EXPRESSION OF C-FOS PROTEIN IN RAT-BRAIN ELICITED BY ELECTRICAL-STIMULATION OF THE PONTINE PARABRACHIAL NUCLEUS

The expression of Fos, the protein product of the primary response gene c-fos, was used metabolically to map the short-term (1 hr) effects of urethane and sodium pentobarbital anesthesia in rat. Subsequently, urethane-anesthetized rats were used to study the integrated response to electrical stimulation (1-1.5 hr) of the pontine parabrachial nucleus (PBN), an important center for relay of autonomic information in the brain. Immunohistochemistry was used to localize Fos-like immunoreactivity (FLI) in the brain. To approximate amounts of FLI in the conscious animal, rats were killed immediately after attaining surgical anesthesia with sodium pentobarbital (50 mg/kg) or urethane (1.21-1.7 gm/kg). No FLI was found in the brains of these rats. In rats killed 1 hr after anesthesia with sodium pentobarbital, FLI was found only in the habenulae. After 1 hr of urethane anesthesia, low levels of FLI were found in the following areas: nucleus of the tractus solitarius (NTS); caudal and rostral ventrolateral medulla (VLM); lateral PBN; ventromedial, paraventricular, and supraoptic nuclei (SON) of the hypothalamus; medial preoptic area; central nucleus of the amygdala (ACE); endopiriform cortex; insular cortex; piriform cortex; and islands of Calleja. Electrical stimulation of the PBN (10 sec on, 10 sec off; 15-50-mu-A at 20 Hz for 60-90 min) in rats anesthetized with urethane led to increases in mean arterial pressure (10-30 mm Hg) and to ipsilateral increases of FLI in the lateral PBN, dorsal division of SON, ACE, endopiriform nucleus, insular cortex, piriform cortex, and islands of Calleja. In two animals, ipsilateral increases were found in the ventromedial hypothalamus and medial amygdaloid nucleus. Finally, consistent bilateral increases in FLI were found in the NTS, caudal and rostral VLM, and area postrema. These data confirm that sodium pentobarbital has a generally depressive effect on neural activity. Expression of Fos within autonomic centers in rats anesthetized with urethane for 1 hr may be due to the consistent drop in arterial pressure that occurs during anesthesia. Ipsilateral increases of FLI in discrete brain regions of rats in which the PBN was stimulated indicate that alterations are likely due to the electrical stimulation whereas bilateral increases (in NTS and VLM) are likely due to the increases in arterial pressure that accompany PBN stimulation.