Mechanisms involved in hippocampal LTP: implications for retrograde messengers

Afferent high strength tetanizations favour potentiation of the NMDA vs. AMPA receptor-mediated component of field EPSP in CA1 hippocampal slices of rats

Long-term potentiation (LTP) of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated components of 'dual-component' field excitatory postsynaptic potentials (fEPSP-A and fEPSP-N) was studied in the CA1 stratum radiatum in hippocampal slices of rats. Relative degrees of LTP of these fEPSP components were compared for tetanizations with low and high strengths. Magnitudes of fEPSP-A and fEPSP-N were estimated in parallel with a least-square fitting of a short-latent (0.1-8.8 ms) fragment of evoked responses by a weighted sum of 'basic' fEPSP-A and fEPSP-N, obtained during a short preliminary application of d-2-amino-5-phosphonovalerate (APV). We found that low-strength tetanizations selectively potentiated fEPSP-A, while high strength tetanizations potentiated both fEPSP components. These results demonstrate in the experiments with parallel measurements of fEPSP-A and fEPSP-N that LTP of these components differ depending on the strength of afferent tetanization. Unequal potentiation of the commissural-collateral and excitatory local-circuit synapses, which presumably contain different amounts of the AMPA and NMDA receptors, is discussed as the most probable explanation for these results.

Neurophysiological analysis of long-term potentiation in mammalian brain

Long-term potentiation (LTP) is a persistent increase in postsynaptic response following a high-frequency presynaptic activation. Characteristic LTP features, including input specificity and associativity, make it a popular model to study memory mechanisms. Mechanisms of LTP induction and maintenance are briefly reviewed. Increased intracellular Ca2+ concentration is shown to be critical for LTP induction. This increase is believed to be based on Ca2+ influx secondary to activation of N-methyl-D-aspartate (NMDA) subtype of glutamate receptors. Existence of other sources of Ca2+ increase and other critical factors is now becoming evident. They include voltage-dependent Ca2+ channels, Ca2+ intracellular stores, metabotropic glutamate receptors, 'modulatory' transmitters. An example of an involvement of voltage-dependent Ca2+ channels is potentiation induced by intracellular depolarizing pulses. LTP can be divided into decremental earlier (E-LTP) and non-decremental late (L-LTP) phases which explains some inconsistencies in studies of LTP mechanisms. E-LTP is suggested to be based on a transient increase in presynaptic release probabilities. A hypothesis is considered which explains L-LTP by suggesting that Ca2+ activates structural changes leading to an increase in the synaptic gap resistance. This enhances positive synaptic electrical feedback and augments release probability. The hypothesis predicts specific morphological changes, synchronous transmitter release of two or several quanta in some central synapses and the amplification of such synchronization following LTP induction. Data are discussed which maintain these predictions.