Extensive lesions of cholinergic basal forebrain neurons do not impair spatial working memory

Spatial Disorientation Under Dark Conditions Across Development in an Alzheimer's Disease Mouse Model

Alzheimer's disease (AD) is associated with hippocampal neuropathology and cognitive impairments, including wandering behavior or becoming lost in a familiar environment. Wandering behavior is severe and manifests early in life for people with specific genetic mutations. Genetic mouse models of AD have been developed to characterize the onset and progression of behavioral deficits that represent human behaviors, such as wandering, to test the efficacy of therapeutics. It is not clear if current assessments of mouse models capture the onset of AD or a snapshot of its progression. Sequential analysis of open field behavior provides a robust, quick test to dissociate navigation cues that contribute to spatial disorientation, a feature of wandering. Despite potential utility in evaluating this feature of AD, little work has been reported using animal models of dementia in this task. Thus, we examined the use of different sources of information to maintain spatial orientation at two prodromal ages in female transgenic CRND8 AD (n = 17) and Control mice (n = 16). These mice exhibit amyloid plaques, a hallmark neuropathological feature of AD, that are associated with cognitive dysfunction at ∼three months of age. Spatial disorientation was observed at two months and more severely at four months under dark conditions, but performance was spared when visual environmental cues were available. This study provides documentation of impaired self-movement cue processing in AD mice, establishing the dark open field as a behavioral tool to characterize spatial disorientation associated with AD. These findings may accelerate future assessments of novel therapeutic interventions for neurological disorders.

Disruption of basal forebrain cholinergic neurons after traumatic brain injury does not compromise environmental enrichment-mediated cognitive benefits

Environmental enrichment (EE) attenuates traumatic brain injury (TBI)-induced loss of medial septal (MS) choline acetyltransferase (ChAT)-cells and enhances spatial learning and memory vs. standard (STD) housing. Whether basal forebrain cholinergic neurons (BFCNs) are important mediators of EE-induced benefits after TBI requires further investigation. Anesthetized female rats were randomly assigned to intraseptal infusions of the immunotoxin 192-IgG-saporin (SAP; 0.22 μg in 1.0 μL) or vehicle (VEH; 1.0 μL IgG) followed immediately by a cortical impact (2.8 mm deformation depth at 4 m/s) or sham injury and divided into EE and STD housing. Spatial learning and memory retention were assessed on post-operative days 14-19. MS ChAT⁺ cells were quantified at 3 weeks. SAP significantly reduced ChAT⁺ cells in both the EE and STD groups. Cognitive performance was improved in the EE groups, regardless of VEH or SAP infusion, vs. the STD-housed groups (p's < 0.05). No cognitive differences were revealed between the TBI + EE + SAP and TBI + EE + VEH groups (p > 0.05) or between the TBI + STD + SAP and TBI + STD + VEH groups (p > 0.05). These data show that despite significant MS ChAT⁺ cell loss, the EE-mediated benefit in cognitive recovery is not compromised.

Cholinergic Signaling Dynamics and Cognitive Control of Attention

The central cholinergic system is one of the most important modulator neurotransmitter system implicated in diverse behavioral processes. Activation of the basal forebrain cortical cholinergic input system represents a critical step in cortical information processing. This chapter explores recent developments illustrating cortical cholinergic transmission mediate defined cognitive operations, which is contrary to the traditional view that acetylcholine acts as a slowly acting neuromodulator that influences arousal cortex-wide. Specifically, we review the evidence that phasic cholinergic signaling in the prefrontal cortex is a causal mediator of signal detection. In addition, studies that support the neuromodulatory role of cholinergic inputs in top-down attentional control are summarized. Finally, we review new findings that reveal sex differences and hormonal regulation of the cholinergic-attention system.

Hippocampal acetylcholine depletion has no effect on anxiety, spatial novelty preference, or differential reward for low rates of responding (DRL) performance in rats

We investigated the role of the septo-hippocampal cholinergic projection in anxiety, spatial novelty preference, and differential reward for low rates of responding (DRL) performance. Cholinergic neurons of the rat medial septum (MS) and the vertical limb of the diagonal band of Broca (VDB) were lesioned using the selective immunotoxin, 192 IgG-saporin. Rats were then tested on several behavioral tests previously shown to be sensitive to either (a) hippocampal lesions or (b) nonselective MS/VDB lesions which target both cholinergic and γ-aminobutyric acid (GABA)-ergic projections, or both. Saporin lesions substantially reduced hippocampal cholinergic innervation, resulting in an absence of acetyl cholinesterase staining and markedly reduced choline acetyltransferase activity (mean reduction: 80 ± 5%; range: 50-97%). However, the saporin-lesioned rats did not differ from control rats in any of the behavioral tests. Thus we found no evidence from these lesion studies that the septo-hippocampal cholinergic projection plays an essential role in anxiety, spatial novelty preference, or DRL.

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.

Harnessing the power of theta: natural manipulations of cognitive performance during hippocampal theta-contingent eyeblink conditioning

Neurobiological oscillations are regarded as essential to normal information processing, including coordination and timing of cells and assemblies within structures as well as in long feedback loops of distributed neural systems. The hippocampal theta rhythm is a 3–12 Hz oscillatory potential observed during cognitive processes ranging from spatial navigation to associative learning. The lower range, 3–7 Hz, can occur during immobility and depends upon the integrity of cholinergic forebrain systems. Several studies have shown that the amount of pre-training theta in the rabbit strongly predicts the acquisition rate of classical eyeblink conditioning and that impairment of this system substantially slows the rate of learning. Our lab has used a brain-computer interface (BCI) that delivers eyeblink conditioning trials contingent upon the explicit presence or absence of hippocampal theta. A behavioral benefit of theta-contingent training has been demonstrated in both delay and trace forms of the paradigm with a two- to four-fold increase in learning speed. This behavioral effect is accompanied by enhanced amplitude and synchrony of hippocampal local field potential (LFP)s, multi-unit excitation, and single-unit response patterns that depend on theta state. Additionally, training in the presence of hippocampal theta has led to increases in the salience of tone-induced unit firing patterns in the medial prefrontal cortex, followed by persistent multi-unit activity during the trace interval. In cerebellum, rhythmicity and precise synchrony of stimulus time-locked LFPs with those of hippocampus occur preferentially under the theta condition. Here we review these findings, integrate them into current models of hippocampal-dependent learning and suggest how improvement in our understanding of neurobiological oscillations is critical for theories of medial temporal lobe processes underlying intact and pathological learning.

Time to pay attention: attentional performance time-stamped prefrontal cholinergic activation, diurnality, and performance

Although the impairments in cognitive performance that result from shifting or disrupting daily rhythms have been demonstrated, the neuronal mechanisms that optimize fixed-time daily performance are poorly understood. We previously demonstrated that daily practice of a sustained attention task (SAT) evokes a diurnal activity pattern in rats. Here, we report that SAT practice at a fixed time produced practice time-stamped increases in prefrontal cholinergic neurotransmission that persisted after SAT practice was terminated and in a different environment. SAT time-stamped cholinergic activation occurred regardless of whether the SAT was practiced during the light or dark phase or in constant-light conditions. In contrast, prior daily practice of an operant schedule of reinforcement, albeit generating more rewards and lever presses per session than the SAT, neither activated the cholinergic system nor affected the animals' nocturnal activity pattern. Likewise, food-restricted animals exhibited strong food anticipatory activity (FAA) and attenuated activity during the dark phase but FAA was not associated with increases in prefrontal cholinergic activity. Removal of cholinergic neurons impaired SAT performance and facilitated the reemergence of nocturnality. Shifting SAT practice away from a fixed time resulted in significantly lower performance. In conclusion, these experiments demonstrated that fixed-time, daily practice of a task assessing attention generates a precisely practice time-stamped activation of the cortical cholinergic input system. Time-stamped cholinergic activation benefits fixed-time performance and, if practiced during the light phase, contributes to a diurnal activity pattern.

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.

Cholinergic modulation of cognitive processing: insights drawn from computational models

Acetylcholine plays an important role in cognitive function, as shown by pharmacological manipulations that impact working memory, attention, episodic memory, and spatial memory function. Acetylcholine also shows striking modulatory influences on the cellular physiology of hippocampal and cortical neurons. Modeling of neural circuits provides a framework for understanding how the cognitive functions may arise from the influence of acetylcholine on neural and network dynamics. We review the influences of cholinergic manipulations on behavioral performance in working memory, attention, episodic memory, and spatial memory tasks, the physiological effects of acetylcholine on neural and circuit dynamics, and the computational models that provide insight into the functional relationships between the physiology and behavior. Specifically, we discuss the important role of acetylcholine in governing mechanisms of active maintenance in working memory tasks and in regulating network dynamics important for effective processing of stimuli in attention and episodic memory tasks. We also propose that theta rhythm plays a crucial role as an intermediary between the physiological influences of acetylcholine and behavior in episodic and spatial memory tasks. We conclude with a synthesis of the existing modeling work and highlight future directions that are likely to be rewarding given the existing state of the literature for both empiricists and modelers.

Decrease of GABAergic markers and arc protein expression in the frontal cortex by intraventricular 192 IgG-saporin

BACKGROUND/AIMS

Previous studies used 192 IgG-saporin to study cholinergic function because of its facility for selective lesioning; however, results varied due to differences in the methods of administration and behavioral tests used. We examined an animal model of dementia using 192 IgG-saporin to confirm its features before applying this model to research of therapeutic drugs or electrical stimulation techniques.

METHODS

Features were verified by the Morris water maze test, immunochemistry, and Western blotting. Animals were examined after intraventricular injection of 192 IgG-saporin (0.63 μg/μl; 6, 8, and 10 μl) or phosphate-buffered saline.

RESULTS

In the acquisition phase of the Morris water maze test, the latencies of the injection groups were significantly delayed, but recovered within 1 week. In the probe test, 2 of 4 indices (time in the platform zone and the number of crossings) were significantly different in the 8-μl injection group. Immunohistochemistry revealed the extent of cholinergic destruction. Activity-regulated cytoskeleton-associated protein and glutamate decarboxylase expression significantly decreased in the frontal cortex (8- and 10-μl groups), but not in the hippocampus.

CONCLUSION

Spatial memory impairment was associated with cholinergic basal forebrain injury as well as frontocortical GABAergic hypofunction and synaptic plasticity deceleration.

Impaired visual search in rats reveals cholinergic contributions to feature binding in visuospatial attention

The visual search task established the feature integration theory of attention in humans and measures visuospatial attentional contributions to feature binding. We recently demonstrated that the neuromodulator acetylcholine (ACh), from the nucleus basalis magnocellularis (NBM), supports the attentional processes required for feature binding using a rat digging-based task. Additional research has demonstrated cholinergic contributions from the NBM to visuospatial attention in rats. Here, we combined these lines of evidence and employed visual search in rats to examine whether cortical cholinergic input supports visuospatial attention specifically for feature binding. We trained 18 male Long-Evans rats to perform visual search using touch screen-equipped operant chambers. Sessions comprised Feature Search (no feature binding required) and Conjunctive Search (feature binding required) trials using multiple stimulus set sizes. Following acquisition of visual search, 8 rats received bilateral NBM lesions using 192 IgG-saporin to selectively reduce cholinergic afferentation of the neocortex, which we hypothesized would selectively disrupt the visuospatial attentional processes needed for efficient conjunctive visual search. As expected, relative to sham-lesioned rats, ACh-NBM-lesioned rats took significantly longer to locate the target stimulus on Conjunctive Search, but not Feature Search trials, thus demonstrating that cholinergic contributions to visuospatial attention are important for feature binding in rats.

Modes and models of forebrain cholinergic neuromodulation of cognition

As indicated by the profound cognitive impairments caused by cholinergic receptor antagonists, cholinergic neurotransmission has a vital role in cognitive function, specifically attention and memory encoding. Abnormally regulated cholinergic neurotransmission has been hypothesized to contribute to the cognitive symptoms of neuropsychiatric disorders. Loss of cholinergic neurons enhances the severity of the symptoms of dementia. Cholinergic receptor agonists and acetylcholinesterase inhibitors have been investigated for the treatment of cognitive dysfunction. Evidence from experiments using new techniques for measuring rapid changes in cholinergic neurotransmission provides a novel perspective on the cholinergic regulation of cognitive processes. This evidence indicates that changes in cholinergic modulation on a timescale of seconds is triggered by sensory input cues and serves to facilitate cue detection and attentional performance. Furthermore, the evidence indicates cholinergic induction of evoked intrinsic, persistent spiking mechanisms for active maintenance of sensory input, and planned responses. Models have been developed to describe the neuronal mechanisms underlying the transient modulation of cortical target circuits by cholinergic activity. These models postulate specific locations and roles of nicotinic and muscarinic acetylcholine receptors and that cholinergic neurotransmission is controlled in part by (cortical) target circuits. The available evidence and these models point to new principles governing the development of the next generation of cholinergic treatments for cognitive disorders.

Chronic cerebral ischaemia forms new cholinergic mechanisms of learning and memory

The purpose of this research was a comparative analysis of cholinergic synaptic organization following learning and memory in normal and chronic cerebral ischaemic rats in the Morris water maze model. Choline acetyltransferase and protein content were determined in subpopulations of presynapses of “light” and “heavy” synaptosomal fractions of the cortex and the hippocampus, and the cholinergic projective and intrinsic systems of the brain structures were taken into consideration. We found a strong involvement of cholinergic systems, both projective and intrinsic, in all forms of cognition. Each form of cognition had an individual cholinergic molecular profile and the cholinergic synaptic compositions in the ischaemic rat brains differed significantly from normal ones. Our data demonstrated that under ischaemic conditions, instead of damaged connections new key synaptic relationships, which were stable against pathological influences and able to restore damaged cognitive functions, arose. The plasticity of neurochemical links in the individual organization of certain types of cognition gave a new input into brain pathology and can be used in the future for alternative corrections of vascular and other degenerative dementias.

Selective disconnection of the hippocampal formation projections to the mammillary bodies produces only mild deficits on spatial memory tasks: implications for fornix function

It is now clear that the integrity of the fornix is important for normal mnemonic function. The fornix, however, is a major white matter tract, carrying numerous hippocampal formation afferents and efferents, and it is not known which specific components support memory processes. Established theories of extended hippocampal function emphasize the sequential pathway from the hippocampal formation (i.e., subicular complex) to the mammillary bodies and, thence, to the anterior thalamus, as pathology in each of these structures is implicated in anterograde amnesia in humans and spatial memory deficits in rats. The specific importance of the hippocampal formation projections that just innervate the mammillary bodies has, however, never been tested. This study isolated these specific projections in the rat by selectively cutting the descending component of the postcommissural fornix. Two successive, cohorts of rats with these tract lesions were tested on working memory tasks in the water-maze, T-maze, and radial-arm maze. Disconnecting the descending postcommissural fornix had only a mild effect or sometimes no apparent effect on the performance of these spatial memory tasks, even though tracing experiments confirmed the loss of hippocampal formation-mammillary projections. One implication is that the spatial deficits found in rats following standard fornix lesions are only partly attributable to the loss of projections from the hippocampal formation to the mammillary bodies. Perhaps more surprising, the behavioral impact of cutting the descending postcommissural fornix in rats appeared appreciably less than the effect of either mammillary body or mammillothalamic tract lesions. The present experiments show that the mammillary bodies can still effectively support spatial memory in the absence of their dense subicular complex inputs, so revealing the importance of the other afferents for sustaining mammillary body function. This new evidence for independent functions shows that the mammillary bodies are more than just a hippocampal relay.

The septo-hippocampal system, learning and recovery of function

We understand this review as an attempt to summarize recent advances in the understanding of cholinergic function in cognition. Such a role has been highlighted in the 1970s by the discovery that dementia patients have greatly reduced cholinergic activity in cortex and hippocampus. A brief anatomical description of the major cholinergic pathways focuses on the basal forebrain and its projections to cortex and hippocampus. From this distinction, compelling evidence suggests that the basal forebrain --> cortex projection regulates the excitability of principal cortical neurons and is thereby critically involved in attention, stimulus detection and memory function, although the biological conditions for these functions are still debated. Similar uncertainties remain for the septo-hippocampal cholinergic system. Although initial lesions of the septum caused memory deficits reminiscent of hippocampal ablations, recent and more refined neurotoxic lesion studies which spared non-cholinergic cells of the basal forebrain failed to confirm these memory impairments in experimental animals despite a near total loss of cholinergic labeling. Yet, a decline in cholinergic markers in aging and dementia still stands as the most central piece of evidence for a link between the cholinergic system and cognition and appear to provide valuable targets for therapeutic approaches.

Cholinergic deafferentation of the neocortex using 192 IgG-saporin impairs feature binding in rats

The binding problem refers to the fundamental challenge of the CNS to integrate sensory information registered by distinct brain regions to form a unified neural representation of a stimulus. Although the human cognitive literature has established that attentional processes in frontoparietal cortices support feature binding, the neurochemical and specific downstream neuroanatomical contributions to feature binding remain unknown. Using systemic pharmacology in rats, it has been shown that the neuromodulator acetylcholine is essential for feature binding at encoding, but the neural source of such critical cholinergic neurotransmission has yet to be identified. Cholinergic efferents from the nucleus basalis magnocellularis (NBM) of the basal forebrain provide the majority of the cholinergic input to the neocortex. Accordingly, it was hypothesized that the NBM is the neural source that provides the critical neuromodulatory support for feature binding. To test this hypothesis, rats received bilateral 192 IgG-saporin lesions of the NBM, and their feature binding performance was tested using a forced-choice digging paradigm. Relative to sham-lesioned rats, NBM-lesioned rats were significantly impaired at acquiring a crossmodal feature conjunction (FC) stimulus set that required feature binding, whereas their ability to retrieve an FC stimulus set and to acquire two crossmodal feature singleton stimulus sets, one of greater difficulty than the other but neither requiring feature binding, remained intact. These behavioral findings, along with histological analyses demonstrating positive relationships between feature-binding acquisition and markers of cholinergic activity in frontoparietal regions, reveal the importance of neocortical cholinergic input from the NBM to feature binding at encoding.

Efficacy of a murine-p75-saporin immunotoxin for selective lesions of basal forebrain cholinergic neurons in mice

Selective lesioning of cholinergic neurons in the basal forebrain provides a tool for examining the functional significance of cholinergic loss, which is associated with a number of developmental and neurodegenerative disorders. A new version of an immunotoxin (murine-p75NTR-saporin) was used to produce a selective loss of cholinergic neurons in the adult basal forebrain of the mouse. This new version of the toxin is significantly more potent and selective than a previously developed version. C57Bl/6J mice (n=30) were given 1 microL of either saline or murine-p75NTR-saporin (0.65 microg/microL or 1.3 microg/microL) into the lateral ventricles, and then sacrificed 10-12 days post-surgery for histological analysis. In contrast to results from the previous version of the toxin, survival of the toxin-treated mice was 100% at both doses. A complete loss of cholinergic neurons was seen in the medial septum (MS) with both doses, while a dose-dependent loss of cholinergic neurons was observed in the nucleus basalis magnocellularis (nBM). The lesions were associated with locomotor hypoactivity and anxiolytic-type behavioral effects. These studies describe the efficacy and selectivity of this new version of murine-p75NTR-saporin, which may be used to provide insight into functional deficits that result from the loss of cholinergic neurons in the mouse basal forebrain.

Increasing hippocampal acetylcholine levels enhance behavioral performance in an animal model of diencephalic amnesia

Pyrithiamine-induced thiamine deficiency (PTD) was used to produce a rodent model of Wernicke-Korsakoff syndrome that results in acute neurological disturbances, thalamic lesions, and learning and memory impairments. There is also cholinergic septohippocampal dysfunction in the PTD model. Systemic (Experiment 1) and intrahippocampal (Experiment 2) injections of the acetylcholinesterase inhibitor physostigmine were administered to determine if increasing acetylcholine levels would eliminate the behavioral impairment produced by PTD. Prior to spontaneous alternation testing, rats received injections of either physostigmine (systemic=0.075 mg/kg; intrahippocampal=20, 40 ng/muL) or saline. In Experiment 2, intrahippocampal injections of physostigmine significantly enhanced alternation rates in the PTD-treated rats. In addition, although intrahippocampal infusions of 40 ng of physostigmine increased the available amount of ACh in both pair-fed (PF) and PTD rats, it did so to a greater extent in PF rats. The increase in ACh levels induced by the direct hippocampal application of physostigmine in the PTD model likely increased activation of the extended limbic system, which was dysfunctional, and therefore led to recovery of function on the spontaneous alternation task. In contrast, the lack of behavioral improvement by intrahippocampal physostigmine infusion in the PF rats, despite a greater rise in hippocampal ACh levels, supports the theory that there is an optimal range of cholinergic tone for optimal behavioral and hippocampal function.

Cholinergic suppression of glutamatergic synaptic transmission in hippocampal region CA3 exhibits laminar selectivity: Implication for hippocampal network dynamics

Acetylcholine may help set the dynamics within neural systems to facilitate the learning of new information. Neural models have shown that if new information is encoded at the same time as retrieval of existing information that is already stored, the memories will interfere with each other. Structures such as the hippocampus have a distinct laminar organization of inputs that allows this hypothesis to be tested. In region CA1 of the rat (Sprague Dawley) hippocampus, the cholinergic agonist carbachol (CCh) suppresses transmission in stratum radiatum (SR), at synapses of the Schaffer collateral projection from CA3, while having lesser effects in stratum lacunosum-moleculare (SLM), the perforant path projection from entorhinal cortex (Hasselmo and Schnell, 1994). The current research extends support of this selectivity by demonstrating laminar effects in region CA3. CCh caused significantly greater suppression in SR than in SLM at low concentrations, while the difference in suppression was not significant at higher concentrations. Differences in paired-pulse facilitation suggest presynaptic inhibition substantially contributes to the suppression and is highly concentration and stratum dependent. This selective suppression of the recurrent excitation would be appropriate to set CA3 dynamics to prevent runaway modification of the synapses of excitatory recurrent collaterals by reducing the influence of previously stored associations and allowing incoming information from the perforant path to have a predominant influence on neural activity.

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.

Elevations of endogenous kynurenic acid produce spatial working memory deficits

Kynurenic acid (KYNA) is a tryptophan metabolite that is synthesized and released by astrocytes and acts as a competitive antagonist of the glycine site of N-methyl-D-aspartate receptors at high concentrations and as a noncompetitive antagonist of the alpha7-nicotinic acetylcholine receptor at low concentrations. The discovery of increased cortical KYNA levels in schizophrenia prompted the hypothesis that elevated KYNA concentration may underlie the working memory dysfunction observed in this population that has been attributed to altered glutamatergic and/or cholinergic transmission. The present study investigated the effect of elevated endogenous KYNA on spatial working memory function in rats. Increased KYNA levels were achieved with intraperitoneal administration of kynurenine (100 mg/kg), the precursor of KYNA synthesis. Rats were treated with either kynurenine or a vehicle solution prior to testing in a radial arm maze task at various delays. Elevations of endogenous KYNA resulted in increased errors in the radial arm maze. In separate experiments, assessment of locomotor activity in an open field and latency to retrieve food reward from one of the maze arms ruled out the possibility that deficits in the maze were attributable to altered locomotor activity or motivation to consume food. These results provide evidence that increased KYNA levels produce spatial working memory deficits and are among the first to demonstrate the influence of glia-derived molecules on cognitive function. The implications for psychopathological conditions such as schizophrenia are discussed.

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

Rats organize their open field behavior into a series of exploratory trips focused around a central location or home base. In addition, differences in movement kinematics have been used to fractionate the exploratory trip into tour (i.e., sequences of linear movement or progressions punctuated by stops) and homeward (i.e., single progression direct to the home base) segments. The observation of these characteristics independent of environmental familiarity and visual cue availability has suggested a role for self-movement information or dead reckoning in organizing exploratory behavior. Although previous work has implicated a role for the septohippocampal system in dead reckoning based navigation, as of yet, no studies have investigated the contribution of the medial septum to dead reckoning. First, the present study examined the organization of exploratory behavior under dark and light conditions in control rats and rats receiving either electrolytic or sham medial septum lesions. Medial septum lesions produced a significant increase in homeward segment path circuity and variability of temporal pacing of linear speeds. Second, as an independent assessment of the effectiveness of the medial septum lesions, rats were trained to locate a hidden platform in the standard water maze procedure. Consistent with previous research, medial septum lesions attenuated learning the location of the hidden platform. These results demonstrate a role for the medial septum in organizing exploratory behavior and provide further support for the role of the septohippocampal system in dead reckoning based navigation.

Effect of selective cholinergic denervation on the serotonergic system: implications for learning and memory

The cholinergic system has been widely implicated in cognitive processes and cholinergic loss is a classical hallmark in Alzheimer disease. Increasing evidence supports a role of the serotonergic system in cognition, possibly through a modulation of cholinergic activity. We compared selective cholinergic denervation by administration of the immunotoxin 192 IgG-saporin in the nucleus basalis of Meynert (NBM) with intracerebroventricular (ICV) lesions of the basal forebrain in male rats 7 days after lesioning. NBM lesions induced significant changes in cholinergic markers in the frontal cortex, whereas ICV lesions produced significant decreases in cholinergic markers both in the frontal cortex and hippocampus. Only ICV lesions lead to memory impairments in passive avoidance and Morris water maze tasks. Both models lead to reductions of serotonin levels in the frontal cortex. Similar changes in 5-hydroxytriptophan levels were observed, suggesting a downregulation of the rate-limiting enzyme for the synthesis of serotonin along with the cholinergic deficit. Neither 5-HT1A nor 5-HT1B receptors seem to mediate this process. These data imply that the serotonergic system in the frontal cortex can compensate for diminished cholinergic function and support the investigation of the serotonergic system as a therapeutic target to treat Alzheimer disease.

Relaxin-3 in GABA projection neurons of nucleus incertus suggests widespread influence on forebrain circuits via G-protein-coupled receptor-135 in the rat

Relaxin-3 (RLX3) is a newly identified member of the relaxin/insulin peptide family that is highly conserved across a range of species from fish to mammals and is highly expressed in rat, mouse and human brain. Extensive pharmacological studies have demonstrated that RLX3 is a high affinity, selective ligand for G-protein-coupled receptor-135 (GPCR135, now classified as relaxin family peptide-3 receptor; RXFP3). In ongoing studies to understand the physiological functions of RLX3, the distribution of RLX3-containing neuronal elements in rat brain was determined by immunohistochemistry, using an affinity-purified polyclonal antiserum raised against a conserved segment of the RLX3 C-peptide (AS-R3(85-101)). Consistent with the distribution of RLX3 mRNA, neurons containing RLX3-like immunoreactivity (LI) were observed in the pontine nucleus incertus and the majority of these cells, which are known to express corticotropin-releasing factor receptor-1, were shown to express glutamic acid decarboxylase-65-immunoreactivity, suggesting a GABA phenotype. Nerve fibers and terminals containing RLX3-LI were observed adjacent to cells in the nucleus incertus and in various forebrain regions known to receive afferents from the nucleus incertus, including cortex, septum, hippocampus, thalamus, hypothalamus and midbrain. Regions that contained highest densities of RLX3-positive fibers included the medial septum, lateral preoptic area, lateral hypothalamus/medial forebrain bundle and ventral hippocampus; and additional fibers were observed in olfactory bulb and olfactory and frontal/cingulate cortices, bed nucleus of the stria terminalis, dorsal endopiriform, intergeniculate, and supramammillary nuclei, and the periaqueductal gray and dorsal raphe. The RLX3-positive network overlapped the regional distribution of GPCR135 mRNA and specific binding sites for an [125I]-GPCR135-selective, chimeric peptide. These anatomical findings further support the proposition that RLX3 is the endogenous ligand for GPCR135 in rat brain and provide evidence for broad modulatory activity of RLX3 in behavioral activation relating to autonomic and neuroendocrine control of metabolism and reproduction and higher-order processes such as stress and cognition.

The consequences of atheoretical, task-driven experimentation: theoretical comment on Paban, Chambon, Jaffard, and Alescio-Lautier (2005)

V. Paban, C. Chambon, M. Jaffard, and B. Alescio-Lautier (2005) concluded that their evidence "demonstrates the independence of aging and cholinergic lesion effects" and stated that "this was quite unexpected" (p. 943). This commentary suggests that the authors' negative findings were predictable, resulting from a focus on behavioral tests and functions unrelated to present evidence and theory concerning the cognitive functions of forebrain cholinergic systems. The article by V. Paban et al. serves as a useful illustration for the importance of theory- and hypothesis-driven assessment of behavioral functions, and for the unproductive consequences of experiments treating behavioral tests as assays used merely for the generation of behavioral correlates.

The septohippocampal cholinergic system and spatial working memory in the Morris water maze

The objective of the present study was to determine whether a systematic optimization of Morris water maze (mwm) testing parameters could reveal a significant role of the septohippocampal cholinergic system in spatial working memory. Young adult rats were lesioned using 192 IgG-saporin infused bilaterally into the medial septum. Lesions were near complete as measured by choline acetyltransferase (ChAT) activity and immunohistochemistry. Behavioral testing was performed in three phases. In the first, lesioned and unlesioned rats were trained in the mwm focusing on working memory, which was tested using novel platform locations daily. In the second phase, the optimal locations were retested with increasing intertrial intervals (ITI). In the third phase, intracerebroventricular infusions of nerve growth factor (NGF) were employed to enhance cholinergic activity of the unlesioned rats and potentially further separate group performance. Neither the standard or increased ITI resulted in a consistent significant difference in spatial working memory between groups. In addition, NGF treatment also failed to induce a significant difference in behavioral performance. In conclusion, impairments in working memory as assessed by the mwm could not be revealed despite a greater than 90% loss of hippocampal ChAT and the use of optimal testing parameters and NGF treatment.

Septohippocampal acetylcholine: involved in but not necessary for learning and memory?

The neurotransmitter acetylcholine (ACh) has been accorded an important role in supporting learning and memory processes in the hippocampus. Cholinergic activity in the hippocampus is correlated with memory, and restoration of ACh in the hippocampus after disruption of the septohippocampal pathway is sufficient to rescue memory. However, selective ablation of cholinergic septohippocampal projections is largely without effect on hippocampal-dependent learning and memory processes. We consider the evidence underlying each of these statements, and the contradictions they pose for understanding the functional role of hippocampal ACh in memory. We suggest that although hippocampal ACh is involved in memory in the intact brain, it is not necessary for many aspects of hippocampal memory function.