Transient emotional changes elicited by intraperitoneal saline injection: effect of naloxone and flumazenil

The effect of the intraperitoneal (i.p.) saline injection was assessed by using the defensive burying (DB) and the elevated plus-maze (EPM) anxiety paradigms in rats. Animals were handled gently by the body, injected i.p. with saline solution, 2 ml/kg, and tested independently in the defensive burying as well as in the elevated plus-maze test at different times after the i.p. injection: 1.5, 3, 5, 10, 15, and 30 min. A transient effect of i.p. saline injection was observed (i.e., increased DB in animals tested 1.5 min after injection) and a decrease in this parameter when studied 3 min after the injection. No changes at 5, 10, 15, and 30 min after the injection were found. To discriminate the putative participation of the opiate peptide and benzodiazepine receptors in the actions of the i.p. injection, flumazenil (5 mg/kg) and nalozone (1 mg/kg) were administered. The increase in DB at 1.5 min was masked by double injection, an effect blocked by naloxone, but not by flumazenil, while both of them reverted the decrease in DB response in animals tested 3 min after injection. A partial action of the i.p. in the animals tested in the elevated plus-maze test was found. Present results are discussed on the basis of behavioral and pharmacological evidence.