'New' memories are initially labile and sensitive to disruption before being consolidated into stable long-term memories. Much evidence indicates that this consolidation involves the synthesis of new proteins in neurons. The lateral and basal nuclei of the amygdala (LBA) are believed to be a site of memory storage in fear learning. Infusion of the protein synthesis inhibitor anisomycin into the LBA shortly after training prevents consolidation of fear memories. Here we show that consolidated fear memories, when reactivated during retrieval, return to a labile state in which infusion of anisomycin shortly after memory reactivation produces amnesia on later tests, regardless of whether reactivation was performed 1 or 14 days after conditioning. The same treatment with anisomycin, in the absence of memory reactivation, left memory intact. Consistent with a time-limited role for protein synthesis production in consolidation, delay of the infusion until six hours after memory reactivation produced no amnesia. Our data show that consolidated fear memories, when reactivated, return to a labile state that requires de novo protein synthesis for reconsolidation. These findings are not predicted by traditional theories of memory consolidation.

The extracellular signal-regulated kinase (ERK) pathway is critical for various forms of learning and memory, and is activated by the potent estrogen 17␤-estradiol (E 2 ). Here, we asked whether E 2 modulates memory via ERK activation and putative membrane-bound estrogen receptors (ERs). Using ovariectomized mice, we first demonstrate that intraperitoneal injection of 0.2 mg/kg E 2 significantly increases dorsal hippocampal levels of phosphorylated ERK protein 1 h after injection. Second, we show that E 2 administered intraperitoneally (0.2 mg/kg) or via intrahippocampal infusion (5.0 g/side) immediately after training in an object recognition task significantly enhances memory retention, and that the beneficial effect of intraperitoneal E 2 is blocked by dorsal hippocampal inhibition of ERK activation. Third, using bovine serum albumin-conjugated 17␤-estradiol (BSA-E 2 ), we demonstrate that E 2 binding at membrane-bound ERs can increase dorsal hippocampal ERK activation and enhance object memory consolidation in an ERK-dependent manner. Fourth, we show that this effect is independent of nuclear ERs, but is dependent on the dorsal hippocampus. By demonstrating that E 2 enhances memory consolidation via dorsal hippocampal ERK activation, this study is the first to identify a specific molecular pathway by which E 2 modulates memory and to demonstrate a novel role for membrane-bound ERs in mediating E 2 -induced improvements in hippocampal memory consolidation.

Although much has been learned about the neurobiological mechanisms underlying Pavlovian fear conditioning at the systems and cellular levels, relatively little is known about the molecular mechanisms underlying fear memory consolidation. The present experiments evaluated the role of the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/ MAPK) signaling cascade in the amygdala during Pavlovian fear conditioning. We first show that ERK/MAPK is transiently activated-phosphorylated in the amygdala, specifically the lateral nucleus (LA), at 60 min, but not 15, 30, or 180 min, after conditioning, and that this activation is attributable to paired presentations of tone and shock rather than to nonassociative auditory stimulation, foot shock sensitization, or unpaired tone-shock presentations. We next show that infusions of U0126, an inhibitor of ERK/MAPK activation, aimed at the LA, dose-dependently impair long-term memory of Pavlovian fear conditioning but leaves short-term memory intact. Finally, we show that bath application of U0126 impairs long-term potentiation in the LA in vitro. Collectively, these results demonstrate that ERK/MAPK activation is necessary for both memory consolidation of Pavlovian fear conditioning and synaptic plasticity in the amygdala. Key words: amygdala; fear conditioning; ERK; MAPK; learning; LTPConsiderable evidence has implicated the lateral and basal nuclei of the amygdala (LBA) in the plastic changes underlying acquisition and retention of Pavlovian fear conditioning. Lesion, tract tracing, and electrophysiological studies suggest that fear conditioning involves transmission of sensory information to the lateral nucleus of the amygdala (LA) where alterations in synaptic transmission are thought to encode key aspects of the learning (Fendt and Fanselow, 1999;Maren, 1999;LeDoux, 2000). However, although fear conditioning has received much attention at the systems and cellular levels, relatively little is known about the molecular mechanisms that underlie consolidation of fear memory in the LA.One relatively recent discovery is the role of the mitogenactivated protein (MAP) family of kinases in synaptic plasticity and memory. These include the p38 MAP kinase (MAPK) and Jun (or stress-activated protein) kinase members, which have been implicated in stress-related cellular responses to injury or inflammation, and also the extracellular signal-regulated kinase (ERK), which has been implicated in cellular growth and differentiation (Kornhauser and Greenberg, 1997;Impey et al., 1999;Oruban et al., 1999). In neurons, ERK/MAPK has been shown to be potently activated by phosphorylation after synaptically driven increases in intracellular Ca 2ϩ (Rosen et al., 1994;Impey et al., 1999). Furthermore, ERK/MAPK has been shown to be activated-phosphorylated in the hippocampus after long-term potentiation (LTP) induction in the Schaffer collateral pathway, an effect that is blocked, along with LTP, by pretreatment with inhibitors of ERK/ MAPK activation Sweatt, 1996, 1997;Imp...

Previous studies have shown that long-term potentiation (LTP) can be induced in the lateral nucleus of the amygdala (LA) after stimulation of central auditory pathways and that auditory fear conditioning modifies neural activity in the LA in a manner similar to LTP. The present experiments examined whether intra-LA administration of inhibitors of protein synthesis or protein kinase A (PKA) activity, treatments that block LTP in hippocampus, interfere with memory consolidation of fear conditioning. In the first series of experiments, rats received a single conditioning trial followed immediately by intra-LA infusions of anisomycin (a protein synthesis inhibitor) or Rp-cAMPS (an inhibitor of PKA activity) and were tested 24 hr later. Results indicated that immediate post-training infusion of either drug dose-dependently impaired fear memory retention, whereas infusions 6 hr after conditioning had no effect. Additional experiments showed that anisomycin and Rp-cAMPS interfered with long-term memory (LTM), but not short-term memory (STM), of fear and that the effect on LTM was specific to memory consolidation processes rather than to deficits in sensory or performance processes. Findings suggest that the LA is essential for memory consolidation of auditory fear conditioning and that this process is PKA and protein-synthesis dependent.