Dissecting the neural circuitry of addiction and psychiatric disease with optogenetics

Establishing the causal relationships between brain function and behavior is one of the most important goals in neuroscience research. Traditionally, this has been accomplished with electrical stimulation or lesioning techniques that nonselectively activate or ablate the neural tissue, or by microinjections of drugs, which often selectively activate or inhibit specific neurons, but do so on a timescale irrelevant to neuronal firing. Although these techniques have been crucial in defining the gross neuroanatomical pathways that mediate behavior, they have had limited success in determining the specific synaptic connections and cell types that mediate a given behavioral response. With the recent advances in the emerging field of optogenetics, it is now possible to selectively introduce light-gated ion channels and pumps into genetically defined populations of neurons to selectively stimulate or inhibit neuronal circuit elements with light. Although optogenetics is still in its infancy, its use has already revealed important and novel information on neural function that was inaccessible with traditional techniques.

For optical excitation of neural tissue, the algae protein, Channelrhodopsin-2 (ChR2), can be introduced into neurons by various genetic techniques, such as viral transfection (Boyden et al, 2005). Targeting neuronal subtypes has been achieved by expressing ChR2 under the control of cell-specific promoters (Adamantidis et al, 2007) or by using cell-specific recombination to drive expression in neuronal subpopulations expressing CRE recombinase (Atasoy et al, 2008; Tsai et al, 2009). Illuminating ChR2 with blue light (∼480 nm) results in the absorption of a photon by the ChR2 cofactor, all-trans-retinal. This leads to a conformational change in the ChR2 complex allowing it to pass monovalent and divalent cations, resulting in the depolarization of neurons at resting membrane potentials. Activation of ChR2 can be used in vitro to excite cell bodies to induce firing of neurons at relatively high frequencies (Figure 1a; Boyden et al, 2005; Tsai et al, 2009). Alternatively, direct activation of axons and synaptic terminals expressing ChR2 can be used to probe afferent-specific synaptic transmission and strength (Figure 1b; Petreanu et al, 2007; Atasoy et al, 2008). In vivo, light can be introduced into specific brain regions expressing ChR2 via a fiber optic cable coupled to a guide cannula to directly excite specific neurons with high temporal resolution during behavior (Figure 1a; Adamantidis et al, 2007; Tsai et al, 2009). For the first time, this allows for activation of genetically defined neuronal subpopulations on a physiologically relevant timescale without directly altering the activity of neighboring cells.

Figure 1

Activation of genetically defined neurons or axon terminals in heterogeneous tissue. (a) Neurons expressing ChR2 (cell type A) are activated upon illumination with blue light. Optical stimulation of neurons expressing ChR2 leads to selective modulation of firing in these neurons, whereas neighboring neurons are unaffected (cell type B). (b) Afferents from neurons expressing ChR2 are directly activated by light, whereas afferents from other neurons in close proximity that are not expressing ChR2 are unaffected. This allows for afferent-specific perturbation of synaptic function in vitro or in vivo.

Transient inhibition of circuit components could potentially be even more valuable for assaying the neural basis of behavior. By introducing the bacteria-derived light-sensitive chloride pump Halorhodopsin (NpHR) into select neurons, optical inhibition of neural activity is now possible on a millisecond timescale (Zhang et al, 2007). NpHR, which is maximally activated by yellow light (∼590 nm), yields an inward chloride conductance capable of hyperpolarizing neurons and inhibiting firing. NpHR can potentially be used in combination with ChR2 to selectively inhibit or excite subpopulations of neurons with different wavelengths of light during behavioral tasks. Taken together, these emerging tools may help reshape systems neuroscience and greatly aid our understanding of the neural circuitry underlying addiction and psychiatric disease.