The earliest evidence for a presynaptic mechanism of expression in LTP, was seen in experim ents in w hich neurotransm itter release was m easured follow ing LTP induction. Pre-accumulated [^H]-D-aspartate release from Schaffer collateral fibres was significantly increased in response to bursts of electrical stimulation (Skrede and
M althe-Sorenssen, 1981). A dditionally, D olphin et al. (1 9 8 2 ) used a push-pull
increase and remain elevated for at least 1 hour. However, others have seen that LTP in the C A l or dentate gyrus regions of the anaesthetised rat is not accompanied by a significant, long-lasting increase in the release of either glutam ate or aspartate
(Aniksztejn et ah, 1989). Therefore, the involvement of increased transm itter release
in LTP is still open to debate.
The m ajority of evidence implicating a presynaptic locus for LTP expression has come from quantal analysis studies. In such experim ents the am plitude of either evoked EPSCs or spontaneous miniature EPSCs is plotted against their frequency of occurrence. These plots reveal peaks thought to correspond to release o f single or multiple quanta. Assuming that the quantal size (the amount of neurotransmitter per vesicle) is consistent, the distance between the peaks corresponds to the quantal amplitude (i.e. the postsynaptic response to a single quantum of neurotransmitter). Changes in the amplitude, frequency or shape of these peaks can provide insights into changes occurring during LTP. A number of studies have attem pted to determine whether quantal size, the number of quanta released or the probability of quantal release, changes after LTP. The majority of papers suggest som e presynaptic component is involved in the expression of LTP, but they report different levels of involvement. Bekkers and Stevens (1990) suggest that LTP is a purely presynaptic mechanism, depending on the probability of transmitter release (PJ. Others report a decrease in the number of failures following LTP, suggesting either an increase in the number of vesicles available for release and/or increases in the probability of release (Malinow and Tsien, 1990), however, postsynaptic modifications are not ruled out. Subsequent studies point to both pre and postsynaptic expression m echanism s (Kullmann and Nicoll, 1992), the relative importance of each depending on the initial
setting of the presynaptic release mechanism (Larkman et al., 1992).
Several studies have used quantal analysis to look at the postsynaptic responses produced by ‘minimal stimulation’, in which only one or a few, presynaptic fibres are stimulated. These methods attempt to look at changes in individual quanta following LTP induction. This approach has shown that LTP induction is associated with a decrease in the number of failures (Malinow, 1991; Stevens and Wang, 1994; Strieker
and Sieglebaum (1995) recording from pairs of pyramidal neurons in hippocampal slices, showed that only a single quantum of transm itter was released from a CA3 presynaptic bouton in response to an action potential. However, the probability of release of the quantum was unique for a particular cell pair. In response to PPF, 2-3 week old rats with low initial P^ synapses (P^ <0.5) showed a decrease in the number of failures to the second pulse, however this decrease did not occur in young animals (4-8 days) that contained high P^ synapses (P^= 0.9). Furthermore, LTP was correlated with an increase in P, in older animals whereas LTP was occluded in young animals, perhaps due to the initial high P^ of the synapses.
Recently, Ca^^ imaging studies have been employed to address the same question.
This approach offers the advantage that only single synapses are studied. Emptage et
al. (2 0 0 0 ) im aged calcium transients in single dendritic spines of hippocam pal
pyram idal cells before and after LTP induction. The probability o f evoking a postsynaptic Ca^^ transient (EPSCaTs) in response to a single synaptic stimulus increased following LTP. Assuming that EPSCaTs reflect the release of transmitter at a synapse, these results show a change in P, following LTP induction. Increases in EPSCaT amplitude were also seen following LTP, perhaps suggesting that LTP is expressed at individual synapses via both pre and postsynaptic mechanisms.
Although the majority of studies in this area support the view that an increase in the probability of neurotransm itter release is a likely presynaptic m echanism in LTP expression, there are other possibilities. In theory, transmitter release could also be influenced by the quantal size or the number o f quanta released. However, for either of these mechanisms to be effective it is essential that the postsynaptic receptors are not saturated by neurotransm itter under conditions of basal transm ission. Early evidence from quantal analysis studies revealed only modest variation in quantal
amplitude which suggested postsynaptic receptor saturation (Larkman et al., 1991;
Liao et al., 1992). Additionally, studies looking at mEPSCs in cultured hippocampal
neurons found that the peak amplitudes of closely timed pairs of mEPSCs (thought to originate from a common release site) were extrem ely sim ilar, also suggesting
receptor saturation (Tang et al., 1994). However, there are results that contradict these
presynaptic bouton inside a loose patch pipette and recording from a lose seal on the associated postsynaptic membrane. They found that the variability of the conductance of any single synapse was larger than could be accounted for by the random opening of receptor channels, suggesting that the postsynaptic receptors were not saturated in response to a quantum of neurotransmitter. In line with these findings, NMDAR- m ediated synaptic calcium transients in single dendritic spines o f C A l pyramidal cells, have been shown to summate in response to two closely paired shocks (occurring <10ms apart). This suggests that a single release event does not saturate
spine NMDARs. (Mainen et al., 1999). Therefore, one cannot rule out the possibility
that a synapse could increase its efficacy by increasing the neurotransmitter content of its vesicles or the number of vesicles released.