Truncated TrkB: The predominant TrkB isoform in nociceptors.

Truncated TrkB (TrkBT1), traditionally considered a dominant-negative regulator of full-length TrkB (TrkBTK+), remains poorly understood in peripheral sensory neurons, particularly nociceptors. Furthermore, sensory neuronal TrkB expression and function has been traditionally associated with non-nociceptive neurons, particularly Aδ low-threshold mechanoreceptors. This study challenges prevailing assumptions by demonstrating that TrkBT1 is the predominant TrkB isoform expressed in trigeminal sensory neurons and plays a functional role in modulating neuronal activity. We demonstrate that TrkBT1 is the predominant isoform expressed in trigeminal nociceptors, identified by markers such as TRPV1, TRPA1, TRPM8 and 5HT3A, as well as non-nociceptors, while the full-length isoform (TrkBTK+) is restricted to non-nociceptive subpopulation. Functionally, we show that acute application of BDNF induces modest calcium influx in nociceptors and prolonged BDNF exposure significantly potentiates capsaicin-induced calcium influx, an effect blocked by the TrkB-specific antagonist ANA12. Additionally, BDNF also promotes survival of both nociceptive and non-nociceptive neurons in culture, an effect dependent on TrkBT1 activity. Our data also reveal that ANA12 inhibits BDNF-mediated neuronal sensitization and survival in a concentration-dependent manner, implicating distinct TrkBT1 signaling pathways in these processes. Collectively, our findings redefine TrkBT1 as a functional modulator of trigeminal nociceptor activity rather than a passive regulator of full-length TrkB. By uncovering its dual roles in nociceptor sensitization and survival, this study provides new insights into the molecular mechanisms of BDNF/TrkB signaling in pain. Future work evaluating the role of TrkBT1 in sensory biology could offer new perspectives on how this receptor contributes to neuronal function and plasticity during chronic pain conditions. PERSPECTIVE: This study redefines TrkB-T1 as a functional modulator of trigeminal nociceptors, challenging the assumption that full-length TrkB is the primary isoform. It reveals TrkB-T1's role in BDNF-induced sensitization and survival, providing new insights into BDNF/TrkB signaling in pain and potential therapeutic interventions.