Stimulation of α2-adrenergic receptors in the central nucleus of the amygdala attenuates stress-induced reinstatement of nicotine seeking in rats

Abstract

Tobacco addiction is a chronic disorder that is characterized by craving for tobacco products, withdrawal upon smoking cessation, and relapse after periods of abstinence. Previous studies demonstrated that systemic administration of α2-adrenergic receptor agonists attenuates stress-induced reinstatement of drug seeking in rats. The aim of the present experiments was to investigate the role of noradrenergic transmission in the central nucleus of amygdala (CeA) in stress-induced reinstatement of nicotine seeking. Rats self-administered nicotine for 14-16 days and then nicotine seeking was extinguished by substituting saline for nicotine. The effect of the intra-CeA infusion of the α2-adrenergic receptor agonists clonidine and dexmedetomidine, the nonselective β1/β2-adrenergic receptor antagonist propranolol, and the α1-adrenergic receptor antagonist prazosin on stress-induced reinstatement of nicotine seeking was investigated. In all the experiments, exposure to footshocks reinstated extinguished nicotine seeking. The administration of clonidine or dexmedetomidine into the CeA attenuated stress-induced reinstatement of nicotine seeking. The administration of propranolol or prazosin into the CeA did not affect stress-induced reinstatement of nicotine seeking. Furthermore, intra-CeA administration of clonidine or dexmedetomidine did not affect operant responding for food pellets. This suggests that the effects of clonidine and dexmedetomidine on stress-induced reinstatement of nicotine seeking were not mediated by motor impairments or sedation. Taken together, these findings indicate that stimulation of α2-adrenergic receptors, but not blockade of α1 or β-adrenergic receptors, in the CeA attenuates stress-induced reinstatement of nicotine seeking. These findings suggest that α2-adrenergic receptor agonists may at least partly attenuate stress-induced reinstatement of nicotine seeking by stimulating α2-adrenergic receptors in the CeA.

Figure 1

Histological overview of bilateral injection sites in the CeA (A). Drug injections were administered within or at the boundaries of the CeA (−1.88 to −2.56 anterior-posterior). The injections sites were evenly distributed over the 3 levels depicted in this figure. The figures are copies from the Paxinos and Watson brain atlas (Paxinos and Watson, 1998). Figure 1B depicts a photomicrograph of a representative cresyl violet-stained brain section at the level of the CeA. The photomicrograph shows the cannula tract and the injector tract that ends within the CeA. Abbreviations: cc, corpus callosum, CA3, hippocampal CA3 region.

Figure 1

Histological overview of bilateral injection sites in the CeA (A). Drug injections were administered within or at the boundaries of the CeA (−1.88 to −2.56 anterior-posterior). The injections sites were evenly distributed over the 3 levels depicted in this figure. The figures are copies from the Paxinos and Watson brain atlas (Paxinos and Watson, 1998). Figure 1B depicts a photomicrograph of a representative cresyl violet-stained brain section at the level of the CeA. The photomicrograph shows the cannula tract and the injector tract that ends within the CeA. Abbreviations: cc, corpus callosum, CA3, hippocampal CA3 region.

Figure 2

Effects of clonidine on stress-induced reinstatement of nicotine seeking. Figure 2A depicts responding on the active and inactive lever during extinction of nicotine self-administration and stress-induced reinstatement of nicotine seeking (n = 15). In figure 2A, asterisks (* P<0.05, ** P<0.01) indicate a decrease in responding on the active lever compared to the last day of nicotine self-administration. The pound sign (# P<0.05) indicates a decrease in response on the inactive lever compared to the last day of nicotine self-administration. Plus signs (+ P<0.05, ++ P<0.01) indicate a footshock-induced increase in responding on the active lever or inactive lever compared to day 7 of extinction training. In figure 2B, Asterisks (P<0.05) indicate a decrease in responding on the active lever compared to rats that were exposed to footshocks and pretreated with vehicle. Abbreviations: B, baseline; Sh, footshocks. Data are expressed as mean ± SEM.

Figure 2

Effects of clonidine on stress-induced reinstatement of nicotine seeking. Figure 2A depicts responding on the active and inactive lever during extinction of nicotine self-administration and stress-induced reinstatement of nicotine seeking (n = 15). In figure 2A, asterisks (* P<0.05, ** P<0.01) indicate a decrease in responding on the active lever compared to the last day of nicotine self-administration. The pound sign (# P<0.05) indicates a decrease in response on the inactive lever compared to the last day of nicotine self-administration. Plus signs (+ P<0.05, ++ P<0.01) indicate a footshock-induced increase in responding on the active lever or inactive lever compared to day 7 of extinction training. In figure 2B, Asterisks (P<0.05) indicate a decrease in responding on the active lever compared to rats that were exposed to footshocks and pretreated with vehicle. Abbreviations: B, baseline; Sh, footshocks. Data are expressed as mean ± SEM.

Figure 3

Effects of dexmedetomidine on stress-induced reinstatement of nicotine seeking. Figure 3A depicts responding on the active and inactive lever during extinction of nicotine self-administration and stress-induced reinstatement of nicotine seeking (n = 17). In figure 3A, asterisks (* P<0.05, ** P<0.01) indicate a decrease in responding on the active lever compared to the last day of nicotine self-administration. The pound signs (# P<0.05, ## P<0.01) indicate an increase in responding on the inactive lever compared to the last day of nicotine self-administration. Plus signs (++ P<0.01) indicate a footshock-induced increase in responding on the active lever or inactive lever compared to day 21 of extinction training. In figure 3B, Asterisks (** P<0.01) indicate a decrease in responding on the active lever compared to rats that were exposed to footshocks and pretreated with vehicle. Abbreviations: B, baseline; Sh, footshocks. Data are expressed as mean ± SEM.

Figure 3

Effects of dexmedetomidine on stress-induced reinstatement of nicotine seeking. Figure 3A depicts responding on the active and inactive lever during extinction of nicotine self-administration and stress-induced reinstatement of nicotine seeking (n = 17). In figure 3A, asterisks (* P<0.05, ** P<0.01) indicate a decrease in responding on the active lever compared to the last day of nicotine self-administration. The pound signs (# P<0.05, ## P<0.01) indicate an increase in responding on the inactive lever compared to the last day of nicotine self-administration. Plus signs (++ P<0.01) indicate a footshock-induced increase in responding on the active lever or inactive lever compared to day 21 of extinction training. In figure 3B, Asterisks (** P<0.01) indicate a decrease in responding on the active lever compared to rats that were exposed to footshocks and pretreated with vehicle. Abbreviations: B, baseline; Sh, footshocks. Data are expressed as mean ± SEM.

Figure 4

Effects of propranolol on stress-induced reinstatement of nicotine seeking. Figure 4A shows responding on the active and inactive lever during extinction of nicotine self-administration and stress-induced reinstatement of nicotine seeking (n = 12). In figure 4A, asterisks (** P<0.01) indicate a decrease in responding on the active lever compared to the last day of nicotine self-administration. Plus signs (++ P<0.01) indicate a footshock-induced increase in responding on the active lever compared to day 9 of extinction training. Abbreviations: B, baseline; Sh, footshocks. Data are expressed as mean ± SEM.

Figure 4

Effects of propranolol on stress-induced reinstatement of nicotine seeking. Figure 4A shows responding on the active and inactive lever during extinction of nicotine self-administration and stress-induced reinstatement of nicotine seeking (n = 12). In figure 4A, asterisks (** P<0.01) indicate a decrease in responding on the active lever compared to the last day of nicotine self-administration. Plus signs (++ P<0.01) indicate a footshock-induced increase in responding on the active lever compared to day 9 of extinction training. Abbreviations: B, baseline; Sh, footshocks. Data are expressed as mean ± SEM.

Figure 5

Effects of prazosin on stress-induced reinstatement of nicotine seeking. Figure 5A shows responding on the active and inactive lever during extinction of nicotine self-administration and stress-induced reinstatement of nicotine seeking (n = 17). In figure 5A, asterisks (** P<0.01) indicate a decrease in responding on the active lever compared to the last day of nicotine self-administration. The pound sign (## P<0.01) indicate an increase in responding on the inactive lever compared to the last day of nicotine self-administration. Plus signs (++ P<0.01) indicate a footshock-induced increase in responding on the active lever compared to day 18 of extinction training. Abbreviations: B, baseline; Sh, footshocks. Data are expressed as mean ± SEM.

Figure 5

Effects of prazosin on stress-induced reinstatement of nicotine seeking. Figure 5A shows responding on the active and inactive lever during extinction of nicotine self-administration and stress-induced reinstatement of nicotine seeking (n = 17). In figure 5A, asterisks (** P<0.01) indicate a decrease in responding on the active lever compared to the last day of nicotine self-administration. The pound sign (## P<0.01) indicate an increase in responding on the inactive lever compared to the last day of nicotine self-administration. Plus signs (++ P<0.01) indicate a footshock-induced increase in responding on the active lever compared to day 18 of extinction training. Abbreviations: B, baseline; Sh, footshocks. Data are expressed as mean ± SEM.

Figure 6

Effects of clonidine (A) and dexmedetomidine (B) on responding for food pellets (n = 10). Data are expressed as mean ± SEM.

Figure 6

Effects of clonidine (A) and dexmedetomidine (B) on responding for food pellets (n = 10). Data are expressed as mean ± SEM.