Inverse agonist properties of the THBC discriminative stimulus: asymmetrical generalization with FG 7142

Discriminative stimulus effects of m-chlorophenylpiperazine as a model of the role of serotonin receptors in anxiety

Serotonin is known to play a role in anxiety. The roles of serotonin reuptake and 5-HT1A receptors have been well characterized, but the contribution of other serotonin receptor subtypes is not as clear. 1-(3-Chlorophenyl)-piperazine (mCPP), which binds non-selectively to a wide range of serotonin receptors, has often been used to produce anxiety in humans and in animal models. Because functional assays indicate that mCPP is significantly more potent at 5-HT2C receptors, it may serve as a tool to investigate the contribution of 5-HT2C receptors to anxiety. This paper reviews the results of behavioral tests using mCPP, including the drug discrimination assay, to model anxiety. Although the discriminative stimulus effects of mCPP do not seem to be a useful screen for general anxiolytics, they do seem to be useful for characterization of the contribution of 5-HT1B and 5-HT2C receptors to the mediation of anxiety-like behaviors.

Evidence for noradrenergic involvement in mediating the FG 7142 discriminative stimulus

Rats were trained to discriminate the stimulus properties of the benzodiazepine receptor partial inverse agonist beta-carboline-3-carboxylate acid methyl amide (FG 7142) (5.0 mg/kg) or the alpha 2-adrenergic receptor antagonist 17 alpha-hydroxyyohimban-16 alpha-carboxylic acid methyl ester (yohimbine) (3.0 mg/kg) from vehicle in a two-lever, food-motivated operant task. These compounds have in common a beta-carboline structure and anxiogenic behavioral profiles. The yohimbine discriminative stimulus was mimicked by the alpha 2-adrenergic receptor antagonist idazoxan and antagonized by the alpha 2-adrenergic receptor agonist clonidine, indicating that the yohimbine stimulus was mediated through the alpha 2-adrenergic receptor. The anxiogenic beta-carbolines FG 7142, 1,2,3,4-tetrahydro-beta-carboline (THBC), and norharmane, the anxiogenic/convulsant agent pentylenetetrazole (PTZ), and two physiological stressors failed to mimic the yohimbine discriminative stimulus. In contrast, both yohimbine and idazoxan dose responsively mimicked the anxiogenic FG 7142 stimulus. The present results demonstrate that an asymmetrical generalization exists between the discriminative stimuli produced by yohimbine and FG 7142. Furthermore, these data suggest that yohimbine can produce a multicomponent discriminative stimulus, part of which may be anxiogenic in nature. The ability of alpha 2-adrenergic receptor antagonists to mimic the FG 7142 cue suggests that activation of the noradrenergic system may underlie cues produced by benzodiazepine receptor inverse agonists.