Development of Cell-Type-Specific Viral Vectors to Tease Apart the Neural Circuitry that Contributes to Drug Addiction - Viral Vector Approaches in Neurobiology and Brain Diseases - page 151

receptors, a G i/o DREADD (D-i), a G s DREADD (D-s), and a G q
DREADD (D-q) (although the nomenclature for these receptors is
still unsettled). Activation of D-i receptors by the synthetic ligand
clozapine- N -oxide (CNO) has been shown to activate potassium
channels in pyramidal neurons of the hippocampus as well as striatal
medium spiny neurons, resulting in membrane hyperpolarization
and transient inhibition of neuronal activity [ 8 , 9 ]. Conversely, acti-
vation of D-s or D-q receptors by CNO increases neuronal activity
through increasing intracellular signaling cascades (D-s) and induc-
ing burst fi ring of neurons (D-q) [ 7 ]. In all of these cases, altered
neuronal fi ring is an important consequence, but the activation of
each of these DREADD receptors changes intracellular signaling
pathways may have more subtle effects on cellular activity and plas-
ticity that have not been fully explored. A similar technique that has
rapidly gained popularity involves engineered optogenetic recep-
tors; viral vectors that express light-activated proteins to control the
activity of neurons have been used to investigate the role of specifi c
cell populations in a number of complex behaviors [ 10 - 12 ]. Some
of the more widely used optogenetic proteins include a class of
light-sensitive ion channels known as rhodopsins. When channel
rhodopsins are exposed to light, these nonselective cation channels
rapidly depolarize neurons, whereas light stimulation of the chlo-
ride pump halorhodopsins or of the proton pump rhodopsins, such
as archaerhodopsin-3, rapidly hyperpolarize neurons [ 13 ]. In addi-
tion, chimeric optogenetic proteins, or OptoXRs, have been devel-
oped which combine rhodopsin proteins with G-protein-coupled
receptors to allow for light-activated control of intracellular signal-
ing cascades [ 13 ]. These viral vector strategies are compelling tools
for manipulating cellular activity because they provides us with a
great degree of anatomical accuracy since the viral vectors do not
widely diffuse in tissue in addition to their absolute pharmacologi-
cal specifi city. Because activation of DREADD receptors is ligand
driven and rhodopsin proteins are light activated, changes in cell
function are transient and can be timed to coincide with different
phases of a behavior; these two technologies each have distinct
advantages and limitations. Thus, the coupling of cell-type-specifi c
viral vectors with either DREADD receptors or optogenetic tools
gives us a novel and powerful strategy for understanding the contri-
bution of select components of neural circuits in addiction. This
approach is an enormously fl exible method to probe behavior and
can be adapted for a range of applications while maintaining its
numerous advantages.

2 Materials

In order to use viral-mediated gene transfer to probe the role of
specifi c cell populations in the neural circuitry of addiction and
reinforcement, viral vectors that express transgenes under control