Neurotransmitter synthesis, storage, and turnover in neonatally deafferented sympathetic neurons

In the superior cervical sympathetic ganglion (SCG) of the rat, a significant amount of morphological and biochemical maturation occurs in the first few postnatal weeks. The specific activity of tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of norepinephrine (NE), increases during this time and is subject to transsynaptic regulation by the preganglionic inputs. In the present study, we examined the normal postnatal development of NE stores in sympathetic neurons and the transsynaptic regulation of this development. NE content undergoes an 8-fold increase from the time of birth, and stabilizes at adult levels at one month. Following neonatal deafferentation, there is a temporary stunting of NE accumulation in sympathetic neurons and a permanent reduction in the activity of TH, whether or not regeneration of the afferents occurs. When regeneration is prevented, the turnover of NE is significantly reduced, while NE levels rise to near normal levels. When regeneration is permitted, however, both the stored amount and turnover of NE attain normal levels. These data suggest that there is a critical period during the first two postnatal weeks when transsynaptic influences from afferents are necessary for the induction of TH in sympathetic neurons. Levels and turnover of transmitter do not have this critical period, but appear to depend solely on the functional integrity of the system.