DEFEAT ENGENDERS PENTYLENETETRAZOLE-APPROPRIATE RESPONDING IN RATS - ANTAGONISM BY MIDAZOLAM

Defeat and the threat of defeat by an aggressive conspecific is stress ful and may engender an anxiety- or fear-like state in animals; the pr esent experiment investigated whether defeat generalized to the discri minative stimulus properties of PTZ and how benzodiazepine receptors w ere involved in this generalization. Separate groups of male Long-Evan s rats (Rattus norvegicus) were trained to discriminate 20 mg/kg penty lenetetrazole (PTZ) or 0.4 mg/kg midazolam (MDZ) from saline in a two- choice drug-discrimination task. After establishing stimulus control, PTZ- and MDZ-trained rats were exposed to an aggressive conspecific wh ich resulted in defeat, as defined by the display of defensive and sub missive postures as well as audible and ultrasonic vocalizations, Admi nistration of saline after defeat resulted in greater than 80 % PTZ le ver selection in 15 out of 25 PTZ-trained rats; this effect was attenu ated through pretreatment with MDZ (1 mg/kg). Furthermore, short-term defeat substitution for the PTZ discriminative stimulus was not accomp anied by long-term changes in the post-defeat generalization curves fo r PTZ and MDZ when compared to pre-defeat generalization curves. Nor d id defeat alter the antagonism of PTZ by diazepam (2.5 mg/kg) or MDZ b y flumazenil (10 mg/kg). In order further to characterize the necessar y features for defeat substitution for the PTZ discriminative stimulus , exposure to a threatening conspecific was also attempted by PTZ-trai ned rats protected from physical contact with a wire mesh cage. In the se tests, saline continued to engender greater than 50 % PTZ lever res ponding in 15 of 25 rats. These results suggest that an anxiety-like s tate is induced during defeat and exposure to a threatening conspecifi c in most rats; this state, as well as the PTZ discriminative stimulus , can be reversed by benzodiazepine receptor agonists. In contrast, sh ort-term defeat substitution for the PTZ discriminative stimulus does not appear to be related to long-term alterations in the benzodiazepin e receptor.