Decreased Signs of Nicotine Withdrawal in Mice Null for the β4 Nicotinic Acetylcholine Receptor Subunit

Introduction

Nicotine, the major addictive component of tobacco, binds to nicotinic acetylcholine receptors (nAChRs). These receptors are usually heteropentamers composed of at least one type of α and one type of β subunit. The α7-α10 subunits are the exception, because they are believed to form α-only homopentamers and heteropentamers. To date, nine α and three β subunits have been cloned. Some α/β subunit combinations, such as α4β2, α3β4, α6β2β3, or α7 only, seem to be favored in neurons. Among these, the most widely expressed are the α4β2- and α7-containing combinations. α4β2 nAChRs are also responsible for most of the high-affinity nicotine binding sites and have been the focus of many studies in the field (Dani and De Biasi, 2001; Hogg et al., 2003).

Determining the roles of each nAChR subtype in physiology and addiction is one of the major goals in nicotine research, and the study of mice carrying null mutations for several nAChR subunits has indicated the involvement of certain subunits in specific nicotinic mechanisms. Examples are as follows: the involvement of α4 in nicotine-dependent antinociception (Marubio et al., 1999), α4, β3, and β4 in anxiety-related behaviors (Booker et al., 2000; Ross et al., 2000; Salas et al., 2003a), β2 in fear conditioning and cocaine-induced nicotine self-administration (Picciotto et al., 1998), α7 in neuronal protection (Laudenbach et al., 2002), and α3, α5, and β4 in nicotine-induced seizures (Salas et al., 2003b, 2004; Kedmi et al., 2004).

The appearance of withdrawal symptoms after chronic nicotine cessation is an important clinical effect of nicotine, because it is one of the major factors precluding people from successfully quitting smoking. The mechanisms underlying the physical and affective symptoms of withdrawal are poorly understood, and which nAChRs subtypes are involved in this aspect of nicotine addiction is still uncertain. Nicotine withdrawal can be precipitated in mice by antagonists with preferential effects on α3β4-, α4β2-, or α7-containing nAChRs (Damaj et al., 2003). The present study addressed the role of β2- and β4-containing nAChRs in mediating the somatic signs of nicotine withdrawal by using mice null for either the β2 or the β4 nAChR subunits.

Materials and Methods

Animals. We studied 2- to 5-month-old wild-type mice and mice lacking the β2 or the β4 subunit of nAChRs (Xu et al., 1999). All mice were backcrossed for 8-10 generations into a C57BL/6J background. Male and female mice were housed under a 12 hr light/dark cycle, with access to food and water ad libitum, and experiments were performed during the light phase. Alzet pumps model 1004 (14 d, flow rate of 0.25 μl/hr; Durect, Cupertino, CA) were subcutaneously implanted according to the instructions of the manufacturer. Pumps were filled with either saline or (-)nicotine tartrate in saline to deliver a 24 mg · kg^-1 · d^-1 dose of nicotine (as free base) for 13 d. The experimenters were blind to the genotypes until data were gathered and analyzed. All procedures were approved by the Baylor College of Medicine Animal Care and Use Committee and followed the guidelines for animal intramural research from the National Institutes of Health.

Nicotine withdrawal. Our mouse nicotine withdrawal protocol was based on the research by Berrendero et al. (2002) and Damaj et al. (2003). After 13 d of either nicotine or saline infusion, mice were scored for somatic signs for a 5 min period to obtain a baseline of behavioral scores during chronic exposure to nicotine. Subsequently, the mice were intraperitoneally injected with 3 mg/kg racemic mecamylamine and immediately placed in the home cage, where withdrawal signs were recorded for 20 min. The following parameters were monitored: grooming, scratching, chewing, shaking, cage scratching, head nodding, and jumping. A pilot study conducted on drug-naive C57BL/6J mice indicated that 3 mg/kg mecamylamine does not increase withdrawal signs under our experimental conditions. Data were analyzed by ANOVA, followed by Newman-Keuls post hoc comparisons. Nicotine tartrate and mecamylamine were purchased from Sigma (St. Louis, MO).

Hyperalgesia. We measured nicotine-induced hyperalgesia on mice undergoing either spontaneous or mecamylamine-induced nicotine withdrawal and found that hyperalgesia was more reliably observed in the mice undergoing withdrawal after depletion of the nicotine infusion pumps. Therefore, tail-flick and hot-plate experiments were performed 2 d after interruption of the nicotine infusion. First, the tail of a mouse was manually inserted into a 48°C water bath, and the time to remove the tail was recorded (tail flick). Immediately after tail flick, mice were placed on a platform at 51°C, and the latency time to lick their paws or to jump was recorded (hot plate).

Epibatidine binding. After nicotine withdrawal symptoms were recorded, mice were killed, and brains removed and rapidly frozen in -20°C isopentane. Binding was performed as described previously (Franceschini et al., 2002). Briefly, fresh frozen 20 μm sections were cut, incubated in binding buffer (in mm: 50 Tris base, pH 7.4, 120 NaCl, 5 KCl, 2.5 CaCl₂, and 1 MgCl₂) for 10 min and then in 500 pm [¹²⁵I]epibatidine (specific activity, 2200 Ci/mmol; NEN, Boston, MA) in binding buffer for 2 hr. Some sections were treated with 500 pm [¹²⁵I]epibatidine plus 100 nm cold cytisine (Sigma) as competitor. Sections were subsequently washed in binding buffer, rinsed in water, dried, and exposed to Biomax film (Eastman Kodak, Rochester, NY) for 1 and 16 hr. Quantification of binding was performed as described previously (Broide et al., 2002). Statistical significance of the difference in signal density of the nicotine versus saline treatment groups was determined by Student's t test, as well as two-way ANOVA.

Results

β4-/- but not β2-/- mice show decreased nicotine withdrawal signs

During the 20 min of observation after mecamylamine injection, control mice chronically treated with nicotine exhibited significantly more somatic signs of withdrawal than saline-treated mice (Fig. 1A). After mecamylamine injection, nicotine-treated β2-/- mice displayed withdrawal scores similar to those of nicotine-treated control mice. Saline-treated β2-/- mice showed no difference compared with saline-treated controls (Fig. 1A). In contrast, after mecamylamine injection, nicotine-treated β4-/- mice showed a decreased amount of symptoms when compared with nicotine-treated control or β2-/- mice (Fig. 1A). The behavior of saline-treated β4-/- mice was indistinguishable from that of saline-treated wild-type and nicotine-treated β4-/- mice (25 ± 6, 22 ± 3, and 21 ± 3 signs, respectively). When each of the four major withdrawal symptoms (shaking, grooming, scratching, and chewing) were studied separately, β4-/- mice scored at lower levels in every case (Fig. 1B). Some behaviors such as “hindpaw digging,” jumping, and cage scratching were seen in very few mice. These behaviors were unique to wild-type and β2-/- mice after mecamylamine, were never observed before mecamylamine injection, and did not appear in β4-/- mice.

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Figure 1.

Mecamylamine-induced withdrawal signs in wild-type, β4-/-, and β2-/- mice. A, Total nicotine withdrawal signs in control mice (+/+, black bars) treated with saline (n = 6) or nicotine (n = 21), β4-/- mice (white bars) treated with saline (n = 4) or nicotine (n = 18), and β2-/- mice (gray bars) treated with saline (n = 7) or nicotine (n = 7) during the 20 min immediately after mecamylamine injection. B, Comparison of different withdrawal signs between wild-type (black bars) and β4-/- mice (white bars). The Other bar refers to other symptoms such as leg shakes, head nods, rear leg digging, and cage scratching. ^*p < 0.05 versus saline for the same genotype (ANOVA and Newman-Keuls post hoc comparisons) (A) versus +/+ (B).

To verify whether nonsomatic signs of withdrawal are also altered in β4-/- mice, we conducted hot-plate and tail-flick experiments. As expected (Damaj et al., 2003), nicotine withdrawal elicited hyperalgesia in the hot-plate test. The latency to lick or jump was 11.9 ± 1.2 sec for saline-treated β4+/+ mice and 8.0 ± 1.0 sec for nicotine-treated β4+/+ mice (n = 7 and 8, respectively; p < 0.05). In contrast, in β4-/- mice, nicotine did not produce hyperalgesia. The latency to lick or jump was 9.4 ± 1.6 sec in saline-treated β4-/- mice and 15.6 ± 2.1 sec in nicotine-treated β4-/- mice (n = 8 and 5, respectively). These results demonstrate that both somatic and nonsomatic signs of nicotine withdrawal are affected in the β4-/- mice. In the tail-flick test, the same trend was observed, but the data did not reach statistical significance in any of the experimental conditions tested.

Upregulation of epibatidine binding does not correlate with the onset of withdrawal signs

Immediately after withdrawal symptoms were scored for each mouse, brains were harvested, rapidly dissected, and frozen to perform [¹²⁵I]epibatidine binding on brain sections. As expected, epibatidine binding was increased during nicotine treatment in control mice (Marks et al., 1983; Schwartz and Kellar, 1983). This increase occurred in brain regions such as cortex, striatum, and hippocampus (Fig. 2A,C,H). In β4-/- mice, epibatidine binding was also increased by chronic nicotine treatment, and binding levels were indistinguishable from those of control mice in areas dominated by α4β2 type receptors, including cortex, striatum, and thalamus (Fig. 2B,D,H). The medial habenula (MHb) and the interpenduncular nucleus (IPN) of β4-/- mice showed lower epibatidine binding than in control mice, because these regions express mainly α3β4 type receptors (Nguyen et al., 2003). To discern between α4β2-containing and non-α4β2-containing nAChRs, we competed the binding of epibatidine with an excess of cold cytisine. Under these conditions, epibatidine binds mainly to α6/β3- and α3/β4-containing receptors. As expected, β4-/- mouse brains contained much lower epibatidine binding activity after cytisine competition in the MHb and IPN (Fig. 2F,G). However, those lower levels of cytisine-competed epibatidine binding were unaffected by nicotine treatment (data not shown). Finally, as previously documented (Picciotto et al., 1995), epibatidine binding was significantly decreased in most areas of β2-/- mouse brains. As shown in Figure 2E, epibatidine binding was only visible on the MHb, medial habenula, superior colliculus, and other regions rich in β4-containing receptors.

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Figure 2.

Epibatidine binding in wild-type and β4-/- mouse brain sections. A-E, MHb level. F, G, IPN level. A, Saline-treated (Veh) β4+/+ mouse. B, Saline-treated (Veh) β4-/- mouse. C, Nicotine-treated (Nic) β4+/+ mouse. D, Nicotine-treated (Nic) β4-/- mouse. E, β2-/- mouse. F, Saline-treated (Veh) β4+/+ mouse; epibatidine binding has been competed with cytisine. G, Saline-treated (Veh) β4-/- mouse; epibatidine binding has been competed with cytisine. H, Quantification of epibatidine binding in striatum (Str), prefrontal cortex (PFC), and dentate gyrus (DG). n = 3-6 per group. ^*p < 0.05; ^#p < 0.1; Student's t test versus corresponding saline. A two-factor ANOVA analysis of the data indicated that there was a significant effect of genotype (Str, PFC, and DG, p < 0.05) and treatment (Str and PFC, p < 0.01; DG, p < 0.05) but no treatment × genotype interaction (p > 0.6 in all cases), confirming that β4-/- mice up regulate epibatidine binding sites during chronic nicotine treatment. Cx, Cortex; SC, superior colliculus.

Discussion

We have shown that the lack of β4-containing nAChRs is enough to substantially decrease the somatic signs of mecamylamine-induced nicotine withdrawal. In addition, the β4 null mutation reduces hyperalgesia, a nonsomatic sign observed during nicotine withdrawal. Interestingly, β2-containing nAChRs, which have been implicated in nicotine self-administration (Picciotto et al., 1998), do not seem to influence the onset of nicotine withdrawal symptoms.

Like other drugs of addiction, initial nicotine use produces positive affective states, but its prolonged use triggers neuroadaptations that contribute to both the phenomenon of dependence and the negative reinforcement aspects of nicotine use (Laviolette and van der Kooy, 2004). Our results imply that the nAChR subtypes that mediate the positive-reinforcing effects of nicotine might be different from those mediating its negative-reinforcing effects. This is of considerable interest because withdrawal symptoms are among the major causes of failure when smokers try to quit (West et al., 1989), and therapeutic measures toward the minimization of withdrawal symptoms might aid in smoking cessation.

In mice, nicotine withdrawal produces somatic signs such as head shakes and paw tremors, as well as nonsomatic signs such as hyperalgesia and anxiety-like responses in the elevated plus maze (Damaj et al., 2003). Mecamylamine, which has a slightly higher selectivity for α3β4-containing nAChRs than for other α/β combinations (Papke et al., 2001), is the most effective antagonist at precipitating both somatic and nonsomatic withdrawal signs. Dihydro-β-erythroidine, a relatively selective antagonist for α4β2-containing nAChRs, can precipitate anxiety-like responses, and methyllycaconitine, an α7-containing nAChR antagonist, selectively precipitates hyperalgesia (Damaj et al., 2003). In rats also, mecamylamine precipitates both somatic and nonsomatic withdrawal signs (Malin et al., 1994). These data, together with our findings, confirm the dominant role of β4- containing nAChRs in mediating the negative-reinforcing properties of nicotine.

A major open question is where does mecamylamine act and which neural circuits underlie nicotine withdrawal. In humans, nicotine withdrawal is characterized by bradycardia, gastrointestinal discomfort, insomnia, increased appetite, (Hughes et al., 1991), depressed mood, irritability, anxiety, frustration, difficulty concentrating, and craving for tobacco (Lewis, 1996), suggesting that both peripheral and central mechanisms contribute to the withdrawal syndrome. Studies conducted in rodents also indicate that the signs of nicotine withdrawal have a central and a peripheral component (Watkins et al., 2000). The mesolimbic dopamine nuclei implicated in the positive-reinforcing effects of nicotine are among the candidate areas mediating some of the centrally mediated aversive effects of withdrawal, because injection of mecamylamine in the ventral tegmental area (VTA) has been shown to elicit withdrawal signs such as gasps, teeth chatter, and hypolocomotion in rats (Hildebrand et al., 1999). However, β4-containing nAChRs are expressed at relatively low densities in the VTA (Klink et al., 2001; Salas et al., 2003a), suggesting that β4-containing nAChRs in other brain areas might be important for nicotine withdrawal. Indeed, besides the olfactory bulb and pineal gland, the β4 subunit is expressed in the CNS at high levels only in the MHb and the IPN (Quick et al., 1999; Klink et al., 2001; Salas et al., 2003a), which are part of the dorsal diencephalic system (Sutherland, 1982). Rich projections from and to the diencephalic system might explain the effect of β4-containing nAChRs on the mesolimbic system. For example, the MHb sends projections to the IPN but also to the VTA, while receiving projections from the nucleus accumbens. In addition, the IPN sends projections to the raphe nuclei, which in turn projects to the VTA (Maisonneuve and Glick, 2003).

The peripheral symptoms of nicotine deprivation are likely to be mediated by the autonomic nervous system, and the β4 subunit is highly expressed in both sympathetic and parasympathetic ganglia (Xu et al., 1999; De Biasi, 2002). The aspects of autonomic function studied so far in the β4-/- mice indicate that the absence of β4 alone does not alter organ function in basal, non-challenged conditions (Xu et al., 1999; Salas et al., 2003a; Wang et al., 2003). However, β4-mediated mechanisms might become relevant during chronic nicotine exposure and/or mecamylamine-induced withdrawal.

Other nAChR subunits are probably implicated in nicotine withdrawal symptoms. α3 heterozygous and α5 null mice have a phenotype similar to that of β4-/- mutants in that they are less sensitive to acute nicotine than controls (Salas et al., 2003b, 2004; Kedmi et al., 2004). In addition, the α3 and α5 subunits are coexpressed with β4 in the MHb, the IPN, and peripheral ganglia. Therefore, it is likely that these two subunits are also involved in the mechanisms of nicotine withdrawal. The role of the α6, α7, and β3 subunits in nicotine withdrawal is yet to be determined. These subunits are expressed in the VTA (Franceschini et al., 2002; Salas et al., 2003a; Wooltorton et al., 2003), which puts them in an ideal position to affect the rewarding effects of nicotine.

One of the most extensively studied effects of long-term nicotine in humans and rodents is the upregulation of receptor expression and function during nicotine treatment (Marks et al., 1983; Schwartz and Kellar, 1983; Nguyen et al., 2003). This effect, which occurs mainly for β2-containing nAChRs, has been hypothesized to be critical for the development of nicotine addiction. Epibatidine binds to several nAChR populations in the brain, including α4β2- and α3β4-containing receptors. We have shown that epibatidine binding is upregulated in β4-/- mice, but these mice show a decrease in the somatic as well as nonsomatic signs of withdrawal. Conversely, β2-/- mice, which lack α4β2 nAChRs, do undergo withdrawal. In addition, the β4 component of epibatidine binding in the brain is not upregulated by nicotine (Nguyen et al., 2003), suggesting that nAChR upregulation is not critical for the appearance of nicotine withdrawal symptoms.

In conclusion, we have shown that the β4 subunit of nAChRs is critical for nicotine withdrawal symptoms in mice, whereas the β2 subunit is not. These data can be of importance in the effort to develop new and more effective drugs to help smoking cessation.