Dmv. Weisenhorn et al., THE ONSET OF PARVALBUMIN-EXPRESSION IN INTERNEURONS OF THE RAT PARIETAL CORTEX DEPENDS UPON EXTRINSIC FACTOR(S), European journal of neuroscience, 10(3), 1998, pp. 1027-1036
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.