Functionally Distinct Circuits Are Linked by Heterocellular Electrical Synapses in the Thalamic Reticular Nucleus

Abstract

The thalamic reticular nucleus (TRN) inhibits sensory thalamocortical relay neurons and is a key regulator of sensory attention as well as sleep and wake states. Recent developments have identified two distinct genetic subtypes of TRN neurons, calbindin-expressing (CB) and somatostatin-expressing (SOM) neurons. These subtypes differ in localization within the TRN, electrophysiological properties, and importantly, targeting of thalamocortical relay channels. CB neurons send inhibition to and receive excitation from first-order thalamic relay nuclei, while SOM neurons send inhibition to and receive excitation from higher-order thalamic areas. These differences create distinct channels of information flow. It is unknown whether TRN neurons form electrical synapses between SOM and CB neurons and consequently bridge first-order and higher-order thalamic channels. Here, we use GFP reporter mice to label and record from CB-expressing and SOM-expressing TRN neurons. We confirm that GFP expression properly differentiates TRN subtypes based on electrophysiological differences, and we identified electrical synapses between pairs of neurons with and without common GFP expression for both CB and SOM types. That is, electrical synapses link both within and across subtypes of neurons in the TRN, forming either homocellular or heterocellular synapses. Therefore, we conclude that electrical synapses within the TRN provide a substrate for functionally linking thalamocortical first-order and higher-order channels within the TRN.

Keywords: electrical synapse; gap junction; thalamocortical; thalamus.

Figure 1.

Example spatial distributions and spiking responses of SOM and CB neurons in the TRN. A, Fluorescence images of a live slice from a SOM×Ai6 mouse. SOM-GFP expression, pseudocolored in magenta here, occupies the shell of the TRN. Boundaries were drawn from corresponding IR image. B, Example responses of SOM neuron to current steps of −100 pA and rheobase for each cell. From bottom to top, V_m= −74.5, −74.0, −77.0 mV. Scale bars, 20 mV, 50 ms. C, CB-GFP expression in the TRN. D, Example responses of CB neuron to current steps of −100 pA and rheobase for each cell. From bottom to top, V_m= −74.9, −72.7, −77.3 mV. Scale bars, 20 mV, 50 ms. See Extended Data Figure 1-1 for an example of a recording at the boundary between the core and shell of the TRN.

Figure 2.

Intrinsic and bursting properties distinguish GFP + SOM and CB subtypes in the TRN. A, R_in for CB (mean, 124.6 ± 4.7 MΩ) and SOM (222.3 ± 7.7 MΩ; p < 0.01; n = 107 CB and 154 SOM) neurons. B, Peak instantaneous firing rate within bursts for SOM (mean, 184.4 ± 7.5 Hz) and CB (312.5 ± 10.6 Hz; p < 0.01) neurons. C, Gain of tonic spiking frequency for SOM and CB neurons (SOM, 0.37 ± 0.03 Hz/pA; CB, 0.30 ± 0.02 Hz/pA; p = 0.5). D, CB neurons fired more spikes for each burst (SOM, 4.8 ± 0.3 spikes; CB, 7.0 ± 0.3 spikes; p < 0.01). E, Spiking within a burst accelerated (expressed as the ratio of fastest to first instantaneous rate within the burst) for CB neurons more than for SOM neurons (SOM: median, 1.0, mean, 1.1 ± 0.03; CB: median, 1.13, mean, 1.20 ± 0.02; p < 0.01). F, Instantaneous firing for each interspike interval (ISI) within bursts, normalized to the first interval. Data are mean ± SEM. G, Cell identity as a function of peak burst rate, input resistance, and acceleration ratio. See Extended Data Figure 2-1 for input resistance across age.

Figure 3.

Homocellular and heterocellular coupling between pairs of TRN neurons. A, Example of coupling between two pairs of SOM neurons. Top, Overlay of fluorescence and DIC images taken during live recordings. Scale bar, 20 µm. Middle, Simultaneous voltage responses of both neurons to −100 pA current steps delivered to one neuron of each pair. Scale bars, 5 mV, 50 ms. Bottom, Spiking responses of each neuron to separate injections of rheobase current. Scale bars, 20 ms, 20 mV. B, As for A, but for pairs of SOM and non-SOM neurons; scale bars in top right are 2.5 mV, 50 ms. C, As for A, but for pairs of CB neurons. D, As for A, but for pairs of CB and non-CB neurons.

Figure 4.

Distribution of coupling within and without SOM and CB cell types. A, Stacked distribution of homocellular electrical synapses in the TRN; coupling was found between 9 out of 44 pairs of CB neurons tested and 30 out of 60 pairs of SOM neurons tested. cc, coupling coefficient. B, Stacked distribution of heterocellular electrical coupling between TRN neurons: synapses were identified between 8 out of 19 pairs of CB and non-CB neurons and 23 out of 34 pairs of SOM and non-SOM neurons tested. C, Schematic of distinct thalamocortical pathways linked by electrical synapses of the TRN. FO, first order; HO, higher order. See Extended Data Figure 4-1 for signal-to-noise of coupling measurements.