Reduced number of intrinsic pulmonary nitrergic neurons in Fawn-Hooded rats as compared to control rat strains

Recent advances in research on nitrergic nerve-mediated vasodilatation

Cerebral vascular resistance and blood flow were widely considered to be regulated solely by tonic innervation of vasoconstrictor adrenergic nerves. However, pieces of evidence suggesting that parasympathetic nitrergic nerve activation elicits vasodilatation in dog and monkey cerebral arteries were found in 1990. Nitric oxide (NO) as a neurotransmitter liberated from parasympathetic postganglionic neurons decreases cerebral vascular tone and resistance and increases cerebral blood flow, which overcome vasoconstrictor responses to norepinephrine liberated from adrenergic nerves. Functional roles of nitrergic vasodilator nerves are found also in peripheral vasculature, including pulmonary, renal, mesenteric, hepatic, ocular, uterine, nasal, skeletal muscle, and cutaneous arteries and veins; however, adrenergic nerve-induced vasoconstriction is evidently greater than nitrergic vasodilatation in these vasculatures. In coronary arteries, neurogenic NO-mediated vasodilatation is not clearly noted; however, vasodilatation is induced by norepinephrine released from adrenergic nerves that activates β1-adrenoceptors. Impaired actions of NO liberated from the endothelium and nitrergic neurons are suggested to participate in cerebral hypoperfusion, leading to brain dysfunction, like that in Alzheimer's disease. Nitrergic neural dysfunction participates in impaired circulation in peripheral organs and tissues and also in systemic blood pressure increase. NO and vasodilator peptides, as sensory neuromediators, are involved in neurogenic vasodilatation in the skin. Functioning of nitrergic vasodilator nerves is evidenced not only in a variety of mammals, including humans and monkeys, but also in non-mammals. The present review article includes recent advances in research on the functional importance of nitrergic nerves concerning the control of cerebral blood flow, as well as other regions, and vascular resistance. Although information is still insufficient, the nitrergic nerve histology and function in vasculatures of non-mammals are also summarized.

Control of systemic and pulmonary blood pressure by nitric oxide formed through neuronal nitric oxide synthase

Nitric oxide formed by neuronal nitric oxide synthase (nNOS) in the brain, autonomic inhibitory (nitrergic) nerves, and heart plays important roles in the control of blood pressure. Activation of nitrergic nerves innervating the systemic vasculature elicits vasodilatation, decreases peripheral resistance, and lowers blood pressure. Impairment of nitrergic nerve function, as well as endothelial dysfunction, results in systemic and pulmonary hypertension and decreased regional blood flow. Blockade of nNOS activity in the brain, particularly the medulla and hypothalamus, causes systemic hypertension. Under hypertensive states, such as those in spontaneously hypertensive and Dahl salt-sensitive rats, the expression of the nNOS gene in the brain is increased; this appears to counteract the activated sympathetic function in the vasomotor center. The present article summarizes information concerning the modulation of systemic and pulmonary hypertension through nNOS-derived nitric oxide produced in the brain and periphery.

Parasympathetic control of airway submucosal glands: central reflexes and the airway intrinsic nervous system

Airway submucosal glands produce the mucus that lines the upper airways to protect them against insults. This review summarizes evidence for two forms of gland secretion, and hypothesizes that each is mediated by different but partially overlapping neural pathways. Airway innate defense comprises low level gland secretion, mucociliary clearance and surveillance by airway-resident phagocytes to keep the airways sterile in spite of nearly continuous inhalation of low levels of pathogens. Gland secretion serving innate defense is hypothesized to be under the control of intrinsic (peripheral) airway neurons and local reflexes, and these may depend disproportionately on non-cholinergic mechanisms, with most secretion being produced by VIP and tachykinins. In the genetic disease cystic fibrosis, airway glands no longer secrete in response to VIP alone and fail to show the synergy between VIP, tachykinins and ACh that is observed in normal glands. The consequent crippling of the submucosal gland contribution to innate defense may be one reason that cystic fibrosis airways are infected by mucus-resident bacteria and fungi that are routinely cleared from normal airways. By contrast, the acute (emergency) airway defense reflex is centrally mediated by vagal pathways, is primarily cholinergic, and stimulates copious volumes of gland mucus in response to acute, intense challenges to the airways, such as those produced by very vigorous exercise or aspiration of foreign material. In cystic fibrosis, the acute airway defense reflex can still stimulate the glands to secrete large amounts of mucus, although its properties are altered. Importantly, treatments that recruit components of the acute reflex, such as inhalation of hypertonic saline, are beneficial in treating cystic fibrosis airway disease. The situation for recipients of lung transplants is the reverse; transplanted airways retain the airway intrinsic nervous system but lose centrally mediated reflexes. The consequences of this for gland secretion and airway defense are poorly understood, but it is possible that interventions to modify submucosal gland secretion in transplanted lungs might have therapeutic consequences.

Ionotropic glutamate receptors in lungs and airways: molecular basis for glutamate toxicity

We earlier showed that the ionotropic glutamate receptor agonist N-methyl D-aspartate (NMDA) induces excitotoxic pulmonary edema, and that endogenous activation of NMDA receptors (NMDAR) could mediate lung injury caused by oxidative stress. In this study, we searched for evidence of NMDAR expression in the rat lung and in the alveolar macrophage (AM) cell line NR8383, and for possible regulation of receptor expression by NMDA. The presence of mRNA for NMDAR 1 and the four known NMDAR 2 subtypes (A, B, C, and D) was examined by reverse transcriptase-polymerase chain reaction using isoform-specific primers. NMDAR 1 was expressed in all lung regions examined (peripheral, midlung, and mainstem), as well as in trachea and the AMs. Expression of NMDAR 2A and 2B subtypes was not detected, whereas NMDAR 2C was present only in peripheral and mid-lung samples. NMDAR 2D was the dominant subtype expressed in the peripheral, gas-exchange zone of lung and in alveolar macrophages, and this expression was upregulated in lungs treated with NMDA. Western blot confirmed the presence of NMDAR 1 protein in all lung regions and in AMs. These findings provide a molecular-biological basis for the excitotoxic actions of glutamate in rat lungs and airways, and raise the question of a possible physiologic role for NMDAR in lung and airway function.

Quantification of neuroepithelial bodies and their innervation in fawn-hooded and Wistar rat lungs

The Fawn-Hooded rat (FHR), a model for primary pulmonary hypertension, shows an unexplained hypersensitivity to airway hypoxia. Because pulmonary neuroepithelial bodies (NEBs) appear to express a functional oxygen-sensing mechanism and an extensive sensory innervation, possible changes in this system should be taken into consideration. In the present study a comparative analysis of NEBs and their selective innervation was performed in FHRs and Wistar control rats. In both rat strains, the number of NEBs was estimated to be around 3,500, approximately 40% of which were innervated by vagal sensory calbindin D28k-immunoreactive (IR) nerve endings and approximately 50% by spinal sensory calcitonin gene-related peptide (CGRP)-IR nerve terminals. The number of intrinsic pulmonary nitrergic neurons and the percentage of pulmonary NEBs revealing a nitrergic innervation were highly significantly lower in FHRs. In both FHRs and Wistar rats, a remarkable morphologic interaction was observed between the intrinsic nitrergic and the CGRP-IR sensory population contacting NEBs. Our findings suggest a possible link between the hypersensitivity to airway hypoxia observed in FHRs and a reduced intrinsic pulmonary nitrergic innervation, possibly via the interaction with pulmonary NEBs and their spinal sensory CGRP-IR innervation.