Differential Acquisition of Specific Components of a Classically Conditioned Arterial Blood Pressure Response in Rat AUTHORS:

Presenting a 15 sec. pulsed tone, the conditional stimulus (CS+), followed by ½ sec. tail shock to a well-trained rat causes a sudden, but transient, pressor response (C 1). 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-) evokes a C 1 , but not a C 2 response. Experiment 1 tests the hypothesis that this BP response pattern does not depend on the nature of the tone (i.e., pulsed vs. steady) used for CS+ and CS-. The tones were reversed from the traditional paradigm, above, in 9 rats. The C 1 BP increase for a steady tone CS+ (+4.8 ± 1.9 mm Hg, mean change ± SEM) and a pulsed CS-(+2.9 ± 1.3 mm Hg) did not differ. Conversely, C 2 showed a clear discrimination (CS+: +5.1 ± 1.2 mm Hg; CS-: +0 .7 ± 0.8 mm Hg; p < 0.05). Experiment 2 tested the hypothesis that the C 1 and C 2 BP responses first appear at different times during training. On training Day 1, five 15 sec. pulsed tones (CS+) were presented to each of 18 rats; the last tone was followed by a tail shock. Likewise, 5 steady CS-tones never followed by shock were given. Training continued for 2 more days with each CS+ followed by shock. At the end of Day 2 CS+ evoked a C 1 BP response (+3.9 ± 0.9 mmHg,) but no C 2 (+0.6 ± 0.4 mmHg, NS vs. pre-tone). By the end of Day 3 CS+ evoked a significant (vs. baseline) C 1 (+7.3 ± 1.4 mmHg) and C 2 (+3.3 ± 0.8 mmHg). Conversely, while CS-evoked a C 1 response (3.5 ± 1.3 mm Hg), there was no C 2 (+0.7 ± 0.5 mmHg; NS). We conclude that (a) C 1 and C 2 are acquired at different rates; (b) early in training C 1 is an orienting response evoked by both tones; and (c) C 2 is only acquired as an animal learns to associate the CS+ tone with shock. This suggests that C 1 and C 2 are controlled by different processes in the brain.