RESUMO
This investigation compared the development of neuronal excitability in the ventral nucleus of the trapezoid body (VNTB) between two strains of mice with differing progression rates for age-related hearing loss. In contrast to CBA/Ca (CBA) mice, the C57BL/6J (C57) strain are subject to hearing loss from a younger age and are more prone to damage from sound over-exposure. Higher firing rates in the medial olivocochlear system (MOC) are associated with protection from loud sounds and these cells are located in the VNTB. We postulated that reduced neuronal firing of the MOC in C57 mice could contribute to hearing loss in this strain by reducing efferent protection. Whole cell patch clamp was used to compare the electrical properties of VNTB neurons from the two strains initially in two age groups: before and after hearing onset at â¼ P9 and â¼P16, respectively. Prior to hearing onset VNTB neurons electrophysiological properties were identical in both strains, but started to diverge after hearing onset. One week after hearing onset VNTB neurons of C57 mice had larger amplitude action potentials but in contrast to CBA mice, their waveform failed to accelerate with increasing age, consistent with the faster inactivation of voltage-gated potassium currents in C57 VNTB neurons. The lower frequency action potential firing of C57 VNTB neurons at P16 was maintained to P28, indicating that this change was not a developmental delay. We conclude that C57 VNTB neurons fire at lower frequencies than in the CBA strain, supporting the hypothesis that reduced MOC firing could contribute to the greater hearing loss of the C57 strain.
Assuntos
Potenciais Evocados Auditivos do Tronco Encefálico , Audição , Presbiacusia/fisiopatologia , Corpo Trapezoide/fisiopatologia , Fatores Etários , Envelhecimento , Animais , Vias Auditivas/metabolismo , Vias Auditivas/fisiopatologia , Núcleo Coclear/metabolismo , Núcleo Coclear/fisiopatologia , Estimulação Elétrica , Camundongos Endogâmicos C57BL , Camundongos Endogâmicos CBA , Neurônios/metabolismo , Núcleo Olivar/metabolismo , Núcleo Olivar/fisiopatologia , Técnicas de Patch-Clamp , Canais de Potássio de Abertura Dependente da Tensão da Membrana/metabolismo , Presbiacusia/metabolismo , Tempo de Reação , Especificidade da Espécie , Fatores de Tempo , Corpo Trapezoide/metabolismoRESUMO
The central auditory brainstem provides an efferent projection known as the medial olivocochlear (MOC) system, which regulates the cochlear amplifier and mediates protection on exposure to loud sound. It arises from neurons of the ventral nucleus of the trapezoid body (VNTB), so control of neuronal excitability in this pathway has profound effects on hearing. The VNTB and the medial nucleus of the trapezoid body are the only sites of expression for the Kv2.2 voltage-gated potassium channel in the auditory brainstem, consistent with a specialized function of these channels. In the absence of unambiguous antagonists, we used recombinant and transgenic methods to examine how Kv2.2 contributes to MOC efferent function. Viral gene transfer of dominant-negative Kv2.2 in wild-type mice suppressed outward K(+) currents, increasing action potential (AP) half-width and reducing repetitive firing. Similarly, VNTB neurons from Kv2.2 knock-out mice (Kv2.2KO) also showed increased AP duration. Control experiments established that Kv2.2 was not expressed in the cochlea, so any changes in auditory function in the Kv2.2KO mouse must be of central origin. Further, in vivo recordings of auditory brainstem responses revealed that these Kv2.2KO mice were more susceptible to noise-induced hearing loss. We conclude that Kv2.2 regulates neuronal excitability in these brainstem nuclei by maintaining short APs and enhancing high-frequency firing. This safeguards efferent MOC firing during high-intensity sounds and is crucial in the mediation of protection after auditory overexposure.
Assuntos
Vias Auditivas/fisiologia , Cóclea/fisiologia , Perda Auditiva/prevenção & controle , Ruído/efeitos adversos , Núcleo Olivar/fisiologia , Canais de Potássio Shab/fisiologia , Potenciais de Ação/efeitos dos fármacos , Potenciais de Ação/genética , Animais , Animais Recém-Nascidos , Linhagem Celular Tumoral , Modelos Animais de Doenças , Potenciais Evocados Auditivos do Tronco Encefálico/fisiologia , Feminino , Regulação da Expressão Gênica/efeitos dos fármacos , Regulação da Expressão Gênica/genética , Perda Auditiva/etiologia , Técnicas In Vitro , Masculino , Camundongos , Camundongos Endogâmicos CBA , Camundongos Transgênicos , Mutação/genética , Neuroblastoma/patologia , Técnicas de Patch-Clamp , Canais de Potássio Shab/deficiência , Canais de Potássio Shaw/metabolismo , TransfecçãoRESUMO
We examined membrane properties and synaptic responses of neurons in the mouse lateral superior olivary nucleus (LSO). Two clear populations were identified consistent with: principal neurons which are involved in detecting interaural intensity differences (IIDs) and efferent neurons of the lateral olivocochlear (LOC) system which project to the cochlea. Principal neurons fired a short latency action potential (AP) often followed by an AP train during maintained depolarization. They possessed sustained outward K(+) currents, with little or no transient K(+) current (I(A)) and a prominent hyperpolarization-activated non-specific cation conductance, I(H). On depolarization, LOC neurons exhibited a characteristic delay to the first AP. These neurons possessed a prominent transient outward current I(A), but had no I(H). Both LOC and principal neurons received glutamatergic and glycinergic synaptic inputs. LOC synaptic responses decayed more slowly than those of principal neurons; the mean decay time constant of AMPA receptor-mediated EPSCs was around 1 ms in principal neurons and 4 ms in LOC neurons. Decay time constants for glycinergic IPSCs were around 5 ms in principal neurons and 10 ms in LOC neurons. We conclude that principal cells receive fast synaptic responses appropriate for integration of IID inputs, while the LOC cells possess excitatory and inhibitory receptors with much slower kinetics.
Assuntos
Vias Auditivas/fisiologia , Nervo Coclear/fisiologia , Potenciais Pós-Sinápticos Excitadores , Potenciais Pós-Sinápticos Inibidores , Neurônios/fisiologia , Núcleo Olivar/fisiologia , Animais , Vias Auditivas/citologia , Vias Auditivas/efeitos dos fármacos , Nervo Coclear/citologia , Nervo Coclear/efeitos dos fármacos , Canais de Cátion Regulados por Nucleotídeos Cíclicos/metabolismo , Potenciais Pós-Sinápticos Excitadores/efeitos dos fármacos , Ácido Glutâmico/metabolismo , Glicina/metabolismo , Canais Disparados por Nucleotídeos Cíclicos Ativados por Hiperpolarização , Técnicas In Vitro , Potenciais Pós-Sinápticos Inibidores/efeitos dos fármacos , Cinética , Camundongos , Camundongos Endogâmicos CBA , Neurônios/efeitos dos fármacos , Neurônios Eferentes/fisiologia , Neurotransmissores/farmacologia , Núcleo Olivar/citologia , Núcleo Olivar/efeitos dos fármacos , Técnicas de Patch-Clamp , Potássio/metabolismo , Canais de Potássio/metabolismo , Tempo de ReaçãoRESUMO
Sound localization by auditory brainstem nuclei relies on the detection of microsecond interaural differences in action potentials that encode sound volume and timing. Neurons in these nuclei express high amounts of the Kv3.1 potassium channel, which allows them to fire at high frequencies with short-duration action potentials. Using computational modeling, we show that high amounts of Kv3.1 current decrease the timing accuracy of action potentials but enable neurons to follow high-frequency stimuli. The Kv3.1b channel is regulated by protein kinase C (PKC), which decreases current amplitude. Here we show that in a quiet environment, Kv3.1b is basally phosphorylated in rat brainstem neurons but is rapidly dephosphorylated in response to high-frequency auditory or synaptic stimulation. Dephosphorylation of the channel produced an increase in Kv3.1 current, facilitating high-frequency spiking. Our results indicate that the intrinsic electrical properties of auditory neurons are rapidly modified to adjust to the ambient acoustic environment.
Assuntos
Tronco Encefálico/citologia , Neurônios/fisiologia , Estimulação Acústica/métodos , Potenciais de Ação/fisiologia , Potenciais de Ação/efeitos da radiação , Animais , Animais Recém-Nascidos , Células CHO/efeitos dos fármacos , Células CHO/metabolismo , Cricetinae , Cricetulus , Relação Dose-Resposta à Radiação , Estimulação Elétrica/métodos , Inibidores Enzimáticos/farmacologia , Lateralidade Funcional/fisiologia , Regulação da Expressão Gênica/fisiologia , Regulação da Expressão Gênica/efeitos da radiação , Imuno-Histoquímica/métodos , Técnicas In Vitro , Indóis/farmacologia , Maleimidas/farmacologia , Técnicas de Patch-Clamp/métodos , Fosforilação , Proteína Quinase C/metabolismo , Ratos , Ratos Sprague-Dawley , Acetato de Tetradecanoilforbol/análogos & derivados , Acetato de Tetradecanoilforbol/farmacologiaRESUMO
Principal neurons of the lateral superior olive (LSO) detect interaural intensity differences by integration of excitatory projections from ipsilateral bushy cells and inhibitory inputs from the medial nucleus of the trapezoid body. The intrinsic membrane currents active around firing threshold will form an important component of this binaural computation. Whole cell patch recording in an in vitro brain slice preparation was employed to study conductances regulating action potential (AP) firing in principal neurons. Current-clamp recordings from different neurons showed two types of firing pattern on depolarization, one group fired only a single initial AP and had low input resistance while the second group fired multiple APs and had a high input resistance. Under voltage-clamp, single-spiking neurons showed significantly higher levels of a dendrotoxin-sensitive, low threshold potassium current (ILT). Block of ILT by dendrotoxin-I allowed single-spiking cells to fire multiple APs and indicated that this current was mediated by Kv1 channels. Both neuronal types were morphologically similar and possessed similar amounts of the hyperpolarization-activated nonspecific cation conductance (Ih). However, single-spiking cells predominated in the lateral limb of the LSO (receiving low frequency sound inputs) while multiple-firing cells dominated the medial limb. This functional gradient was mirrored by a medio-lateral distribution of Kv1.1 immunolabelling. We conclude that Kv1 channels underlie the gradient of LSO principal neuron firing properties. The properties of single-spiking neurons would render them particularly suited to preserving timing information.