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The molecular genetics of panic disorder (PD) with and without agoraphobia (AG) are still largely unknown and progress is hampered by small sample sizes. We therefore performed a genome-wide association study with a dimensional, PD/AG -related anxiety phenotype based on the Agoraphobia Cognition Questionnaire (ACQ) in a sample of 1,370 healthy German volunteers of the CRC TRR58 MEGA study wave 1. A genome-wide significant association was found between ACQ and single non-coding nucleotide variants of the GLRB gene (rs78726293, p=3.3x10 -8 ; rs191260602, p=3.9x10 -8 ). We followed up on this finding in a larger dimensional ACQ sample (N=2,547) and in independent samples with a dichotomous AG phenotype based on the Symptoms Checklist (SCL-90; N=3,845) and a casecontrol sample with the categorical phenotype PD/AG (N combined =1,012) obtaining highly Partial Glrb knockout-mice demonstrated an agoraphobic phenotype. In conjunction with the clinical observation that rare coding GLRB gene mutations are associated with the neurological disorder hyperekplexia characterized by a generalized startle reaction and agoraphobic behavior, our data provide evidence that non-coding, though functional GLRB gene polymorphisms may predispose to PD by increasing startle response and agoraphobic cognitions.

Proliferation of astrocytes plays an essential role during ontogeny and in the adult brain, where it occurs following trauma and in inflammation and neurodegenerative diseases as well as in normal, healthy mammals. The cellular mechanisms underlying glial proliferation remain poorly understood. As dopamine is known to modulate proliferation in different cell populations, we investigated the effects of dopamine on the proliferation of striatal astrocytes in vitro. We found that dopamine reduced proliferation. As proliferation involves, among other things, a change in cell volume, which normally comes with water movement across the membrane, water channels might represent a molecular target of the dopamine effect. Therefore we studied the effect of dopamine on aquaporin 4 (AQP4) expression, the main aquaporin subtype expressed in glial cells, and observed a down-regulation of the AQP4-M23 isoform. This down-regulation was the cause of the dopamine-induced decrease in proliferation as knockdown of AQP4 using siRNA techniques mimicked the effects of dopamine on proliferation. Furthermore, stimulation of glial proliferation by basic fibroblast growth factor was also abolished by knocking down AQP4. In addition, blocking of AQP4 with 10 mum tetraethylammonium inhibited osmotically induced cell swelling and stimulation of glial cell proliferation by basic fibroblast growth factor. These results demonstrate a clear-cut involvement of AQP4 in the regulation of proliferation and implicate that modulation of AQP4 could be used therapeutically in the treatment of neurodegenerative diseases as well as in the regulation of reactive astrogliosis by preventing or reducing the glia scar formation, thus improving regeneration following ischemia or other trauma.

Reactive astrogliosis is the universal response to any brain insult. It is characterized by cellular hypertrophy, up-regulation of the astrocyte marker glial fibrillary acidic protein (GFAP), and proliferation. The source of these proliferating cells is under intense debate. Progenitor cells derived from the subventricular zone (SVZ), cells positive for chondroitin sulfate proteoglycan (NG2(+)), and de-differentiated astrocytes have been proposed as the origin of proliferating cells following injury. We have analyzed the effect of intraventricular-applied 6-hydroxydopamine (6-OHDA) on the proliferation and morphology of astrocytes in rat cortex and striatum by means of immunohistochemistry and confocal laser microscopy. At 4 days post-lesion, GFAP expression increased markedly. A subpopulation of the GFAP(+) cells co-expressed Ki-67, indicating that these cells were proliferating. To investigate whether these cells (1) arose from migrating SVZ progenitor cells, (2) derived from NG2(+) progenitor cells, or (3) de-differentiated from resident astrocytes, we studied the expression of the migration marker doublecortin (Dcx), the oligodendrocyte progenitor marker NG2, and the progenitor markers Nestin and Pax6. The proliferating Ki-67(+) cells co-expressed Nestin and Pax6, whereas no co-expression of Ki-67 with NG2 or the migration marker Dcx was observed. Thus, resident astrocytes de-differentiate, in response to the intraventricular application of 6-OHDA, to a phenotype resembling radial glia cells, which represent transient astrocyte precursors during development. An understanding of the mechanisms of the de-differentiation of mature astrocytes might be useful for designing new approaches to cell therapy in neurodegenerative diseases such as Parkinson's disease.

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