STUDIES ON LONG-TERM POTENTIATION OF THE POPULATION SPIKE COMPONENT OF HIPPOCAMPAL FIELD POTENTIAL BY THE TETANIC STIMULATION OF THE PERFORANT PATH IN RATS - EFFECTS OF A DOPAMINE AGONIST, SKF-38393

Long-term potentiation of the field potentials recorded in the dentate gyrus of the hippocampus was observed in freely-moving rats by delivering a brief tetanic stimulation to the perforant path, and the effects of the D1 agonist. SKF-38393, on it was investigated. The field potential was divided into two components; excitatory postsynaptic potential (EPSP) and population spike. In Expt. I, synaptic stimulus-response (S-R) relationship, spike S-R relationship, and EPSP-spike (E-S) relationship were plotted. The estimated slope of the regression line in the spike S-R relationship was enhanced after delivery of the tetanic stimulation (10 pulse at 400 Hz), where that in synaptic S-R relationship was not enhanced. The estimated slope of the regression line in the E-S relationship was also enhanced by the tetanic stimulation. In Expt. II, time-dependent change of the field potential after tetanic stimulation was investigated. The population spike was enhanced significantly for about 2 h following tetanic stimulation, while pEPSP did not change significantly. These changes following tetanic stimulation in Expt. I and II were significantly inhibited by previous administration of SKF-38393 (10 mg/kg, i.p.), and the effect of this drug was dose-dependently antagonized by the D1 antagonist, SCH-23390 (0.1. 0.2 and 0.5 mg/kg, i.p.). These results suggest that a brief tetanic stimulation of the perforant path induces long-term potentiation of the population spike without potentiating the synaptic input in the perforant path-dentate synapses, and that potentiation of the population spike is inhibited by the dopaminergic D1 mechanism.