Single neuron activity and c-Fos expression in the rat striatum following electrical stimulation of the peripheral vestibular system

Abstract

Connections between the vestibular system and the basal ganglia have been postulated since the early 20th century. However, the results of electrophysiological studies investigating neuronal responses to electrical stimulation of the vestibular system have been inconsistent. The aim of this study was to investigate the effects of electrical stimulation of the vestibular labyrinth on single neuron activity and c-Fos expression in the rat striatum. We used electrical stimulation of the vestibular labyrinth (various intensities delivered to the round window) to examine the electrophysiological response of striatal neurons and c-Fos expression. From 507 single neurons recorded (n = 20 rats), no vestibular-responsive neuron was found at 1× and 2× the nystagmus threshold; however, 6 neurons were found at 3× the threshold. These neurons were found bilaterally, with a response latency of ~50 msec from the end of the stimulus. For the c-Fos study, the number of neurons expressing c-Fos was quantified using stereological methods. Stimulation at 2× the threshold for nystagmus (n = 5 rats) resulted in a significant decrease in the number of neurons expressing c-Fos in the bilateral striatum compared to both the sham control group (n = 5) and the lower stimulus intensity group (n = 5) (P ≤ 0.0001 for both). The results of this study demonstrate that: (1) some single striatal neurons respond to electrical vestibular stimulation, however, these responses are circumscribed and infrequent; (2) electrical stimulation of the vestibular labyrinth results in a decrease in the number of striatal neurons expressing c-Fos, in a current-dependent manner.

Keywords: c-Fos; single neurons; striatum; vestibular.

Figure 1

Possible neuronal pathways connecting the vestibular nucleus complex to the striatum. PFN, parafascicular nucleus; PPT, pedunculopontine tegmental nucleus; SNc, Substantia nigra pars compacta; VNC, vestibular nucleus complex.

Figure 2

(A) Example of an eye movement trace of the ipsilateral eye in response to vestibular stimulation in a single rat. The animal received five 1 s stimulations (100 Hz, 400 μA) with 0.5 s in between. Eye movement was measured as the change in the angle of rotation from the “zero” around the pupil of the eye. (B) Threshold for nystagmus production in urethane‐anesthetized rats undergoing electrical vestibular stimulation at 100 Hz. Data are shown as number of animals with thresholds as specific amplitudes (μA) (n = 22).

Figure 3

Examples of the firing patterns of the 6 single striatal neurons responding to electrical stimulation of the vestibular labyrinth in a phase‐locked manner, with examples of their action potential waveforms (averages of 200 action potentials; mean ± SD in red).

Figure 4

Peri‐stimulus histograms of neuronal responses to electrical vestibular stimulation. (A) Combined histogram of firing of all nonresponsive neurons at 1× (top) and 2× (bottom) the threshold of nystagmus. (B) Combined firing of all 6 responsive neurons, at 3× the threshold of nystagmus, phase‐locked to the stimulus. Red bar represents the stimulation period. Spikes from the stimulus artifact have been removed for clarity. Data are presented as mean (black bars) and standard deviation (gray bars).

Figure 5

Histogram showing the results of 10,000 Bayesian MCMC simulations to estimate the frequency of vestibular‐responsive neurons in the striatum. The Bayesian credible interval was between 0.4% and 2.0%.

Figure 6

Schematic diagram showing the recording sites for the striatal neurons analyzed in the current study and an example of a cresyl violet‐stained section showing a typical electrode track. Scale bar = 20 μm.

Figure 7

Example of c‐Fos labeling in the striatum. In order to be counted, cells had to have clear nuclear labeling. Scale bar = 20 μm.

Figure 8

Estimated number of c‐Fos‐positive cells in the striatum following vestibular stimulation. ***P ≤ 0.0001 for the comparison of the higher current with both the sham groups and the lower current group P ≤ 0.0001) (see Figs 4 and 5).