This study investigated the role of
gamma-aminobutyric acid (
GABA) and
GABA(A) receptors in the spinal cord in the expression of
pain behaviors evoked by injection of
formalin in concentrations ranging from 0.25 to 2.5% in the hindpaw of the rat. Two approaches were used. The first approach compared the effect of
drug treatment to saline at each concentration of
formalin. The second approach examined the effect of
drug treatment on the concentration-response functions of
formalin, i.e., its EC50. Intrathecal (i.t.) pretreatment with 0.03 to 0.3 microg of
bicuculline, a
GABA(A) receptor antagonist, dose-dependently increased the number of flinches and weighted
pain scores in the interphase and phase 2, but did not alter responses in phase 1. In the interphase, the EC50 values of
formalin for number of flinches or weighted
pain score in
bicuculline-pretreated rats were decreased to one-third or one-fourth, respectively, of their values in saline-pretreated rats. In phase 2, the EC50 values of
formalin for number of flinches or weighted
pain score in
bicuculline-pretreated rats were similarly decreased to one-half of their value in saline-pretreated rats. These results suggest that
formalin was a significantly more noxious stimulus in the presence of
bicuculline. Pretreatment with the
GABA(A) receptor agonists,
muscimol (0.3 microg) or
isoguvacine (10 or 30 microg i.t.), significantly decreased the number of flinches in phase 1 and phase 2, but produced only a marginal decrease in the weighted
pain score at the highest doses. These findings suggest that there is little tonic activation of
GABA(A) receptors by
GABA in the spinal cord before or immediately after the injection of
formalin. However, approximately 10 min after the induction of injury by
formalin, there is a release of
GABA and activation of
GABA(A) receptors in the spinal cord that 1) contributes to the period of quiescence between phase 1 and phase 2 and 2) coincidentally diminishes the magnitude of
pain behaviors in phase 2, possibly by limiting the development of central sensitization in the spinal cord.