Differential synaptic inputs to the cell body and proximal dendrites of preganglionic parasympathetic neurons in the rat conus medullaris

Abstract

Preganglionic parasympathetic neurons (PPNs) reside in the intermediolateral (IML) nucleus of the rat lumbosacral spinal cord and contribute to the autonomic control of visceral pelvic organs. PPNs provide the final common pathway for efferent parasympathetic information originating in the spinal cord.We examined the detailed ultrastructure of the type and organization of synaptic inputs to the cell body and proximal dendrites of PPNs in the rat conus medullaris. The PPNs were retrogradely labeled by a systemic administration of the B subunit of cholera toxin conjugated to horseradish peroxidase.We demonstrate four distinct types of synaptic boutons in apposition with PPN somata and proximal dendrites: S-type boutons show clear, spheroid vesicles; F-type boutons show flattened vesicles; DCV-type boutons show a mixture of clear and dense-cored vesicles; L-type boutons were rare, but large, exhibited clear spheroid vesicles, and were only encountered in apposition with the PPN dendrites in our sample. The membrane surface covered by apposed boutons was markedly higher for the proximal dendrites of PPNs, compared to their somata. The inhibitory synaptic influence was markedly higher over the PPN somata compared to their proximal dendrites, as suggested by the higher proportion of putative inhibitory F-type boutons in apposition with the soma and a higher frequency of S-type boutons per membrane length for the proximal dendrites. Our studies suggest that the synaptic input to PPNs originates from multiple distinct sources and is differentially distributed and integrated over the cell membrane surface. KeywordsElectron microscopy; Ultrastructure; Autonomic; Spinal cord; Sacral; Retrograde tracingThe spinal parasympathetic nucleus contributes to the autonomic control of pelvic visceral organs, including the urinary bladder and distal colon (de Groat, 2006;Chung and Emmanuel, 2006). For this purpose, preganglionic parasympathetic neurons (PPNs), interneurons, and spino-bulbar neurons, which are located in the intermediolateral (IML) nucleus of the conus medullaris (Araki, 1994;Hamilton et al., 1995;Araki and de Groat, 1996;Birder et al., 1999), contribute to the reflex control of the pelvic organs, and integrate synaptic inputs from primary visceral afferents, spinal interneurons, and supraspinal nuclei (de Groat, 2006 Early electron microscopic studies on the IML nucleus examined the fine structure of its synaptic inputs and the ultrastructural features of the neuronal somata. For instance, quantitative ultrastructural studies of synaptic profiles in the cat sacral IML nucleus showed the presence of boutons containing clear spherical vesicles, boutons exhibiting a mixture of clear and dense-cored vesicles, and boutons containing flattened vesicles (Nolan and Brown, 1978;Brown and Nolan, 1979). Electron microscopic studies of the IML nucleus in the thoracic spinal cord of the non-human primate Macaca fascicularis identified three main morphological types of neurons based on neuronal size, shape, relative promin...

The soma and proximal dendrites of sympathetic preganglionic neurons innervating the major pelvic ganglion in female rats receive predominantly inhibitory inputs

“…In comparison to previous studies, this coverage was even lower than the complementary pelvic-innervating PPNs in female rats (Persson and Havton, 2008). Our results are consistent with prior studies, which have noted that SPNs at the T1-T4 level receive few somatic inputs, despite many synaptic contacts being made on nearby dendrites in the vicinity the neuropil (Markham and Vaughn, 1990).…”

“…Studies of the vesicular glutamate transporters, such as VGLUT2, are known to densely innervate the thoracolumbar sympathetic IML and the DCN, and boutons immunopositive for the VGLUT2 marker have been found to appose these SPNs associated with pelvic targets in male rats (Llewellyn-Smith et al, 2007). Compared to previous studies of pelvic-innervating PPNs in female rats (Persson and Havton, 2008), thoracolumbar SPNs exhibit far fewer appositions of DCV-type boutons, which are widely regarded as containing neuropeptide transmitters (Hökfelt et al, 1987). …”

“…In addition, nearly all of the input onto the DCN and IML cells come from putative excitatory S-type and putative inhibitory F-type boutons apposing the soma and dendritic membrane, and that direct apposition by neuropeptide-containing DCV-type boutons is rare. The membrane density of these inputs is relatively low on average, even when these cells are compared to their lumbosacral parasympathetic counterparts (Persson and Havton, 2008). The amount and proportion of inputs onto the somata and dendrites are similar, however the SPN somata and proximal dendrites receive more input from F-type boutons than S-type boutons, suggesting that these cells may receive more inhibitory than excitatory influence.…”

“…However, a review of previously published literature on the ultrastructural studies of PPNs in female rats reveal examples of nuclei both with and without nuclear invaginations (Havton and Broman, 2005, Persson and Havton, 2008), though no quantitative assessments of the relative occurrence of these features were conducted. In addition, we have not found any literature addressing this characteristic in the rat dorsolateral nucleus, which contains motoneurons innervating the external urethral sphincter.…”

Neuroprotective Effects of Autophagy Induced by Rapamycin in Rat Acute Spinal Cord Injury Model

“…After anesthetization and perfusion, a 5-mm segment of spinal cord tissue from the injured site was collected for electron microscopy analysis according to the published protocols with modification [26]. Briefly, the tissue was fixed with 3% glutaraldehyde plus 1.5% paraformaldehyde at 4°C for 2 days, followed by incubation of 1% osmium plus 1.5% potassium ferrocyanide for 1.5 h. After PBS wash, the tissue was stained with 70% ethanol-saturated uranyl acetate, dehydrated with ethanol-acetone (series of dilutions), and embedded using epoxy resin 618.…”