Long-Term Potentiation at the Apical and Basal Dendritic Synapses of CA1 After Local Stimulation in Behaving Rats. Leung, L. Stan, and B. Shen. Departments of Physiology and Clinical Neurological Sciences, University of Western Ontario, London N6A 5A5, Canada.
APStracts 2:0058N, 1995.
SUMMARY AND CONCLUSIONS
1 . We recorded the extracellular excitatory postsynaptic potentials (EPSPs) in CA1 in the freely behaving rat after stimulation of the apical dendritic and basal dendritic afferents. The apical dendritic population EPSP was negative at the apical dendritic layers and positive at stratum oriens and alveus. The basal dendritic population EPSP was negative at basal dendritic layer (stratum oriens) and positive at the cell body and apical dendritic layers. 2 . We delivered various tetanic stimulus patterns in an attempt to elicit long-term potentiation (LTP) at the apical dendritic synapse of CA1 pyramidal cells. A 200-Hz high-frequency train (HF) 1 s in duration was more successful than theta-frequency primed bursts (PBs; 1 or 4 priming pulses followed by 10 or 4 pulses at 100 Hz) in eliciting LTP of the apical dendritic population EPSP, recorded either ipsilaterally or contralaterally. 3 . Tetanus applied to the apical dendritic afferents occasionally elicited long- term depression (LTD) at an intensity lower than the LTP threshold. LTD persisted to >/=1 day after a tetanus. 4 . High-intensity tetanic stimulations often elicited hippocampal afterdischarges (ADs). If the AD duration was brief (<15 s), apical dendritic LTP could follow the AD. However, if the AD duration was long, no apical dendritic LTP was observed. Tetanus that evoked an apical dendritic LTP typically evoked a short hippocampal AD <15 s in duration. 5 . LTP of the basal dendritic population EPSP was readily elicited either after an HF or a PB tetanus of low (1.5_2 times threshold) stimulus intensity. For a PB tetanus at the same intensity, LTP at the basal dendrites was significantly easier to elicit than that at the apical dendrites. 6 . The LTP at the apical or basal dendritic synapse was generally input specific. However, heterosynaptic potentiation of the basal dendritic synapse after tetanization of the apical dendritic afferents may occur with or without apical dendritic LTP. 7 . The apical dendritic synaptic enhancement after a single HF train was _30%, which remained stable for the first 3 h. The enhancement declined slightly on day 1 but was still significant 9_12 days after the tetanus. Basal dendritic synaptic enhancement peaked at >80% immediately posttetanus and decayed rapidly in the 1st hour but persisted for >/=7 days. 8 . A slow sustained potential, typically of the same polarity as the respective basal or apical dendritic single-pulse response, developed during the burst period of a theta-frequency PB. The slow potential likely reflected sustained depolarization of the CA1 neurons, and its magnitude correlated significantly with the degree of LTP. 9 . We speculate that apical and basal dendritic LTP in CA1 may serve different functions in the behaving rat. The basal dendritic LTP may be activated by the theta-frequency inputs during spatial behaviors, whereas apical dendritic LTP may be activated during the posttrial periods, when the rat was immobile or in slow-wave sleep, and when CA3 cells fired in high-frequency bursts. 

Received 3 June 1994; accepted in final form 1 February 1995.
APS Manuscript Number J317-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  3 April 1995.