EVIDENCE FOR MONOSYNAPTIC PROJECTIONS FROM THE NUCLEUS RETROAMBIGUOUSTO HINDLIMB MOTONEURONS IN THE CAT

The nucleus retroambiguus (NRA) is a group of premotor neurons at the transition between brainstem and spinal cord. It projects to certain m otoneuronal cell groups, among which is a distinct set of motoneurons in the lumbar enlargement innervating muscles including iliopsoas, add uctor longus, and hamstrings. To find out whether these NRA-motoneuron al projections are monosynaptic, injections of wheat germ-agglutinin h orseradish peroxidase (WGA-HRP) into the NRA were combined with inject ions of cholera toxin subunit b (CTb) into the hamstring muscles. Elec tron microscopical examination revealed that the NRA terminal profiles make monosynaptic contacts with dendrites of motoneurons innervating these muscles. The NRA terminal profiles formed asymmetrical synapses, and contained spherical and a few dense core vesicles. These findings provide evidence of monosynaptic NRA-hindlimb motoneuronal projection s which are likely to be excitatory. (C) 1997 Elsevier Science Ireland Ltd.