Invited Symposium: Behaviour-Induced Neural Events after Brain Injury
Discussion and Conclusion
After 7 days of dehydration, astrocytic processes within the SON were significantly reduced in surface density in comparison to control and dehydration/rehydration animals. In addition, the numerical density of astrocytes in the VGL was not significantly reduced by dehydration. However, these finding together indicate a reduction of astrocyte processes per VGL astrocyte.
We find it of interest that the number of astrocytes in the SON-VGL was unchanged across all groups, as it indicates that the reduced thickness which we have seen in two separate studies (Bobak and Salm, 1996; Hawrylak et al., 1998) is not due to an overall reduction in the number of cells. This is surprising, since we have evidence for cell death in the SON-VGL at both the light and electron microscopic levels using both morphological (Bobak et al., 1997) and TUNEL labeling (Gavrieli and Ben-Sasson, 1992) as criteria. The most reasonable explanation for this is that new astrocytes are being born in the SON and the VGL that are replacing those lost to cell death. Indeed, it has been long known (Murray, 1968; Patterson and Leblond, 1977) that there is proliferation of astrocytes in the activated SON. We have recently reexamined the issue of proliferation with contemporary techniques (Moats and Salm, unpublished observations) and found significant cell proliferation in both the SON and the VGL with dehydration. However, the amount of proliferation in the VGL was much less than that seen in the SON. We currently speculate that some of the astrocytes born in the SON migrate ventrally to take up residence in the VGL. In support of this, we have documented (Singleton and Salm, 1996) a significant reduction in the extracellular matrix protein tenascin, as well as a reduction in the VGL basal lamina (Bobak et al., 1996) in the VGL with dehydration, which could both function to permit cell migration through the sometimes tortuous maze of astrocyte processes found there.
The observed dehydration-associated reduction in astrocyte surface volume per astrocyte provides further support for the hypothesis that when the neurons of the SON are activated, extensive astrocyte process withdrawal from between MNC somata occurs.
We wish to thank Mr. Jeff Altemus, Allison Reed and the WVU Health Sciences Center Imaging Facility for imaging support and Dr. Gerry Hobbs for statistical consultation. Troy Sedlmeyer was supported by a Howard Hughes undergraduate research fellowship through the WVU Department of Biology. David Boone was supported by the WVU Health Sciences Center, High School Research Apprenticeship Program. This work was supported by NSF IBN 951457.
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|Hawrylak, N; Sedlmeyer, T; Boone, D; Salm, AK; (1998). Dehydration and Rehydration Reversibly Alter the Surface Density of Glial Fibrillary Acidic Protein Immunoreactivity (GFAP-IR) of Astrocytes in the Rat Supraoptic Nucleus. Presented at INABIS '98 - 5th Internet World Congress on Biomedical Sciences at McMaster University, Canada, Dec 7-16th. Invited Symposium. Available at URL http://www.mcmaster.ca/inabis98/schallert/hawrylak0850/index.html|
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