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

Substance P and 5-hydroxytryptamine 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-hydroxytryptamine and substance P both at light and electron microscopic levels. At the light microscopic level, substance P immunoreactivity was observed as large dot-like structures in close apposition to the soma and dendrites of presumed alpha-motoneurons. Individual immunoreactive structures were separated from each other. On the other hand, 5-hydroxytryptamine immunoreactivity was observed as many small dot- or fiber-like structures outlining the soma and dendrites of large neurons in the ventral horn. Electron microscopically, 5-hydroxytryptamine 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. These immunocytochemical results suggest that alpha-motoneurons may be directly modulated by substance P and 5-hydroxytryptamine in the chicken.