Determination of the chemical mechanism of neurotransmitter receptor-mediated reactions by rapid chemical kinetic methods

Optogenetic tools for analyzing the neural circuits of behavior

In order to understand how the brain generates behaviors, it is important to be able to determine how neural circuits work together to perform computations. Because neural circuits are made of a great diversity of cell types, it is critical to be able to analyze how these different kinds of cell work together. In recent years, a toolbox of fully genetically encoded molecules has emerged that, when expressed in specific neurons, enables the electrical activity of the targeted neurons to be controlled in a temporally precise fashion by pulses of light. We describe this optogenetic toolbox, how it can be used to analyze neural circuits in the brain and how optogenetics is impacting the study of cognition.

A history of optogenetics: the development of tools for controlling brain circuits with light

Understanding how different kinds of neuron in the brain work together to implement sensations, feelings, thoughts, and movements, and how deficits in specific kinds of neuron result in brain diseases, has long been a priority in basic and clinical neuroscience. “Optogenetic” tools are genetically encoded molecules that, when targeted to specific neurons in the brain, enable their activity to be driven or silenced by light. These molecules are microbial opsins, seven-transmembrane proteins adapted from organisms found throughout the world, which react to light by transporting ions across the lipid membranes of cells in which they are genetically expressed. These tools are enabling the causal assessment of the roles that different sets of neurons play within neural circuits, and are accordingly being used to reveal how different sets of neurons contribute to the emergent computational and behavioral functions of the brain. These tools are also being explored as components of prototype neural control prosthetics capable of correcting neural circuit computations that have gone awry in brain disorders. This review gives an account of the birth of optogenetics and discusses the technology and its applications.

A history of optogenetics: the development of tools for controlling brain circuits with light

Understanding how different kinds of neuron in the brain work together to implement sensations, feelings, thoughts, and movements, and how deficits in specific kinds of neuron result in brain diseases, has long been a priority in basic and clinical neuroscience. "Optogenetic" tools are genetically encoded molecules that, when targeted to specific neurons in the brain, enable their activity to be driven or silenced by light. These molecules are microbial opsins, seven-transmembrane proteins adapted from organisms found throughout the world, which react to light by transporting ions across the lipid membranes of cells in which they are genetically expressed. These tools are enabling the causal assessment of the roles that different sets of neurons play within neural circuits, and are accordingly being used to reveal how different sets of neurons contribute to the emergent computational and behavioral functions of the brain. These tools are also being explored as components of prototype neural control prosthetics capable of correcting neural circuit computations that have gone awry in brain disorders. This review gives an account of the birth of optogenetics and discusses the technology and its applications.

Chemical two-photon uncaging

INTRODUCTIONChemical two-photon uncaging is useful for a wide range of applications, including both mapping of receptor location and localized photostimulation of neurons via activation of excitatory glutamate receptors. Experimental preparations could include brain slices, cultured neurons, and, among other possibilities, whole brains in vivo. This protocol documents the utility of chemical two-photon uncaging in examining glutamate receptors of pyramidal neurons in hippocampal slices.

Molecular dynamics simulation links conformation of a pore-flanking region to hyperekplexia-related dysfunction of the inhibitory glycine receptor

Inhibitory glycine receptors mediate rapid synaptic inhibition in mammalian spinal cord and brainstem. The previously identified hyperekplexia mutation GLRA1(P250T), located within the intracellular TM1-2 loop of the GlyR alpha1 subunit, results in altered receptor activation and desensitization. Here, elementary steps of ion channel function of alpha1(250) mutants were resolved and shown to correlate with hydropathy and molar volume of residue alpha1(250). Single-channel recordings and rapid activation kinetic studies using laser pulse photolysis showed reduced conductance but similar open probability of alpha1(P250T) mutant channels. Molecular dynamics simulation of a helix-turn-helix motif representing the intracellular TM1-2 domain revealed alterations in backbone conformation, indicating an increased flexibility in these mutants that paralleled changes in elementary steps of channel function. Thus, the architecture of the TM1-2 loop is a critical determinant of ion channel conductance and receptor desensitization.

Role of local anesthetics on both cholinergic and serotonergic ionotropic receptors

A great body of experimental evidence indicates that the main target for the pharmacological action of local anesthetics (LAs) is the voltage-gated Na+ channel. However, the epidural and spinal anesthesia as well as the behavioral effects of LAs cannot be explained exclusively by its inhibitory effect on the voltage-gated Na+ channel. Thus, the involvement of other ion channel receptors has been suggested. Particularly, two members of the neurotransmitter-gated ion channel receptor superfamily, the nicotinic acetylcholine receptor (AChR) and the 5-hydroxytryptamine receptor (5-HT3R type). In this regard, the aim of this review is to explain and delineate the mechanism by which LAs inhibit both ionotropic receptors from peripheral and central nervous systems. Local anesthetics inhibit the ion channel activity of both muscle- and neuronal-type AChRs in a noncompetitive fashion. Additionally, LAs inhibit the 5-HT3R by competing with the serotonergic agonist binding sites. The noncompetitive inhibitory action of LAs on the AChR is ascribed to two possible blocking mechanisms. An open-channel-blocking mechanism where the drug binds to the open channel and/or an allosteric mechanism where LAs bind to closed channels. The open-channel-blocking mechanism is in accord with the existence of high-affinity LA binding sites located in the ion channel. The allosteric mechanism seems to be physiologically more relevant than the open-channel-blocking mechanism. The inhibitory property of LAs is also elicited by binding to several low-affinity sites positioned at the lipid-AChR interface. However, there is no clearcut evidence indicating whether these sites are located at either the annular or the nonannular lipid domain. Both tertiary (protonated) and quaternary LAs gain the interior of the channel through the hydrophilic pathway formed by the extracellular ion channel's mouth with the concomitant ion flux blockade. Nevertheless, an alternative mode of action is proposed for both deprotonated tertiary and permanently-uncharged LAs: they may pass from the lipid membrane core to the lumen of the ion channel through a hydrophobic pathway. Perhaps this hydrophobic pathway is structurally related to the nonannular lipid domain. Regarding the LA binding site location on the 5-HT3R, at least two amino acids have been involved. Glutamic acid at position 106 which is located in a residue sequence homologous to loop A from the principal component of the binding site for cholinergic agonists and competitive antagonists, and Trp67 which is positioned in a stretch of amino acids homologous to loop F from the complementary component of the cholinergic ligand binding site.

In vitro selection of RNA molecules that displace cocaine from the membrane-bound nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) controls signal transmission between cells in the nervous system. Abused drugs such as cocaine inhibit this receptor. Transient kinetic investigations indicate that inhibitors decrease the channel-opening equilibrium constant [Hess, G. P. & Grewer, C. (1998) Methods Enzymol. 291, 443-473]. Can compounds be found that compete with inhibitors for their binding site but do not change the channel-opening equilibrium? The systematic evolution of RNA ligands by exponential enrichment methodology and the AChR in Torpedo californica electroplax membranes were used to find RNAs that can displace inhibitors from the receptor. The selection of RNA ligands was carried out in two consecutive steps: (i) a gel-shift selection of high-affinity ligands bound to the AChR in the electroplax membrane, and (ii) subsequent use of nitrocellulose filters to which both the membrane-bound receptor and RNAs bind strongly, but from which the desired RNA can be displaced from the receptor by a high-affinity AChR inhibitor, phencyclidine. After nine selection rounds, two classes of RNA molecules that bind to the AChR with nanomolar affinities were isolated and sequenced. Both classes of RNA molecules are displaced by phencyclidine and cocaine from their binding site on the AChR. Class I molecules are potent inhibitors of AChR activity in BC3H1 muscle cells, as determined by using the whole-cell current-recording technique. Class II molecules, although competing with AChR inhibitors, do not affect receptor activity in this assay; such compounds or derivatives may be useful for alleviating the toxicity experienced by millions of addicts.

Chemical two-photon uncaging: a novel approach to mapping glutamate receptors

Functional mapping of neurotransmitter receptors requires rapid and localized application of transmitter. The usefulness of caged glutamate for this purpose has been limited, because photolysis by unfocused light above and below the target cell limits depth resolution. This problem is eliminated by using a double-caged glutamate that requires absorption of two photons for conversion to active glutamate, resulting in a substantial improvement in spatial resolution over conventional caged glutamate. This method was used to map the distribution of glutamate receptors on hippocampal pyramidal neurons. A higher density of AMPA receptors was found on distal apical dendrites than on basal or primary apical dendrites, suggesting that synaptic efficacy is locally heterogeneous. Such "chemical two-photon uncaging" offers a simple, general, and economical strategy for spatially localized photolysis of caged compounds.

Identification of chemical synapses in the pharynx of Caenorhabditis elegans

The rhythmic contraction of the Caenorhabditis elegans pharynx is unique in that the network of 12 neurons, including two M3 neurons, that regulate the contraction is known. The neurotransmitters secreted by these cells, and the target cells responding to these chemical signals, are not known. Here, we describe an approach to obtain this missing information and use the M3 cells as an example. Electrical recordings (electropharyngeograms) were used in conjunction with temporally and spatially defined application of neurotransmitters via photolysis of inactive, photolabile precursors. To illustrate the technique we used pharyngeal preparations in which the two M3 neurons are intact and preparations in which they were removed by laser irradiation. Removal of M3 neurons results in the loss of the small negative peaks in the electropharyngeograms and an increase in time during which the pharynx remains contracted. We demonstrate that the application of glutamate by photolysis of caged glutamate to a pharynx from which the two M3 neurons were removed produces effects similar to those observed before removal of the M3 neurons. In control experiments, photolytic release from photolabile precursors of carbamoylcholine, a stable and well characterized analog of acetylcholine, or of gamma-aminobutyric acid, from photolabile precursors did not have this effect. The response depended on the amount of glutamate released. By reducing the size of the photolytic beam, glutamate was released at several different locations of the pharynx. Two areas of the pharynx mainly respond to the application of glutamate; one corresponds to the pm4 muscle cells in the metacorpus, and the other to the junction between muscle cells pm5 in the isthmus and pm6 in the terminal bulb.

Pulsing mercury arc lamps for uncaging and fast imaging

It is shown that the light intensity from a mercury short arc light bulb can be boosted to about 100 times its steady-state value for a period of about 1 ms by superimposing a current pulse of up to 100 amperes in magnitude and 1-2 ms in duration on a simmer current of 3 amperes. The output spectrum changes in a remarkable way from a line to a broadband distribution. The radiance delivered during a 1-ms pulse is comparable to what can be obtained from a Xe flash bulb. The lack of a high voltage ignition pulse accounts for an extreme reduction of the electrical artifact seen for the pulsed Hg bulb as compared to a Xe flash lamp. Possible applications include the release of caged compounds, high speed imaging, and wavelength ratio-imaging without filter switching.

Rapid chemical kinetic techniques for investigations of neurotransmitter receptors expressed in Xenopus oocytes

Xenopus laevis oocytes have been used extensively during the past decade to express and study neurotransmitter receptors of various origins and subunit composition and also to express and study receptors altered by site-specific mutations. Interpretations of the effects of structural differences on receptor mechanisms were, however, hampered by a lack of rapid chemical reaction techniques suitable for use with oocytes. Here we describe flow and photolysis techniques, with 2-ms and 100-microseconds time resolution, respectively, for studying neurotransmitter receptors in giant (approximately 20-microns diameter) patches of oocyte membranes, using muscle and neuronal acetylcholine receptors as examples. With these techniques, we find that the muscle receptor in BC3H1 cells and the same receptor expressed in oocytes have comparable kinetic properties. This finding is in contrast to previous studies and raises questions regarding the interpretations of the many studies of receptors expressed in oocytes in which an insufficient time resolution was available. The results obtained indicate that the rapid reaction techniques described here, in conjunction with the oocyte expression system, will be useful in answering many outstanding questions regarding the structure and function of diverse neurotransmitter receptors.