Fast-spiking parvalbumin-expressing GABAergic interneurons play a key role in the function of neuronal networks, such as feedforward inhibition, feedback inhibition, and oscillatory activity (Bartos et al., 2007, Nature Reviews Neurosci. 8:45). Although parvalbumin (PV) is a robust immunocytochemical marker for this type of interneuron, the function of this Ca²⁺ binding protein is poorly understood. One key problem is that the concentration of PV in different subcellular regions of GABAergic interneurons, including soma, dendrites, and axon, has not been quantified.

To measure the concentration of PV in GABAergic interneurons of the dentate gyrus, we combined whole-cell recordings in acute hippocampal slices from 20- to 22-day-old rats with immunohistochemistry, using a mouse monoclonal primary antibody against parvalbumin (#235, Swant, 1:10000) and an Alexa Fluor 568-conjugated goat anti-mouse secondary antibody. Absolute somatic PV concentrations were determined by comparing the fluorescence intensity of several non-recorded PV-expressing cells with that of one cell filled with a defined concentration (100 mM) of recombinant PV via somatic whole-cell recording in the same slice (Müller et al., 2005, J. Neurosci. 25:558). Confocal stacks were acquired, and cytoplasmic fluorescence intensity was quantified in the equatorial region of the soma of each cell. In GABAergic interneurons with somata located in the granule cell layer or adjacent hilus and inner molecular layer, the mean somatic concentration of PV was 38 6 mM [range: 3 – 160 mM; n = 45 cells in 3 slices]. To assess the concentration of PV in dendritic compartments, we measured the fluorescence intensity in different locations of the apical dendrites in a subpopulation of cells with high PV expression. In proximal apical dendrites (~30 mm from the point of origin at the soma), the cytoplasmic concentration of PV was 59.6% [range: 43.3 – 73.9%] of the somatic concentration. In distal apical dendrites (~190 mm from the point of origin at the soma), the concentration of PV was 50.3% [range: 34.6 – 69.7%; n = 4 dendrites in 3 slices] of the somatic concentration. Thus, the concentration of PV in the apical dendrite of PV-expressing GABAergic interneurons decreases as a function of distance from the soma.

In conclusion, our results demonstrate that (1) the concentration of PV differs between soma and dendrites of the same cell and (2) the concentration of PV differs substantially between GABAergic interneurons in the same region. The relatively low concentration of PV in the soma and the dendrites may suggest that PV has smaller effects on interneuron Ca²⁺ signaling than previously suggested (Lisman et al., 2008, TINS 31:234).