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. 2021 Nov;599(22):5085-5101.
doi: 10.1113/JP281711. Epub 2021 Oct 23.

Morphology, physiology and synaptic connectivity of local interneurons in the mouse somatosensory thalamus

Jane Simko  1 , Henry Markram  1   2
Affiliations

Morphology, physiology and synaptic connectivity of local interneurons in the mouse somatosensory thalamus

Jane Simko et al. J Physiol. 2021 Nov.

Abstract

The thalamic reticular nucleus (TRN) neurons, projecting across the external medullary lamina, have long been considered to be the only significant source of inhibition of the somatosensory ventral posterior (VP) nuclei of the thalamus. Here we report for the first time effective local inhibition and disinhibition in the VP. Inhibitory interneurons were found in GAD67-GFP-expressing mice and studied using in vitro multiple patch clamp. Inhibitory interneurons have expansive bipolar or tripolar morphologies, reach across most of the VP nucleus and display low threshold bursting behaviour. They form triadic and non-triadic synaptic connections onto thalamocortical relay neurons and other interneurons, mediating feedforward inhibition and disinhibition. Synaptic inputs arrive before those expected from the TRN neurons, suggesting that local inhibition plays an early and significant role in the functioning of the somatosensory thalamus. KEY POINTS: The physiology and structure of local interneurons in the mouse somatosensory thalamus is described for the first time. Inhibitory interneurons have extensive dendritic arborization providing significant local dendro-dendritic inhibition in the somatosensory thalamus. Triadic and non-triadic synaptic connectivity onto thalamic relay neurons and other interneurons provides both local feedforward inhibition and disinhibition. Interneurons of the somatosensory thalamus provide inhibition before the thalamic reticular nucleus, suggesting they play an important role in sensory perception.

Keywords: inhibition; interneurons; microcircuitry; mouse; somatosensory thalamus; triadic.

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Figures

Figure 1
Figure 1. Morphology of VP interneurons
A, maximum projection of z‐stack images in the VP of a GAD67‐GFP mouse. Interneuron somas are marked with arrowheads. B, bipolar reconstructed interneuron with slice image. C, tripolar reconstructed interneuron with slice image. D, histogram of soma sizes fitted with non‐parametric (continuous grey line) and finite mixture model (dashed grey line) estimations of distribution. Estimated cut‐off for the bimodal distribution is in the center of the red line indicating the 97.5% confidence interval. E, 3D reconstructed morphologies. Likely cut major dendritic branches are noted. Putative axons are coloured in grey and also marked with arrowheads. F, horizontal slicing orientation outlines of the ventral posterolateral (VPL) and ventral posteromedial (VPM) nuclei overlaid with cell positions for 7 of the 8 morphologies shown. One morphology from a coronal slicing orientation was excluded in this overlay.
Figure 2
Figure 2. Partial reconstructions of VP interneurons
3D reconstructed partial morphologies. Putative axons are coloured grey and also marked with arrowhead.
Figure 3
Figure 3. Electrophysiology of VP interneurons
A, left, example 300 μm coronal slice. Right, relevant cell populations and axon tract. B, firing pattern at three different holding potentials, V 1: −60 mV, V 2: −70 mV, V 3: −80 mV. I 1–3: current injected for V 1 (10 pA), V 2 (14.4 pA), V 3 (30 pA). Scale bar: 30 pA. C, hyperpolarizing current steps of 10 pA increments for V 1 and 11 pA increments for V 2 (I 1–2). Scale bar: 70 pA for V 1, 77 pA for V 2. D, example trace of a spontaneously oscillating interneuron. Scale bar: 24 pA. E, feature extraction analyses of one example trace. Red line represents average voltage value of the initial 200 ms. AP peak shown with ‘+’, AP threshold with ‘*’. Interspike intervals at top of trace. AP width shown in zoomed portion of the trace to the right. Scale bar: 31.2 pA. F, 10 active and passive electrophysiological features in histograms (bin = 10) and boxplots. Membrane potential (n = 37 cells), input resistance (n = 52 cells), tau (n = 52 cells), capacitance (n = 52 cells), minimum current (n = 47), AP threshold (n = 53 cells), first AP half‐width (n = 53 cells), last AP half‐width (n = 53 cells), ln(last ISI/first ISI) (n = 50 cells), V sag (n = 25 cells). Non‐parametric estimations of distribution shown as a grey line. Outliers in red x are greater than 1.5 quartiles. None of the distributions are gaussian (Kolmogorov–Smirnov test; P < 0.05). ML, medial lemniscus; TC, thalamocortical relay; TRN, thalamic reticular nucleus; VPi, VP interneuron.
Figure 4
Figure 4. Synaptic connections formed by VP interneurons
A, left, whole‐cell paired recordings of presynaptic VPi, postsynaptic TC. Right, single presynaptic VPi actional potential and postsynaptic TC IPSPs (black: average of 10 traces; current clamp V hold: −50 mV). n = 4 paired recordings current clamp, n = 5 paired recordings voltage clamp. Train of action potentials (black: average of 8 traces). B, connections tested, connections found, and connection probabilities per intersomatic distances for VPi–TC pairs. Red dashed line represents global probability (3.46%). Error bars represent 95% binomial confidence interval. C, left, whole‐cell paired recordings of presynaptic VPi, postsynaptic VPi. Right, single presynaptic VPi action potential and postsynaptic VPi IPSPs (black: average of 10 traces; current clamp V hold: −50 mV). n = 9 paired recordings current clamp. Train of action potentials (black: average of 8 traces). D, presynaptic VPi delaying and preventing an action potential in postsynaptic VPi (n = 2 pairs). E, as in C for VPi–VPi pairs. Red dashed line represents global probability (7.89%). F, normalized IPSP for each stimulation in a 40 Hz train. Trains recorded with other stimulation frequencies were not included (n = 2 VPi–TC pairs, n = 7 VPi–VPi pairs). G, IPSP amplitude: VPi–VPi: −2.00 (0.86) mV, VPi–TC: −0.92 (0.85) mV, 20–80% rise time: VPi–VPi: 4.46 (1.44) ms, VPi–TC: 4.31 (1.09) ms, latency: VPi–VPi: 1.75 (0.75) ms, VPi–TC: 0.97 (0.73) ms, and decay time constants: VPi–VPi: 91.87 (53.39) ms, VPi–TC: 34.20 (18.47) ms. P‐values: 0.0503, IPSP amplitude; 0.9399, 20–80% rise time; 0.1147, latency; 0.0112, decay. n.s.: not significant, Mann–Whitney U‐test, P‐value < 0.05. Data are presented as means (SD).
Figure 5
Figure 5. Locations of paired recordings
A, pre‐ and postsynaptic responses from three simultaneously patched neurons. Presynaptic spikes in black, postsynaptic connections in red. Non‐connected responses in grey. B, partial reconstructions of experiment shown in A. C, partial reconstructions of a VPi to TC pair (left) and VPi to VPi pair (right). Presynaptic neuron in grey, postsynaptic neuron in black. Putative touch locations marked with a red asterisk. D, same as in C, flipped. E, slice of partial reconstructions shown in A and B. F, slice of paired reconstruction shown in C and D left. G, slice of paired reconstruction shown in C and D right.
Figure 6
Figure 6. VP interneurons receive sensory input and provide two forms of feedforward inhibition
A, a bipolar electrode is placed on the medial lemniscus (ML) and a VPi is patched with regular ICS in current clamp. B, EPSPs seen in VPi following ML stimulation. EPSPs are blocked by the application of glutamatergic receptor blockers (GluR block: CNQX, DAP5 10 μM; n = 4 cells). C, same set‐up as in A, with a relay patched in voltage clamp with caesium ICS and TRN separated from VP. D, multiple IPSCs seen with ML pulse stimulation. Delayed IPSCs abolished with GluR block, but first IPSC resulting from direct electrode stimulation remains. E, left, amplitude of EPSC or IPSC against current threshold. Right, example IPSC and EPSC from ML stimulation (overlay). F, left, amplitude of EPSC or IPSC against current threshold. Right, another example as in E. G, time delay difference of IPSCs and EPSCs for same threshold experiments (n = 8 EPSC–IPSC pairs, 0.73 (0.62) ms) and different (diff) threshold experiments (n = 11 EPSC–IPSC pairs, 0.54 (3.54) ms). H, putative circuit arrangement for same threshold experiments. I, putative circuit arrangement for different threshold experiments. Data are expressed as means (SD).
Figure 7
Figure 7. VP interneurons receive input from the reticular nucleus and the cortex
A, schematic representation of experiment with relevant cell populations shown on right. B, slice image of experiment. C, top, IPSCs displayed in patched VPi upon TRN stimulation at 40 Hz with a recovery pulse (average of 4 traces). IPSCs are abolished with gabazine (10 μM; average of 4 traces) and partially recovered during washout (average of 4 traces) (n = 2 cells, block and partial washout). Bottom, first IPSC of both experiments. Five traces in grey, average trace in black. D, normalized IPSC for each stimulation in a 40 Hz train. E, schematic representation of experiment with relevant cell populations and axon tract shown on right. F, slice image of experiment. G, EPSPs displayed in patched VPi upon IC (internal capsule) stimulation at 40 Hz with a recovery pulse (average of 9 traces). EPSPs are abolished with GluR block (CNQX and DAP5; average of 5 traces) and partially recovered during washout (average of 5 traces) (n = 4 cells, block; n = 4 cells, block and partial washout). H, normalized EPSP for each stimulation in a 40 Hz train.
Figure 8
Figure 8. VP interneurons provide two forms of feedforward disinhibition
A, a bipolar electrode is placed on the medial lemniscus (ML) and a VPi is patched with caesium ICS in voltage clamp. B, left, amplitude of EPSC or IPSC against current threshold. Right, example IPSC and EPSC from ML stimulation (overlay). C, left, amplitude of EPSC or IPSC against current threshold. Right, another example as in B. D, time delay difference of IPSCs and EPSCs for same threshold experiments (n = 5 EPSC–IPSC pairs, 1.24 (0.67) ms) vs. different (diff) threshold experiments (n = 14 EPSC–IPSC pairs, 2.59 (2.05) ms). E, putative circuit arrangement for same threshold experiments. F, putative circuit arrangement for different threshold experiments. G and H, normalized EPSC for each stimulation in a 40 Hz train for triadic and non‐triadic synapses in feedforward inhibition (G) and feedforward disinhibition (H) recordings. Trains or paired pulses recorded with other stimulation frequencies were not included (n = 2 ML–TC triad, n = 2 ML–TC synapse, n = 2 ML–VPi triad, n = 4 ML–VPi synapse). Data are expressed as means (SD).
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References

    1. Acuno‐Goycolea C, Brenowitz S & Regehr W (2008). Active dendritic conductances dynamically regulate GABA release from thalamic interneurons. Neuron 57, 420–431. - PubMed
    1. Allen Institute for Brain Science (2004). Allen Mouse Brain Atlas. http://atlas.brain‐map.org/
    1. Arcelli P, Frassoni C, Regondi MC, Biasi SD & Spreafico R (1997). GABAergic neurons in mammalian thalamus: a marker of thalamic complexity? Brain Res Bull 42, 27–37. - PubMed
    1. Babadi B, Casti A, Xiao Y, Kaplan E & Paninski L (2010). A generalized linear model of the impact of direct and indirect inputs to the lateral geniculate nucleus. J Vis 10, 22. - PMC - PubMed
    1. Blitz D & Regehr W (2005). Timing and specificity of feed‐forward inhibition within the LGN. Neuron 45, 917–928. - PubMed

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