Temperature dependence of synaptic responses in guinea pig hippocampal CA1 neurons in vitro

1. Temperature-dependent properties of synaptic transmission were studied by recording orthodromic responses of the population spike and excitatory postsynaptic potential in CA1 pyramidal neurons of guinea pig hippocampal slices. 2. Increasing the temperature of the perfusing medium from 30 to 43degreesC resulted in a decrease in the amplitude of the population spike (A-PS) and a reduced slope of the excitatory postsynaptic potential (S-EPSP). Bath application of the gamma-aminobutyric acid receptor antagonist, picrotoxin, or a change in the calcium concentration of the perfusate did not affect the A-PS during heating. 3. Increasing the strength of the synaptic input to that eliciting a PS with an amplitude 50,75, or 100% of maximal at 30degreesC resulted in a significant increase in the A-PS during the middle phase of hyperthermia (35-39degreesC). 4. The long-term potentiation (LTP) induced at either 30 or 37degreesC showed the same percentage increase in both the amplitude of the population spike and the S-EPSP after delivery of a tetanus (100 Hz, 100 pulses) to CA1 synapses. 5. The results of the present study, therefore, indicate that the decrease in CA1 field potential was linearly related to the temperature of the slice preparation, while LTP was induced in these responses during heating from 30 to 37degreesC.