Cell types within the medial forebrain bundle: A golgi study of preoptic and hypothalamic neurons in the rat

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Abstract
The morphology of lateral preoptic (POL) and lateral hypothalamic (HLA) neurons was studied in 14‐ to 200‐day‐old rats with the chlorate‐formaldehyde modification of the Golgi method. Drawings of 91 POL and HLA neurons revealed three distinct neuronal types within the MFB based on somatic size and shape and dendritic morphology. Class I neurons, which accounted for 75–80% of the neurons in the MFB, had fusiform or multipolar somata averaging 21 × 14 μm and 2–5 sparsely branched dendrites with a moderate number of sticklike spines. The extensive dendritic domains of Class I neurons ranged from 700 to 1,500 μm and were usually oriented perpendicular to the longitudinal fibers of the MFB. Both nonoriented and oriented Class I neurons were encountered. Nonoriented Class I neurons had expansive dendritic arbors which reached nearly all regions of the MFB in the coronal plane. Oriented Class I neurons had dendritic domains which were confined to specific regions (e.g., ventral‐lateral) of the MFB. Class II neurons, which made up approximately 10% of the MFB neurons, had large multipolar somata averaging 30 × 17 μm and 2–5 stout dendrites which were densely covered with hairlike spines. Class II neurons also exhibited spines on their somata and proximal dendritic trunks and had dendritic domains of 700–1,000 μm. Class III neurons had small somata averaging 15 × 12 μm and restricted dendritic arbors of 500–700 μm in diameter. Class III neurons exhibited both spiny and spine‐free dendrites and made up 10% of MFB neurons. Because of the parcellation of chemically coded fiber systems within the MFB, individual POL and HLA neurons may not be homogeneous in the type of afferents they receive from other brain areas.