Reinitiation of directed nerve fiber growth in central monoamine neurons after intraocular maturation.

Abstract

Grafts of central monoamine (MA) neurons and irides were combined in the rat anterior eye chamber to reveal factors regulating nerve fiber growth. Central MA nerve fiber production in host irides and iris grafts was measured by quantitative Falck-Hillarp histochemistry and 3H-metaraminol uptake in sympathetically denervated eyes. 1. Fetal locus coeruleus (LC) grafts produce a dense halo of noradrenaline (NA) fibers on host irides. When this growth ceases after 6 weeks, it covers only 1/3 of the host iris, despite the sympathetic denervation. If an iris graft is then placed in contact with the LC graft, the new iris will reinitiate a vigorous fiber growth in the central NA neurons leading to a rapid and complete reinnervation of the iris graft. There is no simultaneous increase in the halo of nerve fibers on the host iris, showing that this growth response is highly selective. The innervation of the iris grafts and host irides remain essentially unchanged during 1 year without degenerative signs. Grafted dopamine and 5-hydroxytryptamine neurons show a similar response to iris grafts. 2. Addition of a second iris graft, sandwiched on top of the first iris graft which in turn was attached to a LC graft results in complete reinnervation also of the second iris graft. Hence, maturated NA neurons are capable of repeated fiber growth responses and contact between the stimulus (second iris graft) and the stimulated neurons can be indirect (via the first iris graft). 3. Immature LC grafts are also stimulated to additional growth by iris grafts present during ontogenetic maturation of LC in oculo. 4. When iris grafts are placed opposite to LC on the host irides and without contact with the fiber halo on the host irides, there is no LC fiber growth stimulation. 5. When iris grafts are placed in sympathetically denervated eyes first, and fetal LC grafts added to them after 1 month the pattern of NA fibers in the iris grafts becomes similar to that in host irides, i.e. only a halo of fibers around the CNS graft. The growth stimulation exerted by acute iris grafts ceases with time of the iris graft in oculo. 6. Mature intact NA neurons of LC attached to its host iris can survive regrafting to a new eye. The distribution of fibers from the surviving neurons on the old host iris changes from a halo to a wide spread pattern.