Morphological correlates of corticosteroid-induced changes in prefrontal cortex-dependent behaviors

Abstract

Imbalances in the corticosteroid milieu have been implicated in several neuropsychiatric disorders, including depression and schizophrenia. Prefrontal cortex (PFC) dysfunction is also a hallmark of these conditions, causing impairments in executive functions such as behavioral flexibility and working memory. Recent studies have suggested that the PFC might be influenced by corticosteroids released during stress. To test this possibility, we assessed spatial working memory and behavioral flexibility in rats submitted to chronic adrenalectomy or treatment with corticosterone (25 mg/kg) or the synthetic glucocorticoid dexamethasone (300 microg/kg); the behavioral analysis was complemented by stereological evaluation of the PFC (prelimbic, infralimbic, and anterior cingulate regions), the adjacent retrosplenial and motor cortices, and the hippocampal formation. Dexamethasone treatment resulted in a pronounced impairment in working memory and behavioral flexibility, effects that correlated with neuronal loss and atrophy of layer II of the infralimbic, prelimbic, and cingulate cortices. Exposure to corticosterone produced milder impairments in behavioral flexibility, but not in working memory, and reduced the volume of layer II of all prefrontal areas. Interestingly, adrenalectomy-induced deleterious effects only became apparent on the reverse learning task and were not associated with structural alterations in the PFC. None of the experimental procedures influenced the morphology of retrosplenial or motor cortices, but stereological measurements confirmed previously observed effects of corticosteroids on hippocampal structure. Our results describe, for the first time, that imbalances in the corticosteroid environment can induce degeneration of specific layers of the PFC; these changes appear to be the morphological correlate of corticosteroid-induced impairment of PFC-dependent behavior(s).

Figure 1.

Biometric parameters and hormonal determinations. A, Body weights over the course of the study. B, Thymus/body weight ratios. C, Adrenal weights. D, Serum corticosterone levels with respect to time of day (left, 1 h after lights on, 9:00 A.M.; middle, middle of the daily light/dark cycle, 3:00P.M.; right, 1 h after lights out, 9:00 P.M.) after 4 weeks of exposure to the various treatments. CORT and DEX were injected 1 h before lights off; CON and ADX animals received vehicle injections. ^*p < 0.05 versus all other groups; ^#p < 0.05 versus CON and versus ADX; ^##p < 0.05 versus CON and versus CORT; ⁺⁺p < 0.05 versus CON. Error bars represent SEM.

Figure 2.

Behavioral data. A, Schematic representation of the experimental procedure. The position of the platform on each day is also indicated (above). Note that, during the 3 d allowed for memory consolidation (days 4-6), the platform was kept at the same position. B, Learning curve in the working memory task. The higher distances swum by the DEX-treated rats is easily appreciated. C, Results from the reverse task experiment: average distance swum on the four trials in each imaginary quadrant is given as a percentage of the total distance swum. ^*p < 0.05 versus all other groups; ^#p < 0.05 versus CON and versus ADX; ⁺⁺p < 0.05 versus CON. Error bars represent SEM.

Figure 3.

Stereology estimations. A, Boundaries between different regions and layers, traced over brain slices stained with Giemsa (left to right) at +2.2, +0.7, and -3.8 cm from bregma, respectively (magnification, 7.9×). B-D, Average volumes of layers I, II, and III-VI of the five regions studied. E, F, Estimated number of neurons in layers II and III-VI of the five regions studied. Note the different (and broken) scales used and that, with the exception of the numerical values for the IL and PL, which should be read off against the left axis, all other values should be read off against the right one. ^*p < 0.05 versus every other group; ^#p < 0.05 versus CON and versus ADX; ⁺p < 0.05 versus ADX; ⁺⁺p < 0.05 versus CON. Error bars represent SEM.

Figure 4.

Stereological data from the hippocampal formation. A-C, Mean volumes of the different layers of the dentate gyrus, CA3, and CA1 regions, respectively. D, Estimated number of neurons in the granule [dentate gyrus (DG)] and pyramidal (CA3 and CA1) layers. Note that, in A and D, the numerical values for the leftward bars (molecular layer and granule cell layer, respectively) should be read off against the axis on the left, whereas all other data should be read off against the right one. ^*p < 0.05 versus every other group; ^#p < 0.05 versus CON and versus ADX; ⁺⁺p < 0.05 versus CON; ^##p < 0.05 versus CON and versus CORT. Error bars represent SEM.