Evidence that age-induced decline in memory retention is delayed in growth hormone resistant GH-R-KO (Laron) mice
Introduction
It is well documented that serum levels of growth hormone (GH), and subsequently insulin-like growth factor (IGF-I), decline with age in both rodents and humans [1], [2], [3], [4], [5]. It has been estimated that human GH secretion declines as much as 14% with each decade beyond the age of 20 years [6]. Additionally, it has been proposed that the loss of lean body mass and muscle volume, and increased adiposity that normally accompany aging result from GH deficiency [7]. In fact, there is currently much debate as to whether this age-related decrease in GH concentration mimics the conditions of GH deficiency, warranting GH replacement [5]. While GH replacement in elderly patients that are not GH-deficient is controversial, there are studies reporting that elderly subjects treated with GH experienced an increase in lean body mass, reductions in adiposity and in the rate of bone density loss, and improved feelings of well-being [7]. However, others have pointed out that administering GH produces only a minimal increase in bone density while potentially causing severe side effects to include increased risk of cancer [8].
Several animal models have been identified as potential models for elucidating the role of GH in aging, including the Snell dwarf mouse, Ames dwarf mouse and transgenic mice that overexpress GH or have genetic mutations resulting in GH resistance. Recent studies with Ames dwarf mice, which have a primary pituitary deficiency causing them to lack GH, thyroid-stimulating hormone (TSH) and prolactin (PRL), have shown that these mice live significantly longer than their normal siblings and appear to experience delayed physical aging [9]. Similar findings were also reported with Snell dwarf mice, which are phenotypically similar to the Ames dwarfs but have mutation of a different gene [10]. Moreover, increased lifespan was recently shown in GH-R-KO mice [11], animals with targeted disruption of the GH receptor gene and consequent GH resistance [12]. On the other hand, transgenic mice overexpressing GH have been found to have a reduced lifespan and experience some symptoms of accelerated physical aging [13]. It has recently been suggested that in humans the natural reduction in GH during aging may be a causal factor in the decline in the ability to learn and retain various types of information [14], [15], [16]. While the data in this area are contradictory and inconclusive, it seems that learning and retention tests could be useful in assessing age-related changes in animals with GH deficiency or resistance. Studies assessing learning and memory in Ames dwarf animals have supported this notion, in that old (22–29 months) Ames dwarf mice retained their memory for shock in an inhibitory avoidance test, unlike their old normal siblings (20–23 months) who experienced a sharp decline in retention over time (Kinney and Bartke, in press). In fact, the performance of old Ames dwarf mice did not differ from that of young normal and dwarf mice (Kinney and Bartke, in press). These findings were supported by another study in our lab, in which 18–21-month-old caloric restricted and ad lib-fed dwarf mice showed increased retention in the inhibitory avoidance task at 24-h and 7-day retention tests [17]. However, given that the Ames dwarf mice are deficient in multiple pituitary hormones, it is difficult to conclude the causal factor for enhanced retention. On the other hand, studies with 6-month-old transgenic mice overexpressing GH found that these mice performed poorly on the inhibitory avoidance task when compared with their normal siblings and did not differ from 25-month-old normal mice [18].
Several studies have looked at the effects of GH and GH-releasing hormone (GHRH) on behavior and cognitive function in genetically normal young and old rats [19], [20]. One study administered saline, GH or GHRH to young (3 months old) and old (24 months old) rats 24 h prior to training in the passive avoidance task and 24 h prior to each retention trial until extinction had occurred. Results indicated that GH and GHRH facilitated memory in the young rats but not the aged rats [19]. In a separate study, rats were administered daily injections of GHRH or saline for 21 months, followed by testing in the Morris water maze. Results of this test were compared with scores of 6-month-old animals, revealing that the administration of GHRH attenuated the natural age-dependent decrease in memory [20]. Like the studies reported for humans, the animal data also tend to be contradictory and inconclusive. However, the use of animal models with a targeted disruption of the GH receptor could help in alleviating the controversy.
The purpose of these experiments was to assess learning and memory in the GH-R-KO (Laron) mouse. We feel that by comparing the ability of Laron mice and their normal siblings to learn and retain information in the inhibitory avoidance task, a measure known to be sensitive to the effects of aging, we can help elucidate the role of GH in the aging process.
Section snippets
Subjects
All animals were between the ages of 17 and 20 months at the time of testing. Experimental groups consisted of naive male (N=10) and female (N=11) GH-R-KO mice and male (N=7) and female (N=13) controls. Controls were normal siblings of the GH-R-KO animals. In this and all other experiments described here, animals were housed in all male or all female groups in a room with a controlled photoperiod (12:12, lights on at 06:00 hours) and temperature (22±2°C). They were given access to pelletized
Experiment 2
The GH-R-KO mice in Experiment 1 clearly showed superior performance compared to their control siblings in the inhibitory avoidance learning task. However, using a standard shock level regardless of size differences among animals could result in experiential disparities in shock intensity. Given that the GH-R-KO mice are approximately 1/2 the size of their normal siblings and that shock delivery was planned for Experiment 3, it was decided to devise a method for delivering equivalent shock to
Experiment 3
The use of multiple retention tests in the inhibitory avoidance task described in Experiment 1 could be questionable, in that extinction could be another possible interpretation for the results. Therefore, the aim of this study was to determine whether old GH-R-KO mice would continue to show superior performance in this task if independent retention tests were conducted at the same postshock time intervals as those in Experiment 1. Additionally, the present experiment set out to determine
Experiment 4
The purpose of this experiment was to determine whether genotype performance differences observed in the old animals in the inhibitory avoidance test were due to differences in locomotor behavior or emotionality. If the GH-R-KO mice were motorically less active than their normal siblings, then that could be one possible explanation for the longer latencies observed in the inhibitory avoidance test. Likewise, if the GH-R-KO mice were more fearful or emotional than their normal siblings, that
Discussion
Applying the appropriate shock intensity in an avoidance learning task is critical for obtaining a true measure of learning in experimental animals. While early studies focused entirely on the rat, they did provide conclusive evidence regarding the importance of equalizing the amount of voltage applied to all animals [23]. Similar to what we have done here with mice, these investigators obtained the behavioral shock threshold for a group of rats and followed that up by measuring the resistance
Acknowledgements
This work was supported by the Illinois Council for Food and Agricultural Research (C-FAR) and by NIH (HD20001). We thank Dr. R.A. Jensen for use of the inhibitory avoidance apparatus and Dr. N.E. Kinney for comments and suggestions during manuscript preparation.
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