Differential effects of α4β2 nicotinic receptor antagonists and partial-agonists on contextual fear extinction in male C57BL/6 mice

Abstract

Rationale: Numerous studies have attributed the psychopathology of post-traumatic stress disorder (PTSD) to maladaptive behavioral responses such as an inability to extinguish fear. While exposure therapies are mostly effective in treating these disorders by enhancing extinction learning, relapse of PTSD symptoms is common. Although several studies indicated a role for cholinergic transmission and nicotinic acetylcholine receptors (nAChRs) in anxiety and stress disorder symptomatology, very little is known about the specific contribution of nAChRs to fear extinction OBJECTIVES: In the present study, we examined the effects of inhibition and desensitization of α4β2 nAChRs via a full antagonist (Dihydro-beta-erythroidine (DhβE)) and two α4β2 nAChR partial-agonists (varenicline and sazetidine-A) on contextual fear extinction, locomotor activity, and spontaneous recovery of contextual fear in mice.

Methods: We trained and tested the subjects in a contextual fear extinction as well as an open field paradigm and spontaneous recovery following injections of DhβE, varenicline, and sazetidine-A.

Results: Our results demonstrated that lower doses of DhβE (1 mg/kg) and sazetidine-A (0.01 mg/kg) enhanced contextual fear extinction whereas higher doses of varenicline (0.1 mg/kg) and sazetidine-A (0.1 mg/kg) resulted in impaired contextual fear extinction. However, the higher dose of sazetidine-A (0.1 mg/kg) decreased locomotor activity, which may contribute to increased freezing response observed during fear extinction. Finally, we found that the low dose of DhβE, but not sazetidine-A, also decreased spontaneous recovery of contextual fear following fear extinction.

Conclusions: Overall, these results suggest that inhibition and desensitization of α4β2 nAChRs enhance extinction of contextual fear memories. This suggests that modulation of α4β2 nAChRs may be employed as an alternative pharmacological strategy to aid exposure therapies associated with PTSD by augmenting contextual fear extinction processes.

Keywords: Fear extinction; Nicotinic receptors; PTSD; Spontaneous recovery.

Figure 1. The schematic of experimental designs

While each box represents a phase of the experiment, the syringe represents DhβE, varenicline, sazetidine-A, or saline injections and the lightning bolt symbol indicates the presentations of the footshocks.

Figure 2. Effects of DhβE, varenicline, sazetidine-A on contextual fear extinction

Normalized and raw %freezing scores across initial testing and 5 extinctions sessions. Panel A–B: Effects of acute injections of DhβE (DhβE 1 and DhβE 5 mg/kg) and saline on contextual fear extinction. Panel C–D: Effects of acute varenicline (Var 0.01 mg/kg and Var 0.1 mg/kg) and saline injections on contextual fear extinction. Panel E–F: Effects of acute sazetidine-A (Saz-A 0.01 mg/kg and Saz-A 0.1 mg/kg) and saline injections on contextual fear extinction. Error bars show standard error of the mean. (*) denotes significant differences between lower dose drug groups and saline controls at p value <0.05. (#) denotes significant differences between higher dose drug groups and saline controls at p value <0.05.

Figure 3. Locomotor activity following DhβE, varenicline, sazetidine-A

Distance traveled (inches) in an open field paradigm. Panel A: Effects of acute injections of DhβE (DhβE 1 and DhβE 5 mg/kg) and saline on locomotor activity. Panel B: Effects of acute varenicline (Var 0.01 mg/kg and Var 0.1 mg/kg) and saline injections on locomotor activity. Panel C: Effects of acute sazetidine-A (Saz-A 0.01 mg/kg and Saz-A 0.1 mg/kg) and saline injections on locomotor activity. Error bars show standard error of the mean. (*) denotes Tukey post-hoc results showing significant differences between drug groups and saline controls with p value <0.05.

Figure 4. DhβE decreases spontaneous recovery of contextual fear following extinction

%freezing and %Rebound scores during re-testing 7 days following the last fear extinction session Panel A: Effects of low dose DhβE (DhβE 1 mg/kg) on spontaneous recovery of contextual fear when administered prior to re-testing. Panel B: %Rebound values for the DhβE 1 mg/kg and saline controls. Panel C: Effects of low dose sazetidine-A (Saz-A 0.01 mg/kg) on spontaneous recovery of contextual fear. Panel D: %Rebound values for the Saz-A 0.01 mg/kg and saline controls. Error bars show standard error of the mean. (*) denotes significant main effect of Drug with p value <0.05.