FOS EXPRESSION INDUCED BY CHANGES IN ARTERIAL-PRESSURE IS LOCALIZED IN DISTINCT, LONGITUDINALLY ORGANIZED COLUMNS OF NEURONS IN THE RAT MIDBRAIN PERIAQUEDUCTAL GRAY

The distribution of neurons expressing Fos within the periaqueductal g ray (FAG) following pharmacologically induced high or low blood pressu re was examined to determine (1) if FAG neurons are responsive to chan ges in arterial pressure (AP) and (2) the relationship of these cells to the functionally defined hypertensive and hypotensive columns in FA G. Changes in AP differentially induced robust Fos expression in neuro ns confined to discrete, longitudinally organized columns within FAG. Increased AP produced extensive Fos-like immunoreactivity within the l ateral FAG, beginning at the level of the oculomotor nucleus. At the l evel of the dorsal raphe, Fos expression induced by increased AP shift ed dorsally, into the dorsolateral division of FAG; this pattern of Fo s labeling was maintained throughout the caudal one-third of FAG. Doub le-labeling for Fos and nicotinamide adenine dinucleotide phosphate di aphorase confirmed that Fos-positive cells induced by increased AP wer e located in the dorsolateral division of FAG at these caudal levels. Fos positive cells were codistributed, but not colocalized, with nicot inamide adenine dinucleotide phosphate diaphorase-positive cells. Decr eased AP evoked a completely different pattern of Fos expression. Fos- positive cells were predominantly located within the ventrolateral FAG region, extending from the level of the trochlear nucleus through the level of the caudal dorsal raphe. Double-labeling studies for Fos and serotonin indicated that only 1-2 double-labeled cells per section we re present. Saline infusion resulted in very few Foslike immunoreactiv e cells, indicating that volume receptor activation does not account f or Fos expression in FAG evoked by changes in AP. These results indica te that (1) substantial numbers of FAG neurons are excited by pharmaco logically induced changes in AP and (2) excitatory barosensitive FAG n eurons are anatomically segregated based on their responsiveness to a specific directional change in AP. (C) 1995 Wiley-Liss, Inc.