State-Dependent Modification of Sensory Sensitivity via Modulation of Backpropagating Action Potentials

IV neuron activation decreases the frequency of ectopic spike initiation in AGR. A, Schematic of the stomatogastric nervous system. Axonal projections of the paired IV neurons are depicted in red. AGR and its axonal projections are depicted in blue. Nerve names are italicized. Green circles in the CoG represent descending projection neurons. A’, Composite photo of AGR (yellow) and STG (orange) showing the morphology of AGR and its axonal projections in the stn and dgn. AGR was visualized via intracellular injection of Alexa Fluor 568. Neural structures were visualized via bath-application of the voltage-sensitive dye Di-4 ANNEPS. Note that AGR possesses one to three arbors in the STG neuropil (Städele and Stein, 2016) that are not visible here because of high background fluorescence of the STG neuropil. Scale bar, 100 µm. n = neuropil. B, AGR instantaneous ff. (AGR ff., top) and extracellular nerve recordings of the lgn and dgn (bottom) showing the responses of AGR (blue) and the STG gastric mill neurons before and during IV neuron stimulation (gray area). Black bars above the recording visualize the repetitive stimulation of the IV neurons (40 Hz, 10 consecutive trains). IV neurons stimulation elicited a gastric mill rhythm (note the alternating activity of LG on the lgn and DG on the dgn) and a concurrent decrease in AGR ff. by 41%. C, Time course of the average change in AGR ff. during 40 Hz IV neuron stimulation for 10 consecutive trains. AGR ff. was normalized to the frequency measured 100 s before IV neuron stimulation (baseline). Control refers to the frequency measured immediately before the stimulation. Shown are means ± SD. N = 14 preparations. D, Average time course of the change in normalized AGR ff. during IV neuron stimulation with varying stimulation frequency (10–50 Hz). Shown are means. N = 10 preparations. Nerves: ivn: inferior ventricular nerve, ion: inferior esophageal nerve, son: superior esophageal nerve, dpon: dorsal posterior esophageal nerve, stn: stomatogastric nerve, dgn: dorsal gastric nerve; agn: anterior gastric nerve, lvn: lateral ventricular nerve, lgn: lateral gastric nerve. Ganglia: STG: stomatogastric ganglion, CoG: commissural ganglion, brain: supraesophageal ganglion. Neurons: AGR: anterior gastric receptor neuron, IV: inferior ventricular neurons. Panel A adapted from Hedrich and Stein (2008) and Städele et al. (2012).

IV neuron stimulation can stop ectopic spike production. A, Extracellular recording of the dgn (bottom) and AGR instantaneous ff. (top) showing that AGR spike amplitude changed during strong decrease of AGR instantaneous ff. A’, Magnification of the gray area in A. While AGR spikes on the dgn had similar shapes and amplitudes before IV neuron stimulation, spike amplitude continuously changed during IV stimulation. Arrows mark the changes in AGR amplitude. Ectopic APs had negative deflections and are marked blue. APs generated in the periphery had positive deflections and are highlighted in green. Note the different time scales in A and A’. B, Comparison of spike appearance and delay of arrival of AGR APs with negative (Bi), and positive deflection (Bii) at three recording sites along the AGR axon (dgn, stn, and son). APs on the dgn were used for temporal alignment. Shown are single sweeps (left, middle) and an overlay of 20 APs (right) showing the loss in temporal accuracy (jitter). The recording sites where APs appeared first are highlighted in bold. Colors correspond to different AP deflections as shown in A. Gray lines depict the delay in AP appearance between recording sites. C, Example recording showing a complete stop in AGR’s ectopic firing during IV stimulation (gray area). Note the large gap in spike frequency after the 4th IV stimulus train.

The IV neurons exert their effects on the AGR axon via chemical transmission. A, AGR ff. before and during VCN stimulation (gray bar). VCN stimulation did not diminish AGR ff., but started a gastric mill rhythm (see LG activity on the lgn and DG on the dgn). Recordings are from the same experiment as shown in Figure 1B. B, Average time course of normalized AGR ff. in response to VCN stimulation. Shown are mean ± SD. VCN stimulation did not cause a significant change in AGR ff. N = 12 preparations. C, AGR ff. during IV neuron stimulation in the intact nervous system (Ci), after CoG transection (Cii), and after block of chemical transmission via application of low Ca²⁺ saline to the posterior stn (Ciii). Recordings are from the same preparation. D, Analysis of the average change in AGR ff. during IV neuron stimulation in saline (IV stim), after CoG transection (IV stim + CoG cut), and after chemical transmission was blocked (IV stim + low Ca²⁺). Shown are means ± SD. Control refers to the frequency measured immediately before IV stimulation. n.s. = not significant different with p > 0.8, one-way RM ANOVA, N = 6 preparations.

The IV neuron co-transmitter histamine diminishes the AGR ff. mainly via acting on H₂ receptors. A, B, AGR ff. in response to (A) FMRF-like peptide F1 and (B) histamine application to the posterior stn. Colored areas mark the time of drug application. Arrows in B indicate switches of AP initiation to other locations. A’, B’, Analysis of the change in AGR ff. before and during application of (A’) FMRF-like peptide F1 and (B’) histamine. Black circles represent individual experiments. Diamonds represent mean ± SD. N = 6 (FMRF-like peptide F1), N = 13 (HA). C, AGR ff. in response to 40 Hz IV neuron stimulation in saline (Ci) and after blocking of H₂ receptors with cimetidine (Cii). Recordings are from the same preparation and scaled identically. D, Comparison of AGR ff. during H₂ receptor blockade with cimetidine immediately before (control cimet.) and during IV stimulation (IV + cimet.). Circles represent individual experiments. Diamonds represent mean ± SD. n.s. = not significant different, paired t test, t₍₄₎ = 2.66; p = 0.056, N = 5 preparations. E, Average change in AGR ff. in saline (gray) and cimetidine (purple). Shown are means ± SD. One-way RM ANOVA, F_(4,3) = 33.27, p < 0.001, Holm–Sidak post hoc test with p < 0.05, N = 5 preparations. n.s. = not significant different with p = 0.58.

IV neurons only influence AGR when APs are generated ectopically in the stn. A, B, Overlay of multiple AGR APs (72 APs each) for (A) control condition and (B) during HiDi application. Data are from the same preparation. Bold highlighted nerve names mark the recording site where APs appeared first. APs on the dgn were used for temporal alignment. A’, B’, Example recording showing AGR ff. before and during IV neuron stimulation for (A’) control condition and (B’) during HiDi application to the stn. Extracellular recordings of the dgn (bottom) show the gastric mill rhythm (rhythmic firing of DG). Data from the same preparation as shown in A, B.

Antidromic ectopic APs alter sensory sensitivity to muscle stretch. A, Original recordings of AGR’s burst activities at different ectopic spike frequencies (7, 5, 3 Hz). Recordings were taken on a section of the dgn. Sensory bursts were elicited by stretching the gm1 muscles. Ectopic APs were elicited with extracellular stimulation of the posterior part of the stn (siAPs) and are highlighted in light blue. Orthodromic APs of the sensory burst are depicted in green while spontaneous ectopic APs are highlighted in dark blue. B, Overlay of several original traces from dgn, stn, and son recordings plus average showing the directions of AP propagation for the three conditions shown in A. The gray area depicts the stimulus artifact. C–E, Quantification of sensory bursts at siAP frequencies between 3 and 10 Hz. C, Burst duration. D, Number of spikes per burst. E, Average intraburst ff. Shown are means ± SEM. N = 8 preparations each.

Antidromic ectopic APs alter sensory sensitivity to K⁺ application. A, Original recordings of AGR’s burst activities at different ectopic spike frequencies (7, 5, 3 Hz). Recordings were taken on a section of the stn, anterior to the AGR ectopic SIZ. Sensory bursts were elicited with high potassium (K⁺, arrow) in the periphery, ectopic APs with extracellular stimulation of the posterior part of the stn. siAPs are highlighted in light blue. Orthodromic APs of the sensory burst are depicted in green, while spontaneous ectopic APs are highlighted in dark blue. B, Overlay of several original traces from dgn, stn, and son recordings plus average showing the directions of AP propagation for the three conditions shown in A. The gray area depicts the stimulus artifact. C–E, Quantification of sensory bursts at siAP frequencies between 3 and 10 Hz. C, Burst duration. D, Number of spikes per burst. E, Average intraburst ff. The gray area indicates the physiologic range of AGR ff. decrease caused by IV stimulation. Shown are means ± SEM. N = 5 preparations.

Slow ionic conductances determine the effectiveness of ectopic APs on burst activity. A, Burst activity of three models with an I_KS time constant of 2 s. siAP frequency was varied from 7, to 5, to 3 Hz, and compared to 0 Hz (no firing). siAPs are depicted in black, sensory burst APs are highlighted in green. Note that decreasing siAP ff. increased burst duration (gray area). For better visualization, only burst starts and ends are shown. B–D, Analysis of changes in burst structure for different I_KS time constants at different siAP frequencies. B, Burst duration. C, Number of spikes per burst. D, Average intraburst ff.