To test the hypothesis that all locus coeruleus projections are simultaneously activated when the locus coeruleus cells fire, the norepinephrine metabolite 3-methoxy-4-hydroxyphenethyleneglycol was assayed in four regions of the central nervous system innervated by the locus coeruleus after three treatments designed to increase locus coeruleus firing in rats. Electrical stimulation of the locus coeruleus, intraperitoneal piperoxan treatment, and electric footschock all significantly increased MHPG levels in rat cerebral cortex, cerebellum, hippocampus, and spinal cord. The magnitude of MHPG increase was greater after locus coeruleus stimulation than after footshock or piperoxan. No significant differences between increases in the above brain regions were found within each treatment group.
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