Long-term synaptic changes induced by intracellular tetanization of CA3 pyramidal neurons in hippocampal slices from juvenile rats

Minimal excitatory postsynaptic potentials were evoked in CA3 pyramidal neu rons by activation of the mossy fibres in hippocampal slices from seven- to 16-day-old rats. Conditioning intracellular depolarizing pulses were deliv ered as 50- or 100-Hz bursts. A statistically significant depression and po tentiation was induced in four and five of 13 cases, respectively. The init ial state of the synapses influenced the effect: the amplitude changes corr elated with the pretetanic paired-pulse facilitation ratio. Afferent (mossy fibre) tetanization produced a significant depression in four of six input s, and no significant changes in two inputs. Quantal content decreased or i ncreased following induction of the depression or potentiation, respectivel y, whereas no significant changes in quantal size were observed. Compatible with presynaptic maintenance mechanisms of both depression and potentiatio n, changes in the mean quantal content were associated with modifications i n the paired-pulse facilitation ratios, coefficient of variation of respons e amplitudes and number of response failures. Cases were encountered when a pparently "presynaptically silent" synapses were converted into functional synapses during potentiation or when effective synapses became "presynaptic ally silent" when depression was induced, suggesting respective changes in the probability of transmitter release. It is concluded that, in juvenile rats, it is possible to induce lasting po tentiation at the mossy fibre-CAS synapses by purely postsynaptic stimulati on, while afferent tetanization is accompanied by long-lasting depression. The data support the existence not only of a presynaptically induced, but a lso a postsynaptically induced form of long-term potentiation in the mossy fibre-CA3 synapse. Despite a postsynaptic induction mechanism, maintenance of both potentiation and depression is likely to occur presynaptically. (C) 1999 IBRO. Published by Elsevier Science Ltd.