Excitotoxic lesions of the septum produce anxiolytic effects in the elevated plus-maze and the shock-probe burying tests

A projection from the medial prefrontal cortex to the lateral septum modulates coping behavior on the shock-probe test

Effective coping plays an important role in preventing stress-induced neuropsychiatric conditions. The ventromedial prefrontal cortex (vmPFC) has been associated with active, adaptive coping in humans and rodents. Chronic or severe stress has been shown to induce a maladaptive shift from active to passive coping behavior; however, the neural circuits for effective coping strategies remain unclear. In the current study, we demonstrated that neurons in the infralimbic (IL) subregion of rat vmPFC that project to the lateral septum (LS) were recruited by exposure to the shock probe in the shock-probe defensive burying (SPDB) test. Both chemogenetic inhibition of LS-projecting neurons in the IL and optogenetic inhibition of glutamatergic IL terminals in the LS selectively suppressed active burying responses in the SPDB test in non-stressed rats. In contrast, chemogenetic activation of the IL-LS pathway effectively reversed the shift from active coping to passive immobility in the SPDB test induced by chronic unpredictable stress (CUS). These results indicate that top-down regulation of the LS by a projection from the IL cortex is necessary for an active, adaptive behavioral coping response, and is sufficient to restore active coping that has been compromised by chronic stress. More broadly, these results point to the IL-to-LS circuit as a potential substrate underlying maladaptive shifts from active to passive coping behavior that are often associated with stress-related neuropsychiatric disorders.

Selective Medial Septum Lesions in Healthy Rats Induce Longitudinal Changes in Microstructure of Limbic Regions, Behavioral Alterations, and Increased Susceptibility to Status Epilepticus

Septo-hippocampal pathway, crucial for physiological functions and involved in epilepsy. Clinical monitoring during epileptogenesis is complicated. We aim to evaluate tissue changes after lesioning the medial septum (MS) of normal rats and assess how the depletion of specific neuronal populations alters the animals' behavior and susceptibility to establishing a pilocarpine-induced status epilepticus. Male Sprague-Dawley rats were injected into the MS with vehicle or saporins (to deplete GABAergic or cholinergic neurons; n = 16 per group). Thirty-two animals were used for diffusion tensor imaging (DTI); scanned before surgery and 14 and 49 days post-injection. Fractional anisotropy and apparent diffusion coefficient were evaluated in the fimbria, dorsal hippocampus, ventral hippocampus, dorso-medial thalamus, and amygdala. Between scans 2 and 3, animals were submitted to diverse behavioral tasks. Stainings were used to analyze tissue alterations. Twenty-four different animals received pilocarpine to evaluate the latency and severity of the status epilepticus 2 weeks after surgery. Additionally, eight different animals were only used to evaluate the neuronal damage inflicted on the MS 1 week after the molecular surgery. Progressive changes in DTI parameters in both white and gray matter structures of the four evaluated groups were observed. Behaviorally, the GAT1-saporin injection impacted spatial memory formation, while 192-IgG-saporin triggered anxiety-like behaviors. Histologically, the GABAergic toxin also induced aberrant mossy fiber sprouting, tissue damage, and neuronal death. Regarding the pilocarpine-induced status epilepticus, this agent provoked an increased mortality rate. Selective septo-hippocampal modulation impacts the integrity of limbic regions crucial for certain behavioral skills and could represent a precursor for epilepsy development.

Capillary connections between sensory circumventricular organs and adjacent parenchyma enable local volume transmission

Among contributors to diffusible signaling are portal systems which join two capillary beds through connecting veins (Dorland 2020). Portal systems allow diffusible signals to be transported in high concentrations directly from one capillary bed to the other without dilution in the systemic circulation. Two portal systems have been identified in the brain. The first was discovered almost a century ago and connects the median eminence to the anterior pituitary gland (Popa & Fielding 1930). The second was discovered a few years ago, and links the suprachiasmatic nucleus to the organum vasculosum of the lamina terminalis, a sensory circumventricular organ (CVO) (Yao et al. 2021). Sensory CVOs bear neuronal receptors for sensing signals in the fluid milieu (McKinley et al. 2003). They line the surface of brain ventricles and bear fenestrated capillaries, thereby lacking blood brain barriers. It is not known whether the other sensory CVOs, namely the subfornical organ (SFO), and area postrema (AP) form portal neurovascular connections with nearby parenchymal tissue. This has been difficult to establish as the structures lie at the midline and protrude into the ventricular space. To preserve the integrity of the vasculature of CVOs and their adjacent neuropil, we combined iDISCO clearing and light-sheet microscopy to acquire volumetric images of blood vessels. The results indicate that there is a portal pathway linking the capillary vessels of the SFO and the posterior septal nuclei, namely the septofimbrial nucleus and the triangular nucleus of the septum. Unlike the latter arrangement, the AP and the nucleus of the solitary tract share their capillary beds. Taken together, the results reveal that all three sensory circumventricular organs bear specialized capillary connections to adjacent neuropil, providing a direct route for diffusible signals to travel from their source to their targets.

Context-induced renewal of passive but not active coping behaviours in the shock-probe defensive burying task

Renewal is the return of extinguished responding after removal from the extinction context. Renewal has been extensively studied using classical aversive conditioning procedures that measure a passive freezing response to an aversive conditioned stimulus. However, coping responses to aversive stimuli are complex and can be reflected in passive and active behaviours. Using the shock-probe defensive burying task, we investigated whether different coping responses are susceptible to renewal. During conditioning, male, Long-Evans rats were placed into a specific context (Context A) where an electrified shock-probe delivered a 3 mA shock upon contact. During extinction, the shock-probe was unarmed in either the same (Context A) or a different context (Context B). Renewal of conditioned responses was assessed in the conditioning context (ABA) or in a novel context (ABC or AAB). Renewal of passive coping responses, indicated by an increased latency and a decreased duration of shock-probe contacts, was observed in all groups. However, renewal of passive coping, measured by increased time spent on the side of the chamber opposite the shock-probe, was only found in the ABA group. Renewal of active coping responses linked to defensive burying was not observed in any group. The present findings highlight the presence of multiple psychological processes underlying even basic forms of aversive conditioning and demonstrate the importance of assessing a broader set of behaviours to tease apart these different underlying mechanisms. The current findings suggest that passive coping responses may be more reliable indicators for assessing renewal than active coping behaviours associated with defensive burying.

Feeding behavior elicited by mu opioid and GABA receptor activation in the lateral septum

The lateral septum (LS), a brain region typically associated with behaviors involving reward, anxiety-like behavior, learning, and memory, has recently received increased interest due to its potential role in eating behavior. Our current results showed that morphine (5 μg) microinjected into the LS produced a stable feeding response. Specifically, across five days of repeated injections, there was no increase or sensitization effect, nor a decrease in feeding or tolerance. Additionally, we found that pretreatment with the broad-spectrum opioid receptor antagonist naloxone blocked morphine-elicited feeding, further supporting a role for LS opioid receptors in the activation of feeding behaviors. We had previously found that the GABA_A receptor agonist muscimol produces a similar increase in feeding when injected into the LS. Given the involvement of the LS in multiple behaviors, we next evaluated whether other behaviors might be co-occurring with feeding in response to opioid or GABA_A receptor agonist injection into the LS. We assessed eating, drinking, grooming, sleeping, activity levels and resting behavior for 3 h after injection of aCSF, DAMGO, morphine, or muscimol. We found that morphine and muscimol both decreased the latency to eat, and all drugs tested increased food intake. The feeding occurred within 30 min of muscimol injection but was delayed after opioid injections. The absence of increases in other goal-oriented behavior like drinking or grooming or behavioral hyperactivity supports a primary effect of muscimol and the opioids on LS mechanisms of feeding control. SIGNIFICANCE STATEMENT: The LS is interesting because of its role in a wide range of behaviors including defensive behaviors, social behaviors, learning, memory, and motivation. Although the LS was discovered to have a role in feeding stimulation over 30 years ago, only recently has major progress begun to reveal the underlying mechanisms. The present paper contributes by suggesting that LS GABA_A and μ-opioid receptors elicit eating by inhibiting LS neurons that themselves inhibit eating. Importantly, this work informs lateral septal research which may shed light on disordered eating included binge eating and anorexia.

The role of the lateral septum in neuropsychiatric disease

The lateral septum (LS) is a structure in the midline of the brain that is interconnected with areas associated with stress and feeding. This review highlights the role of the LS in anxiety, depression, and eating disorders and their comorbidity. There is a prevailing view that the LS is anxiolytic. This review finds that the LS is both anxiolytic and anxiogenic. Furthermore, the LS can promote and inhibit feeding. Given these shared roles, the LS represents a common site for the comorbidity of neuropsychiatric disorders, and therefore a potential pharmacological target. This is crucial since currently available treatments are not always effective. Corticotrophin-releasing factor 2 antagonists are potential drugs for the treatment of anxiety and anorexia and require further research. Furthermore, other drugs currently in trials for binge eating, such as alpha-adrenergic agonists, may in fact promote food intake. It is hoped that the advancements in chemo- and optogenetic techniques will allow future studies to profile the specific neural connections of the LS and their function. This information could facilitate our understanding of the underlying mechanisms, and therefore pharmacological targets, of these psychiatric conditions.

Lateral septum as a nexus for mood, motivation, and movement

The lateral septum (LS) has been implicated in a wide variety of functions, including emotional, motivational, and spatial behavior, and the LS may regulate interactions between the hippocampus and other regions that mediate goal directed behavior. In this review, we suggest that the lateral septum incorporates movement into the evaluation of environmental context with respect to motivation, anxiety, and reward to output an 'integrated movement value signal'. Specifically, hippocampally-derived contextual information may be combined with reinforcement or motivational information in the LS to inform task-relevant decisions. We will discuss how movement is represented in the LS and the literature on the LS's involvement in mood and motivation. We will then connect these results to LS movement-related literature and hypotheses about the role of the lateral septum. We suggest that the LS may communicate a movement-scaled reward signal via changes in place-, movement-, and reward-related firing, and that the LS should be considered a fundamental node of affect and locomotor pathways in the brain.

A limbic circuitry involved in emotional stress-induced grooming

Prolonged exposure to negative stressors could be harmful if a subject cannot respond appropriately. Strategies evolved to respond to stress, including repetitive displacement behaviours, are important in maintaining behavioural homoeostasis. In rodents, self-grooming is a frequently observed repetitive behaviour believed to contribute to post-stress de-arousal with adaptive value. Here we identified a rat limbic di-synaptic circuit that regulates stress-induced self-grooming with positive affective valence. This circuit links hippocampal ventral subiculum to ventral lateral septum (LSv) and then lateral hypothalamus tuberal nucleus. Optogenetic activation of this circuit triggers delayed but robust excessive grooming with patterns closely resembling those evoked by emotional stress. Consistently, the neural activity of LSv reaches a peak before emotional stress-induced grooming while inhibition of this circuit significantly suppresses grooming triggered by emotional stress. Our results uncover a previously unknown limbic circuitry involved in regulating stress-induced self-grooming and pinpoint a critical role of LSv in this ethologically important behaviour.

Self-grooming is a frequently observed repetitive behaviour in rodents that is believed to contribute to post-stress de-arousal. The authors identified a previously unknown limbic circuit that includes the ventral lateral septum in rats and is involved in regulating stress-induced self-grooming.

Neuroligin-2 Determines Inhibitory Synaptic Transmission in the Lateral Septum to Optimize Stress-Induced Neuronal Activation and Avoidance Behavior

BACKGROUND

Investigations in the neocortex have revealed that the balance of excitatory and inhibitory synaptic transmission (E/I ratio) is important for proper information processing. The disturbance of this balance underlies many neuropsychiatric illnesses, including autism spectrum disorder and schizophrenia. However, little is known about the contribution of E/I balance to the functioning of subcortical brain regions, such as the lateral septum (LS), a structure that plays important roles in regulating anxiety-related behavior.

METHODS

We manipulated E/I balance in the mouse LS by localized conditional deletion of neuroligin-2, a postsynaptic cell adhesion protein located at gamma-aminobutyric acidergic synapses and important for inhibitory synaptic transmission. We then performed analyses of synaptic transmission in the LS, stress-induced expression of immediate early gene c-fos, and anxiety-related and depression-related behavior.

RESULTS

The absence of neuroligin-2 in the LS in the mature mouse brain resulted in postsynaptic impairment of inhibitory synaptic transmission. Importantly, the reduced inhibition and resulting E/I imbalance decreased the responsiveness of LS neurons to stress. Furthermore, this E/I imbalance in the LS was associated with impaired stress-induced activation of downstream hypothalamic nuclei and reduced avoidance behavior of the animals in the elevated plus maze.

CONCLUSIONS

Our results described the synaptic function of neuroligin-2 in the LS, uncovered a positive association between c-Fos-expressing neurons in the LS and downstream hypothalamic areas and avoidance behavior, and demonstrated that intact inhibitory synaptic transmission and proper E/I balance are required for the optimal functioning of this subcortical circuit.

Modification of anxiety-like behaviors by nociceptin/orphanin FQ (N/OFQ) and time-dependent changes in N/OFQ-NOP gene expression following ethanol withdrawal

Anxiety is a key consequence of ethanol withdrawal and important risk factor for relapse. The neuropeptide nociceptin/orphanin FQ (N/OFQ) or agonists at this peptide's receptor (NOP) exert anxiolytic-like and antistress actions. N/OFQ dysfunction has been linked to both a high-anxiety behavioral phenotype and excessive ethanol intake. Recent studies suggest a possible link between genetic polymorphisms of the NOP transcript and alcoholism. Thus, in the present study, the effects of intracerebroventricularly administered N/OFQ were tested for modification of anxiety-like behaviors, using the shock-probe defensive burying and elevated plus-maze tests, in ethanol-dependent versus non-dependent rats, 1 and 3 weeks following termination of ethanol exposure. Additionally, prepro-N/OFQ (ppN/OFQ) and NOP receptor gene expression was measured in the central nucleus of the amygdala, in the bed nucleus of the stria terminalis and in the lateral hypothalamus at the same timepoints in separate subjects. One week post-ethanol, N/OFQ dose-dependently attenuated elevated anxiety-like behavior in ethanol-dependent rats and produced anxiolytic-like effects in non-dependent controls in both behavioral tests. However, 3 weeks post-ethanol, N/OFQ altered behavior consistent with anxiogenic-like actions in ethanol-dependent rats but continued to exert anxiolytic-like actions in non-dependent controls. These findings were paralleled by ethanol history-dependent changes of ppN/OFQ and NOP gene expression that showed a distinctive time course in the examined brain structures. The results demonstrate that ethanol dependence and withdrawal are associated with neuroadaptive changes in the N/OFQ-NOP system, suggesting a role of this neuropeptidergic pathway as a therapeutic target for the treatment of alcohol abuse.

Distinct roles of segregated transmission of the septo-habenular pathway in anxiety and fear

The posterior septum consisting of the triangular septum (TS) and the bed nucleus of the anterior commissure (BAC) is predominantly linked with the medial habenula (MHb) and has been implicated in the control of anxiety and fear responses. However, its anatomical and functional linkage has largely remained elusive. We established a transgenic mouse model in which the TS and BAC projection neurons were visualized by GFP fluorescence and selectively eliminated by immunotoxin-mediated cell targeting. The linkage between the TS/BAC and the MHb constitutes two parallel pathways composed of the TS-ventral MHb, the core part of the interpeduncular nucleus (IPN), and the BAC-dorsal MHb, the peripheral part of the IPN. Ablation of the TS and BAC projection neurons selectively impaired anxiety and enhanced fear responses and learning, respectively. Inputs from the TS and BAC to the MHb are thus segregated by two parallel pathways and play specialized roles in controlling emotional behaviors.

Lateral septal infusions of the neuropeptide Y Y2 receptor agonist, NPY(13-36) differentially affect different defensive behaviors in male, Long Evans rats

The lateral septum has been extensively implicated in regulating anxiety-related defensive behaviors in the rat. Neuropeptide Y (NPY) contributes to anxiety, likely through activity at the NPY Y1 and/or Y2 receptor binding sites. Although the lateral septum contains the highest density of Y2 receptors in brain, the involvement of this receptor in anxiety-related defensive behaviors is not clear. Thus, the purpose of the current study was to characterize lateral septal Y2 receptor contributions to rats' defensive responses to threat and/or potentially threatening environments. We investigated this by infusing the NPY Y2 agonist NPY13-36 into the lateral septum and testing rats across a battery of animal models of anxiety (Experiment 1). To verify the role of Y2 in mediating the observed effects, rats were pre-infused with the potent and highly selective Y2 antagonist BIIE 0246 prior to infusion with NPY13-36 (Experiment 2). Infusions of NPY13-36 into the lateral septum increased rats' open-arm exploration in the elevated plus-maze test (p<0.01) and decreased the proportion of rats' that buried (p<0.05) as well as their latency to initiate burying in the shock-probe burying test (p<0.01). By contrast, NPY13-36 did not affect either anxiety- or appetite-related responses in the novelty-induced suppression of feeding test (all ps>0.3; Experiment 1). Pre-treatment with the Y2 antagonist BIIE 0246 prevented the anxiolytic-like actions of NPY13-36 in the plus-maze but not in the shock-probe test (Experiment 2). Thus, it appears that the anxiolytic-like actions of lateral septal NPY13-36 are mediated by the Y2 receptor in a test-specific manner.

Social status alters defeat-induced neural activation in Syrian hamsters

Although exposure to social stress leads to increased depression-like and anxiety-like behavior, some individuals are more vulnerable than others to these stress-induced changes in behavior. Prior social experience is one factor that can modulate how individuals respond to stressful events. In this study, we investigated whether experience-dependent resistance to the behavioral consequences of social defeat was associated with a specific pattern of neural activation. We paired weight-matched male Syrian hamsters in daily aggressive encounters for 2 weeks, during which they formed a stable dominance relationship. We also included control animals that were exposed to an empty cage each day for 2 weeks. Twenty-four hours after the final pairing or empty cage exposure, half of the subjects were socially defeated in 3, 5-min encounters, whereas the others were not socially defeated. Twenty-four hours after social defeat, animals were tested for conditioned defeat in a 5-min social interaction test with a non-aggressive intruder. We collected brains after social defeat and processed the tissue for c-Fos immunoreactivity. We found that dominants were more likely than subordinates to counter-attack the resident aggressor during social defeat, and they showed less submissive and defensive behavior at conditioned defeat testing compared with subordinates. Also, social status was associated with distinct patterns of defeat-induced neural activation in select brain regions, including the amygdala, prefrontal cortex, hypothalamus, and lateral septum. Our results indicate that social status is an important form of prior experience that predicts both initial coping style and the degree of resistance to social defeat. Further, the differences in defeat-induced neural activation suggest possible brain regions that may control resistance to conditioned defeat in dominant individuals.

Mineralocorticoid receptors in the medial prefrontal cortex and hippocampus mediate rats' unconditioned fear behaviour

Corticosterone is released from the adrenal cortex in response to stress, and binds to glucocorticosteroid receptors (GRs) and mineralocorticosteroid receptors (MRs) in the brain. Areas such as the dorsal hippocampus (DH), ventral hippocampus (VH) and medial prefrontal cortex (mPFC) all contain MRs and have been previously implicated in fear and/or memory. The purpose of the following experiments was to examine the role of these distinct populations of MRs in rats' unconditioned fear and fear memory. The MR antagonist (RU28318) was microinfused into the DH, VH, or mPFC of rats. Ten minutes later, their unconditioned fear was tested in the elevated plus-maze and the shock-probe tests, two behavioral models of rat "anxiety." Twenty-four hours later, conditioned fear of a non-electrified probe was assessed in rats re-exposed the shock-probe apparatus. Microinfusions of RU28318 into each of the three brain areas reduced unconditioned fear in the shock-probe burying test, but only microinfusions into the VH reduced unconditioned fear in the plus-maze test. RU28318 did not affect conditioned fear of the shock-probe 24hr later. MRs in all three areas of the brain mediated unconditioned fear to a punctate, painful stimulus (probe shock). However, only MRs in the ventral hippocampus seemed to mediate unconditioned fear of the more diffuse threat of open spaces (open arms of the plus maze). In spite of the known roles of the hippocampus in spatial memory and conditioned fear memory, MRs within these sites did not appear to mediate memory of the shock-probe.

Infusions of neuropeptide Y into the lateral septum reduce anxiety-related behaviors in the rat

Neuropeptide Y (NPY) is one of the most abundant peptides in mammalian brain and NPY-like-immunoreactivity is highly expressed in the lateral septum, an area extensively involved in anxiety regulation. NPY counteracts the neurochemical and behavioral responses to acute threat in animal models, and intracerebroventricular (i.c.v.) administration of NPY at low doses is anxiolytic. Less is known about the specific contributions of the lateral septum to NPY-mediated anxiety regulation. In Experiment 1, the effects of infusions of NPY (1.5 μg) into the lateral septum were investigated in three animal models of anxiety: the elevated plus-maze, novelty-induced suppression of feeding, and shock-probe burying tests. Experiment 2 examined the role of the NPY Y1 receptor in these models by co-infusing the Y1 antagonist BIBO 3304 (0.15 μg, 0.30 μg) with NPY into the lateral septum. In the elevated plus-maze, there were no changes in rats' open arm exploration, the index of anxiety reduction in this test. In the novelty-induced suppression of feeding test, rats infused with NPY showed decreases in the latency to consume a palatable snack in a novel (but not familiar) environment, suggesting a reduction in anxiety independent of increases in appetite. This anxiolysis was attenuated by co-infusion with BIBO 3304 (0.30 μg) in Experiment 2. Lastly, rats infused with NPY showed decreases in the duration of burying behavior in the shock-probe burying test, also indicative of anxiety reduction. However, unlike in the feeding test, BIBO 3304 did not attenuate the NPY-induced anxiolysis in the shock-probe test. It is concluded that NPY produces anxiolytic-like actions in the lateral septum in two animal models of anxiety: the novelty-induced suppression of feeding, and shock-probe burying tests, and that this anxiolysis is dependent on Y1 receptor activation in the feeding test.

The modulatory role of the lateral septum on neuroendocrine and behavioral stress responses

The lateral septum (LS) has been shown to have a key role in emotional processes and stress responses. However, the exact role of the LS on stress modulation is not clear, as previous lesion studies mostly used electrolytic lesions, thereby destroying the whole septal area, including medial components and/or fibers of passage. The aim of the present study was therefore, to investigate the effects of selective excitotoxic ablation of the LS on neuroendocrine and behavioral stress responses in rats. Bilateral ibotenic acid lesions of the LS increased hypothalamo-pituitary-adrenocortical (HPA) axis responses to forced swim stress indicated by enhanced plasma ACTH and corticosterone responses and higher stress-induced c-Fos-like immunoreactivity in the paraventricular hypothalamic nucleus. Moreover, LS-lesioned animals showed a more passive coping style in the forced swim test indicated by increased floating and reduced struggling/swimming behavior compared with sham-lesioned controls. Interestingly, intraseptal corticosteroid receptor blockade modulated behavioral stress coping but failed to change HPA axis stress responses. Further experiments aimed at elucidating underlying neurochemical mechanisms revealed that intraseptal administration of the selective 5-HT(1A) receptor antagonist WAY-100635 increased and prolonged stress-induced ACTH and corticosterone levels mimicking lesion effects, while the agonist 8-OH-DPAT suppressed HPA axis activity facilitating the inhibitory role of the LS. In addition, 8-OH-DPAT-injected animals showed increased active and decreased passive coping strategies during forced swimming suggesting antidepressant efficacy. Taken together, our data suggest that the LS promotes active stress coping behavior and is involved in a HPA-inhibitory mechanism that is at least in part mediated by septal 5-HT(1A) receptors and does not involve a glucocorticoid mediated feedback mechanism.

Distribution of mRNAs encoding classical progestin receptor, progesterone membrane components 1 and 2, serpine mRNA binding protein 1, and progestin and ADIPOQ receptor family members 7 and 8 in rat forebrain

Several lines of evidence suggest the existence of multiple progestin receptors that may account for rapid and delayed effects of progesterone in the CNS. The delayed effects have been long attributed to activation of the classical progestin receptor (Pgr). Recent studies have discovered novel progestin signaling molecules that may be responsible for rapid effects. These include progesterone receptor membrane component 1 (Pgrmc1), Pgrmc2, progestin and adipoQ receptor 7 (Paqr7) and Paqr8. The functions of these molecules have been investigated extensively in non-neural, but not in neural tissues, partly because it is unclear which are expressed in the brain and where they are expressed. To address these issues, we compared the distributions of mRNAs encoding Pgr, Pgrmc1, Pgrmc2, Paqr7 and Paqr8 using in situ hybridization with radiolabeled oligodeoxynucleotidyl probes in forebrain tissues of estradiol-treated female rats. We also examined the distribution of serpine mRNA binding protein 1 (Serbp1), a putative binding partner of Pgrmc1. Analyses of adjacent brain sections showed that the highest expression of mRNAs encoding Pgr, Pgrmc1, Pgrmc2 and Serbp1 was detected in several hypothalamic nuclei important for female reproduction. In contrast, expression patterns of Paqr7 and Paqr8 were low and homogeneous in the hypothalamus, and more abundant in thalamic nuclei. The neuroanatomical distributions of these putative progestin signaling molecules suggest that Pgrmc1 and Pgrmc2 may play roles in neuroendocrine functions while Paqr7 and Paqr8 are more likely to regulate sensory and cognitive functions.

Inactivation of the dorsal or ventral hippocampus with muscimol differentially affects fear and memory

It was recently found that temporary inactivation of the dorsal hippocampus with lidocaine impaired fear memory, whereas temporary inactivation of the ventral hippocampus did not. These site-specific deficits, however, may have resulted from disruption of axonal signals arriving from structures outside of the hippocampus, or from disruption of axons that pass through the hippocampus entirely. This is problematic because the hippocampus receives extensive afferent input from both the amygdala and the septum, which also play very important roles in fear and fear memory. To mitigate this problem, rats were infused with the GABA(A) receptor agonist muscimol, into either the dorsal or the ventral hippocampus, just after an "acquisition" session in which the rats were shocked from an electrified probe. A "retention" test in the same apparatus was conducted 24h later, when the hippocampus was no longer inactivated, and the probe was no longer electrified. Dorsal hippocampal inactivation just after acquisition impaired conditioned fear behavior (probe avoidance) during the retention test, whereas ventral hippocampal inactivation after acquisition did not. However, muscimol inactivation of the ventral hippocampus during an "acquisition" session selectively impaired unconditioned fear behavior, replicating earlier findings with lidocaine, a sodium channel blocker. Because muscimol hyperpolarizes neurons through a post-synaptic, GABA(A) receptor-mediated increase of chloride conductance-whereas lidocaine produces indiscriminate disruption of all axonal signalling-its effects are more likely to be restricted to intrinsic neurons within the area of infusion. These results provide strong evidence that afferent input from brain structures located outside of the hippocampus is not responsible for the differential effects of dorsal and ventral hippocampal inactivation on fear memory.

Involvement of septal Cdk5 in the emergence of excessive anxiety induced by stress

The aim of the present study was to evaluate whether the activation of Cdk5, a protein that has been suggested to participate in higher cognitive functions, is required for the onset of a sensitized anxiety-related behavior induced by stress. The exposure to restraint enhanced both Cdk5 expression in certain subareas of the septohippocampal system, principally in the lateral septum (LS) and septal Cdk5 kinase activity in rats. Behaviorally, restrained wild type mice showed a behavior indicative of enhanced anxiety in the elevated plus maze (EPM). In contrast, unstressed mice and stressed knockout mice, which lacked the p35 protein, the natural activator of Cdk5, displayed similar anxiety-like behavior in the EPM. Finally, the intra-LS infusion of olomoucine - a Cdk5 inhibitor - blocked the enhanced anxiety in the EPM induced by prior stress in rats. All these data provide evidence that septal Cdk5 is required in the emergence of a sensitized emotional process induced by stress.

Rats with extended access to cocaine exhibit increased stress reactivity and sensitivity to the anxiolytic-like effects of the mGluR 2/3 agonist LY379268 during abstinence

Metabotropic glutamate 2/3 receptors (mGluR2/3) are emerging targets for the reduction of stress that contributes to drug relapse. The effect of a history of cocaine escalation on stress reactivity during abstinence and the role of mGlu2/3 receptors in stress in these animals were tested. Experiment 1-Rats trained to self-administer cocaine, under short (ShA, 1-h) or long (LgA, 6-h) access conditions, or noncaloric food pellets (Ctrl, 1-h), were tested for stress reactivity in the shock-probe defensive burying test following 1, 14, 42, or 84 days of abstinence. Experiment 2-Experimentally naive rats receiving the mGlu2/3 receptor agonist LY379268 (0, 0.3, 1.0, or 3.0 mg/kg) were tested in the defensive burying test to establish the anxiolytic efficacy of this compound in this model. Experiment 3-Rats with a history of ShA vs LgA cocaine self-administration, or a history of operant responding reinforced by noncaloric food pellets, were tested in the defensive burying test, following administration of LY379268 (0.3, 1.0, or 3.0 mg/kg) at 14 days of abstinence. LgA rats exhibited a two- to threefold increase in defensive burying at 1, 14, and 42 days of abstinence compared to ShA or control animals. LY379268 (3.0 mg/kg) reduced burying in all groups, whereas the 1.0-mg/kg dose reduced burying only in the LgA group. A robust and enduring increase in stress reactivity developed in rats with a history of daily 6-h access to cocaine. The anxiolytic-like effects of LY379268 identify mGlu2/3 receptors as targets for ameliorating stress-associated relapse risk, and point toward the possibility that a history of cocaine escalation in rats may modify glutamatergic function.

Subtype-selective corticotropin-releasing factor receptor agonists exert contrasting, but not opposite, effects on anxiety-related behavior in rats

The corticotropin-releasing factor (CRF) system mediates stress responses. Extrahypothalamic CRF1 receptor activation has anxiogenic-like properties, but anxiety-related functions of CRF2 receptors remain unclear. The present study determined the effects of intracerebroventricular administration of a CRF2 agonist, urocortin 3, on behavior of male Wistar rats in the shock-probe, social interaction, and defensive withdrawal tests of anxiety-like behavior. Equimolar doses of stressin1-A, a novel CRF1 agonist, were administered to separate rats. The effects of pyrazolo[1,5-a]-1,3,5-triazin-4-amine,8-[4-(bromo)-2-chlorophenyl]-N, N-bis(2-methoxyethyl)-2,7-dimethyl-(9Cl) (MJL-1-109-2), a CRF1 antagonist, on behavior in the shock-probe test also were studied. Stressin1-A increased anxiety-like behavior in the social interaction and shock-probe tests. Stressin1-A elicited behavioral activation and defensive burying at lower doses (0.04 nmol), but it increased freezing, grooming, and mounting at 25-fold higher (1-nmol) doses. Conversely, systemic administration of MJL-1-109-2 (10 mg/kg) had anxiolytic-like effects in the shock-probe test. Unlike stressin1-A or MJL-1-109-2, i.c.v. urocortin 3 infusion did not alter anxiety-like behavior in the shock-probe test across a range of doses that reduced locomotion and rearing and increased grooming. Urocortin 3 also did not decrease social interaction, but it decreased anxiety-like behavior in the defensive withdrawal test at a 2-nmol dose. Thus, i.c.v. administration of CRF1 and CRF2 agonists produced differential, but not opposite, effects on anxiety-like behavior. Urocortin 3 (i.c.v.) did not consistently decrease or increase anxiety-like behavior, the latter unlike effects seen previously after local microinjection of CRF2 agonists into the septum or raphe. With increasing CRF1 activation, however, the behavioral expression of anxiety qualitatively changes from "coping" to "noncoping" and offensive, agonistic behaviors.

Microinfusion of pituitary adenylate cyclase-activating polypeptide into the central nucleus of amygdala of the rat produces a shift from an active to passive mode of coping in the shock-probe fear/defensive burying test

High concentrations of pituitary adenylate cyclase-activating polypeptide (PACAP) nerve fibers are present in the central nucleus of amygdala (CeA), a brain region implicated in the control of fear-related behavior. This study evaluated PACAPergic modulation of fear responses at the CeA in male Sprague-Dawley rats. PACAP (50–100 pmol) microinfusion via intra-CeA cannulae produced increases in immobility and time the rats spent withdrawn into a corner opposite to the electrified probe compared to controls in the shock-probe fear/defensive burying test. Shock-probe burying and exploration, numbers of shocks received, locomotion distance, and velocity were all reduced by intra-CeA PACAP injection. Further, intra-CeA PACAP effects were manifested only when the animals were challenged by shock, as intra-CeA PACAP injections did not cause significant changes in the behaviors of unshocked rats. Thus, intra-CeA administration of PACAP produces a distinct reorganization of stress-coping behaviors from active (burying) to passive modes, such as withdrawal and immobility. These findings are potentially significant toward enhancing our understanding of the involvement of PACAP and the CeA in the neural basis of fear and anxiety.

Dissociated roles for the lateral and medial septum in elemental and contextual fear conditioning

Extensive evidence indicates that the septum plays a predominant role in fear learning, yet the direction of this control is still a matter of debate. Increasing data suggest that the medial (MS) and lateral septum (LS) would be differentially required in fear conditioning depending on whether a discrete conditional stimulus (CS) predicts, or not, the occurrence of an aversive unconditional stimulus (US). Here, using a tone CS-US pairing (predictive discrete CS, context in background) or unpairing (context in foreground) conditioning procedure, we show, in mice, that pretraining inactivation of the LS totally disrupted tone fear conditioning, which, otherwise, was spared by inactivation of the MS. Inactivating the LS also reduced foreground contextual fear conditioning, while sparing the higher level of conditioned freezing to the foreground (CS-US unpairing) than to the background context (CS-US pairing). In contrast, inactivation of the MS totally abolished this training-dependent level of contextual freezing. Interestingly, inactivation of the MS enhanced background contextual conditioning under the pairing condition, whereas it reduced foreground contextual conditioning under the unpairing condition. Hence, the present findings reveal a functional dissociation between the LS and the MS in Pavlovian fear conditioning depending on the predictive value of the discrete CS. While the requirement of the LS is crucial for the appropriate processing of the tone CS-US association, the MS is crucial for an appropriate processing of contextual cues as foreground or background information.

Noradrenergic facilitation of shock-probe defensive burying in lateral septum of rats, and modulation by chronic treatment with desipramine

We have previously shown that acute stress-induced release of norepinephrine (NE) facilitates anxiety-like behavioral responses to stress, such as reduction in open-arm exploration on the elevated-plus maze and in social behavior on the social interaction test. Since these responses represent inhibition of ongoing behavior, it is important to also address whether NE facilitates a response that represents an activation of behavior. Correspondingly, it is unknown how a chronic elevation in tonic steady-state noradrenergic (NA) neurotransmission induced by NE reuptake blockade might alter this acute modulatory function, a regulatory process that may be pertinent to the anxiolytic effects of NE reuptake blockers such as desipramine (DMI). Therefore, in this study, we investigated noradrenergic modulation of the shock-probe defensive burying response in the lateral septum (LS). In experiment 1, shock-probe exposure induced an acute 3-fold increase in NE levels measured in LS of male Sprague-Dawley rats by microdialysis. Shock-probe exposure also induced a modest rise in plasma ACTH, taken as an indicator of perceived stress, that returned to baseline more rapidly in rats that were allowed to bury the probe compared to rats prevented from burying by providing them with minimal bedding, indicating that the active defensive burying behavior is an effective coping strategy that reduces the impact of acute shock probe-induced stress. In experiment 2, blockade of either alpha(1)- or beta-adrenergic receptors in LS by local antagonist microinjection immediately before testing reduced defensive burying and increased immobility. In the next experiment, chronic DMI treatment increased basal extracellular NE levels in LS, and attenuated the acute shock probe-induced increase in NE release in LS relative to baseline. Chronic DMI treatment decreased shock-probe defensive burying behavior in a time-dependent manner, apparent only after 2 weeks or more of drug treatment. Moreover, rats treated chronically with DMI showed no significant rise of plasma ACTH in response to shock-probe exposure. Thus, acute stress-induced release of NE in LS facilitated defensive burying, an active, adaptive behavioral coping response. Chronic treatment with the NE reuptake blocker and antidepressant drug DMI attenuated acute noradrenergic facilitation of the active burying response, and also attenuated the level of perceived stress driving that response. These results suggest that long-term regulation of the acute modulatory function of NE by chronic treatment with reuptake blockers may contribute to the mechanisms by which such drugs exert their anxiolytic effects in the treatment of stress-related psychiatric conditions, including depression and anxiety.

Delta opioid receptor ligands modulate anxiety-like behaviors in the rat

The role of the delta opioid receptor in regulating anxiety-like behavior in male Sprague-Dawley rats was examined. Using an elevated plus maze, the effects of the selective delta opioid receptor antagonist naltrindole (1 or 5 mg kg(-1)) and agonist SNC80 (1, 5 or 20 mg kg(-1)) on anxiety-like behavior were measured. Anxiety was also measured following administration of diazepam (3 mg kg(-1)) and amphetamine (1 mg kg(-1)) and compared to the effects of SNC80. Locomotor activity following administration of naltrindole, SNC80, diazepam, and amphetamine was measured. Finally, the defensive burying paradigm was used to confirm the findings from the elevated plus maze. Results demonstrated that SNC80 produced dose-dependent anxiolytic effects similar to that of the classical antianxiety agent, diazepam. Administration of naltrindole caused anxiogenic behavior in rats further supporting the involvement of the delta opioid receptor system in regulating anxiety. Naltrindole also blocked the anxiolytic effects of SNC80. Amphetamine had no effect on anxiety-like behavior. SNC80 induced hyperactivity similar to amphetamine at the doses tested, while naltrindole and diazepam did not significantly affect locomotor activity. Although SNC80 can increase locomotor activity, control experiments reported herein indicate that hyperlocomotion is not sufficient to produce an anxiolytic response on the elevated plus maze. Together with the results from the defensive burying paradigm, this suggests that the effects of SNC80 on reducing anxiety are independent of its effects on locomotion. Collectively these data show that the delta opioid receptor system can regulate anxiety-like behavior in an anxiolytic (agonist) and anxiogenic (antagonist) manner.

Allopregnanolone reduces immobility in the forced swimming test and increases the firing rate of lateral septal neurons through actions on the GABAA receptor in the rat

Since allopregnanolone reduces the total time of immobility in rats submitted to the forced swimming test, we decided to explore whether this neuroactive steroid shares other antidepressant-like actions, such as increasing the neuronal firing rate in the lateral septal nucleus (LSN). In order to discard the influence of the oestrous cycle on immobility and on the firing rate of LSN neurons, all Wistar rats used in the study underwent ovariectomy before treatments. A group of rats received different doses of allopregnanolone (0.5, 1.0, 2.0 and 3.0 mg/kg, i.p.) 1 hour before being forced to swim in order to identify the minimum effective dose diminishing immobility. None of the tested doses of allopregnanolone produced significant changes in motor activity in the open-field test. The minimum dose of allopregnanolone producing a significant reduction in the total time of immobility (p<0.05) against the vehicle was 1.0 mg/kg, while 2.0 mg/kg and above also increased the latency to the first period of immobility (p<0.05). The minimum effective dose of allopregnanolone reducing immobility in the forced swimming test (1.0 mg/kg) significantly (p <0.05) produced a higher (twofold) neuronal firing rate in LSN neurons, but did not produce any change in septofimbrial nucleus neurons, which fired at a rate similar to that of vehicle-treated rats. The pretreatment with the non-competitive GABAA receptor antagonist, picrotoxin (1.0 mg/kg), blocked the aforementioned actions of allopregnanolone on both immobility and LSN firing rate. In conclusion, allopregnanolone produces an antidepressant-like effect in the forced swimming test, associated with an increase in the LSN neuronal firing rate, seemingly mediated by the GABAA receptor.

Genetic deletion of muscarinic M4 receptors is anxiolytic in the shock-probe burying model

We used muscarinic M2 and M4 receptor knockout (KO) mice to further explore the role of the cholinergic system in anxiety. Using the shock-probe burying model we were able to both assess anxiety and cognition. In this paradigm, an anxiolytic response is reflected by decreased burying behavior. In addition, retention latency depicts long-term memory performance. Whereas muscarinic M2 receptor KO mice did not differ behaviorally from wild-type mice, muscarinic M4 receptor KO mice showed increased anxiolysis, but normal long-term memory compared to wild-type mice. Therefore, muscarinic M4 receptors are of particular significance in anxiety modulation that seems dissociated from changes in long-term memory.

Fluoxetine disrupts the integration of anxiety and aversive memories

Anxiety disorders may result from an overexpression of aversive memories. Evidence suggests that the hippocampal cholinergic system could be the point of convergence of anxiety and memory. We propose that clinically effective anxiolytics may exert their effect by interfering with this integration mechanism. To assess anxiety and aversive memory, we used the shock-probe burying test. A reduction in anxiety in this test is indicated by decreased burying, whereas impaired cognition is reflected by an increased number of probe-contacts and/or reduced retention latency. Both an aversive stimulus and the memory of that stimulus significantly increased hippocampal acetylcholine (ACh) levels (Experiment 1). In fact, the memory of the event seemed to be more important than the event itself since the aversive memory induced a greater increase in hippocampal ACh. Injections (i.p.) of fluoxetine (Prozac) reduced burying behavior, while not affecting probe contacts or retention latency (Experiment 2). Although injections of fluoxetine did not affect basal hippocampal ACh efflux (Experiment 3), fluoxetine abolished the increase in ACh induced by the aversive stimulus and the memory of that stimulus (Experiment 4), emphasizing the significance of aversive memories in anxiety disorders. These actions may be mediated by a decrease in the event-related enhancement in cholinergic neurotransmission through M1 cholinergic receptors (Experiment 5). Therefore, anxiety disorders may stem from an unopposed formation of aversive memories and clinically effective anxiolytics hinder the association between emotional and cognitive processing. This reduces the emotional impact of aversive memories, thereby opposing consequent anxiety.

Regulation of affect by the lateral septum: implications for neuropsychiatry

Substantial evidence indicates that the lateral septum (LS) plays a critical role in regulating processes related to mood and motivation. This review presents findings from the basic neuroscience literature and from some clinically oriented research, drawing from behavioral, neuroanatomical, electrophysiological, and molecular studies in support of such a role, and articulates models and hypotheses intended to advance our understanding of these functions. Neuroanatomically, the LS is connected with numerous regions known to regulate affect, such as the hippocampus, amygdala, and hypothalamus. Through its connections with the mesocorticolimbic dopamine system, the LS regulates motivation, both by stimulating the activity of midbrain dopamine neurons and regulating the consequences of this activity on the ventral striatum. Evidence that LS function could impact processes related to schizophrenia and other psychotic spectrum disorders, such as alterations in LS function following administration of antipsychotics and psychotomimetics in animals, will also be presented. The LS can also diminish or enable fear responding when its neural activity is stimulated or inhibited, respectively, perhaps through its projections to the hypothalamus. It also regulates behavioral manifestations of depression, with antidepressants stimulating the activity of LS neurons, and depression-like phenotypes corresponding to blunted activity of LS neurons; serotonin likely plays a key role in modulating these functions by influencing the responsiveness of the LS to hippocampal input. In conclusion, a better understanding of the LS may provide important and useful information in the pursuit of better treatments for a wide range of psychiatric conditions typified by disregulation of affective functions.

Selective antagonism of medial prefrontal cortex D4 receptors decreases fear-related behaviour in rats

It is well known that the mesolimbocortical dopamine pathway is highly active during periods of stress and fear. However, very little research has directly examined how dopamine receptors in this pathway influence fear-related behaviour. The present study examined the effects of selective antagonism of D(4), D(1) and D(2) dopamine receptors of the medial prefrontal cortex (MPFC) on rats' fear behaviour in the elevated plus-maze and the shock-probe burying tests. The results demonstrated that bilateral intra-MPFC infusions of the highly selective D(4) antagonist, L-745 870 (0.2, 1 or 10 nmol/0.5 microL), increased the percentage of open-arm entries and open-arm time in the elevated plus-maze test (1 nmol/0.5 microL), and decreased the duration of burying in the shock-probe test (0.2 or 1 nmol/0.5 microL). Furthermore, none of the doses of the D(4) antagonist affected measures of general activity or pain sensitivity. Intra-MPFC infusions of the D(1) antagonist, SCH-23390 (0.2 or 1 nmol/0.5 microL), or the D(2) antagonist, remoxipride (0.2, 1 or 10 nmol/0.5 microL), had no significant behavioural effects in either test. Taken together, these findings suggest that MPFC D(4) receptors may play an important role in the mediation of fear-related behaviour.

Genetic deletion and pharmacological blockade of CB1 receptors modulates anxiety in the shock-probe burying test

Cannabinoids affect various behavioral processes, including emotion, learning and memory, which may be specifically regulated through the CB1 receptors. The exact role CB1 receptors play in anxiety remains unclear. Both genetic and pharmacological blockade of CB1 receptors have produced inconsistent effects on anxiety. However, these studies examined passive avoidance as an index of anxiety. In the present study, both active and passive avoidance were examined using the shock-probe burying test while CB1 receptors were blocked genetically or pharmacologically. In the shock-probe burying test, anxiety is reflected by increased burying (increased active avoidance) and increased freezing (increased passive avoidance). In addition, probe-contacts may reflect cognitive performance and/or passive avoidance. As there have been few studies examining mouse behavior in the shock-probe burying test, experiment 1 was designed to pharmacologically validate this model in mice. Our results indicated that administration (i.p.) of chlordiazepoxide (4 mg/kg) or FG7412 (5 mg/kg) decreased and increased burying behavior, respectively, without affecting freezing or the number of probe contacts. Experiments 2 and 3 showed that both CB1 knockout mice and mice injected (i.p.) with 3 or 10 mg/kg, but not 1 mg/kg, of the CB1 receptor antagonist SR141716A had lower burying scores, fewer contacts with the probe and similar freezing times compared with wild-type mice and mice injected with vehicle (experiments 2 and 3). Collectively, these results suggest that CB1 receptor blockade reduces some, but not all, aspects of anxiety. The decrease in probe contacts induced by CB1 receptor blockade may be due to enhanced cognition.

Chronic stress alters behavior in the conditioned defensive burying test: role of the posterior paraventricular thalamus

In the present studies, we examined the effects of chronic restraint on behavior in the conditioned defensive burying paradigm, a well-validated test of anxiety. This test is based on the findings that rodents tend to cover or bury the source of a noxious or aversive stimulus. However, little is known about whether prior chronic stress exposure can alter this anxiety-related behavior. In the present study, we examined whether chronic restraint affects indices of behavior in the conditioned defensive burying paradigm. Furthermore, since the posterior division of the paraventricular thalamus (pPVTh) regulates neuroendocrine activity specifically in chronically stressed but not control rats, we hypothesized that the pPVTh may also regulate any chronic stress-induced changes in behavior observed in the defensive burying test. Chronically stressed rats (30-min restraint per day for seven consecutive days) exhibited decreased latency to bury compared to control rats regardless of the presence of lesions suggesting increased reactivity to the shock in these animals. Importantly, pPVTh-lesioned chronically stressed rats exhibited increased duration and height of burying compared to control rats with pPVTh lesions, whereas no differences existed between sham-lesioned control and chronically stressed rats. Since both burying height and duration of burying are considered indices of anxiety in the defensive burying test, the present results suggest that the intact pPVTh may be important in dampening behaviors related to anxiety in chronically stressed rats.

Anxiety is functionally segregated within the septo-hippocampal system

Previous lesion studies have suggested that the septal-hippocampal system is involved in fear and anxiety. In this study we examined the effects on anxiety of temporary neuronal inhibition of various aspects of the septo-hippocampal system in rats. Infusions of tetrodotoxin (TTX) were used to induce reversible lesions in the fimbria fornix, medial septum, dorsal hippocampus, and ventral hippocampus. To assess anxiety we used the elevated plus-maze and the shock-probe burying tests. A reduction in anxiety in the elevated plus-maze is indicated by increased open arm exploration, whereas a reduction in anxiety in the shock-probe burying test is indicated by decreased burying behavior or increased contacts with the shock-probe. The results suggested that inhibition of the septal-hippocampal system induced site-specific anxiolytic effects that vary in nature. Tetrodotoxin lesions of the fimbria fornix increased both open arm exploration and the number of shocks taken by the rats, while having no effect on burying behavior. Both septal and ventral hippocampal lesions increased open arm exploration and decreased burying behavior, but had no effect on the number of probe shocks. Finally, TTX lesions of the dorsal hippocampus increased the number of shocks taken by the rats, but did not affect open arm activity or burying behavior. Neuroanatomical studies indicated that the effect on the number of shocks induced by dorsal hippocampal TTX lesions was not likely mediated by the amygdala. Collectively, the data suggest that the control of specific anxiety reactions is functionally segregated within different aspects of the septo-hippocampal system.

Infusions of midazolam into the medial prefrontal cortex produce anxiolytic effects in the elevated plus-maze and shock-probe burying tests

Previous research has shown that lesions of the medial prefrontal cortex (MPFC) inhibit fear-related behavior in rats (Brain Res. 969 (2003) 183-194). However, at present little is known about the role of specific neurotransmitter receptor systems within the MPFC in the mediation of fear and anxiety. For example, extensive research has demonstrated the effectiveness of benzodiazepines in decreasing fear-related behavior. However, no research has yet been published regarding the effects of micro-infusions of benzodiazepines, or any other GABA-A receptor agonist, into the MPFC. In addition, previous work has suggested that there may be functional differences between the dorsal and ventral subregions of the MPFC in regard to fear and anxiety. Therefore, the present study examined the effects of dorsal and ventral MPFC infusions of the benzodiazepine midazolam in two well-validated animal models of anxiety, the elevated plus maze and the shock probe burying test. The results showed that bilateral (5 microg/side) infusions of midazolam into the MPFC produced anxiolytic effects in both behavioural tests, without affecting general activity or pain sensitivity. Furthermore, these anxiolytic effects were found in both the dorsal and ventral regions of the MPFC. The present findings indicate that the benzodiazepine receptors of the MPFC are capable of modulating fear-related behaviors.

Septal GABAergic and hippocampal cholinergic systems interact in the modulation of anxiety

According to Gray [(1982) The neuropsychology of anxiety: an enquiry into the function of the septo-hippocampal system. Oxford: Oxford University Press; (1991) Neural systems, emotion and personality. In: Neurobiology of learning, emotion, and affect (Madden J, ed), pp 273-306. New York: Raven Press; Gray JA, McNaughton N (2000) The neuropsychology of anxiety. Oxford: Oxford University Press], the septum and the hippocampus act in concert to control anxiety. In the present study we examined a possible interaction between septal GABAergic and hippocampal cholinergic systems in the shock-probe burying test, an animal model of anxiety. In experiment 1, we found that a 10-ng infusion of muscimol in the medial septum produced a significant suppression of burying behavior, whereas lower doses (2.5 and 5.0 ng) did not. In experiment 2, we found a significant suppression of burying behavior after a 20-microg infusion of physostigmine into the dorsal hippocampus, but not after lower-dose infusions (5 and 10 microg). In experiment 3, we infused combined sub-effective doses of physostigmine and muscimol in the hippocampus and medial septum respectively. The combination of sub-effective doses of physostigmine (5 microg) and muscimol (2.5 ng) significantly reduced burying of the shock probe. The results indicate that the hippocampal cholinergic and septal GABAergic systems act synergistically in the modulation of anxiety.

The effects of central administration of physostigmine in two models of anxiety

The effects of intracerebroventricular and intraseptal (the medial septum) administration of a prototypical acetylcholinesterase inhibitor (AChE-I), physostigmine, and a classic benzodiazepine midazolam on rat behavior in the open field test of neophobia and in the conditioned fear test (freezing reaction) were examined in rats. In the open field test of neophobia midazolam and physostigmine increased at a limited dose range, rat exploratory activity, after intracerebroventricular injection. Physostigmine produced in addition the hyperlocomotory effect. Following intraseptal injections, only physostigmine selectively prolonged the time spent by animals in the central sector of the open field. In the model of a conditioned fear, both midazolam and physostigmine inhibited rat freezing reaction to the aversively conditioned context after intracerebroventricular, but not after intraseptal, pretrial drug administration. The presented data support the notion about the selective anxiolytic-like effects of some AChE-Is. It appears, therefore, that the calming and sedative effects of AChE-Is observed in patients with Alzheimer's disease may be directly related to their anxiolytic action, independent of an improvement in cognitive functions, which in turn may decrease disorientation-induced distress and anxiety.

Excitotoxic lesions of the medial prefrontal cortex attenuate fear responses in the elevated-plus maze, social interaction and shock probe burying tests

Previous research investigating the effects of medial prefrontal cortex (MPFC) lesions on fear- and anxiety-related behavior has yielded an inconsistent body of findings. Behavioral studies have reported increases, decreases, and no effect on anxiety. In addition, many studies are complicated by the use of lesioning techniques that destroy fibers of passage, and the use of conditioned fear tests, which may introduce the confounding effects of learning and memory. Therefore, the present study examined the effects of ibotenic acid lesions of the MPFC (including prelimbic, infralimbic and anterior cingulate) on three wide-ranging and well-validated behavioral assays of anxiety: the elevated plus maze (EPM), social interaction (SI) and the shock-probe tests (SP). In the EPM test, lesioned rats showed a significantly higher percentage of open arm entries and open arm time than controls. In a version of the SI test sensitive to anxiolytic effects, lesioned rats were found to spend a significantly greater amount of time in active interaction with a conspecific; while another version of the SI test sensitive to anxiogenic effects did not show any differences between lesioned and non-lesioned controls. In the SP test, lesioned rats exhibited significantly lower rates of burying. In contrast, retention of shock probe avoidance was not affected. No effects of lesions on measures of locomotor activity or shock reactivity were found. The concordant anxiolytic-like effects found in the three behavioral assays strongly suggests a general reduction in fear responsiveness in MPFC lesioned rats.

Defensive burying in rodents: ethology, neurobiology and psychopharmacology

Defensive burying refers to the typical rodent behavior of displacing bedding material with vigorous treading-like movements of their forepaws and shoveling movements of their heads directed towards a variety of noxious stimuli that pose a near and immediate threat, such as a wall-mounted electrified shock-prod. Since its introduction 25 years ago by Pinel and Treit [J. Comp. Physiol. Psychol. 92 (1978) 708], defensive (shock-prod) burying has been the focus of a considerable amount of research effort delineating the methodology/ethology, psychopharmacology and neurobiology of this robust and species-specific active avoidance or coping response. The present review gives a summary of this research with special reference to the behavioral (face and construct) and pharmacological (predictive) validity of the shock-prod burying test as an animal model for human anxiety. Emphasis is also placed on some recent modifications of the paradigm that may increase its utility and reliability as to individual differences in expressed emotional coping responses and sensitivity to pharmacological treatments. Overall, the behavioral and physiological responses displayed in the shock-prod paradigm are expressions of normal and functionally adaptive coping patterns and the extremes of either active (i.e., burying) or passive (i.e., freezing) forms of responding in this test cannot simply be regarded as inappropriate, maladaptive or pathological. For this reason, the shock-prod paradigm is not an animal model for anxiety disorder or for any other psychiatric disease, but instead possesses a high degree of face and construct validity for normal and functionally adaptive human fear and anxious apprehension. However, the apparent good pharmacological validation (predictive validity) of this test reinforces the view that normal and pathological anxiety involves, at least partly, common neurobiological substrates. Therefore, this paradigm is not only suitable for screening potential anxiolytic properties of new drugs, but seems to be especially valuable for unraveling the neural circuitry and neurochemical mechanisms underlying the generation of active and passive coping responses as different expressions of anxiety.

Double dissociation of function within the hippocampus: spatial memory and hyponeophagia

Complete and dorsal hippocampal lesions impaired spatial performance on 2 working memory tasks: rewarded alternation on the T maze and matching to position in the water maze. In contrast, ventral hippocampal lesions had no effect on these tasks, even when task difficulty was increased by the introduction of delays. Ventral lesions did resemble complete lesions in reducing anxiety in 3 commonly used tests of anxiety (social interaction, plus-maze, and hyponeophagia). Dorsal lesions also appeared to be anxiolytic in the social interaction and plus-maze tests, but they did not affect hyponeophagia. Complete- and dorsal-lesioned rats displayed hyperactivity, whereas ventral-lesioned rats did not. These results show a double dissociation between dorsal and ventral hippocampal lesions (hyponeophagia vs. spatial memory), suggesting differentiation of function along the septotemporal axis of this structure.

Dorsal and ventral hippocampal cholinergic systems modulate anxiety in the plus-maze and shock-probe tests

There is emerging evidence that increased acetylcholine levels in brain reduce anxiety. More specifically there is evidence that some of these anxiolytic effects of acetylcholine are modulated by the hippocampus. In the present study we examined the roles of the cholinergic systems in the dorsal and ventral hippocampus in two animal models of anxiety: the elevated plus-maze and the shock-probe burying tests. We found that microinfusions (10 microg/0.5 microl) of the acetylcholinesterase inhibitor physostigmine in either the dorsal or the ventral hippocampus increased rats' open arm exploration in the plus-maze test, and decreased burying behavior in the shock-probe test. Interestingly, infusions in the ventral, but not the dorsal hippocampus also increased the number of contacts rats made with the shock-probe. Overall, the results suggest that cholinergic stimulation in the dorsal and ventral hippocampus modulate anxiety, but that only the ventral hippocampal cholinergic system is involved in the passive avoidance of painful stimuli.

Participation of the lateral septal nuclei (LSN) in the antidepressant-like actions of progesterone in the forced swimming test (FST)

The possible participation of lateral septal nuclei (LSN) in the antidepressant-like actions of progesterone was evaluated. The effect of different concentrations of progesterone (0.001, 0.01 and 0.1 M) or saline solution injected directly into the LSN of ovariectomised rats was determined using the forced swimming test (FST). In addition, the temporal course of progesterone (0.1 M) antidepressant-like actions was compared with that of the classical antidepressant imipramine (0.1 M). Finally, in order to establish the possible participation of the GABA(A) receptor in the antidepressant-like action of progesterone, the effect of pre-treatment with the GABA(A) antagonist picrotoxin (0.125 mg/kg, i.p.) was evaluated. Intraseptally administered progesterone produced a concentration-dependent decrease in immobility behaviour but did not modify locomotor activity. These antidepressant-like actions lasted 4 h, while those of imipramine lasted 72 h. Finally, progesterone-induced anti-immobility effect could be blocked by the systemic injection of picrotoxin. Present results reveal that LSN play a role in the antidepressant-like actions of progesterone that appear to be mediated by the GABA(A) receptor.