A Structural and Mutagenic Blueprint for Molecular Recognition of Strychnine and d-Tubocurarine by Different Cys-Loop Receptors

Cys-loop receptors (CLR) are pentameric ligand-gated ion channels that mediate fast excitatory or inhibitory transmission in the nervous system. Strychnine and d-tubocurarine (d-TC) are neurotoxins that have been highly instrumental in decades of research on glycine receptors (GlyR) and nicotinic acetylcholine receptors (nAChR), respectively. In this study we addressed the question how the molecular recognition of strychnine and d-TC occurs with high affinity and yet low specificity towards diverse CLR family members. X-ray crystal structures of the complexes with AChBP, a well-described structural homolog of the extracellular domain of the nAChRs, revealed that strychnine and d-TC adopt multiple occupancies and different ligand orientations, stabilizing the homopentameric protein in an asymmetric state. This introduces a new level of structural diversity in CLRs. Unlike protein and peptide neurotoxins, strychnine and d-TC form a limited number of contacts in the binding pocket of AChBP, offering an explanation for their low selectivity. Based on the ligand interactions observed in strychnine- and d-TC-AChBP complexes we performed alanine-scanning mutagenesis in the binding pocket of the human α1 GlyR and α7 nAChR and showed the functional relevance of these residues in conferring high potency of strychnine and d-TC, respectively. Our results demonstrate that a limited number of ligand interactions in the binding pocket together with an energetic stabilization of the extracellular domain are key to the poor selective recognition of strychnine and d-TC by CLRs as diverse as the GlyR, nAChR, and 5-HT3R

Genetic deletion of α7 nAChRs reduces hippocampal granule and pyramidal cell number in both sexes but impairs pattern separation in males only

IntroductionNeurogenesis within the dentate gyrus is thought to play an important role in cognitive processes such as reversal learning and pattern separation. The α7 nicotinic acetylcholine receptor (α7 nAChR) is expressed early in newly formed granule cells of the dentate gyrus, though its role in neurogenesis and related cognitive function is not fully understood.MethodsTo better characterize relevant function of α7 nAChRs, we performed unbiased stereology to quantify hippocampal granule cells, pyramidal cells, and total volume and used a touchscreen operant spatial discrimination/reversal task to test pattern separation in a global α7 nAChR knockout mouse line.ResultsThe knockout resulted in an ≈22% reduction in granule cells and a ≈ 20% reduction in pyramidal cells in both sexes, with no change in total hippocampal volume. However, the knockout impaired performance in the touchscreen task for males only. The sex-dependent difference in behavioral, but not stereological, results suggest a divergence in the structure–function relationship in males versus females. Detailed analyses revealed males were more biased by the initial reversal contingency relative to females indicating a potential source of the sex-specific interaction with the loss of α7 nAChRs.DiscussionThese findings argue that the α7 nAChR plays a critical role in hippocampal development, not just granule cell neurogenesis, and plays a sex-dependent role in cognitive function

Regulation of nicotinic acetylcholine receptor channel function by acetylcholinesterase inhibitors in rat hippocampal CA1 interneurons. Mol Pharmacol 2004; 66: 658–66

Abstract Neuronal nicotinic acetylcholine receptors (nAChRs) are involved in cognition, and may play a role in Alzheimer's disease (AD). Known inhibitors of acetylcholinesterase (AChE) are used to treat AD, and are known cognitive enhancers, however their mechanism of action relating to AD is not fully understood. We tested several AChE inhibitors, including huperzine A, tacrine, and BW284c51, on nAChRs in rat hippocampal CA1 interneurons in slices using patch-clamp techniques. These interneurons express both α7 and non-α7 subunit-containing nAChRs, and were activated with pressure applications of ACh, choline, or carbachol. These AChE inhibitors had no significant effect on either the amplitude or kinetics of α7 nAChRs activated by ACh, but slowed the rate of recovery from desensitization through an indirect mechanism; responses activated with either choline or carbachol were unaffected. For non-α7 receptors, these inhibitors significantly increased the amplitude and decay phase for responses induced by ACh (but not carbachol), also through an indirect mechanism. Slices preincubated with diisopropylflurophosphate (DFP; to permanently inactivate AChE) mimicked the effect of these AChE inhibitors on both α7 and non-α7 nAChRs. In addition galantamine, which is both an inhibitor of AChE and an allosteric potentiator of nAChRs, had similar effects. Therefore various AChE inhibitors are having significant and indirect effects on nAChRs, through direct inhibition of AChE; this results in an enhanced amount and/or duration of ACh in slices, with no effect on the levels of choline or carbachol. Therefore, drugs that target AChE are likely to be important regulators of cholinergic signaling in the hippocampus

Cholinergic Regulation of Hippocampal Theta Rhythm

Cholinergic regulation of hippocampal theta rhythm has been proposed as one of the central mechanisms underlying hippocampal functions including spatial memory encoding. However, cholinergic transmission has been traditionally associated with atropine-sensitive type II hippocampal theta oscillations that occur during alert immobility or in urethane-anesthetized animals. The role of cholinergic regulation of type I theta oscillations in behaving animals is much less clear. Recent studies strongly suggest that both cholinergic muscarinic and nicotinic receptors do actively regulate type I hippocampal theta oscillations and thus provide the cholinergic mechanism for theta-associated hippocampal learning. Septal cholinergic activation can regulate hippocampal circuit and theta expression either through direct septohippocampal cholinergic projections, or through septal glutamatergic and GABAergic neurons, that can precisely entrain hippocampal theta rhythmicity

Hippocampus and Entorhinal Cortex Recruit Cholinergic and NMDA Receptors Separately to Generate Hippocampal Theta Oscillations

Summary: Although much progress has been made in understanding type II theta rhythm generation under urethane anesthesia, less is known about the mechanisms underlying type I theta generation during active exploration. To better understand the contributions of cholinergic and NMDA receptor activation to type I theta generation, we recorded hippocampal theta oscillations from freely moving mice with local infusion of cholinergic or NMDA receptor antagonists to either the hippocampus or the entorhinal cortex (EC). We found that cholinergic receptors in the hippocampus, but not the EC, and NMDA receptors in the EC, but not the hippocampus, are critical for open-field theta generation and Y-maze performance. We further found that muscarinic M1 receptors located on pyramidal neurons, but not interneurons, are critical for cholinergic modulation of hippocampal synapses, theta generation, and Y-maze performance. These results suggest that hippocampus and EC neurons recruit cholinergic-dependent and NMDA-receptor-dependent mechanisms, respectively, to generate theta oscillations to support behavioral performance. : Gu et al. find that the entorhinal cortex and hippocampus recruit NMDA-receptor-dependent and cholinergic-dependent mechanisms, respectively, to generate hippocampal theta oscillations in freely moving mice. Muscarinic M1 receptors on pyramidal neurons are important for theta generation, providing potential cellular mechanisms underlying theta generation. Keywords: theta, hippocampus, entorhinal cortex, NMDA receptor, cholinergic receptor, muscarinic, medial septum, Y-maz