Leonardo Munari

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.

Heterogeneity of histaminergic neurons in the tuberomammillary nucleus of the rat

Histaminergic neurons of the hypothalamic tuberomammillary nuclei (TMN) send projections to the whole brain. Early anatomical studies described histaminergic neurons as a homogeneous cell group, but recent evidence indicates that histaminergic neurons are heterogeneous and organized into distinct circuits. We addressed this issue using the double‐probe microdialysis in freely moving rats to investigate if two compounds acting directly onto histaminergic neurons to augment cell firing [thioperamide and bicuculline, histamine H3‐ and γ‐aminobutyric acid (GABA)A‐receptor (R) antagonists, respectively] may discriminate groups of histaminergic neurons impinging on different brain regions. Intra‐hypothalamic perfusion of either drug increased histamine release from the TMN and cortex, but not from the striatum. Thioperamide, but not bicuculline, increased histamine release from the nucleus basalis magnocellularis (NBM), bicuculline but not thioperamide increased histamine release from the nucleus accumbens (NAcc). Intra‐hypothalamic perfusion with thioperamide increased the time spent in wakefulness. To explore the local effects of H3‐R blockade in the histaminergic projection areas, each rat was implanted with a single probe to simultaneously administer thioperamide and monitor local changes in histamine release. Thioperamide increased histamine release from the NBM and cortex significantly, but not from the NAcc or striatum. The presence of H3‐Rs on histaminergic neurons was assessed using double‐immunofluorescence with anti‐histidine decarboxylase antibodies to identify histaminergic cells and anti‐H3‐R antibodies. Confocal analysis revealed that all histaminergic somata were immunopositive for the H3‐R. This is the first evidence that histaminergic neurons are organized into functionally distinct circuits that influence different brain regions, and display selective control mechanisms.

Selective brain region activation by histamine H3 receptor antagonist/inverse agonist ABT-239 enhances acetylcholine and histamine release and increases c-Fos expression

Histamine axons originate solely from the tuberomamillary nucleus (TMN) to innervate almost all brain regions. This feature is consistent with a function for histamine over a host of physiological processes, including regulation of appetite, body temperature, cognitive processes, pain perception and sleep-wake cycle. An important question is whether these diverse physiological roles are served by different histamine neuronal subpopulations. Here we report that systemic administration of the non-imidazole histamine H₃ receptor antagonist 4-(2-{2-[(2R)-2-methylpyrrolidinyl]ethyl}-benzofuran-5-yl)benzonitrile (ABT-239, 3 mg/kg) increased c-Fos expression dose-dependently in rat cortex and nucleus basalis magnocellularis (NBM) but not in the nucleus accumbens (NAcc) nor striatum, and augmented acetylcholine and histamine release from rat prefrontal cortex. To further understand functional histaminergic pathways in the brain, dual-probe microdialysis was used to pharmacologically block H₃ receptors in the TMN. Perfusion of the TMN with ABT-239 (10 μM) increased histamine release from the TMN, NBM, and cortex, but not from the striatum or NAcc. When administered locally, ABT-239 increased histamine release from the NBM, but not from the NAcc. Systemic as well as intra-TMN administration of ABT-239 increased c-Fos expression in the NBM, and cortex, but not in the striatum or NAcc. Thus, as defined by their sensitivity to ABT-239, histaminergic neurons establish distinct pathways according to their terminal projections, and can differentially modulate neurotransmitter release in a brain region-specific manner. This implies independent functions of subsets of histamine neurons according to their terminal projections, with relevant consequences for the development of specific compounds that affect only subsets of histamine neurones, thus increasing target specificity.

Histamine in the basolateral amygdala promotes inhibitory avoidance learning independently of hippocampus

Significance Integrity of the brain histaminergic system is necessary for long-term memory (LTM) but not short-term memory of step-down inhibitory avoidance (IA). Histamine depletion in hippocampus or basolateral amygdala (BLA) impairs LTM of that task. Histamine infusion into either structure restores LTM in histamine-depleted rats. The restoring effect in BLA occurs even when hippocampal activity was impaired. Cyclic adenosine monophosphate (cAMP) responsive-element-binding protein phosphorylation correlates anatomically and temporally with histamine-induced memory recall. Thus, histaminergic neurotransmission appears critical to provide the brain with the plasticity necessary for IA through recruitment of alternative circuits. Our findings indicate that the histaminergic system comprises parallel, coordinated pathways that provide compensatory plasticity when one brain structure is compromised. Recent discoveries demonstrated that recruitment of alternative brain circuits permits compensation of memory impairments following damage to brain regions specialized in integrating and/or storing specific memories, including both dorsal hippocampus and basolateral amygdala (BLA). Here, we first report that the integrity of the brain histaminergic system is necessary for long-term, but not for short-term memory of step-down inhibitory avoidance (IA). Second, we found that phosphorylation of cyclic adenosine monophosphate (cAMP) responsive-element-binding protein, a crucial mediator in long-term memory formation, correlated anatomically and temporally with histamine-induced memory retrieval, showing the active involvement of histamine function in CA1 and BLA in different phases of memory consolidation. Third, we found that exogenous application of histamine in either hippocampal CA1 or BLA of brain histamine-depleted rats, hence amnesic, restored long-term memory; however, the time frame of memory rescue was different for the two brain structures, short lived (immediately posttraining) for BLA, long lasting (up to 6 h) for the CA1. Moreover, long-term memory was formed immediately after training restoring of histamine transmission only in the BLA. These findings reveal the essential role of histaminergic neurotransmission to provide the brain with the plasticity necessary to ensure memorization of emotionally salient events, through recruitment of alternative circuits. Hence, our findings indicate that the histaminergic system comprises parallel, coordinated pathways that provide compensatory plasticity when one brain structure is compromised.

Brain Histamine Is Crucial for Selective Serotonin Reuptake Inhibitors‘ Behavioral and Neurochemical Effects

Backgound: The neurobiological changes underlying depression resistant to treatments remain poorly understood, and failure to respond to selective serotonin reuptake inhibitors may result from abnormalities of neurotransmitter systems that excite serotonergic neurons, such as histamine. Methods: Using behavioral (tail suspension test) and neurochemical (in vivo microdialysis, Western-blot analysis) approaches, here we report that antidepressant responses to selective serotonin reuptake inhibitors (citalopram or paroxetine) are abolished in mice unable to synthesize histamine due to either targeted disruption of histidine decarboxylase gene (HDC-/-) or injection of alpha-fluoromethylhistidine, a suicide inhibitor of this enzyme. Results: In the tail suspension test, all classes of antidepressants tested reduced the immobility time of controls. Systemic reboxetine or imipramine reduced the immobility time of histamine-deprived mice as well, whereas selective serotonin reuptake inhibitors did not even though their serotonergic system is functional. In in vivo microdialysis experiments, citalopram significantly increased histamine extraneuronal levels in the cortex of freely moving mice, and methysergide, a serotonin 5-HT1/5-HT2 receptor antagonist, abolished this effect, thus suggesting the involvement of endogenous serotonin. CREB phosphorylation, which is implicated in the molecular mechanisms of antidepressant treatment, was abolished in histamine-deficient mice treated with citalopram. The CREB pathway is not impaired in HDC-/- mice, as administration of 8-bromoadenosine 3’, 5’-cyclic monophosphate increased CREB phosphorylation, and in the tail suspension test it significantly reduced the time spent immobile by mice of both genotypes. Conclusions: Our results demonstrate that selective serotonin reuptake inhibitors selectively require the integrity of the brain histamine system to exert their preclinical responses.

Histaminergic Neurotransmission as a Gateway for the Cognitive Effect of Oleoylethanolamide in Contextual Fear Conditioning.

Abstract Background: The integrity of the brain histaminergic system is necessary for the unfolding of homeostatic and cognitive processes through the recruitment of alternative circuits with distinct temporal patterns. We recently demonstrated that the fat-sensing lipid mediator oleoylethanolamide indirectly activates histaminergic neurons to exerts its hypophagic effects. The present experiments investigated whether histaminergic neurotransmission is necessary also for the modulation of emotional memory induced by oleoylethanolamide in a contextual fear conditioning paradigm. Methods: We examined the acute effect of i.p. administration of oleoylethanolamide immediately posttraining in the contextual fear conditioning test. Retention test was performed 72 hours after training. To test the participation of the brain histaminergic system in the cognitive effect of oleoylethanolamide, we depleted rats of brain histamine with an i.c.v. injection of alpha-fluoromethylhistidine (a suicide inhibitor of histidine decarboxylase) or bilateral intra-amygdala infusions of histamine H1 or H2 receptor antagonists. We also examined the effect of oleoylethanolamide on histamine release in the amygdala using in vivo microdialysis. Results: Posttraining administration of oleoylethanolamide enhanced freezing time at retention. This effect was blocked by both i.c.v. infusions of alpha-fluoromethylhistidine or by intra-amygdala infusions of either pyrilamine or zolantidine (H1 and H2 receptor antagonists, respectively). Microdialysis experiments showed that oleoylethanolamide increased histamine release from the amygdala of freely moving rats. Conclusions: Our results suggest that activation of the histaminergic system in the amygdala has a “permissive” role on the memory-enhancing effects of oleoylethanolamide. Hence, targeting the H1 and H2 receptors may modify the expression of emotional memory and reduce dysfunctional aversive memories as found in phobias and posttraumatic stress disorder.

Experience-induced forgetting by WT1 enables learning of sequential tasks.

Remembering and forgetting are important aspects of normal behavioral adaptation; however, the molecular basis of forgetting has been less studied. Using rat and mouse models we find that WT1, a transcriptional repressor that is activated in the hippocampus by LTP producing stimuli and behavioral memory, enables forgetting. Acute or tonic knockdown of WT1 did not affect short-term memory but enhanced long-term memory and enables a switch from circuit to cellular computation in the hippocampus. A control theory model predicts that WT1 could be a general repressor of memory or a regulator that preserves the ability to remember multiple sequential experiences. Using sequential training for two tasks, mice with non-functional WT1 have better memory for the first task, but show impaired memory for the second task. Taken together, our observations indicate that WT1 mediates an experience-activated forgetting process that preserves the capability of the animal to remember other new experiences. One sentence summary The transcription factor WT1 is a core component of an active forgetting process, and is required for normal behavioral flexibility by allowing LTM for successive experiences.Abstract Under physiological conditions, strength and persistence of memory must be regulated in order to produce behavioral flexibility. In fact, impairments in memory flexibility are associated with pathologies such as post-traumatic stress disorder or autism; however the underlying mechanisms that enable memory flexibility are still poorly understood. Here we identified the transcriptional repressor Wilm’s Tumor 1 (WT1) as a critical synaptic plasticity regulator that decreases memory strength, promoting memory flexibility. WT1 was activated in the hippocampus following induction of long-term potentiation (LTP) or learning. WT1 knockdown enhanced CA1 neuronal excitability, LTP and long-term memory whereas its over-expression weakened memory retention. Moreover, forebrain WT1-deficient mice showed deficits in both reversal, sequential learning tasks and contextual fear extinction, exhibiting impaired memory flexibility. We conclude that WT1 limits memory strength or promotes memory weakening, thus enabling memory flexibility, a process that is critical for learning from new experience.

Brain Histamine Affects Eating and Drinking Behaviours

Functional studies demonstrated that activation of the central histaminergic system alters brain functions in both behavioural and homeostatic contexts, which include sleep and wakefulness, learning and memory, anxiety, locomotion, feeding and drinking and neuroendocrine regulation. These actions are achieved through interactions with other neurotransmitter systems. Hence, numerous laboratories are pursuing novel compounds targeting the brain histaminergic receptors for various therapeutic indications. Preclinical studies are focusing on three major areas of interest and intense research is mainly oriented towards providing drugs for the treatment of sleep, cognitive and feeding disorders. The interest in the histaminergic system as a potential target for the treatment of feeding disorders is driven by the unsatisfactory history of the pharmacotherapy of obesity. The drugs currently available for long-term treatment of obesity (such as sibutramine, a monoamine re-uptake inhibitor, and orlistat, a peripherally acting lipase inhibitor) work by different mechanisms, reflecting the complex etiology of the disease. Often these medications achieve a rather modest degree of weight loss, and psychiatric and cardiovascular diseases are reported as adverse drug reactions. As a consequence, new molecular targets are being probed by academic and industrial research teams in the pursuit of novel treatments that may provide advantages over the currently available ones. The control of food intake and body weight is very complex and depends on the interplay of several central and peripheral neuroendocrine systems, environmental factors, the behavioural state and circadian rhythm, which all concur to alter homeostatic aspects of appetite and energy expenditure. Histaminergic activity shows a clear circadian rhythm with high levels during the active period and low levels during the sleep period and there is consistent evidence supporting a role of brain histamine in food intake and energy metabolism. Furthermore, brain histamine acts in concert with and complementary to reward systems, and learning circuits to influence appetitive and aversive behaviours. Brain histaminergic H1 receptors are crucial for the regulation of the diurnal rhythm of food intake and the regulation of obesity; however, from a therapeutic standpoint, no brain penetrating H1 receptor agonists have been identified that would have anti-obesity effects. Despite conflicting preclinical data, though insight are emerging into the potential role of histaminergic H3 receptor modulation as a target of anti-obesity therapeutics. Aim of this review is to outline the relevance of the histaminergic system in controlling feeding behaviour and to suggest the potential therapeutic use of histaminergic ligands for the treatment of feeding disorders.