Ventral hippocampal histamine increases the frequency of evoked theta rhythm but produces anxiolytic-like effects in the elevated plus maze.

The neurobiological underpinnings of anxiety are of paramount importance to the development of effective therapeutic treatments. To date, there is considerable pharmacological evidence suggesting that the suppression of hippocampal theta frequency is a robust and predictive assay of anxiolytic drug action. Recently, this idea has been challenged using histamine (2-(4-imidazolyl)ethanamine), an endogenous neurotransmitter involved in a number of brain and behavioral functions. Here, we systematically evaluate the effects of dorsal and ventral hippocampal histamine infusions on evoked theta frequency and behavioral anxiety. Given the complex pharmacological profile of histamine and its receptors in the hippocampus, we reasoned that local intra-hippocampal infusions would be a powerful test of the theta suppression model. While dorsal hippocampal infusions of histamine produced neither significant changes in anxious-like behavior in the elevated plus maze nor changes of evoked theta, ventral infusions of histamine produced potent behavioral anxiolysis which corresponded to an increase, and not a decrease, in evoked theta frequency. As a positive neurophysiological control, we demonstrated that diazepam, a proven anxiolytic drug, decreased the frequency of hippocampal theta following both dorsal and ventral hippocampal infusions. Our results further challenge the hippocampal theta frequency suppression model as a measure of anxiolytic drug action. This article is part of the Special Issue entitled 'Histamine Receptors'.

Intrahippocampal Anisomycin Impairs Spatial Performance on the Morris Water Maze.

New memories are thought to be solidified (consolidated) by de novo synthesis of proteins in the period subsequent to learning. This view stems from the observation that protein synthesis inhibitors, such as anisomycin (ANI), administered during this consolidation period cause memory impairments. However, in addition to blocking protein synthesis, intrahippocampal infusions of ANI cause the suppression of evoked and spontaneous neural activity, suggesting that ANI could impair memory expression by simply preventing activity-dependent brain functions. Here, we evaluated the influence of intrahippocampal ANI infusions on allocentric spatial navigation using the Morris water maze, a task well known to require dorsal hippocampal integrity. Young, adult male Sprague Dawley rats were implanted with bilateral dorsal hippocampal cannulae, and their ability to learn the location of a hidden platform was assessed before and following infusions of ANI, TTX, or vehicle (PBS). Before infusion, all groups demonstrated normal spatial navigation (training on days 1 and 2), whereas 30 min following infusions (day 3) both the ANI and TTX groups showed significant impairments in allocentric navigation, but not visually cued navigation, when compared with PBS-treated animals. Spatial navigational deficits appeared to resolve on day 4 in the ANI and TTX groups, 24 h following infusion. These results show that ANI and TTX inhibit the on-line function of the dorsal hippocampus in a similar fashion and highlight the importance of neural activity as an intervening factor between molecular and behavioral processes. SIGNIFICANCE STATEMENT: The permanence of memories has long thought to be mediated by the production of new proteins, because protein synthesis inhibitors can block retrieval of recently learned information. However, protein synthesis inhibitors may have additional detrimental effects on neurobiological function. Here we show that anisomycin, a commonly used protein synthesis inhibitor in memory research, impairs on-line brain function in a way similar to an agent that eliminates electrical neural activity. Since disruption of neural activity can also lead to memory loss, it may be that memory permanence is mediated by neural rehearsal following learning.

ANI inactivation: unconditioned anxiolytic effects of anisomycin in the ventral hippocampus.

Although hippocampal function is typically described in terms of memory, recent evidence suggests a differentiation along its dorsal/ventral axis, with dorsal regions serving memory and ventral regions serving emotion. While long-term memory is thought to be dependent on de novo protein synthesis because it is blocked by translational inhibitors such as anisomycin (ANI), online (moment-to-moment) functions of the hippocampus (such as unconditioned emotional responding) should not be sensitive to such manipulations since they are unlikely to involve neuroplasticity. However, ANI has recently been shown to suppress neural activity which suggests (1) that protein synthesis is critical for neural function and (2) that paradigms using ANI are confounded by its inactivating effects. We tested this idea using a neurobehavioral assay which compared the influence of intrahippocampal infusions of ANI at dorsal and ventral sites on unconditioned emotional behavior of rats. We show that ANI infusions in ventral, but not dorsal, hippocampus produced a suppression of anxiety-related responses in two well-established rodent tests: the elevated plus maze and shock-probe burying tests. These results are similar to those previously observed when ventral hippocampal activity is directly suppressed (e.g., by using sodium channel blockers). The present study offers compelling behavioral evidence for the proposal that ANI adversely affects ongoing neural function and therefore its influence is not simply limited to impairing the consolidation of long-term memories

FG7142, yohimbine, and ßCCE produce anxiogenic-like effects in the elevated plus-maze but do not affect brainstem activated hippocampal theta.

The neurobiological underpinnings of anxiety are of paramount importance to selective and efficacious pharmaceutical intervention. Hippocampal theta frequency in urethane anaesthetized rats is suppressed by all known (and some previously unknown) anti-anxiety (anxiolytic) drugs. Although these findings support the predictive validity of this assay, its construct validity (i.e., whether theta frequency actually indexes anxiety per se) has not been a subject of systematic investigation. We reasoned that if anxiolytic drugs suppress hippocampal theta frequency, then drugs that increase anxiety (i.e., anxiogenic agents) should increase theta frequency, thus providing evidence of construct validity. We used three proven anxiogenic drugs--two benzodiazepine receptor inverse agonists, N-methyl-ß-carboline-3-carboxamide (FG7142) and ß-carboline-3-carboxylate ethyl ester (ßCCE), and one α2 noradrenergic receptor antagonist, 17α-hydroxy-yohimban-16α-carboxylic acid methyl ester (yohimbine) as pharmacological probes to assess the construct validity of the theta model. Although all three anxiogenic drugs significantly increased behavioural measures of anxiety in the elevated plus-maze, none of the three increased the frequency of hippocampal theta oscillations in the neurophysiological model. As a positive control, we demonstrated that diazepam, a proven anxiolytic drug, decreased the frequency of hippocampal theta, as in all other studies using this model. Given this discrepancy between the significant effects of anxiogenic drugs in the behavioural model and the null effects of these drugs in the neurophysiological model, we conclude that the construct validity of the hippocampal theta model of anxiety is questionable.

Intrahippocampal infusion of the Ih blocker ZD7288 slows evoked theta rhythm and produces anxiolytic-like effects in the elevated plus maze.

Hippocampal theta rhythm has been associated with a number of behavioral processes, including learning and memory, spatial behavior, sensorimotor integration and affective responses. Suppression of hippocampal theta frequency has been shown to be a reliable neurophysiological signature of anxiolytic drug action in tests using known anxiolytic drugs (i.e., correlational evidence), but only one study to date (Yeung et al. (2012) Neuropharmacology 62:155-160) has shown that a drug with no known effect on either hippocampal theta or anxiety can in fact separately suppress hippocampal theta and anxiety in behavioral tests (i.e., prima facie evidence). Here, we attempt a further critical test of the hippocampal theta model by performing intrahippocampal administrations of the Ih blocker ZD7288, which is known to disrupt theta frequency subthreshold oscillations and resonance at the membrane level but is not known to have anxiolytic action. Intrahippocampal microinfusions of ZD7288 at high (15 µg), but not low (1 µg) doses slowed brainstem-evoked hippocampal theta responses in the urethane anesthetized rat, and more importantly, promoted anxiolytic action in freely behaving rats in the elevated plus maze. Taken together with our previous demonstration, these data provide converging, prima facie evidence of the validity of the theta suppression model.

The anxiolytic effects of somatostatin following intra-septal and intra-amygdalar microinfusions are reversed by the selective sst2 antagonist PRL2903.

Somatostatin (SST) is a polypeptide with two biological isoforms (SST14, and SST28), and five SST receptor subtypes (sst1-5). Together, they mediate a number of neural and hormonal functions. Recently, we found that intracerebroventricular (ICV), intra-amygdalar, and intra-septal microinfusions of SST14, SST28, and a selective sst2 receptor agonist L-779976 all produced anxiolytic-like effects in the elevated plus-maze, a widely used animal model of anxiety. The receptor specificity of these anxiolytic-like effects, however, has not been conclusively established. Accordingly, the anxiolytic effects of SST in the elevated plus-maze were assessed following intra-septal or intra-amygalar microinfusions of 1) SST (1.5µg per hemisphere), 2) the highly selective sst2 receptor antagonist PRL2903 (1.5µg per hemisphere), or 3) the combination of SST and PRL2903 (each 1.5µg per hemisphere). Antagonism of the anxiolytic effects of SST in the plus-maze by PRL2903 should result in open-arm exploration that is equivalent to that of 4) vehicle-injected control rats. Both intra-septal and intra-amygdalar microinfusions of SST produced anxiolytic effects in the elevated plus-maze, consistent with results found previously after ICV microinfusions (see Engin et al., 2008; Engin and Treit, 2009; Yeung et al., 2011). More importantly, infusion of PRL2903 completely reversed the anxiolytic effects of SST in both the amygdala and the septum. These results show that somatostatin's anxiolytic effects are mediated by sst2 receptors contained in the amygdala and septum of the rat brain.

A critical test of the hippocampal theta model of anxiolytic drug action.

Hippocampal theta rhythms have been associated with a number of behavioural processes, including learning, memory and arousal. Recently it has been argued that the suppression of hippocampal theta is a valid indicator of anxiolytic drug action. Like all such models, however, it has relied almost exclusively on the experimental effects of well-known, clinically proven anxiolytic compounds for validation. The actual predictive validity of putative models of anxiolytic drug action, however, cannot be rigorously tested with this approach alone. The present study provides a stringent test of the predictive validity of the theta suppression model, using the drug phenytoin (50 mg/kg and 10 mg/kg), and a positive comparison compound, diazepam (2 mg/kg). Phenytoin has two important properties that are advantageous for assessing the validity of the theta suppression model: 1) it is a standard antiepileptic drug with no known anxiolytic effects, and 2) its primary mechanism of action is through suppression of the persistent sodium current, an effect that should also suppress hippocampal theta. Because of the latter property, we also directly compared the effects of phenytoin in the theta suppression model with its effects in the most widely tested behavioural model of anxiolytic drug action, the elevated plus-maze. While an anxiolytic-like effect of phenytoin in the theta suppression model might be expected simply due to its suppressive effects on sodium channel currents, anxiolytic effects in both tests would provide strong support for the predictive validity of the theta suppression model. Surprisingly, phenytoin produced clear anxiolytic-like effects in both neurophysiological and behavioural models, thus providing strong evidence of the predictive validity of the theta suppression model. This article is part of a Special Issue entitled 'Anxiety and Depression'.

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.

Anxiolytic-like effects of somatostatin isoforms SST 14 and SST 28 in two animal models (Rattus norvegicus) after intra-amygdalar and intra-septal microinfusions.

RATIONALE AND OBJECTIVES: Somatostatin (SST) isoforms, SST 14 and SST 28, inhibit regulatory hormones in the periphery (e.g., growth hormone) and are widely distributed in the brain. In recent experiments, intracerebroventricular (ICV) SST produced anxiolytic-like effects in both behavioral and electrophysiological models. The sites of action of these anxiolytic effects in the brain, however, and the relative contributions of SST 14 and SST 28 to these effects are unknown. MATERIALS AND METHODS: Anxiolytic effects were assessed in the plus-maze and shock-probe tests after (1) intra-amygdalar microinfusion of SST 14 (0.5 or 3 µg per hemisphere) or SST 28 (3 µg per hemisphere), (2) intra-septal microinfusion of SST 14 (0.5 or 1.5 µg per hemisphere) or SST 28 (1.5 µg per hemisphere), or (3) intra-striatal microinfusion of SST 14 (3 µg per hemisphere). RESULTS: Intra-amygdalar and intra-septal microinfusions of SST 14 and SST 28 produced robust anxiolytic-like effects in the behavioral tests, unlike intra-striatal microinfusions. The magnitude of the anxiolytic effects in the amygdala and septum were comparable to those found previously with ICV SST 14, ICV L-779976, an SST (sst2) receptor agonist, and ICV diazepam, a classical benzodiazepine anxiolytic. CONCLUSIONS: SST receptors in the septum and amygdala are responsive to both SST 14 and SST 28, but not those in the striatum. Although no obvious differences in the anxiolytic-like effects of the isoforms were detected, quantitative or even qualitative differences in their specific anxiolytic effects may occur in different sub-regions of the septum and amygdala, as has been found for benzodiazepine anxiolytics.

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.

Animal models of anxiety and anxiolytic drug action.

Animal models of anxiety attempt to represent some aspect of the etiology, symptomatology, or treatment of human anxiety disorders, in order to facilitate their scientific study. Within this context, animal models of anxiolytic drug action can be viewed as treatment models relevant to the pharmacological control of human anxiety. A major purpose of these models is to identify novel anxiolytic compounds and to study the mechanisms whereby these compounds produce their anxiolytic effects. After a critical analysis of "face," "construct," and "predictive" validity, the biological context in which animal models of anxiety are to be evaluated is specified. We then review the models in terms of their general pharmacological profiles, with particular attention to their sensitivity to 5-HTIA agonists and antidepressant compounds. Although there are important exceptions, most of these models are sensitive to one or perhaps two classes of anxiolytic compounds, limiting their pharmacological generality somewhat, but allowing in depth analysis of individual mechanisms of anxiolytic drug action (e.g., GABAA agonism). We end with a discussion of possible sources of variability between models in response to 5-HTIA agonists and antidepressant drugs.

Anxiolytic and antidepressant actions of somatostatin: the role of sst2 and sst3 receptors.

RATIONALE AND OBJECTIVES: Somatostatin is a cyclic polypeptide that inhibits the release of a variety of regulatory hormones (e.g., growth hormone, insulin, glucagon, and thyrotropin). Somatostatin is also widely distributed within the central nervous system (CNS), acting both as a neurotransmitter and as a neuromodulator. Recently, we showed that intracerebroventricular (i.c.v.) administration of somatostatin reduced anxiety-like and depression-like behaviors in animal models. The somatostatin receptor subtypes that are involved in these behavioral effects, however, have not been investigated. In the CNS, the neurotransmitter actions of somatostatin are mediated through five G-protein coupled receptors (sst1 to sst5). MATERIALS AND METHODS: We examined the behavioral effects of i.c.v. microinfusions of different doses of selective agonists of each of the five somatostatin receptor subtypes. Their behavioral effects were assessed in the elevated plus-maze and the forced swim apparatus, rodent models of anxiolytic and antidepressant drug effects, respectively. RESULTS: Anxiety-like behavior was reduced following i.c.v. infusions of a selective sst2 receptor agonist, but not after infusions of the other four receptor agonists. An antidepressant-like effect was observed following infusions of either sst2 or sst3 agonists. CONCLUSIONS: The results add to our nascent understanding of the role of somatostatin in anxiety- and depression-like behavior and suggest a clinical role for somatostatin agonists for the simultaneous treatment of anxiety and depression, which are often comorbid.

The role of the dorsal and ventral hippocampus in fear and memory of a shock-probe experience.

The roles of the dorsal and ventral hippocampus in fear and memory are unclear. This study examined the effects of temporary inactivation of the dorsal or ventral hippocampus on unconditioned and conditioned fear, using the shock-probe test. In Experiment 1, rats received either dorsal or ventral hippocampal infusions of lidocaine or saline, before exposure to an electrified shock-probe (acquisition I). In Experiment 2, rats received lidocaine or saline infusions after exposure to the shock-probe (acquisition II). In both experiments, a retention test in the same apparatus was given 24 h later, at which time the hippocampus was no longer inactivated, and the probe was disconnected from the shock-source. Because ventral hippocampal inactivation impaired fear behaviour during acquisition, and dorsal hippocampal inactivation impaired fear behaviour (probe avoidance) during retention, we concluded that 1) the ventral hippocampus plays a role in the expression of untrained fear reactions whereas 2) the dorsal hippocampus plays a role in encoding memory of the fearful experience.

Dissociation of the anxiolytic-like effects of Avpr1a and Avpr1b receptor antagonists in the dorsal and ventral hippocampus.

Arginine-vasopressin (AVP) is synthesized and released centrally in several brain structures. AVP is thought to mediate anxiety-related behavior through two central receptor subtypes, Avpr1a and Avpr1b. Although these AVP receptor subtypes are expressed in several brain regions, including the hippocampus, little is known about their explicit role in unconditioned fear or anxiety. This experiment assessed the anxiety-related effects of a selective Avpr1a antagonist ([beta-Mercapto-beta,beta-cyclopentamethylenepropionyl1, O-me-Tyr2, Arg8]-AVP) and a selective Avpr1b antagonist ((2S,4R)-1-[5-chloro-1-[(2,4-dimethoxyphenyl)sulfonyl]-3-(2-methoxy-phenyl)-2-oxo-2,3-dihydro-1H-indol-3-yl]-4-hydroxy-N,N-dimethyl-2-pyrrolidine carboxamide; SSR 149415) microinfused into either the dorsal or ventral sub-regions of the rat hippocampus. Avpr1a antagonism in the ventral, but not the dorsal hippocampus reduced rats' anxiety-like behavior in the elevated plus-maze test. Conversely, Avpr1b antagonism in the dorsal, but not the ventral, hippocampus reduced anxiety in the plus-maze test. Neither antagonist reduced anxiety-like behavior in the shock-probe burying test. Overall, the results show that both receptor subtypes of AVP are involved in anxiety-related responses, but their specific contributions depend on three variables: (1) the anxiety-related response (shock-probe avoidance versus open-arm avoidance), (2) the receptor subtype antagonized (Avpr1a versus Avpr1b), and (3) the area of hippocampus (dorsal versus ventral) into which these antagonists are infused. These dissociations suggest that different fear responses are under the control of specific AVP receptor systems within discrete parts of the hippocampus.

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.

The role of hippocampus in anxiety: intracerebral infusion studies.

Although current models of hippocampal function stress its well-known role in cognitive functions, historically it has also been viewed as a neural mediator of emotion. Here, we review recent evidence from intrahippocampal infusion studies in animals that support a distinctive role of the hippocampus in anxiety, independent of its roles in learning and memory. Specifically, gamma-aminobutyric acid type A receptor agonists, both direct and indirect, reliably inhibit a number of animals' untrained anxiety reactions when microinfused into the hippocampus, whereas gamma-aminobutyric acid type A receptor antagonists do not. Intrahippocampal infusions of glutamatergic, serotonergic and cholinergic compounds also produce statistically reliable antianxiety effects, but the results vary as a function of specific anxiety reactions, and to some extent specific intrahippocampal targets. One hypothesis that may accommodate some of this variability is that anxiety is functionally segregated within the hippocampus, with ventral subregions more involved in anxiety-related processes, and dorsal subregions more involved with cognitive processes. Another possibility is that different hippocampal functions (e.g. memory and anxiety) are mediated by different neurotransmitter systems and/or different receptor subtypes within the hippocampus. Although there is some evidence that supports the latter hypothesis, the evidence for the former is not conclusive. Overall, however, the data clearly suggest that the hippocampus is importantly and directly involved in the mediation of untrained anxiety reactions in animals.

The anxiolytic-like effects of allopregnanolone vary as a function of intracerebral microinfusion site: the amygdala, medial prefrontal cortex, or hippocampus.

Allopregnanolone is a 5alpha-reduced metabolite of progesterone that potentiates gamma-aminobutyric acid type-A (GABA(A)) receptor activity and produces anxiolytic effects in animal models. Little is, however, known about the brain regions that mediate its anxiolytic effects. In this study Sprague-Dawley rats were microinfused with allopregnanolone into the amygdala, medial prefrontal cortex, or hippocampus--brain regions that have been previously implicated in the control of anxiety in animal models. After the microinfusion, the animals were tested on the elevated plus-maze and the shock-probe burying test. In the amygdala, allopregnanolone produced anxiolytic-like effects in both tests; in the medial prefrontal cortex, allopregnanolone produced anxiolytic effects restricted to the plus-maze test; in the hippocampus, allopregnanolone was ineffective in both tests. The results were discussed in terms of differences in the control of specific fear reactions within subregions of each brain area, differences in the 'sensitivity' of behavioral tests to the anxiolytic effects of allopregnanolone, and finally, regional differences in the subunit composition of GABA(A) receptors and their possible relationship to the relative efficacy of steroidal and nonsteroidal GABA(A) agonists.

Inactivation of the medial prefrontal cortex with the GABAA receptor agonist muscimol increases open-arm activity in the elevated plus-maze and attenuates shock-probe burying in rats.

This study examined the effects of infusions of a direct GABA(A) receptor agonist, muscimol, into the medial prefrontal cortex (MPFC), on fear behavior measured in the elevated plus-maze and shock-probe burying tests. Bilateral infusions of either a 0.175 or 4 nmol/0.5 microl dose of muscimol increased the percentage of entries and time spent in the open arms, and attenuated shock-probe burying. These findings indicate that intra-MPFC infusions of muscimol induce anxiolysis, and suggest that the direct stimulation of MPFC GABA(A) receptors attenuates fear-related behavior.

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.

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.