Parasympathetic varicosity proliferation and synaptogenesis in rat eyelid smooth muscle after sympathectomy.
Parasympathetic innervation to eyelid smooth muscle inhibits sympathetic neurotransmission pre-junctionally without appreciable direct post-junctional effects. However, 5 weeks after sympathectomy, parasympathetic stimulation elicits substantial cholinergically mediated contractions. This study examined ultrastructural changes accompanying the conversion to parasympathetic excitation. In intact muscles, 64+/-9 nerve varicosities were encountered per 104 micron2. Most were close to muscle cells and not fully enclosed by supporting cells. Axo-axonal synapses were observed occasionally. Two days following sympathectomy, varicosity numbers were reduced by 97% and, relative to controls, remaining varicosities were farther from muscle cells and more frequently fully enclosed by supporting cells, but contained greater numbers of small spherical and large dense vesicles. By 6 weeks post-sympathectomy, numbers of varicosities per unit muscle volume increased to 14% of controls. These varicosities differed from those at 2 days in being closer to smooth muscle cells, less frequently enclosed, and having fewer small vesicles. These findings indicate that intact eyelid smooth muscle varicosities are predominantly sympathetic, but a small number of parasympathetic varicosities are present, some of which may form pre-junctional synapses with sympathetic nerves. Between 2 days and 6 weeks post-sympathectomy, varicosities increased in number and established appositions with smooth muscle cells. This suggests that parasympathetic nerves are capable of re-innervating an atypical smooth muscle target after sympathectomy, and that parasympathetic synaptogenesis is likely to contribute to conversion from pre-junctional inhibition to post-junctional excitation after sympathectomy.
Smith
P G
PG
Department of Molecular and Integrative Physiology, University of Kansas Medical Center, Kansas City, KS 66160-7401, USA. psmith@kumc.edu
Marzban
F
F
D000818:Animals; D005143:Eyelids / Q000294:innervation*; D008854:Microscopy, Electron; D009130:Muscle, Smooth / Q000294:innervation*; D010275:Parasympathetic Nervous System / Q000473:pathology* / Q000503:physiopathology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013562:Sympathectomy; D013569:Synapses / Q000502:physiology*; D013997:Time Factors