Research report
Performance of α7 nicotinic receptor null mutants is impaired in appetitive learning measured in a signaled nose poke task

https://doi.org/10.1016/j.bbr.2005.03.004Get rights and content

Abstract

Wild-type and mutant mice lacking expression of α5, α7, β2, β3, or β4 neuronal nicotinic cholinergic receptors (nAChRs) were compared on a signaled nose poke task, a multi-phased task used to measure appetitive learning and impulsivity. In the early phases of training, mutants of all nicotinic lines did not differ compared to wild types in the days to reach criterion when mice were required to nose poke for a sucrose reward on FR1 or FR3 schedules, or in their ability to respond to an auditory clicker to receive a sucrose reward. However, mutants lacking α7 nAChRs, but not lines lacking other nAChRs, showed impairments when task difficulty was increased such that an auditory stimulus was presented on a variable schedule and mice were required to withhold their responses until the presentation of the auditory cue to obtain a reward. α7 mutants were impaired compared to wild types in appetitive learning as measured by the percentage of conditioned responses but overcame their deficits with extensive training for 10 days. However, when efficiency ratios were used to measure impulsivity, α7 mutants exhibited lower efficiency ratios even after 10 days of training. These results support a role of the α7 nicotinic receptor in mediating appetitive learning and suggest a potential role for the α7 nAChRs in the regulation of behavioral disinhibition.

Introduction

Nicotine binds with high affinity to neuronal nicotinic cholinergic receptors (nAChRs) throughout the brain and produces a wide range of behavioral responses including enhancement of learning and memory in a number of different animal species [23], [24], [25], [26], [40], [65]. Eleven nAChR subunits (α2–α7, α9, α10, and β2–β4) have been identified within the mammalian brain, with α4β2 and α7-type nAChRs being the most widely expressed subtypes of nAChRs [41].

While nAChRs are clearly required for nicotine to have its behavioral effects, the role of specific nAChR subtypes in regulating nicotine's behavioral effects as well as the roles of these receptors in regulating behavior in the absence of nicotine is not clear. The availability of null mutant mice that lack the expression of specific nAChRs allows the examination of the potential roles of the individual nAChR subtypes in behavioral processes including learning and memory. Examinations of nicotinic receptor subunit null mutants have demonstrated that β2-containing receptors are required for nicotine enhancement of avoidance learning [53], for normal responses to the discriminative stimulus and taste aversion properties of nicotine [62], and nicotine enhancement of contextual fear conditioning [67]. β2-containing nicotinic receptors also participate in mediating contextual learning when training conditions are more difficult [11], [67], passive avoidance learning [33] and some behavioral processes thought to measure executive function [27]. Thus, β2-containing receptors participate in the regulation of several forms of complex learning but less is known concerning the roles of other nAChRs in regulating behavior.

Behavioral studies of cognitive processes and attention also have been conducted using nicotinic antagonists to explore the role of nAChRs [3], [7], [28], [38], [39]. These studies using the nicotinic receptor antagonists dihydro-β-erythroidine (DHβE) and methyllycaconitine (MLA) have implicated β2- and α7-containing receptors in nicotine enhancement of cognition in some forms of learning [39] but these drugs may not be specific for either β2 or α7 subtypes, respectively [32], [48].

Recent studies have shown that α7 nAChRs participate in the mediation of sensitivity to the cognitive impairing effects of ethanol [67], but α7 null mutants are not impaired in contextual learning despite the high levels of expression of α7 in the hippocampus [52], [67]. In addition to its localization in brain regions regulating some types of memory, α7 receptors are unique among nAChRs in that they can function as homomers and have both presynaptic and postsynaptic localizations [44], [57], whereby they modulate the release of glutamate and play a role in synaptic transmission of dopamine, GABA, and glutamate [4], [17]. Additional interest in whether α7 nAChRs mediate behavioral processes also stems from the fact that human genetic studies have found an association of mutations in the α7 gene in families with schizophrenia [18]. Given that schizophrenics exhibit cognitive deficits, the potential role of α7 receptors remains an important issue [68].

Multiple other nAChRs are expressed as heteromers composed of α and β subunits in brain regions known to regulate cognitive processes such as prefrontal cortex, hippocampus, amygdala, and nucleus accumbens [16], [34], [43], [51], [66], [69], [70]. These other nAChRs have presynaptic localizations and function to modulate the release of several neurotransmitters known to be important for learning and memory processes including acetylcholine, dopamine, and norepinephrine ([4], see review [71]). Multiple nAChR subunits can also be expressed in the same neurons. For example, rat basal forebrain cholinergic neurons express α4, α7, and β2 mRNAs [5], mouse GABAergic interneurons in the dorsal hippocampus show immunoreactivity to antibodies to α3, α4, α5, and α7 in addition to β2 and β4 [20], and mouse striatal dopaminergic terminals express as many as four different functional nAChRs [59].

Given that many of the nAChRs may be involved in modulation of cognitive processes, the present study investigated the potential role of several nicotinic receptor subtypes in appetitive learning, measured in a signaled nose poke task. The signaled nose poke task, originally developed by Steinmetz et al. [64] as a barpressing task in rats to measure appetitive learning, was later modified by Logue et al. [42] for mice. Logue et al. [42] and Bowers and Wehner [8] used this task to provide measures of not only appetitive learning, but also behavioral impulsivity. Previous studies of smokers suggest that they exhibit increased impulsivity [21], [47] making this task useful to address whether any of the nAChRs are involved in regulation of appetitive learning as well as behavioral impulsivity.

While the present studies were being conducted, Young et al. [73] demonstrated a role of the α7 nAChR in regulation of sustained attention measured in a version of the 5-choice serial reaction-time (5-CSRT) task [12], another appetitive task affected by nicotine [7], [28], [29], [46], [73]. Whether other nicotinic receptor subtypes in addition to α7 nAChRs participate in the regulation of sustained attention was not determined.

Section snippets

Subjects

In the present study, wild-type and homozygous null mutant mice from each of the α5, α7, β2, β3, and β4 nAChR mouse lines were tested in the signaled nose poke task. A detailed description of the construction of each of these mutant mouse lines and genotyping reactions is available elsewhere [13], [50], [55], [58], [72]. Colonies of each mouse line were established and maintained by heterozygote breeding at the Institute for Behavioral Genetics at the University of Colorado (Boulder, CO). With

Performance of nicotinic null mutant lines on phases 1–3

nAChRs are present throughout the brain and are found in high concentrations in regions that may mediate learning and memory processes as well as reward processes [54]. While previous studies of other forms of learning suggested that α7 and β2-containing receptors would also be likely candidates for mediation of appetitive learning, it is possible that several nAChRs could be involved in the mediation of performance in the signaled nose poke task. Thus, the performances of wild types and

A role for α7 receptors in appetitive learning

These results demonstrate: (1) α7 nAChRs are involved in the regulation of appetitive learning and (2) receptors containing the α5, β2, β3, and β4 subunits do not mediate appetitive learning. Mutants lacking α7 expression showed impaired performance on phase 4 of the signaled nose poke task compared to their wild-type littermates. Since α7 mutants did not significantly differ from wild types, or from other mouse lines with various nicotinic receptor modifications, in phases 1–3, it appears that

Acknowledgements

This work was funded by CTRP grant (2RR-033), AA13018 to J.M.W. and a Conte Center grant to Dr. Robert Freedman (P50 MH068582). We thank Dr. Allan C. Collins for helpful discussions and for providing mice from the NIDA-supported nicotinic mutant colonies (DA15663) as well as Dr. Arthur Beaudet, Dr. Marina Picciotto, and Dr. Steven Heinemann for providing breeding pairs of null mutants to the breeding facility. We thank Estaban Loetz and Jill Miyamoto for assistance in breeding and genotyping

References (73)

  • T.J. Gould et al.

    Nicotine enhancement of contextual fear conditioning

    Behav Brain Res

    (1999)
  • A.J. Grottick et al.

    Effect of subtype selective nicotinic compounds on attention as assessed by the five-choice serial reaction time task

    Behav Brain Res

    (2000)
  • B. Hahn et al.

    Attentional effects of nicotinic agonists in rats

    Neuropharmacology

    (2003)
  • M.R. Picciotto et al.

    Neuronal nicotinic acetylcholine receptor subunit knockout mice: physiological and behavioral phenotypes and possible clinical implications

    Pharmacol Ther

    (2001)
  • B. Schilstrom et al.

    Nicotine and food-induced dopamine release in the nucleus accumbens of the rat: putative role of α7 nicotinic receptors in the ventral tegmental area

    Neuroscience

    (1998)
  • M. Shoaib et al.

    The role of nicotinic receptor beta-2 subunits in nicotine discrimination and conditioned taste aversion

    Neuropharmacology

    (2002)
  • K.E. Stevens et al.

    Genetic correlation of inhibitory gating of hippocampal auditory evoked response and alpha-bungarotoxin-binding nicotinic cholinergic receptors in inbred mouse strains

    Neuropsychopharmacology

    (1996)
  • I.P. Stolerman et al.

    Nicotine in an animal model of attention

    Eur J Pharmacol

    (2000)
  • J.M. Wehner et al.

    Role of neuronal nicotinic receptors in the effects of nicotine and ethanol on contextual fear conditioning

    Neuroscience

    (2004)
  • D.R. Weinberger et al.

    Neurobiology of schizophrenia and the role of atypical antipsychotics

    Biol Pyschiatry

    (2001)
  • S. Wonnacott

    Presynaptic nicotinic ACh receptors

    Trends Neurosci

    (1997)
  • M. Alkondon et al.

    Neuronal nicotinic acetylcholine receptor activation modulates gamma-aminobutyric acid release from CA1 neurons of rat hippocampal slices

    J Pharmacol Exp Ther

    (1997)
  • J.M. Birrell et al.

    Medial frontal cortex mediates perceptual attentional set shifting in the rat

    J Neurosci

    (2000)
  • A. Blondel et al.

    Characterisation of the effects of nicotine in the five-choice serial reaction time task in rats: antagonist studies

    Psychopharmacology

    (2000)
  • B.J. Bowers et al.

    Ethanol consumption and behavioral impulsivity are increased in protein kinase Cγ null mutant mice

    J Neurosci

    (2001)
  • B.J. Bowers et al.

    Deletion of the α7 nicotinic receptor subunit gene results in increased sensitivity to several behavioral effects produced by alcohol

    Alcoholism: Clin Exp Res

    (2005)
  • A.E. Bullock et al.

    Inbred mouse strains differ in the regulation of startle and prepulse inhibition of the startle response

    Behav Neurosci

    (1997)
  • C. Cui et al.

    The beta3 receptor subunit: a component of the alpha-conotoxin MII binding nAChRs which modulate dopamine release and related behaviors

    J Neurosci

    (2003)
  • K.Y. Dineley et al.

    β-Amyloid activated the mitogen-activated protein kinase cascade via hippocampal α7 nicotinic acetylcholine receptors: in vitro and in vivo mechanisms related to Alzheimer's disease

    J Neurosci

    (2001)
  • C.J. Frazier et al.

    Acetylcholine activates an alpha-bungarotoxin-sensitive nicotinic current in rat hippocampal interneurons, but not pyramidal cells

    J Neurosci

    (1998)
  • R. Freedman et al.

    Linkage of a neurophysiological deficit in schizophrenia to a chromosome 15 locus

    Proc Natl Acad Sci USA

    (1997)
  • L.C. Gahring et al.

    Mouse strain-specific nicotinic receptor expression by inhibitory interneurons and astrocytes in the dorsal hippocampus

    J Comp Neurol

    (2004)
  • D.G. Gilbert et al.

    Personality, psychopathology, and nicotine response as mediators of the genetics of smoking

    Behav Genet

    (1995)
  • P.S. Goldman-Rakic

    The prefrontal landscape: implications of functional architecture for understanding human mentation and the central executive

    Philos Trans R Soc Lond B Biol Sci

    (1996)
  • T.J. Gould et al.

    Nicotine enhances latent inhibition and ameliorates ethanol-induced deficits in latent inhibition

    Nicotine Tob Res

    (2001)
  • T.J. Gould et al.

    Nicotine enhances contextual fear conditioning and ameliorates ethanol-induced deficits in contextual fear conditioning

    Behav Neurosci

    (2003)
  • Cited by (55)

    • Nicotine improves probabilistic reward learning in wildtype but not alpha7 nAChR null mutants, yet alpha7 nAChR agonists do not improve probabilistic learning

      2018, European Neuropsychopharmacology
      Citation Excerpt :

      This released dopamine acts preferentially on dopamine D1 receptors (Wonnacott et al., 2005). Consistent with this observation, mice lacking ɑ7 nAChR expression (knockouts; KO), exhibit impaired reward learning (Young et al., 2011,2004; Keller et al., 2005), but normal aversive/punishment associative learning (Young et al., 2011; Paylor et al., 1998). In addition, activation of ɑ7 nAChRs via the full agonist AR-R-17779 improved within-session learning in the radial-arm maze (Levin et al., 1999).

    • NAChR dysfunction as a common substrate for schizophrenia and comorbid nicotine addiction: Current trends and perspectives

      2016, Schizophrenia Research
      Citation Excerpt :

      As impulsivity traits are evident during adolescence (Romer, 2010), and reduced striatal DA D2/D3 receptor function has been linked to trait impulsivity (Dalley et al., 2007), it is possible that dysregulated mesolimbic DA signaling may contribute to increased smoking initiation and restore dopaminergic signaling via α4β2 nAChRs. It must be noted that α7 nAChRs are also known to regulate behavioral inhibition (Keller et al., 2005). Likewise, a polymorphism in CHRNA5/A3/B4 gene cluster is also linked to impulsivity (Stephens et al., 2012).

    • The Role of Nicotine in Schizophrenia

      2015, International Review of Neurobiology
    View all citing articles on Scopus
    View full text