Histaminergic facilitation of electrocorticographic activation: role of basal forebrain, thalamus, and neocortex

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.

The histaminergic tuberomammillary nucleus is critical for motivated arousal

Obtaining food, shelter or water, or finding a mating partner are examples of motivated behaviors, which are essential to preserve the species. The full expression of such behaviors requires a high but optimal arousal state. We tested the idea that tuberomammillary nucleus (TMN) histamine neurons are crucial to generate such motivated arousal, using a model of the appetitive phase of feeding behavior. Hungry rats enticed with food within a wire mesh box showed intense goal-directed motor activity aimed at opening the box, an increase in core temperature, a fast histamine release in the hypothalamus and an early increase in Fos immunoreactivity in TMN and cortical neurons. Enticing with stronger-tasting food induced stronger motor, temperature and Fos immunoreactivity brain responses than ordinary food pellets. TMN lesion greatly decreased all of those responses. We conclude that histamine neurons increase arousal and vegetative activity, allowing the normal unfolding of voluntary, goal-directed behavior such as obtaining food.

The basolateral amygdala modulates specific sensory memory representations in the cerebral cortex

Stress hormones released by an experience can modulate memory strength via the basolateral amygdala, which in turn acts on sites of memory storage such as the cerebral cortex [McGaugh, J. L. (2004). The amygdala modulates the consolidation of memories of emotionally arousing experiences. Annual Review of Neuroscience, 27, 1-28]. Stimuli that acquire behavioral importance gain increased representation in the cortex. For example, learning shifts the tuning of neurons in the primary auditory cortex (A1) to the frequency of a conditioned stimulus (CS), and the greater the level of CS importance, the larger the area of representational gain [Weinberger, N. M. (2007). Associative representational plasticity in the auditory cortex: A synthesis of two disciplines. Learning & Memory, 14(1-2), 1-16]. The two lines of research suggest that BLA strengthening of memory might be accomplished in part by increasing the representation of an environmental stimulus. The present study investigated whether stimulation of the BLA can affect cortical memory representations. In male Sprague-Dawley rats studied under urethane general anesthesia, frequency receptive fields were obtained from A1 before and up to 75min after the pairing of a tone with BLA stimulation (BLAstm: 100 trials, 400ms, 100Hz, 400microA [+/-16.54]). Tone started before and continued after BLAstm. Group BLA/1.0 (n=16) had a 1s CS-BLAstm interval while Group BLA/1.6 (n=5) has a 1.6s interval. The BLA/1.0 group did develop specific tuning shifts toward and to the CS, which could change frequency tuning by as much as two octaves. Moreover, its shifts increased over time and were enduring, lasting 75min. However, group BLA/1.6 did not develop tuning shifts, indicating that precise CS-BLAstm timing is important in the anesthetized animal. Further, training in the BLA/1.0 paradigm but stimulating outside of the BLA did not produce tuning shifts. These findings demonstrate that the BLA is capable of exerting highly specific, enduring, learning-related modifications of stimulus representation in the cerebral cortex. These findings suggest that the ability of the BLA to alter specific cortical representations may underlie, at least in part, the modulatory influence of BLA activity on strengthening long-term memory.

Atomoxetine increases histamine release and improves learning deficits in an animal model of attention-deficit hyperactivity disorder: the spontaneously hypertensive rat

Substantial development in the pharmacological treatment for attention-deficit hyperactivity disorder (ADHD) has been made recently including approval of new non-stimulant agents targeting noradrenergic, histaminergic and dopaminergic systems. Among such, atomoxetine has been widely used, although its mechanism of action is poorly understood. It is known that central nervous system histamine is closely associated with cognition and it was recently shown that both atomoxetine and methylphenidate enhance cortical histamine release in rats. To that end, the aim of our study was to investigate the effect of atomoxetine (2 mg/kg, intraperitoneally) on histamine release using the microdialysis technique in the spontaneously hypertensive rat (SHR), a suitable genetic model for ADHD. Our data confirmed that atomoxetine increases extracellular levels of histamine in the prefrontal cortex, a brain region that is implicated in the pathophysiology of ADHD. Given the tie between histamine neurotransmission and treatment of cognitive dysfunction, we also assessed the effects of atomoxetine on learning and memory as measured by the Morris water maze in SHR. The results indicated that atomoxetine significantly ameliorated performance in the Morris water maze, consistent with its histamine-enhancing profile. In conclusion, the current study provides further support for the notion that the therapeutic effect of atomoxetine could involve activation of histamine neurotransmission within the prefrontal cortex.

Differential effect of cannabinoid agonists and endocannabinoids on histamine release from distinct regions of the rat brain

Cannabinoids exert complex actions on neurotransmitter systems involved in cognition, locomotion, appetite, but no information was available so far on the interactions between the endocannabinoid system and histaminergic neurons that command several, similar behavioural states and memory. In this study, we investigated the effect of cannabimimetic compounds on histamine release using the microdialysis technique in the brain of freely moving rats. We found that systemic administration of the cannabinoid receptors 1 (CB1-r) agonist arachidonyl-2'chloroethylamide/N-(2chloroethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (ACEA; 3 mg/kg) increased histamine release from the posterior hypothalamus, where the histaminergic tuberomamillary nuclei (TMN) are located. Local infusions of ACEA (150 nm) or R(+)-methanandamide (mAEA; 1 microm), another CB1-r agonist, in the TMN augmented histamine release from the TMN, as well as from two histaminergic projection areas, the nucleus basalis magnocellularis and the dorsal striatum. When the endocannabinoid uptake inhibitor AM404 was infused into the TMN, however, increased histamine release was observed only in the TMN. The cannabinoid-induced effects on histamine release were blocked by co-administrations with the CB1-r antagonist AM251. Using double-immunofluorescence labelling and confocal laser-scanning microscopy, CB1-r immunostaining was found in the hypothalamus, but was not localized onto histaminergic cells. The modulatory effect of cannabimimetic compounds on histamine release apparently did not involve inhibition of gamma-aminobutyric acid (GABA)ergic neurotransmission, which provides the main inhibitory input to the histaminergic neurons in the hypothalamus, as local infusions of ACEA did not modify GABA release from the TMN. These profound effects of cannabinoids on histaminergic neurotransmission may partially underlie some of the behavioural changes observed following exposure to cannabinoid-based drugs.

Contribution of Histamine Type-1 Receptor to Metabolic and Behavioral Control of Ventilation

Histaminergic neurons in the hypothalamus are well documented as being involved in the control of autonomic functions, such as the balance of energy metabolism and circadian rhythm. We tested the hypothesis that an activation of the histamine type-1 (H1) receptor is required for the control of ventilation during the course of a day in free-moving mice. Ventilation, aerobic metabolism, and electroencephalogram were measured by a whole-body-plethysmograph, a magnetic-type mass spectrometry system, and a telemetry system, respectively, in H1 receptor-knockout (H1RKO) and wild-type mice. Both genotypes showed daily oscillations in minute ventilation (V(E)) and oxygen consumption (VO(2)), with greater values during the dark period compared to the light period. In the latter, H1RKO mice showed increased V(E) and CO(2) excretion (VCO(2)) relative to wild-type mice, and V(E) was comparable to the VCO(2) increase. However, there was no change in VO(2) in H1RKO mice, suggesting that differences in VCO(2) between genotypes are responsible for differences in V(E) during the light period. During the dark period, VCO(2) was elevated in H1RKO mice compared with WT mice. Because there was no difference in V(E), the ratio of V(E) to VCO(2) was reduced in H1RKO mice. Electroencephalogram results suggested that this might be due to a depressed arousal state in H1RKO mice because the ratio of delta to theta band power spectrum densities was greater in H1RKO mice than in wild-type mice. We concluded that histamine modulates ventilation by affecting metabolism and arousal state via H1 receptors.

Changes in motoric, exploratory and emotional behaviours and neuronal acetylcholine content and 5-HT turnover in histidine decarboxylase-KO mice

Histamine has been implicated, inter alia, in mechanisms underlying arousal, exploratory behaviour and emotionality. Here, we investigated behavioural and neurochemical parameters related to these concepts, including open-field activity, rotarod performance and anxiety, as well as brain acetylcholine and 5-HT concentrations of mice deficient for the histidine decarboxylase (HDC) gene. These mice are unable to synthesize histamine from its precursor histidine. The HDC-knockout mice showed reduced exploratory activity in an open-field, but normal habituation to a novel environment. They behaved more anxious than the controls, as assessed by the height-fear task and the graded anxiety test, a modified elevated plus-maze. Furthermore, motor coordination on the rotarod was superior to controls. Biochemical assessments revealed that the HDC-knockout mice had higher acetylcholine concentrations and a significantly higher 5-HT turnover in the frontal cortex, but reduced acetylcholine levels in the neostriatum. These results are suggestive of important interactions between neuronal histamine and these site-specific neurotransmitters, which may be related to the behavioural changes found in the HDC-deficient animals.