Thin-film micro-electrode stimulation of the cochlea in rats exposed to aminoglycoside induced hearing loss.

The multi-channel cochlear implant (CI) provides sound and speech perception to thousands of individuals who would otherwise be deaf. Broad activation of auditory nerve fibres when using a CI results in poor frequency discrimination. The CI also provides users with poor amplitude perception due to elicitation of a narrow dynamic range. Provision of more discrete frequency perception and a greater control over amplitude may allow users to better distinguish speech in noise and to segregate sound sources. In this research, thin-film (TF) high density micro-electrode arrays and conventional platinum ring electrode arrays were used to stimulate the cochlea of rats administered sensorineural hearing loss (SNHL) via ototoxic insult, with neural responses taken at 434 multiunit clusters in the central nucleus of the inferior colliculus (CIC). Threshold, dynamic range and broadness of response were used to compare electrode arrays. A stronger current was required to elicit CIC threshold when using the TF array compared to the platinum ring electrode array. TF stimulation also elicited a narrower dynamic range than the PR counterpart. However, monopolar stimulation using the TF array produced more localised CIC responses than other stimulation strategies. These results suggest that individuals with SNHL could benefit from micro stimulation of the cochlea using a monopolar configuration which may provide discrete frequency perception when using TF electrode arrays.

Modulation of medial geniculate nucleus neuronal activity by electrical stimulation of the nucleus accumbens.

Dysfunctional sensory gating has been proposed to result in the generation of phantom perceptions. In agreement, it has been recently suggested that tinnitus, a phantom perception of sound commonly associated with hearing loss, is the result of a breakdown of circuitry involving the limbic system and the medial geniculate nucleus (MGN) of the thalamus. In humans with tinnitus, structural changes and abnormal activity have been found to occur in the auditory pathway as well as parts of the limbic system such as the nucleus accumbens (NAc). However, at present, no studies have been conducted on the influence of the NAc on the MGN. We investigated the functional connectivity between the NAc and MGN single neurons. Bipolar electrical stimulation was delivered to the NAc while recording single neuron activity in MGN in anesthetized Wistar rats. Histological analysis was used to confirm placement of electrodes. NAc electrical stimulation generally decreased spontaneous firing rates in MGN neurons and, in a limited number of neurons, caused an increase in firing rate. This suggests that NAc can modulate the activity of auditory neurons in the MGN and may play a role in the development of tinnitus.

Intralaminar stimulation of the inferior colliculus facilitates frequency-specific activation in the auditory cortex.

OBJECTIVE: Auditory midbrain implants (AMI) provide inadequate frequency discrimination for open set speech perception. AMIs that can take advantage of the tonotopic laminar of the midbrain may be able to better deliver frequency specific perception and lead to enhanced performance. Stimulation strategies that best elicit frequency specific activity need to be identified. This research examined the characteristic frequency (CF) relationship between regions of the auditory cortex (AC), in response to stimulated regions of the inferior colliculus (IC), comparing monopolar, and intralaminar bipolar electrical stimulation. APPROACH: Electrical stimulation using multi-channel micro-electrode arrays in the IC was used to elicit AC responses in anaesthetized male hooded Wistar rats. The rate of activity in AC regions with CFs within 3 kHz (CF-aligned) and unaligned CFs was used to assess the frequency specificity of responses. MAIN RESULTS: Both monopolar and bipolar IC stimulation led to CF-aligned neural activity in the AC. Altering the distance between the stimulation and reference electrodes in the IC led to changes in both threshold and dynamic range, with bipolar stimulation with 400 µm spacing evoking the lowest AC threshold and widest dynamic range. At saturation, bipolar stimulation elicited a significantly higher mean spike count in the AC at CF-aligned areas than at CF-unaligned areas when electrode spacing was 400 µm or less. Bipolar stimulation using electrode spacing of 400 µm or less also elicited a higher rate of elicited activity in the AC in both CF-aligned and CF-unaligned regions than monopolar stimulation. When electrodes were spaced 600 µm apart no benefit over monopolar stimulation was observed. Furthermore, monopolar stimulation of the external cortex of the IC resulted in more localized frequency responses than bipolar stimulation when stimulation and reference sites were 200 µm apart. SIGNIFICANCE: These findings have implications for the future development of AMI, as a bipolar stimulation strategy may improve the ability of implant users to discriminate between frequencies.

Midbrain responses to micro-stimulation of the cochlea using high density thin-film arrays.

A broader activation of auditory nerve fibres than normal using a cochlear implant contributes to poor frequency discrimination. As cochlear implants also deliver a restricted dynamic range, this hinders the ability to segregate sound sources. Better frequency coding and control over amplitude may be achieved by limiting current spread during electrical stimulation of the cochlea and positioning electrodes closer to the modiolus. Thin-film high density microelectrode arrays and conventional platinum ring electrode arrays were used to stimulate the cochlea of urethane-anaesthetized rats and responses compared. Neurophysiological recordings were taken at 197 multi-unit clusters in the central nucleus of the inferior colliculus (CIC), a site that receives direct monaural innervation from the cochlear nucleus. CIC responses to both the platinum ring and high density electrodes were recorded and differences in activity to changes in stimulation intensity, thresholds and frequency coding of neural activation were examined. The high density electrode array elicited less CIC activity at nonspecific frequency regions than the platinum ring electrode array. The high density electrode array produced significantly lower thresholds and larger dynamic ranges than the platinum ring electrode array when positioned close to the modiolus. These results suggest that a higher density of stimulation sites on electrodes that effectively 'aim' current, combined with placement closer to the modiolus would permit finer control over charge delivery. This may equate to improved frequency specific perception and control over amplitude when using future cochlear implant devices.

Synaptic responses in cochlear nucleus neurons evoked by activation of the olivocochlear system.

The action of olivocochlear collaterals to the cochlear nucleus is not fully established. Synaptic ultrastructure suggests an excitatory role. Extracellular recordings show spikes evoked by electrical stimulation of olivocochlear axons, but these spikes in the cochlear nucleus may be antidromic (activation of output axons) or orthodromic (synaptic input). We therefore recorded intracellular responses to shocks to olivocochlear axons in anaesthetized guinea pigs. In chopper and primary-like neurons shocks caused either no response or an inhibitory synaptic response (IPSP), but never an excitatory one (EPSP). In contrast, onset neurons never showed IPSPs but showed a variety of other responses; antidromic spikes, EPSPs, orthodromic spikes or no effect. The results agree with earlier extracellular observations in that olivocochlear collaterals provide excitatory input to onset neurons. Because some onset neurons are inhibitory they may be the source of the IPSPs observed in other cochlear nucleus neurons. The data also show that electrical stimulation at the floor of the IVth ventricle results in antidromic spikes as well. However, intracellular recording enabled the orthodromic action to be verified and the presumed olivocochlear action to be better understood. Our data support the hypothesis that olivocochlear collaterals initiate excitatory input onto onset-chopper neurons.

Olivocochlear collaterals evoke excitatory effects in onset neurones of the rat cochlear nucleus.

Axons of medial olivocochlear neurones in the superior olivary complex terminate on the outer hair cells of the cochlea and also give off collaterals that terminate in the cochlear nucleus. Previous work in our laboratory, using extracellular recordings in the cochlear nucleus, has indicated that stimulation of the olivocochlear axons may have an excitatory effect on specific cell populations of the cochlear nucleus, such as onset-choppers, in contrast to the peripheral suppressive action of the same axons. We have investigated whether this excitation is produced by action of the olivocochlear collaterals in the cochlear nucleus or whether it is mediated via the peripheral suppression, by measuring intracellular responses in the rat cochlear nucleus to electrical stimulation of the olivocochlear axons in silence. The results demonstrate that single shocks applied to the olivocochlear axons can evoke excitatory postsynaptic potentials in onset neurones. We observed an inhibitory effect in one chopper only. In the same animals in all other neurones investigated (i.e. three primary-like neurones and eight choppers) the same stimulation was without any effect on cell membrane potential. We conclude that the excitatory effects in onset neurones are not caused by suppression in the auditory peripheral organ, but by activation of olivocochlear collaterals in the cochlear nucleus.

Delay analysis in the auditory brainstem of the rat: comparison with click latency.

Many cells in the auditory brainstem 'phase lock' to tone stimuli. From the changing phase relationship between the stimulus and the neural response in phase-locking cells, the delay between them can be estimated. This delay, however, is consistently greater than the latency measured in response to click stimuli, an important discrepancy. In this paper the different measures of delay, namely phase delay, group delay and signal-front delay are re-examined. An improved method for computing the average group delay is presented, which accounts for the cyclical nature of the phase data. Data were collected from units in successive processing sites of auditory pathway: the auditory nerve, the cochlear nucleus, the trapezoid body and the medial nucleus of the trapezoid body. Low-characteristic frequency (CF) units gave multimodal post-stimulus-time histograms in response to clicks, and showed stepwise decreases in latency with increasing intensity, with the appearance of earlier peaks in the response, rather than shifts in the timing of the peaks. The separation of peaks corresponded to the inverse of the unit's CF. High-CF units also showed a decline in click latency with intensity, but to a lesser degree than low CF units. We present an analysis which explains the difference between click latency and delay, and which in contrast to previous accounts is experimentally testable. We demonstrate that this new framework accounts for the discrepancy between the two measures of delay, and in addition accounts for the observed stepwise shifts in click latency for low-CF units.

Temporal processing from the auditory nerve to the medial nucleus of the trapezoid body in the rat.

This investigation examines temporal processing through successive sites in the rat auditory pathway: auditory nerve (AN), anteroventral cochlear nucleus (AVCN) and the medial nucleus of the trapezoid body (MNTB). The degree of phase-locking, measured as vector strength, varied with intensity relative to the cell's threshold, and saturated at a value that depended upon stimulus frequency. A typical pattern showed decline in the saturated vector strength from approximately 0.8 at 400 Hz to about 0.3 at 2000 Hz, with similar profiles in units with a range of characteristic frequencies (480-32,000 Hz). A new expression for temporal dispersion indicates that this variation corresponds to a limiting degree of temporal imprecision, which is relatively consistent between different cells. From AN to AVCN, an increase in vector strength was seen for frequencies below 1000 Hz. At higher frequencies, a decrease in vector strength was observed. From AVCN to MNTB a tendency for temporal coding to be improved below 800 Hz and degraded further above 1500 Hz was seen. This change in temporal processing ability could be attributed to units classified as primary-like with notch (PL(N)). PL(N) MNTB units showed a similar vector strength distribution to PL(N) AVCN units. Our results suggest that AVCN PL(N) units, representing globular bushy cells, are specialised for enhancing the temporal code at low frequencies and relaying this information to principal cells of the MNTB.

Intracellular responses of onset chopper neurons in the ventral cochlear nucleus to tones: evidence for dual-component processing.

Intracellular responses of onset chopper neurons in the ventral cochlear nucleus to tones: evidence for dual-component processing. The ventral cochlear nucleus (VCN) contains a heterogeneous collection of cell types reflecting the multiple processing tasks undertaken by this nucleus. This in vivo study in the rat used intracellular recordings and dye filling to examine membrane potential changes and firing characteristics of onset chopper (OC) neurons to acoustic stimulation (50 ms pure tones, 5 ms r/f time). Stable impalements were made from 15 OC neurons, 7 identified as multipolar cells. Neurons responded to characteristic frequency (CF) tones with sustained depolarization below spike threshold. With increasing stimulus intensity, the depolarization during the initial 10 ms of the response became peaked, and with further increases in intensity the peak became narrower. Onset spikes were generated during this initial depolarization. Tones presented below CF resulted in a broadening of this initial depolarizing component with high stimulus intensities required to initiate onset spikes. This initial component was followed by a sustained depolarizing component lasting until stimulus cessation. The amplitude of the sustained depolarizing component was greatest when frequencies were presented at high intensities below CF resulting in increased action potential firing during this period when compared with comparable high intensities at CF. During the presentation of tones at or above the high-frequency edge of a cell's response area, hyperpolarization was evident during the sustained component. The presence of hyperpolarization and the differences seen in the level of sustained depolarization during CF and off CF tones suggests that changes in membrane responsiveness between the initial and sustained components may be attributed to polysynaptic inhibitory mechanisms. The dual-component processing resulting from convergent auditory nerve excitation and polysynaptic inhibition enables OC neurons to respond in a unique fashion to intensity and frequency features contained within an acoustic stimulus.

Muscimol suppression of the dorsal cochlear nucleus impairs frequency discrimination in rats.

The cochlear nucleus is composed of three sub-nuclei: the dorsal (DCN), anteroventral (AVCN) and posteroventral cochlear nucleus (PVCN). In addition to connections between these sub-nuclei, each nucleus receives frequency specific tonotopically organised input from the cochlea. Evidence suggests that connections from the DCN to the AVCN are inhibitory and organised tonotopically but the functional significance of this pathway has yet to be elucidated. The possible role of this pathway in frequency discrimination using a T-maze behavioural paradigm and DCN suppression was examined. Five rats were trained on a two choice frequency discrimination task. Once frequency difference limens for 10-30% performance above chance were determined, rats had cannulae implanted bilaterally over the DCN. After recovery rats were tested on the behavioural task with nothing, saline and the GABA agonist muscimol injected into the DCN via the cannulae. Muscimol alone significantly reduced the rats ability to perform the task. This performance decrease was attributed to an inability to discriminate high frequency and not low frequency tones suggesting that place and not temporal coding of sound was compromised by DCN suppression. These results are consistent with the hypothesis that inhibitory drive from the DCN to AVCN may be crucial for the fine tuning of frequency information.

Intracellular responses of the rat anteroventral cochlear nucleus to intracochlear electrical stimulation.

The anteroventral cochlear nucleus (AVCN) is the first central processing site for acoustic information. The influence and extent of convergent auditory nerve input to AVCN neurons was investigated using brief (<0.2 ms) intracochlear electrical activation of spiral ganglion cells. In 40 neurons recorded in vivo, the major intracellular response to stimulation was an excitatory postsynaptic potential (EPSP) with short latency (approximately 1 ms) and fast rise time (<1 ms). Graduated EPSP amplitude increases were also seen with increasing stimulation strength resulting in spike generation. Hyperpolarization followed excitation in most neurons, its extent distinguished three response types: Type I showed no hyperpolarization; Type II and Type III displayed short (<10 ms) and long (>19 ms) duration hyperpolarization, respectively. Hyperpolarization was attributed to an inhibitory postsynaptic potential (IPSP) in addition to spike after hyperpolarization. Neurobiotin filling identified Type I and II neurons as stellate and Type III as bushy cells. These results suggests that AVCN neurons receive direct, possibly convergent, excitatory input from auditory nerves emanating from spiral ganglion cells with hyperpolarization resulting from polysynaptic inhibitory input.

Muscimol suppression of the dorsal cochlear nucleus modifies frequency tuning in rats.

The cochlear nucleus is composed of three sub-nuclei: the dorsal (DCN), anteroventral (AVCN) and posteroventral cochlear nucleus (PVCN). Intrinsic connections from the DCN to the AVCN are inhibitory and organised tonotopically. In this investigation, this pathway and its possible role in frequency tuning was examined using in vivo extracellular recordings. Extracellular recordings were made from 191 units in the AVCN, 69 of which were recorded after suppression of DCN by application of the GABA agonist Muscimol (15 ng, 0.26 mM). Tuning curves were plotted and characteristic frequency (CF) and response threshold (measured in dB SPL) were determined for each unit. Units recorded post-Muscimol showed significantly broader tuning characteristics and lower thresholds. Primary-like and transient chopper neurons contributed to this decrease in threshold suggesting that they receive 'on' CF inhibitory drive from the DCN. Sustained chopper units did not show a significant decrease in response threshold after Muscimol; however, there was a tendency for broader tuning and a significant increase in CF tone evoked maximum discharge rate and chopping frequency suggesting that the DCN may play a role in regulating the temporal firing of these units in addition to providing lateral inhibition. These results suggest that the DCN to AVCN pathway may aid in fine tuning frequency information.

Intracellular responses of the rat cochlear nucleus to sound and its role in temporal coding.

The anteroventral cochlear nucleus (AVCN), the first centre of the central auditory pathway, contains globular bushy cells, which are unique in their ability to produce fast excitatory post-synaptic potentials (EPSPs). Using in vivo intracellular recordings in the rat AVCN we examined these fast EPSPs in relation to temporal coding. At frequencies up to 2.5 kHz, EPSPs were evoked on successive sine waves of the stimulus with EPSP summation limited. This one-to-one relationship between the EPSPs and the sound wave period was present at higher frequencies and over a greater intensity range than for action potentials. These results suggest that temporal coding is possible in globular bushy neurones by their ability to extract temporal information through fast processing of convergent presynaptic input.

Intracellular recording of magnocellular preoptic neuron responses to olfactory brain.

The magnocellular preoptic nucleus of the rat supplies centrifugal input to the olfactory bulb as well as projecting to other olfactory-related areas. The extent to which the piriform and entorhinal cortices can influence the activity of magnocellular preoptic neurons and hence that of the olfactory bulb were examined using intracellular in vivo recording. Stable recordings were obtained in 58 neurons impaled in the magnocellular preoptic nucleus. Antidromic responses occurred on stimulating olfactory bulb (15), piriform cortex (14), or entorhinal area (eight). Monosynaptic excitation was evoked by piriform (27 of 37 tested) and entorhinal cortex (15 of 32 tested) stimulation with polysynaptic inhibition occurring in seven and five neurons, respectively. Polysynaptic as well as antidromic excitation by olfactory bulb stimulation occurred in four; a further 28 tested responded polysynaptically. No response to olfactory bulb stimulation was monosynaptic. In stable impalements, 29 neurons discharged spontaneously in the absence of applied current. Lucifer Yellow and Neurobiotin were used to label 16 cells. All but one had smooth dendrites with soma diameters ranging from 8 to 24 microm. These results provide a framework in which magnocellular preoptic neurons can influence olfactory processing by direct action on the olfactory bulb, which action can be boosted by positive feedback from the bulb through the olfactory piriform and entorhinal cortices.

Effects of inactivation of the magnocellular preoptic nucleus of olfactory bulb processing.

The magnocellular nucleus (MCPO) was inactivated in anaesthetized rats, using muscimol, a gamma-amino butyric acid ergic agonist, in order to examine the effect of suppression of its tonic activity on extracellular unit firing in the granular (GRL), mitral (MCL) and external plexiform (EPL) layers of the olfactory bulb (OB). In GRL there was a significant increase in unit activity during the first hour after muscimol injection (30 ng), followed by a significant decrease in activity during the following hour. No effect on activity in MCL was seen after muscimol injection into the MCPO. Unit activity in EPL increased during the second hour post-injection. It was concluded that MCPO plays an important part in regulating the balance between granule and tufted cell activity.

Lesions in the magnocellular preoptic nucleus decrease olfactory investigation in rats.

The nuclear complex of the horizontal limb of the diagonal band and the magnocellular preoptic nucleus, components of the basal forebrain magnocellular system affected in Alzheimer-type dementia, supply centrifugal innervation to the olfactory bulb. The lateral magnocellular preoptic nucleus provides significant GABAergic input. Since its stimulation may facilitate olfactory bulb mitral cells, we have investigated the effect of sub-total electrolytic lesions in this nucleus on performance in a simple test of olfactory investigation and its habituation. Two groups of rats used with lesions which occupied restricted volumes, approximately 30 and 15% of the magnocellular preoptic nucleus. Behaviorally, there was interference with olfactory investigation, with increased investigation latency and decreased investigation times, the group with larger lesions at 6 and 16 days after operation. There was no significant effect of the smaller lesions. No effects on patterns of olfactory habituation or discrimination were seen. The impairment of olfactory investigation could not be explained by interruption of medial forebrain bundle fibres traversing the nucleus. It is suggested that bilateral partial destruction of magnocellular preoptic neurones may produce significant deficits in either olfactory sensitivity or olfactory motivation.

Effects of lesions in the horizontal diagonal band nucleus on olfactory habituation in the rat.

The nucleus of the horizontal limb of the diagonal band, a component of the basal forebrain magnocellular complex affected in Alzheimer type dementia, supplies centrifugal innervation to the olfactory bulb. We have tested the hypothesis that horizontal limb of the diagonal band lesions will interfere with olfactory memory in a simple olfactory test paradigm. Lesions occupied a restricted volume, approximately 20%, of medial horizontal limb of the diagonal band. There was interference with habituation of investigation latency and duration, six and 16 days after lesioning. It is concluded that bilateral partial lesions of the medial nucleus of the horizontal limb of the diagonal band interfere with habituation memory for odours.