Rate representation of tones in noise in the inferior colliculus of decerebrate cats

Neurons in the central nucleus of the inferior colliculus (ICC) of decerebrate cats show three major response patterns when tones of different frequencies and sound-pressure levels (SPLs) are presented to the contralateral ear. The frequency response maps of type I units are uniquely defined by a narrow excitatory area at best frequency (BF: a unit's most sensitive frequency) and surrounding inhibition at higher and lower frequencies. As a result of this receptive field organization, type I units exhibit strong excitatory responses to BF tones but respond only weakly to broadband noise (BBN). These response characteristics predict that type I units are well suited to encode narrowband signals in the presence of background noise. To test this hypothesis, the dynamic range properties of ICC unit types were measured under quiet conditions and in multiple levels of continuous noise. As observed in previous studies of the auditory nerve and cochlear nucleus, type I units showed upward threshold shifts and discharge rate compression in background noise that partially degraded the dynamic range properties of neural representations at high noise levels. Although the other two unit types in the ICC showed similar trends in threshold shift and noise compression, their ability to encode auditory signals was compromised more severely in increasing noise levels. When binaural masking effects were simulated, only type I units showed an enhanced representation of spatially separated signals and maskers that was consistent with human perceptual performance in independent psychoacoustic observations. These results support the interpretation that type I units play an important role in the auditory processing of narrowband signals in background noise and suggest a physiological basis for spatial factors that govern signal detection under free-field listening conditions.

Modulation of audiogenic seizures by histamine and adenosine receptors in the inferior colliculus

Susceptibility to behaviorally similar audiogenic seizures (AGS) occurs genetically and is inducible during ethanol withdrawal (ETX). Comparisons between AGS mechanisms of genetically epilepsy-prone rats (GEPR-9s) and ethanol-withdrawn rats (ETX-Rs) are yielding information about general pathophysiological mechanisms of epileptogenesis. The inferior colliculus (IC) is the AGS initiation site. Excitatory amino acid (EAA) abnormalities in the IC are implicated in AGS, and histamine and adenosine receptor activation each reduce EAA release and inhibit several seizure types. Previous studies indicate that focal infusion of an adenosine receptor agonist into the IC blocked AGS in GEPR-9s, but the effects of adenosine receptor activation in the IC on AGS in ETX-Rs are unknown. The effects of histamine receptor activation on either form of AGS are also unexamined. The present study evaluated effects of histamine or a nonselective adenosine A(1) agonist, 2-chloroadenosine, on AGS by focal microinjection into the IC. Ethanol dependence and AGS susceptibility were induced in normal rats by intragastric ethanol. Histamine (40 or 60 nmol/side) significantly reduced AGS in GEPR-9s, but histamine in doses up to 120 nmol/side did not affect AGS in ETX-Rs. 2-Chloroadenosine (5 or 10 nmol/side) did not affect AGS in ETX-Rs, despite the effectiveness of lower doses of this agent in GEPR-9s reported previously. Thus, histamine and adenosine receptors in the IC modulate AGS of GEPR-9s, but do not modulate ETX-induced AGS. The reasons for this difference may involve the chronicity of AGS susceptibility in GEPR-9s, which may lead to more extensive neuromodulation as compensatory mechanisms to limit the seizures compared to the acute AGS of ETX-Rs.

Effects of intracochlear factors on spiral ganglion cells and auditory brain stem response after long-term electrical stimulation in deafened kittens

Using an animal model, we have studied the response of the auditory brain stem to cochlear implantation and the effect of intracochlear factors on this response. Neonatally, pharmacologically deafened cats (100 to more than 180 days old) were implanted with a 4-electrode array in both cochleas. Then, the left cochlea of each cat was electrically stimulated for total periods of up to 1000 hours. After a terminal (14)C-2-deoxyglucose (2DG) experiment, the fraction of the right inferior colliculus with a significant accumulation of 2DG label was calculated. Using 3-dimensional computer-aided reconstruction, we examined the cochleas of these animals for spiral ganglion cell (SGC) survival and intracochlear factors such as electrode positions, degeneration of the organ of Corti, and the degree of fibrosis of the scala tympani. The distribution of each parameter was calculated along the organ of Corti from the basal end. There was a positive correlation between SGC survival and the level of fibrosis in the scala tympani, and a negative correlation between SGC survival and the degree of organ of Corti degeneration. Finally, there was a negative correlation between the 2DG-labeled inferior colliculus volume fraction and the degree of fibrosis, particularly in the 1-mm region nearest the pair of electrodes, and presumably in the basal turn.

Lateralized tinnitus studied with functional magnetic resonance imaging: abnormal inferior colliculus activation

Tinnitus, the perception of sound in the absence of external stimuli, is a common and often disturbing symptom that is not understood physiologically. This paper presents an approach for using functional magnetic resonance imaging (fMRI) to investigate the physiology of tinnitus and demonstrates that the approach is effective in revealing tinnitus-related abnormalities in brain function. Our approach as applied here included 1) using a masking noise stimulus to change tinnitus loudness and examining the inferior colliculus (IC) for corresponding changes in activity, 2) separately considering subpopulations with particular tinnitus characteristics, in this case tinnitus lateralized to one ear, 3) controlling for intersubject differences in hearing loss by considering only subjects with normal or near-normal audiograms, and 4) tailoring the experimental design to the characteristics of the tinnitus subpopulation under study. For lateralized tinnitus subjects, we hypothesized that sound-evoked activation would be abnormally asymmetric because of the asymmetry of the tinnitus percept. This was tested using two reference groups for comparison: nontinnitus subjects and nonlateralized tinnitus subjects. Binaural noise produced abnormally asymmetric IC activation in every lateralized tinnitus subject (n = 4). In reference subjects (n = 9), activation (i.e., percent change in image signal) in the right versus left IC did not differ significantly. Compared with reference subjects, lateralized tinnitus subjects showed abnormally low percent signal change in the IC contralateral, but not ipsilateral, to the tinnitus percept. Consequently, activation asymmetry (i.e., the ratio of percent signal change in the IC ipsilateral versus contralateral to the tinnitus percept) was significantly greater in lateralized tinnitus subjects as compared with reference subjects. Monaural noise also produced abnormally asymmetric IC activation in lateralized tinnitus subjects. Two possible models are presented to explain why IC activation was abnormally low contralateral to the tinnitus percept in lateralized tinnitus subjects. Both assume that the percept is associated with abnormally high ("tinnitus-related") neural activity in the contralateral IC. Additionally, they assume that either 1) additional activity evoked by sound was limited by saturation or 2) sound stimulation reduced the level of tinnitus-related activity as it reduced the loudness of (i.e., masked) the tinnitus percept. In summary, this work demonstrates that fMRI can provide objective measures of lateralized tinnitus and tinnitus-related activation can be interpreted at a neural level.

Neonatal sensorineural hearing loss affects neurone size in cat auditory midbrain

We examined the effect of a neonatal sensorineural hearing loss on the soma area of neurones in the central nucleus of the inferior colliculus (ICC) in adult cats to evaluate the role of auditory experience on neuronal atrophy within the auditory midbrain. Three groups of animals were used: bilaterally deafened, unilaterally deafened and normal hearing controls. Soma area measurements were made from the laminated central and medial divisions of the ICC of eight deafened and two normal hearing cats. A small but significant reduction in soma area was evident for bilaterally deafened animals compared with normal hearing controls (P<0.05, Dunnett's test). In contrast, there was no significant difference in mean soma area between normal hearing and unilaterally deafened animals (P0.05) irrespective of whether the ICC examined was ipsi- or contralateral to the deafened ear. These results demonstrate that the reduction in soma area of auditory brainstem neurones reported following a sensorineural hearing loss is also evident at the level of the auditory midbrain.

Inner hair cell loss leads to enhanced response amplitudes in auditory cortex of unanesthetized chinchillas: evidence for increased system gain

Carboplatin preferentially destroys inner hair cells (IHCs) in the chinchilla inner ear, while retaining a near-normal outer hair cell (OHC) population. The present study investigated the functional consequences of IHC loss on the compound action potential (CAP), inferior colliculus potential (ICP) and auditory cortex potential (ACP) recorded from chronically implanted electrodes. IHC loss led to a reduction in CAP amplitude that was roughly proportional to IHC loss. The ICP amplitude was typically reduced by IHC loss, but the magnitude of this reduction was generally less than that observed for the CAP. In contrast to the CAP and ICP, ACP amplitudes were generally not reduced following IHC loss. In some animals, the ACP amplitude remained at pre-carboplatin values despite substantial IHC loss. However, in other animals, IHC loss led to an increase ('enhancement') of ACP amplitude. ACP enhancement was greatest at 1-2 weeks post-carboplatin, returning towards baseline amplitudes at 5 weeks post-carboplatin. In other animals, the ACP remained enhanced up to 5 weeks post-carboplatin. We interpret the transient and sustained enhancement of ACP amplitude following partial IHC loss as evidence of functional reorganization occurring at or below the level of the auditory cortex. These results suggest that the gain of the central auditory pathway increases following IHC loss to compensate for the reduced input from the cochlea.

Myoclonic and tonic seizures elicited by microinjection of cholinergic drugs into the inferior colliculus

The inferior colliculus (IC) is the initiation site in the neuronal network for the epileptic audiogenic seizure (AGS). The present study investigates the effects of alteration of IC cholinergic transmission on the elicitation of epileptic seizures. Unilateral microinjections of carbachol (3 and 6 micrograms/0.2 microliter) into the IC elicited intense locomotor activity, contraversive rotations and myoclonic seizures. This result indicates that the IC is the initiation site for the induction of myoclonic seizures and suggests that these myoclonic seizures may result from activation of m1 muscarinic receptors. Microinjections of the nicotinic-muscarinic antagonist, gallamine (2 and 6 micrograms/0.2 microliter), into the IC induced AGS susceptibility. However, microinjections of muscarinic antagonists, atropine (15 micrograms/0.2 microliter) and scopolamine (12 and 20 micrograms/0.2 microliter), or the nicotinic antagonist, hexamethonium (12 and 20 micrograms/0.2 microliter), into the IC have no effect. Gallamine-induced AGS susceptibility may result from a selective blockade of m2 muscarinic receptors.

Does electrical stimulation of deaf cochleae prevent spiral ganglion degeneration?

Thirty-six drug deafened guinea pigs were studied to determine how electrical stimulation of the cochlea affects spiral ganglion cell (SGC) survival. Animals were divided into two groups, extracochlear and intracochlear stimulation, and each group was further divided into four stimulus subgroups: no stimulation (implanted controls), the inferior colliculus electrically evoked potential (ICEEP) threshold-2 dB, ICEEP threshold+2 dB, and ICEEP threshold+6 dB. Stimuli consisted of 200 micros/phase charge balanced biphasic current pulses presented at 100 pulses per second using monopolar stimulation. Animals were stimulated 5 h/day, 5 days per week, for 8 weeks. The animals were then perfused and the cochleae serially sectioned at 4 microm saving every 8th section. We counted the number of intact SGCs, those containing a nucleus with chromatin, in each 20% segment of the cochlea and also measured SGC densities (number of neurons per mm2 of Rosenthal's canal). The number of surviving spiral ganglion neurons was not significantly different (P > 0.05) between the implanted and the unimplanted ears in any of the experimental groups. However, the spiral ganglion neuron densities were significantly elevated in the electrically stimulated ears (P < 0.001) but not in the implanted but not chronically stimulated ears (P > 0.05). We measured the volume of Rosenthal's canal in one subgroup (ICEEP threshold+2 dB) and found a decrease in this volume in the stimulated ear compared to the unstimulated ear (P < 0.01). These findings support the hypothesis that chronic monopolar electrical intracochlear or extracochlear stimulation is not a neurotrophic factor, increasing spiral ganglion neuron survival, but instead causes a narrowing of Rosenthal's canal that accounts for the increased spiral ganglion neuronal densities seen in the stimulated cochleae.

Differential roles in the neuronal network for audiogenic seizures are observed among the inferior colliculus subnuclei and the amygdala

The inferior colliculus (IC) is established as the initiation site within the neuronal network for audiogenic seizures (AGS), but the relative importance of the IC subnuclei in AGS is controversial. The lateral and basolateral subdivisions of the amygdala are implicated in the expansion of the AGS network that occurs during AGS kindling. However, the role of the amygdala in the AGS network in nonkindled AGS is unknown. NMDA receptors are implicated in modulation of AGS and in neurotransmission in both the IC and amygdala. Therefore, changes in AGS severity in genetically epilepsy-prone rats (GEPR-9s) were examined after bilateral focal microinjection into IC subnuclei or lateral/basolateral subdivisions of the amygdala of a competitive NMDA receptor antagonist, 3-((+)-2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (CPP). Blockade of AGS in IC central nucleus (ICc) and external cortex (ICx) was observed at identical doses of CPP, but these doses were ineffective in IC dorsal cortex (ICd). Microinjection of CPP into the amygdala did not produce significant changes in AGS severity except at doses 20 times those effective in IC. The latter data contrast with the anticonvulsant effects of amygdala microinjections on seizure severity in kindled AGS reported previously. The present data in concord with neuronal recording studies of these nuclei suggest that the ICc is the most critical site in AGS initiation, the ICx in propagation, and that the ICd plays a lesser role in the AGS network. The amygdala does not appear to play a requisite role in the neuronal network for AGS in animals that have not been subjected to AGS kindling.

Modulation of the audiogenic seizure network by noradrenergic and glutamatergic receptors of the deep layers of superior colliculus

Recent studies suggest that the deep layers of superior colliculus (DLSC) play a role in the network for audiogenic seizures (AGS) in genetically epilepsy-prone rats (GEPR-9s). The present study examined the role of glutamatergic and noradrenergic receptors in DLSC in modulation of AGS susceptibility. The study examined effects of a competitive NMDA receptor antagonist [dl-2-amino-7-phosphonoheptanoic acid (AP7)] or an alpha1 noradrenergic agonist (phenylephrine) focally microinjected into DLSC as compared to effects in the inferior colliculus (IC) and pontine reticular formation (PRF), which are major established components of the AGS network. The results demonstrated that blockade of NMDA receptors in DLSC suppressed AGS susceptibility. AP7 microinjection was effective at relatively low doses in IC, but required higher doses in DLSC and PRF. The DLSC was relatively more sensitive to seizure reduction by the alpha1 noradrenergic agonist as compared to the IC and PRF. The anticonvulsant effect of AP7 was longer-lasting than phenylephrine in the DLSC and IC but not in the PRF. These data suggest that neurons in the DLSC are a requisite component for the neuronal network for AGS in GEPR-9s and that NMDA and alpha1 adrenoreceptors in this site may play important roles in the modulation of AGS propagation. The relatively greater sensitivity of DLSC to phenylephrine as compared to IC and PRF indicates that norepinephrine may be more important in the modulation of AGS in DLSC, which contrasts to the role of glutamate modulation. These data support recent neuronal recording data, which indicate that DLSC neurons play a critical role in AGS.

Intracochlear infusion of buthionine sulfoximine potentiates carboplatin ototoxicity in the chinchilla

The aim of this experiment was to determine if buthionine sulfoximine (BSO), an inhibitor of glutathione (GSH) synthesis, enhances the ototoxicity of carboplatin. Osmotic pumps were used to infuse BSO into the right cochleas of 12 adult chinchillas for 14 days. The left cochleas served as controls. Animals were assigned to three groups: a drug control group that did not receive carboplatin, a group that received a single dose of carboplatin (25 mg/kg i.p.), and a group that received a double dose of carboplatin (25 mg/kg i.p. x 2), with 4 days between injections. Carboplatin was administered after three days of BSO pre-treatment. Ototoxicity was assessed with evoked potentials recorded from electrodes implanted in the inferior colliculi (ICPs), distortion product otoacoustic emissions (DPOAEs), and cochleograms. BSO infusion itself caused no long-term functional or morphological changes. One of four animals treated with it single dose of carboplatin showed a significant loss of inner hair cells (IHCs), with greater loss in the BSO-treated ear. All animals in the double-dose carboplatin group showed marked differences between BSO-treated and control ears. Average IHC losses were 59% in BSO-treated ears vs. 18% in control ears. Moreover, BSO-treated ears sustained significantly greater outer hair cell (OHC) losses than control ears (37% vs. 2%, respectively). ICP and DPOAE response amplitudes were reduced slightly in BSO-treated ears relative to control ears, consistent with their greater hair cell loss. The results clearly show that BSO can enhance carboplatin ototoxicity in the chinchilla, supporting a role of GSH and reactive oxygen species in platinum ototoxicity.

Sensorineural hearing loss during development: morphological and physiological response of the cochlea and auditory brainstem

We have investigated the effects of sensorineural hearing loss on the cochlea and central auditory system of profoundly deafened cats. Seventeen adult cats were used: four had normal hearing; 12 were deafened neonatally for periods of < 2.5 years (five bilaterally, seven unilaterally); and one animal had a long-term (approximately 8 years) profound bilateral hearing loss. Bipolar scala tympani stimulating electrodes were bilaterally implanted in each animal, and electrically evoked auditory brainstem responses (EABRs) were recorded in an acute study to evaluate the basic physiologic response properties of the deafened auditory pathway. The cochleae and cochlear nuclei (CN) of each animal were examined with light microscopy. Spiral ganglion cell density in neonatally deafened cochleae was 17% of normal, and only 1.5% of normal in the long-term deaf animal. There was a 46% reduction in total CN volume in neonatally deafened animals compared to normal, and a 60% reduction in the long-term deaf animal. Neural density in the anteroventral CN of bilaterally deafened animals was 37% higher than normal; 44% higher in the long-term deaf animal. Significantly, however, we saw no evidence of a loss of neurones within the anteroventral CN in any deafened animal. There was a significant increase in EABR threshold and wave IV latency in the deafened animals, and a significant decrease in response amplitude and input/output function gradient. Again, these changes were more extensive in the long-term deaf animal. These data show that a sensorineural hearing loss can evoke significant morphological and physiological changes within the cochlea and auditory brainstem, and these changes become greater with duration of deafness. It remains to be seen whether these changes can be reversed following the introduction of afferent activity via chronic electrical stimulation of the auditory nerve.

Epileptic discharge of cortical, subcortical and spinal neurons in penicillin induced experimental epilepsy

The sensitivity and electrophysiological patterns of paroxysmal activity induced in different brain structures by topical application of penicillin-G were evaluated in the rat. Recordings were carried out in five groups of animals, in telencephalon, diencephalon, mesencephalon, rombencephalon and spinal cords. The following analysis were carried out: frequency distribution histograms, latency and time course duration of paroxysmal activity, duration and amplitude of epileptic bursts. The results obtained showed that the nervous structures tested with penicillin-G had a different epileptogenic sensitivity and response pattern which significantly changed along the cerebral cortex-spinal cord axis. The highest epileptic sensitivity was observed in somatosensory cortex (SI) at 500-600 microns depth; in the other cortical layers, a significant lenghtening in latency was observed. Among the other structures, the spinal cord seemed to be the most sensitive target to the epileptogenic action of penicillin-G, whereas in the remaining structures, sensitivity significantly decreased in rostro-caudal direction. As far as the features of the paroxysmal activity are concerned, significant differences among tested structures were observed. In particular, within the SI cortex, the main differences were represented by the gradual increase in burst frequency and voltage from the surface to the IVth layer and by their subsequent decrease in deeper layers (V-VI). In the diencephalon, the paroxysmal activity was similar to that observed in more superficial and deeper cortical layers even though epileptic bursts showed a lower amplitude. Mesencephalon and rombencephalon displayed a paroxysmal activity with a distinctive feature, characterized by long lasting bursts of low amplitude, although bulbar outbursts showed a shorter duration than the mesencephalic ones. In the spinal cord, the epileptiform activity displayed a different paroxysmal pattern, characterized by the longest duration and the highest amplitude. The different sensitivities of the investigated brain structures to penicillin-G and the characteristics of the induced paroxysmal activity have been extensively discussed.

Effects of recurrent hypoglycemia on brainstem function in diabetic BB rats: protective adaptation during acute hypoglycemia

To determine whether antecedent recurrent hypoglycemia protects the brain from the adverse effects of a standardized hypoglycemic stimulus, we implanted electrodes in the inferior colliculi of diabetic rats to directly record inferior colliculi auditory-evoked potentials (ICEPs). Awake, chronically catheterized BB rats were studied after 2 weeks of insulin therapy designed to produce either chronic hyperglycemia (hyper-DM, glycated hemoglobin 7.6 +/- 0.4%) or recurrent hypoglycemia (hypo-DM, glycated hemoglobin 6.2 +/- 0.7%), and the results were compared with those observed in nondiabetic rats. When plasma glucose was lowered to and clamped at 2.8 mmol/l, the release of catecholamines was suppressed in the hypo-DM rats (epinephrine: 2.5 +/- 0.4 nmol/l) as compared with hyper-DM and the nondiabetic rats (9.3 +/- 2.3 and 32.7 +/- 6.1 nmol/l, respectively). ICEP latency was significantly delayed in hyper-DM and nondiabetic rats (P < 0.001), but it was unchanged in hypo-DM rats. A more pronounced reduction in plasma glucose (2.0 mmol/l), however, provoked a greater adrenergic response than that seen at 2.8 mmol/l and delayed ICEP latency by 23% in a separate group of hypo-DM animals. These data demonstrate that antecedent recurrent hypoglycemia attenuates the brainstem dysfunction associated with mild to moderate, but not severe, hypoglycemia in diabetic rats. This phenomenon may contribute to the alterations in hypoglycemia counterregulation seen in diabetic patients during intensive insulin therapy.

Defective auditory recognition after small hemorrhage in the inferior colliculi

We report the case of a male patient with a traumatic small hemorrhage partially involving the bilateral inferior colliculi without evidence of a temporal lobe lesion. He was unable to comprehend spoken words although he had intact speech production, reading and writing abilities. Comprehension of environmental sounds was also affected. Among the receptive musical abilities, discrimination of intensity, tone and rhythm were preserved, while recognition of melody was impaired. Audiometry showed normal thresholds for pure tone. Waves I-IV of brainstem auditory evoked potentials were elicited normally, whereas the wave V was elicited with reduced amplitude and prolonged latencies on both sides. The main component of middle latency auditory evoked potentials, which is evoked over both hemispheres by monaural stimulation to either side in normal subjects, was elicited only over the hemisphere contralateral to the ear receiving stimulation. Our patient's auditory findings were similar to those usually found in generalized auditory agnosia. Auditory agnosia is usually considered as a sign of a bitemporal cortical or subcortical disorder, but, in our patient, a brainstem disorder caused a disturbance of auditory recognition similar to auditory agnosia due to a bitemporal lesion. Our patient's auditory findings may belong to the category of a brainstem auditory-processing disorder brought on by a small hemorrhage in the inferior colliculi. In addition, the impairment in our patient implies that, in the neural processing of musical parameters, the decoding of intensity, tone and rhythm is accomplished at the level of inferior colliculus, whereas further cortical processing is necessary for the appropriate recognition of melody.

[Running fit and generalized tonic-clonic seizure are differently controlled by different subtype receptors in the brainstem]

Rats neonatally treated with 0.02% propylthiouracil (PTU) through mother's milk showed a high incidence of audiogenic seizures after maturation. These audiogenic seizures were differently modified by MK-801 and NBQX; while intraperitoneal MK-801 equally inhibited running fit (RF) and generalized tonic-clonic seizure (GTCS), NBQX administered into cisterna ambiens significantly inhibited RF but not GTCS. The possible involvement of glutamate receptors in the inferior colliculus was further investigated using naive Sprague-Dawley rats injected with NMDA, AMPA or cyclothiazide, known as an inhibitor of desensitization of AMPA action. All drugs tested successfully induced RF followed by GTCS, resembling audiogenic seizures in PTU-treated rats. However, sound stimulation could augment AMPA-induced, but not NMDA-induced GTCS. Systemic administration with MK-801 potently blocked GTCS induced by AMPA/cyclothiazide, but the same drug failed to block RF after intracisternal injection with AMPA/cyclothiazide. Furthermore, intracisternal administration with NBQX significantly inhibited only RF induced by AMPA/cyclothiazide. The present study suggests that: 1) glutamate receptors in the brainstem, possible in the inferior colliculus, play a crucial role in audiogenic seizures, namely the initiation of RF and propagation into GTCS; and 2) the initiation mechanism is regulated by both NMDA and AMPA receptors, whereas propagation is mainly controlled by NMDA receptors.

[Effects of lidocaine on salicylate-induced discharge of auditory neurons in the inferior colliculus of the guinea pig]

The effects of lidocaine (a local anesthetic known to relieve tinnitus) in a guinea pig animal model of tinnitus in which spontaneous discharge of inferior colliculus (IC) neurons was augmented by intravenous application of salicylate (200 mg/kg) were studied by extracellular recording. The salicylate induced discharge was inhibited by intravenous injection of lidocaine at a concentration (lmg/kg) used clinically for treating tinnitus. IC neurons could be divided into two groups according to the difference in sensitivity to lidocaine, weakly sensitive neurons and highly sensitive neurons. In weakly sensitive neurons, the lidocaine effect lasted for less than five min, and the inhibiton of the discharge of neurons was increased as the latency of response to the sound stimulus became longer. It was considered that lidocaine inhibited the propagation of action potentials by blocking Na+ channels. In highly sensitive neurons, on the other hand, the activity of neurons was almost completely inhibited for longer than 30 min and it might involved different mechanisms. Furthermore, the inhibitory action of lidocaine was stronger in IC neurons discharging at higher frequencies, suggesting a use dependent blocking action of Na+ channels, by lidocaine.

Pathophysiological mechanisms of genetic absence epilepsy in the rat

Generalized non-convulsive absence seizures are characterized by the occurrence of synchronous and bilateral spike and wave discharges (SWDs) on the electroencephalogram, that are concomitant with a behavioral arrest. Many similarities between rodent and human absence seizures support the use of genetic rodent models, in which spontaneous SWDs occur. This review summarizes data obtained on the neurophysiological and neurochemical mechanisms of absence seizures with special emphasis on the Genetic Absence Epilepsy Rats from Strasbourg (GAERS). EEG recordings from various brain regions and lesion experiments showed that the cortex, the reticular nucleus and the relay nuclei of the thalamus play a predominant role in the development of SWDs. Neither the cortex, nor the thalamus alone can sustain SWDs, indicating that both structures are intimely involved in the genesis of SWDs. Pharmacological data confirmed that both inhibitory and excitatory neurotransmissions are involved in the genesis and control of absence seizures. Whether the generation of SWDs is the result of an excessive cortical excitability, due to an unbalance between inhibition and excitation, or excessive thalamic oscillations, due to abnormal intrinsic neuronal properties under the control of inhibitory GABAergic mechanisms, remains controversial. The thalamo-cortical activity is regulated by several monoaminergic and cholinergic projections. An alteration of the activity of these different ascending inputs may induce a temporary inadequation of the functional state between the cortex and the thalamus and thus promote SWDs. The experimental data are discussed in view of these possible pathophysiological mechanisms.

Effects of noise on inferior colliculus evoked potentials and cochlear anatomy in young and aged chinchillas

Like many aging humans, the aging chinchilla tends to lose high-frequency sensitivity at a faster rate than low-frequency sensitivity. This feature, combined with its excellent low-frequency hearing, makes the chinchilla attractive as an animal model for studying the relationship between noise-induced hearing loss (NIHL) and age-related hearing loss (AHL). In the present study, we examined susceptibility to noise in 15 aged (10-15 years old) and 15 young chinchillas. Two levels of noise were used, with the aim of determining whether age-related differences exist in the magnitude and rate of recovery from temporary threshold shifts produced by a moderate-level (95 dB) noise exposure, or in susceptibility to permanent threshold shifts and cochlear damage caused by a high-level (106 dB) noise exposure. Thresholds and response amplitudes at 0.5, 1, 2, 4, 8 and 16 kHz were determined from evoked potentials recorded from the inferior colliculus. Cochlear histology was performed on animals exposed to high-level noise. The results suggest that older animals are equally vulnerable to moderate-level noise, but may be slightly more vulnerable to high-level noise. For moderate-level exposures, there appears to be a simple additive relationship (in dB) between AHL and NIHL. For high-level exposures, the relationship may be more complex.

Microstimulation of auditory nerve for estimating cochlear place of single fibers in a deaf ear

Multielectrode cochlear prostheses seek to approximate the cochlea's normal frequency-place mapping through spatial segregation of stimulus currents. Various electrode configurations have been employed to achieve such segregation. Direct measurements of stimulation regions among single auditory nerve (AN) fibers has been possible only when normal hearing is preserved, such that each fiber's cochlear place can be inferred from its tuning curve. This precludes measurements in deafened ears, or ears compromised by implantation of the electrodes. Data presented here demonstrate that the cochlear place of an AN fiber can be estimated without acoustic sensitivity, using electrical microstimulation through a recording pipette in the AN bundle. The procedure exploits cochleotopic projection to isofrequency laminae within the contralateral inferior colliculus (IC). Microstimulation excites a small group of fibers neighboring the recorded fiber, generating centrally propagated volleys along a narrow frequency-specific pathway. Evoked potential recordings at varying depths are made to identify the ICC lamina where the response to AN microstimulation is greatest. Preliminary data are also presented for an alternative method of identifying the lamina using a frequency domain measure of binaural interactions within the IC.

Similarities between chronic pain and tinnitus

OBJECTIVE

The aim of this study is to review hypotheses about the mechanisms of chronic pain and to compare them with that of tinnitus. Hypotheses about the pathophysiology of severe tinnitus and chronic pain have been of mainly two kinds: one of which claims that pathology located in the periphery (the ear for tinnitus, and peripheral nerves for pain) can explain the symptoms, while the other claims that the symptoms are caused by changes in the function of nuclei of the central nervous system.

DATA SOURCES

A search of the literature from the past 35 years was used.

CONCLUSIONS

There is considerable evidence that both chronic pain and some forms of tinnitus are caused by changes in the central nervous system and that the anatomic location of the physiologic abnormality causing the symptoms of chronic pain and some forms of tinnitus is not the same location to which the symptoms are referred, i.e., the ear for tinnitus and the location of injury for pain. Such changes in the central nervous system may have been induced by peripheral processes such as tissue damage, but the changes can persist a long time after complete healing of a peripheral lesion. Different forms of tinnitus may respond to different treatments as is the case for chronic pain. If the different forms of tinnitus cannot be separated, then the results of studies of the efficacy of different kinds of drugs may be misleading.

Combined effects of a simultaneous exposure to noise and toluene on hearing function

To study the combined effects of noise and toluene on auditory function, three experimental groups of Long-Evans adult rats were used. The first group was exposed to toluene (2000 ppm, 6 h/day, 5 days/week, 4 weeks), the second group to an octave band of noise centered at 8 kHz (92 dB SPL), and the last group to a simultaneous exposure to toluene and noise. Auditory function was tested by recording brainstem (inferior colliculus) auditory-evoked potentials. The auditory deficit induced by the combined exposure exceeded the summated losses caused by toluene alone and by noise alone within the range (2-32 kHz) of test frequencies. The nature of the cochlear damage induced by noise alone (injured stereocilia) or by toluene alone (outer hair cells loss) is different.

Audiogenic seizures after neck tourniquet-induced cerebral ischemia in the rat

Development of audiogenic seizures (AGS) and their correlation with neurodegeneration were studied after 7.5 min of whole-brain ischemia. One day post-ischemia, all animals became hyperreactive and responded to auditory stimulation by generalized seizures. Neuronal necrosis developed already 6 h post-ischemia in inferior colliculi, reticular thalamic nucleus and hippocampal hilar region. Repeated ischemia did not induce any neurological changes, suggesting that the neurological effects are consequences of selective neuronal injury.

Afferent projection patterns in the auditory brainstem in normal and congenitally deaf white cats

Cochlear implantation in congenitally deaf children is developing to a successful medical tool. Little is known, however, on morphology and pathophysiology of the central auditory system in these auditory deprived children. One form of congenital hearing loss, that seen in the deaf white cat, was investigated to see if there are differences in the afferent pathways from the cochlear nuclei to the inferior colliculus. The retrogradely transported fluorescent tracer diamidino yellow (DY) was injected into different parts of the central nucleus of the inferior colliculus (ICC) of normal cats and deaf white cats. It was found that the main afferent projection patterns in deaf white cats were unchanged in spite of congenital auditory deprivation; minor differences were seen.