The role of tonic preganglionic neuron firing in the turnover of the large dense-cored vesicle store in sympathetic preganglionic nerve terminals

Abstract

Large dense-cored vesicles are transported centrifugally in the cervical sympathetic trunk and are depleted in a calcium-dependent manner from synaptic boutons of the cat superior cervical ganglion during orthodromic stimulation at 20–40 Hz [P. Weldon et al. (1993) Neuroscience 55, 1045–1054]. In the present study, we tested in awake cats whether the normal tonic firing of the sympathetic preganglionic neuron contributes to the turnover of large dense-cored vesicles in synaptic boutons of the superior cervical ganglion. Tetrodotoxin was applied with a mini-osmotic pump to one cervical sympathetic trunk, while vehicle alone was applied to the contralateral cervical sympathetic trunk, for two, four or seven days. The appearance of Horner syndrome ipsilateral to the tetrodotoxin application demonstrated block of action potential propagation. Both superior cervical ganglia were excised and processed for electron microscopy. The number of large dense-cored vesicles per bouton cross-section was higher in the ganglion with tetrodoxin-treated input than in the control. The content at four days was higher than at two days; the content at seven days was similar to that at four days. The number of lysosomes per bouton profile also increased in the ganglion with tetrodotoxin-treated input. No changes were observed in size of bouton profiles, number of boutons or of synapses per grid square and length of the presynaptic densities in the ganglion with tetrodotoxin-treated input. The increased large dense-cored vesicle content of the boutons in the ganglion with tetrodotoxin-treated input is likely the result of lack of release while supply is maintained and suggests that under normal conditions there is a steady turnover of large dense-cored vesicles by an action potential-dependent mechanism, presumably involving exocytosis. The lack of a further increase in large dense-cored vesicle content after four days of tetrodotoxin block may be the result of action potential-independent release and/or decreased supply and/or increased intraterminal breakdown. These observations are consistent with the hypothesis that, in sympathetic preganglionic axon terminals, the components of large, dense-cored vesicle turnover are supplied from the soma and released by the neuron tonic firing which, in this preparation, has a mean rate of 1–2 Hz.