Interactions between medial prefrontal cortex and dorsomedial striatum are necessary for odor span capacity in rats: role of GluN2B-containing NMDA receptors

Nucleus accumbens core dopamine D2 receptors are required for performance of the odor span task in male rats

RATIONALE

The nucleus accumbens (NAc) core gates motivationally relevant behavioral action sequences through afferents from cortical and subcortical brain regions. While the role of the NAc core in reward and effort-based decision making is well established, its role in working memory (WM) processes is incompletely understood. The odor span task (OST) has been proposed as a measure of non-spatial working memory capacity (WMC) as it requires rodents to select a novel odor from an increasing number of familiar odors to obtain a food reward.

OBJECTIVE

To assess the role of the NAc core in the OST using (1) reversible chemical inactivation and (2) selective blockade of dopamine D1 and D2 receptors in the area.

METHODS

Well-trained male rats were tested on the OST following intra-NAc core infusions of muscimol/baclofen, the D1 receptor antagonist SCH-23390 (1 μg/hemisphere) and the D2 receptor antagonist eticlopride (1 μg/hemisphere). Behavioral measurements included the average odor span, maximum odor span, choice latency, searching vigor, and patterns of responding during foraging that may relate to impulsivity.

RESULTS

Chemical inactivation of the NAc core significantly decreased odor span relative to sham and vehicle conditions. Selective antagonism of D2, but not D1, receptors in the NAc core also produced deficits in odor span. We found that secondary behavioral measures of choice latency, searching vigor, and responding to the first odor stimulus encountered were largely unaffected by treatment.

CONCLUSIONS

These findings suggest that D2 receptors in the NAc core are required for OST performance.

Intrahippocampal injection of a selective blocker of NMDA receptors containing the GluN2B subunit, Ro25-6981, increases glutamate neurotransmission and induces antidepressant-like effects

Major depressive disorder (MDD) is a serious public health problem, as it is the most common psychiatric disorder worldwide. Antidepressant drugs increase adult hippocampal neurogenesis, which is required to induce some behavioral effects of antidepressants. Adult-born granule cells in the dentate gyrus (DG) and the glutamate receptors subunits 2 (GluN2B) subunit of N-methyl-D-aspartate (NMDA) ionotropic receptors play an important role in these effects. However, the precise neurochemical role of the GluN2B subunit of the NMDA receptor on adult-born GCs for antidepressant-like effects has yet to be elucidated. The present study aims to explore the contribution of the GluN2B-containing NMDA receptors in the ventral dentate gyrus (vDG) to the antidepressant drug treatment using a pharmacological approach. Thus, (αR)-(4-hydroxyphenyl)-(βS)-methyl-4-(phenylmethyl)-1-piperidinepropanol (Ro25-6981), a selective antagonist of the GluN2B subunit, was acutely administered locally into the ventral DG (vDG, 1 μg each side) following a chronic fluoxetine (18 mg/kg/day) treatment-known to increase adult hippocampal neurogenesis-in a mouse model of anxiety/depression. Responses in a neurogenesis-dependent task, the novelty suppressed feeding (NSF), and neurochemical consequences on extracellular glutamate and gamma-aminobutyric acid (GABA) levels in the vDG were measured. Here, we show a rapid-acting antidepressant-like effect of local Ro25-6981 administration in the NSF independent of fluoxetine treatment. Furthermore, we revealed a fluoxetine-independent increase in the glutamatergic transmission in the vDG. Our results suggest behavioral and neurochemical effects of GluN2B subunit independent of serotonin reuptake inhibition.

Olfactory Dysfunction in Schizophrenia: Evaluating Olfactory Abilities Across Species

Though understudied relative to perturbations in the auditory and visual domains, olfactory dysfunction is a common symptom of schizophrenia. Over the past two decades, the availability of standardized assessments to quantify human olfactory abilities, and enhance understanding of the neurophysiology supporting olfaction, has increased, enabling a more thorough characterization of these deficits. In contrast to other psychiatric conditions for which olfactory dysfunction has been observed (e.g., major depressive disorder, bipolar disorder, Alzheimer's disease), the impairments observed in schizophrenia are particularly global and profound. At this level, such deficits in olfactory abilities likely impact the enjoyment of food, detection of environmental hazards, and influence social relationships. More broadly, the study of olfactory phenotypes in schizophrenia presents new avenues for detection of those at-risk for the condition, identification of therapeutic targets for treatment development, and for the characterization of novel animal models relevant to schizophrenia and psychosis. This review will consider the olfactory performance of individuals with schizophrenia in domains for which standardized assessments are available (odor sensitivity, discrimination, identification, and memory). Paradigms available for assessing these abilities in rodents will also be discussed with the aim of facilitating translation. Thus, future studies will be able to include cross-species translation of mechanisms relevant to olfactory function and cognition, what has gone awry in the disease state, and test potential therapeutics.

Male and female impairments in odor span are observed in a rat model of PTSD

Posttraumatic stress disorder (PTSD) is associated with neural and behavioral alterations in response to trauma exposure, including working memory impairments. Rodent models of PTSD have not fully investigated chronic or reactive working memory deficits, despite clinical relevance. The present study uses footshock to induce a posttraumatic stress state in male and female rats and evaluates the effect of footshock and trauma-paired odor cues on working memory performance in the odor span task. Results demonstrate the emergence of chronic deficits in working memory among animals exposed to footshock by 3 wk after traumatic stress. The presentation of a trauma-paired odor cue was associated with further decrement in working memory performance for male animals. Furthermore, anxiety-like behaviors associated with the PTSD-like phenotype could predict the degree of working memory impairment in response to the trauma-paired odor cue. This study enhances validation of an existing rodent model of PTSD through replication of the clinical observations of working memory deficits associated with PTSD and provides novel insight into effects in female rodents. This will facilitate work to probe underlying mechanistic dysregulation of working memory following footshock trauma exposure and future development of novel treatment strategies.

The rodent medial prefrontal cortex and associated circuits in orchestrating adaptive behavior under variable demands

Emerging evidence implicates rodent medial prefrontal cortex (mPFC) in tasks requiring adaptation of behavior to changing information from external and internal sources. However, the computations within mPFC and subsequent outputs that determine behavior are incompletely understood. We review the involvement of mPFC subregions, and their projections to the striatum and amygdala in two broad types of tasks in rodents: 1) appetitive and aversive Pavlovian and operant conditioning tasks that engage mPFC-striatum and mPFC-amygdala circuits, and 2) foraging-based tasks that require decision making to optimize reward. We find support for region-specific function of the mPFC, with dorsal mPFC and its projections to the dorsomedial striatum supporting action control with higher cognitive demands, and ventral mPFC engagement in translating affective signals into behavior via discrete projections to the ventral striatum and amygdala. However, we also propose that defined mPFC subdivisions operate as a functional continuum rather than segregated functional units, with crosstalk that allows distinct subregion-specific inputs (e.g., internal, affective) to influence adaptive behavior supported by other subregions.

Altered Cortico-Subcortical Network After Adolescent Alcohol Exposure Mediates Behavioral Deficits in Flexible Decision-Making

Behavioral flexibility, the ability to modify behavior according to changing conditions, is essential to optimize decision-making. Deficits in behavioral flexibility that persist into adulthood are one consequence of adolescent alcohol exposure, and another is decreased functional connectivity in brain structures involved in decision-making; however, a link between these two outcomes has not been established. We assessed effects of adolescent alcohol and sex on both Pavlovian and instrumental behaviors and resting-state functional connectivity MRI in adult animals to determine associations between behavioral flexibility and resting-state functional connectivity. Alcohol exposure impaired attentional set reversals and decreased functional connectivity among cortical and subcortical regions-of-interest that underlie flexible behavior. Moreover, mediation analyses indicated that adolescent alcohol-induced reductions in functional connectivity within a subnetwork of affected brain regions statistically mediated errors committed during reversal learning. These results provide a novel link between persistent reductions in brain functional connectivity and deficits in behavioral flexibility resulting from adolescent alcohol exposure.

Olfactory Information Storage Engages Subcortical and Cortical Brain Regions That Support Valence Determination

The olfactory bulb (OB) delivers sensory information to the piriform cortex (PC) and other components of the olfactory system. OB-PC synapses have been reported to express short-lasting forms of synaptic plasticity, whereas long-term potentiation (LTP) of the anterior PC (aPC) occurs predominantly by activating inputs from the prefrontal cortex. This suggests that brain regions outside the olfactory system may contribute to olfactory information processing and storage. Here, we compared functional magnetic resonance imaging BOLD responses triggered during 20 or 100 Hz stimulation of the OB. We detected BOLD signal increases in the anterior olfactory nucleus (AON), PC and entorhinal cortex, nucleus accumbens, dorsal striatum, ventral diagonal band of Broca, prelimbic–infralimbic cortex (PrL-IL), dorsal medial prefrontal cortex, and basolateral amygdala. Significantly stronger BOLD responses occurred in the PrL-IL, PC, and AON during 100 Hz compared with 20 Hz OB stimulation. LTP in the aPC was concomitantly induced by 100 Hz stimulation. Furthermore, 100 Hz stimulation triggered significant nuclear immediate early gene expression in aPC, AON, and PrL-IL. The involvement of the PrL-IL in this process is consistent with its putative involvement in modulating behavioral responses to odor experience. Furthermore, these results indicate that OB-mediated information storage by the aPC is embedded in a connectome that supports valence evaluation.

Transcranial direct current stimulation relieves visceral hypersensitivity via normalizing GluN2B expression and neural activity in anterior cingulate cortex

Irritable bowel syndrome (IBS) is one of the most common challenging diseases for clinical treatment. The aim of this study is to investigate whether transcranial direct current stimulation (tDCS) has analgesic effect on visceral hypersensitivity (VH) in an animal model of IBS as well as the underlying mechanism. As the activation of GluN2B in anterior cingulate cortex (ACC) takes part in VH, we examined whether and how GluN2B in ACC takes part in the effect of tDCS. Neonatal maternal deprivation (NMD), a valuable experimental model to study the IBS pathophysiology, was used to induce visceral hypersensitivity of rats. We quantified VH as colorectal distention threshold and performed patch-clamp recordings of ACC neurons. The expression of GluN2B were determined by RT-qPCR and Western blotting. The GluN2B antagonist Ro 25-6981 was microinjected into the rostral and caudal ACC. tDCS was performed for 7 consecutive days. It was found that NMD decreased expression of GluN2B, which could be obviously reversed by tDCS. Injection of Ro 25-6981 into rostral and caudal ACC of normal rats induced VH and also reversed the analgesic effect of tDCS. Our data sheds light on the nonpharmacological therapy for chronic VH in pathological states such as IBS.NEW & NOTEWORTHY Irritable bowel syndrome (IBS) is a gastrointestinal disease characterized by visceral hypersensitivity. This study showed a decrease of GluN2B expression and neural activity in ACC of IBS-model rats, which could be obviously reversed by tDCS. In addition, blockade of GluN2B in rostral and caudal ACC induced VH of normal rats. Furthermore, analgesic effect of tDCS on NMD rats was reversed by GluN2B antagonist.

Effects of caloric restriction on monoaminergic neurotransmission, peripheral hormones, and olfactory memory in aged rats

Aging is associated with a reduced ability to identify and discriminate scents, and olfactory dysfunction has been linked to preclinical stages of neurodegenerative diseases in humans. Moreover, emerging evidence suggests that smell-driven behaviors are regulated by hormones like insulin or leptin, and by metabolic parameters like glucose, which in turn may influence monoaminergic neurotransmission in brain areas related to cognition. Several studies have suggested that dietary interventions like caloric restriction (CR) can mitigate the age-induced decline in memory by modifying metabolic parameters and brain monoaminergic levels. The present study explored the effects of CR on age-dependent olfactory memory deficits, as well as their relationship with peripheral leptin, insulin and glucose levels, and brain monoamines. To this end, aged rats (24-months-old) fed on a CR diet or with ad libitum access to food, and adult rats (3-4 months), were trained in an odor discrimination task (ODT). The peripheral plasma levels of insulin, leptin, and glucose, and of monoamines and metabolites/precursors in brain areas related to olfactory learning and memory processes, such as the striatum and frontal cortex (FC), were determined. The data obtained indicated that CR attenuated the age-dependent decline in olfactory sensitivity in old animals fed ad libitum, which was correlated with the performance in ODT retention trial, as well as with leptin plasma levels. CR enhanced dopamine levels in the striatum, while it attenuated the age-related decline in serotonin levels in the striatum and FC. Such findings support a positive effect of CR on age-dependent olfactory sensitivity decline and dysfunctions in brain monoamine levels.

Successive incrementing non-matching-to-samples in rats: An automated version of the odor span task

The odor span task is a procedure frequently used to study remembering of multiple stimuli in rodents. A large arena is used and odor stimuli are presented using scented cups. Selection of each odor is reinforced when first presented, but not on subsequent presentations; correct selections depend on remembering which stimuli were previously presented. The use of an arena setting with manual stimulus presentation makes the odor span task labor-intensive and limits experimental control; thus, an automated version of the task would be of value. The present study used an operant chamber equipped with an olfactometer and trained rats using successive conditional discrimination procedures under an incrementing non-matching-to-samples contingency. High rates of responding developed to odor stimuli when they were session-novel with low rates of responding to subsequent presentations of that odor. Additional experiments assessed variations of the procedure to determine the role of the frequency of odor presentation and the retention interval separating sample and comparison. Discrimination was impaired with long retention intervals suggesting the importance of this variable. These findings confirmed that rats differentiate between stimuli that are session-novel and those previously encountered and support the use of an automated procedure as an alternative to the odor span task.

Type 3 adenylyl cyclase in the main olfactory epithelium participates in depression-like and anxiety-like behaviours

BACKGROUND

Deficiency of olfaction is thought to be associated with depression, and type 3 adenylyl cyclase (AC₃) genetic knockout and forebrain knockout mice show depression-like behaviours. AC₃ is expressed in the main olfactory epithelium (MOE) and hippocampus, which plays an important role in olfactory signal transduction. However, it is unclear whether AC₃ in the MOE also plays a role in the pathogenesis of depression. Thus, we aimed to study the relationship between AC₃ in the MOE and the pathogenesis of depression.

METHODS

We obtained anosmic mice by intranasal perfusion of zinc sulphate (ZnSO₄) (ZnSO₄ mice), and distinctively knocked down AC₃ in the MOE (AC_3KD/MOE mice) by CRISPR/cas9 technology. Behavioural tests related to depression and anxiety were employed to evaluate the depression- and anxiety-like behaviours of mice. The mRNA and protein expressions of tyrosine hydroxylase (TH), dopamine receptors (Drds), and N-Methyl D-aspartate receptor subunit 2B (GluN2B) in the hippocampus of mice were investigated by qPCR and western blotting to explore the mechanism of depression and anxiety caused by AC₃ in the MOE, preliminarily.

RESULTS

Compared with NaCl mice, ZnSO₄ mice exhibited depression-like behaviours in tail suspension tests (TST), forced swimming tests, and social (FST) interaction tests (SIT), but showed no anxiety-like behaviours in anxiety-related behavioural tests. The mRNA and protein expressions of Drd3 and GluN2B in the hippocampus of ZnSO₄ mice were significantly downregulated. Compared with the negative control mice (NC mice), AC_{3KD / MOE} mice showed depression-like behaviours in TST, FST, and SIT tests, anxiety-like behaviours in light/dark transition test, elevated-plus maze test, and novelty-suppressed feeding test. The protein expressions of Drd3, TH, and GluN2B were significantly downregulated in the hippocampus.

LIMITATIONS

We did not further demonstrate that AC₃ in the MOE causes depression through the dopaminergic nervous system with dopamine or dopamine receptor agonists.

CONCLUSIONS

Our data demonstrate that intranasal infusion of ZnSO₄ can cause depression-like behaviours and has no effect on anxiety-like behaviours. Specific knockdown of AC₃ in the MOE can cause depression-like and anxiety-like behaviours. The behavioural changes caused by intranasal ZnSO₄ and specific knockdown of AC₃ in the MOE can be related to the significant downregulation of dopaminergic system and GluN2B expressions in the hippocampus of mice.

Roles of the medial prefrontal cortex, mediodorsal thalamus, and their combined circuit for performance of the odor span task in rats: analysis of memory capacity and foraging behavior

Working memory (WM), the capacity for short-term storage of small quantities of information for immediate use, is thought to depend on activity within the prefrontal cortex. Recent evidence indicates that the prefrontal neuronal activity supporting WM is driven by thalamocortical connections arising in mediodorsal thalamus (mdThal). However, the role of these connections has not been studied using olfactory stimuli leaving open the question of whether this circuit extends to all sensory modalities. Additionally, manipulations of the mdThal in olfactory memory tasks have yielded mixed results. In the present experiment, we investigated the role of connections between the rat medial prefrontal cortex (mPFC) and mdThal in the odor span task (OST) using a pharmacological contralateral disconnection technique. Inactivation of either the mPFC or mdThal alone both significantly impaired memory performance in the OST, replicating previous findings with the mPFC and confirming that the mdThal plays an essential role in intact OST performance. Contralateral disconnection of the two structures impaired OST performance in support of the idea that the OST relies on mPFC-mdThal connections, but ipsilateral control infusions also impaired performance, complicating this interpretation. We also performed a detailed analysis of rats’ errors and foraging behavior and found a dissociation between mPFC and mdThal inactivation conditions. Inactivation of the mdThal and mPFC caused a significant reduction in the number of approaches rats made per odor, whereas only mdThal inactivation or mPFC-mdThal disconnection caused significant increases in choice latency. Our results confirm that the mdThal is necessary for performance of the OST and that it may critically interact with the mPFC to mediate OST performance. Additionally, we have provided evidence that the mPFC and mdThal play dissociable roles in mediating foraging behavior.

Effects of NMDA antagonist dizocilpine (MK-801) are modulated by the number of distractor stimuli in the rodent odor span task of working memory

The rodent odor span task (OST) uses an incrementing non-matching to sample procedure in which a series of odors is presented and selection of the session-novel odor is reinforced. An OST is frequently used to test the effects of neurobiological variables on memory capacity as the number of odors to remember increases during the course of the session. In this regard, one important finding has been that NMDA receptor antagonists selectively impair OST performance at doses that spare accuracy on control tasks. However, in many versions of the odor span task the number of stimuli to remember is confounded with the number of distractor odors presented to the rat on each trial. The present study compared the effects of the NMDA antagonist dizocilpine when the number of choices was held constant at two (one novel odor-S+ and one previously presented distractor odor-S-) and when the number of choice stimuli was permitted to increase up to 10 (one S+ and 9 S-). Dizocilpine impaired OST accuracy at doses that had no effect on a reference memory control task in both 2-choice and 10-choice conditions; however, the dose-response function was shifted to the left in the 10-choice tests. The impairments produced by dizocilpine were exacerbated as the memory load increased in both 2- and 10-choice conditions. These findings support the hypothesis that NMDA antagonism reduces the number of stimuli that rats can remember accurately, but the interaction between the effective DZP dose and the number of distractors shows that drug effects on OST performances may involve attentional factors in addition to memory capacity. The findings also demonstrate that variations in number of OST distractors can be used to alter sensitivity of the task.

The Rat Medial Prefrontal Cortex Exhibits Flexible Neural Activity States during the Performance of an Odor Span Task

Medial prefrontal cortex (mPFC) activity is fundamental for working memory (WM), attention, and behavioral inhibition; however, a comprehensive understanding of the neural computations underlying these processes is still forthcoming. Toward this goal, neural recordings were obtained from the mPFC of awake, behaving rats performing an odor span task of WM capacity. Neural populations were observed to encode distinct task epochs and the transitions between epochs were accompanied by abrupt shifts in neural activity patterns. Putative pyramidal neuron activity increased earlier in the delay for sessions where rats achieved higher spans. Furthermore, increased putative interneuron activity was only observed at the termination of the delay thus indicating that local processing in inhibitory networks was a unique feature to initiate foraging. During foraging, changes in neural activity patterns associated with the approach to a novel odor, but not familiar odors, were robust. Collectively, these data suggest that distinct mPFC activity states underlie the delay, foraging, and reward epochs of the odor span task. Transitions between these states likely enables adaptive behavior in dynamic environments that place strong demands on the substrates of working memory.

Cognitive Phenotypes for Biomarker Identification in Mental Illness: Forward and Reverse Translation

Psychiatric illness has been acknowledged for as long as people were able to describe behavioral abnormalities in the general population. In modern times, these descriptions have been codified and continuously updated into manuals by which clinicians can diagnose patients. None of these diagnostic manuals have attempted to tie abnormalities to neural dysfunction however, nor do they necessitate the quantification of cognitive function despite common knowledge of its ties to functional outcome. In fact, in recent years the National Institute of Mental Health released a novel transdiagnostic classification, the Research Domain Criteria (RDoC), which utilizes quantifiable behavioral abnormalities linked to neurophysiological processes. This reclassification highlights the utility of RDoC constructs as potential cognitive biomarkers of disease state. In addition, with RDoC and cognitive biomarkers, the onus of researchers utilizing animal models no longer necessitates the recreation of an entire disease state, but distinct processes. Here, we describe the utilization of constructs from the RDoC initiative to forward animal research on these cognitive and behavioral processes, agnostic of disease. By linking neural processes to these constructs, identifying putative abnormalities in diseased patients, more targeted therapeutics can be developed.