Effects of naltrindole (NAL, 1 or 5 mg kg−1; (a, b), SNC80 (1, 5 or 20 mg kg−1; (c, d) and SNC80 (5 mg kg−1) antagonism by naltrindole (1 or 5 mg kg−1) (e, f) on the elevated plus maze. Data are expressed as means±s.e.m. of open arm entries (a, c, e) and time on open arms (b, d, f) expressed as a percentage of the total time or number of entries (n=7–10/group). *P<0.05, **P<0.01, and ***P<0.001.