Immediate early genes are crucial intermediates in a cascade linking membra
ne stimulation to long-term alterations of neuronal activity. In the presen
t experiment, we performed immunohistochemistry for c-Fos to determine the
effects of persistent pain on cells of the hippocampus of male and female r
ats. Animals were subcutaneously injected with formalin (50 mu l, 10%) and
perfused: 2 h later, time 2; 24 h later, time 24; 24 h later after 20 min o
f the open-field test, time 24/OF. Controls were left undisturbed. In contr
ol, c-Fos was higher in females than in males in all hippocampal fields. In
males at time 2, formalin increased c-Fos in the dentate gyrus (DG) and CA
3 fields; at time 24, c-Fos returned to the control level; at time 24/OF, c
-Fos was higher than in control in the DG, but not in the other fields. In
the formalin-treated females at time 2 and at time 24, c-Fos levels were lo
wer, or tended to be lower, than in control in all hippocampal fields; at t
ime 24/OF, c-Fos levels in the DG were higher than in control and in males.
In conclusion, persistent pain had different effects on c-Fos in the hippo
campal subfields, depending on the time after treatment and the sex of the
subject. (C) 1999 Elsevier Science Inc.