Restoration of the nasopharyngeal response after bilateral sectioning of the anterior ethmoidal nerve in the rat

Abstract

In response to stimulation of the nasal passages with volatile ammonia vapors, the nasopharyngeal reflex produces parasympathetically mediated bradycardia, sympathetically mediated increased peripheral vascular tone, and apnea. The anterior ethmoidal nerve (AEN), which innervates the anterior nasal mucosa, is thought to be primarily responsible for providing the sensory afferent signals that initiate these protective reflexes, as bilateral sectioning causes an attenuation of this response. However, recent evidence has shown cardiovascular responses to nasal stimulation with ammonia vapors are fully intact 9 days after bilateral AEN sectioning, and are similar to control animals without bilaterally sectioned AENs. To investigate this restoration of the nasopharyngeal response, we recorded the cardiorespiratory responses to nasal stimulation with ammonia vapors immediately after, and 3 and 9 days after, bilateral AEN sectioning. We also processed brainstem tissue for Fos to determine how the restoration of the nasopharyngeal response would affect the activity of neurons in the medullary dorsal horn (MDH), the part of the ventral spinal trigeminal nucleus caudalis region that receives primary afferent signals from the nose and nasal passages. We found 3 days after bilateral AEN sectioning the cardiorespiratory responses to nasal stimulation are partially restored. The bradycardic response to nasal stimulation is significantly more intense 3 days after AEN sectioning compared to Acute AEN sectioning. Surprisingly, 3 days after AEN sectioning the number of Fos-positive neurons within MDH decreased, even though the cardiorespiratory responses to nasal stimulation intensified. Collectively these findings indicate that, besides the AEN, there are alternate sensory pathways that can activate neurons within the trigeminal nucleus in response to nasal stimulation. The findings further suggest trigeminal neuronal plasticity involving these alternate sensory pathways occurs in as few as 3 days after bilateral AEN sectioning. Finally, activation of even a significantly reduced number of MDH neurons is sufficient to initiate the nasopharyngeal response.

Keywords: Anterior ethmoidal nerve; Fos; medullary dorsal horn; nasal stimulation; nasopharyngeal response.

Figure 1

Cardiorespiratory responses to repetitive stimulation of the nasal passages with ammonia vapors in rats with AENs intact (A, Sham) and with AENs cut bilaterally (B, Acute; C, 3 Day; D, 9 Day). Sham received nasal stimulation immediately after sham AEN surgery. Acute received nasal stimulation immediately after bilateral AEN sectioning. Other rats experienced a 3 or 9 day delay between bilateral AEN sectioning and nasal stimulation. In each panel, from top: heart rate, pulsatile arterial blood pressure, and respiration (Resp; up is inspiration (Insp) and down is expiration (Exp). Solid bar underneath each tracing indicates period of nasal stimulation.

Figure 2

Mean Heart Rate (HR), arterial Blood Pressure (MAP), and respiratory rate responses (±SE) during nasal stimulation with ammonia vapors in rats with AENs intact (Sham) and with AENs cut bilaterally (Acute, 3 Day and 9 Day). Solid bar: pretrial; Open bar: trial; Cross‐hatched bar: Lowest heart rate during the trial. Nasal stimulation causes (A) a significant decrease in heart rate; (B) a significant increase in mean arterial blood pressure; and (C) a significant decrease in respiratory rate. Using two‐way ANOVAs: 1 = HR responses to nasal stimulation significantly different in Acute compared with Sham, 3 and 9 day rats; 2 = trial respiratory rate was significantly greater in Acute than in Sham. N = 5 for each of the four groups.

Figure 3

Brightfield photomicrographs of Fos‐positive neurons within the ventral trigeminal region at the level of calamus scriptorius after repetitive nasal stimulation with ammonia vapors in rats with AENs intact (A, Sham) and with AENs cut bilaterally (B, Acute; C, 3 Day; and D, 9 Day). Fos‐positive neurons were present in the ventral tip of the superficial laminae of the medullary dorsal horn (MDH), as well as in the ventral paratrigeminal nuclei (Para) located within the spinal trigeminal tract (sp5). Scale bar in A is 100 μm for panels A‐D. Insets are examples of Fos‐positive neurons at 100x magnification of indicated rectangular region. All four panels correspond to Figure 77 from Paxinos and Watson (1998).

Figure 4

Total Fos‐positive neuron counts (mean ± SE) within the ventral (A) paratrigeminal nucleus and (B) MDH after repetitive nasal stimulation with ammonia vapors. Sham received nasal stimulation immediately after sham AEN sectioning. Acute received nasal stimulation immediately after bilateral AEN sectioning. Other rats experienced a 3 or 9 day delay between bilateral AEN sectioning and nasal stimulation. Using one‐way ANOVA: 1 = significantly different from Sham. Using t‐tests: a = significantly different from Sham; b = significantly different from Acute. N = 5 for each of the four groups.

Figure 5

Counts of Fos‐positive neurons (mean ± SE) within the ventral (A) paratrigeminal nucleus and (B) medullary dorsal horn at each of six caudal‐rostral levels. Filled square (▴; N = 5): Sham; Open circle (○; N = 5): Acute; Filled down triangle (▾; N = 5): 3 Day; and Open up triangle (▵; N = 5): 9 Day. Using two‐way ANOVA: 1 = Sham is significantly different from 3 Day. Using one‐way ANOVAs at individual caudal‐rostral levels: 2 = significantly different from other three groups; 3 = significantly different from 3 Day.