Presenting a 15-s pulsed tone, the conditional stimulus (CS ϩ ), followed by 0.5-s tail shock, to a well-trained rat causes a sudden, but transient, pressor response (C1). Blood pressure (BP) then drops before increasing again (C 2). A steady tone of the same frequency never followed by a shock (a discriminative stimulus, or CS Ϫ ) evokes a C1 but not a C2 response. Experiment 1 tested the hypothesis that this BP response pattern does not depend on the nature of the tone (i.e., pulsed vs.

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... ) used for CS ϩ and CS Ϫ . The tones were reversed from the traditional paradigm, above, in nine rats. The C1 BP increase for a steady-tone CS ϩ (ϩ4.8 Ϯ 1.9 mmHg, mean change Ϯ SE) and a pulsed CS Ϫ (ϩ2.9 Ϯ 1.3 mmHg) did not differ. Conversely, C2 showed a clear discrimination (CS ϩ : ϩ5.1 Ϯ 1.2 mmHg, CS Ϫ : ϩ0 .7 Ϯ 0.8 mmHg; P Ͻ 0.05). Experiment 2 tested the hypothesis that the C1 and C2 BP responses first appear at different times during training. On training day 1, five 15-s pulsed tones (CS ϩ ) were presented to each of 18 rats; the last tone was followed by a tail shock. Likewise, five steady CS Ϫ tones never followed by shock were given. Training continued for 2 more days, with each CS ϩ followed by shock. At the end of day 2, CS ϩ evoked a C1 BP response (ϩ3.9 Ϯ 0.9 mmHg) but no C2 (ϩ0.6 Ϯ 0.4 mmHg, not significant vs. pretone). By the end of day 3, CS ϩ evoked a significant (vs. baseline) C1 (ϩ7.3 Ϯ 1.4 mmHg) and C2 (ϩ3.3 Ϯ 0.8 mmHg). Conversely, although CS Ϫ evoked a C1 response (3.5 Ϯ 1.3 mmHg), there was no C2 (ϩ0.7 Ϯ 0.5 mmHg; not significant). We conclude that 1) C1 and C2 are acquired at different rates, 2) early in training C1 is an orienting response evoked by both tones, and 3) C2 is only acquired as an animal learns to associate the CS ϩ tone with shock. This suggests that C1 and C2 are controlled by different processes in the brain.