Factors regulating growth of catecholamine-containing nerves, as revealed by transplantation and explantation studies

Intraocular grafting of various types of neuron and target tissue shows that peripheral and central noradrenergic neurons may substitute morphologically and functionally for each other in certain, but not all, target tissues. The morphology of growing adrenergic nerve terminals, their patterning, and the number of fibres are completely determined by the target tissues. Thus, the sympathetic adrenergic neuron of an adult organism is a highly plastic unit which may, for instance, double or even triple its terminal field in response to new demands from the environment. Several differences exist between central and peripheral adrenergic nerves: locus coeruleus will not innervate heart grafts; sympathetic fibres will not innervate the spinal cord; and central adrenergic neurons are not sensitive to nerve growth factor (NGF). Chromaffin cells can be made to innervate peripheral and central targets. No NGF is detected in normal adult iris. The iris responds to grafting, explantation, and to sensory or sympathetic denervation with rapid production of NGF as shown by bioassays on chick embryonic ganglia. This iris also contains a potent stimulatory factor for the ciliary ganglion. Trauma to the iris or to the anterior eye chamber may cause it to become hyperinnervated. Heavy metals have characteristic and different effects on the sympathetic nerves of the iris: lead and manganese causes hyperinnervation; cadmium does not change the number of nerves; while mercury causes severe terminal degeneration followed by regeneration.