LTP IN HIPPOCAMPAL CA1 OF URETHANE-NARCOTIZED RATS REQUIRES STRONGER TETANIZATION PARAMETERS

Rats with chronically implanted electrodes in the hippocampal CA1 region were tested in their capacity to express and maintain long-term potentiation (LTP) of the population spike (PS) or of the field excitatory postsynaptic potential (fEPSP). Two different states were compared: a) freely moving animals; b) urethane-anesthetized animals (1 g/kg, IP). We found that a short, high-frequency tetanus (six bursts of 15 pulses; 200 Hz; double-pulse width; interburst interval 10 s) increased PS amplitudes and fEPSP slopes up to 300% in response to test stimuli in the awake rat. The PS amplitude slowly decreased in time, returning to baseline levels 4 h posttetanically, whereas the fEPSP slope remained at higher values for 24 h. Urethane injection reduced the fEPSP slope and abolished the PS to normal test pulses. We thus increased the strength of the test stimuli until we again recorded magnitudes of PSs and fEPSPs comparable to those in the awake animal. In conjunction with these stronger stimuli, tetanus-induced LTP was elicited that for the PS was increased in magnitude and prolonged in duration compared to the untreated control group. Although, stronger tetanic stimuli were applied to the narcotized fEPSP group too, no difference was found compared to controls. These results suggest that urethane narcosis influences the sensitivity of CA1 neurons to express LTP. Stronger stimulation was required to induce and maintain a long-lasting potentiation of the fEPSP slope and PS amplitude.