Thyroid hormone dependency in immature but not mature grafted locus coeruleus neurons. Evidence from intraocular innervation of iris transplants.

Fetal brain tissue pieces containing locus coeruleus noradrenaline neurons were combined with sequentially or simultaneously grafted irides in the anterior eye chamber of thyroidectomized or normal host rats. The aim was to reveal possible morphological alterations in the adrenergic fibres innervating the iris grafts, induced by thyroid hormone deficiency, and to compare that possible hormone dependency with what has been found before in host irides innervated by locus coeruleus neurons. Nerve fibre outgrowth was evaluated in iris whole mounts, using Falck Hillarp fluorescence histochemistry. The distribution of locus-coeruleus-derived fibres on host irides was markedly altered in the thyroidectomized hosts. The number of fluorescent axon bundles was significantly decreased, and the intermingled varicose nerve fibre plexus contained numerous accumulations of fluorescent material. In the iris grafts from normal or thyroidectomized hosts reinnervated by matyured locus coeruleus neurons there was no difference in distribution or morphology between the two groups. This clearly shows that matured locus coeruleus neurons are not thyroid hormone dependent during a reinnervation process, after the stimulus elicited by a sensory denervation of iris. When immature locus coeruleus neurons were made to innervate iris grafts by simultaneous grafting of brain tissue and an iris to the eye a clearly reduced number of axon bundles was formed in the iris grafts of the thyroidectomized group. The morphological discrepancies between the two groups were, however, markedly smaller for iris grafts than for corresponding host irides. This might indicate that the potent growth stimulus elicited by sensory denervation of irides partly counteract the inhibition of axon bundle formation by immature grafted locus coeruleus neurons in irides during thyroid hormone deficiency.