Degeneration of myelinated sympathetic nerve fibres following treatment with guanethidine.

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The specificity and characteristics of the degeneration of myelinated axons after chronic guanethidine treatment have been investigated in sympathetic and non-sympathetic nerves. Adult male Sprague-Dawley rats aged approximately 43 weeks were treated with guanethidine sulphate (50 mg per kg body weight per day) for between ten days and six weeks. Tissues were examined by qualitative and quantitative light and electron microscopy. In the superior cervical (sympathetic) ganglion (SCG), guanethidine treatment produced a 78% decrease (P = 0.009) in the mean number of myelinated fibres at a standard level of section, compared to the contralateral control ganglion which was removed surgically prior to drug treatment. This reduction in the treated SCG was apparent after 10 days, though complete degeneration of nerve cell bodies was not widespread at this stage. Degeneration of unmyelinated axons was extensive. Degenerating myelinated fibres were consistently small in diameter (up to approximately 3 microns). In individual myelinated fibres the earliest signs of degeneration involved disruption of axonal organelles, particularly the cytoskeleton, and focal widening of the periaxonal space. Myelin breakdown followed these events; degeneration of myelin still associated with a structurally intact axon was not observed. Myelin breakdown appeared to take place initially within the Schwann cell, at least to the stage of 'loosened' membranes. However, infiltrating cells were also involved in myelin phagocytosis. At all stages of treatment some small diameter myelinated fibres remained intact, and there was no evidence of degeneration of the larger diameter fibres (up to approximately 15 microns) which are consistently present in small numbers in the SCG. In the cervical sympathetic trunk, which carries preganglionic axons to the SCG and the vagus and sciatic nerves, degeneration only of unmyelinated axons was detected. These results indicate that guanethidine does not exert a primary degenerative influence on myelin or myelinating Schwann cells and that the myelin degeneration observed in the SCG is a secondary result of the previously documented selectively destructive effect of guanethidine on postganglionic sympathetic neurons. Surviving, small diameter myelinated fibres in the SCG could be either preganglionic or processes of resistant postganglionic neurons, while the larger diameter fibres are likely to be somatic. While the cervical sympathetic trunk, vagus and sciatic nerves all contain postganglionic sympathetic fibres it appears that few of these are myelinated, at least at the levels sampled in this study.