The anterior cingulate cortex is a target structure for the anxiolytic-like effects of benzodiazepines assessed by repeated exposure to the elevated plus maze and Fos immunoreactivity

Differential modulation of the anterior cingulate and insular cortices on anxiogenic-like responses induced by empathy for pain

Mice cohabiting with a conspecific in chronic pain display anxiogenesis in the elevated plus-maze (EPM). Given that the anterior cingulate (ACC) and insular (InC) cortices play a role in the modulation of anxiety, pain, and emotional contagion, we investigated (a) the FosB activation in both brain areas and (b) the effects of intra-ACC or -InC injection of cobalt chloride (CoCl₂, a synaptic blocker), on the anxiety of mice cohabiting with a cagemate suffering pain. Twenty-one days after birth, male Swiss mice were housed in pairs for 14 days to establish familiarity. On the 14th day, mice were divided into two groups: cagemate sciatic nerve constriction (CNC; i.e., one animal of each pair was subjected to sciatic nerve constriction), and cagemate sham (CS; i.e., a similar procedure but without suffering nerve constriction). After that, both groups were housed again with the same pairs for the other 14 days. On the 28th day, mice had their brains removed for the immunoassays analyses (Exp. 1). For experiments 2 and 3, on the 23rd day, the cagemates received guide cannula implantation bilaterally in the ACC or InC and, on the 28th day, they received local injections of saline or CoCl₂, and then were exposed to the EPM. Results showed that cohabitation with a conspecific with chronic pain decreases and increases neuronal activation (FosB) within the ACC and InC, respectively. Intra-ACC or InC injection of CoCl₂ reversed the anxiogenic effect in those animals that cohabited with a conspecific in chronic pain. ACC and InC seem to modulate anxiety induced by emotional contagion in animals cohabitating with a conspecific suffering pain.

Distinct patterns of brain Fos expression in Carioca High- and Low-conditioned Freezing Rats

BACKGROUND

The bidirectional selection of high and low anxiety-like behavior is a valuable tool for understanding the neurocircuits that are responsible for anxiety disorders. Our group developed two breeding lines of rats, known as Carioca High- and Low-conditioned Freezing (CHF and CLF), based on defensive freezing in the contextual fear conditioning paradigm. A random selected line was employed as a control (CTL) comparison group for both CHF and CLF lines of animals. The present study performed Fos immunochemistry to investigate changes in neural activity in different brain structures among CHF and CLF rats when they were exposed to contextual cues that were previously associated with footshock.

RESULTS

The study indicated that CHF rats expressed high Fos expression in the locus coeruleus, periventricular nucleus of the hypothalamus (PVN), and lateral portion of the septal area and low Fos expression in the medial portion of the septal area, dentate gyrus, and prelimbic cortex (PL) compared to CTL animals. CLF rats exhibited a decrease in Fos expression in the PVN, PL, and basolateral nucleus of the amygdala and increase in the cingulate and perirhinal cortices compared to CTL animals.

CONCLUSIONS

Both CHF and CLF rats displayed Fos expression changes key regions of the anxiety brain circuitry. The two bidirectional lines exhibit different pattern of neural activation and inhibition with opposing influences on the PVN, the main structure involved in regulating the hypothalamic-pituitary-adrenal neuroendocrine responses observed in anxiety disorders.

Fecal transplant prevents gut dysbiosis and anxiety-like behaviour after spinal cord injury in rats

Secondary manifestations of spinal cord injury beyond motor and sensory dysfunction can negatively affect a person's quality of life. Spinal cord injury is associated with an increased incidence of depression and anxiety; however, the mechanisms of this relationship are currently not well understood. Human and animal studies suggest that changes in the composition of the intestinal microbiota (dysbiosis) are associated with mood disorders. The objective of the current study is to establish a model of anxiety following a cervical contusion spinal cord injury in rats and to determine whether the microbiota play a role in the observed behavioural changes. We found that spinal cord injury caused dysbiosis and increased symptoms of anxiety-like behaviour. Treatment with a fecal transplant prevented both spinal cord injury-induced dysbiosis as well as the development of anxiety-like behaviour. These results indicate that an incomplete unilateral cervical spinal cord injury can cause affective disorders and intestinal dysbiosis, and that both can be prevented by treatment with fecal transplant therapy.

Effects of plus-maze experience and chlordiazepoxide on anxiety-like behavior and serotonin neural activity in the dorsal raphe nucleus in rats

The extent to which rats express anxiety-like behavior on the elevated plus-maze (EPM) depends on their previous maze experience. Open-arm avoidance develops in maze-experienced rats, and is often accompanied by a diminished anxiolytic response to benzodiazepines. Regions of the dorsal raphe nucleus (DRN) were examined in male Sprague-Dawley rats using c-Fos and serotonin immunohistochemistry following a single exposure, a second exposure or no exposure to the EPM. We then examined the effect of the benzodiazepine anxiolytic chlordiazepoxide (CDP, 5 mg/kg) on EPM behavior and DRN neural activity. Enhanced open-arm avoidance was evident on the second EPM trial in both experiments. The observed pattern of c-Fos expression suggests that the first exposure to the maze activates serotonin cells in the rostral and dorsal regions of the DRN and that only the dorsal subregion is activated by a second exposure. CDP increased open-arm exploration during the first trial, which corresponded to decreased 5-hydroxytryptamine (5-HT) activity in the rostral and ventral subregions of the DRN. However, 5-HT activity in the DRN was reduced in rats on the second maze trial compared with the first trial, when CDP had no effect on open-arm exploration. These results suggest that open-arm avoidance in maze-experienced rats can be characterized as a coping response that is mediated by specific populations of 5-HT neurons in the DRN.

Repeated elevated plus maze trials as a measure for tracking within-subjects behavioral performance in rats (Rattus norvegicus)

The elevated plus maze (EPM) is routinely used in neuroscience research to evaluate emotional behavior in rodents by measuring general exploratory performance and avoidance of the aversive open arms of the maze. According to standard practice, behavior on the EPM is evaluated during a single trial to avoid the possibility of habituation to the apparatus that would result in lost sensitivity of key outcome measures. However, this possibility has not been systematically evaluated across repeated trials or across different environmental conditions. In the current study, we assessed within-subject behavior on the EPM in adult male rats over thirteen trials (tested twice weekly) repeated under identical conditions. We also assessed within-subject behavior on the EPM in adult male rats under dim (1 lux in the closed arm) and lit (246 lux in the closed arm) environmental conditions. We found that measures of general performance (basic movements and total distanced travelled throughout the maze) were stable across repeated trials and environmental conditions. We found that measures of open arm avoidance (distance travelled in, time spent in and entries in to the open arm) varied across trials and environmental conditions and were sensitive to the lighting conditions of the initial test. Though measures of open arm avoidance did show a linear trend indicative of habituation across repeated trials, this effect was variable across trials. Notably, preference for the open arm over the closed arm (measured as % of time spent in the open arm) assessed among individual animals occurred rarely and was never observed on the group level across the thirteen repeated trials. Together, these data demonstrate that measures of general performance such as basic movements and total distance traveled are robust to repeated testing and changing environmental lighting conditions. In contrast, measures of open arm avoidance show habituation with repeated testing and are sensitive to changing environmental lighting conditions. Based on these results, we suggest that within-subjects repeated testing on the EPM is valid in well-controlled studies that include an untreated control group to account for inter-trial variability and habituation.

Oxytocin in the medial prefrontal cortex attenuates anxiety: Anatomical and receptor specificity and mechanism of action

Numerous studies in animals and humans have established that oxytocin (OT) reduces anxiety. In rats, the prelimbic (PL) subregion of the medial prefrontal cortex (mPFC) is among the brain areas implicated in the anxiolytic actions of OT. However, questions remain about the anatomical and receptor specificity of OT and its mechanism of action. Here we assessed whether the regulation of anxiety by mPFC OT is restricted to the PL subregion and evaluated whether oxytocin receptor (OTR) activation is required for OT to have an anxiolytic effect. We also examined whether OT interacts with GABA in the mPFC to reduce anxiety and investigated the extent to which OT in the mPFC affects activation of mPFC GABA neurons as well as neuronal activation in the amygdala, a primary target of the mPFC which is part of the neural network regulating anxiety. We found that OT reduced anxiety-like behavior when delivered to the PL, but not infralimbic or anterior cingulate subregions of the mPFC. The anxiolytic effect of OT in the PL mPFC was blocked by pretreatment with an OTR, but not a vasopressin receptor, antagonist as well as with a GABA_A receptor antagonist. Lastly, administration of OT to the PL mPFC was accompanied by increased activation of GABA neurons in the PL mPFC and altered neuronal activation of the amygdala following anxiety testing. These results demonstrate that OT in the PL mPFC attenuates anxiety-related behavior and may do so by engaging GABAergic neurons which ultimately modulate downstream brain regions implicated in anxiety.

Neonatal alcohol exposure reduces number of parvalbumin-positive interneurons in the medial prefrontal cortex and impairs passive avoidance acquisition in mice deficits not rescued from exercise

Developmental alcohol exposure causes a host of cognitive and neuroanatomical abnormalities, one of which is impaired executive functioning resulting from medial prefrontal cortex (mPFC) damage. This study determined whether third-trimester equivalent alcohol exposure reduced the number of mPFC GABAergic parvalbumin-positive (PV+) interneurons, hypothesized to play an important role in local inhibition of the mPFC. The impact on passive avoidance learning and the therapeutic role of aerobic exercise in adulthood was also explored. Male C57BL/6J mice received either saline or 5g/kg ethanol (two doses, two hours apart) on PD 5, 7, and 9. On PD 35, animals received a running wheel or remained sedentary for 48days before behavioral testing and perfusion on PD 83. The number of PV+ interneurons was stereologically measured in three separate mPFC subregions: infralimbic, prelimbic and anterior cingulate cortices (ACC). Neonatal alcohol exposure decreased number of PV+ interneurons and volume of the ACC, but the other regions of the mPFC were spared. Alcohol impaired acquisition, but not retrieval of passive avoidance, and had no effect on motor performance on the rotarod. Exercise had no impact on PV+ cell number, mPFC volume, or acquisition of passive avoidance, but enhanced retrieval in both control and alcohol-exposed groups, and enhanced rotarod performance in the control mice. Results support the hypothesis that part of the behavioral deficits associated with developmental alcohol exposure are due to reduced PV+ interneurons in the ACC, but unfortunately exercise does not appear to be able to reverse any of these deficits.

Bidirectional modulation of anxiety-related and social behaviors by amygdala projections to the medial prefrontal cortex

The basolateral amygdala (BLA) and the medial prefrontal cortex (mPFC) modulate anxiety and social behaviors. It remains to be elucidated, however, whether direct projections from the BLA to the mPFC play a functional role in these behaviors. We used optogenetic approaches in behaving mice to either activate or inhibit BLA inputs to the mPFC during behavioral assays that assess anxiety-like behavior and social interaction. Channelrhodopsin-2 (ChR2)-mediated activation of BLA inputs to the mPFC produced anxiogenic effects in the elevated plus maze and open field test, whereas halorhodopsin (NpHR)-mediated inhibition produced anxiolytic effects. Furthermore, activation of the BLA-mPFC pathway reduced social interaction in the resident-intruder test, whereas inhibition facilitated social interaction. These results establish a causal relationship between activity in the BLA-mPFC pathway and the bidirectional modulation of anxiety-related and social behaviors.

A comparison of neural responses to appetitive and aversive stimuli in humans and other mammals

Distinguishing potentially harmful or beneficial stimuli is necessary for the self-preservation and well-being of all organisms. This assessment requires the ongoing valuation of environmental stimuli. Despite much work on the processing of aversive- and appetitive-related brain signals, it is not clear to what degree these two processes interact across the brain. To help clarify this issue, this report used a cross-species comparative approach in humans (i.e. meta-analysis of imaging data) and other mammals (i.e. targeted review of functional neuroanatomy in rodents and non-human primates). Human meta-analysis results suggest network components that appear selective for appetitive (e.g. ventromedial prefrontal cortex, ventral tegmental area) or aversive (e.g. cingulate/supplementary motor cortex, periaqueductal grey) processing, or that reflect overlapping (e.g. anterior insula, amygdala) or asymmetrical, i.e. apparently lateralized, activity (e.g. orbitofrontal cortex, ventral striatum). However, a closer look at the known value-related mechanisms from the animal literature suggests that all of these macroanatomical regions are involved in the processing of both appetitive and aversive stimuli. Differential spatiotemporal network dynamics may help explain similarities and differences in appetitive- and aversion-related activity.

Long-term behavioral programming induced by peripuberty stress in rats is accompanied by GABAergic-related alterations in the Amygdala

Stress during childhood and adolescence is a risk factor for psychopathology. Alterations in γ-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the brain, have been found following stress exposure and fear experiences and are often implicated in anxiety and mood disorders. Abnormal amygdala functioning has also been detected following stress exposure and is also implicated in anxiety and social disorders. However, the amygdala is not a unitary structure; it includes several nuclei with different functions and little is known on the potential differences the impact of early life stress may have on this system within different amygdaloid nuclei. We aimed here to evaluate potential regional differences in the expression of GABAergic-related markers across several amygdaloid nuclei in adult rats subjected to a peripuberty stress protocol that leads to enhanced basal amygdala activity and psychopathological behaviors. More specifically, we investigated the protein expression levels of glutamic acid decarboxylase (GAD; the principal synthesizing enzyme of GABA) and of GABA-A receptor subunits α2 and α3. We found reduced GAD and GABA-A α3, but not α2, subunit protein levels throughout all the amygdala nuclei examined (lateral, basolateral, basomedial, medial and central) and increased anxiety-like behaviors and reduced sociability in peripubertally stressed animals. Our results identify an enduring inhibition of the GABAergic system across the amygdala following exposure to early adversity. They also highlight the suitability of the peripuberty stress model to investigate the link between treatments targeting the dysfunctional GABAergic system in specific amygdala nuclei and recovery of specific stress-induced behavioral dysfunctions.

D(1)-like receptors in the nucleus accumbens shell regulate the expression of contextual fear conditioning and activity of the anterior cingulate cortex in rats

Although dopamine-related circuits are best known for their roles in appetitive motivation, consistent data have implicated this catecholamine in some forms of response to stressful situations. In fact, projection areas of the ventral tegmental area, such as the amygdala and hippocampus, are well established to be involved in the acquisition and expression of fear conditioning, while less is known about the role of the medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) in these processes. In the present study, we initially investigated the involvement of the mPFC and NAc in the expression of conditioned fear, assessing freezing behaviour and Fos protein expression in the brains of rats exposed to a context, light or tone previously paired with footshocks. Contextual and cued stimuli were able to increase the time of the freezing response while only the contextual fear promoted a significant increase in Fos protein expression in the mPFC and caudal NAc. We then examined the effects of specific dopaminergic agonists and antagonists injected bilaterally into the posterior medioventral shell subregion of the NAc (NAcSh) on the expression of contextual fear. SKF38393, quinpirole and sulpiride induced no behavioural changes, but the D1-like receptor antagonist SCH23390 increased the freezing response of the rats and selectively reduced Fos protein expression in the anterior cingulate cortex and rostral NAcSh. These findings confirm the involvement of the NAcSh in the expression of contextual fear memories and indicate the selective role of NAcSh D1-like receptors and anterior cingulate cortex in this process.

Conditioned fear is modulated by CRF mechanisms in the periaqueductal gray columns

The periaqueductal gray (PAG) columns have been implicated in controlling stress responses through corticotropin-releasing factor (CRF), which is a neuropeptide with a prominent role in the etiology of fear- and anxiety-related psychopathologies. Several studies have investigated the involvement of dorsal PAG (dPAG) CRF mechanisms in models of unconditioned fear. However, less is known about the role of this neurotransmission in the expression of conditioned fear memories in the dPAG and ventrolateral PAG (vlPAG) columns. We assessed the effects of ovine CRF (oCRF 0.25 and 1.0 μg/0.2 μL) locally administered into the dPAG and vlPAG on behavioral (fear-potentiated startle and freezing) and autonomic (arterial pressure and heart rate) responses in rats subjected to contextual fear conditioning. The lower dose injected into the columns promoted proaversive effects, enhanced contextual freezing, increased the blood pressure and heart rate and decreased tail temperature. The lower dose of oCRF into the vlPAG, but not into the dPAG, produced a pronounced enhancement of the fear-potentiated startle response. The results imply that the PAG is a heterogeneous structure that is involved in the coordination of distinct behaviors and autonomic control, suggest PAG involvement in the expression of contextual fear memory as well as implicate the CRF as an important modulator of the neural substrates of fear in the PAG.

Enhanced anxiety observed in cocaine withdrawn rats is associated with altered reactivity of the dorsomedial prefrontal cortex

Discontinuation of drug intake in cocaine abusers commonly produces a variety of adverse withdrawal symptoms among which anxiety and depression-related behavior are prevailing during the initial period of abstinence. The aim of this study was to provide further insight into the neurobiological dysregulations that might contribute to these pathological states. Rats were treated with cocaine or saline for 14 days (20 mg/kg; i.p) and anxiety-related behavior was assessed in different paradigms (elevated plus-maze (EPM), confinement to an open arm of the EPM and shock-probe burying tests) for up to 4 weeks after withdrawal. Depression-like behavior was assessed by the forced swim test and sucrose preference test. Altogether our results demonstrated that cocaine withdrawal induced persistent heightened levels of anxiety that last for at least 28 days but did not affect depression-like behavior. We then used Fos immunohistochemistry to map neuronal activation patterns in withdrawn rats confined to one open arm of an EPM, and a double labeling procedure using Fos immunohistochemistry and in situ hybridization of glutamic acid decarboxylase or vesicular glutamate transporter mRNAs to identify the phenotype of the activated neurons. Our data showed that the exacerbated anxiety observed in cocaine withdrawn rats exposed to an elevated open arm was accompanied by an altered reactivity of the dorsal part of the medial prefrontal cortex (anterior cingulate and dorsal prelimbic cortices), the paraventricular thalamic nucleus and the lateral and anterior areas of the hypothalamus. In the medial prefrontal cortex, we evidenced a negative correlation between Fos expression in its dorsal part and open arm-induced freezing in NaCl-treated rats but not in cocaine withdrawn rats. We also found that more than 65% of activated neurons were glutamatergic projection neurons. The present study provides new insights into the neuroanatomical regions and neuronal cell types that may underlie pathological anxiety during cocaine withdrawal.

Common brain activations for painful and non-painful aversive stimuli

BACKGROUND

Identification of potentially harmful stimuli is necessary for the well-being and self-preservation of all organisms. However, the neural substrates involved in the processing of aversive stimuli are not well understood. For instance, painful and non-painful aversive stimuli are largely thought to activate different neural networks. However, it is presently unclear whether there is a common aversion-related network of brain regions responsible for the basic processing of aversive stimuli. To help clarify this issue, this report used a cross-species translational approach in humans (i.e. meta-analysis) and rodents (i.e. systematic review of functional neuroanatomy).

RESULTS

Animal and human data combined to show a core aversion-related network, consisting of similar cortical (i.e. MCC, PCC, AI, DMPFC, RTG, SMA, VLOFC; see results section or abbreviation section for full names) and subcortical (i.e. Amyg, BNST, DS, Hab, Hipp/Parahipp, Hyp, NAc, NTS, PAG, PBN, raphe, septal nuclei, Thal, LC, midbrain) regions. In addition, a number of regions appeared to be more involved in pain-related (e.g. sensory cortex) or non-pain-related (e.g. amygdala) aversive processing.

CONCLUSIONS

This investigation suggests that aversive processing, at the most basic level, relies on similar neural substrates, and that differential responses may be due, in part, to the recruitment of additional structures as well as the spatio-temporal dynamic activity of the network. This network perspective may provide a clearer understanding of why components of this circuit appear dysfunctional in some psychiatric and pain-related disorders.

Risk assessment behaviors associated with corticosterone trigger the defense reaction to social isolation in rats: role of the anterior cingulate cortex

The extent to which the hypothalamic-pituitary-adrenal axis is activated by short-term and long-term consequences of stress is still open to investigation. This study aimed to determine (i) the correlation between plasma corticosterone and exploratory behavior exhibited by rats subjected to the elevated plus maze (EPM) following different periods of social isolation, (ii) the effects of the corticosterone synthesis blocker, metyrapone, on the behavioral consequences of isolation, and (iii) whether corticosterone produces its effects through an action on the anterior cingulate cortex, area 1 (Cg1). Rats were subjected to 30-min, 2-h, 24-h, or 7-day isolation periods before EPM exposure and plasma corticosterone assessments. Isolation for longer periods of time produced greater anxiogenic-like effects on the EPM. However, stretched attend posture (SAP) and plasma corticosterone concentrations were increased significantly after 30 min of isolation. Among all of the behavioral categories measured in the EPM, only SAP positively correlated with plasma corticosterone. Metyrapone injected prior to the 24 h isolation period reversed the anxiogenic effects of isolation. Moreover, corticosterone injected into the Cg1 produced a selective increase in SAP. These findings indicate that risk assessment behavior induced by the action of corticosterone on Cg1 neurons initiates a cascade of defensive responses during exposure to stressors.

Identifying a network of brain regions involved in aversion-related processing: a cross-species translational investigation

The ability to detect and respond appropriately to aversive stimuli is essential for all organisms, from fruit flies to humans. This suggests the existence of a core neural network which mediates aversion-related processing. Human imaging studies on aversion have highlighted the involvement of various cortical regions, such as the prefrontal cortex, while animal studies have focused largely on subcortical regions like the periaqueductal gray and hypothalamus. However, whether and how these regions form a core neural network of aversion remains unclear. To help determine this, a translational cross-species investigation in humans (i.e., meta-analysis) and other animals (i.e., systematic review of functional neuroanatomy) was performed. Our results highlighted the recruitment of the anterior cingulate cortex, the anterior insula, and the amygdala as well as other subcortical (e.g., thalamus, midbrain) and cortical (e.g., orbitofrontal) regions in both animals and humans. Importantly, involvement of these regions remained independent of sensory modality. This study provides evidence for a core neural network mediating aversion in both animals and humans. This not only contributes to our understanding of the trans-species neural correlates of aversion but may also carry important implications for psychiatric disorders where abnormal aversive behavior can often be observed.

Midazolam reduces the selective activation of the rhinal cortex by contextual fear stimuli

Independent brain circuits appear to underlie different forms of conditioned fear, depending on the type of conditioning used, such as a context or explicit cue paired with footshocks. Several clinical reports have associated damage to the medial temporal lobe (MTL) with retrograde amnesia. Although a number of studies have elucidated the neural circuits underlying conditioned fear, the involvement of MTL components in the aversive conditioning paradigm is still unclear. To address this issue, we assessed freezing responses and Fos protein expression in subregions of the rhinal cortex and ventral hippocampus of rats following exposure to a context, light or tone previously paired with footshock (Experiment 1). A comparable degree of freezing was observed in the three types of conditioned fear, but with distinct patterns of Fos distribution. The groups exposed to cued fear conditioning did not show changes in Fos expression, whereas the group subjected to contextual fear conditioning showed selective activation of the ectorhinal (Ect), perirhinal (Per), and entorhinal (Ent) cortices, with no changes in the ventral hippocampus. We then examined the effects of the benzodiazepine midazolam injected bilaterally into these three rhinal subregions in the expression of contextual fear conditioning (Experiment 2). Midazolam administration into the Ect, Per, and Ent reduced freezing responses. These findings suggest that contextual and explicit stimuli endowed with aversive properties through conditioning recruit distinct brain areas, and the rhinal cortex appears to be critical for storing context-, but not explicit cue-footshock, associations.

Measuring behavior in mice with chronic stress depression paradigm

Many studies with chronic stress, a common depression paradigm, lead to inconsistent behavioral results. We are introducing a new model of stress-induced anhedonia, which provides more reproducible induction and behavioral measuring of depressive-like phenotype in mice. First, a 4-week stress procedure induces anhedonia, defined by decreased sucrose preference, in the majority of but not all C57BL/6 mice. The remaining 30-50% non-anhedonic animals are used as an internal control for stress effects that are unrelated to anhedonia. Next, a modified sucrose test enables the detection of inter-individual differences in mice. Moreover, testing under dimmed lighting precludes behavioral artifacts caused by hyperlocomotion, a major confounding factor in stressed mice. Finally, moderation of the stress load increases the reproducibility of anhedonia induction, which otherwise is difficult to provide because of inter-batch variability in laboratory mice. We believe that our new mouse model overcomes some major difficulties in measuring behavior with chronic stress depression models.