Small conductance calcium-activated potassium channels (SK channels) mediate apamin sensitive afterhyperpolarising (AHP) currents in neurons following action potentials, and as such are responsible for controlling neuronal excitability and regulating spike frequency adaptation (Stocker, 2004). Functional SK channels consist of a-subunit tetramers, of which three family members have been cloned SK1, SK2 and SK3. For each a-subunit a differential distribution pattern has been shown in the adult rat brain. SK1 and SK2 display an overlapping distribution pattern with a clear localisation to the cortex and hippocampus. SK3 expression is almost absent from the cortex and the hippocampus, whereas high levels of SK3 expression are observed in subcortical regions such as the thalamus and basal ganglia. In certain brain regions the presence of SK channels correlates with the presence of apamin sensitive AHP currents, in other regions where SK channel a-subunits have been found, apamin sensitive calcium-activated currents have yet to be demonstrated. In this study the distribution of the SK3 a-subunit in rat brain at different developmental stages (postnatal days 1, 3, 6, 12 and 25) is shown using an affinity purified anti-CSK3 antibody. The specificity of the anti-CSK3 antibody was tested by western blot and immunocytochemistry on SK channels heterologously expressed in HEK293 (human embryonic kidney) cells. No cross-reactivity was observed for the SK channel family members. Next, the time course of SK3 expression was investigated by Western blot analysis at different developmental stages. These immunoblots showed a single band, specific for SK3, because it was not detectable in pre-absorption controls and the estimated molecular weight (75 kDa) was in accordance with the predicted molecular weight of 81 kDa. The level of SK3 protein expression appears to remain constant and the molecular weight does not change throughout the analyzed developmental stages. The anti-CSK3 antibody was then used for immunohistochemistry on rat brain sections during development. The distribution and level of SK3 expression remains constant throughout development and is overall in accordance with the pattern previously described for adult rat brain. SK3 is highly expressed in many different neuronal populations already at early postnatal stages. Of particular interest are the high levels of expression seen in monoaminergic nuclei such as the substantia nigra pars compacta, locus coeruleus and the dorsal raphe nucleus. The expression of SK3 in the monoaminergic neurons has been shown at postnatal day 25 by double immunostaining with dopaminergic and noradrenergic markers.