The cytoarchitecture, histochemistry and projections of the tuberomammillary nucleus in the rat

Structural Basis for Histaminergic Regulation of Neural Circuits in the Mouse Olfactory Bulb

Odor information is modulated by centrifugal inputs from other brain regions to the olfactory bulb (OB). Neurons containing monoamines, such as serotonin, acetylcholine, and noradrenaline, are well known as centrifugal inputs; however, the role of histamine, which is also present in the OB, is not well understood. In this study, we examined the histaminergic neurons projecting from the hypothalamus to the OB. We used an antibody against histidine decarboxylase (HDC), a synthesizing enzyme of histamine, to identify histaminergic neurons and assess their localization within the OB and the ultrastructure of their fibers and synapses using multiple immunostaining laser microscopy, ultra-high voltage electron microscopy (EM), and EM to confirm their relationships with other neurons. To further identify the origin nucleus of the histaminergic neurons projecting to the OB, we injected the retrograde tracer FluoroGold and analyzed the pathway to the OB anterogradely. HDC-immunoreactive (-ir) fibers were abundant in the olfactory nerve (ON) layer compared to other monoamines. HDC-ir neurons received asymmetrical synapses from ONs and formed synapses containing pleomorphic vesicles with variable postsynaptic densities to non-ON elements, thus forming serial synapses. We also confirmed that histaminergic neurons project from the rostral ventral tuberomammillary nucleus to the granule cell layer of the OB and, for the first time, successfully visualized their axons from the hypothalamus to the OB. These findings indicate that histamine may regulate odor discrimination in the OB, suggesting a regulatory relationship between hypothalamic function and olfaction. We thus elucidate morphological mechanisms with tuberomammillary nucleus-derived histaminergic neurons involved in olfactory information.

Topographic organization of bidirectional connections between the cingulate region (infralimbic area and anterior cingulate area, dorsal part) and the interbrain (diencephalon) of the adult male rat

The medial prefrontal cortex [cingulate region (Brodmann, 1909) (CNG)] in the rat is a connectionally and functionally diverse structure. It harbors cerebral nuclei that use long-range connections to promote adaptive changes to ongoing behaviors. The CNG is often described across functional and anatomical gradients, a dorsalventral gradient being the most prominent. Topographic organization is a general feature of the nervous system, and it is becoming clear that such spatial arrangements can reflect connectional, functional, and cellular differences. Portions of the CNG are known to form reciprocal connections with cortical areas and thalamus; however, these connectional features have not been described in detail or mapped to standardized rat brain atlases. Here, we used co-injected anterograde (Phaseolus vulgaris leucoagglutinin) and retrograde (cholera toxin B subunit) tracers throughout the CNG to identify zones of reciprocal connectivity in the diencephalon [or interbrain (Baer, 1837) (IB)]. Tracer distributions were observed using a Nissl-based atlas-mapping approach that facilitates description of topographic organization. This draft report describes CNG connections of the infralimbic area (Rose & Woolsey, 1948) (ILA) and the anterior cingulate area, dorsal part (Krettek & Price, 1977) (ACAd) throughout the IB. We found that corticothalamic connections are predominantly reciprocal, and that ILA and ACAd connections tended to be spatially segregated with minimal overlap. In the hypothalamus (Kuhlenbeck, 1927), we found dense and specific ILA-originating terminals in the following Brain Maps 4.0 atlas territories: dorsal region (Swanson, 2004) (LHAd) and suprafornical region (Swanson, 2004) (LHAs) of the lateral hypothalamic area (Nissl, 1913), parasubthalamic nucleus (Wang & Zhang, 1995) (PSTN), tuberal nucleus, terete part (Petrovich et al., 2001) (TUte), and an ill-defined dorsal cap of the medial mammillary nucleus (Gudden, 1881) (MM). We discuss these findings in the context of feeding behaviors.

Histamine in the neocortex: Towards integrating multiscale effectors

Histamine is a modulatory neurotransmitter, which has received relatively less attention in the central nervous system than other neurotransmitters. The functional role of histamine in the neocortex, the brain region that controls higher-order cognitive functions such as attention, learning and memory, remains largely unknown. This article focuses on the emerging roles and mechanisms of histamine release in the neocortex. We describe gaps in current knowledge and propose the application of interdisciplinary tools to dissect the detailed multiscale functional logic of histaminergic action in the neocortex ranging from sub-cellular, cellular, dendritic and synaptic levels to microcircuits and mesoscale effects.

Histaminergic regulation of food intake

Histamine is a biogenic amine that acts as a neuromodulator within the brain. In the hypothalamus, histaminergic signaling contributes to the regulation of numerous physiological and homeostatic processes, including the regulation of energy balance. Histaminergic neurons project extensively throughout the hypothalamus and two histamine receptors (H1R, H3R) are strongly expressed in key hypothalamic nuclei known to regulate energy homeostasis, including the paraventricular (PVH), ventromedial (VMH), dorsomedial (DMH), and arcuate (ARC) nuclei. The activation of different histamine receptors is associated with differential effects on neuronal activity, mediated by their different G protein-coupling. Consequently, activation of H1R has opposing effects on food intake to that of H3R: H1R activation suppresses food intake, while H3R activation mediates an orexigenic response. The central histaminergic system has been implicated in atypical antipsychotic-induced weight gain and has been proposed as a potential therapeutic target for the treatment of obesity. It has also been demonstrated to interact with other major regulators of energy homeostasis, including the central melanocortin system and the adipose-derived hormone leptin. However, the exact mechanisms by which the histaminergic system contributes to the modification of these satiety signals remain underexplored. The present review focuses on recent advances in our understanding of the central histaminergic system's role in regulating feeding and highlights unanswered questions remaining in our knowledge of the functionality of this system.

The activation of GABAergic neurons in the hypothalamic tuberomammillary nucleus attenuates sevoflurane and propofol-induced anesthesia in mice

Background: The histaminergic neurons in the hypothalamic tuberomammillary nucleus (TMN) have been suggested to play a vital role in maintaining a rising state. But the neuronal types of the TMN are in debate and the role of GABAergic neurons remains unclear. Methods: In the present study, we examined the role of TMN GABAergic neurons in general anesthesia using chemogenetics and optogenetics strategies to regulate the activity of TMN GABAergic neurons. Results: The results indicated that either chemogenetic or optogenetic activation of TMN GABAergic neurons in mice decreased the effect of sevoflurane and propofol anesthesia. In contrast, inhibition of the TMN GABAergic neurons facilitates the sevoflurane anesthesia effect. Conclusion: Our results suggest that the activity of TMN GABAergic neurons produces an anti-anesthesia effect in loss of consciousness and analgesia.

The Role of the Central Histaminergic System in Behavioral State Control

Histamine is a small monoamine signaling molecule that plays a role in many peripheral and central physiological processes, including the regulation of wakefulness. The tuberomammillary nucleus is the sole neuronal source of histamine in the brain, and histamine neurons are thought to promote wakefulness and vigilance maintenance - under certain environmental and/or behavioral contexts - through their diffuse innervation of the cortex and other wake-promoting brain circuits. Histamine neurons also contain a number of other putative neurotransmitters, although the functional role of these co-transmitters remains incompletely understood. Within the brain histamine operates through three receptor subtypes that are located on pre- and post-synaptic membranes. Some histamine receptors exhibit constitutive activity, and hence exist in an activated state even in the absence of histamine. Newer medications used to reduce sleepiness in narcolepsy patients in fact enhance histamine signaling by blunting the constitutive activity of these histamine receptors. In this chapter, we provide an overview of the central histamine system with an emphasis on its role in behavioral state regulation and how drugs targeting histamine receptors are used clinically to treat a wide range of sleep-wake disorders.

Targeting Histamine and Histamine Receptors for the Precise Regulation of Feeding

Histamine has long been accepted as an anorexigenic agent. However, lines of evidence have suggested that the roles of histamine in feeding behaviors are much more complex than previously thought, being involved in satiety, satiation, feeding motivation, feeding circadian rhythm, and taste perception and memory. The functional diversity of histamine makes it a viable target for clinical management of obesity and other feeding-related disorders. Here, we update the current knowledge about the functions of histamine in feeding and summarize the underlying molecular and neural circuit mechanisms. Finally, we review the main clinical studies about the impacts of histamine-related compounds on weight control and discuss insights into future research on the roles of histamine in feeding. Despite the recent progress in histamine research, the histaminergic feeding circuits are poorly understood, and it is also worth verifying the functions of histamine receptors in a more spatiotemporally specific manner.

Different Peas in the Same Pod: The Histaminergic Neuronal Heterogeneity

The histaminergic neuronal system is recently receiving increasing attention, as much has been learned over the past 25 years about histamine role as a neurotransmitter. Indeed, this amine is crucial in maintaining arousal and provides important contributions to regulate circadian rhythms, energy, endocrine homeostasis, motor behavior, and cognition. The extent to which these distinct physiological functions are operated by independent histamine neuronal subpopulation is unclear. In the rat brain histamine neuronal cell bodies are grouped within the tuberomamillary nucleus of the posterior hypothalamus in five clusters, E1-E5, each sending overlapping axons throughout the entire central nervous system with no strict topographical pattern. These features lead to the concept that histamine regulation of a wide range of functions in the central nervous system is achieved by the histaminergic neuronal system as a whole. However, increasing experimental evidence suggesting that the histaminergic system is organized into distinct pathways modulated by selective mechanisms challenges this view. In this review, we summarized experimental evidence supporting the heterogeneity of histamine neurons, and their organization in functionally distinct circuits impinging on separate brain regions and displaying selective control mechanisms. This implies independent functions of subsets of histaminergic neurons according to their respective origin and terminal projections with relevant consequences for the development of specific compounds that affect only subsets of histamine neurons, thus increasing the target specificity.

Histaminergic neurons in the tuberomammillary nucleus as a control centre for wakefulness

Histamine plays pleiotropic roles as a neurotransmitter in the physiology of brain function, this includes the maintenance of wakefulness, appetite regulation and memory retrieval. Since numerous studies have revealed an association between histaminergic dysfunction and diverse neuropsychiatric disorders, such as Alzheimer's disease and schizophrenia, a large number of compounds acting on the brain histamine system have been developed to treat neurological disorders. In 2016, pitolisant, which was developed as a histamine H₃ receptor inverse agonist by Schwartz and colleagues, was launched for the treatment of narcolepsy, emphasising the prominent role of brain histamine on wakefulness. Recent advances in neuroscientific techniques such as chemogenetic and optogenetic approaches have led to remarkable progress in the understanding of histaminergic neural circuits essential for the control of wakefulness. In this review article, we summarise the basic knowledge about the histaminergic nervous system and the mechanisms underlying sleep/wake regulation that are controlled by the brain histamine system. LINKED ARTICLES: This article is part of a themed issue on Neurochemistry in Japan. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.4/issuetoc.

Electrophysiological Properties of Genetically Identified Histaminergic Neurons

Histaminergic neurons of the tuberomammillary nucleus (TMN) are important regulators of behavioral and homeostatic processes. Previous work suggested that histaminergic neurons exhibit a characteristic electrophysiological signature, allowing for their identification in brain slice preparations. However, these previous investigations focused on neurons in the ventral subregion of the TMN of rats. Consequently, it remains unclear whether such electrophysiological properties extend to mice, including other subregions of the TMN, and the potential for differences between males and females. To further characterize the electrophysiological properties of histaminergic neurons, we performed whole-cell patch-clamp recordings on transgenic mice expressing Cre recombinase in histidine decarboxylase (HDC)-expressing cells; the sole enzyme for histamine synthesis (Hdc-cre::tdTomato). Despite similarities with the electrophysiological properties reported in rats, we observed considerable variability in mouse HDC neuron passive membrane properties, action potential firing, and intrinsic subthreshold active membrane properties. Overall, the electrophysiological properties of HDC neurons appeared similar across subregions of the TMN, consistent with a lack of topographical organization in this nucleus. Moreover, we found no obvious sex differences in the electrical excitability of HDC neurons. However, our data reveal a diversity in the electrophysiological properties of genetically identified histaminergic neurons from mice not previously appreciated from rat studies. Thus, these data highlight the utility of mouse genetics to target the widespread histaminergic neuronal population within the TMN and support the idea that histaminergic neurons are a heterogeneous neuronal population.

Histamine: neural circuits and new medications

Histamine was first identified in the brain about 50 years ago, but only in the last few years have researchers gained an understanding of how it regulates sleep/wake behavior. We provide a translational overview of the histamine system, from basic research to new clinical trials demonstrating the usefulness of drugs that enhance histamine signaling. The tuberomammillary nucleus is the sole neuronal source of histamine in the brain, and like many of the arousal systems, histamine neurons diffusely innervate the cortex, thalamus, and other wake-promoting brain regions. Histamine has generally excitatory effects on target neurons, but paradoxically, histamine neurons may also release the inhibitory neurotransmitter GABA. New research demonstrates that activity in histamine neurons is essential for normal wakefulness, especially at specific circadian phases, and reducing activity in these neurons can produce sedation. The number of histamine neurons is increased in narcolepsy, but whether this affects brain levels of histamine is controversial. Of clinical importance, new compounds are becoming available that enhance histamine signaling, and clinical trials show that these medications reduce sleepiness and cataplexy in narcolepsy.

Brain histamine modulates recognition memory: possible implications in major cognitive disorders

Several behavioural tests have been developed to study and measure emotionally charged or emotionally neutral memories and how these may be affected by pharmacological, dietary or environmental manipulations. In this review, we describe the experimental paradigms used in preclinical studies to unravel the brain circuits involved in the recognition and memorization of environmentally salient stimuli devoid of strong emotional value. In particular, we focus on the modulatory role of the brain histaminergic system in the elaboration of recognition memory that is based on the judgement of the prior occurrence of an event, and it is believed to be a critical component of human declarative memory. The review also addresses questions that may help improve the treatment of impaired declarative memory described in several affective and neuropsychiatric disorders such as ADHD, Alzheimer's disease and major neurocognitive disorder. LINKED ARTICLES: This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc.

Sleep and Sedative States Induced by Targeting the Histamine and Noradrenergic Systems

Sedatives target just a handful of receptors and ion channels. But we have no satisfying explanation for how activating these receptors produces sedation. In particular, do sedatives act at restricted brain locations and circuitries or more widely? Two prominent sedative drugs in clinical use are zolpidem, a GABA_A receptor positive allosteric modulator, and dexmedetomidine (DEX), a selective α2 adrenergic receptor agonist. By targeting hypothalamic neuromodulatory systems both drugs induce a sleep-like state, but in different ways: zolpidem primarily reduces the latency to NREM sleep, and is a controlled substance taken by many people to help them sleep; DEX produces prominent slow wave activity in the electroencephalogram (EEG) resembling stage 2 NREM sleep, but with complications of hypothermia and lowered blood pressure—it is used for long term sedation in hospital intensive care units—under DEX-induced sedation patients are arousable and responsive, and this drug reduces the risk of delirium. DEX, and another α2 adrenergic agonist xylazine, are also widely used in veterinary clinics to sedate animals. Here we review how these two different classes of sedatives, zolpidem and dexmedetomideine, can selectively interact with some nodal points of the circuitry that promote wakefulness allowing the transition to NREM sleep. Zolpidem enhances GABAergic transmission onto histamine neurons in the hypothalamic tuberomammillary nucleus (TMN) to hasten the transition to NREM sleep, and DEX interacts with neurons in the preoptic hypothalamic area that induce sleep and body cooling. This knowledge may aid the design of more precise acting sedatives, and at the same time, reveal more about the natural sleep-wake circuitry.

Histamine H3 Heteroreceptors Suppress Glutamatergic and GABAergic Synaptic Transmission in the Rat Insular Cortex

Histamine H₃ receptors are autoreceptors that regulate histamine release from histaminergic neuronal terminals. The cerebral cortex, including the insular cortex (IC), expresses abundant H₃ receptors; however, the functions and mechanisms of H₃ receptors remain unknown. The aim of this study was to elucidate the functional roles of H₃ in synaptic transmission in layer V of the rat IC. Unitary excitatory and inhibitory postsynaptic currents (uEPSCs and uIPSCs) were obtained through paired whole-cell patch-clamp recording in cerebrocortical slice preparations. The H₃ receptor agonist, R-α-methylhistamine (RAMH), reduced the uEPSC amplitude obtained from pyramidal cell to pyramidal cell or GABAergic interneuron connections. Similarly, RAMH reduced the uIPSC amplitude in GABAergic interneuron to pyramidal cell connections. RAMH-induced decreases in both the uEPSC and uIPSC amplitudes were accompanied by increases in the failure rate and paired-pulse ratio. JNJ 5207852 dihydrochloride or thioperamide, H₃ receptor antagonists, inhibited RAMH-induced suppression of uEPSCs and uIPSCs. Unexpectedly, thioperamide alone increased the uIPSC amplitude, suggesting that thioperamide was likely to act as an inverse agonist. Miniature EPSC or IPSC recordings support the hypothesis that the activation of H₃ receptors suppresses the release of glutamate and GABA from presynaptic terminals. The colocalization of H₃ receptors and glutamate decarboxylase or vesicular glutamate transport protein 1 in presynaptic axon terminals was confirmed through double pre-embedding microscopy, using a combination of pre-embedding immunogold and immunoperoxidase techniques. The suppressive regulation of H₃ heteroreceptors on synaptic transmission might mediate the regulation of sensory information processes, such as gustation and visceral sensation, in the IC.

Histamine facilitates GABAergic transmission in the rat entorhinal cortex: Roles of H1 and H2 receptors, Na+ -permeable cation channels, and inward rectifier K+ channels

In the brain, histamine (HA) serves as a neuromodulator and a neurotransmitter released from the tuberomammillary nucleus (TMN). HA is involved in wakefulness, thermoregulation, energy homeostasis, nociception, and learning and memory. The medial entorhinal cortex (MEC) receives inputs from the TMN and expresses HA receptors (H1 , H2 , and H3 ). We investigated the effects of HA on GABAergic transmission in the MEC and found that HA significantly increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) with an EC50 of 1.3 µM, but failed to significantly alter sIPSC amplitude. HA-induced increases in sIPSC frequency were sensitive to tetrodotoxin (TTX), required extracellular Ca2+ , and persisted when GDP-β-S, a G-protein inactivator, was applied postsynaptically via the recording pipettes, indicating that HA increased GABA release by facilitating the excitability of GABAergic interneurons in the MEC. Recordings from local MEC interneurons revealed that HA significantly increased their excitability as determined by membrane depolarization, generation of an inward current at -65 mV, and augmentation of action potential firing frequency. Both H1 and H2 receptors were involved in HA-induced increases in sIPSCs and interneuron excitability. Immunohistochemical staining showed that both H1 and H2 receptors are expressed on GABAergic interneurons in the MEC. HA-induced depolarization of interneurons involved a mixed ionic mechanism including activation of a Na+ -permeable cation channel and inhibition of a cesium-sensitive inward rectifier K+ channel, although HA also inhibited the delayed rectifier K+ channels. Our results may provide a cellular mechanism, at least partially, to explain the roles of HA in the brain. © 2017 Wiley Periodicals, Inc.

Mastication as a Stress-Coping Behavior

Exposure to chronic stress induces various physical and mental effects that may ultimately lead to disease. Stress-related disease has become a global health problem. Mastication (chewing) is an effective behavior for coping with stress, likely due to the alterations chewing causes in the activity of the hypothalamic-pituitary-adrenal axis and autonomic nervous system. Mastication under stressful conditions attenuates stress-induced increases in plasma corticosterone and catecholamines, as well as the expression of stress-related substances, such as neurotrophic factors and nitric oxide. Further, chewing reduces stress-induced changes in central nervous system morphology, especially in the hippocampus and hypothalamus. In rodents, chewing or biting on wooden sticks during exposure to various stressors reduces stress-induced gastric ulcer formation and attenuates spatial cognitive dysfunction, anxiety-like behavior, and bone loss. In humans, some studies demonstrate that chewing gum during exposure to stress decreases plasma and salivary cortisol levels and reduces mental stress, although other studies report no such effect. Here, we discuss the neuronal mechanisms that underline the interactions between masticatory function and stress-coping behaviors in animals and humans.

Cholinergic interneurons in the dorsal and ventral striatum: anatomical and functional considerations in normal and diseased conditions

Striatal cholinergic interneurons (ChIs) are central for the processing and reinforcement of reward-related behaviors that are negatively affected in states of altered dopamine transmission, such as in Parkinson's disease or drug addiction. Nevertheless, the development of therapeutic interventions directed at ChIs has been hampered by our limited knowledge of the diverse anatomical and functional characteristics of these neurons in the dorsal and ventral striatum, combined with the lack of pharmacological tools to modulate specific cholinergic receptor subtypes. This review highlights some of the key morphological, synaptic, and functional differences between ChIs of different striatal regions and across species. It also provides an overview of our current knowledge of the cellular localization and function of cholinergic receptor subtypes. The future use of high-resolution anatomical and functional tools to study the synaptic microcircuitry of brain networks, along with the development of specific cholinergic receptor drugs, should help further elucidate the role of striatal ChIs and permit efficient targeting of cholinergic systems in various brain disorders, including Parkinson's disease and addiction.

Enhanced histamine H2 excitation of striatal cholinergic interneurons in L-DOPA-induced dyskinesia

Levodopa is the most effective therapy for the motor deficits of Parkinson's disease (PD), but long term treatment leads to the development of L-DOPA-induced dyskinesia (LID). Our previous studies indicate enhanced excitability of striatal cholinergic interneurons (ChIs) in mice expressing LID and reduction of LID when ChIs are selectively ablated. Recent gene expression analysis indicates that stimulatory H2 histamine receptors are prefentially expressed on ChIs at high levels in the striatum, and we tested whether a change in H2 receptor function might contribute to the elevated excitability in LID. Using two different mouse models of PD (6-hydroxydopamine lesion and Pitx3^ak/ak mutation), we chronically treated the animals with either vehicle or L-DOPA to induce dyskinesia. Electrophysiological recordings indicate that histamine H2 receptor-mediated excitation of striatal ChIs is enhanced in mice expressing LID. Additionally, H2 receptor blockade by systemic administration of famotidine decreases behavioral LID expression in dyskinetic animals. These findings suggest that ChIs undergo a pathological change in LID with respect to histaminergic neurotransmission. The hypercholinergic striatum associated with LID may be dampened by inhibition of H2 histaminergic neurotransmission. This study also provides a proof of principle of utilizing selective gene expression data for cell-type-specific modulation of neuronal activity.

Pharmacology of cognitive enhancers for exposure-based therapy of fear, anxiety and trauma-related disorders

Pathological fear and anxiety are highly debilitating and, despite considerable advances in psychotherapy and pharmacotherapy they remain insufficiently treated in many patients with PTSD, phobias, panic and other anxiety disorders. Increasing preclinical and clinical evidence indicates that pharmacological treatments including cognitive enhancers, when given as adjuncts to psychotherapeutic approaches [cognitive behavioral therapy including extinction-based exposure therapy] enhance treatment efficacy, while using anxiolytics such as benzodiazepines as adjuncts can undermine long-term treatment success. The purpose of this review is to outline the literature showing how pharmacological interventions targeting neurotransmitter systems including serotonin, dopamine, noradrenaline, histamine, glutamate, GABA, cannabinoids, neuropeptides (oxytocin, neuropeptides Y and S, opioids) and other targets (neurotrophins BDNF and FGF2, glucocorticoids, L-type-calcium channels, epigenetic modifications) as well as their downstream signaling pathways, can augment fear extinction and strengthen extinction memory persistently in preclinical models. Particularly promising approaches are discussed in regard to their effects on specific aspects of fear extinction namely, acquisition, consolidation and retrieval, including long-term protection from return of fear (relapse) phenomena like spontaneous recovery, reinstatement and renewal of fear. We also highlight the promising translational value of the preclinial research and the clinical potential of targeting certain neurochemical systems with, for example d-cycloserine, yohimbine, cortisol, and L-DOPA. The current body of research reveals important new insights into the neurobiology and neurochemistry of fear extinction and holds significant promise for pharmacologically-augmented psychotherapy as an improved approach to treat trauma and anxiety-related disorders in a more efficient and persistent way promoting enhanced symptom remission and recovery.

Reorganization of supramammillary-hippocampal pathways in the rat pilocarpine model of temporal lobe epilepsy: evidence for axon terminal sprouting

In mesial temporal lobe epilepsy (MTLE), spontaneous seizures likely originate from a multi-structural epileptogenic zone, including several regions of the limbic system connected to the hippocampal formation. In this study, we investigate the structural connectivity between the supramammillary nucleus (SuM) and the dentate gyrus (DG) in the model of MTLE induced by pilocarpine in the rat. This hypothalamic nucleus, which provides major extracortical projections to the hippocampal formation, plays a key role in the regulation of several hippocampus-dependent activities, including theta rhythms, memory function and emotional behavior, such as stress and anxiety, functions that are known to be altered in MTLE. Our findings demonstrate a marked reorganization of DG afferents originating from the SuM in pilocarpine-treated rats. This reorganization, which starts during the latent period, is massive when animals become epileptic and continue to evolve during epilepsy. It is characterized by an aberrant distribution and an increased number of axon terminals from neurons of both lateral and medial regions of the SuM, invading the entire inner molecular layer of the DG. This reorganization, which reflects an axon terminal sprouting from SuM neurons, could contribute to trigger spontaneous seizures within an altered hippocampal intrinsic circuitry.

Brain-wide map of efferent projections from rat barrel cortex

The somatotopically organized whisker barrel field of the rat primary somatosensory (S1) cortex is a commonly used model system for anatomical and physiological investigations of sensory processing. The neural connections of the barrel cortex have been extensively mapped. But most investigations have focused on connections to limited regions of the brain, and overviews in the literature of the connections across the brain thus build on a range of material from different laboratories, presented in numerous publications. Furthermore, given the limitations of the conventional journal article format, analyses and interpretations are hampered by lack of access to the underlying experimental data. New opportunities for analyses have emerged with the recent release of an online resource of experimental data consisting of collections of high-resolution images from 6 experiments in which anterograde tracers were injected in S1 whisker or forelimb representations. Building on this material, we have conducted a detailed analysis of the brain wide distribution of the efferent projections of the rat barrel cortex. We compare our findings with the available literature and reports accumulated in the Brain Architecture Management System (BAMS₂) database. We report well-known and less known intracortical and subcortical projections of the barrel cortex, as well as distinct differences between S1 whisker and forelimb related projections. Our results correspond well with recently published overviews, but provide additional information about relative differences among S1 projection targets. Our approach demonstrates how collections of shared experimental image data are suitable for brain-wide analysis and interpretation of connectivity mapping data.

Effect of dorsal and ventral hippocampal lesions on contextual fear conditioning and unconditioned defensive behavior induced by electrical stimulation of the dorsal periaqueductal gray

The dorsal (DH) and ventral (VH) subregions of the hippocampus are involved in contextual fear conditioning. However, it is still unknown whether these two brain areas also play a role in defensive behavior induced by electrical stimulation of the dorsal periaqueductal gray (dPAG). In the present study, rats were implanted with electrodes into the dPAG to determine freezing and escape response thresholds after sham or bilateral electrolytic lesions of the DH or VH. The duration of freezing behavior that outlasted electrical stimulation of the dPAG was also measured. The next day, these animals were subjected to contextual fear conditioning using footshock as an unconditioned stimulus. Electrolytic lesions of the DH and VH impaired contextual fear conditioning. Only VH lesions disrupted conditioned freezing immediately after footshock and increased the thresholds of aversive freezing and escape responses to dPAG electrical stimulation. Neither DH nor VH lesions disrupted post-dPAG stimulation freezing. These results indicate that the VH but not DH plays an important role in aversively defensive behavior induced by dPAG electrical stimulation. Interpretations of these findings should be made with caution because of the fact that a non-fiber-sparing lesion method was employed.

Orexin receptor antagonists as therapeutic agents for insomnia

Insomnia is a common clinical condition characterized by difficulty initiating or maintaining sleep, or non-restorative sleep with impairment of daytime functioning. Currently, treatment for insomnia involves a combination of cognitive behavioral therapy (CBTi) and pharmacological therapy. Among pharmacological interventions, the most evidence exists for benzodiazepine (BZD) receptor agonist drugs (GABAA receptor), although concerns persist regarding their safety and their limited efficacy. The use of these hypnotic medications must be carefully monitored for adverse effects. Orexin (hypocretin) neuropeptides have been shown to regulate transitions between wakefulness and sleep by promoting cholinergic/monoaminergic neural pathways. This has led to the development of a new class of pharmacological agents that antagonize the physiological effects of orexin. The development of these agents may lead to novel therapies for insomnia without the side effect profile of hypnotics (e.g., impaired cognition, disturbed arousal, and motor balance difficulties). However, antagonizing a system that regulates the sleep-wake cycle may create an entirely different side effect profile. In this review, we discuss the role of orexin and its receptors on the sleep-wake cycle and that of orexin antagonists in the treatment of insomnia.

Synchronized electrical stimulation of the rat medial forebrain bundle and perforant pathway generates an additive BOLD response in the nucleus accumbens and prefrontal cortex

To study how a synchronized activation of two independent pathways affects the fMRI response in a common targeted brain region, blood oxygen dependent (BOLD) signals were measured during electrical stimulation of the right medial forebrain bundle (MFB), the right perforant pathway (PP) and concurrent stimulation of the two fiber systems. Repetitive electrical stimulations of the MFB triggered significant positive BOLD responses in the nucleus accumbens (NAcc), septum, anterior cingulate cortex/medial prefrontal cortex (ACC/mPFC), ventral tegmental area/substantia nigra (VTA/SN), right entorhinal cortex (EC) and colliculus superior, which, in general, declined during later stimulation trains. At the same time, negative BOLD responses were observed in the striatum. Thus, the same stimulus caused region-specific hemodynamic responses. An identical electrical stimulation of the PP generated positive BOLD responses in the right dentate gyrus/hippocampus proper/subiculum (DG/HC), the right entorhinal cortex and the left entorhinal cortex, which remained almost stable during consecutive stimulation trains. Co-stimulation of the two fiber systems resulted in an additive activation pattern, i.e., the BOLD responses were stronger during the stimulation of the two pathways than during the stimulation of only one pathway. However, during the simultaneous stimulation of the two pathways, the development of the BOLD responses to consecutive trains changed. The BOLD responses in regions that were predominantly activated by MFB stimulation (i.e., NAcc, septum and ACC/mPFC) did not decline as fast as during pure MFB stimulation, thus an additive BOLD response was only observed during later trains. In contrast, in the brain regions that were predominantly activated by PP stimulation (i.e., right EC, DG/HC), co-stimulation of the MFB only resulted in an additive effect during early trains but not later trains. Consequently, the development of the BOLD responses during consecutive stimulations indicates the presence of an interaction between the two pathways in a target region, whereas the observed averaged BOLD responses do not.

Thyrotropin-releasing hormone-containing axons innervate histaminergic neurons in the tuberomammillary nucleus

Recent studies indicate that the effect of thyrotropin-releasing hormone (TRH) on the regulation of food intake may be mediated by histaminergic neurons. To elucidate the anatomical basis for a functional relationship between TRH- and histamine-synthesizing neuronal systems, double-labeling immunocytochemistry was performed on the tuberomammillary nucleus (TMN) of rats, the exclusive location of histaminergic neurons. TRH-immunoreactive (IR) innervation of the histaminergic neurons were detected in all five subnuclei (E1-5) of the TMN, but was most prominent in the E4 and E5 subnuclei where 100% of the histamine-IR neurons were contacted. The number of TRH-IR varicosities in contact with histamine-IR neurons was also greatest in the E4 and E5 subnuclei, averaging 27.0±1.2 in E4 and 7.9±0.5 in E5. Somewhat fewer histamine-IR neurons were juxtaposed by TRH-IR varicosities in E2 and E3 and contacted by 6.3±0.2 and 6.8±0.2 varicosities/innervated cell, respectively. The number of juxtapositions of TRH-IR axon varicosities with histamine-IR neurons was the lowest in the E1 subnucleus (85.7±0.9%; 4.0±0.2 varicosities/innervated cell). Ultrastructural analysis demonstrated that TRH-IR axons established both asymmetric and symmetric type synapses on the perikaryon and dendrites of the histamine-IR neurons, although the majority of synapses were asymmetric type. These data demonstrate that TRH neurons heavily innervate histaminergic neurons in all subdivisions of the TMN, with the densest innervation in the E4 and E5 subdivisions, and are likely to exert activating effects.

Identification of a histaminergic circuit in the caudal hypothalamus: an evidence for functional heterogeneity of histaminergic neurons

It is well established that histaminergic neurons in the posterior hypothalamus make connections with whole brain areas and regulate several functions. Recent evidence indicates that histaminergic neurons are heterogeneous cell group and organized into distinct circuits. However, functional circuits of histaminergic neurons have not been fully mapped so far. To address this issue, we have investigated antihistamine-sensitive neuronal activation in the hypothalamus to determine the hypothalamic region primarily innervated by histaminergic neurons. Here we review our recent findings showing the existence of the heterogeneous subpopulations of histaminergic neurons in the TMN that innervated distinct regions to regulate particular functions. We have identified the caudal part of the arcuate nucleus of hypothalamus (cARC) as a target region of histaminergic neurons in food-restricted rats by assessing suppression of c-Fos expression by pretreatment with antihistamines. Histaminergic neurons in the tuberomammillary nucleus (TMN) are morphologically subdivided into five groups (E1-E5). Among the subdivisions, the E3 group was found to be activated corresponding to the activation of cARC neurons. Our findings suggest that this subpopulation selectively innervate cARC neurons. Accumulating reports have also described c-Fos expression in other TMN subpopulations. Various stress challenge induced c-Fos expression primarily in E4 and E5 subpopulations. Motivation- and drug-induced arousal elicited in common activation of ventrolateral part of the TMN containing E1 and E2 subdivisions, which receive projections from wake-active orexin neurons and sleep-active GABA neurons. These lines of evidence support the hypothesis that there are heterogeneous subpopulations in the TMN that innervated distinct regions to regulate particular functions.

Cognitive demands induce selective hippocampal reorganization: Arc expression in a place and response task

Place cells in the hippocampus can maintain multiple representations of a single environment and respond to physical and/or trajectory changes by remapping. Within the hippocampus there are anatomical, electrophysiological, and behavioral dissociations between the dorsal and ventral hippocampus and within dorsal CA1. Arc expression was used to measure the recruitment of ensembles across different hippocampal subregions in rats trained to utilize two different cognitive strategies while traversing an identical trajectory. This behavioral paradigm allowed for the measurement of remapping in the absence of changes in external cues, trajectory traversed (future/past), running speed, motivation, or different stages of learning. Changes in task demands induced remapping in only some hippocampal regions: reorganization of cell ensembles was observed in dorsal CA1 but not in dorsal CA3. Moreover, a gradient was found in the degree of remapping within dorsal CA1 that corresponds to entorhinal connectivity to this region. Remapping was not seen in the ventral hippocampus: neither ventral CA1 nor CA3 exhibited ensemble changes with different cognitive demands. This contrasts with findings of remapping in both the dorsal and ventral dentate gyrus using this task. The results suggest that the dorsal pole of the hippocampus is more sensitive to changes in task demands.

Histamine neurons in the tuberomamillary nucleus: a whole center or distinct subpopulations?

Histamine axons originate from a single source, the tuberomamillary nucleus (TMN) of the posterior hypothalamus, to innervate almost all central nervous system (CNS) regions. This feature, a compact cell group with widely distributed fibers, resembles that of other amine systems, such as noradrenaline or serotonin, and is consistent with a function for histamine over a host of physiological processes, including the regulation of the sleep-wake cycle, appetite, endocrine homeostasis, body temperature, pain perception, learning, memory, and emotion. An important question is whether these diverse physiological roles are served by different histamine neuronal subpopulation. While the histamine system is generally regarded as one single functional unit that provides histamine throughout the brain, evidence is beginning to accumulate in favor of heterogeneity of histamine neurons. The aim of this review is to summarize experimental evidence demonstrating that histamine neurons are heterogeneous, organized into functionally distinct circuits, impinging on different brain regions, and displaying selective control mechanisms. This could imply independent functions of subsets of histamine neurons according to their respective origin and terminal projections.

In vivo electrochemical evidence for simultaneous 5-HT and histamine release in the rat substantia nigra pars reticulata following medial forebrain bundle stimulation

Exploring the mechanisms of serotonin [5-hydroxytryptamine (5-HT)] in the brain requires an in vivo method that combines fast temporal resolution with chemical selectivity. Fast-scan cyclic voltammetry is a technique with sufficient temporal and chemical resolution for probing dynamic 5-HT neurotransmission events; however, traditionally it has not been possible to probe in vivo 5-HT mechanisms. Recently, we optimized fast-scan cyclic voltammetry for measuring 5-HT release and uptake in vivo in the substantia nigra pars reticulata (SNR) with electrical stimulation of the dorsal raphe nucleus (DRN) in the rat brain. Here, we address technical challenges associated with rat DRN surgery by electrically stimulating 5-HT projections in the medial forebrain bundle (MFB), a more accessible anatomical location. MFB stimulation elicits 5-HT in the SNR; furthermore, we find simultaneous release of an additional species. We use electrochemical and pharmacological methods and describe physiological, anatomical and independent chemical analyses to identify this species as histamine. We also show pharmacologically that increasing the lifetime of extracellular histamine significantly decreases 5-HT release, most likely because of increased activation of histamine H-3 receptors that inhibit 5-HT release. Despite this, under physiological conditions, we find by kinetic comparisons of DRN and MFB stimulations that the simultaneous release of histamine does not interfere with the quantitative 5-HT concentration profile. We therefore present a novel and robust electrical stimulation of the MFB that is technically less challenging than DRN stimulation to study 5-HT and histamine release in the SNR.

Histamine receptors in the CNS as targets for therapeutic intervention

Histamine has long been known to trigger allergic reactions and gastric acid secretion. However, it was later discovered that, in the brain, histamine regulates basic homeostatic and higher functions, including cognition, arousal, circadian and feeding rhythms. The sole source of brain histamine is neurons localized in the hypothalamic tuberomammillary nuclei. These neurons project axons to the whole brain, are organized into functionally distinct circuits influencing different brain regions and display selective control mechanisms. Although all histamine receptors (H1R, H2R, H3R and H4R) are expressed in the brain, only the H3R has become a drug target for the treatment of neurologic and psychiatric disorders, such as sleep disturbances and cognitive deficits. In this review, we discuss recent developments in the pharmacological manipulation of H3Rs and the implications for H3R-related therapies for neurological and psychiatric disorders. The legacy of Sir James Black.

The waking brain: an update

Wakefulness and consciousness depend on perturbation of the cortical soliloquy. Ascending activation of the cerebral cortex is characteristic for both waking and paradoxical (REM) sleep. These evolutionary conserved activating systems build a network in the brainstem, midbrain, and diencephalon that contains the neurotransmitters and neuromodulators glutamate, histamine, acetylcholine, the catecholamines, serotonin, and some neuropeptides orchestrating the different behavioral states. Inhibition of these waking systems by GABAergic neurons allows sleep. Over the past decades, a prominent role became evident for the histaminergic and the orexinergic neurons as a hypothalamic waking center.

Histamine in the regulation of wakefulness

The histaminergic system is exclusively localized within the posterior hypothalamus with projection to almost all the major regions of the central nervous system. Strong and consistent evidence exist to suggest that histamine, acting via H₁ and/or H₃ receptor has a pivotal role in the regulation of sleep-wakefulness. Administration of histamine or H₁ receptor agonists induces wakefulness, whereas administration of H₁ receptor antagonists promotes sleep. The H₃ receptor functions as an auto-receptor and regulates the synthesis and release of histamine. Activation of H₃ receptor reduces histamine release and promotes sleep. Conversely, blockade of H₃ receptor promotes wakefulness. Histamine release in the hypothalamus and other target regions is highest during wakefulness. The histaminergic neurons display maximal activity during the state of high vigilance, and cease their activity during non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. The cerebrospinal levels of histamine are reduced in diseased states where hypersomnolence is a major symptom. The histamine deficient L-histidine decarboxylase knockout (HDC KO) mice display sleep fragmentation and increased REM sleep during the light period along with profound wakefulness deficit at dark onset, and in novel environment. Similar results have been obtained when histamine neurons are lesioned. These studies strongly implicate the histaminergic neurons of the TMN to play a critical role in the maintenance of high vigilance state during wakefulness.

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 neuronal system as a therapeutic target for the treatment of cognitive disorders

Much has been learned over the past 20 years about the role of histamine as a neurotransmitter. This brief article attempts to evaluate the progress accomplished in this field, and discusses the therapeutic potential of the H3 receptor (H3R). All histaminergic neurons are localized in the tuberomammillary nucleus of the posterior hypothalamus and project to almost all regions of the CNS. Histamine exerts its effect via interaction with specific receptors (H1R, H2R, H3R and H4R). Antagonists of both H1R and H2R have been successful as blockbuster drugs for treating allergic conditions and gastric ulcers. H4R is still awaiting better functional characterization, but the H3R is an attractive target for potential therapies of CNS disorders. Indeed, considerable interest was raised by reports that pharmacological blockade of H3Rs exerted procognitive effects in a variety of animal tasks analyzing different types of memory. In addition, blockade of H3Rs increased wakefulness and reduced bodyweight in animal models. Such findings hint at the potential use of H3R antagonists/inverse agonists for the treatment of Alzheimer’s disease and other dementias, attention-deficit hyperactivity disorder, obesity and sleep disorders. As a result, an increasing number of H3R antagonists/inverse agonists progress through the clinic for the treatment of a variety of conditions, including attention-deficit hyperactivity disorder, cognitive disorders, narcolepsy and schizophrenia. Moreover, the use of H3R antagonists/inverse agonists that weaken traumatic memories may alleviate disorders such as post-traumatic stress syndrome, panic attacks, specific phobias and generalized anxiety. The use of H3R ligands for the treatment of neurodegenerative disorders is demonstrated in several studies, indicating a role of the histamine neurons and H3Rs in neuroprotection. Recently, direct evidence demonstrated that histaminergic neurons are organized into functionally distinct circuits, impinging on different brain regions, and displaying selective control mechanisms. This could imply independent functions of subsets of histaminergic neurons according to their respective origin and terminal projections. The possibility that H3Rs control only some of those functions implies that H3R-directed therapies may achieve selective effects, with minimal side effects, and this may increase the interest regarding this class of drugs.

Regional differential effects of the novel histamine H3 receptor antagonist 6-[(3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-yl)oxy]-N-methyl-3-pyridinecarboxamide hydrochloride (GSK189254) on histamine release in the central nervous system of freely moving rats

After oral administration, the nonimidazole histamine H(3) receptor antagonist, 6-[(3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-yl)oxy]-N-methyl-3-pyridinecarboxamide hydrochloride (GSK189254), increased histamine release from the tuberomammillary nucleus, where all histaminergic somata are localized, and from where their axons project to the entire brain. To further understand functional histaminergic circuitry in the brain, dual-probe microdialysis was used to pharmacologically block H(3) receptors in the tuberomammillary nucleus, and monitor histamine release in projection areas. Perfusion of the tuberomammillary nucleus with GSK189254 increased histamine release from the tuberomammillary nucleus, nucleus basalis magnocellularis, and cortex, but not from the striatum or nucleus accumbens. Cortical acetylcholine (ACh) release was also increased, but striatal dopamine release was not affected. When administered locally, GSK189254 increased histamine release from the nucleus basalis magnocellularis, but not from the striatum. Thus, defined by their sensitivity to GSK189254, histaminergic neurons establish distinct pathways according to their terminal projections, and can differentially modulate neurotransmitter release in a brain region-specific manner. Consistent with its effects on cortical ACh release, systemic administration of GSK189254 antagonized the amnesic effects of scopolamine in the rat object recognition test, a cognition paradigm with important cortical components.

Distribution of a splice variant of choline acetyltransferase in the trigeminal ganglion and brainstem of the rat: comparison with calcitonin gene-related peptide and substance P

Rat trigeminal ganglion neurons have been shown to contain a splice variant of choline acetyltransferase (pChAT). Here we report the distribution pattern of pChAT-containing afferents from the trigeminal ganglion to the brainstem, compared with that of calcitonin gene-related peptide (CGRP) and substance P (SP), by use of the immunohistochemical techniques in the rat. Most of CGRP(+) SP(+) ganglion cells contain pChAT, whereas half of the pChAT(+) ganglion cells possess neither CGRP nor SP. In the brainstem, pChAT(+) nerve fibers are found exclusively in the trigeminal and solitary systems, although the distribution pattern differs from that of CGRP(+) or SP(+) fibers. First, the ventral portion of the principal sensory nucleus contains many pChAT(+) fibers, with few CGRP(+) or SP(+) fibers. Because this portion receives projections of nociceptive corneal afferents, a subpopulation of pChAT(+) CGRP(-) SP(-) primary afferents is most probably nonpeptidergic nociceptors innervating the cornea. Second, the superficial laminae of the medullary dorsal horn, the main target of nociceptive afferents, contain dense CGRP(+) and SP(+) fibers but sparse pChAT(+) fibers. Because pChAT occurs in most CGRP(+) SP(+) ganglion cells, such sparseness of pChAT(+) fibers implies poor transportation of pChAT to axon branchlets. Another important finding is that pChAT(+) axons are smooth and nonvaricose, whereas CGRP(+) or SP(+) fibers possess numerous varicosities. Our confocal microscopy suggests colocalization of these three markers in the same single axons in some brainstem regions. The difference in morphological appearance, nonvaricose or varicose, appears to reflect the difference in intraaxonal distribution between pChAT and CGRP or SP.

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.

Low-frequency stimulation of the tuberomammillary nucleus facilitates electrical amygdaloid-kindling acquisition in Sprague-Dawley rats

Histamine plays a suppressive role in seizure. The tuberomammillary nucleus (TM) is the only locus of histaminergic neurons in the brain. To determine whether deep brain stimulation (DBS) of the TM provides protection against seizures, we tested the effects of low-frequency stimulation (LFS, 1 Hz), high frequency stimulation (HFS, 100 Hz), and electrolytic lesions of the TM on seizures generated by amygdaloid kindling, pentylenetetrazol (PTZ) and maximal electroshock (MES) in rats. LFS of TM accelerated the progression of behavioral seizure stage and increased the mean afterdischarge duration (ADD) during acquisition of amygdaloid-kindling seizures, but had no considerable anticonvulsive effect in fully kindled animals. It augmented the MES-induced seizures as well, but had no appreciable effects on PTZ-kindled seizures. In addition, both HFS and bilateral lesions of the TM exacerbated the progression of amygdaloid-kindling seizures. These results suggest that specific negative sites for DBS exist in the brain, such as the TM. This study indicates that it is crucial to choose a suitable target for DBS in the clinical treatment of epilepsy.

Dipsogenic potentiation by sodium chloride but not by sucrose or polyethylene glycol in tuberomammillary-mediated polydipsia

The aim of this study was to examine the dipsogenic mechanisms involved in the recently discovered tuberomammillary (TM)-mediated polydipsia. Rats with bilateral electrolytic lesions of each TM subnucleus underwent several dipsogenic treatments, both osmotic and volemic. Animals with ventral (E2) or medial TM lesions (E3 or E4) showed a potentiated hyperdipsic response to hypertonic sodium chloride administration but not to sucrose or polyethylene glycol treatments. The increase in response to sodium chloride was significantly greater in groups E3/E4 and E2 than in the non-lesioned group and in animals with polydipsia induced by lesion of the median eminence. As previously reported, hyperphagia was induced by lesion to ventral TM nuclei (E1 or E2), confirming a possible role for the TM complex in food intake. However, lesions in medial nuclei (E3 or E4) did not produce this increase in food intake. These results are interpreted in relation to the hypothalamic systems involved in food and water intake.