2. Marco conceptual
2.3. Desarrollo Infantil
MSO neurons (as well as low frequency LSO cells) represent an extreme example of temporal precision, both in that the kinetics of glycinergic inhibition in these neurons are the fastest reported so far and in that these neurons are sensitive to ITDs in the microsecond range (Goldberg and Brown, 1969;Finlayson and Caspary, 1991;Magnusson et al, 2005, Smith et al 2000). We showed in our pharmacological experiments that the glycinergic inputs to the MSO were crucial for defining the binaural coincidence window of the MSO neurons (the range of ITDs that are able to elicit responses), most probably by acting in sub- millisecond events (i.e. in cycle-by-cycle manner). In our proposed scenario, the glycinergic inputs interact with the synaptic integration of the excitatory inputs and thereby refine the width of the coincidence window. Therefore, glycinergic inhibition in the MSO is not only well- timed itself, but it also increases the overall temporal sensitivity of MSO neurons. This functional role of controlling temporal sensitivity and defining the coincidence window of excitatory inputs is a recurring functional role of inhibition also in other neuronal systems, albeit on less extreme time scales. In the cerebellum, for instance, it was shown that the temporal summation of excitatory inputs from parallel fibers onto Purkinje cells is restrained by a delayed GABAergic input stemming from an interneuron that is activated by the same parallel fibers (Mittmann et al., 2005). This feed-forward inhibition is able to refine the coincidence window of the Purkinje cell from <30ms to less than 2ms, thereby establishing the temporal precision of the cerebellar output that is crucial for the coordination of movements. A similar feed-forward inhibitory pathway is also found in cortical and hippocampal pyramidal cells and here, likewise to the cerebellum, GABAergic, trailing inhibitory inputs refine the coincidence window of multiple excitatory inputs to a range of a few milliseconds (Buzsaki 1984;Pouille and Scanziani 2001; Gabernet et al., 2005). Notably, feed-forward inhibitory inputs in the hippocampus synapse mainly onto somatic regions of the pyramidal cell, reminiscent to the arrangement found in the MSO (Pouille and Scanziani 2001;Kapfer et al., 2002). However, the requirements in temporal precision of the glycinergic
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inputs to the MSO seem to be an order of magnitude higher than in the compared GABAergic feed-forward pathways.
Next to feed-forward inhibition to increase temporal sensitivity, another prominent role of inhibition is gain control. Gain control is typically achieved by a negative feedback loop to control the excitatory input strength, thereby assuring a broad dynamic range regardless of the stimulus intensity. Such robustness against changes in intensity is particularly important in ITD processing because the ability of the detector neurons to remain unresponsive to out- of-phase ITDs independent of their input strength is crucial for neuronal coincidence detection. However, the amplitude of a net EPSP of all incoming inputs increases with increasing intensities, as more inputs are recruited at higher intensities (Reed and Durbeck 1995;Pena et al., 1996). At some point the depolarization that is produced by the net EPSP from either the ipsi- or the contralateral ear alone might be supra-threshold and elicit spikes. Hence, the neuron could potentially lose ITD sensitivity at high intensities if it also responded to out-of-phase ITDs (Reed and Durbeck 1995). In the avian ITD detection system, to prevent these so-called “monaural responses”, a gain control circuit consisting of GABAergic inhibitory inputs to the NL reduces the amplitude of excitatory inputs and shortens their duration, thereby ensuring consistent ITD sensitivity across intensity levels (Pena et al., 1996;Dasika et al., 2005). Importantly, the inhibition in NL neurons is not timed (it is decoupled from the phase-locked excitation; Yang et al., 1999), and it actually has a depolarizing effect on the NL cells (due to a high intracellular Cl- concentration), which in turn activates low-threshold potassium-channels that lead to shunting of the cell (Hyson et al., 1995;Yang et al., 1999;Burger et al., 2005b). Moreover, the superior olivary nucleus (SON), which is the source of this GABAergic inhibition in the NL (Lachica et al., 1994), shares the same input source with the NL and is additionally innervated by NL neurons, creating a differential gain control circuit for the NL (Monsivais and Rubel, 2001;Burger et al 2005a). In the MSO, the inhibitory inputs seem not to fulfill a classical gain control function. Our results in chapter 1 suggest that the phase-locked glycinergic inputs create short intervals of hyper-polarization that are precisely aligned relative to the excitatory intervals and thereby refine the range of ITDs at which incoming EPSPs are able to exceed spiking threshold. However, by refining the binaural coincidence window, the inhibition should also be able to prevent out-of-phase responses at any given intensity. Recent studies in the lab of Benedikt Grothe showed that ITD sensitivity in the MSO is indeed very robust against changes in intensity (Pecka et al., 2008, abstract), yet it is an open question what fraction of this robustness can be attributed to the effects of the timed inhibition rather than to the specific morphology and channel composition of MSO neurons. It was shown that the presence of low-threshold potassium channels in MSO neurons mediate the generation of very fast and small action potentials that only allow supra-threshold depolarization when multiple PSPs
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coincide at the spike-initiating zone (Svirskis et al., 2003;Scott et al., 2005). Modeling studies additionally suggested that the distinct dendritic morphology of the MSO (and NL) neurons promotes binaural coincidences and reduces the summation of monaural inputs (Grau-Serrat et al., 2003, Svirskis et al., 2003;Dasika et al., 2005). It has also been suggested that synaptic depression of the excitatory inputs helps to preserve ITD sensitivity across intensities (Cook et al., 2003). Nevertheless, at least in NL neurons, which have similar cellular specializations, a gain control mechanism seems to be fundamental for maintaining ITD sensitivity at high intensities (Pena et al., 1996;Dasika et al., 2005). Therefore, it seems likely that glycinergic inhibition may account not only for tuning ITD sensitivity in the MSO, but also contributes to establish robustness of the coincidence mechanism against changes in intensity. Both of these functional roles are accomplished by the preservation of a defined width of the ITD window, allowing supra-threshold depolarization only by coincidence of binaural inputs.
Finally, it is noteworthy that glycinergic inhibition in the MSO serves an additional functional role during development that is not accomplished by the tonic inhibition in the NL: During maturation of an animal, the correct neuronal representation of sound sources in space demands a learning mechanism that calibrates the system. In both the MSO and the avian auditory system, such experience-dependent developmental plasticity is conveyed by inhibition, but the mechanisms are very different. In the avian system, the auditory topographic map that is created in the midbrain is eventually merged with the visual map in the optic tectum (review: Knudsen, 2002). Thereby the accuracy of the auditory map can be supervised during development by visual input and, if necessary, re-calibrated. Knudsen and colleagues showed in owls that the ability to shift the auditory maps to match the visual maps is dependent on GABAergic inputs that are created during a critical period of plasticity (Zheng and Knudson 1999, 2001). Importantly, these GABAergic inputs are formed in the midbrain, not in the NL. Hence, the spatial sensitivity of single NL cells is determined only by their specific excitatory innervation (i.e. the offset of the delay lines), while the inhibition- mediated calibration takes place at the level of the midbrain, the site of convergence with the visual map. In contrast, the auditory localization ability of mammals is not based on maps in which activity of single neurons is read out, but information is encoded in the overall activity level in a population of neurons (McAlpine et al., 2001;Hancock and Delgutte, 2004). Consequently, all neurons of a particular nucleus need to be tuned to assure monotonic response modulation across the range of physiological ITDs. This tuning mechanism is provided already at the level of the MSO by the glycinergic inhibition, as was explicitly demonstrated in this thesis (chapter 1). It is important to mention that this tuning is also experience dependent and restricted to a critical period: ITD functions in the gerbil DNLL were shown to be adjusted to the physiological range during a period shortly after hearing
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onset that coincided with a period in which a refinement of inhibitory synapses to the somatic area of MSO neurons takes place (Seidl and Grothe 2005;Kapfer et al 2002). These maturation processes were absent when animals did not experience directional auditory input during the critical period.
In summary, the glycinergic inputs to the MSO and the GABAergic inputs to the NL both generate a gain control for the coincidence-detection mechanism. However, only glycinergic inhibition in the MSO has the essential function of refining ITD sensitivity. The underlying mechanisms facilitating the functional roles in the two systems are, accordingly, very different for the two systems and strongly coupled to the different receptor kinetics of the respective transmitters. The effectiveness of the inhibitory inputs in the MSO is crucially linked to their precise timing relative to the excitatory inputs and, hence, demands very fast kinetics as found for glycinergic inhibition (Gingrich et al, 1995;Smith et al., 2000;Legendre, 2001;Magnusson et al, 2005). In contrast, in the avian ITD detection system a sustained activity of inhibition rather than timing is required, promoting GABA as inhibitory transmitter. Next, the role of GABAergic transmission in mediating sustained inhibition of activity will be examined in more detail.