Ultrastructural and optogenetic dissection of V1 corticotectal terminal synaptic properties

Abstract

The superior colliculus (SC) is a major site of sensorimotor integration in which sensory inputs are processed to initiate appropriate motor responses. Projections from the primary visual cortex (V1) to the SC have been shown to exert a substantial influence on visually induced behavior, including “freezing.” However, it is unclear how V1 corticotectal terminals affect SC circuits to mediate these effects. To investigate this, we used anatomical and optogenetic techniques to examine the synaptic properties of V1 corticotectal terminals. Electron microscopy revealed that V1 corticotectal terminals labeled by anterograde transport primarily synapse (93%) on dendrites that do not contain gamma aminobutyric acid (GABA). This preference was confirmed using optogenetic techniques to photoactivate V1 corticotectal terminals in slices of the SC maintained in vitro. In a mouse line in which GABAergic SC interneurons express green fluorescent protein (GFP), few GFP‐labeled cells (11%) responded to activation of corticotectal terminals. In contrast, 67% of non‐GABAergic cells responded to activation of V1 corticotectal terminals. Biocytin‐labeling of recorded neurons revealed that wide‐field vertical (WFV) and non‐WFV cells were activated by V1 corticotectal inputs. However, WFV cells were activated in the most uniform manner; 85% of these cells responded with excitatory postsynaptic potentials (EPSPs) that maintained stable amplitudes when activated with light trains at 1–20 Hz. In contrast, in the majority of non‐WFV cells, the amplitude of evoked EPSPs varied across trials. Our results suggest that V1 corticotectal projections may initiate freezing behavior via uniform activation of the WFV cells, which project to the pulvinar nucleus.

GABAergic cell types in the superficial layers of the mouse superior colliculus

“…Selected sections were stained for the presence of GABA. A postembedding immunocytochemical protocol described previously (Bickford et al, ; Masterson et al, ; Zhou, Masterson, Damron, Guido, & Bickford, ) was employed. Briefly, we used a 0.25 μg/ml concentration of a rabbit polyclonal antibody against GABA (Sigma‐Aldrich, St. Louis, MO, catalogue #A2052, RRID:AB_477652).…”

“…For analysis of retinotectal synaptic connections, ultrathin sections obtained from C57BL/6J mice were stained for GABA and examined using an electron microscope. Retinotectal terminals were identified by their unique “pale” mitochondria with widened cristae; RLP profiles, (Bickford et al, ; Bickford, Zhou, Krahe, Govindaiah, & Guido, ; Boka et al, ; Masterson et al, ), and those involved in a synapse were imaged. The areas of the presynaptic and postsynaptic profiles were measured using Image J, RRID: nif‐000‐30467, or Maxim DL© 5 software) and the gold particles were counted to calculate the gold density overlying each profile.…”

“…The presence or absence of synaptic vesicles in postsynaptic profiles was also noted. As previously described (Masterson et al, ), profiles were identified as GABAergic if the gold particle density overlying them was greater than the maximum density overlying RLP profiles ( n = 124; average 9.27 ± 8.05 gold particles/μm 2 ). This analysis revealed that in the C57BL/6J tissue, GABAergic profiles could be identified by a density of >30 gold particles/μm 2 .…”

“…These efforts are aided by a variety of transgenic lines that can be used to specifically label and manipulate GABAergic cell types. Most notably, a mouse line that expresses green fluorescent protein (GFP) in cells that contain the 67 kDa isoform of glutamic acid decarboxylase (GAD67‐GFP), and a line that expresses Cre‐recombinase in cells that contain glutamic acid decarboxylase (GAD; GAD2‐cre), have been used to isolate GABAergic cells of the SC (Endo, Yanagawa, Obata, & Isa, ; Gale & Murphy, , ; Masterson, Zhou, Akers, Dang, & Bickford, ; Villalobos et al, ; Zingg et al, ). Such studies have revealed that GABAergic SC cells can exhibit a variety of morphologies, physiological properties, synaptic inputs and outputs, as well as the presence or absence of the calcium binding protein parvalbumin (PV).…”

Parallel organization of the avian sensorimotor arcopallium: Tectofugal visual pathway in the pigeon (Columba livia)

“…Specifically, the tectofugalrecipient cortex (area TP or POR), densely projects to the superficial layers of the SC (Bennett et al, 2019;Wang & Burkhalter, 2013) where, as we here suggest for the AI, they may contact the TGC neurons. In the case of the mouse V1, descending axons do make direct synaptic contacts upon TGC dendrites, potentiating the visual responses transmitted by the TGCs to the mouse caudal pulvinar (Fredes, Vega-Zuniga, Karten, & Mpodozis, 2012;Masterson, Zhou, Akers, Dang, & Bickford, 2018). Furthermore, V1 stimulation can drive TGCs activity and provoke defensive behaviors presumably mediated by the TGC-Rt pathway (Liang et al, 2015;Zhao et al, 2014;Zhou, Masterson, Damron, Guido, & Bickford, 2018;Zingg et al, 2017).…”