Structural synaptic remodelling in the perirhinal cortex of adult and old rats following object-recognition training
C. Bertoni-Freddari, D. Platano, P. Fattoretti, B. Giorgetti, Y Grossi, M. Balietti, T. Casoli, G. Di Stefano, G. Aicardi
Neurobiology of Aging Laboratory, INRCA Research Department, Via Birarelli 8, 60121 Ancona, Italy
A computer-assisted morphometric study has been carried out on the
synaptic junctional areas of perirhinal cortex isolated from adult (4-6
month-old) and old (25-27 month-old) Wistar rats exposed to an
object-recognition training. The paradigm consisted of 6 sessions in
which the animals could explore two identical objects. Rats were
divided into the following groups: trained animals: adult trained (AT)
and old trained (OT); untrained animals: adult control (AC) and old
control (OC). The ultrastructural parameters measured on synapses
preferentially stained by the ethanol phosphotungstic acid preferential
technique (E-PTA) were: numeric density (number of synapses/mm3 of
tissue: Nv), synaptic average area (size: S), and surface density (area
of the synaptic contact zones/mm3 of tissue: Sv). Nv was significantly
higher in OT, and AT vs. OC groups. S was significantly higher in OC
vs. OT, AT and AC animals. Sv was significantly higher only in OT vs.
AT. No significant difference was found as regards the percent of
perforated synapses among the groups analysed. A percent distribution
of S showed that in all the old animals (i.e. OC and OT) the fraction
of oversized contacts (S>0.5 μm2) was higher than in the groups of
adult rats (i.e. AC and AT). Moreover, in the OC group the fraction of
these megasynapses accounted for about 8% vs. 3% in the OT group.
Accordingly, the fraction of smaller junctional areas (S<0.15 μm2)
accounted for 43% in OC and 53.5% in OT. The fraction of oversized
junctions (S>0.5 μm2) was almost the same in the two adult groups
(it ranged from about 1 to 2%), while the small junctions (S<0.15
μm2) accounted for 52% in AC and 64% in AT. The inverse balance
between Nv and S, also clearly envisaged by several other authors,
tends to maintain a constant Sv value. Nv, S and Sv reflect specific
aspects of synaptic ultrastructure; when taken together per
experimental group, these values may be considered a more reliable
index of the morphological adaptive response of the synaptic junctional
areas to environmental stimulation. We observed a clear increasing
trend of Nv values together with a decreasing trend of S values in
trained vs. control conditions in both adults and old animals. This
indicates that a consistent rearrangement of the synaptic network has
occurred after visual learning. The present findings confirm that the
brain plastic condition is retained in adult and old animals.
Key words:
Synaptic plasticity, Object-recognition test, Perirhinal cortex, E-PTA, Synaptic morphometry
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