Pulvinar Projections to the Striatum and Amygdala in the Tree Shrew

Visually guided movement is possible in the absence of conscious visual perception, a phenomenon referred to as “blindsight.” Similarly, fearful images can elicit emotional responses in the absence of their conscious perception. Both capabilities are thought to be mediated by pathways from the retina through the superior colliculus (SC) and pulvinar nucleus. To define potential pathways that underlie behavioral responses to unperceived visual stimuli, we examined the projections from the pulvinar nucleus to the striatum and amygdala in the tree shrew (Tupaia belangeri), a species considered to be a prototypical primate. The tree shrew brain has a large pulvinar nucleus that contains two SC-recipient subdivisions; the dorsal (Pd) and central (Pc) pulvinar both receive topographic (“specific”) projections from SC, and Pd receives an additional non-topographic (“diffuse”) projection from SC (Chomsung et al., 2008). Anterograde and retrograde tract tracing revealed that both Pd and Pc project to the caudate and putamen, and Pd, but not Pc, additionally projects to the lateral amygdala. Using immunocytochemical staining for substance P (SP) and parvalbumin (PV) to reveal the patch/matrix organization of tree shrew striatum, we found that SP-rich/PV-poor patches interlock with a PV-rich/SP-poor matrix. Confocal microscopy revealed that tracer-labeled pulvino-striatal terminals preferentially innervate the matrix. Electron microscopy revealed that the postsynaptic targets of tracer-labeled pulvino-striatal and pulvino-amygdala terminals are spines, demonstrating that the pulvinar nucleus projects to the spiny output cells of the striatum matrix and the lateral amygdala, potentially relaying: (1) topographic visual information from SC to striatum to aid in guiding precise movements, and (2) non-topographic visual information from SC to the amygdala alerting the animal to potentially dangerous visual images

Diffuse and specific tectopulvinar terminals in the tree shrew: synapses, synapsins, and synaptic potentials.

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 20 Hz. 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 immunocytochemistry. 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

<i>In vitro</i> recording methods.

<p>A) A parasaggital section of the tree shrew brain stained for the type 2 vesicular glutamate transporter (vGLUT2) illustrates the location of the whole cell recordings in the caudal pulvinar nucleus and the location of the 8 electrode stimulus array in the stratum griseum superficial (SGS) and stratum opticum (SO) of the superior colliculus. B, C) High magnification views of the Pc (B) and Pd (C) illustrated in panel A. Immunohistochemical staining for vGLUT2 is a marker for tectopulvinar terminals <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0023781#pone.0023781-Chomsung2" target="_blank">[9]</a> and reveals the distinct arrangements of tectal terminals in the dorsal (Pd) central (Pc) pulvinar nucleus. The Pd contains dense clusters of tectal terminals and tubular clusters line long lengths of dendrites (C). In contrast, the Pc contains smaller more sparsely distributed clusters of tectal terminals (B). D) Voltage fluctuations recorded in response to the injection of depolarizing or hyperpolarizing current steps of varying size revealed that all recorded cells fired with both tonic action potentials, and low threshold calcium bursts. E) Drawing of a biocytin-labeled cell recorded in the juvenile pulvinar, F) Drawing of a biocytin-labeled cell recorded in the adult pulvinar. Scale in A = 1 mm. Scale in C = 30 µm and also applies to B. Scales in E and F = 10 µm. dLGN, dorsal lateral geniculate nucleus.</p

Two types of EPSPs.

<p>A) With increasing stimulation intensity, tecto-pulvinar EPSPs in the Pd show a graded increase in amplitude. B) Average first type EPSP amplitudes and latencies as a function of stimulation intensity (n = 17), graph show a graded increase in peak amplitude correlate to the increase in stimulation current but the latency of the EPSP is not relative to stimulation current. C) Average second type EPSP amplitudes and latencies as a function of stimulation intensity (n = 8), second type EPSP show a graded increase in peak amplitude and no change in latency with increasing stimulation intensity, but the threshold amplitude was smaller and latency was longer (p<0.05).</p

Distribution of synapsins and vGLUTs in the dLGN.

<p>Tree shrew dLGN tissue was labeled with antibodies against synapsin I or II (purple), and vGLUT1 or 2 (green). Profiles that are labeled with two antibodies appear white. Representative confocal images (single 0.2 µm scan with a 100x objective) are illustrated. Corticogeniculate terminals (RS profiles labeled with the vGLUT1 antibody) contain synapsin I (A), but not synapsin II (C). Retinogeniculate terminals (RL profiles labeled with the vGLUT2 antibody) do not contain either synapsin I (B) or synapsin II (D). Scale  =  10 µm.</p

Distribution of synapsins and vGLUTs in the pulvinar nucleus.

<p>Tree shrew pulvinar tissue was labeled with antibodies against synapsin I or II (purple), and vGLUT1 or 2 (green). Profiles that are labeled with two antibodies appear white. Representative confocal images of the Pd (single 0.2 µm scan with a 100x objective) are illustrated. Corticopulvinar terminals (RS profiles labeled with the vGLUT1 antibody) contain synapsin I (A), but not synapsin II (C). Tectopulvinar terminals (large clusters of RM profiles labeled with the vGLUT2 antibody) contain both synapsin I (B) and synapsin II (D). Scale  =  10 µm.</p