NOXIOUS HEAT-EVOKED FOS-LIKE IMMUNOREACTIVITY IN THE RAT MEDULLA, WITH EMPHASIS ON THE CATECHOLAMINE CELL GROUPS

The objectives of the present study were 1) to utilize Fos immunohisto chemistry as a marker for neuronal activity in order to examine the po pulation of neurons in the medulla that is engaged by activation of no ciceptive peripheral afferents and 2) to determine whether catecholami ne-containing neurons in the medulla also express noxious heat-evoked Fos-like immunoreactivity. Noxious heating of the hindpaw evoked speci fic patterns of Fos-like immunoreactivity in the medulla in regions kn own to be involved in both nociceptive processing and cardiovascular r egulation. Noxious heating of the hindpaw significantly increased the mean number of neurons expressing Fos-like immunoreactivity in the con tralateral ventrolateral medulla. Increased numbers of Fos-positive ne urons also were observed in both the ipsilateral and the contralateral Al catecholamine cell groups. Similarly, in the contralateral medulla ry dorsal reticular fields, noxious heating of the hindpaw significant ly increased the mean number of neurons expressing Fos-like immunoreac tivity. In contrast, in the paramedian reticular nucleus, noxious heat ing of the hindpaw resulted in a significant decrease in the mean numb er of neurons expressing Fos-like immunoreactivity. No significant dif ferences in the mean numbers of neurons expressing Fos-like immunoreac tivity were noted in the A2, C1, or C2/C3 medullary catecholamine cell groups. These results suggest that noxious stimuli affect pools of ne urons in the medulla with multiple physiological functions. (C) 1995 W iley-Liss, Inc.