Cholinergic modulation of spontaneous activity in rat dorsal cochlear nucleus

Amino acid and acetylcholine chemistry in mountain beaver cochlear nucleus and comparisons to pocket gopher, other rodents, and cat

The mountain beaver and pocket gopher are two rodents that live mostly underground in tunnel systems. Previous studies have suggested that their cochlear nucleus structure, particularly that of the dorsal cochlear nucleus (DCN), differs significantly from that of other mammals, that the hearing ability of the pocket gopher is deficient compared to that of other rodents, and that the DCN of the mountain beaver is more responsive to slow oscillations of air pressure than to sounds. We conducted some electrophysiological recordings from mountain beaver DCN and then used microchemical methods to map in mountain beaver cochlear nuclei the distributions of amino acids, including the major neurotransmitters of the brain, and enzyme activities related to the metabolism of the neurotransmitter acetylcholine, which functions in centrifugal pathways to the cochlear nucleus. Similar measurements were made for a pocket gopher cochlear nucleus. Responses to tonal stimuli were found in mountain beaver DCN. Distributions and magnitudes of neurotransmitter and related amino acids within mountain beaver and pocket gopher cochlear nuclei were not very different from those of other rodents and cat. However, the enzyme of synthesis for acetylcholine, choline acetyltransferase, had only low activities in the DCN of both mountain beaver and pocket gopher. The chemical distributions in the mountain beaver DCN support a conclusion that it corresponds to just the superficial DCN portion of other mammals. High correlations between the concentrations of γ-aminobutyrate (GABA) and glycine were found for both mountain beaver and pocket gopher cochlear nuclei, suggesting that their co-localization in cochlear nucleus synapses may be especially prominent in these animals. Previous evidence suggests convergence of somatosensory and auditory information in the DCN, and this may be especially true in animals spending most of their time underground. Our results suggest that the enlarged DCN of the mountain beaver and that of the pocket gopher are not very different from those of other rodents with respect to involvement of amino acid neurotransmitters, but they appear to have reduced cholinergic innervation.

The Role of the Brainstem in Generating and Modulating Tinnitus

Purpose The purpose of this work is to present a perspective article summarizing ideas pertaining to the brainstem's role in generating and modulating tinnitus. It is organized in 4 sections: Part 1, the role of the brainstem as a tinnitus generator; Part 2, the role of the brainstem in modulating tinnitus; Part 3, the role of the brainstem in nonauditory comorbid conditions associated with tinnitus; and Part 4, clinical implications. In Part 1, well-established neurophysiological models are discussed providing the framework of evidence that auditory brainstem nuclei play a role in generating tinnitus. In Part 2, ideas are presented explaining modulatory effects on tinnitus related to underlying pathways originating from or projecting to brainstem auditory and nonauditory nuclei. This section addresses multiple phenomena including somatic-related, attention-mediated, and emotion-mediated changes in the tinnitus percept. In Part 3, the role of the brainstem in common nonauditory comorbidities that occur in patients with tinnitus is discussed. Part 4 presents clinical implications of these new ideas related to the brainstem's involvement in generating and modulating tinnitus. Impact Knowledge of the brainstem's involvement in generating and modulating tinnitus provides a context for health care professionals to understand the temporal relationship between tinnitus and common nonauditory comorbid conditions.

Muscarinic acetylcholine receptors control baseline activity and Hebbian stimulus timing-dependent plasticity in fusiform cells of the dorsal cochlear nucleus

Cholinergic modulation contributes to adaptive sensory processing by controlling spontaneous and stimulus-evoked neural activity and long-term synaptic plasticity. In the dorsal cochlear nucleus (DCN), in vitro activation of muscarinic acetylcholine receptors (mAChRs) alters the spontaneous activity of DCN neurons and interacts with N-methyl-d-aspartate (NMDA) and endocannabinoid receptors to modulate the plasticity of parallel fiber synapses onto fusiform cells by converting Hebbian long-term potentiation to anti-Hebbian long-term depression. Because noise exposure and tinnitus are known to increase spontaneous activity in fusiform cells as well as alter stimulus timing-dependent plasticity (StTDP), it is important to understand the contribution of mAChRs to in vivo spontaneous activity and plasticity in fusiform cells. In the present study, we blocked mAChRs actions by infusing atropine, a mAChR antagonist, into the DCN fusiform cell layer in normal hearing guinea pigs. Atropine delivery leads to decreased spontaneous firing rates and increased synchronization of fusiform cell spiking activity. Consistent with StTDP alterations observed in tinnitus animals, atropine infusion induced a dominant pattern of inversion of StTDP mean population learning rule from a Hebbian to an anti-Hebbian profile. Units preserving their initial Hebbian learning rules shifted toward more excitatory changes in StTDP, whereas units with initial suppressive learning rules transitioned toward a Hebbian profile. Together, these results implicate muscarinic cholinergic modulation as a factor in controlling in vivo fusiform cell baseline activity and plasticity, suggesting a central role in the maladaptive plasticity associated with tinnitus pathology.NEW & NOTEWORTHY This study is the first to use a novel method of atropine infusion directly into the fusiform cell layer of the dorsal cochlear nucleus coupled with simultaneous recordings of neural activity to clarify the contribution of muscarinic acetylcholine receptors (mAChRs) to in vivo fusiform cell baseline activity and auditory-somatosensory plasticity. We have determined that blocking the mAChRs increases the synchronization of spiking activity across the fusiform cell population and induces a dominant pattern of inversion in their stimulus timing-dependent plasticity. These modifications are consistent with similar changes established in previous tinnitus studies, suggesting that mAChRs might have a critical contribution in mediating the maladaptive alterations associated with tinnitus pathology. Blocking mAChRs also resulted in decreased fusiform cell spontaneous firing rates, which is in contrast with their tinnitus hyperactivity, suggesting that changes in the interactions between the cholinergic and GABAergic systems might also be an underlying factor in tinnitus pathology.

Slow Cholinergic Modulation of Spike Probability in Ultra-Fast Time-Coding Sensory Neurons

Sensory processing in the lower auditory pathway is generally considered to be rigid and thus less subject to modulation than central processing. However, in addition to the powerful bottom-up excitation by auditory nerve fibers, the ventral cochlear nucleus also receives efferent cholinergic innervation from both auditory and nonauditory top-down sources. We thus tested the influence of cholinergic modulation on highly precise time-coding neurons in the cochlear nucleus of the Mongolian gerbil. By combining electrophysiological recordings with pharmacological application in vitro and in vivo, we found 55-72% of spherical bushy cells (SBCs) to be depolarized by carbachol on two time scales, ranging from hundreds of milliseconds to minutes. These effects were mediated by nicotinic and muscarinic acetylcholine receptors, respectively. Pharmacological block of muscarinic receptors hyperpolarized the resting membrane potential, suggesting a novel mechanism of setting the resting membrane potential for SBC. The cholinergic depolarization led to an increase of spike probability in SBCs without compromising the temporal precision of the SBC output in vitro. In vivo, iontophoretic application of carbachol resulted in an increase in spontaneous SBC activity. The inclusion of cholinergic modulation in an SBC model predicted an expansion of the dynamic range of sound responses and increased temporal acuity. Our results thus suggest of a top-down modulatory system mediated by acetylcholine which influences temporally precise information processing in the lower auditory pathway.

Cochlear damage affects neurotransmitter chemistry in the central auditory system

Tinnitus, the perception of a monotonous sound not actually present in the environment, affects nearly 20% of the population of the United States. Although there has been great progress in tinnitus research over the past 25 years, the neurochemical basis of tinnitus is still poorly understood. We review current research about the effects of various types of cochlear damage on the neurotransmitter chemistry in the central auditory system and document evidence that different changes in this chemistry can underlie similar behaviorally measured tinnitus symptoms. Most available data have been obtained from rodents following cochlear damage produced by cochlear ablation, intense sound, or ototoxic drugs. Effects on neurotransmitter systems have been measured as changes in neurotransmitter level, synthesis, release, uptake, and receptors. In this review, magnitudes of changes are presented for neurotransmitter-related amino acids, acetylcholine, and serotonin. A variety of effects have been found in these studies that may be related to animal model, survival time, type and/or magnitude of cochlear damage, or methodology. The overall impression from the evidence presented is that any imbalance of neurotransmitter-related chemistry could disrupt auditory processing in such a way as to produce tinnitus.

Cholinergic modulation of large-conductance calcium-activated potassium channels regulates synaptic strength and spine calcium in cartwheel cells of the dorsal cochlear nucleus

Acetylcholine is a neuromodulatory transmitter that controls synaptic plasticity and sensory processing in many brain regions. The dorsal cochlear nucleus (DCN) is an auditory brainstem nucleus that integrates auditory signals from the cochlea with multisensory inputs from several brainstem nuclei and receives prominent cholinergic projections. In the auditory periphery, cholinergic modulation serves a neuroprotective function, reducing cochlear output under high sound levels. However, the role of cholinergic signaling in the DCN is less understood. Here we examine postsynaptic mechanisms of cholinergic modulation at glutamatergic synapses formed by parallel fiber axons onto cartwheel cells (CWCs) in the apical DCN circuit from mouse brainstem slice using calcium (Ca) imaging combined with two-photon laser glutamate uncaging onto CWC spines. Activation of muscarinic acetylcholine receptors (mAChRs) significantly increased the amplitude of both uncaging-evoked EPSPs (uEPSPs) and spine Ca transients. Our results demonstrate that mAChRs in CWC spines act by suppressing large-conductance calcium-activated potassium (BK) channels, and this effect is mediated through the cAMP/protein kinase A signaling pathway. Blocking BK channels relieves voltage-dependent magnesium block of NMDA receptors, thereby enhancing uEPSPs and spine Ca transients. Finally, we demonstrate that mAChR activation inhibits L-type Ca channels and thus may contribute to the suppression of BK channels by mAChRs. In summary, we demonstrate a novel role for BK channels in regulating glutamatergic transmission and show that this mechanism is under modulatory control of mAChRs.

Suppression of noise-induced hyperactivity in the dorsal cochlear nucleus following application of the cholinergic agonist, carbachol

Increased spontaneous firing (hyperactivity) is induced in fusiform cells of the dorsal cochlear nucleus (DCN) following intense sound exposure and is implicated as a possible neural correlate of noise-induced tinnitus. Previous studies have shown that in normal hearing animals, fusiform cell activity can be modulated by activation of parallel fibers, which represent the axons of granule cells. The modulation consists of a transient excitation followed by a more prolonged period of inhibition, presumably reflecting direct excitatory inputs to fusiform cells and an indirect inhibitory input to fusiform cells from the granule cell-cartwheel cell system. We hypothesized that since granule cells can be activated by cholinergic inputs, it might be possible to suppress tinnitus-related hyperactivity of fusiform cells using the cholinergic agonist, carbachol. To test this hypothesis, we recorded multiunit spontaneous activity in the fusiform soma layer (FSL) of the DCN in control and tone-exposed hamsters (10 kHz, 115 dB SPL, 4h) before and after application of carbachol to the DCN surface. In both exposed and control animals, 100 μM carbachol had a transient excitatory effect on spontaneous activity followed by a rapid weakening of activity to near or below normal levels. In exposed animals, the weakening of activity was powerful enough to completely abolish the hyperactivity induced by intense sound exposure. This suppressive effect was partially reversed by application of atropine and was usually not associated with significant changes in neural best frequencies (BF) or BF thresholds. These findings demonstrate that noise-induced hyperactivity can be pharmacologically controlled and raise the possibility that attenuation of tinnitus may be achievable by using an agonist of the cholinergic system.

Activation of muscarinic receptors increases the activity of the granule neurones of the rat dorsal cochlear nucleus--a calcium imaging study

Acetylcholine modulates the function of the cochlear nucleus via several pathways. In this study, the effects of cholinergic stimulation were studied on the cytoplasmic Ca(2+) concentration of granule neurones of the rat dorsal cochlear nucleus (DCN). Ca(2+) transients were recorded in Oregon-Green-BAPTA 1-loaded brain slices using a calcium imaging technique. For the detection, identification and characterisation of the Ca(2+) transients, a wavelet analysis-based method was developed. Granule cells were identified on the basis of their size and localisation. The action potential-coupled character of the Ca(2+) transients of the granule cells was established by recording fluorescence changes and electrical activity simultaneously. Application of the cholinergic agonist carbamyl-choline (CCh) significantly increased the frequency of the Ca(2+) transients (from 0.37 to 6.31 min(-1), corresponding to a 17.1-fold increase; n = 89). This effect was antagonised by atropine, whereas CCh could still evoke an 8.3-fold increase of the frequency of the Ca(2+) transients when hexamethonium was present. Using immunolabelling, the expression of both type 1 and type 3 muscarinic receptors (M1 and M3 receptors, respectively) was demonstrated in the granule cells. Application of 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide (an M3-specific antagonist) prevented the onset of the CCh effect, whereas an M1-specific antagonist (pirenzepine) was less effective. We conclude that cholinergic stimulation increases the activity of granule cells, mainly by acting on their M3 receptors. The modulation of the firing activity of the granule cells, in turn, may modify the firing of projection neurones and may adjust signal processing in the entire DCN.

Multiple origins of cholinergic innervation of the cochlear nucleus

Acetylcholine (Ach) affects a variety of cell types in the cochlear nucleus (CN) and is likely to play a role in numerous functions. Previous work in rats suggested that the acetylcholine arises from cells in the superior olivary complex, including cells that have axonal branches that innervate both the CN and the cochlea (i.e. olivocochlear cells) as well as cells that innervate only the CN. We combined retrograde tracing with immunohistochemistry for choline acetyltransferase to identify the source of ACh in the CN of guinea pigs. The results confirm a projection from cholinergic cells in the superior olivary complex to the CN. In addition, we identified a substantial number of cholinergic cells in the pedunculopontine tegmental nucleus (PPT) and the laterodorsal tegmental nucleus (LDT) that project to the CN. On average, the PPT and LDT together contained about 26% of the cholinergic cells that project to CN, whereas the superior olivary complex contained about 74%. A small number of additional cholinergic cells were located in other areas, including the parabrachial nuclei.The results highlight a substantial cholinergic projection from the pontomesencephalic tegmentum (PPT and LDT) in addition to a larger projection from the superior olivary complex. These different sources of cholinergic projections to the CN are likely to serve different functions. Projections from the superior olivary complex are likely to serve a feedback role, and may be closely tied to olivocochlear functions. Projections from the pontomesencephalic tegmentum may play a role in such things as arousal and sensory gating. Projections from each of these areas, and perhaps even the smaller sources of cholinergic inputs, may be important in conditions such as tinnitus as well as in normal acoustic processing.

Targets, receptors and effects of muscarinic neuromodulation on giant neurones of the rat dorsal cochlear nucleus

Although cholinergic modulation of the cochlear nucleus (CN) is functionally important, neither its cellular consequences nor the types of receptors conveying it are precisely known. The aim of this work was to characterise the cholinergic effects on giant cells of the CN, using electrophysiology and quantitative polymerase chain reaction. Application of the cholinergic agonist carbachol increased the spontaneous activity of the giant cells; which was partly the consequence of the reduction in a K(+) conductance. This effect was mediated via M4 and M3 receptors. Cholinergic modulation also affected the synaptic transmission targeting the giant cells. Excitatory synaptic currents evoked by the stimulation of the superficial and deep regions of the CN were sensitive to cholinergic modulation: the amplitude of the first postsynaptic current was reduced, and the short-term depression was also altered. These changes were mediated via M3 receptors alone and via the combination of M4, M2 and M3 receptors, when the superficial and deep layers, respectively, were activated. Inhibitory synaptic currents evoked from the superficial layer showed short-term depression, but they were unaffected by carbachol. In contrast, inhibitory currents triggered by the activation of the deep parts exhibited no significant short-term depression, but they were highly sensitive to cholinergic activation, which was mediated via M3 receptors. Our results indicate that pre- and postsynaptic muscarinic receptors mediate cholinergic modulation on giant cells. The present findings shed light on the cellular mechanisms of a tonic cholinergic modulation in the CN, which may become particularly important in evoking contralateral excitatory responses under certain pathological conditions.

The potassium channel KCNQ5/Kv7.5 is localized in synaptic endings of auditory brainstem nuclei of the rat

KCNQ, also called Kv7, is a family of voltage-dependent potassium channels with important roles in excitability regulation. Of its five known subunits, KCNQ5/Kv7.5 is extensively expressed in the central nervous system and it contributes to the generation of M-currents. The distribution of KCNQ5 was analyzed in auditory nuclei of the rat brainstem by high-resolution immunocytochemistry. Double labeling with anti-KCNQ5 antibodies and anti-synaptophysin or anti-syntaxin, which mark synaptic endings, or anti-microtubule-associated protein 2 (MAP2) antibodies, which mark dendrites, were used to analyze the subcellular distribution of KCNQ5 in neurons in the cochlear nucleus, superior olivary complex, nuclei of the lateral lemniscus, and inferior colliculus. An abundance of KCNQ5 labeling in punctate structures throughout auditory brainstem nuclei along with colocalization with such synaptic markers suggests that a preferred localization of KCNQ5 is in synaptic endings in these auditory nuclei. Punctate KCNQ5 immunoreactivity virtually disappeared from the cochlear nucleus after cochlea removal, which strongly supports localization of this channel in excitatory endings of the auditory nerve. Actually, neither glycinergic endings, labeled with an anti-glycine transporter 2 (GlyT2) antibody, nor gamma-aminobutyric acid (GABA)ergic endings, labeled with an anti-glutamic acid decarboxylase (GAD65) antibody, contained KCNQ5 immunoreactivity, suggesting that KCNQ5 is mostly in excitatory endings throughout the auditory brainstem. Overlap of KCNQ5 and MAP2 labeling indicates that KCNQ5 is also targeted to dendritic compartments. These findings predict pre- and postsynaptic roles for KCNQ5 in excitability regulation in auditory brainstem nuclei, at the level of glutamatergic excitatory endings and in dendrites.

Intense sound-induced plasticity in the dorsal cochlear nucleus of rats: Evidence for cholinergic receptor upregulation

Previous studies in a number of species have demonstrated that spontaneous activity in the dorsal cochlear nucleus (DCN) becomes elevated following exposure to intense sound. This condition of hyperactivity has aroused considerable interest because it may represent an important neural correlate of tinnitus. There is some evidence that neurons in the superficial DCN, such as cartwheel, stellate and fusiform cells, may contribute to the level of hyperactivity induced by intense sound, although the relative importance of these different cell types is unknown. In the present study, we sought to determine the effect of intense sound exposure on multiunit spontaneous activity both at the DCN surface and in the fusiform cell layer and to examine the influence of cholinergic input to DCN circuits on the level of activity in the fusiform cell layer. Rats were studied in two groups, one of which had been exposed to a continuous intense sound (10 kHz 127 dB SPL) for 4h while the other group served as unexposed controls. Between 30 and 52 days post-exposure, recordings of multiunit activity were performed at the DCN surface as well as in the middle of the fusiform cell layer. Changes in fusiform cell layer activity were also studied in response to superficial applications of the cholinergic agonist, carbachol, either alone or following pre-application of the cholinergic antagonist, atropine. The results demonstrated that multiunit spontaneous activity in the rat DCN was generally much higher in both control and exposed animals relative to that which has been observed in other species. This activity was significantly higher at the DCN surface of sound-exposed animals than that of controls. In contrast, hyperactivity could not be demonstrated in the fusiform cell layer of sound-exposed animals. Carbachol administration most commonly caused suppression of fusiform cell layer activity. However, this suppression was considerably stronger in the DCN of sound-exposed animals than in controls. These findings suggest that, hyperactivity at the DCN surface of exposed rats may arise as a consequence of more highly activated neurons in the molecular layer, such as cartwheel and/or stellate cells, and that the lack of hyperactivity in the fusiform cell layer may be the result of inhibition of fusiform cells by these inhibitory interneurons. Although this finding does not rule out fusiform cells as possible sources of hyperactivity in other species, or even in the rat after short post-exposure recovery periods, the enhanced sensitivity of the fusiform cell layer to cholinergic stimulation suggests that in the rat, at least after prolonged post-exposure recovery periods, increased inhibition of activity in this layer by more superficially located neurons may result from an upregulation of receptors for cholinergic input. This upregulation may be greater in rats than in other species due to the relatively heavy cholinergic input that exists in the cochlear nucleus of this species.

Effects of intense tone exposure on choline acetyltransferase activity in the hamster cochlear nucleus

Choline acetyltransferase (ChAT) activity has been mapped in the cochlear nucleus (CN) of control hamsters and hamsters that had been exposed to an intense tone. ChAT activity in most CN regions of hamsters was only a third or less of the activity in rat CN, but in granular regions ChAT activity was similar in both species. Eight days after intense tone exposure, average ChAT activity increased on the tone-exposed side as compared to the opposite side, by 74% in the anteroventral CN (AVCN), by 55% in the granular region dorsolateral to it, and by 74% in the deep layer of the dorsal CN (DCN). In addition, average ChAT activity in the exposed-side AVCN and fusiform soma layer of DCN was higher than in controls, by 152% and 67%, respectively. Two months after exposure, average ChAT activity was still 53% higher in the exposed-side deep layer of DCN as compared to the opposite side. Increased ChAT activity after intense tone exposure may indicate that this exposure leads to plasticity of descending cholinergic innervation to the CN, which might affect spontaneous activity in the DCN that has been associated with tinnitus.

Effects of cochlear ablation on muscarinic acetylcholine receptor binding in the rat cochlear nucleus

Cholinergic synapses in the cochlear nucleus (CN) have been reported to modulate spontaneous activity via muscarinic acetylcholine receptors. In this study, muscarinic receptor binding was measured as specific binding of 1-[N-methyl-(3)H]scopolamine in CN regions of control rats and 7 days, 1 month, and 2 months after unilateral cochlear ablation. In control rats, the strongest binding was found in granular regions, followed in order by fusiform soma, molecular, and deep layers of the dorsal cochlear nucleus (DCN), with much lower binding in the anteroventral CN (AVCN) and posteroventral CN (PVCN). After unilateral cochlear ablation, binding in the AVCN, PVCN, and their associated granular regions on the lesion side became progressively greater than on the control side through 2 months after lesion. A significant asymmetry, with binding higher on the lesion side, was also found in the DCN fusiform soma layer at 7 days, and there and in the DCN deep layer at 1 and 2 months after lesion. There was also evidence of increased binding on the control side in most CN regions. By contrast, binding in the ipsilateral facial nucleus decreased, compared with the control side, by 7 days after the lesion and showed some recovery toward symmetry by 2 months after lesion, and there was no evidence for contralateral changes. These muscarinic receptor binding changes reflect receptor plasticity after loss of auditory nerve innervation. Such plasticity may underlie some of the central auditory functional changes that occur following peripheral lesions, such as tinnitus and hyperacusis.

Effects of cochlear ablation on choline acetyltransferase activity in the rat cochlear nucleus and superior olive

Using microdissection and quantitative microassay, choline acetyltransferase (ChAT) activity was mapped in the cochlear nucleus (CN) and in the source nuclei of the olivocochlear bundle, the lateral superior olive and ventral nucleus of the trapezoid body. In control rats, gradients of ChAT activity were found within the major subdivisions of the CN and in the lateral superior olive. These gradients correlated with the known tonotopic organizations, with higher activities corresponding to locations representing higher sound frequencies. No gradient was found in the ventral nucleus of the trapezoid body. In rats surviving 7 days or 1 or 2 months after cochlear ablation, ChAT activity was increased 1 month after ablation in the anteroventral CN by 30-50% in most parts of the lesion-side and by 40% in the contralateral ventromedial part. ChAT activity in the lesion-side posteroventral CN was increased by approximately 40-50% at all survival times. Little change was found in the dorsal CN. Decreases of ChAT activity were also found ipsilaterally in the lateral superior olive and bilaterally in the ventral nucleus of the trapezoid body. Our results suggest that cholinergic neurons are involved in plasticity within the CN and superior olive following cochlear lesions.

Changes in [14C]-2-deoxyglucose uptake in the auditory pathway of hamsters previously exposed to intense sound

The current study evaluated changes in [14C]-2-deoxyglucose (2-DG) uptake along the auditory pathways of hamsters that were exposed unilaterally to intense sound. The measurement of the acoustically evoked auditory brainstem responses indicated that intense sound exposure caused asymmetrical hearing loss. The 2-DG results revealed some changes in metabolic activity in exposed animals, as compared to unexposed animals. Significant decreases in 2-DG uptake were found in the ipsilateral anteroventral and posteroventral cochlear nucleus, with respect to the exposed left ears. Exposed animals also showed significant increases in the ipsilateral nucleus of the lateral lemniscus, central nucleus of inferior colliculus and medial geniculate body. No significant changes in uptake were observed in the ipsilateral dorsal cochlear nucleus, superior olivary complex, auditory cortex and any contralateral structures. The mechanisms for the observed changes in 2-DG uptake are discussed.

Cisplatin-induced hyperactivity in the dorsal cochlear nucleus and its relation to outer hair cell loss: relevance to tinnitus

Cisplatin causes both acute and chronic forms of tinnitus as well as increases in spontaneous neural activity (hyperactivity) in the dorsal cochlear nucleus (DCN) of hamsters. It has been hypothesized that the induction of hyperactivity in the DCN may be a consequence of cisplatin's effects on cochlear outer hair cells (OHCs); however, systematic studies testing this hypothesis have yet to appear in the literature. In the present investigation, the relationship between hyperactivity and OHC loss, induced by cisplatin, was examined in detail. Hamsters received five treatments of cisplatin at doses ranging from 1.5 to 3 mg. kg(-1). day(-1), every other day. Beginning 1 mo after initiation of treatment, electrophysiological recordings were carried out on the surface of the DCN to measure spontaneous multiunit activity along a set of coordinates spanning the medial-lateral (tonotopic) axis of the DCN. After recordings, cochleas were removed and studied histologically using a scanning electron microscope. The results revealed that cisplatin-treated animals with little or no loss of OHCs displayed levels of activity similar to those seen in saline-treated controls. In contrast, the majority (75%) of cisplatin-treated animals with severe OHC loss displayed well-developed hyperactivity in the DCN. The induced hyperactivity was seen mainly in the medial (high-frequency) half of the DCN of treated animals. This pattern was consistent with the observation that OHC loss was distributed mainly in the basal half of the cochlea. In several of the animals with severe OHC loss and hyperactivity, there was no significant damage to IHC stereocilia nor any observable irregularities of the reticular lamina that might have interfered with normal IHC function. Hyperactivity was also observed in the DCN of animals showing severe losses of OHCs accompanied by damage to IHCs, although the degree of hyperactivity in these animals was less than in animals with severe OHC loss but intact IHCs. These results support the view that loss of OHC function may be a trigger of tinnitus-related hyperactivity in the DCN and suggest that this hyperactivity may be somewhat offset by damage to IHCs.

Spontaneous activity in rat vestibular nuclei in brain slices and effects of acetylcholine agonists and antagonists

Extracellular recording was used to investigate spontaneously active neurons in all four major nuclei of the rat vestibular nuclear complex (VNC) in brainstem slices. The density of spontaneously active neurons was highest in the medial vestibular nucleus (MVN), slightly lower in the superior (SuVN) and spinal (SpVN) nuclei, and lowest in the lateral vestibular nucleus (LVN). We compared the effects of acetylcholine agonists and antagonists on spontaneously discharging neurons in MVN, SuVN, and SpVN with those in the nearby dorsal cochlear nucleus (DCN). The proportion of neurons responding to carbachol was greatest in DCN and smallest in SpVN. Unlike in DCN, some neurons in MVN, SuVN, and SpVN showed decreased firing during carbachol or muscarine. Magnitudes of responses to carbachol and muscarine were closely correlated (P<0.01). MVN neurons possessed nicotinic as well as muscarinic receptors. Activation of either type was unaffected by blocking synaptic transmission. The IC(50) values for the muscarinic subtype-preferential antagonists were compared, and tropicamide, preferential for M(4), was the most potent. Our results suggest that: (1) the relative numbers of spontaneously active neurons in rat VNC differ among nuclei; (2) acetylcholine agonists elicit changes in mean firing rates of neurons in MVN, SuVN and SpVN, but fewer neurons respond, and responses are smaller than in DCN; (3) both muscarinic and nicotinic acetylcholine receptors are present on MVN neurons, but muscarinic receptors may be more prominent.

Effects of acoustic trauma on dorsal cochlear nucleus neuron activity in slices

Previous studies found increased multi-unit spontaneous activity in the dorsal cochlear nucleus (DCN) of animals that had been exposed to intense sound. Such activity may be related to tinnitus. Our study examined effects of previous exposure to intense sound on single neurons in the DCN, by measuring spontaneous activities and sensitivities to acetylcholine, an important neurotransmitter of centrifugal pathways to the cochlear nucleus, in brain slices. Spontaneous discharges were recorded extracellularly in the DCN portion of brain slices from control and intense-tone-exposed rats. Slices from exposed rats showed increased prevalence of bursting and decreased regular spontaneous activity. Since regular neurons include fusiform cells, and bursting neurons include cartwheel cells, intense tone exposure may lead to increased activity of DCN cartwheel cells and decreased activity of fusiform cells. Alternatively, the activity of some fusiform cells might change to bursting. Intense tone exposure also appeared to increase bursting neuron sensitivity to carbachol. This suggests that changes in DCN cartwheel cell spontaneous activity may reflect changes in effects of cholinergic centrifugal pathways following intense tone exposure. We conclude that acoustic trauma may lead to changes in the physiology and pharmacology of DCN neurons. These changes may be related to underlying mechanisms of central tinnitus.

Sodium pentobarbital abolishes bursting spontaneous activity of dorsal cochlear nucleus in rat brain slices

There is evidence that pentobarbital, a commonly used anesthetic, can affect neuronal activity, but its effects on particular neurons of the dorsal cochlear nucleus (DCN) are not well known. Bursting (complex spiking) spontaneous activity has been observed in the DCN in brain slice preparations and in recordings from unanesthetized decerebrate animals, but seldom in experiments with anesthetized animals. This study investigated the effects of pentobarbital on spontaneous activity in the DCN in brain slices. Most extracellularly recorded bursting neurons decreased firing rates and reversibly changed their firing to simple spiking with irregular intervals during pentobarbital. Some reversibly stopped firing after the change to an irregular pattern. Most neurons with regular spontaneous activity (simple spiking) showed decreased firing rates and more irregular intervals during pentobarbital. The results also suggest some involvement of gamma-aminobutyric acid type A receptors in the pentobarbital effects.

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.

Glutamergic transmission of neuronal responses to carbachol in rat dorsal cochlear nucleus slices

This study found that glutamate receptor antagonists block the excitatory effects of carbachol, a cholinergic agonist, on bursting neurons in the dorsal cochlear nucleus of rat brain slices. Among antagonists for glutamate receptor subtypes, those for non-N-methyl-D-aspartate ionotropic glutamate receptors were more potent than those for N-methyl-D-aspartate receptors. The glutamate receptor antagonists did not block the effects of carbachol on regularly firing neurons in the dorsal cochlear nucleus of the same slices. Antagonists for GABA or glycine receptors did not alter the effects of carbachol on bursting neurons. Effects of carbachol on bursting activity could be mimicked by application of glutamate or its agonist, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, whose effects were not blocked by synaptic blockade. During carbachol application, increased release of glutamate and glycine from the dorsal cochlear nucleus part of brain slices was measured using high-performance liquid chromatography. Release of other amino acids showed no significant change. The results suggest that, in rat dorsal cochlear nucleus, cholinergic effects on regular and bursting spontaneous firing occur through different mechanisms. Cholinergic effects on regular neurons (which include fusiform cells) are direct, through muscarinic receptors. Cholinergic effects on bursting neurons (which include cartwheel cells) are indirect and involve glutamatergic neurotransmission, mostly via non-N-methyl-D-aspartate ionotropic receptors. The granule cell-parallel fiber pathway may be involved in this glutamatergic transmission.

Immunolocalization of alpha4 and alpha7 subunits of nicotinic receptor in rat cochlear nucleus

The rat cochlear nucleus (CN) is known to receive cholinergic input. To investigate the prevalence of nicotinic acetylcholine receptor (nAChR), immunohistochemistry for alpha4 and alpha7 subunits, which represent nAChRs with high binding affinities for nicotine and alpha-bungarotoxin, respectively, was performed on perfusion-fixed rat brain sections. Microscopic observations and densitometric measurements show dense labeling for alpha7 but not alpha4. Within the CN, alpha7 receptors are found in all subregions, with relatively high densities in granular regions. The distribution of alpha7 within the CN appears to correlate more closely with that of acetylcholinesterase than with mAChR or choline acetyltransferase. Our results suggest a role of nicotinic cholinergic transmission in the rat CN associated with high affinity for alpha-bungarotoxin.

Effects of nicotine and mecamylamine on rat dorsal raphe neurons

This study investigates the hypothesis that serotonin mediates certain nicotine effects, such as mood improvement and the suppression of the ponto-geniculo-occipital spikes of rapid eye movement sleep. The influence of nicotine (10-300 microM) on the firing rate of dorsal raphe neurons and on serotonin release was therefore, studied in rat midbrain slices. Nicotine increased the firing rate, 10-90%, in 67.5% recorded neurons and decreased it, 8-100%, in the remaining 32.5%. Serotonin release increased 2-7 times after nicotine administration, regardless of firing frequency, but the absolute value of serotonin release was 3 times higher during the decreases than during the increases in firing rate. Mecamylamine (1-20 microM) transiently stimulated the dorsal raphe neurons and competitively antagonized the nicotine-induced serotonin release. The results support the working hypothesis and additionally show that mecamylamine also stimulates dorsal raphe neurons.

Nicotinic acetylcholine receptors in rat cochlear nucleus: [125I]-alpha-bungarotoxin receptor autoradiography and in situ hybridization of alpha 7 nAChR subunit mRNA

The cochlear nucleus (CN) is the first site in the central nervous system (CNS) for processing auditory information. Acetylcholine in the CN is primarily extrinsic and is an important neurotransmitter in efferent pathways thought to provide CNS modulation of afferent signal processing. Although muscarinic acetylcholine receptors have been studied in the CN, the role of nicotinic receptors has not. We examined the distribution of one nicotinic acetylcholine receptor subtype, the alpha-bungarotoxin receptor (alpha Bgt), in the CN. Quantitative autoradiography was used to localize receptors and in situ hybridization was used to localize alpha 7 mRNA in CN neurons that express the alpha Bgt receptor. Binding sites for alpha Bgt are abundant in the anterior ventral, posterior ventral, and dorsal divisions of the CN, and receptor density is low in the granule cell layer and interstitial nucleus. Heterogeneity in CN subregions is described. Four distinct patterns of alpha Bgt binding were observed: (1) binding over and around neuronal cell bodies, (2) receptors locally surrounding neurons, (3) dense punctate binding in the dorsal CN (DCN) not associated with neuronal cell bodies, and (4) diffuse fields of alpha Bgt receptors prominent in the DCN molecular layer, a field underlying the granule cell layer and in the medial sheet. The perikaryial receptors are abundant in the ventral CN (VCN) and are always associated with neurons expressing mRNA for the receptor. Other neurons in the VCN also express alpha 7 mRNA, but without alpha Bgt receptor expression associated with the cell body. In general, alpha Bgt receptor distribution parallels cholinergic terminal distribution, except in granule cell regions rich in cholinergic markers but low in alpha Bgt receptors. The findings indicate that alpha Bgt receptors are widespread in the CN but are selectively localized on somata, proximal dendrites, or distal dendrites depending on the specific CN subregion. The data are consistent with the hypothesis that descending cholinergic fibers modulate afferent auditory signals by regulating intracellular Ca2+ through alpha Bgt receptors.

Effects of endogenous acetylcholine on spontaneous activity in rat dorsal cochlear nucleus slices

We have examined the contribution of endogenous acetylcholine (ACh) release to the spontaneous firing of both regular (probably fusiform cells) and bursting neurons (probably cartwheel cells) in the dorsal cochlear nucleus (DCN) in rat brainstem slices. The muscarinic antagonists atropine, scopolamine, and tropicamide (1-2 microM) caused substantial decreases of firing rates in a majority of the neurons. Reversible acetylcholinesterase (AChE) inhibitors typically caused large transient increases in firing that decayed more slowly than responses to carbachol. The irreversible AChE inhibitor diisopropyl fluorophosphate (DFP) usually caused a sustained increase, with an initial peak followed by a gradual change to a final level higher than before DFP. Tropicamide caused large decreases in firing after DFP, confirming sustained ACh release. Both neostigmine and DFP applied after AChE inhibition by DFP sometimes elicited a transient response. We conclude that the level of sustained response to DFP is determined by the rate of endogenous ACh release, and that DFP and reversible AChE inhibitors exert an initial transient agonist effect that overlaps the initial effect of acetylcholinesterase inhibition. The slice experiments provide a model for cholinergic mechanisms in vivo, confirm that the release of endogenous ACh increases the firing rates of regular and bursting neurons in superficial DCN, and support the hypothesis that spontaneous firing of DCN neurons is sustained in part by cholinergic inputs.

Evidence of stimulus-dependent correlated activity in the dorsal cochlear nucleus of decerebrate gerbils

Cross-correlation analysis of simultaneously recorded spike trains was used to study the internal organization of the dorsal cochlear nucleus (DCN) of unanesthetized decerebrate Mongolian gerbils. The goal was to test the model (adapted from cat) that its principal cells (type III and type IV units) receive three sources of shared auditory input: excitatory input from the auditory nerve; inhibitory input from DCN interneurons (vertical cells; type II and type II-i units) that respond vigorously to tones; and inhibitory input from ventral cochlear nucleus principal cells (D-stellate cells; wideband inhibitors) that conversely respond vigorously to noise. Records of spontaneous and/or driven activities (to long-duration tones and frozen broadband noise) were obtained for 51 pairs consisting of type II, type III, and type IV units; type III units inhibited by low-level noise were subclassified as type III-i units. Pairs were isolated with two electrodes to study the effect of differences in unit best frequencies (BFs) on correlation. All correlated pairs composed of type III and type IV units (17 of 31 pairs) showed central mounds (CMs), indicative of shared input, in their cross-correlograms. These data exhibited two important properties: pairs with similar BFs were more likely to show CMs, and the shape of the CMs was stimulus dependent. That is, CM width typically changed sharply from wide to narrow with increasing level; significantly, transition-level CMs were either a composite of these shapes or not present. The transition to only narrow CMs occurred above the thresholds of type II and type III-i units to tones, but below their thresholds to noise. Cross-correlograms derived from the tone-evoked activities of pairs involving type II units (3 of 6 pairs) showed inhibitory troughs (ITs); unexpectedly, type III-i units were involved in both IT and CM pairs, suggesting that this unit type may reflect recordings from both vertical and principal cells. Overall, the results are interpretable in terms of the model of gerbil DCN that was adapted from cat, suggesting that the model generalizes across species. Compared with cat, however, gerbil principal cell responses (predominantly type III unit properties) are less dominated by inhibition.

Densitometric evaluation of markers for cholinergic transmission in rat superior olivary complex

Cholinergic transmission in the rat superior olivary complex (SOC) was assessed by densitometric measurements of labeling with several markers: acetylcholinesterase (AChE) histochemistry, choline acetyltransferase (ChAT) immunohistochemistry, and muscarinic acetylcholine receptor (M35) immunohistochemistry and N-methylscopolamine (NMS) binding autoradiography, as well as its subtype 2 (m2) immunohistochemistry. The markers which may occur in cholinergic neurons (ChAT, m2 receptor and AChE) showed dense labeling in the ventral nucleus of the trapezoid body (VNTB), in line with other evidence that it contains cholinergic neuron somata. The markers which are predominantly in cholinoceptive neurons (M35 and NMS) showed less labeling in the SOC, suggesting few muscarinic cholinergic inputs. These results are compared with those for the cochlear nucleus.

Immunolocalization of muscarinic acetylcholine subtype 2 receptors in rat cochlear nucleus

It has been suggested that cholinergic effects in the rat cochlear nucleus (CN) are mediated by muscarinic acetylcholine receptors. In this study, immunohistochemistry for muscarinic subtype 2 (m2) receptors using a monoclonal subtype-specific antibody (Levey et al. [1995] J. Comp. Neurol. 351:339-356) revealed an m2-like system in the rat CN. A prominent lamina of m2-immunoreactive fibers and puncta was located in a subgranular layer of the caudal anteroventral cochlear nucleus (AVCN) and the posteroventral cochlear nucleus (PVCN). The superficial granular layer of the rostral AVCN and the medial sheet region also contained notable immunoreactivity for m2. Some labeled somata and their processes were found in magnocellular regions of the ventral CN. A network of neurites and puncta was located in the fusiform soma and deep layers of the dorsal CN. The olivocochlear bundle and its branches to the CN were also m2 immunoreactive and possibly contributed m2-labeled fibers and terminals to the CN. Some similarities and some differences were found between this m2 receptor distribution pattern and previous results for choline acetyltransferase (ChAT), acetylcholinesterase (AChE), and muscarinic acetylcholine receptor immunohistochemistry and binding in the CN. The results suggest that m2 receptors that are located both pre- and postsynaptically mediate many cholinergic effects in the rat CN.

Effects of parallel fiber stimulation on neurons of rat dorsal cochlear nucleus

We have compared the effects of parallel fiber stimuli on extracellularly recorded neurons showing regular or bursting spontaneous activity patterns in the dorsal cochlear nucleus of rat brainstem slices. Ninety percent of regular neurons failed to respond to stimulus currents (1.4 +/- 0.28 mA, mean +/- SEM) significantly greater than those (0.4 +/- 0.07 mA) that elicited responses from 96% of bursting neurons. Responses of bursting neurons were elicited from widely separated loci along the molecular layer. Kynurenic acid and CNQX or DNQX blocked both spontaneous firing and responses to parallel fiber stimuli of bursting neurons. The same agents also blocked responses of regular neurons but had little or no effect on their spontaneous firing rates. AP-5 caused small decreases in spontaneous rates of both bursting and regular neurons but did not appear to affect responses to stimuli. The data support the hypothesis that the responses of both regular and bursting neurons to parallel fiber stimulation are mediated by glutamate, acting mainly through non-NMDA receptors. Spontaneous activity of bursting, but not regular, neurons also requires non-NMDA glutamatergic transmission, suggesting that the spontaneous firing of bursting neurons, consisting largely of cartwheel cells, may depend upon granule cell activity.

Muscarinic receptor subtypes in rat dorsal cochlear nucleus

We previously reported that responses of spontaneously active rat dorsal cochlear nucleus (DCN) neurons to cholinergic agonists are mediated predominantly by muscarinic receptors. We have now tested the effects of 7 antagonists with differing affinities for the muscarinic receptor subtypes M1-M4 on the responses to constant, submaximal doses of carbachol in rat brainstem slices. Each slice was exposed to one or more concentrations of one antagonist applied during extracellular recording of a DCN neuron. The concentrations yielding 50% reduction of test responses (IC50) of regular and bursting neurons were estimated for each antagonist. Correlation coefficients were calculated between log(IC50) values and log(Ki) values of the drugs for the receptor subtypes. Correlation coefficients for both regular and bursting neurons were not significant (P > 0.05) for M1 and M3, but were significant (P < 0.02) for M4. Bursting but not regular neurons also showed a significant correlation for M2 (P < 0.05). Our results suggest that (1) M4 contributes to the cholinergic responses in DCN and M2 may also contribute to the responses of bursting neurons, but the contribution of other subtypes cannot be completely excluded; (2) muscarinic subtypes in DCN probably differ from those reported for cochlea and some brain regions.

Immunohistochemistry of muscarinic acetylcholine receptors in rat cochlear nucleus

Neurons of the cochlear nucleus (CN) receive extrinsic and intrinsic cholinergic inputs, the effects of which appear to be mediated primarily by muscarinic acetylcholine receptors (mAChRs). To investigate the distribution of mAChRs and the correlation of pre- and post-synaptic cholinergic markers in CN, we used a monoclonal anti-mAChR antibody (M35) and a monoclonal antibody against choline acetyltransferase (ChAT) to perform immunohistochemistry on rat brain sections, and we also carried out histochemistry for acetylcholinesterase (AChE) activity. The density distributions of ChAT immunohistochemistry and AChE activity histochemistry agreed well with previous quantitative measurements of the activity distributions of these enzymes. A generally close correlation between densities of M35 and ChAT immunoreactivities was found across subregions of rat CN. The localizations of M35 and ChAT immunoreactivities suggest that cholinergic synapses are mostly axosomatic in ventral CN and possibly mostly axodendritic in dorsal CN. Prominent differences in density and appearance between M35 immunoreactivity and AChE activity were found in the regions of the rat CN where granule cells predominate.