CONVERGING INPUTS TO THE ENTORHINAL CORTEX FROM THE PIRIFORM CORTEX AND MEDIAL SEPTUM: CURRENT SOURCE DENSITY ANALYSIS IN VIVO, AND SYNAPTIC RESPONSE FACILITATION IN BEHAVING ANIMALS. C. Andrew Chapman and Ronald J. Racine. Department of Psychology, McMaster University, Hamilton, Ontario, CANADA L8S 4K1.
APStracts 4:140N, 1997.
ABSTRACT
The entorhinal cortex receives sensory inputs from the piriform cortex and modulatory inputs from the medial septum. To examine short-term synaptic facilitation effects in these pathways, current source density (CSD) analysis was first used to localize the membrane currents in the entorhinal cortex which generate field potentials evoked by stimulation of these afferents. Field potentials were recorded at 50 æm intervals through the medial entorhinal cortex in urethane-anesthetized rats and the one-dimensional CSD was calculated. Piriform cortex stimulation evoked a surface-negative, deep- positive field potential component in the entorhinal cortex with mean onset and peak latencies of 10.4 and 18.4 msec. The component followed brief 100 Hz stimulation, consistent with a monosynaptic response. CSD analysis linked the component to a current sink which often began in layer I before peaking in layer II. A later, surface-positive field potential component peaked at latencies near 45 msec and was associated with a current source in layer II. Medial septal stimulation evoked positive and negative field potential components which peaked at latencies near 7 and 16 msec, respectively. A weaker and more prolonged surface-negative, deep-positive component peaked at latencies near 25 msec. The early components were generated by currents in the hippocampal formation, and the late surface-negative component was generated by currents in layers II to IV of the entorhinal cortex. Short-term facilitation effects in conscious animals were examined using electrodes chronically implanted near layer II of the entorhinal cortex. Paired-pulse stimulation of the piriform cortex at interpulse intervals of 30 and 40 msec caused the largest facilitation (248%) of responses evoked by the second pulse. Responses evoked by medial septal stimulation were also maximally facilitated (59%) by a piriform cortex conditioning pulse delivered 30 to 40 msec earlier. Paired pulse stimulation of the medial septum caused the largest facilitation (149%) at intervals of 70 msec, but piriform cortex evoked responses were maximally facilitated (46%) by a septal conditioning pulses 100 to 200 msec earlier. Frequency potentiation effects were maximal during 12-18 Hz stimulation of either the piriform cortex or medial septum. Occlusion tests suggested that piriform cortex and medial septal efferents activate the same neurons. The CSD analysis results show that evoked field potential methods can be used effectively in chronically-prepared animals to examine synaptic responses in the converging inputs from the piriform cortex and medial septum to the entorhinal cortex. The short-term potentiation phenomena observed here suggest that low-frequency activity in these pathways during endogenous oscillatory states may enhance entorhinal cortex responsivity to olfactory inputs. 

Received 5 March 1997; accepted in final form 2 July 1997.
APS Manuscript Number J192-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 27 August 1997