Increased neurodegeneration during ageing in mice lacking high-affinity nicotine receptors

Nicotinic receptor function: new perspectives from knockout mice

Knockout mice, in which one or more genes of interest are silenced, provide unique opportunities to analyse diverse aspects of gene function in vivo. In particular, the contribution of the encoded protein(s) in complex behaviours can be assessed. Since the first targeted disruption in 1995 of the gene encoding the beta2-subunit of the nicotinic acetylcholine receptor (nAChR), all but a few of the mammalian nAChR subunits have been disrupted (i.e. alpha7, alpha4, alpha3, alpha9, beta4 and beta3). Recent advances brought by genetically modified mice to our understanding of the endogenous composition and role of nAChRs in the nervous system, and of the diverse pharmacological actions of nicotine regarding learning, analgesia, reinforcement, development and aging in the brain will be discussed.

Acetylcholine receptors and myasthenia

Much progress has been made in the 26 years since initial studies of the first purified acetylcholine receptors (AChRs) led to the discovery that an antibody-mediated autoimmune response to AChRs causes the muscular weakness and fatigability characteristic of myasthenia gravis (MG) and its animal model, experimental autoimmune myasthenia gravis (EAMG). Now, the structure of muscle AChRs is much better known. Monoclonal antibodies to muscle AChRs, developed as model autoantibodies for studies of EAMG, were used for initial purifications of neuronal AChRs, and now many homologous subunits of neuronal nicotinic AChRs have been cloned. There is a basic understanding of the pathological mechanisms by which autoantibodies to AChRs impair neuromuscular transmission. Immunodiagnostic assays for MG are used routinely. Nonspecific approaches to immunosuppressive therapy have been refined. However, fundamental mysteries remain regarding what initiates and sustains the autoimmune response to muscle AChRs and how to specifically suppress this autoimmune response using a practical therapy. Many rare congenital myasthenic syndromes have been elegantly shown to result from mutations in muscle AChRs. These studies have provided insights into AChR structure and function as well as into the pathological mechanisms of these diseases. Evidence has been found for autoimmune responses even to some central nervous system neurotransmitter receptors, but only one neuronal AChR has so far been implicated in an autoimmune disease. Thus far, only two neuronal AChR mutations have been found to be associated with a rare form of epilepsy, but many more neuronal AChR mutations will probably be found to be associated with disease in the years ahead.

Nicotinic acetylcholine receptor knockout mice as animal models for studying receptor function

Nicotinic acetylcholine receptors are pentameric ligand-gated ion channels, which are involved in a wide range of neuronal functions. During the past decade, a large number of nicotinic acetylcholine receptor subunits have been cloned and showed a discreet yet overlapping distribution pattern. Recently, several groups have produced mutant mice lacking specific nicotinic acetylcholine receptor subunits. In this review, we focus on how the study of these knockout mouse models has advanced our understanding of the role individual nicotinic acetylcholine receptor subunits play in the function and composition of endogenous receptors and the diverse pharmacological actions of nicotine in the mammalian nervous system.

Structural aspects of high affinity ligands for the alpha 4 beta 2 neuronal nicotinic receptor

Neuronal nicotinic acetylcholine receptors (nAChRs) are a heterogeneous family of related ion channels that are widely distributed throughout the central and peripheral nervous systems. They all share a common architecture of five subunit proteins that combine at the cell surface to create a ligand-gated cation permeable pore. Significant effort is currently being expended by medicinal chemistry teams to synthesize ligands that exhibit selectivity for central over peripheral nAChR subtypes. Within the CNS, multiple nAChR subtypes are recognized, and the discovery of ligands exhibiting selectivity among these subtypes offers an opportunity for the development of novel therapeutic agents. The alpha 4 beta 2 subtype is one of the most abundant nAChR subtypes within the CNS, and has been the primary focus of high affinity ligand design. Nicotine (1), and more recently, epibatidine (2) have served as structural templates for the design of the majority of active compounds. Although the diversity of nAChR ligands is growing, the structural requirements necessary for high affinity binding with the alpha 4 beta 2 receptor remain poorly understood. The putative pharmacophoric elements common to all potent alpha 4 beta 2 ligands include (1) a basic or quaternized nitrogen atom, and (2) a less basic nitrogen or a carbonyl oxygen that presumably interact with electron rich and electron deficient sites on the receptor, respectively. The family of currently known high affinity analogs consists of a diverse array of azacycles containing a basic amine. Several additional basic amine fragments have been identified, including the pyrrolizidine nucleus (exemplified by 8) and the 2-azabicyclo[2.2.1]heptane skeleton (exemplified by 9). In addition, we have found that the furo[2,3-b]pyridine heterocycle (compound 10) serves as useful bioisosteric replacement for the pyridyl substituent of nicotine. A preliminary pharmacophore model is proposed in which a reasonable superposition of the putative pharmacophoric elements of the diverse array of high affinity ligands for the alpha 4 beta 2 nAChR reported herein may be accommodated.

Neurotrophic effects of central nicotinic receptor activation

A growing number of data have shown that compounds interacting with neuronal nicotinic acetylcholine receptors (nAChRs) have, both in vivo and in vitro, the potential to be neuroprotective and that treatment with nAChR agonists elicit long-lasting improvement of cognitive performance in a variety of behavioural tests in rats, monkeys and humans. Epidemiological and clinical studies suggested also a potential neuroprotective/trophic role of (-)-nicotine in neurodegenerative disease, such as Alzheimer's disease and Parkinson's disease. This neuroprotective/trophic role of nAChR activation has been mainly mediated by alpha7 and alpha4beta2 nAChR subtypes, as evidenced using selective nAChR antagonists, and by potent nAChR agonists recently found displaying efficacy and/or larger selective affinities than (-)-nicotine for neuronal nAChR subtypes. A neurotrophic factor gene regulation by nAChR signalling has been taken into consideration as a possible mechanism involved in neuroprotective/trophic effects of nAChR activation and has given evidence that the fibroblast growth factor (FGF-2) gene is a target for nAChR signalling. These findings suggested that FGF-2 could be involved, in view of its neurotrophic functions, in nAChR mechanisms mediating neuronal survival, trophism and plasticity.

Adolescent nicotine exposure causes persistent upregulation of nicotinic cholinergic receptors in rat brain regions

Whereas numerous studies have explored the consequences of fetal or adult nicotine exposure, little or no basic research has been conducted for nicotine exposure during adolescence, the developmental period in which regular cigarette use typically begins. We administered nicotine to adolescent rats on postnatal days 30-47 via continuous infusion with implanted osmotic minipumps, using a dose rate (3-6 mg kg-1 day-1) set to achieve plasma nicotine levels found in smokers; results were compared to exposure of adult rats. During and after exposure, we assessed nicotinic cholinergic receptor binding in the midbrain, cerebral cortex, and hippocampus, using [3H]cytisine. Robust receptor upregulation was observed with both adolescent and adult nicotine exposure but there were major differences in the regional specificity and persistence of effect. In adolescents, upregulation was uniform across all regions during the infusion period, whereas in adults, there was a distinct regional hierarchy: midbrain < cerebral cortex < hippocampus; accordingly, receptors in the adolescent midbrain were upregulated far more than with adult exposure. In addition, adolescent nicotine treatment produced long-lasting effects on the receptors, with significant increases still apparent in male rats 1 month after the termination of drug exposure. We also obtained evidence for hippocampal cell damage in adolescent female rats exposed to nicotine, characterized by increases in total membrane protein concentration indicative of a decrease in overall cell size. Adolescent nicotine exposure thus elicits region- and gender-selective effects that differ substantially from those in adults, effects that may contribute to increased addictive properties and lasting deficits in behavioral performance.

Multiorgan autonomic dysfunction in mice lacking the beta2 and the beta4 subunits of neuronal nicotinic acetylcholine receptors

Transcripts for the beta2 and the beta4 nicotinic acetylcholine receptor (nAChR) subunits are found throughout the CNS and the peripheral nervous system. These two beta subunits can form heteromultimeric channels with any of the alpha2, alpha3, alpha4, or alpha5 subunits in heterologous expression systems. Nonetheless, the subunit composition of native nAChRs and the role of different nAChR subtypes in vivo remain unclear. We prepared null mutations for the beta2 and the beta4 genes and bred beta2-/-beta4-/- mice by mating mice of identical beta2-/-beta4+/- or beta2+/-beta4-/- genotype. The beta2-/- and the beta4-/- single-mutant mice grow to adulthood with no visible phenotypic abnormalities. The beta2-/-beta4-/- double mutants survive to birth but have impaired growth and increased perinatal mortality. They also present enlarged bladders with dribbling urination and develop urinary infection and bladder stones. The ocular pupils are widely dilated and do not constrict in response to light. Histological studies revealed no significant abnormalities of brain or peripheral tissues except for hyperplasia in the bladder mucosa of beta4-/- and beta2-/-beta4-/- mutants. Bladder strips from beta2-/-beta4-/- mice did not respond to nicotine but contracted when stimulated with a muscarinic agonist or electric field stimulation. Bladder strips from beta4 mutants did not respond to nicotine despite the absence of major bladder dysfunction in vivo. Acetylcholine-activated whole-cell currents were absent in superior cervical ganglion neurons from beta2-/-beta4-/- mice and reduced in neurons from beta4-/- mice. Although there is apparent redundancy and a superficially normal phenotype in beta2-/- and beta4-/- mice, physiological studies indicate major deficits in the beta4-/- mice. Our previous description of a similar phenotype in alpha3-/- mice and the current data suggest that the alpha3 and the beta4 subunits are major components in autonomic nAChRs. The phenotype of the beta2-/-beta4-/- and alpha3-/- mice resembles the autosomal recessive megacystis-microcolon-hypoperistalsis syndrome in humans.

Multiorgan autonomic dysfunction in mice lacking the beta2 and the beta4 subunits of neuronal nicotinic acetylcholine receptors

Transcripts for the beta2 and the beta4 nicotinic acetylcholine receptor (nAChR) subunits are found throughout the CNS and the peripheral nervous system. These two beta subunits can form heteromultimeric channels with any of the alpha2, alpha3, alpha4, or alpha5 subunits in heterologous expression systems. Nonetheless, the subunit composition of native nAChRs and the role of different nAChR subtypes in vivo remain unclear. We prepared null mutations for the beta2 and the beta4 genes and bred beta2-/-beta4-/- mice by mating mice of identical beta2-/-beta4+/- or beta2+/-beta4-/- genotype. The beta2-/- and the beta4-/- single-mutant mice grow to adulthood with no visible phenotypic abnormalities. The beta2-/-beta4-/- double mutants survive to birth but have impaired growth and increased perinatal mortality. They also present enlarged bladders with dribbling urination and develop urinary infection and bladder stones. The ocular pupils are widely dilated and do not constrict in response to light. Histological studies revealed no significant abnormalities of brain or peripheral tissues except for hyperplasia in the bladder mucosa of beta4-/- and beta2-/-beta4-/- mutants. Bladder strips from beta2-/-beta4-/- mice did not respond to nicotine but contracted when stimulated with a muscarinic agonist or electric field stimulation. Bladder strips from beta4 mutants did not respond to nicotine despite the absence of major bladder dysfunction in vivo. Acetylcholine-activated whole-cell currents were absent in superior cervical ganglion neurons from beta2-/-beta4-/- mice and reduced in neurons from beta4-/- mice. Although there is apparent redundancy and a superficially normal phenotype in beta2-/- and beta4-/- mice, physiological studies indicate major deficits in the beta4-/- mice. Our previous description of a similar phenotype in alpha3-/- mice and the current data suggest that the alpha3 and the beta4 subunits are major components in autonomic nAChRs. The phenotype of the beta2-/-beta4-/- and alpha3-/- mice resembles the autosomal recessive megacystis-microcolon-hypoperistalsis syndrome in humans.

The alpha7 nicotinic acetylcholine receptor in neuronal plasticity

A growing body of evidence indicates that neuronal nicotinic acetylcholine receptors (nAChRs), in addition to promoting fast cholinergic transmission, may modulate other neuronal activities within the central nervous system (CNS). In particular, the alpha7 nAChR is highly permeable to Ca2+ and may serve a distinct role in regulating neuronal plasticity. By elevating intracellular Ca2+ levels in discrete neuronal locations, these ligand-gated ion channels may influence numerous physiological processes in developing and adult CNS. In this article, we review evidence that both pre- and postsynaptic alpha7 nAChRs modulate transmitter release in the brain and periphery through Ca2+-dependent mechanisms. The possible role of alpha7 nAChRs in regulating neuronal growth and differentiation in developing CNS is also evaluated. We consider an interaction between cholinergic and glutamatergic transmission and propose a hypothesis on the possible coregulation of intracellular Ca2+ by N-methyl-D-aspartate (NMDA) receptors and alpha7 nAChRs. Finally, the clinical significance of alterations in the normal function of alpha7 nAChRs is discussed as it pertains to prenatal nicotine exposure, schizophrenia, and epilepsy.