Cellular basis of temporal synaptic signalling: an in vitro electrophysiological study in rat auditory thalamus

Auditory thalamus dysfunction and pathophysiology in tinnitus: a predictive network hypothesis

Tinnitus is the perception of a 'ringing' sound without an acoustic source. It is generally accepted that tinnitus develops after peripheral hearing loss and is associated with altered auditory processing. The thalamus is a crucial relay in the underlying pathways that actively shapes processing of auditory signals before the respective information reaches the cerebral cortex. Here, we review animal and human evidence to define thalamic function in tinnitus. Overall increased spontaneous firing patterns and altered coherence between the thalamic medial geniculate body (MGB) and auditory cortices is observed in animal models of tinnitus. It is likely that the functional connectivity between the MGB and primary and secondary auditory cortices is reduced in humans. Conversely, there are indications for increased connectivity between the MGB and several areas in the cingulate cortex and posterior cerebellar regions, as well as variability in connectivity between the MGB and frontal areas regarding laterality and orientation in the inferior, medial and superior frontal gyrus. We suggest that these changes affect adaptive sensory gating of temporal and spectral sound features along the auditory pathway, reflecting dysfunction in an extensive thalamo-cortical network implicated in predictive temporal adaptation to the auditory environment. Modulation of temporal characteristics of input signals might hence factor into a thalamo-cortical dysrhythmia profile of tinnitus, but could ultimately also establish new directions for treatment options for persons with tinnitus.

Stimulus-Specific Adaptation in Auditory Thalamus Is Modulated by the Thalamic Reticular Nucleus

A striking property of the auditory system is its capacity for the stimulus-specific adaptation (SSA), which is the reduction of neural response to repeated stimuli but a recuperative response to novel stimuli. SSA is found in both the medial geniculate body (MGB) and thalamic reticular nucleus (TRN). However, it remains unknown whether the SSA of MGB neurons is modulated by inhibitory inputs from the TRN, as it is difficult to investigate using the extracellular recording method. In the present study, we performed intracellular recordings in the MGB of anesthetized guinea pigs and examined whether and how the TRN modulates the SSA of MGB neurons with inhibitory inputs. This was accomplished by using microinjection of lidocaine to inactivate the neural activity of the TRN. We found that (1) MGB neurons with hyperpolarized membrane potentials exhibited SSA at both the spiking and subthreshold levels; (2) SSA of MGB neurons depends on the interstimulus interval (ISI), where a shorter ISI results in stronger SSA; and (3) the long-lasting hyperpolarization of MGB neurons decreased after the burst firing of the TRN was inactivated. As a result, SSA of these MGB neurons was diminished after inactivation of the TRN. Taken together, our results revealed that the SSA of the MGB is strongly modulated by the neural activity of the TRN, which suggests an alternative circuit mechanism underlying the SSA of the auditory thalamus.

Sodium salicylate potentiates the GABAB-GIRK pathway to suppress rebound depolarization in neurons of the rat's medial geniculate body

Rebound depolarization (RD) is a voltage response to the offset from pre-hyperpolarization of neuronal membrane potential, which manifests a particular form of the postsynaptic membrane potential response to inhibitory presynaptic inputs. We previously demonstrated that sodium salicylate (NaSal), a tinnitus inducer, can drastically suppress the RD in neurons of rat medial geniculate body (MGB) (Su et al, 2012; PLoS ONE 7, e46969). The purpose of the present study was to investigate the underlying cellular mechanism by using whole-cell patch-clamp recordings in rat MGB slices. NaSal (1.4 mM) had no effects on the current mediated by T-type Ca(2+) channels, indicating that it does not target these channels to suppress the RD. Instead, NaSal was shown to hyperpolarize the resting membrane potential to suppress the RD. NaSal had no effects on the current mediated by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, indicating that it does not target these channels to hyperpolarize the resting membrane potential. NaSal induced an outward leak current that could be abolished by CGP55845, a GABAB receptor blocker, or respectively by Ba(2+) and Tertiapin-Q, blockers for G-protein-gated inwardly rectifying potassium (GIRK) channels, indicating that NaSal potentiates the GABAB-GIRK pathway to hyperpolarize the resting membrane potential. Our study demonstrates that NaSal targets GABAB receptors to alter functional behaviors of MGB neurons, which may be implicated in NaSal-induced tinnitus.

T-type calcium channels cause bursts of spikes in motor but not sensory thalamic neurons during mimicry of natural patterns of synaptic input

Although neurons within intact nervous systems can be classified as ‘sensory’ or ‘motor,’ it is not known whether there is any general distinction between sensory and motor neurons at the cellular or molecular levels. Here, we extend and test a theory according to which activation of certain subtypes of voltage-gated ion channel (VGC) generate patterns of spikes in neurons of motor systems, whereas VGC are proposed to counteract patterns in sensory neurons. We previously reported experimental evidence for the theory from visual thalamus, where we found that T-type calcium channels (TtCCs) did not cause bursts of spikes but instead served the function of ‘predictive homeostasis’ to maximize the causal and informational link between retinogeniculate excitation and spike output. Here, we have recorded neurons in brain slices from eight sensory and motor regions of rat thalamus while mimicking key features of natural excitatory and inhibitory post-synaptic potentials. As predicted by theory, TtCC did cause bursts of spikes in motor thalamus. TtCC-mediated responses in motor thalamus were activated at more hyperpolarized potentials and caused larger depolarizations with more spikes than in visual and auditory thalamus. Somatosensory thalamus is known to be more closely connected to motor regions relative to auditory and visual thalamus, and likewise the strength of its TtCC responses was intermediate between these regions and motor thalamus. We also observed lower input resistance, as well as limited evidence of stronger hyperpolarization-induced (‘H-type’) depolarization, in nuclei closer to motor output. These findings support our theory of a specific difference between sensory and motor neurons at the cellular level.

GABAergic inhibition shapes SAM responses in rat auditory thalamus

Auditory thalamus (medial geniculate body [MGB]) receives ascending inhibitory GABAergic inputs from inferior colliculus (IC) and descending GABAergic projections from the thalamic reticular nucleus (TRN) with both inputs postulated to play a role in shaping temporal responses. Previous studies suggested that enhanced processing of temporally rich stimuli occurs at the level of MGB, with our recent study demonstrating enhanced GABA sensitivity in MGB compared to IC. The present study used sinusoidal amplitude-modulated (SAM) stimuli to generate modulation transfer functions (MTFs), to examine the role of GABAergic inhibition in shaping the response properties of MGB single units in anesthetized rats. Rate MTFs (rMTFs) were parsed into "bandpass (BP)", "mixed (Mixed)", "highpass (HP)" or "atypical" response types, with most units showing the Mixed response type. GABAA receptor blockade with iontophoretic application of the GABAA receptor (GABAAR) antagonist gabazine (GBZ) selectively altered the response properties of most MGB neurons examined. Mixed and HP units showed significant GABAAR-mediated SAM-evoked rate response changes at higher modulation frequencies (fms), which were also altered by N-methyl-d-aspartic acid (NMDA) receptor blockade (2R)-amino-5-phosphonopentanoate (AP5). BP units, and the lower arm of Mixed units responded to GABAAR blockade with increased responses to SAM stimuli at or near the rate best modulation frequency (rBMF). The ability of GABA circuits to shape responses at higher modulation frequencies is an emergent property of MGB units, not observed at lower levels of the auditory pathway and may reflect activation of MGB NMDA receptors (Rabang and Bartlett, 2011; Rabang et al., 2012). Together, GABAARs exert selective rate control over selected fms, generally without changing the units' response type. These results showed that coding of modulated stimuli at the level of auditory thalamus is at least, in part, strongly controlled by GABA neurotransmission, in delicate balance with glutamatergic neurotransmission.

Effects of location and timing of co-activated neurons in the auditory midbrain on cortical activity: implications for a new central auditory prosthesis

OBJECTIVE

An increasing number of deaf individuals are being implanted with central auditory prostheses, but their performance has generally been poorer than for cochlear implant users. The goal of this study is to investigate stimulation strategies for improving hearing performance with a new auditory midbrain implant (AMI). Previous studies have shown that repeated electrical stimulation of a single site in each isofrequency lamina of the central nucleus of the inferior colliculus (ICC) causes strong suppressive effects in elicited responses within the primary auditory cortex (A1). Here we investigate if improved cortical activity can be achieved by co-activating neurons with different timing and locations across an ICC lamina and if this cortical activity varies across A1.

APPROACH

We electrically stimulated two sites at different locations across an isofrequency ICC lamina using varying delays in ketamine-anesthetized guinea pigs. We recorded and analyzed spike activity and local field potentials across different layers and locations of A1.

RESULTS

Co-activating two sites within an isofrequency lamina with short inter-pulse intervals (<5 ms) could elicit cortical activity that is enhanced beyond a linear summation of activity elicited by the individual sites. A significantly greater extent of normalized cortical activity was observed for stimulation of the rostral-lateral region of an ICC lamina compared to the caudal-medial region. We did not identify any location trends across A1, but the most cortical enhancement was observed in supragranular layers, suggesting further integration of the stimuli through the cortical layers.

SIGNIFICANCE

The topographic organization identified by this study provides further evidence for the presence of functional zones across an ICC lamina with locations consistent with those identified by previous studies. Clinically, these results suggest that co-activating different neural populations in the rostral-lateral ICC rather than the caudal-medial ICC using the AMI may improve or elicit different types of hearing capabilities.

The organization and physiology of the auditory thalamus and its role in processing acoustic features important for speech perception

The auditory thalamus, or medial geniculate body (MGB), is the primary sensory input to auditory cortex. Therefore, it plays a critical role in the complex auditory processing necessary for robust speech perception. This review will describe the functional organization of the thalamus as it relates to processing acoustic features important for speech perception, focusing on thalamic nuclei that relate to auditory representations of language sounds. The MGB can be divided into three main subdivisions, the ventral, dorsal, and medial subdivisions, each with different connectivity, auditory response properties, neuronal properties, and synaptic properties. Together, the MGB subdivisions actively and dynamically shape complex auditory processing and form ongoing communication loops with auditory cortex and subcortical structures.

Neural integration and enhancement from the inferior colliculus up to different layers of auditory cortex

While the cochlear implant has successfully restored hearing to many deaf patients, it cannot benefit those without a functional auditory nerve or an implantable cochlea. As an alternative, the auditory midbrain implant (AMI) has been developed and implanted into deaf patients. Consisting of a single-shank array, the AMI is designed for stimulation along the tonotopic gradient of the inferior colliculus (ICC). Although the AMI can provide frequency cues, it appears to insufficiently transmit temporal cues for speech understanding because repeated stimulation of a single site causes strong suppressive and refractory effects. Applying the electrical stimulation to at least two sites within an isofrequency lamina can circumvent these refractory processes. Moreover, coactivation with short intersite delays (<5 ms) can elicit cortical activation which is enhanced beyond the summation of activity induced by the individual sites. The goal of our study was to further investigate the role of the auditory cortex in this enhancement effect. In guinea pigs, we electrically stimulated two locations within an ICC lamina or along different laminae with varying interpulse intervals (0-10 ms) and recorded activity in different locations and layers of primary auditory cortex (A1). Our findings reveal a neural mechanism that integrates activity only from neurons located within the same ICC lamina for short spiking intervals (<6 ms). This mechanism leads to enhanced activity into layers III-V of A1 that is further magnified in supragranular layers. This integration mechanism may contribute to perceptual coding of different sound features that are relevant for improving AMI performance.

Altered neuronal intrinsic properties and reduced synaptic transmission of the rat's medial geniculate body in salicylate-induced tinnitus

Sodium salicylate (NaSal), an aspirin metabolite, can cause tinnitus in animals and human subjects. To explore neural mechanisms underlying salicylate-induced tinnitus, we examined effects of NaSal on neural activities of the medial geniculate body (MGB), an auditory thalamic nucleus that provides the primary and immediate inputs to the auditory cortex, by using the whole-cell patch-clamp recording technique in MGB slices. Rats treated with NaSal (350 mg/kg) showed tinnitus-like behavior as revealed by the gap prepulse inhibition of acoustic startle (GPIAS) paradigm. NaSal (1.4 mM) decreased the membrane input resistance, hyperpolarized the resting membrane potential, suppressed current-evoked firing, changed the action potential, and depressed rebound depolarization in MGB neurons. NaSal also reduced the excitatory and inhibitory postsynaptic response in the MGB evoked by stimulating the brachium of the inferior colliculus. Our results demonstrate that NaSal alters neuronal intrinsic properties and reduces the synaptic transmission of the MGB, which may cause abnormal thalamic outputs to the auditory cortex and contribute to NaSal-induced tinnitus.

Thalamocortical projections to rat auditory cortex from the ventral and dorsal divisions of the medial geniculate nucleus

The ventral and dorsal medial geniculate (MGV and MGD) constitute the major auditory thalamic subdivisions providing thalamocortical inputs to layer IV and lower layer III of auditory cortex. No quantitative evaluation of this projection is available. Using biotinylated dextran amine (BDA)/biocytin injections, we describe the cortical projection patterns of MGV and MGD cells. In primary auditory cortex the bulk of MGV axon terminals are in layer IV/lower layer III with minor projections to supragranular layers and intermediate levels in infragranular layers. MGD axons project to cortical regions designated posterodorsal (PD) and ventral (VA) showing laminar terminal distributions that are quantitatively similar to the MGV-to-primary cortex terminal distribution. At the electron microscopic level MGV and MGD terminals are non-γ-aminobutyric acid (GABA)ergic with MGD terminals in PD and VA slightly but significantly larger than MGV terminals in primary cortex. MGV/MGD terminals synapse primarily onto non-GABAergic spines/dendrites. A small number synapse on GABAergic structures, contacting large dendrites or cell bodies primarily in the major thalamocortical recipient layers. For MGV projections to primary cortex or MGD projections to PD or VA, the non-GABAergic postsynaptic structures at each site were the same size regardless of whether they were in supragranular, granular, or infragranular layers. However, the population of MGD terminal-recipient structures in VA were significantly larger than the MGD terminal-recipient structures in PD or the MGV terminal-recipient structures in primary cortex. Thus, if terminal and postsynaptic structure size indicate strength of excitation then MGD to VA inputs are strongest, MGD to PD intermediate, and MGV to primary cortex the weakest.

Projections from the inferior colliculus to the tectal longitudinal column in the rat

We have used the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) to study with albino rats the projections from the inferior colliculus (IC) to the tectal longitudinal column (TLC), a newly discovered nucleus that spans the midbrain tectum longitudinally, on each side of the midbrain, immediately above the periaqueductal gray matter. We studied the projections of the medial IC, which includes the classical central nucleus (CNIC) and the dorsal cortex (DCIC), and those of the lateral IC, equivalent to the classical external cortex (ECIC). Following unilateral injections of PHA-L into the medial IC, numerous terminal fibers are labeled bilaterally in the TLC. The ipsilateral projection is denser and targets the entire nucleus, whereas the contralateral projection targets significantly only the caudal half or two-thirds of the TLC. Fibers from the medial IC reach the TLC by two routes: as collaterals of axons that travel in the commissure of the IC and as collaterals of thick ipsilateral colliculogeniculate axons; the latter travel through the deep superior colliculus on their way to the TLC. Within the TLC, individual IC fibers tend to run longitudinally. The injection of PHA-L into the lateral IC indicates that this subdivision sends a weak, bilateral projection to the TLC whose trajectory, morphology and distribution are similar to those of the projection from the medial IC. These results demonstrate that all subdivisions of the IC send projections to the TLC, suggesting that the IC may be one of the main sources of auditory input to this tectal nucleus.

Topography and physiology of ascending streams in the auditory tectothalamic pathway

Auditory information is relayed from the cochlea along parallel pathways and reaches the inferior colliculus (IC) and the medial geniculate body (MGB) en route to the cortex. Although the ascending tectothalamic pathway to the ventral division of the MGB is regarded as a high-fidelity information-bearing channel, the roles of the pathways to the dorsal and medial divisions are more opaque. Here, we show fundamental differences between these ascending pathways using an in vitro slice preparation. Using photostimulation, we found three main patterns of input (excitatory, inhibitory, and mixed) that differed in each pathway. Furthermore, electrical stimulation of the central nucleus of the IC evoked a depressing response in the MGB with no metabotropic glutamate (mGlu) receptor component, whereas stimulation of the lateral cortex of the IC evoked a facilitating response with an mGlu receptor component. These data suggest that the ascending tectothalamic pathways are functionally distinct from one another.

Axonal projections of auditory cells with short and long response latencies in the medial geniculate nucleus: distinct topographies in the connection with the thalamic reticular nucleus

The thalamic reticular nucleus (TRN) is a crucial anatomical node of thalamocortical connectivity for sensory processing. In the rat auditory system, we determined features of thalamic projections to the TRN, using juxtacellular recording and labeling techniques. Two types of auditory cells (short latency, SL, and long latency, LL), exhibiting unit discharges to noise burst stimuli (duration, 100 ms) with short (< 50 ms) and long (> 100 ms) response latencies, were obtained from the ventral division of the medial geniculate nucleus (MGV). Both SL and LL cells had a propensity to exhibit reverberatory discharges in response to sound stimuli. The primary discharges of SL cells were mostly single spikes while the non-primary discharges of SL cells and the whole discharges of LL cells were mostly burst spikes. SL cells sent topographic projections to the TRN along the dorsoventral and rostrocaudal neural axes while LL cells only along the rostrocaudal axis. As tonotopy-related cortical projections to the TRN are topographic primarily along the dorsoventral extent of the TRN and the MGV is tonotopically organized along the dorsoventral axis, SL cells, directly activated by ascending auditory inputs, may be closely involved in tonotopic thalamocortical connectivity. On the other hand, LL cells, which are suppressed by ascending inputs and could be driven to discharge by corticofugal inputs, are assumed to activate the TRN in a manner less related to tonotopic organization. There may exist heterogeneous projections from the MGV to the TRN, which, in conjunction with corticofugal connections, could constitute distinct channels of auditory processing.

Functional architecture and spike timing properties of corticofugal projections from rat ventral temporal cortex

Sensory association and parahippocampal cortex in the ventral temporal lobe plays an important role in sensory object recognition and control of top-down attention. Although layer V neurons located in high-order cortical structures project to multiple cortical and subcortical regions, the architecture and functional organization of this large axonal network are poorly understood. Using a large in vitro slice preparation, we examined the functional organization and spike timing properties of the descending layer V axonal network. We found that most, if not all, layer V neurons in this region can form multiple axonal pathways that project to many brain structures, both proximal and remote. The conduction velocities of different axonal pathways are highly diverse and can vary up to more than threefold. Nevertheless for those axonal projections on the ipsilateral side, the speeds of axonal conduction appear to be tuned to their length. As such, spike delivery becomes nearly isochronic along these pathways regardless of projection distance. In contrast, axons projecting to the contralateral hemisphere are significantly slower and do not participate in this lateralized isochronicity. These structural and functional features of layer V network from the ventral temporal lobe may play an important role in top-down control of sensory cue processing and attention.

Theta, alpha and beta burst transcranial magnetic stimulation: brain modulation in tinnitus

INTRODUCTION

Some forms of tinnitus are considered to be auditory phantom phenomena related to reorganization and hyperactivity of the auditory central nervous system. Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive tool capable of modulating human brain activity, using single pulse or burst stimuli. Burst rTMS has only been performed in the theta range, and has not been used clinically. The authors analyze whether burst TMS at theta (5 Hz), alpha (10 Hz) and beta (20 Hz) frequencies can temporarily suppress narrow band noise/white noise tinnitus, which has been demonstrated to be intractable to tonic stimulation.

METHODS

rTMS is performed both in tonic and burst mode in 46 patients contralateral to the tinnitus side, at 5, 10 and 20 Hz. Fourteen placebo negative rTMS responders are further analyzed.

RESULTS

In 5 patients, maximal tinnitus suppression is obtained with theta, in 2 with alpha and in 7 with beta burst stimulation. Burst rTMS suppresses narrow band/white tinnitus much better than tonic rTMS t(13)=6.4, p<.000. Women experience greater suppression of their tinnitus with burst stimulation than men, t(12)=2.9, p<.05. Furthermore left sided tinnitus is perceived as more distressing on the TQ than right sided tinnitus, t(12)=3.2, p<.01. The lower the tinnitus pitch the more effectively rTMS suppresses tinnitus(r=-0.65, p<0.05).

DISCUSSION

Burst rTMS can be used clinically, not only theta burst, but also alpha and beta burst. Burst rTMS is capable of suppressing narrow band/white noise tinnitus very much better than tonic rTMS. This could be due the simple fact that burst neuromodulation is more powerful than tonic neuromodulation or to a differential effect of burst and tonic stimulation on the lemniscal and extralemniscal auditory system. In some patients only alpha or beta burst rTMS is capable of suppressing tinnitus, and theta burst not. Therefore in future rTMS studies it could be worthwhile not to limit burst stimulation to theta burst rTMS.

Unique combination of anatomy and physiology in cells of the rat paralaminar thalamic nuclei adjacent to the medial geniculate body

The medial geniculate body (MGB) has three major subdivisions, ventral (MGV), dorsal (MGD), and medial (MGM). MGM is linked with paralaminar nuclei that are situated medial and ventral to MGV/MGD. Paralaminar nuclei have unique inputs and outputs compared with MGV and MGD and have been linked to circuitry underlying some important functional roles. We recorded intracellularly from cells in the paralaminar nuclei in vitro. We found that they possess an unusual combination of anatomical and physiological features compared with those reported for "standard" thalamic neurons seen in the MGV/MGD and elsewhere in the thalamus. Compared with MGV/MGD neurons, anatomically, 1) paralaminar cell dendrites can be long, branch sparingly, and encompass a much larger area; 2) their dendrites may be smooth but can have well defined spines; and 3) their axons can have collaterals that branch locally within the same or nearby paralaminar nuclei. When compared with MGV/MGD neurons, physiologically, 1) their spikes are larger in amplitude and can be shorter in duration; 2) their spikes can have dual afterhyperpolarizations with fast and slow components; and 3) they can have a reduction or complete absence of the low-threshold, voltage-sensitive calcium conductance that reduces or eliminates the voltage-dependent burst response. We also recorded from cells in the parafascicular nucleus, a nucleus of the posterior intralaminar nuclear group, because they have unusual anatomical features that are similar to those of some of our paralaminar cells. As with the labeled paralaminar cells, parafascicular cells had physiological features distinguishing them from typical thalamic neurons.

Electrophysiological properties and thermosensitivity of mouse preoptic and anterior hypothalamic neurons in culture

Responses of mouse preoptic and anterior hypothalamic neurons to variations of temperature are key elements in regulating the setpoint of homeotherms. The goal of the present work was to assess the relevance of culture preparations for investigating the cellular mechanisms underlying thermosensitivity in hypothalamic cells. Our working hypothesis was that some of the main properties of preoptic/anterior hypothalamic neurons in culture are similar to those reported by other authors in slice preparations. Indeed, cultured preoptic/anterior hypothalamic neurons share many of the physiological and morphological properties of neurons in hypothalamic slices. They display heterogenous dendritic arbors and somatic shapes. Most of them are GABAergic and their activity is synaptically driven by the activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptors. Active membrane properties include a depolarizing "sag" in response to hyperpolarization, and a low threshold spike, which is present in a majority of cells and is generated by T-type Ca2+ channels. In a fraction of the cells, the low threshold spike repeats rhythmically, either spontaneously, or in response to depolarization. The background synaptic noise in cultured neurons is characterized by the presence of numerous postsynaptic potentials which can be easily distinguished from the baseline, thus providing an opportunity for assessing their possible roles in thermosensitivity. An unexpected finding was that GABA-A receptors can generate both hyper- and depolarizing postsynaptic potentials in the same neuron. About 20% of the spontaneously firing preoptic/anterior hypothalamic neurons are warm-sensitive. Warming (32-41 degrees C) depolarizes some cells, a phenomenon which is Na+-dependent and tetrodotoxin-insensitive. The increased firing rate of warm-sensitive cells in response to warming can be prepotential and/or synaptically driven. Overall, our data suggest that a warm-sensitive phenotype is already developed in cultured cells. Therefore, and despite obvious differences in their networks, cultured and slice preparations of hypothalamic neurons can complement each other for further studies of warm-sensitivity at the cellular and molecular level.

Burst firing induces a slow after hyperpolarization in rat auditory thalamus

The Ca2+-activated slow after hyperpolarization (sAHP) is found in many CNS regions where it may induce post-spike suppression of neuronal firing over many seconds. Nevertheless, the presence of sAHP in sensory thalamus remains uncertain. Here we show that a robust sAHP could be evoked in the rat medial geniculate body of auditory thalamus in vitro following a low-threshold Ca2+ spike and burst firing. The evoked sAHP exhibited kinetic and pharmacological features similar to that found elsewhere in the CNS. The sAHP was resistant to TTX or apamin but eliminated by muscarine. Furthermore, activation of low-threshold Ca2+ conductance alone is sufficient to induce the sAHP. Therefore, the membrane conductance underlying sAHP is functionally expressed in lemniscal thalamic relay neurons which may be preferably activated during burst firing.

In vivo intracellular responses of the medial geniculate neurones to acoustic stimuli in anaesthetized guinea pigs

In the present study, we investigated the auditory response features of the medial geniculate neurones, using in vivo intracellular recordings in anaesthetized guinea pigs. Of the 76 neurones examined, 9 showed 'off' or 'on-off' responses to an acoustic stimulus and thus were defined as 'off' or 'on-off' neurones. Among the remaining 67 neurones, 42 showed an excitatory postsynaptic potential (EPSP) to acoustic stimuli and 25 showed either a pure inhibitory postsynaptic potential (IPSP, 7 neurones), or an IPSP preceded by an EPSP (EPSP-IPSP type, 18 neurones). The EPSP responses exhibited a mean latency of 15.7 +/- 6.1 ms, which was significantly shorter than that of the IPSP responses (21.3 +/- 8.6 ms, P < 0.01). The IPSP responses also showed a significantly greater duration than the EPSP responses (208.5 +/- 128.2 ms versus 122.4 +/- 84.8 ms, P < 0.05), while there were no significant differences between the amplitudes of IPSP and EPSP (8.3 +/- 3.2 mV versus 8.7 +/- 5.3 mV). Of the 11 neurones that showed EPSP responses to acoustic stimuli and were histologically labelled, 7 were located in the lemniscal medial geniculate body (MGB) and 4 in the non-lemniscal MGB. Another 6 labelled neurones that showed IPSP responses to acoustic stimuli were located in the non-lemniscal MGB. With a membrane potential of above -72 mV, the neurones showed greater EPSP or IPSP to an acoustic stimulus when their membrane potential was depolarized. However, upon hyperpolarization to below -74 mV, the neurones shifted to low-threshold calcium spikes (LTS)/LTS bursts. In response to auditory stimuli of different durations, 'off' neurones that responded to the offset of the acoustic stimulus and were located in the non-lemniscal MGB showed different response latencies or deviations of latencies in addition to exhibiting different numbers of spikes, suggesting that the timing of the spikes could be another component utilized by thalamic neurones to encode information on the stimulus. Given that some non-lemniscal neurones are multisensory and project to the entire auditory cortex, the selective corticofugal inhibition in the non-lemniscal MGB would enable the ascending pathway to prepare the auditory cortex to receive subsequent auditory information, avoiding the interference of other sensory inputs.

Corticofugal gating of auditory information in the thalamus: an in vivo intracellular recording study

In the present study, we investigated the auditory responses of the medial geniculate (MGB) neurons, through in vivo intracellular recordings of anesthetized guinea pigs, while the auditory cortex was electrically activated. Of the 63 neurons that received corticofugal modulation of the membrane potential, 30 received potentiation and 33 received hyperpolarization. The corticofugal potentiation of the membrane potential (amplitude, mean +/- SD, 8.6 +/- 5.5 mV; duration, 125.5 +/- 75.4 msec) facilitated the auditory responses and spontaneous firing of the MGB neurons. The hyperpolarization of -11.3 +/- 4.9 mV in amplitude and 210.0 +/- 210.1 msec in duration suppressed the auditory responses and spontaneous firing of the MGB neurons. Four of the five neurons that were histologically confirmed to be located in the lemniscal MGB received corticofugal facilitatory modulation, and all of the four neurons that were confirmed to be located in the non-lemniscal MGB received corticofugal inhibitory modulation. The present intracellular recording provides novel results on how the corticofugal projection gates the sensory information in the thalamus: via the spatially selective depolarization of lemniscal MGB neurons and hyperpolarization of non-lemniscal MGB neurons. It is speculated that the systematic selectivity of facilitation and inhibition over the lemniscal and non-lemniscal MGB is related to the attention shift within the auditory modality and across the sensory modalities.

Auditory Thalamus Bursts in Anesthetized and Non-Anesthetized States: Contribution to Functional Properties

Over the last 10 years, high-frequency bursts of action potentials have been the subject of intense researches to understand their potential role in information encoding. Based on recordings from auditory thalamus neurons ( n = 302) collected during anesthesia (pentobarbital, urethan, or ketamine/xylazine), waking (W), and slow-wave sleep (SWS), we investigated how bursts participate to frequency tuning, intensity-function, response latency (and latency variability), and stimulus detectability. Although present in all experimental conditions, bursts never dominated the cells mode of discharge: the highest proportion was found during ketamine/xylazine anesthesia (22%), the lowest during waking (4.5%). In all experimental conditions, bursts preferentially occurred at or around the cells best frequency (BF), thus increasing the frequency selectivity. This effect was observed at both the intensities producing the highest and the lowest evoked responses. Testing the intensity-functions indicated that for most of the cells, there was no systematic relationship between burst proportion and responses strength. Under several conditions (W, SWS, and urethan), when cells exhibited bursts >20%, the variability of their response latency was reduced in burst mode compared with single-spike mode. During W, this effect was accompanied by a reduction of the response latency. Finally, a receiver operating characteristic analysis indicated no particular relation between bursts and stimulus detectability. Compared with single-spike mode, which is present for broader frequency ranges, the prominence of bursts at the BF should contribute to filter information reaching the targets of medial geniculate cells at both cortical and subcortical levels.

Distinct forms of cholinergic modulation in parallel thalamic sensory pathways

Mammalian thalamus is a critical site where early perception of sensorimotor signals is dynamically regulated by acetylcholine in a behavioral state-dependent manner. In this study, we examined how synaptic transmission is modulated by acetylcholine in auditory thalamus where sensory relay neurons form parallel lemniscal and nonlemniscal pathways. The former mediates tonotopic relay of acoustic signals, whereas the latter is involved in detecting and transmitting auditory cues of behavioral relevance. We report here that activation of cholinergic muscarinic receptors had opposite membrane effects on these parallel synaptic pathways. In lemniscal neurons, muscarine induced a sustained membrane depolarization and tonic firing by closing a linear K(+) conductance. In contrast, in nonlemniscal neurons, muscarine evoked a membrane hyperpolarization by opening a voltage-independent K(+) conductance. Depending on the level of membrane hyperpolarization and the strength of local synaptic input, nonlemniscal neurons were either suppressed or selectively engaged in detecting and transmitting synchronized synaptic input by firing a high-frequency spike burst. Immunohistochemical and Western blotting experiments showed that nonlemniscal neurons predominantly expressed M2 muscarinic receptors, whereas lemniscal cells had a significantly higher level of M1 receptors. Our data indicate that cholinergic modulation in the thalamus is pathway-specific. Enhanced cholinergic tone during behavioral arousal or attention may render synaptic transmission in nonlemniscal thalamus highly sensitive to the context of local synaptic activities.

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.

Slow oscillation in non-lemniscal auditory thalamus

In the present study, we investigated the oscillatory behavior of the auditory thalamic neurons through in vivo intracellular and extracellular recordings in anesthetized guinea pigs. Repeated acoustic stimulus and cortical electrical stimulation were applied to examine their modulatory effects on the thalamic oscillation. The time course of the spike frequency over each trial was obtained by summing all spikes in the onset period and/or the last time period of 100 or 200 msec in the raster display. Spectral analysis was made on the time course of the spike frequency. A slow-frequency oscillation ranging from 0.03 to 0.25 Hz (mean +/- SD, 0.11 +/- 0.05 Hz) was found in the medial geniculate body (MGB) together with a second rhythm of 5-10 Hz. The oscillation neurons had a mean auditory response latency of 17.3 +/- 0.3 msec, which was significantly longer than that of the non-oscillation neurons in lemniscal MGB (9.0 +/- 1.5 msec, p < 0.001, ANOVA) and similar to the non-oscillation neurons in the non-lemniscal MGB (17.6 +/- 5.4 msec, p = 0.811). They were located in the non-lemniscal nuclei of the auditory thalamus. Cortical stimulation altered the thalamic oscillation, leading to termination of the oscillation or to acceleration of the rhythm of the oscillation (the average rhythm changed from 0.07 +/- 0.03 to 0.11 +/- 0.04 Hz, n = 8, p = 0.066, t test). Acoustic stimulation triggered a more regular rhythm in the oscillation neurons. The present results suggest that only the non-lemniscal auditory thalamus is involved in the slow thalamocortical oscillation. The auditory cortex may control the oscillation of the auditory thalamic neurons.

Slow oscillation in non-lemniscal auditory thalamus

In the present study, we investigated the oscillatory behavior of the auditory thalamic neurons through in vivo intracellular and extracellular recordings in anesthetized guinea pigs. Repeated acoustic stimulus and cortical electrical stimulation were applied to examine their modulatory effects on the thalamic oscillation. The time course of the spike frequency over each trial was obtained by summing all spikes in the onset period and/or the last time period of 100 or 200 msec in the raster display. Spectral analysis was made on the time course of the spike frequency. A slow-frequency oscillation ranging from 0.03 to 0.25 Hz (mean +/- SD, 0.11 +/- 0.05 Hz) was found in the medial geniculate body (MGB) together with a second rhythm of 5-10 Hz. The oscillation neurons had a mean auditory response latency of 17.3 +/- 0.3 msec, which was significantly longer than that of the non-oscillation neurons in lemniscal MGB (9.0 +/- 1.5 msec, p < 0.001, ANOVA) and similar to the non-oscillation neurons in the non-lemniscal MGB (17.6 +/- 5.4 msec, p = 0.811). They were located in the non-lemniscal nuclei of the auditory thalamus. Cortical stimulation altered the thalamic oscillation, leading to termination of the oscillation or to acceleration of the rhythm of the oscillation (the average rhythm changed from 0.07 +/- 0.03 to 0.11 +/- 0.04 Hz, n = 8, p = 0.066, t test). Acoustic stimulation triggered a more regular rhythm in the oscillation neurons. The present results suggest that only the non-lemniscal auditory thalamus is involved in the slow thalamocortical oscillation. The auditory cortex may control the oscillation of the auditory thalamic neurons.

Hyperpolarization-activated (Ih) conductances affect brainstem auditory neuron excitability

Blockade of the hyperpolarization-activated cyclic-nucleotide-gated mixed-cationic conductance (I(h)) by ZD7288 markedly reduces excitability of neurons in the superior olivary complex (SOC), in vivo. Following pressure ejection application of 100 microM ZD7288, extracellular recorded single unit responses of 47/47 SOC neurons to monaural or binaural pure tone best frequency (BF) stimuli (30 dB above threshold) decreased by 49.7+/-19%, and background activity decreased by 56.3+/-18.1%. Pressure ejection of the vehicle did not affect excitability. The dose- and time-dependence of ZD7288 (10-100 microM) effects are consistent with specific blockade of I(h) currents. SOC neuron responses to pressure-ejected glutamate were also decreased following application of 100 microM ZD7288 by 76.7+/-28.0%, which suggests a predominant direct effect of ZD7288 on auditory cell excitability. The considerable variability in the magnitude of ZD7288 effects between cells was only partially accounted for by greater effects on neurons with BFs greater than 16 kHz. Therefore, I(h) channels significantly contribute to auditory brainstem neuron excitability, affecting their response level to acoustic stimuli. The variability in the ZD7288 reduction in excitability and its variation with the BF of units could be an indication of regulation and plasticity in neuronal encoding of sounds.

Spermine modulates neuronal excitability and NMDA receptors in juvenile gerbil auditory thalamus

Medial geniculate body (MGB) neurons process synaptic inputs from auditory cortex. Corticothalamic stimulation evokes glutamatergic excitatory postsynaptic potentials (EPSPs) that vary markedly in amplitude and duration during development. The EPSP decay phase is prolonged during second postnatal week but then shortens, significantly, until adulthood. The EPSP prolongation depends on spermine interactions with a polyamine-sensitive site on receptors for N-methyl-D-aspartate (NMDA). We examined effects of spermine application on EPSPs, firing modes, and membrane properties in gerbil MGB neurons during the P14 period of highest polyamine sensitivity. Spermine slowed EPSP decay and promoted firing on EPSPs, without changing passive membrane properties. Spermine increased membrane rectification on depolarization, which is mediated by tetrodotoxin (TTX)-sensitive, persistent Na(+) conductance. As a result, spermine lowered threshold and increased tonic firing evoked with current injection by up to approximately 150%. These effects were concentration-dependent (ED(50)=100 microM), reversible, and eliminated by NMDA receptor antagonist, 2-amino-5-phosphonovalerate (APV). In contrast, spermine increased dV/dt of the low threshold Ca(2+) spike (LTS) and burst firing, evoked from hyperpolarized potentials. LTS enhancement was greater at -55 mV than at hyperpolarized potentials and did not result from persistent Na(+) conductance or glutamate receptor mechanisms. In summary, spermine increased excitability by modulating NMDA receptors in juvenile gerbil neurons.

Corticofugal modulation on both ON and OFF responses in the nonlemniscal auditory thalamus of the guinea pig

Corticofugal modulation on both ON and OFF responses in various nuclei in the medial geniculate body (MGB) was examined by locally activating the auditory cortex and looking for effects on the neuronal responses to acoustic stimuli. In contrast with a major corticofugal facilitatory effect on the ON neurons in the lemniscal nucleus of the MGB of the guinea pigs, of 132 ON neurons tested in three conditions with cortical activation through each of three implanted electrodes, the majority of the tested conditions (319/396) that were sampled from the nonlemniscal nuclei of the MGB received inhibitory modulation from the activated cortex. This inhibitory effect was >50% for 99 cases while the auditory cortex was activated. Most of the OFF and ON-OFF MGB neurons (44/54) showed a facilitatory effect of 111.4 +/- 99.9%, and three showed a small inhibitory effect of 25.7 +/- 5.8% on their OFF responses. Thirty neurons in the border region between the lemniscal and nonlemniscal MGB showed mainly facilitatory corticofugal effects on both ON and OFF responses. Meanwhile, cortical stimulation induced almost exclusive inhibitory effects on the ON response and facilitatory effects on the OFF response in the MGcm. It is suggested that the OFF response is produced as a disinhibition from the inhibitory input of the auditory stimulus. The present results provide a possible explanation for selective gating of the auditory information through the lemniscal MGB while switching off other unwanted sensory signals and the interference from the limbic system, leaving the other auditory cortex prepared to process only the auditory signal.

OFF responses in the auditory thalamus of the guinea pig

ON and OFF auditory responses were examined in the medial geniculate body (MGB) of the guinea pig. Single- and multiunit recordings were carried out on 12 anesthetized animals, and noise-burst or pure-tone stimuli were applied to the ear contralateral to the recording hemisphere. One hundred and thirty-five OFF or ON-OFF neurons and 160 ON neurons were studied, and the tuning curves of 21 ON-OFF or OFF neurons were examined from various nuclei of the MGB. The mean minimum threshold of the OFF responses (40.8 +/- 20.0 dB SPL, mean +/- SD; range: 0-80 dB SPL) was significantly higher than that of the ON responses (28.5 +/- 17.6 dB SPL, range: 0-60 dB SPL; n = 17, P < 0.001). Of 10 ON-OFF neurons that showed identifiable tuning frequencies for both ON and OFF responses, 7 showed a higher OFF than ON best frequency (BF), 2 showed the same BF for both ON and OFF, and only 1 showed a slightly lower OFF than ON BF. Most OFF responses sampled from the borders of the ventral (MGv) and the rostromedial (MGrm) nuclei of the MGB showed single-peaked tuning curves, similar to those of the ON responses in the MGv. The neurons located in the shell (MGs) and dorsal (MGd) nuclei of the MGB showed complicated-either multi-peaked or broad-tuning curves. All OFF responses showed long-duration-selectivity for acoustic stimuli: the mean half-maximum duration was 116.5 +/- 114.8 ms (n = 19, range: 27-411 ms). The latencies of 135 OFF responses were studied in various divisions of the MGB. The ventral border region of MGv showed the shortest latency, followed by the dorsal border region of the MGv, the MGrm, and the caudomedial nucleus (MGcm) of the MGB. The posterior nucleus of the thalamus (Po), the MGd, and the MGs showed much longer mean latencies of >30 ms (P < 0.05 compared with the border regions of the MGv, ANOVA), with Po showing the greatest mean latency of 60.3 ms and the greatest deviation of 25.5 ms). The latency of the OFF response (29.0 +/- 14.0 ms, n = 135) was significantly greater than that of the ON response (15.6 +/- 9.6 ms, n = 160, P < 0.001). The present results provide valuable information about the threshold, frequency tuning characteristics, minimal response latency, and duration selectivity of OFF neurons in the auditory thalamus.

Differential distribution of burst and single-spike responses in auditory thalamus

The medial geniculate body (MGB) of the auditory thalamus comprises lemniscal and nonlemniscal neurons that project to the primary auditory cortex and limbic structures, respectively. Here we show that in anesthetized guinea pigs, MGB responses to a noise-burst stimulus exhibit distinct and synaptic pathway-specific firing patterns. The majority of nonlemniscal MGB cells exhibited bursting responses, whereas lemniscal neurons discharged mainly single or spike doublets. The burst firing is delayed in nonlemniscal neurons and exhibited several features that are characteristics of those mediated by low-threshold Ca(2+) spikes. Such a synaptic pathway-specific allocation of bursting and single-spike firing patterns is consistent with the notion of parallel processing of auditory information in thalamocortical system.

Modulatory effect of cortical activation on the lemniscal auditory thalamus of the Guinea pig

In the present study, we investigated the point-to-point modulatory effects from the auditory cortex to the thalamus in the guinea pig. Corticofugal modulation on thalamic neurons was studied by electrical activation of the auditory cortex. The modulation effect was sampled along the frontal or sagittal planes of the auditory thalamus, focusing on the ventral division (MGv) of the medial geniculate body (MGB). Electrical activation was targeted at the anterior and dorsocaudal auditory fields, to which the MGv projects and from which it assumptively receives reciprocal projections. Of the 101 MGv neurons examined by activation of the auditory cortex through passing pulse trains of 100-200 microA current into one after another of the three implanted electrodes (101 neurons x 3 stimulation sites = 303 cases), 208 cases showed a facilitatory effect, 85 showed no effect, and only 10 cases (7 neurons) showed an inhibitory effect. Among the cases of facilitation, 63 cases showed a facilitatory effect >100%, and 145 cases showed a facilitatory effect from 20-100%. The corticofugal modulatory effect on the MGv of the guinea pig showed a widespread, strong facilitatory effect and very little inhibitory effect. The MGv neurons showed the greatest facilitations to stimulation by the cortical sites, with the closest correspondence in BF. Six of seven neurons showed an elevation of the rate-frequency functions when the auditory cortex was activated. The comparative results of the corticofugal modulatory effects on the MGv of the guinea pig and the cat, together with anatomical findings, hint that the strong facilitatory effect is generated through the strong corticothalamic direct connection and that the weak inhibitory effect might be mainly generated via the interneurons of the MGv. The temporal firing pattern of neuronal response to auditory stimulus was also modulated by cortical stimulation. The mean first-spike latency increased significantly from 15.7 +/- 5.3 ms with only noise-burst stimulus to 18.3 +/- 4.9 ms (n = 5, P < 0.01, paired t-test), while the auditory cortex was activated with a train of 10 pulses. Taking these results together with those of previous experiments conducted on the cat, we speculate that the relatively weaker inhibitory effect compared with that in the cat could be due to the smaller number of interneurons in the guinea pig MGB. The corticofugal modulation of the firing pattern of the thalamic neurons might enable single neurons to encode more auditory information using not only the firing rate but also the firing pattern.

Parvalbumin and calbindin are differentially distributed within primary and secondary subregions of the mouse auditory forebrain

The calcium binding proteins parvalbumin and calbindin are thought to differentially regulate physiological functions and often show complementary distributions in the CNS. Our goal was to determine parvalbumin and calbindin distributions in the different subdivisions of mouse auditory thalamus and auditory cortex. Following fixation, FVB mouse brains (postnatal days 38-80) were sectioned along coronal and horizontal planes, then processed for parvalbumin and calbindin immunohistochemistry (antibodies: parvalbumin pa-235, calbindin-d-28k cl-300). Strong complementary differences in calcium binding protein distributions were found in mouse auditory thalamus. The ventral division of the medial geniculate, which is the principal relay to primary auditory cortex, exhibited dense parvalbumin but weak calbindin immunoreactivity. In contrast, most of the 'secondary' auditory thalamic regions surrounding the ventral division showed strong calbindin and lighter parvalbumin levels. Thus, the mouse auditory thalamus is composed of a parvalbumin positive 'core' surrounded by a calbindin positive 'shell'. Parvalbumin immunoreactivity was also more prominent in the primary auditory cortex than in the secondary belt auditory cortex. Calbindin immunoreactivity in auditory cortex was less clearly divided along primary/secondary lines, especially in supragranular layers. However, within infragranular layers, there was heavier staining in belt areas than in primary auditory cortex. In auditory thalamus, parvalbumin labeling was largely confined to the neuropil, whereas calbindin labeling involved somata and neuropil. In auditory cortex, somata and neuropil were positive for both proteins.In summary, the calcium binding proteins parvalbumin and calbindin were found to be differentially distributed within the primary and non-primary regions of mouse auditory forebrain. These differences in protein distribution may contribute to the distinct types of physiological responses that occur in the primary vs. non-primary areas.

Comparison of the fine structure of cortical and collicular terminals in the rat medial geniculate body

Neurons throughout the rat medial geniculate body, including the dorsal and ventral divisions, display a variety of responses to auditory stimuli. To investigate possible structural determinants of this variability, measurements of axon terminal profile area and postsynaptic dendrite diameter were made on inferior colliculus and corticothalamic terminal profiles in the medial geniculate body identified by anterograde tracer labeling following injections into the inferior colliculus or cortex. Over 90% of the synapses observed were axodendritic, with few axosomatic synapses. Small (<0.5 microm(2)) and large (>1.0 microm(2)) collicular profiles were found throughout the medial geniculate, but were smaller on average in the dorsal division (0.49+/-0.49 microm(2)) than in the ventral division (0.70+/-0.64 microm(2)). Almost all corticothalamic profiles were small and ended on small-caliber dendrites (0.57+/-0.25 microm diameter) throughout the medial geniculate. A few very large (>2.0 microm(2)) corticothalamic profiles were found in the dorsal division and in the marginal zone of the medial geniculate. GABA immunostaining demonstrated the presence of GABAergic profiles arising from cells in the inferior colliculus. These profiles were compared with GABAergic profiles not labeled with anterograde tracer, which were presumed to be unlabeled inferior colliculus profiles or thalamic reticular nucleus profiles. The distributions of dendritic diameters postsynaptic to collicular, cortical and unlabeled GABAergic profiles were compared with dendritic diameters of intracellularly labeled medial geniculate neurons from rat brain slices. Our results demonstrate a corticothalamic projection to medial geniculate body that is similar to other sensory corticothalamic projections. However, the heterogeneous distributions of excitatory inferior collicular terminal sizes and postsynaptic dendritic diameters, along with the presence of a GABAergic inferior collicular projection to dendrites in the medial geniculate body, suggest a colliculogeniculate projection that is more complex than the ascending projections to other sensory thalamic nuclei. These findings may be useful in understanding some of the differences in the response characteristics of medial geniculate neurons in vivo.

Tone-evoked oscillations in the rat auditory cortex result from interactions between the thalamus and reticular nucleus

This study investigates the origins of tone-evoked oscillations (5-13 Hz) in the thalamo-cortical auditory system of anaesthetized rats. In three separate experiments, the auditory sector of the reticular nucleus (RE), the auditory cortex and the auditory thalamus were inactivated by local applications of muscimol (1 mg/mL). To assess the efficacy of this procedure, recordings were performed in the inactivated structure in each experiment; and to determine the extent of the drug diffusion autoradiographic experiments were carried out. The evolution of the strength of the oscillations was followed using power spectra during the whole recording session. In the first experiment, muscimol injection in the auditory RE totally suppressed the tone-evoked oscillations in the auditory thalamus and cortex. In the second experiment, inactivation of the auditory cortex did not interfere with the presence of tone-evoked oscillations in the auditory RE. In the third experiment, inactivation of the auditory thalamus impaired the oscillations produced by cortical stimulation in the auditory RE. From these results, it appears that both the auditory thalamus and the auditory sector of the RE, but not the auditory cortex, are involved in the generation of stimulus-evoked oscillations in the thalamo-cortical auditory system.

Auditory thalamus neurons during sleep: changes in frequency selectivity, threshold, and receptive field size

The present study describes how the frequency receptive fields (RF) of auditory thalamus neurons are modified when the state of vigilance of an unanesthetized animal naturally fluctuates among wakefulness (W), slow-wave sleep (SWS), and paradoxical sleep (PS). Systematic quantification of several RF parameters-including strength of the evoked responses, response latency, acoustic threshold, shape of rate-level function, frequency selectivity, and RF size-was performed while undrugged, restrained guinea pigs presented spontaneous alternances of W, SWS, and PS. Data are from 102 cells recorded during W and SWS and from 53 cells recorded during W, SWS, and PS. During SWS, thalamic cells behaved as an homogeneous population: as compared with W, most of them (97/102 cells) exhibited decreased evoked spike rates. The frequency selectivity was enhanced and the RF size was reduced. In contrast during PS, two populations of cells were identified: one (32/53 cells) showed the same pattern of changes as during SWS, whereas the other (21/53 cells) expressed values of evoked spike rates and RF properties that did not significantly differ from those in W. These two populations were equally distributed in the different anatomical divisions of the auditory thalamus. Last, during both SWS and PS, the responses latency was longer and the acoustic threshold was higher than in W but the proportion of monotonic versus nonmonotonic rate-level functions was unchanged. During both SWS and PS, no relationship was found between the changes in burst percentage and the changes of the RF properties. These results point out the dual aspect of sensory processing during sleep. On the one hand, they show that the auditory messages sent by thalamic cells to cortical neurons are reduced both in terms of firing rate at a given frequency and in terms of frequency range. On the other hand, the fact that the frequency selectivity and the rate-level function are preserved suggests that the messages sent to cortical cells are not deprived of informative content, and that the analysis of complex acoustic sounds should remain possible. This can explain why, although attenuated, reactivity to biologically relevant stimuli is possible during sleep.

Characteristics of reliable tone-evoked oscillations in the rat thalamo-cortical auditory system

Tone-evoked oscillations were studied from simultaneous recordings collected in the auditory cortex, auditory thalamus and auditory sector of the reticular nucleus in urethane anesthetized rats. These oscillations were precisely time-locked to tone onset and were easily observed on peristimulus time histograms (PSTHs). Visual inspection of PSTHs and rasters led us to distinguish between 'reliable' oscillations (which exhibited oscillatory patterns in more than 50% of the trials) and 'labile' oscillations (which exhibited oscillations in less than 50% of the trials). Systematic quantification of oscillations based on several indices derived from power spectra confirmed this distinction. 'Reliable' stimulus-locked oscillations were observed in 51/184 (28%) of the recordings from auditory cortex, 9/55 (17%) of the recordings from auditory thalamus and 11/26 (42%) of the recordings from the auditory sector of the reticular nucleus. The frequency range of these oscillations was the same in the three structures (5-14 Hz). Within the same animal, when one electrode exhibited oscillations, there was a high probability of detecting similar oscillations from electrodes located in the same structure, but not from electrodes located in the other structures. These oscillations were observed for pure tone frequency (or for clicks) whatever the tone duration (1 s, 100 ms, 10 ms). The inter-tone interval (ITI) was found to be the critical factor controlling the occurrence of these oscillations: they were present for ITIs of 2 s or longer, but were absent for ITIs of 1 s or less. In contrast, the occurrence of the oscillations was a function neither of the strength of the 'on' evoked response nor of the animal's temperature. However, lowering the animal's temperature from 37-38 degrees C to 35-36 degrees C systematically led to a decrease in the frequency and an increase in the duration of the tone-evoked oscillations. These results suggest that, even in well defined conditions (temperature, EEG, ITI, level of anesthesia), the oscillations triggered by presentation of the same stimulus can be stable or unstable. This temporal instability of stimulus-evoked oscillations has to be taken into account before stating percentages of oscillations in a given brain structure. They also suggest that some general factors such as the animals temperature or the inter-stimulus interval can considerably affect their characteristics and/or their occurrence.

Effects of metabotropic glutamate receptor activation in auditory thalamus

Metabotropic glutamate receptors (mGluRs) are expressed predominantly in dendritic regions of neurons of auditory thalamus. We studied the effects of mGluR activation in neurons of the ventral partition of medial geniculate body (MGBv) using whole cell current- and voltage-clamp recordings in brain slices. Bath application of the mGluR-agonist, 1S,3R-1-aminocyclopentan-1,3-dicarboxylic acid or 1S,3R-ACPD (5-100 microM), depolarized MGBv neurons (n = 67), changing evoked response patterns from bursts to tonic firing as well as frequency responses from resonance ( approximately 1 Hz) to low-pass filter characteristics. The depolarization was resistant to Na(+)-channel blockade with tetrodotoxin (TTX; 300 nM) and Ca(2+)-channel blockade with Cd(2+) (0.1 mM). The application of 1S, 3R-ACPD did not change input conductance and produced an inward current (I(ACPD)) with an average amplitude of 84.2 +/- 5.3 pA (at -70 mV, n = 22). The application of the mGluR antagonist, (RS)-alpha-methyl-4-carboxyphenylglycine (0.5 mM), reversibly blocked the depolarization or I(ACPD). During intracellular application of guanosine 5'-O-(3-thiotriphosphate) from the recording electrode, bath application of 1S,3R-ACPD irreversibly activated a large amplitude I(ACPD). During intracellular application of guanosine 5'-O-(2-thiodiphosphate), application of 1S, 3R-ACPD evoked only a small I(ACPD). These results implicate G proteins in mediation of the 1S,3R-ACPD response. A reduction of external [Na(+)] from 150 to 26 mM decreased I(ACPD) to 32.8 +/- 10. 3% of control. Internal applications of a Ca(2+) chelator, 1, 2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA; 10 mM), suppressed I(ACPD), implying a contribution of a Ca(2+) signal or Na(+)/Ca(2+) exchange. However, partial replacement of Na(+) with Li(+) (50 mM) did not significantly change I(ACPD). Therefore it seemed less likely that a Na(+)/Ca(2+) exchange current was a major participant in the response. A reduction of extracellular [K(+)] from 5.25 to 2.5 mM or external Ba(2+) (0.5 mM) or Cs(+) (2 mM) did not significantly change I(ACPD) between -40 and -85 mV. Below -85 mV, 1S,3R-ACPD application reversibly attenuated an inward rectification, displayed by 11 of 20 neurons. Blockade of an inwardly rectifying K(+) current with Ba(2+) (1 mM) or Cs(+) (2-3 mM) occluded the attenuation. In the range positive to -40 mV, 1S, 3R-ACPD application activated an outward current which Cs(+) blocked; this unmasked a voltage dependence of the inward I(ACPD) with a maximum amplitude at approximately -30 mV. The I(ACPD) properties are consistent with mGluR expression as a TTX-resistant, persistent Na(+) current in the dendritic periphery. We suggest that mGluR activation changes the behavior of MGBv neurons by three mechanisms: activation of a Na(+)-dependent inward current; activation of an outward current in a depolarized range; and inhibition of the inward rectifier, I(KIR). These mechanisms differ from previously reported mGluR effects in the thalamus.

Anatomic, intrinsic, and synaptic properties of dorsal and ventral division neurons in rat medial geniculate body

Anatomic, intrinsic, and synaptic properties of dorsal and ventral division neurons in rat medial geniculate body. Presently little is known about what basic synaptic and cellular mechanisms are employed by thalamocortical neurons in the two main divisions of the auditory thalamus to elicit their distinct responses to sound. Using intracellular recording and labeling methods, we characterized anatomic features, membrane properties, and synaptic inputs of thalamocortical neurons in the dorsal (MGD) and ventral (MGV) divisions in brain slices of rat medial geniculate body. Quantitative analysis of dendritic morphology demonstrated that tufted neurons in both divisions had shorter dendrites, smaller dendritic tree areas, more profuse branching, and a greater dendritic polarization compared with stellate neurons, which were only found in MGD. Tufted neuron dendritic polarization was not as strong or consistent as earlier Golgi studies suggested. MGV and MGD cells had similar intrinsic properties except for an increased prevalence of a depolarizing sag potential in MGV neurons. The sag was the only intrinsic property correlated with cell morphology, seen only in tufted neurons in either division. Many MGV and MGD neurons received excitatory and inhibitory inferior colliculus (IC) inputs (designated IN/EX or EX/IN depending on excitation/inhibition sequence). However, a significant number only received excitatory inputs (EX/O) and a few only inhibitory (IN/O). Both MGV and MGD cells displayed similar proportions of response combinations, but suprathreshold EX/O responses only were observed in tufted neurons. Excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) had multiple distinguishable amplitude levels implying convergence. Excitatory inputs activated alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors the relative contributions of which were variable. For IN/EX cells with suprathreshold inputs, first-spike timing was independent of membrane potential unlike that of EX/O cells. Stimulation of corticothalamic (CT) and thalamic reticular nucleus (TRN) axons evoked a GABAA IPSP, EPSP, GABAB IPSP sequence in most neurons with both morphologies in both divisions. TRN IPSPs and CT EPSPs were graded in amplitude, again suggesting convergence. CT inputs activated AMPA and NMDA receptors. The NMDA component of both IC and CT inputs had an unusual voltage dependence with a detectable DL-2-amino-5-phosphonovaleric acid-sensitive component even below -70 mV. First-spike latencies of CT evoked action potentials were sensitive to membrane potential regardless of whether the TRN IPSP was present. Overall, our in vitro data indicate that reported regional differences in the in vivo responses of MGV and MGD cells to auditory stimuli are not well correlated with major differences in intrinsic membrane features or synaptic responses between cell types.

Neural architecture of the rat medial geniculate body

The rat medial geniculate body was subdivided using Nissl preparations to establish nuclear boundaries, with Golgi-Cox impregnations to identify projection and local circuit neurons, and in fiber stained material to delineate the fiber tracts and their distribution. Three divisions were recognized (ventral, dorsal and medial): the first two had subdivisions. The ventral division had lateral and medial parts. The main cell type had bushy tufted dendrites which, with the afferent axons, formed fibrodendritic laminae oriented from dorso-lateral to ventro-medial; such laminae were not as regular medially, in the ovoid nucleus. The dorsal division contained several nuclei (dorsal superficial, dorsal, deep dorsal, suprageniculate, and ventrolateral) and neurons with radiating or bushy dendrites; the nuclear subdivisions differed in the concentration of one cell type or another, and in packing density. A laminar organization was present only in the dorsal superficial nucleus. Medial division neurons were heterogeneous in size and shape, ranging from tiny cells to magnocellular neurons; the various cell types intermingled. so that no further subdivision could be made. This parcellation scheme was consistent with, and supported by, the findings from plastic embedded or fiber stained material. There were very few small neurons with locally ramifying axons and which could perform an intrinsic role like that of Golgi type II cells. Their rarity was consistent with the small number of such profiles in plastic embedded or Nissl material and the few GABAergic medial geniculate body neurons seen in prior immunocytochemical work. While similar neuronal types and nuclear subdivisions are recognized in the rat and cat, there may be major interspecific differences with regard to interneuronal organization in the auditory thalamus whose functional correlates are unknown.

Temporal integration and duration tuning in the dorsal zone of cat auditory cortex

The present study examined auditory cortical neurons, the responses of which depended on the duration of noise bursts. We recorded from 150 neurons with response latencies exceeding 30 msec and from 28 neurons with OFF responses to auditory stimuli in the dorsal zone of cat auditory cortex. Of 150 long-latency neurons, 132 displayed some form of duration selectivity. Seventy-eight were classified as selective for long durations. Among the long-duration-selective neurons, 30 responded only to noise burst stimuli with durations longer than a minimal threshold and were classified further as duration threshold neurons. Of 132 duration-selective neurons, 41 responded selectively to noise bursts of short duration; 13 showed maximal responses to noise bursts of a particular duration and could be regarded as duration-tuned neurons. OFF-response neurons included ones that were long-duration-selective, duration-tuned, and nonduration-selective. Duration tuning has been described previously only at the midbrain level in amphibians and bats. The present finding of sensitivity to sound duration in at least one region of cat auditory cortex indicates that this form of neural tuning may be important for hearing in all vertebrates, and for processing of sound at multiple levels in the auditory pathway. The duration tuning in the cat auditory cortex was much broader, and the best duration was distributed over a wider range than in the bat inferior colliculus. We suggest that the duration selectivity of the long-latency neurons results from integration along the time domain of a stimulus during the latent period.

Long-latency neurons in auditory cortex involved in temporal integration: theoretical analysis of experimental data

A previous experimental study (He et al., 1997) found 132 duration-selective neurons with long latencies of greater than 30 ms in the dorsal zone of cat auditory cortex. The mechanism by which such long-latency neurons integrate information during their latent period is investigated by analysis of the temporal relationship between the stimulus and neuronal response. In the present study, we developed a one-layer perceptron to examine the above temporal relationship of the experimental results. The acoustic stimulus was represented as a contiguous series of sequential short time epochs. The perceptron was trained by using the spike data as the desired outputs and the acoustic stimuli (in digital format) as the inputs. The adaptive weights between the outputs and the inputs after training indicated the temporal relationship between neuronal responses and the stimuli. The contribution of each time epoch of the stimulus could be either positive or negative: the positive contribution corresponds to excitatory input and the negative contribution to inhibitory input. Long-duration-selective neurons were found to receive mainly excitatory input along the entire effective stimulus duration. However, duration-tuned neurons received excitatory input for only the time period from the stimulus onset to their best durations, and inhibitory thereafter. The temporal integration pattern of short-duration-selective neurons was similar to duration-tuned neurons. However, short-duration-selective neurons received excitatory input only at the beginning of the stimulus. Each of the duration-threshold neurons integrated auditory information only for a restricted time period of the stimulus, suggesting that they have a time window over the stimulus time domain. Non-duration-threshold neurons have time windows extending from the stimulus onset onward. The assembly of duration-threshold neurons and non-duration-threshold neurons may collectively represent the time axis of the stimulus.

Modulation of bursts and high-threshold calcium spikes in neurons of rat auditory thalamus

Neurons in the ventral partition of the medial geniculate body are able to fire high-threshold Ca2+-spikes. The neurons normally discharge such spikes on low-threshold Ca2+-spikes after the action potentials of a burst. We studied membrane mechanisms that regulate the discharge of high-threshold Ca2+-spikes, using whole-cell recording techniques in a slice preparation of rat thalamus. A subthreshold (persistent) Na+-conductance amplified depolarizing inputs, enhancing membrane excitability in the tonic firing mode and amplifying the low-threshold Ca2+-spike in the burst firing mode. Application of tetrodotoxin blocked the amplification and high-threshold Ca2+-spike firing. A slowly inactivating K+ conductance, sensitive to blockade with 4-aminopyridine (50-100 microM), but not tetraethylammonium (2-10 mM), appeared to suppress excitability and high-threshold Ca2+-spike firing. Application of 4-aminopyridine increased the low-threshold Ca2+-spike and the number of action potentials in the burst, and led to a conversion of the superimposed high-threshold Ca2+-spike into a plateau potential. Application of the Ca2+-channel blocker Cd2+ (50 microM), reduced or eliminated this plateau potential. The tetrodotoxin sensitive, persistent Na+-conductance also sustained plateau potentials, triggered after 4-aminopyridine application on depolarization by current pulses. Our results suggest that high-threshold Ca2+-spike firing, and a short-term influx of Ca2+, are regulated by a balance of voltage-dependent conductances. Normally, a slowly inactivating A-type K+-conductance may reduce high-threshold Ca2+-spike firing and shorten high-threshold Ca2+-spike duration. A persistent Na+-conductance promotes coupling of the low-threshold Ca2+-spike to a high-threshold Ca2+-spike. Thus, the activation of both voltage-dependent conductances would affect Ca2+ influx into ventral medial geniculate neurons. This would alter the quality of the different signals transmitted in the thalamocortical system during wakefulness, sleep and pathological states.

Slow synaptic inhibition in nucleus HVc of the adult zebra finch

Nervous systems process information over a broad range of time scales and thus need corresponding cellular mechanisms spanning that range. In the avian song system, long integration times are likely necessary to process auditory feedback of the bird's own vocalizations. For example, in nucleus HVc, a center that contains both auditory and premotor neurons and that is thought to act as a gateway for auditory information into the song system, slow inhibitory mechanisms appear to play an important role in the processing of auditory information. These long-lasting processes include inhibitory potentials thought to shape auditory selectivity and a vocalization-induced inhibition of auditory responses lasting several seconds. To investigate the possible cellular mechanisms of these long-lasting inhibitory processes, we have made intracellular recordings from HVc neurons in slices of adult zebra finch brains and have stimulated extracellularly within HVc. A brief, high-frequency train of stimuli (50 pulses at 100 Hz) could elicit a hyperpolarizing response that lasted 2-20 sec. The slow hyperpolarization (SH) could still be elicited in the presence of glutamate receptor blockers, suggesting that it does not require polysynaptic excitation. Three major components contribute to this activity-induced SH: a long-lasting GABAB receptor-mediated IPSP, a slow afterhyperpolarization requiring action potentials but not Ca2+ influx, and a long-lasting IPSP, the neurotransmitter and receptor of which remain unidentified. These three slow hyperpolarizing events are well placed to contribute to the observed inhibition of HVc neurons after singing and could shape auditory feedback during song learning.

Slow synaptic inhibition in nucleus HVc of the adult zebra finch

Nervous systems process information over a broad range of time scales and thus need corresponding cellular mechanisms spanning that range. In the avian song system, long integration times are likely necessary to process auditory feedback of the bird's own vocalizations. For example, in nucleus HVc, a center that contains both auditory and premotor neurons and that is thought to act as a gateway for auditory information into the song system, slow inhibitory mechanisms appear to play an important role in the processing of auditory information. These long-lasting processes include inhibitory potentials thought to shape auditory selectivity and a vocalization-induced inhibition of auditory responses lasting several seconds. To investigate the possible cellular mechanisms of these long-lasting inhibitory processes, we have made intracellular recordings from HVc neurons in slices of adult zebra finch brains and have stimulated extracellularly within HVc. A brief, high-frequency train of stimuli (50 pulses at 100 Hz) could elicit a hyperpolarizing response that lasted 2-20 sec. The slow hyperpolarization (SH) could still be elicited in the presence of glutamate receptor blockers, suggesting that it does not require polysynaptic excitation. Three major components contribute to this activity-induced SH: a long-lasting GABAB receptor-mediated IPSP, a slow afterhyperpolarization requiring action potentials but not Ca2+ influx, and a long-lasting IPSP, the neurotransmitter and receptor of which remain unidentified. These three slow hyperpolarizing events are well placed to contribute to the observed inhibition of HVc neurons after singing and could shape auditory feedback during song learning.