STIMULUS PARAMETERS AFFECTING PAIRED-PULSE DEPRESSION OF DENTATE GRANULE CELL FIELD POTENTIALS .2. LOW-FREQUENCY STIMULATION

Low frequency (1 Hz) stimulation of the perforant path produces a depr ession in the population spike (PS) of dentate granule cell field pote ntials and also may affect the strength of paired pulse depression. Th e effects of 1 Hz stimulation (30 s train) on paired pulse depression (20 and 200 ms interpulse intervals, IPI) were evaluated in the unanes thetized rat under two conditions: (i) when the stimulus intensity of both pulses was increased simultaneously (5-100%); and (ii) when the s timulus intensity of the first (conditioning) pulse was increased (5-1 00%), while the stimulus intensity of the second (test) pulse was held constant (50%). The test PS amplitude was predicted based upon either the conditioning PS amplitude at the end of the 1 Hz train or upon th e additive effects of paired pulse depression and I Hz stimulation. Th ese predicted values then were assessed for the best fit to observed v alues following 1 Hz trains. Under both stimulus conditions, the 1 Hz depression in the conditioning PS amplitude exhibited characteristics that were identical to late paired pulse depression recorded before th e train. A decrease in the test PS amplitude also was observed followi ng 1 Hz stimulation at the 20 and 200 ms IPIs. The best fit to observe d values of the test PS at the end of 1 Hz trains was provided by esti mates based upon the additive effects of 1 Hz stimulation and paired p ulse depression. These results indicate that the strength of paired pu lse depression in the unanesthetized rat is unchanged following 1 Hz s timulation, and further, that the 1 Hz depression in dentate granule c ell field potentials most likely reflects the cumulative influence of late paired pulse depression.