Orexin-A Excites Airway Vagal Preganglionic Neurons via Activation of Orexin Receptor Type 1 and Type 2 in Rats.

Airway vagal nerves play a predominant role in the neural control of the airway, and augmented airway vagal activity is known to play important roles in the pathogenesis of some chronic inflammatory airway diseases. Several lines of evidence indicate that dysfunctional central orexinergic system is closely related to the severity of airway diseases, however, whether orexins affect airway vagal activity is unknown. This study investigates whether and how orexin-A regulates the activity of medullary airway vagal preganglionic neurons (AVPNs). The expression of orexin receptor type 1 (OX1R) and type 2 (OX2R) was examined using immunofluorescent staining. The effects of orexin-A on functionally identified inspiratory-activated AVPNs (IA-AVPNs), which are critical in the control of airway smooth muscle, were examined using patch-clamp in medullary slices of neonatal rats. Airway vagal response to injection of orexin-A into the magna cisterna was examined using plethysmography in juvenile rats. The results show that retrogradely labeled AVPNs were immunoreactive to anti-OX1R antibody and anti-OX2R antibody. Orexin-A dose-dependently depolarized IA-AVPNs and increased their firing rate. In synaptically isolated IA-AVPNs, the depolarization induced by orexin-A was blocked partially by OX1R antagonist SB-334867 or OX2R antagonist TCS OX2 29 alone, and completely by co-application of both antagonists. The orexin-A-induced depolarization was also mostly blocked by Na+/Ca2+ exchanger inhibitor KB-R7943. Orexin-A facilitated the glutamatergic, glycinergic and GABAergic inputs to IA-AVPNs, and the facilitation of each type of input was blocked partially by SB-334867 or TCS OX2 29 alone, and completely by co-application of both antagonists. Injection of orexin-A into the magna cisterna of juvenile rats significantly increased the inspiratory and expiratory resistance of the airway and consequently decreased the dynamic compliance of the lungs, all of which were prevented by atropine sulfate or bilateral vagotomy. These results demonstrate that orexin-A excites IA-AVPNs via activation of both OX1R and OX2R, and suggest that increased central synthesis/release of orexins might participate in the pathogenesis of airway diseases via over-activation of AVPNs.

Inspiratory-Activated Airway Vagal Preganglionic Neurones Excited by Thyrotropin-Releasing Hormone via Multiple Mechanisms in Neonatal Rats.

The airway vagal preganglionic neurons (AVPNs) providing projections to intrinsic tracheobronchial ganglia are considered to be crucial to modulation of airway resistance in physiological and pathological states. AVPNs classified into inspiratory-activated AVPNs (IA-AVPNs) and inspiratory-inhibited AVPNs (II-AVPNs) are regulated by thyrotropin-releasing hormone (TRH)-containing terminals. TRH causes a direct excitatory current and attenuates the phasic inspiratory glycinergic inputs in II-AVPNs, however, whether and how TRH influences IA-AVPNs remains unknown. In current study, TRH regulation of IA-AVPNs and its mechanisms involved were investigated. Using retrogradely fluorescent labeling method and electrophysiology techniques to identify IA-AVPNs in brainstem slices with rhythmic inspiratory hypoglossal bursts recorded by a suction electrode, the modulation of TRH was observed with patch-clamp technique. The findings demonstrate that under voltage clamp configuration, TRH (100 nM) caused a slow excitatory inward current, augmented the excitatory synaptic inputs, progressively suppressed the inhibitory synaptic inputs and elicited a distinctive electrical oscillatory pattern (OP). Such a current and an OP was independent of presynaptic inputs. Carbenoxolone (100 µM), a widely used gap junction inhibitor, fully suppressed the OP with persistence of TRH-induced excitatory slow inward current and augment of the excitatory synaptic inputs. Both tetrodotoxin (1 µM) and riluzole (20 µM) functioned to block the majority of the slow excitatory inward current and prevent the OP, respectively. Under current clamp recording, TRH caused a slowly developing depolarization and continuously progressive oscillatory firing pattern sensitive to TTX. TRH increased the firing frequency in response to injection of a square-wave current. The results suggest that TRH excited IA-AVPNs via the following multiple mechanisms: (1) TRH enhances the excitatory and depresses the inhibitory inputs; (2) TRH induces an excitatory postsynaptic slow inward current; (3) TRH evokes a distinctive OP mediated by gap junction.

Corticotropin-releasing hormone modulates airway vagal preganglionic neurons of Sprague-Dawley rats at multiple synaptic sites via activation of its type 1 receptors: Implications for stress-associated airway vagal excitation.

Corticotropin-releasing hormone release is the final common pathway of stress-associated neuroendocrine responses. This study tested how corticotropin-releasing hormone modulates airway vagal preganglionic neurons. Airway vagal preganglionic neurons in neonatal rats were retrogradely labeled with fluorescent dye and identified in medullary slices, and their responses to corticotropin-releasing hormone (200nmolL-1) were examined using whole-cell patch clamp. The results show that under current clamp, corticotropin-releasing hormone (200nmolL-1) depolarized airway vagal preganglionic neurons and significantly increased the rate of their spontaneous firing. Under voltage clamp, corticotropin-releasing hormone caused a tonic inward current and significantly facilitated the spontaneous glutamatergic and GABAergic inputs of these neurons. Corticotropin-releasing hormone had no impact on the spontaneous glycinergic inputs of these neurons. In the preexistence of tetrodotoxin (1µmolL-1), corticotropin-releasing hormone had no impact on the miniature excitatory or inhibitory postsynaptic currents, but still induced a tonic inward current and significantly increased the input resistance. The responses induced by corticotropin-releasing hormone were prevented by Antalarmin hydrochloride (50µmolL-1), an antagonist of type 1 corticotropin-releasing hormone receptors, but insensitive to Astressin 2B (200nmolL-1), an antagonist of type 2 corticotropin-releasing hormone receptors. These results suggest that corticotropin-releasing hormone excites airway vagal preganglionic neurons via activation of its type 1 receptors at multiple sites, which includes a direct postsynaptic excitatory action and presynaptic facilitation of both glutamatergic and GABAergic inputs. In stress, corticotropin-releasing hormone might be able to activate the airway vagal nerves and, consequently, participate in induction or exacerbation of airway disorders.

Activation of α1-adrenoceptors facilitates excitatory inputs to medullary airway vagal preganglionic neurons.

In mammals, the neural control of airway smooth muscle is dominated by a subset of airway vagal preganglionic neurons in the ventrolateral medulla. These neurons are physiologically modulated by adrenergic/noradrenergic projections, and weakened α2-adrenergic inhibition of them is indicated to participate in the pathogenesis and exacerbation of asthma. This study tests whether these neurons are modulated by α1-adrenoceptors, and if so, how. In anesthetized adult rats, microinjection of the α1A-adrenoceptor agonist A61603 (1 pmol) unilaterally into the medullary region containing these neurons caused a significant increase in airway resistance, which was prevented by intraperitoneal atropine (0.5 mg/kg). In rhythmically firing medullary slices of newborn rats, A61603 (10 nM) caused depolarization in both the inspiratory-activated and inspiratory-inhibited airway vagal preganglionic neurons that were retrogradely labeled, and a significant increase in the spontaneous firing rate. Under voltage clamp, A61603 significantly enhanced the spontaneous excitatory inputs to both types of neurons and caused a tonic inward current in the inspiratory-activated neurons along with significantly increased peak amplitude of the inspiratory inward currents. The responses in vitro were prevented by α1A-adrenoceptor antagonist RS100329 (1 µM), which alone significantly inhibited the spontaneous excitatory inputs to both types of the neurons. After pretreatment with tetrodotoxin (1 µM), A61603 (10 or 100 nM) had no effect on either type of neuron. We conclude that in rats, activation of α1-adrenoceptors in the medullary region containing airway vagal preganglionic neurons increases airway vagal tone, and that this effect is primarily mediated by facilitation of the excitatory inputs to the preganglionic neurons.

Inspiratory-activated and inspiratory-inhibited airway vagal preganglionic neurons in the ventrolateral medulla of neonatal rat are different in intrinsic electrophysiological properties.

This study investigates the firing properties of the inspiratory-activated and inspiratory-inhibited airway vagal preganglionic neurons located in the external formation of the nucleus ambiguus. The results showed that inspiratory-activated and inspiratory-inhibited neurons are distributed with different density and site preference in this area. Inspiratory-inhibited neurons exhibit significantly more positive resting membrane potential, more negative voltage threshold and lower minimal current required to evoke an action potential under current clamp. The afterhyperpolarization in inspiratory-activated neurons was blocked by apamin, a blocker of the small-conductance Ca(2+)-activated K(+) channels; and that in inspiratory-inhibited neurons by charybdotoxin, a blocker of the large-conductance Ca(2+)-activated K(+) channels. Under voltage clamp, depolarizing voltage steps evoked tetrodotoxin-sensitive rapid inward sodium currents, 4-aminopyridine-sensitive outward potassium transients and lasting outward potassium currents. 4-Aminopyridine partially blocked the lasting outward potassium currents of inspiratory-activated neurons but was ineffective on those of inspiratory-inhibited neurons. These findings suggest that inspiratory-activated and inspiratory-inhibited neurons are differentially organized and express different types of voltage-gated ion channels.

Diffusion tensor imaging detects Wallerian degeneration of the corticospinal tract early after cerebral infarction.

To investigate the feasibility and time window of early detection of Wallerian degeneration in the corticospinal tract after middle cerebral artery infarction, 23 patients were assessed using magnetic resonance diffusion tensor imaging at 3.0T within 14 days after the infarction. The fractional anisotropy values of the affected corticospinal tract began to decrease at 3 days after onset and decreased in all cases at 7 days. The diffusion coefficient remained unchanged. Experimental findings indicate that diffusion tensor imaging can detect the changes associated with Wallerian degeneration of the corticospinal tract as early as 3 days after cerebral infarction.

Beta-asarone inhibits synaptic inputs to airway preganglionic parasympathetic motoneurons.

Therapeutic application of Asarum, a herbal medicine that has been used for centuries, reportedly causes acute respiratory disturbance. The responsible constituents, the sites of action, and the mechanisms involved in this side effect are unclear. We investigated the effects of ß-asarone, a volatile constituent of Asarum, on neurotransmission in the medullary respiratory neuronal network using extracellular recording of the rhythmic hypoglossal activity and voltage clamp recordings of the postsynaptic activity of the airway preganglionic parasympathetic motoneurons (APPMs) in vitro. ß-Asarone caused progressive decreases in the duration and area of the hypoglossal bursts in a concentration-dependent manner. The frequency and amplitude of the bursts were initially unaltered or temporarily increased, but were then inhibited progressively after prolonged exposure. As with the inhibition of the hypoglossal bursts, the tonic and the phasic excitatory and inhibitory postsynaptic currents in the APPMs were attenuated. These data suggest that the Asarum-caused acute respiratory disturbance involves ß-asarone-induced inhibition of neurotransmission in the medullary respiratory neuronal network.