Fos-like immunoreactivity in the auditory brainstem evoked by bipolar intracochlear electrical stimulation: effects of current level and pulse duration

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.

Current focusing and steering: modeling, physiology, and psychophysics

Current steering and current focusing are stimulation techniques designed to increase the number of distinct perceptual channels available to cochlear implant (CI) users by adjusting currents applied simultaneously to multiple CI electrodes. Previous studies exploring current steering and current focusing stimulation strategies are reviewed, including results of research using computational models, animal neurophysiology, and human psychophysics. Preliminary results of additional neurophysiological and human psychophysical studies are presented that demonstrate the success of current steering strategies in stimulating auditory nerve regions lying between physical CI electrodes, as well as current focusing strategies that excite regions narrower than those stimulated using monopolar configurations. These results are interpreted in the context of perception and speech reception by CI users. Disparities between results of physiological and psychophysical studies are discussed. The differences in stimulation used for physiological and psychophysical studies are hypothesized to contribute to these disparities. Finally, application of current steering and focusing strategies to other types of auditory prostheses is also discussed.

Selectivity of neural stimulation in the auditory system: a comparison of optic and electric stimuli

Pulsed, mid-infrared lasers were recently investigated as a method to stimulate neural activity. There are significant benefits of optically stimulating nerves over electrically stimulating, in particular the application of more spatially confined neural stimulation. We report results from experiments in which the gerbil auditory system was stimulated by optical radiation, acoustic tones, or electric current. Immunohistochemical staining for the protein c-FOS revealed the spread of excitation. We demonstrate a spatially selective activation of neurons using a laser; only neurons in the direct optical path are stimulated. This pattern of c-FOS labeling is in contrast to that after electrical stimulation. Electrical stimulation leads to a large, more spatially extended population of labeled, activated neurons. In the auditory system, optical stimulation of nerves could have a significant impact on the performance of cochlear implants, which can be limited by the electric current spread.

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.

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.

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

The aim of this study was to establish the use of c-Fos immunoreactivity as a marker for functional mapping in the auditory system in response to direct electrical stimulation of the cochlear nerve in the cerebellopontine angle. In rats the cochlear nerve was electrically stimulated with a biphasic current (120-250 microA, 5 Hz) for 30 min using a bipolar concentric Tungsten electrode. Bilateral cochlectomy was performed in a control group in order to investigate basal expression of c-Fos in the auditory brainstem nuclei. The response of auditory brainstem nuclei to electrical stimulation and the completeness of cochlear ablations were electrophysiologically verified. After the experiments, the animals were prepared for cryotomy and c-Fos immunohistochemistry. The results were morphologically analyzed and statistically compared among groups. In anesthetized animals with unilateral electrical stimulation of the cochlear nerve increased expression of c-Fos was detected in the ipsilateral ventral (VCN) and bilateral dorsal cochlear nucleus (DCN), whereas the VCN of the contralateral side revealed only few immunoreactive cells. In animals with bilateral cochlear ablation the number of c-Fos reactive cell nuclei representing basal expression was generally low in the VCN and DCN of both sides. Our data show that electrical stimulation of the cochlear nerve leads to increased expression of c-Fos in the cochlear nucleus. It also confirms bilateral connections between the cochlear nuclei. These experimental results suggest that c-Fos immunoreactivity mapping provides a powerful tool for functional investigations on the cellular level after direct electrical stimulation of the cochlear nerve. Future functional studies analyzing the effect of electrical stimulation of the central auditory system as performed by auditory brainstem implants could be investigated in detail by mapping c-Fos expression on cellular level.

Influence of auditory deprivation upon the tonopic organization in the inferior colliculus: a Fos immunocytochemical study in the rat

The frequency organization in the inferior colliculus of neonatally-deafened rats was investigated using electrical stimulation of the cochlea and immunoreactivity for Fos as a marker of neuronal activity. An electrode implanted either at the base or at the apex of the right cochlea delivered a unique 45-min stimulation at two different level intensities and at two time points, i.e. either at 4 weeks or at 4 months. In 4-week-old rats stimulated at 5x threshold, a site-for-site organization was observed since basal or apical stimulation induced a strong labelling in the ventro-medial or in the dorsolateral part of the left inferior colliculus, respectively. In 4-month-old rats, stimulation of the base induced an extremely weak Fos labelling without any specific location in the left inferior colliculus while stimulation of the apex induced a diffuse labelling with two discrete bands being distinguishable in the left inferior colliculus. In 4-week-old rats stimulated at 15x threshold, basal stimulation elicited a diffuse Fos-like immunoreactivity in the left inferior colliculus while apical stimulation yielded a response restricted to the dorsal part of the left inferior colliculus. In 4-month-old rats, no response was detected in the left inferior colliculus after stimulation of the basal part of the cochlea. Stimulation of the apex could still induce a labelling in the dorsolateral left inferior colliculus. Thus, the inferior colliculus exhibits an adult-like tonotopic organization early on independently of any acoustic stimulation. Prolonged absence of auditory input dramatically alters this organization in the inferior colliculus, especially for high frequencies. From a clinical standpoint, these results could argue for early implantation in deaf children.

Acoustic trauma induces reemergence of the growth- and plasticity-associated protein GAP-43 in the rat auditory brainstem

We explored the consequences of unilateral acoustic trauma to intracochlear and central nervous system structures in rats. An acoustic trauma, induced by applying click stimuli of 130 dB (sound pressure level; SPL) for 30 minutes, resulted in an instant and permanent threshold shift of 95.92 +/- 1.08 dB (SEM) in the affected ear. We observed, as a consequence, a structural deterioration of the organ of Corti. Deprivation-dependent changes of neurons of the auditory brainstem were determined using antibodies against neurofilament and the growth-associated protein GAP-43 and compared with those following cochleotomy, studied earlier. By 231 days posttrauma, spiral ganglion cell bodies and their processes were almost entirely lost from all cochlear regions with destroyed organ of Corti. In the lateral superior olive (LSO) ipsilateral to the trauma, cell bodies of lateral olivocochlear neurons turned transiently GAP-43 positive within the first 1.5 years posttrauma. The time course of emergence and disappearance of this population of neurons was similar to that found after cochleotomy. Additionally, after noise trauma, principal cells in contralateral LSO and in medial superior olive (MSO) on both sides of the brainstem developed an expression of GAP-43 that began 3 and 16 days posttrauma, respectively, and lasted for at least 1 year. Such cells were rarely observed after cochleotomy. An unequivocal rise in GAP-43 immunoreactivity was also found in the neuropil of the inferior colliculus and the ventral cochlear nucleus, both preferentially on the acoustically damaged side. We conclude that the degree and specific cause of sudden unilateral deafness entail specific patterns of plasticity responses in the auditory brainstem, possibly to prevent the neural network dedicated to locate sounds in the environment from delivering erroneous signals centralward.

Transcription factor modulation and expression in the rat auditory brainstem following electrical intracochlear stimulation

Neuronal activity in sensory organs elicited by adequate or electrical stimulation not only invokes fast electrical responses but may also trigger complex molecular changes inside central neurons. Following electrical intracochlear stimulation with a cochlear implant under urethane anesthesia, we observed changes in the phosphorylation state of the cAMP response element binding protein (CREB) and the expression of the immediate-early genes c-fos and egr-1, molecules known to act as transcription factors, in a tonotopically precise pattern in central auditory neurons. These neurons resided in the posteroventral and anteroventral cochlear nucleus, the dorsal cochlear nucleus, the lateral superior olive, the medial nucleus of the trapezoid body, the dorsal and ventral nucleus of the lateral lemniscus, and the central nucleus of the inferior colliculus. Moreover, effects of electrical stimulation were identified in the medial vestibular nucleus and the lateral parabrachial nucleus. Regionally, CREB was dephosphorylated wherever immediate-early gene expression went up. These massive stimulation-dependent modulations of transcription factors in the ascending auditory system are indicative of ongoing changes that modify the chemistry and structure of the affected cells and, consequently, their response characteristics to subsequent stimulation of the inner ear.

Vomeronasal organ: electrical stimulation activates Fos in mating pathways and in GnRH neurons

Electrical stimulation of the vomeronasal organ in male hamsters activated Fos expression in neurons of the chemosensory pathways, as in experiments where animals were stimulated with female chemical stimuli. Fos was also activated in gonadotropin hormone releasing hormone (GnRH, or LHRH) neurons in the rostral medial preoptic region of the brain, a possible substrate for GnRH influence on chemosensory-dependent reproductive behavior.

Inner ear lesion alters acoustically induced c-Fos expression in the rat auditory rhomboencephalic brainstem

The pattern of c-Fos expression was mapped in the adult rat's brain following unilateral cochlear lesions. In normal and cochlear lesioned rats, c-Fos expression was induced with sound stimuli. Acoustic stimulation consisted of pulses of four tones. An additional control group consisted of non-stimulated rats. In the cochlear nuclei (CN), c-Fos activation was scarce in isolated rats and increased strongly following sound stimulation. Following unilateral cochlear lesion, acoustically driven expression was decreased in all CN in both the lesioned and the untreated sides. The ventromedial periolivary nucleus and the rostral periolivary nucleus showed c-Fos activation in isolated conditions and were strongly activated following sound stimulation. The rest of the superior olivary complex showed no c-Fos activation in isolated rats and a weak activation following sound stimulation. Following unilateral cochlear lesions, acoustically driven expression was decreased in some, but not all superior olivary nuclei in both the lesioned and the untreated sides. In the lateral lemniscus complex, c-Fos activation was scarce in isolated rats and increased strongly after stimulation. Following unilateral cochlear lesion, acoustically driven expression decreased bilaterally in all nuclei. We have found that unilateral inner ear lesions lead to bilateral impairment of the capability of acoustic pathway neurons, to being c-Fos-activated following sound stimulation.

Cochlear electrical stimulation: influence of age of implantation on Fos immunocytochemical reactions in inferior colliculi and dorsal cochlear nuclei of the rat

The influence of age at the time of implantation of a stimulating electrode unilaterally in the inner ear on central auditory pathways was investigated in rats deafened shortly after birth. Immunoreactivity for Fos served as a functional marker of neuronal activity. Electrodes were implanted in the left cochlea of rats aged 3 weeks or 4 months. Stimulation lasted 45 minutes, then rats were sacrificed and tissues processed for immunocytochemistry. The younger animals showed significantly more neurons with Fos immunoreactivity bilaterally in the dorsal cochlear nuclei (DCN) and inferior colliculi (IC) than the older rats or control animals with normal hearing receiving the same stimulation. Activity was more prominent in the left DCN and right IC. The results show that electrical stimulation of the inner ear is more effective in younger animals in eliciting gene expression associated with development of a functional network in the auditory pathways. This suggests that deaf children should be provided with cochlear implants as early as possible.

Modulation of P-CREB and expression of c-fos in cochlear nucleus and superior olive following electrical intracochlear stimulation

Investigating activity-dependent plasticity in the auditory brain stem of the adult rat, we observed that electrical intracochlear stimulation led to a tonotopically localized modulation of the phosphorylation of the cAMP response element binding protein (CREB) and an equally localized expression of the immediate early gene product c-Fos in cochlear nucleus and superior olive. As P-CREB is thought to act as transcription factor on one promoter site of the c-fos gene, we compared immunolabeling for P-CREB and c-fos in adjacent brain sections. Following 2h sustained stimulation in previously deafened animals, labelling for P-CREB declined in regions where c-Fos labelling increased. This suggests that the level or state of P-CREB (e.g. whether it is phosphorylated or not) are affected by intracochlear stimulation in a process that appears to be linked to the stimulation-dependent expression of c-Fos in auditory brain stem nuclei.

Effects of chronic stimulation on auditory nerve survival in ototoxically deafened animals

For almost 10 years, chronic stimulation has been known to affect spiral ganglion cell (SGC) survival in the deaf ear. However, the reported effects of chronic stimulation vary across preparations and studies. In this review, the effects of chronic stimulation on the deafened auditory periphery are examined, and variables that may impact on the efficacy of chronic stimulation are identified. The effects of deafening on the unstimulated peripheral and central auditory system are also described, as the deafened, unstimulated system is the canvas upon which stimulation-mediated effects are imposed. Discrepancies in the effects of chronic stimulation across studies may be attributable in large part to the combined effects of the deafening method and the post-deafening delay prior to chronic stimulation, which vary across studies. Emphasis is placed on the need to consider the natural progression of SGC loss following deafening in the absence of chronic stimulation, as the rate of SGC loss almost certainly affects both the efficacy of stimulation, and the impact of any delay between deafening and initiation of stimulation. The differences across preparations complicate direct comparison of protective efficacy of stimulation. At the same time, these differences can be used to our advantage, aiding characterization of the effects of different factors on the efficacy of chronic stimulation as a neuroprotective intervention.

Plasticity of the superior olivary complex

The superior olivary complex (SOC) is part of the auditory brainstem of the vertebrate brain. Residing ventrally in the rhombencephalon, it receives sensory signals from both cochleae through multisynaptic pathways. Neurons of the SOC are also a target of bilateral descending projections. Ascending and descending efferents of the SOC affect the processing of auditory signals on both sides of the brainstem and in both organs of Corti. The pattern of connectivity indicates that the SOC fulfills functions of binaural signal integration serving sound localization. But whereas many of these connectional features are shared with the inferior colliculus (with the important exception of a projection to the inner ear), cellular and molecular investigations have shown that cells residing in SOC are unique in several respects. Unlike those of other auditory brainstem nuclei, they specifically express molecules known to be involved in development, plasticity, and learning (e.g., GAP-43 mRNA, specific subunits of integrin). Moreover, neurons of the SOC in adult mammals respond to various kinds of hearing impairment with the expression of plasticity-related substances (e.g., GAP-43, c-Jun, c-Fos, cytoskeletal elements), indicative of a restructuring of auditory connectivity. These observations suggest that the SOC is pivotal in the developmental and adaptive tuning of binaural processing in young and adult vertebrates.

Changes in cochlear electrical stimulation induced Fos expression in the rat inferior colliculus following deafness

Fos immunoreactive (IR) staining was used to examine changes in excitatory neuronal activity in the rat inferior colliculus (IC) between normal hearing and 21 day deaf rats evoked by basal or apical monopolar cochlear electrical stimulation. The location of evoked Fos IR neurons was consistent with expected tonotopic areas. The number of Fos IR cells increased as stimulation intensity increased in both normal and 21 day deaf animals. Stimulation at 1. 5x threshold evoked fewer Fos IR cells in 21 day deafened animals compared to normal hearing animals. At 5x and above, however, significantly increased numbers of Fos IR neurons (in a larger grouping) were evoked in 21 day deafened animals compared to normal hearing animals. Another group of animals had 7 days of deafness followed by 14 days of chronic basal cochlear electrical stimulation. In this group basal monopolar stimulation at 5x evoked not only a greater number of Fos IR neurons, compared to normal hearing animals, but the location of their grouping was slightly shifted to a more dorso-lateral region in the contralateral IC, compared to the normal hearing and 21 day deaf groups. These observations indicate that both deafness and chronic electrical stimulation may alter central auditory processing.

Cochleotopic fos immunoreactivity in cochlea and cochlear nuclei evoked by bipolar cochlear electrical stimulation

Fos-like immunoreactivity evoked by basal, second or apical turn bipolar intracochlear electrical stimulation was evaluated in the spiral ganglion and cochlear nuclei. At stimulation levels of six times the electrically evoked auditory brain stem response thresholds, immunoreactive neurons were observed at appropriate discrete cochleotopic regions relative to stimulation site. The number of neurons increased with stimulus level and closely correlated to wave I amplitude. At 10 times thresholds, some spread in fos-like immunoreactivity to adjacent cochlear turns was found. However, fos-like immunoreactivity at this high level of stimulation still clearly showed a differential distribution in density of expression. These results indicated that the restricted topographic distribution of activity evoked by high levels of electrical stimulation is initiated at first order primary neurons of the system. For the profoundly deaf with cochlear implants, this indicates that place (channel) information can be maintained in the spiral ganglion and central nervous system even at very high levels of electrical stimulation. Together with our previous studies, these results indicate that fos provides a marker which can be used for evaluation of extent and pattern of cellular activation throughout the central auditory pathways, including activation of auditory nerve cells.

Effects of stimulus level on electrode-place discrimination in human subjects with cochlear implants

Effects of stimulus level on discrimination of one stimulation site from another were examined in 15 human subjects with Nucleus-22 cochlear implant systems. Bipolar stimulation was used in all cases with electrodes in the bipolar pair separated by 1.5 mm (center to center). Subjects were first tested at a medium loudness level, using an adaptive tracking procedure, to determine the regions of the electrode array where electrode-place discrimination was best and the regions where it was poorest. Electrode-place discrimination was then tested at three regions distributed throughout the array, which included the regions of best and poorest discrimination. At each region, electrode-place discrimination was tested at three levels: 25%, 50%, and 75% of the dynamic range. For each of these nine conditions (3 sites x 3 levels), the test-electrode pairs were loudness balanced with the reference-electrode pairs. A two-interval forced-choice same-different procedure was then used to determine discriminability of the reference-electrode pair from the nearest, apical, test-electrode pair. If P(C)max was <0.707 at all three levels, additional testing was done using the next, more apical, electrode pair as the test-electrode pair. A tendency toward better discrimination at more apical regions of the array was observed. Electrode pairs with poor discrimination typically had smaller dynamic ranges than those with good discrimination. There was a weak tendency toward better discrimination at higher levels of stimulation. However, effects of level on electrode-place discrimination were less pronounced and less consistent than previously observed effects of level on temporal discriminations. These results suggest interactions between current spread and the condition of the implanted cochlea as underlying mechanisms.