Effect of maternal care on hearing onset induced by developmental changes in the auditory periphery.

Handling (H) and cross-fostering (CF) rodent pups during postnatal development triggers changes in maternal behavior which in turn trigger long-term physiological changes in the offspring. However, less is known about the short-term effects of H and CF on infant development. In this study we hypothesized that manipulations of maternal care affect the onset of hearing in Wistar rats. To test this hypothesis we obtained auditory brainstem responses (ABRs) and micro-CT x-ray scans to measure changes in the development of the auditory periphery in H and CF pups manipulated at postnatal day (P)1, P5, or P9. We found evidence of changes in hearing development in H and CF pups compared with naive pups, including changes in the percentage of animals with ABRs during development, a decrease in ABR thresholds between P13 and P15, and anatomical results consistent with an accelerated formation of the middle ear cavity and opening of the ear canal. Biochemical measurements showed elevated levels of thyroid hormone in plasma from naive and CF pups. These results provide evidence that manipulations of maternal care accelerate hearing onset in Wistar rats. Understanding the mechanisms by which maternal care affects hearing onset opens new opportunities to study experience-dependent development of mammalian hearing.

Stimulus-specific and stimulus-nonspecific firing synchrony and its modulation by sensory adaptation in the whisker-to-barrel pathway.

The stimulus-evoked response of a cortical neuron depends on both details of the afferent signal and the momentary state of the larger network in which it is embedded. Consequently, identical sensory stimuli evoke highly variable responses. Using simultaneous recordings of thalamic barreloid and/or cortical barrel neurons in the rat whisker-to-barrel pathway, we determined the extent to which the responses of pairs of cells covary on a trial-by-trial basis. In the thalamus and cortical layer IV, a substantial component of trial-to-trial variability is independent of the specific parameters of the stimulus, probed here using deflection angle. These stimulus-nonspecific effects resulted in greater-than-chance similarities in trial-averaged angular tuning among simultaneously recorded pairs of barrel neurons. Such effects were not observed among simultaneously recorded thalamic and cortical barrel neurons, suggesting strong intracortical mechanisms of synchronization. Sensory adaptation produced by prior whisker deflections reduced response magnitudes and enhanced the joint angular tuning of simultaneously recorded neurons. Adaptation also decorrelated stimulus-evoked responses, rendering trial-by-trial responses of neuron pairs less similar to each other. Adaptation-induced decorrelation coupled with sharpened joint tuning could enhance the saliency of cells within thalamus or cortex that continue to fire synchronously during ongoing tactile stimulation associated with active touch.

Angularly nonspecific response suppression in rat barrel cortex.

Response modulation by prior sensory stimulation is a common property of cortical neurons. The degree to which effects are specific to the adapting stimulus provides insights into properties of the underlying circuitry. Here, we examined the effects of an adapting whisker deflection's angle on the angular tuning of layer IV barrel neurons and their major input source, thalamic barreloid neurons. In both barrel regular-spike units (RSUs) and fast-spike units (FSUs), presumed excitatory and inhibitory neurons, prior whisker deflections suppressed subsequent test deflections in a largely angularly nonspecific manner, that is, adaptation in one direction reduced responses for test deflections of all angles. FSUs were poorly tuned for deflection angle and remained so after adaptation. In adapted RSUs, responses to suboptimal directions were suppressed most and angular preferences remained constant; tuning therefore became sharper. Adaptation effects in RSUs and FSUs do not appear to reflect corresponding changes in thalamic neurons. The angularly nonspecific suppression of barrel neurons is likely mediated by local intrabarrel suppressive interactions, such as broadly tuned inhibition and/or short-term synaptic depression of excitatory connections. The dominance of angularly nonspecific suppression suggests that barrel neurons interact largely in an angularly nonspecific manner to reinforce stimulus preferences encoded by their synchronously firing thalamic inputs.

Thalamocortical angular tuning domains within individual barrels of rat somatosensory cortex.

In the rodent somatosensory cortex, whisker-related barrels in layer IV are morphological counterparts of functional cortical columns that extend throughout the cortical depth. We used microelectrode recordings and spike-triggered averaging of field potentials evoked by single thalamic barreloid neurons to investigate functional thalamocortical microcircuits. The function of such circuits was probed by deflecting the principal whisker of a barrel in different angular directions. We found that individual barrels contain minicolumns of neurons preferring the same deflection angle. Angular tuning domains are established by convergent inputs from thalamocortical cells with corresponding angular preferences. Processing within such domains may depend on local connectivity among vertically aligned barrel neurons.