Golgi-like immunoperoxidase staining of hypothalamic magnocellular neurons that contain vasopressin, oxytocin or neurophysin in the rat

Abstract

Hypothalamic magnocellular neurons that contain vasopressin, oxytocin, and neurophysin were stained in 120 μm thick frozen sections using specific antisera and a modified immunoperoxidase technique yielding a Golgi-like image allowing detailed analysis of neuromorphology. Various peptide-containing processes were observed arising from magnocellular neurons. These could be divided into three basic groups: (1) beaded processes, (2) dendritic processes, and (3) thick peptide-containing processes. Beaded processes with an appearance similar to the classical descriptions of beaded neurosecretory axons, and which exhibited a morphology similar to the processes comprising the major efferent tracts in the present study, were considered to be axons. Beaded axonal processes were found arising from neurons in several ways: directly from the cell body, from the basal portion of a dendritic stem, from the proximal portion of a thicker process, and as the direct extension of a thicker process. Short, finelybeaded processes of an as yet unknown nature were found arising from distal portions of processes considered dendrites. A number of short, beaded, collateral branches and occasional long, beaded, collateral branches, were observed arising from axons close to the parent neurons in certain areas, particularly in the dorsal caudal division of the paraventricular nucleus. Most of the dendritic processes appeared as short, unbranching, lightly-stained extensions of the perikaryon. In some cases, dendrites gave rise to one, or rarely, several, smooth tapering peptide-containing branches. Most dendrites were between 30 μm and 300 μm long, however, a number extended over 500 μm. Some dendritic processes gave rise to short beaded processes far distal to the perikaryon, and some appeared to terminate in a plexus of beaded fibers. A third descriptive group of processes was formed to accommodate processes which fit neither the definitions of axons nor of dendrites. These thick peptide-containing processes appeared as processes of irregular caliber, forming thick, intensely-stained swellings which often attained diameters equal to or greater than that of the perikaryon. Often they became very thickly, coarsely beaded. Most were from 150–300 μm long, terminating either freely in the surrounding neuropil, or within one of the magnocellular neural cell groups, and occasionally adjacent to another immunopositively-stained neuron. In addition, a number of thick peptide-containing processes appeared to terminate in contact with the ependyma of the third ventricle. Throughout the hypothalamus, the perikarya of most magnocellular neurons appeared somewhat rounded or oval with diameters ranging from 20–35 μm. A number of spindle-shaped perikarya were also noted, with cross-section widths of 10–14 μm and lengths of 30–40 μm. Most hypothalamic magnocellular neurons appeared to be multipolar, although some bipolar neurons were found. Various types of neurons were observed as having different combinations of processes. The most frequent combination was one axon plus several dendritic processes. Other combinations were one axon plus one thick peptide-containing process; or one axon, one dendrite, plus one thick peptide-containing process. Occasionally, neurons were found giving rise to 2 beaded axon-like processes plus several dendritic processes. Differences in the distribution within the hypothalamus of the various types of magnocellular neurons were also noted. The basic characteristics described were found for both vasopressin- and oxytocin-containing neurons, and there appeared to be no major differences between these two types of neuron. In this study we have specifically stained peptidergic hypothalamic magnocellular neurons using a modified immunoperoxidase procedure yielding a Golgi-like image. The preparations obtained were comparable with classical Golgi staining, allowing detailed cytological analysis and revealing that these neurons constitute a heterogeneous population, exhibiting a much more varying and complex morphology than has previously been appreciated. This heterogeneity of morphology may reflect the involvement of these neurons in several different functions.