Effect of hyperthermia on hippocampal synaptic transmission and CA3 kindling in developing rats

The effect of hyperthermia on excitatory synaptic transmission in the hippocampal CA1 area in response to contralateral CA3 stimuli at 23-26 days of age and the influence of hyperthermia-induced seizures (HS) on the kindling phenomenon induced by CA3 stimulation at 27-29 days of age were investigated in developing rats. When hyperthermia (43.6 +/- 0.5 degrees C) did not induced seizures in conscious unrestrained rats, transient (< 1 h) potentiation was observed in electrically evoked synaptic responses (EPSP and population spikes). When generalized seizures were induced by hyperthermia (43.3 +/- 0.4 degrees C), long-term potentiation (LTP) was observed over 24 h. The difference in time course of the potentiation depended on whether high-voltage multispikes on the EEG, which sustained for longer than 20-30 s and associated with behavioral convulsions, appeared or not. In the following kindling session, the threshold intensity required to produce afterdischarges (ADs) in the HS rats (187 +/- 16 microA) was significantly lower than in the rats without HS (293 +/- 41 microA). However, there was no clear difference between the development of the kindling phenomenon to repeated tetanus at the threshold intensity in the rats with and without HS. It was concluded that potentiation of synaptic responses consists of two different components, transient potentiation induced by hyperthermia alone and LTP induced by HS, and that developing rats were susceptible to kindling epilepsy at the lower AD threshold intensity when experienced HS.