Inverse agonist properties of the THBC discriminative stimulus: asymmetrical generalization with FG 7142.
Male rats were trained to discriminate the stimulus properties of the beta-carbolines 1,2,3,4-tetrahydro-beta-carboline (THBC) (15.0 mg/kg) or FG 7142 (5.0 mg/kg) from vehicle in a two-lever, food-motivated operant task. Consistent with the serotonergic properties of THBC, administration of the 5HT1B agonists TFMPP and mCPP to THBC-trained rats resulted in THBC-appropriate responding. Norharmane, a beta-carboline metabolite of THBC, also mimicked the THBC discriminative stimulus. In contrast, the benzodiazepine receptor partial inverse agonist FG 7142, the anxiogenic/convulsant pentylenetetrazole (PTZ), two physiological stressors and the alpha 2 adrenergic antagonists yohimbine and idazoxan failed to produce THBC-appropriate responding. In the FG 7142-trained rats, THBC and norharmane dose-dependently mimicked the FG 7142 discriminative stimulus. This generalization was not based upon the serotonergic properties of THBC and norharmane since administration of the serotonin agonist mCPP to FG 7142-trained rats failed to produce FG 7142-appropriate responding. The ability of THBC to substitute for the FG 7142 discriminative stimulus was antagonized by the benzodiazepine receptor mixed agonist/antagonist CGS 9896 and the benzodiazepine receptor antagonist RO 15-1788, indicating that THBC produces an inverse agonist stimulus in FG 7142-trained rats. These results suggest that THBC produces a discriminative stimulus which consists of both serotonergic and inverse agonist components.
Leidenheimer
N J
NJ
Department of Pharmacology, Northeastern Ohio Universities College of Medicine, Rootstown 44272.
Schechter
M D
MD
D000818:Animals; D001362:Avoidance Learning / Q000187:drug effects; D002243:Carbolines / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D004193:Discrimination Learning / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D005795:Generalization, Stimulus / Q000187:drug effects*; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D011963:Receptors, GABA-A / Q000187:drug effects*
D002243:Carbolines; D011963:Receptors, GABA-A; C035874:FG 7142; C009804:tryptoline