Appearance of the noradrenergic markers tyrosine hydroxylase and neuropeptide Y in cholinergic nerves of the iris following sympathectomy

Selective autonomic denervations of the iris have been used to study the possible redistribution of adrenergic markers within adult nerve fiber systems and to reveal the cellular origin of a nonsympathetic fiber plexus induced to express such markers. The presence and distribution of fibers showing neuropeptide Y (NPY)- and tyrosine hydroxylase (TH)-like immunoreactivity was studied in the rat iris using stretch-prepared whole mounts. Normal irides contained a dense regular network of NPY-positive varicose fibers. Such fibers were regularly seen innervating blood vessels. The choroid membrane had a high number of fluorescent fibers. A similar, although slightly denser TH-positive fiber system was visualized in the iris. One or 2 days after surgical removal of the superior cervical ganglion, almost all NPY- and TH-positive fibers had disappeared, suggesting that most, if not all, NPY-positive fibers in the iris originate in the superior cervical ganglion. In irides from long-term sympathectomized animals, a high number of TH- and NPY-immunoreactive fibers had reappeared, while such irides were devoid of catecholamine-containing fibers, as evidenced by Falck-Hillarp histochemistry. The appearance of TH- and NPY-positive fibers in sympathetically denervated irides was clearly time dependent. The distribution of fluorescent fibers in irides from intact and sympathectomized animals showed obvious dissimilarities such as a lower fluorescence intensity and fewer varicose fibers in denervated irides. Furthermore, in irides from sympathectomized rats, TH- and NPY-positive fibers were not associated with blood vessels. Unilateral removal of the parasympathetic ciliary ganglion, which supplies the iris with cholinergic fibers, 3 days prior to sacrifice in animals bilaterally sympathectomized 1 month earlier, led to a drastic reduction in numbers of TH- and NPY-positive iris fibers on the ciliarectomized/sympathectomized side as compared to the sympathectomized-alone side. The present experiments thus suggest that adult cholinergic neurons in vivo are capable of expressing adrenergic characteristics under experimental conditions.