Muscarinic receptor blockade in ventral hippocampus and prelimbic cortex impairs memory for socially transmitted food preference

Cannabidiol enhances socially transmitted food preference: a role of acetylcholine in the mouse basal forebrain

RATIONALE AND OBJECTIVE

Rodents acquire food information from their conspecifics and display a preference for the conspecifics' consumed food. This social learning of food information from others promotes the survival of a species, and it is introduced as the socially transmitted food preference (STFP) task. The cholinergic system in the basal forebrain plays a role in the acquisition of STFP. Cannabidiol (CBD), one of the most abundant phytocannabinoids, exerts its therapeutic potential for cognitive deficits through versatile mechanisms of action, including its interaction with the cholinergic system. We hypothesize a positive relationship between CBD and STFP because acetylcholine (ACh) is involved in STFP, and CBD increases the ACh levels in the basal forebrain.

MATERIALS AND METHODS

Male C57BL/6J mice were trained to acquire the STFP task. We examined whether CBD affects STFP memory by administering CBD (20 mg/kg, i.p.) before the STFP social training. The involvement of cholinergic system in CBD's effect on STFP was examined by knockdown of brain acetylcholinesterase (AChE), applying a nonselective muscarinic antagonist SCO (3 mg/kg, i.p.) before CBD treatment, and measuring the basal forebrain ACh levels in the CBD-treated mice.

RESULTS

We first showed that CBD enhanced STFP memory. Knockdown of brain AChE also enhanced STFP memory, which mimicked CBD's effect on STFP. SCO blocked CBD's memory-enhancing effect on STFP. Our most significant finding is that the basal forebrain ACh levels in the CBD-treated mice, but not their control counterparts, were positively correlated with mice's STFP memory performance.

CONCLUSION

This study indicates that CBD enhances STFP memory in mice. Specifically, those which respond to CBD by increasing the muscarinic-mediated ACh signaling perform better in their STFP memory.

The cortical amygdala consolidates a socially transmitted long-term memory

Social communication guides decision-making, which is essential for survival. Social transmission of food preference (STFP) is an ecologically relevant memory paradigm in which an animal learns a desirable food odour from another animal in a social context, creating a long-term memory1,2. How food-preference memory is acquired, consolidated and stored is unclear. Here we show that the posteromedial nucleus of the cortical amygdala (COApm) serves as a computational centre in long-term STFP memory consolidation by integrating social and sensory olfactory inputs. Blocking synaptic signalling by the COApm-based circuit selectively abolished STFP memory consolidation without impairing memory acquisition, storage or recall. COApm-mediated STFP memory consolidation depends on synaptic inputs from the accessory olfactory bulb and on synaptic outputs to the anterior olfactory nucleus. STFP memory consolidation requires protein synthesis, suggesting a gene-expression mechanism. Deep single-cell and spatially resolved transcriptomics revealed robust but distinct gene-expression signatures induced by STFP memory formation in the COApm that are consistent with synapse restructuring. Our data thus define a neural circuit for the consolidation of a socially communicated long-term memory, thereby mechanistically distinguishing protein-synthesis-dependent memory consolidation from memory acquisition, storage or retrieval.

Observational learning in rats: Interplay between demonstrator and observer behavior

BACKGROUND

Observational learning is a vital skill for survival. This form of learning has been seen in humans and certain non-human animals. However, the neural circuitry underlying this form of learning is still poorly understood.

NEW METHOD

To better understand the factors underlying successful observation in rats, we employed a task where an observer must base its behavior on that of a demonstrator rat to identify a reward location. A comparison was made of behavior during a social and non-social observation condition.

RESULTS

Observers oriented more, responded faster and omitted less responses in the social compared to the non-social condition. Observer performance was also linked to initial orientation, proximity, and the manner in which the demonstrator rat performed the task.

COMPARISON WITH EXISTING METHOD

Previous work on observational learning encompassed multiple exposures to a single solution over days or weeks. The current method provides data from multiple individual novel observational learning trials, leading to much faster and more robust social learning. This method provides a clearly defined interval in which observation must take place. Allowing for precise tracking of both the observer and demonstrator behavior during the learning period.

CONCLUSIONS

This study highlights observer and demonstrator interplay in successful observational learning and provides a novel method for analyzing social behavior in rodents.

Ventral hippocampus-lateral septum circuitry promotes foraging-related memory

Remembering the location of a food or water source is essential for survival. Here, we reveal that spatial memory for food location is reflected in ventral hippocampus (HPCv) neuron activity and is impaired by HPCv lesion. HPCv mediation of foraging-related memory involves communication to the lateral septum (LS), as either reversible or chronic disconnection of HPCv-to-LS signaling impairs spatial memory retention for food or water location. This neural pathway selectively encodes appetitive spatial memory, as HPCv-LS disconnection does not affect spatial memory for escape location in a negative reinforcement procedure, food intake, or social and olfactory-based appetitive learning. Neural pathway tracing and functional mapping analyses reveal that LS neurons recruited during the appetitive spatial memory procedure are primarily GABAergic neurons that project to the lateral hypothalamus. Collective results emphasize that the neural substrates controlling spatial memory are outcome specific based on reinforcer modality.

Linking Social Cognition to Learning and Memory

Many mammals have evolved to be social creatures. In humans, the ability to learn from others' experiences is essential to survival; and from an early age, individuals are surrounded by a social environment that helps them develop a variety of skills, such as walking, talking, and avoiding danger. Similarly, in rodents, behaviors, such as food preference, exploration of novel contexts, and social approach, can be learned through social interaction. Social encounters facilitate new learning and help modify preexisting memories throughout the lifespan of an organism. Moreover, social encounters can help buffer stress or the effects of negative memories, as well as extinguish maladaptive behaviors. Given the importance of such interactions, there has been increasing work studying social learning and applying its concepts in a wide range of fields, including psychotherapy and medical sociology. The process of social learning, including its neural and behavioral mechanisms, has also been a rapidly growing field of interest in neuroscience. However, the term "social learning" has been loosely applied to a variety of psychological phenomena, often without clear definition or delineations. Therefore, this review gives a definition for specific aspects of social learning, provides an overview of previous work at the circuit, systems, and behavioral levels, and finally, introduces new findings on the social modulation of learning. We contextualize such social processes in the brain both through the role of the hippocampus and its capacity to process "social engrams" as well as through the brainwide realization of social experiences. With the integration of new technologies, such as optogenetics, chemogenetics, and calcium imaging, manipulating social engrams will likely offer a novel therapeutic target to enhance the positive buffering effects of social experiences or to inhibit fear-inducing social stimuli in models of anxiety and post-traumatic stress disorder.

Observational learning of a shifting goal location in rats: Impact of distance, observed performance, familiarity, and delay

BACKGROUND

Observational learning allows for learning without direct exposure to danger or energetic demands. This form of learning is seen in humans, other primates, and other species such as rodents. The neurobiology behind social learning has been studied mostly in rats, specifically focusing on social transmission of food preference and fear. However, less is known regarding the neural circuitry behind social learning of a foraging scenario.

NEW METHOD

The current study examined observational learning in a working memory Tmaze task. The food location changed daily such that the observing animal had to learn the correct location anew each day. This delineated the time frame when an animal learned by observation, making the phenomenon easier to study.

RESULTS

Rats learned the location of a food reward by observing a conspecific. Furthermore, the distance of the rats from the maze affected performance. Additionally, performance was affected by whether the performer made mistakes. This memory could persist for at least five minutes. Lastly, performance was not affected by observer-demonstrator familiarity COMPARISON WITH EXISTING METHODS: Previous rodent foraging studies typically exposed observers to the same behavior over many observation sessions. In this scenario, it is difficult to determine when and how an animal learns through observation. The current task delineates the period of observation in each session, allowing manipulations during the observation period.

CONCLUSIONS

The current paradigm allows for repeated examinations of observational learning and provides an alternative method for neurobiological studies of social learning.

Loss of Function of Phosphodiesterase 11A4 Shows that Recent and Remote Long-Term Memories Can Be Uncoupled

Systems consolidation is a process by which memories initially require the hippocampus for recent long-term memory (LTM) but then become increasingly independent of the hippocampus and more dependent on the cortex for remote LTM. Here, we study the role of phosphodiesterase 11A4 (PDE11A4) in systems consolidation. PDE11A4, which degrades cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), is preferentially expressed in neurons of CA1, the subiculum, and the adjacently connected amygdalohippocampal region. In male and female mice, deletion of PDE11A enhances remote LTM for social odor recognition and social transmission of food preference (STFP) despite eliminating or silencing recent LTM for those same social events. Measurement of a surrogate marker of neuronal activation (i.e., Arc mRNA) suggests the recent LTM deficits observed in Pde11 knockout mice correspond with decreased activation of ventral CA1 relative to wild-type littermates. In contrast, the enhanced remote LTM observed in Pde11a knockout mice corresponds with increased activation and altered functional connectivity of anterior cingulate cortex, frontal association cortex, parasubiculum, and the superficial layer of medial entorhinal cortex. The apparent increased neural activation observed in prefrontal cortex of Pde11a knockout mice during remote LTM retrieval may be related to an upregulation of the N-methyl-D-aspartate receptor subunits NR1 and NR2A. Viral restoration of PDE11A4 to vCA1 alone is sufficient to rescue both the LTM phenotypes and upregulation of NR1 exhibited by Pde11a knockout mice. Together, our findings suggest remote LTM can be decoupled from recent LTM, which may have relevance for cognitive deficits associated with aging, temporal lobe epilepsy, or transient global amnesia.

Murine genetic variance in muscarinic cholinergic receptor antagonism of acquisition and expression of sucrose-conditioned flavor preferences in three inbred mouse strains

Conditioned flavor preferences (CFP) are elicited by sucrose relative to saccharin in inbred mice with both the robustness of the preferences and sensitivity to pharmacological receptor antagonists sensitive to genetic variance. Dopamine, opioid and N-methyl-d-aspartate receptor antagonists differentially interfere with the acquisition (learning) and expression (maintenance) of sucrose-CFP in BALB/c and SWR inbred mice. Further, the muscarinic cholinergic receptor antagonist, scopolamine (SCOP) more potently reduces both sucrose and saccharin intake in BALB/c and C57BL/6 relative to SWR inbred mice. The present study examined whether SCOP altered the expression and acquisition of sucrose-CFP in BALB/c, C57BL/6 and SWR mice. In expression experiments, food-restricted mice alternately consumed a flavored (CS+, e.g., cherry, 5 sessions) 16% sucrose solution and a differently-flavored (CS-, e.g., grape, 5 sessions) 0.05% saccharin solution. Two-bottle CS choice tests with the two flavors mixed in 0.2% saccharin solutions occurred following vehicle or SCOP at doses of 1 or 5 mg/kg. SCOP significantly reduced the magnitude of the expression of sucrose-CFP in BALB/c, but not either C57BL/6 or SWR mice. In acquisition experiments, separate groups of BALB/c, C57BL/6 and SWR mice were treated prior to acquisition training sessions with vehicle or 2.5 or 5 mg/kg SCOP doses that was followed by six two-bottle CS choice tests without injections. SCOP dose-dependently reduced (1 mg/kg) and eliminated (2.5 mg/kg) the acquisition of sucrose-CFP in BALB/c mice, and reduced the magnitude of acquisition of sucrose-CFP in SWR mice. In contrast, neither SCOP dose affected the acquisition of sucrose-CFP in C57BL/6 mice. Thus, muscarinic cholinergic receptor signaling is essential for the learning of sucrose-CFP in BALB/c mice, to a lesser degree in SWR mice, but not in C57BL/6 mice. Murine genetic variance differentially modulates muscarinic cholinergic receptor control of sweet intake per se relative to learned conditioned flavor preferences of sweets.

Gut vagal sensory signaling regulates hippocampus function through multi-order pathways

The vagus nerve is the primary means of neural communication between the gastrointestinal (GI) tract and the brain. Vagally mediated GI signals activate the hippocampus (HPC), a brain region classically linked with memory function. However, the endogenous relevance of GI-derived vagal HPC communication is unknown. Here we utilize a saporin (SAP)-based lesioning procedure to reveal that selective GI vagal sensory/afferent ablation in rats impairs HPC-dependent episodic and spatial memory, effects associated with reduced HPC neurotrophic and neurogenesis markers. To determine the neural pathways connecting the gut to the HPC, we utilize monosynaptic and multisynaptic virus-based tracing methods to identify the medial septum as a relay connecting the medial nucleus tractus solitarius (where GI vagal afferents synapse) to dorsal HPC glutamatergic neurons. We conclude that endogenous GI-derived vagal sensory signaling promotes HPC-dependent memory function via a multi-order brainstem–septal pathway, thereby identifying a previously unknown role for the gut–brain axis in memory control.

Feeding-relevant vagal signaling occurs between the gastrointestinal tract and the brain, but it is unclear if this pathway influences cognitive processes. This study shows that endogenous gastrointestinal derived vagal sensory signaling promotes hippocampal-dependent memory function via a multi-order brainstem–septal pathway.

Hippocampus ghrelin receptor signaling promotes socially-mediated learned food preference

Social cues are potent regulators of feeding behavior, yet the neurobiological mechanisms through which social cues influence food intake are poorly understood. Here we investigate the hypothesis that the appetite-promoting gut-derived hormone, ghrelin, signals in the hippocampus to promote learned social aspects of feeding behavior. We utilized a procedure known as 'social transmission of food preference' (STFP) in which rats ('Observers') experience a social interaction with another rat ('Demonstrators') that recently consumed flavored/scented chow. STFP learning in Observer rats is indicated by a significant preference for the Demonstrator paired flavor of chow vs. a novel unpaired flavor of chow in a subsequent consumption choice test. Our results show that relative to vehicle treatment, ghrelin targeted to the ventral CA1 subregion of the hippocampus (vHP) enhanced STFP learning in rats. Additionally, STFP was impaired following peripheral injections of l-cysteine that reduce circulating ghrelin levels, suggesting that vHP ghrelin-mediated effects on STFP require peripheral ghrelin release. Finally, the endogenous relevance of vHP ghrelin receptor (GHSR-1A) signaling in STFP is supported by our data showing that STFP learning was eliminated following targeted viral vector RNA interference-mediated knockdown of vHP GHSR-1A mRNA. Control experiments indicate that vHP ghrelin-mediated STFP effects are not secondary to altered social exploration and food intake, nor to altered food preference learning based on nonsocial olfactory cues. Overall these data reveal a novel neurobiological system that promotes conditioned, social aspects of feeding behavior.

Murine genetic variance in muscarinic cholinergic receptor antagonism of sucrose and saccharin solution intakes in three inbred mouse strains

Nutritive (e.g., sucrose) and non-nutritive (e.g., saccharin) sweeteners stimulate intake in inbred mouse strains. BALB/c, SWR and C57BL/6 mice differ in the ability of dopamine (DA) D1 (SCH23390) and opioid (naltrexone) receptor antagonism to alter sucrose intake. Whereas SCH23390 comparably reduced cumulative sucrose intake in all three strains, naltrexone reduced cumulative sucrose intake maximally in C57/BL/6 mice, in intermediate fashion in BALB/c mice, but not in SWR mice. Whereas cumulative saccharin intake was reduced by DA D1 receptor antagonism in BALB/c and SWR mice, naltrexone was more potent in SWR relative to BALB/c mice. The present study first examined whether SCH23390 (50-1600nmol/kg) and naltrexone (0.01-5mg/kg) altered saccharin intake in C57BL/6 mice. Given that scopolamine (SCOP), a muscarinic cholinergic receptor antagonist, reduces sweet intake in outbred rats, a second experiment examined whether SCOP (0.1-10mg/kg) altered 0.2% saccharin and 10% sucrose intakes in BALB/c, SWR and C57BL/6 mice. Cumulative saccharin intake was significantly reduced by SCH23390 (200-1600nmol/kg; ID₄₀=175nmol/kg) and naltrexone (0.1-5mg/kg; ID₄₀>5mg/kg) in C57BL/6 mice. Cumulative sucrose intake was significantly reduced following SCOP in C57BL/6 (0.1-10mg/kg; ID₄₀=2.32mg/kg) and BALB/c (2.5-10mg/kg; ID₄₀=0.52mg/kg) mice. In contrast, SWR mice (ID₄₀=41.61mg/kg) only displayed transient (15min) reductions in sucrose intake following SCOP (2.5-10mg/kg). Cumulative saccharin intake was significantly reduced following SCOP in C57BL/6 and BALB/c mice (0.1-10mg/kg; ID₄₀<0.1mg/kg). In contrast, SWR mice (ID₄₀=2.28mg/kg) displayed smaller significant reductions in saccharin intake following SCOP (0.1-10mg/kg). These data indicate that although both nutritive and non-nutritive sweet intakes are governed by muscarinic cholinergic receptor signaling, this process is subject to murine genetic variance with greater sensitivity observed in C57BL/6 and BALB/c relative to SWR inbred mouse strains.

Expression of P301L-hTau in mouse MEC induces hippocampus-dependent memory deficit

Intracellular accumulation of abnormally phosphorylated tau in different types of neurons is a pathological characteristic of Alzheimer's disease (AD). While tau modification and associated neuronal loss and hypometabolism start in the entorhinal cortex (EC) in early AD patients, the mechanism by which mutant P301L hTau leads to dementia is not fully elucidated. Here, we studied the effects of P301L hTau transduction in the medial EC (MEC) of mice on tau phosphorylation and accumulation, and cognitive deficit. We found that the exogenous mutant tau protein was restricted in MEC without spreading to other brain regions at one month after transduction. Interestingly, expression of the mutant tau in MEC induces endogenous tau hyperphosphorylation and accumulation in hippocampus and cortex, and inhibits neuronal activity with attenuated PP-DG synapse plasticity, leading to hippocampus-dependent memory deficit with intact olfactory function. These findings suggest a novel neuropathological mechanism of early AD, which is initiated by tau accumulation in MEC, and demonstrate a tau pathological model of early stage AD.

Impaired cholinergic transmission in patients with Parkinson's disease and olfactory dysfunction

Olfactory dysfunction represents a frequent and disturbing non-motor manifestation of Parkinson's disease (PD). The pathophysiology of olfactory dysfunction in PD is still poorly understood. Experimental evidence suggests that olfactory impairment could be related to central cholinergic dysfunction. Short latency afferent inhibition (SAI) technique gives the opportunity to test an inhibitory cholinergic circuit in the human cerebral motor cortex. The objective of the study was to assess the cholinergic function, as measured by SAI, in PD patients with different degrees of olfactory dysfunction. We applied SAI technique in 31 patients with PD. These patients also underwent Olfactory Event-Related Potentials (OERPs) studies to objectively evaluate the olfactory system and a battery of neuropsychological tests to assess the cognitive functions. Absent OERPs indicated a severe olfactory dysfunction in 13 subjects. The presence of OERPs with an alteration in latency and/or amplitude can be considered as a borderline condition of slight alteration of smell and was found in other 15 patients. Only 3 patients showed normal OERPs. SAI was significantly reduced in the PD patients with absent OERPs compared with those with present but abnormal OERPs. Neuropsychological examination showed a mild cognitive impairment in 12 out of 13 PD patients with severe olfactory dysfunction, and in 3 out of the 15 patients with borderline olfactory dysfunction. SAI abnormalities and presence of severe olfactory impairment strongly support the hypothesis of cholinergic dysfunction in some patients with PD, who will probably develop a dementia. Longitudinal studies are required to verify whether SAI abnormalities in PD patients with olfactory dysfunction can predict a future severe cognitive decline.

Estrogen involvement in social behavior in rodents: Rapid and long-term actions

This article is part of a Special Issue ("Estradiol and cognition"). Estrogens have repeatedly been shown to influence a wide array of social behaviors, which in rodents are predominantly olfactory-mediated. Estrogens are involved in social behavior at multiple levels of processing, from the detection and integration of socially relevant olfactory information to more complex social behaviors, including social preferences, aggression and dominance, and learning and memory for social stimuli (e.g. social recognition and social learning). Three estrogen receptors (ERs), ERα, ERβ, and the G protein-coupled ER 1 (GPER1), differently affect these behaviors. Social recognition, territorial aggression, and sexual preferences and mate choice, all requiring the integration of socially related olfactory information, seem to primarily involve ERα, with ERβ playing a lesser, modulatory role. In contrast, social learning consistently responds differently to estrogen manipulations than other social behaviors. This suggests differential ER involvement in brain regions important for specific social behaviors, such as the ventromedial and medial preoptic nuclei of the hypothalamus in social preferences and aggression, the medial amygdala and hippocampus in social recognition, and the prefrontal cortex and hippocampus in social learning. While the long-term effects of ERα and ERβ on social behavior have been extensively investigated, our knowledge of the rapid, non-genomic, effects of estrogens is more limited and suggests that they may mediate some social behaviors (e.g. social learning) differently from long-term effects. Further research is required to compare ER involvement in regulating social behavior in male and female animals, and to further elucidate the roles of the more recently described G protein-coupled ERs, both the GPER1 and the Gq-mER.

D-cycloserine prevents relational memory deficits and suppression of long-term potentiation induced by scopolamine in the hippocampus

Previous research has demonstrated that systemic D-cycloserine (DCS), a partial agonist of the N-methyl-D-aspartate receptor (NMDAR), enhances memory processes in different learning paradigms and attenuates mnemonic deficits produced by diverse pharmacological manipulations. In the present study two experiments were conducted in rats to investigate whether DCS administered in the hippocampus may rescue relational memory deficits and improve deficient synaptic plasticity, both induced by an intracerebral injection of the muscarinic receptor antagonist scopolamine (SCOP). In experiment 1, we assessed whether DCS would prevent SCOP-induced amnesia in an olfactory learning paradigm requiring the integrity of the cholinergic system, the social transmission of food preference (STFP). The results showed that DCS (10 μg/site) injected into the ventral hippocampus (vHPC) before STFP acquisition compensated the 24-h retention deficit elicited by post-training intra-vHPC SCOP (40 μg/site), although it did not affect memory expression in non-SCOP treated rats. In experiment 2, we evaluated whether the perfusion of DCS in hippocampal slices may potentiate synaptic plasticity in CA1 synapses and thus recover SCOP-induced deficits in long-term potentiation (LTP). We found that DCS (50 µM and 100 µM) was able to rescue SCOP (100 µM)-induced LTP maintenance impairment, in agreement with the behavioral findings. Additionally, DCS alone (50 µM and 100 µM) enhanced field excitatory postsynaptic potentials prior to high frequency stimulation, although it did not significantly potentiate LTP. Our results suggest that positive modulation of the NMDAR, by activation of the glycine-binding site, may compensate relational memory impairments due to hippocampal muscarinic neurotransmission dysfunction possibly through enhancements in LTP maintenance.

Rapid oestrogenic regulation of social and nonsocial learning

Much research on oestrogens has focused on their long-term action, exerting behavioural effects within hours to days through gene transcription. Oestrogens also affect behaviour on a much shorter time scale. These rapid effects are assumed to occur through cell signalling and can elicit a behavioural effect as early as 15 min after treatment. These effects on behaviour have primarily been explored through the action of oestradiol at three well-known oestrogen receptors (ERs): ERα, ERβ and the more recently described G protein-coupled ER1 (GPER1). The rapid effects of oestradiol and ER agonists have been tested on both social and nonsocial learning paradigms. Social learning refers to a paradigm in which an animal acquires information and modifies its behaviour based on observation of another animal, commonly studied using the social transmission of food preferences paradigm. When administered shortly before testing, oestradiol rapidly improves social learning on this task, although no ER agonist has definitive, comparable improving effects. Some evidence points to GPER1, whereas ERα impairs, and ERβ activation has no effect on social learning. Conversely, ERα and GPER1 play a larger role than ERβ in the rapid improving effect of oestrogens on nonsocial learning, including social and object recognition. In addition, when administered immediately post-acquisition, oestrogens also rapidly improve memory consolidation in a variety of learning paradigms: object recognition, object placement, inhibitory avoidance and the Morris water maze, indicating that oestradiol affects the consolidation of multiple types of memory. Evidence suggests that these improvements are the result of oestrogens acting in the dorsal hippocampus where selective activation of all three ERs shows rapid improving effects on spatial learning comparable to oestradiol. However, the hippocampus is not necessary for rapid oestradiol improvements on social recognition. Although acute treatment with oestradiol enhances learning and memory on various social and nonsocial learning paradigms, the specific ERs play different roles in each type of learning. Future research should aim to further determine the roles of ERs with respect to the enhancing effects of oestradiol on learning and memory, and also determine where in the brain oestradiol acts to affect social and nonsocial learning.

D-cycloserine in prelimbic cortex reverses scopolamine-induced deficits in olfactory memory in rats

A significant interaction between N-methyl-D-aspartate (NMDA) and muscarinic receptors has been suggested in the modulation of learning and memory processes. The present study further investigates this issue and explores whether d-cycloserine (DCS), a partial agonist at the glycine binding site of the NMDA receptors that has been regarded as a cognitive enhancer, would reverse scopolamine (SCOP)-induced amnesia in two olfactory learning tasks when administered into the prelimbic cortex (PLC). Thus, in experiment 1, DCS (10 µg/site) was infused prior to acquisition of odor discrimination (ODT) and social transmission of food preference (STFP), which have been previously characterized as paradigms sensitive to PLC muscarinic blockade. Immediately after learning such tasks, SCOP was injected (20 µg/site) and the effects of both drugs (alone and combined) were tested in 24-h retention tests. To assess whether DCS effects may depend on the difficulty of the task, in the STFP the rats expressed their food preference either in a standard two-choice test (experiment 1) or a more challenging three-choice test (experiment 2). The results showed that bilateral intra-PLC infusions of SCOP markedly disrupted the ODT and STFP memory tests. Additionally, infusions of DCS alone into the PLC enhanced ODT but not STFP retention. However, the DCS treatment reversed SCOP-induced memory deficits in both tasks, and this effect seemed more apparent in ODT and 3-choice STFP. Such results support the interaction between the glutamatergic and the cholinergic systems in the PLC in such a way that positive modulation of the NMDA receptor/channel, through activation of the glycine binding site, may compensate dysfunction of muscarinic neurotransmission involved in stimulus-reward and relational learning tasks.

The role of medial prefrontal cortex in memory and decision making

Some have claimed that the medial prefrontal cortex (mPFC) mediates decision making. Others suggest mPFC is selectively involved in the retrieval of remote long-term memory. Yet others suggests mPFC supports memory and consolidation on time scales ranging from seconds to days. How can all these roles be reconciled? We propose that the function of the mPFC is to learn associations between context, locations, events, and corresponding adaptive responses, particularly emotional responses. Thus, the ubiquitous involvement of mPFC in both memory and decision making may be due to the fact that almost all such tasks entail the ability to recall the best action or emotional response to specific events in a particular place and time. An interaction between multiple memory systems may explain the changing importance of mPFC to different types of memories over time. In particular, mPFC likely relies on the hippocampus to support rapid learning and memory consolidation.

The role of medial prefrontal cortex in memory and decision making

Some have claimed that the medial prefrontal cortex (mPFC) mediates decision making. Others suggest mPFC is selectively involved in the retrieval of remote long-term memory. Yet others suggests mPFC supports memory and consolidation on time scales ranging from seconds to days. How can all these roles be reconciled? We propose that the function of the mPFC is to learn associations between context, locations, events, and corresponding adaptive responses, particularly emotional responses. Thus, the ubiquitous involvement of mPFC in both memory and decision making may be due to the fact that almost all such tasks entail the ability to recall the best action or emotional response to specific events in a particular place and time. An interaction between multiple memory systems may explain the changing importance of mPFC to different types of memories over time. In particular, mPFC likely relies on the hippocampus to support rapid learning and memory consolidation.

D-cycloserine in the basolateral amygdala prevents extinction and enhances reconsolidation of odor-reward associative learning in rats

It is well established that D-cycloserine (DCS), a partial agonist of the NMDA receptor glycine site, enhances learning and memory processes. Although the effects of DCS have been especially elucidated in the extinction and reconsolidation of aversive behavioral paradigms or drug-related behaviors, they have not been clearly determined in appetitive tasks using natural reinforcers. The current study examined the effects of pre-retrieval intra-basolateral amygdala (BLA) infusions of DCS on the extinction and reconsolidation of an appetitive odor discrimination task. Rats were trained to discriminate between three odors, one of which was associated with a palatable food reward, and, 20 min prior to extinction learning (experiment 1) or reactivation (experiment 2), they received bilateral intra-BLA infusions of DCS or vehicle. In experiment 1, DCS infusion reduced the rate of extinction learning, weakened extinction retention in a post-extinction test and enhanced reacquisition of the ODT task. In experiment 2, DCS improved subsequent memory expression in the reconsolidation test performed one day after the reactivation session. Such results indicate the involvement of BLA NMDA receptors in odor-food reward associative memory and suggest that DCS may potentiate the persistence or strength of the original memory trace.

Acetylcholine release in the hippocampus and prelimbic cortex during acquisition of a socially transmitted food preference

Interference with cholinergic functions in hippocampus and prefrontal cortex impairs learning and memory for social transmission of food preference, suggesting that acetylcholine (ACh) release in the two brain regions may be important for acquiring the food preference. This experiment examined release of ACh in the hippocampus and prefrontal cortex of rats during training for social transmission of food preference. After demonstrator rats ate a food with novel flavor and odor, a social transmission of food preference group of rats was allowed to interact with the demonstrators for 30 min, while in vivo microdialysis collected samples for later measurement of ACh release with HPLC methods. A social control group observed a demonstrator that had eaten food without novel flavor and odor. An odor control group was allowed to smell but not ingest food with novel odor. Rats in the social transmission but not control groups preferred the novel food on a trial 48 h later. ACh release in prefrontal cortex, with probes that primarily sampled prelimbic cortex, did not increase during acquisition of the social transmission of food preference, suggesting that training-initiated release of ACh in prelimbic cortex is not necessary for acquisition of the food preference. In contrast, ACh release in the hippocampus increased substantially (200%) upon exposure to a rat that had eaten the novel food. Release in the hippocampus increased significantly less (25%) upon exposure to a rat that had eaten normal food and did not increase significantly in the rats exposed to the novel odor; ACh release in the social transmission group was significantly greater than that of the either of the control groups. Thus, ACh release in the hippocampus but not prelimbic cortex distinguished well the social transmission vs. control conditions, suggesting that cholinergic mechanisms in the hippocampus but not prelimbic cortex are important for acquiring a socially transmitted food preference.

Odor identification deficits identify Parkinson's disease patients with poor cognitive performance

Olfactory dysfunction is a prodromal and prevalent nonmotor symptom of Parkinson's disease. Unlike olfactory dysfunction in Alzheimer's disease, it is believed to be unrelated to cognitive impairment. However, recent research has implicated cholinergic denervation in Parkinson's disease hyposmia and linked it to verbal memory. This research hypothesized that severe odor identification deficits may identify patients with Parkinson's disease at risk for cognitive impairment. The current study tested this hypothesis by comparing 24 functionally anosmic, nondemented patients with Parkinson's disease and 39 nonanosmic, nondemented patients with Parkinson's disease with 29 healthy control participants on composite scores of memory, processing speed, executive function, and language. The functionally anosmic group had significantly poorer visual and verbal memory than the nonanosmic group, which was indistinguishable from the control group. Furthermore, the functionally anosmic group had reduced processing speed compared with the nonanosmic patients with Parkinson's disease, who, in turn, were outperformed by the control group. On the composite language score, the score of the functionally anosmic group was significantly reduced compared with that of the control group, whereas the nonanosmic group scored in the medium range. The 2 patient groups did not differ on executive functioning. These findings demonstrate co-occurrence between reduced cognitive function and olfactory deficits in functionally anosmic patients with Parkinson's disease and support the notion of more severe cognitive deficits in this group.

Consolidation of an olfactory memory trace in the olfactory bulb is required for learning-induced survival of adult-born neurons and long-term memory

BACKGROUND

It has recently been proposed that adult-born neurons in the olfactory bulb, whose survival is modulated by learning, support long-term olfactory memory. However, the mechanism used to select which adult-born neurons following learning will participate in the long-term retention of olfactory information is unknown. We addressed this question by investigating the effect of bulbar consolidation of olfactory learning on memory and neurogenesis.

METHODOLOGY/PRINCIPAL FINDINGS

Initially, we used a behavioral ecological approach using adult mice to assess the impact of consolidation on neurogenesis. Using learning paradigms in which consolidation time was varied, we showed that a spaced (across days), but not a massed (within day), learning paradigm increased survival of adult-born neurons and allowed long-term retention of the task. Subsequently, we used a pharmacological approach to block consolidation in the olfactory bulb, consisting in intrabulbar infusion of the protein synthesis inhibitor anisomycin, and found impaired learning and no increase in neurogenesis, while basic olfactory processing and the basal rate of adult-born neuron survival remained unaffected. Taken together these data indicate that survival of adult-born neurons during learning depends on consolidation processes taking place in the olfactory bulb.

CONCLUSION/SIGNIFICANCE

We can thus propose a model in which consolidation processes in the olfactory bulb determine both survival of adult-born neurons and long-term olfactory memory. The finding that adult-born neuron survival during olfactory learning is governed by consolidation in the olfactory bulb strongly argues in favor of a role for bulbar adult-born neurons in supporting olfactory memory.

Olfactory dysfunction, central cholinergic integrity and cognitive impairment in Parkinson's disease

Olfactory dysfunction is common in subjects with Parkinson’s disease. The pathophysiology of such dysfunction, however, remains poorly understood. Neurodegeneration within central regions involved in odour perception may contribute to olfactory dysfunction in Parkinson’s disease. Central cholinergic deficits occur in Parkinson’s disease and cholinergic neurons innervate regions, such as the limbic archicortex, involved in odour perception. We investigated the relationship between performance on an odour identification task and forebrain cholinergic denervation in Parkinson’s disease subjects without dementia. Fifty-eight patients with Parkinson’s disease (mean Hoehn and Yahr stage 2.5 ± 0.5) without dementia (mean Mini-Mental State Examination, 29.0 ± 1.4) underwent a clinical assessment, [¹¹C]methyl-4-piperidinyl propionate acetylcholinesterase brain positron emission tomography and olfactory testing with the University of Pennsylvania Smell Identification Test. The diagnosis of Parkinson’s disease was confirmed by [¹¹C]dihydrotetrabenazine vesicular monoamine transporter type 2 positron emission tomography. We found that odour identification test scores correlated positively with acetylcholinesterase activity in the hippocampal formation (r = 0.56, P < 0.0001), amygdala (r = 0.50, P < 0.0001) and neocortex (r = 0.46, P = 0.0003). Striatal monoaminergic activity correlated positively with odour identification scores (r = 0.30, P < 0.05). Multiple regression analysis including limbic (hippocampal and amygdala) and neocortical acetylcholinesterase activity as well as striatal monoaminergic activity, using odour identification scores as the dependent variable, demonstrated a significant regressor effect for limbic acetylcholinesterase activity (F = 10.1, P < 0.0001), borderline for striatal monoaminergic activity (F = 1.6, P = 0.13), but not significant for cortical acetylcholinesterase activity (F = 0.3, P = 0.75). Odour identification scores correlated positively with scores on cognitive measures of episodic verbal learning (r = 0.30, P < 0.05). These findings indicate that cholinergic denervation of the limbic archicortex is a more robust determinant of hyposmia than nigrostriatal dopaminergic denervation in subjects with moderately severe Parkinson's disease. Greater deficits in odour identification may identify patients with Parkinson's disease at risk for clinically significant cognitive impairment.

Role of the hippocampus in goal-oriented tasks requiring retrieval of spatial versus non-spatial information

The role of the hippocampus in non-spatial memory has been issue of some controversy. To investigate the nature of dorsal hippocampus engagement in spatial and non-spatial memory we performed discrete excitotoxic lesions of this region before mice (C57/BL6) were trained in one of two tasks that required the animals to retrieve a hidden food reward. In the visuospatial task animals had to remember a particular spatial location, independent of odor cues. In contrast, in a non-spatial olfactory task animals had to remember a particular odor, independent of spatial location. The mice were trained in one of these tasks over a period of three days. We found that lesions restricted to the dorsal hippocampus affected performance only in the spatial task. In contrast, lesions that also encompassed a larger portion of the ventral hippocampus caused a moderate deficit in the olfactory task. These results are consistent with the role of the dorsal hippocampus in long-term spatial episodic memory, and support the involvement of larger portions of the hippocampus on the encoding of non-spatial olfactory representations.

Prefrontal cortex: role in acquisition of overlapping associations and transitive inference

"Transitive inference" refers to the ability to judge from memory the relationships between indirectly related items that compose a hierarchically organized series, and this capacity is considered a fundamental feature of relational memory. Here we explored the role of the prefrontal cortex in transitive inference by examining the performance of mice with selective damage to the medial prefrontal cortex. Damage to the infralimbic and prelimbic regions resulted in significant impairment in the acquisition of a series of overlapping odor discrimination problems, such that animals with prefrontal lesions required twice as many trials to learn compared to sham-operated controls. Following eventually successful acquisition, animals with medial prefrontal lesions were severely impaired on a transitive inference probe test, whereas they performed as well as controls on a test that involved a nontransitive judgment from a novel odor pairing. These results suggest that the prefrontal cortex is part of an integral hippocampal-cortical network essential for relational memory organization.