Intrinsic neurons in the duck choroid are contacted by CGRP-immunoreactivenerve fibres: Evidence for a local pre-central reflex arc in the eye

Intrinsic choroidal neurons represent peripherally displaced autonomic nerv e cells supposed to work as a local integrative network similar to the ente ric nervous system, to control choroidal vasculature and stromal smooth mus cle. A typical feature of such intramural neuronal networks is the innervat ion by primary afferent collaterals expressing peptides, e.g, CGRP. The pre sent study was aimed at determining primary afferent contacts on nitrergic intrinsic choroidal neurons (ICN) in the duck eye. In addition, a sympathet ic innervation of ICN was assessed. Choroids were immunohistochemically pro cessed for the following markers: neuronal nitric oxide synthase (nNOS), ga lanin (GAL), calcitonin gene-related peptide (CGRP), and tyrosine hydroxyla se (TH). For evaluation, fluorescence as well as confocal laser scanning mi croscopy were used. For electron microscopy, immunoperoxidase staining for CGRP in combination with NADPH-diaphorase histochemistry was applied. ICN i mmunoreactive for nNOS or GAL spread over the entire choroid, although they were concentrated in an equatorial zone passing obliquely from naso-crania l to temporo-caudal, About 40 % of ICN showed close relationships with CGRP -immunoreactive nerve fibres, originating most likely in the trigeminal gan glion, as seen in the fluorescence and confocal laserscanning microscope. T hese appositions could be ultrastructurally defined as both synapses and cl ose contacts without synaptic specialization. Some ICN endowed with CGRP-po sitive fibres also received TH-immunoreactive boutons. CGRP-immunoreactive profiles were also detected in close relationship to choroidal non-vascular smooth muscle cells and collagen fibres connected to them. In many instanc es, they were intercalated between smooth muscle cells and processes of ICN forming triads. These results suggest that ICN, similar to other intramura l autonomic systems integrate signals from trigeminal primary afferent coll aterals. The 'sensory' terminals of these primary afferents may be located in the anterior eye segment but also within the smooth muscle stroma of the choroid itself. Thus, ocular homeostasis may be regulated via intraocular pre-central reflexes which are probably subject to sympathetic modulation. (C) 2001 Academic Press.