Ursula Koch

Functional properties of neurons derived from in vitro reprogrammed postnatal astroglia

With the exception of astroglia-like cells in the neurogenic niches of the telencephalic subependymal or hippocampal subgranular zone, astroglia in all other regions of the adult mouse brain do not normally generate neurons. Previous studies have shown, however, that early postnatal cortical astroglia in culture can be reprogrammed to adopt a neuronal fate after forced expression of Pax6, a transcription factor (TF) required for proper neuronal specification during embryonic corticogenesis. Here we show that also the proneural genes neurogenin-2 and Mash1 (mammalian achaete schute homolog 1) possess the ability to reprogram astroglial cells from early postnatal cerebral cortex. By means of time-lapse imaging of green fluorescent astroglia, we provide direct evidence that it is indeed cells with astroglial characteristics that give rise to neurons. Using patch-clamp recordings in culture, we show that astroglia-derived neurons acquire active conductances and are capable of firing action potentials, thus displaying hallmarks of true neurons. However, independent of the TF used for reprogramming, astroglia-derived neurons appear to mature more slowly compared with embryonic-born neurons and fail to generate a functional presynaptic output within the culturing period. However, when cocultured with embryonic cortical neurons, astroglia-derived neurons receive synaptic input, demonstrating that they are competent of establishing a functional postsynaptic compartment. Our data demonstrate that single TFs are capable of inducing a remarkable functional reprogramming of astroglia toward a truly neuronal identity.

Maturation of glycinergic inhibition in the gerbil medial superior olive after hearing onset

The neurones of the medial superior olive (MSO) are the most temporally sensitive neurones in the brain. They respond to the arrival time difference of sound at the two ears with a microsecond resolution; these interaural time differences are used to localize low‐frequency sounds. In addition to the excitatory inputs from each ear, the MSO neurones also receive binaural glycinergic projections, which have a critical role in sound localization processing. Recently, it was shown that the glycinergic input to the MSO undergoes an experience‐dependent structural reorganization after hearing onset. To explore the maturation of inhibition during the development of sound localization on a cellular level, glycinergic currents and potentials were measured in gerbil MSO principal cells from postnatal (P) day P12–P25 by whole‐cell patch‐clamp recordings. The synaptic glycinergic currents accelerated to rapid decay kinetics (∼2 ms) and rise times (∼0.4 ms) after hearing onset, reaching maturity around P17. Since the kinetics of miniature glycinergic currents did not change with age, it is likely that a higher degree of transmitter release synchrony is the underlying mechanism influencing the acceleration of the kinetics. During the same period, the synaptic glycinergic potentials accelerated four‐fold, largely as a result of a prominent decrease in input resistance. In accordance with a reorganization of the glycinergic inputs, the evoked peak conductances decreased more than two‐fold, together with a three‐fold reduction in the frequency of miniature events after hearing onset. These age‐dependent changes were absent in animals that had been reared in omni‐directional noise, indicating that an experience‐dependent pruning of synaptic inputs is important for the maturation of functional inhibition in the MSO. Taken together, these striking developmental adjustments of the glycinergic inhibition in the MSO most probably reflect an adaptation to improve the encoding of auditory cues with great temporal precision and fidelity during the maturation of sound localization behaviour.

Retrograde GABA Signaling Adjusts Sound Localization by Balancing Excitation and Inhibition in the Brainstem

Central processing of acoustic cues is critically dependent on the balance between excitation and inhibition. This balance is particularly important for auditory neurons in the lateral superior olive, because these compare excitatory inputs from one ear and inhibitory inputs from the other ear to compute sound source location. By applying GABA(B) receptor antagonists during sound stimulation in vivo, it was revealed that these neurons adjust their binaural sensitivity through GABA(B) receptors. Using an in vitro approach, we then demonstrate that these neurons release GABA during spiking activity. Consequently, GABA differentially regulates transmitter release from the excitatory and inhibitory terminals via feedback to presynaptic GABA(B) receptors. Modulation of the synaptic input strength, by putative retrograde release of neurotransmitter, may enable these auditory neurons to rapidly adjust the balance between excitation and inhibition, and thus their binaural sensitivity, which could play an important role as an adaptation to various listening situations.

Experience-Dependent Refinement of the Inhibitory Axons Projecting to the Medial Superior Olive

Neurons in the medial superior olive (MSO) analyze interaural time differences (ITDs) by comparing the arrival times of the two excitatory inputs from each ear using a coincidence detection mechanism. They also receive a prominent inhibitory, glycinergic projection from the ipsilateral medial nucleus of the trapezoid body (MNTB), which contributes to the fine‐tuning of ITD analysis. Here, we investigated developmental changes of the axonal arborisation pattern of single Microruby‐labeled MNTB neurons projecting to the MSO region. During the first 2 weeks after hearing onset, the axonal arborisation of MNTB neurons was significantly refined resulting in a narrowed projection area across the tonotopic axis of the MSO and a redistribution of the axonal endsegments to a mostly somatic location. Rearing the animals in omnidirectional noise prevented the structural changes of single MNTB projections. These results indicate that the functional elimination of inhibitory inputs on MSO neurons after hearing onset, as described previously, is paralleled by a structural, site‐specific refinement of the inputs and is dependent on the normal acoustic experience of the animal.

Unconventional GABA release: mechanisms and function

GABA is the main inhibitory neurotransmitter in the mammalian brain, and perturbed GABA signalling is the underlying cause of many neurological and psychiatric disorders. Synaptic release of GABA and the functional consequences of its receptor activation have been studied extensively. However, GABA can also be released in unconventional ways. For example, GABA can provide a retrograde signal, released from parts of the neuron other than the axon terminal. Alternatively, GABA can be released from neurons or glial cells by the reversal of transporters, or by other non-vesicular release mechanisms. In this review, we provide an overview of the recent findings regarding the mechanisms and functions of unconventionally released GABA and discuss the physiological significance of such neural regulation.

Sensory deprivation regulates the development of the hyperpolarization-activated current in auditory brainstem neurons

Hyperpolarization‐activated and cyclic nucleotide‐gated (HCN) channels are highly expressed in the superior olivary complex, the primary locus for binaural information processing. This hyperpolarization‐activated current (Ih) regulates the excitability of neurons and enhances the temporally precise analysis of the binaural acoustic cues. By using the whole‐cell patch‐clamp technique, we examined the properties of Ih current in neurons of the lateral superior olive (LSO) and the medial nucleus of the trapezoid body (MNTB) before and after hearing onset. Moreover, we tested the hypothesis that Ih currents are actively regulated by sensory input activity by performing bilateral and unilateral cochlear ablations before hearing onset, resulting in a chronic auditory deprivation. The results show that after hearing onset, Ih currents are rapidly upregulated in LSO neurons, but change only marginally in neurons of the MNTB. We also found a striking difference in maximal current density, voltage dependence and activation time constant between the LSO and the MNTB in mature‐like animals. Following bilateral cochlear ablations before hearing onset, the Ih currents were scaled up in the LSO and scaled down in the MNTB. Consequently, in the LSO this resulted in a depolarized resting membrane potential and a lower input resistance of these neurons. This type of activity‐dependent homeostatic change could thus result in an augmented response to the remaining inputs.

Dynamics of binaural processing in the mammalian sound localization pathway – The role of GABAB receptors

The initial binaural processing in the superior olive represents the fastest computation known in the entire mammalian brain. Although the binaural system has to perform under very different and often highly dynamic acoustic conditions, the integration of binaural information in the superior olivary complex (SOC) has not been considered to be adaptive or dynamic itself. Recent evidence, however, shows that the initial processing of interaural level and interaural time differences relies on well-adjusted interactions of both the excitatory and the inhibitory projections, respectively. Under static conditions, these inputs seem to be tightly balanced, but may also require dynamic adjustment for proper function when the acoustic environment changes. GABA(B) receptors are at least one mechanism rendering the system more dynamic than considered so far. A comprehensive description of how binaural processing in the SOC is dynamically regulated by GABA(B) receptors in adults and in early development is important for understanding how spatial auditory processing changes with acoustic context.

The Mammalian Interaural Time Difference Detection Circuit Is Differentially Controlled by GABAB Receptors during Development

Throughout development GABAB receptors (GABABRs) are widely expressed in the mammalian brain. In mature auditory brainstem neurons, GABABRs are involved in the short-term regulation of the strength and dynamics of excitatory and inhibitory inputs, thus modulating sound analysis. During development, GABABRs also contribute to long-term changes in input strength. Using a combination of whole-cell patch-clamp recordings in acute brain slices and immunostainings in gerbils, we characterized developmental changes in GABABR-mediated regulation of synaptic inputs to neurons in the medial superior olive (MSO), an auditory brainstem nucleus that analyzes interaural time differences (ITDs). Here, we show that, before hearing onset, GABABR-mediated depression of transmitter release is much stronger for excitation than inhibition, whereas in mature animals GABABRs mainly control the inhibition. During the same developmental period, GABABR immunoreactivity shifts from the dendritic to the somatic region of the MSO. Furthermore, only before hearing onset (postnatal day 12), stimulation of the fibers originating in the medial and the lateral nucleus of the trapezoid body (MNTB and LNTB) activates GABABRs on both the inhibitory and the excitatory inputs. After hearing onset, GAD65-positive endings devoid of glycine transporter reactivity suggest GABA release from sources other than the MNTB and LNTB. At this age, pharmacological increase of spontaneous synaptic release activates GABABRs only on the inhibitory inputs. This indicates not only a profound inhibitory effect of GABABRs on the major inputs to MSO neurons in neonatal animals but also a direct modulatory role of GABABRs for ITD analysis in the MSO of adult animals.

Comparative posthearing development of inhibitory inputs to the lateral superior olive in gerbils and mice.

Interaural intensity differences are analyzed in neurons of the lateral superior olive (LSO) by integration of an inhibitory input from the medial nucleus of the trapezoid body (MNTB), activated by sound from the contralateral ear, with an excitatory input from the ipsilateral cochlear nucleus. The early postnatal refinement of this inhibitory MNTB-LSO projection along the tonotopic axis of the LSO has been extensively studied. However, little is known to what extent physiological changes at these inputs also occur after the onset of sound-evoked activity. Using whole-cell patch-clamp recordings of LSO neurons in acute brain stem slices, we analyzed the developmental changes of inhibitory synaptic currents evoked by MNTB fiber stimulation occurring after hearing onset. We compared these results in gerbils and mice, two species frequently used in auditory research. Our data show that neither the number of presumed input fibers nor the conductance of single fibers significantly changed after hearing onset. Also the amplitude of miniature inhibitory currents remained constant during this developmental period. In contrast, the kinetics of inhibitory synaptic currents greatly accelerated after hearing onset. We conclude that tonotopic refinement of inhibitory projections to the LSO is largely completed before the onset of hearing, whereas acceleration of synaptic kinetics occurs to a large part after hearing onset and might thus be dependent on proper auditory experience. Surprisingly, inhibitory input characteristics, as well as basic membrane properties of LSO neurons, were rather similar in gerbils and mice.