Development of tonotopy in the inferior colliculus II: 2-DG measurements in the kitten

Development of intrinsic connectivity in the central nucleus of the mouse inferior colliculus

The inferior colliculus (IC) in the mammalian midbrain is the major subcortical auditory integration center receiving ascending inputs from almost all auditory brainstem nuclei as well as descending inputs from the thalamus and cortex. In addition to these extrinsic inputs, the IC also contains a dense network of local, intracollicular connections, which are thought to provide gain control and contribute to the selectivity for complex acoustic features. However, in contrast to the organization of extrinsic IC afferents, the development and functional organization of intrinsic connections in the IC has remained poorly understood. Here we used laser-scanning photostimulation with caged glutamate to characterize the spatial distribution and strength of local synaptic connections in the central nucleus of the inferior colliculus of newborn mice until after hearing onset (P2-P22). We demonstrate the presence of an extensive excitatory and inhibitory intracollicular network already at P2. Excitatory and inhibitory synaptic maps to individual IC neurons formed continuous maps that largely overlapped with each other and that were aligned with the presumed isofrequency axis of the central nucleus of the IC. Although this characteristic organization was present throughout the first three postnatal weeks, the size of input maps was developmentally regulated as input maps underwent an expansion during the first week that was followed by a dramatic refinement after hearing onset. These changes occurred in parallel for excitatory and inhibitory input maps. However, the functional elimination of intrinsic connections was greater for excitatory than for inhibitory connections, resulting in a predominance of intrinsic inhibition after hearing onset.

Development of contralateral and ipsilateral frequency representations in ferret primary auditory cortex

Little is known about the maturation of functional maps in the primary auditory cortex (A1) after the onset of sensory experience. We used intrinsic signal imaging to examine the development of the tonotopic organization of ferret A1 with respect to contralateral and ipsilateral tone stimulation. Sound-evoked responses were recorded as early as postnatal day (P) 33, a few days after hearing onset. From P36 onwards, pure tone stimuli evoked restricted, tonotopically organized patches of activity. There was an age-dependent increase in the cortical area representing each octave, with a disproportionate expansion of cortical territory representing frequencies > 4 kHz after P60. Similar tonotopic maps were observed following stimulation of the contralateral and ipsilateral ears. During the first few weeks following hearing onset, no differences were found in the area of cortical activation or in the magnitude of the optical responses evoked by stimulation of each ear. In older animals, however, contralateral stimuli evoked stronger responses and activated a larger A1 area than ipsilateral stimuli. Our findings indicate that neither the tonotopic organization nor the representation of inputs from each ear reach maturity until approximately 1 month after hearing onset. These results have important implications for cortical signal processing in juvenile animals.

Functional development of the inferior colliculus (IC) and its relationship with the auditory brainstem response (ABR) in the tammar wallaby (Macropus eugenii)

To discover the developmental relationship between the auditory brainstem response (ABR) and the focal inferior colliculus (IC) response, 32 young tammar wallabies were used, by the application of simultaneous ABR and focal brainstem recordings, in response to acoustic clicks and tone bursts of seven frequencies. The IC of the tammar wallaby undergoes a rapid functional development from postnatal day (PND) 114 to 160. The earliest (PND 114) auditory evoked response was recorded from the rostral IC. With development, more caudal parts of the IC became functional until age about PND 127, when all parts of the IC were responsive to sound. Along a dorsoventral direction, the duration of the IC response decreased, the peak latency shortened, while the amplitude increased, reaching a maximum value at the central IC, then decreased. After PND 160, the best frequency (BF) of the ventral IC was the highest, with values between 12.5 and 16 kHz, the BF of the dorsal IC was the lowest, varying between 3.2 and 6.4 kHz, while the BF of the central IC was between 6.4 and 12.5 kHz. Between PND 114 and 125, the IC response did not have temporal correlation with the ABR. Between PND 140 and 160, only the early components of the responses from the ventral and central IC correlated with the P4 waves of the ABR. After PND 160, responses recorded from different depths of the IC had a temporal correlation with the ABR.

Tonotopic organization of the human auditory cortex as detected by BOLD-FMRI

Functional magnetic resonance imaging is a noninvasive and nonradioactive method for the detection of focal brain activity. In the present study the auditory cortex was investigated in nine normal subjects who were binaurally stimulated using pulsed sine tones of 500 Hz and 4000 Hz. The BOLD (blood oxygenation level dependent) signal change coincided with the stimulation paradigm and was detected in the plane of the superior temporal gyrus. The comparison of the spatial distribution of activated areas revealed a different behavior for the two frequencies. The present findings underline the existence of a frequency specific organization in the medio-lateral, fronto-occipital and cranio-caudal extension in both hemispheres of the auditory cortex in human. The activated areas for the high tone were found more frontally and medially orientated than the low tone stimulated areas. Furthermore, a slight cranio-caudal shift was observed for the higher frequency, more pronounced in the right than in the left temporal lobe. Finally, for most of the subjects investigated the BOLD activation area of the 500 Hz sine tone was larger than that of the 4000 Hz stimulation. Both frequencies showed a lateralization of signal response to the left temporal lobe.

Depolarization selectively increases the expression of the Kv3.1 potassium channel in developing inferior colliculus neurons

The Kv3.1 channel subunit, when expressed heterologously, gives rise to a high-threshold noninactivating potassium current. Experiments with auditory neurons have suggested that the presence of this channel subunit enables them to fire action potentials at high frequencies. We have found that the expression levels of Kv3.1 transcripts increase in inferior colliculus neurons before the onset of hearing and then remain relatively constant. Because spontaneous neuronal activity plays an important role in modulating neuronal excitability during development, we examined the effects of depolarization with an elevated concentration of external potassium ions on the expression of Kv3.1 channel subunits in immature inferior colliculus neurons. Elevated potassium produced a marked increase in Kv3.1 mRNA levels and in the amplitude of a high-threshold, noninactivating current before the onset of hearing. This increase was prevented by the presence of a calcium channel blocker, indicating that calcium influx mediated the depolarization-induced increase in this current. In contrast, treatment with an elevated external potassium concentration caused only a moderate increase in the peak amplitude of a lower-threshold inactivating current. The repolarization of action potentials in the high-potassium-treated cells was more rapid and complete than in the control cells. Computer simulations confirmed that this change could be explained by a change in Kv3.1-like current of the same magnitude as recorded in voltage-clamp experiments. Thus, depolarization and calcium influx may alter the excitability of immature inferior colliculus neurons by selectively increasing the levels of a Kv3. 1-like potassium current.

Modification of tonotopic representation in the auditory system during development

During the early development of the bird and the mammalian peripheral auditory system, a restricted range of low--mid frequencies is recorded in immature animals. These early recordings are correlated to the base or mid-basal region of the cochlea which codes high frequencies in the adult. In order to reconcile the functional observations with anatomical ones, two main hypotheses have been put forward: one called the development of the place principle derived from observations of acoustic trauma in chick cochlea and a second derived from auditory nerve fiber recordings in kittens. Whatever the theories, the tonotopic shift during development is a well-established phenomenon in both birds and mammals that could be explained by a synthetic theory including active and passive cochlear processes. The tonotopic shift observed in the central auditory system mimics quite closely the frequency representation of the peripheral auditory system. The same trend is observed in all auditory nuclei including the cortex, except that the frequency representation is more complex because it shows tonotopic maps that can be twisted in three dimensions. From current observations, there is a simultaneous onset of tonotopic maps across auditory nuclei up to the cortex. A hypothesis is presented related to the frequency changes observed in the cochlea that affect the central auditory pathway, along with possible consequences on auditory behavior.