Diffuse and Specific Tectopulvinar Terminals in the Tree Shrew: Synapses, Synapsins, and Synaptic Potentials Haiyang Wei1, Sean P. Masterson 1 , Heywood M. Petry2, Martha E. Bickford1* 1 Department of Anatomical Sciences and Neurobiology, University of Louisville, Louisville, Kentucky, United States of America, 2 Department of Psychological and Brain Sciences, University of Louisville, Louisville, Kentucky, United States of America Abstract The pulvinar nucleus of the tree shrew receives both topographic (specific) and nontopographic (diffuse) projections from superior colliculus (SC), which form distinct synaptic arrangements. We characterized the physiological properties of these synapses and describe two distinct types of excitatory postsynaptic potentials (EPSPs) that correlate with structural properties of the specific and diffuse terminals. Synapses formed by specific terminals were found to be significantly longer than those formed by diffuse terminals. Stimulation of these two terminal types elicited two types of EPSPs that differed in their latency and threshold amplitudes. In addition, in response to repetitive stimulation (0.5–20 Hz) one type of EPSP displayed frequency-dependent depression whereas the amplitudes of the second type of EPSP were not changed by repetitive stimulation of up to 20Hz. To relate these features to vesicle release, we compared the synapsin content of terminals in the pulvinar nucleus and the dorsal lateral geniculate (dLGN) by combining immunohistochemical staining for synapsin I or II with staining for the type 1 or type 2 vesicular glutamate transporters (markers for corticothalamic and tectothalamic/retinogeniculate terminals, respectively). We found that retinogeniculate terminals do not contain either synapsin I or synapsin II, corticothalamic terminals in the dLGN and pulvinar contain synapsin I, but not synapsin II, whereas tectopulvinar terminals contain both synapsin I and synapsin II. Finally, both types of EPSPs showed a graded increase in amplitude with increasing stimulation intensity, suggesting convergence; this was confirmed using a combination of anterograde tract tracing and immunocytochemisty. We suggest that the convergent synaptic arrangements, as well as the unique synapsin content of tectopulvinar terminals, allow them to relay a dynamic range of visual signals from the SC. Citation: Wei H, Masterson SP, Petry HM, Bickford ME (2011) Diffuse and Specific Tectopulvinar Terminals in the Tree Shrew: Synapses, Synapsins, and Synaptic Potentials. PLoS ONE 6(8): e23781. doi:10.1371/journal.pone.0023781 Editor: Izumi Sugihara, Tokyo Medical and Dental University, Japan Received February 18, 2011; Accepted July 25, 2011; Published August 15, 2011 Copyright: 2011 Wei et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: Grant sponsor: National Institutes of Health, Grant Number EY01655 and Grant Number NS35377. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: martha.bickford@louisville.edu Introduction Three main types of glutamatergic terminals have been identified in the visual dorsal thalamus of rodents, carnivores, primates, and tree shrews. All contain round synaptic vesicles, but can be distinguished based on their size as small (RS), medium (RM) and large (RL) profiles. RS profiles originate from cortical layer VI [1,2,3,4,5,6], RM profiles originate from the superior colliculus [7,8,9,10] and RL profiles are contributed by primary sensory inputs, cortical layer V, or thalamocortical axon collaterals [1,2,7,11,12,13,14,15,16,17,18,19]. RL profiles, such as those originating from the retina, have been described as ‘‘drivers’’ because the receptive field properties of postsynaptic neurons are dependent on activation of these inputs. In contrast, corticothalamic RS profiles have been described as ‘‘modulators’’ because their activation changes the overall responsiveness of postsynaptic neurons, but does not dramatically change receptive field properties [20]. The role of tectothalamic RM profiles is less clear, but they appear to form a third functional class of terminals [21,22]; it has been suggested that the collective activities of multiple convergent RM inputs are critical for the formation of the receptive field properties of postsynaptic neurons. Comparisons of the excitatory postsynaptic potentials (EPSPs) elicited by stimulation of RS or RL profiles have revealed distinct forms of short-term synaptic plasticity, which are thought to reflect differences in the probability of glutamate release from each terminal type [23]. Each RL profile makes multiple synaptic contacts [7,14,16,17,24], and their stimulation elicits large amplitude EPSPs that depress when the stimulation is repeated at high frequency [25,26,27,28]. In contrast, RS profiles make single synaptic contacts [16,17,24] and their stimulation elicits smaller amplitude EPSPs that facilitate with high frequency stimulation [23,25,28,29,30]. Recently, these features have been linked to the distribution of synapsin I and synapsin II, proteins that tether a reserve pool of synaptic vesicles [31,32,33,34,35]. These proteins are found in corticogeniculate (RS) terminals but not retinogeniculate (RL) terminals, and in synapsin I/II knockout mice the short-term plasticity of corticogeniculate EPSPs is altered, while retinogeniculate EPSPs are unchanged [36]. Unlike RS and RL profiles, the amplitudes of EPSPs elicited by stimulation of tectothalamic terminals (RM profiles) remain relatively stable at stimulation frequencies of 1–10 Hz [21,22]. To investigate potential mechanisms responsible for the stability of RM EPSPs, we correlated morphological features of tectopulvinar terminals with the postsynaptic responses elicited by their stimulation in tissue obtained from tree shrews (Tupaia belangeri). In this species, tectopulvinar terminals are classified as RM profiles, but they exhibit two different types of synaptic PLoS ONE | www.plosone.org 1 August 2011 | Volume 6 | Issue 8 | e23781