DREADDs for Neuroscientists

Abstract

To understand brain function, it is essential that we discover how cellular signaling specifies normal and pathological brain function. In this regard, chemogenetic technologies represent valuable platforms for manipulating neuronal and non-neuronal signal transduction in a cell-type-specific fashion in freely moving animals. Designer Receptors Exclusively Activated by Designer Drugs (DREADD)-based chemogenetic tools are now commonly used by neuroscientists to identify the circuitry and cellular signals that specify behavior, perceptions, emotions, innate drives, and motor functions in species ranging from flies to nonhuman primates. Here I provide a primer on DREADDs highlighting key technical and conceptual considerations and identify challenges for chemogenetics going forward.

Keywords: DREADD; chemical biology; chemogenetic; synthetic biology.

Figure 1. A Modified and Extended Ternary Complex Model of GPCR Action

As shown in the top panel GPCRs (R) may interact with ligands (L), hetereotrimeric G proteins (G), and arrestins (βArr) and thereby form a variety of inactive (green boxes), active (orange and red boxes), and signaling complexes (blue and red boxes). The bottom panel shows a cartoon of the various signaling complexes for canonical G protein signaling (L) and β-Arrestin signaling (R).

Figure 2. How Receptor Reserve and Constitutive Activity may Modify DREADD Actions In Vitro and In Vivo

(A) Simulations of receptor activity using a standard four-parameter logistic equation for GPCR activation, and variable receptor expression (DeLean et al., 1978) was used to simulate the effects of over-expression of a DREADD with constitutive activity (red circles); high receptor reserve, minimal constitutive activity (blue circles); high receptor reserve + desensitization (green circles); low expression and no receptor reserve (purple circles); and low expression, no receptor reserve, and desensitization (orange circles). (B and C) Potential pharmacokinetic parameters of CNO following high (B) and lower (C) doses. The dotted red line indicates the threshold concentration required for activation of the DREADD in situ.

Figure 3. Available DREADDs and Chemical Actuators

(A) The available DREADDs, their current accepted nomenclature, and the potential downstream neuronal effects of activation. (B and C) (B) Shows the structures of currently available chemical actuators for CNO-based DREADDs, while (C) shows the structure of the KORD ligand salvinorin B.

Figure 4. Potential Approaches for Cell- and Projection-Specific Modulation of Neuronal Activity Using DREADDs

(A) The standard approach whereby DREADDs are expressed in a cell-type-specific manner (either virally or transgenically) and then activated by systemic administration of chemical actuator. (B) How a combination of cell-type-specific expression (e.g., localized injection of AAV-FLEX-hSyn-DREADD) and projection-specific infusion of CAV-Cre allows for the projection-specific expression and activation of DREADDs. (C) How local infusion of a chemical actuator provides for projection-specific effects with a limited area of activation. (D) How distinct DREADDs may be expressed in a cell-type-specific fashion to afford multiplexed chemogenetic modulation of neural activity and physiology.