Serotonin nerve terminals in the locus coeruleus of adult rat: a radioautographic study.

Serotonin (5-HT) nerve terminals in the locus coeruleus (LC) of adult rat were visualized by high-resolution radioautography, in order to examine their distribution, fine structural features and intimate relationships with norepinephrine neurons. In animals pretreated with a monoamine oxidase inhibitor, prolonged intraventricular perfusion of 10(-4) M [3H]5-HT resulted in a specific identification of most if not all 5-HT axonal varicosities in LC. These terminals were equally distributed between the dorsal and ventral divisions of the nucleus. Their density was approximated at 10(7) per cu.mm within the middle third of the LC. In electron microscope radioautographs, the labeled 5-HT varicosities averaged 0.9 micron in diameter. They all exhibited a distinctive storage organelle, in the form of microvesicles and microcanaliculi (15-25 nm in diameter) partly filled with electron-dense material and usually aggregated in association with several large dense-core vesicles. While this finding of intrinsic morphological characteristics appeared compatible with a special cellular origin or regional differentiation, it was also suggestive of particular functional properties and/or mode of action. In a sample comprised of some 500 sectional profiles from labeled 5-HT varicosities in LC, a small proportion only (less than 10%) exhibited morphologically defined synaptic junctions. These rare contacts were invariably made with dendritic processes and never observed on the noradrenergic perikarya. It is therefore concluded that, in the LC, non-synaptic as well as synaptic mechanisms might be involved in the modulation and transneuronal regulation of norepinephrine neurons by 5-HT afferents.