A morphometric analysis of functionally defined subpopulations of neurons in the paraventricular nucleus of the rat with observations on the effects of colchicine.

Two populations of neurons in the paraventricular nucleus of the hypothalamus that have different efferent projections and physiological roles in the regulation of visceral responses were characterized morphologically with a combined intracellular filling, retrograde tracer, and immunohistochemical method. Neuroendocrine cells were retrogradely labeled by an intravenous injection of Fast blue, and distinguished from descending neurons that were retrogradely labeled by an injection of fluorogold into the spinal cord. Retrogradely labeled neurons were selectively penetrated and filled intracellularly with Lucifer yellow to visualize detailed features of their morphology. Corticotropin-releasing hormone (CRH)-containing neurons were distinguished from other neuroendocrine cells by immunostaining the tissue with an antiserum to rat CRH. Morphometric features of defined populations of neurons were then quantified and reconstructed graphically to generate multicellular montage drawings that demonstrate their spatial organization. Descending neurons were further separated into dorsal and ventral medial parvicellular components, while the neuroendocrine population was differentiated into parvicellular and magnocellular groups. The mean somal areas, total dendritic lengths, and spine densities were compared between groups of neurons, and these showed significant differences across cell types. These measures were also dramatically affected by colchicine, which appears to induce the formation of new dendritic appendages, swelling of the soma, and reduction of dendritic length. Whether colchicine is acting directly upon cytoskeletal structures or indirectly by altering the physiology of the cell is unclear. However, the precise effects of colchicine on mean somal area, total dendritic length, and spine density appear to be dependent upon individual cell type. Colchicine may therefore act in a nonspecific, but nonetheless highly selective, manner in disrupting an endogenous mechanism regulating the number, morphology, and location of spines.