Protein kinase C in central auditory pathways of the rat

Age-related increase in PKC gamma expression in the cochlear nucleus of hearing impaired C57BL/6J and BALB/c mice

Age-dependent alteration in cellular signaling is implicated in the onset of age-related hearing loss (presbycusis). The gamma subtype of protein kinase C (PKCγ) is a PKC isoenzyme exclusively expressed in central nervous system but its potential role in the presbycusis remains unclear. Using two presbycusis-like animal models (C57BL/6J strain and BALB/c strain), the auditory thresholds were assessed by auditory brainstem response (ABR) in young (2-month-old), adult (8-month-old) and old (24-month-old) groups, and the localization and expression of PKCγ in the cochlear nucleus (CN) was examined by immunohistochemistry, Western blotting and Real-Time PCR. The results showed that PKCγ immmunoreactive (-ir) neurons were mainly concentrated in the molecular layer and fusiform layer of the dorsal CN (DCN) and their number was increased significantly with aging (p<0.05). Moreover, compared with 2-month-old mice, PKCγ expression in the CN at both protein and mRNA levels was significantly increased in the 8-month-old mice and 24-month-old mice (p<0.05). Thus our findings demonstrate a potential link between the increased PKCγ expression and the age-related hearing loss in these mice, suggesting novel strategies for the prevention and therapy of age-associated auditory disorders.

P2 receptor-mediated signaling in spherical bushy cells of the mammalian cochlear nucleus

Purinoreceptors of the P2 family contribute strongly to signaling in the cochlea, but little is known about the effects of purinergic neurotransmission in the central auditory system. Here we examine P2 receptor-mediated signaling in the large spherical bushy cells (SBCs) of Mongolian gerbils around the onset of acoustically evoked signal processing (P9-P14). Brief adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS) application evoked inward current, membrane depolarization, and somatic Ca2+ signals. Moreover, ATPgammaS changed the SBCs firing pattern from phasic to tonic, when the application was synchronized with depolarizing current injection. This bursting discharge activity was dependent on [Ca2+]i and Ca2+-dependent protein kinase (PKC) activity and is presumably caused by modulation of low-threshold K+ conductance. Activation of P2Y1 receptors could not evoke these changes per se, thus it was concluded that the involvement of P2X receptors seems to be necessary. Ca2+ imaging data showed that both P2X and P2Y1 receptors mediate Ca2+ signals in SBCs where P2Y1 receptors most likely activate the PLC-IP3 (inositol trisphosphate) pathway and release Ca2+ from internal stores. Immunohistochemical staining confirmed the expression of P2X2 and P2Y1 receptor proteins in SBCs, providing additional evidence for the involvement of both receptors in signal transduction in these neurons. Purinergic signaling might modulate excitability of SBCs and thereby contribute to regulation of synaptic strength. Functionally, the increase in firing rate mediated by P2 receptors could reduce temporal precision of the postsynaptic firing, e.g., phase locking, which has an immediate effect on signal processing related to sound localization. This might provide a mechanism for adaptation to the ambient acoustic environment.

Activation of protein kinase CbetaI constitutes a new neurotrophic pathway for deafferented spiral ganglion neurons

In mammals, degeneration of peripheral auditory neurons constitutes one of the main causes of sensorineural hearing loss. Unfortunately, to date, pharmacological interventions aimed at counteracting this condition have not presented complete effectiveness in protecting the integrity of cochlear neural elements. In this context, the protein kinase C (PKC) family of enzymes are important signalling molecules that play a role in preventing neurodegeneration after nervous system injury. The present study demonstrates, for the first time, that the PKC signalling pathway is directly neurotrophic to axotomised spiral ganglion neurons (SGNs). We found that PKCbetaI was strictly expressed by postnatal and adult SGNs both in situ and in vitro. In cultures of SGNs, we observed that activators of PKC, such as phorbol esters and bryostatin 1, induced neuronal survival and neurite regrowth in a manner dependent on the activation of PKCbetaI. The neuroprotective effects of PKC activators were suppressed by pre-treatment with LY294002 (a PI3K inhibitor) and with U0126 (a MEK inhibitor), indicating that PKC activators promote the survival and neurite outgrowth of SGNs by both PI3K/Akt and MEK/ERK-dependent mechanisms. In addition, whereas combining the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) was shown to provide only an additive effect on SGN survival, the interaction between PKC and neurotrophin signalling gave rise to a synergistic increase in SGN survival. Taken together, the data indicate that PKCbetaI activation represents a key factor for the protection of the integrity of neural elements in the cochlea.

Chemical divisions in the medial geniculate body and surrounding paralaminar nuclei of the rat: quantitative comparison of cell density, NADPH diaphorase, acetyl cholin esterase and basal expression of c-fos

Quantitative methods of cell density, the intensities of both acetyl cholinesterase (AChE) and NADPH diaphorase (NADPHd), as well as the basal expression of c-fos, have been carried out in order to study the anatomical divisions of the medial geniculate body (MGB) and the group of nuclei located ventromedially to the MGB called the paralaminar complex (PL). The MGB was composed of the dorsal (MGd), and the ventral (MGv) divisions. We included the medial, or the magnocellular division (MGm), in the PL complex. MGd was composed of a dorsolateral (DL) core and a belt. The belt was composed of the suprageniculate (SG), the deep dorsal (DD), the caudo-medial (CM) and the caudo-dorsal (CD) nuclei. In the MGv, the basal expression of c-fos was the only way to trace a clear boundary between the ovoid (Ov) and the ventrolateral (VL) divisions. However, the marginal zone (MZ) was clearly and contrastingly different. The PL was considered to be composed of: the MGm, the posterior intralaminar nucleus (PIN), the peripeduncular nucleus (PP) and the nucleus subparafascicularis lateralis (SPFL). The MGm and the PIN share most of the chemical features, meanwhile both SPFL and PP displayed different patterns of NADPHd reactivity. The study of cell density on Giemsa stained sections confirmed main divisions of the area. AChE and NADPHd methods allowed the main MGB divisions to be discriminated. The differences between subdivisions were emphasized when cell density and c-fos activity were quantified in each nucleus. Each MGB division displayed a different pattern of c-fos activity under basal conditions. Thus, c-fos basal expression was a particular feature in each MGB or PL nucleus.

Morphine actions in the rat forebrain: role of protein kinase C

Acute administration of morphine induces expression of the immediate-early gene (IEG) c-Fos in dorsomedial striatum, portions of cerebral cortex, and in several midline-intralaminar thalamic nuclei, partly via a trans-synaptic mechanism that involves activation of glutamate receptors. Because activation of protein kinase C (PKC) may occur following the activation of glutamate receptors, we determined whether pharmacological inhibition of PKC would attenuate morphine-induced c-Fos expression, and whether acute administration of morphine would induce translocation of PKC. The selective PKC antagonist NPC 15437 given 30 min prior to morphine significantly decreased morphine-induced c-Fos expression in striatum and cingulate cortex, but not in centrolateral thalamus. In another experiment, rats were given an acute dose of morphine, and immunocytochemical analysis was performed for the betaI and betaII isoforms of PKC. Morphine induced a rapid and transient translocation of PKC betaII, but not betaI, from perinuclear spots to plasma membrane in numerous cortical and striatal neurons. Prior administration of naloxone blocked this response. Ultrastructural studies confirmed translocation from Golgi apparatus to plasma membrane 15 min after morphine injection. Double immunocytochemistry at the light microscopic level demonstrated co-localization of translocated PKC betaII and c-Fos in some cortical neurons 90 min after morphine injection. These results support a role for PKC, especially PKC betaII, in the rapid effects of morphine on the brain.

Protein kinase C mediates potentiation of synaptic transmission by phorbol ester at parallel fibers in the dorsal cochlear nucleus

Many cells in the outer two layers of the dorsal cochlear nucleus (DCN) express high levels of the phospholipid-activated, calcium dependent kinase, protein kinase C (PKC), an enzyme that can phosphorylate numerous proteins involved in neurotransmission and postsynaptic signaling. We investigated the effects of stimulating PKC with phorbol esters (phorbol 12-13 diacetate; PDAc) on parallel fiber synaptic transmission in brain slices of the guinea pig DCN. Phorbol esters increased the amplitude of the postsynaptic components of the field potential, including the excitatory post-synaptic field potential (fEPSP) and the population spike following electric stimulation of parallel fibers. Phorbol esters simultaneously decreased paired-pulse facilitation, suggesting that transmitter release mechanisms were affected. Potentiation of synaptic transmission and diminished paired-pulse potentiation were also observed in intracellular recordings of DCN neurons. The effects of phorbol esters were antagonized by the specific PKC blockers bisindolylmaleimide and calphostin C. Although modulation of the synaptic potentials appears to be mediated by presynaptic PKC, the differential effects of PDAc on the fEPSP and the population spike also suggest the involvement of postsynaptic PKC and postsynaptic targets. These experiments demonstrate that protein kinase C is capable of profoundly modulating synaptic transmission at parallel fiber synapses in the DCN.

Immunolocalization of PICK1 in the ascending auditory pathways of the adult rat

Protein that interacts with C-kinase alpha (PICK1) is a PDZ domain protein that interacts with many binding partners in the central nervous system (CNS), including activated protein kinase Calpha and subunits of the AMPA subtype of glutamate receptor. Almost nothing is known about the anatomic distribution of PICK1 in the intact adult CNS. By using PICK1 antisera and peroxidase immunocytochemistry, we report on the distribution of PICK1 in the ascending pathways of the central auditory system of the adult rat. PICK1-immunoreactivity (ir) was observed in many component nuclei of the central auditory system, including the dorsal cochlear nucleus, anteroventral cochlear nucleus, posteroventral cochlear nucleus, some divisions of the superior olivary complex, inferior colliculus, medial geniculate body, and primary auditory cortex. The general staining pattern for PICK1-immunoreactivity was somatodendritic with scattered puncta in neuropil and somatodendritic regions. The distribution of PICK1 partially overlaps with PKCalpha and glutamate receptor subunits such as GluR2. These data suggest that PICK1 may function in the regulation of PKCalpha and GluR2 localization in components of the rat auditory system, which may be a fundamental mechanism of synaptic transmission and/or plasticity. J. Comp. Neurol.

Protein kinase C regulates [3H]D-aspartate release in auditory brain stem nuclei

We previously found that unilateral cochlear ablation altered transmitter release from glutamatergic synaptic endings in several brain stem auditory nuclei. To determine if this release activity could be regulated by protein kinase C (PKC), which has been associated with regulation of transmitter release, the electrically evoked release of [3H]d-aspartate ([3H]d-Asp) was quantified in vitro as an index of exocytosis from glutamatergic presynaptic endings in the major subdivisions of the cochlear nucleus (CN) and in the main nuclei of the superior olivary complex (SOC). Treating dissected tissues with a PKC activator, such as phorbol 12,13-diacetate (PDA) or phorbol 12,13-dibutyrate (PDBu) (3 microM), elevated the evoked release of [3H]d-Asp by 1.5- to 3.3-fold. The PKC inhibitor Ro31-8220 (50 nM) did not alter the evoked release but blocked the stimulatory effects of PDA and PDBu. These findings suggested that PKC could positively regulate transmitter release from glutamatergic presynaptic endings in brain stem auditory pathways. Seven days after unilateral cochlear ablation, when cochlear nerve endings had degenerated in the ipsilateral CN, PDBu elevated the evoked release bilaterally in each CN subdivision and SOC nucleus, implying that PKC could regulate glutamatergic release in the noncochlear pathways remaining in the ipsilateral CN and in the other pathways after unilateral hearing loss. After 145 postlesion days, Ro31-8220 blocked endogenous elevations in the evoked release in the ipsilateral SOC but did not alter the elevated or upregulated release in the other tissues. This suggested that the elevations of glutamatergic release activity in the ipsilateral SOC that appeared after unilateral cochlear ablation depended on endogenous activation of PKC.

A physiologically based model of discharge pattern regulation by transient K+ currents in cochlear nucleus pyramidal cells

Pyramidal cells in the dorsal cochlear nucleus (DCN) show three characteristic discharge patterns in response tones: pauser, buildup, and regular firing. Experimental evidence suggests that a rapidly inactivating K+ current (I(KIF)) plays a critical role in generating these discharge patterns. To explore the role of I(KIF), we used a computational model based on the biophysical data. The model replicated the dependence of the discharge pattern on the magnitude and duration of hyperpolarizing prepulses, and I(KIF) was necessary to convey this dependence. Phase-plane and perturbation analyses show that responses to depolarization are critically controlled by the amount of inactivation of I(KIF). Experimentally, half-inactivation voltage and kinetics of I(KIF) show wide variability. Varying these parameters in the model revealed that half-inactivation voltage, and activation and inactivation rates, controls the voltage and time dependence of the model cell discharge. This suggests that pyramidal cells can adjust their sensitivity to different temporal patterns of inhibition and excitation by modulating the kinetics of I(KIF). Overall, I(KIF) is a critical conductance controlling the excitability of DCN pyramidal cells.

Deafferentation-induced changes in protein kinase C expression in the rat cochlear nucleus

Isoforms of the signal transducing molecule, protein kinase C (PKC), may play a role in neural plasticity following sensory deafferentation. To explore the role of PKC in central auditory plasticity, we studied the effect of auditory deafferentation on the expression of PKC betaI, betaII, gamma, and delta in the rat dorsal (DCN) and ventral cochlear nucleus (VCN), using immunocytochemistry. Male rats were treated with kanamycin and furosemide to induce hair cell loss. At various intervals post-treatment, brains were perfusion-fixed and processed for immunocytochemistry. Following deafferentation, we observed a gradual increase in PKC betaI immunoreactivity (ir) in the deepest layers of the DCN, possibly representing synapses of primary afferents or parallel fibers on unlabeled neurons. Correlated with this, we observed an increase in the number of neurons in the deep DCN that showed PKC delta ir. In controls, we observed PKC gamma ir in small ovoid cells concentrated in the middle layer of the DCN. From days 4 through 14 after deafferentation, we found an increase in the intensity of staining of these cells, with a return toward control levels by day 28. Finally, Purkinje-like cells (PLC) in the VCN, which express only PKC delta in control rats, began to express PKC gamma after deafferentation, correlated with increased expression of calbindin D28k in PLC. Thus PKC isoforms are differentially regulated in the CN following deafferentation, supporting a role for PKC in auditory plasticity.

Distribution of protein kinase C in the brain of Apteronotus leptorhynchus as revealed by phorbol ester binding

An antibody to the mammalian protein kinase C alpha (PKCalpha) subunit and brain dissection was used for immunoblot analysis of this protein in various brain regions of Apteronotus leptorhynchus. Western blots revealed that the antibody labeled a band of the expected molecular mass (approximately 80 kDa) for this enzyme in mammalian cortex and electric fish brain, suggesting that this protein is also found in gymnotiform brain. The 80-kDa band was enriched in fish forebrain and cerebellum compared with hypothalamus and brainstem areas. [3H]Phorbol 12,13-dibutyrate ([3H]PDBu) binding was used as a marker for the distribution of protein kinase C (PKC). [3H]PDBu binding was nearly completely displaced by excess cold PDBu; specific [3H]PDBu binding sites were heterogenously distributed with high densities in some gray matter regions and negligible densities in fiber tracts. A very high density of [3H]PDBu binding sites were found in the dorsal forebrain with far lower densities in most ventral forebrain nuclei. Low binding densities were observed in preoptic and hypothalamic areas with the exception of the nucleus diffusus and nucleus tuberis anterior. The thalamus and midbrain also had only low levels of binding. The cerebellar molecular layer had dense binding, in contrast to the granule cell layer where binding was negligible. In the electrosensory lateral line lobe (ELL), there was moderate binding in the dorsal molecular layer, which contains cerebellar parallel fibers; the other layers of the ELL had far lower binding densities.

Glycine receptors in adult guinea pig brain stem auditory nuclei: regulation after unilateral cochlear ablation

In young adult guinea pigs, the effects of unilateral cochlear ablation were determined on the specific binding of [3H]strychnine measured in subdivisions of the cochlear nucleus (CN), the superior olivary complex, and the auditory midbrain, after 2, 7, 31, 60, and 147 postlesion days. Changes in binding relative to that in age-matched controls were interpreted as altered activity and/or expression of synaptic glycine receptors. Postlesion binding declined ipsilaterally in most of the ventral CN and in the lateral superior olive (LSO). Binding was modestly deficient in the ipsilateral dorsal CN and in the anterior part of the contralateral anteroventral CN. Binding was elevated in the contralateral LSO. Transient changes also occurred. Binding was elevated transiently, between 2 and 31 days, contralaterally in parts of the anteroventral CN, bilaterally in the medial superior olive (MSO), and bilaterally in most of the midbrain nuclei. Binding was deficient transiently, at 60 days, in most of the contralateral CN and bilaterally in the midbrain nuclei. The present findings, together with previously reported postlesion changes in glycine release, were consistent with persistently weakened glycinergic inhibitory transmission ipsilaterally in the ventral CN and the LSO and bilaterally in the dorsal CN. Glycinergic inhibitory transmission was strengthened in the contralateral LSO and transiently strengthened in the MSO bilaterally. A hypothetical model of the findings suggested that glycine receptor regulation may depend on excitatory and glycinergic input to auditory neurons. The present changes in glycine receptor activity may contribute to altered auditory functions, which often accompany hearing loss.

Plastic changes in glycine and GABA release and uptake in adult brain stem auditory nuclei after unilateral middle ear ossicle removal and cochlear ablation

[i] In young adult guinea pigs, the effects of unilateral ossicle removal and unilateral cochlear ablation were determined on [14C]glycine or [14C]GABA release and uptake measured in subdivisions of the cochlear nucleus (CN), the superior olivary complex, and the auditory midbrain, after 2 or 5, 59, and 145 postlesion days. Activities were compared to those of age-matched, unlesioned controls. [ii] [14C]Glycine release declined bilaterally in the anteroventral and dorsal CN after ossicle removal and in the dorsal CN after cochlear ablation. [iii] Transient elevations of release occurred at 59 days in the ipsilateral posteroventral CN ([14C]glycine) and bilaterally in the ventral nucleus of the lateral lemniscus ([14C]GABA) after ossicle removal, and bilaterally in the medial superior olive ([14C]glycine) after cochlear ablation. [iv] In the medial nucleus of the trapezoid body, [14C]GABA release was depressed bilaterally 5 days after ossicle removal, but was elevated at 145 days contralaterally after ossicle removal and ipsilaterally after cochlear ablation. [v] In the contralateral central nucleus of the inferior colliculus, [14C]GABA release was elevated persistently after ossicle removal. After cochlear ablation, release was elevated at 5 days, near the control at 59 days, and elevated again at 145 days. [vi] After both lesions, [14C]glycine uptake was elevated bilaterally in the CN and medial superior olive. [14C]GABA uptake became depressed by 59 or 145 days bilaterally in the auditory midbrain. [vii] These changes may stem from regulation and may contribute to mechanisms that generate symptoms such as loudness recruitment and tinnitus, which often accompany hearing loss.