Interactions of metergoline with diazepam, quipazine, and hallucinogenic drugs on a conflict behavior in the rat

Conflict Test Battery for Studying the Act of Facing Threats in Pursuit of Rewards

Survival depends on the ability of animals to avoid threats and approach rewards. Traditionally, these two opposing motivational systems have been studied separately. In nature, however, they regularly compete for the control of behavior. When threat- and reward-eliciting stimuli (learned or unlearned) occur simultaneously, a motivational conflict emerges that challenges individuals to weigh available options and execute a single behavioral response (avoid or approach). Most previous animal models using approach/avoidance conflicts have often focused on the ability to avoid threats by forgoing or delaying the opportunity to obtain rewards. In contrast, behavioral tasks designed to capitalize on the ability to actively choose to execute approach behaviors despite threats are scarce. Thus, we developed a behavioral test battery composed of three conflict tasks to directly study rats confronting threats to obtain rewards guided by innate and conditioned cues. One conflict task involves crossing a potentially electrified grid to obtain food on the opposite end of a straight alley, the second task is based on the step-down threat avoidance paradigm, and the third one is a modified version of the open field test. We used diazepam to pharmacologically validate conflict behaviors in our tasks. We found that, regardless of whether competing stimuli were conditioned or innate, a low diazepam dose decreased risk assessment and facilitated taking action to obtain rewards in the face of threats during conflict, without affecting choice behavior when there was no conflict involved. Using this pharmacologically validated test battery of ethologically designed innate/learned conflict tasks could help understand the fundamental brain mechanisms underlying the ability to confront threats to achieve goals.

A macrophage-based screen identifies antibacterial compounds selective for intracellular Salmonella Typhimurium

Salmonella Typhimurium (S. Tm) establishes systemic infection in susceptible hosts by evading the innate immune response and replicating within host phagocytes. Here, we sought to identify inhibitors of intracellular S. Tm replication by conducting parallel chemical screens against S. Tm growing in macrophage-mimicking media and within macrophages. We identify several compounds that inhibit Salmonella growth in the intracellular environment and in acidic, ion-limited media. We report on the antimicrobial activity of the psychoactive drug metergoline, which is specific against intracellular S. Tm. Screening an S. Tm deletion library in the presence of metergoline reveals hypersensitization of outer membrane mutants to metergoline activity. Metergoline disrupts the proton motive force at the bacterial cytoplasmic membrane and extends animal survival during a systemic S. Tm infection. This work highlights the predictive nature of intracellular screens for in vivo efficacy, and identifies metergoline as a novel antimicrobial active against Salmonella.

Reconciling the role of central serotonin neurons in human and animal behavior

Animal research suggests that central serotonergic neurons are involved in behavioral suppression, particularly anxiety-related inhibition. The hypothesis linking decreased serotonin transmission to reduced anxiety as the mechanism in the anxiolytic activity of benzodiazepines conflicts with most clinical observations. Serotonin antagonists show no marked capacity to alleviate anxiety. On the other hand, clinical signs of reduced serotonergic transmission (low 5-HIAA levels in the cerebrospinal fluid) are frequently associated with aggressiveness, suicide attempts, and increased anxiety. The target article attempts to reconcile such human and animal findings by investigating whether anxiety reduction or increased impulsivity is more Likely to account for animal behavioral changes associated with decreased serotonergic transmission. The effects of manipulating central serotonin in experimental anxiety paradigms in animals (punishment, extinction, novelty) are reviewed and compared with the effects of antianxiety drugs. Anxiety seems neither necessary nor sufficient to induce control by serotonergic neurons on behavior. Further evidence suggests that behavioral effects of anxiolytics thought to be mediated by decreases in anxiety are not caused by the ability of these drugs to reduce serotonin transmission. Blockade of serotonin transmission, especially at the level of the substantia nigra, results in a shift of behavior toward facilitation of responding. This behavioral shift is particularly marked when there is competition between acting and restraining response tendencies and when obstacles prevent the immediate attainment of an anticipated reward. It is proposed that serotonergic neurons are involved not only in behavioral arousal but also in enabling the organism to arrange or tolerate delay before acting. Decreases in serotonin transmission seem to be associated with the increased performance of behaviors that are usually suppressed, though not necessarily because of the alleviation of anxiety, which might contribute to the suppression.

5-Hydroxytryptamine-interacting drugs in animal models of anxiety disorders: more than 30 years of research

An overview of the behavioral data arising from the vast literature concerning the involvement of 5-hydroxytryptamine (5-HT) neurotransmission in the regulation of anxiety is presented. More than 1300 experiments were carried out in this area and they provide evidence that: (1) results obtained in ethologically based animal models of anxiety with drugs stimulating 5-HT transmission are most consistent with the classic 5-HT hypothesis of anxiety in that they show an increase in animals' emotional reactivity; (2) no category of anti-anxiety models are selectively sensitive to the anxiolytic-like effects of drugs targetting 5-HT1A, 5-HT2A or 5-HT2C receptor subtypes; (3) anxiolytic-like effects of 5-HT3 receptor antagonists, in the great part, are revealed by models based on spontaneous behaviors. Taken together, these observations lead to the conclusion that different 5-HT mechanisms, mediated by different receptor subtypes, are involved in the genesis of anxiety.

Modulation of 5HT1A receptors in the hippocampus and the raphe area of rats treated with clonazepam

1. Clonazepam is one of the most potent benzodiazepines known to decrease the activity of the central serotonergic systems. The acute and subchronic administration of clonazepam reduced serotonin (5HT) turnover rate in the hippocampus of the rat, as determined by the ratio of the monoamine and its metabolite, 5-hydroxyindoleacetic acid. 2. The modulation of 5HT binding sites and 5HT1A receptors by the administration of clonazepam for various periods of time were studied in the hippocampus and the raphe area by experiments with radioligands. 3. The density of [3H]5HT recognition sites increased in the hippocampus of clonazepam-treated rats in a dose- and time-dependent manner. This increase was impaired by the simultaneous administration of the 5HT agonist 5-methoxy-N,N-dimethyltryptamine. The affinity of this binding did not significantly change. This observation might indicate an increase in some of the 5HT receptors or an increase of the uptake site. 4. The binding parameters for [3H]DPAT, Bmax and Kd, decreased in the hippocampus but not in the raphe area of clonazepam-treated rats. It seems that the presynaptic reduction in 5HT function, resulting in the decrease of its availability at the synaptic space, modifies the corresponding 5HT recognition sites. 5. These changes could be related to the anxyolitic activity or the withdrawal symptoms of benzodiazepines.

Conflict behavior in Maudsley reactive and nonreactive rats: effects of noradrenergic neuronal destruction

The present studies were designed to examine the effects of treatment with the noradrenergic neurotoxin N-(2-chloroethyl)-n-ethyl-2-bromobenzylamine HCl (DSP4; 65 mg/kg, IP) on conflict behavior in the Maudsley reactive (MR) and nonreactive (MNRA) rat strains. In daily 10-min sessions, water-restricted rats were trained to drink water from a tube that was occasionally electrified; electrification was signaled by the presence of a tone (7-s duration; ISI = 30 s). Consistent with previous reports, the number of shocks accepted by rats of the MR and MNRA strains did not differ initially, but MNRA rats exhibited a dramatic increase in punished responding relative to their MR counterparts over the course of several weeks of conflict testing. This MR vs. MNRA strain difference in punished responding did not exhibit extinction following discontinuation of CSD conflict behavior testing for a period of 6 weeks. Whether it was administered after conflict training or before, DSP4 treatment did not reduce the MR vs. MNRA strain difference in conflict behavior; rather, DSP4 treatment tended to increase the magnitude of the MR vs. MNRA difference in conflict behavior. The effects of DSP4 on norepinephrine (NE) and 5-hydroxytrypamine (5-HT) concentrations in the pons medulla region were determined in one group of conflict-experienced MR and MNRA rats (35 weeks after administration) and in a second group of naive MR and MNRA rats (3 weeks after administration). There were no MR vs. MNRA strain differences in NE or 5-HT concentrations in vehicle-treated rats. DSP4 treatment significantly reduced NE, but not 5-HT, concentrations when compared to control values; rats that were sacrificed 3 weeks following DSP4 administration exhibited a greater NE depletion than did rats sacrificed 35 weeks after DSP4 administration. Finally, there were no significant correlations between pons medulla region NE concentrations and conflict behavior in either strain alone or when the data from the two strains were combined. The present results are not consistent with the hypothesis that the MR vs. MNRA strain difference in conflict behavior is the result of strain differences in brain NE function.

Anxiolytic effects of serotonergic interventions in the shock-probe burying test and the elevated plus-maze test

Although serotonergic neural systems have been implicated in the control of anxiety for a number of years, evidence in favour of this role is controversial. The present experiments were designed to further characterize the putative role of serotonin (5-HT) in anxiety, using two pharmacologically validated animal models: the elevated plus-maze and the shock-probe burying tests. If the integrity of 5-HT neural systems is necessary for the expression of 'anxious' behaviors, then disruption of 5-HT systems should produce effects in the plus-maze and shock-probe tests that are similar to those of anxiolytic drugs. In the present experiments, serotonergic function was disrupted in rats, either by chemical depletion using the synthesis inhibitor p-CPA, by inhibitory autoreceptor activation using the selective 5-HT1A receptor ligand 8-OH-DPAT, or by electrolytic lesions of the serotonin-containing, dorsal raphe nucleus. p-CPA and dorsal raphe lesions produced robust anxiolytic effects in the elevated plus-maze and the shock-probe burying tests, whereas 8-OH-DPAT produced anxiolytic effects only in the shock-probe burying test, and 'anxiogenic' effects in the elevated plus-maze test. Although these results generally support the view that serotonin plays a role in the expression of 'anxious' behavior, the opposite effects of 8-OH-DPAT in the two behavioral paradigms suggest that the 5-HT1A receptor subtype exerts differential control over different types of experimental anxiety.

Antagonism of the anti-conflict effects of phenobarbital, but not diazepam, by the A-1 adenosine agonist l-PIA

The present study examined the effects of the anxiolytics diazepam and phenobarbital, the A-1 adenosine agonist N6-R-phenylisopropyladenosine (l-PIA), and the A-2 adenosine agonist 5'-N-ethylcarboxamidoadenosine (NECA) on conflict behavior. Water-restricted rats were trained to drink from a tube that was electrified (0.5 mA intensity) on a FI-29s schedule, electrification being signaled by a tone. After 3 weeks of daily 10-min sessions, the animals accepted a stable number of shocks (punished responding) and consumed a consistent volume of water (unpunished responding) per session. Different doses of l-PIA and NECA were then tested separately at weekly intervals. In addition, the effects of diazepam and phenobarbital were determined in animals pretreated with saline, l-PIA, or NECA. Neither l-PIA (15-250 nmole/kg) nor NECA (2.5-20 nmole/kg) produced a significant anti-conflict effect when administered alone. Diazepam (1.25-10 mg/kg) or phenobarbital (10-40 mg/kg) administration to saline-pretreated rats resulted in a dose-dependent increase in punished responding (shocks received) with minimal effects on unpunished responding (water intake). Neither l-PIA nor NECA pretreatment reliably altered the effects of diazepam on conflict behavior. Pretreatment with l-PIA, but not NECA, significantly reduced the anti-conflict effects of phenobarbital on conflict behavior. These data suggest that phenobarbital, but not diazepam, anti-conflict responses may involve interactions with A-1 adenosine receptors.

The psychopharmacology of 5-HT3 receptors

The review presents evidence that 5-HT3 receptors within the brain may contribute to the control of behavior. 5-HT3 receptor antagonists GR38032F, zacopride, ICS 205-930 and other agents are very potent in reducing mesolimbic dopamine hyperactivity caused by the injection of amphetamine or infusion of dopamine into the rat nucleus accumbens and amygdala, and the ventral striatum of the marmoset. Such actions are distinguished from those of neuroleptic agents by a failure to reduce normal levels of activity or to induce a rebound hyperactivity after discontinuation of treatment. Indeed, the 5-HT3 receptor antagonists can prevent the neuroleptic-induced rebound hyperactivity. Further evidence that 5-HT3 receptors moderate limbic dopamine function is shown by their ability to reduce both the behavioral hyperactivity and changes in limbic dopamine metabolism caused by DiMe-C7 injection into the ventral tegmental area. The 5-HT3 receptor antagonists also have an anxiolytic profile in the social interaction test in the rat, the light/dark exploration test in the mouse, the marmoset human threat test and behavioral observations in the cynomolgus monkey. They differ from the benzodiazepines by an absence of effect in the rat water lick conflict test and a withdrawal syndrome. Importantly, the 5-HT3 receptor antagonists are highly effective to prevent the behavioral syndrome following withdrawal from treatment with diazepam, nicotine, cocaine and alcohol. Intracerebral injection techniques in the mouse indicate that the dorsal raphe nucleus and amygdala may be important sites of 5-HT3 receptor antagonist action to inhibit aversive behavior. Studies with GR38032F indicate an additional effect in reducing alcohol consumption in the marmoset. The identification and distribution of 5-HT3 receptors in the brain using a number of 5-HT3 receptor ligands, [3H]65630, [3H]zacopride and [3H]ICS 205-930 correlates between studies, and the 5-HT3 recognition sites in cortical, limbic and other areas meet the criteria for 5-HT3 receptors to mediate the above behavioral effects. Thus the use of 5-HT3 receptor antagonists reveals an important role for 5-hydroxytryptamine in the control of disturbed behavior in the absence of effect on normal behavior. The profile of action of the 5-HT3 receptor antagonists has generated a major clinical interest in their potential use for schizophrenia, anxiety and in the control of drug abuse.

MR/Har and MNRA/Har Maudsley rat strains: differential response to chlordiazepoxide in a conflict task

The Maudsley Reactive (MR/Har) and Non-Reactive (MNRA/Har) rat strains, selectively bred for differences in open field defecation, have also been shown to differ in their baseline behavior in the Conditioned Suppression of Drinking (CSD) procedure, a second "model" behavior for the study of anxiety and/or emotionality in rats. The present studies were designed to compare the responsiveness of these two strains to the typical antianxiety agent chlordiazepoxide in the CSD paradigm. In daily 10-minute sessions, water-deprived rats were trained to drink from a tube that was occasionally electrified (0.5 mA), electrification being signaled by a tone. Consistent with previous reports, after several weeks of CSD testing, MNRA/Har rats accepted significantly more shocks than did MR/Har rats during control (nondrug) sessions. In both strains, the number of shocks accepted was inversely related to the intensity of the shock used (0.25-1.0 mA), with MNRA/Har rats accepting significantly more shocks than MR/Har rats at all intensities examined. The effects of various doses (1.25-28.4 mg/kg, IP) of chlordiazepoxide were determined in subjects of the MNRA/Har strain at the original training intensity (0.5 mA), while a lower intensity (0.25 mA) was utilized in MR/Har rats. Although punished responding in control (i.e., nondrug) CSD sessions did not differ under these conditions, MNRA/Har rats were found to be more responsive to the anticonflict effects of chlordiazepoxide than rats of the MR/Har strain. This strain difference in anticonflict efficacy of chlordiazepoxide was quite dramatic, with MNRA/Har rats accepting twice as many shocks as MR/Har rats following maximally effective doses of chlordiazepoxide. Low doses of chlordiazepoxide increased water intake slightly, while higher doses decreased water intake. Surprisingly, the chlordiazepoxide-induced depression of water intake was greater in rats of the MR/Har strain. Thus, these Maudsley Reactive and Non-Reactive rat strains, bred originally for their differences in open field behavior, also differ markedly in their responsiveness to chlordiazepoxide in the CSD paradigm. These findings further support the hypothesis that the MR/Har and MNRAHar rat strains may represent a genetically-based "animal model" for the study of emotionality and/or anxiety.

The effects of beta-antagonists and anxiolytics on conflict behavior in the rat

The present studies were designed to evaluate the effects of beta-adrenoceptor antagonists and traditional anxiolytics (phenobarbital and diazepam), alone and in combination, on behavior in the Conditioned Suppressioned of Drinking (CSD) conflict paradigm, an "animal model" for the study of anxiety and antianxiety agents. In daily 10-minute sessions, water-deprived rats were trained to drink from a tube which was occasionally electrified (0.5 mA), electrification being signalled by the presence of a tone. Within 2-3 weeks, control responding had stabilized (10-15 shocks/session and 10-15 ml water/session); drug tests were then conducted at weekly intervals. As expected, diazepam (0.6-10 mg/kg) and phenobarbital (10-40 mg/kg) administration resulted in a marked and dose-dependent increase in punished responding at doses which did not markedly alter background responding (water intake). Neither propranolol (0.5-8 mg/kg) nor the beta-1-selective antagonist atenolol (1-16 mg/kg) significantly affected punished responding in the CSD. Both propranolol and atenolol produced significant beta-1-adrenoceptor blockade, as evidenced by the production of significant bradycardic effects in conscious rats at the doses employed. Pretreatment with 2.0 mg/kg propranolol did not alter the anticonflict effects of diazepam (0.6-10 mg/kg) or phenobarbital (10-40 mg/kg). Further, reduction of the shock intensity to 0.125 mA (i.e., decreased suppression) failed to alter the behavioral response to propranolol (1.5-5 mg/kg) or the interaction of 2.0 mg/kg propranolol with diazepam. Finally, chronic administration of propranolol (2.0 mg/kg, twice daily) did not affect punished responding over the course of 5 weeks of treatment. These data suggest that the CSD paradigm, although an effective "animal model" for the study of benzodiazepine and barbiturate anticonflict effects, cannot serve as an "animal model" for the study of the situation-specific (i.e., phobic) anxiety for which propranolol and related agents are presently used.

Potentiation of the anticonflict effects of diazepam, but not pentobarbital and phenobarbital, by aminooxyacetic acid (AOAA)

The Conditioned Suppression of Drinking (CSD) paradigm is an "animal model" for anxiety which has been used to study the anticonflict effects of the benzodiazepines. It has been postulated that benzodiazepines produce their effects through interactions with GABA. The present study examined this potential GABA-BZ interaction on CSD behavior. In daily 10-minute sessions, water-deprived rats were trained to drink from a tube which was occasionally electrified (0.5 mA), electrification being signalled by a tone. Within 2-3 weeks control CSD responding had stabilized (16-24 shocks session and 10-14 ml water/session); drug tests were conducted at weekly intervals. As expected, diazepam (0.3-20.0 mg/kg), pentobarbital (0.6-10.0 mg/kg) and phenobarbital (10.0-40.0 mg/kg) alone markedly increased the number of shocks received at doses which did not depress background responding (i.e., water intake). Treatment with the GABA-transaminase inhibitor aminooxyacetic acid (AOAA: 2.5-10.0 mg/kg, 10- or 60-minute pretreatment) alone had no anticonflict effect on CSD behavior. However, pretreatment (60-minute) with 10.0 mg/kg AOAA significantly potentiated the effects of diazepam, as indicated by a significant shift to the left in the diazepam dose-response curve relative to diazepam alone. By contrast, the anticonflict effects of pentobarbital and phenobarbital were unaffected by this AOAA pretreatment. Thus, while increases in GABA transmission alone do not appear to affect CSD behavior, the anticonflict effect of benzodiazepines, but not barbiturates, appear to be potentiated by increases in GABA transmission.

Convulsant versus typical barbiturates: effects on conflict behavior in the rat

Typical barbiturates produce a spectrum of behavioral effects, including anti-convulsant, muscle relaxant, sedative hypnotic and anti-anxiety actions. In contrast to these typical barbiturates, there exists a group of barbiturates which are pro-, rather than anti-convulsant. The effects of these convulsant barbiturates on anxiety-related behaviors have not been examined. Therefore, the present studies were designed to compare the effects of the convulsant barbiturate CHEB to those of a number of typical barbiturates in the Conditioned Suppression of Drinking (CSD) paradigm, an "animal model" for the study of anxiety and anti-anxiety agents. In daily 10-minute sessions, water-deprived rats were trained to drink from a tube which was occasionally electrified (0.5 mA), electrification being signalled by a tone. Within 3-4 weeks control responding had stabilized (10-15 shocks and 10-15 ml water/session); drug tests were then conducted at weekly intervals. Consistent with previous reports, typical barbiturates (pentobarbital, secobarbital, phenobarbital) produced dose-dependent increases in the number of shocks received at doses which did not depress background responding (water intake). In contrast, sub-convulsant doses of CHEB (0.3-2.5 mg/kg) produced a dose-dependent depression of both punished responding and background responding. Finally, it was found that pre-treatment with 1.25 mg/kg CHEB did not alter the anti-conflict effects of pentobarbital. These results suggest that (1) convulsant and typical barbiturates have markedly different effects on conflict behavior in the rat and (2) CHEB appears not to possess any "barbiturate antagonist" qualities.

Competition for sucrose-pellets in triads of male Wistar rats: effects of three serotonergic drugs

1. Within triads of rats, individuals can be discerned according to their competition for sucrose-pellets as a high-, a medium- and a poor-performing animal. 2. The competition-rates, expressed as scores, are affected by serotonergic drugs: upon inhibiting tryptophan-hydroxylase, the characteristic abstention of the poor-performing rats can temporarily be overcome; quipazine, on the other hand, leads to a dose-dependent decrease in the competition-rate of high-performing rats. 3. Such findings are indicative for a regulatory effect of some serotonergic mechanisms on a competition-behavior evoked within the familiar social context.

Serotonin does not mediate anxiolytic effects of buspirone in the fear-potentiated startle paradigm: comparison with 8-OH-DPAT and ipsapirone

The present study evaluated the role of various neurotransmitter systems in mediating buspirone's blockade of the fear-potentiated startle effect, where acoustic startle amplitude is normally increase in the presence of a light previously paired with a shock. Large lesions of the dorsal and median raphe nuclei or IP injections of the serotonin antagonists cinanserin (10 mg/kg) or cyproheptadine (5 mg/kg) did not alter fear-potentiated startle, nor did these treatments prevent buspirone (5 or 10 mg/kg SC) from blocking fear-potentiated startle. The 5-HT 1A agonist 8-OH-DPAT (2.5-10.0) did not block fear-potentiated startle even at doses that produced a marked "5-HT syndrome". Another 5-HT 1A agonist, ipsapirone (10-20 mg/kg), blocked potentiated startle only at a very high dose (40 mg/kg). p-Chlorophenylalanine and p-chloroamphetamine did not alter fear-potentiated startle. Finally, pretreatment with the benzodiazepine receptor antagonist RO-15-1788 (1 mg/kg); the opiate antagonist naloxone (2 mg/kg) or the alpha 2-adrenergic antagonist yohimbine (5 mg/kg) did not reduce fear-potentiated startle, nor did they prevent buspirone from blocking fear-potentiated startle. Taken together, the data do not support the hypothesis that buspirone's anxiolytic effects are mediated by actions at 5-HT 1A receptors and more generally indicate that serotonergic neurons do not play an important role in fear-potentiated startle.

Selective agonists and antagonists for 5-hydroxytryptamine receptor subtypes, and interactions with yohimbine and FG 7142 using the elevated plus-maze test in the rat

The effects of some 5-HT receptor ligands were investigated on measures of anxiety in an elevated plus-maze test in the rat. Quipazine (2 and 4 mg kg-1), a non-specific 5-HT agonist and ritanserin (0.25-10 mg kg-1), a 5-HT2 receptor antagonist displayed anxiogenic profiles by reducing both of the measures of anxiety used in this test. Two 5-HT1A receptor ligands, buspirone (4 and 8 mg kg-1) and ipsapirone (2.5-10 mg kg-1) and the 5-HT1 agonist, RU 24969 (0.1875-1.5 mg kg-1) significantly reduced only the percentage of time spent on the open arms. (-)-Propranolol (5 and 10 mg kg-1), a 5-HT1 receptor antagonist significantly reduced only the percentage of entries made onto the open arms. Metergoline (4 mg kg-1), a non-specific 5-HT antagonist displayed anxiolytic effects in this test by increasing both measures of anxiety. The 5-HT1A receptor agonist, 8-OH-DPAT (0.0625-0.25 mg kg-1) had no effect on either of the measures of anxiety. The results from the non-specific ligands (quipazine and metergoline) are consistent with the theory that a reduction in 5-HT function reduces anxiety. However, in spite of their more selective effects on 5-HT receptors the results in this test from the more specific ligands are not consistent with a strong involvement of any single receptor subtype. The interaction studies with yohimbine and FG 7142 (beta-carboline-3-carboxylate methylamide) provided no clear evidence for a major role of 5-HT pathways in the mediation of their anxiogenic effects.

Buspirone effects in an animal conflict procedure: comparison with diazepam and phenobarbital

Buspirone has been introduced as a novel non-benzodiazepine anti-anxiety agent. The Conditioned Suppression of Drinking (CSD) paradigm is an "animal model" for anxiety which provides information on both the relative potency and relative efficacy of anti-conflict agents. The present study compared the anti-conflict effects of buspirone to those of more "classical" anti-anxiety agents, diazepam and phenobarbital. In daily 10-minute sessions, water-deprived rats were trained to drink from a tube which was occasionally electrified (0.5 mA), electrification being signalled by a tone. Within 2-3 weeks control CSD responding had stabilized (approximately 15-20 shocks/session and 10-15 ml water/session); drug tests were conducted at weekly intervals. Diazepam and phenobarbital markedly (400-500%) increased the number of shocks received at doses which did not depress background responding (i.e., water intake). A number of agents, most notably morphine and ethanol, did not reliably affect punished responding in the CSD. Administered IP, low doses (0.25-1 mg/kg) of buspirone increased punished responding only slightly (less than 100% increase); higher doses (2, 4 mg/kg) depressed background responding. Administered SC, buspirone (0.125-1.0 mg/kg) had more potent effects on both punished and unpunished responding; again, anti-conflict efficacy was only marginal. These results suggest that buspirone might be less effective than the benzodiazepines in the management of anxiety.

5-HT and anxiety: promises and pitfalls

This review examines the evidence implicating serotonergic systems in the control of anxiety. The effects of manipulations of 5-HT, including 5-HT lesions, pharmacological reduction and enhancement of serotonergic function in animal tests of anxiety are reviewed. Biochemical and behavioral evidence implicating serotonergic pathways in the anxiolytic action of benzodiazepines is presented. In each section the promises of a serotonergic involvement, as indicated by positive findings, and the pitfalls, evidenced by inconsistent and conflicting results, are discussed. Finally the dangers of a superficial interpretation of the behavioral and biochemical findings is stressed.

Serotonergic mechanisms in the behavioral effects of buspirone and gepirone

The literature describing the role of serotonin (5-HT) in the mediation of anxiety is a controversial one. Serotonergic involvement in the mechanism of action of two nonbenzodiazepine anxiolytics, buspirone and gepirone, supports a role for serotonin in anxiety. The anticonflict effect of both drugs is blocked by serotonin lesions, and gepirone induces the serotonin syndrome. A shift in the gepirone dose-response curve to the left in serotonin lesioned rats suggests that this may be 5-HT-receptor mediated. Both buspirone and gepirone enhance the acoustic startle response and gepirone's effect is attenuated in serotonin lesioned animals. While other components of buspirone's mechanism of action may suppress the behavioral expression of its serotonergic interactions, results from these studies suggest that serotonin agonist-like activity may be an important mechanism in the actions of a clinically proven nonbenzodiazepine anxiolytic (buspirone), and anxiolytic candidate (gepirone).

Maudsley reactive and non-reactive rat strains: differential performance in a conflict task

Based on open field defecation rates, distinct strains of Maudsley rats have been identified. The Maudsley Reactive strains exhibit high levels of open field defecation, while the Maudsley Non-Reactive strains exhibit low levels of open field defecation. This difference in open field behavior has been interpreted as an indicator of "emotionality." The present studies were designed to compare the performance of two Maudsley strains, MR/Har and MNRA/Har, in the conditioned suppression of drinking (CSD) paradigm, another "model" behavior for the study of emotionality and/or anxiety in animals. In daily 10-minute sessions, water-deprived rats were trained to drink from a tube that was occasionally electrified (0.5 mA), electrification being signalled by a tone. In this procedure, MNRA/Har rats accepted significantly more shocks than did MR/Har rats. This difference between MR/Har and MNRA/Har strains, initially somewhat slight, became quite impressive over weeks of repeated testing. Although present in both male and female subjects, this strain difference was more prominent in the males. Thus, the MR/Har and MNRA/Har rat strains, bred originally for their differences in open field behavior, also differ markedly in their performance in the CSD paradigm. These findings suggest that the Maudsley rat strains may represent a useful genetically-based "animal model" for the study of emotionality and/or anxiety.

Effect of metergoline on human anxiety

In order to assess the role played by serotonin (5-HT) in subjective anxiety, three groups of 12 healthy volunteers were given 12 mg metergoline (MET), 10 mg diazepam (DZ) or placebo (PB), under double-blind conditions, and submitted to a simulated public speaking (SPS) test. MET increased state-anxiety scores, measured by Spielberg's State-Trait Anxiety Inventory. The effect of MET was significantly different from both the PB and DZ groups immediately before the SPS test (prestress) as well as 24 h after medication, and from the DZ group only, 2.5 h after the test (poststress). In contrast, DZ did not significantly affect subjective anxiety. The SPS test significantly increased anxiety in DZ- or PB-treated subjects as compared to prestress scores, whereas the increases in the MET group were not significant, probably because pretest levels were already high. No drug effect on heart rate, skin electrical conductance and quality of sleep during the night following medication was found. In addition, the drugs did not cause bodily symptoms that could secondarily affect mood. Since MET is a 5-HT receptor antagonist, active on the central nervous system, an inhibitory role of 5-HT on subjective anxiety might be suggested.

Animal models for the study of anti-anxiety agents: a review

Animal models for the study of anxiolytic agents are reviewed and evaluated according to pharmacological and behavioral criteria. Although there are important exceptions, in general, most early animal models have not provided a reliable basis for identifying compounds with potential anxiolytic action, or for delineating the mechanisms of anxiolytic drug action. The possibility that phylogenetically 'prepared' forms of defensive learning might serve as a basis for the study of anxiolytic agents is introduced.

Behavioral and 5-HT antagonist effects of ritanserin: a pure and selective antagonist of LSD discrimination in rat

The newly synthesized compound and putative 5-HT2 antagonist ritanserin, but not the structurally related compound R 56413, resembles pirenperone in that it acts as a pure antagonist in an LSD-saline drug discrimination assay in the rat. Ritanserin exceeded pirenperone in terms of behavioral specificity; the lowest effective dose of ritanserin in antagonizing LSD was one order of magnitude higher than that of pirenperone, but the compound depressed rate of operant responding only at doses that were about 1000-fold higher than those at which pirenperone was effective. Ritanserin exerted effects in an open field test which were reminiscent of anxiolytic drug activity in the rat; its effects were greater than those of pirenperone, R 56413 and the benzodiazepines chlordiazepoxide and diazepam. The results of experiments on antagonism of 5-HT-induced hypothermia and of the 5-HTP-induced head-twitch response fail to support the possibility that the putative anxiolytic effects of ritanserin in the rat can be ascribed simply to a pharmacologically defined action at 5-HT receptors.

Lack of central 5-hydroxytryptamine influence on the anticonflict activity of diazepam

This study examined the effects of various drug treatments (IP injections) proposed to modify central 5-hydroxytryptamine (5-HT) activity on a conditioned suppression of drinking behavior in water-deprived rats. The subjects were trained to drink their daily water requirement during a 10-min session. Intermittent tone periods of 7 s were then introduced, the last 5 s of which the drinking tube was electrified. The animals gradually suppressed tube contacts during the tone to a low constant level within 2 weeks of training. Diazepam increased punished responding dramatically. The 5-HT antagonists methysergide (1 - 18 mg/kg), cyproheptadine (1 - 18 mg/kg), metergoline (0.25 - 2.0 mg/kg) and cinanserin (10 - mg/kg) failed to induce large, reliable increases in punished responding. When a low dose of diazepam was combined with 5-HT antagonists, only one treatment, methysergide at 3 mg/kg, potentiated the anticonflict activity of diazepam. Acute or chronic treatment with PCPA increased behavior suppressed by punishment, but this effect was weak, brief, and poorly related to the depletion of brain 5-HT. LSD (0.3 - 100 microgram/kg) administered 1, 10, or 30 min before the test was ineffective in overcoming suppression by punishment. Mescaline (6 - 30 mg/kg) had no significant effect on punished responding. 5-HTP (18 mg/kg) decreased the number of shocks accepted, but not after pretreating with carbidopa. Pretreatment with carbidopa plus 5-HTP potentiated the anticonflict effect of diazepam. The 5-HT agonist mCPP (0.25 - 2.0 mg/kg) enhanced suppression due to punishment, but only in doses that interfered with unpunished responding. The 5-HT-releasing agent fenfluramine (0.25 - 1.0 mg/kg) did not affect this behavior. Amitriptyline pretreatment in a dose not affecting unpunished behavior (5.6 mg/kg) potentiated the diazepam-induced increase in punished responding. These results are difficult to reconcile with the proposal that suppression of behavior consequent to punishment is related to brain 5-HT activity.