Characterization and manipulation of Corticothalamic neurons in associative cortices using Syt6‐Cre transgenic mice

Abstract

Corticothalamic interactions between associative cortices and higher order thalamic nuclei are involved in high‐cognitive functions such as decision‐making and working memory. Corticothalamic neurons (CTn) in the prefrontal cortex and other associative areas have been much less studied than their counterparts in the primary sensory areas. The availability of characterized transgenic tools to study CTn in associative areas will facilitate their study and contribute to overcome the scarcity of data about their properties, network dynamics, and contribution to cognitive functions. Here, we characterized the Syt6‐Cre (KI148Gsat/Mmud) transgenic mouse line, by tracking expression of a Cre‐mediated reporter. In this line, Cre‐reporter is strongly expressed in the prefrontal, motor, cingulate, and retrosplenial cortices, as well as in other brain areas including the cerebellum and the olfactory tubercle. Cortical expression starts embryonically and reaches the adult expression pattern by postnatal day 15. In the cortex, Cre‐reporter is expressed by layer 6‐CTn and by layer 5‐CTn to a lesser extent. We quantified Syt6‐Cre+ CTn axon varicosities to estimate the distribution and density of putative corticothalamic driver and modulator inputs to thalamic nuclei in the medial, midline, intralaminar, anterior, and motor groups. Also, we characterized the effect of optogenetic stimulation of Syt6‐Cre+ neurons in the activity of the prefrontal cortex. CTn stimulation in the prefrontal cortex induces an oscillatory activity in the local field potential that resembles the cortical downstates typically observed during slow‐wave sleep or quiet wake.

Chrna5 and lynx prototoxins identify acetylcholine super-responder subplate neurons

“…Syt6 has been extensively used as a marker to characterize layer 6 corticothalamic neurons and their function during PFC-dependent tasks ( 28, 30, 31, 51 ). Yet, the extent of overlap between Chrna5 and Syt6 expressing deep-layer pyramidal neuron populations is unclear.…”

“…To further probe the distinct cholinergic responses of prefrontal Chrna5+ neurons, we examined factors differentiating these cells from neurons labeled by Syt6 expression (Syt6+), which has hereto been the predominant marker of layer 6 corticothalamic neurons used to characterize their function during PFC-dependent tasks (Harris et al, 2019; Nakayama et al, 2018; Nectow et al, 2017b; Vaasjo et al, 2021). Since the extent of overlap between Chrna5 and Syt6 expressing deep-layer pyramidal neuron populations was unclear, we adopted an imaging strategy to visualize the distribution of Chrna5+ and Syt6+ neurons in the PFC and determine the exact proportion of distinctive non-overlapping Chrna5+ neurons.…”

“…Previously, the deep-layer cell types expressing Chrna5 were uncharacterized, and generally thought to include L6CT neurons (Kassam et al, 2008). Investigation of L6CT neurons have relied on Syt6 -Cre and Ntsr1 -Cre mouse lines that label similar sets of neurons (Harris et al, 2014, 2019; Nectow et al, 2017a), with only Syt 6-Cre mice successfully labeling this population in prefrontal cortex (Vaasjo et al, 2021). L6CT neurons labeled by Syt6 or Ntsr1 expression are excited by acetylcholine (Sundberg et al, 2018; Tian et al, 2014), but the degree to which their nicotinic response relied on Chrna5 expression was unclear.…”

“…Previously, the deep-layer cell types expressing Chrna5 were uncharacterized, and generally thought to include L6CT neurons (8). Investigation of L6CT neurons have relied on Syt6-Cre and Ntsr1-Cre mouse lines that label similar sets of neurons (27)(28)(29), with only Syt6-Cre mice successfully labeling this population in prefrontal cortex (31). L6CT neurons act to control attention through their thalamic projections (67,68), as well as their intracortical excitatory and inhibitory outputs (69)(70)(71).…”

“…Chrna5 is predicted to be expressed in L6CT neurons (Heath et al, 2010; Kassam et al, 2008), which are usually identified by their expression of Syt6 , a conserved L6CT neuronal marker (Harris et al, 2014, 2019; Nectow et al, 2017a). Syt6 -Cre mice have been widely used to characterize prefrontal L6CT neurons and their cholinergic properties (Nakayama et al, 2018; Tian et al, 2014; Vaasjo et al, 2021). However, it is unclear whether these are the same neurons expressing Chrna5 .…”