Medial septum lesions disrupt exploratory trip organization: Evidence for septohippocampal involvement in dead reckoning

Temporal dynamics of cholinergic activity in the septo-hippocampal system

Cholinergic projection neurons in the medial septum and diagonal band of Broca are the major source of cholinergic modulation of hippocampal circuit functions that support neural coding of location and running speed. Changes in cholinergic modulation are known to correlate with changes in brain states, cognitive functions, and behavior. However, whether cholinergic modulation can change fast enough to serve as a potential speed signal in hippocampal and parahippocampal cortices and whether the temporal dynamics in such a signal depend on the presence of visual cues remain unknown. In this study, we use a fiber-photometric approach to quantify the temporal dynamics of cholinergic activity in freely moving mice as a function of the animal's movement speed and visual cues. We show that the population activity of cholinergic neurons in the medial septum and diagonal band of Broca changes fast enough to be aligned well with changes in the animal's running speed and is strongly and linearly correlated to the logarithm of the animal's running speed. Intriguingly, the cholinergic modulation remains strongly and linearly correlated to the speed of the animal's neck movements during periods of stationary activity. Furthermore, we show that cholinergic modulation is unaltered during darkness. Lastly, we identify rearing, a stereotypic behavior where the mouse stands on its hindlimbs to scan the environment from an elevated perspective, is associated with higher cholinergic activity than expected from neck movements on the horizontal plane alone. Taken together, these data show that temporal dynamics in the cholinergic modulation of hippocampal circuits are fast enough to provide a potential running speed signal in real-time. Moreover, the data show that cholinergic modulation is primarily a function of the logarithm of the animal's movement speed, both during locomotion and during stationary activity, with no significant interaction with visual inputs. These data advance our understanding of temporal dynamics in cholinergic modulation of hippocampal circuits and their functions in the context of neural coding of location and running speed.

Basal Forebrain Impairment: Understanding the Mnemonic Function of the Septal Region Translates in Therapeutic Advances

The basal forebrain is one of the three major brain circuits involved in episodic memory formation together with the hippocampus and the diencephalon. The dysfunction of each of these regions is known to cause anterograde amnesia. While the hippocampal pyramidal neurons are known to encode episodic information and the diencephalic structures are known to provide idiothetic information, the contribution of the basal forebrain to memory formation has been exclusively associated with septo-hippocampal cholinergic signaling. Research data from the last decade broadened our understanding about the role of septal region in memory formation. Animal studies revealed that septal neurons process locomotor, rewarding and attentional stimuli. The integration of these signals results in a systems model for the mnemonic function of the medial septum that could guide new therapeutic strategies for basal forebrain impairment (BFI). BFI includes the disorders characterized with basal forebrain amnesia and neurodegenerative disorders that affect the basal forebrain. Here, we demonstrate how the updated model of septal mnemonic function can lead to innovative translational treatment approaches that include pharmacological, instrumental and behavioral techniques.

Sexually dimorphic organization of open field behavior following moderate prenatal alcohol exposure

BACKGROUND

Prenatal alcohol exposure (PAE) can produce deficits in a wide range of cognitive functions but is especially detrimental to behaviors requiring accurate spatial information processing. In open field environments, spatial behavior is organized such that animals establish "home bases" marked by long stops focused around one location. Progressions away from the home base are circuitous and slow, while progressions directed toward the home base are non-circuitous and fast. The impact of PAE on the organization of open field behavior has not been experimentally investigated.

METHODS

In the present study, adult female and male rats with moderate PAE or saccharin exposure locomoted a circular high walled open field for 30 minutes under lighted conditions.

RESULTS

The findings indicate that PAE and sex influence the organization of open field behavior. Consistent with previous literature, PAE rats exhibited greater locomotion in the open field. Novel findings from the current study indicate that PAE and sex also impact open field measures specific to spatial orientation. While all rats established a home base on the periphery of the open field, PAE rats, particularly males, exhibited significantly less clustered home base stopping with smaller changes in heading between stops. PAE also impaired progression measures specific to distance estimation, while sex alone impacted progression measures specific to direction estimation.

CONCLUSIONS

These findings support the conclusion that adult male rats have an increased susceptibility to the effects of PAE on the organization of open field behavior.

Differential organization of open field behavior in mice following acute or chronic simulated GCR exposure

Astronauts undertaking deep space travel will receive chronic exposure to the mixed spectrum of particles that comprise Galactic Cosmic Radiation (GCR). Exposure to the different charged particles of varied fluence and energy that characterize GCR may impact neural systems that support performance on mission critical tasks. Indeed, growing evidence derived from years of terrestrial-based simulations of the space radiation environment using rodents has indicated that a variety of exposure scenarios can result in significant and long-lasting decrements to CNS functionality. Many of the behavioral tasks used to quantify radiation effects on the CNS depend on neural systems that support maintaining spatial orientation and organization of rodent open field behavior. The current study examined the effects of acute or chronic exposure to simulated GCR on the organization of open field behavior under conditions with varied access to environmental cues in male and female C57BL/6 J mice. In general, groups exhibited similar organization of open field behavior under dark and light conditions. Two exceptions were noted: the acute exposure group exhibited significantly slower and more circuitous homeward progressions relative to the chronic group under light conditions. These results demonstrate the potential of open field behavior organization to discriminate between the effects of select GCR exposure paradigms.

Path integration in normal aging and Alzheimer's disease

In this review we discuss converging evidence from human and rodent research demonstrating how path integration (PI) is impaired in healthy aging and Alzheimer's disease (AD), and point to the neural mechanisms that underlie these deficits. Importantly, we highlight that (i) the grid cell network in the entorhinal cortex is crucial for PI in both humans and rodents, (ii) PI deficits are present in healthy aging and are significantly more pronounced in patients with early-stage AD, (iii) compromised entorhinal grid cell computations in healthy older adults and in young adults at risk of AD are linked to PI deficits, and (iv) PI and grid cell deficits may serve as sensitive markers for pathological decline in early AD.

Organization of exploratory behavior under dark conditions in female and male rats

Sexually dimorphic performance has been observed across humans and rodents in many spatial tasks. In general, these spatial tasks do not dissociate the use of environmental and self-movement cues. Previous work has demonstrated a role for self-movement cue processing in organizing open field behavior; however, these studies have not directly compared female and male movement characteristics. The current study examined the organization of open field behavior under dark conditions in female and male rats. Significant differences between female and male rats were observed in the location of stopping behavior relative to a cue and the topography exhibited during lateral movements. In contrast, no sex differences were observed on measures used to detect self-movement cue processing deficits. These results provide evidence that female and male rats are similar in their use of self-movement cues to organize open field behavior; however, other factors may be contributing to differences in performance.

A Brainstem Locomotor Circuit Drives the Activity of Speed Cells in the Medial Entorhinal Cortex

Locomotion activates an array of sensory inputs that may help build the self-position map of the medial entorhinal cortex (MEC). In this map, speed-coding neurons are thought to dynamically update representations of the animal’s position. A possible origin for the entorhinal speed signal is the mesencephalic locomotor region (MLR), which is critically involved in the activation of locomotor programs. Here, we describe, in rats, a circuit connecting the pedunculopontine tegmental nucleus (PPN) of the MLR to the MEC via the horizontal limb of the diagonal band of Broca (HDB). At each level of this pathway, locomotion speed is linearly encoded in neuronal firing rates. Optogenetic activation of PPN cells drives locomotion and modulates activity of speed-modulated neurons in HDB and MEC. Our results provide evidence for a pathway by which brainstem speed signals can reach cortical structures implicated in navigation and higher-order dynamic representations of space.

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A speed-coding multisynaptic circuit connects PPN to MEC via HDB

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Each level of the PPN-HDB-MEC pathway contains cells with linear speed coding

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Optogenetic stimulation of PPN elicits activity in HDB and MEC speed cells

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In MEC, locomotor inputs from PPN mainly target speed-modulated interneurons

The Firing Rate Speed Code of Entorhinal Speed Cells Differs across Behaviorally Relevant Time Scales and Does Not Depend on Medial Septum Inputs

The firing rate of speed cells, a dedicated subpopulation of neurons in the medial entorhinal cortex (MEC), is correlated with running speed. This correlation has been interpreted as a speed code used in various computational models for path integration. These models consider firing rate to be linearly tuned by running speed in real-time. However, estimation of firing rates requires integration of spiking events over time, setting constraints on the temporal accuracy of the proposed speed code. We therefore tested whether the proposed speed code by firing rate is accurate at short time scales using data obtained from open-field recordings in male rats and mice. We applied a novel filtering approach differentiating between speed codes at multiple time scales ranging from deciseconds to minutes. In addition, we determined the optimal integration time window for firing-rate estimation using a general likelihood framework and calculated the integration time window that maximizes the correlation between firing rate and running speed. Data show that these time windows are on the order of seconds, setting constraints on real-time speed coding by firing rate. We further show that optogenetic inhibition of either cholinergic, GABAergic, or glutamatergic neurons in the medial septum/diagonal band of Broca does not affect modulation of firing rates by running speed at each time scale tested. These results are relevant for models of path integration and for our understanding of how behavioral activity states may modulate firing rates and likely information processing in the MEC.SIGNIFICANCE STATEMENT Path integration is the most basic form of navigation relying on self-motion cues. Models of path integration use medial septum/diagonal band of Broca (MSDB)-dependent MEC grid-cell firing patterns as the neurophysiological substrate of path integration. These models use a linear speed code by firing rate, but do not consider temporal constraints of integration over time for firing-rate estimation. We show that firing-rate estimation for speed cells requires integration over seconds. Using optogenetics, we show that modulation of firing rates by running speed is independent of MSDB inputs. These results enhance our understanding of path integration mechanisms and the role of the MSDB for information processing in the MEC.

Making waves: Comparing Morris water task performance in rats and prairie voles

Spatial processing is a critical component for survival. This domain of information processing has been extensively studied in rats and mice. Limited work has examined the capacity of other rodent species, like the prairie vole (Microtus ochrogaster), to process spatial information. The Morris water task (MWT) is a classic spatial task that has been used to examine spatial cognition in rodents. This task involves an animal developing configural relationships between extra-maze cues and the location of a hidden platform to successfully escape from a pool of water. The current study compared performance in the MWT between rats and prairie voles. Rats were observed to outperform prairie voles in key aspects of the task including latency to find the platform, directness of swim paths to the platform, and degrees of heading error. These results may be attributed to potential interspecies differences in spatial cognition, stress reactivity, physiology, or motivation. This study provides the foundation for future work investigating the spatial cognition of prairie voles and the factors that contribute to water task performance in rodents.

Method for Studies of Orientation and Exploratory Behavior in Humans. Effects of Emotional Stress

We developed a new experimental model for studies of orientation and exploratory behavior in humans. This method allows analysis of variants of orientation and exploratory behavior in humans, including risky behavior, and cognitive and individual typological parameters. The model allows distinguishing examinees with high and low orientation and exploratory activity, which helps to determine their psychoemotional status. Surprisingly, individuals in emotional strain demonstrated longer orientation and exploratory behavior. This allows them to achieve the results more rapidly during purposeful behavior under similar conditions. This model provides the tool for evaluation of the electrophysiological, autonomic, and biochemical mechanisms of orientation and exploratory behavior of humans.

Differential Representation of Landmark and Self-Motion Information along the CA1 Radial Axis: Self-Motion Generated Place Fields Shift toward Landmarks during Septal Inactivation

Spatial location in the environment can be defined in relation to specific landmarks or in relation to the global context, and is estimated from both the sensing of landmarks and the inner sense of cumulated locomotion referred to as path-integration. The respective contribution of landmark and path-integration to place-cell activity in the hippocampus is still unclear and complicated by the fact that the two mechanisms usually overlap. To bias spatial mechanisms toward landmark or path-integration, we use a treadmill equipped with a long belt on which male mice run sequentially through a zone enriched and a zone impoverished in visual-tactile cues. We show that inactivation of the medial septum (MS), which is known to disrupt the periodic activity of grid cells, impairs mice ability to anticipate the delivery of a reward in the cue-impoverished zone and transiently alter the spatial configuration of place fields in the cue-impoverished zone selectively: following MS inactivation, place fields in the cue-impoverished zone progressively shift backward and stabilize near the cues, resulting in the contraction of the spatial representation around cues; following MS recovery, the initial spatial representation is progressively restored. Furthermore, we found that place fields in the cue-rich and cue-impoverished zones are preferentially generated by cells from the deep and superficial sublayers of CA1, respectively. These findings demonstrate with mechanistic insights the contribution of MS to the spread of spatial representations in cue-impoverished zones, and indicate a segregation of landmark-based and path-integration-assisted spatial mechanisms into deep and superficial CA1, respectively.SIGNIFICANCE STATEMENT Cells encoding a cue-impoverished zone and the vicinity of landmarks responded differentially to septal inactivation and resided in distinct sublayers of CA1. These findings provide new insights on place field mechanisms: septal activity is critical for maintaining the spread of place fields in cue-impoverished areas, but not for the generation of place fields; Following MS inactivation, trial-by-trial network modifications by activity-dependent mechanisms are responsible for the gradual collapse of spatial representations. Furthermore, the findings suggest parallel coding streams for landmark and self-motion information. Superficial CA1 cells are better suited for encoding global position via the assist of path-integration, whereas deep CA1 cells can support spatial memory processes on an object-specific basis.

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.

Disentangling the Role of the MEC and LEC in the Processing of Spatial and Non-Spatial Information: Contribution of Lesion Studies

It is now widely accepted that the entorhinal cortex (EC) plays a pivotal role in the processing of spatial information and episodic memory. The EC is segregated into two sub-regions, the medial EC (MEC) and the lateral EC (LEC) but a comprehensive understanding of their roles across multiple behavioral contexts remains unclear. Considering that it is still useful to investigate the impact of lesions of EC on behavior, we review the contribution of lesion approach to our knowledge of EC functions. We show that the MEC and LEC play different roles in the processing of spatial and non-spatial information. The MEC is necessary to the use of distal but not proximal landmarks during navigation and is crucial for path integration, in particular integration of linear movements. Consistent with predominant hypothesis, the LEC is important for combining the spatial and non-spatial aspects of the environment. However, object exploration studies suggest that the functional segregation between the MEC and the LEC is not as clearly delineated and is dependent on environmental and behavioral factors. Manipulation of environmental complexity and therefore of cognitive demand shows that the MEC and the LEC are not strictly necessary to the processing of spatial and non-spatial information. In addition we suggest that the involvement of these sub-regions can depend on the kind of behavior, i.e., navigation or exploration, exhibited by the animals. Thus, the MEC and the LEC work in a flexible manner to integrate the “what” and “where” information in episodic memory upstream the hippocampus.

Speed and Oscillations: Medial Septum Integration of Attention and Navigation

Several cortical and diencephalic limbic brain regions incorporate neurons that fire in correlation with the speed of whole-body motion, also known as linear velocity. Besides the field mapping and head-directional information, the linear velocity is among the major signals that guide animal’s spatial navigation. Large neuronal populations in the same limbic regions oscillate with theta rhythm during spatial navigation or attention episodes; and the frequency of theta also correlates with linear velocity. A functional similarity between these brain areas is that their inactivation impairs the ability to form new spatial memories; whereas an anatomical similarity is that they all receive projections from medial septum-diagonal band of Broca complex. We review recent findings supporting the model that septal theta rhythm integrates different sensorimotor signals necessary for spatial navigation. The medial septal is described here as a circuitry that mediates experience-dependent balance of sustained attention and path integration during navigation. We discuss the hypothesis that theta rhythm serves as a key mechanism for the aligning of intrinsic spatial representation to: (1) rapid change of position in the spatial environment; (2) continuous alteration of sensory signals throughout navigation; and (3) adapting levels of attentional behavior. The synchronization of these spatial, somatosensory and neuromodulatory signals is proposed here to be anatomically and physiologically mediated by the medial septum.

Otolith dysfunction alters exploratory movement in mice

The organization of rodent exploratory behavior appears to depend on self-movement cue processing. As of yet, however, no studies have directly examined the vestibular system's contribution to the organization of exploratory movement. The current study sequentially segmented open field behavior into progressions and stops in order to characterize differences in movement organization between control and otoconia-deficient tilted mice under conditions with and without access to visual cues. Under completely dark conditions, tilted mice exhibited similar distance traveled and stop times overall, but had significantly more circuitous progressions, larger changes in heading between progressions, and less stable clustering of home bases, relative to control mice. In light conditions, control and tilted mice were similar on all measures except for the change in heading between progressions. This pattern of results is consistent with otoconia-deficient tilted mice using visual cues to compensate for impaired self-movement cue processing. This work provides the first empirical evidence that signals from the otolithic organs mediate the organization of exploratory behavior, based on a novel assessment of spatial orientation.

If I had a million neurons: Potential tests of cortico-hippocampal theories

Considerable excitement surrounds new initiatives to develop techniques for simultaneous recording of large populations of neurons in cortical structures. This chapter focuses on the potential value of large-scale simultaneous recording for advancing research on current issues in the function of cortical circuits, including the interaction of the hippocampus with cortical and subcortical structures. The review describes specific research questions that could be answered using large-scale population recording, including questions about the circuit dynamics underlying coding of dimensions of space and time for episodic memory, the role of GABAergic and cholinergic innervation from the medial septum, the functional role of spatial representations coded by grid cells, boundary cells, head direction cells, and place cells, and the fact that many models require cells coding movement direction.

Otolithic information is required for homing in the mouse

Navigation and the underlying brain signals are influenced by various allothetic and idiothetic cues, depending on environmental conditions and task demands. Visual landmarks typically control navigation in familiar environments but, in the absence of landmarks, self-movement cues are able to guide navigation relatively accurately. These self-movement cues include signals from the vestibular system, and may originate in the semicircular canals or otolith organs. Here, we tested the otolithic contribution to navigation on a food-hoarding task in darkness and in light. The dark test prevented the use of visual cues and thus favored the use of self-movement information, whereas the light test allowed the use of both visual and non-visual cues. In darkness, tilted mice made shorter-duration stops during the outward journey, and made more circuitous homeward journeys than control mice; heading error, trip duration, and peak error were greater for tilted mice than for controls. In light, tilted mice also showed more circuitous homeward trips, but appeared to correct for errors during the journey; heading error, trip duration, and peak error were similar between groups. These results suggest that signals from the otolith organs are necessary for accurate homing performance in mice, with the greatest contribution in non-visual environments.

Theta rhythm and the encoding and retrieval of space and time

Physiological data demonstrates theta frequency oscillations associated with memory function and spatial behavior. Modeling and data from animals provide a perspective on the functional role of theta rhythm, including correlations with behavioral performance and coding by timing of spikes relative to phase of oscillations. Data supports a theorized role of theta rhythm in setting the dynamics for encoding and retrieval within cortical circuits. Recent data also supports models showing how network and cellular theta rhythmicity allows neurons in the entorhinal cortex and hippocampus to code time and space as a possible substrate for encoding events in episodic memory. Here we discuss these models and relate them to current physiological and behavioral data.

Lesions of the basal forebrain cholinergic system in mice disrupt idiothetic navigation

Loss of integrity of the basal forebrain cholinergic neurons is a consistent feature of Alzheimer's disease, and measurement of basal forebrain degeneration by magnetic resonance imaging is emerging as a sensitive diagnostic marker for prodromal disease. It is also known that Alzheimer's disease patients perform poorly on both real space and computerized cued (allothetic) or uncued (idiothetic) recall navigation tasks. Although the hippocampus is required for allothetic navigation, lesions of this region only mildly affect idiothetic navigation. Here we tested the hypothesis that the cholinergic medial septo-hippocampal circuit is important for idiothetic navigation. Basal forebrain cholinergic neurons were selectively lesioned in mice using the toxin saporin conjugated to a basal forebrain cholinergic neuronal marker, the p75 neurotrophin receptor. Control animals were able to learn and remember spatial information when tested on a modified version of the passive place avoidance test where all extramaze cues were removed, and animals had to rely on idiothetic signals. However, the exploratory behaviour of mice with cholinergic basal forebrain lesions was highly disorganized during this test. By contrast, the lesioned animals performed no differently from controls in tasks involving contextual fear conditioning and spatial working memory (Y maze), and displayed no deficits in potentially confounding behaviours such as motor performance, anxiety, or disturbed sleep/wake cycles. These data suggest that the basal forebrain cholinergic system plays a specific role in idiothetic navigation, a modality that is impaired early in Alzheimer's disease.

Infusion of GAT1-saporin into the medial septum/vertical limb of the diagonal band disrupts self-movement cue processing and spares mnemonic function

Degeneration of the septohippocampal system is associated with the progression of Dementia of the Alzheimer's type (DAT). Impairments in mnemonic function and spatial orientation become more severe as DAT progresses. Although evidence supports a role for cholinergic function in these impairments, relatively few studies have examined the contribution of the septohippocampal GABAergic component to mnemonic function or spatial orientation. The current study uses the rat food-hoarding paradigm and water maze tasks to characterize the mnemonic and spatial impairments associated with infusing GAT1-Saporin into the medial septum/vertical limb of the diagonal band (MS/VDB). Although infusion of GAT1-Saporin significantly reduced parvalbumin-positive cells in the MS/VDB, no reductions in markers of cholinergic function were observed in the hippocampus. In general, performance was spared during spatial tasks that provided access to environmental cues. In contrast, GAT1-Saporin rats did not accurately carry the food pellet to the refuge during the dark probe. These observations are consistent with infusion of GAT1-Saporin into the MS/VDB resulting in spared mnemonic function and use of environmental cues; however, self-movement cue processing was compromised. This interpretation is consistent with a growing literature demonstrating a role for the septohippocampal system in self-movement cue processing.

Hippocampal damage produces retrograde but not anterograde amnesia for a cued location in a spontaneous exploratory task in rats

Performance in several memory tasks is known to be unaffected by hippocampal damage sustained before learning, but is severely disrupted if the same damage occurs after learning. Memories for preferred locations, or home bases, in exploratory tasks can be formed by rats with hippocampal damage, but it is unknown if the memory for a home base survives hippocampal damage. To examine this question, for 30 min each day for five consecutive days, rats explored a circular open field containing one local cue. By Day 5 the rats preferentially went directly to that location, spent the majority of their time at that location, made rapid direct trips to that location when returning from an excursion and so demonstrated that the location was a home base. Memory for the cued location was examined after a 24 h or 14-day interval with the cue removed. In Experiments 1 and 2, control rats and rats with prior N-methyl-D-aspartic acid hippocampal lesions demonstrated memory of the home base location by making direct trips to that location. In Experiment 3, rats that had first explored the open field and cue and then received hippocampal lesions showed no memory for the cued location. The absence of anterograde impairment vs. the presence of retrograde impairment for memory of a spatial home base confirms a role for the hippocampus in the retention of spatial memory acquired during exploration.

Reduction of theta rhythm dissociates grid cell spatial periodicity from directional tuning

Grid cells recorded in the medial entorhinal cortex of freely moving rats exhibit firing at regular spatial locations and temporal modulation with theta rhythm oscillations (4 to 11 hertz). We analyzed grid cell spatial coding during reduction of network theta rhythm oscillations caused by medial septum (MS) inactivation with muscimol. During MS inactivation, grid cells lost their spatial periodicity, whereas head-direction cells maintained their selectivity. Conjunctive grid-by-head-direction cells lost grid cell spatial periodicity but retained head-direction specificity. All cells showed reduced rhythmicity in autocorrelations and cross-correlations. This supports the hypothesis that spatial coding by grid cells requires theta oscillations, and dissociates the mechanisms underlying the generation of entorhinal grid cell periodicity and head-direction selectivity.

A morphometric analysis of the septal nuclei in schizophrenia and affective disorders: reduced neuronal density in the lateral septal nucleus in bipolar disorder

The septal nuclei are assumed to play a significant role in the pathophysiology of schizophrenia and affective disorders. The aim of this study was to morphometrically characterize the septal nuclei in patients with schizophrenia, bipolar disorder, and major depressive disorder, when compared with healthy control subjects. We analyzed the septal nuclei by determining the density and size of the neurons in postmortem brains in 17 patients with schizophrenia, 8 patients with bipolar disorder, 7 patients with major depressive disorder, and 14 control subjects matched for age and gender. There was a significant reduction in the neuronal density, but not in the mean cross-sectional area, in the lateral septal nucleus (P = 0.013) in patients with bipolar disorder when compared with control subjects. There were no significant changes in the neuronal density of the septal nuclei of the medial and lateral cell groups in patients with schizophrenia and major depressive disorder when compared with control subjects. There was a significant negative correlation between neuronal density in the lateral septal nucleus and disease duration in patients with major depressive disorder (P = 0.037, r = -0.9). The histopathological abnormality of the decreased neuronal density in the lateral septal nucleus, which is an important limbic region involved in emotions, might be a neuropathological correlate of bipolar disorder.

Organization of food protection behavior is differentially influenced by 192 IgG-saporin lesions of either the medial septum or the nucleus basalis magnocellularis

Converging lines of evidence have supported a role for the nucleus basalis magnocellularis (NB) in attentional mechanisms; however, debate continues regarding the role of the medial septum in behavior (MS). Recent studies have supported a role for the septohippocampal system in the online processing of internally generated cues. The current study was designed to investigate a possible double dissociation in rat food protection behavior, a natural behavior that has been shown to depend on external and internal sources of information. The study examined the effects of intraparenchymal injections of 192 IgG-saporin into either the MS or NB on the organization of food protection behavior. NB cholinergic lesions reduced the number of successful food protection behaviors while sparing the temporal organization of food protection behavior. In contrast, MS cholinergic lesions disrupted the temporal organization of food protection behavior while sparing the ability to successfully protect food items. These observations are consistent with a double dissociation of NB and MS cholinergic systems' contributions to processing external and internal sources of information and provide further evidence for the septohippocampal system's involvement in processing internally generated cues.

Fractionating dead reckoning: role of the compass, odometer, logbook, and home base establishment in spatial orientation

Rats use multiple sources of information to maintain spatial orientation. Although previous work has focused on rats' use of environmental cues, a growing number of studies have demonstrated that rats also use self-movement cues to organize navigation. This review examines the extent that kinematic analysis of naturally occurring behavior has provided insight into processes that mediate dead-reckoning-based navigation. This work supports a role for separate systems in processing self-movement cues that converge on the hippocampus. The compass system is involved in deriving directional information from self-movement cues; whereas, the odometer system is involved in deriving distance information from self-movement cues. The hippocampus functions similar to a logbook in that outward path unique information from the compass and odometer is used to derive the direction and distance of a path to the point at which movement was initiated. Finally, home base establishment may function to reset this system after each excursion and anchor environmental cues to self-movement cues. The combination of natural behaviors and kinematic analysis has proven to be a robust paradigm to investigate the neural basis of spatial orientation.

Selective hippocampal cholinergic deafferentation impairs self-movement cue use during a food hoarding task

Investigations using selective lesion techniques suggest that the septohippocampal cholinergic system may not be critical for spatial orientation. These studies employ spatial tasks that provide the animal with access to both environmental and self-movement cues; therefore, intact performance may reflect spared spatial orientation or compensatory mechanisms associated with one class of spatial cues. The present study investigated the contribution of the septohippocampal cholinergic system to spatial behavior by examining performance in foraging tasks in which cue availability was manipulated. Thirteen female Long-Evans rats received selective lesions of the medial septum/vertical band with 192 IgG saporin, and 11 received sham surgeries. Rats were trained to forage for hazelnuts in an environment with access to both environmental and self-movement cues (cued condition). Manipulations include altering availability of environmental cues associated with the refuge (uncued probe), removing all visual environmental cues (dark probe), and placing environmental and self-movement cues into conflict (reversal probe). Medial septum lesions disrupted homeward segment topography only under conditions in which self-movement cues were critical for organizing food hoarding behavior (dark and reversal). These results are consistent with medial septum lesions producing a selective impairment in self-movement cue processing and suggest that these rats were able to compensate for deficits in self-movement cue processing when provided access to environmental cues.