Abstract
The present study evaluated associative and non-associative synaptic interactions between the medial (MPP) and lateral (LPP) perforant path projections to the ipsilateral dentate gyrus (DG) in barbiturate anaesthetized rats. Associative interactions were investigated using 5 Hz bursts of short 100 Hz trains delivered to one path, in association with single pulses administered to the other path, and spaced equidistantly between the bursts. Non-associative interactions were studied using the
conditioning trains or the single pulses alone. In some animals, paired-pulse facilitation was used to assess changes in presynaptic transmitter release that might accompany the induction of LTP or LTD in the medial and lateral perforant pathways. In these experiments, pairs of pulses were administered, using a 50 ms interpulse interval, prior to and following the induction of LTP and LTD.
The present results indicate that input specific, NMDA-dependent LTP may be exhibited in both lateral and medial path elicited field potentials. In addition, both forms of synaptic plasticity were shown not to inhibit the expression of further plasticity. LTP was found to be exhibited more robustly by the lateral pathway in naive pathways, and 5 Hz synaptic priming was found to selectively reduce the amount of LTP exhibited by the lateral path. The differential expression of LTP in the medial and lateral paths was suggested to involve the differential release of transmitters by the lateral and medial pathways. Evidence was also presented that presynaptic changes accompany LTP in the lateral pathway, as reductions in paired-pulse facilitation were recorded 30 minutes following the induction of LTP.
The lateral path exhibited an associative form of synaptic depression that differed from non-associative LTD in that it was: 1) only elicited by the associative protocol following 5 Hz priming; 2) NMDA-independent; and 3) accompanied by changes in paired-pulse facilitation. The 5 Hz priming
stimuli appear to have exerted their effect via an NMDA-dependent mechanism as associative LTD was not attained when NMDA-receptor antagonism preceded synaptic priming. The medial path did not readily exhibit either associative or non-associative LTD .