Abstract

The contents of norepinephrine (NE) in cell bodies (superior cervical ganglia) and terminals (submaxillary salivary glands) of postganglionic sympathetic neurons were determined in cats during, immediately after and two and six hours after cessation of three hours of preganglionic electrical stimulation. In ganglia, neither the NE contents nor the rates of formation of NE-¹⁴C from tyrosine-^l4C or dopamine-¹⁴C were altered during the stimulation or poststimulation periods. Stimulation for one hour reduced the amount of NE in terminals to 40% of control; this reduced content was maintained when stimulation continued for three hours. The stimulus-induced reduction of NE in terminals was enhanced if either synthesis or reuptake of NE was blocked by α-methyltyrosine or desmethylimipramine, respectively. During the poststimulation period the amount of NE in the terminals increased to 65% of control within two hours, and then remained at this reduced value during the next four hours. α-Methyltyrosine, but not desmethylimipramine, prevented this partial recovery of NE. The synthesis of NE-¹⁴C from tyrosine-¹⁴C increased during and immediately after cessation of stimulation, but six hours later the conversion of tyrosine-¹⁴C to NE-¹⁴C was the same in stimulated and non-stimulated terminals. The failure of NE to return to control values after stimulation was not the result of reduced activity of dopamine-β-hydroxylase because six hours after cessation of stimulation the conversion of dopamine-¹⁴C to NE-¹⁴C was accelerated. Thus, recovery of NE stores in nerve terminals after depletion by preganglionic stimulation is effected by an increased rate of synthesis of this amine from both tyrosine, probably involving release of feedback inhibition, and from dopamine, involving an unknown mechanism.