Glucose attenuates impairments in memory and CREB activation produced by an α4β2 but not an α7 nicotinic receptor antagonist

Abstract

Glucose improves memory for a variety of tasks when administered to rats and mice near the time of training. Prior work indicates glucose may enhance memory by increasing the synthesis and release of the neurotransmitter acetylcholine in the brain. To investigate if specific acetylcholine receptor subtypes may mediate some of the memory-enhancing actions of glucose, we examined the effects of subtype-specific nicotinic acetylcholine receptor antagonists on memory in Fischer-344 rats and also examined the ability of glucose to reverse drug-induced impairments. Pre-training peripheral injections of methyllycaconitine (MLA) or dihydro-beta-erythroidine (DHβE), which are specific α7 and α4β2 nicotinic receptor antagonists, respectively, dose-dependently impaired retention latencies in an inhibitory avoidance task when tested 7-days but not 1 h after training. Immediate post-training glucose injections attenuated the impairments, but were more effective in attenuating the DHβE-induced impairments. Likewise, peripheral or direct intrahippocampal injections of MLA or DHβE dose-dependently impaired spatial working memory scores on a spontaneous alternation task. Concurrent administration of glucose reversed DHβE- but not MLA-induced impairments. CREB phosphorylation downstream of cholinergic signaling was assessed 30 min after spontaneous alternation testing and intrahippocampal drug infusions. Both MLA and DHβE impaired hippocampal CREB phosphorylation; glucose reversed DHβE- but not MLA-induced deficits. The effectiveness of glucose in reversing DHβE- but not MLA-induced impairments in behavioral performance and CREB phosphorylation suggests that activation of α7 receptors may play an important role in memory enhancement by glucose.

Figure 1

Effects of peripherally-administered nicotinic antagonists on inhibitory avoidance memory tested 7 days after training. (A) MLA doses of 2 and 5, but not 0.5 mg/kg, significantly impaired memory. (*) ps < .05 vs. saline. Ns = 8. (B) DHβE doses of 2 and 5, but not 0.5 mg/kg, significantly impaired memory. (*) ps < .05 vs. saline. Ns = 8.

Figure 2

Glucose attenuation of drug-induced memory impairments after inhibitory avoidance training. Pre-training administration of MLA or DHβE (2 mg/kg), together with post-training saline injections, produced deficits in memory tested at 7 days after training. Post-training glucose attenuated the drug-induced memory impairments, but was more effective at reversing the impairments induced by DHβE. (*) ps < .05 vs. saline-saline. Ns = 8.

Figure 3

Effects of peripherally-administered nicotinic antagonists on spontaneous alternation spatial working memory scores. (A) An MLA dose of 2, but not 0.5 or 5 mg/kg, significantly impaired alternation scores. (*) p < .05 vs. saline. Ns = 8. (B) DHβE doses of 2 and 5, but not 0.5 mg/kg, significantly impaired alternation scores. (*) ps < .05 vs. saline. Ns = 8.

Figure 4

Glucose attenuation of drug-induced working memory impairments in the spontaneous alternation task. MLA or DHβE (2 mg/kg) produced deficits in alternation scores. Co-administration of glucose with the antagonists attenuated DHβE but not MLA-induced deficits. (*) ps < .05 vs. saline. Ns = 8.

Figure 5

Effects of intrahippocampally-administered nicotinic antagonists on working memory assessed in a spontaneous alternation task. (A) MLA doses of 6.75 and 13.5, but not 27 μg/side, significantly impaired alternation scores. (*) ps < .05 vs. aCSF. N = 9. (B) DHβE doses of 8 and 20, but not 4 μg/side, significantly impaired alternation scores. (*) ps < .05 vs. aCSF. N = 9. To the right of each graph is an illustration of the infusion sites targeting the dorsal hippocampus for each experiment. 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 6

Glucose attenuation of impairments in spontaneous alternation memory scores produced by intrahippocampally-administered antagonists. MLA (6.75 μg/side) or DHβE (8 μg/side) produced deficits in alternation scores. Co-administration of glucose with the antagonists attenuated DHβE but not MLA-induced deficits. There was a trend for enhancement in alternation scores following administration of glucose alone. (*) ps < .05 vs. aCSF. Ns = 8 for aCSF and GLU. Ns = 7 for all other groups.

Figure 7

Differences in pCREB immunoreactivity in area CA1 following spontaneous alternation testing with intrahippocampal administration of drugs. (A) MLA (6.75 μg/side) or DHβE (8 μg/side) reduced pCREB levels. Co-administration of glucose with the antagonists attenuated DHβE but not MLA-induced pCREB deficits. (*) ps < .05 vs. aCSF. Ns = 8 for aCSF and GLU. Ns = 7 for all other groups. (B) Representative photomicrographs of pCREB immunostaining in area CA1. Scale bar = 100 microns.

Figure 8

Correlations between area CA1 pCREB immunostaining and spontaneous alternation scores. There was a significant correlation in rats receiving intrahippocampal injections of MLA-glucose (p < .01), and a trend for a correlation in rats receiving MLA alone (p = .09).