Tectopulvinar Synapses and Synaptic Potentials Figure 8. Two types of EPSPs display distinct short-term plasticity by paired-pulse stimulation. A) 1st and 2nd EPSP evoked by paired pulse stimulation of first type tecto-pulvinar fiber at 200 ms time interval, traces were recorded at resting membrane potential at 258 mV (upper panel) or 270 mV under voltage-clamp mode (lower panel), both traces showed paired-pulse depression. B) 1st and 2nd EPSP evoked by paired pulse stimulation of second type tecto-pulvinar fiber at 200ms time interval, traces were recorded at resting membrane potential at -59mV (upper panel) or 270 mV under voltage-clamp mode (lower panel), both traces showed stable EPSP amplitudes. C) Plots of paired-pulse ratio (EPSP2 amplitude/EPSP1 amplitude) as a function of inter-stimulus intervals (0.1 s, 0.2 s, 0.3 s, 0.4 s, 0.6 s, 0.8 s, 1 s, 2 s) by stimulation of first type tecto- pulvinar fiber, showed gradual increase of paired-pulse ratio. D) Plots of paired-pulse ratio (EPSP2 amplitude/EPSP1 amplitude) as a function of inter- stimulus intervals (0.1 s, 0.2 s, 0.3 s, 0.4 s, 0.6 s, 0.8 s, 1 s, 2 s) by stimulation of second type tecto-pulvinar fiber, showed stable paired-pulse ratio. doi:10.1371/journal.pone.0023781.g008 Synapsin distribution varies in glutamatergic terminals of the thalamus Synapsins have been associated with the tethering of synaptic vesicles to cytoskeletal elements [47,48,49,50,51,52,53,54] and are thought to regulate the equilibrium of a reserve pool of synaptic vesicles and a population of vesicles that are docked for ready release [55,56]. It has been hypothesized that synapsins are necessary to sustain the release of neurotransmitter at high rates of synaptic transmission [50,51,52]. We found that, within the visual thalamus, tectothalamic terminals are the only glutamatergic terminals that contain both synapsin I and synapsin II. Retinogeniculate terminals contained neither synapsin, and corticothalamic terminals con- tained synapsin I, but not synapsin II. These distributions provide further support for the division of thalamic glutamatergic terminals into 3 functional classes, and are presumably related to the distinct synaptic properties of these terminal types. Retinogeniculate EPSPs exhibit strong frequency-dependent depression [25,26,27], cortico- thalamic EPSPs exhibit strong frequency-dependent facilitation [25,29,30], while tectothalamic EPSP amplitudes remain relatively stable during high frequency stimulation [22]. Even the tectopulvi- nar EPSPs that showed a small decrease in amplitude with high stimulation frequencies (29.42% decrease at 10Hz) stimulation are more stable than retinogeniculate EPSPs which can depress by as much as 60% at 10Hz [25]. Recent studies have suggested that synapsin IIa is a key regulator of the reserve pool of synaptic vesicles. Gitler et al. [35] Figure 9. ‘‘Mixed’’ EPSP show characteristics of two types of EPSPs. A) ‘‘Mixed’’ EPSPs show small amplitude with small stimulation currents, followed by a large increase in amplitude and corresponding decrease in latency at larger stimulation currents. B) Plots of ‘‘mixed’’ EPSP amplitudes and latencies as a function of stimulation intensity, showed graded increase in amplitude with small stimulation currents followed by a large increase in amplitude and corresponding decrease in latency at larger stimulation currents. doi:10.1371/journal.pone.0023781.g009 PLoS ONE | www.plosone.org 11 August 2011 | Volume 6 | Issue 8 | e23781