Formalin induces biphasic activity in C-fibers in the rat

Neurosci Lett. 1996 Apr 12;208(1):45-8. doi: 10.1016/0304-3940(96)12552-0.


While the formalin test is a widely used behavioral model of tonic chemogenic pain, little is known about the responses of primary afferent nociceptors to formalin. Formalin (2.5%, 50 microliters) was injected either directly in or adjacent to the mechanical receptive fields of single C-fibers isolated from the saphenous nerve of pentobarbital-anesthetized rats. The average formalin-evoked response in C-fibers (n = 29) over time was biphasic. This biphasic time course of the C-fiber response to formalin is similar to that of the behavioral response in the awake animal and is compatible with the hypothesis that increased C-fiber activity contributes to the behavioral response in phase 2, as well as in phase 1 of the formalin test.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Action Potentials / drug effects
  • Animals
  • Electrophysiology
  • Formaldehyde / pharmacology*
  • Male
  • Nerve Fibers / drug effects
  • Nerve Fibers / physiology*
  • Neural Conduction / drug effects
  • Neural Conduction / physiology
  • Neurons, Afferent / drug effects
  • Neurons, Afferent / physiology
  • Nociceptors / drug effects*
  • Nociceptors / physiology
  • Rats
  • Rats, Sprague-Dawley


  • Formaldehyde