Sympathetic response to intracranial NGF infusion in the absence of afferent input: axonal sprouting without neurotransmitter production.

The anatomical relationships between postganglionic sympathetic neurons, their targets, and their afferent inputs provide an opportunity to experimentally distinguish between the anterograde and the retrograde influences on neuronal responsivity to growth factors. In the present study, the effect of decentralization of the superior cervical ganglion (SCG) on the NGF-induced sympathetic sprouting response by mature cerebrovascular axons was assessed in young adult rats. Growth by the perivascular axons associated with the intradural segment of the internal carotid artery was quantified using electron microscopy and changes in norepinephrine (NE) levels were monitored using high-performance liquid chromatography coupled with electrochemical detection. The mean number of perivascular axons observed in the treatment group receiving decentralization of the SCG prior to NGF infusion was not significantly different from that in NGF-infused cases, suggesting that central input was not required for axonal growth of intact sympathetic neurons. However, decentralization prevented the typical NGF-induced increase in perivascular NE associated with the ICA, indicating that afferent input was necessary for the neurotransmitter increase to occur. Thus, afferent input appears to play a role in the regulation of neurotransmitter expression of the sprouted axons but is not required for trophic factor-induced axonal growth.