SUMMARY1. Electrophysiological studies showed that injury of post-ganglionic nerve fibres leads to severe and prolonged depression of synaptic transmission through the rat superior cervical ganglion, beginning within 24 h. This is in line with the results of previous studies in other species and upon other neurones.2. Electron microscopy after post-ganglionic axotomy revealed nerve endings of presynaptic type with all the specialized membrane-related features of a synaptic zone, but which were not apposed to any postsynaptic nervous element. These unusual profiles were interpreted as detached presynaptic nerve endings. In normal and control ganglia, such profiles formed at most 05 % of-all vesicle-containing profiles of presynaptic type; in ganglia with all major post-ganglionic branches cut the proportion rose to approximately 7 %, between 3 and 7 d post-operatively.Over this period,' the mean incidence of chromatolytic neurones was 74.6%.3. Concomitantly, the -incidence of synapses within the ganglion fell by about 75 %, reaching its lowest levels between 3 and 7 d post-operatively.There was strikingly little evidence of persistence of post-synaptic membrane specializations ('membrane thickenings') following detachment of synapses. 4. At longer survival intervals the incidence of synapses gradually increased, and that of detached nerve endings gradually decreased; recovery was well advanced by 42 d.5. The fall in the incidence of synapses was closely paralleled by a fall in the incidence of desmosome-like attachments in the ganglion; the * M.R.C. Scholar. 92 MARGARET R. MATTHEWS AND VICTORIA H. NELSON incidence of such attachments was found to be correlated to a significant degree with that of synapses.6. It is concluded that most or all of the synapses upon sympathetic neurones become physically dissociated during the chromatolytic reaction of these neurones to axotomy. The failure to persist of ultrastructurally specialized post-synaptic sites, and the loss of desmosomes (particularly marked for those involving purely post-ganglionic nervous elements) suggest that the post-ganglionic neurone is losing all its specializations for attachment. 7. Some evidence suggests that the satellite cells may effect the final separation between pre-and post-synaptic structures.

The presence of substance P-immunoreactive *(SPI) varicose nerve networks and nerve fiber bundles in guinea pig prevertebral sympathetic ganglia has been confirmed by fluorescence immunohistochemistry. No SPI, neurons have been found in sympathetic ganglia, including lumbar paravertebral ganglia. Peroxidase-antiperoxidase immunocytochemical methods have shown that SPI nerve terminal varicosities in the inferior mesenteric ganglion (IMG) form morphologically identifiable synapses on dendritic shafts. Cutting the intermesenteric nerve produces no obvious change in SP immunoreactivity in the IMG; cutting the lumbar splanchnic nerves produces nearly total depletion which becomes virtually complete if the two lesions are combined; SP immunoreactivity accumulates in the central ends of the lumbar-splanchnic nerves and in the cranial end ofthe intermesenteric nerve. Cutting hypogastric nerves or.colonic branches of the IMG leads to accumulation of SP immunoreactivity in their ganglionic stumps and to build-up (colonic nerve lesion) rather than depletion of SP immunoreactivity in the IMG. Capsaicin treatment leads to total loss of SP immunoreactivity from the prevertebral ganglia and dorsal root ganglia, severe depletion in laminae I and II and dorsolateral fasciculus of the spinal cord, and total loss from penvascular and paravascular networks of the ileum and mesentery, with sparing of the SP immunoreactivity of the enteric.nerve plex. uses. Capsaicin is.thought 'to deplete sensory neurons selectively. Removal of the spinal cord belowT7 without damage to the dorsal root ganglia leaves the intraganglionic SPI nerve networks and bundles intact. We conclude' that these are derived from peripheral processes of sensory neurons and we propose that the SPI synapses in the IMG arise from collateral branches of these sensory peripheral processes. This implies a novel role for these processes, in forming intraganglionically in the prevertebral ganglia synapses which may take part in the reflex control of the viscera, independently of the centralnervous system.In sympathetic ganglia of the guinea pig, rat, and cat, Hokfelt et al. (1) have demonstrated bundles and varicose networks of nerve fibers immunoreactive for substance P (SP). These are particularly abundant in the prevertebral ganglia (celiac, superior mesenteric, inferior mesenteric). There is strong electrophysiological evidence that SP plays a transmitter role in the prevertebral ganglia (2). Interest has recently been focused on the possibility ofpeptidergic transmission in autonomic ganglia after the demonstration by Kuffler and associates (3) that luteinizing hormone-releasing hormone has a transmitter function in a bullfrog sympathetic ganglion. Mammalian prevertebral ganglia have been shown to contain nerve networks immunoreactive for enkephalin and for vasoactive intestinal' polypeptide, in addition to the substance P-immunoreactive (SPI) networks (4, 5). In the case of the bullfrog sympathetic ganglion, the luteinizing hormone-releasing hormone has been...

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