The formalin test: effects of formalin concentration and short-term halothane inhalation.
OBJECTIVE
In order to evaluate the effects of formalin concentration and inhalational anesthetics in formalin tests, we injected 5 concentrations of formalin into awake and anesthetized rats and investigated their behavior and the c-fos immunoreactivity of their spinal cords.


METHODS
Sixty adult male Sprague-Dawley rats were divided into 10 experimental groups. Each rat was either manually restrained or subdued through several minutes of 2% to 2.5% halothane inhalation, and then 1 of 5 concentrations of 0%, 5%, 10%, 27%, or 100% formalin (0%, 1.85%, 3.7%, 10%, or 37% formaldehyde solutions) was injected into its rear paw. Nociceptive behaviors were checked for 1 hour, 6 times for each 5-minute period. Two hours after the formalin injections, the rats were killed and c-fos immunoreactivity was measured.


RESULTS
Typical responses were observed in the 5% and 10% formalin (1.85% and 3.7% formaldehyde) groups, and nociceptive behaviors were lower in the 27% and 100% formalin (10% and 37% formaldehyde) groups. The number of c-fos-positive cells increased as the formalin concentration increased. Halothane inhalation affected the results of both the behavior and the c-fos immunoreactivity, especially in the 10% formalin (3.7% formaldehyde) group.


CONCLUSIONS
It is desirable to describe both formalin and formaldehyde concentrations simultaneously and to do formalin tests without inhalational anesthetics.
Fukuda
T
T
Department of Anesthesiology, Institute of Clinical Medicine, Tsukuba University, Tsukuba City, Ibaraki, Japan. taekof@md.tsukuba.ac.jp
Nishimoto
C
C
Shiga
Y
Y
Toyooka
H
H
D000280:Administration, Inhalation; D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D005557:Formaldehyde / Q000494:pharmacology*; D006221:Halothane / Q000494:pharmacology*; D008297:Male; D010146:Pain / Q000139:chemically induced* / Q000378:metabolism; D016760:Proto-Oncogene Proteins c-fos / Q000032:analysis / Q000276:immunology; D051381:Rats; D017207:Rats, Sprague-Dawley
D018685:Anesthetics, Inhalation; D016760:Proto-Oncogene Proteins c-fos; D005557:Formaldehyde; D006221:Halothane