Nerve fiber production by central noradrenaline (NA), dopamine (DA), and 5-hydroxytryptamine (5-HT) neurons was studied using immature brain tissue containing locus coeruleus, substantia nigra, or ventro-caudal medulla oblongata respectively, homologously grafted to the anterior chambers of rat eyes. A method was developed for quantitation of the fiber growth that occurs on the sympathetically denervated host irides as observed in whole mounts using Falck-Hillarp fluorescence histochemistry and by the uptake of 3H-metaraminol into the irides. Survival and growth in oculo of the three different areas were characterized by direct observations through the cornea in vivo for a number of pre- and postnatal stages of development of the donors, and the findings correlated to the degree of monoamine nerve fiber production on the host irides. The growth of fetal locus coeruleus transplants on irides was quantified using both fluorescence microscopical measurements of innervated areas and uptake of 3H-metaraminol. The uptake was well correlated to the histochemical measurements on individual irides, thus validating the fluorescence microscopical measurements of fiber production. The fiber growth of fetal locus coeruleus grafts on irides was followed for 20 weeks. The nerves increased in number and uptake capacity approximately linearly for 6 weeks whereafter the increase rapidly levelled off. On average, the final amount of nerve production by fetal locus grafts did not cover more than 1/3 of the host iris surface, and the average uptake of 3H-metaraminol by these nerves did not exceed 60% of that found in sympathetically intact control irides. The locus grafts produced a similar amount of fluorescent fibers in the host iris independent of the crown-rump length stage of the donor fetus and the final size of the transplants in oculo.
The survival and growth of NA, DA and 5-HT neurons grafted from various postnatal donor rats was also followed by fluorescence microscopy. Locus coeruleus grafts produced markedly more fibers than the two other types of grafts when the donor was one week old or less, and DA grafts produced the least fibers of the three. Even from one month old donors some MA neurons survived grafting. Also, using prenatal donars, the locus coeruleus grafts produced many more fibers on the irides than did the DA grafts. It was concluded that the intraocular transplantation technique is very suitable for quantitative studies of nerve fiber production by immature monoamine neurons, and that it should be possible to study many other neuron systems in similar ways with this technique.
Central monoamine neurons Fiber growth Locus coeruleus Quantitation Fluorescence histochemistry