Microinjection of naltrexone into the central, but not the basolateral, amygdala blocks the anxiolytic effects of diazepam in the plus maze

Unconventional anxiety pharmacology in zebrafish: Drugs beyond traditional anxiogenic and anxiolytic spectra

Anxiety is the most prevalent brain disorder and a common cause of human disability. Animal models are critical for understanding anxiety pathogenesis and its pharmacotherapy. The zebrafish (Danio rerio) is increasingly utilized as a powerful model organism in anxiety research and anxiolytic drug screening. High similarity between human, rodent and zebrafish molecular targets implies shared signaling pathways involved in anxiety pathogenesis. However, mounting evidence shows that zebrafish behavior can be modulated by drugs beyond conventional anxiolytics or anxiogenics. Furthermore, these effects may differ from human and/or rodent responses, as such 'unconventional' drugs may affect zebrafish behavior despite having no such profiles (or exerting opposite effects) in humans or rodents. Here, we discuss the effects of several putative unconventional anxiotropic drugs (aspirin, lysergic acid diethylamide (LSD), nicotine, naloxone and naltrexone) and their potential mechanisms of action in zebrafish. Emphasizing the growing utility of zebrafish models in CNS drug discovery, such unconventional anxiety pharmacology may provide important, evolutionarily relevant insights into complex regulation of anxiety in biological systems. Albeit seemingly complicating direct translation from zebrafish into clinical phenotypes, this knowledge may instead foster the development of novel CNS drugs, eventually facilitating innovative treatment of patients based on novel 'unconventional' targets identified in fish models.

Modulatory action of environmental enrichment on hormonal and behavioral responses induced by chronic stress in rats: Hypothalamic renin-angiotensin system components

Environmental enrichment (EE) has been studied as a protocol that can improve brain plasticity and may protect against negative insults such as chronic stress. The aim of this study was to evaluate the effects of EE on the hormonal and behavioral responses induced by chronic mild unpredictable stress (CMS) in rats, considering the involvement of the renin-angiotensin system. Male adult rats were divided into 4 groups: control, CMS, EE, and CMS + EE, and the experimental protocol lasted for 7 weeks. EE was performed during 7 weeks, 5 days per week, 2 h per day. CMS was applied during weeks 3, 4, and 5. After the CMS (week 6), depression-like behavior was evaluated by forced swimming and sucrose consumption tests, anxiety level was evaluated using the elevated plus-maze test, and memory was evaluated using the Y-maze test. On week 7, the animals were euthanized and basal plasma levels of corticosterone and catecholamines were determined. The hypothalamus was isolated and tissue levels of angiotensin peptides were evaluated. CMS increased plasma corticosterone, norepinephrine, and epinephrine basal concentrations, induced depression-like behaviors, impaired memory, and increased hypothalamic angiotensin I, II, and IV concentrations. EE decreased stress hormones secretion, depression-like behaviors, memory impairment, and hypothalamic angiotensin II induced by stress. Reductions of anxiety-like behavior and norepinephrine secretion were observed in both stressed and unstressed groups. The results indicated that EE seemed to protect adult rats against hormonal and behavioral CMS effects, and that the reduction of angiotensin II could contribute to these effects.

Opioid system and human emotions

Emotions are states of vigilant readiness that guide human and animal behaviour during survival-salient situations. Categorical models of emotions posit neurally and physiologically distinct basic human emotions (anger, fear, disgust, happiness, sadness and surprise) that govern different survival functions. Opioid receptors are expressed abundantly in the mammalian emotion circuit, and the opioid system modulates a variety of functions related to arousal and motivation. Yet, its specific contribution to different basic emotions has remained poorly understood. Here, we review how the endogenous opioid system and particularly the μ receptor contribute to emotional processing in humans. Activation of the endogenous opioid system is consistently associated with both pleasant and unpleasant emotions. In general, exogenous opioid agonists facilitate approach-oriented emotions (anger, pleasure) and inhibit avoidance-oriented emotions (fear, sadness). Opioids also modulate social bonding and affiliative behaviour, and prolonged opioid abuse may render both social bonding and emotion recognition circuits dysfunctional. However, there is no clear evidence that the opioid system is able to affect the emotions associated with surprise and disgust. Taken together, the opioid systems contribute to a wide array of positive and negative emotions through their general ability to modulate the approach versus avoidance motivation associated with specific emotions. Because of the protective effects of opioid system-mediated prosociality and positive mood, the opioid system may constitute an important factor contributing to psychological and psychosomatic resilience.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Nucleus accumbens cocaine-amphetamine regulated transcript mediates food intake during novelty conflict

Obesity is a persistent and pervasive problem, particularly in industrialized nations. It has come to be appreciated that the metabolic health of an individual can influence brain function and subsequent behavioral patterns. To examine the relationship between metabolic phenotype and central systems that regulate behavior, we tested rats with divergent metabolic phenotypes (Low Capacity Runner: LCR vs. High Capacity Runner: HCR) for behavioral responses to the conflict between hunger and environmental novelty using the novelty suppressed feeding (NSF) paradigm. Additionally, we measured expression of mRNA, for peptides involved in energy management, in response to fasting. Following a 24-h fast, LCR rats showed lower latencies to begin eating in a novel environment compared to HCR rats. A 48-h fast equilibrated the latency to begin eating in the novel environment. A 24-h fast differentially affected expression of cocaine-amphetamine regulated transcript (CART) mRNA in the nucleus accumbens (NAc), where 24-h of fasting reduced CART mRNA in LCR rats. Bilateral microinjections of CART 55-102 peptide into the NAc increased the latency to begin eating in the NSF paradigm following a 24-h fast in LCR rats. These results indicate that metabolic phenotype influences how animals cope with the conflict between hunger and novelty, and that these differences are at least partially mediated by CART signaling in the NAc. For individuals with poor metabolic health who have to navigate food-rich and stressful environments, changes in central systems that mediate conflicting drives may feed into the rates of obesity and exacerbate the difficulty individuals have in maintaining weight loss.

Activation of corticotropin releasing factor-containing neurons in the rat central amygdala and bed nucleus of the stria terminalis following exposure to two different anxiogenic stressors

Rats exposed to the odor of a predator or to the elevated plus maze (EPM) express unique unconditioned fear behaviors. The extended amygdala has previously been demonstrated to mediate the response to both predator odor and the EPM. We seek to determine if divergent amygdalar microcircuits are associated with the different behavioral responses. The current experiments compared activation of corticotropin-releasing factor (CRF)-containing neuronal populations in the central amygdala and bed nucleus of the stria terminalis (BNST) of rats exposed to either the EPM (5 min) versus home cage controls, or predator (ferret) odor versus butyric acid, or no odor (30 min). Sections of the brains were prepared for dual-labeled immunohistochemistry and counts of c-Fos co-localized with CRF were made in the centrolateral and centromedial amygdala (CLA and CMA) as well as the dorsolateral (dl), dorsomedial (dm), and ventral (v) BNST. Ferret odor-exposed rats displayed an increase in duration and a decrease in latency of defensive burying versus control rats. Exposure to both predator stress and EPM induced neuronal activation in the BNST, but not the central amygdala, and similar levels of neuronal activation were seen in both the high and low anxiety groups in the BNST after EPM exposure. Dual-labeled immunohistochemistry showed a significant increase in the percentage of CRF/c-Fos co-localization in the vBNST of ferret odor-exposed rats compared to control and butyric acid-exposed groups as well as EPM-exposed rats compared to home cage controls. In addition, an increase in the percentage of CRF-containing neurons co-localized with c-Fos was observed in the dmBNST after EPM exposure. No changes in co-localization of CRF with c-Fos was observed with these treatments in either the CLA or CMA. These results suggest that predator odor and EPM exposure activates CRF neurons in the BNST to a much greater extent than CRF neurons of the central amygdala, and indicates unconditioned anxiogenic stimuli may activate unique anatomical circuits in the extended amygdala.

Investigating the effect of hydro-alcoholic extract of Salix aegyptiaca on anxiety in male rat

Background:

Anxiety disorders are frequently common neuropsychiatric disorders. Herbal medicines are widespread and used universal as a treatment compound for anxiety. The present study investigated the effects of hydro-alcoholic extract of Salix aegyptiaca blossom on rat behavior in the elevated plus-maze (EPM) and compared results with the effects of diazepam, as a positive control drug.

Materials and Methods:

Seventy adult male Wistar rats were divided into seven groups (N = 10). Animals received S. aegyptiaca extract (25, 50, 100 mg/kg) or Diazepam (0.3, 0.6, or 1.2 mg/kg) intraperitoneally and the control group was given the vehicle (10 ml/kg) 30 min before submitting into plus-maze test. The number of entries into the open and closed arms, the percentage of entries into the open arms of the EPM, and the time spent in the open arms were recorded.

Results:

The results revealed significant increases in percentage of entries into the open arms (P < 0.01) and in the time spent in the open arms (P < 0.01) after administration of diazepam (0.3, 0.6) and S. aegyptiaca (50, 100 mg/kg) in compare with control group. S. aegyptiaca extract has no effects on the total distance covered by animals and number of closed arms entries, whereas diazepam decreased these parameters. The locomotor activity was not significantly changed by S. aegyptiaca.

Conclusion:

Single-session administration of optimum doses of total extract of S. aegyptiaca has anxiolytic effects in rat similar to the low dose of diazepam. More research is needed for better understanding of anxiolytic properties and neurobiological mechanisms of action and probable interactions of S. aegyptiaca extract with neurotransmitters.

Study of the effect of extract of Thymus vulgaris on anxiety in male rats

There is some evidence in traditional medicine for the effectiveness of Thymus vulgaris (百里香 bǎi lǐ xiāng) in the treatment of anxiety in humans. The elevated plus-maze (EPM) has broadly been used to investigate anxiolytic and anxiogenic compounds. The present study investigated the effects of extract of T. vulgaris on rat behavior in the EPM. In the present study, the data were obtained from male Wistar rats. Animals were divided into four groups: saline group and T. vulgaris groups (50 mg/kg, 100 mg/kg, and 200 mg/kg infusion for 7 days by feeding). During the test period, the total distance covered by animals, the number of open- and closed-arm entries, and the time spent in open and closed arms of the EPM were recorded. T. vulgaris increased open-arm exploration and open-arm entry in the EPM, whereas extract of this plant has no effects on the total distance covered by animals and the number of closed-arm entries. The results of the present experiment indicate that T. vulgaris may have an anxiolytic profile in rat behavior in the EPM test, which is not influenced by the locomotor activity. Further research is required to determine the mechanisms by which T. vulgaris extract exerts an anxiolytic effect in rats.

Comparison of the activation of somatostatin- and neuropeptide Y-containing neuronal populations of the rat amygdala following two different anxiogenic stressors

Rats exposed to the odor of a predator or to the elevated plus maze express fear behaviors without a prior exposure to either stimulus. The expression of innate fear provides for an ideal model of anxiety which can aid in the elucidation of brain circuits involved in anxiety-related behaviors. The current experiments compared activation of neuropeptide-containing neuronal populations in the amygdala of rats exposed to either the elevated plus maze (EPM; 5 min) versus home cage controls, or predator ferret odor versus butyric acid, or no odor (30 min). Sections of the brains were prepared for dual-labeled immunohistochemistry and counts of c-Fos co-localized with somatostatin (SOM) or neuropeptide Y (NPY) were made in the basolateral (BLA), central (CEA), medial (MEA) nuclei of the amygdala. Ferret odor and butyric acid exposure significantly decreased the percentage of SOM-positive neurons also immunoreactive for c-Fos in the anterior BLA compared to controls, whereas EPM exposure yielded a significant increase in the activation of SOM-positive neurons versus home cage controls. In the CEA, ferret odor and butyric exposure significantly decreased the percentage of SOM-positive neurons also immunoreactive for c-Fos compared to no-odor controls whereas EPM exposure yielded no change versus controls. In the MEA, both ferret odor exposure and EPM exposure resulted in increased SOM co-localized with c-Fos compared to control groups whereas NPY co-localized with c-Fos occurred following ferret odor exposure, but not EPM exposure. These results indicate that phenotypically distinct neuronal populations of the amygdala are differentially activated following exposure to different anxiogenic stimuli. These studies further elucidate the fundamental neurocircuitry of anxiety and could possibly explain the differential behavioral effects of predator versus novelty-induced stress.

Activation of amygdalar metabotropic glutamate receptors modulates anxiety, and risk assessment behaviors in ovariectomized estradiol-treated female rats

Anxiety disorders are more prevalent in females than males. The underlying reasons for this gender difference are unknown. Metabotropic glutamate receptors (mGluRs) have been linked to anxiety and it has been shown that interaction between estrogen receptors and mGluRs modulate sexual receptivity in rats. We investigated the role of mGluRs in anxiety-related behaviors in ovariectomized female rats with (OVX+EB) or without (OVX) estradiol implants. We centrally infused (s)-3,5-dihydroxyphenylglycine (DHPG), a group I mGluR agonist, into the basolateral amygdala (BLA) of OVX+EB and OVX rats at 0.1 and 1.0 μM. Male rats that normally have low estradiol levels were used to compare with OVX rats. Generalized anxiety, explorative activity and detection and analysis of threat were analyzed in the elevated plus maze (EPM) and risk assessment behaviors (RABs). DHPG (1.0 μM) increased the percentage of time spent in- and entries into- the open arms in OVX+EB, but not in OVX or male rats. Flat-back approaches and stretch-attend postures, two RABs, were significantly reduced by DHPG (0.1 and 1.0 μM) in OVX+EB rats only. DHPG did not modulate rearing and freezing, behaviors related to exploration and fear-like behavior, respectively. However, DHPG (1.0 μM) increased head dipping and decreased grooming behaviors in OVX rats, suggesting a weak explorative modulation. The effects of DHPG observed in OVX+EB, were blocked by 50 μM of (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA), a mGluR1 antagonist. AIDA and/or estradiol did not modulate anxiety and or RABs. Our results show that intra-BLA infusion of DHPG exerts an anxiolytic-like effect in OVX+EB, but not in OVX or male rats. This effect seems to depend upon mGluR1 subtype activation. Our findings led us to suggest that the effects observed in OVX+EB rats might be due to an interaction at the membrane level of estrogen receptors with mGlu1 within the BLA.

A food predictive cue must be attributed with incentive salience for it to induce c-fos mRNA expression in cortico-striatal-thalamic brain regions

Cues associated with rewards acquire the ability to engage the same brain systems as rewards themselves. However, reward cues have multiple properties. For example, they not only act as predictors of reward capable of evoking conditional responses (CRs), but they may also acquire incentive motivational properties. As incentive stimuli they can evoke complex emotional and motivational states. Here we sought to determine whether the predictive value of a reward cue is sufficient to engage brain reward systems, or whether the cue must also be attributed with incentive salience. We took advantage of the fact that there are large individual differences in the extent to which reward cues are attributed with incentive salience. When a cue (conditional stimulus, CS) is paired with delivery of food (unconditional stimulus, US), the cue acquires the ability to evoke a CR in all rats; that is, it is equally predictive and supports learning the CS-US association in all. However, only in a subset of rats is the cue attributed with incentive salience, becoming an attractive and desirable incentive stimulus. We used in situ hybridization histochemistry to quantify the ability of a food cue to induce c-fos mRNA expression in rats that varied in the extent to which they attributed incentive salience to the cue. We found that a food cue induced c-fos mRNA in the orbitofrontal cortex, striatum (caudate and nucleus accumbens), thalamus (paraventricular, intermediodorsal and central medial nuclei), and lateral habenula, only in rats that attributed incentive salience to the cue. Furthermore, patterns of "connectivity" between these brain regions differed markedly between rats that did or did not attribute incentive salience to the food cue. These data suggest that the predictive value of a reward cue is not sufficient to engage brain reward systems-the cue must also be attributed with incentive salience.

Opioids and anxiety

The opioid system plays a crucial role in the neural modulation of anxiety. The involvement of opioid ligands and receptors in physiological and dysfunctional forms of anxiety is supported by findings from a wide range of preclinical and clinical studies, including clinical trials, experimental research, and neuroimaging, genetic, and epidemiological data. In this review we provide a summary of studies from a variety of research disciplines to elucidate the role of the opioid system in the neurobiology of anxiety. First, we report data from preclinical studies using animal models to examine the modulatory role of central opioid system on defensive responses conducive to fear and anxiety. Second, we summarize the human literature providing evidence that clinical and experimental human studies are consistent with preclinical models. The implication of these data is that activation of the opioid system leads to anxiolytic responses both in healthy subjects and in patients suffering from anxiety disorders. The role of opioids in suppressing anxiety may serve as an adaptive mechanism, collocated in the general framework of opioid neurotransmission blunting acute negative and distressing affective responses.

Neuroanatomical characterization of endogenous opioids in the bed nucleus of the stria terminalis

Numerous neuroanatomical data indicate that the bed nucleus of the stria terminalis (BST) provides an interface between cortical and amygdaloid neurons, and effector neurons modulating motor, autonomic and neuroendocrine responses. Distinct divisions of the BST may be involved in stress response, homeostatic regulation, nociception, and motivated behaviors. Endogenous opioid peptides and receptors are expressed in the BST, but their exact distribution is poorly characterized. The present study used in situ hybridization in order to characterize the endogenous opioid system of the BST, focusing on both enkephalin and dynorphin neuropeptides, as well as their respective receptors (mu, delta, and kappa opioid receptors). We report that preprodynorphin mRNA is observed in distinct nuclei of the BST, namely the fusiform, oval and anterior lateral nuclei. In contrast, there is a widespread expression of preproenkephalin mRNA in both anterior and posterior divisions of the BST. Similarly, mu and kappa opioid receptors are broadly expressed in the BST, whereas delta opioid receptor mRNA was observed only in the principal nucleus. For further characterization of enkephalin-expressing neurons of the BST, we performed a double fluorescent in situ hybridization in order to reveal the coexpression of enkephalin peptides and markers of GABAergic and glutamatergic neurons. Although most neurons of the BST are GABAergic, there is also a modest population of glutamatergic cells expressing vesicular glutamate transporter 2 (VGLUT2) in specific nuclei of the BST. Finally, we identified a previously unreported population of enkephalinergic neurons expressing VGLUT2, which is principally located in the posterior BST.

Long-term effects of neonatal handling on mu-opioid receptor levels in the brain of the offspring

Neonatal handling is an experimental paradigm of an early experience which permanently alters hypothalamic-pituitary-adrenal axis function resulting in increased ability to cope with stress, and decreased emotionality. In the present work we investigated the effect of neonatal handling on adult rat brain mu-opioid receptor levels, since the opioid system is known to play an important role in emotional processing, anxiety and stress responses. Neonatal handling resulted in increased levels of mu-opioid receptors in the basolateral and central amygdaloid nuclei, in the CA3 and CA4 hippocampal areas, in the ventral tegmental area, the nucleus accumbens and the prefrontal cortex. Handled animals of both sexes had lower anxiety as measured in the elevated plus maze. The increased mu receptor levels could participate in the molecular mechanisms underlying the well-documented decreased stress and anxiety responses of handled animals.

Oxytocin enhances the inhibitory effects of diazepam in the rat central medial amygdala

Oxytocin is a neuropeptide that can reduce neophobia and improve social affiliation. In vitro, oxytocin induces a massive release of GABA from neurons in the lateral division of the central amygdala which results in inhibition of a subpopulation of peripherally projecting neurons in the medial division of the central amygdala (CeM). Common anxiolytics, such as diazepam, act as allosteric modulators of GABA(A) receptors. Because oxytocin and diazepam act on GABAergic transmission, it is possible that oxytocin can potentiate the inhibitory effects of diazepam if both exert their pre, - respectively postsynaptic effects on the same inhibitory circuit in the central amygdala. We found that in CeM neurons in which diazepam increased the inhibitory postsynaptic current (IPSC) decay time, TGOT (a specific oxytocin receptor agonist) increased IPSC frequency. Combined application of diazepam and TGOT resulted in generation of IPSCs with increased frequency, decay times as well as amplitudes. While individual saturating concentrations of TGOT and diazepam each decreased spontaneous spiking frequency of CeM neurons to similar extent, co-application of the two was still able to cause a significantly larger decrease. These findings show that oxytocin and diazepam act on different components of the same GABAergic circuit in the central amygdala and that oxytocin can facilitate diazepam effects when used in combination. This raises the possibility that neuropeptides could be clinically used in combination with currently used anxiolytic treatments to improve their therapeutic efficacy.

GABA and opioid mechanisms of the central amygdala underlie the withdrawal-potentiated startle from acute morphine

Anxiety is an affective symptom common to withdrawal from acute or chronic opiate treatment. Although the potentiation of the acoustic startle reflex has been proposed as an index of increased anxiety, there are variable effects of the opiate withdrawal on the startle reflex in chronic dependence models. On the other hand, withdrawal from acute morphine treatment consistently potentiates the acoustic startle reflex, a response that seems to be mediated by the central nucleus of the amygdala (CeA). However, the underlying neurochemical mechanisms have not been elucidated yet. In the present study, we firstly made a comparison between the effects of the withdrawal from both acute and chronic treatments with morphine on the motor activity and the anxiety-like behavior of rats tested in two experimental models, the acoustic startle reflex and the open-field tests. Our second objective was to investigate the role of GABAergic and opioid mechanisms of the CeA in the modulation of the withdrawal-potentiated startle as a measure of anxiety induced by morphine withdrawal. For the production of chronic dependence, rats received morphine injections (10 mg/kg; s.c.) twice daily during 10 days. Forty-eight hours after the interruption of this treatment, independent groups were probed in the startle reflex and open-field tests. For the acute dependence model, groups of rats were tested in the open field and startle tests under control conditions and under withdrawal from a single injection of morphine (10 mg/kg; s.c.) precipitated by naltrexone injections (0.1 mg/kg; s.c.). The results obtained showed that withdrawal from chronic and acute morphine treatments produced anxiety-like behavior in the open field test, although the anxiogenic-like effects could not be dissociated from the motor effects in the acute dependence model. On the other hand, only the withdrawal from acute morphine treatment significantly potentiated the startle response. Next, we examined the effects of intra-CeA microinjections of muscimol-a GABA(A) receptors agonist-and DAMGO-a mu-opioid receptors agonist-on the potentiated startle induced by acute morphine withdrawal. The results obtained showed that intra-CeA injections of muscimol (1 nmol) and DAMGO (0.5 and 1 nmol) significantly inhibited this response. These findings suggest that the acute dependence model is more suitable to study the aversive effects of morphine withdrawal on the acoustic startle response than the chronic opiate dependence model. Besides, mechanisms mediated by mu- and GABA(A)-receptors in the CeA appear to exert an inhibitory influence on the anxiety-like behavior induced by withdrawal from acute morphine treatment.

The role of amygdalar mu-opioid receptors in anxiety-related responses in two rat models

Amygdala opioids such as enkephalin appear to play some role in the control of anxiety and the anxiolytic effects of benzodiazepines, although the opioid receptor subtypes mediating such effects are unclear. This study compared the influences of mu-opioid receptor (MOR) activation in the central nucleus of the amygdala (CEA) on unconditioned fear or anxiety-like responses in two models, the elevated plus maze, and the defensive burying test. The role of MORs in the anxiolytic actions of the benzodiazepine agonist diazepam was also examined using both models. Either the MOR agonist [D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO), or the MOR antagonists Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP) or beta-funaltrexamine (FNA) were bilaterally infused into the CEA of rats before testing. The results show that microinjection of DAMGO in the CEA decreased open-arm time in the plus maze, whereas CTAP increased open-arm behaviors. In contrast, DAMGO injections in the CEA reduced burying behaviors and increased rearing following exposure to a predator odor, suggesting a shift in the behavioral response in this context. Amygdala injections of the MOR agonist DAMGO or the MOR antagonist CTAP failed to change the anxiolytic effects of diazepam in either test. Our results demonstrate that MOR activation in the central amygdala exerts distinctive effects in two different models of unconditioned fear or anxiety-like responses, and suggest that opioids may exert context-specific regulation of amygdalar output circuits and behavioral responses during exposure to potential threats (open arms of the maze) vs discrete threats (predator odor).

The effects of intra-cerebral drug infusions on animals' unconditioned fear reactions: a systematic review

Intra-cerebral (i.c.) microinfusion of selective receptor agonists and antagonists into behaving animals can provide both neuroanatomical and neurochemical insights into the neural mechanisms of anxiety. However, there have been no systematic reviews of the results of this experimental approach that include both a range of unconditioned anxiety reactions and a sufficiently broad theoretical context. Here we focus on amino acid, monoamine, cholinergic and peptidergic receptor ligands microinfused into neural structures previously implicated in anxiety, and subsequent behavioral effects in animal models of unconditioned anxiety or fear. GABAA receptor agonists and glutamate receptor antagonists produced the most robust anxiolytic-like behavioral effects, in the majority of neural substrates and animal models. In contrast, ligands of the other receptor systems had more selective, site-specific anti-anxiety effects. For example, 5-HT1A receptor agonists produced anxiolytic-like effects in the raphe nuclei, but inconsistent effects in the amygdala, septum, and hippocampus. Conversely, 5-HT3 receptor antagonists produced anxiolytic-like effects in the amygdala but not in the raphe nuclei. Nicotinic receptor agonists produced anxiolytic-like effects in the raphe and anxiogenic effects in the septum and hippocampus. Unexpectedly, physostigmine, a general cholinergic agonist, produced anxiolytic-like effects in the hippocampus. Neuropeptide receptors, although they are popular targets for the development of selective anxiolytic agents, had the least reliable effects across different animal models and brain structures, perhaps due in part to the fact that selective receptor ligands are relatively scarce. While some inconsistencies in the microinfusion data can easily be attributed to pharmacological variables such as dose or ligand selectivity, in other instances pharmacological explanations are more difficult to invoke: e.g., even the same dose of a known anxiolytic compound (midazolam) with a known mechanism of action (the benzodiazepine-GABAA receptor complex), can selectively affect different fear reactions depending upon the different subregions of the nucleus into which it is infused (CeA versus BLA). These particular functional dissociations are important and may depend on the ability of a GABAA receptor agonist to interact with distinct isoforms and combinations of GABAA receptor subunits (e.g., alpha1-6, beta1-3, Upsilon1-2, delta), many of which are unevenly distributed throughout the brain. Although this molecular hypothesis awaits thorough evaluation, the microinfusion data overall give some support for a model of "anxiety" that is functionally segregated along different levels of a neural hierarchy, analogous in some ways to the organization of sensorimotor systems.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Neotic preferences in laboratory rodents: Issues, assessment and substrates

Neotic preference refers to the extent to which animals prefer stimuli of differing novelty value. Degree of novelty is determined by within- and between-trials habituation and amount of temporal (novelty) and spatial change (complexity) in stimulation which in turn will determine the amount of curiosity-based approach (neophilia) or fear-based avoidance (neophobia) of novel stimuli. Tests of genuine neotic preferences enable direct assessments of responsiveness to temporal and spatial changes and include measurements of novel versus familiar locations (such as novelty-related location preferences), responsiveness to stimulus complexity (such as object exploration) and learning for exploratory rewards (such as light-contingent bar-pressing). Effects of brain lesions and peripherally administered drugs have implicated several brain areas and neurotransmitters that subserve memory, fear and reward in neotic preferences namely the hippocampus and ACh (memory), the amygdala, GABA and 5-HT (fear), and the mesolimbic DA reward system. However, more attention should be paid to the complexity of interactions between different brain and neurotransmitter systems and improvements in methodology before conclusions should be drawn about the neurobiological basis of neotic preferences.

The role of delta opioid receptors in the anxiolytic actions of benzodiazepines

The anxiolytic effects of benzodiazepines appear to involve opioid processes in the amygdala. In previous experiments, overexpression of enkephalin in the amygdala enhanced the anxiolytic actions of the benzodiazepine agonist diazepam in the elevated plus maze. The effects of systemically administered diazepam are also blocked by injections of naltrexone into the central nucleus of the amygdala. The current studies investigated the role of delta opioid receptors in the anxiety-related effects of diazepam. Three days following bilateral stereotaxic injections of viral vectors containing cDNA encoding proenkephalin or beta-galactosidase (control vector), the delta opioid receptor antagonist naltrindole (10 mg/kg, s.c.) attenuated the enhanced anxiolytic effects of 1-2 mg/kg diazepam in rats overexpressing preproenkephalin in the amygdala. Despite this effect, naltrindole failed to attenuate the anxiolytic action of higher diazepam doses (3 mg/kg) in animals with normal amygdalar enkephalin expression. Similarly, the mu opioid receptor antagonist, beta-funaltrexamine (20 mg/kg, s.c.), had no effect on the anxiolytic effect of diazepam alone. These data support a role for delta opioid receptors in the opioid-enhanced anxiolytic effects of diazepam.