Behavioural experience (e.g. chronic stress, environmental enrichment) can have long-lasting effects on cognitive functions.
Because activity-dependent persistent changes in synaptic strength are believed to mediate memory processes in brain areas such
as hippocampus, we tested whether behaviour has also long-lasting effects on synaptic plasticity by examining the induction of longterm
potentiation (LTP) and long-term depression (LTD) in slices of hippocampal CA1 obtained from rats either 7–9 months after
social defeat (behavioural stress) or 3–5 weeks after 5-week exposure to environmental enrichment. Compared with age-matched
controls, defeated rats showed markedly reduced LTP. LTP was even completely impaired but LTD was enhanced in defeated and,
subsequently, individually housed (during the 7–9-month period after defeat) rats. However, increasing stimulus intensity during 100-
Hz stimulation resulted in significant LTP. This suggests that the threshold for LTP induction is still raised and that for LTD lowered
several months after a short stressful experience. Both LTD and LTP were enhanced in environmentally enriched rats, 3–5 weeks
after enrichment, as compared with age-matched controls. Because enrichment reduced paired-pulse facilitation, an increase in
presynaptic release, facilitating both LTD and LTP induction, might contribute to enhanced synaptic changes. Consistently,
enrichment reduced the number of 100-Hz stimuli required for inducing LTP. But enrichment may also actually enhance the range of
synaptic modification. Repeated LTP and LTD induction produced larger synaptic changes in enriched than in control rats. These
data reveal that exposure to very different behavioural experiences can produce long-lasting effects on the susceptibility to synaptic
plasticity, involving pre- and postsynaptic processes.