The Postnatal Development of Post-Activation Potentiation in the Neocortex and Dentate Gyrus of the Rat

Abstract

<p>The postnatal development of short-term potentiation (STP) and long-term potentiation (LTP) were examined in the neocortex and dentate gyrus of the rat. It was found that STP and LTP develop during narrowly defined time periods in both systems. This functional synaptic development did not appear to correspond well with known structural development in either system (e.g. synaptogenesis or dendritic spine formation).</p> <p>Stimulation of callosal fibers produced a biphasic, positive-negative, transcallosal response (TCR) recorded near the surface of the anterior neocortex in all ages tested. The TCR showed a decrease in threshold, latency and halfwidth, and an increase in peak amplitude with age. STP and LTP of the TCR could not be reliably detected until after PN16 and PN18, respectively. The magnitude of STP and LTP was initially small out approached adult levels rapidly after their initial appearance.</p> <p>Stimulation of perforant path fibers produced a positive excitatory post-synaptic potential (EPSP) with a super-imposed negative population spike recorded in the dentate hilus. The EPSP showed a decrease in threshold, latency and halfwidth, and an increase in peak amplitude with age. STP and LTP (of the EPSP and/or population spike) could not be reliably detected until the second postnatal week, with STP appearing prior to LTP. Again, STP and LTP approached adult levels rapidly after their initial appearance. These results could not be explained by differential effects of anesthesia on immature animals, nor by higher STP/LTP thresholds in immature animals.</p> <p>The maturation of evoked response morphology (e.g. threshold and latency) did not correspond closely with STP/LTP development in either the hippocampal or neocortical system. Also, the correspondence between STP/LTP development and structural developments such as synaptogenesis, spine formation, or myelinogenesis (as reported in the literature) was not particularly strong in either system.</p> <p>These results suggest that the postnatal development of STP and LTP, and thus the mechanism of potentiation effects in mature animals, may not depend so much on the maturation of specific structures (e.g. dendritic spines as on the maturation of neurochemical processes (e.g. receptor or protease development). Possible PAP-mechanisms were discussed.</p>