Growth interaction between locus coeruleus and trigeminal ganglion after intraocular double grafting

Fetal trigeminal ganglia were combined in the anterior chamber of eyes of rat recipients with sequentially grafted fetal locus coeruleus transplants. After maturation of both grafts in oculo the growth pattern of locus coeruleus derived noradrenaline fibres in the iris was examined with Falck-Hillarp fluorescence histochemistry. Contrary to what was expected from earlier studies, which revealed a radiating halo of fibres around the CNS tissue on the iris, the locus coeruleus-derived fibres radiated to a large extent from the adjacent trigeminal ganglion attachment. Thus, the presence of a grafted trigeminal ganglion changed the distribution of the central monoamine nerves growing out in the iris. The possible cause of this changed distribution is discussed. Maturated intraocular trigeminal ganglion transplants were retransplanted, together with the whole iris to which it was attached, into a new eye which already harboured a maturated locus coeruleus graft attached to its host iris. The expected reinitiated locus coeruleus fibre ingrowths into the iris transplants were then compared in those iris transplants that had trigeminal ganglia and controls that had not. The surface area of the iris grafts covered by newly formed locus coeruleus noradrenaline fibres was significantly smaller (27% reduction) if trigeminal ganglia were situated on them during the reinnervation process. This finding strongly supports our earlier studies, which suggested that the presence of sensory nerves in the iris inhibits growth of locus coeruleus in that receptor tissue, and furthermore, that the sensory nerves responsible for this inhibitory interaction have their origin within the trigeminal ganglion itself.