The use of chemogenetics in behavioural neuroscience: receptor variants, targeting approaches and caveats

Abstract

The last decade has seen major advances in neuroscience tools allowing us to selectively modulate cellular pathways in freely moving animals. Chemogenetic approaches such as designer receptors exclusively activated by designer drugs (DREADDs) permit the remote control of neuronal function by systemic drug administration. These approaches have dramatically advanced our understanding of the neural control of behaviour. Here, we review the different techniques and genetic approaches available for the restriction of chemogenetic receptors to defined neuronal populations. We highlight the use of a dual virus approach to target specific circuitries and the effectiveness of different routes of administration of designer drugs. Finally, we discuss the potential caveats associated with DREADDs including off-target effects of designer drugs, the effects of chronic chemogenetic receptor activation and the issue of collateral projections associated with DREADD activation and inhibition. AbbreviationsAAVs, adeno-associated viral vectors; BBB, blood-brain barrier; CAMKII, calmodulin-dependent protein kinase II; CAVs, canine adenoviruses; CNO, clozapine N-oxide; DREADDs, designer receptors exclusively activated by designer drugs; hM3D, human M 3 muscarinic DREADD; hM4D, human M 4 muscarinic DREADD; HSV, herpes simplex viral; hSyn, human synapsin; KORD, κ-opioid-based receptor designer receptor exclusively activated by designer drugs; LGICs, ligand-gated ion channels; NAc, nucleus accumbens; RASSL, receptor activated solely by a synthetic ligand; SalB, salvinorin B; VTA, ventral tegmental area IntroductionDuring the last decade, there has been a revolution in neuroscience techniques that have resulted in increasingly precise methods to manipulate neural systems in awake, behaving animals. Understanding the relationship between brain function and behaviour is critical for the advancement of both neuroscience research and targeted medication development. Chemogenetics refers to the technique that allows for the reversible remote control of cell populations and neural circuitry via systemic injection or microinfusion of an activating ligand (Alexander et al., 2009;Armbruster et al., 2007). The chemogenetic technique uses engineered receptors and biologically inert ligands to achieve this aim. Unlike optogenetics, which has the ability to control cells and neural circuitry with light, the use of designer drugs makes chemogenetics simple to use, removing the need for optical fibre probes and tethers. While the temporal resolution of chemogenetics is lower than optogenetics, this relatively non-invasive technique is still effective for functional mapping, cell-type-specific manipulations and multiplexed control of neurons.In this review, we will evaluate the different strategies that have been used to restrict chemogenetic receptors to defined neuronal populations. We will also highlight the use of a dual virus approach at targeting projection neurons and the effectiveness of different routes of administration of designer drugs. Fin...

Context-dependent limb movement encoding in neuronal populations of motor cortex

“…One suggested possibility that activation of DREADDs in vivo is likely to be mediated by metabolism of CNO to clozapine 69 , which readily crosses the blood brain barrier, but has also affinity for serotonergic and dopaminergic receptors 71 . However, the affinity of clozapine for muscarinic-based DREADDs is substantially higher than for native receptors 68 . That the observed decline of forelimb performance in the expert M2-off phase was actually generated by effects of low metabolized clozapine doses on native receptors, is therefore in our opinion highly unlikely, especially since clozapine is known as anti-psychotic drug with minimal motor side effects 71 .…”

“…Mouse 6 and 7 were additionally injected with AAV-6 Cre-dependent hM4D(Gi)-mCherry virus into lower layer 2/3 as well as upper layer 5 of M2 (1.5 mm anterior, 0.5 mm lateral to Bregma, 500 μm below pial surface) and with AAV-6 Cre virus into M1 L2/3 (0.1 mm anterior, 1.9 mm lateral from Bregma, 300 μm below pial surface). hM4D(Gi) is a DREADD ("designer receptor exclusively activated by designer drug") 68 that we used in the second experimental series to chemogenetically silence M2 neurons with axonal projections to M1 L2/3, found mainly in lower L2/3 and upper L5 35 . hM4D(Gi)-DREADDs are activated by the otherwise pharmacologically inert synthetic ligand clozapine-N-oxide (CNO), resulting in membrane hyperpolarization and silencing of the infected neurons [52][53][54] .…”

Compound 21, a two-edged sword with both DREADD-selective and off-target outcomes in rats

“…Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) are chemogenetic tools that represent one of the major breakthroughs of the last ten years in integrative neuroscience (Roth, 2016;Campbell and Marchant, 2018). Combining the precision of genetics with pharmacology, DREADDs provide a remote, prolonged and reversible control of neuronal or extra-neuronal subpopulations via conditional expression and allow the study of complex phenomena in awake animals.…”

Molecular basis for and chemogenetic modulation of comorbidities in GABRG2‐deficient epilepsies

“…Neuromodulation of a specific group of neurons to control seizures or comorbidities such as anxiety would be an ideal choice for epilepsy as it would avoid side effect from broad alteration of brain activity in drug treatment which globally decreases excitation 37 . Despite lack of precise temporal control afforded by optogenetics, DREADDs have proven to be well suited for translation due to the slow temporal control and chronic modulation as well as its feasibility 38;39 . It is highly attractive for treatment development using DREADD as CNO can be delivered orally 40;41 or via other safe routes such as topical administration 42 .…”

“…38 Despite lack of precise temporal control afforded by optogenetics, DREADDs have proven to be well suited for translation due to the slow temporal control and chronic modulation as well as their feasibility. 39,40 Treatment development using DREADD is highly attractive, as CNO can be delivered orally 41,42 or via other safe routes such as topical administration. 43 This suggests that in addition to pharmacological approaches, selectively increased inhibition in CeA via neuromodulation could be a feasible treatment option for anxiety.…”