Classification of subpopulations of neurons in the cochlear nuclei of the kangaroo rat

Inhibitory neurotransmission, plasticity and aging in the mammalian central auditory system

Aging and acoustic trauma may result in partial peripheral deafferentation in the central auditory pathway of the mammalian brain. In accord with homeostatic plasticity, loss of sensory input results in a change in pre- and postsynaptic GABAergic and glycinergic inhibitory neurotransmission. As seen in development, age-related changes may be activity dependent. Age-related presynaptic changes in the cochlear nucleus include reduced glycine levels, while in the auditory midbrain and cortex, GABA synthesis and release are altered. Presumably, in response to age-related decreases in presynaptic release of inhibitory neurotransmitters, there are age-related postsynaptic subunit changes in the composition of the glycine (GlyR) and GABA(A) (GABA(A)R) receptors. Age-related changes in the subunit makeup of inhibitory pentameric receptor constructs result in altered pharmacological and physiological responses consistent with a net down-regulation of functional inhibition. Age-related functional changes associated with glycine neurotransmission in dorsal cochlear nucleus (DCN) include altered intensity and temporal coding by DCN projection neurons. Loss of synaptic inhibition in the superior olivary complex (SOC) and the inferior colliculus (IC) likely affect the ability of aged animals to localize sounds in their natural environment. Age-related postsynaptic GABA(A)R changes in IC and primary auditory cortex (A1) involve changes in the subunit makeup of GABA(A)Rs. In turn, these changes cause age-related changes in the pharmacology and response properties of neurons in IC and A1 circuits, which collectively may affect temporal processing and response reliability. Findings of age-related inhibitory changes within mammalian auditory circuits are similar to age and deafferentation plasticity changes observed in other sensory systems. Although few studies have examined sensory aging in the wild, these age-related changes would likely compromise an animal's ability to avoid predation or to be a successful predator in their natural environment.

Aged-related loss of temporal processing: altered responses to amplitude modulated tones in rat dorsal cochlear nucleus

Loss of temporal processing is characteristic of age-related loss of speech understanding observed in the elderly. Inhibitory glycinergic circuits provide input onto dorsal cochlear nucleus (DCN) projection neurons which likely serve to modulate excitatory responses to time-varying complex acoustic signals. The present study sought to test the hypothesis that age-related loss of inhibition would compromise the ability of output neurons to encode sinusoidally amplitude modulated (SAM) tones. Extracellular recordings were obtained from young and aged FBN rat DCN putative fusiform cells. Stimuli were SAM tones at three modulation depths (100, 50, and 20%) at 30 dB hearing level with the carrier frequency set to the unit's characteristic frequency. Discharge rate and synchrony were calculated to describe SAM responses. There were significant age-related changes in the shape and peak vector strength [best modulation frequency (BMF)] of temporal modulation transfer functions (tMTFs), with no significant age-related changes in rate modulation transfer functions (rMTFs) at BMF. Young neurons exhibited band-pass tMTFs for most SAM conditions while aged fusiform cells exhibited significantly more low-pass or double-peaked tMTFs. There were significant differences in tMTFs between buildup, pauser-buildup, and wide-chopper temporal response types. Young and aged wide-choppers displayed significantly lower vector strength values than the other two temporal DCN response types. Age-related decreases in the number of pauser-buildup response types and increases in wide-chopper types reported previously, could account, in part, for the observed loss of temporal coding of the aged fusiform cell. Age-related changes in SAM coding were similar to changes observed with receptor blockade of glycinergic inhibition onto fusiform cells and consistent with previously observed age-related loss of endogenous glycine levels and changes in normal adult glycine receptor function. DCN changes in SAM coding could, in part, underpin temporal processing deficits observed in the elderly.

Age-related changes in the inhibitory response properties of dorsal cochlear nucleus output neurons: role of inhibitory inputs

Age-related hearing loss frequently results in a loss in the ability to discriminate speech signals, especially in noise. This is attributable, in part, to a loss in temporal resolving power and ability to adjust dynamic range. Circuits in the adult dorsal cochlear nucleus (DCN) have been shown to preserve signal in background noise. Fusiform cells, major DCN output neurons, receive focused glycinergic inputs from tonotopically aligned vertical cells that also project to the ventral cochlear nucleus. Glycine-mediated inhibition onto fusiform cells results in decreased tone-evoked activity as intensity is increased at frequencies adjacent to characteristic frequency (CF). DCN output is thus shaped by glycinergic inhibition, which can be readily assessed in recordings from fusiform cells. Previous DCN studies suggest an age-related loss of markers for glycinergic neurotransmission. The present study postulated that response properties of aged fusiform cells would show a loss of inhibition, resembling conditions observed with glycine receptor blockade. The functional impact of aging was examined by comparing response properties from units meeting fusiform-cell criteria in young and aged rats. Fusiform cells in aged animals displayed significantly higher maximum discharge rates to CF tones than those recorded from young-adult animals. Fusiform cells of aged rats displayed significantly fewer nonmonotonic CF rate-level functions and an age-related change in temporal response properties. These findings are consistent with an age-related loss of glycinergic input, likely from vertical cells, and with findings from other sensory aging studies suggesting a selective age-related decrement in inhibitory amino acid neurotransmitter function.

Hazard functions and expected spike density functions for neuron spike activity in the cochlear nucleus of the cat

The goal of these experiments was to evaluate the effect of stimulus evoked input and post spike refractoriness on the shapes of post stimulus time histograms (PSTHs). The time courses of spontaneous and/or evoked activity were studied in 153 neurons located predominantly in the dorsal cochlear nucleus in cats anesthetized with Nembutal. Tone bursts were presented to the ipsilateral ear in a free sound field. About half the cells were characterized by the pauser/build-up type of PSTH. Marked refractoriness was evidenced by relatively long recovery times of the hazard functions of spontaneous and tone-evoked spike activity. On presentation of tonal bursts, the time dependence of the probability of the first spike in the absence of a preceding spike (expected spike density function) was greater than the PSTH (actual spike density function). The initial PSTH peak with pause was shaped primarily by stimulus evoked input, whereas refractoriness tended to diminish the build-up portion of the PSTH. In chopper cells, PSTH peaks were usually not reflected in expected spike density functions showing that post spike refractoriness plays a major role in shaping the PSTH. In primary-like cells, refractoriness was small and had little effect on the shape of the PSTH. Some presumptively inhibitory cells showed a tendency to burst discharges with non-monotonic hazard functions. A very small number of cells showed a tendency to internal tuning to a defined signal periodicity.

Parallel auditory pathways: projection patterns of the different neuronal populations in the dorsal and ventral cochlear nuclei

The cochlear nuclear complex gives rise to widespread projections to nuclei throughout the brainstem. The projections arise from separate, well-defined populations of cells. None of the cell populations in the cochlear nucleus projects to all brainstem targets, and none of the targets receives inputs from all cell types. The projections of nine distinguishable cell types in the cochlear nucleus-seven in the ventral cochlear nucleus and two in the dorsal cochlear nucleus-are described in this review. Globular bushy cells and two types of spherical bushy cells project to nuclei in the superior olivary complex that play roles in sound localization based on binaural cues. Octopus cells convey precisely timed information to nuclei in the superior olivary complex and lateral lemniscus that, in turn, send inhibitory input to the inferior colliculus. Cochlear root neurons send widespread projections to areas of the reticular formation involved in startle reflexes and autonomic functions. Type I multipolar cells may encode complex features of natural stimuli and send excitatory projections directly to the inferior colliculus. Type II multipolar cells send inhibitory projections to the contralateral cochlear nuclei. Fusiform cells in the dorsal cochlear nucleus appear to be important for the localization of sounds based on spectral cues and send direct excitatory projections to the inferior colliculus. Giant cells in the dorsal cochlear nucleus also project directly to the inferior colliculus; some of them may convey inhibitory inputs to the contralateral cochlear nucleus as well.

Elevated fusiform cell activity in the dorsal cochlear nucleus of chinchillas with psychophysical evidence of tinnitus

Chinchillas with psychophysical evidence of chronic tinnitus were shown to have significantly elevated spontaneous activity and stimulus-evoked responses in putative fusiform cells of the dorsal cochlear nuclei (DCN). Chinchillas were psychophysically trained and tested before and after exposure to a traumatic unilateral 80 dB (sound pressure level) 4 kHz tone. Before exposure, two groups were matched in terms of auditory discrimination performance (noise, and 1, 4, 6, and 10 kHz tones). After exposure, a single psychophysical difference emerged between groups. The exposed group displayed enhanced discrimination of 1 kHz tones (p = 0.00027). Postexposure discrimination of other stimuli was unaffected. It was hypothesized that exposed animals experienced a chronic subjective tone (i.e., tinnitus), resulting from their trauma, and that features of this subjective tone were similar enough to 1 kHz to affect discrimination of 1 kHz objective signals. After psychophysical testing, single-unit recordings were obtained from each animal's DCN fusiform cell layer. Putative fusiform cells of exposed animals showed significantly (p = 0.0136) elevated spontaneous activity, compared with cells of unexposed animals. Putative fusiform cells of exposed animals showed a greater stimulus-evoked response to tones at 1 kHz (p = 0.0000006) and at characteristic-frequency (p = 0.0000009). This increased activity was more pronounced on the exposed side. No increase in stimulus-evoked responses was observed to other frequencies or noise. These parallel psychophysical and electrophysiological results are consistent with the hypothesis that chronic tonal tinnitus is associated with, and may result from, trauma-induced elevation of activity of DCN fusiform cells.

Modulation of spontaneous activity by acetylcholine receptors in the rat dorsal cochlear nucleus in vivo

In vitro studies have implicated muscarinic acetylcholine receptors (mAChRs) in the modulation of spontaneous activity (SA) of neurons in the rat dorsal cochlear nucleus (DCN) (Chen et al., 1994,1998). Early studies suggest that cholinergic pathways also modulate SA in vivo, but these effects have not been investigated pharmacologically. The purpose of the present study was to determine whether multiunit SA can be modulated in vivo by application of cholinergic agents to the surface of the DCN. Sprague Dawley rats were used in the current experiment. The influence of cholinergic activation on SA was tested by applying carbachol (5-500 microM) to the DCN surface while recording multiunit SA at a depth of 250 microm. Out of a total of 32 sites tested, all but 2 (94%) showed well-defined responses to carbachol, characterized by suppression, activation or a combination of both (two-component responses). The most common responses were pure suppression and suppression accompanied by transient activation. Both the proportion of sites showing suppressive responses and the magnitude of suppression averaged across sites increased with dose. Although the proportion of sites showing pure activation in response to carbachol decreased with dose, there was no clear trend in the magnitude of activation with dose. The suppressive responses to high doses of carbachol were blocked by pre-application of atropine. These results extend previous work by suggesting that muscarinic receptors play an important role in the modulation of SA in vivo.

Exposure to low frequency noise during rearing induces spongiform lesions in gerbil cochlear nucleus: high frequency exposure does not

Spongiform lesions of the gerbil cochlear nucleus are reduced in number and extent by rearing in acoustic isolation compared with rearing while exposed to normal colony low-frequency background noise. This study tested whether rearing under exposure to noise bands of moderate intensity would increase the number and extent of cochlear nucleus spongiform lesions. Gerbils were reared from weaning to young adulthood in acoustic isolation chambers while continually exposed to moderately intense bands of either high frequency or low frequency noise. Exposure to low frequency noise resulted in lesion number and area densities that were more than twice those seen in gerbils exposed to high frequency noise. Lesion extent in the low frequency group was similar to that in colony-reared gerbils; lesion extent in the high frequency group was similar to gerbils reared in acoustic isolation. Comparisons within the posterior ventral cochlear nucleus revealed that the differences in lesion extent were most pronounced in the middle and dorsal-medial portions, the regions that are most responsive to middle and high frequencies. These finding suggest that the regional restriction of spongiform lesions within the cochlear nucleus does not have a tonotopic basis.

Glycine response in isolated dorsal cochlear nucleus of C57BL/6J mouse

Pharmacological properties of glycine (Gly)-induced Cl- current (ICl) in the dorsal cochlear nucleus (DCN) neurons acutely dissociated from C57BL/6J mouse were investigated in the whole-cell configuration of the patch-clamp technique. Gly-induced ICl increased in a sigmoidal manner with higher Gly concentrations. Strychnine blocked the Gly response competitively at low and non-competitively at high concentrations. Both glutamate (Glu) and N-methyl-D-aspartate (NMDA) responses were augmented by adding 10(-6) M Gly, at which concentration Gly did not induce any ICl. This facilitation was not affected by strychnine. Our results clearly show the existence of strychnine-sensitive and -insensitive glycine receptors in the DCN neurons.

Encoding of amplitude modulation in the gerbil cochlear nucleus: II. Possible neural mechanisms

Rapid changes in sound amplitude--amplitude modulation (AM)--comprise an important feature of biologically-relevant sounds, including speech. In the companion paper, a hierarchy of enhancement for AM processing was demonstrated for unit types of the gerbil ventral cochlear nucleus (VCN) [Frisina, et al., Hear. Res. 44, 1990]. In the present report additional neurophysiological findings are presented as an initial test of alternative hypotheses of how VCN unit types amplify or enhance AM information, and how they accomplish this over a wide intensity range. These hypotheses invoke mechanisms such as off-CF excitatory or inhibitory inputs, input from high-threshold auditory-nerve fibers, amplification of residual AM responses of auditory-nerve fibers at high intensities, or post-synaptic cell feedback. From consideration of VCN unit response properties such as onset and steady-state rate-intensity functions, pure-tone tuning, and non-CF responses to AM, it is concluded that: Off-CF excitatory inputs do not play a significant role in VCN AM encoding; Off-CF inhibitory inputs could work in conjunction with one or more of the other proposed mechanisms to account for differential enhancement of AM by VCN neurons.

Responses of single units in the anteroventral cochlear nucleus of the guinea pig

Single unit responses have been recorded from the anteroventral cochlear nucleus of the anaesthetised guinea-pig. For each unit a response profile was obtained consisting of spike waveform shape, suprathreshold post-stimulus time histogram at characteristic frequency, frequency/intensity response area, a measure of phase-locking and where possible variation in post-stimulus time histogram shape as a function of position within the response area. Units were classified according to schemes based on both post-stimulus time histogram shape and response area. The majority of units with Type I response areas were primarylike and most with Type III response areas were choppers. One-to-one correspondence between the two classification schemes was found for units which were classified as onset by the post-stimulus time histogram scheme and Type I/III by the response area scheme. Primarylike units with a prepotential in their spike waveform most faithfully preserved the temporal information (as measured by phase-locking) present in the auditory nerve input. Primarylike units in which a prepotential was not detected showed varying abilities to phase-lock. Non-primarylike units do not phase-lock as well as auditory nerve fibres in the same species. Nonmonotonic rate-level functions for tones at characteristic frequency were observed across all unit types (with the exception of onset units) classified by the post-stimulus time histogram scheme. An unexpected finding was a small number of primarylike units characterised by reduced driven discharge rates within their response areas. We hypothesize that the mechanism for this reduction is centre-band inhibition.

Encoding of amplitude modulation in the gerbil cochlear nucleus: I. A hierarchy of enhancement

The main goal of the present study was to investigate the encoding of a biologically-relevant acoustic feature--amplitude modulation (AM)--in single neurons of the auditory nerve and ventral cochlear nucleus (VCN). In the anesthetized gerbil auditory-nerve fibers and VCN units show strong synchronous responses to low-intensity, low-frequency AM. As frequency increases, the strength of the synchronous response decreases. In the auditory nerve the strength of the synchronous response is substantially less at high intensities than at low intensities and does not change significantly with AM frequency at high intensities. In contrast to the auditory nerve, VCN units show strong responses at high intensities. They have a particular AM frequency to which they are maximally responsive, and this frequency varies from unit to unit. Therefore, VCN units transform their ascending inputs by enhancing the synchronous response to AM. A correlation exists between a unit's ability to encode AM and its responses to simple sounds. Specifically, onset units show the strongest synchronous responses, followed in order by chopper, primarylike-with-notch and primarylike units. This enhancement is greatest at high intensities and can occur up to 90 dB above a unit's threshold. Thus, a hierarchy of enhancement for AM processing exists in the most peripheral nucleus of the central auditory system.

Synaptic potentials of chinchilla lateral superior olivary neurons

Neurons in the lateral superior olive (LSO) were characterized in vivo, by extracellular and intracellular recordings. Principal neurons of the LSO are excited by ipsilateral auditory stimuli and exhibit binaural inhibition, as observed in extracellular recordings. In subsequent intracellular recordings, ipsilateral acoustic stimuli evoked robust excitatory postsynaptic potentials (epsps), while contralateral stimuli evoked large inhibitory postsynaptic potentials (ipsps). The contralaterally evoked ipsps were reversed when the cell was polarized below resting membrane potential and when current was injected into neurons recorded with chloride-filled electrodes. The ipsp is probably a reflection of contralaterally evoked release of glycine acting through glycinergic receptors on the somata and proximal dendrites of these neurons. The properties of the epsps are consistent with data suggesting that ipsilaterally evoked excitation may be mediated by an excitatory amino acid-like substance acting through quisqualate or kainate receptors at dendritic locations.

A degenerative disorder of the central auditory system of the gerbil

A previously unidentified disorder which affects primarily the cochlear nucleus was observed in two species of gerbils, Meriones unguiculatus and M. libycus. Unusual lesions were observed in the cochlear nucleus bilaterally in all animals examined. In light and electron microscopic specimens these lesions were characterized by the presence of microcysts and vacuolar neuronal degeneration. The microcysts resembled large holes, containing trabeculae, organelles and cellular remnants. Also observed were light and dark degeneration of neuronal perikarya and degenerated axons, dendrites, and synapses, accompanied by phagocytosis. Astrocytosis was not conspicuous. In the one cochlea examined, no microcysts were observed. In young animals the microcysts were prevalent in the cochlear nerve root region and the posteroventral cochlear nucleus. In older animals the microcysts increased in number and area. In the oldest animals, the microcysts had spread to other central auditory structures, including the superior olivary complex, the nuclei of the lateral lemniscus, and the inferior colliculus. Other regions of the brain were largely free of microcysts. The etiology and behavioral manifestations of this disorder are unknown, although it is clearly neurodegenerative and perhaps genetically determined.

Strychnine alters the fusiform cell output from the dorsal cochlear nucleus

Anatomical and physiological evidence suggests that fusiform cells, the major output neurons of the dorsal cochlear nucleus (DCN), receive significant inhibitory input. Fusiform cells often display strongly non-monotonic rate-intensity functions and pauser-buildup or buildup tone-evoked temporal responses, patterns which may be mediated by inhibitory neurotransmitters. Other neurons located within the fusiform cell layer or in the more superficial molecular layer display varied rate-intensity functions and temporal responses. Neurons displaying response properties characteristic of fusiform cells are sensitive to iontophoretic application of the inhibitory amino acid neurotransmitter, glycine. Application of the glycine receptor antagonist, strychnine, alters the non-monotonic portion of the rate-intensity function at doses which do not alter spontaneous activity or near-threshold tone-evoked responses. These neurons are also sensitive to GABA and the GABAB agonist, (-)-baclofen, but are insensitive to the GABAA antagonist, bicuculline. DCN neurons which display monotonic rate-intensity functions and temporal response properties different than those associated with fusiform cells are sensitive to bicuculline, (-)-baclofen, and GABA. These data suggest that a glycinergic input onto fusiform cells may control the non-monotonic nature of the response of these neurons near characteristic frequency and therefore may contribute significantly to the nature of the output of the DCN.

Spectral and temporal response patterns of single units in the chinchilla dorsal cochlear nucleus

Spectral and temporal response patterns to pure-tone stimuli were collected from single units in the dorsal cochlear nucleus of anesthetized chinchillas. The spectral response profiles were divisible into groups based on the balance of excitation and inhibition. Temporal responses were characterized in chloralose-anesthetized animals by collecting PST-histograms. There appeared to be no simple one-to-one relationship between a unit's spectral and its temporal response pattern. Excitatory spectral responses were generally sharply tuned areas resembling those of auditory nerve fibers. However, unlike the latter, the majority of these had chopper or pauser/buildup temporal responses. Inhibitory spectral responses were of two distinct types: one included lateral inhibitory areas flanking the tuned excitatory areas which occasionally invaded the latter creating a nonmonotonic excitatory response at the unit's characteristic frequency. The other included sharply tuned inhibitory areas. The characteristic frequencies of these units were found to be in close correspondence with those of sharply tuned excitatory units from the same penetration suggesting that these inhibitory units were tonotopically mapped in the same register as tuned excitatory units. The spectral response patterns were studied with three types of anesthesia: ketamine/xylazine, dial/urethane, and chloralose. In each of these groups the patterns were similar. However, the proportions of units showing inhibition was strongly dependent on the choice of anesthetic agent with chloralose yielding the highest proportions (59%) and ketamine/xylazine yielding the lowest (29%).

Ventral cochlear nucleus neural discharge characteristics in the absence of outer hair cells

The role of the cochlear outer hair cell (OHC) in auditory processing remains poorly understood. The OHCs possess an independent afferent innervation which constitutes 5-10% of cochlear afferent neurons and which appears to project to the cochlear nucleus (CN). Whether the OHCs contribute to the processing of auditory signals in the CN has not been determined. To address this question, kanamycin ototoxicity was used to produce selective OHC loss while leaving the inner hair cell (IHC) population largely intact, in the basal portion of the cochlea of chinchillas. Single unit responses were then recorded in the ventral cochlear nucleus (VCN), and compared to responses in untreated subjects. Many of the changes observed in VCN neural responses reflected changes which have previously been reported in the VIIIth nerve. However, frequency tuning curve tip segments which were normal in both bandwidth and length were observed in approximately 22% of the units associated with regions of complete OHC loss and preservation of IHCs. This has not been reported in previous OHC lesion studies. Also, first spike latency was found to be significantly lengthened for units associated with the OHC free regions. Those features of VCN neural responses which first arise within the CN, such as non-primary-like post-stimulus-time histogram response patterns, were unaffected by OHC loss. These results suggest that afferent fibers associated with OHCs do not play a major role in signal processing in the VCN.

Functional organization of the cochlear nucleus of rufous horseshoe bats (Rhinolophus rouxi): frequencies and internal connections are arranged in slabs

The functional organization of the cochlear nucleus (CN) was studied with physiological recording and anatomical tracing techniques. Recordings were made from single CN neurons to examine their temporal firing patterns to tone burst stimuli and their frequency tuning characteristics. Recording loci of individual neurons were carefully monitored in order to understand how the functional properties of a cell relate to its location within the CN. We found that tonal frequencies were systematically represented in each of the three CN divisions (anteroventral, AVCN; posteroventral, PVCN; dorsal, DCN). Eight temporal response patterns were observed in CN neurons when stimulated at units' best excitatory frequencies (BF). With a few exceptions, neurons in each CN division could generate all eight firing patterns with different distributions for the three division. A focal injection of horseradish peroxidase (HRP), at the end of the physiological study, to a group of neurons possessing a similar BF in one CN division resulted in anterograde labeling of nerve terminals in the other two divisions at precisely the areas where the same frequency band was processed in these divisions. Labeled terminals in each division were closely congregated in the form of a thin slab. The slab orientation was division specific whereas its location was frequency specific, which could be predicted on the basis of physiological data. HRP injections into the DCN also resulted in retrograde labeling of somata in the AVCN and PVCN. On the other hand, only DCN neurons were retrogradely labeled when HRP was injected into the AVCN or the PVCN. These data showed how the three CN divisions are internally connected. Furthermore, retrogradely labeled cells occupied the same slabs where we found anterogradely labeled nerve terminals. Additionally, in a group of bats, HRP was injected into various functionally (i.e., BF) identified regions of the central nucleus of the inferior coliculus (IC) to clarify the type and location of CN projecting neurons. Retrogradely labeled cells in individual CN divisions likewise were arranged in slabs whose locations in the CN nuclei depended on the BFs of neurons at the injection site in the IC. These results show that slabs represent units of functional organization (i.e., tonal frequency, local connection and central projection) in the CN.

Baclofen reduces tone-evoked activity of cochlear nucleus neurons

Recent evidence suggests that an excitant amino acid may be a neurotransmitter at acoustic nerve synapses in cochlear nucleus (CN). Release of excitant amino acids is reportedly reduced by baclofen, a lipophilic GABA-mimetic used to treat the spasticity of multiple sclerosis and spinal injury. Microiontophoresis of (-)baclofen suppressed spontaneous and tone-evoked activity in CN neurons. GABA inhibited the responses of most neurons responsive to (-)baclofen. However, iontophoresis of these two substances onto the same CN neuron resulted in dramatic differences in time course to maximum effect and to recovery. Onset and offset of (-)baclofen-induced firing reduction were gradual at all doses (currents), but even the highest doses rarely caused total suppression of firing. Inhibition of firing by GABA was abrupt, and total suppression was frequently observed over the range of doses used. GABA desensitization (fading) commonly occurred while the (-)baclofen response never faded. The same CN neurons were also suppressed by D-alpha-aminoadipate, which blocks certain excitatory amino acid receptors, while the GABA antagonist bicuculline had no effect on the (-)baclofen response. These findings support the hypothesis that an excitant amino acid may be a transmitter at acoustic nerve synapses in CN.

Effects of acetylcholine on cochlear nucleus neurons

Iontophoretic application of acetylcholine (ACh) onto neurons in the dorsal cochlear nucleus (DCN) resulted in an inhibition of the tone-evoked responses of 85% of neurons which were affected. That effect in the DCN contrasts with the predominance of excitatory effects of ACh seen in ventral cochlear nucleus (VCN) neurons. The ACh-induced inhibition in the DCN had a considerably slower onset and time course of recovery than that seen with glycine-induced inhibition. The degree of ACh effects was constant with increasing intensity or attained a maximum effect at 20 to 30 dB above best-frequency threshold in contrast to glycine, which had effects that were relatively greater at low intensities. These findings suggested a modulatory role for ACh at DCN and VCN synapses.

Physiological response properties of cells labeled intracellularly with horseradish peroxidase in cat dorsal cochlear nucleus

The physiology and morphology of fusiform cells in the dorsal cochlear nucleus were studied using extracellular and intracellular recording and intracellular injection of horseradish peroxidase. Fusiform cells displayed a variety of responses to tone pips presented at the characteristic frequency; most often these cells exhibited the pauser/buildup pattern defined in earlier studies. The response pattern of each neuron was dependent on frequency and sound-pressure level. Tone pips evoked short-lasting depolarizations of about 10 mV and long-lasting hyperpolarizations of about 10 mV in cells whose resting potentials were -50 to -65 mV. The time courses of both the excitation and the inhibition depended on frequency and sound-pressure level. Generally the depolarization was sustained for the duration of the tone pip, whereas the hyperpolarization could last as long as 600 ms after the end of the tone pip. Often a neuron exhibited a sustained chopper pattern after microelectrode impalement. This was probably a result of a decrease in membrane potential which altered the relative effectiveness of the excitatory and inhibitory inputs. The large, bitufted fusiform cells had many apical dendrites, which branched one to five times and were covered with spines, and fewer basal dendrites, which exhibited little branching and had few appendages. The morphology of fusiform cells varied systematically as a function of location within the dorsal cochlear nucleus. Response patterns for tone pips were not exclusive to individual cell types as two nonfusiform cells were found to exhibit a buildup pattern. Axons of injected neurons left the nucleus via the dorsal acoustic stria and 14 of 15 had collaterals within the dorsal cochlear nucleus.

Synaptic excitation of the second and third order auditory neurons in the avian brain stem

Synaptic potentials were examined in the second- and third-order auditory neurons of nucleus magnocellularis and nucleus laminaris in the chick. Brain stems of mature chick embryos were explanted and maintained in vitro for 4 to 8 h. Field potentials, extracellular spike potentials and intracellular potentials evoked by 8th-nerve stimulation were examined. Eighth-nerve stimulation reliability elicited four identifiable field potentials which could be attributed to: (i) the afferent volley of the 8th-nerve axons, (ii) postsynaptic responses of n. magnocellularis neurons, and (iii) ipsilaterally and, (iv) contralaterally-evoked n. laminaris postsynaptic responses. Intracellular-recorded postsynaptic potentials were characterized by a rapid rise time and short duration. They were apparently monosynaptic with a synaptic delay of 0.4 ms. In each n. magnocellularis neuron the 'fast' excitatory postsynaptic potentials were composed of 1 to 3 all-or-none components. 'Slow' excitatory postsynaptic potentials were characterized by a longer latency, a longer duration and graded amplitude variation in proportion to the intensity of 8th-nerve stimulation. Both 'fast' and 'slow' excitatory postsynaptic potentials had similar reversal potentials. Since the 8th nerve makes monosynaptic connection with n. magnocellularis neurons, it is likely that at this synapse the 'fast' excitatory postsynaptic potentials were produced, while the 'slow' potential may be attributable to the convergence of many boutonal synapses of unknown origin. Intracellular injections of horseradish peroxidase into n. magnocellularis revealed that its efferents bifurcate below the nucleus and send one axon to the contralateral n. laminaris while the other axon forms a highly divergent projection to the ipsilateral laminar nucleus. The intracellular records obtained from n. laminaris are consistent with this anatomical finding in that graded excitatory postsynaptic potentials were elicited by 8th-nerve stimulation.

Anatomy and physiology of the gerbil cochlear nucleus: an improved surgical approach for microelectrode studies

A new, improved surgical approach to the cochlear nucleus is developed in the gerbil. This new approach involves making a small hole in the lateral wall of the temporal bone located within the perimeter of the superior semicircular canal. Microelectrodes are passed through the intact parafloccular lobe of the cerebellum to the cochlear nucleus. One advantage of the new approach is that no removal of any CNS vasculature or neural tissue is necessary. Relations between the bulla, temporal bone and cochlear nucleus are presented in detail. The new approach is demonstrated by making single unit recordings from the cochlear nucleus and classifying response patterns as measured in PST histograms. All of the response types found in cat are found in the gerbil.

Glutamate and aspartate: alteration of thresholds and response patterns of auditory neurons

Iontophoretic application of the excitant amino acids glutamate and aspartate onto neurons in the chinchilla cochlear nucleus results in a lowering of the threshold of response to auditory stimuli. Neurons that display 'on'-type phasic responses to toneburst stimuli may become tonic, sustained responders with iontophoretic application of glutamate or aspartate. The ability of either glutamate or aspartate to effect changes in thresholds and response patterns of cochlear nucleus neurons is further evidence that one of these amino acids may be the afferent transmitter of the auditory nerve. The effects seen with these excitant amino acids may also provide insight into the underlying synaptic events involved in the generation of a particular response pattern.

Neural phase-locking properties in the absence of cochlear outer hair cells

A combined regimen of kanamycin sulfate treatment (175 mg/kg/day) and behavioral evaluation of resulting audiometric threshold shifts was used to produce selective outer hair cells (OHC) loss in chinchillas. This protocol resulted in a 3-7 mm region in the cochlear base in which OHCs were completely absent and inner hair cells (IHCs) were largely resent and normal at both light and electron microscopic levels. Partial OHC loss was associated with audiometric threshold shifts in excess of 15 dB, while complete OHC loss was associated with audiometric threshold shifts in excess of 40 dB. After recovery periods of at least three weeks, phase-locking was examined across frequency for auditory nerve (VIIIth nerve) and ventral cochlear nucleus (VCN) neurons. The frequency range for neural phase-locking in normal subjects extended up to approximately 4 kHz for VIIIth nerve fibers and 3 kHz for VCN neurons. Following kanamycin intoxication, however, the frequency range for neural phase-locking in both of these auditory regions varied with characteristic frequency (CF): neurons whose CF corresponded to normal cochlear regions exhibited phase-locking throughout the normal frequency range; neurons whole CF corresponded to cochlear regions with selective OHC loss exhibited a marked reduction in the frequency range over which they could phase-lock.

Transmission delay of phase-locked cells in the medial geniculate body

Over 4000 single unit recordings were obtained from the medial geniculate body (MGB) of nitrous oxide anaesthetized cats. Out of 1600 cells sensitive to tone bursts below 4 kHz, 10% were responding in a sustained manner. From these, 121 were tested for phase-locked responses. The general characteristics of these units have been described in a previous report. The central tendency of the discharges distribution within the period or mean phase angle was studied for many frequencies in 24 phase-locked units. For each of them, the mean phase angle shifts linearly with the frequency. The slope of these phase versus frequency lines is an accurate measure of the transmission delay from the cochlea to the MGB. This delay is a function of the unit's characteristic frequency and shows that the time spread introduced by the cochlea between the high and low frequency components of an acoustic signal is preserved up to the MGB. Subtracting the cochlear delay from this overall delay, the neural delay from the eighth nerve to the MGB was found to be 6.4 ms for neurons having a CF above 300 Hz; it was greater by 3 ms for cells with a CF below that frequency.

Frequency-following responses in primary auditory and reticular formation structures

The responses of the cat brain to tonal stimuli were recorded from the inferior colliculus, medial geniculate, reticular formation and the far field. The response consisted of an onset component and a frequency-following response (FFR) component in the inferior colliculus (IC) and the far field. In contrast to previous work, the FFR was also observed in the reticular formation. The response in the reticular formation was abolished at lower doses of pentobarbital and at lower relative intensities of masking than that in the IC and far field. The amplitude of the FFR increased and the latency decreased with progressive ventral movement of the electrode through the IC. The onset component of the response in IC was more easily masked than the FFR component, while the FFR component was depressed to a somewhat greater extent by pentobarbital administration. These findings suggest that the different components of the response to tonal stimuli are generated by different mechanisms.

Differentiation of nerve endings in the cochlear nucleus on morphological and experimental basis

The axo-somatic terminals which synapse with the large spherical cells in the antero-ventral cochlear nucleus of the guinea pig are subdivided into three groups by means of different stereological form of their synaptic vesicles. Unilateral stimulation as well as long-term degeneration of the cochlear nerve produce significant changes of the cross sectional area of the terminals and the number of their mitochondria, when compared to the contralateral control. Both parameters are increased after stimulation and decreased following degeneration. These changes occur in different extent in the three groups of terminals. The discussion of these findings has led to the following conclusions: (1) The volume of synaptic terminals and the number of the enclosed mitochondria display the most pronounced plastic changes. Therefore these parameters seem to be very indicative for the study of function-related changes. (2) The volume of terminals, the quantity of their mitochondria, and the total sum of membranes may be increased under stimulation by actual acceleration of the axoplasmatic flow rate and by incorporation of the preterminal portion of the axon. (3) The presence of at least three types of terminals is evident from the different form of their synaptic vesicles and in addition, from their different response to our experimental conditions. It is not decided from the present findings, whether the bouton-like terminals and bulbs of the afferent cochlear nerve axons contain the same type of synaptic vesicles or whether these axons produce heterotypic terminals.

Survey of intracellular recording in the cochlear nucleus of the cat

Intracellular recordings were made in the cochlear nucleus of anesthetized cats. In anterior passes, one never obtained sustained depolarizations from 'primary-like' units. For 'chopper' units, however, it was possible to record sustained depolarizations accompaneid by spikes that lasted as long as the tone burst. 'Pauser, 'buildup' and 'on' units also had spike responses that could be accompanied by sustained depolarizations. For 'pauser', 'buildup' and 'on' units, hyperpolarization was not seen during the times when no spike discharges appeared so long as the tone bursts were at the characteristic frequency of the units.

Response characteristics of cochlear nucleus neurons to vowel sounds

Most studies in auditory neurophysiology have utilized tonal stimuli to determine the coding properties of neurons in the cochlear nuclei. In this investigation of the kangaroo rat, cochlear nuclei, neuronal responses to vowel sounds, as well as tones, were studied. The vowel sounds, each about 40 msec in duration were: see article. Five were linked together to form a 200 msec stimulus and various combinations of five vowel sounds provided us with 18 different stimuli. The results show that neurons in the cochlear nuclei are remarkably sensitive and selective to vowel sounds. Furthermore, the responses of these neurons to pure tones do not provide a complete basis to predict the types of responses to the vowel sounds. More significant is the finding that the neural discharge rate and pattern of discharge to a particular vowel may depend on where the vowel appears in the stimulus and what other vowel precedes it. This vowel positional effect is not the same for every neuron. We have called this phenomenon a neural "set".

Intracellular study of synaptic events related to phase-locking responses of cat cochlear nucleus cells to low frequency tones

In this study intracellular recording techniques were used to study the synaptic events related to phase-locking of cochlear nucleus cells to low frequency stimuli. A variable degree of phase-locking was noted even with units of the same low characteristic frequency. With low frequency phase-locking units an excitatory postsynaptic potential (EPSP) occurred in response to each period of the frequency stimulus, but the probability of an action potential occurring decreased as the frequency of the stimulus was raised. Complex units were described which phase-locked at lower frequencies of stimulation but did not at higher frequencies where the temporal pattern of firing to tone burst stimulation changed as well. Results are discussed as they relate to the frequency following response recorded with gross electrodes in the lower auditory pathway and the relationship to frequency coding in the auditory pathway.

Auditory fatigue: retrocochlear components

Changes in auditory sensitivity were measured at the VII nerve, cochlear nucleus, and inferior colliculus after a fatiguing sound exposure. Losses in sensitivity progressively increased from peripheral to central auditory sites. The results suggest that there is a retrocochlear component to auditory fatigue when it is induced by low-level sounds of short duration.

Single unit activity in the posteroventral cochlear nucleus of the cat

Single unit activity in the posteroventral cochlear nucleus (PVCN) was recorded for a variety of stimulus conditions. The units were classified according to their response characteristics. The locations of units were plotted onto a three-dimensional block model of the cochlear nucleus. Certain types of units that responded best to the onsets of stimuli were located predominantly in the octopus cell region of the PVCN. The remainder of the PVCN, which contains a rather heterogeneous collection of small and multipolar cells, was found to contain several types of units with the dominant type being "chopper" units.

A block model of the cat cochlear nucleus

A three-dimensional block model of the cochlear nucleus of the cat was constructed from histologic sections. Boundaries of various subdivisions, based on cytoarchitectonic criteria, were included in the model. Usage of the block model in correlating physiological and anatomical data is illustrated by localizing characteristic waveforms of gross evoked responses and characteristic frequencies of single units.

Single unit activity in the dorsal cochlear nucleus of the cat

Single unit activity was examined in three component layers of the dorsal cochlear nucleus (DCN): the molecular layer, the fusiform cell layer, and the polymorphic layer (deep DCN). Electrophysiological units were classified into types on the basis of their activity under a variety of stimulus conditions. In the molecular layer spike activity was small and difficult to isolate. Almost all units in the fusiform cell layer could be classified as either "pauser" or "buildup" units. Classification of units in the deep DCN was sometimes difficult, but "pauser," "chopper," and some "on" units were found. The "on" types of units tended to be located in the more superficial part of the deep DCN. Unit locations were referred to a three-dimensional block model of the cochlear nucleus.

Postnatal maturation of the cochlear neclei in the cat: a neurophysiological study

The study of the postnatal maturation of the ventral cochlear nuclei (VCN) and the dorsal cochlear nuclei (DCN) was carried out on the cat by means of recordings of the extra-cellular neuronal activity. At birth it is already possible to obtain toneburst responses in the VCN and DCN. At this age the responses are characterised by a small number of spikes grouped in three bursts when the tone-bursts lasted 500 msec. Subsequently, the number of bursts increases until, from 9 or 10 days onwards the responses become sustained. These responses do not acquire their adult characteristics until more than a month after birth. During postnatal maturation of the cochlear nuclei, the VCN is distinguishable from the DCN by the greater number of units revealing spontaneous activity. Another criterion, such as latency, differentiates the VCN from the DCN from the point of view of the kinetics of maturation.