Tectopulvinar Synapses and Synaptic Potentials Figure 7. Two types of EPSPs display distinct frequency-dependent short-term plasticity. A) Train stimulation consisted of 8 pulses at 1 Hz, 1.25 Hz, 2.5 Hz, 5 Hz and 10 Hz, 1st, 3rd and 5th EPSPs from the train stimulation of first type tecto-pulvinar fiber showed frequency-dependent depression, recorded at resting membrane potential of -58mV. C) 1st, 3rd and 5th EPSPs from the train stimulation of second type tecto-pulvinar fiber produced EPSPs with stable amplitudes, recorded at resting membrane potential of 259 mV. B and D) Normalized average EPSPs evoked in 14 cells by 8 pulses at 1 Hz, 1.25 Hz, 2.5 Hz, 5 Hz and 10 Hz, each point was normalized by dividing the EPSP amplitude of that pulse in the train (EPSPn) to the amplitude of the first EPSP of the train (EPSP1), B) stimulation of the first type tecto-pulvinar fiber showed change in EPSP peak amplitudes of the 8 EPSPs evoked by the stimulus train at various frequencies. D) stimulation of the second type tecto-pulvinar fiber showed stable EPSP peak amplitudes of the 8 EPSPs evoked by the stimulus train at various frequencies. doi:10.1371/journal.pone.0023781.g007 for ‘‘mixed’’ EPSPs the amplitudes were unaffected by stimulus frequency when stimuli were delivered with threshold current levels, but frequency-dependent depression was observed when the stimuli were delivered with current levels that elicited the larger amplitude EPSPs. Discussion Diffuse and specific EPSPs Based on differences in their convergence patterns, we suggest that the slower latency, smaller amplitude EPSPs represent activation of diffuse tectopulvinar projections, while the faster, larger amplitude EPSPs represent activation of specific tectopulvi- nar projections. This suggestion is also based on comparisons to our previous study of EPSPs elicited by stimulation of tectotha- lamic axons in the rat. Tectothalamic projections in the rodents are nontopographic [46] and EPSP amplitudes remain stable at stimulation frequencies of up to 20 Hz [22]. The lack of short- term frequency-dependent plasticity in the rat tectothalamic pathway compares to the slower latency, smaller amplitude tectopulvinar EPSPs of the tree shrew. In contrast, the faster, larger amplitude tectopulvinar EPSPs of the tree shrew exhibited a small frequency-dependent depression (29.42% at 10Hz), a feature which was not observed in our studies of the rat tectothalamic EPSPs. This difference does not appear to be a function of age because we detected these two types of tetctopulvinar EPSPs in both juvenile and adult tree shrew tissue. At threshold, specific EPSPs may be larger than diffuse EPSPs because stimulation of a single specific axon activates clusters of terminals that innervate the same dendrite [9], while stimulation of a single diffuse axon activates unitary inputs. In addition, relative to diffuse terminals, the glutamate release is likely to be higher for specific terminals because their synaptic zones are significantly larger. This conclusion is tentative since we measured the length of specific synapses in the Pc, while we recorded EPSPs in the Pd. However, assuming specific synapses are similar in the Pc and Pd, their longer length relative to diffuse synapses would help to explain why we observed frequency- dependent depression in one group of EPSPs; the pool of synaptic vesicles ready for release in specific tectopulvinar terminals may not be fully replenished during short interstimulus intervals. Diffuse tectopulvinar EPSPs may exhibit relative stability due to a lower glutamate release probability. Since we detected no difference in the overall size of specific and diffuse terminals, a lower probability of neurotransmitter release would imply less vesicle depletion. PLoS ONE | www.plosone.org 9 August 2011 | Volume 6 | Issue 8 | e23781