M. De Biasi

Evaluating the suitability of nicotinic acetylcholine receptor antibodies for standard immunodetection procedures.

Nicotinic acetylcholine receptors play important roles in numerous cognitive processes as well as in several debilitating central nervous system (CNS) disorders. In order to fully elucidate the diverse roles of nicotinic acetylcholine receptors in CNS function and dysfunction, a detailed knowledge of their cellular and subcellular localizations is essential. To date, methods to precisely localize nicotinic acetylcholine receptors in the CNS have predominantly relied on the use of anti‐receptor subunit antibodies. Although data obtained by immunohistology and immunoblotting are generally in accordance with ligand binding studies, some discrepancies remain, in particular with electrophysiological findings. In this context, nicotinic acetylcholine receptor subunit‐deficient mice should be ideal tools for testing the specificity of subunit‐directed antibodies. Here, we used standard protocols for immunohistochemistry and western blotting to examine the antibodies raised against the α3‐, α4‐, α7‐, β2‐, and β4‐nicotinic acetylcholine receptor subunits on brain tissues of the respective knock‐out mice. Unexpectedly, for each of the antibodies tested, immunoreactivity was the same in wild‐type and knock‐out mice. These data imply that, under commonly used conditions, these antibodies are not suited for immunolocalization. Thus, particular caution should be exerted with regards to the experimental approach used to visualize nicotinic acetylcholine receptors in the brain.

Inactivation determined by a single site in K+ pores

An N-terminus peptide or a C-terminus mechanism involving a single residue in transmembrane segment 6 produces inactivation in voltage-dependent K+ channels. Here we show that a single position in the pore of K+ channels can produce inactivation having characteristics distinct from either N- or C-type inactivation. In a chimeric K+ channel (CHM), the point reversion CHM V 369I produced fast inactivation and CHM V 369S had the additional effect of halving K+ conductance consistent with a position in the pore. The result was not restricted to CHM; mutating position 369 in the naturally occurring channel Kv2.1 also produced fast inactivation. Like N- and C-types of inactivation, pore or P-type inactivation was characterized by short bursts terminated by rapid entry into the inactivated state. Unlike C-type inactivation, in which external tetraethylammonium (TEA) produced a simple blockade that slowed inactivation and reduced currents, in P-type inactivation external TEA increased currents. Unlike N-type inactivation, internal TEA produced a simple reduction in current and K+ occupancy of the pore had no effect. External TEA was not the only cation to increase current; external K+ enhanced channel availability and recovery from inactivation. Additional features of P-type inactivation were residue-specific effects on the extent of inactivation and removal of inactivation by a point reversion at position 374, which also regulates conductance. The demonstration of P-type inactivation indicates that pore residues in K+ channels may be part of the inactivation gating machinery.

Progesterone modulation of α5 nAChR subunits influences anxiety‐related behavior during estrus cycle

Smokers often report an anxiolytic effect of cigarettes. In addition, stress‐related disorders such as anxiety, post‐traumatic stress syndrome and depression are often associated with chronic nicotine use. To study the role of the α5 nicotinic acetylcholine receptor subunit in anxiety‐related responses, control and α5 subunit null mice (α5−/−) were subjected to the open field activity (OFA), light–dark box (LDB) and elevated plus maze (EPM) tests. In the OFA and LDB, α5−/− behaved like wild‐type controls. In the EPM, female α5−/− mice displayed an anxiolytic‐like phenotype, while male α5−/− mice were undistinguishable from littermate controls. We studied the hypothalamus–pituitary–adrenal axis by measuring plasma corticosterone and hypothalamic corticotropin‐releasing factor. Consistent with an anxiolytic‐like phenotype, female α5−/− mice displayed lower basal corticosterone levels. To test whether gonadal steroids regulate the expression of α5, we treated cultured NTera 2 cells with progesterone and found that α5 protein levels were upregulated. In addition, brain levels of α5 mRNA increased upon progesterone injection into ovariectomized wild‐type females. Finally, we tested anxiety levels in the EPM during the estrous cycle. The estrus phase (when progesterone levels are low) is anxiolytic‐like in wild‐type mice, but no cycle‐dependent fluctuations in anxiety levels were found in α5−/− females. Thus, α5‐containing neuronal nicotinic acetylcholine receptors may be mediators of anxiogenic responses, and progesterone‐dependent modulation of α5 expression may contribute to fluctuations in anxiety levels during the ovarian cycle.

Histidine substitution identifies a surface position and confers Cs+ selectivity on a K+ pore.

The amino acid located at position 369 is a key determinant of the ion conduction pathway or pore of the voltage-gated K+ channels, Kv2.1 and a chimeric channel, CHM, constructed by replacing the pore region of Kv2.1 with that of Kv3.1. To determine the orientation of residue 369 with respect to the aqueous lumen of the pore, the nonpolar Ile at 369 in Kv2.1 was replaced with a basic His. This substitution produced a Cs(+)-selective channel with Cs+:K+ permeability ratio of 4 compared to 0.1 in the wild type. Block by external tetraethylammonium (TEA) was reduced about 20-fold, while block by internal TEA was unaffected. External protons and Zn2+, that are known to interact with the imidazole ring of His, blocked the mutant channel much more effectively than the wild type channel. The blockade by Zn2+ and protons was voltage-independent, and the proton blockade had a pKa of about 6.5, consistent with the pKa for His in solution. The histidyl-specific reagent diethylpyrocarbonate produced greatly exaggerated blockade of the mutated channel compared to the wild type. The residue at position 369 appears to form part of the binding site for external TEA and to influence the selectivity for monovalent cations. We suggest that the imidazole side-chain of His369 is exposed to the aqueous lumen at a surface position near the external mouth of the pore.

Regulation of K+/Rb+ selectivity and internal TEA blockade by mutations at a single site in K+ pores

A conservative reversion at position 374 in a chimeric K+ pore, CHM, switched the preferred ionic conductance from K+ to Rb+. To understand how selectivity was switched, codons for 18 different amino acids were substituted at position 374 in each of two different K+ channels CHM and Kv2.1, the host channel for CHM. After injection of cRNA into Xenopus oocytes, less than half of the substituted mutants expressed functional channels. In both CHM and Kv2.1, channels with the substituted hydrophobic residues Val or Ile expressed Rb+-preferring pores while channels with the substituted polar residues Thr or Ser expressed K+-preferring pores. Val or Ile stabilized while Thr or Ser destabilized blockade by internal tetraethylammonium (TEA) confirming the importance of hydrophobic interactions for blockade. TEA blockade was dependent upon the charge carrier and was more effective in the presence of the ion having the larger conductance. The results are consistent with a model in which the side chains at position 374 form a filter for K+ and Rb+ ions and a site for blockade by internal TEA.

The α3β4* nicotinic ACh receptor subtype mediates physical dependence to morphine: mouse and human studies.

Recent data have indicated that α3β4* neuronal nicotinic (n) ACh receptors may play a role in morphine dependence. Here we investigated if nACh receptors modulate morphine physical withdrawal.

Genetic variation within the Chrna7 gene modulates nicotine reward-like phenotypes in mice.

Mortality from tobacco smoking remains the leading cause of preventable death in the world, yet current cessation therapies are only modestly successful, suggesting new molecular targets are needed. Genetic analysis of gene expression and behavior identified Chrna7 as potentially modulating nicotine place conditioning in the BXD panel of inbred mice. We used gene targeting and pharmacological tools to confirm the role of Chrna7 in nicotine conditioned place preference (CPP). To identify molecular events downstream of Chrna7 that may modulate nicotine preference, we performed microarray analysis of α7 knock‐out (KO) and wild‐type (WT) nucleus accumbens (NAc) tissue, followed by confirmation with quantitative polymerase chain reaction (PCR) and immunoblotting. In the BXD panel, we found a putative cis expression quantitative trait loci (eQTL) for Chrna7 in NAc that correlated inversely to nicotine CPP. We observed that gain‐of‐function α7 mice did not display nicotine preference at any dose tested, whereas conversely, α7 KO mice demonstrated nicotine place preference at a dose below that routinely required to produce preference. In B6 mice, the α7 nicotinic acetylcholine receptor (nAChR)‐selective agonist, PHA‐543613, dose‐dependently blocked nicotine CPP, which was restored using the α7 nAChR‐selective antagonist, methyllycaconitine citrate (MLA). Our genomic studies implicated a messenger RNA (mRNA) co‐expression network regulated by Chrna7 in NAc. Mice lacking Chrna7 demonstrate increased insulin signaling in the NAc, which may modulate nicotine place preference. Our studies provide novel targets for future work on development of more effective therapeutic approaches to counteract the rewarding properties of nicotine for smoking cessation.

Nicotinic Receptors: Autonomic Neurons

This article focuses on the role of nicotinic acetycholine receptors (nAChRs) in the control of organ systems innervated by the autonomic nervous system. As the main mediators of fast synaptic transmission in ganglia, nAChRs are key molecules for the processing of neural commands directed to the peripheral organs. The article also briefly summarizes the function of nAChRs found in organ and tissues that do not receive direct autonomic innervation. The contribution of nicotinic cholinergic mechanisms to normal physiology and disease is discussed in the light of the results available from the analysis of null mutations targeted to the nAChRs subunits expressed in the autonomic nervous system and peripheral organs.