TIME-DEPENDENT REVERSAL OF LONG-TERM POTENTIATION BY BRIEF COOLING SHOCKS IN RAT HIPPOCAMPAL SLICES

Using a recording chamber built with peltier elements, we studied the effects of fast and brief reductions in temperature on synaptic transm ission and plasticity in area CA1 of rat hippocampal slices. Cooling s hocks consisted of a drop in temperature from 33-degrees-C to 30-degre es-C, 27-degrees-C or 24-degrees-C for 2-5 min. Equilibrium to the new temperature was reached in about 30 s. During these cooling episodes, marked modifications of the size and time course of synaptic response s were observed. Changing the temperature for 4-5 min from 33-degrees- C to 24-degrees-C resulted in a 75% reduction in amplitude and 158% pr olongation of the rise time of excitatory postsynaptic potentials (EPS Ps). These changes were followed by a complete recovery of synaptic tr ansmission. This recovery was very fast for the EPSP rise time (about 30 s), but much slower for the amplitude or initial slope (20-30 min). This slow recovery was correlated with changes in size of the presyna ptic fiber volley, thereby indicating transient modifications of cell excitability. Application of cooling episodes of 4-5 min from 33-degre es-C to 24-degrees-C during the first 20 min that followed induction o f long-term potentiation resulted in a complete reversal of synaptic p otentiation. The LTP abolished by a cooling shock could be re-instated by re-applying high frequency trains. Several sequential induction/ab olition effects could thus be obtained. In contrast, cooling episodes applied later than 25 min after LTP induction did not affect synaptic potentiation. These results show that fast and transient modifications in temperature can be used to explore mechanisms of synaptic transmis sion and plasticity, and they indicate that LTP can be fully reversed by cooling shocks applied during a specific time window after high fre quency stimulation.