Unbalanced synaptic inhibition can create intensity-tuned auditory cortex neurons.

Intensity-tuned auditory cortex neurons have spike rates that are nonmonotonic functions of sound intensity: their spike rate initially increases and peaks as sound intensity is increased, then decreases as sound intensity is further increased. They are either "unbalanced," receiving disproportionally large synaptic inhibition at high sound intensities; or "balanced," receiving intensity-tuned synaptic excitation and identically tuned synaptic inhibition which neither creates enhances nor creates intensity-tuning. It has remained unknown if the synaptic inhibition received by unbalanced neurons enhances intensity-tuning already present in the synaptic excitation, or if it creates intensity-tuning that is not present in the synaptic excitation. Here we show, using in vivo whole cell recordings in pentobarbital-anesthetized rats, that in some unbalanced intensity-tuned auditory cortex neurons synaptic inhibition enhances the intensity-tuning; while in others it actually creates the intensity-tuning. The lack of balance between synaptic excitation and inhibition was not always apparent in their peak amplitudes, but could sometimes be revealed only by considering their relative timing. Since synaptic inhibition is essentially cortical in origin, the unbalanced neurons in which inhibition creates intensity-tuning provide examples of auditory feature-selectivity arising de novo at the auditory cortex.

Functional congruity in local auditory cortical microcircuits.

Functional columns of primary auditory cortex (AI) are arranged in layers, each composed of highly connected fine-scale networks. The basic response properties and interactions within these local subnetworks have only begun to be assessed. We examined the functional diversity of neurons within the laminar microarchitecture of cat AI to determine the relationship of spectrotemporal processing between neighboring neurons. Neuronal activity was recorded across the cortical layers while presenting a dynamically modulated broadband noise. Spectrotemporal receptive fields (STRFs) and their nonlinear input/output functions (nonlinearities) were constructed for each neuron and compared for pairs of neurons simultaneously recorded at the same contact site. Properties of these local neuron pairs showed greater similarity than non-paired neurons within the same column for all considered parameters including firing rate, envelope-phase precision, preferred spectral and temporal modulation frequency, as well as for the threshold and transition of the response nonlinearity. This higher functional similarity of paired versus non-paired neurons was most apparent in infragranular neuron pairs, and less for local supragranular and granular pairs. The functional similarity of local paired neurons for firing rate, best temporal modulation frequency and two nonlinearity aspects was laminar dependent, with infragranular local pair-wise differences larger than for granular or supragranular layers. Synchronous spiking events between pairs of neurons revealed that simultaneous 'Bicellular' spikes, in addition to carrying higher stimulus information than non-synchronized spikes, encoded faster modulation frequencies. Bicellular functional differences to the best matched of the paired neurons could be substantial. Bicellular nonlinearities showed that synchronous spikes act to transmit stimulus information with higher fidelity and precision than non-synchronous spikes of the individual neurons, thus, likely enhancing stimulus feature selectivity in their target neurons. Overall, the well-correlated and temporally precise processing within local subnetworks of cat AI showed laminar-dependent functional diversity in spectrotemporal processing, despite high intra-columnar congruity in frequency preference.

Acoustic injury in mice: 129/SvEv is exceptionally resistant to noise-induced hearing loss.

129/SvEv is an inbred mouse strain popular for use in genetic knockout studies. Here, we compare normal auditory function and vulnerability to acoustic injury in wild-type mice of the 129/SvEv vs. CBA/CaJ strains. Compound action potentials (CAPs) and distortion product otoacoustic emissions (DPOAEs) showed slightly higher thresholds for 129/SvEv re CBA/CaJ, especially at frequencies >20 kHz. Middle-ear motion (i.e. umbo velocity) was similar in the two strains; although frequencies >20 kHz could not be evaluated. Permanent threshold shift (PTS) and hair cell losses, measured 1 week after high-intensity exposure to an 8-16 kHz noise band, were smaller in129/SvEv at all exposure levels and durations from 97 dB SPLx2 h to 106 dB SPLx8 h. Furthermore, PTS growth with increasing exposure energy was slower in 129/SvEv (<2 dB/dB) than CBA/CaJ (9 dB/dB). These data suggest that the vulnerability differences lie in the inner ear, not the middle ear. Several 129/Sv substrains show age-related hearing loss (AHL): 129/SvEv has not yet been evaluated (Zheng, Q.Y., Johnson, K. R., Erway, L.C., 1999. Assessment of hearing in 80 inbred strains of mice by ABR threshold analyses. Hear. Res. 130, 94-107). Thus, although other strains with AHL, e.g. C57Bl/6J, show increased vulnerability to noise-induced hearing loss (NIHL), pairing of AHL and NIHL vulnerabilities may not be obligatory.