Low-frequency stimulation induces homosynaptic depotentiation but not long-term depression of synaptic transmission in the adult anaesthetized and awake rat hippocampus in vivo.

The induction of homosynaptic long-term depression and depotentiation of previously established long-term potentiation was investigated in the CA1 hippocampal region of anaesthetized and awake adult rats following prolonged ipsilateral low-frequency stimulation of the Schaffer collateral/ commissural pathway. Prolonged low-frequency stimulation at 1-10 Hz failed to induce long-term depression of field excitatory postsynaptic potentials in the anaesthetized or awake adult rat. However, prolonged low-frequency stimulation at 5 and 10 Hz, although not at 1 or 2 Hz, did induce depotentiation of previously established long-term potentiation in anaesthetized animals. Thus, in the anaesthetized animals, 900 pulses at 10 Hz induced a depotentiation of 68%, 59% and 66% when given 10, 30 and 40 min following long-term potentiation induction. Depotentiation could also be induced at much longer times following the induction of long-term potentiation. Thus, in anaesthetized rats, depotentiation measuring 34% was induced by 10-Hz stimulation 4 h following long-term potentiation induction, and depotentiation measuring 60% was induced in two sets of experiments 24 h after long-term potentiation induction in awake animals. The results of the present study show that homosynaptic long-term depression was not induced in the adult hippocampus in vivo using stimulation protocols which are effective in hippocampal slices. However, erasure of long-term potentiation by the process of depotentiation has been shown to occur in the adult hippocampus in vivo, both at short times and at prolonged times after the induction of long-term potentiation.