Coding and Plasticity in the Mammalian Thermosensory System

Abstract

Perception of the thermal environment begins with the activation of peripheral thermosensory neurons innervating the body surface. To understand how temperature is represented in vivo, we used genetically encoded calcium indicators to measure temperature-evoked responses in hundreds of neurons across the trigeminal ganglion. Our results show how warm, hot, and cold stimuli are represented by distinct population responses, uncover unique functional classes of thermosensory neurons mediating heat and cold sensing, and reveal the molecular logic for peripheral warmth sensing. Next, we examined how the peripheral somatosensory system is functionally reorganized to produce altered perception of the thermal environment after injury. We identify fundamental transformations in sensory coding, including the silencing and recruitment of large ensembles of neurons, providing a cellular basis for perceptual changes in temperature sensing, including heat hypersensitivity, persistence of heat perception, cold hyperalgesia, and cold analgesia.

Keywords: TRP channels; changes in peripheral coding; injury; sensory coding; temperature representation; warmth sensing.

Figure 1. Functional imaging of peripheral thermosensory neurons

A) Shown are whole mount immunofluorescent staining for GCaMP5 in trigeminal ganglia from six driver lines tested for thermosensory sensitivity. Populations labeled by Th^Cre and Parv^Cre exhibited low-threshold mechanosensory responses but did not respond reliably to thermal stimuli. Scale bar is 100 micron. B) Schematic diagram illustrating the methodology employed to record the responses of trigeminal sensory neurons to thermal stimuli (stimuli were applied in the oral cavity. C) A top-down view of the trigeminal ganglion imaged at 5x magnification, highlighting the dense fields of neurons (see also Fig S1) and the three branches of the trigeminal nerve (image was obtained using basal GCaMP fluorescence and is a composite of Z-projections through the trigeminal ganglion). Scale bar is 500 micron. D) Color-coded heat map of thermosensory responses. The upper left panel shows neurons responding to cold stimulation, and the right panel those responding to hot stimuli. Shown below the panels are the temperature ramps used for stimulation; color scale indicates percent ΔF/F. The lower panel depicts a color-coded map of the same field, showing distribution of hot, cold, and bimodal thermosensory neurons. Scale bar is 100 micron, color scale indicates maximal percent ΔF/F. E) Traces of GCaMP fluorescence changes for three neurons detected in a single imaging field, representing typical response profiles for hot, cold, and bimodal neurons. Bars to left of traces indicate 5% ΔF/F, and the graph above illustrate our standard stimulus series, with a baseline of 32° C and temperature ramps ranging from 4 to 50° C in 19 steps.

Figure 2. Representation of heat and warmth

A) Composite map of neurons representing heat in a single trigeminal ganglion, labeling the high-threshold noxious heat sensory neurons (red) and the low-threshold warmth sensors (yellow). B) Heat map of calcium responses in 122 heat neurons and 14 warm neurons from a single TrpV1^Cre;Rosa^GCaMP5 imaging field to a series of heating steps. The vast majority of neurons are only recruited at noxious temperatures. Color scale indicates ΔF/F. C) Maps of heat representations in a trigeminal imaging field as a function of temperature. Innocuous temperatures (39° and 42° C) are represented by warm neurons (shown in yellow) while painful heat stimuli recruit a large population of noxious heat neurons (shown in red). Intensity of labeling indicates normalized calcium response (ΔF/F as percent of maximal response for each neuron). D) Temperature coding properties of warmth and heat neurons. Shown above are examples of stimulus- normalized response plots for three warmth sensing neurons (left panel) and three noxious heat neurons (right panel). Lower panels display scatter plots of responses from 22 warm neurons (left) and 187 heat neurons (right) to the same stimulus series. E) Warmth neurons encode temperature over a wide dynamic range, defined as temperature differential between 10% and 90% response levels, while heat neurons exhibit much narrower ranges; mean and SEM of dynamic range is plotted for each population. F–H) Warmth neurons, like noxious-heat sensors, are activated by capsaicin. Shown in F is the population response elicited by capsaicin in a trigeminal imaging field (displayed as maximal projection of ΔF/F after capsaicin stimulation) Warm neurons displayed in panels G are circled in magenta, noxious heat neurons in red. G,H) Calcium traces of capsaicin-activated warmth and heat cells from panel C, cells 1–3 are high-threshold heat neurons, cells 4–6 are warmth-sensing neurons; temperature ramps for stimulation are shown above the traces. Bars (to the left of traces) indicate 5% ΔF/F′; scale bars is 100 micron for all images.

Figure 3. TRPV1 channel is required for warmth sensation

A) Pharmacological inhibition of the TRPV1 ion channel abolishes responses of warm responding neurons to warming stimuli. Shown are responses of four warmth neurons before (left, black traces) and after (right, red traces) blockade of TrpV1 channel with JNJ-17203212. Bars (at left of traces) indicate 5% ΔF/F′. B) Quantification of responses before and after administration of JNJ-17203212. For B, data are shown for 16 warm sensing neurons from 4 mice. Note the complete loss of responses to the 43° C test stimulus. Recordings from TrpV1^Cre;Rosa^GCaMP5 mice; values are maximal ΔF/F. C) TrpV1 knockout animals maintain responses to noxious heat (see Fig S6F for responses to cold). Raster plot of 40 heat-responding neurons from a TrpV1 knockout animal to temperatures between 2–55° C (left panel) and to stimulation with 5mM capsaicin (CAPS, right panel). Importantly, while neurons from the TrpV1 knockouts still respond to temperatures above 45° C, they are no longer responsive to capsaicin or innocuous warmth; the traces below show the mean normalized responses from all heat neurons. Color scale indicates percent ΔF/F. D) Graph illustrating the cumulative number of neurons responding to increasing temperatures in wild-type (black trace) and TrpV1 KO mice (red trace). Note that warm responding neurons (low-threshold) are missing in the absence of TrpV1 channel. Data were obtained from ΔRosa^GCaMP mice expressing calcium indicator in all neurons (see Methods), n=3 mice for each genotype. E) Pharmacological inhibition of TRPV1 impairs warmth discrimination. Mice trained to associate a warm (40° C) cue with the location of a water reward (top diagram) chose the correct side in ~90% of trials; after pharmacological inhibition of TRPV1, these mice displayed a significant deficit in recognition of the warm stimulus. N =16 mice for inhibition with JNJ-17203212, N=11 for vehicle control, mean and SEM of performance index are plotted for each condition, p-values by Student’s T-test. We note that “warmth sensing” also involves cold sensors (Pogorzala et al., 2013), therefore a loss of warm-sensing neurons may not be expected to produce a complete loss of warm detection.

Figure 4. Functional and genetic characterization of cold sensing neurons

A) Calcium imaging of Wnt1^Cre and TrpV1^Cre (shown here) trigeminal neurons identified three classes of cold-selective responders, color coded within a representative imaging field. Shown are the immediate response to cooling (left panels) and the population response after 12 seconds of stimulus application at 3 different stimulus intensities (9°, 19° and 26° C, stimuli shown above). Intensity of labeling indicates normalized response for each neuron at each temperature and time point (calculated as percentage of maximal calcium response to any cold stimulus). B) Heat-map of calcium responses from all cold-sensing neurons labeled in panel A, illustrating cold-adapting (Type I) neurons, a smaller population of non-adapting cold neurons (Type II), and a rare subset exhibiting a bi-phasic response (Type III). C) Cold sensing neurons exhibit distinct response kinetics, as seen in traces from exemplary neurons of each class. D) Calcium responses from a single Trpm8^Cre imaging field, showing that Trpm8^Cre neurons represent all three classes of cold sensing neuron. Gray bars represent 5% ΔF/F. All scale bars are 100 microns, color scales represent % ΔF/F. E) Maps of GCaMP population responses to intense heating and cooling stimuli show that Trpm8^Cre neurons (upper panels) are overwhelming selective for cold stimuli, while the vast majority of Trpa1^Cre neurons (lower panels) respond only to heat (see Fig. S8 for quantification).

Figure 5. Injury evokes ongoing activity that is suppressed by cooling

A) Left panel shows responses of neurons before injury, and right panel after injury. Temperature was kept constant at 32° C throughout trials; data were obtained from a TrpV1^Cre;Rosa^GCaMP5 animal, color scale indicates percent ΔF/F. B) Representative calcium imaging traces of neurons from the imaging field shown in panel A, illustrating activity while exposed to neutral temperature (32° C). Note that the neurons are silent at neutral temperature before injury but exhibit sporadic episodes of activity after injury. C) Distribution of cell types exhibiting spontaneous activity induced by injury. The majority of active neurons were noxious heat sensors, but all thermosensory classes, displayed some degree of spontaneous activity. D) Ongoing injury-induced activity can be inhibited by cooling; the spontaneous responses of noxious heat sensors (labeled with GCaMP with the TrpA1^Cre driver) after injury can be abolished by brief cooling ramps. Temperature ramps of cooling stimulation are shown above traces; bar to left indicates 5% ΔF/F. D,E) Quantification of mean fluorescence in 11 TrpA1^Cre labeled neurons from a single imaging field, showing no detectable baseline activity before injury at 32° C, an increase after injury while held at 32° C, and suppression of this activity upon cooling to 18° C.

Figure 6. Transformation of heat sensation by injury

A) Injury induces a dramatic recruitment of neurons that now respond to warm temperatures. The upper panel shows a field of neurons stimulated with innocuous warmth (41° C), before (green) and after (red) induction of burn injury (image is an overlay of the maximal projections of GCaMP fluorescence ratio); the panels below show the same field before (left) and after (right) injury. Color scale indicates percent ΔF/F, red traces show the temperature ramps, scale bar is 100 micron. B) Representative calcium responses of 5 individual neurons before (left) and after injury (right), demonstrating their profound shift in sensitivity, from the noxious range (>43° C, pink box) to normally innocuous temperatures (<43° C, yellow box). C) Scatter plot of calcium responses for 125 noxious heat neurons before and after thermal injury, illustrating that injury produces a large-scale shift in the thermal sensitivity and preference of this population into the innocuous warm temperature range. D, E) Injury induced heat sensitization is dependent on TrpV1. Panel D shows representative calcium responses for warmth and noxious heat neurons to mild heating (42°) before injury, after injury, and after subsequent pharmacological inhibition of TRPV1 with JNJ-17203212. Note warmth neuron responses to mild heating are unchanged by injury. Panel E shows quantification of sensitization before and after TRPV1 inhibition for all 21 heat neurons tested.

Figure 7. Injury induces the desensitization of innocuous cold neurons

A) Responses to innocuous cold are silenced by injury; upper panel shows neurons activated by moderate cooling before (cyan) and after injury (magenta), lower panels shows responses of the same fields of neurons pre- (left) and post-injury (right); color scales indicate percent ΔF/F, scale bar is 100 μm. B) Representative traces of the differential effect of burn injury on the different types of Trpm8-neurons; Type I cold sensors (top traces) exhibit a selective loss of responses. By contrast, the responses of Type II (bottom traces) are largely unaltered. Type III cold neurons (not shown) exhibit desensitization, similar to Type I cells. Temperature ramps of cooling stimuli are shown below traces; bar to left indicates 5% ΔF/F.

Figure 8. Injury activates a population of silent thermosensory neurons

A) Comparison of population responses to cold reveal a rare class of neurons that are only activated post-injury (white arrowheads). Upper panels shows responses of cold sensitive neurons before (left) and after (right) injury. The lower panel shows the superimposed fields. Pseudo-colored bars to the right indicate percentage ΔF/F; data are from TrpV1^Cre;Rosa^GCaMP5 animals. B) Representative calcium traces of silent thermosensors demonstrating that these neurons are completely insensitive to a broad temperature range (from 1° to 56° C) prior to injury, yet respond robustly and reliably to cold after injury. Sample data are from a Trpm8^Cre; Rosa^GCaMP5 mouse. C) Silent thermosensor cold responses (green traces) adapt at all stimulus intensities; compare response of a typical silent thermosensor to a Type I cold response (blue traces), which adapts to intense stimuli but sustains during mild cooling. D) Quantification of response adaptation as a function of stimulus intensity for Type I, Type II, and silent cold sensors, expressed as duration of the response divided by duration of the cooling stimulus, mean and SEM plotted. E) Scatter plots of cold responses (ΔF/F) before and after injury for Trpm8^Cre neurons (N = 289 neurons) and TrpA1^Cre expressing neurons (N = 153 neurons). Analysis was performed on the maximal calcium responses to noxious cold stimuli (~4° C) before and after injury. Note that responses of silent cold-thermosensory cells are clustered along the y-axis. There are no TrpA1^Cre-neurons sensitized to noxious cold after injury. F) Burn enhances behavioral sensitivity to cooling. Plotted are the latencies of paw reflex withdrawal to a cooling stimulus, before and after injury for 6 mice tested at two baseline temperatures (18°, p=0.0052 and 22° C, p=0.0008, paired Student’s T-test).