We examined synaptic plasticity at intrasuprachiasmatic nucleus (SCN) γ-aminobutyric acid (GABA)ergic synapses by measuring the paired-pulse ratio between pairs of evoked inhibitory postsynaptic currents (IPSCs). Interstimulus intervals were chosen to represent the range of spontaneous action potential firing frequencies found in SCN neurons. A majority of synapses studied during the day exhibited paired-pulse depression (PPD), whereas a majority of synapses studied during the night showed no PPD. Two types of PPD were found. Type 1 PPD expresses the greatest inhibition at shorter interstimulus intervals, is predominant in the early morning and is likely to be a result of vesicle depletion. Type 2 showed the greatest inhibition at interstimulus intervals between 175 and 225 ms, is found throughout the day yet rarely at night and is likely to be a result of a Ca2+-dependent mechanism that is independent of pertussis toxin-sensitive G-proteins. Thus, multiple mechanisms of synaptic plasticity modulate intra-SCN communication throughout the diurnal cycle.
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