Sympathetic axons surround nerve growth factor-immunoreactive trigeminal neurons: observations in mice overexpressing nerve growth factor.

It has been postulated that the aberrant projection of sympathetic axons to individual primary sensory neurons may provide the morphological basis for pain-related behaviors in rat models of chronic pain syndrome. Since nerve growth factor (NGF) can elicit the collateral sprouting of noradrenergic sympathetic terminals, it might be predicted that NGF plays a role in mediating the sprouting of sympathetic axons into sensory ganglia. Using a line of transgenic mice overexpressing NGF among glial cells, it was first found that trigeminal ganglia from adult transgenic mice possessed significantly higher levels of NGF protein in comparison to age-matched wild-type mice; as well, detectable levels of NGF mRNA transgene expression were present in both the ganglia and brain stem. Within the trigeminal ganglia, a small proportion of the sensory neuronal population stained immunohistochemically for NGF; a higher percentage of NGF-positive neurons was evident in transgenic mice. New sympathetic axons extended into the trigeminal ganglia of transgenic mice only and formed perineuronal plexuses surrounding only those neurons immunostained for NGF. In addition, such plexuses were accompanied by glial processes from nonmyelinating Schwann cells. From these data, we propose that accumulation of glial-derived NGF by adult sensory neurons and its putative release into the ganglionic environment induce the directional growth of sympathetic axons to the source of NGF, namely, the cell bodies of primary sensory neurons.