Quantitative analysis of ultrastructural changes in synapses of the rat hippocampal field CA3 in vitro in different functional states.

Transverse slices (250-350 microns) of the rat hippocampus were used for estimation of quantitative correlations between the ultrastructure and function of giant spinous synapses localized in stratum lucidum of the field CA3. Spontaneous and evoked spike discharges were used to determine the following five functional states of the neurons: "control"; "depletion" was achieved by long-term continuous stimulation (30-50 Hz for 1 h and longer); "recovery" when the slices rested after "depletion" till the evoked response was recovered; long-term potentiation I was achieved by short-term tetanic stimulation (5-15 s, 50-70 Hz); long-term potentiation II was achieved by a similar tetanic stimulation as for long-term potentiation I after the "recovery". For quantitative analysis of ultrastructural changes in the giant spinous synapses the following parameters were used: density of presynaptic vesicles determined as a ratio between the number of vesicles located within the giant bouton and the area of the latter (number of vesicles per 1 micron2); vesicle diameter distribution; area and length of the postsynaptic densities. A correlation of these parameters with the functional state of CA3 neurons was found. The area and length of postsynaptic densities are the most statistically significant parameters of the giant spinous synapses in different functional states. In contrast to other states, an increase in the length and the area of postsynaptic densities in long-term potentiation was found. A hypothesis on postsynaptic densities' role in long-term potentiation formation is suggested. The role of presynaptic and postsynaptic ultrastructural rearrangements is discussed as a possible mechanism determining the efficiency of synaptic transmission.