Chemical and genetic engineering of selective ion channel-ligand interactions

How do neurons sense a spike burst?

In this issue of Neuron, Xu et al. (2012) show that knock down of Syt1, a major Ca(2+) sensor, impairs synaptic transmission similarly in different brain regions but with unexpected, region-specific behavioral outcomes.

Transient activation of specific neurons in mice by selective expression of the capsaicin receptor

The ability to control the electrical activity of a neuronal subtype is a valuable tool in deciphering the role of discreet cell populations in complex neural circuits. Recent techniques that allow remote control of neurons are either labor intensive and invasive or indirectly coupled to neural electrical potential with low temporal resolution. Here we show the rapid, reversible and direct activation of genetically identified neuronal subpopulations by generating two inducible transgenic mouse models. Confined expression of the capsaicin receptor, TRPV1, allows cell-specific activation after peripheral or oral delivery of ligand in freely moving mice. Capsaicin-induced activation of dopaminergic or serotonergic neurons reversibly alters both physiological and behavioural responses within minutes, and lasts ~10 min. These models showcase a robust and remotely controllable genetic tool that modulates a distinct cell population without the need for invasive and labour-intensive approaches.

Mapping out hippocampal inhibition

A synthesis of opto- and pharmacogenetics is beginning to reveal how various interneuron types direct incoming traffic in the hippocampus to help create a map of the environment.

Virus-mediated swapping of zolpidem-insensitive with zolpidem-sensitive GABA(A) receptors in cortical pyramidal cells

Recently developed pharmacogenetic and optogenetic approaches, with their own advantages and disadvantages, have become indispensable tools in modern neuroscience. Here, we employed a previously described knock-in mouse line (GABA_ARγ2^77Ilox) in which the γ2 subunit of the GABA_A receptor (GABA_AR) was mutated to become zolpidem insensitive (γ2^77I) and used viral vectors to swap γ2^77I with wild-type, zolpidem-sensitive γ2 subunits (γ2^77F). The verification of unaltered density and subcellular distribution of the virally introduced γ2 subunits requires their selective labelling. For this we generated six N- and six C-terminal-tagged γ2 subunits, with which cortical cultures of GABA_ARγ2^−/− mice were transduced using lentiviruses. We found that the N-terminal AU1 tag resulted in excellent immunodetection and unimpaired synaptic localization. Unaltered kinetic properties of the AU1-tagged γ2 (^AU1γ2^77F) channels were demonstrated with whole-cell patch-clamp recordings of spontaneous IPSCs from cultured cells. Next, we carried out stereotaxic injections of lenti- and adeno-associated viruses containing Cre-recombinase and the ^AU1γ2^77F subunit (Cre-2A-^AU1γ2^77F) into the neocortex of GABA_ARγ2^77Ilox mice. Light microscopic immunofluorescence and electron microscopic freeze-fracture replica immunogold labelling demonstrated the efficient immunodetection of the AU1 tag and the normal enrichment of the ^AU1γ2^77F subunits in perisomatic GABAergic synapses. In line with this, miniature and action potential-evoked IPSCs whole-cell recorded from transduced cells had unaltered amplitudes, kinetics and restored zolpidem sensitivity. Our results obtained with a wide range of structural and functional verification methods reveal unaltered subcellular distributions and functional properties of γ2^77I and ^AU1γ2^77F GABA_ARs in cortical pyramidal cells. This transgenic–viral pharmacogenetic approach has the advantage that it does not require any extrinsic protein that might endow some unforeseen alterations of the genetically modified cells. In addition, this virus-based approach opens up the possibility of modifying multiple cell types in distinct brain regions and performing alternative recombination-based intersectional genetic manipulations.

Neuron transplantation partially reverses an obesity disorder in mice

Mice lacking leptin receptors are grossly obese and diabetic, in part due to dysfunction in brain circuits important for energy homeostasis. Transplantation of leptin receptor-expressing hypothalamic progenitor neurons into the brains of leptin receptor deficient mice led to integration into neural circuits, reduced obesity, and normalized circulating glucose levels.

Regulation of neuronal input transformations by tunable dendritic inhibition

Transforming synaptic input into action potential output is a fundamental function of neurons. The pattern of action potential output from principal cells of the mammalian hippocampus encodes spatial and nonspatial information, but the cellular and circuit mechanisms by which neurons transform their synaptic input into a given output are unknown. Using a combination of optical activation and cell type-specific pharmacogenetic silencing in vitro, we found that dendritic inhibition is the primary regulator of input-output transformations in mouse hippocampal CA1 pyramidal cells, and acts by gating the dendritic electrogenesis driving burst spiking. Dendrite-targeting interneurons are themselves modulated by interneurons targeting pyramidal cell somata, providing a synaptic substrate for tuning pyramidal cell output through interactions in the local inhibitory network. These results provide evidence for a division of labor in cortical circuits, where distinct computational functions are implemented by subtypes of local inhibitory neurons.

How inhibition shapes cortical activity

Cortical processing reflects the interplay of synaptic excitation and synaptic inhibition. Rapidly accumulating evidence is highlighting the crucial role of inhibition in shaping spontaneous and sensory-evoked cortical activity and thus underscores how a better knowledge of inhibitory circuits is necessary for our understanding of cortical function. We discuss current views of how inhibition regulates the function of cortical neurons and point to a number of important open questions.

Chemical genetics: receptor-ligand pairs for rapid manipulation of neuronal activity

Toward the functional dissection of neuronal circuits, a number of new genetic tools have been developed that enable rapid and reversible manipulation of genetically defined neuronal subtypes in intact mammalian brain circuits. Alongside the breakthrough technology of optogenetics, receptor-ligand pairs provide complementary approaches to modulate neuronal activity using chemical genetics.

Visualization and manipulation of neural activity in the developing vertebrate nervous system

Neural activity during vertebrate development has been unambiguously shown to play a critical role in sculpting circuit formation and function. Patterned neural activity in various parts of the developing nervous system is thought to modulate neurite outgrowth, axon targeting, and synapse refinement. The nature and role of patterned neural activity during development has been classically studied with in vitro preparations using pharmacological manipulations. In this review we discuss newly available and developing molecular-genetic tools for the visualization and manipulation of neural activity patterns specifically during development.