C. Bertoni-Freddari, P. Fattoretti, T. Casoli, G. Di Stefano, B. Giorgetti, Y. Grossi, M. Balietti, G. Perretta

Senile plaques (SP) are alterations of the senile brain particularly prominent in Alzheimers disease (AD). The classical plaque appears as a roughly spherical area in the neuropil consisting of a compact core or of narrow bundles of beta amyloid (bA), between abnormal neurites. Although SP formation is reported to occur over years, in the human brain this process can be studied only at autopsy and this constitutes a great limit to obtain results specifically regarding the steps of SP build up. Monkeys are reported to develop SP progressively from the age of 15 years, thus these animals represent a good animal model to investigate SP formation dynamics over time and the eventual age-associated changes favouring this process. Synaptic junctions have been documented to undergo significant alterations both in aging and AD, thus an impaired synaptic function may constitute a precocious step in the development and progression of SP. On the basis of this assumption, a computer-assisted morphometric study has been carried out on ethanol phosphotungstic acid (E-PTA) stained synaptic junctions in the frontal and temporal brain cortex of adult and aged monkeys (Macaca fascicularis) with the aim of identifying early signs of synaptic pathology in aging. The average synaptic size (area: S), the synaptic numeric density (Nv: number of contacts/mm3 of tissue), the synaptic surface density (Sv: overall area of synaptic contacts/mm3 of tissue) and the number of synapses/neurone (Syn/Neur) were the measured ultrastructural parameters. At a comparison with adult animals, both in frontal and temporal cortex of aged animals Nv was decreased and S was increased at a not significant extent, while Sv was unchanged in the temporal cortex and not significantly increased in the temporal cortex. In aged monkeys Syn/Neur was decreased by 16.4% in the temporal cortex and it was increased by 12.3% in the frontal cortex. The present findings confirm and extend previous results from the human brain in normal aging and AD, i.e. the age-related loss of synaptic contacts is associated with an increase in synaptic size. A percent distribution of S showed that in the frontal cortex of old animals the fraction of enlarged contact zones (0.2mm2>) accounts for more than 27.5% vs. 16.9% found in adult monkeys, while in the temporal cortex the larger contacts in old animals account for more than 32.5% vs. 21.3% found in adults. The opposite results of Syn/Neur observed in the two areas of the brain cortex deserves a specific comment. Since synapses and neurones are counted in the same area of the brain cortex, the Syn/Neur value is independent from any change in tissue volume (e.g. shrinkage) due either to age or experimental processing; thus, this parameter reliably reports on the real situation in the tissue. Accordingly, the present findings suggest that the temporal cortex appears to be more vulnerable to aging than the frontal cortex. Tenable interpretations of the present data must take into account the sequence of steps currently suggested to occur in the functional remodelling of synaptic ultrastructure. Namely, as a consequence of repeated stimulation, synaptic junctions may undergo a two, three-fold enlargement of their normal size (S) and, if stimulation proceeds, the larger contact zones perforate and split into smaller junctional areas that, in turn, can be reinforced or degraded according to the functional conditions of the neural network where they are located. This cycle of events is reported to be an ongoing adaptive process purported to modulate synaptic number and size according to stimulations coming from the individuals experiential framework; thus, any impairment and/or delay in the rate of this physiological synaptic turnover may result in a functional decay. On the basis of this cycle of events, the consistent increase of the fraction of enlarged contacts in the brain cortex of aged animals suggests that the synaptic remodelling process proceeds at a slower rate in aging or is halted at the early step of the enlargement of the junctional areas. Moreover, we found that in the temporal cortex the percent of perforated junctions is the same in both the age groups (3.8% and 4.0%), while in the frontal cortex of old monkeys it is twice as much the value of the adults (7.1% vs. 3.5%, respectively): according to the remodelling cycle described above, this increased percent of the fraction of synapses at the step of perforation suggests that in the frontal cortex synaptic rearrangement appears to occur at a higher extent or to proceed at a faster rate. The temporal cortex of the aged Macaca fascicularis is reported to be particularly sensitive to SP formation and our data confirm this vulnerability, thus lending further support to the involvement of synaptic subtle pathology in the development and progression of these alterations typical of the human and primate brain cortex.

Keywords (Optional): 
Alzheimer disease
Macaca fascicularis