Modulation of the vestibulo-ocular reflex by serotonin in the rat

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

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.

5-HT1A receptor-mediated attenuation of synaptic transmission in rat medial vestibular nucleus impacts on vestibular-related motor function

KEY POINTS

Chemogenetic activation of medial vestibular nucleus-projecting 5-HT neurons resulted in deficits in vestibular-mediated tasks, including negative geotaxis, balance beam and rota-rod tests. The 5-HT_1A receptor mediates the vestibular-related behavioural effects of 5-HT in the vestibular nucleus. 5-HT_1A receptor activation attenuated evoked excitatory postsynaptic currents and evoked inhibitory postsynaptic currents via a presynaptic mechanism in the vestibular nucleus.

ABSTRACT

While the anxiolytic effects of serotonergic neuromodulation are well studied, its role in sensorimotor coordination and postural control is unclear. In this study, we show that an increase of serotonin (5-hydroxytryptamine, 5-HT) at the medial vestibular nucleus (MVN), a brainstem centre for vestibulospinal coordination, by either direct cannula administration or chemogenetic stimulation of MVN-projecting serotonergic neurons, adversely affected performance of rats in vestibular-mediated tasks, including negative geotaxis, balance beam and rota-rod tests. Application of the 5-HT₁ and 5-HT₇ receptor co-agonist 8-hydroxy-2-(di-n-propylamino) tetralin recapitulated the effect of 5-HT, while co-administration of the specific 5-HT_1A receptor antagonist WAY 100135 effectively abolished all 5-HT-induced behavioural deficits. This indicated that 5-HT_1A receptors mediated the effects of 5-HT in the rat MVN. Using whole-cell patch-clamp recording, we demonstrated that 5-HT_1A receptor activation attenuated both evoked excitatory and evoked inhibitory postsynaptic currents through a presynaptic mechanism in the rat MVN. The results thus highlight the 5-HT_1A receptor as the gain controller of vestibular-related brainstem circuits for posture and balance.

Organization of projections from the raphe nuclei to the vestibular nuclei in rats

Previous anatomic and electrophysiological evidence suggests that serotonin modulates processing in the vestibular nuclei. This study examined the organization of projections from serotonergic raphe nuclei to the vestibular nuclei in rats. The distribution of serotonergic axons in the vestibular nuclei was visualized immunohistochemically in rat brain slices using antisera directed against the serotonin transporter. The density of serotonin transporter-immunopositive fibers is greatest in the superior vestibular nucleus and the medial vestibular nucleus, especially along the border of the fourth ventricle; it declines in more lateral and caudal regions of the vestibular nuclear complex. After unilateral iontophoretic injections of Fluoro-Gold into the vestibular nuclei, retrogradely labeled neurons were found in the dorsal raphe nucleus (including the dorsomedial, ventromedial and lateral subdivisions) and nucleus raphe obscurus, and to a minor extent in nucleus raphe pallidus and nucleus raphe magnus. The combination of retrograde tracing with serotonin immunohistofluorescence in additional experiments revealed that the vestibular nuclei receive both serotonergic and non-serotonergic projections from raphe nuclei. Tracer injections in densely innervated regions (especially the medial and superior vestibular nuclei) were associated with the largest numbers of Fluoro-Gold-labeled cells. Differences were observed in the termination patterns of projections from the individual raphe nuclei. Thus, the dorsal raphe nucleus sends projections that terminate predominantly in the rostral and medial aspects of the vestibular nuclear complex, while nucleus raphe obscurus projects relatively uniformly throughout the vestibular nuclei. Based on the topographical organization of raphe input to the vestibular nuclei, it appears that dense projections from raphe nuclei are colocalized with terminal fields of flocculo-nodular lobe and uvula Purkinje cells. It is hypothesized that raphe-vestibular connections are organized to selectively modulate processing in regions of the vestibular nuclear complex that receive input from specific cerebellar zones. This represents a potential mechanism whereby motor activity and behavioral arousal could influence the activity of cerebellovestibular circuits.

Monoamines (norepinephrine, dopamine, serotonin) in the rat medial vestibular nucleus: endogenous levels and turnover

Monoamine (norepinephrine, dopamine, serotonin) and metabolite endogenous levels were determined in the rat medial vestibular nucleus (MVN) using HPLC with electrochemical detection. As a comparison, the locus coeruleus (LC) and dorsal raphe nucleus (RD) which contain the cell bodies of MVN noradrenergic and serotoninergic neurons respectively were also analyzed. Norepinephrine (NE) and serotonin (5-HT) basal levels of MVN were high (33.8 and 39.2pmol/mg protein respectively) but lesser than in LC or RD. Great amounts of MHPG and 5-HIAA were also present in the MVN. The turnover of NE assessed both from the ratio MHPG/NE and by the decrease in the NE content after treatment with alpha-methylparatyrosine was faster in the MVN (half-life: 1.5h) than in LC (half-life: 3.6h). On the other hand, the ratio 5-HIAA/5-HT was lower in the MVN (0.58) than in the RD (0.85) indicating a smaller 5-HT turnover in the MVN. In addition, like LC and RD, the MVN contained meaningful amounts of dopamine (DA) and DOPAC. The high ratio DA/NE (0.27) suggests the presence of non precursor specific dopaminergic pools. However, individualized dopaminergic neurons have not yet been demonstrated. The data are discussed in line with the possible neurotransmitter function of monoamines in the MVN.

Chronic recording of the vestibulo-ocular reflex in the restrained rat using a permanently implanted scleral search coil

A technique is described which allows accurate long-term monitoring of eye movements in the rat using permanently implanted scleral search coils. Search coils permanently sutured around the sclera yield vestibulo-ocular reflex (VOR) gain and phase values which are comparable to those reported previously in the literature using acutely implanted coils or electrooculographic electrodes. Considerations related to strain, sex and surgical procedures which permit measurement of responses in the chronically restrained rat are described. VOR gain and phase show a time course to their recovery following the implant surgery, with asymptotic performance typically attained approximately 10 days post-surgically. This technique, with the ability to monitor eye movements over weeks to months, appears ideal for development of rodent models of reflex adaptation which require observation of reflex behavior over extended periods of time. Development of a chronic procedure for monitoring eye movement in rodents is especially important given their initial response to restraint (extensive struggling). Finally, adaptation of this technique to smaller species (e.g., mouse) appears technically feasible which should permit the application of transgenic and knockout techniques to the determination of various vestibular reflex functions requiring long-term monitoring.

Aging effects on monoamines in rat medial vestibular and cochlear nuclei

Noradrenaline (NA), dopamine (DA); serotonin (5-HT) and their metabolites-3-methoxy, 4-hydroxyphenylglycol (MHPG), 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA)-were determined using HPLC in medial vestibular nucleus (MVN), anteroventral cochlear nucleus (AVCN), dorsal+posteroventral cochlear nucleus (DCN+PVCN), locus coeruleus (LC) and raphe dorsalis of Dark Agouti-Hanovre (DA-HAN) rats aged 4, 21 and 24 months. In older rats, the main noradrenergic changes were a decrease of NA content with an increase of the MHPG/NA ratio in MVN and a selective NA increase in AVCN. 5-HT and 5-HIAA levels were increased in all the brainstem nuclei except raphe dorsalis. DA and DOPAC remained unchanged. These data show that noradrenergic neurons in sensory nuclei are differently affected by aging whereas serotonergic activation occurs in most of them possibly as a compensatory response to dysfunction of sensory input and processing. The increase of NA stores in the AVCN of aged rats is in line with the elevated auditory brainstem threshold reported in old rats and could improve the signal-to-noise ratio. Noradrenergic neurons in the MVN seem to be more sensitive to age effect than cochlear nuclei; however, even if neuronal loss occurs, NA synthesis and/or metabolism increase to ensure normal or increased noradrenergic activity.

Excitation and inhibition of rat medial vestibular nucleus neurones by 5-hydroxytryptamine

The effects of 5-hydroxytryptamine (5-HT) and related compounds on the discharge rate of tonically active medial vestibular nucleus (MVN) neurones were studied in an in vitro slice preparation of the dorsal brainstem of the rat. The majority (87 of 107, 82%) of MVN neurones were excited by 5-HT. Nine cells (8%) showed a biphasic response to 5-HT, which consisted of a brief inhibition followed by excitation. Eleven cells (10%) were inhibited by 5-HT. The excitatory effects of 5-HT were mimicked by alpha-methyl-5-HT and antagonised by ketanserin and ritanserin, indicating the involvement of the 5-HT2 subtype of 5-HT receptor. In biphasic cells, blockade of 5-HT2 receptors by ketanserin reduced the excitatory component of the response and revealed an enhanced initial inhibition. The inhibitory effects in biphasic cells, and in cells that showed a pure inhibition in response to 5-HT, were blocked by pindobind-5-HT and mimicked by 8-hydroxy-2-(di-n-propylamino)-tetralin indicating the involvement of 5-HT1A receptors. The significance of these findings in relation to the effects of 5-HT on vestibular reflex function is discussed.

Changes in motor performance and rubral single unit activity in cats after microinjections of serotonin into the red nucleus area

The serotonergic control exerted on the red nucleus (RN) was studied in unrestrained cats during the performance of a simple reaction time task which consisted of releasing a lever in response to an auditory go-signal. The effects of microinjections of serotonin-oxalate salt into the rubral area on the motor activity and on the firing of neurons recorded concomitantly in the red nucleus were investigated. Injections of serotonin (5-HT) (200-400 ng) into the red nucleus or its dorsal border induced subtle alterations in the conditioned motor performances but had no major effects on the spontaneous motor behavior. The changes in the conditioned motor output (an increase in the static force exerted on the lever and a speeding up of the lever release) are reminiscent of the facilitatory influence of serotonin on various motor reflexes previously reported. Changes in the neuronal activity were observed concomitantly with the effects on the motor output: 5-HT either enhanced or reduced the firing rate of the rubral neurons. These effects were apparently dependent on the discharge pattern of the neurons during the static motor activity. The results suggest that the serotonergic input to the red nucleus may participate in motor control by exerting a dual modulatory action on the activity of rubral neurons.