Formalin-Induced Paw Edema [105]. This model is based
upon the ability of test drug to inhibit the edema produced
in the hind paw of the mice after injection of formalin. The
nociceptive effect of formalin is biphasic, an early neurogenic
component followed by a later tissue-mediated response.
In the first phase, there is release of histamine, 5-HT, and
kinin, while the second phase is related to the release of
prostaglandins. Procedure. The animals are divided into three
groups (n = 6), and inflammation is produced by subplantar
injection of 20 mL of freshly prepared 2% formalin in
the right hind paw. The paw thickness is measured by
Plethysmometrically 1 h before and after formalin injection.
The drug treatment is continued for 6 consecutive days.
The increase in paw thickness and percentage inhibition are
calculated and compared with control group.