Figure 6: Synaptic strength and functional convergence of glabrous skin- and hairy skin-innervating Aβ-LTMR synapses onto thalamic projection neurons in the brainstem.

A. Genetic strategies for assessing synaptic connections between glabrous skin- or hairy skin-innervating LTMRs and thalamic projection neurons in acute brainstem slices. (left) Glabrous skin Aβ RA-LTMRs that send ascending projections through the dorsal column and synapse in the GN express ReaChR in TrkB^CreER; Avil^FlpO; R26^{LsL-FsF-ReaChR::mCitrine} mice. (right) Hairy skin-innervating Aβ RA-LTMRs selectively express ChR2 under a Ret^CreER ON, Pvalb^FlpO OFF strategy.

B. Representative whole-cell recording from a retrogradely labeled thalamic projection neuron in the GN. (left) Minimal optical stimulation of glabrous Aβ RA-LTMRs axons in the dorsal column (1–4 mm from the GN) isolated putative single fiber-evoked EPSCs as stereotyped currents accompanied by failures. (right) Histograms of EPSC amplitudes for these representative recordings are consistent with single fiber stimulation.

C. EPSC evoked by maximal stimulation of axons ascending the dorsal column, from the same recording as B.

D. The biocytin fill of a recorded cell (red) shows apposition to VGlut1⁺ (blue), ReaChR::mCitrine+ (green) presynaptic boutons, consistent with a direct synaptic connection (scale bar: 1 μm).

E. Representative whole-cell recording from a retrogradely labeled thalamic projection neuron in the GN postsynaptic to hairy skin innervating Aβ RA-LTMRs, similar to B.

F. EPSC evoked by maximal stimulation of all axons ascending the dorsal column in the same recording as E.

G. Biocytin filled dendrites apposed to ChR2:YFP+ (green) presynaptic axons, similar to D (scale bar: 1 μm).

H. Optogenetic activation of individual glabrous skin innervating Aβ RA-LTMRs neurons evokes a larger EPSC than activation of individual hairy skin-innervating Aβ RA-LTMRs (p < .005, Mann Whitney U-test).

I. The estimated heterotypic convergence for thalamic projection neurons receiving synaptic input from glabrous skin- (red) and hairy skin-innervating (blue) Aβ RA-LTMRs computed by dividing the maximal EPSC evoked by electrical stimulation of the dorsal column by the genotype-specific single fiber strength. Hairy skin Aβ RA-LTMRs synaptic connections exhibit a greater estimated convergence rate than glabrous skin Aβ RA-LTMRs (p < .05, Mann-Whitney U-test).

J. A model of differential synaptic expansion consistent with synaptic strength and convergence measurements presented in (A-I). Glabrous skin LTMR circuits thus have a lower functional convergence ratio while maintaining comparable synaptic input to thalamic PNs, resulting in synaptic expansion that enlarges the representation of glabrous skin relative to hairy skin.