Increased surface P2X4 receptor regulates anxiety and memory in P2X4 internalization-defective knock-in mice.

ATP signaling and surface P2X4 receptors are upregulated selectively in neurons and/or glia in various CNS disorders including anxiety, chronic pain, epilepsy, ischemia, and neurodegenerative diseases. However, the cell-specific functions of P2X4 in pathological contexts remain elusive. To elucidate P2X4 functions, we created a conditional transgenic knock-in P2X4 mouse line (Floxed P2X4mCherryIN) allowing the Cre activity-dependent genetic swapping of the internalization motif of P2X4 by the fluorescent mCherry protein to prevent constitutive endocytosis of P2X4. By combining molecular, cellular, electrophysiological, and behavioral approaches, we characterized two distinct knock-in mouse lines expressing noninternalized P2X4mCherryIN either exclusively in excitatory forebrain neurons or in all cells natively expressing P2X4. The genetic substitution of wild-type P2X4 by noninternalized P2X4mCherryIN in both knock-in mouse models did not alter the sparse distribution and subcellular localization of P2X4 but increased the number of P2X4 receptors at the surface of the targeted cells mimicking the pathological increased surface P2X4 state. Increased surface P2X4 density in the hippocampus of knock-in mice altered LTP and LTD plasticity phenomena at CA1 synapses without affecting basal excitatory transmission. Moreover, these cellular events translated into anxiolytic effects and deficits in spatial memory. Our results show that increased surface density of neuronal P2X4 contributes to synaptic deficits and alterations in anxiety and memory functions consistent with the implication of P2X4 in neuropsychiatric and neurodegenerative disorders. Furthermore, these conditional P2X4mCherryIN knock-in mice will allow exploring the cell-specific roles of P2X4 in various physiological and pathological contexts.

Synaptic rearrangement of NMDA receptors controls memory engram formation and malleability in the cortex.

Initially hippocampal dependent, memory representations rely on a broadly distributed cortical network as they mature over time. How these cortical engrams acquire stability during systems-level memory consolidation without compromising their dynamic nature remains unclear. We identified a highly responsive "consolidation switch" in the synaptic composition of N-methyl-d-aspartate receptors (NMDARs), which dictates the progressive embedding and persistence of enduring memories in the rat cortex. Cortical GluN2B subunit-containing NMDARs were preferentially recruited upon encoding of associative olfactory memory to support neuronal allocation of memory engrams. As consolidation proceeds, a learning-induced redistribution of GluN2B subunit-containing NMDARs outward synapses increased synaptic GluN2A subunit contribution and enabled stabilization of remote memories. In contrast, synaptic reincorporation of GluN2B subunits occurred during subsequent forgetting. By manipulating the surface distribution of GluN2A and GluN2B subunit-containing NMDARs at cortical synapses, we uncovered that the rearrangement of GluN2B-containing NMDARs constitutes an essential tuning mechanism that determines the fate of cortical memory engrams and controls their malleability.

Effects of Head-Only Exposure to 900 MHz GSM Electromagnetic Fields in Rats: Changes in Neuronal Activity as Revealed by c-Fos Imaging without Concomitant Cognitive Impairments.

Over the last two decades, animal models have been used to evaluate the physiological and cognitive effects of mobile phone exposure. Here, we used a head-only exposure system in rats to determine whether exposure to 900 MHz GSM electromagnetic fields (EMFs) induces regional changes in neuronal activation as revealed by c-Fos imaging. In a first study, rats were exposed for 2 h to brain average specific absorption rates (BASARs) ranging from 0.5 to 6 W/kg. Changes in neuronal activation were found to be dose-dependent, with significant increases in c-Fos expression occurring at BASAR of 1 W/kg in prelimbic, infralimbic, frontal, and cingulate cortices. In a second study, rats were submitted to either a spatial working memory (WM) task in a radial maze or a spatial reference memory (RM) task in an open field arena. Exposures (45 min) were conducted before each daily training session (BASARs of 1 and 3.5 W/kg). Control groups included sham-exposed and control cage animals. In both tasks, behavioral performance evolved similarly in the four groups over testing days. However, c-Fos staining was significantly reduced in cortical areas (prelimbic, infralimbic, frontal, cingulate, and visual cortices) and in the hippocampus of animals engaged in the WM task (BASARs of 1 and 3.5 W/kg). In the RM task, EMF exposure-induced decreases were limited to temporal and visual cortices (BASAR of 1 W/kg). These results demonstrate that both acute and subchronic exposures to 900 MHz EMFs can produce region-specific changes in brain activity patterns, which are, however, insufficient to induce detectable cognitive deficits in the behavioral paradigms used here.

Physical exercise restores adult neurogenesis deficits induced by simulated microgravity.

Cognitive impairments have been reported in astronauts during spaceflights and documented in ground-based models of simulated microgravity (SMG) in animals. However, the neuronal causes of these behavioral effects remain largely unknown. We explored whether adult neurogenesis, known to be a crucial plasticity mechanism supporting memory processes, is altered by SMG. Adult male Long-Evans rats were submitted to the hindlimb unloading model of SMG. We studied the proliferation, survival and maturation of newborn cells in the following neurogenic niches: the subventricular zone (SVZ)/olfactory bulb (OB) and the dentate gyrus (DG) of the hippocampus, at different delays following various periods of SMG. SMG exposure for 7 days, but not shorter periods of 6 or 24 h, resulted in a decrease of newborn cell proliferation restricted to the DG. SMG also induced a decrease in short-term (7 days), but not long-term (21 days), survival of newborn cells in the SVZ/OB and DG. Physical exercise, used as a countermeasure, was able to reverse the decrease in newborn cell survival observed in the SVZ and DG. In addition, depending on the duration of SMG periods, transcriptomic analysis revealed modifications in gene expression involved in neurogenesis. These findings highlight the sensitivity of adult neurogenesis to gravitational environmental factors during a transient period, suggesting that there is a period of adaptation of physiological systems to this new environment.

Therapeutic efficacy of the BKCa channel opener chlorzoxazone in a mouse model of Fragile X syndrome.

Fragile X syndrome (FXS) is an X-linked neurodevelopmental disorder characterized by several behavioral abnormalities, including hyperactivity, anxiety, sensory hyper-responsiveness, and autistic-like symptoms such as social deficits. Despite considerable efforts, effective pharmacological treatments are still lacking, prompting the need for exploring the therapeutic value of existing drugs beyond their original approved use. One such repurposed drug is chlorzoxazone which is classified as a large-conductance calcium-dependent potassium (BKCa) channel opener. Reduced BKCa channel functionality has been reported in FXS patients, suggesting that molecules activating these channels could serve as promising treatments for this syndrome. Here, we sought to characterize the therapeutic potential of chlorzoxazone using the Fmr1-KO mouse model of FXS which recapitulates the main phenotypes of FXS, including BKCa channel alterations. Chlorzoxazone, administered either acutely or chronically, rescued hyperactivity and acoustic hyper-responsiveness as well as impaired social interactions exhibited by Fmr1-KO mice. Chlorzoxazone was more efficacious in alleviating these phenotypes than gaboxadol and metformin, two repurposed treatments for FXS that do not target BKCa channels. Systemic administration of chlorzoxazone modulated the neuronal activity-dependent gene c-fos in selected brain areas of Fmr1-KO mice, corrected aberrant hippocampal dendritic spines, and was able to rescue impaired BKCa currents recorded from hippocampal and cortical neurons of these mutants. Collectively, these findings provide further preclinical support for BKCa channels as a valuable therapeutic target for treating FXS and encourage the repurposing of chlorzoxazone for clinical applications in FXS and other related neurodevelopmental diseases.

Interaction between αCaMKII and GluN2B controls ERK-dependent plasticity.

Understanding how brief synaptic events can lead to sustained changes in synaptic structure and strength is a necessary step in solving the rules governing learning and memory. Activation of ERK1/2 (extracellular signal regulated protein kinase 1/2) plays a key role in the control of functional and structural synaptic plasticity. One of the triggering events that activates ERK1/2 cascade is an NMDA receptor (NMDAR)-dependent rise in free intracellular Ca(2+) concentration. However the mechanism by which a short-lasting rise in Ca(2+) concentration is transduced into long-lasting ERK1/2-dependent plasticity remains unknown. Here we demonstrate that although synaptic activation in mouse cultured cortical neurons induces intracellular Ca(2+) elevation via both GluN2A and GluN2B-containing NMDARs, only GluN2B-containing NMDAR activation leads to a long-lasting ERK1/2 phosphorylation. We show that αCaMKII, but not ßCaMKII, is critically involved in this GluN2B-dependent activation of ERK1/2 signaling, through a direct interaction between GluN2B and αCaMKII. We then show that interfering with GluN2B/αCaMKII interaction prevents synaptic activity from inducing ERK-dependent increases in synaptic AMPA receptors and spine volume. Thus, in a developing circuit model, the brief activity of synaptic GluN2B-containing receptors and the interaction between GluN2B and αCaMKII have a role in long-term plasticity via the control of ERK1/2 signaling. Our findings suggest that the roles that these major molecular elements have in learning and memory may operate through a common pathway.

Early Administration of the Phytocannabinoid Cannabidivarin Prevents the Neurobehavioral Abnormalities Associated with the Fmr1-KO Mouse Model of Fragile X Syndrome.

Phytocannabinoids, including the non-addictive cannabis component cannabidivarin (CBDV), have been reported to hold therapeutic potential in several neurodevelopmental disorders (NDDs). Nonetheless, the therapeutic value of phytocannabinoids for treating Fragile X syndrome (FXS), a major NDD, remains unexplored. Here, we characterized the neurobehavioral effects of CBDV at doses of 20 or 100 mg/kg in the Fmr1-knockout (Fmr1-KO) mouse model of FXS using two temporally different intraperitoneal regimens: subchronic 10-day delivery during adulthood (Study 1: rescue treatment) or chronic 5-week delivery at adolescence (Study 2: preventive treatment). Behavioral tests assessing FXS-like abnormalities included anxiety, locomotor, cognitive, social and sensory alterations. Expression of inflammatory and plasticity markers was investigated in the hippocampus and prefrontal cortex. When administered during adulthood (Study 1), the effects of CBDV were marginal, rescuing at the lower dose only the acoustic hyper-responsiveness of Fmr1-KO mice and at both doses their altered hippocampal expression of neurotrophins. When administered during adolescence (Study 2), CBDV at both doses prevented the cognitive, social and acoustic alterations of adult Fmr1-KO mice and modified the expression of several inflammatory brain markers in both wild-type littermates and mutants. These findings warrant the therapeutic potential of CBDV for preventing neurobehavioral alterations associated with FXS, highlighting the relevance of its early administration.

Context-dependent modulation of hippocampal and cortical recruitment during remote spatial memory retrieval.

According to systems consolidation, as hippocampal-dependent memories mature over time, they become additionally (or exclusively) dependent on extra-hippocampal structures. We assessed the recruitment of hippocampal and cortical structures on remote memory retrieval in a performance-degradation resistant (PDR; no performance degradation with time) versus performance-degradation prone (PDP; performance degraded with time) context. Using a water-maze task in two contexts with a hidden platform and three control conditions (home cage, visible platform with or without access to distal cues), we compared neuronal activation (c-Fos imaging) patterns in the dorsal hippocampus and the medial prefrontal cortex (mPFC) after the retrieval of recent (5 days) versus remote (25 days) spatial memory. In the PDR context, the hippocampus exhibited greater c-Fos protein expression on remote than recent memory retrieval, be it in the visible or hidden platform group. In the PDP context, hippocampal activation increased at the remote time point and only in the hidden platform group. In the anterior cingulate cortex, c-Fos expression was greater for remote than for recent memory retrieval and only in the PDR context. The necessity of the mPFC for remote memory retrieval in the PDR context was confirmed using region-specific lidocaine inactivation, which had no impact on recent memory. Conversely, inactivation of the dorsal hippocampus impaired both recent and remote memory in the PDR context, and only recent memory in the PDP context, in which remote memory performance was degraded. While confirming that neuronal circuits supporting spatial memory consolidation are reorganized in a time-dependent manner, our findings further indicate that mPFC and hippocampus recruitment (i) depends on the content and perhaps the strength of the memory and (ii) may be influenced by the environmental conditions (e.g., cue saliency, complexity) in which memories are initially formed and subsequently recalled.

Recruitment of adult-generated neurons into functional hippocampal networks contributes to updating and strengthening of spatial memory.

The dentate gyrus (DG), a hippocampal subregion, continuously produces new neurons in the adult mammalian brain that become functionally integrated into existing neural circuits. To what extent this form of plasticity contributes to memory functions remains to be elucidated. Using mapping of activity-dependent gene expression, we visualized in mice injected with the birthdating marker 5-bromo-2'-deoxyuridine the recruitment of new neurons in a set of controlled water maze procedures that engage specific spatial memory processes and require hippocampal-cortical networks. Here, we provide new evidence that adult-generated hippocampal neurons make a specific but differential contribution to the processing of remote spatial memories. First, we show that new neurons in the DG are recruited into neuronal networks that support retrieval of remote spatial memory and that their activation is situation-specific. We further reveal that once selected, new hippocampal neurons are durably incorporated into memory circuits, and also that their recruitment into hippocampal networks contributes predominantly to the updating and strengthening of a previously encoded memory. We find that initial spatial training during a critical period, when new neurons are more receptive to surrounding neuronal activity, favors their subsequent recruitment upon remote memory retrieval. We therefore hypothesize that new neurons activated during this critical period become tagged so that once mature, they are preferentially recruited into hippocampal networks underlying remote spatial memory representation when encountering a similar experience.

Targeting Pdzrn3 maintains adult blood-brain barrier and central nervous system homeostasis.

Blood brain barrier (BBB) disruption is a critical component of the pathophysiology of cognitive impairment of vascular etiology (VCI) and associated with Alzheimer's disease (AD). The Wnt pathway plays a crucial role in BBB maintenance, but there is limited data on its role in cognitive pathologies. The E3 ubiquitin ligase PDZRN3 is a regulator of the Wnt pathway. In a murine model of VCI, overexpressing Pdzrn3 in endothelial cell (EC) exacerbated BBB hyperpermeability and accelerated cognitive decline. We extended these observations, in both VCI and AD models, showing that EC-specific depletion of Pdzrn3, reinforced the BBB, with a decrease in vascular permeability and a subsequent spare in cognitive decline. We found that in cerebral vessels, Pdzrn3 depletion protects against AD-induced Wnt target gene alterations and enhances endothelial tight junctional proteins. Our results provide evidence that Wnt signaling could be a molecular link regulating BBB integrity and cognitive decline under VCI and AD pathologies.

The formation of recent and remote memory is associated with time-dependent formation of dendritic spines in the hippocampus and anterior cingulate cortex.

Although hippocampal-cortical interactions are crucial for the formation of enduring declarative memories, synaptic events that govern long-term memory storage remain mostly unclear. We present evidence that neuronal structural changes, i.e., dendritic spine growth, develop sequentially in the hippocampus and anterior cingulate cortex (aCC) during the formation of recent and remote contextual fear memory. We found that mice placed in a conditioning chamber for one 7 min conditioning session and exposed to five footshocks (duration, 2 s; intensity, 0.7 mA; interstimulus interval, 60 s) delivered through the grid floor exhibited robust fear response when returned to the experimental context 24 h or 36 d after the conditioning. We then observed that their fear response at the recent, but not the remote, time point was associated with an increase in spine density on hippocampal neurons, whereas an inverse temporal pattern of spine density changes occurred on aCC neurons. At each time point, hippocampal or aCC structural alterations were achieved even in the absence of recent or remote memory tests, thus suggesting that they were not driven by retrieval processes. Furthermore, ibotenic lesions of the hippocampus impaired remote memory and prevented dendritic spine growth on aCC neurons when they were performed immediately after the conditioning, whereas they were ineffective when performed 24 d later. These findings reveal that gradual structural changes modifying connectivity in hippocampal-cortical networks underlie the formation and expression of remote memory, and that the hippocampus plays a crucial but time-limited role in driving structural plasticity in the cortex.

The intralaminar thalamic nuclei contribute to remote spatial memory.

Recent studies have shown that the anterior (ATN) and lateral thalamic nuclei (including the intralaminar nuclei; ILN/LT) play different roles in memory processes. These nuclei have prominent direct and indirect connections with the hippocampal system and/or the prefrontal cortex and may thus participate in the time-dependent reorganization of memory traces during systems-level consolidation. We investigated whether ATN or ILN/LT lesions in rats influenced acquisition and subsequent retrieval of spatial memory in a Morris water maze. Retrieval was assessed with a probe trial after a short (5 d, recent memory) or a long (25 d, remote memory) postacquisition delay. The ATN group showed impaired acquisition compared with the Sham controls and ILN/LT groups, which did not differ during acquisition, and exhibited no preference for the target quadrant during the recent or remote memory probe trials. In contrast, probe trial performance in rats with ILN/LT lesions differed according to the age of the memory, with accurate spatial retrieval for the recent memory probe trial but impaired retrieval during the remote memory one. These findings confirm that ATN but not ILN/LT lesions disrupt the acquisition of spatial memory and provide new evidence that the ILN/LT region contributes to remote memory processing. Thus, the lateral thalamus may modulate some aspects of remote memory formation and/or retrieval during the course of systems-level consolidation.

The Tyrosine Phosphatase STEP Is Involved in Age-Related Memory Decline.

Cognitive disabilities that occur with age represent a growing and expensive health problem. Age-associated memory deficits are observed across many species, but the underlying molecular mechanisms remain to be fully identified. Here, we report elevations in the levels and activity of the striatal-enriched phosphatase (STEP) in the hippocampus of aged memory-impaired mice and rats, in aged rhesus monkeys, and in people diagnosed with amnestic mild cognitive impairment (aMCI). The accumulation of STEP with aging is related to dysfunction of the ubiquitin-proteasome system that normally leads to the degradation of STEP. Higher level of active STEP is linked to enhanced dephosphorylation of its substrates GluN2B and ERK1/2, CREB inactivation, and a decrease in total levels of GluN2B and brain-derived neurotrophic factor (BDNF). These molecular events are reversed in aged STEP knockout and heterozygous mice, which perform similarly to young control mice in the Morris water maze (MWM) and Y-maze tasks. In addition, administration of the STEP inhibitor TC-2153 to old rats significantly improved performance in a delayed alternation T-maze memory task. In contrast, viral-mediated STEP overexpression in the hippocampus is sufficient to induce memory impairment in the MWM and Y-maze tests, and these cognitive deficits are reversed by STEP inhibition. In old LOU/C/Jall rats, a model of healthy aging with preserved memory capacities, levels of STEP and GluN2B are stable, and phosphorylation of GluN2B and ERK1/2 is unaltered. Altogether, these data suggest that elevated levels of STEP that appear with advancing age in several species contribute to the cognitive declines associated with aging.

Assessing recent and remote associative olfactory memory in rats using the social transmission of food preference paradigm.

Rats have the ability to learn about potential food sources by sampling their odors on the breath of conspecifics. Although this ethologically based social behavior has been transposed to the laboratory to probe nonspatial associative olfactory memory, only a few studies have taken full advantage of its unique features to examine the organization of recently and remotely acquired information. We provide a set of standardized procedures and technical refinements that are particularly useful in achieving this goal while minimizing confounding factors. These procedures, built upon a three-stage protocol (odor exposure, social interaction and preference test), are designed to optimize performance across variable retention delays, thus enabling the reliable assessment of recent and remote memory, and underlying processes, including encoding, consolidation, retrieval and forgetting. The different variants of the social transmission of food preference paradigm, which take a few days to several weeks to perform, make it an attractive and versatile tool that can be coupled to many applications in CNS research. The paradigm can be easily implemented in a typical rodent facility by personnel with standard animal behavioral expertise.

Soluble amyloid beta oligomers block the learning-induced increase in hippocampal sharp wave-ripple rate and impair spatial memory formation.

Post-learning hippocampal sharp wave-ripples (SWRs) generated during slow wave sleep are thought to play a crucial role in memory formation. While in Alzheimer's disease, abnormal hippocampal oscillations have been reported, the functional contribution of SWRs to the typically observed spatial memory impairments remains unclear. These impairments have been related to degenerative synaptic changes produced by soluble amyloid beta oligomers (Aßos) which, surprisingly, seem to spare the SWR dynamics during routine behavior. To unravel a potential effect of Aßos on SWRs in cognitively-challenged animals, we submitted vehicle- and Aßo-injected mice to spatial recognition memory testing. While capable of forming short-term recognition memory, Aß mice exhibited faster forgetting, suggesting successful encoding but an inability to adequately stabilize and/or retrieve previously acquired information. Without prior cognitive requirements, similar properties of SWRs were observed in both groups. In contrast, when cognitively challenged, the post-encoding and -recognition peaks in SWR occurrence observed in controls were abolished in Aß mice, indicating impaired hippocampal processing of spatial information. These results point to a crucial involvement of SWRs in spatial memory formation and identify the Aß-induced impairment in SWRs dynamics as a disruptive mechanism responsible for the spatial memory deficits associated with Alzheimer's disease.

Cognitive enhancing properties and tolerability of cholinergic agents in mice: a comparative study of nicotine, donepezil, and SIB-1553A, a subtype-selective ligand for nicotinic acetylcholine receptors.

Several studies have demonstrated the importance of nicotinic mechanisms in the pathophysiology of neurodegenerative and cognitive disorders, warranting the search and development of novel nicotinic ligands as potential therapeutic agents. The present study was designed to assess whether the subtype-selective nicotinic acetylcholine receptor (nAChR) ligand SIB-1553A [(+/-)-4-([2-(1-methyl-2-pyrrolidinyl)ethyl]thio)phenol hydrochloride], with predominant agonist activity at beta4 subunit-containing human nAChRs, and no activity at muscle nAChR subtypes, could enhance cognitive performance in rodents with a more desirable safety/tolerability profile as compared to the nonselective prototypic nAChR ligand nicotine. SIB-1553A was equi-efficacious to nicotine in improving working memory performance in scopolamine-treated mice as measured by increased alternation in a T-maze, and was more efficacious than nicotine in improving the baseline cognitive performance of aged mice. This effect on working memory was confirmed in a delayed nonmatching to place task using the eight-arm radial maze. SIB-1553A produced dose-dependent side effects (ie motor deficits and seizures), although these effects were observed at doses 12 to 640-fold above those required to increase cognitive performance. Overall, SIB-1553A was significantly less potent than nicotine in eliciting these undesirable effects. Thus, the subtype-selective profile of SIB-1553A appears to translate into a more efficacious and better tolerated nAChR ligand as compared to nicotine. In the present studies, cognitive enhancement induced by SIB-1553A was similar in magnitude to that produced by the clinically efficacious acetylcholinesterase inhibitor donepezil. Taken together, the present data confirm the importance of nAChR subtypes in modulating cognitive processes, and suggest that activation of nAChR subtypes by selective nAChR ligands may be a viable approach to enhance cognitive performance.

Selective immunolesioning of the basal forebrain cholinergic neurons in rats: effect on attention using the 5-choice serial reaction time task.

RATIONALE: Excitotoxic lesions of the nucleus basalis magnocellularis (nbm) in rats produce deficits in performance of the 5-choice serial reaction time (5CSRT) task, suggesting that basal forebrain cholinergic projections to the neocortex play an important role in visuospatial attention. However, non-selective damage induced by excitotoxins may have confounded the interpretation of the specific contribution of the corticopetal cholinergic neurons of the nbm to attentional processes. OBJECTIVE: The purpose of the present study was to produce selective immunolesions of the cholinergic neurons of the nbm in order to examine more precisely the role of the cholinergic projections of the basal forebrain on attentional performance in a 5CSRT task. METHODS: Rats received bilateral injections of the selective cholinergic immunotoxin 192 IgG-saporin (0.067 microg/ microl, 1 microl) into the nbm after baseline training in the 5CSRT task. Performance of sham and nbm lesion groups was then assessed during baseline and increased task difficulty conditions. RESULTS: Contrary to results previously reported, accuracy of responding and behavioral inhibition were unaffected by the immunotoxin. Rats with nbm lesions showed, however, significant increases in omissions relative to control rats, most markedly during sessions with increased difficulty of signal detection, e.g., decreased stimulus intensity or duration. Magazine and correct latencies were unaffected, suggesting that the lesion-induced omissions were not due to changes in motivation. Omissions were highly correlated with percentage of choline acetylcholine transferase depletion. Reduced premature responses were also observed when the target stimulus was made less predictable. CONCLUSIONS: Although the 192 IgG-saporin lesion produced a different array of behavioral deficits than previously reported, these effects nevertheless are consistent with an important role of the basal forebrain cholinergic system in attentional function, in particular with accurate timing of stimulus presentation and target detection.

Reduced cytochrome oxidase activity in the retrosplenial cortex after lesions to the anterior thalamic nuclei.

The anterior thalamic nuclei (ATN) make a critical contribution to hippocampal system functions. Growing experimental work shows that the effects of ATN lesions often resemble those of hippocampal lesions and both markedly reduce the expression of immediate-early gene markers in the retrosplenial cortex, which still appears normal by standard histological means. This study shows that moderate ATN damage was sufficient to produce severe spatial memory impairment as measured in a radial-arm maze. Furthermore, ATN rats exhibited reduced cytochrome oxidase activity in the most superficial cortical layers of the granular retrosplenial cortex, and, to a lesser extent, in the anterior cingulate cortex. By contrast, no change in cytochrome oxidase activity was observed in other limbic cortical regions or in the hippocampal formation. Altogether our results indicate that endogenous long-term brain metabolic capacity within the granular retrosplenial cortex is compromised by even limited ATN damage.