The tuberomammillary nucleus projections in the control of learning, memory and reinforcement processes: evidence for an inhibitory role

Targeting Histamine and Histamine Receptors for Memory Regulation: An Emotional Perspective

Histamine has long been accepted as a pro-cognitive agent. However, lines of evidence have suggested that the roles of histamine in learning and memory processes are much more complex than previously thought. When explained by the spatial perspectives, there are many contradictory results. However, using emotional memory perspectives, we suspect that the histaminergic system may interplay with stress, reward inhibition, and attention to modulate emotional memory formation. The functional diversity of histamine makes it a viable target for clinical management of neuropsychiatric disorders. Here, we update the current knowledge about the functions of histamine in emotional memory and summarize the underlying molecular and neural circuit mechanisms. Finally, we review the main clinical studies about the impacts of histamine-related compounds on memory and discuss insights into future research on the roles of histamine in emotional memory. Despite the recent progress in histamine research, the histaminergic emotional memory circuits are poorly understood, and it is also worth verifying the functions of histamine receptors in a more spatiotemporally specific manner.

Copper as a Collaborative Partner of Zinc-Induced Neurotoxicity in the Pathogenesis of Vascular Dementia

Copper is an essential trace element and possesses critical roles in various brain functions. A considerable amount of copper accumulates in the synapse and is secreted in neuronal firings in a manner similar to zinc. Synaptic copper and zinc modulate neuronal transmission and contribute to information processing. It has been established that excess zinc secreted during transient global ischemia plays central roles in ischemia-induced neuronal death and the pathogenesis of vascular dementia. We found that a low concentration of copper exacerbates zinc-induced neurotoxicity, and we have demonstrated the involvement of the endoplasmic reticulum (ER) stress pathway, the stress-activated protein kinases/c-Jun amino-terminal kinases (SAPK/JNK) signaling pathway, and copper-induced reactive oxygen species (ROS) production. On the basis of our results and other studies, we discuss the collaborative roles of copper in zinc-induced neurotoxicity in the synapse and the contribution of copper to the pathogenesis of vascular dementia.

The Lateral Hypothalamus: An Uncharted Territory for Processing Peripheral Neurogenic Inflammation

The roles of the hypothalamus and particularly the lateral hypothalamus (LH) in the regulation of inflammation and pain have been widely studied. The LH consists of a parasympathetic area that has connections with all the major parts of the brain. It controls the autonomic nervous system (ANS), regulates feeding behavior and wakeful cycles, and is a part of the reward system. In addition, it contains different types of neurons, most importantly the orexin neurons. These neurons, though few in number, perform critical functions such as inhibiting pain transmission and interfering with the reward system, feeding behavior and the hypothalamic pituitary axis (HPA). Recent evidence has identified a new role for orexin neurons in the modulation of pain transmission associated with several inflammatory diseases, including rheumatoid arthritis and ulcerative colitis. Here, we review recent findings on the various physiological functions of the LH with special emphasis on the orexin/receptor system and its role in mediating inflammatory pain.

Brain anatomy of the 4-day-old European rabbit

The European rabbit (Oryctolagus cuniculus) is a widely used model in fundamental, medical and veterinary neurosciences. Besides investigations in adults, rabbit pups are relevant to study perinatal neurodevelopment and early behaviour. To date, the rabbit is also the only species in which a pheromone - the mammary pheromone (MP) - emitted by lactating females and active on neonatal adaptation has been described. The MP is crucial since it contributes directly to nipple localisation and oral seizing in neonates, i.e. to their sucking success. It may also be one of the non-photic cues arising from the mother, which stimulates synchronisation of the circadian system during pre-visual developmental stages. Finally, the MP promotes neonatal odour associative and appetitive conditioning in a remarkably rapid and efficient way. For these different reasons, the rabbit offers a currently unique opportunity to determine pheromonal-induced brain processing supporting adaptation early in life. Therefore, it is of interest to create a reference work of the newborn rabbit pup brain, which may constitute a tool for future multi-disciplinary and multi-approach research in this model, and allow comparisons related to the neuroethological basis of social and feeding behaviour among newborns of various species. Here, in line with existing experimental studies, and based on original observations, we propose a functional anatomical description of brain sections in 4-day-old rabbits with a particular focus on seven brain regions which appear important for neonatal perception of sensory signals emitted by the mother, circadian adaptation to the short and single daily nursing of the mother in the nest, and expression of specific motor actions involved in nipple localisation and milk intake. These brain regions involve olfactory circuits, limbic-related areas important in reward, motivation, learning and memory formation, homeostatic areas engaged in food anticipation, and regions implicated in circadian rhythm and arousal, as well as in motricity.

Histamine H1 receptor occupancy by the new-generation antipsychotics olanzapine and quetiapine: a positron emission tomography study in healthy volunteers

RATIONALE

Histamine H1 antagonists have hypnotic, appetite-promoting, and sedative side effects. Most second-generation antipsychotics have potent antagonistic effects on histamine H1 receptor (H1R). Positron emission tomography (PET) can measure the H1R occupancy (H1RO) in vivo, although there are no reports regarding antipsychotics.

OBJECTIVES

We studied the H1RO of olanzapine and quetiapine in vivo with respect to their plasma concentrations and subjective drowsiness by performing human PET imaging studies with [(11)C]doxepin, a potent PET ligand of H1R.

METHODS

Six healthy Japanese male volunteers were enrolled. Cross-randomized PET imaging was performed after a single oral administration of olanzapine (2.5 mg), quetiapine (25 mg), or placebo. PET data were analyzed by region of interest and voxel-by-voxel analysis. We concurrently measured plasma drug concentrations by liquid chromatography/tandem mass spectrometry and evaluated subjective sleepiness.

RESULTS

The binding potential ratios of olanzapine and quetiapine in the cerebral cortex were significantly lower than that of the placebo. The H1RO values of olanzapine and quetiapine in the cortex were approximately 61-80 and 56-81%, respectively. The binding potential ratios of the drugs were significantly lower than that of the placebo in the dorsolateral prefrontal and lateral temporal cortices, and anterior and posterior cingulate gyri. The H1RO values in the cortex were significantly correlated with subjective sleepiness but not plasma drug concentrations.

CONCLUSIONS

Olanzapine and quetiapine have high H1RO values in the human brain under their clinical minimum doses. This study provides a foundation of the properties by which new-generation antipsychotics block the central histaminergic system in humans.

Histamine is required for H₃ receptor-mediated alcohol reward inhibition, but not for alcohol consumption or stimulation

BACKGROUND AND PURPOSE

Conflicting data have been published on whether histamine is inhibitory to the rewarding effects of abused drugs. The purpose of this study was to clarify the role of neuronal histamine and, in particular, H₃ receptors in alcohol dependence-related behaviours, which represent the addictive effects of alcohol.

EXPERIMENTAL APPROACH

Alcohol-induced conditioned place preference (alcohol-CPP) was used to measure alcohol reward. Alcohol-induced locomotor stimulation, alcohol consumption and kinetics were also assessed. mRNA levels were quantified using radioactive in situ hybridization.

KEY RESULTS

Low doses of H₃ receptor antagonists, JNJ-10181457 and JNJ-39220675, inhibited alcohol reward in wild-type (WT) mice. However, these H₃ receptor antagonists did not inhibit alcohol reward in histidine decarboxylase knock-out (HDC KO) mice and a lack of histamine did not alter alcohol consumption. Thus H₃ receptor antagonists inhibited alcohol reward in a histamine-dependent manner. Furthermore, WT and HDC KO mice were similarly stimulated by alcohol. The expression levels of dopamine D₁ and D₂ receptors, STEP61 and DARPP-32 mRNA in striatal subregions were unaltered in HDC KO mice. No differences were seen in alcohol kinetics in HDC KO compared to WT control animals. In addition, JNJ-39220675 had no effect on alcohol kinetics in WT mice.

CONCLUSIONS AND IMPLICATIONS

These data suggest that histamine is required for the H₃ receptor-mediated inhibition of alcohol-CPP and support the hypothesis that the brain histaminergic system has an inhibitory role in alcohol reward. Increasing neuronal histamine release via H₃ receptor blockade could therefore be a novel way of treating alcohol dependence.

Histamine H₁ receptor occupancy by the new-generation antidepressants fluvoxamine and mirtazapine: a positron emission tomography study in healthy volunteers

RATIONALE

Histamine H₁ antagonists have hypnotic, appetite-promoting, and sedative effects. The affinities of various antidepressants for histamine receptors have only been partially determined in vitro and animal study. Positron emission tomography (PET) can clarify the in vivo dynamics of antidepressants at histamine receptors.

OBJECTIVES

We performed human PET imaging with [¹¹C]doxepin, a selective PET ligand of the histamine H₁ receptor (H₁R), to study the in vivo affinities of fluvoxamine and mirtazapine for the H₁R.

METHODS

The subjects were five male healthy Japanese volunteers. We performed cross-randomized PET imaging after single oral administration of fluvoxamine (25mg), mirtazapine (15 mg), or placebo. PET data were analyzed by region-of-interest and voxel-by-voxel analysis. We concurrently measured plasma drug concentrations, using liquid chromatography/tandem mass spectrometry and subjective sleepiness.

RESULTS

The binding potential ratio of mirtazapine in brain cortex was significantly lower than that of fluvoxamine or placebo. Fluvoxamine did not occupy the H₁R, whereas H₁R occupancy (H₁RO) of mirtazapine reached 80-90 % in the cerebral neocortex. In the voxel-by-voxel analysis, the binding potential of mirtazapine was significantly lower than placebo in the dorsolateral prefrontal cortex, lateral temporal cortex, anterior cingulate gyrus, and posterior cingulate gyrus. The H₁RO of mirtazapine depended on the plasma drug concentration (AUC(0-180 min)) and was related to subjective sleepiness.

CONCLUSIONS

Our results demonstrate a low affinity of fluvoxamine and a very high affinity of mirtazapine for the human brain H₁R in vivo. This study provides a basis for investigating the efficacy of new-generation antidepressants in central histamine systems.

JNJ-39220675, a novel selective histamine H3 receptor antagonist, reduces the abuse-related effects of alcohol in rats

RATIONALE

A few recent studies suggest that brain histamine levels and signaling via H(3) receptors play an important role in modulation of alcohol stimulation and reward in rodents.

OBJECTIVE

The present study characterized the effects of a novel, selective, and brain penetrant H(3) receptor antagonist (JNJ-39220675) on the reinforcing effects of alcohol in rats.

METHODS

The effect of JNJ-39220675 on alcohol intake and alcohol relapse-like behavior was evaluated in selectively bred alcohol-preferring (P) rats using the standard two-bottle choice method. The compound was also tested on operant alcohol self administration in non-dependent rats and on alcohol-induced ataxia using the rotarod apparatus. In addition, alcohol-induced dopamine release in the nucleus accumbens was tested in freely moving rats.

RESULTS

Subcutaneous administration of the selective H(3) receptor antagonist dose-dependently reduced both alcohol intake and preference in alcohol-preferring rats. JNJ-39220675 also reduced alcohol preference in the same strain of rats following a 3-day alcohol deprivation. The compound significantly and dose-dependently reduced alcohol self-administration without changing saccharin self-administration in alcohol non-dependent rats. Furthermore, the compound did not change the ataxic effects of alcohol, alcohol elimination rate, nor alcohol-induced dopamine release in nucleus accumbens.

CONCLUSIONS

These results indicate that blockade of H(3) receptor should be considered as a new attractive mechanism for the treatment of alcoholism.

Effects of histamine H3 receptor ligands on the rewarding, stimulant and motor-impairing effects of ethanol in DBA/2J mice

Histamine H3 receptor (H3R) antagonists are currently being investigated for the possible therapeutic use in various cognitive deficits such as those in schizophrenia, attention deficit hyperactivity disorder and Alzheimer's disease. Our previous studies suggest a role for H3Rs in ethanol-related behaviors in rat and mice. Here we have examined the role of different H3R ligands on the effects of ethanol in conditioned place preference (CPP) paradigm, stimulation of locomotor activity and motor impairment in rotarod and balance beam in male DBA/2J mice. We found that H3R antagonists ciproxifan and JNJ-10181457 inhibited the ethanol-evoked CPP whereas H3R agonist immepip did not alter ethanol-induced place preference. Acute stimulatory response by ethanol was also modulated by H3R ligands. Ciproxifan increased ethanol activation when ethanol was given 1g/kg but not at 1.5g/kg dose. Immepip pretreatment diminished ethanol stimulation and increased motor-impairing effects of ethanol on the balance beam. In conclusion, these findings give further evidence of the involvement of H3R in the regulation of the effects of ethanol. The inhibition of ethanol reward by H3R antagonism implies that H3R might be a possible target to suppress compulsory ethanol seeking. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Involvement of the brain histaminergic system in addiction and addiction-related behaviors: a comprehensive review with emphasis on the potential therapeutic use of histaminergic compounds in drug dependence

Neurons that produce histamine are exclusively located in the tuberomamillary nucleus of the posterior hypothalamus and send widespread projections to almost all brain areas. Neuronal histamine is involved in many physiological and behavioral functions such as arousal, feeding behavior and learning. Although conflicting data have been published, several studies have also demonstrated a role of histamine in the psychomotor and rewarding effects of addictive drugs. Pharmacological and brain lesion experiments initially led to the proposition that the histaminergic system exerts an inhibitory influence on drug reward processes, opposed to that of the dopaminergic system. The purpose of this review is to summarize the relevant literature on this topic and to discuss whether the inhibitory function of histamine on drug reward is supported by current evidence from published results. Research conducted during the past decade demonstrated that the ability of many antihistaminic drugs to potentiate addiction-related behaviors essentially results from non-specific effects and does not constitute a valid argument in support of an inhibitory function of histamine on reward processes. The reviewed findings also indicate that histamine can either stimulate or inhibit the dopamine mesolimbic system through distinct neuronal mechanisms involving different histamine receptors. Finally, the hypothesis that the histaminergic system plays an inhibitory role on drug reward appears to be essentially supported by place conditioning studies that focused on morphine reward. The present review suggests that the development of drugs capable of activating the histaminergic system may offer promising therapeutic tools for the treatment of opioid dependence.

Histamine and H3 receptor-dependent mechanisms regulate ethanol stimulation and conditioned place preference in mice

RATIONALE

Neuronal histamine has a prominent role in sleep-wake control and body homeostasis, but a number of studies suggest that histamine has also a role in higher brain functions including drug reward.

OBJECTIVE

The present experiments characterized the involvement of histamine and its H3 receptor in ethanol-related behaviors in mice.

MATERIALS AND METHODS

Male histidine decarboxylase knockout (HDC KO) and control mice were used to study the role of histamine in ethanol-induced stimulation of locomotor activity, impairment of motor coordination, and conditioned place preference (CPP). Male C57BL/6Sca mice were used to study the effects of H3 receptor antagonist in the effects of ethanol on locomotor activity.

RESULTS

The HDC KO mice displayed a weaker stimulatory response to acute ethanol than the wild-type (WT) mice. No differences between genotypes were found after ethanol administration on accelerating rotarod. The HDC KO mice showed stronger ethanol-induced CPP than the WT mice. Binding of the GABA(A) receptor ligand [(3)H]Ro15-4513 was not markedly changed in HDC KO mouse brain and thus could not explain altered responses in KO mice. Ethanol increased the activity of C57BL/6Sca mice, and H3 receptor antagonist ciproxifan inhibited this stimulation. In CPP paradigm ciproxifan, an H3 receptor inverse agonist potentiated ethanol reward.

CONCLUSIONS

Histaminergic neurotransmission seems to be necessary for the stimulatory effect of ethanol to occur, whereas lack of histamine leads to changes that enhance the conditioned reward by ethanol. Our findings also suggest a role for histamine H3 receptor in modulation of the ethanol stimulation and reward.

Histamine H1 receptor knockout mice exhibit impaired spatial memory in the eight-arm radial maze

BACKGROUND AND PURPOSE

In the mammalian brain, histaminergic neurotransmission is mediated by the postsynaptic histamine H1 and H2 receptors and the presynaptic H3 autoreceptor, which also acts as a heteroreceptor. The H1 receptor has been implicated in spatial learning and memory formation. However, pharmacological and lesion studies have revealed conflicting results. To examine the involvement of histamine H1 receptor in spatial reference and working memory formation, H1 receptor knockout mice (KO) were tested in the eight-arm radial maze. Previously, we found that the H1 receptor-KO mice showed reduced emotionality when confronted with spatial novelty. As it is known that emotions can have an impact on spatial learning and memory performance, we also evaluated H1 receptor-KO mice in terms of emotional behaviour in the light-dark box.

EXPERIMENTAL APPROACH

Mice lacking the H1 receptor and wild-type mice (WT) were tested for spatial reference and working memory in an eight-arm radial maze with three arms baited and one trial per day. Emotional behaviour was measured using the light-dark test.

KEY RESULTS

The H1 receptor-KO mice showed impaired spatial reference and working memory in the radial maze task. No significant differences between H1 receptor-KO and WT mice were observed in the light-dark test.

CONCLUSIONS AND IMPLICATIONS

The spatial memory deficits of the H1 receptor-KO mice might be due to the reported changes in cholinergic neurochemical parameters in the frontal cortex and the CA1 subregion of the hippocampus, to impaired synaptic plasticity in the hippocampus, and/or to a dysfunctional brain reward/reinforcement system.

Neuromodulation and the orchestration of the respiratory rhythm

The respiratory system is continuously modulated by numerous aminergic and peptidergic substances that act at all levels of integration: from the sensory level to the level of central networks and motor nuclei. The same neuronal networks receive inputs from multiple modulators released locally as well as from distal nuclei. All parameters of respiratory control are controlled by multiple neuromodulators. By partly converging onto similar G-proteins and second messenger systems, acetylcholine, norepinephrine, histamine, serotonin (5-HT), dopamine, ATP, substance P, cholecystokinin (CCK) can increase frequency, regularity and amplitude of respiratory activity. Yet, the same modulator can also exert differential effects on respiratory activity by acting on different receptors partly in the same neurons. In the pre-Bötzinger complex (pre-BötC) modulators can differentially modulate frequency and amplitude in different types of pacemaker neurons. Similarly motoneurons located in different motor nuclei receive differential amplitude modulation from different modulators. Thus, modulators are capable of orchestrating and modulating different parameters of respiratory activity by differentially targeting different cellular targets. A disturbance in modulatory control may lead to Sudden Infant Death Syndrome (SIDS) and erratic breathing.

Chlorpheniramine impairs functional recovery in Carassius auratus after telencephalic ablation

We determined the effect of an H1 receptor antagonist on the functional recovery of Carassius auratus submitted to telencephalic ablation. Five days after surgery the fish underwent a spatial-choice learning paradigm test. The fish, weighing 6-12 g, were divided into four groups: telencephalic ablation (A) or sham lesion (S) and saline (SAL) or chlorpheniramine (CPA, ip, 16 mg/kg). For eight consecutive days each animal was trained individually in sessions separated by 24 h (alternate days). Training trials (T1-T8) consisted of finding the food in one of the feeders, which were randomly blocked for each subject. Animals received an intraperitoneal injection of SAL or CPA 10 min after the training trials. The time spent by the animals in each group to find the food (latency) was analyzed separately at T1 and T8 by the Kruskal-Wallis test, followed by the Student Newman-Keuls test. At T1 the latencies (mean +/- SEM) of the A-SAL (586.3 +/- 13.6) and A-CPA (600 +/- 0) groups were significantly longer than those of the S-SAL (226.14 +/- 61.15) and S-CPA (356.33 +/- 68.8) groups. At T8, the latencies of the A-CPA group (510.11 +/- 62.2) remained higher than those of the other groups, all of which showed significantly shorter latencies (A-SAL = 301.91 +/- 78.32; S-CPA = 191.58 +/- 73.03; S-SAL = 90.28 +/- 41) compared with T1. These results support evidence that training can lead to functional recovery of spatial-choice learning in telencephalonless fish and also that the antagonist of the H1 receptor impairs it.

H3 receptor antagonists reverse delay-dependent deficits in novel object discrimination by enhancing retrieval

RATIONALE

Accumulated evidence suggests a role for histamine in cognition and the use of H3 receptor antagonists in the treatment of learning and memory disorders.

OBJECTIVES

The aim of the current study was to investigate the cognition enhancing properties of ciproxifan, an H3 receptor antagonist, after natural forgetting in normal adult rats.

MATERIALS AND METHODS

The novel object discrimination task, a recognition memory test based on spontaneous exploratory behaviour, was used. Briefly, rats exposed to two identical objects during an acquisition trial can discriminate between a novel object and a familiar one during a subsequent choice trial after a short delay but not after a 24-h inter-trial interval.

RESULTS

The scopolamine (0.5 mg/kg, i.p.)-induced impairment after a short delay was abolished by ciproxifan (p < 0.001). Natural forgetting was prevented by a single administration of ciproxifan (3 mg/kg) prior to the retention test (p < 0.001) but not when administered before or immediately after the acquisition trial (schedule effect p < 0.05), demonstrating a specific activity on memory retrieval. Pretreatment with either pyrilamine (10 mg/kg), an H1 antagonist, or zolantidine (10 mg/kg), an H2 antagonist, prevented the retrieval enhancement effect of ciproxifan (p < 0.05 and p < 0.001, respectively).

CONCLUSIONS

Histamine H3 receptor antagonists restore the performance of rats impaired by scopolamine and enhance recognition memory after acute administration before the retrieval phase via a mechanism dependent on H1 and H2 receptor activation.

The role of histamine on cognition

Histamine was intensively studied at the beginning of the 20th century because of its important role in allergic and inflammation processes. In those days it was very difficult that researchers could envisage another impacting function for the imidazolamine in the living systems. Once the imidazolamine was found located in neuron compartment in the brain, increasing evidence supported many regulatory functions including its possible role in memory and learning. The specific participation of histamine in cognitive functions followed a slow and unclear pathway because the many different experimental learning models, pharmacologic approaches, systemic and localized applications of the histamine active compounds into the brain used by researchers showed facilitating or inhibitory effects on learning, generating an active issue that has extended up to present time. In this review, all these aspects are analyzed and discussed considering the many intracellular different mechanisms discovered for histamine, the specific histamine receptors and the compartmentalizing proprieties of the brain that might explain the apparent inconsistent effects of the imidazolamine in learning. In addition, a hypothetical physiologic role for histamine in memory is proposed under the standard theories of learning in experimental animals and humans.

The histamine H3 receptor: an attractive target for the treatment of cognitive disorders

The histamine H3 receptor, first described in 1983 as a histamine autoreceptor and later shown to also function as a heteroreceptor that regulates the release of other neurotransmitters, has been the focus of research by numerous laboratories as it represents an attractive drug target for a number of indications including cognition. The purpose of this review is to acquaint the reader with the current understanding of H3 receptor localization and function as a modulator of neurotransmitter release and its effects on cognitive processes, as well as to provide an update on selected H3 antagonists in various states of preclinical and clinical advancement. Blockade of centrally localized H3 receptors by selective H3 receptor antagonists has been shown to enhance the release of neurotransmitters such as histamine, ACh, dopamine and norepinephrine, among others, which play important roles in cognitive processes. The cognitive-enhancing effects of H3 antagonists across multiple cognitive domains in a wide number of preclinical cognition models also bolster confidence in this therapeutic approach for the treatment of attention deficit hyperactivity disorder, Alzheimer's disease and schizophrenia. However, although a number of clinical studies examining the efficacy of H3 receptor antagonists for a variety of cognitive disorders are currently underway, no clinical proof of concept for an H3 receptor antagonist has been reported to date. The discovery of effective H3 antagonists as therapeutic agents for the novel treatment of cognitive disorders will only be accomplished through continued research efforts that further our insights into the functions of the H3 receptor.

Levodopa ameliorates learning and memory deficits in a murine model of Alzheimer's disease

Dopamine plays an important role in learning and memory processes. A deficit of this neurotransmitter as it is apparent in Alzheimer's disease (AD) may contribute to cognitive decline, a major symptom of AD patients. The aim of this study was to elucidate whether or not stimulation of the dopaminergic system leads to an improvement of cognitive function and reduction of non-cognitive behavioral alterations in a murine model of AD. Transgenic and wild type male mice of the TgCRND8 line were treated either with the dopamine precursor levodopa or vehicle and tested in two learning tasks, the object-recognition task and the Barnes maze test. Additionally 24 h spontaneous behavior in the home cage was analyzed. In both memory tasks wild type mice performed significantly better than transgenics. However, transgenics treated with levodopa showed a significant object recognition memory and improved acquisition of spatial memory in the Barnes maze compared to vehicle treated transgenics. Concerning spontaneous behavior transgenic mice performed much more stereotypies than wild types. However, there was a trend for reduced stereotypies in the levodopa group in the time the drug was active. Neurochemical analysis revealed elevated levels of dopamine in the neostriata and frontal cortices and reduced levels in the hippocampi of transgenic mice compared to wild types. Thus cognitive deficits and stereotypies may be due to changes in the dopaminergic system as they could be ameliorated by levodopa treatment, that might also have a therapeutic significance for AD.

Organization of histamine-immunoreactive, tuberomammillary neurons projecting to the dorsal tier of the substantia nigra compacta in the rat

Following the injection of a retrograde tracer, gold-conjugated and inactivated wheatgerm agglutinin-horseradish peroxidase (WGA-apo-HRP-gold), into the dorsal tier of substantia nigra compacta (SNCD), histamine immunostaining was performed for the tuberomammillary nucleus (TMN) in order to investigate the projection from the TMN to the SNCD. At the rostral pole of the TMN, the retrograde labeling in the dorsomedial subdivision following medial SNCD injections was predominantly ipsilateral (78%), whereas only a few cells were located bilaterally following lateral SNCD injections. Following tracer injections along the medio-lateral location along the SNCD, the labeling at the ventrolateral TMN was bilateral with slightly ipsilateral (58-61%) dominance. At rostral and caudal TMN levels, clusters of labeled neurons were localized within two discrete columns of the ventrolateral TMN. At rostral TMN level, a lateral column of cells was located at the lateral tip of the ventrolateral TMN just medial to the internal capsule, while the medial column was close to the protruded region along the ventral, pial border. At the caudal TMN level, two columns were located on either side of the lateral mammillary nucleus. Taken together, the present study suggests that ventrolateral as well as dorsomedial TMN might provide arousal-related information to medial, intermediate, and lateral regions of the SNCD, which in turn influence extrapyramidal, behavioral functions performed by the substantia nigra compacta.

A lamprey striatal brain slice preparation for patch-clamp recordings

Striatum, the input layer of the basal ganglia is important for functions such as the selection of motor behaviour. The lamprey, a lower vertebrate, is particularly well suited as a model system for the control of motor functions as its central nervous system is similar to that of higher vertebrates and exhibits a lower level of complexity. Therefore, studies in lamprey preparations enable cellular and synaptic mechanisms to be correlated with behaviour. The lamprey brain slice preparation presented has been developed to study the striatal microcircuits and input/output systems with patch-clamp recordings. The method involves dissection of the central nervous system, brain slice preparation, identification of the striatum, visual identification of striatal neurons and patch-clamp recordings. By combining studies in the slice preparation presented here and other lamprey preparations such as the semi-intact lamprey, we will be able to correlate striatal mechanisms on the cellular, synaptic and network levels with striatal output and motor behaviour. The method can be adapted to produce similar slice preparations from other areas of the lamprey brain.

The psychostimulant and rewarding effects of cocaine in histidine decarboxylase knockout mice do not support the hypothesis of an inhibitory function of histamine on reward

RATIONALE AND OBJECTIVES

Lesion studies have shown that the tuberomammillary nucleus (TM) exerts inhibitory effects on the brain reward system. To determine whether histamine from the TM is involved in that reward inhibitory function, we assessed the stimulant and rewarding effects of cocaine in knockout mice lacking histidine decarboxylase (HDC KO mice), the histamine-synthesizing enzyme. If histamine actually plays an inhibitory role in reward, then it would be expected that mice lacking histamine would be more sensitive to the behavioral effects of cocaine.

MATERIALS AND METHODS

The first experiment characterized spontaneous locomotion and cocaine-induced hyperactivity (0, 8, and 16 mg/kg, i.p.) in wild-type and HDC KO mice. The rewarding effects of cocaine were investigated in a second experiment with the place-conditioning technique.

RESULTS

The first experiment demonstrated that histidine decarboxylase mice showed reduced exploratory behaviors but normal habituation to the test chambers. After habituation to the test chambers, HDC KO mice were slightly, but significantly, less stimulated by cocaine than control mice. This finding was replicated in the second experiment, when cocaine-induced activity was monitored with the place-conditioning apparatus. Furthermore, a significant place preference was present in both genotypes for 8 and 16 mg/kg cocaine, but not for 2 and 4 mg/kg.

CONCLUSIONS

Our data confirm previous results demonstrating that HDC KO mice show reduced exploratory behaviors. However, contrary to the hypothesis that histamine plays an inhibitory role in reward, histamine-deficient mice were not more responsive to the psychostimulant effects of cocaine.

Endogenous histamine inhibits the development of morphine-induced conditioned place preference

AIM

The conditioned place preference (CPP) paradigm was used to investigate the effects of endogenous histamine on the processes leading to morphine-induced reward-seeking behavior in Sprague-Dawley rats.

METHODS

The model of CPP was used to assess the rewarding effect of morphine. The levels of histamine, glutamate, gamma-aminobutyric acid (GABA), dopamine (DA) and 3, 4-dihydroxyphenylacetic acid (DOPAC) in rat brains were measured with high-performance liquid chromatography. Immunohistochemistry technique was used to observe the morphological changes of neurons.

RESULTS

Intraperitoneal injection of morphine (2, 5 or 10 mg/kg) induced the development of CPP in a dose-dependent manner. In addition, morphine administrations (10 mg/kg) decreased the histamine content and reduced the number and size of histaminergic neurons in the tubero-mammillary nucleus (TM), as well as markedly increasing the DOPAC/DA ratios in the ventral tegmental area (VTA) and nucleus accumbens (NAc). Intraperitoneal injection of histidine (50, 100 or 200 mg/kg) dose-dependently inhibited the development of morphine-induced CPP. Bilateral lesions of the TM, which decreased the histamine levels in the VTA and NAc, potentiated the development of CPP induced by morphine (1 mg/kg, a dose that produced no appreciable effect when given alone) and increased the DOPAC/DA ratios in the VTA and NAc, but did not change the glutamate or GABA levels in these nuclei. Histidine reversed the effects of the TM lesions.

CONCLUSION

These results indicate that endogenous histamine plays a role in inhibiting the development of morphine-induced reward-seeking behavior, and the inhibition may involve the modulation of dopaminergic activity.

Selective cognitive dysfunction in mice lacking histamine H1 and H2 receptors

Previous pharmacological experiments provide conflicting findings that describe both facilitatory and inhibitory effects of neuronal histamine on learning and memory. Here, we examined learning and memory and synaptic plasticity in mice with a null mutation of gene coding histamine H1 or H2 receptor in order to clarify the role of these receptors in learning and memory processes. Learning and memory were evaluated by several behavioral tasks including object recognition, Barnes maze and fear conditioning. These behavioral tasks are highly dependent on the function of prefrontal cortex, hippocampus or amygdala. Object recognition and Barnes maze performance were significantly impaired in both H1 receptor gene knockout (H1KO) and H2 receptor gene knockout (H2KO) mice when compared to the respective wild-type (WT) mice. Conversely, both H1KO and H2KO mice showed better auditory and contextual freezing acquisition than their respective WT mice. Furthermore, we also examined long-term potentiation (LTP) in the CA1 area of hippocampus in H1KO and H2KO mice and their respective WT mice. LTP in the CA1 area of hippocampus was significantly reduced in both H1KO and H2KO mice when compared with their respective WT mice. In conclusion, our results demonstrate that both H1 and H2 receptors are involved in learning and memory processes for which the frontal cortex, amygdala and hippocampus interact.

Involvement of histamine receptors in the acquisition of inhibitory avoidance in Carassius auratus

This study investigated the involvement of H(1) and H(2) histaminegic receptors on the acquisition of a new task in Carassius auratus by using an inhibitory avoidance paradigm in which the animals had to learn to avoid an aversive stimulus. Before training, the fish received injections of H(2) antagonist zolantidine at a dose of 20 mg/kg, or H(1) antagonist chlorpheniramine at a dose of 4 or 16 mg/kg. Control animals were injected with distilled water. A facilitatory effect of chlorpheniramine was observed at the dose of 16 mg/kg. On the other hand, the administration of 20 mg/kg of zolantidine inhibited acquisition. Place preference conditioning was used to observe the aversive or reinforcing effects of the drugs, which could interfere with the inhibitory avoidance procedure; however, no effects were observed. Thus, it can be suggested that both receptors, H(1) and H(2), are involved in the acquisition of a new task in this species.

Histaminergic receptors of medial septum and conditioned place preference: D1 dopamine receptor mechanism

In the present study, the effects of intra-medial septum injections of histamine and/or the histamine H1 or H2 receptor antagonists on the acquisition of conditioned place preference (CPP) in male Wistar rats have been investigated. Our data showed that the conditioning treatments with intra-medial septum injection of different doses of histamine (0.5-15 microg/rat) induced a significant CPP for the drug-associated place. Using a 3-day schedule of conditioning, it was found that the histamine H1 receptor antagonist, pyrilamine (10 and 15 microg/rat, intra-medial septum) also induced a significant place preference. In addition, pyrilamine inhibited the histamine (7.5 microg/rat)-induced place preference. Intra-medial septum administration of the histamine H2 receptor antagonist, ranitidine (5-15 microg/rat) alone or in combination with histamine did not produce a significant place preference or place aversion. On the other hand, intra-medial septum administration of the dopamine D1 receptor antagonist, SCH 233390 (0.5, 0.75 and 1 microg/rat) inhibited the histamine (7.5 microg/rat) or pyrilamine (15 microg/rat)-induced place preference in a dose-dependent manner, but no effect was observed for the dopamine D2 receptor antagonist, sulpiride on the histamine or pyrilamine response. The administration of histamine (2.5-15 microg/rat) or pyrilamine (10 and 15 microg/rat) during acquisition increased locomotor activity of the animals on the testing days. The results suggest that histaminergic receptors of the medial septum may be involved in CPP and thus it is postulated that dopamine D1 receptors may play an important role in this effect.

Influence of morphine- or apomorphine-induced sensitization on histamine state-dependent learning in the step-down passive avoidance test

Effects of morphine- or apomorphine-induced sensitization on histamine state-dependent memory of passive avoidance task were examined in mice. Pre-training intracerebroventricular (i.c.v.) administration of histamine (20 microg/mouse) decreased the learning of a one-trial passive avoidance task. Pre-test administration of histamine (10 and 20 microg/mouse) reversed amnesia induced by pre-training of histamine, with maximum response at 20 microg/mouse. Pre-training histamine-induced amnesia was also reversed in morphine- or apomorphine-sensitized mice that had previously received once daily injections of morphine (20 and 30 mg/kg) or apomorphine (0.5 and 1 mg/kg) for 3 days. The reversion of histamine-induced amnesia in morphine-sensitized mice was decreased by once daily administration of naloxone (0.5 and 1 mg/kg), SCH 23390 (0.05 and 0.1 mg/kg) or sulpiride (25, 50 and 100 mg/kg) prior to injection of morphine (30 mg/kg/day, 3 days). Furthermore, once daily administration of sulpiride (50 and 100 mg/kg) but not SCH 23390 (0.01, 0.05 and 0.1 mg/kg) prior to apomorphine (1 mg/kg, for 3 days) decreased the reversion of pre-training histamine-induced amnesia by apomorphine. The results suggest that apomorphine or morphine sensitization affects the impairment of memory induced by histamine and thus it is postulated that opioid and dopamine receptors may play an important role in this effect.

Histamine enhances inhibitory avoidance memory consolidation through a H2 receptor-dependent mechanism

Several evidences suggest that brain histamine is involved in memory consolidation but the actual contribution of the hippocampal histaminergic system to this process remains controversial. Here, we show that when infused into the CA1 region of the dorsal hippocampus immediately after training in an inhibitory avoidance task, but not later, histamine induced a dose-dependent promnesic effect without altering locomotor activity, exploratory behavior, anxiety state or retrieval of the avoidance response. The facilitatory effect of intra-CA1 histamine was mimicked by the histamine N-methyltransferase inhibitor SKF-91844 as well as by the H2 receptor agonist dimaprit and it was blocked completely by the H2 receptor antagonist ranitidine. Conversely, the promnesic action of histamine was unaffected by the H1 receptor antagonist pyrilamine, the H3 receptor antagonist, thioperamide, and the NMDAr polyamine-binding site antagonist ifenprodil. By themselves, ranitidine, pyrilamine, thioperamide, and ifenprodil did not affect IA memory consolidation. Our data indicate that, when given into CA1, histamine enhances memory consolidation through a mechanism that involves activation of H2 receptors; however, endogenous CA1 histamine does not seem to participate in the consolidation of IA memory at least at the post-training times analyzed.

Distribution of MT1 melatonin receptor immunoreactivity in the human hypothalamus and pituitary gland: Colocalization of MT1 with vasopressin, oxytocin, and corticotropin-releasing hormone

Melatonin is implicated in numerous physiological processes, including circadian rhythms, stress, and reproduction, many of which are mediated by the hypothalamus and pituitary. The physiological actions of melatonin are mainly mediated by melatonin receptors. We here describe the distribution of the melatonin receptor MT1 in the human hypothalamus and pituitary by immunocytochemistry. MT1 immunoreactivity showed a widespread pattern in the hypothalamus. In addition to the area of the suprachiasmatic nucleus (SCN), a number of novel sites, including the paraventricular nucleus (PVN), periventricular nucleus, supraoptic nucleus (SON), sexually dimorphic nucleus, the diagonal band of Broca, the nucleus basalis of Meynert, infundibular nucleus, ventromedial and dorsomedial nucleus, tuberomamillary nucleus, mamillary body, and paraventricular thalamic nucleus were observed to have neuronal MT1 receptor expression. No staining was observed in the nucleus tuberalis lateralis and bed nucleus of the stria terminalis. The MT1 receptor was colocalized with some vasopressin (AVP) neurons in the SCN, colocalized with some parvocellular and magnocellular AVP and oxytocine (OXT) neurons in the PVN and SON, and colocalized with some parvocellular corticotropin-releasing hormone (CRH) neurons in the PVN. In the pituitary, strong MT1 expression was observed in the pars tuberalis, while a weak staining was found in the posterior and anterior pituitary. These findings provide a neurobiological basis for the participation of melatonin in the regulation of various hypothalamic and pituitary functions. The colocalization of MT1 and CRH suggests that melatonin might directly modulate the hypothalamus-pituitary-adrenal axis in the PVN, which may have implications for stress conditions such as depression.

Cross state-dependent retrieval between histamine and lithium

Histamine and lithium state-dependent (StD) retrieval of passive avoidance task and their interactions was examined in mice. The pre-training or pre-test intracerebroventricular (i.c.v.) injection of histamine (20 microg/mouse) impaired retrieval when it was tested 24 h later. In the animals, in which retrieval was impaired due to histamine pre-training administration, pre-test administration of histamine, with the same dose, restored retrieval. The H1 blocker, pyrilamine (20 microg/mouse, i.c.v.), but not the H(2) blocker; ranitidine prevented the restoration of retrieval by pre-test histamine. The pre-training (5 and 10 mg/kg) or pre-test (5 mg/kg) injection of lithium also impaired retrieval, when it was tested 24 h later. In the animals that received lithium (5 mg/kg) or histamine (20 microg/mouse) as pre-training treatment, administration of histamine, clobenpropit or lithium, respectively, resulted in restoration of memory retrieval. Neither pyrilamine nor ranitidine prevented the restoration of retrieval by pre-test lithium. In conclusion, histamine or lithium can induce state-dependent retrieval and a cross-StD exists between these drugs, which may be mediated through the inositol pathway.

Influence of Intracerebroventricular Administration of Histaminergic Drugs on Morphine State-Dependent Memory in the Step-Down Passive Avoidance Test

The effects of histaminergic drugs on morphine state-dependent memory of a passive avoidance task were examined in mice. Pre-training administration of morphine (5 mg/kg) led to state-dependent learning with impaired memory recall on the test day which was reversed by pre-test administration of the same dose of the opioid. The pre-test intracerebroventricular (i.c.v.) administration of the H(1) blocker (pyrilamine) prevented the restoration of memory by morphine. The H(2) blocker (ranitidine) was ineffective in this regard and the H(3) blocker (clobenpropit) potentiated the effect of morphine on memory recall. The pre-test i.c.v. administration of histamine alone (5, 10, and 20 microg/mouse) not only mimicked the effect of pre-test morphine treatment, but also increased this action of the opioid. The effect of histamine on memory recall was not changed by the pre-test administration of mu-opioid receptor antagonist, naloxone. In conclusion, the improvement of memory recall by morphine treatment, on the test day, seems to be, at least in part, through the release of histamine followed by the stimulation of H(1) receptors. Histamine by itself, when administered on the test day, mimicked morphine-induced memory improvement by a mechanism independent of the mu-opioid receptors.

Drug interaction between methamphetamine and antihistamines: behavioral changes and tissue concentrations of methamphetamine in rats

Methamphetamine is a psychomotor stimulant, whereas first generation antihistamines cause sedation. Several studies have demonstrated that first generation antihistamines potentiate methamphetamine-induced psychomotor activation and two possible mechanisms have been postulated. One is blockage of the central histaminergic neuron system and the other is inhibition of dopamine reuptake. However, the exact mechanism is still controversial. In this study, we examined in behavioral tests the effects of selected antihistamines on methamphetamine-induced psychomotor activation in rats, and measured plasma and brain tissue concentrations of methamphetamine. We found that some antihistamines significantly potentiate methamphetamine-induced psychomotor activation in rats and that plasma and brain tissue concentrations of methamphetamine in rats treated with methamphetamine in combination with D-chlorpheniramine were markedly higher than those in rats treated with methamphetamine alone. These results suggest that the potentiating effects of antihistamines are due to not only central effects but also the alteration of the pharmacokinetics of methamphetamine.

Thr105Ile, a Functional Polymorphism of Histamine N-Methyltransferase, Is Associated with Alcoholism in Two Independent Populations

BACKGROUND

Histamine is expressed in cortical and limbic areas that are involved in emotion and cognition and modulates these behaviors. H1 receptor antagonists are sedative. Histamine N-methyltransferase (HNMT) catalyzes the Ntau methylation of histamine, the sole pathway for termination of the neurotransmitter action of histamine in mammalian brain. A common and functionally significant polymorphism, a C314T transition in exon 4 of the HNMT gene results in a Thr105Ile substitution of the protein encoded. The Thr105 allele is associated with approximately 2-fold higher enzyme activity, leading to the prediction that it might be associated with diminished histamine levels, resulting in differences in anxiety, cognition, and sedation that play important roles in alcoholism. In two ethnically distinct populations, we tested whether the Thr105Ile polymorphism was associated with alcoholism and with harm avoidance, a dimensional measure of anxious personality.

METHODS

A 5' exonuclease assay (TaqMan) was used to genotype Thr105Ile in psychiatrically interviewed Finnish Caucasian (n = 218) and Plains American Indian (n = 186) alcoholics, along with ethnically matched, psychiatrically interviewed, controls (Finns: n = 313, Plains Indian: n = 140).

RESULTS

Ile105 allele frequencies were significantly lower in alcoholics compared with nonalcoholics in both populations (Finns: 0.12 vs. 0.17, chi(2) = 6, p = 0.015; Plains Indians: 0.03 vs. 0.08, chi(2) = 5, p = 0.023). Genotype distributions also differed significantly. In Finns, Ile105 showed borderline significance for an association with lower harm avoidance (p = 0.070) after correcting for alcoholism diagnosis.

CONCLUSIONS

Decreased levels of brain histamine consequent to the Thr105 allele may result in higher levels of anxiety and, as a consequence, vulnerability to alcoholism.

Histamine H3 antagonist thioperamide dose-dependently enhances memory consolidation and reverses amnesia induced by dizocilpine or scopolamine in a one-trial inhibitory avoidance task in mice

In the literature, there is some evidence indicating that H3 histamine receptor antagonists, in particular thioperamide, can facilitate learning and memory retrieval in laboratory rodents. The present study aimed at verifying whether this also holds for memory consolidation, a phase of memory for which there is scarcity of convincing data on the effects of H3 receptor antagonists given systemically. To that end, memory consolidation was assessed in C57BL/6J mice using the one-trial step-through inhibitory avoidance task, the compounds being injected immediately after training (foot-shock) and performance measured 24 h later. More specifically, the following effects of thioperamide (1.25-20 mg/kg) were dose-dependently analysed: (1) its potential direct effects on memory consolidation; (2) its potential reversing effects on retrograde amnesia induced by the NMDA antagonist dizocilpine (MK-801, 0.5 mg/kg) and (3) its potential reversing effects on the well-known amnesia induced by the muscarinic antagonist scopolamine (0.25 mg/kg). We found that thioperamide exerted a dose-dependent facilitative effect on memory consolidation. Furthermore, the H3 receptor antagonist reversed scopolamine- and especially dizocilpine-induced amnesia. The results strongly support the view that the brain mechanisms of memory consolidation involve a functional interaction between the NMDA and the H3 sites.