HIGH-YIELD, SINGLE-UNIT RESOLUTION OF THE NOCICEPTOR RESPONSE TO FORMALIN <EM>IN VIVO</EM>

The formalin test is a mainstay in pain research, yet it’s debated whether nociceptors drive both behavioural phases. Addressing this is challenging, as traditional single-unit recording methods are too low-yield to survey multiple nociceptor populations. To overcome this, we used a 32-channel multi-contact Neuronexus electrode to capture single-unit activity from the rat saphenous nerve in vivo.
Adult rats were anesthetised, the saphenous nerve exposed, and the Neuronexus electrode inserted. Nerve fibres were characterised using electrically-evoked activity dependent slowing (ADS), and responses to mechanical stimuli. Saline or formalin (30µl, 2.5%) was injected subcutaneously adjacent to the receptive field, and latencies to 0.25 Hz suprathreshold electrical stimulation recorded for 1 hour. Mechanical and ADS testing was then repeated.
Across 20 rats, 86 well-isolated A- and C-fibres were recorded (27 and 59, respectively). Human ADS classification corroborated with rat classification: 39% C-fibres were mechanically-insensitive nociceptors (CMi), 25% mechanically-sensitive nociceptors (CM), and 36% non-nociceptors. A-fibres showed distinct ADS profiles, though further work is needed to characterise subtypes. Following formalin injection, 39% C-fibres and 25% A-fibres were activated, compared with 17% C-fibres and 13% A-fibres after saline. Among formalin-activated C-fibres, 5 were CMi, 6 CM, and 3 non-nociceptors. More responded during the early phase (64%) than the late phase (29%), with one CMi fibre active during.
This high-yield recording approach enhances efficiency of peripheral nerve studies and provides new insight into nociceptor contributions during the formalin behavioural phases. Nociceptor activity contributes to early-phase behaviours, and sustained activity may also contribute to late-phase behaviours.