Septohippocampal pathways contribute to system consolidation of a spatial memory: sequential implication of GABAergic and cholinergic neurons

Selective Medial Septum Lesions in Healthy Rats Induce Longitudinal Changes in Microstructure of Limbic Regions, Behavioral Alterations, and Increased Susceptibility to Status Epilepticus

Septo-hippocampal pathway, crucial for physiological functions and involved in epilepsy. Clinical monitoring during epileptogenesis is complicated. We aim to evaluate tissue changes after lesioning the medial septum (MS) of normal rats and assess how the depletion of specific neuronal populations alters the animals' behavior and susceptibility to establishing a pilocarpine-induced status epilepticus. Male Sprague-Dawley rats were injected into the MS with vehicle or saporins (to deplete GABAergic or cholinergic neurons; n = 16 per group). Thirty-two animals were used for diffusion tensor imaging (DTI); scanned before surgery and 14 and 49 days post-injection. Fractional anisotropy and apparent diffusion coefficient were evaluated in the fimbria, dorsal hippocampus, ventral hippocampus, dorso-medial thalamus, and amygdala. Between scans 2 and 3, animals were submitted to diverse behavioral tasks. Stainings were used to analyze tissue alterations. Twenty-four different animals received pilocarpine to evaluate the latency and severity of the status epilepticus 2 weeks after surgery. Additionally, eight different animals were only used to evaluate the neuronal damage inflicted on the MS 1 week after the molecular surgery. Progressive changes in DTI parameters in both white and gray matter structures of the four evaluated groups were observed. Behaviorally, the GAT1-saporin injection impacted spatial memory formation, while 192-IgG-saporin triggered anxiety-like behaviors. Histologically, the GABAergic toxin also induced aberrant mossy fiber sprouting, tissue damage, and neuronal death. Regarding the pilocarpine-induced status epilepticus, this agent provoked an increased mortality rate. Selective septo-hippocampal modulation impacts the integrity of limbic regions crucial for certain behavioral skills and could represent a precursor for epilepsy development.

Pontine parabrachial nucleus-basal forebrain circuitry regulating cortical and hippocampal arousal

INTRODUCTION

The basal forebrain (BF) and the medial septum (MS) respectively drive neuronal activity of cerebral cortex and hippocampus (HPC) in sleep-wake cycle. Our previous studies of lesions and neuronal circuit tracing have shown that the pontine parabrachial nucleus (PB) projections to the BF and MS may be a key circuit for cortical and HPC arousal.

AIMS

This study aims to demonstrate that PB projections to the BF and MS activate the cerebral cortex and HPC.

RESULTS

By using chemogenetic stimulation of the BF, the PB-BF and the PB-MS pathway combined with electroencephalogram (EEG) Fast Fourier Transformation (FFT) analysis in rats, we demonstrated that chemogenetic stimulation of the BF or PB neurons projecting to the BF activated the cerebral cortex while chemogenetic stimulation of the MS or PB neurons projecting to the MS activated HPC activity, in sleep and wake state. These stimulations did not significantly alter sleep-wake amounts.

CONCLUSIONS

Our results support that PB projections to the BF and MS specifically regulating cortical and HPC activity.

Hypothalamic supramammillary neurons that project to the medial septum modulate wakefulness in mice

The hypothalamic supramammillary nucleus (SuM) plays a crucial role in controlling wakefulness, but the downstream target regions participating in this control process remain unknown. Here, using circuit-specific fiber photometry and single-neuron electrophysiology together with electroencephalogram, electromyogram and behavioral recordings, we find that approximately half of SuM neurons that project to the medial septum (MS) are wake-active. Optogenetic stimulation of axonal terminals of SuM-MS projection induces a rapid and reliable transition to wakefulness from non-rapid-eye movement or rapid-eye movement sleep, and chemogenetic activation of SuMMS projecting neurons significantly increases wakefulness time and prolongs latency to sleep. Consistently, chemogenetically inhibiting these neurons significantly reduces wakefulness time and latency to sleep. Therefore, these results identify the MS as a functional downstream target of SuM and provide evidence for the modulation of wakefulness by this hypothalamic-septal projection.

Medial septum activation improves strategy switching once strategies are well-learned via bidirectional regulation of dopamine neuron population activity

Strategy switching is a form of cognitive flexibility that requires inhibiting a previously successful strategy and switching to a new strategy of a different categorical modality. It is dependent on dopamine (DA) receptor activation and release in ventral striatum and prefrontal cortex, two primary targets of ventral tegmental area (VTA) DA projections. Although the circuitry that underlies strategy switching early in learning has been studied, few studies have examined it after extended discrimination training. This may be important as DA activity and release patterns change across learning, with several studies demonstrating a critical role for substantia nigra pars compacta (SNc) DA activity and release once behaviors are well-learned. We have demonstrated that medial septum (MS) activation simultaneously increased VTA and decreased SNc DA population activity, as well as improved reversal learning via these actions on DA activity. We hypothesized that MS activation would improve strategy switching both early in learning and after extended training through its ability to increase VTA DA population activity and decrease SNc DA population activity, respectively. We chemogenetically activated the MS of male and female rats and measured their performance on an operant-based strategy switching task following 1, 10, or 15 days of discrimination training. Contrary to our hypothesis, MS activation did not affect strategy switching after 1 day of discrimination training. MS activation improved strategy switching after 10 days of training, but only in females. MS activation improved strategy switching in both sexes after 15 days of training. Infusion of bicuculline into the ventral subiculum (vSub) inhibited the MS-mediated decrease in SNc DA population activity and attenuated the improvement in strategy switching. Intra-vSub infusion of scopolamine inhibited the MS-mediated increase in VTA DA population activity but did not affect the improvement in strategy switching. Intra-vSub infusion of both bicuculline and scopolamine inhibited the MS-mediated effects on DA population activity in both the SNc and VTA and completely prevented the improvement in strategy switching. These data indicate that MS activation improves strategy switching once the original strategy has been sufficiently well-learned, and that this may occur via the MS's regulation of DA neuron responsivity.

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.

Progesterone rapidly alters the use of place and response memory during spatial navigation in female rats

17β-Estradiol (E2) and progesterone (P) influence place and response memory in female rats in spatial navigation tasks. Use of these memory systems is associated with the hippocampus and the dorsal striatum, respectively. Injections of E2 result in a well-established bias to use place memory, while much less is understood about the role of P. A total of 120 ovariectomized female rats were tested within a dual-solution T-maze task and treated with either low E2 (n = 24), high E2 (10 μg/kg; n = 24), or high E2 in combination with P (500 μg/kg) at three time points before testing: 15 min (n = 24), 1 h (n = 24), and 4 h (n = 24). Given alone, high E2 biases rats to the use of place memory, but this effect is reversed when P is given 1 h or 4 h before testing. This indicates that P may be playing an inhibitory role in the hippocampus during spatial tasks, which is consistent with past findings. Our findings show that P acts rapidly (within an hour) to affect performance during spatial tasks.

Age-related changes in medial septal cholinergic and GABAergic projection neurons and hippocampal neurotransmitter receptors: relationship with memory impairment

The hippocampus, which provides cognitive functions, has been shown to become highly vulnerable during aging. One important modulator of the hippocampal neural network is the medial septum (MS). The present study attempts to determine how age-related mnemonic dysfunction is associated with neurochemical changes in the septohippocampal (SH) system, using behavioral and immunochemical experiments performed on young-adult, middle-aged and aged rats. According to these behavioral results, the aged and around 52.8% of middle-aged rats (within the "middle-aged-impaired" sub-group) showed both impaired spatial reference memory in the Morris water maze and habituation in the open field. Immunohistochemical studies revealed a significant decrease in the number of MS choline acetyltransferase immunoreactive cells in the aged and all middle-aged rats, in comparison to the young; however the number of gamma-aminobutyric acid-ergic (GABAergic) parvalbumin immunoreactive cells was higher in middle-aged-impaired and older rats compared to young and middle-aged-unimpaired rats. Western Blot analysis moreover showed a decrease in the level of expression of cholinergic, GABAergic and glutamatergic receptors in the hippocampus of middle-aged-impaired and aged rats in contrast to middle-aged-unimpaired and young rats. The present results demonstrate for the first time that a decrease in the expression level of hippocampal receptors in naturally aged rats with impaired cognitive abilities occurs in parallel with an increase in the number of GABAergic neurons in the MS, and it highlights the particular importance of inhibitory signaling in the SH network for memory function.

The Medial Septum as a Potential Target for Treating Brain Disorders Associated With Oscillopathies

The medial septum (MS), as part of the basal forebrain, supports many physiological functions, from sensorimotor integration to cognition. With often reciprocal connections with a broad set of peers at all major divisions of the brain, the MS orchestrates oscillatory neuronal activities throughout the brain. These oscillations are critical in generating sensory and emotional salience, locomotion, maintaining mood, supporting innate anxiety, and governing learning and memory. Accumulating evidence points out that the physiological oscillations under septal influence are frequently disrupted or altered in pathological conditions. Therefore, the MS may be a potential target for treating neurological and psychiatric disorders with abnormal oscillations (oscillopathies) to restore healthy patterns or erase undesired ones. Recent studies have revealed that the patterned stimulation of the MS alleviates symptoms of epilepsy. We discuss here that stimulus timing is a critical determinant of treatment efficacy on multiple time scales. On-demand stimulation may dramatically reduce side effects by not interfering with normal physiological functions. A precise pattern-matched stimulation through adaptive timing governed by the ongoing oscillations is essential to effectively terminate pathological oscillations. The time-targeted strategy for the MS stimulation may provide an effective way of treating multiple disorders including Alzheimer's disease, anxiety/fear, schizophrenia, and depression, as well as pain.

Regulation of hippocamposeptal input within the medial septum/diagonal band of Broca

The medial septum/diagonal band of Broca (MS/DBB) receives direct GABAergic input from the hippocampus via hippocamposeptal (HS) projection neurons as part of a reciprocal loop that mediates cognition and is altered in Alzheimer's disease. Cholinergic and GABAergic interactions occur throughout the MS/DBB, but it is not known how HS GABA release is impacted by these circuits. Most HS neurons contain somatostatin (SST), so to evoke HS GABA release we expressed Cre-dependent mCherry/channelrhodopisin-2 (ChR2) in the hippocampi of SST-IRES-Cre mice and then used optogenetics to stimulate HS fibers while performing whole-cell patch clamp recordings from MS/DBB neurons in acute slices. We found that the acetylcholine receptor (AChR) agonist carbachol and the GABA_B receptor (GABA_BR) agonist baclofen significantly decreased HS GABA release in the MS/DBB. Carbachol's effects were blocked by eliminating local GABAergic activity or inhibiting GABA_BRs, indicating that it was indirectly decreasing HS GABA release by increasing GABAergic tone. There was no effect of acute exposure to amyloid-β on HS GABA release. Repetitive stimulation of HS fibers increased spontaneous GABA release in the MS/DBB, revealing that HS projections can modulate local GABAergic tone. These results show that HS GABA release has far-reaching impacts on overall levels of inhibition in the MS/DBB and is under regulatory control by cholinergic and GABAergic activity. This bidirectional modulation of GABA release from local and HS projections in the MS/DBB will likely have profound impact not only on activity within the MS/DBB, but also on output to the hippocampus and hippocampal-dependent learning and memory.

Modulation of spatial memory and expression of hippocampal neurotransmitter receptors by selective lesion of medial septal cholinergic and GABAergic neurons

The medial septum (MS) is an important modulator of hippocampal function. The degree of damage in which the particular set of septo-hippocampal projections contributes to the deficits of spatial memory with concomitant changes of hippocampal receptors expression has not been studied till present. Therefore, we investigated spatial memory and the expression level of cholinergic (α7 nACh and M1), GABAergic (α1 subunit of GABAA) and glutamatergic (NR2B subunit of NMDA and GluR 1 subunit of AMPA) receptors in the hippocampus following selective lesions of cholinergic and GABAergic septo-hippocampal projection. Learning process and long-term spatial memory were assessed using a Morris water maze. The obtained results revealed that in contrast to cholinergic lesions, rats with MS GABAergic lesions exhibit a retention deficit in 3 days after training. Western blot analyses revealed the MS cholinergic lesions have significant effect on the expression level of the M1 mACh receptors, while MS GABAergic lesions induce dramatic modulations of hippocampal glutamatergic, cholinergic and GABAergic receptors expression. These results for the first time demonstrated that selective lesions of MS cholinergic and GABAergic neurons differentially affect long-term spatial memory and the memory deficit after MS GABAergic lesion is paralleled with significant changes of hippocampal glutamate, GABA and acetylcholine receptors expression.

Medial septum activation produces opposite effects on dopamine neuron activity in the ventral tegmental area and substantia nigra in MAM vs. normal rats

The medial septum (MS) differentially impacts midbrain dopamine (DA) neuron activity via the ventral hippocampus, a region implicated in DA-related disorders. However, whether MS regulation of ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) is disrupted in a developmental disruption model of schizophrenia is unknown. Male Sprague-Dawley rats were exposed at gestational day 17 to methylazoxymethanol (MAM) or saline. As adults, NMDA (0.75 µg/0.2 µL) was infused into the MS, and either DA neuron activity in the VTA and SNc (7-9 anesthetized rats per group) or amphetamine-induced hyperlocomotion (AIH, 11-13 rats per group) was measured. MS activation produced a 58% increase in the number of spontaneously active DA neurons in VTA and a 37% decrease in SNc in saline rats. However, MS activation produced opposite effects on DA population activity in MAM rats, decreasing VTA DA activity by 51% and increasing SNc DA activity by 47%. MS activation also increased AIH by 113% in MAM rats, opposite of what is seen in intact rats. The effect in behavioral output may be due to disrupted GABAergic regulation of SNc as bicuculline infusion into vSub, which selectively prevented the MS activation-induced decrease in SNc DA activity in intact rats, prevented the increase in AIH and SNc DA activity in MAM rats. These findings demonstrate that the regulation of midbrain DA neurons by the MS is disrupted in this well-validated animal model, suggesting that it could be a potential locus for pharmacological intervention in disorders such as schizophrenia.

Cholinergic modulation of spatial learning, memory and navigation

Spatial learning, including encoding and retrieval of spatial memories as well as holding spatial information in working memory generally serving navigation under a broad range of circumstances, relies on a network of structures. While central to this network are medial temporal lobe structures with a widely appreciated crucial function of the hippocampus, neocortical areas such as the posterior parietal cortex and the retrosplenial cortex also play essential roles. Since the hippocampus receives its main subcortical input from the medial septum of the basal forebrain (BF) cholinergic system, it is not surprising that the potential role of the septo-hippocampal pathway in spatial navigation has been investigated in many studies. Much less is known of the involvement in spatial cognition of the parallel projection system linking the posterior BF with neocortical areas. Here we review the current state of the art of the division of labour within this complex 'navigation system', with special focus on how subcortical cholinergic inputs may regulate various aspects of spatial learning, memory and navigation.

Medial septum differentially regulates dopamine neuron activity in the rat ventral tegmental area and substantia nigra via distinct pathways

The medial septum (MS) impacts hippocampal activity and the hippocampus, in turn, regulates midbrain dopamine (DA) neuron activity. However, it remains to be determined how MS activation impacts midbrain DA activity. This question was addressed by infusing NMDA (0.75 µg/0.2 µL) into the medial septum of anesthetized male Sprague-Dawley rats and recording dopamine neuron activity in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). MS activation increased (71%) the number of spontaneously active DA neurons in the VTA, and decreased (40%) the number of active DA neurons in the SNc. Effects in both the VTA and SNc required the ventral subiculum, but were differentially dependent on cholinergic and GABAergic mechanisms within the vSub and rostral and caudal subregions of the ventral pallidum, respectively. MS activation also decreased amphetamine-induced locomotor behavior, which was dependent on GABAergic inputs to the hippocampus. These findings demonstrate that the MS differentially regulates meso-striatal DA transmission via distinct pathways.

Increased acetylcholine and glutamate efflux in the prefrontal cortex following intranasal orexin-A (hypocretin-1)

Orexin/hypocretin neurons of the lateral hypothalamus and perifornical area are integrators of physiological function. Previous work from our laboratory and others has shown the importance of orexin transmission in cognition. Age-related reductions in markers of orexin function further suggest that this neuropeptide may be a useful target for the treatment of age-related cognitive dysfunction. Intranasal administration of orexin-A (OxA) has shown promise as a therapeutic option for cognitive dysfunction. However, the neurochemical mechanisms of intranasal OxA administration are not fully understood. Here, we use immunohistochemistry and in vivo microdialysis to define the effects of acute intranasal OxA administration on: (i) activation of neuronal populations in the cortex, basal forebrain, and brainstem and (ii) acetylcholine (ACh) and glutamate efflux in the prefrontal cortex (PFC) of Fischer 344/Brown Norway F1 rats. Acute intranasal administration of OxA significantly increased c-Fos expression, a marker for neuronal activation, in the PFC and in subpopulations of basal forebrain cholinergic neurons. Subsequently, we investigated the effects of acute intranasal OxA on neurotransmitter efflux in the PFC and found that intranasal OxA significantly increased both ACh and glutamate efflux in this region. These findings were independent from any changes in c-Fos expression in orexin neurons, suggesting that these effects are not resultant from direct activation of orexin neurons. In total, these data indicate that intranasal OxA may enhance cognition through activation of distinct neuronal populations in the cortex and basal forebrain and through increased neurotransmission of ACh and glutamate in the PFC.

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.

The Serotonin Receptor 6 Antagonist Idalopirdine and Acetylcholinesterase Inhibitor Donepezil Have Synergistic Effects on Brain Activity-A Functional MRI Study in the Awake Rat

The 5-HT₆ receptor is a promising target for cognitive disorders, in particular for Alzheimer's disease (AD) and other CNS disorders. The high-affinity and selective 5-HT₆ receptor antagonist idalopirdine (Lu AE58054) is currently in development for mild-moderate AD as adjunct therapy to acetylcholinesterase inhibitors (AChEIs). We studied the effects of idalopirdine alone and in combination with the AChEI donepezil on brain activity using BOLD (Blood Oxygen Level Dependent) functional magnetic resonance imaging (fMRI) in the awake rat. Idalopirdine (2 mg/kg, i.v.) alone had a modest effect on brain activity, resulting in activation of eight brain regions at the peak response. Of these, the cholinergic diagonal band of Broca, the infralimbic cortex, the ventral pallidum, the nucleus accumbens shell, and the magnocellular preoptic area were shared with the effects of donepezil (0.3 mg/kg, i.v.). Donepezil alone activated 19 brain regions at the peak response, including several cortical regions, areas of the septo-hippocampal system and the serotonergic raphe nucleus. When idalopirdine and donepezil were combined, there was a robust stimulation pattern with activation of 36 brain regions spread across the extended-amygdala-, striato-pallidal, and septo-hippocampal networks as well as the cholinergic system. These findings indicate that, whilst idalopirdine and donepezil recruit a number of overlapping regions including one of the forebrain cholinergic nuclei, the synergistic effect of both compounds extends beyond the cholinergic system and the effects of donepezil alone toward recruitment of multiple neural circuits and neurotransmitter systems. These data provide new insight into the mechanisms via which idalopirdine might improve cognition in donepezil-treated AD patients.

Identifying the appropriate time for deep brain stimulation to achieve spatial memory improvement on the Morris water maze

BACKGROUND

The possibility of using deep brain stimulation (DBS) for memory enhancement has recently been reported, but the precise underlying mechanisms of its effects remain unknown. Our previous study suggested that spatial memory improvement by medial septum (MS)-DBS may be associated with cholinergic regulation and neurogenesis. However, the affected stage of memory could not be distinguished because the stimulation was delivered during the execution of all memory processes. Therefore, this study was performed to determine the stage of memory affected by MS-DBS. Rats were administered 192 IgG-saporin to lesion cholinergic neurons. Stimulation was delivered at different times in different groups of rats: 5 days before the Morris water maze test (pre-stimulation), 5 days during the training phase of the Morris water maze test (training-stimulation), and 2 h before the Morris water maze probe test (probe-stimulation). A fourth group of rats was lesioned but received no stimulation. These four groups were compared with a normal (control) group.

RESULTS

The most effective memory restoration occurred in the pre-stimulation group. Moreover, the pre-stimulation group exhibited better recall of the platform position than the other stimulation groups. An increase in the level of brain derived neurotrophic factor (BDNF) was observed in the pre-stimulation group; this increase was maintained for 1 week. However, acetylcholinesterase activity in the pre-stimulation group was not significantly different from the lesion group.

CONCLUSION

Memory impairment due to cholinergic denervation can be improved by DBS. The improvement is significantly correlated with the up-regulation of BDNF expression and neurogenesis. Based on the results of this study, the use of MS-DBS during the early stage of disease may restore spatial memory impairment.

The effect of nucleus basalis magnocellularis deep brain stimulation on memory function in a rat model of dementia

Background

Deep brain stimulation has recently been considered a potential therapy in improving memory function. It has been shown that a change of neurotransmitters has an effect on memory function. However, much about the exact underlying neural mechanism is not yet completely understood.

We therefore examined changes in neurotransmitter systems and spatial memory caused by stimulation of nucleus basalis magnocellularis in a rat model of dementia.

Methods

We divided rats into four groups: Normal, Lesion, Implantation, and Stimulation. We used 192 IgG-saporin for degeneration of basal forebrain cholinergic neuron related with learning and memory and it was injected into all rats except for the normal group. An electrode was ipsilaterally inserted in the nucleus basalis magnocellularis of all rats of the implantation and stimulation group, and the stimulation group received the electrical stimulation. Features were verified by the Morris water maze, immunochemistry and western blotting.

Results

All groups showed similar performances during Morris water maze training. During the probe trial, performance of the lesion and implantation group decreased. However, the stimulation group showed an equivalent performance to the normal group. In the lesion and implantation group, expression of glutamate acid decarboxylase65&67 decreased in the medial prefrontal cortex and expression of glutamate transporters increased in the medial prefrontal cortex and hippocampus. However, expression of the stimulation group showed similar levels as the normal group.

Conclusion

The results suggest that nucleus basalis magnocellularis stimulation enhances consolidation and retrieval of visuospatial memory related to changes of glutamate acid decarboxylase65&67 and glutamate transporter.

Excitatory Transmission to the Lateral Habenula Is Critical for Encoding and Retrieval of Spatial Memory

The lateral habenula (LHb) is viewed as a relay between the limbic system, the basal ganglia (BG), and monoaminergic neurons of the midbrain. If a prominent role has been evidenced in BG-mediated functions such as value-based decision-making, very little is known about the involvement of the LHb in limbic functions such as memory processing. In the present study, we used two pharmacological approaches-LHb reversible inactivation with intra-LHb infusion of muscimol, an agonist of the GABA-A receptor, or blockade of excitatory inputs with intra-LHb infusion of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an antagonist of the glutamatergic AMPA receptor-to investigate the involvement of the LHb in encoding, consolidation, and retrieval of spatial memory in the water maze (WM) in rats. We found that intra-LHb infusion of muscimol or CNQX prevented encoding and retrieval, but not consolidation of spatial information. In addition, muscimol but not CNQX induced impairments during a cued version of the WM task, and marked anxiety in the elevated plus maze. These results confirm the involvement of the LHb in higher cognitive functions. They further suggest a dichotomy between the role of glutamatergic and other inputs to the LHb in hippocampus-dependent memory processing, as well as in emotional aspects of goal-directed behaviors.

Volumetric analysis of the diagonal band of Broca in patients with schizophrenia and affective disorders: A post-mortem study

The human diagonal band of Broca is connected to other parts of the limbic system, such as the hippocampus, that are involved in the pathology of schizophrenia. This study aimed to characterize the volume and anterior-to-posterior distance of the human diagonal band of Broca (vertical limb) from post-mortem brains obtained from three groups: healthy control subjects (N = 17), patients with schizophrenia (N = 26), and patients with affective disorders (N = 12). There were no significant differences in the volume or anterior-to-posterior distance in the patients with schizophrenia or affective disorders compared with the healthy control subjects. To date, this is the first post-mortem investigation measuring the volume and the anterior-to-posterior distance of the diagonal band of Broca (vertical limb) in patients with schizophrenia or affective disorders compared with healthy control subjects.

A Novel 1,4-Dihydropyridine Derivative Improves Spatial Learning and Memory and Modifies Brain Protein Expression in Wild Type and Transgenic APPSweDI Mice

Ca²⁺ blockers, particularly those capable of crossing the blood-brain barrier (BBB), have been suggested as a possible treatment or disease modifying agents for neurodegenerative disorders, e.g., Alzheimer’s disease. The present study investigated the effects of a novel 4-(N-dodecyl) pyridinium group-containing 1,4-dihydropyridine derivative (AP-12) on cognition and synaptic protein expression in the brain. Treatment of AP-12 was investigated in wild type C57BL/6J mice and transgenic Alzheimer’s disease model mice (Tg APP_SweDI) using behavioral tests and immunohistochemistry, as well as mass spectrometry to assess the blood-brain barrier (BBB) penetration. The data demonstrated the ability of AP-12 to cross the BBB, improve spatial learning and memory in both mice strains, induce anxiolytic action in transgenic mice, and increase expression of hippocampal and cortical proteins (GAD67, Homer-1) related to synaptic plasticity. The compound AP-12 can be seen as a prototype molecule for use in the design of novel drugs useful to halt progression of clinical symptoms (more specifically, anxiety and decline in memory) of neurodegenerative diseases, particularly Alzheimer’s disease.

Septo-hippocampal signal processing: breaking the code

The septo-hippocampal connections appear to be a key element in the neuromodulatory cholinergic control of the hippocampal neurons. The cholinergic neuromodulation is well established in shifting behavioral states of the brain. The pacemaker role of medial septum in the limbic theta rhythm is demonstrated by lesions and pharmacological manipulations of GABAergic neurons, yet the link between the activity of different septal neuronal classes and limbic theta rhythm is not fully understood. We know even less about the information transfer between the medial septum and hippocampus--is there a particular kind of processed information that septo-hippocampal pathways transmit? This review encompasses fundamental findings together with the latest data of septo-hippocampal signal processing to tackle the frontiers of our understanding about the functional significance of medial septum to the hippocampal formation.

Exploratory behavior and recognition memory in medial septal electrolytic, neuro- and immunotoxic lesioned rats

In the present study, the effect of the medial septal (MS) lesions on exploratory activity in the open field and the spatial and object recognition memory has been investigated. This experiment compares three types of MS lesions: electrolytic lesions that destroy cells and fibers of passage, neurotoxic - ibotenic acid lesions that spare fibers of passage but predominantly affect the septal noncholinergic neurons, and immunotoxin - 192 IgG-saporin infusions that only eliminate cholinergic neurons. The main results are: the MS electrolytic lesioned rats were impaired in habituating to the environment in the repeated spatial environment, but rats with immuno- or neurotoxic lesions of the MS did not differ from control ones; the MS electrolytic and ibotenic acid lesioned rats showed an increase in their exploratory activity to the objects and were impaired in habituating to the objects in the repeated spatial environment; rats with immunolesions of the MS did not differ from control rats; electrolytic lesions of the MS disrupt spatial recognition memory; rats with immuno- or neurotoxic lesions of the MS were normal in detecting spatial novelty; all of the MS-lesioned and control rats clearly reacted to the object novelty by exploring the new object more than familiar ones. Results observed across lesion techniques indicate that: (i) the deficits after nonselective damage of MS are limited to a subset of cognitive processes dependent on the hippocampus, (ii) MS is substantial for spatial, but not for object recognition memory - the object recognition memory can be supported outside the septohippocampal system; (iii) the selective loss of septohippocampal cholinergic or noncholinergic projections does not disrupt the function of the hippocampus to a sufficient extent to impair spatial recognition memory; (iv) there is dissociation between the two major components (cholinergic and noncholinergic) of the septohippocampal pathway in exploratory behavior assessed in the open field - the memory exhibited by decrements in exploration of repeated object presentations is affected by either electrolytic or ibotenic lesions, but not saporin.

The integrated role of ACh, ERK and mTOR in the mechanisms of hippocampal inhibitory avoidance memory

The purpose of this review is to summarize the present knowledge on the interplay among the cholinergic system, Extracellular signal-Regulated Kinase (ERK) and Mammalian Target of Rapamycin (mTOR) pathways in the development of short and long term memories during the acquisition and recall of the step-down inhibitory avoidance in the hippocampus. The step-down inhibitory avoidance is a form of associative learning that is acquired in a relatively simple one-trial test through several sensorial inputs. Inhibitory avoidance depends on the integrated activity of hippocampal CA1 and other brain areas. Recall can be performed at different times after acquisition, thus allowing for the study of both short and long term memory. Among the many neurotransmitter systems involved, the cholinergic neurons that originate in the basal forebrain and project to the hippocampus are of crucial importance in inhibitory avoidance processes. Acetylcholine released from cholinergic fibers during acquisition and/or recall of behavioural tasks activates muscarinic and nicotinic acetylcholine receptors and brings about a long-lasting potentiation of the postsynaptic membrane followed by downstream activation of intracellular pathway (ERK, among others) that create conditions favourable for neuronal plasticity. ERK appears to be salient not only in long term memory, but also in the molecular mechanisms underlying short term memory formation in the hippocampus. Since ERK can function as a biochemical coincidence detector in response to extracellular signals in neurons, the activation of ERK-dependent downstream effectors is determined, in part, by the duration of ERK phosphorylation itself. Long term memories require protein synthesis, that in the synapto-dendritic compartment represents a direct mechanism that can produce rapid changes in protein content in response to synaptic activity. mTOR in the brain regulates protein translation in response to neuronal activity, thereby modulating synaptic plasticity and long term memory formation. Some studies demonstrate a complex interplay among the cholinergic system, ERK and mTOR. It has been shown that co-activation of muscarinic acetylcholine receptors and β-adrenergic receptors facilitates the conversion of short term to long term synaptic plasticity through an ERK- and mTOR-dependent mechanism which requires translation initiation. It seems therefore that the complex interplay among the cholinergic system, ERK and mTOR is crucial in the development of new inhibitory avoidance memories in the hippocampus.

The nonspecific thalamus: A place in a wedding bed for making memories last?

We summarize anatomical, electrophysiological and behavioral evidence that the rostral intralaminar (ILN) and the reuniens and rhomboid (ReRh) nuclei that belong to the nonspecific thalamus, might be part of a hippocampo-cortico-thalamic network underlying consolidation of enduring declarative(-like) memories at systems level. The first part of this review describes the anatomical and functional organization of these thalamic nuclei. The second part presents the theoretical models supporting the active systems-level consolidation, a process that relies upon sleep specific field-potential oscillations occurring during both slow-wave sleep (SWS) and rapid eye movement (REM) sleep. The last part presents data in the rat showing that the lesion of the rostral ILN or of the ReRh specifically hinders the formation of remote spatial memories without affecting task acquisition or retrieval of a recent memory. These results showing a critical role of the ILN and ReRh nuclei in the transformation of a recent memory into a remote one are discussed in the context of their control of cortical arousal (ARAS) and of thalamo-cortico-thalamic synchronization.

Cholinergic modulation of hippocampal network function

Cholinergic septohippocampal projections from the medial septal area to the hippocampus are proposed to have important roles in cognition by modulating properties of the hippocampal network. However, the precise spatial and temporal profile of acetylcholine release in the hippocampus remains unclear making it difficult to define specific roles for cholinergic transmission in hippocampal dependent behaviors. This is partly due to a lack of tools enabling specific intervention in, and recording of, cholinergic transmission. Here, we review the organization of septohippocampal cholinergic projections and hippocampal acetylcholine receptors as well as the role of cholinergic transmission in modulating cellular excitability, synaptic plasticity, and rhythmic network oscillations. We point to a number of open questions that remain unanswered and discuss the potential for recently developed techniques to provide a radical reappraisal of the function of cholinergic inputs to the hippocampus.

Medial ganglionic eminence-like cells derived from human embryonic stem cells correct learning and memory deficits

Dysfunction of basal forebrain cholinergic neurons (BFCNs) and γ-aminobutyric acid (GABA) interneurons, derived from medial ganglionic eminence (MGE), is implicated in disorders of learning and memory. Here we present a method for differentiating human embryonic stem cells (hESCs) to a nearly uniform population of NKX2.1(+) MGE-like progenitor cells. After transplantation into the hippocampus of mice in which BFCNs and some GABA neurons in the medial septum had been destroyed by mu P75-saporin, human MGE-like progenitors, but not ventral spinal progenitors, produced BFCNs that synaptically connected with endogenous neurons, whereas both progenitors generated similar populations of GABA neurons. Mice transplanted with MGE-like but not spinal progenitors showed improvements in learning and memory deficits. These results suggest that progeny of the MGE-like progenitors, particularly BFCNs, contributed to learning and memory. Our findings support the prospect of using human stem cell-derived MGE-like progenitors in developing therapies for neurological disorders of learning and memory.

Osthole Improves Spatial Memory Deficits in Rats via Hippocampal α 1-Adrenergic and D 1 /D 2 Receptors

The present study evaluated the effect of osthole, an active ingredient isolated from Cnidium monnieri L. Cusson, on spatial memory deficits caused by central neurotoxins using the Morris water maze in rats. The involvement of catecholaminergic receptors on the memory-enhancing effect of osthole in rat hippocampus was further investigated by intrahippocampal injection of catecholaminergic receptor antagonists. Intracisternal injection of osthole (10  μ g/brain) improved the spatial performance and working memory impairments caused by the catecholaminergic neurotoxin 6-hydroxydopamine. No significant differences in swimming speeds were observed among sham, neurotoxin-induced, and osthole-treated groups. Intracisternal osthole injection also attenuated the spatial performance and working memory impairments caused by the α 1 receptor antagonist phenoxybenzamine, the D1 receptor antagonist SCH 23390, and the D2 receptor antagonist sulpiride. Therefore, we demonstrated that the effect of osthole on improving spatial memory deficits may be related to the activation of hippocampal α 1 and D1/D2 receptors.

Spatial navigation in normal aging and the prodromal stage of Alzheimer's disease: insights from imaging and behavioral studies

Normal aging and mild Alzheimer's disease (AD) are associated with declines in navigational skills, including allocentric and egocentric representations, cognitive mapping, landmark processing, and spatial memory. These changes, however, are associated with different patterns of structural and functional alterations in the neural network of navigation. In AD, these changes occur in the hippocampus, parahippocampal gyrus, parietal lobe, retrosplenial cortex, prefrontal cortex, and caudate nucleus, whereas in aging, modifications occur mainly in the prefrontal cortex and the hippocampus. The navigation abilities of patients with mild cognitive impairment (MCI) have been found to show different performance patterns, depending on their cognitive profiles. Since patients with MCI do not uniformly develop dementia of the Alzheimer type, it is important to identify reliable early cognitive markers of conversion to AD dementia. In this review, we propose that navigation deficits may help distinguish patients at higher risk of developing AD dementia from individuals with normal cognitive aging and those with other neurodegenerative diseases.

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.

Accelerated aging of the GABAergic septohippocampal pathway and decreased hippocampal rhythms in a mouse model of Alzheimer's disease

Patients with Alzheimer's disease (AD) display altered functioning of cortical networks, including altered patterns of synchronous activity and a serious deficit in cholinergic septohippocampal (SH) innervation. However, the mechanisms underlying these alterations and the implication of the GABAergic SH component in AD are largely unknown. In addition, the GABAergic septohippocampal pathway (SHP) is believed to regulate synchronous hippocampal activity by controlling the activity of interneurons. Here we show, using well-characterized pathway tracing experiments, that innervation of the GABAergic SHP decreases during normal aging. Furthermore, in an AD mouse model (hAPP(Sw,Ind); J20 mice), the GABAergic SHP shows a dramatic and early onset of this decrease in 8-mo-old mice. This decline is not caused by neuronal loss, but by the reduced number and complexity of GABAergic SH axon terminals. Finally, we demonstrate that hippocampal θ and γ rhythm power spectra are markedly diminished in 8-mo-old behaving mice expressing mutated hAPP. In addition to the well-known loss of cholinergic input to the hippocampus in AD, these data suggest that the altered patterns of synchronous activity seen in patients with AD could be caused by the loss of GABAergic SH axons, which modulate hippocampal network activities.

Septohippocampal GABAergic neurons mediate the altered behaviors induced by n-methyl-D-aspartate receptor antagonists

We hypothesize that selective lesion of the septohippocampal GABAergic neurons suppresses the altered behaviors induced by an N-methyl-D-aspartate (NMDA) receptor antagonist, ketamine or MK-801. In addition, we hypothesize that septohippocampal GABAergic neurons generate an atropine-resistant theta rhythm that coexists with an atropine-sensitive theta rhythm in the hippocampus. Infusion of orexin-saporin (ore-SAP) into the medial septal area decreased parvalbumin-immunoreactive (GABAergic) neurons by ~80%, without significantly affecting choline-acetyltransferase-immunoreactive (cholinergic) neurons. The theta rhythm during walking, or the immobility-associated theta induced by pilocarpine, was not different between ore-SAP and sham-lesion rats. Walking theta was, however, more disrupted by atropine sulfate in ore-SAP than in sham-lesion rats. MK-801 (0.5 mg/kg i.p.) induced hyperlocomotion associated with an increase in frequency, but not power, of the hippocampal theta in both ore-SAP and sham-lesion rats. However, MK-801 induced an increase in 71-100 Hz gamma waves in sham-lesion but not ore-SAP lesion rats. In sham-lesion rats, MK-801 induced an increase in locomotion and an impairment of prepulse inhibition (PPI), and ketamine (3 mg/kg s.c.) induced a loss of gating of hippocampal auditory evoked potentials. MK-801-induced behavioral hyperlocomotion and PPI impairment, and ketamine-induced auditory gating deficit were reduced in ore-SAP rats as compared to sham-lesion rats. During baseline without drugs, locomotion and auditory gating were not different between ore-SAP and sham-lesion rats, and PPI was slightly but significantly increased in ore-SAP as compared with sham lesion rats. It is concluded that septohippocampal GABAergic neurons are important for the expression of hyperactive and psychotic symptoms an enhanced hippocampal gamma activity induced by ketamine and MK-801, and for generating an atropine-resistant theta. Selective suppression of septohippocampal GABAergic activity is suggested to be an effective treatment of some symptoms of schizophrenia.

The ventral hippocampus is necessary for expressing a spatial memory

Current views posit the dorsal hippocampus (DHipp) as contributing to spatial memory processes. Conversely, the ventral hippocampus (VHipp) modulates stress, emotions and affects. Arguments supporting this segregation include differences in (i) connectivity: the DHipp is connected with the entorhinal cortex which receives visuospatial neocortical inputs; the VHipp is connected with both the amygdala and hypothalamus, (ii) electrophysiological characteristics: there is a larger proportion of place cells in the DHipp than in the VHipp, and an increasing dorsoventral gradient in the size of place fields, suggesting less refined spatial coding in the VHipp, and (iii) consequences of lesions: spatial memory is altered after DHipp lesions, less dramatically, sometimes not, after VHipp lesions. Using reversible inactivation, we report in rats, that lidocaine infusions into the DHipp or VHipp right before a probe trial impair retrieval performance in a water-maze task. This impairment was found at two post-acquisition delays compatible with recent memory (1 and 5 days). Pre-training blockade of the VHipp did not prevent task acquisition and drug-free retrieval, on the contrary to pre-training blockade of DHipp, which altered performance in a subsequent drug-free probe trial. Complementary experiments excluded possible locomotor, sensorimotor, motivational or anxiety-related biases from data interpretation. Our conclusion is that a spatial memory can be acquired with the DHipp, less efficiently with the VHipp, and that the retrieval of such a memory and/or the expression of its representation engages the dorsoventral axis of the hippocampus when the task has been learnt with an entirely functional hippocampus.

Fos expression in arousal and reward areas of the brain in grass rats following induced wakefulness

In the diurnal grass rat nocturnal voluntary wakefulness induces Fos expression in specific cellular populations of arousal and reward areas of the brain. Here, we evaluated whether involuntary wakefulness would result in similar patterns of Fos expression. We assessed this question using male grass rats that were sleep deprived for 6h by gentle stimulation (SD group), starting 2h before lights off (12:12 LD cycle). Then, we examined expression of Fos in cholinergic cells of the basal forebrain (BF), as well as in dopaminergic cells of the reward system, and compared these results to those obtained from an undisturbed control group. Different from previous results with grass rats that were voluntary awake, the BF of SD animals only showed a significant increase in Fos expression in non-cholinergic neurons of the medial septum (MS). These observations differ from reports for nocturnal rodents that are sleep deprived. Thus, our results show that voluntary and induced wakefulness have different effects on neural systems involved in wakefulness and reward, and that the effects of sleep deprivation are different across species. We also investigated whether other arousal promoting regions and circadian and stress related areas responded to sleep deprivation by changing the level of Fos expression. Among these areas, only the lateral hypothalamus (LH) and the ventro lateral preoptic area showed significant effects of sleep deprivation that dissipated after a 2h period of sleep recovery, as it was also the case for the non-cholinergic MS. In addition, we found that Fos expression in the LH was robustly associated with Fos expression in other arousal and reward areas of the brain. This is consistent with the view that the arousal system of the LH modulates neural activity of other arousal regions of the brain, as described for nocturnal rodents.

Spatial memory alterations by activation of septal 5HT 1A receptors: no implication of cholinergic septohippocampal neurons

INTRODUCTION

In rats, activation of medial septum (MS) 5-HT(1A) receptors with the 5-HT(1A)/5-HT(7) receptor agonist 8-OH-DPAT disrupts encoding and consolidation, but not retrieval of a spatial memory in the water maze task. These findings might be explained by an action of 8-OH-DPAT on 5-HT(1A) receptors located on cholinergic neurons which the drug could transiently hyperpolarise. If so, selective damage of these neurons should mimic the effects of 8-OH-DPAT, or, at least, synergistically interfere with them.

METHODS

To test this hypothesis, rats were subjected to intraseptal infusions of 8-OH-DPAT (or phosphate-buffered saline) during acquisition of a water maze task before and/or after 192 IgG-saporin-induced MS cholinergic lesion (vs. sham-operated).

RESULTS

We confirmed that only pre-acquisition intraseptal 8-OH-DPAT infusions prevented learning and subsequent drug-free retrieval of the platform location in intact rats and found that (1) the cholinergic lesion did not prevent recall of the platform location, and (2) the impairing effects of 8-OH-DPAT were similar in sham-operated and lesioned rats, whether naïve or not, to the task before lesion surgery.

CONCLUSIONS

An action of 8-OH-DPAT on only MS cholinergic neurons is not sufficient to account for the drug-induced memory impairments. A concomitant 8-OH-DPAT-induced hyperpolarisation of cholinergic and/or GABAergic and/or glutamatergic neurons (intact rats), or of only GABAergic and/or glutamatergic ones after cholinergic lesion, might be necessary to obliterate task acquisition, confirming that, in the MS, (1) the three neuronal populations could cooperate to process hippocampal-dependent information, and (2) non-cholinergic septohippocampal neurons might be more important than cholinergic ones in serotonin-induced modulation of hippocampus-dependent memory processing.

Acetylcholine and attention

Historically, ACh has been implicated in learning and short-term memory functions. However, more recent studies have provided support for a role of cortical ACh in attentional effort, orienting and the detection of behavioral significant stimuli. The current review article summarizes studies in animals and humans which have investigated the role of ACh in attention and cognition. An attempt has been made to differentiate between brain regions involved in attentional processes versus those important for other cognitive functions. To this purpose, various experimental methods and interventions were used. Animal behavioral studies have injected the selective immunotoxin IgG-saporin to induce specific cholinergic lesions, employed electrochemical techniques such as microdialysis, or have administered cholinergic compounds into discrete parts of the brain. Human studies that give some indication on the link between central cholinergic signaling and cognition are obviously confined to less invasive, imaging methods such as fMRI. The brain areas that are deemed most important for intact attentional processing in both animals and humans appear to be the (pre)frontal, parietal and somatosensory (especially visual) regions, where ACh plays a vital role in the top-down control of attentional orienting and stimulus discrimination. In contrast, cholinergic signaling in the septohippocampal system is suggested to be involved in memory processes. Thus, it appears that the role of ACh in cognition is different per brain region and between nicotinic versus muscarinic receptor subtypes.