changes indicate a modulatory role of H(3) receptor in the sensory and motor relays, and might be involved in possible neuroprotective and compensatory mechanisms after KA administration. However, short-term increases in the H(3) receptor binding appeared earlier (72 h) than the increases of H(3) mRNA expression (1-4 w). The elevations in H(3) binding were evident in the intralaminar area, laterodorsal, lateral posterior, posterior and geniculate nuclei, and were likely to be related to the cortical and subcortical inputs to thalamus.

Specific circular organization of the neurons of human interthalamic adhesion and of periventricular thalamic region

Interthalamic adhesion between the medial surfaces of the left and right thalamus is a variable structure and contains the midline thalamic nuclei, which are not much developed in humans. The research has been done on 6 human brains obtained during routine autopsy (age 45 to 65; 4 male and 2 female). Every tenth 10 microm thick frontal section was stained according to Klüver-Barrera method. In all cases the authors found a specific organization of certain groups of neurons within the interthalamic adhesion (IA) in form of circles on frontal sections. These circular groups were present on all sections but only 1-2 in each. The larger mean diameter of these circular arrangements was R = 229.4 microm, and smaller was r = 203.1 microm. These circular groups within the human IA were formed in average by 7.29 neurons. In periventricular region (PVR) of thalamus similar circular groups of neurons also were present in all cases as in IA. These neuronal groups in PVR were of smaller size than in the IA, with larger mean diameter R = 201.4, smaller mean diameter r = 181.2 microm and they contained fewer neurons, 6.69 on average. All three values (both diameters of circular arrangements, and number of neurons forming them) were significantly smaller in PVR (p < .01). Morphological types and sizes of neurons in both investigated structures (IA and PV) were not different. The circular neuronal groups in IA were formed in 61% of fusiform neurons and in PVR in 48% of fusiform neurons. According to their subependymal localization, size and form, these circular groups can represent in vivo correlates of neurospheres.

Functional effects of single dose first- and second-generation antipsychotic administration in subjects with schizophrenia

Using PET with (15)O water, we characterized the time course of functional brain changes following the acute administration of a first- and a second-generation antipsychotic. Volunteers with schizophrenia were scanned while drug-free (baseline) and after single dose administration of haloperidol (n=6) or olanzapine (n=6) during a time course adapted to their plasma kinetics. To obtain brain location information, we contrasted each post-drug scan to baseline-acquired scans. We plotted the regional cerebral blood flow (rCBF) extracted in these locations and calculated the kinetic characteristics of the curves. Further, we compared and contrasted the rCBF changes induced by the drugs over the first 4 h post-drug administration. Dorsal and ventral striatum, thalamus and anterior cingulate cortex were activated with haloperidol, while frontal, temporal and cerebellum regions evidenced reduced flow. With olanzapine, ventral striatum, anterior cingulate and temporal cortices evidenced increases, and thalamus and lingual cortex decreases, in rCBF. Both drugs activated the caudate nucleus. Haloperidol induced greater activation of the dorsal striatum than did olanzapine. These data reveal important differences in patterns of brain activation between the drugs. Differences in the involvement in basal ganglia parallel known differences between the drugs in the emergence of acute EPS upon emergency administration.

The laminar histamine receptor system in human prefrontal cortex suggests multiple levels of histaminergic regulation

Human prefrontal cortex is essential for high brain functions and its activity is modulated by multiple neurotransmitters, including histamine. However, the histamine receptors in this brain area have not been systematically studied so far. In situ hybridization and receptor binding autoradiography were employed to map and quantify the mRNA expression and receptor binding of three of the four histamine receptors (H(1), H(2), H(3)). mRNA expression and receptor binding of these three histamine receptors displayed characteristic laminar distribution patterns. Both H(1) and H(3) receptor mRNAs were mainly expressed in the deeper layers (H(1) in laminae V and VI; H(3) in lamina V), where most of the corticothalamic projections originate, whereas H(2) receptor mRNA was primarily expressed in the superficial layer II. Receptor ligand binding of these three histamine receptors displayed relatively even distribution patterns throughout the gray matter. However, higher densities of H(1) and H(3) receptor radioligand binding sites were seen in the middle layers III and IV that receive abundant thalamic inputs and where some of the apical dendrites of the deep-layer pyramidal neurons terminate, whereas higher density of H(2) receptor radioligand binding sites was seen in the superficial layers I-III. The results, together with data on histaminergic regulation of thalamic oscillations suggest that histamine regulates both cortico-cortical and thalamo-cortical circuits. As histamine receptors are also abundant in thalamus, histamine may be involved also in human diseases of the thalamocortical system.