Enkephalin-like immunoreactive axon terminals make synapses with alpha-motoneurons in the chicken.

Methionine (Met)-enkephalin (Enk)-like immunoreactive axon terminals on HRP-labeled alpha-motoneurons were studied using the indirect antibody peroxidase--anti-peroxidase technique in the chicken. Electron microscopically, Met-Enk-like immunoreactive axon terminals containing dense-cored vesicles and clear vesicles made axosomatic and axodendritic synapses with PLD alpha-motoneurons. Dense-cored vesicles were sometimes observed to be adjacent to the presynaptic membrane accompanied by the postsynaptic density. These results suggest morphologically that alpha-motoneurons may be directly modulated by enkephalins.