Long-term potentiation of Ca2+ signal in the rat auditory cortex

Tinnitus: Models and mechanisms

Over the past decade, there has been a burgeoning of scientific interest in the neurobiological origins of tinnitus. During this period, numerous behavioral and physiological animal models have been developed which have yielded major clues concerning the likely neural correlates of acute and chronic forms of tinnitus and the processes leading to their induction. The data increasingly converge on the view that tinnitus is a systemic problem stemming from imbalances in the excitatory and inhibitory inputs to auditory neurons. Such changes occur at multiple levels of the auditory system and involve a combination of interacting phenomena that are triggered by loss of normal input from the inner ear. This loss sets in motion a number of plastic readjustments in the central auditory system and sometimes beyond the auditory system that culminate in the induction of aberrant states of activation that include hyperactivity, bursting discharges and increases in neural synchrony. This article will review was has been learned about the biological origins of these alterations, summarize where they occur and examine the cellular and molecular mechanisms that are most likely to underlie them.

Normal hearing is required for the emergence of long-lasting inhibitory potentiation in cortex

Long-term synaptic plasticity is a putative mechanism for learning in adults. However, there is little understanding of how synaptic plasticity mechanisms develop or whether their maturation depends on experience. Since inhibitory synapses are particularly malleable to sensory stimulation, long-lasting potentiation of inhibitory synapses was characterized in auditory thalamocortical slices. Intracortical high-frequency electrical stimulation led to a 67% increase in inhibitory synaptic currents. In the absence of stimulation, inhibitory potentiation was induced by a brief exposure to exogenous brain-derived neurotrophic factor (BDNF). BDNF exposure occluded any additional potentiation by high-frequency afferent stimulation, suggesting that BDNF signaling is sufficient to account for inhibitory potentiation. Moreover, inhibitory potentiation was reduced significantly by extracellular application of a BDNF scavenger or by intracellular blockade of BDNF receptor [tropomyosin-related kinase B (TrkB)] signaling. In contrast, glutamatergic or GABAergic antagonists did not prevent the induction of inhibitory potentiation. Since BDNF and TrkB expression are influenced strongly by activity, we predicted that inhibitory potentiation would be diminished by manipulations that decrease central auditory activity, such as hearing loss. Two forms of hearing loss were examined: conductive hearing loss in which the cochleae are not damaged or sensorineural hearing loss in which both cochleae are removed. Both forms of hearing loss were found to reduce significantly the magnitude of inhibitory potentiation. These data indicate that early experience is necessary for the normal development of BDNF-mediated long-lasting inhibitory potentiation, which may be associated with perceptual deficits at later ages.

Long-term depression induced by local tetanic stimulation in the rat auditory cortex

In sensory cortices, synaptic plasticities such as long-term potentiation (LTP) and long-term depression (LTD) have important roles in the development of neural circuits and sensory information processing. However, the differential roles and mechanisms of the various types of LTP and LTD are not clear. In the present study, we investigated LTP and two types of LTD in slices obtained from the rat auditory cortex. Supragranular field potentials elicited by layer VI stimulation were recorded through a metal electrode. Transsynaptic field potentials exhibited marked LTP after tetanic stimulation (TS, 100 Hz for 1 s) was applied to layer VI. The same field potential components exhibited LTD after low-frequency stimulation (LFS, 1 Hz for 900 s) was applied to layer VI. LTD of supragranular field potentials was also induced by local TS applied to supragranular layers 0.3 mm from the recording site. Neither LTP nor LTD of either type was induced in the presence of 50 muM d-(-)-2-amino-5-phosphonovalerate (APV), an NMDA receptor antagonist. However, 500 muM (+)-alpha-methyl-4-carboxyphenylglycine (MCPG), an antagonist of metabotropic glutamate receptors, had no effect. LTD induced by LFS and that induced by local TS were suppressed in the presence of 3 muM bicuculline, an antagonist of GABA(A) receptors. Each of these forms of LTD occluded the other. These results and intracellular recordings in supragranular pyramidal neurons during LFS and local TS strongly suggest that the two types of LTD share common neural circuits for their induction.

Developmental hearing loss eliminates long-term potentiation in the auditory cortex

Severe hearing loss during early development is associated with deficits in speech and language acquisition. Although functional studies have shown a deafness-induced alteration of synaptic strength, it is not known whether long-term synaptic plasticity depends on auditory experience. In this study, sensorineural hearing loss (SNHL) was induced surgically in developing gerbils at postnatal day 10, and excitatory synaptic plasticity was examined subsequently in a brain slice preparation that preserves the thalamorecipient auditory cortex. Extracellular stimuli were applied at layer 6 (L6), whereas evoked excitatory synaptic potentials (EPSPs) were recorded from L5 neurons by using a whole-cell current clamp configuration. In control neurons, the conditioning stimulation of L6 significantly altered EPSP amplitude for at least 1 h. Approximately half of neurons displayed long-term potentiation (LTP), whereas the other half displayed long-term depression (LTD). In contrast, SNHL neurons displayed only LTD after the conditioning stimulation of L6. Finally, the vast majority of neurons recorded from control prehearing animals (postnatal days 9-11) displayed LTD after L6 stimulation. Thus, normal auditory experience may be essential for the maturation of synaptic plasticity mechanisms.

Experience-dependent formation of activity propagation patterns at the somatosensory S1 and S2 boundary in rat cortical slices

Somatosensory information is serially processed by the primary (S1) and secondary (S2) cortices, which can be identified in fresh cortical slices. We visualized activity propagation between S1 and S2 in rat cortical slices using flavoprotein fluorescence imaging. When S1 was stimulated, fluorescence responses extended into S2, while responses hardly propagated to S1 following S2 stimulation. The dominant activity propagation pattern from S1 to S2 was not affected by antagonists of glutamate or GABA(A) receptors. Ca(2+) imaging and electrophysiological recordings confirmed the anisotropic activity propagation pattern. This pattern could be formed as a result of serial information processing in S1 and S2. To test this hypothesis, activity propagation was investigated in cortical slices prepared 2 weeks or 3 days after trimming contralateral whiskers that provide massive inputs to S1. Supragranular activities in the barrel cortex were clearly suppressed. Furthermore, activities elicited in the rostral small vibrissae/mouth area of S1 near the border between S1 and S2 spread into the adjacent barrel cortex rather than into S2. Behavioral effects of whisker trimming were evaluated using a test, in which rats chose one of two bridges that had a wall on the right or left side only. Immediately after hemilateral whisker trimming, rats preferred to use the bridge with a wall close to the intact side. However, this preference disappeared 3 days after trimming. Modified activities observed in cortical slices after whisker trimming might be mechanisms for this behavioral compensation. These findings suggest experience-dependent formation of activity propagation patterns in the somatosensory cortex.

Functional local connections with differential activity-dependence and critical periods surrounding the primary auditory cortex in rat cerebral slices

Sensory information is processed in neural networks connecting the primary sensory cortices with surrounding higher areas. Here, we investigated the properties of local connections between the primary auditory cortex (area 41) and surrounding areas (areas 20, 36, 18a and 39) in rat cerebral slices. Neural activities elicited by repetitive electrical stimulation were visualized using the activity-dependent changes in endogenous fluorescence derived from mitochondrial flavoproteins, which mostly reflect activities produced by polysynaptic glutamatergic transmission. Polysynaptic feedforward propagation was dominant compared with the corresponding polysynaptic feedback propagation between the primary (area 41) and secondary (areas 20 and 36) auditory cortices, while such a tendency was less clear in other pathways. Long inter-areal (>1 mm) propagation with the same dominancy was observed after layer V stimulation between areas 41 and 20, and was not affected by cutting the underlying white matter. Activity-dependent changes in neural activities induced by low-frequency stimulation in the presence of 1 microM bicuculline were investigated using Ca2+ imaging. Significant potentiation of the polysynaptic Ca2+ activities was only observed in polysynaptic feedforward pathways from the primary to secondary auditory cortices. Experience-dependence of the connections between areas 41 and 20 was investigated using flavoprotein fluorescence imaging. The activities from areas 41 to 20 were reduced by cochlear lesions produced at P12 but not at P28, while the activities from areas 20 to 41 were reduced by the lesions at P28, suggesting the critical period for the polysynaptic feedforward connection was before P28, while for the polysynaptic feedback connection was after P28.

Effects of Agonists and Antagonists of NMDA and ACh Receptors on Plasticity of Bat Auditory System Elicited by Fear Conditioning

In big brown bats, tone-specific plastic changes [best frequency (BF) shifts] of cortical and collicular neurons can be evoked by auditory fear conditioning, repetitive acoustic stimuli or cortical electric stimulation. It has been shown that acetylcholine (ACh) plays an important role in evoking large long-term cortical BF shifts. However, the role of N-methyl-d-aspartate (NMDA) receptors in evoking BF shifts has not yet been studied. We found 1) NMDA applied to the auditory cortex (AC) or inferior colliculus (IC) augmented the auditory responses, as ACh did, whereas 2-amino-5-phosphovalerate (APV), an antagonist of NMDA receptors, reduced the auditory responses, as atropine did; 2) although any of these four drugs did not evoke BF shifts, they influenced the development of the long-term cortical and short-term collicular BF shifts elicited by conditioning; 3) like ACh, NMDA augmented the cortical and collicular BF shifts regardless of whether it was applied to the AC or IC; 4) endogenous ACh of the AC and IC is necessary to produce the long-term cortical and short-term collicular BF shifts; 5) blockade of collicular NMDA receptors by APV abolished the development of the collicular BF shift and made the cortical BF shift small and short-term; 6) blockade of cortical NMDA receptors by APV reduced the cortical and collicular BF shifts and made the cortical BF shift short-term; and 7) conditioning with NMDA + atropine applied to the AC evoked the small, short-term cortical BF shift, whereas conditioning with APV + ACh applied to the AC evoked the small, but long-term cortical BF shift.

Sound sequence discrimination learning is dependent on cholinergic inputs to the rat auditory cortex

In rat auditory cortex (AC) slices, synaptic potentiation following heterosynaptic stimulation is affected by the stimulus sequence used for induction. It was hypothesized that this sequence-dependent plasticity might be partly involved in the cellular mechanisms underlying sound sequence discrimination. Sequence dependence is abolished by muscarinic receptor antagonists. Therefore, dependence of sound sequence discrimination learning on cholinergic inputs to the rat AC was investigated. Rats were trained to discriminate the sequences of two sound components and a licking behavior in response to one of two possible sequences was rewarded with water. Atropine, a muscarinic receptor antagonist, attenuated sound sequence discrimination learning. The acquired sound sequence discrimination was not affected by atropine. Injections of the cholinergic immunotoxin 192IgG-saporin into the AC suppressed sound sequence discrimination learning, while discrimination between the two sound components was not affected. An inhibitor of M-current, linopirdine, restores the sequence dependence of synaptic potentiation in the AC slices suppressed by atropine. In this study, sound sequence discrimination learning attenuated by 192IgG-saporin was also restored by linopirdine. These similarities between sequence dependent plasticity in the AC slices and sound sequence discrimination learning support the hypothesis that the former is involved in the cellular mechanisms underlying the latter.

Short-term plasticity visualized with flavoprotein autofluorescence in the somatosensory cortex of anaesthetized rats

In the present study, short-term plasticity of somatosensory neural responses was investigated using flavoprotein autofluorescence imaging in rats anaesthetized with urethane (1.5 g/kg, i.p.) Somatosensory neural activity was elicited by vibratory skin stimulation (50 Hz for 1 s) applied on the surface of the left plantar hindpaw. Changes in green autofluorescence (lambda = 500-550 nm) in blue light (lambda = 450-490 nm) were elicited in the right somatosensory cortex. The normalised maximal fluorescence responses (deltaF/F) was 2.0 +/- 0.1% (n = 40). After tetanic cortical stimulation (TS), applied at a depth of 1.5-2.0 mm from the cortical surface, the responses elicited by peripheral stimulation were significantly potentiated in both peak amplitude and size of the responsive area (both P < 0.02; Wilcoxon signed rank test). This potentiation was clearly observed in the recording session started 5 min after the cessation of TS, and returned to the control level within 30 min. However, depression of the responses was observed after TS applied at a depth of 0.5 mm. TS-induced changes in supragranular field potentials in cortical slices showed a similar dependence on the depth of the stimulated sites. When TS was applied on the ipsilateral somatosensory cortex, marked potentiation of the ipsilateral responses and slight potentiation of the contralateral responses to peripheral stimulation were observed after TS, suggesting the involvement of commissural fibers in the changes in the somatosensory brain maps. The present study clearly demonstrates that functional brain imaging using flavoprotein autofluorescence is a useful technique for investigating neural plasticity in vivo.

Activity-dependent persisting modification of polysynaptic neural circuits involving layer V pyramidal neurons in rat auditory cortex in vitro

Synaptic plasticity in polysynaptic neural circuits permits modulation of the dynamic properties of these circuits. We investigated the properties of polysynaptic potentiation in pyramidal neurons in layer V of rat auditory cortex (AC) slices using the perforated patch clamp technique. The GABAA receptor inhibitor bicuculline was used to facilitate polysynaptic activity. The amplitude and duration of the polysynaptic activity were both gradually potentiated with repetitive stimulation (RS) at 12 s intervals. Potentiation was saturated within 10 min of the onset of RS. After the cessation of RS, the polysynaptic responses returned to control levels within 30 min. RS-induced potentiation was confirmed by fluorescence imaging of slices loaded with the Ca2+ indicator rhod-2. Such potentiation was not induced by stimulation at 60 s intervals. The magnitude of the RS-induced potentiation in layer V pyramidal neurons in the AC was greater than that in either layer II/III pyramidal neurons in the AC or layer V pyramidal neurons in the visual cortex. The NMDA receptor antagonist APV (100 microm), inhibited RS-induced potentiation. When stimulated at 1 Hz, the potentiated response appeared rapidly. In the absence of bicuculline, RS consisting of five pulses at 30 ms intervals, repeated at 12 s intervals for 10 min, elicited potentiation of firing activity, suggesting that the potentiation is independent of bicuculline. The present study demonstrates the dynamic properties of polysynaptic circuits involving layer V pyramidal neurons in the AC are strongly affected by activity-dependent synaptic potentiation.

Polysynaptic slow depolarization and spiking activity elicited after induction of long-term potentiation in rat auditory cortex

Polysynaptic activity was recorded in supragranular pyramidal neurons before and after the induction of long-term potentiation (LTP) in slices obtained from rat auditory cortex. LTP was induced by tetanic stimulation of layer IV. In the pyramidal neurons exhibiting LTP, repetitive stimulation at 50 Hz with 15 pulses triggered a slow 15-35 mV depolarization lasting 0.5-2 s with two to five spike discharges. There was no such response before the induction of LTP or in the neurons that did not exhibit LTP. Slow depolarization with spike discharges was blocked by an NMDA receptor antagonist but not by a metabotropic glutamate receptor antagonist. The reversal potential of the slow depolarization was approximately -7 mV and the membrane resistance decreased during slow depolarization, suggesting that the slow depolarization was produced by polysynaptic excitatory post-synaptic potentials. LTP was also induced by low frequency stimulation paired with a depolarizing current injection. In the pyramidal neurons exhibiting LTP after the paired stimulation, the slow depolarization amplitude was small and repetitive stimulation did not trigger spike discharges. Tetanic stimulation is expected to induce LTP in the polysynaptic neural circuits connecting many pyramidal neurons. The present findings suggest that polysynaptic activity can be generated in the potentiated neural circuits. Such activity might serve to read out the memory stored in polysynaptic neural circuits in the cerebral cortex.

A model synapse that incorporates the properties of short- and long-term synaptic plasticity

We propose a general computer model of a synapse, which incorporates mechanisms responsible for the realization of both short- and long-term synaptic plasticity-the two forms of experimentally observed plasticity that seem to be very significant for the performance of neuronal networks. The model consists of a presynaptic part based on the earlier 'double barrier synapse' model, and a postsynaptic compartment which is connected to the presynaptic terminal via a feedback, the sign and magnitude of which depend on postsynaptic Ca(2+) concentration. The feedback increases or decreases the amount of neurotransmitter which is in a ready for release state. The model adequately reproduced the phenomena of short- and long-term plasticity observed experimentally in hippocampal slices for CA3-CA1 synapses. The proposed model may be used in the investigation of certain real synapses to estimate their physiological parameters, and in the construction of realistic neuronal networks.

Dynamic imaging of somatosensory cortical activity in the rat visualized by flavoprotein autofluorescence

We used autofluorescence of mitochondrial flavoproteins to image cortical neural activity in the rat. Green autofluorescence in blue light was examined in slices obtained from rat cerebral cortex. About half of the basal autofluorescence was modulated by the presence or absence of O2 or glucose in the medium. Repetitive electrical stimulation at 20 Hz for 1 s produced a localized fluorescence increase in the slices. The amplitude of the increase was 27 +/- 2 % (mean +/- S.D., n = 35). Tetrodotoxin or diphenyleneiodonium, an inhibitor of flavoproteins, blocked the autofluorescence responses. The autofluorescence responses were not observed in slices perfused with calcium-, glucose- or O2-free medium. In the primary somatosensory cortex of rats anaesthetized with urethane (1.5 g kg-1, I.P.), an activity-dependent increase in autofluorescence of 20 +/- 4 % (n = 6) was observed after electrical cortical stimulation at 100 Hz for 1 s, and an increase of 2.6 +/- 0.5 % (n = 33) after vibratory skin stimulation at 50 Hz for 1 s applied to the plantar hindpaw. These responses were large enough to allow visualization of the neural activity without having to average a number of trials. The distribution of the fluorescence responses after electrical or vibratory skin stimulation was comparable to that of the cortical field potentials in the same rats. The fluorescence responses were followed by an increase in arterial blood flow. The former were resistant to an inhibitor of nitric oxide synthase, while the latter was inhibited. Thus, activity-dependent changes in the autofluorescence of flavoproteins are useful for functional brain imaging in vivo.

Anisotropic functional connections between the auditory cortex and area 18a in rat cerebral slices

We developed a new method to visualize the myeloarchitecture in fresh slices, and investigated the properties of the functional neural connections around the boundary between the primary auditory cortex (area 41) and area 18a in rat cerebral slices. A fresh slice illuminated by near-vertical light was observed with a CCD camera. The translucent images of the slice showed contrast patterns very similar to myeloarchitecture. The boundary between these areas was identified by the well-developed layer IV/V in area 41 but not in area 18a. Antidromic/presynaptic components of the field potentials stimulated and recorded across the areal boundary showed symmetric distribution, while the postsynaptic field potentials in the direction from area 41 to 18a were more prominent than those in the opposite direction in layer II/III. In contrast, the dominant direction of propagation of postsynaptic potentials was from area 18a to 41 in layer V. In the presence of 1 microM bicuculline, an inhibitor of GABA(A) receptors, the polysynaptic activities propagating from area 18a into 41 via layer V were elicited by stimulation of area 18a. The propagation measured by Ca(2+) imaging or field potential recordings was potentiated after both areas 18a and 41 were alternately stimulated several times.

Differential dependence of LTD on glutamate receptors in the auditory cortical synapses of cortical and thalamic inputs

Pyramidal neurons in the auditory cortex (AC) receive glutamatergic inputs from the medial geniculate body (MGB inputs) and other pyramidal neurons (pyramidal inputs). We found that the induction of long-term depression (LTD) in supragranular layers was only partially suppressed by 50 microM D-(-)-2-amino-5-phosphonovalerate (APV), an antagonist of N-methyl-D-aspartate (NMDA) receptors (NMDARs), and 500 microM (+)-alpha-methyl-4-carboxyphenylglycine (MCPG), an antagonist of metabotropic glutamate receptors (mGluRs). However, LTD was not observed in the mixture of APV and MCPG. We hypothesized that the mixed dependence of LTD on glutamate receptors could be attributed to the heterogeneity of MGB inputs and pyramidal inputs. To test this hypothesis, the angle of slicing and other recording conditions were adjusted so that postsynaptic potentials were recorded in normal slices, but not in the slices prepared from the rats with MGB lesion. In these experiments, LTD was suppressed by MCPG alone. The conditions were adjusted to minimize the contribution of MGB inputs in field potentials. In these experiments, the induction of LTD was suppressed by APV alone. Interestingly, the induction of LTD was partially suppressed by 20 microM nifedipine, a blocker of L-type Ca(2+) channels, in the slices prepared from the rats with MGB lesions, but not in normal slices. These findings suggest that the induction of LTD requires activation of mGluRs in the synapses of MGB inputs and of NMDARs in the synapses of pyramidal inputs.

Sequence dependence of post-tetanic potentiation after sequential heterosynaptic stimulation in the rat auditory cortex

1. To investigate the mechanisms for the coding stimulus sequence in the auditory cortex (AC), post-tetanic potentiation (PTP) was recorded after sequentially combined heterosynaptic stimulation was applied in rat AC slices. 2. Brief tetanic stimulation (TS) was applied at two sites on AC slices at intervals of 0.5-10 s. PTP of field potentials was induced by the earlier TS, rather than the later TS. PTP was followed by sequence-dependent long-term potentiation (LTP). 3. Using Ca(2+) imaging in the slices loaded with rhod-2, a Ca(2+) indicator, a sequence-dependent distribution of PTP was found in AC slices. 4. The sequence-dependent PTP in excitatory postsynaptic potentials (EPSPs) was observed in supragranular pyramidal neurons. 5. The sequence dependence of PTP was not significantly affected by 1 microM bicuculline, an antagonist of GABA(A) receptors, or 100 microM 2-hydroxysaclofen, an antagonist of GABA(B) receptors. 6. Depolarization and firing recorded in pyramidal neurons during the later TS were less vigorous than when the slices were incubated in the control medium. However, this suppression of the responses during the later TS was not observed in the presence of 50 microM atropine, an antagonist of muscarinic receptors. 7. PTP was induced by the earlier and later TS in the presence of 50 microM atropine, so that the sequence dependence of PTP was abolished. Pirenzepine (50 microM), an antagonist of muscarinic M1 receptors, but not methoctramine (30 microM), an antagonist of M2 receptors, eliminated the sequence dependence of PTP. 8. These findings suggest that the sequence dependence of PTP in AC might have a role in the temporal processing of auditory information on the scale of seconds.

Tetanic stimulation and metabotropic glutamate receptor agonists modify synaptic responses and protein kinase activity in rat auditory cortex

We investigated whether tetanic-stimulation and activation of metabotropic glutamate receptors (mGluRs) can modify field-synaptic-potentials and protein kinase activity in rat auditory cortex, specifically protein kinase A (PKA) and protein kinase C (PKC). Tetanic stimulation (50 Hz, 1 s) increases PKA and PKC activity only if the CNQX-sensitive field-EPSP (f-EPSP) is also potentiated. If the f-EPSP is unchanged, then PKA and PKC activity remains unchanged. Tetanic stimulation decreases a bicuculline-sensitive field-IPSP (f-IPSP), and this occurs whether the f-EPSP is potentiated or not. Potentiation of the f-EPSP is blocked by antagonists of mGluRs (MCPG) and PKC (calphostin-C, tamoxifen), suggesting that the potentiation of the f-EPSP is dependent on mGluRs and PKC. PKC antagonists block the rise in PKC and PKA activity, which suggests that these may be coupled. In contrast, ACPD (agonist at mGluRs) decreases both the f-EPSP and the f-IPSP, but increases PKC and PKA activity. Quisqualate (group I mGluR agonist), decreases the f-IPSP, and increases PKA activity, suggesting that the increase in PKA activity is a result of activation of group I mGluRs. Additionally, the increase in PKC and PKA activity appears to be independent of the decrease of the f-EPSP and f-IPSP, because PKC antagonists block the increase in PKC and PKA activity levels but do not block ACPD's effect on the f-EPSP or f-IPSP. These data suggest that group I mGluRs are involved in potentiating the f-EPSP by a PKC and possibly PKA dependent mechanism which is separate from the mechanism that decreases the f-EPSP and f-IPSP.