Surgical incision can alter capsaicin-induced central sensitization in rat brainstem nociceptive neurons

Surgical trauma can affect spinal neuronal excitability, but there have been no studies of the effects of surgical cutaneous injury on central nociceptive processing of deep afferent inputs evoked by noxious stimuli such as capsaicin. Thus our aim was to test the effect of surgical cutaneous incision in influencing central sensitization induced by capsaicin injection into the temporomandibular joint (TMJ). The activity of single nociceptive neurons activated by noxious mechanical stimulation of the TMJ was recorded in the trigeminal subnucleus caudalis/upper cervical cord of halothane-anesthetized rats. The cutaneous mechanoreceptive field (RF), cutaneous mechanical activation threshold (MAT) and TMJ MAT of neurons before and after both surgical cutaneous incision alone and capsaicin injection were compared with results of incision and lidocaine pretreatment of the facial skin overlying the TMJ and capsaicin injection into the TMJ. Incision itself induced a barrage of neuronal spikes and excitability increases reflecting central sensitization (cutaneous RF expansion, cutaneous MAT reduction) in most neurons tested whereas lidocaine pretreatment significantly attenuated the barrage and central sensitization. Capsaicin injection into the TMJ induced cutaneous RF expansion, cutaneous MAT reduction and TMJ MAT reduction following lidocaine pretreatment of the cutaneous incision site whereas capsaicin injection following incision alone not only failed to induce further central sensitization but also decreased the existing incision-induced central sensitization (no cutaneous RF expansion, increased cutaneous MAT and TMJ MAT) in most neurons tested. These findings suggest that central sensitization induced by capsaicin alone or by cutaneous incision alone can readily occur in TMJ-responsive nociceptive neurons and that following incision-induced excitability increases, capsaicin may result in a temporary suppression of nociceptive neuronal changes reflecting central sensitization.