Involvement of hippocampal cAMP/cAMP-dependent protein kinase signaling pathways in a late memory consolidation phase of aversively motivated learning in rats

Abstract

cAMP/cAMP-dependent protein kinase (PKA) signaling pathway has been recently proposed to participate in both the late phase of long term potentiation in the hippocampus and in the late, protein synthesis-dependent phase of memory formation. Here we report that a late memory consolidation phase of an inhibitory avoidance learning is regulated by an hippocampal cAMP signaling pathway that is activated, at least in part, by D1/D5 receptors. Bilateral infusion of SKF 38393 (7.5 microg/side), a D1/D5 receptor agonist, into the CA1 region of the dorsal hippocampus, enhanced retention of a step-down inhibitory avoidance when given 3 or 6 h, but not immediately (0 h) or 9 h, after training. In contrast, full retrograde amnesia was obtained when SCH 23390 (0.5 microg/side), a D1/D5 receptor antagonist, was infused into the hippocampus 3 or 6 h after training. Intrahippocampal infusion of 8Br-cAMP (1.25 microg/side), or forskolin (0.5 microg/side), an activator of adenylyl cyclase, enhanced memory when given 3 or 6 h after training. KT5720 (0.5 microg/side), a specific inhibitor of PKA, hindered memory consolidation when given immediately or 3 or 6 h posttraining. Rats submitted to the avoidance task showed learning-specific increases in hippocampal 3H-SCH 23390 binding and in the endogenous levels of cAMP 3 and 6 h after training. In addition, PKA activity and P-CREB (phosphorylated form of cAMP responsive element binding protein) immunoreactivity increased in the hippocampus immediately and 3 and 6 h after training. Together, these findings suggest that the late phase of memory consolidation of an inhibitory avoidance is modulated cAMP/PKA signaling pathways in the hippocampus.

Figure 1

Effects of bilateral intrahippocampal infusion of 7.5 μg of SKF 38393 (A) or 0.5 μg of SCH 23390 (B) at different time intervals after training, on test session step down latency. Data are expressed as median (interquartile range). ∗, Significant difference from saline at P < 0.002. Number of animals ranged between 10 and 12 per group.

Figure 2

Effects of 8Br-_CAMP (1.25 μg/side) administered by bilateral infusion into the CA₁ region on step down latency. ∗, Significant difference from test session values of saline group at P < 0.002. All training test latency differences were significant at P < 0.001. Number of animals ranged between 10 and 12 per group.

Figure 3

Endogenous cAMP levels in the hippocampus of naive, shocked, or trained animals killed at different time intervals after training. Data are expressed as mean ± SE pmol/mg tissue of six to nine independent experiments done by triplicate. ∗, P < 0.01; ∗∗, P < 0.001 compared with naive controls, Tukey–Kramer test after ANOVA.

Figure 4

Effects of bilateral intrahippocampal infusion of vehicle or forskolin (0.5 μg/side) on step down latency. Data are expressed as in Fig. 1. Number of rats ranged between 9 and 12 per group. Training test differences were significant at P < 0.001 by Mann–Withney U tests (two tailed). ∗, P < 0.02; ∗∗, P < 0.002 in comparison with test session values of vehicle group (Mann–Whitney U test).

Figure 5

Effects of KT5720 on memory of step down inhibitory avoidance learning in rats that received a bilateral infusion of vehicle or KT5720 (0.5 μg/side) into the CA₁ region 0, 3, 6, or 9 h after training. Data are expressed as in Fig. 1; n = 10–11 animals per group. Training test latency differences significant at P < 0.001. ∗, P < 0.002 compared with vehicle values (Mann–Whitney U tests).

Figure 6

Effect of an inhibitory avoidance learning on hippocampal PKA activity. Data are expressed as mean ± SE nmol/min/mg tissue of seven independent experiments done by triplicate. ∗, P < 0.01; ∗∗, P < 0.001 in comparison with naive control values in Newman–Keuls test after ANOVA.

Figure 7

Quantitative densitometric analysis of the immunocytochemistry of P-CREB in CA₁ region of the dorsal hippocampus of naive, shocked, or trained rats. Data are expressed as percentage of control values [mean ± SE of relative OD (R.O.D.) per mm²]. n = five independent experiments done by triplicate. ∗, P < 0.01, in comparison with naive control values, in Newman–Keuls test after ANOVA.