No abstract

Over time, memory retrieval is thought to transfer from the hippocampus to a distributed network of neocortical sites. Of these sites, the retrosplenial cortex (RSC) is robustly activated during retrieval of remotely acquired, emotionally-valenced memories. It is unclear, however, whether RSC is specifically involved in memory storage or retrieval, and which neurotransmitter receptor mechanisms serve its function. We addressed these questions by inhibiting N-methyl-d-aspartate receptors (NMDAR) in RSC via infusions of APV prior to tests for context fear in male mice. Anterior cingulate cortex (ACC) and dorsal hippocampus (DH), which have been implicated in the retrieval of remote and recent memory, respectively, served as neuroanatomical controls. Surprisingly, infusion of APV only into RSC, but not ACC or DH, abolished retrieval of remote memory, as revealed by lack of freezing to the conditioning context. APV infused into RSC also impaired retrieval of recent memory, but had no effect on conditioning or memory storage. Within-subject experiments confirmed that the role of RSC in memory retrieval is not time-limited. RSC-dependent context fear memory retrieval was mediated by NR2A, but not NR2B, subunit-containing NMDAR. Collectively, these data are the first demonstration that NMDAR in RSC are necessary for the retrieval of remote and recent memories of fear-evoking contexts. Dysfunction of RSC may thereby contribute significantly to the re-experiencing of traumatic memories in patients with post-traumatic stress disorder (PTSD).

Learning processes mediating conditioning and extinction of contextual fear require activation of several key signaling pathways in the hippocampus. Principal hippocampal CA1 neurons respond to fear conditioning by a coordinated activation of multiple protein kinases and immediate early genes, such as cFos, enabling rapid and lasting consolidation of contextual fear memory. The extracellular signalregulated kinase (Erk) additionally acts as a central mediator of fear extinction. It is not known however, whether these molecular events take place in overlapping or nonoverlapping neuronal populations. By using mouse models of conditioning and extinction of fear, we set out to determine the time course of cFos and Erk activity, their cellular overlap, and regulation by afferent cholinergic input from the medial septum. Analyses of cFos ϩ and pErk ϩ cells by immunofluorescence revealed predominant nuclear activation of either protein during conditioning and extinction of fear, respectively. Transgenic cFos-LacZ mice were further used to label in vivo Fos ϩ hippocampal cells during conditioning followed by pErk immunostaining after extinction. The results showed that these signaling molecules were activated in segregated populations of hippocampal principal neurons. Furthermore, immunotoxin-induced lesions of medial septal neurons, providing cholinergic input into the hippocampus, selectively abolished Erk activation and extinction of fear without affecting cFos responses and conditioning. These results demonstrate that extinction mechanisms based on Erk signaling involve a specific population of CA1 principal neurons distinctively regulated by afferent cholinergic input from the medial septum.

General or brain region-specific decreases in spine number or morphology accompany major neuropsychiatric disorders. It is unclear however, whether changes in spine density are specific for an individual mental process or disorder, and if so, which molecules confer such specificity. Here we identify the scaffolding protein IQGAP1 as a key regulator of dendritic spine number with a specific role in cognitive but not emotional or motivational processes. We show that IQGAP1 is an important component of N-methyl-D-aspartate receptor (NMDAR) multiprotein complexes and functionally interacts with the NR2A subunits and the extracellular signal-regulated kinases 1 and 2 (ERK) signaling pathway. Mice lacking the IQGAP1 gene exhibited significantly lower levels of surface NR2A and impaired ERK activity compared to their wild type littermates. Accordingly, primary hippocampal cultures of IQGAP1−/− neurons exhibited reduced surface expression of NR2A and disrupted ERK signaling in response to NR2A-dependent NMDAR stimulation. These molecular changes were accompanied by region-specific reductions of dendritic spine density in key brain areas involved in cognition, emotion and motivation. IQGAP1 knockouts exhibited marked long-term memory deficits accompanied by impaired hippocampal long-term potentiation (LTP) in a weak cellular learning model; in contrast, LTP was unaffected when induced with stronger stimulation paradigms. Anxiety- and depression-like behavior remained intact. On the basis of these findings, we propose that a dysfunctional IQGAP1 gene contributes to the cognitive deficits in brain disorders characterized by fewer dendritic spines.

Activation of NMDA receptors (NMDAR) in the hippocampus is essential for the formation of contextual and trace memory. However, the role of individual NMDAR subunits in the molecular mechanisms contributing to these memory processes is not known. Here we demonstrate, using intrahippocampal injection of subunit-selective compounds, that the NR2A-preferring antagonist impaired contextual and trace fear conditioning as well as learning-induced increase of the nuclear protein c-Fos. The NR2B-specific antagonist, on the other hand, selectively blocked trace fear conditioning without affecting c-Fos levels. Studies with cultured primary hippocampal neurons, further showed that synaptic and extrasynaptic NR2A and NR2B differentially regulate the extracellular signal-regulated kinase 1 and 2/mitogen-and stress-activated protein kinase 1 (ERK1/2/MSK1)/c-Fos pathway. Activation of the synaptic population of NMDAR induced cytosolic, cytoskeletal and perinuclear phosphorylation of ERK1/2 (pERK1/2). The nuclear propagation of pERK1/2 signals, revealed by up-regulation of the downstream nuclear targets pMSK1 and c-Fos, was blocked by a preferential NR2A but not by a specific NR2B antagonist. Conversely, activation of total (synaptic and extrasynaptic) NMDAR engaged receptors with NR2B subunits, and resulted in membrane retention of pERK1/2 without inducing pMSK1 and cFos. Stimulation of extrasynaptic NMDAR alone was consistently ineffective at activating ERK signaling. The discrete contribution of synaptic and total NR2A-and NR2B-containing NMDAR to nuclear transmission versus membrane retention of ERK signaling may underlie their specific roles in the formation of contextual and trace fear memory.