LOW-FREQUENCY STIMULATION INDUCES HOMOSYNAPTIC DEPOTENTIATION BUT NOTLONG-TERM DEPRESSION OF SYNAPTIC TRANSMISSION IN THE ADULT ANESTHETIZED AND AWAKE RAT HIPPOCAMPUS IN-VIVO

The induction of homosynaptic long-term depression and depotentiation of previously established long-term potentiation was investigated in t he CA1 hippocampal region of anaesthetized and awake adult rats follow ing prolonged ipsilateral low-frequency stimulation of the Schaffer co llateral/commissural pathway. Prolonged low-frequency stimulation at 1 -10Hz failed to induce long-term depression of field excitatory postsy naptic potentials in the anaesthetized or awake adult rat. However, pr olonged low-frequency stimulation at 5 and 10 Hz, although not al 1 or 2 Hz, did induce depotentiation of previously established long-term p otentiation in anaesthetized animals. Thus, in the anaesthetized anima ls, 900 pulses at 10 Hz induced a depotentiation of 68%, 59% and 66% w hen given 10, 30 and 40 min following long-term potentiation induction . Depotentiation could also be induced al 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 foll owing long-term potentiation induction, and depotentiation measuring 6 0% was induced in two sets of experiments 24 h after long-term potenti ation induction in awake animals. The results of the present study sho w 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 hippoc ampus in vivo, both al short times and at prolonged times after the in duction of long-term potentiation. Copyright (C) 1997 IBRO.