Abstract

Introduction

Materials & Methods

Results

Discussion & Conclusion

References

Discussion
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Astrocytes the major glial type in the SON, are closely apposed to and interact with the magnocellular neuroendocrine cells (MNCs) responsible for the production and release of the peptides vasopressin (VP) and oxytocin (OX). Activation of the SON that results in the production of VP and OX occurs in response to a wide range of stimuli including dehydration, parturition, suckling and maternal behavior. Most studies of plasticity in the SON have examined the responses to dehydration/rehydration and lactation (Salm et al., 1985), conditions that induce coordinated and reversible structural alteration of the astrocytes and neurons.

The somata of the MNCs are mostly separated from their neighbors by fine astrocytic processes when demand for the peptides is low. With activation of the SON there is a noticeable absence of astrocytic processes that normally separate the MNCs, their dendrites and synapses (Hatton, 1990; Hatton, 1997; Theodosis and Poulain, 1993). Removal of astrocytic processes results in an increase in the amount of directly opposed membranes between MNC somata (Theodosis et al., 1981; Tweedle and Hatton, 1976; Tweedle and Hatton, 1977) their dendrites (Perlmutter et al., 1985). and synaptic reorganization (Theodosis et al., 1981; Tweedle and Hatton, 1984). All of these changes are associated with increases in excitability in the SON and are thought to facilitate the release of hormone (Hatton, 1990; Hatton, 1997; Theodosis and Poulain, 1993), through enhancing neuronal communication via synapses, gap junctions and ephaptic interactions (Hatton, 1997). When hormone demand returns to basal levels, astrocytic processes are once again observed between the neuronal elements (Hatton et al., 1984; Tweedle and Hatton, 1984). These findings of neuronal changes, such as the formation and elimination of synapses, that temporally correspond to the retraction and extension of astrocytic processes (Tweedle and Hatton, 1976; Tweedle and Hatton, 1977) provide evidence for the hypothesis that astrocytes alter their physical relationships with neurons in order to regulate neuronal communication in a use-dependent manner. (See reviews by : Hatton, 1990; 1997; Theodosis and Poulain, 1993).

While a few stellate astrocytes are scattered throughout the dorsal portion of the SON that contains the MNCs, most of the glial processes that are normally interposed between adjacent MNCs appear to originate from astrocytic cell bodies that lie near the pial surface in the ventral glial limitans subjacent to the SON (SON-VGL). Recent experiments in our laboratory using electron and light microscopic measures of astrocytes in the SON-VGL, demonstrated that the thickness of the SON-VGL was significantly decreased in 9-day dehydrated animals when compared with controls (Bobak and Salm, 1996). This reversed toward control levels with rehydration. An electron microscopic evaluation of individual astrocytes in the same material revealed that a reversible overall reorientation of VGL astrocytes, from a direction perpendicular to the pial surface to one parallel to the pial surface also occurred with stimulation. To further assess the possible active rearrangement of astrocytic processes in the SON, we investigated immunoreactivity for the major astrocytic cytoskeletal constituent, glial fibrillary acidic protein (GFAP), reasoning that changes in GFAP might accompany gross structural changes of these cells. The optical density of GFAP immunoreactivity (GFAP-IR) in the dorsal region of the SON was significantly decreased in the dehydrated group compared to controls and subsequently returned to control levels with rehydration (Hawrylak et al., 1998). In addition, the SON-VGL which appeared as a solid region of GFAP-IR in this material, was selectively reduced in size in dehydrated animals, a change that was reversed with rehydration.

Two important questions related to the astrocytic structural plasticity in dehydrated animals are addressed in the present study. First, how does the reversible change of GFAP-IR as measured by optical density relate to the morphology of astrocytic processes in the SON? Second, can changes in the number of astrocytes in the VGL account for the reversible decrease in VGL thickness with dehydration? Through the use of stereological techniques to estimate the surface density (Sv) of astrocytic processes in the SON and glial cell number (Nv) in the VGL we report here that dehydration and rehydration resulted in significant and reversible changes in Sv in the SON while the number of cells in the VGL remained constant.

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