Differential mechanisms involved in the anticonflict action of benzodiazepines injected into the central amygdala and mammillary body

Role of the GABAa and GABAb receptors of the central nucleus of the amygdala in compulsive cocaine-seeking behavior in male rats

RATIONALE

Compulsive cocaine use, defined as the continued use despite the dire consequences, is a hallmark of cocaine addiction. Thus, understanding the brain mechanism regulating the compulsive cocaine-seeking and cocaine-taking behaviors is essential to understand cocaine addiction and the key to identification of the molecular targets for the development of medications against this condition.

OBJECTIVE

This study aimed to determine how the GABAa and GABAb receptors of the central nucleus of the amygdala (CeA) regulate the compulsive cocaine-seeking behavior.

METHODS

Male Wistar outbred rats were trained to self-administer intravenous cocaine (0.4 mg/kg/infusion) under a chained schedule. The compulsive cocaine-seeking behavior was measured as the cocaine-seeking behavior in the face of footshock punishment. The role of the GABA receptors of CeA in the regulation of such behavior was determined by measuring the dose-dependent effects of the GABAa agonist muscimol or the GABAb agonist baclofen bilaterally microinjected into the CeA on the punished cocaine-seeking behavior.

RESULTS

The cocaine-seeking behavior was inhibited by footshock punishment in an intensity-dependent manner. Both muscimol and baclofen dose-dependently increased the punished cocaine-seeking behavior. However, the potency of muscimol but not baclofen was negatively correlated with the effects of punishment.

CONCLUSION

These data indicate that the CeA GABAa receptors play a key role in the regulation of the compulsive cocaine-seeking behavior and suggest that an increase in the function of the GABAa receptors possibly induced by cocaine or genetic factors may be an important mechanism involved in the development of or vulnerability to the compulsive cocaine use and addiction.

Opposite Effects of Basolateral Amygdala Inactivation on Context-Induced Relapse to Cocaine Seeking after Extinction versus Punishment

Studies using the renewal procedure showed that basolateral amygdala (BLA) inactivation inhibits context-induced relapse to cocaine-seeking after extinction. Here, we determined whether BLA inactivation would also inhibit context-induced relapse after drug-reinforced responding is suppressed by punishment, an animal model of human relapse after self-imposed abstinence due to adverse consequences of drug use. We also determined the effect of central amygdala (CeA) inactivation on context-induced relapse.We trained rats to self-administer cocaine for 12 d (6 h/d) in Context A and then exposed them to either extinction or punishment training for 8 d in Context B. During punishment, 50% of cocaine-reinforced lever-presses produced an aversive footshock of increasing intensity (0.1-0.5 or 0.7 mA). We then tested the rats for relapse to cocaine seeking in the absence of cocaine or shock in Contexts A and B after BLA or CeA injections of vehicle or GABA agonists (muscimol-baclofen). We then retrained the rats for cocaine self-administration in Context A, repunished or re-extinguished lever pressing in Context B, and retested for relapse after BLA or CeA inactivation.BLA or CeA inactivation decreased context-induced relapse in Context A after extinction in Context B. BLA, but not CeA, inactivation increased context-induced relapse in Context A after punishment in Context B. BLA or CeA inactivation provoked relapse in Context B after punishment but not extinction. Results demonstrate that amygdala's role in relapse depends on the method used to achieve abstinence and highlights the importance of studying relapse under abstinence conditions that more closely mimic the human condition.SIGNIFICANCE STATEMENT Relapse to drug use during abstinence is often provoked by re-exposure to the drug self-administration environment or context. Studies using the established extinction-reinstatement rodent model of drug relapse have shown that inactivation of the basolateral amygdala inhibits context-induced drug relapse after extinction of the drug-reinforced responding. Here, we determined whether basolateral amygdala inactivation would also inhibit relapse after drug-reinforced responding is suppressed by punishment, a model of human relapse after self-imposed abstinence. Unexpectedly, we found that basolateral amygdala inactivation had opposite effects on relapse provoked by re-exposure to the drug self-administration environment after extinction versus punishment. Our results demonstrate that depending on the historical conditions that lead to abstinence, amygdala activity can either promote or inhibit relapse.

Re-evaluating the role of the mammillary bodies in memory

Although the mammillary bodies were among the first brain regions to be implicated in amnesia, the functional importance of this structure for memory has been questioned over the intervening years. Recent patient studies have, however, re-established the mammillary bodies, and their projections to the anterior thalamus via the mammillothalamic tract, as being crucial for recollective memory. Complementary animal research has also made substantial advances in recent years by determining the electrophysiological, neurochemical, anatomical and functional properties of the mammillary bodies. Mammillary body and mammillothalamic tract lesions in rats impair performance on a number of spatial memory tasks and these deficits are consistent with impoverished spatial encoding. The mammillary bodies have traditionally been considered a hippocampal relay which is consistent with the equivalent deficits seen following lesions of the mammillary bodies or their major efferents, the mammillothalamic tract. However, recent findings suggest that the mammillary bodies may have a role in memory that is independent of their hippocampal formation afferents; instead, the ventral tegmental nucleus of Gudden could be providing critical mammillary body inputs needed to support mnemonic processes. Finally, it is now apparent that the medial and lateral mammillary nuclei should be considered separately and initial research indicates that the medial mammillary nucleus is predominantly responsible for the spatial memory deficits following mammillary body lesions in rats.

Involvement of the GABA/benzodiazepine receptor in the axiolytic-like effect of nociceptin/orphanin FQ

We investigated the mechanism underlying the anxiolytic actions of the neuropeptide nociceptin/orphanin FQ (N/OFQ) with an elevated plus-maze test. In mice, intracerebroventricular (i.c.v.) infusions of N/OFQ (0.1 and 0.32 nmol) led to an increase in time spent in the open arms (anxiolytic-like effects). A non-peptidyl N/OFQ receptor (NOP) antagonist, J-113397(1-{(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl}-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one), (1.0 and 3.2 mg/kg, s.c.) blocked the increase induced by N/OFQ. On the other hand, a benzodiazepine receptor antagonist, flumazenil, (10 mg/kg, i.p.) and a GABAA receptor antagonist, (+)-bicuculline, (5.6 mg/kg, i.p.) also inhibited the increase induced by N/OFQ. In rats, microinfusions of N/OFQ (10 and 32 pmol) into the amygdala led to an increase in time spent in the open arms. However, intracranial infusions of N/OFQ (10-100 pmol) into the dorsal hippocampus did not affect the time spent in the open arms. These findings suggest that the anxiolytic-like effects of N/OFQ may be related to the GABA/benzodiazepine system in the amygdala.

Exploratory behaviour of rats in the elevated plus-maze is differentially sensitive to inactivation of the basolateral and central amygdaloid nuclei

The amygdala has a crucial role in detecting motivationally significant inputs and in communicating relevant information to other limbic structures. Behavioural studies have shown that the central (CeA) and basolateral (BLA) nuclei of amygdala differentially regulate conditioned and unconditioned fear. Indeed, much evidence has accumulated suggesting that regulatory mechanisms in the BLA serve as a filter for unconditioned and conditioned aversive information that ascends to higher structures from the brainstem, whereas the CeA is the main output for the autonomic and somatic components of fear reaction through major projections to other limbic regions. It is still unclear, however, how amygdaloid nuclei function in high and open spaces so as to determine the characteristic exploratory behaviour of rats submitted to the elevated plus-maze test (EPM). In the present study, we carried out an ethopharmacological analysis of the behaviour of rats submitted to the elevated plus-maze test together with analysis of the tissue content of monoamine dopamine (DA) and serotonin (5-HT) and their metabolites in the dorsal hippocampus (DH), nucleus accumbens (NAC) and dorsal striatum (DS) of animals injected with saline or muscimol (1.0 nmol/0.2 microL) into the BLA or CeA. The data obtained show that injections of muscimol into the CeA, but not into the BLA, caused anxiolytic-like effects in the EPM. Such effects of muscimol into the CeA were accompanied by increases in 5-HT content of the DH, whereas corresponding injections into the BLA caused a reduction in the DA content of the NAC. There was no change in the turnover rates of these monoamines. These data suggest that the BLA and CeA have distinct roles in the exploratory behaviour of rodents in the EPM. While BLA appears to be related to the detection and validation of threatening stimuli, the CeA appears to be involved in the expression of fear behaviours in the EPM.

Differential roles of amygdaloid nuclei in the anxiolytic- and antidepressant-like effects of the V1b receptor antagonist, SSR149415, in rats

RATIONALE

SSR149415 ((2S, 4R)-1-[5-chloro-1-[(2,4-dimethoxyphenyl)sulfonyl]-3-(2-methoxyphenyl)-2-oxo-2,3-dihydro-1H-indol-3-yl]-4-hydroxy-N,N-dimethyl-2-pyrrolidinecarboxamide), the first selective nonpeptide vasopressin V1b receptor antagonist has been shown to induce antidepressant-and anxiolytic-like effects following systemic administration, whereas intraseptal infusion of the drug engender antidepressant-but not anxiolytic-like effects.

OBJECTIVES

Based on recent evidence that V1b receptors are located within the amygdaloid complex, a structure which is well known for its modulatory role of emotional processes, the possible involvement of the different amygdaloid nuclei in the anxiolytic- and/or antidepressant-like effects of SSR149415 was examined.

METHODS

Male Sprague-Dawley or Wistar rats were infused with SSR149415 into the central (CeA), the basolateral (BlA), or the medial (MeA) nucleus of the amygdala and tested 10 min after microinjection in the elevated plus-maze or the forced-swimming test, two models typically used for assessing the anxiolytic and antidepressant effects of drugs, respectively.

RESULTS

Microinjection of SSR149415 into the BlA (1-10 ng), but not into the CeA or the MeA, increased the percentage of time spent in the open arms of the elevated plus-maze, indicating anxiolytic-like effects. Furthermore, in the forced-swimming test, microinjection of the drug into the CeA (1, 10, and 100 ng), BlA (1-10 ng), or MeA (100 ng) decreased immobility, an effect which is indicative of an antidepressant-like action. Together, these findings indicate that while the antidepressant-like effects of SSR149415 are mediated by different amygdaloid nuclei, its anxiolytic-like effects appear to involve only the basolateral nucleus of the amygdala. Moreover, these results add further evidence to the role of extrahypothalamic vasopressinergic systems in the control of emotional responses.

Gabaergic modulation of the stress response in frontal cortex and amygdala

GABAergic neurotransmission is thought to play an important role in the modulation of the central response to stress. In the present study we evaluate the influence of a brief restraint exposure on GABA-stimulated chloride influx in diverse brain areas presumed to have a major role in the mediation of emotional behaviors following aversive stimulation. A reduced chloride uptake after stress exposure was only observed in frontal cortex and amygdala. Moreover, rats subjected to such stressor performed an anxiogenic behavior when exposed later to the elevated plus-maze. A comparable behavior in the elevated plus-maze was observed between animals that were allowed to chew during the restraint experience and those without any stressful manipulation, suggesting that chewing served as an efficient coping behavioral strategy during such threatening situations. In order to explore if chewing during the restraint experience could suppress the reduction in GABA-stimulated chloride uptake induced by this stressor, rats were allowed or not to chew during restraint and in both cases GABA-stimulated chloride influx was assayed in frontal cortex and amygdala. The finding of this experiment showed that restrained rats that have the possibility to chew exhibited a similar GABA-stimulated chloride uptake in cortical tissue to that shown by control, unstressed rats. Moreover, chewing in response to restraint attenuated the reduction of GABA-stimulated chloride uptake in amygdala, supporting the notion that chewing is an effective coping response to restraint. These experiments suggest that a reduced GABAergic inhibitory control in these areas could be implicated in the emotional sequelae generated by this uncontrollable stressor and that the suppression of this reduction seems to be associated with the occurrence of coping behavioral response to such fear-inducing stimulus.

The interaction of morphine and gamma-aminobutyric acid (GABA)ergic systems in anxiolytic behavior: using mu-opioid receptor knockout mice

We investigated the interaction of morphine and gamma-aminobutyric acid (GABA)ergic systems in anxiolytic action using mu-opioid receptor knockout and wild-type female mice. An elevated plus-maze test was used to assess anxiolytic behavior. The anxiolytic activities were monitored after experimental animals were pretreated with morphine [15 mg/kg, subcutaneous (s.c.)] and 3 h later received a single dose of muscimol (0.5 mg/kg, s.c.). In another experiment, mice received a single dose of opioid antagonist, naloxone [1.0 mg/kg, intraperitoneal (i.p.)], or GABA(A) receptor antagonist, (+)bicucullin (2.0 mg/kg, i.p.), 2.5 h after the morphine and 30 min before the muscimol injection. Control mice received vehicle only. The results show that morphine enhanced muscimol-induced staying time in open arms by 160% in wild-type mice. Moreover, the effect of morphine in the wild type was inhibited by the pretreatment of either naloxone or (+)bicucullin. Autoradiographic analysis indicated that morphine-administration raised [(3)H]muscimol binding by around 25% of the basal level in posterior thalamic, mediodorsal thalamic, and amygdaloid areas, but not in the hippocampal area in wild-type mice. In contrast, morphine administration did not alter the [(3)H]muscimol binding in mu-opioid receptor knockout mice. The present results reveal that mu-opioid receptor may play a role in the modification of anxiolytic behavior regulated by GABAergic neurotransmission.

Regulation of benzodiazepine receptor binding and GABA(A) subunit mRNA expression by punishment and acute alprazolam administration

Quantitative autoradiography of benzodiazepine (BZ) receptors and competitive reverse transcription-polymerase chain reaction were used to characterize changes in BZ binding and GABA(A) receptor subunit transcription levels associated with the anxiolytic effects of alprazolam. Effects were assessed on punished and non-suppressed water consumption using a lick suppression (Vogel) paradigm. Alprazolam had no effect on non-suppressed licking, [(3)H]Ro 15-1788 binding or receptor subunit transcript levels, compared to non-drug controls. When each fifth lick produced a shock (0-0.5 mA), responding was suppressed in an intensity-related manner. The highest intensity significantly decreased licking (85%), [(3)H]Ro 15-1788 binding (12%) and alpha1 transcript levels (63%) in the basolateral nucleus of the amygdala, and [(3)H]Ro 15-1788 binding in the mediodorsal thalamic nucleus (15%), compared to non-punished controls. Punishment increased the ratio of gamma2L/S transcripts in the basolateral nucleus of the amygdala. Alprazolam blocked or reversed each of these effects. These results show that punishment has similar effects on BZ binding and GABA(A) receptor subunit expression and that alprazolam can block or reverse those effects. Such changes may be related to the anxiolytic effects of alprazolam.

Genetic inactivation of the Serotonin(1A) receptor in mice results in downregulation of major GABA(A) receptor alpha subunits, reduction of GABA(A) receptor binding, and benzodiazepine-resistant anxiety

Anxiety is a common psychiatric illness often treated by benzodiazepines (BZs). BZs, such as Valium, bind to the alpha subunit of the pentameric GABA(A) receptor and increase inhibition in the CNS. There is considerable evidence for abnormal GABA(A) receptor function in anxiety, and a significant proportion of anxiety patients has a reduced sensitivity to BZs. Here, we show that serotonin(1A) (5-HT(1A)) receptor knock-out mice display BZ-resistant anxiety. Consistent with this finding, binding of both BZ and non-BZ GABA(A) receptor ligands were reduced and GABAergic inhibition was impaired in mutant mice. These changes were reflected by abnormal alpha subunit expression in the amygdala and hippocampus, two important limbic regions involved in fear and anxiety. These data suggest a pathological pathway, initiated by a 5-HT(1A) receptor deficit, leading to abnormalities in GABA(A) receptor composition and level, which in turn result in BZ-insensitivity and anxiety. This model mechanistically links together the 5-HT and GABA systems, which both have been implicated in anxiety. A related mechanism may underlie reduced BZ sensitivity in certain forms of anxiety.

Differential expression of EGR-1 mRNA in the amygdala following diazepam in contextual fear conditioning

The amygdaloid complex is thought to be a major site of action of anxiolytic benzodiazepine agonists. To investigate whether activity in the amygdaloid complex is altered with anxiolytic effects of diazepam, mRNA expression of the immediate-early gene EGR-1 was examined in the amygdala following blockade of fear conditioning by diazepam. It was previously shown that mRNA expression of EGR-1 (also called, NGFI-A, Zif 268, Krox 24) increases in the lateral nucleus of the amygdala (LA) shortly following contextual fear conditioning. It was therefore hypothesized that diazepam would block both contextual fear and the concomitant increase in EGR-1 mRNA expression in the LA induced by fear conditioning. Rats administered systemic diazepam before fear conditioning displayed both anxiolytic effects during the post-shock period and amnesic effects during a retention test 24 h later. Diazepam blocked the fear-conditioning-induced increase in EGR-1 expression in the LA. In addition, diazepam significantly increased EGR-1 mRNA expression in the central nucleus of the amygdala (CeA) in a dose-dependent manner. The results reveal differential regulation of EGR-1 by diazepam in the central and lateral nuclei of the amygdala suggesting that these two amygdala nuclei act in a reciprocal manner during the anxiolytic and amnesic action of the benzodiazepine agonist.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala: a review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

Anxiolytic effect of intra-amygdala injection of midazolam and 8-hydroxy-2-(di-n-propylamino)tetralin in the elevated T-maze

The effects of intra-amygdala injection of midazolam (20 nmol) and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 8 and 16 nmol) were investigated in rats submitted to the elevated T-maze, a new animal model of anxiety. This test allows the measurement, in the same rat, of conditioned and unconditioned fear/anxiety responses. Both drugs impaired inhibitory avoidance of the open arms of the T-maze (task representing conditioned fear), indicating an anxiolytic effect, but did not change escape performance from one of the open arms (representing unconditioned fear). The results further implicate gamma-aminobutyric acid (GABA)/benzodiazepine and serotonergic systems within the basolateral/lateral amygdala in the modulation of conditioned anxiety responses.

Alterations of GABAA receptor subunit mRNA levels associated with increases in punished responding induced by acute alprazolam administration: an in situ hybridization study

Changes in the mRNA encoding alpha1, alpha2, beta2 and gamma2 subunits of the GABAA receptor associated with the anxiolytic effects of alprazolam were measured in 20 brain regions using in situ hybridization techniques. Compared to non-punished controls, punishment decreased alpha1 mRNA levels in two nuclei of the amygdala, the cerebral cortex, and the mediodorsal thalamic nucleus and decreased alpha2 mRNA levels in the hippocampus. Punishment increased beta2 mRNA levels in ventroposterior thalamic nucleus and gamma2 mRNA levels in the CA2 area of the hippocampus. All of these effects were reversed when alprazolam increased punished responding, while alprazolam alone had no effect on either non-punished responding or GABAA receptor subunit regulation in these brain regions. Some brain regions that were unaffected by punishment were altered by alprazolam plus punishment. These results demonstrate that punishment and alprazolam can produce reciprocal changes in the mRNA levels for some subunits of the GABAA receptor. These changes may alter GABAergic synaptic inhibition by altering the density of GABAA receptors or their efficacy to bind drugs. They suggest that the underlying mechanisms by which drugs affect behavior can depend upon the conditions under which behavior is assessed.

Ritanserin facilitates anxiety in a simulated public-speaking paradigm

The effects of ritanserin, a 5-HT2A/2C (5-hydroxytryptamine) antagonist, have been investigated in simulated public speaking with healthy volunteers. The aim was to investigate the role of 5-HT in subjective experimental anxiety. There were three experimental groups each comprising four or five males and 11 females. Subjects received placebo, ritanserin 2.5 or 10 mg, p.o. They rated themselves using the Spielberger State-Trait Anxiety Inventory and visual analogue scales factored into anxiety, sedation and discontentment scores. Autonomic measures included skin conductance and heart rate. Subjects were told, 75 min after drug or placebo ingestion, without prior warning, to prepare a 4-min speech. Measures were taken before, during and after the speech. Ritanserin prolonged the anxiety induced by the procedure on the subjective ratings but had minimal effect on autonomic responses to the procedure. The result contrasts with an anxiolytic-like effect of ritanserin on aversively conditioned autonomic responses. The present finding is compatible with animal behavioural evidence that 5-HT has distinct and opposing roles in modulating conditioned and unconditioned anxiety.

Muscarinic acetylcholine receptor immunoreactivity in the amygdala--II. Fear-induced plasticity

Changes in the distribution of muscarinic acetylcholine receptor-immunoreactive neurons were examined in the amygdaloid complex at different time-intervals following a single training session of active shock avoidance in a two-way shuttle-box. Muscarinic acetylcholine receptors were visualized using M35, a monoclonal antibody raised against purified muscarinic acetylcholine receptor protein. Both in naive animals and 2 h after active shock avoidance training, muscarinic acetylcholine receptor immunoreactivity was high in the central nucleus, and only low to moderate in other amygdaloid regions. Twenty-four hours after training, however, the muscarinic acetylcholine receptor immunoreactivity distribution pattern was reversed, showing a dramatic increase in the corticomedial nucleus, while in contrast, in other amygdaloid regions including the central nucleus, muscarinic acetylcholine receptor immunoreactivity was reduced to only a few scattered neurons. Additional studies with a modified experimental design indicated that fear conditioning mechanisms in association with the severity of the aversive stimuli, and not the learning of the avoidance response, may account for the changes in muscarinic acetylcholine receptor immunoreactivity in the amygdala. These results are consistent with the prominent role of the central nucleus in the conditioning and expression of the fear response. A closer examination revealed that 8 h after training the changes in both the central and corticomedial nuclei became significant. The differences still existed after 25 days, but three months after the training session the receptor distribution was returned to normal. The long-lasting, but reversible nature of these changes indicates that fear conditioning is accompanied by a dynamic plasticity of muscarinic acetylcholine receptor immunoreactivity in the amygdaloid complex.

Muscarinic acetylcholine receptor immunoreactivity in the amygdala--I. Cellular distribution correlated with fear-induced behavior

This study examined the distribution of muscarinic acetylcholine receptor-immunoreactive neurons in the amygdaloid complex of the rat, with emphasis on the central nucleus. The monoclonal antibody M35 raised against purified muscarinic acetylcholine receptor protein was used to visualize muscarinic acetylcholine receptor-immunoreactive cells. Muscarinic acetylcholine receptor immuno-reactivity was high in the central nucleus and low to moderate in all other regions of the amygdaloid complex. Within the central nucleus, the muscarinic acetylcholine receptor-immunoreactive neurons were found predominantly in the lateral subdivision. This region contained medium-sized neurons (largest diameter ranging from 10 to 15 microns), with a round or slightly ovoid cell shape. At the subcellular level, however, the labeled neurons revealed relatively few muscarinic acetylcholine receptor-immunoreactive postsynaptic densities. Immunofluorescent double-labeling demonstrated that nearly all of the muscarinic acetylcholine receptor-immunoreactive neurons (98.6%) in the central nucleus expressed abundant amounts of nicotinic acetylcholine receptors, further substantiating the cholinoceptive character of these cells. In addition, the vast majority of these muscarinic acetylcholine receptor-immunoreactive neurons (94.3%) were GABAergic neurons. The muscarinic acetylcholine receptor-immunoreactive neurons expressed moderate levels of protein kinase gamma, one of the likely intracellular mediators between muscarinic acetylcholine receptors and their elicited physiological response. The number and staining intensity of muscarinic acetylcholine receptor-immunoreactive neurons in the central nucleus varied dramatically among rats. This individual variation correlated positively with the rat's expression of conditioned immobility and correlated negatively with active shock avoidance performance. These results suggest that the GABAergic/cholinoceptive neuronal elements in the central nucleus are involved in the expression of fear-induced behaviors. This interpretation is further elaborated in a forthcoming paper.

Involvement of the opioid system in the anxiolytic effect of diazepam in mice

In the present study, the anticonflict effect of diazepam was significantly abolished by pretreatment with naloxone, beta-funaltrexamine or nor-binaltorphimine but not naltrindole, using a Vogel-type conflict paradigm in mice. However, naloxone alone had a significant proconflict effect, and beta-funaltrexamine alone tended to produce a proconflict effect. Spontaneous drinking behavior was not affected by treatment with diazepam and nor-binaltorphimine. In addition, nor-binaltorphimine had no effect on diazepam-induced motor incoordination, hypothermia or anticonvulsant action, respectively. Moreover, the stable dynorphin analog E2078 ([N-methyl-Tyr1, N-alpha-methyl-Arg7-D-Leu8]dynorphin A-(1-8) ethylamide) and the highly selective kappa-opioid receptor agonist U50,488H (trans-3,4-dichloro-N-(2-(1-pyrrolidinyl)cyclohexyl)benzenacetamide++ + methanesulfonate hydrochloride) produced a significant anticonflict effect, which was completely antagonized by pretreatment with nor-binaltorphimine. These findings suggested that the kappa-opioid system may play an important role in the anxiolytic effect of benzodiazepine and the regulation of anxiety.

Regulation of anxiety by GABAA receptors in the rat amygdala

Blockade of GABAergic inhibition in the region of the anterior basolateral amygdala (BLA) of rats elicits physiologic changes associated with a defense reaction. The present study was undertaken to determine whether GABA receptors in the BLA might be involved in regulating experimental anxiety using the social interaction (SI) and conflict test. Guide cannulae were stereotaxically implanted bilaterally in the BLA of rats for intracerebral microinjections. In the BLA, injection of the GABAA receptor antagonists bicuculline methiodide (BMI) and picrotoxin (PIC) produced anxiogenic-like effects in the SI paradigm, as did BMI injection using the conflict paradigm. Injection of the GABAA agonist muscimol (MUS) into the central nucleus of the amygdala (Ce) produced anxiolytic-like effects in the SI test. Microinjection of MUS, baclofen (GABAB agonist), 2OH-saclofen (GABAB antagonist) or strychnine (glycine antagonist) into the BLA or BMI into the Ce elicited no change in experimental anxiety as measured by the SI test. These results suggest that endogenous GABA acts tonically at GABAA receptors in the BLA to inhibit anxiety responses.

Brain sites involved in mu-opioid receptor-mediated actions: a 2-deoxyglucose study

Brain regions that may be functionally involved in the neuropharmacological actions of mu-opioid agonists have been examined in conscious rats using the quantitative [14C]2-deoxyglucose autoradiographic technique. At 0.5 microgram and 1 microgram intracerebroventricularly the highly selective mu-opioid receptor agonist D-Ala2, MePhe4, Gly-ol5-enkephalin effected statistically significant increases as well as statistically significant decreases in regional glucose utilization: in limbic structures, such as hippocampal formation, medial amygdala and lateral septum, glucose utilization was most prominently increased after D-Ala2, MePhe4, Gly-ol5-enkephalin; glucose utilization was further increased in the lateral habenular nucleus, the hypothalamus, ventromedial nucleus and dorsal raphe; whereas decreases were found in the mamillary body and anterior thalamus. Glucose utilization in structures associated with somatosensory and nociceptive processing was increased in the central gray of the midbrain and decreased in the nucleus gelatinosus. Only increases in glucose utilization were produced by D-Ala2; MePhe4, Gly-ol5-enkephalin in brain regions involved in motor control, including the globus pallidus, the substantia nigra, pars reticulata, the nucleus ruber and the cerebellum, and brain regions involved in visual processing--the visual cortex and superior colliculus deep layer. It is concluded that this pattern of regional changes underlies the mu-opioid receptor-mediated antinociceptive-, epileptogenic-, memory- and mood-modulating actions of mu-opioid agonists.

The central and basolateral amygdala differentially mediate the anxiolytic effects of benzodiazepines

Microinfusions of the benzodiazepine anxiolytic midazolam into the central or basolateral amygdaloid nuclei produced different anxiolytic effects in two tests of rat 'anxiety'. Infusions into the basolateral nucleus impaired open-arm avoidance in the elevated plus-maze test, but did not impair shock-probe avoidance in the shock-probe burying test. In contrast, infusions into the central nucleus impaired shock-probe avoidance, but did not impair open-arm avoidance. Both of these site-specific, midazolam-induced anxiolytic effects were blocked by a pre-infusion of the benzodiazepine receptor antagonist Ro 15-1788 (flumazenil). None of the treatments affected defensive burying. These results suggest that benzodiazepine receptors in the central and basolateral amygdaloid nuclei differentially mediate the anti-anxiety effects of benzodiazepine anxiolytics.

The septum and amygdala differentially mediate the anxiolytic effects of benzodiazepines

Microinfusions of a benzodiazepine anxiolytic (midazolam) into the septum or the amygdala suppressed different fear reactions in two tests of rat "anxiety". Septal infusions increased open-arm activity in the plus-maze test and decreased burying behavior in the shock-probe test whereas amygdaloid infusions produced neither of these antianxiety effects. Amygdaloid infusions, however, dramatically impaired shock-probe avoidance, an antianxiety effect not produced by the septal infusions. Infusions of the benzodiazepine receptor antagonist Ro 15-1788 (flumazenil) blocked each of these specific, anti-fear effects of midazolam without producing intrinsic effects by itself. These results suggest that benzodiazepine receptor systems within the amygdala and the septum differentially mediate specific fear reactions.

The MSH/ACTH(4-9) analog Org2766 counteracts isolation-induced enhanced social behavior via the amygdala

MSH/ACTH-like peptides influence social behavior induced by isolation It has been previously demonstrated that changes in locomotor activity as a result of isolation can be counteracted by Org2766 via the amygdala. The present study investigates whether isolation-induced changes in social behavior can also be affected by this peptide via the amygdala. A fully automated observation system was applied for detailed registration and analysis of movements of group-housed and 7-day isolated rats in a social interaction test. Administration of the MSH/ACTH(4-9) analog into the central nucleus of the amygdala elicited decreased locomotion, approach, and avoidance behaviors after isolation as compared to placebo-treated controls. However, general activity and social interest of group-housed rats were not affected by the MSH/ACTH(4-9) fragment. It is hypothesized that the amygdala is a site of action for neuropeptides in modulating social behavior.

Cyproheptadine prevents the initial occurrence of successive negative contrast

Rats shifted from 32% to 4% sucrose make fewer licks for 4% sucrose than rats having only experienced the lower reward. In Experiment 1, the occurrence of this contrast effect was prevented by the administration of the nonspecific serotonin antagonist cyproheptadine (3.0 or 6.0 mg/kg). These results of Experiments 2 and 3 demonstrated that the contrast-reducing action of cyproheptadine was not mediated by the antiserotonergic properties of the drug since systemic administration of the serotonin synthesis inhibitor, PCPA (150 or 300 mg/kg), failed to influence either the occurrence of contrast or the attenuation of contrast by cyproheptadine. The results of Experiment 4 indicated that the contrast-reducing action of cyproheptadine was not mediated by the antihistaminergic properties of the drug since the antihistamine, pyrilamine (6 or 12 mg/kg), also failed to prevent the occurrence of contrast. Finally, the contrast-reducing action of cyproheptadine was not due to rate-dependent and/or appetite stimulating effects since cyproheptadine did not serve to increase lick frequency in rate-dependent controls.

Effects of ritanserin on aversive classical conditioning in humans

According to one formulation of the behavioural functions of 5HT, aversive conditioned stimuli mediate their behavioural and emotional effects through activation of 5HT projections from dorsal raphe nucleus to receptors of the 5HT2 family in amygdala and elsewhere. To test this theory in humans, groups of ten normal volunteers received placebo, the 5HT2 lc antagonist ritanserin (10 mg PO) and no pill. Ritanserin had no effect on skin conductance level, variability (spontaneous fluctuations) or habituation to a sequence of ten neutral tones. After a conditioning trial in which tone 11 was followed by an aversive white noise, skin conductance responses to a further ten tones were enhanced. This effect was abolished by ritanserin. The results indicate a selective involvement of 5HT2/lc receptors in modulating aversively conditioned skin conductance responses.

Differential effect of lesioning of the central amygdala on the bradycardiac and behavioral response of the rat in relation to conditioned social and solitary stress

The central nucleus of the amygdala (CEA) is considered to be involved in stress-dependent regulation of autonomic functions. In the present study the effects of a bilateral electrolytic lesion of the CEA on the cardiac response and on the duration of immobility behavior in two conditioned stress situations have been analysed in Wistar rats. Fear of a previously received electric footshock elicited in almost all animals a profound early bradycardia and a marked immobility behavior in comparison to non-stressed freely moving controls. Lesioning of the CEA completely abolished the bradycardiac response. Immobility behavior was slightly diminished. The conditioned fear of a dominant rat following social defeat was accompanied by a bradycardiac response in about 50% of the rats. This bradycardiac response was not accompanied by significant immobility behavior. The rest of the defeated rats--i.e. cardiac 'non-responders'--displayed an early increase in immobility behavior. The CEA lesion abolished again the bradycardiac response, but the immobility behavior remained unchanged. In the cardiac 'non-responders' CEA lesion failed to affect immobility behavior, and the heart rate also remained unchanged. These results suggest that the CEA is involved in the organization of conditioned parasympathetic responses independent of the nature of stress. In contrast, the role of this nucleus in the organization of the immobility behavior seems to depend on the nature of stress and other, probably individual characteristics of the rats' behavior.

Local cerebral glucose utilization following subacute and chronic diazepam pretreatment: differential tolerance

Local cerebral glucose utilization (LCGU) was determined in parallel groups of conscious rats receiving diazepam (0.3 mg/kg i.v.) either acutely or following subacute (5 mg/kg i.p. daily for 3 days) or chronic (5 mg/kg i.p. daily for 28 days) diazepam pretreatment, using 2-deoxyglucose quantitative autoradiography. Acute administration of diazepam reduced LCGU in 44 of the 66 structures examined compared to vehicle-treated controls. These included limbic, cortical and extrapyramidal structures, and areas associated with sensory processing. These data are consistent with many brain regions being functionally involved in the diverse acute pharmacological effects of diazepam and with the widespread distribution of benzodiazepine receptors throughout the neuroaxis. Following subacute treatment, when animals were tolerant to the sedative effects of diazepam, glucose use remained depressed in the majority of areas studied. However, in the locus coeruleus, dorsal tegmental nucleus and most structures associated with auditory processing, tolerance to the depressant effect of diazepam upon glucose use had occurred suggesting the importance of these structures in the sedative effect of diazepam. The most striking feature of the patterns of LCGU after chronic diazepam treatment was that tolerance had occurred in the mammillary body and subiculum. However, glucose use remained depressed in hippocampal layers and in structures that provide input to the hippocampus (e.g. raphe nuclei). These data suggest that the outflow of neuronal activity from the hippocampus to the mammillary body via the subiculum is restored after chronic treatment, and may implicate these pathways in the anxiolytic action of diazepam. Overall, it would appear that different neuroanatomical substrates underlie the various pharmacological effects of diazepam and that there may be regional differences in tolerance mechanisms.

Conflict behavior and dynamics of monoamines of various brain nuclei in rats

The present study was an attempt to clarify the role of noradrenaline (NA) and of the 5-hydroxytryptamine (5-HT) system in various nuclei in brain, as a component of a proposed neural circuit in the mediation of conflict behavior and the anticonflict action of anxiolytics. The authors investigated changes in the concentrations of NA, 3-methoxy-4-hydroxyphenylethyleneglycol, 5-HT and 5-hydroxyindoleacetic acid in discrete regions of the brain in rats, in correlation with conflict behavior and also the effects of diazepam and suriclone. Noradrenergic neural activity diminished with a conflict situation, in the frontal cortex, central amygdala, mammillary body and dorsal hippocampus. 5-Hydroxytryptaminergic neural activity increased with a conflict situation in the frontal cortex, central amygdala, basolateral amygdala and medial septum. These changes in the frontal cortex, central amygdala, mammillary body and dorsal hippocampus were not observed when diazepam 20 mg/kg (p.o.) and suriclone 40 mg/kg (p.o.) produced anticonflict action. Suriclone normalized the increased 5-HT-ergic activity in the medial septum. The suppression of NA-ergic and the activation of 5-HT-ergic (except for the mammillary body) neural activity in the frontal cortex, central amygdala, mammillary body and dorsal hippocampus seemed to be linked to the mediation of conflict behavior. The facilitatory and inhibitory action on NA and 5-HT (except for the mammillary body) neurons, respectively, in these regions of the brain, may be involved in mechanisms underlying the anticonflict action of anxiolytics (diazepam or suriclone).

Effects of benzodiazepine and GABA antagonists on anticonflict effects of antianxiety drugs injected into the rat amygdala in a water-lick suppression test

In order to elucidate the role of the amygdala in rat conflict behavior in a water lick suppression test, we examined the effect of lesions of various nuclei of the amygdaloid complex on this behavior. An anticonflict effect was produced by a lesion of the anterior part of central and basolateral amygdala, and lesion to the posterior part of the central amygdala, but not by posterior of the basolateral amygdala or medial amygdala lesions. These results suggest that the amygdala, especially the anterior part of the central and basolateral nuclei, plays an important role in conflicting behavior of rats in the water lick test. In a second experiment, the effects of benzodiazepine- and GABA-antagonists on the anticonflict action of diazepam, zopiclone, and phenobarbital injected into the anterior part of central and basolateral amygdala were examined, also using a water lick suppression test. A dose-dependent anticonflict action was produced by systemic administration as well as by intra-amygdala injection of diazepam, zopiclone, lormetazepam, flurazepam and phenobarbital. The order of potency was lormetazepam greater than zopiclone greater than or equal to diazepam greater than flurazepam greater than or equal to phenobarbital for both routes of injection. The anticonflict effects of diazepam and zopiclone injected into the amygdala were completely reversed by Ro15-1788 and beta-CCM but not by bicuculline, while the anticonflict effect of phenobarbital was reversed by beta-CCM but not by Ro15-1788 or bicuculline. The present results strongly suggest that the anterior nuclei of central and basolateral amygdala are important sites of action of antianxiety drugs, and that an anticonflict action produced by intra-amygdala injection of benzodiazepines or barbiturate is mediated through the different receptor mechanisms.

The anti-conflict effect of cyproheptadine is not mediated by its 5-hydroxytryptamine antagonistic property

Cyproheptadine, a 5HT2 receptor antagonist with prominent antimuscarinic and anti-histaminic properties, was shown to have an anti-conflict effect in rats using a modified Geller-Seifter test and also enhanced extinction of conflict behaviour. The selective 5HT2 receptor antagonist ritanserin, however, had neither an anti-conflict effect nor an effect on extinction of conflict behaviour. The muscarinic receptor antagonist scopolamine, on the other hand, was active in both paradigms. The effect of cyproheptadine on extinction of conflict behaviour was decreased by co-administration of physostigmine, an acetylcholinesterase inhibitor, but not affected by the concomitant administration of the muscarine receptor agonist oxotremorine. The results suggest that the anti-conflict effect of cyproheptadine has to be ascribed to its anti-muscarinic activity and is not due to its 5HT2 antagonism.

The effects of FG 7142 upon local cerebral glucose utilization suggest overlap between limbic structures important in anxiety and convulsions

The effects of the beta-carboline benzodiazepine receptor ligand FG 7142 upon local cerebral glucose utilization have been examined in conscious rats using the quantitative [14C]2-deoxyglucose autoradiographic technique. FG 7142 (1-10 mg/kg i.v.) produced behavioural changes consistent with an anxiogenic action. At the largest dose of FG 7142 (10 mg/kg) 30% of the animals experienced overt convulsions. In the data analysis animals were divided according to the behavioural response elicited by the drug. In animals not expressing convulsions, FG 7142 (1-10 mg/kg) effected increases in glucose utilization in 33 of the 65 regions examined. The majority of changes were confined to limbic structures with pronounced effects occurring in the mammillary body, anterior thalamic nuclei, septal nuclei and the oriens and molecular layers of the hippocampus. Glucose use in other structures associated with auditory and visual processing, such as the medial and lateral geniculate body, and associated cortical areas, was also significantly increased. However, brain regions involved in motor control were minimally affected. The patterns of local cerebral glucose use in animals expressing FG 7142-induced convulsions were contrasted with those from an equivalent non-seizure group. Some limbic structures which were significantly affected by FG 7142 (non-seizure group) displayed a further increase in glucose utilization during convulsions. These included the mammillary body and septum. Many other limbic structures (anterior thalamic nuclei, CA fields of the hippocampus and basolateral amygdala) did not display this further rise in glucose utilization. In the cortical amygdala, lateral preoptic area of the hypothalamus, nucleus accumbens and lateral elevations in glucose utilization were restricted to those animals experiencing overt convulsions.(ABSTRACT TRUNCATED AT 250 WORDS)