R. John Rodgers

Defense system psychopharmacology: an ethological approach to the pharmacology of fear and anxiety.

Defensive behaviors comprise a set of flexible and adaptive responses to threatening situations and stimuli. In semi-natural situations affording a wide variety of responses, defensive behaviors change over time in response to information about the presence of danger, acquired through risk assessment activities. Two test batteries, a Fear/Defense Test Battery (F/DTB) measuring defensive behaviors to present, approaching predators, and an Anxiety/Defense Test Battery (A/DTB) measuring reactions to potential threat, have been used in conjunction with administration of potentially anxiolytic drugs. Results suggest that the F/DTB behaviors are not systematically responsive to anxiolytics. However, on the A/DTB, anxiolytic benzodiazepines produce a profile of effects primarily involving risk assessment activities. Very similar profiles of effect are seen also with some 5-HT1A compounds, alcohol, imipramine and MK-801, but not for a variety of additional compounds. A consistent pattern of gender differences are obtained with the A/DTB, with females more defensive than males. These results indicate that particular patterns of defensive behaviors may provide a very appropriate animal model for the analysis of pharmacological effects on anxiety.

Anti-obesity drugs: past, present and future

The ideal anti-obesity drug would produce sustained weight loss with minimal side effects. The mechanisms that regulate energy balance have substantial built-in redundancy, overlap considerably with other physiological functions, and are influenced by social, hedonic and psychological factors that limit the effectiveness of pharmacological interventions. It is therefore unsurprising that anti-obesity drug discovery programmes have been littered with false starts, failures in clinical development, and withdrawals due to adverse effects that were not fully appreciated at the time of launch. Drugs that target pathways in metabolic tissues, such as adipocytes, liver and skeletal muscle, have shown potential in preclinical studies but none has yet reached clinical development. Recent improvements in the understanding of peptidergic signalling of hunger and satiety from the gastrointestinal tract mediated by ghrelin, cholecystokinin (CCK), peptide YY (PYY) and glucagon-like peptide-1 (GLP-1), and of homeostatic mechanisms related to leptin and its upstream pathways in the hypothalamus, have opened up new possibilities. Although some have now reached clinical development, it is uncertain whether they will meet the strict regulatory hurdles required for licensing of an anti-obesity drug. However, GLP-1 receptor agonists have already succeeded in diabetes treatment and, owing to their attractive body-weight-lowering effects in humans, will perhaps also pave the way for other anti-obesity agents. To succeed in developing drugs that control body weight to the extent seen following surgical intervention, it seems obvious that a new paradigm is needed. In other therapeutic arenas, such as diabetes and hypertension, lower doses of multiple agents targeting different pathways often yield better results than strategies that modify one pathway alone. Some combination approaches using peptides and small molecules have now reached clinical trials, although recent regulatory experience suggests that large challenges lie ahead. In future, this polytherapeutic strategy could possibly rival surgery in terms of efficacy, safety and sustainability of weight loss.

Risk Assessment Behaviour: Evaluation of Utility in the Study of 5-HT-Related Drugs in the Rat Elevated Plus-Maze Test

The present study compared the effects of a wide range of 5-hydroxytryptamine (5-HT)-modulating and potential anxiolytic agents in the rat elevated plus-maze using spatiotemporal (i.e., open arm time and entries) and ethologically derived measures (i.e., risk assessment activities and directed exploration). The drugs used were 5-HT1A receptor partial (buspirone and ipsapirone) and full (8-OH-DPAT and flesinoxan) agonists, mixed 5-HT2A/2C receptor antagonists (ritanserin, ketanserin, mianserin, and pirenperone), selective 5-HT3 receptor antagonists (ICS 205-930, MDL 72222, ondansetron, and (RS)-zacopride), and selective (fluoxetine, fluvoxamine, and zimelidine) and nonselective (imipramine) 5-HT reuptake inhibitors. Only buspirone and mianserin produced effects indicative of an anxiolytic-like action on the spatiotemporal measures. However, all 5-HT1A receptor ligands, as well as mianserin, ketanserin, ondansetron, and zacopride, decreased the number of aborted attempts at entry into open arms (risk assessment). In addition, buspirone, mianserin, and zacopride increased head-dipping (directed exploration). Among the 5-HT reuptake inhibitors, zimelidine reduced head-dipping and total entries. The present findings demonstrate that risk assessment responses are sensitive to the action of 5-HT1A receptor ligands, but their modulation by drugs targetting 5-HT2A, 5-HT2C, and 5-HT3 receptors was not convincingly established.

MK-801 produces a reduction in anxiety-related antipredator defensiveness in male and female rats and a gender-dependent increase in locomotor behavior

The present study investigated the effects of the non-competitive NMDA antagonist MK-801 (0.04–0.16 mg/kg), on antipredator defensive reactions of male and female rats in three paradigms comprising the Anxiety/Defense Test Battery (A/DTB). In order to facilitate interpretation of data from the above study, the behavioral effects of the compound were also assessed in the non-threatening environment of the home cage. The data indicate a marked gender difference in the locomotor effects of the compound with females, but not males, showing a dose-dependent increase in general locomotor activity, a decrease in freezing, and a loss of balance at the highest dose, in both non-threatening and threatening contexts. The behavioral profile for males in the A/DTB included decreased orientation to and proxemic avoidance of the cat stimulus or stimulus site, and increased transits and eating in the cat situation. Contacts with the cat odor stimulus were increased, as was normal, curved back, locomotion in this test. In the absence of non-specific locomotor effects for males, this profile for the A/DTB provides convincing evidence for anxiety/fear reduction with MK-801. While locomotor effects tended to mask the putative anxiolytic properties of the compound in females, evidence remains from behavioral changes not attributable to a locomotor influence to indicate anxiety/fear reduction in this sex.

The effects of compounds varying in selectivity as 5-HT1A receptor antagonists in three rat models of anxiety

Compounds varying in selectivity as 5-HT(1A) receptor antagonists have recently been reported to produce benzodiazepine-like antianxiety effects in mice. To assess the cross-species generality of these findings, the present experiments compared the effects of diazepam (0.625-5 mg/kg) with those of several non-selective (MM-77, 0.03-1 mg/kg and pindobind-5-HT(1A), 0.1-5 mg/kg) and selective (WAY100635, 0.01-10 mg/kg, p-MPPI, 0.01-3 mg/kg and SL88.0338, 0.3-10 mg/kg) 5-HT(1A) receptor antagonists in three well-validated anxiolytic screening tests in rats: punished lever-pressing, punished drinking, and the elevated plus-maze. In the punished lever-pressing conflict test, none of the 5-HT(1A) receptor antagonists modified rates of punished responding, whereas in the punished drinking test, WAY100635 (0.3-1 mg/kg), SL88.0338 (3-10 mg/kg), p-MPPI (1 mg/kg), MM-77 (0.03-0.3 mg/kg), but not pindobind-5-HT(1A), produced clear anticonflict activity. However, the increase in punished responding with the 5-HT(1A) compounds was smaller than that produced by diazepam, indicating weaker anxiolytic-like activity. In the elevated plus-maze test, WAY100635 (0.1-0.3 mg/kg), SL88.0338 (0.3-10 mg/kg), MM-77 (0.01-3 mg/kg), pindobind-5-HT(1A) (0.1-3 mg/kg), but not p-MPPI, showed anxiolytic-like activity on traditional behavioral indices, increasing the percentage of time spent in open arms and the percentage of open arm entries. As was the case in the punished drinking test, the magnitude of the positive effects of the 5-HT(1A) compounds was generally smaller than that of diazepam. Of the ethological measures recorded in the plus-maze, all compounds markedly decreased risk assessment (i.e. attempts) over the entire dose-range, but only diazepam clearly increased directed exploration (i.e. head-dipping). Although the present results demonstrate that 5-HT(1A) receptor antagonists elicit anxiolytic-like effects in rats, this action appears to be test-specific and, unlike previous findings in mice, smaller than that observed with benzodiazepines. The data are discussed in relation to the possible relevance of species differences in 5-HT(1A) receptor function and the nature of the anxiety response studied.

Benzodiazepine and Serotonergic Modulation of Antipredator and Conspecific Defense

The mammalian defense repertory comprises an array of individual behaviors that are extraordinarily sensitive to relevant features of the threat stimulus and the situation in which it occurs. In parallel with increasing awareness of the specificity and complexity of defensive behaviors and of their potential relevance to psychopathologies (e.g. anxiety, panic, and depression) is an escalating use of natural threat stimuli such as attacking conspecifics or predators in paradigms aimed at evaluating drug effects on defense. A review of the literature on benzodiazepine (BZ) and serotonin (5-HT) effects on conspecific and antipredator defense, including defensive analgesia, indicates that both types of stimuli elicit a wide array of relevant defensive behaviors. These studies suggest specificity of drug effects on particular behaviors, rather than a general alteration of all aspects of defense. However, stimulus variability and possible confounding of effects are a considerable problem with conspecific defense paradigms, while antipredator paradigms utilizing human experimenters as the predator may be difficult to use with the domesticated laboratory animal subjects. In addition, sensitivity to the organization of defensive behaviors and to differences between species in defense patterns is necessary to adequate interpretation of results. Nonetheless, these paradigms have permitted major advancements in analysis of the behavioral defense systems and their sensitive use in drug studies will greatly facilitate an understanding of the physiology of defense.

Prior exposure to the elevated plus-maze sensitizes mice to the acute behavioral effects of fluoxetine and phenelzine.

A single undrugged experience of the elevated plus-maze modifies future drug responses in the test. The present study investigated the effects of maze-experience on the acute behavioral effects of the monoamine oxidase inhibitor phenelzine and the serotonin reuptake inhibitor fluoxetine. Phenelzine (2.5-12.5 mg/kg) had no clear effect on plus-maze behavior in test-naive Swiss Webster mice, but dose-dependently increased anxiety-like behavior in maze-experienced subjects. Similarly, fluoxetine (5-20 mg/kg) produced non-significant trends for increased anxiety-like behavior in maze-naive mice, but significantly and dose-dependently increased anxiety-like behavior and suppressed locomotor activity in maze-experienced mice. The anxiogenic effects of the benzodiazepine receptor inverse agonist N-methyl-beta-carboline-3-carboxamide (FG 7142) (20 mg/kg) was abolished by prior test experience, suggesting an alteration in gamma-aminobutyric acid (GABA)/benzodiazepine receptor function with maze-experience. However, the benzodiazepine receptor antagonist flumazenil (5-20 mg/kg) produced a silent profile regardless of maze-experience. Present findings provide further evidence demonstrating that prior test history is a critical consideration in mouse studies of anxiety-related behavior.

Behavioural profiles in the mouse defence test battery suggest anxiolytic potential of 5-HT(1A) receptor antagonists.

Abstract  Rationale: Compounds varying in selectivity as 5-HT1A receptor antagonists have recently been reported to produce anxiolytic-like effects comparable to those of benzodiazepines in the mouse elevated plus-maze procedure. Objective: In view of the potential clinical significance of these findings, the present experiments compared the behavioural effects of diazepam (0.5–3.0 mg/kg) with those of several non-selective 5-HT1A receptor antagonists [NAN-190, 0.1–3.0 mg/kg, MM-77, 0.03–1.0 mg/kg, (S)-UH-301, 0.3–3.0 mg/kg and pindobind-5-HT1A, 0.03–1.0 mg/kg], and three selective 5-HT1A receptor antagonists (WAY100635, 0.01–3.0 mg/kg, p-MPPI, 0.1–3.0 mg/kg and SL88.0338, 0.3–3.0 mg/kg) in the mouse defence test battery (MDTB). Methods: In this well-validated anxiolytic screening test, Swiss mice are directly confronted with a natural threat (a rat) as well as situations associated with this threat. Primary measures taken during and after rat confrontation were flight, risk assessment (RA), defensive threat/attack and escape attempts. Results: Diazepam significantly decreased flight reactions after the rat was introduced into the runway, reduced RA activities of mice chased by the rat, increased RA responses displayed when subjects were constrained in a straight alley and reduced defensive upright postures and biting upon forced contact. All the selective 5-HT1A receptor antagonists and NAN-190 also reduced flight, RA in the chase test, and defensive threat and attack behaviours. (S)-UH-301 and pindobind-5-HT1A reduced RA in the chase test, but only partially modified defensive threat and attack. Unlike the other drugs tested, MM-77 produced significant effects only at doses which also markedly reduced spontaneous locomotor activity, suggesting a behaviourally non-specific action. In contrast to diazepam, the 5-HT1A receptor ligands failed to affect RA in the straight alley test. Following removal of the rat from the test area, only diazepam and (S)-UH-301 reduced escape behaviour (contextual defence) at doses which did not decrease locomotion. Overall, the present findings indicate that except for one RA behaviour and escape responses, the 5-HT1A receptor ligands studied modified the same defensive behaviours as diazepam, suggesting potential therapeutic efficacy in the management of anxiety disorders. However, the magnitude of the effects of the 5-HT1A compounds on defence was generally smaller than that of the benzodiazepine. Conclusion: As all of the 5-HT1A compounds tested in this series share antagonistic activity in models of postsynaptic 5-HT1A receptor function, it is proposed that this action accounts for their effects on defence.

“Paradoxical” effects of morphine on antipredator defense reactions in wild and laboratory rats

In a Fear/Defense Test Battery, measuring defensive reactions to a present, approaching and contacting predator, the highest dose of morphine tested (7.5 mg/kg) reliably reduced vocalization to dorsal contact, to vibrissae stimulation, and to an anesthetized conspecific in laboratory-bred wild R. norvegicus. Except for a dose-dependent reduction in flinch/jump reactions to dorsal contact (taps), other defensive behaviors (flight, freezing, etc.) were not reliably altered by morphine treatment (0, 1.0, 2.5, 7.5 mg/kg). Vocalization responses to vibrissae stimulation in wild-trapped R. rattus were reliably increased following naloxone (1.0 and 10.0 mg/kg) administration, lending support for opiate receptor involvement in the mediation of defensive vocalization. In the Anxiety/Defense Test Battery, measuring defensive reactions to situations associated with a predator (cat) or with cat odor, laboratory rats showed no decrease in defensive behavior with morphine (0, 1.0, 5.0 mg/kg). In direct contrast to the above findings, the effects of morphine treatment in this test battery suggested a generalized increase in defensiveness to noncontacting and nonpainful threat stimuli. These effects included a decrease in time spent near the cat compartment, with a complementary increase in time spent at maximum distance, a decrease in transits between these sections, an increase in crouching, and a decrease in grooming and rearing. This pattern of results suggests that morphine may have two opposing effects on defensive behavior, a generalized enhancement, together with a more specific reduction of responses to tactile or painful stimulation. A very widespread pattern of reliable sex or sex x drug effects in the Anxiety/Defense Test Battery was in good agreement with previous reports of sex differences in these tests, with females generally more defensive than males. Consonant with previous findings, no reliable sex differences were found with the Fear/Defense Test Battery, although several values approached an acceptable level of statistical significance.

Effects of scopolamine on antipredator defense reactions in wild and laboratory rats

Two experiments were designed to investigate the effects of scopolamine hydrobromide (0.25-1.0 mg/kg), and its methyl derivative, on the defensive reactions of rats to nonpainful threat stimuli. In the first experiment, over the dose range studied neither compound significantly altered avoidance, freezing, defensive threat or attack in wild Rattus rattus confronted by the experimenter and other predator-related stimuli. Scopolamine hydrobromide did, however, produce a dose-dependent increase in flight distance; this effect was not seen with the methyl compound, confirming central cholinergic mediation. In the second experiment, no dose of either compound significantly altered the behaviour of Long-Evans rats prior to cat exposure. During cat exposure, however, scopolamine hydrobromide (but not methyl scopolamine) increased the amount of time spent in the vicinity of the cat, increased scanning and rearing, and reduced grooming behaviour. Although reliable, the latter effects were not pronounced. Together, these data do not support a major involvement of central muscarinic receptor mechanisms in the regulation of defensive patterns in wild or laboratory rats.