Loss of nigral excitation of cholinergic interneurons contributes to parkinsonian motor impairments.

Progressive loss of dopamine inputs in Parkinson's disease leads to imbalances in coordinated signaling of dopamine and acetylcholine (ACh) in the striatum, which is thought to contribute to parkinsonian motor symptoms. As reciprocal interactions between dopamine inputs and cholinergic interneurons (ChIs) control striatal dopamine and ACh transmission, we examined how partial dopamine depletion in an early-stage mouse model for Parkinson's disease alters nigral regulation of cholinergic activity. We found region-specific alterations in how remaining dopamine inputs regulate cholinergic excitability that differ between the dorsomedial (DMS) and dorsolateral (DLS) striatum. Specifically, we found that dopamine depletion downregulates metabotropic glutamate receptors (mGluR1) on DLS ChIs at synapses where dopamine inputs co-release glutamate, abolishing the ability of dopamine inputs to drive burst firing. This loss underlies parkinsonian motor impairments, as viral rescue of mGluR1 signaling in DLS ChIs was sufficient to restore circuit function and attenuate motor deficits in early-stage parkinsonian mice.

A conserved arginine with non-conserved function is a key determinant of agonist selectivity in α7 nicotinic ACh receptors.

BACKGROUND AND PURPOSE: The α7 and α4ß2* ("*" denotes possibly assembly with another subunit) nicotinic acetylcholine receptors (nAChRs) are the most abundant nAChRs in the mammalian brain. These receptors are the most targeted nAChRs in drug discovery programmes for brain disorders. However, the development of subtype-specific agonists remains challenging due to the high degree of sequence homology and conservation of function in nAChRs. We have developed C(10) variants of cytisine, a partial agonist of α4ß2 nAChR that has been used for smoking cessation. The C(10) methyl analogue used in this study displays negligible affinity for α7 nAChR, while retaining high affinity for α4ß2 nAChR. EXPERIMENTAL APPROACH: The structural underpinning of the selectivity of 10-methylcytisine for α7 and α4ß2 nAChRs was investigated using molecular dynamic simulations, mutagenesis and whole-cell and single-channel current recordings. KEY RESULTS: We identified a conserved arginine in the ß3 strand that exhibits a non-conserved function in nAChRs. In α4ß2 nAChR, the arginine forms a salt bridge with an aspartate residue in loop B that is necessary for receptor expression, whereas in α7 nAChR, this residue is not stabilised by electrostatic interactions, making its side chain highly mobile. This lack of constrain produces steric clashes with agonists and affects the dynamics of residues involved in agonist binding and the coupling network. CONCLUSION AND IMPLICATIONS: We conclude that the high mobility of the ß3-strand arginine in the α7 nAChR influences agonist binding and possibly gating network and desensitisation. The findings have implications for rational design of subtype-selective nAChR agents.

Molecular function of the novel α7ß2 nicotinic receptor.

The α7 nicotinic receptor is a promising drug target for neurological and inflammatory disorders. Although it is the homomeric member of the family, a novel α7ß2 heteromeric receptor has been discovered. To decipher the functional contribution of the ß2 subunit, we generated heteromeric receptors with fixed stoichiometry by two different approaches comprising concatenated and unlinked subunits. Receptors containing up to three ß2 subunits are functional. As the number of ß2 subunits increases in the pentameric arrangement, the durations of channel openings and activation episodes increase progressively probably due to decreased desensitization. The prolonged activation episodes conform the kinetic signature of α7ß2 and may have an impact on neuronal excitability. For activation of α7ß2 receptors, an α7/α7 binding-site interface is required, thus indicating that the three ß2 subunits are located consecutively in the pentameric arrangement. α7-positive allosteric modulators (PAMs) are emerging as novel therapeutic drugs. The presence of ß2 in the pentamer affects neither type II PAM potentiation nor activation by an allosteric agonist whereas it impairs type I PAM potentiation. This first single-channel study provides fundamental basis required to decipher the role and function of the novel α7ß2 receptor and opens doors to develop selective therapeutic drugs.

Exploring the positive allosteric modulation of human α7 nicotinic receptors from a single-channel perspective.

Enhancement of α7 nicotinic receptor (nAChR) function by positive allosteric modulators (PAMs) is a promising therapeutic strategy to improve cognitive deficits. PAMs have been classified only on the basis of their macroscopic effects as type I, which only enhance agonist-induced currents, and type II, which also decrease desensitization and reactivate desensitized nAChRs. To decipher the molecular basis underlying these distinct activities, we explored the effects on single-α7 channel currents of representative members of each type and of less characterized compounds. Our results reveal that all PAMs enhance open-channel lifetime and produce episodes of successive openings, thus indicating that both types affect α7 kinetics. Different PAM types show different sensitivity to temperature, suggesting different mechanisms of potentiation. By using a mutant α7 receptor that is insensitive to the prototype type II PAM (PNU-120596), we show that some though not all type I PAMs share the structural determinants of potentiation. Overall, our study provides novel information on α7 potentiation, which is key to the ongoing development of therapeutic compounds.