Distribution of [3H]noradrenaline and [3H]serotonin in photophores of porichthys notatus. An electron-microscopic radioautographic analysis.

Photophores of Porichthys notatus were examined by electron-microscopic radioautography following incubation in tritiated noradrenaline ( [3H]NA) or serotonin ( [3H]5-HT). Nerve varicosities surrounding the photocytes were found to accumulate [3H]NA but not [3H]5-HT, providing compelling evidence for the catecholaminergic nature of the monoaminergic innervation of photophores. The photocytes themselves appeared selectively labelled with both tracers, but the intensity of labelling after [3H]5-HT incubation was considerably greater than after [3H]NA. Stereological sampling of organelle content in photocytes showed ultrastructural differences between [3H]NA- and [3H]5-HT-labelled cells, probably related to light emission induced by NA. The main changes noted after incubation with [3H]NA were mitochondrial swelling and disorganization, increased coalescence of photocytic vesicles and extrusion of vesicular material into the extracellular matrix. With respect to the subcellular localization of [3H]NA and [3H]5-HT within the photocytes, statistical analysis of the distribution of silver grains disclosed a preferential affinity of both labels for appositional zones between mitochondria and coalescent vesicles. Moreover, in the case of 5-HT, selective affinity was also exhibited by sites comprising vesicular membrane and adjacent cytoplasm, suggesting binding of this biogenic amine to the entire membrane of photocytic vesicles.