GABAergic synapses of the suprachiasmatic nucleus exhibit a diurnal rhythm of short‐term synaptic plasticity

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Abstract
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.