High-affinity α4β2 nicotinic receptors mediate the impairing effects of acute nicotine on contextual fear extinction

Abstract

Previously, studies from our lab have shown that while acute nicotine administered prior to training and testing enhances contextual fear conditioning, acute nicotine injections prior to extinction sessions impair extinction of contextual fear. Although there is also strong evidence showing that the acute nicotine’s enhancing effects on contextual fear conditioning require high-affinity α4β2 nicotinic acetylcholine receptors (nAChRs), it is unknown which nAChR subtypes are involved in the acute nicotine-induced impairment of contextual fear extinction. In this study, we investigated the effects of acute nicotine administration on contextual fear extinction in knock-out (KO) mice lacking α4, β2 or α7 subtypes of nAChRs and their wild-type (WT) littermates. Both KO and WT mice were first trained and tested for contextual fear conditioning and received a daily contextual extinction session for 4 days. Subjects received intraperitoneal injections of nicotine (0.18 mg/kg) or saline 2–4 mins prior to each extinction session. Our results showed that the that mice lack α4 and β2 subtypes of nAChRs showed normal contextual fear extinction but not the acute nicotine-induced impairment while the mice that lack the α7 subtype showed both normal contextual extinction and nicotine-induced impairment of contextual extinction. In addition, control experiments showed that acute nicotine-induced impairment of contextual fear extinction persisted when nicotine administration was ceased and repeated acute nicotine administrations alone did not induce freezing behavior in the absence of context-shock learning. These results clearly demonstrate that high-affinity α4β2 nAChRs are necessary for the effects of acute nicotine on contextual fear extinction.

Acute nicotine enhances spontaneous recovery of contextual fear and changes c-fos early gene expression in infralimbic cortex, hippocampus, and amygdala

“…Subjects were observed every 10 sec for a duration of 1 sec and scored as active or freezing (Blanchard and Blanchard 1972). Following the background contextual fear conditioning procedure used in our previous studies Kutlu et al 2016), in Experiment 1, mice received contextual fear conditioning training, in which they were first placed in the conditioning chambers and baseline freezing was assessed for 120 sec, and then the mice received two white noise-foot shock pairings where the offset of the white noise coincided with the offset of the 2-sec foot shock. (see Fig.…”

“…In addition, there is evidence showing that acute nicotine intake increases the likelihood of intrusive traumatic memories in healthy nonsmokers (Hawkins and Cougle 2013). Moreover, evidence from multiple animal studies showed that acute nicotine enhances contextual fear learning (Gould and Higgins 2003;Gould and Lommock 2003;Davis et al 2006) and disrupts extinction of fear memories Kutlu et al 2016; for review, see Kutlu and Gould 2015). Therefore, given the higher rates of smoking initiation in PTSD patients, the period of acute nicotine intake following a traumatic event may be especially critical for the development and relapse of fear in PTSD.…”

Acute nicotine disrupts consolidation of contextual fear extinction and alters long-term memory-associated hippocampal kinase activity

“…Injection volumes were 10 mL/kg as in previous studies (e.g., Kutlu & Gould 2014). The 0.18 mg/kg dose of acute nicotine was chosen based on our previous reports showing impaired contextual fear extinction at this dose (Kutlu & Gould, 2014; Kutlu et al, 2016a). For the subsequent western blotting experiments, the same dose of nicotine or saline was administered i.p.…”

“…The dissection time point was chosen based on a previous report from our laboratory examining acute nicotine’s effects on ERK1/2 phosphorylation during contextual fear conditioning (Gould et al, 2014). We examined ERK1/2 and JNK1 activation at the end of the 1 st and 3 rd extinction day based on our previous studies showing that the effect of nicotine is the largest on the 3 rd day of extinction as the saline group usually shows complete extinction by that day (Kutlu & Gould, 2014; Kutlu et al, 2016a). Therefore, the 1 st extinction day analysis served as a control as we did not expect to see a large difference in terms of long-term memory-associated kinase activity.…”

“…In contrast to the enhancing effects of acute nicotine on contextual fear conditioning, we recently showed that acute nicotine administered prior to extinction impaired contextual fear extinction (Kutlu & Gould, 2014) and this effect was dependent on high-affinity α4β2 nAChRs (Kutlu et al, 2016a). Fear extinction is a process whereby new inhibitory memories are created, which results in a reduction of learned responses (Myers & Davis, 2007).…”