The Effects of Unilateral Labyrinthectomy on Monoamine Neurotransmitters in the Medial Vestibular Nucleus of Rats

Abstract

Background: This study aimed to investigate the effects of unilateral labyrinthectomy (UL) on monoamine neurotransmitters in the medial vestibular nucleus (MVN) of rats.

Methods: Adult Sprague-Dawley rats were utilized for the vestibular impaired animal model through UL. The success of the model establishment and the recovery process were evaluated using vestibular behavioral tests, including spontaneous nystagmus, postural asymmetry, and balance beam test. Additionally, the expression levels of c-Fos protein in the MVN were assessed by immunofluorescence. Furthermore, changes in the expression levels of monoamine neurotransmitters, including 5-hydroxytryptamine (5-HT), norepinephrine (NE), dopamine (DA), and histamine in the MVN, were analyzed using high-performance liquid chromatography (HPLC) at different time points after UL (4 h, 8 h, 1 day, 2 days, 4 days, and 7 days).

Results: Compared to the sham control group, the UL group exhibited the most pronounced vestibular impairment symptoms at 4 h post-UL, which significantly decreased at 4 days and almost fully recovered by 7 days. Immunofluorescence results showed a notable upregulation of c-Fos expression in the MVN subsequent to the UL-4 h, serving as a reliable indicator of heightened neuronal activity. In comparison with the sham group, HPLC analysis showed that the levels of 5-HT and NE in the ipsilesional MVN of the UL group were significantly elevated within 4 days after UL, and peaked on 1 day and 2 days, respectively. DA showed an increasing trend at different time points up to 7 days post-UL, while histamine levels significantly increased only at 1 day post-UL.

Conclusions: UL-induced dynamic changes in monoamine neurotransmitters during the early compensation period in the rat MVN may be associated with the regulation of the central vestibular compensation mechanism by the MVN.

Keywords: high-performance liquid chromatography; medial vestibular nucleus; monoamine neurotransmitters; unilateral labyrinthectomy; vestibular compensation.

Figure 1

Establishment of the unilateral acute vestibular dysfunction model in rats. (A) Assessment of vestibular behavioral symptoms in rats at 4 h, 8 h, 1 day, 2 days, 4 days, and 7 days after UL, including spontaneous nystagmus, postural asymmetry, and balance beam test. MVNs were collected at different time points after UL for immunofluorescence and high-performance liquid chromatography analysis. (B) Illustration of UL in rats, exposing the bulla, indicated by the red line. (C,F) Postural asymmetry for evaluating postural and locomotor changes in rats, n = 6/group (F (3.06, 30.56) = 102.2, p < 0.0001). (E) Spontaneous nystagmus for evaluating eye movement function after model induction, n = 6/group (F (3.18, 31.76) = 72.17, p < 0.0001). (D,G) Balance beam test for evaluating the balance function in rats, n = 6/group (F (3.80, 38.02) = 38.08, p < 0.0001). Two-way analysis of variance (ANOVA) followed by Bonferroni’s post hoc test was used for comparisons between the UL and sham-operated groups. Data are presented as mean ± standard error of the mean (SEM). ** p < 0.01, *** p < 0.001, **** p < 0.0001, n.s. indicates no statistical difference. The black triangle in the figures (E–G) represent the behavioral score of each rat. UL: unilateral labyrinthectomy; MVN: medial vestibular nucleus.

Figure 2

Massive activation of MVN neurons at 4 h after UL. (A) In the sham-operated group, a small number of c-Fos-positive neurons were activated in the ipsilesional medial vestibular nucleus (Isp-MVN) compared to the contralesional MVN (Con-MVN), n = 3/group, two-tailed Student’s t-test, t = 2.68, p = 0.055. (B) In the UL group, a significant increase in c-Fos-positive neurons was observed in both sides of the MVN, with the ips-MVN showing the most significant activation at 4 h after UL, n = 3, two-tailed Student’s t-test, t = 4.13. Arrows in white indicate c-Fos-positive neurons. Data are presented as mean ± SEM. ** p < 0.01, n.s. indicates no statistical difference. UL: unilateral labyrinthectomy; MVN: medial vestibular nucleus; 4V: fourth ventricle. Scale bar, 100 μm.

Figure 3

HLPC analysis reveals elevated levels of monoamine neurotransmitters after UL. (A) Levels of 5-HT significantly increased at 4 h, 8 h, 1 day, 2 days, and 4 days after UL, peaking at 1 day postoperatively and gradually decreasing around 4 days after UL, returning to preoperative levels by 7 days, n = 3/group (F (6, 14) = 32.39, p < 0.0001). (B) NE levels significantly increased at 8 h, 1 day, and 2 days after UL, peaking at 2 days postoperatively, and gradually decreasing around 4 days after surgery, returning to preoperative levels by 7 days, n = 3/group (F (6, 14) = 18.74, p < 0.0001). (C) DA levels significantly increased at 8 h, 1 day, 2 days, 4 days, and 7 days after UL, peaking at 2 days postoperatively, and gradually decreasing around 7 days after surgery, n = 3/group (F (6, 14) = 15.97, p < 0.0001). (D) Histamine levels significantly increased at 1 day after UL and gradually decreased to preoperative levels around 2 days after surgery, n = 3/group (F (6, 14) = 3.87, p = 0.017). One-way analysis of variance (ANOVA) followed by Bonferroni’s post hoc test was used for comparisons between the UL and sham-operated groups. Data are presented as mean ± SEM. * p < 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.0001, n.s. indicates no statistical difference. HLPC: High-performance liquid chromatography; UL: unilateral labyrinthectomy; 5-HT: 5-hydroxytryptamine (serotonin); NE, norepinephrine; DA, dopamine.