Our models of synaptic action are still very incomplete and will benefit from more empirical research. In this direction, Antonova et al (here) cultured relatively young ( 2 – 3 week old) hippocampal neurons from one day old rats. They placed the neurons in either glutamate in Mg2+-free bath solution or control solution. They then tagged synaptic puncta with fluorescent markers and visualized them with a laser confocal scanning system coupled to an inverted microscope. Synaptic puncta were indentified as between 0.5 and 5 micrometers in diameter.
As expected, applying the glutamate led to evoked EPSCs in the cultured neurons. There was a corresponding increase in the average quantity of synaptic puncta:
Note that inhibiting protein translation via anisomycin caused a downregulation in the quantity of synaptic puncta mainly at 60 and 180 mins, but also a slight decrease at 10 mins. Here is the model they use to explain these changes:
The rapid (< 10 min) outgrowth is probably due to the activation of dormant appositions, as it is actin polymerization-dependent but not protein translation-dependent. On the other hand the gradual (3 hr) increase might be due to an increase in actual appositions between pre and post synaptic neurons that is protein-translation dependent. In their conclusion they draw an analogy between this process and Hebbian learning on the individual puncta level:
In addition to the increase in sites, some existing presynaptic puncta and structures were stabilized and stopped disassembling and reassembling. These two processes are reminiscent of two stages of synaptogenesis during late stage development: exuberant growth of new synapses followed by activity-dependent stabilization of some and pruning of others. Stabilization of existing puncta and structures during potentiation suggests a Hebb-type activity-dependent learning rule, in which puncta and structures that are present during the induction of potentiation (and therefore might contribute to it) are made more permanent.
Antonova I, Lu F-M, Zablow L, Udo H, Hawkins RD (2009) Rapid and Long-Lasting Increase in Sites for Synapse Assembly during Late-Phase Potentiation in Rat Hippocampal Neurons. PLoS ONE 4(11): e7690. doi:10.1371/journal.pone.0007690