Treatment strategies targeting excess hippocampal activity benefit aged rats with cognitive impairment

Abstract

Excess neural activity in the CA3 region of the hippocampus has been linked to memory impairment in aged rats. We tested whether interventions aimed at reducing this excess activity would improve memory performance. Aged (24 to 28 months old) male Long-Evans rats were characterized in a spatial memory task known to depend on the functional integrity of the hippocampus, such that aged rats with identified memory impairment were used in a series of experiments. Overexpression of the inhibitory neuropeptide Y 13-36 in the CA3 via adeno-associated viral transduction was found to improve hippocampal-dependent long-term memory in aged rats, which had been characterized with impairment. Subsequent experiments with two commonly used antiepileptic agents, sodium valproate and levetiracetam, similarly produced dose-dependent memory improvement in such aged rats. Improved spatial memory with low doses of these agents was observed in both appetitve and aversive spatial tasks. The benefits of these different modalities of treatment are consistent with the concept that excess activity in the CA3 region of the hippocampus is a dysfunctional condition that may have a key role underlying age-related impairment in hippocampal-dependent memory processes. Because increased hippocampal activation occurs in age-related memory impairment in humans as observed in functional neuroimaging, the current findings also suggest that low doses of certain antiepileptic drugs in cognitively impaired elderly humans may have therapeutic potential and point to novel targets for this indication.

Figures

Figure 1

Background behavioral characterization. The cognitive status of aged and young rats was assessed in a standardized water maze task before intervention experiments. Higher learning index scores signify worse performance by reflecting search at a greater distance from the escape location during memory probe tests. Aged rats that performed beyond the normative range of young adult rats in this study population were selected for the present studies. *p=0.001.

Figure 2

Behavioral assessment of NPY treatment. Memory-impaired aged rats that received AAV–NPY 13–36 injections into the CA3 of the hippocampus 2 weeks before, performed better than control rats during water maze training (a), and showed stronger memory retention for the escape platform location in a probe trial 24 h after training (b). See text for details of data analysis.

Figure 3

AAV–NPY transduction in the hippocampus. Photomicrographs of coronal hippocampal sections of aged rats showing FIB–NPY 13–36 mRNA (right panel) was present in the NPY group, and the absence of specific probe hybridization in the brain of a control rat (left panel). In testing for the FIB–NPY in situ probe, we also generated and hybridized a sense control probe. This control probe showed very little signal as expected (not shown). DG, dentate gyrus.

Figure 4

VPA assessment in the water maze. Memory-impaired aged rats that received chronic treatment with VPA or saline performed at a comparable level at the end of the water maze training phase (Trial 6), but those treated with VPA at 100 mg/kg/day (VPA 100) showed less forgetting as measured by path length (a) and escape latency (b) after a 6-h delay (Retention Trial) compared with their counterparts treated with saline (VEH) or VPA at 50 mg/kg/day (VPA 50). See text for details of data analysis.

Figure 5

VPA and LEV assessment in the radial maze. Efficacious doses of VPA and LEV to improve memory performance of aged rats with cognitive deficit were determined using a within-subject design in an eight-arm radial maze. Over the course of the study, the same rats were tested repeatedly with vehicle saline (0 mg/kg), VPA, and LEV. Post-delay memory errors were significantly lower than vehicle baseline when rats were treated with VPA at 100 and 200 mg/kg, as well as with LEV at 5 and 10 mg/kg. *p<0.050 and #p=0.051 compared to vehicle treatment (0 mg/kg).

Figure 6

Combination VPA/LEV assessment in the radial maze. Combination therapy consisting of VPA and LEV was assessed in the radial arm maze after each drug was tested individually over a range of doses. (a) Baseline performance was established with combination saline injections, and sub-threshold doses of each drug (VPA at 25 or 50 mg/kg, and LEV at 1.25 or 2.5 mg/kg) were administered in combination. To facilitate interpretation, effective combinations were then plotted on an isobologram (b). The diagonal straight line is the line of additivity, anchored on each axis by the lowest effective doses of VPA and LEV when assessed individually. Efficacious combinations at lower sub-threshold doses that fall on the line are interpreted accordingly as a simple additive effect, as is the case for the treatment of VPA 50 mg/kg combined with LEV 2.5 mg/kg. The dose combination of VPA 50 mg/kg and LEV 1.25 mg/kg falls below the line of additivity, suggesting super-additive or synergistic interaction between the two drugs.

Figure 7

LEV assessment in the water maze. Memory-impaired aged rats were injected with LEV at 10 mg/kg or saline vehicle before training in the water maze (a). Better memory retention was observed under LEV than saline treatment 24 h later in a probe test (b). *p<0.050 compared with percent time spent in target quadrant under saline treatment.

Figure 8

LEV treatment in young rats. Young rats were treated with LEV at varying doses before testing in the radial arm maze using a 5-h retention delay. No differences in memory performance were found as a function of dose.