The autonomic facial nerve pathway in birds: a tracing study in chickens

In birds, the parasympathetic innervation of the choroid is via the ciliary (cranial nerve III) and pterygopalatine (cranial nerve VII) ganglia, the latter consisting of a chain of microganglia within the orbit. Because of the scattered nature of these microganglia, lesions of this nerve pathway in birds have not been attempted, making interpretation of the functional contribution of this parasympathetic input to the avian eye uncertain. The purpose of this study was to find an extraorbital approach to the preganglionic part of cranial nerve VII and to reveal its peripheral terminals and its site of origin in the brain stem.

The radix autonomica cranial nerve VII was accessed via the tympanic cavity and injected with dextran coupled to Texas red (DTxR). Orbital structures and the brain stem were prepared for tracer detection and immunohistochemistry for neuronal nitric oxide synthase (nNOS), choline acetyl transferase (ChAT), vasoactive intestinal polypeptide (VIP), calcitonin gene-related peptide (CGRP), galanin (GAL), and somatostatin (SOM). For documentation, fluorescence and confocal laser scanning microscopy were used.

Anterogradely labeled DTxR-positive nerve fibers were detected within the orbital pterygopalatine microganglionic chain, forming boutons closely associated with nNOS-positive neurons. Retrogradely labeled DTxR-positive neurons with cell diameters of approximately 20 microm were found in the brain stem. These were positive for ChAT, but negative for nNOS, VIP, SOM, GAL, and CGRP. They most likely represent the preganglionic neurons of the superior salivatory nucleus. In close proximity, there were larger (40 microm) unlabeled neurons that were positive for ChAT and CGRP, but negative for GAL. These most likely represent motoneurons of the facial nerve.

This surgical approach offers excellent opportunities for lesioning experiments for the study of the autonomic facial nerve pathway in birds in terms of both its anatomic organization and its function.