Dendritic arborization and axon trajectory of neurons in the hypothalamic arcuate nucleus of the rat

Abstract

Dendritic arborization patterns of neurons in the hypothalamic arcuate nucleus and the course of their axons outside the median eminence were studied using Golgi material, electron microscopic detection of degenerated axon terminals following surgical interference, and horseradish peroxidase technique. Two kinds of neurons can be distinguished in the arcuate nucleus: small fusiform and somewhat larger polygonal cells. Fusiform neurons having two, sparsely arborizing stem dendrites are localized mainly in the medial and dorsal parts of the nucleus. A variant of the fusiform neurons (pear shaped cells) has only one dendritic stem, and the axon originates at the other pole of the neuron. These latter cells are found along the ependymal wall of the third ventricle. Polygonal neurons which occupy the ventral and lateral portions of the nucleus have 4–5 repeatedly branching stem dendrites. In addition to the well known course of the axons of the arcuate neurons towards the median eminence, there are axons of both fusiform and polygonal cells which leave the nucleus in lateral or dorsolateral direction. Axons having this course frequently issue a collateral branch along the first 100–150 μm of their trajectory. A knife-cut through the arcuate nucleus or along its lateral border resulted in degeneration of axon terminals in the area between the arcuate and ventromedial nuclei, and in a ventromedial sector of the ventromedial nucleus itself. A cut located in the area between the arcuate and ventromedial nuclei caused degenerated axon terminals over a large part of the ventromedial nucleus with a considerable increase in the ventromedial portion. Labeled fusiform and polygonal neurons can be seen in the arcuate nucleus following the injection of horseradish peroxidase into the lateral hypothalamus. The results suggest unequivocally that the neurons in the arcuate nucleus have efferent connections not only towards the median eminence but also to the hypothalamic ventromedial nucleus and lateral hypothalamic area.