The professional circadian pacemaker located in the suprachiasmatic nucleus (SCN) is entrained by light intensityCdependent signals transmitted via the retinohypothalamic tract (RHT). related during subjective day and night and decreased with increasing temp. Paired-pulse activation (PPS) and voltage-dependent Ca2+ channel (VDCC) blockers were used to characterize a presynaptic SMN launch mechanism. Facilitation was present in 30% and major depression in 70% of analyzed neurons during PPS. Synaptic transmission was reduced by obstructing both N- and P/Q-type presynaptic VDCCs, but only the N-type CC-5013 irreversible inhibition channel blocker significantly relieved SD. Aniracetam inhibited AMPA receptor desensitization but did not alter SD. Therefore 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. Intro The expert circadian oscillator located in the suprachiasmatic nucleus (SCN) is definitely entrained by light. Intrinsically photosensitive retinal ganglion cells (ipRGCs) project axons to the SCN comprising the retinohypothalamic tract (RHT) (Berson et al. 2002; Warren et al. 2003). Depolarization of ipRGCs by light induces glutamate launch from RHT axon terminals. The glutamate binds to = 8]. To compare synaptic major depression under different conditions and between different neurons the amplitude of each subsequent eEPSC (eEPSCexp[?(? is definitely a constant, is the given time, = 0), and (tau) is the time constant. The extra sum of squares = 7, Fig. 1= 7) but did not follow 200 Hz stimulation. The time required for the eEPSC CC-5013 irreversible inhibition amplitude to reach steady state was shorter at higher stimulation frequencies and was characterized by a specific time constant () (see in methods). For example, the was 329 53 ms at 2 Hz (= 30), 220 24 ms at 5 Hz (= 31), 83 6 ms at 25 Hz (= 31), 49 5 ms at 50 Hz (= 7), and CC-5013 irreversible inhibition 28 3 ms at 100 Hz (= 7). Although the plateau was reached faster at higher stimulus frequencies, more stimulus pulses were required to reach the steady state: 3.7 0.6 stimuli at 2 Hz (steady state: 55.3 3.0% of control), 5.5 0.5 stimuli at 5 Hz (steady state: 37.3 3.3% of control), and 7.6 0.8 stimuli at 25 Hz (steady state: 22.3 2.8% of control, = 24). The eEPSC amplitude recovered to control values during about 40 s after completion of the stimulus train. Open in a separate window Fig. 1. Frequency dependence of synaptic depression during repetitive stimulation of the optic chiasm. = CC-5013 irreversible inhibition 3), 10 Hz (= 7), 50 Hz (= 10). Note: these records are not shown on a timescale (the dots show the stimuli number). Dashed line is the steady-state eEPSC amplitude (mean of last 10 eEPSCs in the train). = 10) and night (ZT: 13.5C17.0; = 7). = 5). 0.001, = 4 (paired and = 10) and night (ZT: 13.5C17.0, = 7) was compared. The frequency dependence of steady-state eEPSC amplitude was similar in both conditions [= 0.78, Fig. 1 0.31 (unpaired = 4) required for the recording chamber temperature to stabilize. Increasing the temperature from 28 to 36C increased the mean steady-state eEPSC amplitude at 0.08 Hz from 216.3 14.5 to 252.0 20 pA (ratio 1.16), at 5 Hz from 105.0 7.8 to 193.3 14.0 pA (ratio 1.84), and at 25 Hz from 66.9 4.0 to 142.2 9.6 pA (ratio 2.12; = 4). The amplitude of each eEPSC was normalized to the first eEPSC in the train and the estimated steady-state amplitude at each temperature was compared (Fig. 1 0.00017]. Synaptic depression was observed in 95 of 99 neurons (96%) studied during 0.5C100 Hz repetitive stimulation of the optic chiasm. However, in 4 neurons (4%) synaptic depression was observed only during 0.5C5 Hz stimulation and a progressive increase of the steady-state eEPSC amplitude was revealed during 10C25 Hz (160% at 25 Hz). CC-5013 irreversible inhibition The increase of steady-state eEPSC amplitude did not result from an increase in the series resistance. The SE of the series resistance for recorded neurons was in the range 0.9C10.7% (mean 4.5%, = 4). In neurons that demonstrated synaptic depression during 0.5C100 Hz stimulus trains the ratio of the amplitude of the second eEPSC to the first one (eEPSC2/eEPSC1) was used to estimate the initial release probability. Initial facilitation (ratio 1) appeared in 5% (2 of 40 neurons) and in 14% (3 of 21 neurons) during 2 or.