An ultrastructural and biochemical analysis of norepinephrine-containing varicosities in the cerebral cortex of the turtlepseudemys

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Abstract
The fine structure and norepinephrine content of small granular vesicle‐containing profiles were studied in normal and norepinephrine‐depleted cerebral cortex of the turtle, Pseudemys. The cortex was fixed for electron microscopy with the KMnO4 procedure of Koda and Bloom ('77), while the norepinephrine content was assayed with the radioenzymatic method of Coyle and Henry ('73). Green fluorescent fibers have been described by Parent and Poitras ('74) as located almost exclusively in the outer half of the molecular layer in turtle cortex. Small granular vesicle‐containing profiles are found down to 100 μm below the pial surface, but over 50% lie within 20 μm of the surface. Within the outer 100 μm of cortex, the frequency of labeled varicosities is 1.39/1,000 μm2. The average area of the norepinephrine‐containing varicosities is 0.61 μm2, and there is a mean of 18.4 vesicles per single section. The average number of large plus small vesicles in an entire varicosity was estimated to be 72. Synaptic membranes are not well‐preserved with KMnO4 fixation, but good examples were found of small granular vesicle‐containing profiles forming both symmetrical and asymmetrical membrane differentations. Only a small percentage of the small granular vesicle profiles were associated with a synaptic membrane differentiation in single sections. When norepinephrine‐fiber synapses are seen, they usually share a postsynaptic element with another unlabeled vesicle‐containing profile. Normal turtle cortex contains an average norepinephrine concentration of 1.95 μg/gr, which is about eight times higher than in rat cortex. The ratio of norepinephrine to dopamine is about 18 to one, suggesting that dopamine is present predominantly in a precursor pool for norepinephrine. Small granular vesicle‐containing profiles were eliminated after tratment with reserpine and 6‐hydroxydopamine in concentrations that were shown to reducenorepinephrine concentration by 94% and 86%, respectively. The labeled varicosities were partially depleted by midbrain hemisection and by an inhibitor of dopamine‐beta‐hydroxylase (FLA‐63). The norepinephrine‐containing varicosities are remarkably coextensive with the distribution of thalamic fibers, both in the total extent of cortex where they are found and in the depth of cortex where they terminate. The results support the idea that there is a close structural and functional association between locus coeruleus and thalamic fibers in cerebral cortex, and the apparent difference in frequency of synapses suggests that each fiber system exerts its influence on cortical cells in a different way.