Stimulation of dorsal root afferents increases the excitability of ascending sensory axons in the isolated spinal cord of mature mice

The phenomenon of windup has often been used to assess excitability increases of spinal neurons induced by repetitive stimulation of nociceptive afferents. Windup has been studied in individual spinal cord neurons and in spinal motor reflexes neither of which accurately reflect the forward transmission of nociceptive signals to the brain. In addition, most in vitro studies of spinal windup have been conducted on immature or juvenile animals and it is challenging to extrapolate these results to the adult spinal cord. In the present study, we have used an in vitro whole spinal cord preparation from functionally mature mice (up to 8 weeks old) to record windup activity in ascending axons in the mid-thoracic region evoked by electrical stimulation of a lumbar or sacral dorsal root. Windup responses were observed in axons in the ipsi- and contralateral dorsolateral funiculus (iDLF and cDLF) and in the contralateral ventrolateral funiculus (cVLF). No windup responses were evoked in postsynaptic axons of the ipsilateral dorsal columns (iDC) and no postsynaptic responses were elicited in the ipsilateral ventrolateral funiculus (iVLF) or contralateral dorsal columns (cDC). Between 40% and 45% of all axons in the DLF and cVLF that responded to a single dorsal root stimulus also showed windup. The NMDA receptor antagonist MK-801 reversibly blocked such windup responses. These results illustrate that windup can be consistently recorded from ascending pathways in the mature spinal cord in vitro but also show that windup can only be elicited in a proportion of sensory axons projecting through some, but not all, ascending spinal cord pathways.

Research Highlights
► Wind-up was recorded from ascending pathways in the mature spinal cord in vitro.
► Wind-up was recorded in sensory axons of the dorsolateral and ventrolateral funiculi.
► No wind-up responses were evoked in axons of the ipsilateral dorsal columns.
► 45% of axons that responded to a single dorsal root stimulus also showed wind-up.
► The NMDA receptor antagonist MK-801 reversibly blocked wind-up responses.