Effects of synaptic and antidromic stimulation on tyrosine hydroxylase activity in the rat superior cervical ganglion.

1. The effects of orthodromic and antidromic stimulation of the rat superior cervical ganglion on the specific activity of the enzyme tyrosine hydroxylase have been studied. 2. Orthodromic stimulation of the ganglion via the sympathetic trunk produced an increase in the activity of tyrosine hydroxylase when measured 3 days later while causing no change in the protein content of the ganglion. This increase in the specific activity of tyrosine hydroxylase was blocked by administration of the nicotinic antagonist, hexamethonium. 3. Antidromic stimulation of the superior cervical ganglion by stimulating the internal carotid nerve, the external carotid nerve or both nerves simultaneously produced no change in the specific activity of tyrosine hydroxylase. 4. Parallel increases in tyrosine hydroxylase activity and protein content per ganglion were seen when the internal carotid nerve was stimulated but similar changes were seen in "sham-stimulated" animals. These "non-specific" changes were apparently produced by the trauma involved in the extensive dissection necessary to position electrodes on this nerve trunk. 5. We conclude that an increase frequency of firing in post-ganglionic neurones is not a sufficient stimulus to elevate the specific activity of tyrosine hydroxylase. Rather some other aspect of nicotinic receptor stimulation seems to be required.