Combined laryngeal inflammation and trauma mediate long-lasting immunoreactivity response in the brainstem sensory nuclei in the rat.

Somatosensory feedback from the larynx plays a critical role in regulation of normal upper airway functions, such as breathing, deglutition, and voice production, while altered laryngeal sensory feedback is known to elicit a variety of pathological reflex responses, including persistent coughing, dysphonia, and laryngospasm. Despite its clinical impact, the central mechanisms underlying the development of pathological laryngeal responses remain poorly understood. We examined the effects of persistent vocal fold (VF) inflammation and trauma, as frequent causes of long-lasting modulation of laryngeal sensory feedback, on brainstem immunoreactivity in the rat. Combined VF inflammation and trauma were induced by injection of lipopolysaccharide (LPS) solution and compared to VF trauma alone from injection of vehicle solution and to controls without any VF manipulations. Using a c-fos marker, we found significantly increased Fos-like immunoreactivity (FLI) in the bilateral intermediate/parvicellular reticular formation (IRF/PCRF) with a trend in the left solitary tract nucleus (NTS) only in animals with combined LPS-induced VF inflammation and trauma. Further, FLI in the right NTS was significantly correlated with the severity of LPS-induced VF changes. However, increased brainstem FLI response was not associated with FLI changes in the first-order neurons of the laryngeal afferents located in the nodose and jugular ganglia in either group. Our data indicate that complex VF alterations (i.e., inflammation/trauma vs. trauma alone) may cause prolonged excitability of the brainstem nuclei receiving a direct sensory input from the larynx, which, in turn, may lead to (mal)plastic changes within the laryngeal central sensory control.

Effects of dopamine D1 and D2 receptor antagonists on laryngeal neurophysiology in the rat.

Hypophonia is an early symptom in Parkinson's disease (PD) that involves an increase in laryngeal muscle activity, interfering with voice production. Our aim was to use an animal model to better understand the role of different dopamine receptor subtypes in the control of laryngeal neurophysiology. First, we evaluated the combined effects of SCH23390-a D(1) receptor antagonist with a D(2) receptor antagonist (eticlopride) on laryngeal neurophysiology, and then tested the separate effects of selective receptor antagonists. Thyroarytenoid (TA) and gastrocnemius (GN) muscle activity was measured at rest and while stimulating the internal branch of superior laryngeal nerve to elicit the laryngeal adductor response (LAR) in alpha-chloralose-anesthetized rats. Paired stimuli at different interstimulus intervals between 250 and 5,000 ms measured central conditioning of the LAR. Changes in resting muscle activity, response latency, amplitude, and LAR conditioning after each drug were compared with the saline control. SCH23390 alone increased the resting TA muscle activity (P < 0.05). With the combined SCH23390 + eticlopride or SCH23390 alone, response latency decreased (P < 0.01), amplitude increased (P < 0.01), and the test LAR was reduced at 2,000-ms ISI (P < 0.01). No LAR changes occurred when eticlopride was administered alone at a low dose and only a tendency to suppress responses was found at a high dose. No changes in GN muscle activity occurred in any of the groups. The results suggest that a loss of stimulation of D(1) receptors plays a significant role in laryngeal pathophysiology in PD.

Laryngeal reflex responses are not modulated during human voice and respiratory tasks.

The laryngeal adductor response (LAR) is a protective reflex that prevents aspiration and can be elicited either by electrical stimulation of afferents in the superior laryngeal nerve (SLN) or by deflection of mechanoreceptors in the laryngeal mucosa. We hypothesized that because this reflex is life-sustaining, laryngeal muscle responses to sensory stimuli would not be suppressed during volitional laryngeal tasks when compared to quiet respiration. Unilateral electrical superior laryngeal nerve stimulation was used to elicit early (R1) and late (R2) responses in the ipsilateral thyroarytenoid muscle in 10 healthy subjects. The baseline levels of muscle activity before stimulation, R1 and R2 response occurrence and the integrals of responses were measured during each task: quiet inspiration, prolonged vowels, humming, forced inhalation and effort closure. We tested whether R1 response integrals during tasks were equal to either: (1) baseline muscle activity during the task added to the response integral at rest; (2) the response integral at rest minus the baseline muscle activity during the task; or (3) the response integral at rest. R1 response occurrence was not altered by task from rest while fewer R2 responses occurred only during effort closure and humming compared to rest. Because the R1 response integrals did not change from rest, task increases in motor neuron firing did not alter the LAR. These findings demonstrate that laryngeal motor neuron responses to sensory inputs are not gated during volitional tasks confirming the robust life-sustaining protective mechanisms provided by this airway reflex.

The three-neuron corneal reflex circuit and modulation of second-order corneal responsive neurons.

Neurons located in the border region between the interpolaris and caudalis subdivisions of the spinal trigeminal nucleus (Vi/Vc) are second order neurons of the corneal reflex, receiving corneal afferents and projecting to the lid closing, orbicularis oculi (OO) motoneurons. Recordings of Vi/Vc neurons identified by antidromic activation from stimulation of the facial nucleus and non-identified Vi/Vc neurons reveal two neuron types, phasic and tonic. Corneal stimulation elicits Adelta latency action potentials that occur early enough to initiate OO contraction and C-fiber latency action potentials that can modulate the end of the blink in phasic Vi/Vc neurons. Tonic Vi/Vc neurons exhibit a constant irregular, low frequency discharge as well as the cornea-evoked activity exhibited by phasic neurons. For both phasic and tonic neurons, blink amplitude increases with the total number of spikes evoked by the corneal stimulus. Peak firing frequency predicts peak orbicularis oculi EMG activity. Paradigms that suppress cornea-evoked blinks differentially affect Vi/Vc neurons. Microstimulation of the border region between the spinal trigeminal caudalis subdivision and the C1 spinal cord (Vc/C1) significantly reduces the number of spikes evoked by corneal stimulation and suppresses blink amplitude. In the paired stimulus paradigm, a blink evoked by a corneal stimulus 150 ms after an identical corneal stimulus is significantly smaller than the blink elicited by the first stimulus. Vi/Vc neuron discharge, however, is slightly larger for the second blink. Our data indicate that second-order Vi/Vc neurons do not determine the specific pattern of OO muscle activity; rather Vi/Vc neurons initiate OO motoneuron discharge and program the activity of another circuit that generates the late phase of the blink. The Vc/C1 suppression of Vi/Vc neurons suggests that the Vc/C1 region provides an "internal model" of the intended blink.

Modification of cornea-evoked reflex blinks in rats.

Although maintaining the tear film on the cornea is the most important role of blinking, information about the organization and modification of cornea-evoked blinks is sparse. This study characterizes cornea-evoked blinks and their modification in urethane-anesthetized rats. Cornea-evoked blinks typically begin 16.2 ms after an electrical stimulus to the cornea and last an average of 50.2 ms. In anesthetized rats, the blink only occurs ipsilateral to the stimulus. In response to cornea stimulation, the orbicularis oculi EMG activity typically exhibits two bursts that correlate with the arrival of A delta and C-fiber inputs to the spinal trigeminal complex. In the paired-stimulus paradigm, suppression of the blink evoked by the second cornea stimulus occurs for interstimulus intervals less than 300 ms and is exclusively unilateral. Stimulation of the contralateral cornea does not affect subsequent blinks evoked from stimulation of the ipsilateral cornea. To determine whether activation of cornea-related neurons in the border region between the spinal trigeminal caudalis subdivision and the C1 spinal cord (Vc/C1) inhibits the second blink in the paired-stimulus paradigm, we examine the suppression of cornea-evoked blinks caused by microstimulation in this region. This suppression of orbicularis oculi EMG activity begins 8.3 ms after Vc/C1 stimulation. Activation of this region, however, is unlike suppression in the paired-stimulus paradigm because Vc/C1 activation bilaterally inhibits cornea-evoked blinks. Thus, activation of Vc/C1 is a previously unidentified mechanism for modulating cornea-evoked blinks.