Extinction of conditioned fear is thought to form a long-term memory of safety, but the neural mechanisms are poorly understood. Consolidation of extinction learning in other paradigms requires protein synthesis, but the involvement of protein synthesis in extinction of conditioned fear remains unclear. Here, we show that rats infused intraventricularly with the protein synthesis inhibitor anisomycin extinguished normally within a session but were unable to recall extinction the following day. Anisomycin-treated rats showed no savings in the rate of re-learning of extinction, consistent with amnesia for extinction training. The identical effect was observed when anisomycin was microinfused into the medial prefrontal cortex (mPFC) but not the insular cortex. Furthermore, we observed that extinction training increased c-Fos levels in the mPFC but not in the insular cortex, consistent with extinction-induced gene expression in the mPFC. These findings extend previous lesion and unit-recording data by demonstrating that the mPFC is a critical storage site for extinction memory, rather than simply a pathway for expression of extinction. Understanding consolidation of fear extinction could lead to new treatments for anxiety disorders in which fear extinction is thought to be compromised.

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Nurr1 expression is up-regulated in the brain following associative learning experiences, but its relevance to cognitive processes remains unclear. In these studies, rats initially received bilateral hippocampal infusions of control or antisense oligodeoxynucleotides (ODNs) 1 h prior to training in a holeboard spatial discrimination task. Such pre-training infusions of nurr1 antisense ODNs caused a moderate effect in learning the task and also impaired LTM tested 7 d later. In a second experiment, ODN infusions were given immediately after the animals had received two sessions of training, during which all animals showed normal learning. Although antisense treated rats were significantly impaired during the post-infusion stages of acquisition of the task, no group differences were observed during the LTM test given 7 d later. These animals were subjected 3 d later to reversal training in the same maze in the absence of any additional treatments. Remarkably, rats previously treated with antisense ODNs displayed perseveration: The animals were fixated with the previously learned pattern of baited holes, causing them to be significantly impaired in the extinction of acquired spatial preferences and future learning. We postulate that Nurr1 function in the hippocampus is important for normal cognitive processes.Learning requires the continued ability of the brain to establish new synaptic configurations and could be viewed as the continuation of the developmental process. Nurr1 (also known as HZF-3) (Peña de Ortiz and Jamieson Jr. 1996; Peña de Ge et al. 2003) is a member of the inducible nuclear receptor family of transcription factors (Law et al. 1992;Xing et al. 1997;Honkaniemi and Sharp 1999;Wang et al. 2003) that has been shown to play a key role in neural development. Specifically, expression of nurr1 is critical for the development of mesencephalic dopaminergic precursor neurons (Zetterstrom et al. 1997;Castillo et al. 1998;Saucedo-Cardenas et al. 1998 On the other hand, recent studies showed that nurr1 mutant heterozygous mice subjected to post-weaning isolation display a disruption of prepulse inhibition (a model of sensorimotor gating in the brain that is disrupted in patients with schizophrenia) (for review, see Van den Buuse et al. 2003), which is possibly related to decreased dopamine neurotransmission in the mutant heterozygous genotype (Eells et al. 2006). Disordered cognitive processes stemming from impaired hippocampal function are an important part of the positive symptoms in schizophrenia (Harvey et al. 2001;Antonova et al. 2004;Sweatt 2004;Johnson 2005). The holeboard spatial discrimination task was initially developed to study thought disorders in animal models of schizophrenia (Oades and Isaacson 1978). In this hippocampal-dependent task (Oades 1981), animals learn to discriminate between relevant versus irrelevant stimuli, a cognitive process that is impaired in schizophrenia (Gray and Snowden 2005). Our previous studies showed increased rat hippocampal nurr1 mRNA levels after the first and thi...

Recent evidence suggests that DNA double strand breaks (DSBs) are introduced in neurons during the course of normal development, and that repair of such DSBs is essential for neuronal survival. Here we describe a non-homologous DNA end joining (NHEJ) system in the adult rat brain that may be used to repair DNA DSBs. In the brain NHEJ system, blunt DNA ends are joined with lower efficiency than cohesive or non-matching protruding ends. Moreover, brain NHEJ is blocked by DNA ligase inhibitors or by dATP and can occur in the presence or absence of exogenously added ATP. Comparison of NHEJ activities in several tissues showed that brain and testis share similar mechanisms for DNA end joining, whereas the activity in thymus seems to utilize different mechanisms than in the nervous system. The developmental profile of brain NHEJ showed increasing levels of activity after birth, peaking at postnatal day 12 and then gradually decreasing along with age. Brain distribution analysis in adult animals showed that NHEJ activity is differentially distributed among different regions. We suggest that the DNA NHEJ system may be utilized in the postnatal brain for the repair of DNA double strand breaks introduced within the genome in the postnatal brain. Keywords: dATP inhibition, brain distribution and development, DNA repair, double strand breaks (DSBs), non-homologous DNA end joining (NHEJ), V(D)J recombination.

Voluntary running is a robust inducer of adult hippocampal neurogenesis. Given that fatty acid synthase (FASN), the key enzyme for de novo fatty acid biosynthesis, is critically involved in proliferation of embryonic and adult neural stem cells, we hypothesized that FASN could mediate both exercise-induced cell proliferation in the subgranular zone (SGZ) of the dentate gyrus (DG) and enhancement of spatial learning and memory. In 20 week-old male mice, voluntary running-induced hippocampal-specific upregulation of FASN was accompanied also by hippocampal-specific accumulation of palmitate and stearate saturated fatty acids. In experiments addressing the functional role of FASN in our experimental model, chronic intracerebroventricular (i.c.v.) microinfusions of C75, an irreversible FASN inhibitor, and significantly impaired exercise-mediated improvements in spatial learning and memory in the Barnes maze. Unlike the vehicle-injected mice, the C75 group adopted a non-spatial serial escape strategy and displayed delayed escape latencies during acquisition and memory tests. Furthermore, pharmacologic blockade of FASN function with C75 resulted in a significant reduction, compared to vehicle treated controls, of the number of proliferative cells in the DG of running mice as measured by immunoreactive to Ki-67 in the SGZ. Taken together, our data suggest that FASN plays an important role in exercise-mediated cognitive enhancement, which might be associated to its role in modulating exercise-induced stimulation of neurogenesis.