Histamine, histidine decarboxylase and histamine-N-methyltransferase in brain areas of spontaneously hypertensive rats

Brain mapping: topography of neurons and their transmitters involved in various brain functions

Use of the demanding techniques microdialysis or push-pull superfusion makes it possible to identify neurons in distinct brain areas involved in central control of peripheral functions, thus enabling brain mapping. Investigations with the push-pull superfusion technique have shown that mainly catecholaminergic neurons of the posterior and anterior hypothalamus, the locus coeruleus, and the nucleus of the solitary tract are of crucial importance for blood pressure regulation. Experimentally induced blood pressure changes also modify the release of histamine, glutamate, and taurine in the posterior hypothalamus and of serotonin in the locus coeruleus. Furthermore, histaminergic neurons of the nucleus accumbens are involved in memory, serotonergic neurons of the locus coeruleus in response to noxious stimuli, while nitric oxide of striatum has been implicated in neurotoxicity elicited by amphetamines. The involvement of several neurons in one brain function is discussed.

Effects of Tianshu Capsule on Spontaneously Hypertensive Rats as Revealed by 1H-NMR-Based Metabolic Profiling

Hypertension is one of the most common cardiovascular diseases, resulting in serious complications such as cardiovascular damage and chronic kidney disease. Tianshu capsule (TSC), composed of Chuanxiong (Ligusticum chuanxiong Hort) and Tianma (Gastrodiaelata Blume), has been widely used to treat the blood stasis type of headache and migraine in clinic. Results of previous research showed its antihypertensive effects, but the underlying mechanisms were still unclear. The purpose of this study was to evaluate the antihypertensive effect of TSC on spontaneously hypertensive rats by ¹H NMR-based metabonomics and enzyme-linked immunosorbent assay (ELIAS), explore potential biomarkers and targets, and probe the potential mechanism of TSC on antihypertensive treatment. The results showed that TSC could decrease the product of oxidative stress (MDA) and enhance the activities of SOD and GSH-Px, down-regulate the expression of enzymes (LDHA, PKM2 and HK2) related to glycolysis, and perturb the levels of a series of amino acids (isoleucine, alanine, asparagine, citrate, etc.) and pathways. Multivariate statistical analyses showed remarkable changes in some endogenous metabolites after administrating TSC related to oxidative stress, amino acid metabolism and energy metabolism disturbances. Some enzymes (alanine-glyoxylate aminotransferase-2, tyrosine hydroxylase, dopa decarboxylase, etc.) related to metabolic biomarkers were predicted as the potential targets of TSC treatment on SHRs. The discoveries are helpful to understand the antihypertensive mechanism of TSC and provide theoretical evidence for its future research, development and clinical use.

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.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

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.

Similar deficits of central histaminergic system in patients with Down syndrome and Alzheimer disease

In order to study whether Alzheimer-like neuropathological changes involve the central histaminergic system we measured the concentration of histamine, its precursor histidine as well as the activity of histidine decarboxylase (HDC) and histamine-N-methyl-transferase (HMT) in frontal cortex of aging Down syndrome (DS) patients, Alzheimer patients and control individuals. The study populations were also investigated for choline acetyltransferase (ChAT) activity, since reduced ChAT activity is an established biochemical hallmark in DS and Alzheimer disease (AD). HDC and ChAT activity were reduced in brains of both DS and Alzheimer patients versus control patients. Additionally, we observed a significant decrease of histamine levels in the DS group. Histamine levels in AD brains tended to be decreased. Histidine concentrations and HMT activities were comparable between the three groups. Thus, our results for the first time show histaminergic deficits in brains of patients with DS resembling the neurochemical pattern in AD. Neuropathological changes may be responsible for similar neurochemical alterations of the histaminergic system in both dementing disorders.

Inhibition of histamine synthesis influences the development of hypertension in spontaneously hypertensive rats

Young (3-week old) and adult (7-week old) spontaneously hypertensive rats (SHR) and normotensive rats (WKY) were treated with alpha-fluoromethylhistidine (alpha-FMH) for 29 and 13 days, respectively. Treatment of SHR and WKY with alpha-FMH led to a pronounced decrease in the histidine decarboxylase activity and in the histamine concentration in all brain areas studied. In adult SHR, the development of hypertension was not influenced by alpha-FMH. In young SHR, alpha-FMH elicited a transient delay in the development of hypertension followed by a short-lasting tendency for increased blood pressure. It is concluded that histaminergic neurons of the brain play, if at all, only a secondary role in the development of hypertension in SHR.