In Vivo Imaging of Neuronal Activity Shifts in the Somatosensory Cortex After Morphine Tolerance in Mice

Morphine is widely used to treat severe pain, but its analgesic effect diminishes with repeated use due to the development of tolerance. Reversing this tolerance remains a clinical challenge, as its underlying mechanisms are complex and not fully understood. Although the involvement of multiple central nervous system regions in morphine tolerance has been established, the role of the primary somatosensory cortex (S1)-a key region for sensory perception-remains unclear. In this study, we used in vivo two-photon calcium imaging to longitudinally track neuronal activity in S1 during the induction of morphine tolerance in mice. Mice received daily morphine injections (10 mg/kg, s.c.) for 7 days. Behavioural assays confirmed the development of tolerance, as shown by diminished analgesic responses. While total neuronal activity in S1 remained stable after the first morphine injection, a significant increase was observed on Day 7. At the single-neuron level, three response patterns were identified: increased, decreased and stable firing following morphine administration. Notably, these subpopulations were dynamically restructured after tolerance was established. Our findings reveal that morphine tolerance is accompanied by network-level reorganization in the somatosensory cortex, suggesting a cortical contribution to altered sensory processing during chronic opioid exposure.