CO2-INDUCED C-FOS EXPRESSION IN THE CNS CATECHOLAMINERGIC NEURONS

In these studies we examined c-fos expression in catecholaminergic neu rons following exposure of unanesthetized rats to hypercapnic stress. Breathing a gas mixture with elevated CO2 (15% CO2, 21% O-2 and 64% N- 2, or 15% CO2 balance O-2) for 60 min, induced activation of the c-fos gene in widespread regions of the CNS, as indicated by the expression of Fos-like immunoreactive protein (Fos). Similar results were obtain ed in carotid body denervated animals. Colocalization studies of tyros ine hydroxylase (TH) and Fos protein revealed that in the brainstem, 7 3 to 85% of noradrenaline-containing cells expressed Fos immunoreactiv ity. Double-labeled neurons were found in the ventrolateral medullary reticular formation (A1 noradrenaline cells), in the dorsal aspect of medulla oblongata (A2 noradrenaline cells), in the ventrolateral pens (A5 noradrenaline cells), and in the locus coeruleus (A6 noradrenaline cells). However, over 90% of TH-immunoreactive neurons in the mesence phalon and diencephalon (dopaminergic cells) did not express Fos-like immunoreactivity in response to CO2. These results indicate that the b rainstem noradrenaline-containing neurons are part of the neuronal net works that react to hypercapnic exposure.