When rats are exposed to unknown environments where novelty and fear-inducing characteristics are present (conflictive environments), some specific behaviors are induced and exploration is apparently modulated by fear. In our laboratory, a new type of plus-maze was designed as a model of conflictive exploration. The maze is composed of four arms with different geometrical characteristics, differing from each other by the presence or absence of walls. The degree of asymmetry was as follows: NW, no wall arm; SW, a single high wall present; HL, a low and a high wall present, and HH, two high walls present. The four arms were arranged at 90o angles and the apparatus was called the elevated asymmetric plus-maze (APM). The purpose of the present study was to assess the behavioral profile of rats exposed for a single time to the APM with or without treatment with benzodiazepine. Increasing doses of diazepam were injected intraperitoneally in several groups of male, 90-day-old Holtzman rats. Distilled water was injected in control animals. Thirty minutes after treatment all rats were exposed singly to a 5-min test in the APM. Diazepam induced a biphasic modification of exploration in the NW and SW arms. The increase in the exploration score was evident at low doses of diazepam (0.25-1.0 mg/kg body weight) and the decrease in exploration was found with the higher doses of diazepam (2.0-3.0 mg/kg body weight). Non-exploratory behaviors (permanency) were not affected by benzodiazepine treatment. In the HL arm, exploration was not modified but permanency was increased in a dose-dependent manner. In the HH arm, exploration and permanency were not affected. Results are compatible with the idea that exploration-processing mechanisms in conflictive environments are modulated by fear-processing mechanisms of the brain.

The possible role of histamine receptors in the hippocampal formation on the exploratory motivation and emotionality of the rat was studied. An elevated asymmetric plus-maze composed of 4 different arms (no walls, single high wall, high and low walls and two high walls) arranged at 90o angles was used. The exploration score, considered to be an index of exploratory motivation, and the permanency score, considered to be an index of emotionality (anxiety), were determined. Histamine was administered locally into the ventral hippocampus at three different doses (9, 45 and 90 nmol). Another group of rats was also microinjected with 45 nmol of pyrilamine (a histamine H1 receptor antagonist) or ranitidine (a histamine H2 receptor antagonist) in addition to 9 nmol of histamine in order to identify the possible type of histamine receptor involved. Histamine administration significantly inhibited the exploration score and increased the permanency score at the doses of 9 and 45 nmol in two of four arms. These effects were completely blocked by the administration of either histamine receptor antagonist. The present results suggest that in the hippocampal formation histamine inhibits exploratory motivation and decreases emotionality by activating both types of histamine receptors. Also, the elevated asymmetric plus-maze appears to be a suitable technique to quantify exploration and possibly" anxiety"