Histamine: effect on self-stimulation

The Diverse Network of Brain Histamine in Feeding: Dissect its Functions in a Circuit-Specific Way

Feeding is an intrinsic and important behavior regulated by complex molecular, cellular and circuit-level mechanisms, one of which is the brain histaminergic network. In the past decades, many studies have provided a foundation of knowledge about the relationship between feeding and histamine receptors, which are deemed to have therapeutic potential but are not successful in treating feeding- related diseases. Indeed, the histaminergic circuits underlying feeding are poorly understood and characterized. This review describes current knowledge of histamine in feeding at the receptor level. Further, we provide insight into putative histamine-involved feeding circuits based on the classic feeding circuits. Understanding the histaminergic network in a circuit-specific way may be therapeutically relevant for increasing the drug specificity and precise treatment in feeding-related diseases.

Reduction of nicotine self-administration by chronic nicotine infusion with H1 histamine blockade in female rats

RATIONALE

Chronic nicotine infusion via transdermal patches has been widely shown to assist with smoking cessation. In particular, transdermal nicotine treatment prior to quitting smoking helps reduce ad libitum smoking and aids cessation Rose et al. (Nicotine Tob Res 11:1067-75, 2009). However, despite this success, the majority of smokers who use transdermal nicotine fail to permanently quit smoking. Additional treatments are needed. Tobacco addiction does not just depend on nicotinic receptor systems; a variety of neural systems are involved, including dopamine, norepinepherine, serotonin, and histamine.

OBJECTIVES

Given the involvement of a variety of neural systems in the circuits of addiction, combination therapy may offer improved efficacy for successful smoking cessation beyond single treatments alone. We have found that pyrilamine, an H1 histamine antagonist, significantly decreases nicotine self-administration in rats.

METHODS

The current study was conducted to confirm the effect of chronic nicotine infusion on ongoing nicotine self-administration and resumed access after enforced abstinence and to determine the interaction of chronic nicotine with an H1 antagonist treatment.

RESULTS

Chronic nicotine infusion via osmotic minipump (2.5 and 5 mg/kg/day for 28 days) significantly reduced nicotine self-administration in a dose-dependent manner. Chronic nicotine infusion also reduced the resumption of nicotine self-administration after enforced abstinence. Chronic pyrilamine infusion (25 mg/kg/day for 14 days) also significantly reduced nicotine self-administration.

CONCLUSION

The combination of chronic nicotine and pyrilamine reduced nicotine self-administration to a greater extent than treatment with either drug alone.

Histamine H3 receptor antagonist JNJ-39220675 modulates locomotor responses but not place conditioning by dopaminergic drugs

RATIONALE

Brain histaminergic system is involved in the regulation of the dopaminergic circuitry. The role of histamine H3 receptor (H3R) in behaviors linked to amphetamine addiction and other behaviors induced by dopaminergic compounds has remained unclear.

OBJECTIVE

Our aim was to study whether H3R antagonist JNJ-39220675 inhibits amphetamine-induced stimulation and reward. The effects of JNJ-39220675 on dopamine D2-like receptor (D2R-like) agonist quinpirole-induced behaviors were also investigated in order to clarify whether the possible effects of H3R antagonists are D2R-like dependent.

METHODS

The effects of JNJ-39220675 on amphetamine and quinpirole-induced behavioral responses in mice were studied assessing the locomotor activation after both acute and repeated administrations of amphetamine and quinpirole. The place conditioning paradigm was also used as a measure of reward or aversion.

RESULTS

JNJ-39220675 inhibited amphetamine-induced stimulation acutely but not after repeated administrations. Amphetamine (2 mg/kg) induced conditioned place preference that was not affected by either of the tested doses of JNJ-39220675 (1 and 10 mg/kg). Quinpirole (0.5 mg/kg) induced conditioned place aversion to which the pretreatment by JNJ-39220675 (10 mg/kg) had no effect. In repeated administration, JNJ-39220675 did, however, inhibit quinpirole-induced tolerance to hypokinesia.

CONCLUSIONS

Our results show that although H3R antagonists inhibit ethanol reward, they may not possess the same ability on psychostimulants, such as amphetamine. However, if H3R antagonists will become clinically available, it is of importance that these compounds potentiate neither the rewarding nor aversive effects of other drugs.

Update on 3-iodothyronamine and its neurological and metabolic actions

3-iodothyronamine (T1AM) is an endogenous amine, that has been detected in many rodent tissues, and in human blood. It has been hypothesized to derive from thyroid hormone metabolism, but this hypothesis still requires validation. T1AM is not a ligand for nuclear thyroid hormone receptors, but stimulates with nanomolar affinity trace amine-associated receptor 1 (TAAR1), a G protein-coupled membrane receptor. With a lower affinity it interacts with alpha2A adrenergic receptors. Additional targets are represented by apolipoprotein B100, mitochondrial ATP synthase, and membrane monoamine transporters, but the functional relevance of these interactions is still uncertain. Among the effects reported after administration of exogenous T1AM to experimental animals, metabolic and neurological responses deserve special attention, because they were obtained at low dosages, which increased endogenous tissue concentration by about one order of magnitude. Systemic T1AM administration favored fatty acid over glucose catabolism, increased ketogenesis and increased blood glucose. Similar responses were elicited by intracerebral infusion, which inhibited insulin secretion and stimulated glucagon secretion. However, T1AM administration increased ketogenesis and gluconeogenesis also in hepatic cell lines and in perfused liver preparations, providing evidence for a peripheral action, as well. In the central nervous system, T1AM behaved as a neuromodulator, affecting adrenergic and/or histaminergic neurons. Intracerebral T1AM administration favored learning and memory, modulated sleep and feeding, and decreased the pain threshold. In conclusion T1AM should be considered as a component of thyroid hormone signaling and might play a significant physiological and/or pathophysiological role. T1AM analogs have already been synthetized and their therapeutical potential is currently under investigation. 3-iodothyronamine (T1AM) is a biogenic amine whose structure is closely related to that of thyroid hormone (3,5,3′-triiodothyronine, or T3). The differences with T3 are the absence of the carboxylate group and the substitution of iodine with hydrogen in 5 and 3′ positions (Figure 1). In this paper we will review the evidence supporting the hypothesis that T1AM is a chemical messenger, namely that it is an endogenous substance able to interact with specific receptors producing significant functional effects. Special emphasis will be placed on neurological and metabolic effects, which are likely to have physiological and pathophysiological importance.

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.

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 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.

Association of THR105Ile, a functional polymorphism of histamine N-methyltransferase (HNMT), with alcoholism in German Caucasians

BACKGROUND

CNS histamine has been shown to have an inhibitory effect on reward and it is implicated in the etiology of addiction and stress. Histamine N-methyltransferase (HNMT) is believed to be the sole pathway for termination of the neurotransmitter action of histamine in mammalian brain. A common, functional polymorphism, a C314T transition in the HNMT gene, results in a Thr105Ile substitution of the protein encoded. A recent study has shown that the frequency of the Ile105 allele was significantly lower in alcoholics compared to that in non-alcoholics in Finns and Plains American Indians. Following up these results, we tested whether the Thr105Ile polymorphism was associated with alcoholism in German Caucasians.

METHODS

Thr105Ile was genotyped in n=366 psychiatrically interviewed German Caucasian ICD-10 lifetime alcoholics, along with n=200 ethnically matched controls.

RESULTS

No significant difference was found in the frequency of the Ile105 allele between alcoholics (0.11) and controls (0.10) (chi(2)=0.21, d.f.=1, p=0.647). Likewise, genotype distributions did not differ significantly. However, the frequency of the Ile105 allele was significantly lower in male alcoholics with a family history of alcoholism compared to that in male alcoholics without a family history of alcoholism (chi(2)=4.07, d.f.=1, p=0.044).

CONCLUSIONS

In German Caucasians the association of the HNMT Thr105Ile polymorphism with alcoholism was not replicated per se, but a congruent association was found between the Ile105 allele and family history of alcoholism supporting the protective role of the Ile105 allele against alcoholism.

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.

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.

The H3 antagonist thioperamide reveals conditioned preference for a context associated with an inactive small dose of cocaine in C57BL/6J mice

The histaminergic system has been speculated to be involved in the inhibitory control of drug reward, H1 and H2 antagonists having been found to potentiate conditioned place preference induced by morphine or cocaine. In contrast, the role of H3 receptors in cocaine-induced place preference is still unknown. The present study tested the effects of thioperamide (0, 10 and 20 mg/kg, i.p.), an H3 autoreceptor antagonist, on the development of a conditioned place preference induced by cocaine (0, 2 and 8 mg/kg, i.p.) in C57BL/6J mice. Thioperamide was injected 10 min before each cocaine-pairing session. The activity scores recorded on the first cocaine-pairing session were also used to test the effects of thioperamide on cocaine-induced locomotor activity. Thioperamide alone had no reinforcing effects and did not affect the conditioned place preference induced by 8 mg/kg cocaine. However, thioperamide dose-dependently revealed a conditioned place preference induced by 2 mg/kg cocaine, a dose that was inactive per se. Finally, thioperamide dose-dependently potentiated the stimulant effects of cocaine, in spite of its slight hypolocomotor effect when given alone. Our results strongly suggest that H3 antagonists potentiate the stimulant and reinforcing effects of cocaine in mice.

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.

Modulation of cellular activity and synaptic transmission in the ventral tegmental area

The mesolimbic dopamine system, of which the cell bodies are located in the ventral tegmental area, has been implicated in the physiology of reward and the related pathophysiology of drug abuse. This area has been a site of significant interest to study the effects of drugs of abuse and neurotransmitter systems implicated in the rewarding effects of these compounds. One important aspect of synaptic transmission is the ability of synapses to strengthen or weaken their connection as a consequence of synaptic activity. Recently, it has become apparent that this phenomenon is also present in the ventral tegmental area and that this may bear important functional consequences for the ways in which drugs of abuse assert their effect. Here, we will review the effects of neurotransmitter systems and drugs of abuse on cellular activity and synaptic transmission in the ventral tegmental area.

Histamine excites GABAergic cells in the rat substantia nigra and ventral tegmental area in vitro

We have investigated the effect of histamine (HA) on spontaneous firing of dopaminergic (DA) and GABAergic neurons in the substantia nigra (SN) and the ventral tegmental area (VTA) of the rat in vitro. Single-unit extracellular recordings were obtained and drugs were bath applied. In both regions application of HA (10 and 100 microM) did not affect the firing frequency of DAergic cells, but increased the firing of GABAergic neurons. The histamine-induced excitation was blocked by the H(1) receptor antagonist mepyramine (1 microM), but was unaffected by application of the H(2) antagonist cimetidine (50 microM) or the H(3) antagonist thioperamide (10 microM). Our results suggest that histamine does not directly inhibit dopaminergic neurons in SN and VTA, but rather that this inhibition is mediated through histamine-induced excitation of GABAergic neurons.

The physiology of brain histamine

Histamine-releasing neurons are located exclusively in the TM of the hypothalamus, from where they project to practically all brain regions, with ventral areas (hypothalamus, basal forebrain, amygdala) receiving a particularly strong innervation. The intrinsic electrophysiological properties of TM neurons (slow spontaneous firing, broad action potentials, deep after hyperpolarisations, etc.) are extremely similar to other aminergic neurons. Their firing rate varies across the sleep-wake cycle, being highest during waking and lowest during rapid-eye movement sleep. In contrast to other aminergic neurons somatodendritic autoreceptors (H3) do not activate an inwardly rectifying potassium channel but instead control firing by inhibiting voltage-dependent calcium channels. Histamine release is enhanced under extreme conditions such as dehydration or hypoglycemia or by a variety of stressors. Histamine activates four types of receptors. H1 receptors are mainly postsynaptically located and are coupled positively to phospholipase C. High densities are found especially in the hypothalamus and other limbic regions. Activation of these receptors causes large depolarisations via blockade of a leak potassium conductance, activation of a non-specific cation channel or activation of a sodium-calcium exchanger. H2 receptors are also mainly postsynaptically located and are coupled positively to adenylyl cyclase. High densities are found in hippocampus, amygdala and basal ganglia. Activation of these receptors also leads to mainly excitatory effects through blockade of calcium-dependent potassium channels and modulation of the hyperpolarisation-activated cation channel. H3 receptors are exclusively presynaptically located and are negatively coupled to adenylyl cyclase. High densities are found in the basal ganglia. These receptors mediated presynaptic inhibition of histamine release and the release of other neurotransmitters, most likely via inhibition of presynaptic calcium channels. Finally, histamine modulates the glutamate NMDA receptor via an action at the polyamine binding site. The central histamine system is involved in many central nervous system functions: arousal; anxiety; activation of the sympathetic nervous system; the stress-related release of hormones from the pituitary and of central aminergic neurotransmitters; antinociception; water retention and suppression of eating. A role for the neuronal histamine system as a danger response system is proposed.

Dopaminergic effects of histamine administration in the nucleus accumbens and the impact of H1-receptor blockade

The mesolimbic dopamine system is thought to play a critical role in reward-related processes. A number of studies have shown that lesion or inhibition of histaminergic neurons acting through H1 receptors can potentiate the effects of drug-induced reward (e.g., psychostimulants and opioids) and can enhance the reinforcing effects of electrical stimulation of the brain. Since dopamine transmission in the nucleus accumbens is thought to provide a crucial link in these histaminergic actions, we examined the effects of local histamine application (0.1, 1.0 and 10.0 micromol/l) on dopamine and its metabolites in the nucleus accumbens of anesthetized rats by means of unilateral reverse dialysis. To study the influence of H1 receptors, we also applied the H1-receptor antagonist pyrilamine (10.0 and 20.0 mg/kg, intraperitoneally) 20 min before histamine administration (1 mmol/l). Finally, pyrilamine (0.1, 1.0 and 10.0 micromol/l) was locally administered into the nucleus accumbens. The data show that histamine can enhance extracellular dopamine levels in the nucleus accumbens in a dose-dependent way. This increase was partially antagonized by prior peripheral administration of 10 mg/kg, and was completely blocked by 20 mg/kg, of pyrilamine. Finally, intra-accumbens administration of pyrilamine locally decreased dopamine and increased dihydroxyphenylacetic acid and homovanillic acid levels. These data are discussed with respect to the possible interactions between dopaminergic and histaminergic mechanisms in the mesolimbic system and their relation to mechanisms of reinforcement.

Food-deprived activity stress decreased the activity of the histaminergic neuron system in rats

The hypothalamus, which is rich in histaminergic neurons, is highly sensitive to aversive stimuli such as stress. Histamine H3 receptors, which regulate histamine release from the presynaptic site, are associated with stress-induced brain activity. In this study, we investigated the changes of histamine content and histamine H1 and H3 receptors in the brains of rats subjected to stress induced through food deprivation and physical activity on a running wheel (food-deprived activity stress). For purposes of comparison, we also examined the stressful effects of forced swimming on the histaminergic neuron system of rats. The H3 receptor density rapidly declined in the acute phase of stress but gradually returned to the control level in the chronic phase. On the other hand, the H1 receptor slowly decreased and remained at a low level during the chronic phase. These results reveal that there is a discrepancy between the levels of H1 and H3 receptors in the acute and chronic phases of stress. Brain histamine content gradually increased during the late phase of both food-deprived activity stress and forced swimming stress. These changes presumably resulted in the inhibition of histaminergic neuronal activity in the chronic stress condition. In accordance with this hypothesis, the intraventricular administration of histamine significantly reduced the hyperactivity caused by food-deprived activity stress. Since extensive exercise and restricted feeding are thought to be associated with anorexia nervosa, the abnormalities in the histaminergic neuron system might contribute to trait status in anorexia nervosa.

Unilateral lesion of dorsal hippocampus enhances reinforcing lateral hypothalamic stimulation in the contralateral hemisphere

Whereas convincing evidence exists for an important role of the hippocampus in mechanisms underlying memory and encoding of location in space, the contribution of the hippocampus to the system underlying central processes of reinforcement is less well established. Scattered data suggesting that hippocampal ablation increases the effectiveness of positive reinforcers have alternatively been interpretated in terms of general and unspecific behavioral disinhibition, which results in higher levels of activity and rates of responding. In the present experiment, 22 Wistar rats were either given a neurotoxic or a sham lesion in the CA1 region of the hippocampus, and the effect on lateral hypothalamic self-stimulation behavior was assessed. To control for nonspecific performance effects rates of lever pressing were assessed ipsi- and contralateral to the lesioned hemisphere as well as under condition of extinction (current set to zero). Following the neurotoxic lesion the animals displayed significant higher rates of self-stimulation at the electrode sites in the hypothalamus situated contralateral but not ipsilateral to the hemisphere with the lesion compared with controls. The increase in self-stimulation commenced on the third day postlesion and was maintained over the 8 days of testing. The lesion did not change the animals' behavior under extinction. Thus, the hippocampal lesion led to an amplification of rewarding lateral hypothalamic self-stimulation behavior, indicative of a lesion induced disinhibition of the brain's reinforcement system.

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

The tuberomammillary nucleus (TM), a cluster of magnocellular cells in the posterior hypothalamus, is the main source of neuronal histamine in the brain. Although this nucleus is well described in terms of anatomy and neurochemistry, only little is known about its function. Our earlier work showed that the TM projection system may be involved in behavioral asymmetries and behavioral recovery after unilateral manipulations of the brain. Using horseradish peroxidase (HRP) labeling we found an increase in strength (structure and/or activity) in the crossed and uncrossed tuberomammillary-striatal projections in the course of recovery from behavioral asymmetries produced by unilateral removal of the rats' vibrissae, which were in the same direction as the asymmetries found in projections from the substantia nigra to the striatum. Experiments performed with unilateral lesions of the TM region provide evidence for an involvement of the TM system in reinforcement mechanisms. Unilateral destruction of the TM with direct current (DC) or ibotenic acid was found to increase the rate of lateral hypothalamic self-stimulation ipsilateral to the lesion site, suggesting that the TM (particularly the E2 subgroup in its rostral part) may function as a reinforcement inhibiting neural substrate. Experiments performed with bilateral DC or ibotenic acid lesions of the TM region suggest a role of the nucleus in learning and mnemonic processes. A bilateral electrolytic or neurotoxic lesion of the TM region was found to facilitate the performance of adult and behaviorally impaired aged rats in a variety of learning tasks, including a habituation paradigm, aversively motivated learning tasks and water mazes. Examination of the site of the neurotoxic lesion in the TM region with immunohistochemical techniques revealed a marked decline of histamine-staining neurons mainly in the rostral part of the TM nucleus, suggesting that the facilitatory effects on reinforcement and mnemonic processes might be related to the destruction of TM intrinsic histaminergic cells. In summary, the present results indicate that the TM nucleus is involved in neural plasticity and functional recovery following damage to the CNS and may function as an inhibitory neural substrate in the control of reinforcement and mnemonic processes.

Lateralized attenuation of hypothalamic self-stimulation after injecting histamine synthesis blocker alpha-FMH into the E2 tuberomammillary subnucleus

The tuberomammillary nucleus (TM), located in the posterior hypothalamic region, is the only known source of neuronal histamine. Unilateral lesions in the rostroventral part of this nucleus enhanced ipsihemispheric lateral hypothalamic self-stimulation behavior, suggesting that this region exerts inhibitory control over the neuronal systems related to reward or reinforcement processes. To examine whether the amplification of reinforcing stimulation following lesions of histamine synthesizing neurons is indeed histamine mediated, we blocked histamine synthesis unilaterally by injection of 200 microg alpha-fluoromethylhistidine into the E2 region of the TM, and assessed the effects on electrical self-stimulation behavior in the lateral hypothalamus (LH) of rats. Based on the finding that TM lesions facilitated such self-stimulation behavior, we hypothesized that this treatment would have similar effects. Unexpectedly, there was a sharp decrease in the rate of ipsihemispheric lateral hypothalamic self-stimulation following the injection of alpha-FMH compared to the contralateral hemisphere of treated animals as well as compared to the vehicle group. Response rates were most strikingly attenuated 1 h postinjection, but remained low over the whole 7 days of testing. Opposite behavioral effects of TM lesions and alpha-FMH application have been reported previously, and the effectiveness of alpha-FMH in reducing brain histamine levels is known to differ between brain regions. The fact that the alpha-FMH injection affected self-stimulation only in the ipsilateral hemisphere rules out an interpretation of the results in terms of unspecific effects of the treatment on arousal and other performance variables, and, instead, indicates a functional interaction with a subsystem linked to lateral hypothalamic reinforcement processes.

The tuberomammillary nucleus region as a reinforcement inhibiting substrate: facilitation of ipsihypothalamic self-stimulation by unilateral ibotenic acid lesions

The tuberomammillary nucleus (TM), located in the posterior hypothalamic region, consists of five subgroups and is the only known source of brain histamine. Knowledge about the function of this nucleus is still scarce. In a previous study we found an increase in the rate of ipsihemispheric hypothalamic self-stimulation following a dc lesion in the rostroventral part of this nucleus, suggesting that this region has an inhibitory action on a neuronal reward system or on the brain's reinforcement mechanism. In the present study we examined whether this facilitating effect on reinforcement was due to the destruction of fibers passing through the lesion area or of intrinsic cells, by lesioning subgroups of the TM with ibotenic acid, an excitatory amino acid, that selectively destroys neural cell bodies, leaving fibers largely intact. Following such lesions in the rostroventral part of the TM the operant response rates increased over the six days of testing when the animals stimulated themselves in the lateral hypothalamus in the hemisphere located ipsilateral but not contralateral to the lesion. No significant changes in response rate occurred following the lesion in the caudal part of the ventral TM. The results indicate that the region influenced by the lesion exerts inhibitory control over lateral hypothalamic self-stimulation, and that it is possible that histamine-containing neurons are involved in this effect.

Amplification of rewarding hypothalamic stimulation following a unilateral lesion in the region of the tuberomammillary nucleus

The tuberomammillary nucleus, a cluster of cells in the posterior hypothalamus, is the only known source of brain histamine. Although this nucleus is well described in terms of anatomy and neurochemistry, only little is known about its function. In the present study, the effect of a lesion in the region of this nucleus on intracranial self-stimulation was examined. Rats were implanted bilaterally with stimulating electrodes in the lateral hypothalamus and unilaterally with one lesion electrode in the region of this nucleus. After three days of baseline testing, half of the animals were given an electrolytic lesion. The animals were retested for six consecutive days, and thereafter weekly for another seven weeks. From the second day postlesion on, we unexpectedly found a gradual increase in response rate, which peaked on day 13 in the ipsilateral hemisphere only. Although there was no further increase over subsequent days, response rates remained elevated during the following seven weekly tests. The observed increase in lateral hypothalamic self-stimulation after an electrolytic lesion of the tuberomammillary nucleus is discussed in terms of an inhibitory system, possibly located in the region of this nucleus which, when removed by the lesion, increased reinforcing effects of the electrical brain stimulation. The fact that the effects on self-stimulation were lateralized to one hemisphere rules out an interpretation in terms of unspecific "performance" variables that could influence rate of lever pressing.

L-histidine attenuates the effects of pentazocine on rewarding brain-stimulation

Previous research has indicated that the antihistamine tripelennamine potentiates the threshold lowering effects of pentazocine on brain stimulation reward, a model of drug-induced euphoria. To determine the importance of histamine in this interaction, we studied the effects of co-administration of L-histidine and pentazocine on the threshold for brain stimulation reward. Pentazocine (2.5-10.0 mg/kg) lowered the threshold for rewarding stimulation to the medial forebrain bundle-lateral hypothalamus in male F344 rats. L-histidine (500 and 750 mg/kg) by itself had no significant effects, yet antagonized the threshold lowering effects of pentazocine. These doses of L-histidine are known to significantly raise brain histamine concentrations. Our results suggest that histamine may play a tonic inhibitory role, at least in part, on the neural systems responsible for the reinforcing properties of pentazocine.

Responses of hypothalamic paraventricular neurons in vitro to norepinephrine and other feeding-relevant agents

To investigate paraventricular hypothalamic neuronal actions responsible for the effects of neurotransmitters on feeding, and to test the notion that a single population of cells there could account for feeding effects, hypothalamic slices containing the paraventricular nucleus (PVN) were prepared from rats. Electrophysiological responses of individual PVN neurons to feeding-inducing agents norepinephrine (NE) and gamma-aminobutyric acid (GABA), and to anorexic agents serotonin (5-HT) and histamine (Hist) were examined. NE inhibited neuronal activity through alpha 2-adrenergic receptors, and excited through alpha 1-receptors. alpha 2-receptors are known to mediate the behavioral effect of NE. NE inhibited most clearly those neurons that otherwise fired continuously in this type of in vitro preparation. GABA affected the activity of 37% of the neurons tested, primarily by inhibition. The inhibitory action of GABA can be related to its feeding-inducing effect. GABA in PVN can also attenuate excitatory responses and enhance inhibitory responses to NE or 5-HT. 5-HT caused excitatory and inhibitory responses with the former action outnumbering the latter by approximately 3 to 1. Since this would result in a net excitation, it appears that 5-HT in PVN inhibits feeding mainly by exciting neuronal activity. Hist excited 72% and inhibited only 2% of PVN neurons. The excitation was blocked by H1-antagonists, which have been shown to mediate Hist effect on feeding. Comparing across neurons, the inhibitory response to NE was correlated with that to GABA, but not with any responses to 5-HT or Hist. The excitatory responses to Hist correlated with 5-HT responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioural effects of histamine and its antagonists: a review

This review focuses on the behavioural effects of histamine and drugs which affect histaminergic function, particularly the H1- and H2-receptors antagonists. Research in this area has assumed considerable importance with increasing interest in the role of brain histamine, the clinical use of both H1 and H2 antagonists and evidence of nonmedical use of H1 antagonists. Results from a number of studies show that H1 and H2 antagonists have clear, but distinct subjective effects and that H1 antagonists have discriminative effects in animals. While H1 antagonists are reinforcers in certain conditions, histamine itself is a punisher. Moderate doses of H1 antagonists affect psychomotor performance in some situations, but the results are variable. The exceptions are terfenadine and astemizole, which do not seem to penetrate the blood-brain barrier readily. In studies of schedule-controlled behaviour, marked changes in response rate have been observed following administration of H1 antagonists, with the magnitude and direction dependent on the dose and the baseline behaviour. Histamine reduces avoidance responding, an effect mediated via H1-receptors. Changes in drinking and aggressive behaviour have also been observed following histamine administration and distinct roles for H1- and H2-receptors have been delineated. Separate H1- and H2-receptor mechanisms have also been suggested to account for changes in activity level. While the H2 antagonists do not always have strong behavioural effects when administered peripherally, there is evidence that cimetidine has a depressant effect on sexual function. These and other findings reveal an important role for histaminergic systems in a wide range of behaviour.

Food intake and selection after peripheral tryptophan

Two studies investigated the effects of peripheral (IP) administration of the dietary indispensible amino acid tryptophan, on food intake and macronutrient selection in rats adapted to a 12 hr nocturnal feeding period and a choice of 10% and 60% casein diets. In a dose-response study (35, 55, 75, 95, 115 mg/kg), the threshold dose of 75 mg/kg produced a significant reduction in total food intake (3.6 to 2.3 g, p less than 0.05) during the first hour of feeding. The reduction in carbohydrate intake (2.1 vs. 1.2 g, p less than 0.05) was greater than that for protein intake (1.6 vs. 1.1 g, p less than 0.05). Twelve hr total food intake was also decreased (20.9 to 19.5 g, p less than 0.05) and this was attributable to decreased carbohydrate intake (13.2 to 11.8 g, p less than 0.05). In a second study designed to determine if tryptophan's effects were mediated by the central nervous system, brain tryptophan uptake was blocked by co-injecting valine with tryptophan. The significant reduction in first hour total food intake by tryptophan was not prevented by co-injection of an equal quantity of valine (3.5 to 1.8 g, p less than 0.05). Again the suppression of carbohydrate intake (2.0 to 0.9 g p less than 0.05) was greater than that for protein intake (1.5 to 0.9 g, p less than 0.05). This dose of valine significantly reduced brain tryptophan uptake by 16% (21.3 to 17.8 micrograms/g, p less than 0.05) and when administered alone did not affect first hour total food intake (3.1 vs 3.2 g).(ABSTRACT TRUNCATED AT 250 WORDS)

Food intake suppression by histidine

Following injection of histidine (as 1-histidine monohydrochloride, 500 mg/kg, IP) rats showed a suppression of total food intake within the first 2 hours of a 12 hour daily feeding period but not if the rats were adapted to a 4 hour daily feeding period. Furthermore, rats adapted to a nocturnal as compared to a diurnal 12 hour feeding period showed a greater response (50% vs. 20% suppression of feeding) to histidine. Overall, within an experiment, food intake suppression correlated with the histidine dose (0, 125, 250, 375 and 500 mg/kg; for mean response r(3) = 0.90, p less than 0.05) although the lowest dose measured to be effective in a cross-over design experiment was 375 mg/kg. No differential effect upon protein or carbohydrate intake was observed in any of the studies. The effects of injections of 250 and 500 mg/kg histidine on food intake were associated with significant elevations of brain histidine and histamine. We conclude that histidine, possibly by changes in brain histidine, influences total food intake but not macronutrient selection.

Further evidence that histamine in hippocampus affects the exploratory behavior in the rat

The effect of histamine (HA) and 3-methyl-histamine (3-MHA) in the hippocampus on hole-board behavior was studied. Male rats were microinjected stereotaxically into the hippocampus with 1 microliter of saline solution containing 9, 45 or 90 nMol of HA or 3-MHA. Five min later, all rats were tested for 5 min in a hole-board and locomotor, rearing, grooming and head-dipping activities were measured by direct observation. HA inhibited rearing and grooming at all the doses used but it affected locomotor activity only at 45 and 90 nMol doses. There was no effect on head-dipping behavior. 3-MHA instead increased locomotor and head-dipping activities at all the doses used and it did not change rearing and grooming. The present results give a further support for a physiological participation of HA in the hippocampus.

The effect of a vasopressin antagonist on the pressor response to histamine administered centrally

The mechanism for the pressor response to intracerebroventricularly (i.c.v.) administered histamine was studied. Histamine (HA), when injected intracerebroventricularly in rats, produced a dose-dependent increase in mean arterial pressure (MAP) which was subject to tachyphylaxis. Spinal transection at C-7 in the anesthetized rat did not attenuate the rise in blood pressure. The possibility that the release of 8-arginine vasopressin was responsible for the rise in blood pressure was explored. By pretreating conscious freely-moving rats with a specific antagonist to the vasopressor action of vasopressin viz., [1-beta-mercapto-beta, beta-cyclopentamethyleneproprionic acid), 2-(O-methyl) tyrosine] arginine-vasopressin, there was a statistically-significant attenuation of the pressor response to intracerebroventricularly administered histamine. The antagonist however, did not totally abolish the pressor response, regardless of the dose employed. Concomitant administration of hexamethonium and the vasopressin antagonist did not further attenuate the response. Previous adrenal demedullation did not diminish the response to intracerebroventricularly administered histamine, nor was there any evidence for release of angiotensin II since pretreatment with saralasin did not attenuate the cardiovascular response. These findings suggest that vasopressin along with other as yet undefined substances, are released from the central nervous system to produce the increase in blood pressure after intracerebroventricularly administered histamine.

Brain histamine-plasma corticosterone interactions

Significant elevation in plasma corticosterone of rats achieved by intraperitoneal (i.p.) administration of corticosterone (2.4 mg/kg) was associated with a rapid (2.5 min) and significant increase in hypothalamic histamine (HA) levels which persisted for 60 min. Midbrain and cortical HA concentrations were not affected. Significant and prolonged elevation of hypothalamic, midbrain and cortical HA levels was achieved by L-histidine administration (500 mg/kg i.p.). The most significant increase was noted in the hypothalamus and persisted for 10 hours. The elevated brain HA levels were associated with significant increase in plasma corticosterone levels which lasted for 120 mins. Present data supports the involvement of central HA in endocrine function.

Discriminating between reward and performance: a critical review of intracranial self-stimulation methodology

Despite numerous pharmacological investigations of intracranial self-stimulation (ICSS), the substrates of this behavior have yet to be completely understood. In view of the likelihood that inadequate methodology has hindered the quest for these substrates, the present review was undertaken. Criteria for ICSS methodology should include not only the ability to discriminate reward from gross performance deficit, but also adequate capacity (ability to generate experimental data at a reasonable rate). For numerous reasons, bar-pressing on a continuous reinforcement schedule fails the first criterion despite its ease and rapidity. The use of partial reinforcement schedules may alleviate some of these shortcomings. Analysis of drug-induced response decrement patterns can discriminate gross motoric incapacity from other variables, although the question of subtle response maintenance deficits remains to be answered. Measurements of response rates using alternative operants do not differentiate reward and performance adequately. More promising, "rate-free" measures using locomotion as an operant include the two-platform method of Valenstein and the "locus of rise" method. Comparison of drug effects on ICSS with those on alternate tasks are fraught with pitfalls including the problems of assuring equivalent rates and patterns of responding. The use of differential electrode placements is ideally suited for neurochemically well-characterized drugs, particularly if "double dissociations" can be established during studies of multiple placements. Presentation of different current intensities or frequencies permits the compilation of rate-intensity functions, and drug-induced shifts in these functions have considerable analytical power. Self-regulation of current intensity constitutes a powerful tool that has yet to realize its full potential in the pharmacological study of ICSS. Extensive studies involving self-regulation of stimulation duration ("shuttlebox" studies) suggest that this method may be highly versatile despite several practical difficulties. It is concluded that at least six of these methods appear to do a reasonable job of excluding gross performance deficit. However, the possible influences of other factors, such as subtle response maintenance deficit, incentive or arousal, remain to be resolved in view of the multifactorial nature of ICSS. Multiple tests for ICSS drug or lesion studies are advocated whenever feasible, as no single test appears capable of resolving all theoretical complexities.

Response of local blood flow in the caudate nucleus of the cat to intraventricular administration of histamine

The effect of intraventricular histamine on blood flow in the caudate nucleus of the cat was studied by means of the hydrogen clearance technique. Bilateral ventriculo-cisternal perfusion was installed. After a control period during which both lateral ventricles were perfused with mock CSF with the same composition, the drug under study was added to one side (experimental side) while the other side was perfused further with the control mock SCF (control side). At each point in time, blood flow at the experimental side was compared to that at the control side. Histamine (10(-3) M) caused a severe vasodilatation and this effect was completely antagonised by the H2-receptor blocker cimetidine (10(-2) M). Cimetidine had no vasoactive effects of itself in the concentration used. The H2-receptor agonist Dimaprit (10(-3) M) had a vasodilator effect although less important than histamine. Indirect evidence was gained that H1-receptors are not active in the vascular bed under study.

Effects of histamine microinjections into the hippocampus on open-field behavior in rats

Increasing evidence supports the concept that histamine would subserve neurotransmitter functions in the brain. In hippocampus, some cases of histamine receptors coupled to adenylate cyclase has been described. In our laboratory we were interested in studying the possible physiological role of these histamine receptors and behavior. Unilateral cannulae were stereotaxically implanted in the caudal hippocampus in adult male rats. Forty-eight hours later, after an acute microinjection of histamine (1 microgram), 3-methyl-histamine (1 microgram) or saline (1 microliter), the following behavioral parameters were measured by an observer in an open-field: locomotor, rearing, sniffing, grooming, social sniffing and holding activities. It was observed that histamine inhibited locomotor and rearing activities; whereas, 3-methyl-histamine induced an increase in almost all behaviors. Results support the concept of a probable function of histamine in exploratory behavior in the rat.

Effect of diphenhydramine on stress-induced changes in brain histidine decarboxylase activity, histamine and plasma corticosterone levels

Exposure of rats to platform stress induced a significant elevation in hypothalamic histamine levels. Air blast-stress resulted in a significant increase in hypothalamic histamine concentration and in histidine decarboxylase activity. No significant changes were noted either in the enzyme activity or in histamine levels in the midbrain or cortex of stressed rats. In the nonstressed rats, diphenhydramine (7.5 mg/kg intragastrically), a H1-receptor antagonist, did not influence histidine decarboxylase activity or histamine concentration in any of the three brain regions investigated. However, diphenhydramine pretreatment prevented the increase in histidine decarboxylase activity induced by air blasts. In untreated rats, plasma corticosterone levels were significantly elevated following either platform stress (4.5-fold) or air blasts (7.8-fold). A significant increase was also noted in saline and diphenhydramine-treated animals following these stressors, however, the increase in saline or diphenhydramine treated rats following air blasts was significantly less than that seen in untreated stressed controls.

Effect of histaminergic drugs on footshock-induced aggressive behaviour in rats

Histamine and 2-pyridylethylamine decreased and 4-methylhistamine increased footshock-induced aggression after their intracerebroventricular administration to rats. Mepyramine and cimetidine had no effect on the fighting response but blocked the effects of the respective agonists. Histamine when given after mepyramine increased and after cimetidine decreased the fighting score, this decrease being more than that with histamine alone. It is inferred that central H1-receptors are inhibitory and H2-receptors facilitatory in footshock aggression and that this is independent of catecholaminergic mechanisms.

Effect of diazepam on stress induced changes in brain histamine

Acute treatment with diazepam (2.5, 5 and 10 mg/kg) did not affect the basal histamine concentration in the hypothalamus, midbrain or in the cortex of the rat. The increase in the hypothalamic histamine level caused by 15 min of "platform stress" was significantly attenuated by diazepam (5 or 10 mg/kg) pretreatment, but the elevation induced by 15 min of air blasts remained unchanged. Diazepam significantly reduced the rise in plasma corticosterone concentration in response to air blast stress but did not affect the increase caused by "platform stress." Thus, in addition to the already known effects of diazepam on stress induced changes in other central neurotransmitters or neuromodulators, diazepam may also affect the hypothalamic histamine elevation induced by certain type of stress.

Brain histamine: plasma corticosterone spontaneous locomotor activity and temperature

Hypothalamic histamine exhibited circadian fluctuations in male Sprague-Dawley rats; low values were found during the dark period when spontaneous locomotor activity (S.L.A.) and temperature were elevated. A relatively high hypothalamic histamine level was observed during the early period of the light cycle and was associated with decreased S.L.A. and temperature. Histamine concentration was high when corticosterone levels were low at the end of the dark cycle and during the morning hours (4 a.m.-1 p.m.); but histamine levels were relatively constant while corticosterone concentration dropped during afternoon and early night hours (4 p.m.-10 p.m.). Furthermore, the lowest hypothalamic histamine level (at 1 a.m.) was associated with the average plasma corticosterone value, thus no consistent relationship between histamine and corticosterone levels could be observed. Circadian fluctuations in brain histamine may support its role in brain function.

Single and repeated air blast stress and brain histamine

Exposure of rats to air blasts for 1, 5 and 15 min resulted in a significant increase in plasma corticosterone level and in the hypothalamic histamine concentration. Midbrain histamine content was increased after 1 and 5 min of exposure but cortical histamine increased following 1 min of exposure only. Stress of longer duration (30 mins did not significantly affect histamine concentration in any of the three brain regions investigated, although plasma corticosterone level remained very significantly (14.5-fold) elevated. Repeated exposure of rats to air blasts of 15 min duration resulted in a significant elevation of hypothalamic histamine concentration while midbrain and cortical histamine was not significantly altered. Plasma corticosterone level was again very significantly (10-fold) increased. Present results suggest the involvement of brain histamine in the response to stress.

Effect of stress on brain histamine

Stress of short duration (5 min) resulted in a significant increase in plasma corticosterone level and a significant decrease in the midbrain histamine concentration in rats. Exposure to 15 min stress caused a significant elevation in the hypothalamic histamine level. Stress of longer duration (30 or 60 min) did not affect hypothalamic, cortical or midbrain histamine concentration although plasma corticosterone level remained elevated. Repeated exposures of rats of 15 min stress did not significantly alter histamine concentration of any of the brain regions studied. Plasma corticosterone concentration was only 28% of that observed in animals exposed to single 15 min stress. Present data suggest a role of brain histamine in response to stress.

Cholinergic link in the histamine-mediated increase in homovanillic acid in the rat striatum

Administration of 500 microng of histamine into the lateral brain ventricle of the rat produced a 50% increase in striatal HVA. The DA level remained unchanged. The histamine-mediated elevation of HVA was abolished by pre-treatment with the antihistaminic drug - mepyramine. Moreover atropine significantly reduced the histamine-induced increase of HVA. It is concluded that histamine primarily activates cholinergic neurons and that the changes in DA metabolism are a consequence of an increased cholinergic activity.