Progression of Cortical Layer 6 Input to First-Order Thalamic Nuclei and the Thalamic Reticular Nucleus During...

One of the largest excitatory projections to the thalamus arises from Layer 6 (L6) neurons of the neocortex. As corticothalamic (CT) projections descend, they pass through the thalamic reticular nucleus (TRN), forming collateral connections with GABAergic neurons, which provide feedback inhibition onto thalamocortical neurons. This arrangement allows for modulation of thalamocortical signalling in a modality-specific and state-dependent manner. Little is known about how L6 projections make functional connections with neurons in TRN and thalamic nuclei during development. We used an L6-specific mouse line (Ntsr1-Cre) crossed onto Cre-dependent reporters to examine when L6 CT axons innervate and form functional connections in TRN, as well as first-order nuclei including the ventrobasal complex (VB) and dorsal lateral geniculate nucleus (dLGN). In Ntsr1-Cre::Ai9 mice, tdTomato-labelled CT axons were present in TRN and latero-ventral VB at postnatal day (P)2-3. By P7, CT fibers occupied all of VB and began to innervate the ventral half of dLGN and eventually progressed dorsally to encompass dLGN by P12-14. Using optogenetics in acute slice recordings in Ntsr1-Cre::Ai32 mice showed that excitatory postsynaptic responses followed a similar sequence, first appearing in TRN (P7), then in VB (P7-10), and lastly in dLGN (P10-14). Initially, responses were weak and failed to follow low rates of repetitive stimulation. As the incidence of responses increased with age, so did synaptic strength, with responses to stimulus trains showing synaptic facilitation. These studies suggest that L6 cortical innervation of the thalamus is highly coordinated, with connections in TRN maturing prior to those in first-order nuclei.