Adolescent Intermittent Alcohol Exposure: Dysregulation of Thrombospondins and Synapse Formation are Associated with Decreased Neuronal Density in the Adult Hippocampus

Abstract

Background Adolescent intermittent alcohol exposure (AIE) has profound effects on neuronal function. We have previously shown that AIE causes aberrant hippocampal structure and function that persists into adulthood. However, the possible contributions of astrocytes and their signaling factors remain largely unexplored. We investigated the acute and enduring effects of AIE on astrocytic reactivity and signaling on synaptic expression in the hippocampus, including the impact of the thrombospondin (TSP) family of astrocyte-secreted synaptogenic factors and their neuronal receptor, alpha2delta-1 (α2δ-1). Our hypothesis is that some of the influences of AIE on neuronal function may be secondary to direct effects on astrocytes. Methods We conducted Western blot analysis on TSPs 1–4 and α2δ-1 from whole hippocampal lysates 24hrs after the 4th and 10th dose of AIE, then 24 days after the last dose (in adulthood). We used immunohistochemistry to assess astrocyte reactivity (i.e. morphology) and synaptogenesis (i.e. co-localization of pre- and post-synaptic puncta). Results AIE reduced α2δ-1 expression, and co-localized pre- and post-synaptic puncta after the 4th ethanol dose. By the 10th dose, increased TSP2 levels were accompanied by an increase in co-localized pre- and post-synaptic puncta, while α2δ-1 returned to control levels. 24 days after the last ethanol dose (i.e. adulthood), TSP2, TSP4, and α2δ-1 expression were all elevated. Astrocyte reactivity, indicated by increased astrocytic volume and area, was also observed at that time. Conclusions Repeated ethanol exposure during adolescence results in long-term changes in specific astrocyte signaling proteins and their neuronal synaptogenic receptor. Continued signaling by these traditionally developmental factors in adulthood may represent a compensatory mechanism whereby astrocytes reopen the synaptogenic window and repair lost connectivity, and consequently contribute to the enduring maladaptive structural and functional abnormalities previously observed in the hippocampus after AIE.

Sustained alterations in neuroimmune gene expression after daily, but not intermittent, alcohol exposure

“…Intermittent access to drugs of abuse results in altered behavioral responses in comparison to continued administration (Marec et al, 2011), and intermittent exposure administration procedures are commonly used to study alcohol effects (Pandey et al, 2015; Risher et al, 2015). Having assessed adaptations in cytokine responses that occur with repeated once-daily exposure to ethanol, we next examined whether the schedule of ethanol exposure would impact cytokine expression patterns in the CNS.…”

Glial and Neuroimmune Mechanisms as Critical Modulators of Drug Use and Abuse

“…However, there is some evidence that alcohol may uniquely affect astrocytes during this developmental period. Intermittent alcohol exposure during adolescence is associated with changes in TSP expression within the hippocampus (Risher et al, 2015b). As previously mentioned, TSPs are glycoproteins secreted in part by astrocytes which mediate cell and matrix adhesion and are critical regulators of synaptogenesis during CNS development (Christopherson et al, 2005;Eroglu et al, 2009).…”

Consequences of adolescent use of alcohol and other drugs: Studies using rodent models

“…A similar enhanced vulnerability of adolescents was also reported by Broadwater et al (2014) who observed lasting disruptions in neurogenesis and increases in cell death in adult rats after EtOH exposure during adolescence but not following equivalent exposure in adulthood. At the synaptic level, there is recent evidence that adolescent exposure to EtOH results in expression of a greater proportion of immature, more “plastic” synaptic spines in the adult HPC (Risher et al, 2015). Alterations in brain activity in this region were evident in terms of long-term potentiation (LTP), with adults exposed to EtOH during adolescence exhibiting more robust LTP than non-exposed animals, suggesting a surprising enhanced state of synaptic plasticity (Risher et al, 2015).…”

“…At the synaptic level, there is recent evidence that adolescent exposure to EtOH results in expression of a greater proportion of immature, more “plastic” synaptic spines in the adult HPC (Risher et al, 2015). Alterations in brain activity in this region were evident in terms of long-term potentiation (LTP), with adults exposed to EtOH during adolescence exhibiting more robust LTP than non-exposed animals, suggesting a surprising enhanced state of synaptic plasticity (Risher et al, 2015). Electrophysiological studies have revealed long-lasting decreases in slow-wave sleep, as well as attenuations in the P3 component of evoked potentials (Ehlers & Criado, 2010).…”