Substance P and 5-hydroxytryptamine immunoreactive presynaptic boutons on presumed .ALPHA.-motoneurons in the chicken ventral horn.

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Abstract
Substance P and 5-hydroxytryptamine [5-HT] containing presynaptic boutons on presumed .alpha.-motoneurons were studied in the chicken ventral horn using the indirect antibody peroxidase-antiperoxidase technique in conjunction with monoclonal antibodies to 5-HT and substance P at light and EM levels. At the light microscopic level, substance P immunoreactivity was observed as large dot-like structures in close apposition to the soma and dendrities of presumed .alpha.-motoneurons. Individual immunoreactive structures were separated from each other. 5-HT immunoreactivity was observed as many small dot- or fiber-like structures outlining the soma and dendrities of large neurons in the ventral horn. Electron microscopically, 5-HT immunoreactive presynaptic boutons were identified as the type which contained elongated dense cored vesicles concomitant with flattened clear vesicles, while the substance P immunoreactivity was observed in presynaptic boutons which contained spherical dense cored vesicles (diameter range 60-110 nm) together with spherical clear vesicles. Both types of dense cored vesicles were often adjacent to the presynaptic membrane of synaptic specialization. .alpha.-motoneurons evidently may be directly modulated by substance P and 5-HT in the chicken.

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