Organization of retrosplenial cortical projections to the anterior cingulate, motor, and prefrontal cortices in the rat

Basal Forebrain Projections to the Retrosplenial and Cingulate Cortex in Rats

The basal forebrain (BF) plays a crucial role in modulating cortical activation through its widespread projections across the cortical mantle. Previous anatomical studies have demonstrated that each cortical region receives a specific projection from the BF. In this study, we examined BF cholinergic and non-cholinergic projections to the retrosplenial cortex (RSC) and anterior cingulate cortex (ACC) using two retrograde tracers, Fast Blue (FB) and Fluoro-Gold (FG), in combination with choline acetyltransferase (ChAT) immunostaining in rats. The RSC and ACC receive cholinergic and non-cholinergic projections mainly from the medial part of the horizontal limb of the diagonal band (HDB) and the vertical limb of the diagonal band (VDB). The main difference of BF projections to the RSC, ACC, and prelimbic cortex (PL) is that the ACC and PL receive projections from the rostral half of the medial globus pallidus (GP), whereas the RSC receives stronger non-cholinergic projections from the VDB and medial septum (MS). As the injection site shifts from rostral (PL) to caudal (RSC) through the ACC, the strong GP and weak MS/VDB projections of non-cholinergic neurons are reversed. Cholinergic projection neurons make up a similar proportion of the total projection neurons in both ACC (37%) and RSC (33%) injections. Double retrograde tracer injections in the RSC and ACC revealed a small number of double-labeled projection neurons in the MS/VDB and HDB. These findings indicate that the ACC and RSC receive both spatially overlapping and differential projections from the BF, with the cholinergic and non-cholinergic projections varying between BF subregions and different rostrocaudal cortical regions.

Psilocybin reduces functional correlation and the encoding of spatial information by neurons in mouse retrosplenial cortex

Psychedelic drugs have profound effects on perception, cognition and mood. How psychedelics affect neural signaling to produce these effects remains poorly understood. We investigated the effect of the classic psychedelic psilocybin on neural activity patterns and spatial encoding in the retrosplenial cortex of head-fixed mice navigating on a treadmill. The place specificity of neurons to distinct locations along the belt was reduced by psilocybin. Moreover, the stability of place-related activity across trials decreased. Psilocybin also reduced the functional correlation among simultaneously recorded neurons. The 5-HT2AR (serotonin 2A receptor) antagonist ketanserin blocked these effects. These data are consistent with proposals that psychedelics increase the entropy of neural signaling and provide a potential neural mechanism contributing to disorientation frequently reported by humans after taking psychedelics.

Functional architecture of the forebrain cholinergic system in rodents

The basal forebrain cholinergic system (BFCS) participates in functions that are global across the brain, such as sleep-wake cycles, but also participates in capacities that are more behaviorally and anatomically specific, including sensory perception. To better understand the underlying organization principles of the BFCS, more and higher quality anatomical data and analysis is needed. Here, we created a "virtual Basal Forebrain", combining data from numerous rats with cortical retrograde tracer injections into a common 3D reference coordinate space and developed a "spatial density correlation" methodology to analyze patterns in BFCS cortical projection targets, revealing that the BFCS is organized into three principal networks: somatosensory-motor, auditory, and visual. Within each network, clusters of cholinergic cells with increasing complexity innervate cortical targets. These networks represent hierarchically organized building blocks that may enable the BFCS to coordinate spatially selective signaling, including parallel modulation of multiple functionally interconnected yet diverse groups of cortical areas.

The clock gene Per1 is necessary in the retrosplenial cortex-but not in the suprachiasmatic nucleus-for incidental learning in young and aging male mice

Aging impairs both circadian rhythms and memory, though the relationship between these impairments is not fully understood. Circadian rhythms are largely dictated by clock genes within the body's central pacemaker, the suprachiasmatic nucleus (SCN), though these genes are also expressed in local clocks throughout the body. As circadian rhythms can directly affect memory performance, one possibility is that memory deficits observed with age are downstream of global circadian rhythm disruptions stemming from the SCN. Here, we demonstrate that expression of clock gene Period1 within a memory-relevant cortical structure, the retrosplenial cortex (RSC), is necessary for incidental learning, and that age-related disruption of Period1 within the RSC-but not necessarily the SCN-contributes to cognitive decline. These data expand the known functions of clock genes beyond maintaining circadian rhythms and suggests that age-associated changes in clock gene expression modulates circadian rhythms and memory performance in a brain region-dependent manner.

Rethinking retrosplenial cortex: Perspectives and predictions

The last decade has produced exciting new ideas about retrosplenial cortex (RSC) and its role in integrating diverse inputs. Here, we review the diversity in forms of spatial and directional tuning of RSC activity, temporal organization of RSC activity, and features of RSC interconnectivity with other brain structures. We find that RSC anatomy and dynamics are more consistent with roles in multiple sensorimotor and cognitive processes than with any isolated function. However, two more generalized categories of function may best characterize roles for RSC in complex cognitive processes: (1) shifting and relating perspectives for spatial cognition and (2) prediction and error correction for current sensory states with internal representations of the environment. Both functions likely take advantage of RSC's capacity to encode conjunctions among sensory, motor, and spatial mapping information streams. Together, these functions provide the scaffold for intelligent actions, such as navigation, perspective taking, interaction with others, and error detection.

Parallel Pathways Provide Hippocampal Spatial Information to Prefrontal Cortex

Long-range synaptic connections define how information flows through neuronal networks. Here, we combined retrograde and anterograde trans-synaptic viruses to delineate areas that exert direct and indirect influence over the dorsal and ventral prefrontal cortex (PFC) of the rat (both sexes). Notably, retrograde tracing using pseudorabies virus (PRV) revealed that both dorsal and ventral areas of the PFC receive prominent disynaptic input from the dorsal CA3 (dCA3) region of the hippocampus. The PRV experiments also identified candidate anatomical relays for this disynaptic pathway, namely, the ventral hippocampus, lateral septum, thalamus, amygdala, and basal forebrain. To determine the viability of each of these relays, we performed three additional experiments. In the first, we injected the retrograde monosynaptic tracer Fluoro-Gold into the PFC and the anterograde monosynaptic tracer Fluoro-Ruby into the dCA3 to confirm the first-order connecting areas and revealed several potential relay regions between the PFC and dCA3. In the second, we combined PRV injection in the PFC with polysynaptic anterograde viral tracer (HSV-1) in the dCA3 to reveal colabeled connecting neurons, which were evident only in the ventral hippocampus. In the third, we combined retrograde adeno-associated virus (AAV) injections in the PFC with an anterograde AAV in the dCA3 to reveal anatomical relay neurons in the ventral hippocampus and dorsal lateral septum. Together, these findings reveal parallel disynaptic pathways from the dCA3 to the PFC, illuminating a new anatomical framework for understanding hippocampal-prefrontal interactions. We suggest that the representation of context and space may be a universal feature of prefrontal function.SIGNIFICANCE STATEMENT The known functions of the prefrontal cortex are shaped by input from multiple brain areas. We used transneuronal viral tracing to discover multiple prominent disynaptic pathways through which the dorsal hippocampus (specifically, the dorsal CA3) has the potential to shape the actions of the prefrontal cortex. The demonstration of neuronal relays in the ventral hippocampus and lateral septum presents a new foundation for understanding long-range influences over prefrontal interactions, including the specific contribution of the dorsal CA3 to prefrontal function.

Comparison of the retrosplenial cortex size between the degu (Octodon degus) and the Wistar rat (Rattus norvegicus)

The degu (Octodon degus) is a rodent that normally constructs burrows for nesting and rearing. To navigate inside these burrows, degus may use idiothetic and/or sensory cues more than visual information, which is less effective in burrows. Spatial information for navigation is processed in several key brain regions including the retrosplenial cortex (RS). However, the structural characteristics of the degu RS have not been previously reported. The present study measured the sizes of the RS and constituent areas 29 and 30 in the degu, and compared these to those found in the rat, which is a terrestrial rodent. The proportion of the rostrocaudal length of the entire RS relative to that of the entire cortex was significantly larger in degus versus rats. The proportion of the rostrocaudal length of the RS at levels rostral to the splenium of the corpus callosum relative to that of the entire cortex was also significantly larger in degus versus rats. Furthermore, the ratio of the estimated volume of area 29 relative to that of area 30 was significantly larger in degus versus rats. These results show that the degu has a rostrocaudally longer rostral RS with a larger area 29 compared to the rat, which suggests that these structural features may be relevant to differences in spatial information processing between the fossorial degu and terrestrial rat.

Examining a role for the retrosplenial cortex in age-related memory impairment

Aging is often characterized by changes in the ability to form and accurately recall episodic memories, and this is especially evident in neuropsychiatric conditions including Alzheimer's disease and dementia. Memory impairments and cognitive decline associated with aging mirror the impairments observed following damage to the retrosplenial cortex, suggesting that this region might be important for continued cognitive function throughout the lifespan. Here, we review lines of evidence demonstrating that degeneration of the retrosplenial cortex is critically involved in age-related memory impairment and suggest that preservation of function in this region as part of a larger circuit that supports memory maintenance will decrease the deleterious effects of aging on memory processing.

Peering into the Brain through the Retrosplenial Cortex to Assess Cognitive Function of the Injured Brain

The retrosplenial cortex (RSC) is a posterior cortical area that has been drawing increasing interest in recent years, with a growing number of studies studying its contribution to cognitive and sensory functions. From an anatomical perspective, it has been established that the RSC is extensively and often reciprocally connected with the hippocampus, neocortex, and many midbrain regions. Functionally, the RSC is an important hub of the default-mode network. This endowment, with vast anatomical and functional connections, positions the RSC to play an important role in episodic memory, spatial and contextual learning, sensory-cognitive activities, and multi-modal sensory information processing and integration. Additionally, RSC dysfunction has been reported in cases of cognitive decline, particularly in Alzheimer's disease and stroke. We review the literature to examine whether the RSC can act as a cortical marker of persistent cognitive dysfunction after traumatic brain injury (TBI). Because the RSC is easily accessible at the brain's surface using in vivo techniques, we argue that studying RSC network activity post-TBI can shed light into the mechanisms of less-accessible brain regions, such as the hippocampus. There is a fundamental gap in the TBI field about the microscale alterations occurring post-trauma, and by studying the RSC's neuronal activity at the cellular level we will be able to design better therapeutic tools. Understanding how neuronal activity and interactions produce normal and abnormal activity in the injured brain is crucial to understanding cognitive dysfunction. By using this approach, we expect to gain valuable insights to better understand brain disorders like TBI.

Functional connectivity of anterior retrosplenial cortex in object recognition memory

Recognition memory can rely on three components: "what", "where" and "when". Recently we demonstrated that the anterior retrosplenial cortex (aRSC), like the perirhinal cortex (PRH) and unlike the hippocampus (HP), is required for consolidation of the "what" component. Here, we aimed at studying which brain structures interact with the aRSC to process object recognition (OR) memory in rats. We studied the interaction of six brain structures that are connected to the aRSC during OR memory processing: PRH, medial prefrontal cortex (mPFC), anteromedial thalamic nuclei (AM), medial entorhinal cortex (MEC), anterior cingulate cortex (ACC) and the dorsal HP (dHP). We previously described the role of the PRH and dHP, so we first studied the participation of the mPFC, AM, MEC and ACC in OR memory consolidation by bilateral microinfusions of the GABA_A receptor agonist muscimol. We observed an impairment in OR long-term memory (LTM) when inactivating the mPFC, the AM and the MEC, but not the ACC. Then, we studied the functional connections by unilateral inactivation of the aRSC and each one of the six structures in the same (ipsilateral) or the opposite (contralateral) hemisphere. Our results showed an amnesic LTM effect in rats with ipsilateral inactivations of aRSC-PRH, aRSC-mPFC, aRSC-AM, or aRSC-MEC. On the other hand, we observed memory impairment when aRSC-ACC were inactivated in opposite hemispheres, and no effect when the aRSC-dHP connection was inactivated. Thus, our ipsilateral inactivation findings reveal that the aRSC and, at least one brain region required in OR LTM processing are essential to consolidate OR memory. In conclusion, our results show that several cortico-cortical and cortico-thalamic pathways are important for OR memory consolidation.

The anterior retrosplenial cortex encodes event-related information and the posterior retrosplenial cortex encodes context-related information during memory formation

The retrosplenial cortex (RSC) is extensively interconnected with the dorsal hippocampus and has several important roles in learning and memory. Recent work has demonstrated that certain types of context-dependent learning are selectively impaired when the posterior, but not the anterior, region of the RSC is damaged, suggesting that the role of the RSC in memory formation may not be uniform along its rostro-caudal axis. The current experiments tested the idea that the anterior and posterior portions of the rat RSC contribute to different aspects of memory formation. We first confirmed that brief optogenetic inhibition of either the anterior or posterior RSC resulted in decreased local cellular activity as indexed by immediate early gene zif268 expression and that this decrease was restricted to the target region within RSC. We then found that silencing the anterior or posterior RSC during trace fear training trials had different effects on memory: While inhibiting neural activity in the anterior RSC had a selective impact on behavior evoked by the auditory CS, inhibition of the posterior RSC selectively impaired memory for the context in which training was conducted. These results contribute to a growing literature that supports functionally distinct roles in learning and memory for subregions of the RSC.

Retrosplenial Cortical Connectivity with Frontal Basal Ganglia Networks

Previous studies of the retrosplenial cortex (RSC) have focused on its role in navigation and memory, consistent with its well-established medial temporal connections, but recent evidence also suggests a role for this region in reward and decision making. Because function is determined largely by anatomical connections, and to better understand the anatomy of RSC, we used tract-tracing methods to examine the anatomical connectivity between the rat RSC and frontostriatal networks (canonical reward and decision-making circuits). We find that, among frontal cortical regions, RSC bidirectionally connects most strongly with the anterior cingulate cortex, but also with an area of the central-medial orbito-frontal cortex. RSC projects to the dorsomedial striatum, and its terminal fields are virtually encompassed by the frontal-striatal projection zone, suggestive of functional convergence through the basal ganglia. This overlap is driven by anterior cingulate cortex, prelimbic cortex, and orbito-frontal cortex, all of which contribute to goal-directed decision making, suggesting that the RSC is involved in similar processes.

Differential Contributions of Glutamatergic Hippocampal→Retrosplenial Cortical Projections to the Formation and Persistence of Context Memories

Learning to associate stressful events with specific environmental contexts depends on excitatory transmission in the hippocampus, but how this information is transmitted to the neocortex for lasting memory storage is unclear. We identified dorsal hippocampal (DH) projections to the retrosplenial cortex (RSC), which arise mainly from the subiculum and contain either the vesicular glutamate transporter 1 (vGlut1) or vGlut2. Both vGlut1+ and vGlut2+ axons strongly excite and disynaptically inhibit RSC pyramidal neurons in superficial layers, but vGlut2+ axons trigger greater inhibition that spreads to deep layers, indicating that these pathways engage RSC circuits via partially redundant, partially differentiated cellular mechanisms. Using contextual fear conditioning in mice to model contextual associative memories, together with chemogenetic axonal silencing, we found that vGlut1+ projections are principally involved in processing recent context memories whereas vGlut2+ projections contribute to their long-lasting storage. Thus, within the DH→RSC pathway, engagement of vGlut1+ and vGlut2+ circuits differentially contribute to the formation and persistence of fear-inducing context memories.

Locomotion-dependent remapping of distributed cortical networks

The interactions between neocortical areas are fluid and state-dependent, but how individual neurons couple to cortex-wide network dynamics remains poorly understood. We correlated the spiking of neurons in primary visual (V1) and retrosplenial (RSP) cortex to activity across dorsal cortex, recorded simultaneously by widefield calcium imaging. Neurons were correlated with distinct and reproducible patterns of activity across the cortical surface; while some fired predominantly with their local area, others coupled to activity in distal areas. The extent of distal coupling was predicted by how strongly neurons correlated with the local network. Changes in brain state triggered by locomotion strengthened affiliations of V1 neurons with higher visual and motor areas, while strengthening distal affiliations of RSP neurons with sensory cortices. Thus, the diverse coupling of individual neurons to cortex-wide activity patterns is restructured by running in an area-specific manner, resulting in a shift in the mode of cortical processing during locomotion.

Functional and anatomical relationships between the medial precentral cortex, dorsal striatum, and head direction cell circuitry. II. Neuroanatomical studies

An animal's directional heading within its environment is encoded by the activity of head direction (HD) cells. In rodents, these neurons are found primarily within the limbic system in the interconnected structures that form the limbic HD circuit. In our accompanying report in this issue, we describe two HD cell populations located outside of this circuit in the medial precentral cortex (PrCM) and dorsal striatum (DS). These extralimbic areas receive their HD signals from the limbic system but do not provide critical input or feedback to limbic HD cells (Mehlman ML, Winter SS, Valerio S, Taube JS. J Neurophysiol 121: 350-370, 2019.). In this report, we complement our previous lesion and recording experiments with a series of neuroanatomical tracing studies in rats designed to examine patterns of connectivity between the PrCM, DS, limbic HD circuit, and related spatial processing circuitry. Retrograde tracing revealed that the DS receives direct input from numerous structures known to contain HD cells and/or other spatially tuned cell types. Importantly, these projections preferentially target and converge within the most medial portion of the DS, the same area in which we previously recorded HD cells. The PrCM receives direct input from a subset of these spatial processing structures. Anterograde tracing identified indirect pathways that could permit the PrCM and DS to convey self-motion information to the limbic HD circuit. These tracing studies reveal the anatomical basis for the functional relationships observed in our lesion and recording experiments. Collectively, these findings expand our understanding of how spatial processing circuitry functionally and anatomically extends beyond the limbic system into the PrCM and DS. NEW & NOTEWORTHY Head direction (HD) cells are located primarily within the limbic system, but small populations of extralimbic HD cells are found in the medial precentral cortex (PrCM) and dorsal striatum (DS). The neuroanatomical tracing experiments reported here explored the pathways capable of transmitting the HD signal to these extralimbic areas. We found that projections arising from numerous spatial processing structures converge within portions of the PrCM and DS that contain HD cells.

The cingulum bundle: Anatomy, function, and dysfunction

The cingulum bundle is a prominent white matter tract that interconnects frontal, parietal, and medial temporal sites, while also linking subcortical nuclei to the cingulate gyrus. Despite its apparent continuity, the cingulum's composition continually changes as fibres join and leave the bundle. To help understand its complex structure, this review begins with detailed, comparative descriptions of the multiple connections comprising the cingulum bundle. Next, the impact of cingulum bundle damage in rats, monkeys, and humans is analysed. Despite causing extensive anatomical disconnections, cingulum bundle lesions typically produce only mild deficits, highlighting the importance of parallel pathways and the distributed nature of its various functions. Meanwhile, non-invasive imaging implicates the cingulum bundle in executive control, emotion, pain (dorsal cingulum), and episodic memory (parahippocampal cingulum), while clinical studies reveal cingulum abnormalities in numerous conditions, including schizophrenia, depression, post-traumatic stress disorder, obsessive compulsive disorder, autism spectrum disorder, Mild Cognitive Impairment, and Alzheimer's disease. Understanding the seemingly diverse contributions of the cingulum will require better ways of isolating pathways within this highly complex tract.

Estradiol Protects White Matter of Male C57BL6J Mice against Experimental Chronic Cerebral Hypoperfusion

BACKGROUND AND PURPOSE

Estradiol is a sex steroid hormone known to protect the brain against damage related to transient and global cerebral ischemia. In the present study, we leverage an experimental murine model of bilateral carotid artery stenosis (BCAS) to examine the putative effects of estradiol therapy on chronic cerebral hypoperfusion. We hypothesize that long-term estradiol therapy protects against white matter injury and declarative memory deficits associated with chronic cerebral hypoperfusion.

METHODS

Adult male C57BL/6J mice underwent either surgical BCAS or sham procedures. Two days after surgery, the mice were given oral estradiol (Sham+E, BCAS+E) or placebo (Sham+P, BCAS+P) treatments daily for 31-34 days. All mice underwent Novel Object Recognition (NOR) testing 31-34 days after the start of oral treatments. Following sacrifice, blood was collected and brains fixed, sliced, and prepared for histological examination of white matter injury and extracellular signal-regulated kinase (ERK) expression.

RESULTS

Animals receiving long-term oral estradiol therapy (BCAS-E2 and Sham-E2) had higher plasma estradiol levels than those receiving placebo treatment (BCAS-P and Sham-P). BCAS-E2 mice demonstrated less white matter injury (Klüver-Barrera staining) and performed better on the NOR task compared to BCAS-P mice. ERK expression in the brain was increased in the BCAS compared to sham cohorts. Among the BCAS mice, the BCAS-E2 cohort had a greater number of ERK + cells.

CONCLUSION

This study demonstrates a potentially protective role for oral estradiol therapy in the setting of white matter injury and declarative memory deficits secondary to murine chronic cerebral hypoperfusion.

Role of retrosplenial cortex in processing stress-related context memories

This work summarizes evidence for the role of RSC in processing fear-inducing context memories. Specifically, we discuss molecular, cellular, and network mechanisms by which RSC might contribute the processing of contextual fear memories. We focus on glutamatergic and cholinergic mechanisms underlying encoding, retrieval, and extinction of context-dependent fear. RSC mechanisms underlying retrieval of recently and remotely acquired memories are compared to memory mechanisms of anterior cortices. Due to the strong connectivity between hippocampus and RSC, we also compare the extent to which their mechanisms of encoding, retrieval, and extinction show overlap. At a theoretical level, we discuss the role of RSC in the framework of systems consolidation as well as retrieval-induced memory modulation. Lastly, we emphasize the implication of these findings for psychopathologies associated with neurological and psychiatric disorders. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Neural mechanisms of navigation involving interactions of cortical and subcortical structures

Animals must perform spatial navigation for a range of different behaviors, including selection of trajectories toward goal locations and foraging for food sources. To serve this function, a number of different brain regions play a role in coding different dimensions of sensory input important for spatial behavior, including the entorhinal cortex, the retrosplenial cortex, the hippocampus, and the medial septum. This article will review data concerning the coding of the spatial aspects of animal behavior, including location of the animal within an environment, the speed of movement, the trajectory of movement, the direction of the head in the environment, and the position of barriers and objects both relative to the animal's head direction (egocentric) and relative to the layout of the environment (allocentric). The mechanisms for coding these important spatial representations are not yet fully understood but could involve mechanisms including integration of self-motion information or coding of location based on the angle of sensory features in the environment. We will review available data and theories about the mechanisms for coding of spatial representations. The computation of different aspects of spatial representation from available sensory input requires complex cortical processing mechanisms for transformation from egocentric to allocentric coordinates that will only be understood through a combination of neurophysiological studies and computational modeling.

Lesions of retrosplenial cortex spare immediate-early gene activity in related limbic regions in the rat

The retrosplenial cortex forms part of a network of cortical and subcortical structures that have particular importance for spatial learning and navigation in rodents. This study examined how retrosplenial lesions affect activity in this network by visualising the expression of the immediate-early genes c-fos and zif268 after exposure to a novel location. Groups of rats with extensive cytotoxic lesions (areas 29 and 30) and rats with lesions largely confined to area 30 (dysgranular cortex) were compared with their respective control animals for levels of c-fos expression measured by immunohistochemistry. These cortical lesions had very limited effects on distal c-fos activity. Evidence of a restricted reduction in c-fos activity was seen in the septal dentate gyrus (superior blade) but not in other hippocampal and parahippocampal subareas, nor in the anterior cingulate and prelimbic cortices. Related studies examined zif268 activity in those cases with combined area 29 and 30 lesions. The only clear evidence for reduced zif268 activity following retrosplenial cell loss came from the septal CA3 area. The confined impact of retrosplenial tissue loss is notable as, by the same immediate-early gene measures, retrosplenial cortex is itself highly sensitive to damage in related limbic areas, showing a marked c-fos and zif268 hypoactivity across all of its subareas. This asymmetry in covert pathology may help to explain the apparent disparity between the severity of learning deficits after retrosplenial cortex lesions and after lesions in either the hippocampus or the anterior thalamic nuclei.

Putting fear in context: Elucidating the role of the retrosplenial cortex in context discrimination in rats

The retrosplenial cortex (RSC), which receives visuo-spatial sensory input and interacts with numerous hippocampal memory system structures, has a well-established role in contextual learning and memory. While it has been demonstrated that RSC function is necessary to learn to recognize a single environment that is directly paired with an aversive event, the role of the RSC in discriminating between two different contexts remains largely unknown. To address this, first order (Experiment 1) and higher order (Experiment 2) fear conditioning paradigms were conducted with sham and RSC-lesioned rats. In Experiment 1 rats were exposed to one context in which shock was delivered and to a second, distinct context without shock. Their ability to discriminate between the contexts was assessed during a re-exposure test. In a second experiment, a new cohort of RSC-lesioned rats was exposed to two contexts made distinct through visual, olfactory and auditory stimuli. In a subsequent conditioning phase, the salience of one of the auditory stimuli was paired to an aversive footshock while the other was not. Similar to Experiment 1, freezing behavior was analyzed upon re-exposure to the contexts in the absence of both the auditory cue and the footshock. The results revealed that RSC is not necessary for rats to use contextual information to successfully discriminate between two contexts under the relatively simple demands involved in this first order conditioning paradigm but that context discrimination is impaired when the processing of complex and/or ambiguous contextual stimuli is required to select appropriate behavioral responses.

Granger causal connectivity dissociates navigation networks that subserve allocentric and egocentric path integration

Studies on spatial navigation demonstrate a significant role of the retrosplenial complex (RSC) in the transformation of egocentric and allocentric information into complementary spatial reference frames (SRFs). The tight anatomical connections of the RSC with a wide range of other cortical regions processing spatial information support its vital role within the human navigation network. To better understand how different areas of the navigational network interact, we investigated the dynamic causal interactions of brain regions involved in solving a virtual navigation task. EEG signals were decomposed by independent component analysis (ICA) and subsequently examined for information flow between clusters of independent components (ICs) using direct short-time directed transfer function (sdDTF). The results revealed information flow between the anterior cingulate cortex and the left prefrontal cortex in the theta (4-7 Hz) frequency band and between the prefrontal, motor, parietal, and occipital cortices as well as the RSC in the alpha (8-13 Hz) frequency band. When participants prefered to use distinct reference frames (egocentric vs. allocentric) during navigation was considered, a dominant occipito-parieto-RSC network was identified in allocentric navigators. These results are in line with the assumption that the RSC, parietal, and occipital cortices are involved in transforming egocentric visual-spatial information into an allocentric reference frame. Moreover, the RSC demonstrated the strongest causal flow during changes in orientation, suggesting that this structure directly provides information on heading changes in humans.

Oral haloperidol or olanzapine intake produces distinct and region-specific increase in cannabinoid receptor levels that is prevented by high fat diet

Clinical studies show higher levels of cannabinoid CB1 receptors (CB1R) in the brain of schizophrenic patients while preclinical studies report a significant functional interaction between dopamine D2 receptors and CB1Rs as well as an upregulation of CB1Rs after antipsychotic treatment. These findings prompted us to study the effects of chronic oral intake of a first and a second generation antipsychotic, haloperidol and olanzapine, on the levels and distribution of CB1Rs in the rat brain. Rats consumed either regular chow or high-fat food and drank water, haloperidol drinking solution (1.5mg/kg), or olanzapine drinking solution (10mg/kg) for four weeks. Motor and cognitive functions were tested at the end of treatment week 3 and upon drug discontinuation. Two days after drug discontinuation, rats were euthanized and brains were processed for in vitro receptor autoradiography. In chow-fed animals, haloperidol and olanzapine increased CB1R levels in the basal ganglia and the hippocampus, in a similar, but not identical pattern. In addition, olanzapine had unique effects in CB1R upregulation in higher order cognitive areas, in the secondary somatosensory cortex, in the visual and auditory cortices and the geniculate nuclei, as well as in the hypothalamus. High fat food consumption prevented antipsychotic-induced increase in CB1R levels in all regions examined, with one exception, the globus pallidus, in which they were higher in haloperidol-treated rats. The results point towards the hypothesis that increased CB1R levels could be a confounding effect of antipsychotic medication in schizophrenia that is circumveneted by high fat feeding.

A Corticocortical Circuit Directly Links Retrosplenial Cortex to M2 in the Mouse

Retrosplenial cortex (RSC) is a dorsomedial parietal area involved in a range of cognitive functions, including episodic memory, navigation, and spatial memory. Anatomically, the RSC receives inputs from dorsal hippocampal networks and in turn projects to medial neocortical areas. A particularly prominent projection extends rostrally to the posterior secondary motor cortex (M2), suggesting a functional corticocortical link from the RSC to M2 and thus a bridge between hippocampal and neocortical networks involved in mnemonic and sensorimotor aspects of navigation. We investigated the cellular connectivity in this RSC→M2 projection in the mouse using optogenetic photostimulation, retrograde labeling, and electrophysiology. Axons from RSC formed monosynaptic excitatory connections onto M2 pyramidal neurons across layers and projection classes, including corticocortical/intratelencephalic neurons (reciprocally and callosally projecting) in layers 2-6, pyramidal tract neurons (corticocollicular, corticopontine) in layer 5B, and, to a lesser extent, corticothalamic neurons in layer 6. In addition to these direct connections, disynaptic connections were made via posterior parietal cortex (RSC→PPC→M2) and anteromedial thalamus (RSC→AM→M2). In the reverse direction, axons from M2 monosynaptically excited M2-projecting corticocortical neurons in the RSC, especially in the superficial layers of the dysgranular region. These findings establish an excitatory RSC→M2 corticocortical circuit that engages diverse types of excitatory projection neurons in the downstream area, suggesting a basis for direct communication from dorsal hippocampal networks involved in spatial memory and navigation to neocortical networks involved in diverse aspects of sensorimotor integration and motor control.

SIGNIFICANCE STATEMENT

Corticocortical pathways interconnect cortical areas extensively, but the cellular connectivity in these pathways remains largely uncharacterized. Here, we show that a posterior part of secondary motor cortex receives corticocortical axons from the rostral retrosplenial cortex (RSC) and these form monosynaptic excitatory connections onto a wide spectrum of excitatory projection neurons in this area. Our results define a cellular basis for direct communication from RSC to this medial frontal area, suggesting a direct link from dorsal hippocampal networks involved in spatial cognition and navigation (the "map") to sensorimotor networks involved the control of movement (the "motor").

The rat retrosplenial cortex as a link for frontal functions: A lesion analysis

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Retrosplenial cortex lesions do not reproduce the pattern of effects of medial frontal damage.

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Retrosplenial cortex lesions spare tests of behavioural flexibility.

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Effort-based decision making does not require the retrosplenial cortex.

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Reveals specific conditions when nonspatial tasks engage retrosplenial cortex.

Cohorts of rats with excitotoxic retrosplenial cortex lesions were tested on four behavioural tasks sensitive to dysfunctions in prelimbic cortex, anterior cingulate cortex, or both. In this way the study tested whether retrosplenial cortex has nonspatial functions that reflect its anatomical interactions with these frontal cortical areas. In Experiment 1, retrosplenial cortex lesions had no apparent effect on a set-shifting digging task that taxed intradimensional and extradimensional attention, as well as reversal learning. Likewise, retrosplenial cortex lesions did not impair a strategy shift task in an automated chamber, which involved switching from visual-based to response-based discriminations and, again, included a reversal (Experiment 2). Indeed, there was evidence that the retrosplenial lesions aided the initial switch to response-based selection. No lesion deficit was found on an automated cost-benefit task that pitted size of reward against effort to achieve that reward (Experiment 3). Finally, while retrosplenial cortex lesions affected matching-to-place task in a T-maze, the profile of deficits differed from that associated with prelimbic cortex damage (Experiment 4). When the task was switched to a nonmatching design, retrosplenial cortex lesions had no apparent effect on performance. The results from the four experiments show that many frontal tasks do not require the retrosplenial cortex, highlighting the specificity of their functional interactions. The results show how retrosplenial cortex lesions spare those learning tasks in which there is no mismatch between the internal and external representations used to guide behavioural choice. In addition, these experiments further highlight the importance of the retrosplenial cortex in solving tasks with a spatial component.

Fast voltage-sensitive dye imaging of excitatory and inhibitory synaptic transmission in the rat granular retrosplenial cortex

Rodent granular retrosplenial cortex (GRS) has dense connections between the anterior thalamic nuclei (ATN) and hippocampal formation. GRS superficial pyramidal neurons exhibit distinctive late spiking (LS) firing property and form patchy clusters with prominent apical dendritic bundles. The aim of this study was to investigate spatiotemporal dynamics of signal transduction in the GRS induced by ATN afferent stimulation by using fast voltage-sensitive dye imaging in rat brain slices. In coronal slices, layer 1a stimulation, which presumably activated thalamic fibers, evoked propagation of excitatory synaptic signals from layers 2-4 to layers 5-6 in a direction perpendicular to the layer axis, followed by transverse signal propagation within each layer. In the presence of ionotropic glutamate receptor antagonists, inhibitory responses were observed in superficial layers, induced by direct activation of inhibitory interneurons in layer 1. In horizontal slices, excitatory signals in deep layers propagated transversely mainly from posterior to anterior via superficial layers. Cortical inhibitory responses upon layer 1a stimulation in horizontal slices were weaker than those in the coronal slices. Observed differences between coronal and horizontal planes suggest anisotropy of the intracortical circuitry. In conclusion, ATN inputs are processed differently in coronal and horizontal planes of the GRS and then conveyed to other cortical areas. In both planes, GRS superficial layers play an important role in signal propagation, which suggests that superficial neuronal cascade is crucial in the integration of multiple information sources.NEW & NOTEWORTHY Superficial neurons in the rat granular retrosplenial cortex (GRS) show distinctive late-spiking (LS) firing property. However, little is known about spatiotemporal dynamics of signal transduction in the GRS. We demonstrated LS neuron network relaying thalamic inputs to deep layers and anisotropic distribution of inhibition between coronal and horizontal planes. Since deep layers of the GRS receive inputs from the subiculum, GRS circuits may work as an integrator of multiple sources such as sensory and memory information.

Postnatal changes of interleukin-18 receptor immunoreactivity in neurons of the retrosplenial cortex in wild-type and interleukin-18 knock out mice

Interleukin-18 (IL-18), which is involved in the inflammatory response, is also found in the cerebral cortex. IL-18 receptor-immunoreactive (IL-18R-ir) neurons are present in layer V of the retrosplenial cortex (RSC). In the adult IL-18 knock out (KO) mice, no IL-18R-ir neurons but many degenerated neurons are present in layer V of the RSC, suggesting that any changes in the neurons of layer V have occurred during postnatal development. We examined changes of IL-18R expression during postnatal development. In the wild-type mice, many IL-18R-ir neurons were present in layers II, III and VI of the RSC in 2-week-old mice, whereas they were sparsely observed in only layer III in 3-week-old mice. No IL-18R-ir neurons were present in 4- and 5-week-old mice. In older than 6-week-old mice, many IL-18R-ir neurons were present in layers V and VI. The IL-18KO mice showed IL-18R-ir neurons in layers II, III and VI at 2-weeks-old, and a few in layer III at 3-week-old mice, similar to that in the wild-type mice. No IL-18R-ir neurons were found in mice older than 4 weeks of age. Thus, IL-18 or IL-18R seem to be involved in the construction of neural circuits corresponding to events after 3-weeks of age.

Fine structure of interleukin 18 (IL-18) receptor-immunoreactive neurons in the retrosplenial cortex and its changes in IL18 knockout mice

Interleukin 18 (IL-18) participates in the inflammatory immune response of lymphocytes. Delay in learning or memory are common in the IL-18 knockout mouse. Many IL-18-immunoreactive neurons are found in the retrosplenial cortex (RSC) and the subiculum. These neurons also contain the IL-18 receptor. We determined the location and the ultrastructure of the IL-18 receptor-immunoreactive neurons in the RSC and observed changes in the IL-18 receptor-immunoreactive neurons of the IL-18 knockout mouse. The IL-18 receptor-immunoreactive neurons were found specifically in layer V of the granular RSC. They were medium-sized neurons with a light oval nucleus and had little cytoplasm with many free ribosomes, rough endoplasmic reticulum and many mitochondria, but no Nissl bodies. The number of axosomatic terminals was about six per section. The IL-18 receptor-immunoreactive neurons were not found in the RSC in the IL-18 knockout mouse at 5 or 9 weeks of age. However, many small electron-dense neurons were found in layer V. Both the nucleus and cytoplasm were electron-dense, but not necrotic. The mitochondria and rough endoplasmic reticulum were swollen. The IL-18 receptor-immunoreactive neurons were presumed to be degenerating. The degeneration of the IL18-receptor-immunoreactive neurons in the RSC may cause the abnormal behaviors of the IL-18 knockout mice.

Which way and how far? Tracking of translation and rotation information for human path integration

Path integration, the constant updating of the navigator's knowledge of position and orientation during movement, requires both visuospatial knowledge and memory. This study aimed to develop a systems-level understanding of human path integration by examining the basic building blocks of path integration in humans. To achieve this goal, we used functional imaging to examine the neural mechanisms that support the tracking and memory of translational and rotational components of human path integration. Critically, and in contrast to previous studies, we examined movement in translation and rotation tasks with no defined end-point or goal. Navigators accumulated translational and rotational information during virtual self-motion. Activity in hippocampus, retrosplenial cortex (RSC), and parahippocampal cortex (PHC) increased during both translation and rotation encoding, suggesting that these regions track self-motion information during path integration. These results address current questions regarding distance coding in the human brain. By implementing a modified delayed match to sample paradigm, we also examined the encoding and maintenance of path integration signals in working memory. Hippocampus, PHC, and RSC were recruited during successful encoding and maintenance of path integration information, with RSC selective for tasks that required processing heading rotation changes. These data indicate distinct working memory mechanisms for translation and rotation, which are essential for updating neural representations of current location. The results provide evidence that hippocampus, PHC, and RSC flexibly track task-relevant translation and rotation signals for path integration and could form the hub of a more distributed network supporting spatial navigation. Hum Brain Mapp 37:3636-3655, 2016. © 2016 Wiley Periodicals, Inc.

Intra- and Interhemispheric Propagation of Electrophysiological Synchronous Activity and Its Modulation by Serotonin in the Cingulate Cortex of Juvenile Mice

Disinhibition of the cortex (e.g., by GABA -receptor blockade) generates synchronous and oscillatory electrophysiological activity that propagates along the cortex. We have studied, in brain slices of the cingulate cortex of mice (postnatal age 14–20 days), the propagation along layer 2/3 as well as the interhemispheric propagation through the corpus callosum of synchronous discharges recorded extracellularly and evoked in the presence of 10 μM bicuculline by electrical stimulation of layer 1. The latency of the responses obtained at the same distance from the stimulus electrode was longer in anterior cingulate cortex (ACC: 39.53 ± 2.83 ms, n = 7) than in retrosplenial cortex slices (RSC: 21.99 ± 2.75 ms, n = 5; p<0.05), which is equivalent to a lower propagation velocity in the dorso-ventral direction in ACC than in RSC slices (43.0 mm/s vs 72.9 mm/s). We studied the modulation of this propagation by serotonin. Serotonin significantly increased the latency of the intracortical synchronous discharges (18.9% in the ipsilateral hemisphere and 40.2% in the contralateral hemisphere), and also increased the interhemispheric propagation time by 86.4%. These actions of serotonin were mimicked by the activation of either 5-HT_1B or 5-HT_2A receptors, but not by the activation of the 5-HT_1A subtype. These findings provide further knowledge about the propagation of synchronic electrical activity in the cerebral cortex, including its modulation by serotonin, and suggest the presence of deep differences between the ACC and RSC in the structure of the local cortical microcircuits underlying the propagation of synchronous discharges.

There and Back Again: Hippocampus and Retrosplenial Cortex Track Homing Distance during Human Path Integration

Path integration, the updating of position and orientation during movement, often involves tracking a home location. Here, we examine processes that could contribute to successful location tracking in humans. In particular, we investigate a homing vector model of path integration, whereby a navigator continuously tracks a trajectory back to the home location. To examine this model, we developed a loop task for fMRI, in which participants viewed movement that circled back to a home location in a sparse virtual environment. In support of a homing vector system, hippocampus, retrosplenial cortex, and parahippocampal cortex were responsive to Euclidean distance from home. These results provide the first evidence of a constantly maintained homing signal in the human brain. In addition, hippocampus, retrosplenial cortex, and parahippocampal cortex, as well as medial prefrontal cortex, were recruited during successful path integration. These findings suggest that dynamic processes recruit hippocampus, retrosplenial cortex, and parahippocampal cortex in support of path integration, including a homing vector system that tracks movement relative to home.

SIGNIFICANCE STATEMENT

Path integration is the continual updating of position and orientation during navigation. Animal studies have identified place cells and grid cells as important for path integration, but underlying models of path integration in humans have rarely been studied. The results of our novel loop closure task are the first to suggest that a homing vector tracks Euclidean distance from the home location, supported by the hippocampus, retrosplenial cortex, and parahippocampal cortex. These findings suggest a potential homing vector mechanism supporting path integration, which recruits hippocampus and retrosplenial cortex to track movement relative to home. These results provide new avenues for computational and animal models by directing attention to homing vector models of path integration, which differ from current movement-tracking models.

Reduced cortical volume and elevated astrocyte density in rats chronically treated with antipsychotic drugs-linking magnetic resonance imaging findings to cellular pathology

BACKGROUND

Increasing evidence suggests that antipsychotic drugs (APD) might affect brain structure directly, particularly the cerebral cortex. However, the precise anatomical loci of these effects and their underlying cellular basis remain unclear.

METHODS

With ex vivo magnetic resonance imaging in rats treated chronically with APDs, we used automated analysis techniques to map the regions that show maximal impact of chronic (8 weeks) treatment with either haloperidol or olanzapine on the rat cortex. Guided by these imaging findings, we undertook a focused postmortem investigation with stereology.

RESULTS

We identified decreases in the volume and thickness of the anterior cingulate cortex (ACC) after chronic APD treatment, regardless of the APD administered. Postmortem analysis confirmed these volumetric findings and demonstrated that chronic APD treatment had no effect on the total number of neurons or S100β+ astrocytes in the ACC. In contrast, an increase in the density of these cells was observed.

CONCLUSIONS

This study demonstrates region-specific structural effects of chronic APD treatment on the rat cortex, primarily but not exclusively localized to the ACC. At least in the rat, these changes are not due to a loss of either neurons or astrocytes and are likely to reflect a loss of neuropil. Although caution needs to be exerted when extrapolating results from animals to patients, this study highlights the power of this approach to link magnetic resonance imaging findings to their histopathological origins.

Localization of α7 nicotinic acetylcholine receptor immunoreactivity on GABAergic interneurons in layers I-III of the rat retrosplenial granular cortex

The rat retrosplenial granular cortex (RSG) receives cholinergic input from the medial septum-diagonal band (MS-DB) of the cholinergic basal forebrain (CBF), with projections terminating in layers I-III of RSG. The modulatory effects of acetylcholine (ACh) on cortical GABAergic interneurons in these layers are mediated by α7 nicotinic acetylcholine receptors (α7nAChRs). α7nAChRs are most abundant in the cerebral cortex and are largely localized to GABAergic interneurons. However, the CBF projection to the RSG has not been studied in detail, and the cellular or subcellular distribution of α7nAChRs in the rat RSG remains unclear. The main objective of this study was to test that α7nAChRs reside on GABAergic interneurons in CBF terminal fields of the rat RSG. First, we set out to define the characteristics of CBF projections from the MS-DB to layers of the RSG using anterograde neural tracing and immunohistochemical labeling with cholinergic markers. These results revealed that the pattern of axon terminal labeling in layer Ia, as well as layer II/III of the RSG is remarkably similar to the pattern of cholinergic axons in the RSG. Next, we investigated the relationship between α7nAChRs, labeled using either α-bungarotoxin or α7nAChR antibody, and the local neurochemical environment by labeling surrounding cells with antibodies against glutamic acid decarboxylase (GAD), parvalbumin (PV) and reelin (a marker of the ionotropic serotonin receptor-expressing GABAergic interneurons). α7nAChRs were found to be localized on both somatodendritic and neuronal elements within subpopulations of GABAergic PV-, reelin-stained and non PV-stained neurons in layers I-III of the RSG. Finally, electron microscopy revealed that α7nAChRs are GAD- and PV-positive cytoplasmic and neuronal elements. These results strongly suggest that ACh released from CBF afferents is transmitted via α7nAChR to GAD-, PV-, and reelin-positive GABAergic interneurons in layers I-III of the RSG.

Pyramidal neurons in the superficial layers of rat retrosplenial cortex exhibit a late-spiking firing property

The rodent granular retrosplenial cortex (GRS) is reciprocally connected with the hippocampus. It is part of several networks implicated in spatial learning and memory, and is known to contain head-direction cells. There are, however, few specifics concerning the mechanisms and microcircuitry underlying its involvement in spatial and mnemonic functions. In this report, we set out to characterize intrinsic properties of a distinctive population of small pyramidal neurons in layer 2 of rat GRS. These neurons, as well as those in adjoining layer 3, were found to exhibit a late-spiking (LS) firing property. We established by multiple criteria that the LS property is a consequence of delayed rectifier and A-type potassium channels. These were identified as Kv1.1, Kv1.4 and Kv4.3 by Genechip analysis, in situ hybridization, single-cell reverse transcriptase-polymerase chain reaction, and pharmacological blockade. The LS property might facilitate comparison or integration of synaptic inputs during an interval delay, consistent with the proposed role of the GRS in memory-related processes.

Role of the anterior thalamic nucleus in the motor hyperactivity induced by systemic MK-801 administration in rats

Non-competitive N-methyl-D-aspartate receptor (NMDA-R) antagonists have been extensively used in rodents to model psychotic symptoms of schizophrenia. Although the motor syndrome induced by acute and systemic administration of low doses of dizocilpine (MK-801) has been extensively characterized, its neurobiological basis is not fully understood. NMDA-R antagonists can disinhibit excitatory inputs in certain brain areas, but the precise circuitry is not fully known. We examined the involvement of the anterior thalamic nucleus (ATN) in hyperlocomotion and other related behaviors (stereotypies, ataxia signs) induced after acute systemic administration of MK-801. Since GABAergic neurons of the reticular thalamic nucleus (RTN) exert the main inhibitory control on thalamic projection neurons, we hypothesized that systemically injected MK-801 might block NMDA-R on RTN GABAergic neurons. This effect would subsequently result in disinhibition of GABAergic inputs onto ATN projections to cortical motor areas, thereby inducing behavioral effects. We evaluated the behavioral syndrome induced by the systemic administration MK-801 (0.2 mg/kg) in control rats and in rats subjected to a bilateral stereotaxic infusion of the GABA(A) agonist muscimol (0.2 μl of 2.5 and 5.0 mM; 0.5-1 nmol per application, respectively) into the ATN. As previously reported, MK-801-induced hyperlocomotion in parallel with disorganized movements (e.g. not guided by normal exploration) slight ataxia signs and stereotypies. All responses were antagonized by pre-infusion of muscimol but not saline into the ATN. According to our results we suggest that the ATN plays a role on hyperlocomotion evoked by MK-801 and could involve a thalamic GABAergic disinhibition mechanism.

Hippocampal, retrosplenial, and prefrontal hypoactivity in a model of diencephalic amnesia: Evidence towards an interdependent subcortical-cortical memory network

The medial diencephalon is vital for memory, but it is not known why. The present study tested between the predictions of current hypotheses as to why this region is critical for memory. Lesions were made in the rat mammillothalamic tract, the only diencephalic structure consistently associated with amnesia in humans after ischemia. Decreased activity, as measured by immediate-early gene expression (c-fos), was found in three key sites associated with memory function: the hippocampus, the prefrontal cortex, and the retrosplenial cortex. The specificity of these changes was confirmed by the qualitatively different patterns of immediately-early gene changes seen after amygdala lesions, e.g., hypoactivity in the hippocampus and retrosplenial cortex following mammillothalamic tract lesions but not following amygdala lesions. The mammillothalamic lesion results unify substrates linked to diencephalic and temporal lobe amnesia, and thereby support a new account of diencephalic amnesia that emphasizes multiple dysfunctions across hippocampal, retrosplenial, and prefrontal areas. This account suggests a role for the mammillary bodies that is independent of their hippocampal inputs.

Demonstration of a neural circuit critical for imprinting behavior in chicks

Imprinting behavior in birds is elicited by visual and/or auditory cues. It has been demonstrated previously that visual cues are recognized and processed in the visual Wulst (VW), and imprinting memory is stored in the intermediate medial mesopallium (IMM) of the telencephalon. Alteration of neural responses in these two regions according to imprinting has been reported, yet direct evidence of the neural circuit linking these two regions is lacking. Thus, it remains unclear how memory is formed and expressed in this circuit. Here, we present anatomical as well as physiological evidence of the neural circuit connecting the VW and IMM and show that imprinting training during the critical period strengthens and refines this circuit. A functional connection established by imprint training resulted in an imprinting behavior. After the closure of the critical period, training could not activate this circuit nor induce the imprinting behavior. Glutamatergic neurons in the ventroposterior region of the VW, the core region of the hyperpallium densocellulare (HDCo), sent their axons to the periventricular part of the HD, just dorsal and afferent to the IMM. We found that the HDCo is important in imprinting behavior. The refinement and/or enhancement of this neural circuit are attributed to increased activity of HDCo cells, and the activity depended on NR2B-containing NMDA receptors. These findings show a neural connection in the telencephalon in Aves and demonstrate that NR2B function is indispensable for the plasticity of HDCo cells, which are key mediators of imprinting.

Granular and dysgranular retrosplenial cortices provide qualitatively different contributions to spatial working memory: evidence from immediate-early gene imaging in rats

The present study revealed striking task-dependent differences in immediate-early gene activity in the two main subregions (granular and dysgranular) of the retrosplenial cortex. In addition, there were activity differences along the rostro-caudal axis of both subregions. Two groups of rats were trained on a working memory task in a radial-arm maze, one group in the light, the other in the dark. Each working memory group had two sets of yoked controls. Working memory consistently increased retrosplenial immediate-early gene activity (c-fos and zif268 ), although systematic differences occurred in the granular and dysgranular subregions. Both c-fos and zif268 expression increased in granular cortex irrespective of whether the spatial memory task was in the light or dark. In contrast, only in the light did spatial memory increase dysgranular cortex activation. Correlations based on the counts of Fos-positive cells helped to reinforce the particular association between the dysgranular retrosplenial cortex and radial-arm maze performance in the light. These results provide clear evidence for proposed functional differences between the major retrosplenial subregions: the granular cortex contributes to spatial learning and navigation based on both internal and external cues (light and dark), while dysgranular cortex is more selectively involved when distal visual cues control performance (light only).

Differential topography of the bilateral cortical projections to the whisker and forepaw regions in rat motor cortex

Whisker and forelimb movements in rats have distinct behavioral functions that suggest differences in the neural connections of the brain regions that control their movements. To test this hypothesis, retrograde tracing methods were used to characterize the bilateral distribution of the cortical neurons that project to the whisker and forelimb regions in primary motor (MI) cortex. Tracer injections in each MI region revealed labeled neurons in more than a dozen cortical areas, but most labeling was concentrated in the sensorimotor areas. Cortical projections to the MI forepaw region originated primarily from the primary somatosensory (SI) cortex in the ipsilateral hemisphere. In contrast, most projections to the MI whisker region originated from the MI whisker region in the contralateral hemisphere. Tracer injections in the MI whisker region also revealed a higher proportion of labeled neurons in the claustrum and in the posterior parietal cortex. Injections of different tracers into the MI whisker and forepaw regions of some rats revealed a topographic organization of neuronal labeling in several sensorimotor regions. Collectively, these findings indicate that the MI whisker and forepaw regions receive different sets of cortical inputs. Whereas the MI whisker region is most strongly influenced by callosal projections, presumably to mediate bilateral coordination of the whiskers, the MI forepaw region is influenced mainly by ipsilateral SI inputs that convey somatosensory feedback.

Organization of intrinsic connections of the retrosplenial cortex in the rat

The retrosplenial cortex consists of areas 29a-d, each of which has different connections with other cortical and subcortical regions. Although these areas also make complex interconnections that constitute part of a neural circuit subserving various functions, such as spatial memory and navigation, the details of such interconnections have not been studied comprehensively. In the study reported here, we investigated the organization of associational and commissural connections of areas 29a-d within the retrosplenial cortex in the rat, using the retrograde tracer cholera toxin B subunit and anterograde tracer biotinylated dextran amine. The results demonstrated that each of these areas has a distinct set of interconnections within the retrosplenial cortex. Each area interconnects strongly along the transverse axis of the retrosplenial cortex: area 29a, area 29b, caudal area 29c, and caudal area 29d connect with each other, and rostral area 29c and rostral area 29d connect with each other. In the longitudinal direction, rostral-to-caudal projections from rostral areas 29c and 29d to areas 29a and 29b and caudal areas 29c and 29d are strong, whereas reciprocal caudal-to-rostral projections are relatively weak. Although most of the intrinsic connections are homotopical, contralateral connections are weaker and less extensive than ipsilateral connections. These findings suggest that each retrosplenial area may not only process specific information somewhat independently but that it may also integrate and transmit such information through intrinsic connections to other areas in order to achieve retrosplenial cortical functions, such as spatial memory and learning.

Coupling of theta oscillations between anterior and posterior midline cortex and with the hippocampus in freely behaving rats

Theta oscillations in the hippocampus support cognitive processing. Theta-range rhythmicity has also been reported in frontal and posterior cortical areas--where it tends to show consistent phase-relations with hippocampal rhythmicity. Theta-range rhythmicity may, then, be important for cortico-cortical and/or cortico-hippocampal interactions. Here, we surveyed the rat frontal and posterior midline cortices for theta-related oscillations and examined their relationships with hippocampal activity in freely moving rats. Variation in electroencephalography across 4 general classes of spontaneous behavior demonstrated different profiles of theta-like activities through the rat midline cortices. Analysis of cortico-cortical and cortico-hippocampal coherences showed distinct, behavior-dependent, couplings of theta and delta oscillations. Increased theta coherence between structures was most obvious during nonautomatic behaviors and least during immobility or grooming. Extensive coupling of theta oscillations throughout the rat midline cortices and hippocampus occurred during rearing, and exploratory behavior. Such increases in coherence could reflect binding of cortico-hippocampal pathways into temporary functional units by behavioral demands. Extensive coupling of frontal delta, which lacked coherence with posterior areas (including the hippocampus), suggests that different frequencies of rhythmicity may act to bind groups of structures into different functional circuits on different occasions.

Organization of anterior cingulate and frontal cortical projections to the retrosplenial cortex in the rat

The retrosplenial cortex (areas 29a-d), which plays an important role in spatial memory and navigation, is known to provide massive projections to frontal association and motor cortices, which are also essential for spatial behavior. The reciprocal projections originating from these frontal cortices to areas 29a-d, however, have been analyzed to only a limited extent. Here, we report an analysis of the anatomical organization of projections from anterior cingulate area 24 and motor and prefrontal cortices to areas 29a-d in the rat, using the axonal transport of cholera toxin B subunit and biotinylated dextran amine. Area 29a receives projections from rostral area 24a, area 24b, the ventral orbital area, and the caudal secondary motor area. Rostral area 29b receives projections from caudal area 24a, whereas caudal area 29b receives projections from rostral area 24a. Area 29b also receives projections from area 24b and the ventral orbital area. Areas 29c and 29d receive projections from areas 24a and 24b and the secondary motor area in a topographic manner such that the rostrocaudal axis of areas 29c and 29d corresponds to the caudorostral axis of areas 24a and 24b and the secondary motor area. Rostral areas 29c and 29d also receive projections from the caudal primary motor area, and area 29d receives projections from the ventral, lateral, and medial orbital areas. These differential frontal cortical projections to each area of the retrosplenial cortex suggest that each area may contribute to different aspects of retrosplenial cortical function such as spatial memory and behavior.

GABAergic projections from the hippocampus to the retrosplenial cortex in the rat

The retrosplenial cortex (RS) in rats has been implicated in a wide range of behaviors, including spatial navigation and memory. Relevant to this, the RS is closely interconnected with the hippocampus by multiple direct and indirect routes. Here, by injecting the retrograde tracer cholera toxin subunit B conjugated with Alexa488 (CTB-Alexa488) in the granular retrosplenial cortex (GRS), we demonstrate a moderately dense non-pyramidal projection from CA1. Neurons are in several layers, but mainly (about 65%) at the border of the stratum radiatum (SR) and stratum lacunosum moleculare (SLM). In particular, by double-labeling with GAD67 or gamma-aminobutyric acid (GABA), we establish that these neurons are GABAergic. Further immunocytochemical screening for calcium-binding proteins, somatostatin (SS) or cholecystokinin (CCK) failed to identify additional neurochemical subgroups; but a small subset (about 14%) is positive for the m2 muscarinic acetylcholine receptor (M2R). Terminations target layer 1 of the GRS, as shown by biotinylated dextran amine (BDA) injections into CA1 and confirmed by a very superficial injection of CTB-Alexa488 in GRS. The superficial injection shows that there is a sparse GABAergic projection from the subiculum to layer 1 of the GRS, in addition to the dense excitatory connections to layer 3. The role of these dual inhibitory-excitatory pathways - within the subiculum, and in parallel from CA1 and the subiculum - remains to be determined, but may be related to synchronized oscillatory activity in the hippocampal complex and GRS, or to the generation of rhythmic activity within the GRS.