Intrinsic choroidal neurons in the duck eye receive sympathetic input: anatomical evidence for adrenergic modulation of nitrergic functions in the choroid

Intrinsic choroidal neurons (ICN) in the duck eye form an intramural gangli onic plexus that may subserve complex integrative functions. A key feature of such ganglia is an innervation by sympathetic postganglionic neurons. Th e present study was thus aimed at determining the sympathetic postganglioni c innervation of ICN. Choroids were processed for double immunofluorescence labelling with the following markers: tyrosine-hydroxylase (TH)/nitric oxi de synthase (nNOS), TH/galanin (GAL), dopamine-beta -hydroxylase (DBH)/vaso active intestinal polypeptide (VIP), TH/DBH and DBH/alpha -smooth-muscle ac tin (alpha SMA), and for triple immunofluorescence labelling with VIP/DBH/T H. Epifluorescence and confocal laser scanning microscopy were used for eva luation. Immunoperoxidase staining for TH or DBH in combination with NADPH- diaphorase histochemistry was applied for electron microscopy. ICN spread o ver the entire choroid but were concentrated in an equatorial zone passing obliquely from naso-cranial to temporocaudal. More than 80% of nNOS-positiv e ICN showed close appositions of TH/DBH-immunoreactive varicose nerve fibr es at the light-microscopic level, as could be confirmed by confocal laser scanning microscopy. Ultrastructurally, these appositions could be defined as both synapses or close contacts without synaptic specialisation. Vascula r and non-vascular smooth muscle fibres also received TH/DBH-immunopositive innervation. Our findings suggest that most ICN receive a sympathetic inpu t that might modulate their nitrergic effects upon vascular and non-vascula r smooth muscle fibres in the chorold and that they may have more complex f unctions than merely being a simple parasympathetic relay.