Spike generation from dorsal roots and cutaneous afferents by hypoxia or hypercapnia in the rat in vivo

The present study aimed at investigating the responsiveness of different pa rts of the primary afferent neurones to a brief hypoxia, hypercapnia or isc haemia under in vivo conditions. Action potentials were recorded in separat e groups of anaesthetized rats from (i) the peripheral end of the central s tump of the cut L3, L4 or L5 dorsal root (dorsal root preparation); (ii) th e central end of the peripheral stump of the cut saphenous nerve (saphenous -receptor preparation); (iii) the distal end of a segment of the saphenous nerve cut at both ends (axon preparation). In paralysed animals interruptio n of artificial ventilation for 20-60 s elicited or increased the frequency of action potentials in both the dorsal root and saphenous-receptor prepar ations. Activation of these preparations was also achieved by inspiration o f gas mixtures containing 10-0% oxygen (mixed with nitrogen) or 20-50% carb on dioxide (mixed with oxygen) which elicited in the blood a decrease in P- O2 or an increase in P-CO2 with a fall in pH. Occlusion of the femoral arte ry for 3 min also caused spike generation in the saphenous-receptor prepara tions with little alteration in blood pressure. All these stimuli failed to evoke action potentials in the axon preparations. Systemic (300 mg kg(-1) s.c.) or perineural (2%) capsaicin pretreatment failed to inhibit the effec t of hypoxia, hypercapnia or ischaemia, indicating a significant contributi on of capsaicin-insensitive neurones to the responses. It is concluded that central and peripheral terminals but not axons of primary afferent neurone s are excited by a brief hypoxia or hypercapnia and the peripheral terminal s by a short local ischaemia as well. Excitation of central terminals by hy poxia or hypercapnia revealed in this way an antidromic activation of dorsa l roots in response to natural chemical stimuli.