Temperature modifies potentiation but not depotentiation in bidirectional hippocampal plasticity of Syrian hamsters (Mesocricetus auratus)

Previous studies have shown that one form of neuroplasticity, population spike (PS) potentiation, can be established in the hamster hippocampus at temperatures above 20 °C. Here, we tested three related hypotheses; namely, that in Syrian hamsters: (1) PS potentiation can be elicited below 20 °C and that at any constant temperature, potentiation can be described by a pair of sigmoidal functions matched to input/output curves; (2) potentiation can be partially reversed by depotentiation (a second and distinctive form of neuroplasticity); and (3) tetanus evokes long-term potentiation in slices from animals housed under conditions corresponding to various stages of the annual hibernation cycle. To test these hypotheses, we measured PS amplitudes and fEPSP slopes in CA1 pyramidal cells in hippocampal slices. We found that sigmoidal functions before and after tetanus showed PS enhancement at 18 °C and a larger enhancement at 28 °C, thereby supporting hypothesis 1. We also found that low-frequency stimulation reduced the amplitude of the potentiated PS by ∼29% at both 18 °C and 28 °C, consistent with hypothesis 2; and that slices from nonhibernating hamsters on long and short photoperiods and from hamsters in hibernation all showed at least 40% increases in fEPSP slope following tetanus at a slice temperature of 23 °C, supporting hypothesis 3. Thus, bidirectional plasticity is present in hamsters. That is, both potentiation and depotentiation were readily evoked at 28 °C; potentiation was muted, while depotentiation (the reversal of the potentiation) remained robust at 18 °C. Moreover, potentiated responses could be elicited in slices from animals housed under diverse conditions.