Two-photon in vivo calcium imaging of L4 dorsal root ganglion (DRG) thermoreceptors was used to map how primary afferent neurons encode innocuous cool and warm temperatures in both awake and isoflurane-anesthetized mice, across a dense stimulus space of target temperatures (12°C–42°C) from four adapted temperatures (22°C, 27°C, 32°C, 37°C).
Of 499 thermally responsive neurons (out of 20,561 recorded under anesthesia; 212/2,221 in awake mice), ~74% were Cool-ON, ~17% bidirectional Cool-ON/Warm-OFF, and only ~7% Warm-ON. Over 99% of cool-responsive neurons showed graded (not combinatorial) encoding. Both cooling and warming were encoded as absolute temperature rather than relative change. TRPM8 blockade abolished cool-evoked ON responses and warm-evoked suppression; varying TRPM8 conductance in a leaky integrate-and-fire model fully reproduced both Cool-ON and Cool-ON/Warm-OFF profiles.
- Only glabrous hindpaw skin of one lumbar DRG (L4) was studied; findings may not generalize to hairy skin or other DRG levels. - Calcium imaging cannot confirm that OFF responses reflect reduced action potential firing (not directly verified electrophysiologically in the same preparation). - The TRPV1::cre driver line broadly labels sensory afferents and may not capture all thermosensory populations.
This is a basic neuroscience study in mice with no direct clinical application at this stage. The finding that a single ion channel (TRPM8) drives both cool activation and warm suppression may inform future analgesic or thermosensory disorder drug targets.
