Sensory and autonomic innervation of the rat eyelid: Neuronal origins and peptide phenotypes

Abstract

Neuronal origins, peptide phenotypes and target distributions were determined for sensory and autonomic nerves projecting to the eyelid. The retrograde tracer, Fluoro-Ruby, was injected into the superior tarsal muscle and meibomian gland of Sprague-Dawley rats. Labelled neurons were observed within the pterygopalatine (31 ± 6 of a total of 8238 ± 1610 ganglion neurons), trigeminal (173 ± 43 of 62 082 ± 5869) and superior cervical ganglia (184 ± 35 of 21 900 ± 1741). Immunostaining revealed vasoactive intestinal polypeptide immunoreactivity (VIP-ir) in nearly all Fluoro-Ruby-labelled pterygopalatine ganglion neurons (86 ± 5%) but only rarely in trigeminal (0.3 ± 0.3%) or superior cervical (1.4 ± 1.4%) ganglion neurons. Calcitonin gene-related peptide (CGRP)-ir was not observed in pterygopalatine or superior cervical ganglion somata, but was present in 24 ± 4% of trigeminal neurons. Bright dopamine β-hydroxylase (DBH) immunofluorescence was observed in the majority of eyelid-projecting neurons within the superior cervical ganglia (65 ± 5%) and lighter staining was detected in pterygopalatine neurons (63 ± 3%), but no DBH-ir was observed in trigeminal neurons. Examination of eyelid sections revealed dense VIP-ir innervation of meibomian gland acini and vasculature and modest distribution within tarsal muscle. CGRP-ir fibers surrounded ductal and vascular elements of the meibomian gland and the perimeter of tarsal muscle. DBH-ir fibers were associated with meibomian gland blood vessels and acini, and were more densely distributed within tarsal muscle. This study provides evidence for prominent meibomian gland innervation by parasympathetic pterygopalatine ganglion VIP-ir neurons, with more restricted innervation by sensory trigeminal CGRP-ir and sympathetic neurons. Tarsal muscle receives abundant sympathetic innervation, as well as moderate parasympathetic and sensory CGRP-ir projections. The eyelid contains substantial non-CGRP-ir sensory innervation, the targets of which remain undetermined. The distribution of identified autonomic and sensory fibers is consistent with the idea that meibomian gland function, as well as that of the tarsal muscle, is regulated by peripheral innervation.