Benzodiazepine (omega) receptor partial agonists and the acquisition of conditioned fear in mice

A mouse model of high trait anxiety shows reduced heart rate variability that can be reversed by anxiolytic drug treatment

Increasing evidence suggests that specific physiological measures may serve as biomarkers for successful treatment to alleviate symptoms of pathological anxiety. Studies of autonomic function investigating parameters such as heart rate (HR), HR variability and blood pressure (BP) indicated that HR variability is consistently reduced in anxious patients, whereas HR and BP data show inconsistent results. Therefore, HR and HR variability were measured under various emotionally challenging conditions in a mouse model of high innate anxiety (high anxiety behaviour; HAB) vs. control normal anxiety-like behaviour (NAB) mice. Baseline HR, HR variability and activity did not differ between mouse lines. However, after cued Pavlovian fear conditioning, both elevated tachycardia and increased fear responses were observed in HAB mice compared to NAB mice upon re-exposure to the conditioning stimulus serving as the emotional stressor. When retention of conditioned fear was tested in the home cage, HAB mice again displayed higher fear responses than NAB mice, while the HR responses were similar. Conversely, in both experimental settings HAB mice consistently exhibited reduced HR variability. Repeated administration of the anxiolytic NK1 receptor antagonist L-822429 lowered the conditioned fear response and shifted HR dynamics in HAB mice to a more regular pattern, similar to that in NAB mice. Additional receiver-operating characteristic (ROC) analysis demonstrated the high specificity and sensitivity of HR variability to distinguish between normal and high anxiety trait. These findings indicate that assessment of autonomic response in addition to freezing might be a useful indicator of the efficacy of novel anxiolytic treatments.

Study of the modulatory activity of BZ (omega) receptor ligands on defensive behaviors in mice: evaluation of the importance of intrinsic efficacy and receptor subtype selectivity

1. This study examined the hypothesis that the anxiolytic effects of benzodiazepine (BZ (omega)) receptor ligands may be associated with actions at a defined receptor subtype and/or their level of intrinsic activity using the mouse defense test battery. 2. This test has been designed to assess defensive reactions of Swiss mice confronted with a natural threat (a rat) and situations associated with this threat. Primary measures taken before, during and after rat confrontation were escape attempts, flight, risk assessment and defensive threat and attack. 3. The drugs used were the non-selective BZ (omega) receptor full agonist diazepam, the non-selective BZ (omega) receptor partial agonist bretazenil and the beta-carboline abecarnil which acts as a full agonist on GABAA receptors containing the alpha 1- and the alpha 3-subunits and as a partial agonist at receptors containing the alpha 2- and the alpha 5-subunits. The drugs were given alone and diazepam was co-administered with either bretazenil or abecarnil. 4. When administered alone, diazepam attenuated several defensive responses including risk assessment activities, defensive threat/attack reactions upon forced contact with the rat and escape attempts following the removal of the rat from the apparatus. Unlike diazepam, bretazenil was devoid of significant activity on defense and abecarnil displayed depressant activity. 5. Bretazenil blocked all behavioral effects of diazepam on defense behaviors. The co-administration of diazepam and abecarnil produced a behavioral profile similar to that observed when diazepam was administered alone, indicating that abecarnil did not influence the effects of diazepam on defense. By contrast, diazepam completely antagonized the sedative effects of abecarnil. 6. These findings indicate that only BZ (omega) ligands with high intrinsic efficacy at all BZ (omega) receptor subtypes display clear and specific effects on defensive behaviors in mice, and suggest that GABAA receptors containing the alpha 3 subunit might represent the primary target involved in the modulatory action of diazepam on defensive behaviors.

Further evidence for differences between non-selective and BZ-1 (omega 1) selective, benzodiazepine receptor ligands in murine models of "state" and "trait" anxiety

The behavioural effects of several BZ (omega) receptor ligands were compared in mice using the light/dark choice task, an animal model of "state" anxiety, and the free-exploration test, which has been proposed as an experimental model of "trait" anxiety. The drugs used included non-selective full (alprazolam, clorazepate, chlordiazepoxide and diazepam), partial agonists (bretazenil, imidazenil and Ro 19-8022) and BZ-1 (omega 1) selective receptor ligands (abecarnil, CL 218,872 and zolpidem). In the light/dark choice task, non-selective full agonists elicited clear anxiolytic-like effects increasing time spent in the lit box and simultaneously reducing attempts at entry into the illuminated cage followed by withdrawal responses, a measure of risk assessment. With the exception of abecarnil, both non-selective partial agonists and BZ-1 (omega 1) selective receptor ligands displayed reduced efficacy compared to the full agonists as they decreased risk assessment responses without altering time in the lit box. In addition, the weak anxiolytic-like actions displayed by selective BZ-1 (omega 1) agents were evident only at doses which reduced locomotor activity, indicating that this effect may be non-specific. In the free-exploration test, non-selective BZ (omega) receptor agonists markedly increased the percentage of time spent in the novel compartment and reduced the number of attempts to enter whereas selective BZ-1 (omega 1) receptor ligands displayed a weaker neophobia-reducing effect as they reduced risk assessment responses only. As was the case in the light/dark choice task, this latter effect was observed at locomotor depressant doses. These findings indicate that while both full and partial BZ (omega) receptor agonists are equally effective against "trait" anxiety, full agonists may be superior in reducing "state" anxiety. In addition, the lack of specific effects of selective BZ-1 (omega 1) receptor ligands in reducing both types of anxiety suggests that the BZ-1 (omega 1) receptor subtype cannot be considered as the primary target mediating the anxiolytic action of drugs interacting with the GABAA benzodiazepine receptor complex.