The master circadian pacemaker located in the suprachiasmatic nucleus (SCN) is entrained by light intensity-dependent signals transmitted via the retinohypothalamic tract (RHT). Short-term plasticity at glutamatergic RHT-SCN synapses was studied using stimulus frequencies that simulated the firing of light sensitive retinal ganglion cells. The evoked excitatory postsynaptic current (eEPSC) was recorded from SCN neurons located in hypothalamic brain slices. The eEPSC amplitude was stable during 0.08 Hz stimulation and exhibited frequency-dependent short-term synaptic depression (SD) during 0.5 to 100 Hz stimulus trains in 95 of 99 (96%) recorded neurons. During SD the steady-state eEPSC amplitude decreased, whereas the cumulative charge transfer increased in a frequency-dependent manner and saturated at 20 Hz. SD was similar during subjective day and night and decreased with increasing temperature. Paired-pulse stimulation (PPS) and voltage-dependent Ca2+ channel (VDCC) blockers were used to characterize a presynaptic release mechanism. Facilitation was present in 30% and depression in 70% of studied neurons during PPS. Synaptic transmission was reduced by blocking both N- and P/Q-type presynaptic VDCCs, but only the N-type channel blocker significantly relieved SD. Aniracetam inhibited AMPA receptor desensitization but did not alter SD. Thus we concluded that SD is the principal form of short-term plasticity at RHT synapses, which presynaptically and frequency-dependently attenuates light-induced glutamatergic RHT synaptic transmission protecting SCN neurons against excessive excitation.
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