Enhanced nociceptive behaviour following conditioning injection of formalin in the perioral area of the rat.
The possible existence of long-term modifications in response to a transient nociceptive conditioning stimulation was investigated in the rat in three experiments. (1) A nociceptive conditioning stimulus was delivered in the form of a s.c. formalin injection (conditioning injection) in the left upper lip. Evaluation of the nociceptive behaviour triggered by another formalin injection (testing injection) made in the controlateral right upper lip was carried out in distinct groups of rats 7, 14 or 28 days after the conditioning. An enhanced nociceptive response at day 7 and 14 and a return to the baseline at day 28 were observed. (2) A similar protocol was developed with formalin used for both conditioning and testing but an anaesthetic blockade of the infraorbital nerve was performed just before the conditioning injection to suppress the initial barrage. The change observed at day 7 was suppressed by the nerve block. (3) A conditioning nociceptive stimulus was applied either ipsilaterally to the right lower lip or to the tail. An increased nociceptive response was observed when the conditioning stimulus was applied to the same side as the test stimulus but no increase in the formalin test response was detected when the conditioning stimulus was applied to the tail. These results indicated that, after a single formalin injection in the left upper lip, a hyperexcitability developed that depended on the initial barrage, lasted for at least 2 weeks, was no longer present at 4 weeks and might rely on a segmental mechanism. The hypothesis of a central sensitization triggered by an initial barrage and maintained by an ongoing input induced from the periphery is discussed.
Cadet
R
R
Laboratoire de Physiologie Orofaciale, Faculté Chirurgie Dentaire, Clermont-Ferrand, France.
Aigouy
L
L
Woda
A
A
D000818:Animals; D002045:Bupivacaine / Q000494:pharmacology; D003213:Conditioning, Psychological / Q000187:drug effects* / Q000502:physiology; D005557:Formaldehyde / Q000008:administration & dosage / Q000494:pharmacology*; D007839:Functional Laterality / Q000502:physiology; D006930:Hyperalgesia / Q000139:chemically induced; D007279:Injections, Subcutaneous; D008046:Lip / Q000187:drug effects* / Q000502:physiology; D008297:Male; D009407:Nerve Block; D009473:Neuronal Plasticity / Q000187:drug effects / Q000502:physiology; D009619:Nociceptors / Q000187:drug effects* / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D013997:Time Factors
D005557:Formaldehyde; D002045:Bupivacaine