Nitric oxide may underlie learned fear in the elevated T-maze

The present study evaluated the role of nitric oxide (NO) in learned and innate fear in rats submitted to the elevated T-maze (ETM). Learned and innate fear were evaluated through the inhibitory avoidance and escape behaviour from the open arms, respectively. Rats treated with the inhibitor of NO synthesis N-omega-nitro-L-Arginine methyl ester (L-NAME; 5, 10, and 50 mg (.) kg(-1)) were able to learn the inhibitory avoidance. However, L-NAME (50 mg (.) kg(-1)), but not its inert isomer N-omega-nitro-D-arginine methyl ester (D-NAME, 50 mg (.) kg(-1)), impaired the inhibitory avoidance 2 with no change in the baseline values, thus suggesting an anxiolytic-like effect without locomotor impairment. All treatments with L-NAME were able to induce increased mean arterial pressure (MAP), measured indirectly through the animal's tail. The treatment with L-NAME (5 and 10 mg kg-l) failed to induce anxiolysis but significantly increased the MAP of the animals, which indicates that hypertension per se, did not underlie anxiolysis induced by L-NAME. L-Arginine, the precursor molecule for NO synthesis, facilitated the inhibitory avoidance and counteracted the L-NAME (50 mg (.) kg(-1)) induced anxiolysis. Neither previous treatment was able to change the escape behaviour. The results indicate that NO may underlie learned, but not innate, fear in the ETM. (C) 2001 Elsevier Science Inc.