Immunolocalization of neurotransmitter-synthesizing enzymes and neuropeptides with associated receptors in the photophores of the hatchetfish, Argyropelecus hemigymnus Cocco, 1829.

Anatomical and functional studies of the autonomic innervation of the photophores of luminescent
fishes are scarce. The present immunohistochemical study demonstrated the presence of nerve fibers
in the luminous epithelium and lens epithelium of the photophores of the hatchet fish, Argyropelecus
hemigymnus and identified the immunoreactive elements of this innervation. Phenylethanolanine
N-methyltransferase (PNMT) and catecholamine (CA)-synthesizing enzymes were detected in nerve varicosities
inside the two epithelia. Neuropeptides were localized in neuropeptide Y (NPY) and substance
P (SP)- and its NK11 receptor-immunopositive nerves in the lens epithelium. Neuropeptides were also
localized in non-neural cell types such as the lens cells, which displayed immunoreactivities for pituitary
adenylate cyclase activating peptide (PACAP) and their receptors R-12 and 93093-3. This reflects the
ability of the neuropeptide-containing nerves and lens cells to turn on and off the expression of selected
messengers. It appears that the neuropeptide-containing nerves demonstrated in this study may be sensory.
Furthermore, neuronal nitric oxide synthase-immunopositive axons associated with photocytes
in the luminous epithelium have previously been described in this species. Whereas it is clear that the
photophores receive efferent (motor) fibers of spinal sympathetic origin, the origin of the neuropeptide
sensory innervation remains to be determined. The functional roles of the above neuropeptides or their
effects on the bioluminescence or the chemical nature of the terminals, either sensory or postganglionic
neurons innervating the photophores, are still not known.