Isomorphic activation of astrocytes in the somatosensory thalamus

Structural recovery in the rat somatosensory thalamus after the loss of one of its major inputs provided a model for studying the changes in astrocytes associated with reactive synaptogenesis. The temporal separation of the initiation of Wallerian degeneration and reactive synaptogenesis permitted astrocytic changes to be correlated either with the removal of degeneration, early in the recovery sequence, or with synaptogenesis, later in recovery. Over a period of post-lesion times ranging from 3 days to 13.5 months, GFAP-positive astrocytic fibers were quantified and the population density of S-100-positive astrocytic cell bodies was determined in the ventral posterolateral nucleus (VPL). The relative area of astrocytic cell bodies was measured at an early peak of the increased GFAP immunoreactivity (4–5 days post-lesion). The normal side of VPL (c-VPL) was compared to the deafferented side of VPL (d-VPL) and the ratio d-VPL/c-VPL was determined. Astrocytes in d-VPL underwent a minimal isomorphic activation with little or no hypertrophy or proliferation but with a large increase in GFAP immunoreactivity. Prior to the initiation of synaptogenesis, there was a decrease both in GFAP immunoreactivity and in the population density of VPL astrocytes. The decreases in the recovery curves suggested that a suppression of the influence of astrocytes may have been important for sprouting and/or synaptogenesis. In other systems, where synaptogenesis was initiated early in the recovery sequence, the suppression of astrocytes that was related to synaptogenesis may have been masked by astrocytic changes related to the removal of degeneration.