Chronic Unpredictable Stress Reduces Immunostaining for Connexins 43 and 30 and Myelin Basic Protein in the Rat Prelimbic and Orbitofrontal Cortices.

Background: Astrocytes and oligodendrocytes are pathologically altered in dorsolateral prefrontal and orbitofrontal cortices in major depressive disorder. In rat models of stress (major depressive disorder risk factor) astrocyte gap junction protein connexin 43 (Cx43) is reduced in the prelimbic cortex. Astrocyte connexins are recognized to strongly influence myelin maintenance in the central nervous system. However, it is unknown whether stress-related changes in Cx43 and the other major astrocyte connexin, Cx30, occur in the orbitofrontal cortex, or whether connexin changes are concurrent with disturbances in myelination.

Methods: Frozen sections containing prelimbic cortex and orbitofrontal cortex of rats subjected to 35 days of chronic unpredictable stress and controls (n = 6/group) were immunolabeled for Cx43, Cx30, and myelin basic protein. Density of Cx43 or Cx30 immunoreactive puncta and area fraction of myelin basic protein immunoreactivity were measured in prelimbic cortex and orbitofrontal cortex and results analyzed with test or Pearson correlations.

Results: Density of Cx43- and Cx30-positive puncta in both prelimbic cortex and orbitofrontal cortex was lower in chronic unpredictable stress-treated than in control rats. In both regions, the area fraction of myelin basic protein immunoreactivity was also lower in chronic unpredictable stress animals. Myelin basic protein area fraction was positively correlated with the density of Cx43-positive puncta in orbitofrontal cortex, and with Cx30 puncta in prelimbic cortex.

Conclusion: Low Cx43 and Cx30 after chronic unpredictable stress in rat prelimbic cortex and orbitofrontal cortex suggests that reduced astrocytic gap junction density may generalize to the entire prefrontal cortex. Concurrent reduction of Cx43-, Cx30-, and myelin basic protein-immunolabeled structures is consistent with a mechanism linking changes in astrocyte gap junction proteins and disturbed myelin morphology in depression.