Abstract
Wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) injection into the hypoglossal nerve mainly resulted in retrograde labeling in the superior ganglia of the glossopharyngeal and vagal nerves ipsilaterally. Anterogradely labeled fibers were found in lamina I of the ipsilateral upper cervical spinal cord with a few distribution to laminae IV-V and VII-VIII. WGA-HRP injection into the PBN revealed intensive labeling of lamina I neurons of the upper cervical spinal cord ipsilaterally. These light microscopic observations appear to indicate the hypoglossal sensory inputs to the PBN through the spinal cord. In order to investigate the synaptic nature of this spinal relay, electron microscopic observations were carried out on lamina I of the first and second cervical spinal cord after cutting the hypoglossal nerve and WGA-HRP injection into the PBN in the same animal. The spinoparabrachial projection neurons were demonstrated to show a low cytoplasmic/nuclear ratio and have an oval or deeply indented nucleus with a centrally located nucleolus. Furthermore, dark and light type degenerating fibers were observed to make synaptic contacts with HRP-labeled somata and dendritic profiles.
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