Infralimbic activity during REM sleep facilitates fear extinction memory

Rapid eye movement (REM) sleep is known to facilitate fear extinction and play a protective role against fearful memories.1,2 Consequently, disruption of REM sleep after a traumatic event may increase the risk for developing PTSD.3,4 However, the underlying mechanisms by which REM sleep promotes extinction of aversive memories remain largely unknown. The infralimbic cortex (IL) is a key brain structure for the consolidation of extinction memory.5 Using calcium imaging, we found in mice that most IL pyramidal neurons are intensively activated during REM sleep. Optogenetically suppressing the IL specifically during REM sleep within a 4-h window after auditory-cued fear conditioning impaired extinction memory consolidation. In contrast, REM-specific IL inhibition after extinction learning did not affect the extinction memory. Whole-cell patch-clamp recordings demonstrated that inactivating IL neurons during REM sleep depresses their excitability. Together, our findings suggest that REM sleep after fear conditioning facilitates fear extinction by enhancing IL excitability and highlight the importance of REM sleep in the aftermath of traumatic events for protecting against traumatic memories.

Distinct engrams control fear and extinction memory

Memories are stored in engram cells, which are necessary and sufficient for memory recall. Recalling a memory might undergo reconsolidation or extinction. It has been suggested that the original memory engram is reactivated during reconsolidation so that memory can be updated. Conversely, during extinction training, a new memory is formed that suppresses the original engram. Nonetheless, it is unknown whether extinction creates a new engram or modifies the original fear engram. In this study, we utilized the Daun02 procedure, which uses c-Fos-lacZ rats to induce apoptosis of strongly activated neurons and examine whether a new memory trace emerges as a result of a short or long reactivation, or if these processes rely on modifications within the original engram located in the basolateral amygdala (BLA) and infralimbic (IL) cortex. By eliminating neurons activated during consolidation and reactivation, we observed significant impacts on fear memory, highlighting the importance of the BLA engram in these processes. Although we were unable to show any impact when removing the neurons activated after the test of a previously extinguished memory in the BLA, disrupting the IL extinction engram reactivated the aversive memory that was suppressed by the extinction memory. Thus, we demonstrated that the IL cortex plays a crucial role in the network involved in extinction, and disrupting this specific node alone is sufficient to impair extinction behavior. Additionally, our findings indicate that extinction memories rely on the formation of a new memory, supporting the theory that extinction memories rely on the formation of a new memory, whereas the reconsolidation process reactivates the same original memory trace.

The mouse dorsal peduncular cortex encodes fear memory

The rodent medial prefrontal cortex (mPFC) is functionally organized across the dorsoventral axis, where dorsal and ventral subregions promote and suppress fear, respectively. As the ventral-most subregion, the dorsal peduncular cortex (DP) is hypothesized to function in fear suppression. However, this role has not been explicitly tested. Here, we demonstrate that the DP paradoxically functions as a fear-encoding brain region and plays a minimal role in fear suppression. By using multimodal analyses, we demonstrate that DP neurons exhibit fear-learning-related plasticity and acquire cue-associated activity across learning and memory retrieval and that DP neurons activated by fear memory acquisition are preferentially reactivated upon fear memory retrieval. Further, optogenetic activation and silencing of DP fear-related neural ensembles drive the promotion and suppression of freezing, respectively. Overall, our results suggest that the DP plays a role in fear memory encoding. Moreover, our findings redefine our understanding of the functional organization of the rodent mPFC.

NrCAM-deficient mice exposed to chronic stress exhibit disrupted latent inhibition, a hallmark of schizophrenia

The neuronal cell adhesion molecule (NrCAM) is widely expressed and has important physiological functions in the nervous system across the lifespan, from axonal growth and guidance to spine and synaptic pruning, to organization of proteins at the nodes of Ranvier. NrCAM lies at the core of a functional protein network where multiple targets (including NrCAM itself) have been associated with schizophrenia. Here we investigated the effects of chronic unpredictable stress on latent inhibition, a measure of selective attention and learning which shows alterations in schizophrenia, in NrCAM knockout (KO) mice and their wild-type littermate controls (WT). Under baseline experimental conditions both NrCAM KO and WT mice expressed robust latent inhibition (p = 0.001). However, following chronic unpredictable stress, WT mice (p = 0.002), but not NrCAM KO mice (F < 1), expressed latent inhibition. Analyses of neuronal activation (c-Fos positive counts) in key brain regions relevant to latent inhibition indicated four types of effects: a single hit by genotype in IL cortex (p = 0.0001), a single hit by stress in Acb-shell (p = 0.031), a dual hit stress x genotype in mOFC (p = 0.008), vOFC (p = 0.020), and Acb-core (p = 0.032), and no effect in PrL cortex (p > 0.141). These results indicating a pattern of differential effects of genotype and stress support a complex stress × genotype interaction model and a role for NrCAM in stress-induced pathological behaviors relevant to schizophrenia and other psychiatric disorders.

Transcranial Direct Current Stimulation (tDCS) Ameliorates Stress-Induced Sleep Disruption via Activating Infralimbic-Ventrolateral Preoptic Projections

Transcranial direct current stimulation (tDCS) is acknowledged for its non-invasive modulation of neuronal activity in psychiatric disorders. However, its application in insomnia research yields varied outcomes depending on different tDCS types and patient conditions. Our primary objective is to elucidate its efficiency and uncover the underlying mechanisms in insomnia treatment. We hypothesized that anodal prefrontal cortex stimulation activates glutamatergic projections from the infralimbic cortex (IL) to the ventrolateral preoptic area (VLPO) to promote sleep. After administering 0.06 mA of electrical currents for 8 min, our results indicate significant non-rapid eye movement (NREM) enhancement in naïve mice within the initial 3 h post-stimulation, persisting up to 16-24 h. In the insomnia group, tDCS enhanced NREM sleep bout numbers during acute stress response and improved NREM and REM sleep duration in subsequent acute insomnia. Sleep quality, assessed through NREM delta powers, remains unaffected. Interference of the IL-VLPO pathway, utilizing designer receptors exclusively activated by designer drugs (DREADDs) with the cre-DIO system, partially blocked tDCS's sleep improvement in stress-induced insomnia. This study elucidated that the activation of the IL-VLPO pathway mediates tDCS's effect on stress-induced insomnia. These findings support the understanding of tDCS effects on sleep disturbances, providing valuable insights for future research and clinical applications in sleep therapy.

Prefrontal cortex melanocortin 4 receptors (MC4R) mediate food intake behavior in male mice

BACKGROUND

Melanocortin 4 receptor (MC4R) activity in the hypothalamus is crucial for regulation of metabolism and food intake. The peptide ligands for the MC4R are associated with feeding, energy expenditure, and also with complex behaviors that orchestrate energy intake and expenditure, but the downstream neuroanatomical and neurochemical targets associated with these behaviors are elusive. In addition to strong expression in the hypothalamus, the MC4R is highly expressed in the medial prefrontal cortex, a region involved in executive function and decision-making.

METHODS

Using viral techniques in genetically modified male mice combined with molecular techniques, we identify and define the effects on feeding behavior of a novel population of MC4R expressing neurons in the infralimbic (IL) region of the cortex.

RESULTS

Here, we describe a novel population of MC4R-expressing neurons in the IL of the mouse prefrontal cortex that are glutamatergic, receive input from melanocortinergic neurons, and project to multiple regions that coordinate appetitive responses to food-related stimuli. The neurons are stimulated by application of MC4R-specific peptidergic agonist, THIQ. Deletion of MC4R from the IL neurons causes increased food intake and body weight gain and impaired executive function in simple food-related behavior tasks.

CONCLUSION

Together, these data suggest that MC4R neurons of the IL play a critical role in the regulation of food intake in male mice.

Synaptic and transcriptomic features of cortical and amygdala pyramidal neurons predict inefficient fear extinction

Fear-related disorders arise from inefficient fear extinction and have immeasurable social and economic costs. Here, we characterize mouse phenotypes that spontaneously show fear-independent behavioral traits predicting adaptive or maladaptive fear extinction. We find that, already before fear conditioning, specific morphological, electrophysiological, and transcriptomic patterns of cortical and amygdala pyramidal neurons predispose to fear-related disorders. Finally, by using an optogenetic approach, we show the possibility to rescue inefficient fear extinction by activating infralimbic pyramidal neurons and to impair fear extinction by activating prelimbic pyramidal neurons.

Ventral Hippocampal Input to Infralimbic Cortex Is Necessary for the Therapeutic-Like Effects of Extinction in Stressed Rats

BACKGROUND

Posttraumatic stress disorder is characterized by deficits in cognitive flexibility related to dysfunction of the medial prefrontal cortex (mPFC). Exposure therapy can effectively reverse these deficits. Fear extinction in rodents bears similarity to exposure therapy. Extinction reverses chronic stress-induced deficits in cognitive flexibility on the attentional set-shifting test (AST), an mPFC-mediated process. This therapeutic effect requires activity of pyramidal neurons and brain derived neurotrophic factor (BDNF) signaling in infralimbic cortex (IL). However, the circuit mechanisms governing BDNF-mediated plasticity initiated by extinction in IL are unknown. The ventral hippocampus (vHipp) plays a role in regulating IL activity during extinction, and plasticity in vHipp is necessary for extinction memory consolidation. Therefore, we investigated the role of vHipp input to IL in the effects of extinction in reversing stress-induced cognitive deficits.

METHODS

vHipp input to IL was silenced using a Gi-Designer Receptors Exclusively Activated by Designer Drugs (DREADD) via local infusion of clozapine-N-oxide (CNO) into IL before extinction. A day later, rats were tested on AST. In a separate experiment, we tested whether vHipp input to the IL induces BDNF signaling to exert therapeutic effects. We activated the vHipp using a Gq-DREADD, and injected an anti-BDNF neutralizing antibody into IL. Rats were tested on the AST 24 hours later.

RESULTS

Silencing the vHipp input to IL prevented the beneficial effects of extinction in reversing stress-induced cognitive deficits. Activating vHipp input to IL in the absence of extinction was sufficient to reverse stress-induced deficits in set-shifting. The beneficial effects were blocked by local infusion of a neutralizing anti-BDNF antibody into IL.

CONCLUSIONS

vHipp-driven BDNF signaling in IL is critical for extinction to counteract the deleterious cognitive effects of chronic stress.

Prefrontal representation of affective stimuli: importance of stress, sex, and context

Stress-related disorders such as depression and anxiety exhibit sex differences in prevalence and negatively impact both mental and physical health. Affective illness is also frequently accompanied by changes in ventromedial prefrontal cortical (vmPFC) function. However, the neurobiology that underlies sex-specific cortical processing of affective stimuli is poorly understood. Although rodent studies have investigated the prefrontal impact of chronic stress, postmortem studies have focused largely on males and yielded mixed results. Therefore, genetically defined population recordings in behaving animals of both sexes were used to test the hypothesis that chronic variable stress (CVS) impairs the neural processing of affective stimuli in the rodent infralimbic region. Here, we targeted expression of a calcium indicator, GCaMP6s, to infralimbic pyramidal cells. In males, CVS reduced infralimbic responses to social interaction and restraint stress but increased responses to novel objects and food reward. In contrast, females did not have CVS-induced changes in infralimbic activity, which was partially dependent on the ovarian status. These results indicate that both male and female vmPFC cells encode social, stress, and reward stimuli. However, chronic stress effects are sex-dependent and behavior-specific. Ultimately, these findings extend the understanding of chronic stress-induced prefrontal dysfunction and indicate that sex is a critical factor for cortical processing of affective stimuli.

Sex bias in social deficits, neural circuits and nutrient demand in Cttnbp2 autism models

Autism spectrum disorders caused by both genetic and environmental factors are strongly male-biased neuropsychiatric conditions. However, the mechanism underlying the sex bias of autism spectrum disorders remains elusive. Here, we use a mouse model in which the autism-linked gene Cttnbp2 is mutated to explore the potential mechanism underlying the autism sex bias. Autism-like features of Cttnbp2 mutant mice were assessed via behavioural assays. C-FOS staining identified sex-biased brain regions critical to social interaction, with their roles and connectivity then validated by chemogenetic manipulation. Proteomic and bioinformatic analyses established sex-biased molecular deficits at synapses, prompting our hypothesis that male-biased nutrient demand magnifies Cttnbp2 deficiency. Accordingly, intakes of branched-chain amino acids (BCAA) and zinc were experimentally altered to assess their effect on autism-like behaviours. Both deletion and autism-linked mutation of Cttnbp2 result in male-biased social deficits. Seven brain regions, including the infralimbic area of the medial prefrontal cortex (ILA), exhibit reduced neural activity in male mutant mice but not in females upon social stimulation. ILA activation by chemogenetic manipulation is sufficient to activate four of those brain regions susceptible to Cttnbp2 deficiency and consequently to ameliorate social deficits in male mice, implying an ILA-regulated neural circuit is critical to male-biased social deficits. Proteomics analysis reveals male-specific downregulated proteins (including SHANK2 and PSD-95, two synaptic zinc-binding proteins) and female-specific upregulated proteins (including RRAGC) linked to neuropsychiatric disorders, which are likely relevant to male-biased deficits and a female protective effect observed in Cttnbp2 mutant mice. Notably, RRAGC is an upstream regulator of mTOR that senses BCAA, suggesting that mTOR exerts a beneficial effect on females. Indeed, increased BCAA intake activates the mTOR pathway and rescues neuronal responses and social behaviours of male Cttnbp2 mutant mice. Moreover, mutant males exhibit greatly increased zinc demand to display normal social behaviours. Mice carrying an autism-linked Cttnbp2 mutation exhibit male-biased social deficits linked to specific brain regions, differential synaptic proteomes and higher demand for BCAA and zinc. We postulate that lower demand for zinc and BCAA are relevant to the female protective effect. Our study reveals a mechanism underlying sex-biased social defects and also suggests a potential therapeutic approach for autism spectrum disorders.

Competing contextual processes rely on the infralimbic and prelimbic medial prefrontal cortices in the rat

Ambiguous relationships between events may be established using interference procedures such as latent inhibition, extinction or counterconditioning. Under these conditions, the retrieval of individual associations between a stimulus and outcome is affected by contextual cues. To examine the roles of the dorsal (prelimbic) and ventral (infralimbic) medial prefrontal cortex in the contextual modulation of such associations, we investigated the context specificity of latent inhibition. Male Lister hooded rats were pre-exposed to two separate stimuli, one in each of two distinct contexts. Both stimuli were then paired with the delivery of mild foot-shock in the same one of these contexts. Finally, the strength of the resultant conditioned emotional response (CER) to each stimulus was assessed in each context. For the sham-operated control rats, the CER was attenuated for each stimulus when it was tested in the context in which it had been pre-exposed. Rats who had received lesions to the infralimbic cortex showed this effect only in the conditioning context, whereas rats with lesions to the prelimbic cortex showed the effect only in the context in which conditioning had not taken place. These findings indicate that infralimbic and prelimbic cortices play distinct, and competing, roles in the contextual modulation of initial and later learning.

Differential Modulation of Dorsal Raphe Serotonergic Activity in Rat Brain by the Infralimbic and Prelimbic Cortices

The reciprocal connectivity between the medial prefrontal cortex (mPFC) and the dorsal raphe nucleus (DR) is involved in mood control and resilience to stress. The infralimbic subdivision (IL) of the mPFC is the rodent equivalent of the ventral anterior cingulate cortex, which is intimately related to the pathophysiology/treatment of major depressive disorder (MDD). Boosting excitatory neurotransmission in the IL-but not in the prelimbic cortex, PrL-evokes depressive-like or antidepressant-like behaviors in rodents, which are associated with changes in serotonergic (5-HT) neurotransmission. We therefore examined the control of 5-HT activity by both of the mPFC subdivisions in anesthetized rats. The electrical stimulation of IL and PrL at 0.9 Hz comparably inhibited 5-HT neurons (53% vs. 48%, respectively). However, stimulation at higher frequencies (10-20 Hz) revealed a greater proportion of 5-HT neurons sensitive to IL than to PrL stimulation (86% vs. 59%, at 20 Hz, respectively), together with a differential involvement of GABA_A (but not 5-HT_1A) receptors. Likewise, electrical and optogenetic stimulation of IL and PrL enhanced 5-HT release in DR in a frequency-dependent manner, with greater elevations after IL stimulation at 20 Hz. Hence, IL and PrL differentially control serotonergic activity, with an apparent superior role of IL, an observation that may help to clarify the brain circuits involved in MDD.

Infralimbic YTHDF1 is necessary for the beneficial effects of acute mild exercise on auditory fear extinction retention

Exposure therapy is the most effective approach of behavioral therapy for anxiety and post-traumatic stress disorder (PTSD). But fear is easy to reappear even after successful extinction. So, identifying novel strategies for augmenting exposure therapy is rather important. It was reported that exercise had beneficial effects on cognitive and memory deficits. However, whether exercise could affect fear memory, especially for fear extinction remained elusive. Here, our results showed that exposure to acute mild exercise 1 or 2 h before extinction training can augment recent fear extinction retention and 2 h for the remote fear extinction retention. These beneficial effects could be attributed to increased YTHDF1 expression in medial prefrontal cortex (mPFC). Furthermore, by using an AAV-shRNA-based approach to silence YTHDF1 expression via stereotactic injection in prelimbic cortex (PL) or infralimbic cortex (IL), respectively, we demonstrated that silence YTHDF1 in IL, but not in PL, blunted augmentation of exposure therapy induced by acute mild exercise and accompanied with decreased NR2B and GluR1 expression. Moreover, YTHDF1 modulated dendritic spines remodeling of pyramidal neuron in IL. Collectively, our findings suggested that acute mild exercise acted as an effective strategy in augmenting exposure therapy with possible implications for understanding new treatment underlying PTSD.

Second-order fear conditioning involves formation of competing stimulus-danger and stimulus-safety associations

How do animals process experiences that provide contradictory information? The present study addressed this question using second-order fear conditioning in rats. In second-order conditioning, rats are conditioned to fear a stimulus, S1, through its pairings with foot-shock (stage 1); and some days later, a second stimulus, S2, through its pairings with the already-conditioned S1 (stage 2). However, as foot-shock is never presented during conditioning to S2, we hypothesized that S2 simultaneously encodes 2 contradictory associations: one that drives fear to S2 (S2-danger) and another that reflects the absence of the expected unconditioned stimulus and partially masks that fear (e.g. S2-safety). We tested this hypothesis by manipulating the substrates of danger and safety learning in the brain (using a chemogenetic approach) and assessing the consequences for second-order fear to S2. Critically, silencing activity in the basolateral amygdala (important for danger learning) reduced fear to S2, whereas silencing activity in the infralimbic cortex (important for safety learning) enhanced fear to S2. These bidirectional changes are consistent with our hypothesis that second-order fear conditioning involves the formation of competing S2-danger and S2-safety associations. More generally, they show that a single set of experiences can produce contradictory associations and that the brain resolves the contradiction by encoding these associations in distinct brain regions.

Compensatory Contextual Fear Memory Pathways Develop in the Infralimbic Cortex within 3 Days after the First Test in the Absence of the Dorsal Hippocampus

The dorsal hippocampus (DH) is primarily involved in the formation of contextual fear-conditioned (CxFC) memory. However, CxFC memory can be formed even in the absence of the DH. In addition to the DH, the infralimbic cortex (IL), a sub-region of the medial prefrontal cortex (mPFC), also plays an important role in the consolidation of CxFC memory. However, role of IL in the development of compensatory CxFC memory is not known. Here, we have examined (a) the development of the compensatory circuitry of CxFC memory within 3 days after the first test in the absence of the DH and (b) the role of IL in the induction of compensatory CxFC memory in the absence of the DH. The DH-lesioned rats re-trained for CxFC 1 day after the first testing exhibited significantly less freezing compared to the control group. However, the DH-lesioned rats, re-trained for CxFC 3 days after the first testing, showed a robust freezing response. It suggests that the fully functional compensatory circuitry of contextual fear memory develops after multiple training separated by 3 days. Furthermore, we observed that reversible inactivation of the IL of the DH-lesioned rats during the first training waned the formation of compensatory CxFC. It suggests that (a) the IL receives contextual fear memory information during the first trial in the absence of the DH and (b) perturbation in fear memory information encoding in the IL during the first trial impairs the development of the compensatory network in the absence of the DH.

Neuronal activity of the medial prefrontal cortex, nucleus accumbens, and basolateral amygdala in conditioned taste aversion and conditioned place preference induced by different doses of morphine administrations in rats

To re-examine the paradoxical effect hypothesis of abused drugs, the present study concerned whether different doses of morphine disparately affect neuronal activity and associations among the subareas of the medial prefrontal cortex (mPFC: cingulate cortex 1-Cg1, prelimbic cortex-PrL, infralimbic cortex-IL), the subregions of the nucleus accumbens (NAc; both core and shell), and the basolateral amygdala (BLA) following conditioned taste aversion (CTA) and conditioned place preference (CPP). All rats were given a 0.1% saccharin solution for 15-min, and they were intraperitoneally injected with saline or 20, 30, or 40 mg/kg morphine to form the aversive CTA learning. Later, half of the rats were tested for CPP (including the CTA and then CPP tests) for 30-min. Finally, the immunohistochemical staining with c-Fos was conducted after the behavioral test. After the CTA test, c-Fos (%) in the Cg1 and PrL (but not the IL) was more in 20-40 mg/kg of the morphine groups; c-Fos (%) in the NAc core, NAc shell, and BLA was more in the 30-40 mg/kg morphine group. After the CPP test, the Cg1, PrL, IL, and BLA showed more c-Fos (%) in 20 mg/kg morphine; the NAc core showed fewer in c-Fos (%) in the 30-40 mg/kg morphine groups. The mPFC subregions (e.g., Cg1, PrL, and IL), NAc subareas (e.g., NAc core and NAc shell), and BLA were involved in the different doses of morphine injections. The correlation analysis showed that a positive correlation was observed between PrL and IL with NAc core with low doses of morphine and with NAc shell with increasing doses of morphine after the CTA test. After the CPP, an association between PrL and NAc core and NAc shell at low doses and between IL and BLA and NAc shell with increasing doses of morphine. Therefore, different neural substrates and the neural connectivity are observed following different doses of morphine and after the CTA and CPP tests. The present data extend the paradoxical effect hypothesis of abused drugs.

Differential processing of decision information in subregions of rodent medial prefrontal cortex

Decision-making involves multiple cognitive processes requiring different aspects of information about the situation at hand. The rodent medial prefrontal cortex (mPFC) has been hypothesized to be central to these abilities. Functional studies have sought to link specific processes to specific anatomical subregions, but past studies of mPFC have yielded controversial results, leaving the precise nature of mPFC function unclear. To settle this debate, we recorded from the full dorso-ventral extent of mPFC in each of 8 rats, as they performed a complex economic decision task. These data revealed four distinct functional domains within mPFC that closely mirrored anatomically identified subregions, including novel evidence to divide prelimbic cortex into dorsal and ventral components. We found that dorsal aspects of mPFC (ACC, dPL) were more involved in processing information about active decisions, while ventral aspects (vPL, IL) were more engaged in motivational factors.

Optogenetic stimulation in the medial prefrontal cortex modulates stimulus valence from rewarding and aversive to neutral states

Introduction

Understanding the modulations of the medial prefrontal cortex (mPFC) in the valence of the stimulus from rewarding and aversive status to neutral status is crucial for the development of novel treatments for drug addiction. This study addressed this issue and examined whether optogenetic ChR2 photostimulation in the cingulate, prelimbic, and infralimbic cortices of the mPFC regulated the valence of saccharin solution consumption from the rewarding property, the aversive property induced by morphine's conditioning, and the neutral states via saccharin extinction processes after morphine's conditioning.

Methods

All rats received virus infection, buried optical fiber, optical stimulation, water deprivation, and saccharin solution consumption phases. In Experiment 1, rats were given ChR2 virus infection into the cingulate cortex (Cg1), prelimbic cortex (PrL), and infralimbic cortex (IL) to influence the rewarding saccharin solution consumption under photostimulation. In Experiment 2, rats were given ChR2 or EYFP virus infection into the Cg1, PrL, and IL to alter the saccharin solution consumption in the morphine-induced aversively conditioned taste aversion (CTA) and the saccharin solution consumption in the neutral state following the extinction process under photostimulation. Later, the immunohistochemical staining with c-Fos protein was performed for the Cg1, IL, PrL, nucleus accumbens core, nucleus accumbens shell, central amygdala, basolateral amygdala, ventral tegmental area, and dentate gyrus.

Results

The results showed that optogenetic PrL stimulation decreased the rewarding valence of saccharin solution consumption and increased the morphine-induced, aversive valence of saccharin solution consumption. PrL stimulation decreased the neutral valence of saccharin solution consumption via the extinction process. Cg1 optogenetic stimulation increased the rewarding valence of saccharin solution consumption and the aversive valence of saccharin solution consumption induced by morphine in conditioning. Optogenetic IL stimulation increased the aversive valence of saccharin solution consumption induced by morphine via conditioning.

Conclusion

Altogether, optogenetic stimulation in the subareas of the mPFC modulated the reward, aversion, and neutral valences of the stimulus and altered neuronal activity in the mPFC, amygdala, nucleus accumbens, and hippocampus. Notably, the change of valence was temporary alternation during light-on related to the light-off periods. However, the findings may provide insights in the development of novel treatments for addictive symptoms.

Projections of the insular cortex to orbitofrontal and medial prefrontal cortex: A tracing study in the rat

The dense fiber pathways that connect the insular cortex with frontal cortices are thought to provide these frontal areas with interoceptive information, crucial for their involvement in executive functions. Using anterograde neuroanatomical tracing, we mapped the detailed organization of the projections from the rat insular cortex to its targets in orbitofrontal (OFC) and medial prefrontal (mPFC) cortex. In OFC, main insular projections distribute to lateral and medial parts, avoiding ventral parts. Whereas projections from the primary gustatory cortex densely innervate dorsolateral OFC, likely corresponding to what in primates is known as the secondary gustatory cortex, these projections avoid mPFC. Instead, mPFC is targeted almost exclusively by projections from agranular fields of the insular cortex. Finally, "parietal" domains of the insular cortex project specifically to the dorsolateral OFC, and strongly innervate ventral portions of mPFC, i.e., the dorsal peduncular cortex.

Social buffering diminishes fear response but does not equal improved fear extinction

Social support during exposure-based psychotherapy is believed to diminish fear and improve therapy outcomes. However, some clinical trials challenge that notion. Underlying mechanisms remain unknown, hindering the understanding of benefits and pitfalls of such approach. To study social buffering during fear extinction, we developed a behavioral model in which partner's presence decreases response to fear-associated stimuli. To identify the neuronal background of this phenomenon, we combined behavioral testing with c-Fos mapping, optogenetics, and chemogenetics. We found that the presence of a partner during fear extinction training causes robust inhibition of freezing; the effect, however, disappears in subjects tested individually on the following day. It is accompanied by lowered activation of the prelimbic (PL) and anterior cingulate (ACC) but not infralimbic (IL) cortex. Accordingly, blocking of IL activity left social buffering intact. Similarly, inhibition of the ventral hippocampus-PL pathway, suppressing fear response after prolonged extinction training, did not diminish the effect. In contrast, inhibition of the ACC-central amygdala pathway, modulating social behavior, blocked social buffering. By reporting that social modulation of fear inhibition is transient and insensitive to manipulation of the fear extinction-related circuits, we show that the mechanisms underlying social buffering during extinction are different from those of individual extinction.

A Preliminary Quantitative Electron Microscopic Analysis Reveals Reduced Number of Mitochondria in the Infralimbic Cortex of Rats Exposed to Chronic Mild Stress

Exposure to severe, uncontrollable and long-lasting stress is a strong risk factor for the development of numerous mental and somatic disorders. Animal studies document that chronic stress can alter neuronal morphology and functioning in limbic brain structures such as the prefrontal cortex. Mitochondria are intracellular powerhouses generating chemical energy for biochemical reactions of the cell. Recent findings document that chronic stress can lead to changes in mitochondrial function and metabolism. Here, we studied putative mitochondrial damage in response to chronic stress in neurons of the medial prefrontal cortex. We performed a systematic quantitative ultrastructural analysis to examine the consequences of 9-weeks of chronic mild stress on mitochondria number and morphology in the infralimbic cortex of adult male rats. In this preliminary study, we analyzed 4,250 electron microscopic images and 67000 mitochondria were counted and examined in the brains of 4 control and 4 stressed rats. We found significantly reduced number of mitochondria in the infralimbic cortex of the stressed animals, but we could not detect any significant alteration in mitochondrial morphology. These data support the concept that prolonged stress can lead to mitochondrial loss. This in turn may result in impaired energy production. Reduced cellular energy may sensitize the neurons to additional injuries and may eventually trigger the development of psychopathologies.

Comparison of prefrontal cortex sucrose seeking ensembles engaged in multiple seeking sessions: Context is key

Encoding of memories, including those associated with prior drug or reward, is thought to take place within distinct populations of neurons, termed ensembles. Neuronal ensembles for drug- and reward-seeking have been identified in regions of the medial prefrontal cortex, but much of our understanding of these ensembles is based on experiments that take place in a single reward-associated environment and measure ensemble encoding over short durations of time. In contrast, reward seeking behavior is evident across different reward-associated environments and persists over time. Using TetTag mice and Fos immunohistochemistry, we examined the relationship between persistent sucrose-seeking and ensemble encoding in mice that undergo seeking sessions in the same or different sucrose self-administration contexts 2 weeks apart. We found that prelimbic (PrL) and anterior cingulate cortex ensembles tagged in the first seeking session were highly sensitive to the context in which a second seeking session took place: reactivation of these ensembles was reduced in the same context but elevated in a distinct sucrose self-administration context. Correlational analyses revealed that ensemble reactivation in the PrL was proportional to the persistence of sucrose seeking behavior across sessions in differing ways in female mice. In the same context, reactivation was proportional to the persistence of non-reinforced operant responses, whereas in a distinct context, reactivation was proportional to the persistence of non-reinforced head entries into the sucrose receptacle. This study underlines the importance of the medial prefrontal cortex importance in maintaining a reward-seeking ensemble over time and identifies context-dependent changes in behavioral correlates of ensemble reactivation.

Corticostriatal Suppression of Appetitive Pavlovian Conditioned Responding

The capacity to suppress learned responses is essential for animals to adapt in dynamic environments. Extinction is a process by which animals learn to suppress conditioned responding when an expected outcome is omitted. The infralimbic (IL) cortex to nucleus accumbens shell (NAcS) neural circuit is implicated in suppressing conditioned responding after extinction, especially in the context of operant cocaine-seeking behavior. However, the role of the IL-to-NAcS neural circuit in the extinction of responding to appetitive Pavlovian cues is unknown, and the psychological mechanisms involved in response suppression following extinction are unclear. We trained male Long Evans rats to associate a 10 s auditory conditioned stimulus (CS; 14 trials per session) with a sucrose unconditioned stimulus (US; 0.2 ml per CS) in a specific context, and then following extinction in a different context, precipitated a renewal of CS responding by presenting the CS alone in the original Pavlovian conditioning context. Unilateral, optogenetic stimulation of the IL-to-NAcS circuit selectively during CS trials suppressed renewal. In a separate experiment, IL-to-NAcS stimulation suppressed CS responding regardless of prior extinction and impaired extinction retrieval. Finally, IL-to-NAcS stimulation during the CS did not suppress the acquisition of Pavlovian conditioning but was required for the subsequent expression of CS responding. These results are consistent with multiple studies showing that the IL-to-NAcS neural circuit is involved in the suppression of operant cocaine-seeking, extending these findings to appetitive Pavlovian cues. The suppression of appetitive Pavlovian responding following IL-to-NAcS circuit stimulation, however, does not appear to be an extinction-dependent process.SIGNIFICANCE STATEMENT Extinction is a form of inhibitory learning through which animals learn to suppress conditioned responding in the face of nonreinforcement. We investigated the role of the IL cortex inputs to the NAcS in the extinction of responding to appetitive Pavlovian cues and the psychological mechanisms involved in response suppression following extinction. Using in vivo optogenetics, we found that stimulating the IL-to-NAcS neural circuit suppressed context-induced renewal of conditioned responding after extinction. In a separate experiment, stimulating the IL-to-NAcS circuit suppressed conditioned responding in an extinction-independent manner. These findings can be used by future research aimed at understanding how corticostriatal circuits contribute to behavioral flexibility and mental disorders that involve the suppression of learned behaviors.

Appetitive Behavior in the Social Transmission of Food Preference Paradigm Predicts Activation of Orexin-A producing Neurons in a Sex-Dependent Manner

Orexin-producing cells in the lateral hypothalamic area have been shown to be involved in a wide variety of behavioral and cognitive functions, including the recall of appetitive associations and a variety of social behaviors. Here, we investigated the role of orexin in the acquisition and recall of socially transmitted food preferences in the rat. Rats were euthanized following either acquisition, short-term recall, or long-term recall of a socially transmitted food preference and their brains were processed for orexin-A and c-Fos expression. We found that while there were no significant differences in c-Fos expression between control and experimental subjects at any of the tested timepoints, females displayed significantly more activity in both orexinergic and non-orexinergic cells in the lateral hypothalamus. In the infralimbic cortex, we found that social behavior was significantly predictive of c-Fos expression, with social behaviors related to olfactory exploration appearing to be particularly influential. We additionally found that appetitive behavior was significantly predictive of orexin-A activity in a sex-dependent matter, with the total amount eaten correlating negatively with orexin-A/c-Fos colocalization in male rats but not female rats. These findings suggest a potential sex-specific role for the orexin system in balancing the stimulation of feeding behavior with the sleep/wake cycle.

Profiling prefrontal cortex protein expression in rats exhibiting an incubation of cocaine craving following short-access self-administration procedures

INTRODUCTION

Incubation of drug-craving refers to a time-dependent increase in drug cue-elicited craving that occurs during protracted withdrawal. Historically, rat models of incubated cocaine craving employed extended-access (typically 6 h/day) intravenous drug self-administration (IV-SA) procedures, although incubated cocaine craving is reported to occur following shorter-access IV-SA paradigms. The notoriously low-throughput of extended-access IV-SA prompted us to determine whether two different short-access IV-SA procedures akin to those in the literature result in qualitatively similar changes in glutamate receptor expression and the activation of downstream signaling molecules within prefrontal cortex (PFC) subregions as those reported previously by our group under 6h-access conditions.

METHODS

For this, adult, male Sprague-Dawley rats were trained to intravenously self-administer cocaine for 2 h/day for 10 consecutive days (2-h model) or for 6 h on day 1 and 2 h/day for the remaining 9 days of training (Mixed model). A sham control group was also included that did not self-administer cocaine.

RESULTS

On withdrawal day 3 or 30, rats were subjected to a 2-h test of cue-reinforced responding in the absence of cocaine and a time-dependent increase in drug-seeking was observed under both IV-SA procedures. Immunoblotting of brain tissue collected immediately following the cue test session indicated elevated phospho-Akt1, phospho-CaMKII and Homer2a/b expression within the prelimbic subregion of the PFC of cocaine-incubated rats. However, we failed to detect incubation-related changes in Group 1 metabotropic glutamate receptor or ionotropic glutamate receptor subunit expression in either subregion.

DISCUSSION

These results highlight further a role for Akt1-related signaling within the prelimbic cortex in driving incubated cocaine craving, and provide novel evidence supporting a potential role also for CaMKII-dependent signaling through glutamate receptors in this behavioral phenomenon.