A novel animal model of hippocampal cognitive deficits, slow neurodegeneration, and neuroregeneration

Abstract

Long-term adrenalectomy (ADX) results in an extensive and specific loss of dentate gyrus granule cells in the hippocampus of adult rats. This loss of granule cells extends over a period of weeks to months and ultimately results in cognitive deficits revealed in a number of tasks that depend on intact hippocampal function. The gradual nature of ADX-induced cell death and the ensuing deficits in cognition resemble in some important respects a variety of pathological conditions in humans. Here, we characterize behavioural and cellular processes, including adult neurogenesis, in the rat ADX model. We also provide experimental evidence for a neurogenic treatment strategy by which the lost hippocampal cells may be replaced, with the goal of functional recovery in mind.

Figure 1

Representative Fluoro-Jade B labeled sections taken from a control (a) and adrenalectomized (ADX) rats at 3 days (b), 8 weeks (c), and 23 weeks (d) after surgery. No Fluoro-Jade B positive cells were apparent in control tissue (a). Fluoro-Jade B positive cells were evident by 3 days after ADX (indicated by arrows) and appeared in the superior lateral blade of the dentate gyrus (b). At 8 weeks after ADX surgery, Fluoro-Jade B positive cells were evident throughout the entire granule cell layer but was still most concentrated in the superior, lateral blade (c). Fluoro-Jade B labeling has substantially decreased at the 23-week time point but was still apparent throughout the entire dentate gyrus granule cell layer (d). Scale bar = 100 μm.

Figure 2

Representative DAPI-labeled sections containing the dentate gyrus granule cell layer in a control (a) and an adrenalectomy (ADX) rat (b) after 10 weeks of degeneration. (c) By 10 weeks, ADX rats had lost approximately one-third of the volume of the dentate gyrus granule cell layer when compared to adrenally intact controls. (d) Cell counts using the optical fractionator revealed a loss of roughly half of the dentate gyrus granule cells at the same time point. *indicates significant difference; error bars represent standard error of the mean (adapted from [29]).

Figure 3

Pilot data assessing the electrophysiological response of the dentate gyrus after 10 weeks of adrenalectomy (ADX) followed by one-week of CORT replacement, all ADX rats were on CORT replacement at the time of recording. (a) Chronic ADX resulted in a significantly attenuated fEPSP slope relative to control rats. (b) A tendency for ADX rats to have a lower population spike amplitude compared to intact controls was apparent. Error bars represent standard error of the mean.

Figure 4

Behavioural deficits associated with long-term ADX. (a) ADX rats were impaired relative to controls at locating both novel and familiar platform locations in a moving platform version of the Morris water task. (b) Discrimination ability of ADX rats in the object/context mismatch task was significantly impaired. ADX rats did not discriminate above chance levels. Dashed lines represent chance, error bars represent standard error of the mean.

Figure 5

Regardless of adrenalectomy (ADX), the combined enrichment and exercise treatment significantly increased neurogenesis as indexed by doublecortin (DCX). Representative pictures of DCX positive cells in the dentate gyrus of (a) home cage controls, (b) controls exposed to wheel running and enrichment, (c) ADX rats housed in the home cage, and (d) ADX rats exposed to wheel running and enrichment.

Figure 6

(a) Our alternating treatment of enrichment and wheel running significantly increased the number of doublecortin (DCX) positive cells in the dentate gyrus of adrenalectomy (ADX) and control rats. (b) Chronic ADX significantly reduced the volume of the dentate gyrus granule cell layer. Most importantly, a combination of six weeks of enriched housing significantly increased the volume of the dentate gyrus granule cell layer in ADX rats, compared to their home cage counterparts. Error bars represent standard error of the mean.

Figure 7

The experimental time course. Bilateral adrenalectomy (ADX) is conducted at the onset of the experiment. The granule cell layer of the dentate gryus degenerates significantly over the following ten weeks, during which rats remain in their homecages. A subset of ADX rats are exposed to a number of different treatment strategies, after which behavioural and electrophysiological measures can be taken. Upon completion of the experiment, rats are perfused and the granule cell layer is assessed using immunohistochemistry and unbiased stereological techniques.