Expression of the c-fos-monomeric red fluorescent protein 1 fusion gene in the spinal cord and the hypothalamic paraventricular nucleus in transgenic rats after nociceptive stimulation.

We generated transgenic rats expressing the c-fos and monomeric red fluorescent protein 1 (mRFP1) fusion gene in the central nervous system after adequate stimulation. In the present study, the time-course of the induction patterns of mRFP1 fluorescence in the spinal cord and the paraventricular nucleus (PVN) was compared with that of Fos-like immunoreactivity (LI) within 24 h after subcutaneous (s.c.) injection of 0.9% saline and 5% formalin in both hind paws. Control rats were not treated. In the control and saline/formalin injected rats, scattered mRFP1 fluorescence in the spinal cord and the PVN was observed at 0 min, though there was little Fos-LI in the same region. The mRFP1 fluorescence in the spinal cord and the PVN was increased at 3h after formalin. On the other hand, the changes of Fos-LI in the spinal cord and the PVN were relatively shorter than those of the mRFP1 fluorescence after formalin. These results suggest that the c-fos-mRFP1 fusion gene expression is slightly upregulated in normal conditions and nociceptive stimulation-induced induction of the fusion gene may be maintained longer than the endogenous c-fos gene expression in the spinal cord and the PVN. Next, nocifensive behavior and mRFP1 fluorescence and Fos-LI in the spinal cord and the PVN after s.c. injection of formalin, 4α-phorbol 12,13-didecanoate (4α-PDD) and saline were compared. Although the 4α-PDD injected rats seldom displayed nocifensive behaviors like s.c. saline injection, 4α-PDD injection caused mRFP1 fluorescence and Fos-LI significantly in the spinal cord and the PVN unlike s.c. saline injection.