In my doctoral thesis, I present new information about the developmental expression pattern of the potassium chloride cotransporter KCC2 in the rat auditory brain stem and the morphometrical effects caused by KCC2 gene silencing in mice. The thesis is divided into 3 Chapters. Chapter 1 is a general introduction which gives a brief outline of the primary ascending auditory pathway in mammals. Also, it provides information about the presence of a large number of inhibitory inputs in the auditory system and how these inputs develop; the involvement of inhibition in the acoustic processing is mentioned. In addition, the role of the KCC2 cotransporter in the shift of GABA/glycine transmission, and thus, in maintaining the normal level of inhibition in the mature brain, is described. The focus of Chapter 2 was to investigate the KCC2 immunofluorescent signal from postnatal day (P) 0 to P60 in four major nuclei of the rats superior olivary complex (SOC), namely the medial nucleus of the trapezoid body (MNTB), the medial superior olive (MSO), the lateral superior olive (LSO), and the superior paraolivary nucleus (SPN). The lack of a correlation between the continuous presence of KCC2 mRNA/protein in the postnatal rat brain stem on one side, and the shift in GABA/glycinergic polarity (i.e. KCC2 functionality) on the other side, prompted me to search for a specific cellular expression pattern of the KCC2 protein that might correlate with the switch in GABA/glycine signalling. To do so, the KCC2 immunoreactivity was analysed using high-resolution confocal microscopy in three cellular regions of interest: the soma surface, the soma interior, and the neuropil. In the soma surface, I observed an increase of the KCC2 immunofluorescent signal intensity, yet with a moderate magnitude (1.1 to 1.6-fold). Therefore, I conclude that the change in the soma surface signal is only of minor importance and does not explain the change in KCC2 functionality. The KCC2 signal intensity in the soma interior decreased in all nuclei (1.4 to 2-fold) with the exception of the MNTB where no statistically significant change was found. The decrease in the soma interior was probably related to the increase in the soma surface immunoreactivity and the proposed (weak) intracellular trafficking process of the KCC2 protein. The main developmental reorganization (in qualitative as well as in quantitative aspects) of the KCC2 immunofluorescence in the SOC nuclei was observed in the neuropil. The signal changed its pattern from a diffusely stained neuropil early in development (P0-P4) to a crisp and membrane-confined signal later on (P8-P60), with single dendrites becoming apparent. The exception was found in the MNTB, where the neuropil became almost unlabeled. Quantification revealed a statistically significant decrease (2.2 to 3.8-fold) in the neuropil immunoreactivity in all four nuclei, although the remaining KCC2-stained dendrites became thicker and the signal became stronger. I suppose that, at least in part, the neuropil reorganization can be explained by an age-related reduction of dendritic branches via a pruning mechanism and with the absence of an abnormal Cl- load via extrasynaptic GABAA receptors. This is consistent with the proposed additional role of KCC2, namely to maintain the cellular ionic homeostasis and to prevent dendritic swelling (Gulyás et al., 2001). In conclusion, neither the increase in the KCC2 soma surface signal intensity, nor the reorganization in the neuropil can be strictly related to the developmental switch in the GABA/glycine polarity and the onset of KCC2 function, although some correlation (the appearance of a specific membrane-confined dendritic pattern) between structure and function was found. Further implication of different molecular methods, regarding the proposed posttranslational modification of KCC2, will shed light upon the question of what leads to the functional activation of the cotransporter. In Chapter 3, the advantage of loss-of-function KCC2 mice made it possible, via manipulating the duration of the depolarizing phase of GABA/glycine transmission, to analyse the effect of disturbed Cl- regulation and, thus, the effect of disrupted GABA/glycine neurotransmission (lack of inhibition). I asked the following question: how important is the Cl- homeostasis to maintain general aspects (brain weight) and specific aspects (nucleus volume, neuron number, and soma cross-sectional area) of brain development? Brain stem slices from KCC2 knock-out animals (-/-), with a trace amount of transporter (~5%), as well as from wild type animals (+/+) at P3 and P12 were stained for Nissl substance and the analyses were performed with the help of basic morphometrical and stereological methods. In KCC2 (-/-) animals, body growth impairment was observed, in part related to the seizure activity preventing normal feeding (Woo et al., 2002). However, their brains, in terms of brain weight, were less affected. Therefore, I conclude that Cl- homeostasis is not essential per se to maintain the brain weight. Four auditory nuclei (MNTB, MSO, LSO, and ventral cochlear nucleus (VCN)), were compared with respect to the KCC2 null mutation. The SOC nuclei were not influenced by the lack of KCC2 at P3 considering the morphometric parameters. A difference in the number of neurons occurred in the VCN at P3. I suggest to perform additional immunohistochemical studies of glial presence related to its involvement in the structural and functional support of the neurons and their survival. At P12, the volume of the auditory nuclei in KCC2 (-/-) animals was smaller than in (+/+) animals. However, this is likely to be an epiphenomenon since the brain weight increase was also impaired with the same magnitude. Therefore, I suppose that the Cl- homeostasis is not crucial for the nucleus volume increase in the VCN, the MNTB and the MSO during development. An exception was found for the LSO. Regarding the other morphometric parameters at P12, the four nuclei behaved in a different way: (1) in the VCN, after P3, no parameter underwent a disproportional change due to impaired Cl- homeostasis; (2) the MNTB and the LSO showed less pronounced neuropil in mutants in comparison to age-matched controls and two reasons were proposed: first, the depolarizing GABA/glycine transmission in mutants may contribute to excessive Ca2+ load, excitotoxicity and dendrite damage; second, a decrease of some trophic factors may prevent dendrite development in addition to impaired normal body growth; (3) the MSO neurons in P12 (-/-) animals had smaller soma cross-sectional area than in P12 (+/+) animals. I conclude that the normal Cl- homeostasis is required in the MSO at older ages (P12) to achieve and maintain a proper soma size; (4) the lack of KCC2 did not prevent the process of neuronal differentiation in the VCN and the MNTB during development in both mutant and control animals. In conclusion, the various auditory nuclei have to be discussed independently regarding the influence of Cl- homeostasis on some morphometric parameters. Presumably, this is related to the different time of the shift in the GABA/glycine polarity i.e., the onset of KCC2 function (Srinivasan et al., 2004a). Taken together, my thesis accumulated data about the immunohistological expression pattern of KCC2 in various auditory brain stem nuclei and the influence of impaired Cl- homeostasis on some morphometric features in these nuclei. This information will be helpful for further investigations involved to discover the mechanisms and the events that govern the inhibition and the inhibitory pathway in the central auditory system.