Background: The goal was to assess changes in natural killer (NK) cellactivity and the number of c-Fos-positive cells in hypothalamic structures induced by painful electricalstimulation and to use extremely high-frequency (EHF) irradiation of the skin to modulate these processes.Material/Methods: Experiments were performed on Wistar rats subjected to painful electrical stimulationof the hind limbs combined with EHF irradiation of the skin. The cytotoxic activity of splenic NK cellswas assessed by their ability to lyse K-562 tumor cells in vitro. c-Fos-like protein was detected byan immunoperoxidase technique. Results: Painful electric stimulation was associated with a significantdecrease in splenic NK cytotoxicity and a dramatic increase in c-Fos-positive cell counts in some hypothalamicstructures, particularly in the anterior hypothalamic nucleus (AHN) and the perifornical lateral hypothalamicarea (LHA). Two EHF exposures, one before and one after electric stimulation, prevented the suppressionof splenic NK cell activity and caused a decrease in the number of c-Fos-positive cells expressed inthe ventromedial hypothalamic nucleus (VMH) and basal LHA. Negative correlation was shown between c-Fos-positivecell counts in the AHN and LHA and the cytotoxic activity of NK cells. Conclusions: These results suggestthat painful electric stimulation of the hind limbs of rats causes a reorganization of the central mechanismsthat regulate splenic NK cell activity, resulting in a decrease in their cytotoxicity, and that EHF irradiationof the skin prevents this reorganization, thus protecting splenic NK cell activity from the impairmentinduced by this stressor.

Keywords: Cytotoxicity, Immunologic, Electric Stimulation, Hindlimb, Hypothalamus - metabolism, Killer Cells, Natural - immunology, Pain - metabolism, Proto-Oncogene Proteins c-fos - biosynthesis, Skin - radiation effects