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Explants from rat fetal retina were placed in culture and assayed for fiber outgrowth. In contrast to results obtained with lower vertebrates, nerve growth factor (NGF) does not seem to play a role in this system: NGF is not able to stimulate fiber outgrowth and antibodies to NGF do not block the spontaneously occurring fiber outgrowth. However, an extract prepared from pig brain is able to stimulate fiber outgrowth in a dose-dependent manner. It is suggested that such an extract can be used as a source of putative neurotrophic factors exhibiting in the mammalian central nervous system (CNS) an action similar to that of NGF in the peripheral nervous system (PNS) of mammals and in the CNS of lower vertebrates like fishes and amphibia.

Goldfish (Carassius auratus) retinal explants, whose ganglion cells were 'primed' in vivo by an optic nerve crush at varying intervals prior to culture, were found to respond by enhanced neurite outgrowth to low (ng/ml) concentrations of nerve growth factor (NGF). This in vitro response was dose-dependent and specific for NGF. The spontaneous fiber outgrowth which normally occurs in vitro in response to optic nerve lesion without exogenous NGF in the medium could be reduced by approximately 80% with administration of NGF antiserum. These observations strongly indicate the formation of an NGF-like molecule in the goldfish retina. The magnitude and sensitivity of the NGF response was dependent on the post-crush interval, referred to as days post-axotomy (DPA). Without a prior crush there was no response unless explants remained in culture 1-2 weeks before NGF treatment. NGF elicited the greatest increase in neurite outgrowth when administered to 7 DPA explants. With increasing intervals after axotomy the response decreased until by 35 DPA none could be elicited.

Blockade of axonal transport or transection of the rat sciatic nerve results in transganglionic degenerative atrophy (TDA) of nerve terminals containing fluoride-resistant acid phosphatase (FRAP) in the Rolando substance of the spinal cord. Application of vinblastine (9 micrograms) in a cuff around the sciatic nerve of adult rats blocked the retrograde transport of [125I]NGF in sensory fibers; this amount of vinblastine is identical to the threshold amount that induces TDA. Conversely, application of NGF to the proximal stump of the transected sciatic nerve prevented or delayed the occurrence of TDA as reflected by the maintenance of FRAP in the upper dorsal horn, that otherwise would inevitably disappear following the peripheral nerve lesion. These results suggest that endogenous NGF transported retrogradely in peripheral sensory fibers of the adult rat under normal conditions may be responsible for the regulation of the structural and functional integrity of the central terminals of these FRAP-containing primary sensory neurons and that TDA may be the consequence of the failure of NGF to reach the perikarya of these neurons.

Administration of NGF to newborn and adult rats elicits a selective increase in TH and DBH both in sympathetic ganglia and adrenal medulla. This effect does not depend on intact preganglionic cholinergic fibers. The augmented enzyme activity results from enhanced enzyme synthesis since it can be abolished by cycloheximide and NGF has been shown to enhance the incorporation of [3H]leucine into DBH molecules. The responsiveness of the adrenal medulla to NGF is also supported by light and electron microscopic autoradiograms which show that intravenously injected 125I-NGF is accumulated with high selectivity in adrenal chromaffin as compared to adjacent adrenal cortical cells. In spite of the many similarities between the response of the adrenergic neurons and adrenal chromaffin cells to NGF, there are also two distinct differences. (a) In newborn rats the ratio between the TH increase effected by a single and 10 subsequent daily injections of NGF is 1:2 in the adrenal medulla and 1:7 in the superior cervical ganglia. (b) If adrenal medullae are transferred to organ culture after intravenous injection of NGF, maximal TH response is initiated 60-90 min after NGF administration. In superior cervical ganglia only a half-maximal response is initiated at that time. After a stationary phase a second increase starts after about 6 h to reach the maximum after 12 h. The biphasic time course of the initiation of TH induction by NGF in sympathetic ganglia is in agreement with the time course of 125I-NGF accumulation after intravenous injection27 reflecting the moiety of NGF reaching the cell bodies of the adrenergic neurons directly by the blood stream (initial accumulation) and by retrograde axonal transport (second phase).