Terminals of the major thalamic input to visual cortex are devoid of synapsin proteins.

Synapsins are nerve-terminal proteins that are linked to synaptic transmission and key factors in several forms of synaptic plasticity. While synapsins are generally assumed to be ubiquitous in synaptic terminals, whether they are excluded from certain types of terminals is of interest. In the visual pathway, synapsins are lacking in photoreceptor and bipolar cell terminals as well as in retinogeniculate synapses. These are the terminals of the first three feedforward synapses in the visual pathway, implying that lack of synapsins may be a common property of terminals that provide the primary driver activity onto their postsynaptic neurons. To further investigate this idea, we studied the fourth driver synapse, thalamocortical synapses in visual cortex, using glutamatergic terminal antibody markers anti-VGluT1 and VGluT2, anti-Synapsin I and II, and confocal microscopy to analyze co-localization of these proteins in terminals. We also used pre-embedding immunocytochemical labeling followed by electron microscopy to investigate morphological similarities or differences between terminals containing synapsins or VGluT2. In visual cortex, synapsin coincided extensively with non-TC-neuron marker, VGluT1, while thalamocortical terminal marker VGluT2 and synapsin overlap was sparse. Morphologically, synapsin-stained terminals were smaller than non-stained, while VGluT2-positive thalamocortical terminals constituted the largest terminals in cortex. The size discrepancy between synapsin- and VGluT2-positive terminals, together with the complementary staining patterns, indicates that thalamocortical synapses are devoid of synapsins, and support the hypothesis that afferent sensory information is consistently transmitted without the involvement of synapsins. Furthermore, VGluT2 and synapsins were colocalized in other brain structures, suggesting that lack of synapsins is not a property of VGluT2-containing terminals, but a property of primary driver terminals in the visual system.

A method of combining biocytin tract-tracing with avidin-biotin-peroxidase complex immunocytochemistry for pre-embedding electron microscopic labeling in neonatal tissue.

A new method that allows the combination of avidin-biotin-peroxidase visualization of antigens and silver-intensified gold labeling of biocytin, a rapid tract-tracer, is described. The method provides a practical tool for in vivo and in vitro studies of chemically specified afferent-target relationships and particularly in developing neural pathways where biocytin is invaluable as a rapidly transporting, sensitive tracer requiring little permeabilizing agents. Transported biocytin was first visualized with silver-intensified colloidal gold conjugated to anti-biotin IgG. This was followed by blocking of all unbound biotin groups of biocytin in the tissue with an Avidin-Biotin blocking kit. Finally, a second antigen, neuronal nitric oxide synthase NOS or GluR2/3 subunit of AMPA receptors, was visualized selectively with avidin-biotin-peroxidase/DAB. This protocol allowed visualization of two chromagens that could be distinguished by electron microscopy. The presence of biocytin was evident by silver particles, while accumulation of peroxidase reaction product marked only the antibody labeling: no cross-reaction between biocytin and the avidin-biotin-peroxidase was observed.