Although neuroanatomical and neurophysiological features of neurons in the ferret trachea have been studied, the neural mediators associated with this plexus have not been completely characterized. The purpose of this study was to examine the occurrence of choline acetyltransferase (ChAT), nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP), and substance P(SP) in the intrinsic neurons of this plexus. The distribution of double- and triple-labeled neurons was quantified in cryostat sections and in whole mounted specimens to evaluate the neurochemical profiles. About 85% of the nerve cell bodies with ChAT immunoreactivity (ChAT-IR) were located in ganglia of the longitudinal trunks or the closely associated bridge ganglia. Approximately 15% of ChAT-positive neurons were in ganglia of the superficial muscular plexus. Conversely, VIP-IR neurons were most frequent in the superficial muscular plexus (>75%) and, <10% were observed in the longitudinal trunks or bridge neurons. Most NOS- and SP-IR neurons were also located in the superficial muscular plexus. The following distribution of neurochemical profiles was determined for neurons of the superficial muscular plexus: 11% only NOS, 20% only VIP, 5% only SP, 67% NOS and VIP, and 40% VIP and SP. NOS, VIP, and SP were frequently localized in the same nerve cell body. The occurrence of nerve terminals containing only SP located around the borders of individual NOS/VIP/SP-containing neurons suggests possible sensory innervation to the airway neurons. The results demonstrate that: (1) most cholinergic nerves do not contain VIP, NOS, or SP; (2) cholinergic neurons are predominantly located in the longitudinal trunk ganglia; (3) VIP, NOS, and SP are predominantly located in the superficial muscular plexus ganglia; and (4) nerve terminals containing exclusively SP, suggesting possible sensory origin, are closely associated with some neurons in the plexus.

The neurochemical profiles of neurons in ferret tracheal ganglia has been characterized, but their projections to smooth muscle and epithelium in ferret trachea has not been examined. The purpose of this study is to determine the location of cell bodies that project VIP-, SP-, and NPY-containing fibers to the ferret tracheal smooth muscle and epithelium. Segments of ferret trachea were cultured for 0, 1, 3, or 7 days, some in the presence of 3 microm capsaicin. VIP, SP, or NPY nerve fiber density was measured using morphometric procedures. A retrograde tracer, rhodamine-labeled microspheres, identified neurons projecting to the epithelium. The density of SP fibers in the epithelium was reduced after culture, but VIP innervation was not different. In tracheal smooth muscle, the density of VIP- and SP-IR fibers was not different during the culture period, but NPY fiber density was reduced at all culture times. Capsaicin treatment did not affect nerve fiber density in the tracheal smooth muscle but produced a significant reduction in the density of epithelial VIP- and SP-IR nerve fibers after 1 day. Rhodamine-labeled microspheres were identified in VIP-containing nerve cell bodies of the ferret tracheal plexus. VIP innervation to the airway epithelium in ferret originates both from cell bodies in airway ganglia and cell bodies in sensory ganglia. The pathway from airway ganglia suggest the existence of a local reflex mechanisms initiated by epithelial irritation.