Histamine content and synthesis in central and pheripheral nerve structures during stress

The role of mast cells in migraine pathophysiology

Mast cells are critical players in allergic reactions, but they have also been shown to be important in immunity and recently also in inflammatory diseases, especially asthma. Migraines are episodic, typically unilateral, throbbing headaches that occur more frequently in patients with allergy and asthma implying involvement of meningeal and/or brain mast cells. These mast cells are located perivascularly, in close association with neurons especially in the dura, where they can be activated following trigeminal nerve, as well as cervical or sphenopalatine ganglion stimulation. Neuropeptides such as calcitonin gene-related peptide (CGRP), hemokinin A, neurotensin (NT), pituitary adenylate cyclase activating peptide (PACAP), and substance P (SP) activate mast cells leading to secretion of vasoactive, pro-inflammatory, and neurosensitizing mediators, thereby contributing to migraine pathogenesis. Brain mast cells can also secrete pro-inflammatory and vasodilatory molecules such as interleukin-6 (IL-6) and vascular endothelial growth factor (VEGF), selectively in response to corticotropin-releasing hormone (CRH), a mediator of stress which is known to precipitate or exacerbate migraines. A better understanding of brain mast cell activation in migraines would be useful and could lead to several points of prophylactic intervention.

Acute stress increases permeability of the blood-brain-barrier through activation of brain mast cells

Disruption of the blood-brain-barrier (BBB) is important in the pathophysiology of various inflammatory conditions of the central nervous system (CNS), such as multiple sclerosis (MS), in which breakdown of the BBB precedes any clinical or pathological findings. There is some evidence that relapsing-remitting MS attacks may be correlated with certain types of acute stressful episodes. Stress typically activates the hypothalamic-pituitary-adrenal (HPA) axis through the release of corticotropin releasing hormone (CRH), leading to production of glucocorticoids that down regulate immune responses. However, acute stress also has pro-inflammatory effects that appear to be mediated through activation of mast cells. Here we show that acute stress by immobilization increased permeability of rat BBB to intravenous 99Technetium gluceptate (99Tc). This effect was statistically significant in the diencephalon and the cerebellum, while it was absent in the cerebral cortex where there are not mast cells. Immobilization stress also induced activation of mast cells in diencephalon, the site where most mast cells are found in the rat brain. Both BBB permeability and mast cell activation were inhibited by the 'mast cell stabilizer' disodium cromoglycate (cromolyn). These results expand the pathophysiology of mast cells and implicate them in CNS disorders, that may possibly be induced or exacerbated by stress.

Glucagon-like peptide (GLP-1) is involved in the central modulation of fecal output in rats

In addition to its insulinotropic action, exogenously administered glucagon-like peptide (GLP-1) inhibits gastropancreatic motility and secretion via central pathways. The aims of the present study were to evaluate the effects of exogenous GLP-1-(7-36) amide on fecal output and to investigate the role of endogenous GLP-1 on stress-induced colonic activity. With the use of a stereotaxic instrument, adult male Sprague-Dawley rats weighing 200-250 g were fitted with stainless steel cerebroventricular guide cannulas under ketamine anesthesia. A group of rats were placed in Bollman-type cages to induce restraint stress. Fecal output monitored for 2 h was increased significantly by intracerebroventricular GLP-1 to 500, 1, 000, and 3,000 pmol/rat (P < 0.05-0.01), whereas intraperitoneal GLP-1 had no effect. Intracerebroventricular administration of the GLP-1 receptor antagonist exendin-(9-39) (10 nmol/rat) reversed the increases induced by GLP-1 (500 pmol/rat; P<0.01). Similar results were also observed with the injection of corticotropin-releasing factor receptor antagonist astressin (10 microg/rat icv). The significant increase in fecal pellet output induced by restraint stress was also decreased by both intracerebroventricular exendin (10 nmol/rat) and astressin (10 microg/rat; P<0.01-0.001). These results suggest that GLP-1 participates in the central, but not peripheral, regulation of colonic motility via its own receptor and that GLP-1 is likely to be a candidate brain-gut peptide that acts as a physiological modulator of stress-induced colonic motility.

Role of histaminergic mechanisms in the regulation of some stress responses in rats

The involvement of histaminergic mechanisms in the regulation of some stress responses was studied in rats. The brain neuronal histamine (HA) depletor, alpha-fluoromethyl histidine (alpha-FMH), at doses (50 or 100 mg/kg) which markedly lower brain HA, significantly attenuated the gastric ulcer formation and the elevation in plasma corticosterone in response to cold restraint stress (CRS). alpha-FMH also appreciably reduced gastric mucosal HA content. The H1-antagonist, pheniramine (25 mg/kg), attenuated both the gastric mucosal and endocrine response to CRS, while the effects of the H2-antagonist, cimetidine (200 mg/kg), were on the plasma corticosterone levels. These results are discussed in light of complex HA-ergic mechanisms in the maintenance of physiological homeostasis during stress.

Age-related changes in brain histamine

The effect of age on brain histamine levels and histamine methyltransferase activity (HMT) was investigated. Male Sprague-Dawley rats (12 months old) displayed significantly higher hypothalamic, midbrain and cortical histamine concentrations than three-month-old animals. In contrast, HMT activity was significantly decreased in all three brain regions. The increase in brain histamine concentration of old rats could have been partially attributed to decreased activity of HMT since elevated levels of brain histamine are known to occur following HMT inhibition. Present results indicate that similarly to the reported changes in the concentration, synthesis and/or metabolism of other central neurotransmitters in old rats, brain histamine regulation may also be affected in the process of aging.

Histamine and the hypothalamus

The chemical tools that could be used to examine the function of histamine in the brain are considered together with the evidence linking histamine specifically with the hypothalamus. The distribution of histamine and the enzymes responsible for its synthesis and metabolism is consistent with there being both mast cells and histaminergic nerve terminals within the hypothalamus. Iontophoresis, mepyramine binding and histamine-stimulated adenylate cyclase studies suggest that both histamine H1- and H2- receptors are present in the hypothalamus. In addition, intracerebroventricularly injected histamine receptor agonists and antagonists affect many functions associated with the hypothalamus such as cardiovascular control, food intake, body temperature control, and pituitary hormones whose release is mediated via the hypothalamus, such as corticotropin, growth hormone, thyroid stimulating hormone, prolactin, gonadotropins and vasopressin. However, only in the case of thyroliberin release, prolactin release, body fluid control and blood pressure control is there evidence yet that such effects are mediated via histamine receptors actually in the hypothalamus. The effects of enzyme inhibitors suggest endogenous histamine may be involved in the physiological control of thyroid stimulating hormone, growth hormone and blood pressure, and the effects of receptor antagonists support a role for endogenous histamine in prolactin control. Otherwise, there is little evidence for a physiological role for endogenous, as against exogenous, histamine whether it be from histaminergic terminals or mast cells. In addition, few studies have tried to distinguish possible effects on presynaptic receptors, postsynaptic receptors, hypothalamic blood vessels or the hypophyseal portal blood vessels. It is concluded that although there is good evidence now linking histamine and the hypothalamus more specific studies are required, for instance using microinjection or in vitro techniques and the more specific chemical tools now available, to enable a clearer understanding of the physiological role of histamine in the hypothalamus.

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

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

Effect of diazepam on stress induced changes in brain histamine

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

Effect of Wallerian degeneration on histamine concentration of the peripheral nerve

One sciatic nerve of a White Leghorn hen was severed and the distal portion was allowed to undergo Wallerian degeneration. The change in histamine and DNA concentration and mast cell number was measured at different times following nerve sectioning in the proximal regenerating, distal degenerating, and intact, contralateral nerves. The experimental results revealed a significant accumulation of histamine in the proximal desheathed segment and in the contralateral "functional nerve," whereas the biogenic amine in the distal desheathed nerve significantly decreased. The pattern of change of histamine in the distal and proximal nerve sheaths was different: it dropped at 2 h and then rose in the later stages of Wallerian degeneration. In the distal desheathed nerves and in both the proximal and distal nerve sheaths DNA increased significantly by 14 days. The number of mast cells appeared to be highest in the 14-day distal nerve and in the 7-day proximal nerve sheaths. These results support a dual localization of histamine in the peripheral nerve, and are consistent with the interpretation that the amine has either some role in neurotransmission or in the process of growth and regeneration.

Single and repeated air blast stress and brain histamine

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

Effect of stress on brain histamine

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

Ontogenesis of histamine in the chick nervous system

Tissues from the central and peripheral nervous systems of the chick were analyzed for concentration of histamine (Hm) during development. Of the three CNS organs examined, cerebral hemispheres had the highest Hm content. Expressed on the bases of wet weight, protein, and DNA concentrations, sciatic nerve and the pineal gland had the highest levels of this biogenic amine of the five tissues investigated. The concentration of Hm was higher in the cerebellum, cerebral hemispheres, and thalamus of adult animals than in the 15 to 17-day-old embryos. The level of Hm rose markedly in the sciatic nerve and pineal gland after the 15th day of embryonic development. These data might indicate a possible involvement of Hm in controlling the course of maturation of certain organs in the nervous system.