THE ONSET OF PARVALBUMIN-EXPRESSION IN INTERNEURONS OF THE RAT PARIETAL CORTEX DEPENDS UPON EXTRINSIC FACTOR(S)

Parvalbumin (PV) belongs to the large family of EF-hand calcium-bindin g proteins and is an excellent marker for a subpopulation of GABAergic neocortical interneurons. During cortical development, PV first appea rs on postnatal day (P)8, in the infragranular layers; after P14, it a lso becomes apparent within the supragranular layers. However, nothing is known about the factors controlling its expression, which could in volve functional activity, neuronal connectivity and/or neurotrophic f actors. It being difficult to manipulate these parameters in vivo, the ir role may be more readily assessed in organotypic cultures, which ar e deprived of their subcortical afferents and efferents, and hence of subcortically derived neurotrophic factors and extrinsic functional ac tivity. We prepared slices of the rat brain on P3, P5, P7 and P9, main tained them in culture for 2-5 weeks, and compared the temporal and sp atial distribution pattern of PV-immunoreactivity within these slices with the in vivo situation. We found, first, that during late postnata l in vivo development and ageing, the number of PV-immunoreactive neur ons in the parietal cortex decreases significantly, and second, that t he expression of PV-immunoreactivity in the parietal cortex was marked ly influenced by the phase of postnatal development at which slice cul tures were explanted. In those removed on P7 and P9, the number of PV- immunoreactive cells, as well as the temporal and spatial distribution pattern of PV-immunoreactivity corresponded to the in vivo situation, but in explants obtained on P3 or P5, PV-immunoreactivity remained co nfined to layer V of the cortex, reminiscent of the expression profile manifested at the end of the second postnatal week in vivo. Also, the number of PV-immunoreactive cells in these cultures was significantly lower than in explants at the later stages. Our results indicate that the onset of PV-expression in the parietal cortex depends upon extrin sic cortical factors subsisting prior to P7. Once the production of th is protein has been initiated, such influences are no longer required.