Morphology Of Dendrites Of Peptidergic Neurons

The dendrites of the hypothalamic supraoptic and paraventricular magnocellular neurons which secrete vasopressin or/and oxytocin are, like the dendrites of most peptidergic neurons, relatively simple structures with few branches and few or no classical dendritic spines, but are the only peptidergic dendrites to have been studied in great detail. The simple morphology has been determined by three different light microscopic procedures: Golgi; dye-filling; and immunocytochemical detection of the peptide contained in the dendrites. Magnocellular neurons have been notoriously difficult to stain with Golgi methods. Armstrong (1995) provides a detailed review of the results achieved with this and other methods in the supraoptic nucleus. The majority show that most of the neurons are bipolar, though some have only one dendrite, and very few are multipolar (particularly in the retrochiasmatic (tuberal) part of the nucleus); branching in all types of dendrite is sparse. In all types, however, one dendrite passes ventrally to the ventral glial lamina, a second may pass dorsally. The dendrites are described as varicose, but few of the cells illustrated show dendritic dilatations of the size seen by electron microscopy (see below). Intracellular fills are most easily made in slice preparations. This technique confirmed and amplified results from Golgi impregnations but, like the Golgi impregnations, did not reveal the large dilations. This may be because such dilatations occur at a relatively long distance from the cell body, often after the ventral dendrites have turned laterally within/just above the ventral glial lamina. Intracellular filling of magnocellular neurons by retrograde uptake of cholera toxin-coupled horseradish peroxidase from the posterior pituitary (Ju et al., 1986) revealed a dense plexus of axon-and dendrite-like processes just beneath the ependyma of the third ventricle. Immunocytochemical staining for vasopressin, oxytocin, or their neurophysins (Fig. 1A) again confirms the rather simple relatively unbranched nature of the dendritic tree. In some studies (e.g. Sofroniew and Glassmann, 1981; Ju et al., 1992) larger dilatations of the dendrites are reported (Fig. IB). It has also been shown that there appear to be few consistent differences either between the morphology of dendrites of oxytocin and

Fig. 1 A: Magnocellular neurons in the paraventricular nucleus immunostained for vasopressin neurophysin. The simple dendrites of the cells pass medially and ventrally toward the third ventricle. The fine beaded axons pass laterally toward the fornix. B Isolated multipolar magnocellular neuron immunolabelled for vasopressin neurophysin. The dendrite shows one branch (arrowhead) and a number of smaller and larger dilatations (arrows).

Fig. 1 A: Magnocellular neurons in the paraventricular nucleus immunostained for vasopressin neurophysin. The simple dendrites of the cells pass medially and ventrally toward the third ventricle. The fine beaded axons pass laterally toward the fornix. B Isolated multipolar magnocellular neuron immunolabelled for vasopressin neurophysin. The dendrite shows one branch (arrowhead) and a number of smaller and larger dilatations (arrows).

Dendrites

Fig. 2: Rat supraoptic nucleus immunostained for vasopressin neurophyin. The cell bodies (arrowheads) are seen adjacent to the optic chiasm (OC). The dendrites, which are strongly immunostained, pass laterally in a large sheaf along the ventral surface of the hypothalamus (dendrites). Inset: Rat supraoptic nucleus immunostained for oxytocin neurophysin. Detail of dendrites in the dendrite-rich region adjacent to the ventral glial lamina. Some parts of the dendrites are quite narrow, other parts are more dilated. Immunoreactivity within the dendrite is patchy consistent with the uneven distribution of the hormone-containing dense-cored vesicles.

vasopressin neurons, or between those of neurons in the supraoptic and paraventricular nucleus. The ventral dendrites from most supraoptic neurons pass down into the ventral glial lamina region and then run laterally beneath the surface of the brain, extending in some cases for several hundred microns (Fig. 2). Coronal sections show most dendrites of paraventricular magnocellular neurons running medially and obliquely downward toward the third ventricle (Fig. 1) and apparently ending there. However, horizontal and parasagittal sections reveal that many of these turn abruptly and run for a considerable distance parallel to the ventricular wall. Thus the dendrites of both paraventricular and supraoptic magnocellular neurons project toward and then run along a brain surface in contact with the CSF. The function of this proximity to the CSF has never been resolved. The rapid response of magnocellular neurons to intraventricular administration of osmotic stimuli suggests a sensory role, but equally the presence of vasopressin, oxytocin and their neurophysins in the cerebrospinal fluid is consistent with targeting of secretion to the CSF, though the function of secretion into the CSF remains uncertain.

Electron microscopy is not good for evaluating the overall morphology of peptidergic dendrites, but is very good at showing the fine structure of such dendrites.

Once again, it is the dendrites of the magnocellular neurons that have been most extensively studied. The proximal parts of the dendrites of magnocellular neurons can be followed from the cell body and more distal parts can be recognised when vasopressin- or oxytocin-containing 160nm dense-cored vesicles are present. Electron-microscopic observations are, however, at odds with most light microscopic studies in two respects: electron microscopy reveals large secretory vesicle-filled dilatations of the distal parts of the dendrites particularly in the more lateral parts of the region ventral to the supraoptic nucleus (Fig. 3A); but on the other hand it has failed to reveal classical dendritic spines with afferent boutons and spine apparatus, so the nature of the spine-like processes reported in a number of light microscopic studies remains uncertain. It might be argued that such spines are only present on parts of the dendrites that lack identifying secretory vesicles, but this seems unlikely given the widespread distribution of secretory vesicles in the dendrites.

The other feature that electron microscopy has revealed is that, at or after the end of pregnancy and during lactation (see Russell et al., 2003), the dendrites of oxytocin- but not vasopressin-secreting neurons become bundled together without intervening glia and that many synaptic boutons now contact more than one dendrite or cell body (double or shared synapses); these plastic changes regress after lactation (see Theodosis, 2002) and are caused by oxytocin released from the dendrites (see below).

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