C-Fos immunoreactivity mapping of the auditory system after electrical stimulation of the cochlear nerve in rats

C-Fos mapping and EEG characteristics of multiple mice brain regions in pentylenetetrazol-induced seizure mice model

Purpose: To confirm different local brain activities characterized in pentylenetetrazol (PTZ)-induced seizure model. Methods: we induced seizure response by a single dose of PTZ injection (45 mg/kg, i.p.). Local activity was recorded in different brain regions by EEG in time and c-Fos staining at different time points (0.5 h, 1 h, 2 h, 4 h) after PTZ treatment. Results: EEG recordings showed distinctive features of activation in different brain areas. With the aggravation of behavioral manifestations of seizures, the frequency and amplitude of the discharges on EEG were increasing gradually. The epileptic response on EEG immediately ended after reaching the maximum stage of seizures, followed by a short period of suppression. The labeling of c-Fos was enhanced in the medial prefrontal cortex, the piriform cortex, the amygdala, hippocampal CA1, CA3 and dentate gyrus, but inapparent in the striatum. The most potent changes in c-Fos were observed in cortex, amygdala nuclei, and dentate gyrus. EEG and c-Fos immunolabeling in neuronal activation showed discrepancies in the striatum. For each brain region, the maximum c-Fos labeling was observed at 2 h after injection and diminished at 4 h. The level of c-Fos immunoreactivity was even lower than the control group, which was accompanied by increased labeling of parvalbumin neurons (PVNs). Conclusions: These findings validated PTZ-induced seizure as a seizure model with a specific spatial-temporal profile. Neuronal activity was enhanced and then subsequently inhibited during seizure evolution. Abbreviations: AEDs: anti-epileptic drugs; AF: Alexa Fluor; CA1: Cornu Ammonis area 1; CA3: Cornu Ammonis area 3; DAB, 3: 3P-diaminobenzidine; DAPI: 4',6-diamidino-2-phenylindole; DG: dentate gyrus; EEG: electroencephalogram; GABA: gamma-aminobutyric acid; IEG: immediate early gene; mPFC: medial prefrontal cortex; NAc: nucleus accumbens; PB: phosphate buffer; PBS: phosphate buffered saline; PBST: phosphate buffered saline with Tween; PFA, paraformaldehyde; PTZ: pentylenetetrazol; PVN: parvalbumin neuron; ROI: regions of interest; SE: status epilepticus.

Bilateral Changes of Spontaneous Activity Within the Central Auditory Pathway Upon Chronic Unilateral Intracochlear Electrical Stimulation

OBJECTIVES

In recent years, cochlear implants have been applied successfully for the treatment of unilateral hearing loss with quite surprising benefit. One reason for this successful treatment, including the relief from tinnitus, could be the normalization of spontaneous activity in the central auditory pathway because of the electrical stimulation. The present study, therefore, investigated at a cellular level, the effect of a unilateral chronic intracochlear stimulation on key structures of the central auditory pathway.

DESIGN

Normal-hearing guinea pigs were mechanically single-sided deafened through a standard HiFocus1j electrode array (on a HiRes 90k cochlear implant) being inserted into the first turn of the cochlea. Four to five electrode contacts could be used for the stimulation. Six weeks after surgery, the speech processor (Auria) was fitted, based on tNRI values and mounted on the animal's back. The two experimental groups were stimulated 16 hours per day for 90 days, using a HiRes strategy based on different stimulation rates (low rate (275 pps/ch), high rate (5000 pps/ch)). The results were compared with those of unilateral deafened controls (implanted but not stimulated), as well as between the treatment groups. All animals experienced a standardized free field auditory environment.

RESULTS

The low-rate group showed a significantly lower average spontaneous activity bilaterally in the dorsal cochlear nucleus and the medial geniculate body than the controls. However, there was no difference in the inferior colliculus and the primary auditory cortex. Spontaneous activity of the high-rate group was also reduced bilaterally in the dorsal cochlear nucleus and in the primary auditory cortex. No differences could be observed between the high-rate group and the controls in the contra-lateral inferior colliculus and medial geniculate body. The high-rate group showed bilaterally a higher activity in the CN and the MGB compared with the low-rate group, whereas in the IC and in the AC a trend for an opposite effect could be determined.

CONCLUSIONS

Unilateral intracochlear electrical stimulation seems to facilitate the homeostasis of the network activity, since it decreases the spontaneous activity that is usually elevated upon deafferentiation. The electrical stimulation per se seems to be responsible for the bilateral changes described above, rather than the particular nature of the electrical stimulation (e.g., rate). The normalization effects of electrical stimulation found in the present study are of particular importance in cochlear implant recipients with single-sided deafness.

Neuronal expression of c-Fos after epicortical and intracortical electric stimulation of the primary visual cortex

Electrical stimulation of the primary visual cortex (V1) is an experimental approach for visual prostheses. We here compared the response to intracortical and epicortical stimulation of the primary visual cortex by using c-Fos immunoreactivity as a marker for neuronal activation. The primary visual cortex of male Sprague Dawley rats was unilaterally stimulated for four hours using bipolar electrodes placed either intracortically in layer IV (n=26) or epicortically (n=20). Four different current intensities with a constant pulse width of 200μs and a constant frequency of 10Hz were used, for intracortical stimulation with an intensity of 0μA (sham-stimulation), 10μA, 20μA and 40μA, and for epicortical stimulation 0μA, 400μA, 600μA and 800μA. Subsequently all animals underwent c-Fos immunostaining and c-Fos expression was assessed in layer I-VI of the primary visual cortex within 200μm and 400μm distance to the stimulation site. C-Fos expression was higher after intracortical stimulation compared to epicortical stimulation, even though ten times lower current intensities were applied. Furthermore intracortical stimulation resulted in more focal neuronal activation than epicortical stimulation. C-Fos expression was highest after intracortical stimulation with 20μA compared to all other intensities. Epicortical stimulation showed a linear increase of c-Fos expression with the highest expression at 800μA. Sham stimulation showed similar expression of c-Fos in both hemispheres. The contralateral hemisphere was not affected by intracortical or epicortical stimulation of either intensities. In summary, intracortical stimulation resulted in more focal neuronal activation with less current than epicortical stimulation. This model may be used as a simple but reliable model to evaluate electrodes for microstimulation of the primary visual cortex before testing in more complex settings.

Transient Down-Regulation of Sound-Induced c-Fos Protein Expression in the Inferior Colliculus after Ablation of the Auditory Cortex

We tested whether lesions of the excitatory glutamatergic projection from the auditory cortex (AC) to the inferior colliculus (IC) induce plastic changes in neurons of this nucleus. Changes in neuronal activation in the IC deprived unilaterally of the cortico-collicular projection were assessed by quantitative c-Fos immunocytochemistry. Densitometry and stereology measures of sound-induced c-Fos immunoreactivity in the IC showed diminished labeling at 1, 15, 90, and 180 days after lesions to the AC suggesting protein down-regulation, at least up to 15 days post-lesion. Between 15 and 90 days after the lesion, c-Fos labeling recovers, approaching control values at 180 days. Thus, glutamatergic excitation from the cortex maintains sound-induced activity in neurons of the IC. Subdivisions of this nucleus receiving a higher density of cortical innervation such as the dorsal cortex showed greater changes in c-Fos immunoreactivity, suggesting that the anatomical strength of the projection correlates with effect strength. Therefore, after damage of the corticofugal projection, neurons of the IC down-regulate and further recover sound-induced c-Fos protein expression. This may be part of cellular mechanisms aimed at balancing or adapting neuronal responses to altered synaptic inputs.

Granule cells in the cochlear nucleus sensitive to sound activation detected by Fos protein expression

Granule cells are the smallest neuronal type in the cochlear nucleus (CN). Due to their small size, it is extremely difficult to record their sound-evoked activity with microelectrodes. Compared with large, non-granule cells, much less is known about their response properties to sound stimulation. Here, we use Fos, the nuclear regulatory protein, as a neuronal activity marker to determine the responsiveness of granule cells to sound in comparison to the larger neurons. The present study determined the threshold sensitivity and activation pattern of neurons in the three subdivisions of the CN with free-field sound stimulation in monaural, awake rats. Immunocytochemical localization of Fos was used as our metric for "sound activation." Neuronal types upregulating Fos expression in response to sound stimulation were further identified with Nissl counterstaining. Our results show that most CN cell types can upregulate Fos expression when sound activated and the number of Fos-expressing neurons is directly related to sound intensity. The threshold for Fos activation in granule cells is lower than that for non-granule cells. The number of Fos activated granule cells saturates at high sound intensity, while the number of Fos activated non-granule cells is a monotonic function. By comparing the patterns of sound-induced Fos expression in different CN cell types, it may be possible to predict features of sound-evoked activity in granule cells.

Quantitative changes in calretinin immunostaining in the cochlear nuclei after unilateral cochlear removal in young ferrets

Neurons of the cochlear nuclei receive axosomatic endings from primary afferent fibers from the cochlea and have projections that diverge to form parallel ascending auditory pathways. These cells are characterized by neurochemical phenotypes such as levels of calretinin. To test whether or not early deafferentation results in changes in calretinin immunostaining in the cochlear nucleus, unilateral cochlear ablations were performed in ferrets soon after hearing onset (postnatal day [P]30-P40). Two months later, changes in calretinin immunostaining as well as cell size, volume, and synaptophysin immunostaining were assessed in the anteroventral (AVCN), posteroventral (PVCN), and dorsal cochlear nucleus (DCN). A decrease in calretinin immunostaining was evident ipsilaterally within the AVCN and PVCN but not in the DCN. Further analysis revealed a decrease both in the calretinin-immunostained neuropil and in the calretinin-immunostained area within AVCN and PVCN neurons. These declines were accompanied by significant ipsilateral decreases in volume as well as neuron area in the AVCN and PVCN compared with the contralateral cochlear nucleus and unoperated animals, but not compared with the DCN. In addition, there was a significant contralateral increase in calretinin-immunostained area within AVCN and PVCN neurons compared with control animals. Finally, a decrease in area of synaptophysin immunostaining in both the ipsilateral AVCN and PVCN without changes in the number of boutons was found. The present data demonstrate that unilateral cochlear ablation leads to 1) decreased immunostaining of the neuropil in the AVCN and PVCN ipsilaterally, 2) decreased calretinin immunostaining within AVCN and PVCN neurons ipsilaterally, 3) synaptogenesis in the AVCN and PVCN ipsilaterally, and 4) increased calretinin immunostaining within AVCN and PVCN neurons contralaterally.

Electrical stimulation of the cochlear nerve in rats: analysis of c-Fos expression in auditory brainstem nuclei

We investigated functional activation of central auditory brainstem nuclei in response to direct electrical stimulation of the cochlear nerve using c-Fos immunoreactivity as a marker for functional mapping. The cochlear nerve was stimulated in the cerebellopontine angle of Lewis rats applying biphasic electrical pulses (120-250 muA, 5 Hz) for 30 min. In a control group, bilateral cochlectomy was performed in order to assess the basal expression of c-Fos in the auditory brainstem nuclei. The completeness of cochlear ablations and the response of auditory brainstem nuclei to electrical stimulation were electrophysiologically verified. C-Fos immunohistochemistry was performed using the free floating method. In anaesthetized animals with unilateral electrical stimulation of the cochlear nerve, increased expression of c-Fos was detected in the ipsilateral ventral cochlear nucleus (VCN), in the dorsal cochlear nucleus bilaterally (DCN), in the ipsilateral lateral superior olive (LSO) and in the contralateral inferior colliculus (IC). A bilateral slight increase of c-Fos expression in all subdivisions of the lateral lemniscus (LL) did not reach statistical significance. Contralateral inhibition of the nuclei of the trapezoid body (TB) was observed. Our data show that unilateral electrical stimulation of the cochlear nerve leads to increased expression of c-Fos in most auditory brainstem nuclei, similar to monaural auditory stimulation. They also confirm previous studies suggesting inhibitory connections between the cochlear nuclei. C-Fos immunoreactivity mapping is an efficient tool to detect functional changes following direct electrical stimulation of the cochlear nerve on the cellular level. This could be particularly helpful in studies of differential activation of the central auditory system by experimental cochlear and brainstem implants.