Reticular formation influence on neuronal transmission from perforant pathway through dentate gyrus.

Electrical stimulation of the perforant pathway discharges granule cell synchronously, giving rise to a characteristic evoked potential in the granule cell layer termed here the evoked action potential or EAP. In freely moving rats, we applied 3 pulses of low intensity electrical stimulation to the medullary reticular formation prior to the application of the perforant path pulse. The effect of prior reticular formation stimulation was a marked augmentation of the normal EAP response to the perforant path stimulus. The augmentation was dependent on the behavioral state of the experimental animal (it occurred during slow-wave sleep but not during still, alert behavior) and was eliminated by anesthetic agents. The latency of EAP augmentation effect (minimum effective time interval between application of the reticular formation stimulus and the perforant path pulse) was 13--18 msec. In order to localize the sites in the medullary reticular formation from which EAP augmentation could be elicited, threshold currents for producing the effect were determined during dorso-ventral penetrations of a reticular formation stimulating electrode. EAP augmentation was elicited at low stimulus currents from a relatively broad region of the reticular formation. It was also noted that reticular formation stimulation which produced EAP augmentation always elicited one or more motor responses of the neck, back, face or vibrissae. Subsequent investigation of the pathways underlying these motor responses suggested that the effect of reticular formation stimulation on granule cell excitability was mediated by a polysynaptic pathway, the first segment of which was a projection to cells of nucleus gigantocellularis of the caudal medulla.