Epinephrine and glucose modulate training-related CREB phosphorylation in old rats: relationships to age-related memory impairments

Abstract

Epinephrine enhances memory in young adult rats, in part, by increasing blood glucose levels needed to modulate memory. In old rats, epinephrine is deficient at raising blood glucose levels and thus is only moderately effective at enhancing memory. In contrast, systemic glucose injections improve memory in old rats, with resulting memory performance equal to that of young rats. The diminished response of glucose to training in old rats may blunt downstream neurochemical and molecular mechanisms needed to upregulate memory processes. In the first experiment, young adult and old rats were trained on an inhibitory avoidance task with immediate post-training injections of aCSF or glucose into the dorsal hippocampus. Old rats had significant memory impairments compared to young rats 7 days after training. Intrahippocampal injections of glucose reversed age-related deficits, improving memory scores in old rats to values seen in young rats. A second experiment examined age-related changes in activation of the transcription factor CREB, which is widely implicated in memory formation and may act downstream of hormonal and metabolic signals. Activation was assessed in response to training with systemic injections of epinephrine and glucose at doses known to enhance memory. Young adult and old rats were trained on inhibitory avoidance with immediate post-training systemic injections of saline, epinephrine, or glucose. After training, old rats had significant impairments in CREB phosphorylation in area CA1 and the dentate gyrus region of the hippocampus, and in the basolateral and lateral amygdala. Epinephrine and glucose attenuated age-related deficits in CREB phosphorylation, but were more effective in the amygdala and hippocampus, respectively. Together, these results support the view that age-related changes in blood glucose responses to epinephrine contribute to memory impairments, which may be related to alterations in regional patterns of CREB phosphorylation.

Figure 1

Effects of age and intrahippocampal glucose administration on 7-day inhibitory avoidance retention latencies. (A) There were no significant differences in training latencies across groups. (B) Old rats receiving post-training intrahippocampal injections of aCSF had significantly lower 7-day retention latencies compared to young rats receiving either aCSF or glucose injections. Post-training glucose injections reversed age-related impairments, improving retention latencies in old rats to levels seen in young rats. (*) p < .05 vs. old aCSF group. i.q.r. = interquartile range. (C) Infusion sites targeting the dorsal hippocampus in young and old rats. Filled circles represent tips of infusion tracts. Numbers refer to distance in mm posterior to bregma. Adapted with permission from Paxinos and Watson (2003).

Figure 2

Age-, training-, and treatment-associated differences in pCREB and CREB immunoreactivity in the dentate gyrus of the hippocampus. (A) Representative photomicrographs of pCREB immunostaining. Scale bar = 100 microns. (B) Old rats had training-related deficits in pCREB activation, which were attenuated by epinephrine and glucose. (*) p < .05 vs. young untrained group. (**) ps < .01 vs. young untrained group. (+) ps < .05 vs. old untrained and saline groups. (C) There were significantly lower CREB levels in old compared to young rats. (D) Old rats had training-related deficits in pCREB:CREB ratios, which were attenuated by glucose. (**) ps < .01 vs. young untrained group. (++) ps < .01 vs. old untrained and saline groups.

Figure 3

Age-, training-, and treatment-associated differences in pCREB and CREB immunoreactivity in area CA3 of the hippocampus. (A) Representative photomicrographs of pCREB immunostaining. Scale bar = 100 microns. (B) There were no significant age-, training-, or treatment-related differences in pCREB levels. (C) There were significantly lower CREB levels in old compared to young rats. (D) pCREB:CREB ratios were significantly elevated in old compared to young rats.

Figure 4

Age-, training-, and treatment-associated differences in pCREB and CREB immunoreactivity in area CA1 of the hippocampus. (A) Representative photomicrographs of pCREB immunostaining. Scale bar = 100 microns. (B) Old rats had training-related deficits in pCREB activation, which were attenuated by epinephrine and glucose. (**) ps < .01 vs. young untrained group. (+) ps < .05 vs. old untrained and saline groups. (C) There were significantly lower CREB levels in old compared to young rats. (D) Old rats had training-related deficits in pCREB:CREB ratios, which were attenuated by glucose. (**) ps < .01 vs. young untrained group. (+) ps < .05 vs. old untrained and saline groups.

Figure 5

Age-, training-, and treatment-associated differences in pCREB and CREB immunoreactivity in the basolateral nucleus of the amygdala. (A) Representative photomicrographs of pCREB immunostaining. (B) Old rats had training-related deficits in pCREB activation, which were attenuated by epinephrine. (**) ps < .01 vs. young untrained group. (+) p < .05 vs. old untrained group. Scale bar = 50 microns. (C) There were significantly lower CREB levels in old compared to young rats. (D) Old rats had training-related deficits in pCREB:CREB ratios, which were attenuated by epinephrine. (**) ps < .01 vs. young untrained group. (+) p < .05 vs. old untrained group.