Enriched environment exposure regulates excitability, synaptic transmission, and LTP in the dentate gyrus of freely moving rats.

Performance in hippocampus-dependent and other tasks can be improved by exposure to an enriched environment (EE), but the physiological changes in neural function that may mediate these effects are poorly understood. To date, there have been conflicting reports regarding potential mechanisms, such as an increase in basal synaptic transmission, an increase in cell excitability, or altered synaptic plasticity. Here, we reexamined in freely moving animals the conditions under which varying degrees of EE exposure might lead to increases in synaptic or neural function in the dentate gyrus of the hippocampus. Adult male Sprague-Dawley rats were chronically implanted with stimulating and recording electrodes in the perforant path and dentate gyrus, respectively, and housed singly in standard cages. After stable recordings were established for field excitatory postsynaptic potentials (fEPSPs) and population spikes (PSs), the effects of various degrees of periodic novel environment exposure for 19 days were assessed. Exposure to an EE increased fEPSPs, but only when animals were kept in nominally low-stress housing conditions. An increase in granule-cell excitability, as evidenced by PS increases, was induced by all environmental treatments with the greatest effect being induced by overnight EE exposure. EE exposure did not change the level of long-term potentiation (LTP) induced by a moderate high-frequency tetanus, but continued EE exposure post-tetanus produced a significantly faster decay of LTP relative to control animals. These results suggest that, in adult animals, EE exposure may augment hippocampal information processing, but may also speed turnover of information in the hippocampus during the maintenance period.

Enriched environment exposure alters the input-output dynamics of synaptic transmission in area CA1 of freely moving rats.

We examined in freely moving animals whether EE exposure directly alters synaptic transmission in CA1 of the hippocampus. Adult male Sprague-Dawley rats were chronically implanted with recording and stimulating electrodes in CA1 and housed singly in standard cages. After stable recordings were established for field excitatory postsynaptic potentials (fEPSPs), the effects of three levels of environmental enrichment for 19 days were assessed. A change in fEPSPs was observed only after repeated overnight enriched environment exposure, such that response potentiation occurred at high stimulus intensities but a reduced response was evident at low stimulus intensities. Behavioural tests confirmed that EE exposure was sufficient to facilitate performance on spatial working memory tasks. These results suggest that overnight EE exposure refines the dynamics of CA1 synaptic transmission in a way that may contribute to augmented information processing and thus performance on hippocampus-dependent tasks.