Ultrastructural correlates of luminescence in Porichthys photophores. I. Effects of spinal cord stimulation and exogenous noradrenaline.

Light emission induced by electrical stimulation of the spinal cord is associated with ultrastructural alterations of the varicose endings, photocyte organelles and membranes in Porichthys photophores. Changes in nerve profiles appear first and include alterations of the shape, number and distribution of synaptic vesicles, as well as invaginations of the axolemma and mitochondrial swellings. Protocyte vesicles become markedly coalesced and their membranes are incorporated inside vesicular pools, whereas photocyte microvilli become sharply reduced at the cell periphery. Luminescence obtained by administration of noradrenaline is accompanied by similar, but more acute changes in the ultrastructure of photocytes, whereas no marked alteration of nerve profiles is noted after this treatment. These and other observations, such as the presence of specialized synaptic contacts, are discussed in terms of neural activity and processes within the photocytes which may lead to light production.