Systemic and intra-dorsal periaqueductal gray injections of cholecystokinin sulfated octapeptide (CCK-8s) induce a panic-like response in rats submitted to the elevated T-maze

The state of the art in secondary pharmacology and its impact on the safety of new medicines

Secondary pharmacology screening of investigational small-molecule drugs for potentially adverse off-target activities has become standard practice in pharmaceutical research and development, and regulatory agencies are increasingly requesting data on activity against targets with recognized adverse effect relationships. However, the screening strategies and target panels used by pharmaceutical companies may vary substantially. To help identify commonalities and differences, as well as to highlight opportunities for further optimization of secondary pharmacology assessment, we conducted a broad-ranging survey across 18 companies under the auspices of the DruSafe leadership group of the International Consortium for Innovation and Quality in Pharmaceutical Development. Based on our analysis of this survey and discussions and additional research within the group, we present here an overview of the current state of the art in secondary pharmacology screening. We discuss best practices, including additional safety-associated targets not covered by most current screening panels, and present approaches for interpreting and reporting off-target activities. We also provide an assessment of the safety impact of secondary pharmacology screening, and a perspective on opportunities and challenges in this rapidly developing field.

Sparse genetically defined neurons refine the canonical role of periaqueductal gray columnar organization

During threat exposure, survival depends on defensive reactions. Prior works linked large glutamatergic populations in the midbrain periaqueductal gray (PAG) to defensive freezing and flight, and established that the overarching functional organization axis of the PAG is along anatomically-defined columns. Accordingly, broad activation of the dorsolateral column induces flight, while activation of the lateral or ventrolateral (l and vl) columns induces freezing. However, the PAG contains diverse cell types that vary in neurochemistry. How these cell types contribute to defense remains unknown, indicating that targeting sparse, genetically-defined populations may reveal how the PAG generates diverse behaviors. Though prior works showed that broad excitation of the lPAG or vlPAG causes freezing, we found in mice that activation of lateral and ventrolateral PAG (l/vlPAG) cholecystokinin-expressing (CCK) cells selectively caused flight to safer regions within an environment. Furthermore, inhibition of l/vlPAG-CCK cells reduced predator avoidance without altering other defensive behaviors like freezing. Lastly, l/vlPAG-CCK activity decreased when approaching threat and increased during movement to safer locations. These results suggest CCK cells drive threat avoidance states, which are epochs during which mice increase distance from threat and perform evasive escape. Conversely, l/vlPAG pan-neuronal activation promoted freezing, and these cells were activated near threat. Thus, CCK l/vlPAG cells have opposing function and neural activation motifs compared to the broader local ensemble defined solely by columnar boundaries. In addition to the anatomical columnar architecture of the PAG, the molecular identity of PAG cells may confer an additional axis of functional organization, revealing unexplored functional heterogeneity.

Cholecystokinin octapeptide improves hippocampal glutamatergic synaptogenesis and postoperative cognition by inhibiting induction of A1 reactive astrocytes in aged mice

Aims

Delayed neurocognitive recovery (dNCR) is a common postoperative complication in geriatric surgical patients for which there is no efficacious therapy. Cholecystokinin octapeptide (CCK‐8), an immunomodulatory peptide, regulates memory and learning. Here, we explored the effects and mechanism of action of CCK‐8 on dNCR.

Methods

We applied laparotomy to establish a model of dNCR in aged mice. Morris water maze and fear conditioning tests were used to evaluate cognition. Immunofluorescence was used to detect the density of CCK‐8, A1 reactive astrocytes, glutamatergic synapses, and activation of microglia in the hippocampus. Quantitative PCR was performed to determine mRNA levels of synapse‐associated factors. A1 reactive astrocytes, activated microglia, and glutamatergic synapse‐associated protein levels in the hippocampus were assessed by western blotting.

Results

Administration of CCK‐8 suppressed the activation of microglia, the induction of A1 reactive astrocytes, and the expression of tumor necrosis factor alpha, complement 1q, and interleukin 1 alpha in the hippocampus. Furthermore, it promoted glutamatergic synaptogenesis and neurocognitive recovery in aged dNCR model mice.

Conclusion

Our findings indicated that CCK‐8 alleviated cognitive impairment and promoted glutamatergic synaptogenesis by inhibiting the induction of A1 reactive astrocytes and the activation of microglia. CCK‐8 is, therefore, a potential therapeutic target for dNCR.

The Periaqueductal Gray and Its Extended Participation in Drug Addiction Phenomena

The periaqueductal gray (PAG) is a complex mesencephalic structure involved in the integration and execution of active and passive self-protective behaviors against imminent threats, such as immobility or flight from a predator. PAG activity is also associated with the integration of responses against physical discomfort (e.g., anxiety, fear, pain, and disgust) which occurs prior an imminent attack, but also during withdrawal from drugs such as morphine and cocaine. The PAG sends and receives projections to and from other well-documented nuclei linked to the phenomenon of drug addiction including: (i) the ventral tegmental area; (ii) extended amygdala; (iii) medial prefrontal cortex; (iv) pontine nucleus; (v) bed nucleus of the stria terminalis; and (vi) hypothalamus. Preclinical models have suggested that the PAG contributes to the modulation of anxiety, fear, and nociception (all of which may produce physical discomfort) linked with chronic exposure to drugs of abuse. Withdrawal produced by the major pharmacological classes of drugs of abuse is mediated through actions that include participation of the PAG. In support of this, there is evidence of functional, pharmacological, molecular. And/or genetic alterations in the PAG during the impulsive/compulsive intake or withdrawal from a drug. Due to its small size, it is difficult to assess the anatomical participation of the PAG when using classical neuroimaging techniques, so its physiopathology in drug addiction has been underestimated and poorly documented. In this theoretical review, we discuss the involvement of the PAG in drug addiction mainly via its role as an integrator of responses to the physical discomfort associated with drug withdrawal.

Evidence that increased cholecystokinin (CCK) in the periaqueductal gray (PAG) facilitates changes in Resident-Intruder social interactions triggered by peripheral nerve injury

Individual differences in the effects of a chronic neuropathic injury on social behaviours characterize both the human experience and pre-clinical animal models. The impacts of these changes to the well-being of the individual are often underappreciated. Earlier work from our laboratory using GeneChip® microarrays identified increased cholecystokinin (CCK) gene expression in the periaqueductal gray (PAG) of rats that showed persistent changes in social interactions during a Resident-Intruder encounter following sciatic nerve chronic constriction injury (CCI). In this study, we confirmed these gene regulation patterns using RT-PCR and identified the anatomical location of the CCK-mRNA as well as the translated CCK peptides in the midbrains of rats with a CCI. We found that rats with persistent CCI-induced changes in social behaviours had increased CCK-mRNA in neurons of the ventrolateral PAG and dorsal raphe nuclei, as well as increased CCK-8 peptide expression in terminal boutons located in the lateral and ventrolateral PAG. The functional significance of these changes was explored by microinjecting small volumes of CCK-8 into the PAG of uninjured rats and observing their Resident-Intruder social interactions. Disturbances to social interactions identical to those observed in CCI rats were evoked when injection sites were located in the rostral lateral and ventrolateral PAG. We suggest that CCI-induced changes in CCK expression in these PAG regions contributes to the disruptions to social behaviours experienced by a subset of individuals with neuropathic injury.

Differential effects of cholecystokinin (CCK-8) microinjection into the ventrolateral and dorsolateral periaqueductal gray on anxiety models in Wistar rats

Cholecystokinin (CCK) is one of the main neurohormone peptide systems in the brain, and a major anxiogenic mediator. The periaqueductal gray (PAG) is a key midbrain structure for defensive behaviors, which could include anxiety, fear, or even panic. The CCK system has wide distribution in the PAG, where the dorsolateral region (DL) participates in active defensive behavior and the ventrolateral region (VL) in passive defensive behavior. The aim of this study was to assess the effect of CCK-8 microinjection into DL-PAG or VL-PAG on anxiety-like behavior through two tests: elevated plus maze (EPM) and defensive burying behavior (DBB). CCK-8 (0.5 and 1.0 μg/0.5 μL) presently microinjected into the DL-PAG produced an anxiogenic-like effect on the EPM evidenced by decreasing the time spent/number of entries in open arms compared to vehicle group. Additionally, the latency to burying decreased and burying time increased on the DBB test. Contrarily, CCK-8 microinjected into the VL-PAG resulted in greater open-arm time and more open-arm entries compared to the vehicle-microinjected group. The results on the DBB test confirmed an anxiolytic-like response of CCK-8 into the VL-PAG. In conclusion, CCK-8 microinjected into DL-PAG produced anxiety-like behavior on EPM, and for first time reported on DBB. Contrarily, CCK-8 microinjected into the VL-PAG reduced anxiety-like behavior also for first time reported using both behavioral models EPM and DBB.

The effects of CCK-8S on spatial memory and long-term potentiation at CA1 during induction of stress in rats

Objectives

Cholecystokinin (CCK) has been proposed as a mediator in stress. However, it is still not fully documented what are its effects. We aimed to evaluate the effects of systemic administration of CCK exactly before induction of stress on spatial memory and synaptic plasticity at CA1 in rats.

Materials and Methods

Male Wistar rats were divided into 4 groups: the control, the control-CCK, the stress and the stress-CCK. Restraint stress was induced 6 hr per day, for 24 days. Cholecystokinin sulfated octapeptide (CCK-8S) was injected (1.6 µg/kg, IP) before each session of stress induction. Spatial memory was evaluated by Morris water maze test. Long-term potentiation (LTP) in Schaffer collateral-CA1 synapses was assessed (by 100 Hz tetanization) in order to investigate synaptic plasticity.

Results

Stress impaired spatial memory significantly (P<0.01). CCK in the control rats improved memory (P<0.05), and prevented the impairments in the stress group. With respect to the control group, both fEPSP amplitude and slope were significantly (P<0.05) decreased in the stress group. However, there were no differences between responses of the control-CCK and Stress-CCK groups compared to the control group.

Conclusion

The present results suggest that high levels of CCK-8S during induction of stress can modulate the destructive effects of stress on hippocampal synaptic plasticity and memory. Therefore, the mediatory effects of CCK in stress are likely as compensatory responses.

Effects of repeated treatment with cholecystokinin sulfated octapeptide on passive avoidance memory under chronic restraint stress in male rats

Background:

Cholecystokinin (CCK), a peptide hormone found in the gut is the most abundant peptide neurotransmitter in the brain as well, and its effects on learning, memory, and anxiety have been shown. However, it is not clear whether this substance acts as a mediator for anxiety and stress induction or inhibits them. Hence, the purpose of this study was to evaluate the effects of CCK on memory function under stress conditions.

Materials and Methods:

Male Wistar rats were divided into four groups: The control, the control-CCK, the stress, and stress-CCK. To induce stress, the rats were placed within adjustable restraint chambers for 6 h daily, for 24 days. CCK-8S (cholecystokinin sulfated octapeptide was injected before induction of stress (1.6 μg/kg, intraperitoneal) for 24 days. Passive avoidance learning test was used for evaluation of learning and memory. Rats received foot electrical shock before stress induction and CCK injection and step through latencies were evaluated 1-day after the last session of stress and treatments.

Results:

Stress impaired memory significantly (P < 0.05). Although CCK per se decreased memory (P < 0.05), it prevented the memory impairments in the stress group as there was no significant difference between the control and stress-CCK groups.

Conclusion:

Stress has a profound effect on cognition and CCK probably acts as a mediator for its action. Our results showed that a high concentration of CCK during stress may be helpful in alleviating the effects of stress on the brain.

Prefrontal cortical circuit for depression- and anxiety-related behaviors mediated by cholecystokinin: role of ΔFosB

Decreased medial prefrontal cortex (mPFC) neuronal activity is associated with social defeat-induced depression- and anxiety-like behaviors in mice. However, the molecular mechanisms underlying the decreased mPFC activity and its prodepressant role remain unknown. We show here that induction of the transcription factor ΔFosB in mPFC, specifically in the prelimbic (PrL) area, mediates susceptibility to stress. ΔFosB induction in PrL occurred selectively in susceptible mice after chronic social defeat stress, and overexpression of ΔFosB in this region, but not in the nearby infralimbic (IL) area, enhanced stress susceptibility. ΔFosB produced these effects partly through induction of the cholecystokinin (CCK)-B receptor: CCKB blockade in mPFC induces a resilient phenotype, whereas CCK administration into mPFC mimics the anxiogenic- and depressant-like effects of social stress. We previously found that optogenetic stimulation of mPFC neurons in susceptible mice reverses several behavioral abnormalities seen after chronic social defeat stress. Therefore, we hypothesized that optogenetic stimulation of cortical projections would rescue the pathological effects of CCK in mPFC. After CCK infusion in mPFC, we optogenetically stimulated mPFC projections to basolateral amygdala or nucleus accumbens, two subcortical structures involved in mood regulation. Stimulation of corticoamygdala projections blocked the anxiogenic effect of CCK, although no effect was observed on other symptoms of social defeat. Conversely, stimulation of corticoaccumbens projections reversed CCK-induced social avoidance and sucrose preference deficits but not anxiogenic-like effects. Together, these results indicate that social stress-induced behavioral deficits are mediated partly by molecular adaptations in mPFC involving ΔFosB and CCK through cortical projections to distinct subcortical targets.

Neuronal network of panic disorder: the role of the neuropeptide cholecystokinin

Panic disorder (PD) is characterized by panic attacks, anticipatory anxiety and avoidance behavior. Its pathogenesis is complex and includes both neurobiological and psychological factors. With regard to neurobiological underpinnings, anxiety in humans seems to be mediated through a neuronal network, which involves several distinct brain regions, neuronal circuits and projections as well as neurotransmitters. A large body of evidence suggests that the neuropeptide cholecystokinin (CCK) might be an important modulator of this neuronal network. Key regions of the fear network, such as amygdala, hypothalamus, peraqueductal grey, or cortical regions seem to be connected by CCKergic pathways. CCK interacts with several anxiety-relevant neurotransmitters such as the serotonergic, GABA-ergic and noradrenergic system as well as with endocannabinoids, NPY and NPS. In humans, administration of CCK-4 reliably provokes panic attacks, which can be blocked by antipanic medication. Also, there is some support for a role of the CCK system in the genetic pathomechanism of PD with particularly strong evidence for the CCK gene itself and the CCK-2R (CCKBR) gene. Thus, it is hypothesized that genetic variants in the CCK system might contribute to the biological basis for the postulated CCK dysfunction in the fear network underlying PD. Taken together, a large body of evidence suggests a possible role for the neuropeptide CCK in PD with regard to neuroanatomical circuits, neurotransmitters and genetic factors. This review article proposes an extended hypothetical model for human PD, which integrates preclinical and clinical findings on CCK in addition to existing theories of the pathogenesis of PD.

Anxiolytic- and panicolytic-like effects of Neuropeptide S in the mouse elevated T-maze

Neuropeptide S (NPS) regulates various biological functions by selectively activating the NPS receptor (NPSR). Recently, epidemiological studies revealed an association between NPSR single nucleotide polymorphisms and susceptibility to panic disorders. Here we investigated the effects of NPS in mice subjected to the elevated T maze (ETM), an assay which has been proposed to model anxiety and panic. Diazepam [1 mg/kg, intraperitoneally (i.p.)] elicited clear anxiolytic effects reducing the latency to emerge from the closed to the open (CO) arm without modifying the latencies from the open to the closed (OC) arm. By contrast, chronic fluoxetine (10 mg/kg i.p., once a day for 21 days) selectively increased OC latency, suggesting a panicolytic-like effect. NPS given intracerebroventricularly at 0.001-1 nmol elicited both anxiolytic- and panicolytic-like effects. However, although the NPS anxiolytic dose-response curve displayed the classical sigmoidal shape, the dose-response curve of the putative panicolytic-like effect was bell shaped with peak effect at 0.01 nmol. The behaviour of wild-type [NPSR(+/+)] and receptor knock out [NPSR(-/-)] mice in the ETM task was superimposable. NPS at 0.01 nmol elicited anxiolytic- and panicolytic-like effects in NPSR(+/+) but not in NPSR(-/-) mice. In conclusion, this study demonstrated that NPS, via selective activation of the NPSR, promotes both anxiolytic- and panicolytic-like actions in the mouse ETM.

fMRI fingerprint of unconditioned fear-like behavior in rats exposed to trimethylthiazoline

Unconditioned fear plays an important yet poorly understood role in anxiety disorders, and only few neuroimaging studies have focused on evaluating the underlying neuronal mechanisms. In rodents the predator odor trimethylthiazoline (TMT), a synthetic component of fox feces, is commonly used to induce states of unconditioned fear. In this study, arterial spin labeling-based functional magnetic resonance imaging (fMRI) was applied to detect TMT-induced regional modulations of neuronal activity in Wistar rats. During TMT exposure the rats displayed increased freezing behavior and reduced exploration in the odor-associated area. Neuronal activity was selectively increased in the dorsal periaqueductal gray, superior colliculus and medial thalamus and reduced in the median raphe, locus coeruleus, nucleus accumbens shell, ventral tegmental area, ventral pallidum and entorhinal piriform cortex. This fMRI fingerprint involving distinct neuronal pathways was used to describe a schematic model of fear processing. Key brain areas known to underlie fear and anxiety-related autonomic and behavioral responses as well as centers of motivational processing were identified as being part of this functional circuitry of innate fear. Thus, preclinical fMRI studies based on unconditioned fear methods may provide a valuable translational approach to better characterize etiological and pathological processes underlying anxiety disorders.

The age of anxiety: role of animal models of anxiolytic action in drug discovery

Anxiety disorders are common, serious and a growing health problem worldwide. However, the causative factors, aetiology and underlying mechanisms of anxiety disorders, as for most psychiatric disorders, remain relatively poorly understood. Animal models are an important aid in giving insight into the aetiology, neurobiology and, ultimately, the therapy of human anxiety disorders. The approach, however, is challenged with a number of complexities. In particular, the heterogeneous nature of anxiety disorders in humans coupled with the associated multifaceted and descriptive diagnostic criteria, creates challenges in both animal modelling and in clinical research. In this paper, we describe some of the more widely used approaches for assessing the anxiolytic activity of known and potential therapeutic agents. These include ethological, conflict-based, hyponeophagia, vocalization-based, physiological and cognitive-based paradigms. Developments in the characterization of translational models are also summarized, as are the challenges facing researchers in their drug discovery efforts in developing new anxiolytic drugs, not least the ever-shifting clinical conceptualization of anxiety disorders. In conclusion, to date, although animal models of anxiety have relatively good validity, anxiolytic drugs with novel mechanisms have been slow to emerge. It is clear that a better alignment of the interactions between basic and clinical scientists is needed if this is to change.

Repeated administration of methamphetamine blocked cholecystokinin-octapeptide injection-induced c-fos mRNA expression without change in capsaicin-induced junD mRNA expression in rat cerebellum

In the cerebellum, there are numerous cholecystokinin (CCK-8)-containing fibers. Since systemic CCK-8 injection-induced anxiety (psychological stress) activates the locus coeruleus cells that send mossy fiber inputs to the cerebellum, we examined whether systemic CCK-8 injections activate the rat and mouse cerebellum. First, injections of CCK-8 were found to induce c-fos mRNA expression in a vague patchy pattern that is different from single methamphetamine-induced Zebrin band-like c-fos mRNA expression, suggesting that the CCK-8 activating mossy fibers induce gene expression differently from the dopamine-containing mossy fibers in the ventral tegmental area. Second, since CCK-8 facilitates neural activity of dopamine in the midbrain, we examined whether repeated methamphetamine administration that induced behavioral sensitization had similar effects on the cerebellar CCK system. Repeated administration of methamphetamine suppressed the CCK-8-induced c-fos mRNA expression in the rat cerebellum. Third, capsaicin injections (physical stress) into a hind limb of the rat increased junD mRNA expression with no effect on c-fos mRNA expression, and repeated methamphetamine injections had no effect on the capsaicin-induced expression of junD mRNA. Fourth, either single injection of methamphetamine or CCK-8 to mice increased c-fos mRNA expression in the locus coeruleus, and so noradrenalin, but not dopamine, might interact with CCK-8-activating system. However, we considered the possibility unlikely. Thus, we conclude that repeated methamphetamine administration though dopamine selectively inhibits the c-fos mRNA expression after CCK-8 injection in the cerebellum.

Adenylyl cyclase-5 activity in the nucleus accumbens regulates anxiety-related behavior

Type 5 adenylyl cyclase (AC5) is highly concentrated in the dorsal striatum and nucleus accumbens (NAc), two brain areas which have been implicated in motor function, reward, and emotion. Here we demonstrate that mice lacking AC5 (AC5-/-) display strong reductions in anxiety-like behavior in several paradigms. This anxiolytic behavior in AC5-/- mice was reduced by the D(1) receptor antagonist SCH23390 and enhanced by the D(1) dopamine receptor agonist, dihydrexidine (DHX). DHX-stimulated c-fos induction in AC5-/- mice was blunted in the dorso-lateral striatum, but it was overactivated in the dorso-medial striatum and NAc. The siRNA-mediated inhibition of AC5 levels within the NAc was sufficient to produce an anxiolytic-like response. Microarray and RT-PCR analyses revealed an up-regulation of prodynorphin and down-regulation of cholecystokinin (CCK) in the NAc of AC5-/- mice. Administration of nor-binaltorphimine (a kappa opioid receptor antagonist) or CCK-8s (a CCK receptor agonist) reversed the anxiolytic-like behavior exhibited by AC5-/- mutants. Taken together, these results suggest an essential role of AC5 in the NAc for maintaining normal levels of anxiety.

Significant increase in the aggressive behavior of transgenic mice overexpressing peripheral progastrin peptides: associated changes in CCK2 and serotonin receptors in the CNS

The gastrin precursor peptide, progastrin (PG), is secreted from enteroendocrine cells in the intestine and increased in patients with hypergastrinemia and colorectal cancers. In recent years, we and others have demonstrated an important role of PG peptides in colorectal carcinogenesis, and were surprised to note significant changes in the behaviors of transgenic mice overexpressing PGs. In the present studies, we examined emotional behaviors of transgenic mice overexpressing PG in the intestinal and peripheral circulation. Aggression, locomotor activity and anxiety-like behaviors of the homozygous transgenic (Tg/Tg) mice and the wild-type (WT) littermates were examined by intruder/resident test, open field and elevated plus maze, respectively. A significant increase in the aggression, locomotor activity, and anxiety-like behaviors was detected in the Tg/Tg vs WT mice. As CCK, CCK(2) receptors (CCK(2)R), and 5-HT(1A) receptors (5-HT(1A)R) in the CNS play an important role in these behaviors, possible changes in the expression of CCK and CCK(2)R and the density of CCK(2)R and 5-HT(1A)R were determined by either real-time RT-PCR or autoradiography of ligand binding assays. The results suggest that the expressions of CCK and CCK(2)R were increased in the hypothalamus, and the density of CCK(2)R were increased in the hypothalamus and amygdala of Tg/Tg vs WT mice. Similarly, the density of 5-HT(1A)R was increased in the hypothalamus. Our results suggest that an upregulation of the CCK response system and 5-HT(1A)R in the hypothalamus of Tg/Tg mice may mediate the alterations in the observed behaviors of these mice.

Cholecystokinin-2 receptors modulate freezing and escape behaviors evoked by the electrical stimulation of the rat dorsolateral periaqueductal gray

Systemic injection of the cholecystokinin type 2 (CCK(2)) receptor agonist CCK-4 evokes panic attacks in humans and facilitates the expression of a panic-related defensive behavior, escape, in rats. Given the prominent role attributed to the dorsal periaqueductal gray (dPAG) in the pathophysiology of panic, this midbrain area has been assumed to be one of the key regions mediating these effects of CCK-4. However, only a few studies have directly investigated the role of dPAG CCK(2) receptors in the regulation of panic-related behaviors. Even more disappointingly, the results of these investigations have been far from conclusive. In the present study we further addressed this issue by evaluating the effect of the intra-dorsolateral periaqueductal gray (dlPAG) injection of CCK-4 on two panic-related defensive behaviors, freezing and escape, evoked in male Wistar rats by the electrical stimulation of the dlPAG. The effects of CCK-4 (0.005-0.5 microg/0.2 microl) were compared to those caused by the local microinjection of the CCK(2) receptor antagonist LY225910 (0.001-1.0 microg/0.2 microl). The results showed that whereas CCK-4 facilitated the expression of both freezing and escape behaviors, LY225910 had the opposite effect. Pretreatment with an ineffective dose of LY225910 prevented the panicogenic-like effect of CCK-4. These results strengthen the view that CCK(2) receptors located in the dlPAG are involved in the regulation of panic-related behaviors and may mediate the effect of CCK-4 on panic.

Enhanced reactivity of 5-HT1A receptors in the rat dorsal periaqueductal gray matter after chronic treatment with fluoxetine and sertraline: Evidence from the elevated T-maze

Behavioral evidence indicates that sensitization of 5-HT1A and 5-HT2A receptors in the dorsal periaqueductal gray (DPAG) may underlie the therapeutic effect of serotonin reuptake inhibitors (SRIs) in panic disorder. These results were obtained from studies using animal models that associate escape behavior with panic attacks, such as the elevated T-maze. In this test, chronic administration of the non-selective SRI imipramine enhances the inhibitory effect on escape caused by the intra-DPAG injection of the 5-HT1A receptor agonist 8-OH-DPAT. We here evaluated the generality of this finding by investigating the effect of chronic administration of two selective SRIs (SSRIs), fluoxetine and sertraline, on the reactivity of the rat DPAG 5-HT1A receptors. The results showed that both SSRIs inhibited escape behavior in the elevated T-maze, suggestive of a panicolytic-like effect. Whereas intra-DPAG injection of a low dose of 8-OH-DPAT (0.4nmol) had no effect on escape in control animals, it significantly enhanced the inhibitory effect caused by the SSRIs on this response. Microinjection of 8-OH-DPAT in SSRI-treated rats also inhibited the acquisition of inhibitory avoidance, another defensive response measured by the elevated T-maze. The results indicate that chronic administration of fluoxetine and sertraline sensitizes 5-HT1A receptors in the DPAG. Overall, they support the view that facilitation of 5-HT1A receptor-mediated neurotransmission in the DPAG is implicated in the therapeutic effect of SRIs on panic disorder.

Elevated mazes as animal models of anxiety: effects of serotonergic agents

This article reviews reported results about the effects of drugs that act upon the serotonergic neurotransmission measured in three elevated mazes that are animal models of anxiety. A bibliographic search has been performed in MEDLINE using different combinations of the key words X-maze, plus-maze, T-maze, serotonin and 5-HT, present in the title and/or the abstract, with no time limit. From the obtained abstracts, several publications were excluded on the basis of the following criteria: review articles that did not report original results, species other than the rat, intracerebral drug administration alone, genetically manipulated rats, and animals having any kind of experimental pathology. The reported results indicate that the effect of drugs on the inhibitory avoidance task performed in the elevated T-maze and on the spatio temporal indexes of anxiety measured in the X and plus mazes correlate with their effect in patients diagnosed with generalized anxiety disorder. In contrast, the drug effects on the one-way escape task in the elevated T-maze predict the drug response of panic disorder patients. Overall, the drug effects assessed with the avoidance task in the T-maze are more consistent than those measured through the anxiety indexes of the X and plus mazes. Therefore, the elevated T-maze is a promising animal model of generalized anxiety and panic disorder.

Involvement of dorsolateral periaqueductal gray N-methyl-D-aspartic acid glutamate receptors in the regulation of risk assessment and inhibitory avoidance behaviors in the rat elevated T-maze

The involvement of the dorsolateral periaqueductal gray in the regulation of fear-related behaviors such as escape and freezing is well established. It is still a matter of investigation, however, whether this midbrain area may have a relevant role in the modulation of more subtle defensive responses associated with anxiety such as risk assessment and inhibitory avoidance. By stimulating N-methyl-D-aspartic acid glutamate receptors located in the dorsolateral periaqueductal gray with its prototypical agonist N-methyl-D-aspartic acid (50 pmol), we report here an increase in both risk assessment and inhibitory avoidance behaviors of male Wistar rats tested in the elevated T-maze. These results are indicative of an anxiogenic-like effect. The selective N-methyl-D-aspartic acid receptor antagonist DL-2-amino-7-phosphonoheptanoic acid (2.0 and 4.0 nmol) had the opposite effect on both defensive tasks. Pretreatment with an ineffective dose of DL-2-amino-7-phosphonoheptanoic acid (1.0 nmol) prevented the N-methyl-D-aspartic acid anxiogenic-like effect. At the dose range of DL-2-amino-7-phosphonoheptanoic acid and/or N-methyl-D-aspartic acid tested, neither the escape response from one of the elevated T-maze open arms nor the general exploratory activity as assessed in the open-field test was affected. The present results suggest that the dorsolateral periaqueductal gray column is also involved in the regulation of defensive behaviors related to anxiety, and N-methyl-D-aspartic acid glutamate receptors are recruited for this action.

Neuronal excitability in the periaqueductal grey matter during the estrous cycle in female Wistar rats

Extracellular recordings were made from output neurons in the dorsal half of the periaqueductal gray matter (dPAG) in urethane-anesthetized female Wistar rats. All the neurons were quiescent. A basal level of firing was therefore induced by continuous iontophoretic application of D,L-homocysteic acid (DLH). In the presence of the GABA(A) receptor antagonist bicuculline methiodide (BIC 0-30 nA) the DLH-induced firing increased further, revealing the presence of ongoing GABAergic inhibitory tone on the recorded neurons. The BIC-induced increase in firing rate was significantly greater in neurons recorded during estrus (Est) and late diestrus (LD) compared with proestrus (Pro) and early diestrus (ED) suggesting that GABAergic tone was lower in Est and LD. I.v. injection of the panicogenic cholecystokinin (CCK)(B) receptor agonist pentagastrin (PG, 40 microg kg(-1)) produced an increase in firing rate in 12/17 (70%) of neurons tested in the dPAG. Iontophoretic application of PG (10-30 nA) also produced a current-related increase in firing rate in 73.6% of the neurons tested. The excitatory response was reduced during application of the selective CCK(B) receptor antagonist beta-[2-([2-(8-azaspiro[4.5]dec-8-ylcarbonyl)-4,6-dimethylphenyl]amino)-2-oxoethyl]-(R)-napthalenepropanoic acid (CR2945) (60 nA, n=6). The PG-evoked increase in firing rate was significantly greater in neurons recorded during Est and LD compared with during Pro and ED. Juxtacellular labeling with neurobiotin in eight neurons revealed multipolar cells 12-44 microm diameter with up to six primary dendrites. In three of eight neurons, a filled axon was present and coursed without branching toward the perimeter of the periaqueductal gray matter (PAG). The estrous cycle-related change in responsiveness to BIC and PG suggests that the panic circuitry in the PAG may become more responsive to panicogenic agents during estrus and late diestrus as a consequence of a decrease in the intrinsic level of inhibitory GABAergic tone. The findings may have implications for understanding the neural processes that underlie the development of premenstrual dysphorias in women.

Rats displaying distinct exploratory activity also have different expression patterns of gamma-aminobutyric acid- and cholecystokinin-related genes in brain regions

The aim of the present study was to evaluate the expression of genes in relation to the exploratory activity of rats. Limbic system-associated membrane protein (LsAMP) gene, gamma-aminobutyric acid-(GABA)- and cholecystokinin-(CCK)-related genes were analyzed in the frontal cortex, amygdala and periaqueductal gray (PAG) after exposure of rats to exploratory challenge. Two groups of animals displaying low and high exploratory activity in the elevated plus-maze were selected for gene expression studies from the population of 43 male Wistar rats. Eight rats were taken randomly from the same cages as animals exposed to the plus-maze, but they were not subjected to the exploratory test. This home-cage control group was also used for gene expression analysis in order to explore a possible impact of the plus-maze exposure. Rats with low and high exploratory activity displayed clearly distinct profiles in gene expression. Most prominent alterations were established in the amygdala where almost all GABA-related and CCK receptor genes were two- to five-fold up-regulated in low exploratory activity rats compared to high exploratory activity and home-cage control group. The expression of several GABA-related genes was also increased in the PAG of animals displaying low exploratory activity compared to the other groups. Moreover, we found reduced expression of GABA- and CCK-related genes in all three brain regions in animals with high exploratory activity compared to the home-cage control group. In addition to these findings, we established a significantly increased expression of the LsAMP gene in the amygdala and PAG of low exploratory activity animals compared with the other groups. In conclusion, low and high exploratory activity rats differed not only by their exploratory activity but also displayed opposite gene expression patterns. Low exploratory activity of rats correlated with the up-regulation of LsAMP and GABA-related genes in the amygdala and PAG and with the up-regulation of CCK receptors in the amygdala. High exploratory activity of rats was related to a significant down-regulation of CCK receptor genes in the amygdala and PAG. These rats also displayed the reduced expression of GABA-related genes in the frontal cortex and PAG.

Angiotensin-II is a putative neurotransmitter in lactate-induced panic-like responses in rats with disruption of GABAergic inhibition in the dorsomedial hypothalamus

Intravenous sodium lactate infusions or the noradrenergic agent yohimbine reliably induce panic attacks in humans with panic disorder but not in healthy controls. However, the exact mechanism of lactate eliciting a panic attack is still unknown. In rats with chronic disruption of GABA-mediated inhibition in the dorsomedial hypothalamus (DMH), achieved by chronic microinfusion of the glutamic acid decarboxylase inhibitor L-allylglycine, sodium lactate infusions or yohimbine elicits panic-like responses (i.e., anxiety, tachycardia, hypertension, and tachypnea). In the present study, previous injections of the angiotensin-II (A-II) type 1 receptor antagonist losartan and the nonspecific A-II receptor antagonist saralasin into the DMH of "panic-prone" rats blocked the anxiety-like and physiological components of lactate-induced panic-like responses. In addition, direct injections of A-II into the DMH of these panic-prone rats also elicited panic-like responses that were blocked by pretreatment with saralasin. Microinjections of saralasin into the DMH did not block the panic-like responses elicited by intravenous infusions of the noradrenergic agent yohimbine or by direct injections of NMDA into the DMH. The presence of the A-II type 1 receptors in the region of the DMH was demonstrated using immunohistochemistry. Thus, these results implicate A-II pathways and the A-II receptors in the hypothalamus as putative substrates for sodium lactate-induced panic-like responses in vulnerable subjects.

Lack of interaction between NMDA and cholecystokinin-2 receptor-mediated neurotransmission in the dorsolateral periaqueductal gray in the regulation of rat defensive behaviors

Several neurotransmitters, including GABA, serotonin, glutamate, and cholecystokinin, modulate defensive behaviors in the dorsolateral periaqueductal gray (dlPAG). Although both glutamate and cholecystokinin have been shown to facilitate these behaviors, a possible interaction between them remains to be examined. The present study investigates whether activation or antagonism of N-methyl-D-aspartic acid (NMDA) glutamate and cholecystokinin 2 (CCK(2)) receptors located in the dlPAG would interact in animals tested in the elevated T-maze. The effect of the NMDA (50 pmol) was evaluated in rats pretreated with the CCK(2) receptor antagonist LY225910 (0.05 nmol). In addition, the effect of the CCK(2) receptor agonist CCK-4 (0.08 nmol) was evaluated in rats pretreated with the NMDA receptor antagonist AP-7 (1.0 nmol). Intra-dlPAG injection of NMDA increased risk assessment and inhibitory avoidance behaviors. This NMDA anxiogenic-like effect was unaltered by the pretreatment with LY225910. Similarly, the shortening of escape latencies induced by CCK-4 was unaffected by AP-7. No drug changed the general exploratory activity as assessed in the open-field. These results, showing that the activation of dlPAG NMDA or CCK(2) receptors facilitate anxiety- and fear-related behaviors, further implicate glutamate and cholecystokinin-mediated neurotransmission in this midbrain area on modulation of defensive behaviors. However, the regulatory action of these two excitatory neurotransmitters seems to be exerted through independent mechanisms.

Cardiovascular and respiratory responses to a panicogenic agent in anaesthetised female Wistar rats at different stages of the oestrous cycle

In urethane-anaesthetised female Wistar rats, intravenous injection of the panicogenic CCK(B) receptor agonist pentagastrin (0.002-80 microg/kg) evoked a dose-related increase in blood pressure, heart rate and ventilation. The response was blocked in the presence of the selective CCK(B) receptor antagonist CR2945 (1 mg/kg i.v.). The same pattern of cardiovascular and respiratory changes was evoked by microinjection of pentagastrin (0.3 nmol in 250 nL) into the dorsal half of the periaqueductal grey matter (PAG). The effect of intra-PAG administration of pentagastrin was also abolished following injection of CR2945 (1 mg/kg, i.v.). Responsiveness to systemically administered pentagastrin was enhanced in rats in late dioestrus. At the highest dose tested (80 microg/kg), the pressor response, tachycardia and tachypnoea evoked in rats in late dioestrus was significantly higher than rats in proestrus. For rats in oestrus, the pressor response and tachycardia but not tachypnoea were also significantly larger than the response evoked in rats in early dioestrus. The results suggest that the dorsal half of the PAG (dPAG) plays a key role in mediating the cardiovascular and respiratory responses evoked by systemically administered CCK(B) agonists. The enhanced responsiveness to panicogenic agents during late dioestrus may be related to changes in the functional responsiveness of gamma-aminobutyric acid (GABA)ergic circuitry in the dPAG due to plasticity of GABA(A) receptor subunit expression as a consequence of falling progesterone levels.

Involvement of dorsolateral periaqueductal gray cholecystokinin-2 receptors in the regulation of a panic-related behavior in rats

Cholecystokinin (CCK) has been implicated in anxiety disorders. The midbrain periaqueductal gray (PAG), which modulates anxiety and panic reactions, contains CCK-immunoreactive fibers and CCK(2) receptors. The present study investigated the involvement of CCK(2) receptors of the PAG dorsolateral subdivision (dlPAG) in the regulation of inhibitory avoidance and escape, two defensive behaviors that have been related in terms of psychopathology to generalized-anxiety and panic disorders, respectively. Male Wistar rats were microinjected in the dlPAG with the CCK(2) receptor agonist cholecystokinin-tetrapeptide (CCK-4; 0.08-0.32 nmol/0.2 microL), the CCK(2) receptor antagonist LY-225910 (0.05-0.20 nmol/0.2 microL) or LY-225910 prior to CCK-4. Inhibitory avoidance and escape behaviors were evaluated in the elevated T-maze. Whereas CCK-4 facilitated escape, indicating a panic-like action, LY-225910 had the opposite effect. Pretreatment with a non-effective dose of LY-225910 prevented the panic-eliciting action of CCK-4. Neither CCK-4 nor LY-225910 affected inhibitory avoidance acquisition. The present results substantiate the view that dlPAG CCK(2) receptors modulate panic-related behaviors.