Proceedings of The Physiological Society

Cardiff University (2009) Proc Physiol Soc 17, PC30

Poster Communications

Differential effects of low frequency repetitive stimulation of the major afferent pathways of the rat hippocampal CA3 region in vivo

N. A. Vorobyov1, G. L. Collingridge1, Z. A. Bortolotto1

1. Department of Anatomy, University of Bristol, Bristol, United Kingdom.

The hippocampal CA3 region receives input directly from the entorhinal cortex via entorhinal-CA3 projections and indirectly through mossy fibres from granule cells of the dentate gyrus (DG). Additionally there are recurrent collaterals projecting ipsi- and contra laterally from CA3 neurons. Here we studied CA3 responses to low frequency (1 - 10 Hz) stimulation of these three inputs, in Wistar rats maintained under Nembutal anaesthesia (50 mg/kg, i.p.). We found the following. 1) Low frequency stimulation of CA3 recurrent collaterals was accompanied by short-term potentiation of CA3 responses followed by their quick return to initial values evoked by basal single stimulation (1/30 sec). This effect was observed following the stimulation of contralateral CA3 or ipsilateral CA1 or inpsilateral hilus fibers. 2) Low frequency stimulation of direct entorhinal inputs to CA3 was accompanied by short-term depression of CA3 responses followed by their quick return to initial values. This was observed during stimulation of the subiculum or molecular layer of the DG. 3) Low frequency stimulation of mossy fibres, by stimulation within the granular layer of the DG, was accompanied by short-term depression followed by long-term potentiation (LTP) of CA3 responses. This effect developed as a rebound: suppressed responses during low frequency stimulation were replaced by enhanced responses during basal stimulation. LTP was observed immediately after the low frequency stimulation protocol. The effects described above were clearly dependent on stimulation frequency (the higher frequency the greater the effect) and were NMDA receptor independent, since all of the experiments were done in the presence of ketamine (10 mg/kg). Thus these results suggest functional differences between the three different CA3 inputs, and identifies a form of NMDA receptor independent LTP, which is induced by low frequency stimulation.

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