Figure 2. The analgesic effects of fourth ventricular infusions of morphine (1 micro gram/ml; n = 9, S-nitroso-acetylpenicillamine (SNAP; 10 sup -5 and 10 sup -4 M; n = 7), or nitro-L-arginine (L-NA; 10 sup -8 and 10 sup -5 M; n = 8). The values (means +/- SE) are expressed as a percentage of the baseline nociceptive threshold (see Table 2). *P <0.05 versus baseline.