Effects of nicotine and dietary salt on a learned blood pressure response in Dahl-S rats

We examined the effects of chronic nicotine exposure and dietary salt on the arterial blood pressure (BP) changes learned in response to an acute behavioral stress in the Dahl salt-sensitive rat. Four groups were tested: low salt + vehicle; low salt + nicotine; high salt + vehicle; and high salt + nicotine. Rats were fed a low-salt (0.08% NaCl) or a high-salt (8% NaCl) diet for 4 wk; 2.4 mg · kg−1 · day−1nicotine or vehicle was given via an implanted osmotic minipump for the last 2 wk. All rats were conditioned by following one tone (CS+) with a 0.5-s tail shock; another tone (CS−) was never followed by shock. CS+ in low salt + vehicle and high salt + vehicle-treated rats evoked an initial arterial BP increase (C1), a component of the startle response, and an ensuing, smaller, but more sustained, pressor response (C2), which is acquired with training. In these rats, both C1 and C2 evoked by CS− were significantly smaller than those to CS+, demonstrating that these groups discriminated between the two tests. Conversely, although the low salt + nicotine-treated rats had both the C1 and C2 components of the conditional arterial pressure response, they did not discriminate between CS+ and CS−. Finally, the high salt + nicotine group failed to both discriminate between tones and acquire (i.e., learn) the C2 response. The unconditional response to shock did not differ between groups. We conclude that combined exposure to high salt and to nicotine inhibits the salt-sensitive animal's acquisition of a learned conditional BP response, perhaps because nicotine acts preferentially on those central processes required for associative learning versus those involved in orientating to external stimuli.