title: The effects of a co-application of menthol and capsaicin on nociceptive behaviors of the rat on the operant orofacial pain assessment device.
abstract: BACKGROUND
  Transient receptor potential (TRP) cation channels are involved in the perception of hot and cold pain and are targets for pain relief in humans. We hypothesized that agonists of TRPV1 and TRPM8/TRPA1, capsaicin and menthol, would alter nociceptive behaviors in the rat, but their opposite effects on temperature detection would attenuate one another if combined.
  
  
  METHODS
  Rats were tested on the Orofacial Pain Assessment Device (OPAD, Stoelting Co.) at three temperatures within a 17 min behavioral session (33°C, 21°C, 45°C).
  
  
  RESULTS
  The lick/face ratio (L/F: reward licking events divided by the number of stimulus contacts. Each time there is a licking event a contact is being made.) is a measure of nociception on the OPAD and this was equally reduced at 45°C and 21°C suggesting they are both nociceptive and/or aversive to rats. However, rats consumed (licks) equal amounts at 33°C and 21°C but less at 45°C suggesting that heat is more nociceptive than cold at these temperatures in the orofacial pain model. When menthol and capsaicin were applied alone they both induced nociceptive behaviors like lower L/F ratios and licks. When applied together though, the licks at 21°C were equal to those at 33°C and both were significantly higher than at 45°C.
  
  
  CONCLUSIONS
  This suggests that the cool temperature is less nociceptive when TRPM8/TRPA1 and TRPV1 are co-activated. These results suggest that co-activation of TRP channels can reduce certain nociceptive behaviors. These data demonstrate that the motivational aspects of nociception can be influenced selectively by TRP channel modulation and that certain aspects of pain can be dissociated and therefore targeted selectively in the clinic.
authors.affiliation: Department of Oral and Maxillofacial Surgery, University of Florida College of Dentistry, Gainesville, Florida, United States of America ; Department of Neuroscience, University of Florida College of Medicine, McKnight Brain Institute, Gainesville, Florida, United States of America.
authors.forename: Ethan M
authors.initials: EM
authors.lastname: Anderson
authors.affiliation: Department of Orthodontics, University of Florida, Gainesville, Florida, United States of America.
authors.forename: Alan C
authors.initials: AC
authors.lastname: Jenkins
authors.affiliation: Department of Oral and Maxillofacial Surgery, University of Florida College of Dentistry, Gainesville, Florida, United States of America ; Department of Neuroscience, University of Florida College of Medicine, McKnight Brain Institute, Gainesville, Florida, United States of America.
authors.forename: Robert M
authors.initials: RM
authors.lastname: Caudle
authors.affiliation: Department of Orthodontics, University of Florida, Gainesville, Florida, United States of America.
authors.forename: John K
authors.initials: JK
authors.lastname: Neubert
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D002211:Capsaicin / Q000494:pharmacology*; D003216:Conditioning, Operant; D005157:Facial Pain / Q000188:drug therapy / Q000503:physiopathology*; D008297:Male; D008610:Menthol / Q000494:pharmacology*; D059225:Nociception / Q000187:drug effects* / Q000502:physiology*; D010147:Pain Measurement / Q000379:methods; D051381:Rats; D017207:Rats, Sprague-Dawley; D050053:TRPM Cation Channels / Q000819:agonists; D050916:TRPV Cation Channels / Q000819:agonists; D013696:Temperature
chemical_list: D050053:TRPM Cation Channels; D050916:TRPV Cation Channels; C498947:Trpm8 protein, rat; C493922:Trpv1 protein, rat; D008610:Menthol; D002211:Capsaicin