PHYSICAL EXERCISE AND TESTOSTERONE MODULATE SYNAPTIC ULTRASTRUCTURE IN THE AGING HIPPOCAMPUS

Synaptic plasticity is significantly impaired in the aging hippocampus. In old rodents physical exercise, as well as androgen administration, are reported to improve cognition. We evaluated the effect of forced, mild physical training and testosterone on synaptic ultrastructure in the hippocampus of old (>24 months) mice. Three groups of male C57B1/6 mice were used. The animals run 30 min a day, five days a week for 30 days on a treadmill (belt speed = 8 m/min) without (E, n=5) or with (ET, n=5) a weekly supraphysiological dose of testosterone, or had no exercise (C, n=9). The number of synapses/μm3 of tissue (Nv), the total synaptic area/μm3 of tissue (Sv), the average size (S) of the contact zones and the percentage of perforated junctional zones (PS) were measured in the stratum moleculare of the hippocampal CA1 pyramidal cells (SMCA1) and in the inner supragranular layer of the hippocampal dentate gyrus (DG) with a semiautomatic image analysis system. In SMCA1 of E vs. C and ET mice and in DG of ET vs. C and E, Nv and Sv were significantly increased. No significant difference for S and PS was found among the three groups of mice. The present findings suggest that the synaptic morphological plasticity of the old hippocampus appears to be modulated by both physical training and testosterone administration. Such plasticity seems to be zonespecific, suggesting that different fibre systems may be selectively stimulated by exercise or testosterone.