Central histaminergic neurons regulate rabbit tracheal tension through the cervical sympathetic nerve

Histamine/H1 receptor signaling in the parafacial region increases activity of chemosensitive neurons and respiratory activity in rats

Histaminergic neurons of the tuberomammillary nucleus (TMN) are pH-sensitive and contribute to CO₂/H⁺-dependent behaviors including arousal and respiratory activity. TMN neurons project to several respiratory centers including the ventral parafacial region (pF) where chemosensitive retrotrapezoid (RTN) neurons are located, and since RTN neurons are an important source of CO₂/H⁺-dependent respiratory drive, we wondered whether histamine contributes to RTN chemoreception. To test this, we characterized effects of histamine on mean arterial pressure (MAP) and diaphragm muscle activity (DIA_EMG) in urethane-anaesthetized, vagotomized and artificially ventilated male Wistar rats. Unilateral injection of histamine (25 mM) in the pF increased DIA_EMG amplitude without changing DIA_EMG frequency and MAP. Bilateral pF injections of the H1 receptor antagonist diphenhydramine hydrochloride (DPH; 0.5 mM) decreased baseline DIA_EMG amplitude and frequency and MAP. Despite the strong inhibitory effect of DPH on baseline breathing, the hypercapnic ventilatory response was preserved under these experimental conditions. At the cellular level, chemosensitive RTN neurons showed a dose-dependent excitatory response to histamine that was blunted by DPH and mimicked by the H1 receptor agonist 2-pyridylethylamine dihydrochloride (2PYEA) under both control conditions and when fast neurotransmitter receptors are blocked. We also tested effects of 2PYEA in the presence of serotonin, another wake-on neurotransmitter that activates RTN chemoreceptors partly by activation of Gq-coupled receptors. We found the response to 2PYEA was diminished in serotonin, suggesting RTN neurons have a limited capacity to respond to multiple Gq-coupled modulators. These results suggest histamine can modulate breathing at the pF level by a mechanism involving H1 receptors.

Autonomic neural control of the airways

The airways and lungs are innervated by both sympathetic and parasympathetic nerves. Cholinergic parasympathetic innervation is well conserved in the airways while the distribution of noncholinergic parasympathetic and adrenergic sympathetic nerves varies considerably amongst species. Autonomic nerve function is regulated primarily through reflexes initiated upon bronchopulmonary vagal afferent nerves. Central regulation of autonomic tone is poorly described but some key elements have been defined.

Impaired ventilation and metabolism response to hypoxia in histamine H1 receptor-knockout mice

The role of central histamine in the hypoxic ventilatory response was examined in conscious wild-type (WT) and histamine type1 receptor-knockout (H1RKO) mice. Hypoxic gas (7% O(2) and 3% CO(2) in N(2)) exposure initially increased and then decreased ventilation, referred to as hypoxic ventilatory decline (HVD). The initial increase in ventilation did not differ between genotypes. However, H1RKO mice showed a blunted HVD, in which mean inspiratory flow was greater than that in WT mice. O(2) consumption (V(O2)) and CO(2) excretion were reduced 10min after hypoxic gas exposure in both genotypes, but (V(O2)) was greater in H1RKO mice than in WT mice. The ratio of minute ventilation to (V(O2)) during HVD did not differ between genotypes, indicating that ventilation is adequately controlled according to metabolic demand in both mice. Peripheral chemoreceptor sensitivity did not differ between genotypes. We conclude that central histamine contributes via the H1 receptor to changes in metabolic rate during hypoxia to increase HVD in conscious mice.

CNS determinants of sleep-related worsening of airway functions: implications for nocturnal asthma

This review summarizes the recent neuroanatomical and physiological studies that form the neural basis for the state-dependent changes in airway resistance. Here, we review only the interactions between the brain regions generating quiet (non-rapid eye movement, NREM) and active (rapid eye movement, REM) sleep stages and CNS pathways controlling cholinergic outflow to the airways. During NREM and REM sleep, bronchoconstrictive responses are heightened and conductivity of the airways is lower as compared to the waking state. The decrease in conductivity of the lower airways parallels the sleep-induced decline in the discharge of brainstem monoaminergic cell groups and GABAergic neurons of the ventrolateral periaqueductal midbrain region, all of which provide inhibitory inputs to airway-related vagal preganglionic neurons (AVPNs). Withdrawal of central inhibitory influences to AVPNs results in a shift from inhibitory to excitatory transmission that leads to an increase in airway responsiveness, cholinergic outflow to the lower airways and consequently, bronchoconstriction. In healthy subjects, these changes are clinically unnoticed. However, in patients with bronchial asthma, sleep-related alterations in lung functions are troublesome, causing intensified bronchopulmonary symptoms (nocturnal asthma), frequent arousals, decreased quality of life, and increased mortality. Unquestionably, the studies revealing neural mechanisms that underlie sleep-related alterations of airway function will provide new directions in the treatment and prevention of sleep-induced worsening of airway diseases.

Central histamine contributes to the inspiratory off-switch mechanism via H1 receptors in mice

Central histaminergic neurons are distributed in areas of the medulla and pons concerned with respiratory rhythm generation, but their effects on breathing pattern are unknown. We examined breathing pattern during hypercapnic responses in wild type (WT) and H1 receptor knockout (H1RKO) mice at 9-10 weeks of age before and after vagotomy. Minute ventilation increased with PaCO(2) increase equally in both genotypes; respiratory rate response was lower and tidal volume (V(T)) response higher in H1RKO mice than in WT mice. The V(T)-inspiratory time (T(I)) relation during hypercapnia was hyperbolic in both groups, with the curve in H1RKO mice shifted right-upward. After vagotomy, the V(T)-T(I) relation was a vertical line, which shifted right in H1RKO mice. We conclude that alterations of inspiratory off-switch and respiratory rhythm generation change breathing pattern without affecting central chemosensitivity in H1RKO. Histamine might affect breathing pattern centrally via H1 receptors.

Neuronal histamine release elicited by hyperthermia mediates tracheal dilation and pressor response

Whether brain histaminergic neurons contribute to the regulation of tracheal tone and peripheral vascular tone under hyperthermia was investigated in anesthetized rabbits. Histamine release from the rostral ventrolateral medulla (RVLM), the raphe nuclei, and the solitary nucleus of the medulla oblongata was significantly increased by hyperthermia. The increased histamine was significantly suppressed by 10−6 M tetrodotoxin microdialyzed in each area. Tracheal pressure and mean arterial pressure were significantly decreased and increased by hyperthermia, respectively. An H1-receptor antagonist, 5 × 10−6 M (+)-chlorpheniramine, bilaterally microdialyzed in the RVLM significantly enhanced histamine release in the RVLM as well as significantly suppressed tracheal dilation and pressor response caused by hyperthermia. These data indicate that histamine release in the medulla oblongata is enhanced by hyperthermia. The enhanced histamine is the neuronal origin and the cause of tracheal dilation and pressor response at least via H1 receptors in the RVLM. Brain histaminergic neurons play important roles in tracheal tone and peripheral vascular tone via H1 receptors in the RVLM and homeostasis on body temperature.

Involvement of central histaminergic neurons in polypnea induced by hyperthermia in rabbits

A role of central histamine in the preoptic area/anterior hypothalamus (POA/AH) for the regulation of hyperthermia-induced polypnea was examined in anesthetized, paralyzed, vagotomized and artificially ventilated rabbits. Phrenic nerve activities were recorded to monitor respiratory neuronal output. Hyperthermia increased respiratory frequency by reductions of inspiratory time (T(I)) and expiratory time (T(E)). Pyrilamine, an H1 receptor antagonist, which was applied to the POA/AH reduced polypnea under hyperthermia. The effect of S+alpha-fluoromethylhistidine, a specific inhibitor of histidine decarboxylase, applied in a lateral ventricle was comparable to the effect of pyrilamine on polypnea. Moreover, histamine dihydrochloride applied into the POA/AH at a normal body temperature produced polypnea by reductions of T(I) and T(E). The results suggest that central histamine in the POA/AH contributes to the generation of polypnea in hyperthermia through H1 receptors.

Central histamine contributed to temperature-induced polypnea in mice

Breathing pattern is influenced by body temperature. However, the central mechanism for changing breathing patterns is unknown. Central histamine is involved in heat loss mechanisms in behavioral studies, but little is known about its effect on breathing patterns. We examined first the effect of body temperature on breathing patterns with increasing hypercapnia in conscious mice and then that of the depletion of central histamine by S(+)-alpha-fluoromethylhistidine hydrochloride (alpha-FMH) (100 mg/kg ip), a specific inhibitor of histidine decarboxylase, at normal and raised body temperatures. A raised body temperature increased respiratory frequency with reductions in both inspiratory and expiratory time and decreased tidal volume. On the other hand, alpha-FMH lowered respiratory frequency with a prolongation of expiratory time at the raised temperature; however, this was not observed at a normal temperature. These results indicate that central histamine contributes to an increase in respiratory frequency as a result of a reduction in expiratory time when body temperature is raised.

Lack of temperature-induced polypnea in histamine H1 receptor-deficient mice

Breathing patterns are influenced by body temperature. However, the central mechanism for changes of breathing patterns is unknown. We previously showed that central histamine contributed to temperature-induced polypnea in mice (Izumizaki, M., Iwase, M., Homma, I., Yanai, K., Watanabe, T. and Watanabe, T., Central histamine contributed to the temperature-induced polypnea in mice, Neurosci. Res., 23 (1999) S282). In this study we examined the role of central histamine H1 receptors in temperature-induced polypnea using wild and mutant mice lacking histamine H1 receptors. Breathing patterns were characterized at two different body temperatures during hypercapnia under conscious conditions. In wild mice a raised body temperature increased respiratory frequency mainly due to a reduction in expiratory time, whereas in mutant mice respiratory frequency did not increase even though the body temperature was elevated. These results indicate that central histamine contributes to an increase in respiratory frequency due to a reduction in expiratory time through histamine H1 receptors when body temperature is raised.