Representation of the spectral dominance region of pitch in the steady-state temporal discharge patterns of cochlear nucleus units

Dendritic Degeneration and Altered Synaptic Innervation of a Central Auditory Neuron During Age-related Hearing Loss

Cellular morphology and synaptic configuration are key determinants of neuronal function and are often modified under pathological conditions. In the first nucleus of the central auditory system, the cochlear nucleus (CN), principal bushy neurons specialize in processing temporal information of sound critical for hearing. These neurons alter their physiological properties during aging that contribute to age-related hearing loss (ARHL). The structural basis of such changes remains unclear, especially age-related modifications in their dendritic morphology and the innervating auditory nerve (AN) synapses. Using young (2-5 months) and aged (28-33 months) CBA/CaJ mice of either sex, we filled individual bushy neurons with fluorescent dye in acute brain slices to characterize their dendritic morphology, followed by immunostaining against vesicular glutamate transporter 1 (VGluT1) and calretinin (CR) to identify innervating AN synapses. We found that dendritic morphology of aged bushy neurons had significantly reduced complexity, suggesting age-dependent dendritic degeneration, especially in neurons with predominantly non-CR-expressing synapses on the soma. These dendrites were innervated by AN bouton synapses, which were predominantly non-CR-expressing in young mice but had increased proportion of CR-expressing synapses in old mice. While somatic AN synapses degenerated substantially with age, as quantified by VGluT1-labeled puncta volume, no significant difference was observed in the total volume of dendritic synapses between young and old mice. Consequently, synaptic density on dendrites was significantly higher in old mice. The findings suggest that dendritic degeneration and altered synaptic innervation in bushy neurons during aging may underlie their changed physiological activity and contribute to the development of ARHL.

Synaptic transmission at the endbulb of Held deteriorates during age-related hearing loss

KEY POINTS

Synaptic transmission at the endbulb of Held was assessed by whole-cell patch clamp recordings from auditory neurons in mature (2-4 months) and aged (20-26 months) mice. Synaptic transmission is degraded in aged mice, which may contribute to the decline in neural processing of the central auditory system during age-related hearing loss. The changes in synaptic transmission in aged mice can be partially rescued by improving calcium buffering, or decreasing action potential-evoked calcium influx. These experiments suggest potential mechanisms, such as regulating intraterminal calcium, that could be manipulated to improve the fidelity of transmission at the aged endbulb of Held.

ABSTRACT

Age-related hearing loss (ARHL) is associated with changes to the auditory periphery that raise sensory thresholds and alter coding, and is accompanied by alterations in excitatory and inhibitory synaptic transmission, and intrinsic excitability in the circuits of the central auditory system. However, it remains unclear how synaptic transmission changes at the first central auditory synapses during ARHL. Using mature (2-4 months) and old (20-26 months) CBA/CaJ mice, we studied synaptic transmission at the endbulb of Held. Mature and old mice showed no difference in either spontaneous quantal synaptic transmission or low frequency evoked synaptic transmission at the endbulb of Held. However, when challenged with sustained high frequency stimulation, synapses in old mice exhibited increased asynchronous transmitter release and reduced synchronous release. This suggests that the transmission of temporally precise information is degraded at the endbulb during ARHL. Increasing intraterminal calcium buffering with EGTA-AM or decreasing calcium influx with ω-agatoxin IVA decreased the amount of asynchronous release and restored synchronous release in old mice. In addition, recovery from depression following high frequency trains was faster in old mice, but was restored to a normal time course by EGTA-AM treatment. These results suggest that intraterminal calcium in old endbulbs may rise to abnormally high levels during high rates of auditory nerve firing, or that calcium-dependent processes involved in release are altered with age. These observations suggest that ARHL is associated with a decrease in temporal precision of synaptic release at the first central auditory synapse, which may contribute to perceptual deficits in hearing.

Frequency difference beyond behavioral limen reflected by frequency following response of human auditory Brainstem

Background

The present study investigated whether the frequency-following response (FFR) of the auditory brainstem can represent individual frequency-discrimination ability.

Method

We measured behavioral frequency-difference limens (FDLs) in normal hearing young adults. Then FFRs were evoked by two pure tones, whose frequency difference was no larger than behavioral FDL. Discrimination of FFRs to individual frequencies was conducted as the neural representation of stimulus frequency difference. Participants were 15 Chinese college students (ages 19–25; 3 males, 12 females) with normal hearing characteristics.

Results

According to discriminative neural representations of individual frequencies, FFRs accurately reflected individual FDLs and detected stimulus-frequency differences smaller than behavioral threshold (e.g., 75% of FDL).

Conclusions

These results suggest that when a frequency difference cannot be behaviorally distinguished, there is still a possibility of it being detected physiologically.

Target-specific IPSC kinetics promote temporal processing in auditory parallel pathways

The acoustic environment contains biologically relevant information on timescales from microseconds to tens of seconds. The auditory brainstem nuclei process this temporal information through parallel pathways that originate in the cochlear nucleus from different classes of cells. Although the roles of ion channels and excitatory synapses in temporal processing have been well studied, the contribution of inhibition is less well understood. Here, we show in CBA/CaJ mice that the two major projection neurons of the ventral cochlear nucleus, the bushy and T-stellate cells, receive glycinergic inhibition with different synaptic conductance time courses. Bushy cells, which provide precisely timed spike trains used in sound localization and pitch identification, receive slow inhibitory inputs. In contrast, T-stellate cells, which encode slower envelope information, receive inhibition that is eightfold faster. Both types of inhibition improved the precision of spike timing but engage different cellular mechanisms and operate on different timescales. Computer models reveal that slow IPSCs in bushy cells can improve spike timing on the scale of tens of microseconds. Although fast and slow IPSCs in T-stellate cells improve spike timing on the scale of milliseconds, only fast IPSCs can enhance the detection of narrowband acoustic signals in a complex background. Our results suggest that target-specific IPSC kinetics are critical for the segregated parallel processing of temporal information from the sensory environment.

Ambiguous pitch and the temporal representation of inharmonic iterated rippled noise in the ventral cochlear nucleus

Neural coding of the pitch of complex sounds is vital for animals' ability to communicate and to perceptually organize natural acoustic scenes. Harmonic complex sounds typically have a well defined pitch corresponding to their fundamental frequency, whereas inharmonic sounds can exhibit pitch ambiguity: their pitch can have more than one value. Iterated rippled noise (IRN), a common "pitch stimulus," is generated from broadband noise by a cascade of delay-and-add steps, with the delayed noise phase-shifted by varphi degrees. By varying varphi, the (in)harmonicity, and therefore the pitch ambiguity, of IRN can be manipulated. Recordings were made from single-units in the ventral cochlear nucleus of anesthetized guinea pigs in response to IRN and complex tones, systematically varying the inharmonicity. In their all-order interspike interval distributions, primary-like and chopper units tuned within the phase-locking range of best frequencies represent the waveform temporal fine structure (which varies with varphi). In contrast, those units tuned to higher frequencies represent the temporal-envelope modulation (independent of varphi). We show a temporal representation of ambiguous pitch for IRN and complex tones based on responses to the stimulus fine structure. Within the dominance region for pitch this representation follows the predictions of classic human behavioral experiments and provides a unifying contribution to possible neuro-temporal explanations for the pitch shift and pitch ambiguity associated with many inharmonic sounds.