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Chemogenetics is a powerful and specific method to control and investigate intracellular signaling pathways. Most widely used in neuroscience to control neuronal or glial cell activity, chemogenetic experiments employ compounds that selectively target a genetically-modified G protein-coupled receptor (GPCR; DREADD) or a chimeric ion channel (PSAM).
GPCRs have been at the forefront of chemogenetic techniques, with the first paper outlining GPCRs that respond only to synthetic ligands published in 1998. These receptors, named Receptors Activated Solely by a Synthetic Ligand (RASSL), were successfully used in vivo to enable control of cardiac activity. However, their use in neuroscience is hampered by the pharmacological activity of the ligands in vivo, and the endogenous, constitutive activity of the engineered receptors in the absence of their specific ligand.
More recently, Designer Receptors Activated by Designer Drugs (DREADD) have been developed. The first of these were mutated human muscarinic acetylcholine receptors activated solely by inert ligands. Multiple rounds of mutagenesis and ligand screening identified muscarinic receptors coupled to the Gq intracellular signaling pathway that responded to Clozapine N-oxide (CNO, Cat. No. 4936). All three Gq coupled DREADDs (hM1Dq, hM3Dq and hM5Dq) activate neuronal activity in response to binding of a DREADD ligand. The same research identified the inhibitory DREADDs, hM2Di and hM4Di, which inhibit neuronal activity in response to DREADD ligand binding. An inhibitory DREADD named KORD, has also been developed from the κ-opioid receptor, which inhibits neuronal activity in response to Salvinorin B (SalB, Cat. No. 5611).
The range of chemogenetic tools available to researchers has recently been expanded with the development of chimeric ion channels, termed Pharmacologically Selective Actuator Modules (PSAMs), which are selectively bound by compounds termed Pharmacologically Selective Effector Molecules (PSEMs).
Chemogenetic experiments require the introduction of genetically engineered receptors or ion channels into specific brain areas, via viral vector expression systems. In vivo expression of a chemogenetic receptor can be achieved using an Adeno-associated virus (AAV) encoding the DREADD or PSAM, which is then injected into the brain region. The cell-type that the receptor is expressed in can be controlled using cell-type specific genetic promotors. For example, viral DREADD constructs with a CaMKIIa promotor will drive DREADD expression in neurons, while a viral DREADD construct with a GFAP promoter will drive DREADD expression in glia.
A similar technique for modulation of neuronal activity is optogenetics, which requires the expression of a light-sensitive ion channel, rather than a ligand-gated ion channel or GPCR. In optogenetic techniques, activation or inhibition of neuronal activity is initiated by implanted fiber optics, rather than small molecules. The key features of optogenetics and chemogenetics are summarized in the table below.
|Method of intervention||Inert, small molecule ligands selective for genetically engineered receptors/ion channels||Light-sensitive ion channels activated by implanted fibre optics|
|Is the intervention 'physiological'?||Yes - uses conserved, intracellular signaling pathways, or changes ion channel conductance, to alter neuronal activity||No - patterns of excitation/inhibition are artificially synchronized by light stimulation pattern|
|Is the intervention inert?||Yes - receptors/ion channels lack pharmacological activity without ligands and ligands are pharmacologically inert without specific engineered receptors/ion channels||No - the fibre optic light source can create heat and bacterial light-sensitive channels used can be antigenic|
|Is this method invasive in vivo?||Minimally to no - ligands can be given by intracerebral infusion, intraperitoneal injection or in drinking water, dependent on specific ligand||Yes - inherently invasive due to implantation of fibre optics|
|Is specialized equipment required?||No||Yes - requires implantable fibre optics as light source|
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Tocris offers the following scientific literature for Chemogenetics to showcase our products. We invite you to request* your copy today!
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Produced by Tocris, the chemogenetics research bulletin provides an introduction to chemogenetic methods to manipulate neuronal activity. It outlines the development of RASSLs, DREADDs and PSAMs, and the use of chemogenetic compounds. DREADD ligands and PSEMs available from Tocris are highlighted.
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