Intracerebroventricular 5,7-DHT alters the in vitro function of rat cortical GABAA/benzodiazepine chloride ionophore receptor complexes

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Serotonin-GABA interactions in the modulation of mu- and kappa-opioid analgesia

In the present study, we studied the interaction between serotonergic (5-HTergic) and gamma-aminobutyric acid (GABA)-ergic systems in the modulation of analgesia from morphine, a mu-opioid agonist, and U50,488, a kappa-opioid agonist. All experiments were performed in mice using the 49 degrees C tail-withdrawal assay. The benzodiazepine receptor agonist, diazepam, the serotonin synthesis inhibitor, para-chlorophenylalanine (p-CPA), and the 5-HT(1A) receptor agonist, 8-OH-DPAT, were all found to attenuate morphine and U50,488 analgesia. In each case, the attenuation was itself blocked by treatment with L-5-HTP, a serotonin precursor, bicuculline, a GABA(A) receptor antagonist or picrotoxin, a GABA(A)-gated chloride channel blocker. Neither L-5-HTP nor the GABA(A) receptor antagonists were found to affect morphine or U50,488 analgesia per se. Thus, these findings indicate that a benzodiazepine-GABAergic agent (diazepam) attenuates opioid analgesia through the serotonergic system, and antiserotonergic agents (8-OH-DPAT, p-CPA) attenuate opioid analgesia through the GABAergic system. The intimate interactions between GABA and serotonin in the present study further suggest that these neurotransmitters work in complex ways together rather than alone in the modulation of opioid analgesia.

The Vogel conflict test: procedural aspects, gamma-aminobutyric acid, glutamate and monoamines

A multitude of mechanisms are involved in the control of emotion and in the response to stress. These incorporate mediators/targets as diverse as gamma-aminobutyric acid (GABA), excitatory amino acids, monoamines, hormones, neurotrophins and various neuropeptides. Behavioural models are indispensable for characterization of the neuronal substrates underlying their implication in the etiology of anxiety, and of their potential therapeutic pertinence to its management. Of considerable significance in this regard are conflict paradigms in which the influence of drugs upon conditioned (trained) behaviours is examined. For example, the Vogel conflict test, which was introduced some 30 years ago, measures the ability of drugs to release the drinking behaviour of water-deprived rats exposed to a mild aversive stimulus ("punishment"). This model, of which numerous procedural variants are discussed herein, has been widely used in the evaluation of potential anxiolytic agents. In particular, it has been exploited in the characterization of drugs interacting with GABAergic, glutamatergic and monoaminergic networks, the actions of which in the Vogel conflict test are summarized in this article. More recently, the effects of drugs acting at neuropeptide receptors have been examined with this model. It is concluded that the Vogel conflict test is of considerable utility for rapid exploration of the actions of anxiolytic (and anxiogenic) drugs. Indeed, in view of its clinical relevance, broader exploitation of the Vogel conflict test in the identification of novel classes of anxiolytic agents, and in the determination of their mechanisms of action, would prove instructive.

Disinhibitory behavior and GABA(A) receptor function in serotonin-depleted adult male rats are reduced by gonadectomy

Impulsive and aggressive behaviors in, e.g., personality or substance abuse disorders in man and corresponding behaviors in rats may involve serotonin (5-HT), gamma-amino-butyric acid(A)/benzodiazepine receptor complexes (GABA(A)/BDZ-RC) and steroid hormones, e.g., testosterone. Here, we studied the effect of gonadectomy on disinhibitory behavior in individually housed 5-HT-depleted rats and on GABA(A)/BDZ-RC function in vitro, in corticohippocampal synaptoneurosomes prepared from the brain of these animals. 5-HT depletion by intracerebroventricular 5,7-dihydroxytryptamine (5,7-DHT)-induced disinhibitory behavior in a shock-induced behavioral inhibition model (punished conflict model) 14 days after operation. Gonadectomy in connection with the 5-HT depletion reduced the disinhibitory behavior and testosterone substitution prevented this effect. Shock threshold and drinking motivation were not affected by gonadectomy and/or 5-HT depletion. The relative epididymides weight was increased in 5-HT-depleted as compared to sham-operated rats. However, the serum concentrations of testosterone and the relative testes weights were not different in 5-HT-depleted rats as compared to controls. GABA-induced (30, 100, 300 microM) 36Cl(-)-uptake into synaptoneurosomes was lower in 5,7-DHT+gonadectomized rats compared to the control group. This effect was reversed by substitution with testosterone. These results demonstrate that gonadectomy reduces disinhibitory behavior in 5-HT-depleted rats and that GABA(A)/BDZ-RC may be involved in this effect.

Effects of buspirone, diazepam, and zolpidem on open field behavior, and brain [3H]muscimol binding after buspirone pretreatment

The effects of 5-HT(1A) receptor agonist buspirone, a nonselective (diazepam), and a selective (zolpidem) GABA(A) receptor agonist were compared in the open field test of neophobia. Unhabituated rats were pretreated with the drugs once, prior to a first exposure to the open field, and their behavior was recorded both during this test and during a second trial 24 h later. It has been hypothesized that the decrease in exploratory activity observed during the second test session may be considered an adaptive reaction to the first day aversive experience (neophobia). If so, a selective modulation of 5-HT and GABA systems activity during the test could bring about significant changes in animal behavior on the retest. Buspirone at the lowest dose of 0.3 mg/kg revealed anxiolytic-like properties on the first day, whereas the action of diazepam and zolpidem was modulated by the dose-related sedative effect. At the dose of 2.4 mg/kg buspirone elicited delayed in time anxiolytic-like action, i.e., produced the antithigmotactic effect during the retrial 24 h later. Diazepam and zolpidem failed to exhibit similar profile of action. Autoradiography of [3H]muscimol binding after pretreatment of rats with buspirone showed a significant increase in the selective radioligand binding within the frontal cortex and a similar, near-significant tendency in the dentate gyrus of the hippocampus. The behavioral data validate buspirone as important drug for the treatment of anxiety disorders, devoid of disruptive influence on motor and cognitive processes. The open field test, as modified by us, appeared sensitive in distinguishing the behavioral profiles of action of different anxiolytic compounds, including 5-HT(1A) receptor agonist. The present results support the assumption that reduced turnover of 5-HT due to stimulation of 5-HT(1A) autoreceptors, may bring about changes in GABA(A) receptor system activity, in some brain structures, leading to the anxiolytic effect.

Naloxone reverses disinhibitory/aggressive behavior in 5,7-DHT-lesioned rats; involvement of GABA(A) receptor blockade?

Effective medical treatment for impulsive aggression and several impulse control disorders is needed. Disinhibited, impulsive behavior of e.g. murderers, arsonists, suicidal patients, and patients suffering from antisocial personality or substance abuse disorders has been associated with signs of a deficiency in brain serotonin (5-HT) systems. Depletion of brain 5-HT consistently produces disinhibition and aggression also in experimental animals. The present series of experiments using a modified Vogel's conflict test indicates that the disinhibitory behavior of 5-HT-lesioned rats can be reversed by the commonly used opiate receptor antagonist naloxone at doses (0.1-5.0 mg/kg, s.c.) that do not significantly affect behavior in sham-lesioned controls. Moreover, this effect of naloxone, which resembles that previously observed after administration of negative modulators of gamma-aminobutyric acidA (GABA(A))/benzodiazepine receptor complexes, was reversed by a low inert dose (2.0 mg/kg, i.p.) of amobarbital. Furthermore, both naloxone (5.0 mg/kg, s.c.) and Ro 15-4513 (1.0 mg/kg, p.o.; a partial inverse agonist at benzodiazepine receptors) significantly decreased the number of attacks and the time spent in aggressive acts in 5,7-DHT-lesioned male residents. These results taken together with previous behavioral and neurochemical data suggest that the behavioral effects of naloxone observed here may involve an antagonistic action at brain gamma-aminobutyric acidA (GABA(A))/benzodiazepine receptor complexes. Thus, naloxone, its stable analogue naltrexone or other weak negative modulators of brain GABA(A)/benzodiazepine receptor complexes may represent a new pharmacological principle for the treatment of impulse control disorders.

The effect of serotonin depletion on motor activity habituation, and [3H]muscimol binding in the rat hippocampus

The effect of serotonin depletion (p-chlorophenylalanine pretreatment) on habituation of exploratory motor activity, and on cortical and hippocampal [3H]muscimol binding in vitro, was examined in rats. It appeared that the very strong decrease in serotonin concentration abolished motor habituation in the open field and decreased [3H]muscimol binding to cortical and hippocampal brain slices. The GABA(A) receptor down-regulation was due to a decrease in the apparent affinity of the radioligand for the receptors. p-Chlorophenylalanine-induced biochemical changes were selective and most probably secondary to serotonin depletion, as the serotonin synthesis inhibitor did not displace [3H]muscimol from its binding sites in neural membranes taken from the occipital cortex. It is concluded that there is a functional interaction between brain serotonin and GABA (gamma-aminobutyric acid) systems, both at behavioral and biochemical levels, that is involved in the motor activity habituation process.

In vivo administration of the 5-HT1A receptor agonist 8-OH-DPAT interferes with brain GABA(A)/benzodiazepine receptor complexes

In the present study the influence of in vivo administration, or in vitro addition, of the prototypic 5-HT1A receptor agonist 8-OH-DPAT on in vitro characteristics of GABA(A)/benzodiazepine receptor complexes was examined. In vivo administration of 8-OH-DPAT at a dose (32 microg/kg, s.c. -10') that has been reported to produce anxiolytic-like effects in the elevated plus-maze doubled the Kd for in vitro binding of 3H-flunitrazepam to rat cortical membranes (Bmax was unchanged) and enhanced GABA-stimulated (3, 10, 30 and 100 microM) 36Cl- influx in corticohippocampal synaptoneurosomes. In synaptoneurosomes from vehicle treated rats, diazepam (1, 3 and 10 microM) potentiated GABA-stimulated (3 microM) 36Cl- influx. No such effect was observed in tissue from 8-OH-DPAT treated rats, in which the GABA-stimulated (3 microM) 36Cl- influx was similar to that caused by GABA + diazepam in tissue from vehicle treated rats. When added in vitro, 8-OH-DPAT failed to alter basal or GABA-stimulated 36Cl- uptake. In vivo administration of a low "anxiolytic" dose of 8-OH-DPAT thus appears to interfere with GABA(A)/benzodiazepine receptor complexes, whereas in vitro application does not. The underlying mechanism remains to be elucidated but could involve in vivo release of positive modulators of GABA(A)/benzodiazepine receptor complexes, e.g. GABA, endozepines or neurosteroids.

The yohimbine-induced anticonflict effect in the rat, Part II. Neurochemical findings

In a companion paper the alpha 2-adrenoceptor antagonist yohimbine was found to produce a dose-dependent anticonflict effect in a modified Vogel's conflict test. The behavioral data further indicated that noradrenergic and serotonergic neurons as well as the benzodiazepine (BDZ) receptor may be involved in the anticonflict effect of yohimbine. In the present study the effects on rat brain monoamine neurochemistry and GABAA/BDZ receptor function (36Cl-uptake in corticohippocampal synaptoneurosomes) of a maximally anticonflict producing dose of yohimbine (4.0 mg/kg, i.p.) were studied. The levels of rat brain catecholamines and indoleamines were measured ex vivo using high performance liquid chromatography with electrochemical detection (HPLC-ED). Yohimbine decreased noradrenaline levels both in the hippocampus and the hemispheres but instead increased DOPAC levels in these brain regions as well as in the limbic forebrain. Yohimbine also markedly enhanced DOPA accumulation in the hippocampus and the hemispheres after inhibition of 1-aromatic amino acid decarboxylase by means of NSD 1015, whereas in the limbic system only a modest increase was obtained. The yohimbine-induced effects on the catecholamine synthesis rate were largely abolished in animals severely depleted of NA by means of 6-hydroxy-dopamine (6-OH-DA) pretreatment. Yohimbine decreased both the 5-HIAA/5-HT quotient (an indicator of 5-HT turnover) and 5-HTP accumulation after NSD 1015 in the hemispheres, whereas in the hippocampus and the limbic system only 5-HTP accumulation was decreased. The yohimbine-induced effect on the indoleamine synthesis rate was not influenced by 6-OH-DA pretreatment, whereas this effect and that on the catecholamine synthesis rate were both abolished by reserpine pretreatment. Neither in vivo nor in vitro administration of yohimbine significantly altered baseline or GABA-induced accumulation of 36Cl- in corticohippocampal synaptoneurosomes. In conclusion, the present study provides neurochemical support for the suggestion that yohimbine may exert its anticonflict effect in a modified Vogel's conflict test by increasing and decreasing NA and 5-HT neurotransmission, respectively, whereas no evidence was obtained for a direct interaction of yohimbine with GABAA/BDZ receptor function.

5-Hydroxytryptamine pathways in anxiety and its treatment

The effects of manipulating 5-hydroxytryptamine (5HT) neuronal function in humans and in animals are reviewed. 5HT pathways do not have a unitary function in modulating anxiety. It is proposed that, rather than acting as input or output channels for brain aversive systems, these pathways provide information concerning waking/motor status, which is crucial to the organisation of appropriate responses to threat. Each terminal region can make use of this information in different ways. Globally, the influence of 5HT neurones on higher centres appears predominantly to facilitate information processing relevant to threat, while their major influence on brainstem centres may be a restraining one.