Catecholamine innervation of cervical dendrite bundles: Possible phrenic nucleus innervation

Abstract

The catecholaminergic innervation of three recently described dendrite bundles (midline, central and lateral) in the cervical spinal cord of the adult Long-Evans hooded rat [41] was examined using Golgi impregnation, fluorescence histochemistry for catecholamines, and cholinesterase histochemistry. The midline and lateral bundles were similar in appearance to those described by the Scheibel and Scheibel [50,51], while the central bundle, present in the region of the phrenic nucleus, has not been described previously. Analysis of Golgi-Cox impregnated horizontal sections demonstrated the presence of fine varicose fibers within all three bundles. These profiles entered the bundles at right angles, either singly or within small transverse dendritic subunits, then turned in a rostral or caudal direction, and coursed adjacent to dendrites of motoneurons in the bundles. Catecholamine histofluorescence in horizontal sections revealed abundant varicosities within all three bundles, similar in size and appearance to the varicose fibers seen in Golgi-Cox impregnated sections. Catecholamine fibers entered the dendrite bundles at right angles then turned rostrally or caudally and coursed horizontally within the bundles. Varicose fluorescent profiles formed pericellular rings around the motoneurons and linear profiles adjacent to the dendrites, sometimes outlining the entire proximal portion of primary dendrites. Catecholamine fibers entered the dendrite bundles at right angles then turned rostrally or caudally to course adjacent to the dendrites within the bundles. Cholinesterase histochemistry in alternate sections revealed staining of motoneurons and their dendrites, and confirmed the location of the Catecholamine varicosities within the motoneuron dendrite bundles. The presence of discrete, compact bundles of dendrites associated with clusters of motoneurons suggests that these bundles may serve to synchronize neuronal activity for coordination of groups of muscles involved in particular movements. Synchronization of motoneuron groups, including the phrenic nucleus, is a well documented phenomenon. Dendrite bundles also provide a compact anatomical substrate for receiving and integrating synaptic inputs. The presence of specific transmitter inputs such as Catecholamine varicosities, suggests that motoneuronal activity is influenced by Catecholamine terminations onto groups of dendrites or perikarya within the bundles. These Catecholamine inputs may participate in the coordination of groups of neurons that act as a functional unit, such as the phrenic nucleus.