Phasic firing in dopaminergic neurons is sufficient for behavioral conditioning

At what stage of neural processing does cocaine act to boost pursuit of rewards?

Dopamine-containing neurons have been implicated in reward and decision making. One element of the supporting evidence is that cocaine, like other drugs that increase dopaminergic neurotransmission, powerfully potentiates reward seeking. We analyze this phenomenon from a novel perspective, introducing a new conceptual framework and new methodology for determining the stage(s) of neural processing at which drugs, lesions and physiological manipulations act to influence reward-seeking behavior. Cocaine strongly boosts the proclivity of rats to work for rewarding electrical brain stimulation. We show that the conventional conceptual framework and methods do not distinguish between three conflicting accounts of how the drug produces this effect: increased sensitivity of brain reward circuitry, increased gain, or decreased subjective reward costs. Sensitivity determines the stimulation strength required to produce a reward of a given intensity (a measure analogous to the K_M of an enzyme) whereas gain determines the maximum intensity attainable (a measure analogous to the v_max of an enzyme-catalyzed reaction). To distinguish sensitivity changes from the other determinants, we measured and modeled reward seeking as a function of both stimulation strength and opportunity cost. The principal effect of cocaine was a two-fourfold increase in willingness to pay for the electrical reward, an effect consistent with increased gain or decreased subjective cost. This finding challenges the long-standing view that cocaine increases the sensitivity of brain reward circuitry. We discuss the implications of the results and the analytic approach for theories of how dopaminergic neurons and other diffuse modulatory brain systems contribute to reward pursuit, and we explore the implications of the conceptual framework for the study of natural rewards, drug reward, and mood.

Monosynaptic circuit tracing in vivo through Cre-dependent targeting and complementation of modified rabies virus

We describe a powerful system for revealing the direct monosynaptic inputs to specific cell types in Cre-expressing transgenic mice through the use of Cre-dependent helper virus and a modified rabies virus. We generated helper viruses that target gene expression to Cre-expressing cells, allowing us to control initial rabies virus infection and subsequent monosynaptic retrograde spread. Investigators can use this system to elucidate the connections onto a desired cell type in a high-throughput manner, limited only by the availability of Cre mouse lines. This method allows for identification of circuits that would be extremely tedious or impossible to study with other methods and can be used to build subcircuit maps of inputs onto many different types of cells within the same brain region. Furthermore, by expressing various transgenes from the rabies genome, this system also has the potential to allow manipulation of targeted neuronal circuits without perturbing neighboring cells.

Influence of phasic and tonic dopamine release on receptor activation

Tonic and phasic dopamine release is implicated in learning, motivation, and motor functions. However, the relationship between spike patterns in dopaminergic neurons, the extracellular concentration of dopamine, and activation of dopamine receptors remains unresolved. In the present study, we develop a computational model of dopamine signaling that give insight into the relationship between the dynamics of release and occupancy of D(1) and D(2) receptors. The model is derived from first principles using experimental data. It has no free parameters and offers unbiased estimation of the boundaries of dopaminergic volume transmission. Bursts primarily increase occupancy of D(1) receptors, whereas pauses translate into low occupancy of D(1) and D(2) receptors. Phasic firing patterns, composed of bursts and pauses, reduce the average D(2) receptor occupancy and increase average D(1) receptor occupancy compared with equivalent tonic firing. Receptor occupancy is crucially dependent on synchrony and the balance between tonic and phasic firing modes. Our results provide quantitative insight in the dynamics of volume transmission and complement experimental data obtained with electrophysiology, positron emission tomography, microdialysis, amperometry, and voltammetry.

Linking molecules to mood: new insight into the biology of depression

Major depressive disorder is a heritable psychiatric syndrome that appears to be associated with subtle cellular and molecular alterations in a complex neural network. The affected brain regions display dynamic neuroplastic adaptations to endocrine and immunologic stimuli arising from within and outside the CNS. Depression's clinical and etiological heterogeneity adds a third level of complexity, implicating different pathophysiological mechanisms in different patients with the same DSM diagnosis. Current pharmacological antidepressant treatments improve depressive symptoms through complex mechanisms that are themselves incompletely understood. This review summarizes the current knowledge of the neurobiology of depression by combining insights from human clinical studies and molecular explanations from animal models. The authors provide recommendations for future research, with a focus on translating today's discoveries into improved diagnostic tests and treatments.

Tuning arousal with optogenetic modulation of locus coeruleus neurons

Neural activity in the noradrenergic locus coeruleus correlates with periods of wakefulness and arousal. However, whether tonic or phasic activity in these neurons is necessary or sufficient to induce behavioral state transitions and promote long-term arousal is unresolved. We used optogenetic tools in mice to demonstrate a frequency-dependent, causal relationship between locus coeruleus firing, cortical activity, sleep-to-wake transitions, and general locomotor arousal. Surprisingly, we also found that sustained, high-frequency stimulation of the locus coeruleus at frequencies 5 Hz and above caused reversible behavioral arrests. These results suggest that the locus coeruleus is finely tuned to regulate organismal arousal and that bursts of noradrenergic over-excitation cause behavioral attacks similar to those observed in neuropsychiatric disorders.

Channelrhodopsin-2 localised to the axon initial segment

The light-gated cation channel Channelrhodopsin-2 (ChR2) is a powerful and versatile tool for controlling neuronal activity. Currently available versions of ChR2 either distribute uniformly throughout the plasma membrane or are localised specifically to somatodendritic or synaptic domains. Localising ChR2 instead to the axon initial segment (AIS) could prove an extremely useful addition to the optogenetic repertoire, targeting the channel directly to the site of action potential initiation, and limiting depolarisation and associated calcium entry elsewhere in the neuron. Here, we describe a ChR2 construct that we localised specifically to the AIS by adding the ankyrinG-binding loop of voltage-gated sodium channels (Na_vII-III) to its intracellular terminus. Expression of ChR2-YFP-Na_vII-III did not significantly affect the passive or active electrical properties of cultured rat hippocampal neurons. However, the tiny ChR2 currents and small membrane depolarisations resulting from AIS targeting meant that optogenetic control of action potential firing with ChR2-YFP-Na_vII-III was unsuccessful in baseline conditions. We did succeed in stimulating action potentials with light in some ChR2-YFP-Na_vII-III-expressing neurons, but only when blocking KCNQ voltage-gated potassium channels. We discuss possible alternative approaches to obtaining precise control of neuronal spiking with AIS-targeted optogenetic constructs and propose potential uses for our ChR2-YFP-Na_vII-III probe where subthreshold modulation of action potential initiation is desirable.

Natural and engineered photoactivated nucleotidyl cyclases for optogenetic applications

Cyclic nucleotides, cAMP and cGMP, are ubiquitous second messengers that regulate metabolic and behavioral responses in diverse organisms. We describe purification, engineering, and characterization of photoactivated nucleotidyl cyclases that can be used to manipulate cAMP and cGMP levels in vivo. We identified the blaC gene encoding a putative photoactivated adenylyl cyclase in the Beggiatoa sp. PS genome. BlaC contains a BLUF domain involved in blue-light sensing using FAD and a nucleotidyl cyclase domain. The blaC gene was overexpressed in Escherichia coli, and its product was purified. Irradiation of BlaC in vitro resulted in a small red shift in flavin absorbance, typical of BLUF photoreceptors. BlaC had adenylyl cyclase activity that was negligible in the dark and up-regulated by light by 2 orders of magnitude. To convert BlaC into a guanylyl cyclase, we constructed a model of the nucleotidyl cyclase domain and mutagenized several residues predicted to be involved in substrate binding. One triple mutant, designated BlgC, was found to have photoactivated guanylyl cyclase in vitro. Irradiation with blue light of the E. coli cya mutant expressing BlaC or BlgC resulted in the significant increases in cAMP or cGMP synthesis, respectively. BlaC, but not BlgC, restored cAMP-dependent growth of the mutant in the presence of light. Small protein sizes, negligible activities in the dark, high light-to-dark activation ratios, functionality at broad temperature range and physiological pH, as well as utilization of the naturally occurring flavins as chromophores make BlaC and BlgC attractive for optogenetic applications in various animal and microbial models.

Cortico-basal ganglia circuitry: a review of key research and implications for functional connectivity studies of mood and anxiety disorders

There is considerable evidence that dysfunction of the cortico-basal ganglia circuits may be associated with several mood and anxiety disorders. However, it is unclear whether circuit abnormalities contribute directly either to the neurobiology of these conditions or to the manifestation of symptoms. Understanding the role of these pathways in psychiatric illness has been limited by an incomplete characterization of normal function. In recent years, studies using animal models and human functional imaging have greatly expanded the literature describing normal cortico-basal ganglia circuit function. In this paper, recent key studies of circuit function using human and animal models are reviewed and integrated with findings from other studies conducted over the previous decades. The literature suggests several hypotheses of cortico-basal ganglia circuitry function in mood and anxiety disorders that warrant further exploration. Hypotheses are proposed herein based upon the cortico-basal ganglia mechanisms of: (1) feedforward and feedback control, (2) circuit integration and (3) emotional control. These are presented as models of circuit function, which may be particularly relevant to future investigations using neuroimaging and functional connectivity analyses.

Cell death and sexual differentiation of behavior: worms, flies, and mammals

Sex differences in the nervous system are found throughout the animal kingdom. Here, we discuss three prominent genetic models: nematodes, fruit flies, and mice. In all three, differential cell death is central to sexual differentiation and shared molecular mechanisms have been identified. Our knowledge of the precise function of neural sex differences lags behind. One fruitful approach to the 'function' question is to contrast sexual differentiation in standard laboratory animals with differentiation in species exhibiting unique social and reproductive organizations. Advanced genetic strategies are also addressing this question in worms and flies, and may soon be applicable to vertebrates.

A behavioral genetics approach to understanding D1 receptor involvement in phasic dopamine signaling

Dopamine-producing neurons fire with both basal level tonic patterns and phasic bursts. Varying affinities of the five dopamine receptors have led to a hypothesis that higher affinity receptors are primarily activated by basal level tonic dopamine, while lower affinity receptors may be tuned to be sensitive to higher levels caused by phasic bursts. Genetically modified mice provide a method to begin to probe this hypothesis. Here we discuss three mouse models. Dopamine-deficient mice were used to determine which behaviors require dopamine. These behaviors were then analyzed in mice lacking D1 receptors and in mice with reduced phasic dopamine release. Comparison of the latter two mouse models revealed a similar failure to learn about and respond normally to cues that indicate either a positive or negative outcome, giving support to the hypothesis that phasic dopamine release and the D1 receptor act in the same pathway. However, the D1 receptor likely has additional roles beyond those of phasic dopamine detection, because D1 receptor knockout mice have deficits in addition to what has been observed in mice with reduced phasic dopamine release.

Good vibrations switch attention: an affective function for network oscillations in evolutionary simulations

In the present study, a new hypothesis on the neural mechanisms linking affect to attention was brought forward by evolutionary simulations on agents navigating a virtual environment while collecting food and avoiding predation. The connection strengths between nodes in the networks controlling the agents were subjected to random variation, and the fittest agents were selected for reproduction. Unexpectedly, oscillations of node activations emerged, which drastically enhanced the agent's fitness. We analyzed the mechanisms involved in the modulation of attention and found that oscillations acted on competitive networks. Response selection depended on the connection structure, but the speed and efficacy of switching between selections was modulated by oscillation frequency. The main focus of the present study was the differential emergence of stimulus-specific oscillation frequencies. Oscillations had a higher frequency in an appetitive motivational state than in an aversive state. We suggest that oscillations in biological networks also mediate the affective modulation of attention.

NMDA receptors on non-dopaminergic neurons in the VTA support cocaine sensitization

BACKGROUND

The initiation of behavioral sensitization to cocaine and other psychomotor stimulants is thought to reflect N-methyl-D-aspartate receptor (NMDAR)-mediated synaptic plasticity in the mesolimbic dopamine (DA) circuitry. The importance of drug induced NMDAR mediated adaptations in ventral tegmental area (VTA) DA neurons, and its association with drug seeking behaviors, has recently been evaluated in Cre-loxp mice lacking functional NMDARs in DA neurons expressing Cre recombinase under the control of the endogenous dopamine transporter gene (NR1(DATCre) mice).

METHODOLOGY AND PRINCIPAL FINDINGS

Using an additional NR1(DATCre) mouse transgenic model, we demonstrate that while the selective inactivation of NMDARs in DA neurons eliminates the induction of molecular changes leading to synaptic strengthening, behavioral measures such as cocaine induced locomotor sensitization and conditioned place preference remain intact in NR1(DATCre) mice. Since VTA DA neurons projecting to the prefrontal cortex and amygdala express little or no detectable levels of the dopamine transporter, it has been speculated that NMDA receptors in DA neurons projecting to these brain areas may have been spared in NR1(DATCre) mice. Here we demonstrate that the NMDA receptor gene is ablated in the majority of VTA DA neurons, including those exhibiting undetectable DAT expression levels in our NR1(DATCre) transgenic model, and that application of an NMDAR antagonist within the VTA of NR1(DATCre) animals still blocks sensitization to cocaine.

CONCLUSIONS/SIGNIFICANCE

These results eliminate the possibility of NMDAR mediated neuroplasticity in the different DA neuronal subpopulations in our NR1(DATCre) mouse model and therefore suggest that NMDARs on non-DA neurons within the VTA must play a major role in cocaine-related addictive behavior.

Cholinergic modulation of locomotion and striatal dopamine release is mediated by alpha6alpha4* nicotinic acetylcholine receptors

Dopamine (DA) release in striatum is governed by firing rates of midbrain DA neurons, striatal cholinergic tone, and nicotinic ACh receptors (nAChRs) on DA presynaptic terminals. DA neurons selectively express alpha6* nAChRs, which show high ACh and nicotine sensitivity. To help identify nAChR subtypes that control DA transmission, we studied transgenic mice expressing hypersensitive alpha6(L9'S)* receptors. alpha6(L9'S) mice are hyperactive, travel greater distance, exhibit increased ambulatory behaviors such as walking, turning, and rearing, and show decreased pausing, hanging, drinking, and grooming. These effects were mediated by alpha6alpha4* pentamers, as alpha6(L9'S) mice lacking alpha4 subunits displayed essentially normal behavior. In alpha6(L9'S) mice, receptor numbers are normal, but loss of alpha4 subunits leads to fewer and less sensitive alpha6* receptors. Gain-of-function nicotine-stimulated DA release from striatal synaptosomes requires alpha4 subunits, implicating alpha6alpha4beta2* nAChRs in alpha6(L9'S) mouse behaviors. In brain slices, we applied electrochemical measurements to study control of DA release by alpha6(L9'S) nAChRs. Burst stimulation of DA fibers elicited increased DA release relative to single action potentials selectively in alpha6(L9'S), but not WT or alpha4KO/alpha6(L9'S), mice. Thus, increased nAChR activity, like decreased activity, leads to enhanced extracellular DA release during phasic firing. Bursts may directly enhance DA release from alpha6(L9'S) presynaptic terminals, as there was no difference in striatal DA receptor numbers or DA transporter levels or function in vitro. These results implicate alpha6alpha4beta2* nAChRs in cholinergic control of DA transmission, and strongly suggest that these receptors are candidate drug targets for disorders involving the DA system.

Remote control of ion channels and neurons through magnetic-field heating of nanoparticles

Recently, optical stimulation has begun to unravel the neuronal processing that controls certain animal behaviours. However, optical approaches are limited by the inability of visible light to penetrate deep into tissues. Here, we show an approach based on radio-frequency magnetic-field heating of nanoparticles to remotely activate temperature-sensitive cation channels in cells. Superparamagnetic ferrite nanoparticles were targeted to specific proteins on the plasma membrane of cells expressing TRPV1, and heated by a radio-frequency magnetic field. Using fluorophores as molecular thermometers, we show that the induced temperature increase is highly localized. Thermal activation of the channels triggers action potentials in cultured neurons without observable toxic effects. This approach can be adapted to stimulate other cell types and, moreover, may be used to remotely manipulate other cellular machinery for novel therapeutics.

Start/stop signals emerge in nigrostriatal circuits during sequence learning

Learning new action sequences subserves a plethora of different abilities such as escaping a predator, playing the piano, or producing fluent speech. Proper initiation and termination of each action sequence is critical for the organization of behaviour, and is compromised in nigrostriatal disorders like Parkinson's and Huntington's diseases. Using a self-paced operant task in which mice learn to perform a particular sequence of actions to obtain an outcome, we found neural activity in nigrostriatal circuits specifically signalling the initiation or the termination of each action sequence. This start/stop activity emerged during sequence learning, was specific for particular actions, and did not reflect interval timing, movement speed or action value. Furthermore, genetically altering the function of striatal circuits disrupted the development of start/stop activity and selectively impaired sequence learning. These results have important implications for understanding the functional organization of actions and the sequence initiation and termination impairments observed in basal ganglia disorders.

Optical control of neuronal activity

Advances in optics, genetics, and chemistry have enabled the investigation of brain function at all levels, from intracellular signals to single synapses, whole cells, circuits, and behavior. Until recent years, these research tools have been utilized in an observational capacity: imaging neural activity with fluorescent reporters, for example, or correlating aberrant neural activity with loss-of-function and gain-of-function pharmacological or genetic manipulations. However, optics, genetics, and chemistry have now combined to yield a new strategy: using light to drive and halt neuronal activity with molecular specificity and millisecond precision. Photostimulation of neurons is noninvasive, has unmatched spatial and temporal resolution, and can be targeted to specific classes of neurons. The optical methods developed to date encompass a broad array of strategies, including photorelease of caged neurotransmitters, engineered light-gated receptors and channels, and naturally light-sensitive ion channels and pumps. In this review, we describe photostimulation methods, their applications, and opportunities for further advancement.

Dopaminergic terminals in the nucleus accumbens but not the dorsal striatum corelease glutamate

Coincident signaling by dopamine and glutamate is thought to be crucial for a variety of motivated behaviors. Previous work has suggested that some midbrain dopamine neurons are themselves capable of glutamate corelease, but this phenomenon remains poorly understood. Here, we expressed the light-activated cation channel Channelrhodopsin-2 (ChR2) in genetically defined midbrain dopamine neurons to stimulate exocytosis specifically from dopaminergic terminals in both the nucleus accumbens (NAc) shell and dorsal striatum of brain slices from adult mice. Optical activation resulted in robust glutamate-mediated EPSCs in all medium spiny neurons examined in the NAc shell. In contrast, optically evoked glutamatergic currents were nearly undetectable in the dorsal striatum. Further, we used a conditional knock-out mouse lacking vesicular glutamate transporter 2 (VGLUT2) specifically in dopamine neurons to determine whether VGLUT2 is required for the exocytotic release of glutamate from dopamine neurons. Our data show that conditional knock-out completely abolished all optically evoked glutamate release. These results provide definitive physiological evidence for VGLUT2-mediated glutamate release by mature dopamine neurons projecting to the NAc shell, but not to the dorsal striatum. Thus, the unique ability of NAc-projecting dopamine neurons to synchronously activate both dopamine and glutamate receptors may have crucial implications for the ability to respond to motivationally significant stimuli.

A transgenic mouse line for molecular genetic analysis of excitatory glutamatergic neurons

Excitatory glutamatergic neurons are part of most of the neuronal circuits in the mammalian nervous system. We have used BAC-technology to generate a BAC-Vglut2::Cre mouse line where Cre expression is driven by the vesicular glutamate transporter 2 (Vglut2) promotor. This BAC-Vglut2::Cre mouse line showed specific expression of Cre in Vglut2 positive cells in the spinal cord with no ectopic expression in GABAergic or glycinergic neurons. This mouse line also showed specific Cre expression in Vglut2 positive structures in the brain such as thalamus, hypothalamus, superior colliculi, inferior colliculi and deep cerebellar nuclei together with nuclei in the midbrain and hindbrain. Cre-mediated recombination was restricted to Cre expressing cells in the spinal cord and brain and occurred as early as E 12.5. Known Vglut2 positive neurons showed normal electrophysiological properties in the BAC-Vglut2::Cre transgenic mice. Altogether, this BAC-Vglut2::Cre mouse line provides a valuable tool for molecular genetic analysis of excitatory neuronal populations throughout the mouse nervous system.

Optical activation of lateral amygdala pyramidal cells instructs associative fear learning

Humans and animals can learn that specific sensory cues in the environment predict aversive events through a form of associative learning termed fear conditioning. This learning occurs when the sensory cues are paired with an aversive event occurring in close temporal proximity. Activation of lateral amygdala (LA) pyramidal neurons by aversive stimuli is thought to drive the formation of these associative fear memories; yet, there have been no direct tests of this hypothesis. Here we demonstrate that viral-targeted, tissue-specific expression of the light-activated channelrhodopsin (ChR2) in LA pyramidal cells permitted optical control of LA neuronal activity. Using this approach we then paired an auditory sensory cue with optical stimulation of LA pyramidal neurons instead of an aversive stimulus. Subsequently presentation of the tone alone produced behavioral fear responses. These results demonstrate in vivo optogenetic control of LA neurons and provide compelling support for the idea that fear learning is instructed by aversive stimulus-induced activation of LA pyramidal cells.

In vitro identification and electrophysiological characterization of dopamine neurons in the ventral tegmental area

Dopamine (DA) neurons in the ventral tegmental area (VTA) have been implicated in brain mechanisms related to motivation, reward, and drug addiction. Successful identification of these neurons in vitro has historically depended upon the expression of a hyperpolarization-activated current (I(h)) and immunohistochemical demonstration of the presence of tyrosine hydroxylase (TH), the rate-limiting enzyme for DA synthesis. Recent findings suggest that electrophysiological criteria may be insufficient for distinguishing DA neurons from non-DA neurons in the VTA. In this study, we sought to determine factors that could potentially account for the apparent discrepancies in the literature regarding DA neuron identification in the rodent brain slice preparation. We found that confirmed DA neurons from the lateral VTA generally displayed a larger amplitude I(h) relative to DA neurons located in the medial VTA. Measurement of a large amplitude I(h) (>100 pA) consistently indicated a dopaminergic phenotype, but non-dopamine neurons also can have I(h) current. The data also showed that immunohistochemical TH labeling of DA neurons can render false negative results after relatively long duration (>15 min) whole-cell patch clamp recordings. We conclude that whole-cell patch clamp recording in combination with immunohistochemical detection of TH expression can guarantee positive but not negative DA identification in the VTA.

Two-photon single-cell optogenetic control of neuronal activity by sculpted light

Recent advances in optogenetic techniques have generated new tools for controlling neuronal activity, with a wide range of neuroscience applications. The most commonly used approach has been the optical activation of the light-gated ion channel channelrhodopsin-2 (ChR2). However, targeted single-cell-level optogenetic activation with temporal precessions comparable to the spike timing remained challenging. Here we report fast (< or = 1 ms), selective, and targeted control of neuronal activity with single-cell resolution in hippocampal slices. Using temporally focused laser pulses (TEFO) for which the axial beam profile can be controlled independently of its lateral distribution, large numbers of channels on individual neurons can be excited simultaneously, leading to strong (up to 15 mV) and fast (< or = 1 ms) depolarizations. Furthermore, we demonstrated selective activation of cellular compartments, such as dendrites and large presynaptic terminals, at depths up to 150 microm. The demonstrated spatiotemporal resolution and the selectivity provided by TEFO allow manipulation of neuronal activity, with a large number of applications in studies of neuronal microcircuit function in vitro and in vivo.

Optogenetic control of striatal dopamine release in rats

Optogenetic control over neuronal firing has become an increasingly elegant method to dissect the microcircuitry of mammalian brains. To date, examination of these manipulations on neurotransmitter release has been minimal. Here we present the first in-depth analysis of optogenetic stimulation on dopamine neurotransmission in the dorsal striatum of urethane-anesthetized rats. By combining the tight spatial and temporal resolution of both optogenetics and fast-scan cyclic voltammetry we have determined the parameters necessary to control phasic dopamine release in the dorsal striatum of rats in vivo. The kinetics of optically induced dopamine release mirror established models of electrically evoked release, indicating that potential artifacts of electrical stimulation on ion channels and the dopamine transporter are negligible. Furthermore a lack of change in extracellular pH indicates that optical stimulation does not alter blood flow. Optical control over dopamine release is highly reproducible and flexible. We are able to repeatedly evoke concentrations of dopamine release as small as a single dopamine transient (50 nM). An inverted U-shaped frequency response curve exists with maximal stimulation inducing dopamine effluxes exceeding 500 nM. Taken together, these results have obvious implications for understanding the neurobiological basis of dopaminergic-based disorders and provide the framework to effectively manipulate dopamine patterns.

Glutamatergic signaling by mesolimbic dopamine neurons in the nucleus accumbens

Recent evidence suggests the intriguing possibility that midbrain dopaminergic (DAergic) neurons may use fast glutamatergic transmission to communicate with their postsynaptic targets. Because of technical limitations, direct demonstration of the existence of this signaling mechanism has been limited to experiments using cell culture preparations that often alter neuronal function including neurotransmitter phenotype. Consequently, it remains uncertain whether glutamatergic signaling between DAergic neurons and their postsynaptic targets exists under physiological conditions. Here, using an optogenetic approach, we provide the first conclusive demonstration that mesolimbic DAergic neurons in mice release glutamate and elicit excitatory postsynaptic responses in projection neurons of the nucleus accumbens. In addition, we describe the properties of the postsynaptic glutamatergic responses of these neurons during experimentally evoked burst firing of DAergic axons that reproduce the reward-related phasic population activity of the mesolimbic projection. These observations indicate that, in addition to DAergic mechanisms, mesolimbic reward signaling may involve glutamatergic transmission.

Depressive-like effects of the kappa opioid receptor agonist salvinorin A are associated with decreased phasic dopamine release in the nucleus accumbens

RATIONALE

Kappa opioid receptors (KORs) have been implicated in depressive-like states associated with chronic administration of drugs of abuse and stress. Although KOR agonists decrease dopamine in the nucleus accumbens (NAc), KOR modulation of phasic dopamine release in the core and shell subregions of the NAc-which have distinct roles in reward processing-remains poorly understood.

OBJECTIVES

Studies were designed to examine whether the time course of effects of KOR activation on phasic dopamine release in the NAc core or shell are similar to effects on motivated behavior.

METHODS

The effect of systemic administration of the KOR agonist salvinorin A (salvA)-at a dose (2.0 mg/kg) previously determined to have depressive-like effects-was measured on electrically evoked phasic dopamine release in the NAc core or shell of awake and behaving rats using fast scan cyclic voltammetry. In parallel, the effects of salvA on intracranial self-stimulation (ICSS) and sucrose-reinforced responding were assessed. For comparison, a threshold dose of salvA (0.25 mg/kg) was also tested.

RESULTS

The active, but not threshold, dose of salvA significantly decreased phasic dopamine release without affecting dopamine reuptake in the NAc core and shell. SalvA increased ICSS thresholds and significantly lowered breakpoint on the progressive ratio schedule, indicating a decrease in motivation. The time course of the KOR-mediated decrease in dopamine in the core was qualitatively similar to the effects on motivated behavior.

CONCLUSIONS

These data suggest that the effects of KOR activation on motivation are due, in part, to inhibition of phasic dopamine signaling in the NAc core.

Chasing the cell assembly

Although we know enormous amounts of detailed information about the neurons that make up the cortex, placing this information back into the context of the behaving animal is a serious challenge. The functional cell assembly hypothesis first described by Hebb (The Organization of Behavior; a Neuropsychological Theory. New York: Wiley; 1949) aimed to provide a mechanistic explanation of how groups of neurons, acting together, form a percept. The vast number of neurons potentially involved make testing this hypothesis exceedingly difficult as neither the number nor locations of assembly members are known. Although increasing the number of neurons from which simultaneous recordings are made is of benefit, providing evidence for or against a hypothesis like Hebb's requires more than this. In this review, we aim to outline some recent technical advances, which may light the way in the chase for the functional cell assembly.

Dopamine signals for reward value and risk: basic and recent data

Background

Previous lesion, electrical self-stimulation and drug addiction studies suggest that the midbrain dopamine systems are parts of the reward system of the brain. This review provides an updated overview about the basic signals of dopamine neurons to environmental stimuli.

Methods

The described experiments used standard behavioral and neurophysiological methods to record the activity of single dopamine neurons in awake monkeys during specific behavioral tasks.

Results

Dopamine neurons show phasic activations to external stimuli. The signal reflects reward, physical salience, risk and punishment, in descending order of fractions of responding neurons. Expected reward value is a key decision variable for economic choices. The reward response codes reward value, probability and their summed product, expected value. The neurons code reward value as it differs from prediction, thus fulfilling the basic requirement for a bidirectional prediction error teaching signal postulated by learning theory. This response is scaled in units of standard deviation. By contrast, relatively few dopamine neurons show the phasic activation following punishers and conditioned aversive stimuli, suggesting a lack of relationship of the reward response to general attention and arousal. Large proportions of dopamine neurons are also activated by intense, physically salient stimuli. This response is enhanced when the stimuli are novel; it appears to be distinct from the reward value signal. Dopamine neurons show also unspecific activations to non-rewarding stimuli that are possibly due to generalization by similar stimuli and pseudoconditioning by primary rewards. These activations are shorter than reward responses and are often followed by depression of activity. A separate, slower dopamine signal informs about risk, another important decision variable. The prediction error response occurs only with reward; it is scaled by the risk of predicted reward.

Conclusions

Neurophysiological studies reveal phasic dopamine signals that transmit information related predominantly but not exclusively to reward. Although not being entirely homogeneous, the dopamine signal is more restricted and stereotyped than neuronal activity in most other brain structures involved in goal directed behavior.

Dopamine signals for reward value and risk: basic and recent data

BACKGROUND

Previous lesion, electrical self-stimulation and drug addiction studies suggest that the midbrain dopamine systems are parts of the reward system of the brain. This review provides an updated overview about the basic signals of dopamine neurons to environmental stimuli.

METHODS

The described experiments used standard behavioral and neurophysiological methods to record the activity of single dopamine neurons in awake monkeys during specific behavioral tasks.

RESULTS

Dopamine neurons show phasic activations to external stimuli. The signal reflects reward, physical salience, risk and punishment, in descending order of fractions of responding neurons. Expected reward value is a key decision variable for economic choices. The reward response codes reward value, probability and their summed product, expected value. The neurons code reward value as it differs from prediction, thus fulfilling the basic requirement for a bidirectional prediction error teaching signal postulated by learning theory. This response is scaled in units of standard deviation. By contrast, relatively few dopamine neurons show the phasic activation following punishers and conditioned aversive stimuli, suggesting a lack of relationship of the reward response to general attention and arousal. Large proportions of dopamine neurons are also activated by intense, physically salient stimuli. This response is enhanced when the stimuli are novel; it appears to be distinct from the reward value signal. Dopamine neurons show also unspecific activations to non-rewarding stimuli that are possibly due to generalization by similar stimuli and pseudoconditioning by primary rewards. These activations are shorter than reward responses and are often followed by depression of activity. A separate, slower dopamine signal informs about risk, another important decision variable. The prediction error response occurs only with reward; it is scaled by the risk of predicted reward.

CONCLUSIONS

Neurophysiological studies reveal phasic dopamine signals that transmit information related predominantly but not exclusively to reward. Although not being entirely homogeneous, the dopamine signal is more restricted and stereotyped than neuronal activity in most other brain structures involved in goal directed behavior.

Basolateral amygdala modulates terminal dopamine release in the nucleus accumbens and conditioned responding

BACKGROUND

Dopamine signaling in the nucleus accumbens (NAc) is essential for goal-directed behaviors and primarily arises from burst firing of ventral tegmental area neurons. However, the role of associative neural substrates such as the basolateral amygdala (BLA) in regulating phasic dopamine release in the NAc, particularly during reward seeking, remains unknown.

METHODS

Male Sprague-Dawley rats learned to discriminate two cues: a discriminative stimulus (DS) that predicted sucrose reinforcement contingent upon a lever press and a nonassociated stimulus (NS) that predicted a second lever never reinforced with sucrose. Following training, a test session was completed in which NAc dopamine was measured using fast-scan cyclic voltammetry in conjunction with inactivation of the ipsilateral BLA (gamma-aminobutyric acid agonists; baclofen/muscimol) to determine the contribution of BLA activity to dopamine release in the NAc core during the task.

RESULTS

Under vehicle conditions, DS and NS presentation elicited dopamine release within the NAc core. The DS evoked significantly more dopamine than the NS. Inactivation of the BLA selectively attenuated the magnitude of DS-evoked dopamine release, concurrent with an attenuation of DS-evoked conditioned approaches. Other behavioral responses (e.g., lever pressing) and dopamine release concomitant with those events were unaltered by BLA inactivation. Furthermore, neither ventral tegmental area electrically stimulated dopamine release nor the probability of high concentration dopamine release events was altered following BLA inactivation.

CONCLUSIONS

These results demonstrate that the BLA terminally modulates dopamine signals within the NAc core under specific, behaviorally relevant conditions, illustrating a functional mechanism by which the BLA selectively facilitates responding to motivationally salient environmental stimuli.

Adaptive and aberrant reward prediction signals in the human brain

Theories of the positive symptoms of schizophrenia hypothesize a role for aberrant reinforcement signaling driven by dysregulated dopamine transmission. Recently, we provided evidence of aberrant reward learning in symptomatic, but not asymptomatic patients with schizophrenia, using a novel paradigm, the Salience Attribution Test (SAT). The SAT is a probabilistic reward learning game that employs cues that vary across task-relevant and task-irrelevant dimensions; it provides behavioral indices of adaptive and aberrant reward learning. As an initial step prior to future clinical studies, here we used functional magnetic resonance imaging to examine the neural basis of adaptive and aberrant reward learning during the SAT in healthy volunteers. As expected, cues associated with high relative to low reward probabilities elicited robust hemodynamic responses in a network of structures previously implicated in motivational salience; the midbrain, in the vicinity of the ventral tegmental area, and regions targeted by its dopaminergic projections, i.e. medial dorsal thalamus, ventral striatum and prefrontal cortex (PFC). Responses in the medial dorsal thalamus and polar PFC were strongly correlated with the degree of adaptive reward learning across participants. Finally, and most importantly, differential dorsolateral PFC and middle temporal gyrus (MTG) responses to cues with identical reward probabilities were very strongly correlated with the degree of aberrant reward learning. Participants who showed greater aberrant learning exhibited greater dorsolateral PFC responses, and reduced MTG responses, to cues erroneously inferred to be less strongly associated with reward. The results are discussed in terms of their implications for different theories of associative learning.

Molecular and cellular approaches for diversifying and extending optogenetics

Optogenetic technologies employ light to control biological processes within targeted cells in vivo with high temporal precision. Here, we show that application of molecular trafficking principles can expand the optogenetic repertoire along several long-sought dimensions. Subcellular and transcellular trafficking strategies now permit (1) optical regulation at the far-red/infrared border and extension of optogenetic control across the entire visible spectrum, (2) increased potency of optical inhibition without increased light power requirement (nanoampere-scale chloride-mediated photocurrents that maintain the light sensitivity and reversible, step-like kinetic stability of earlier tools), and (3) generalizable strategies for targeting cells based not only on genetic identity, but also on morphology and tissue topology, to allow versatile targeting when promoters are not known or in genetically intractable organisms. Together, these results illustrate use of cell-biological principles to enable expansion of the versatile fast optogenetic technologies suitable for intact-systems biology and behavior.

Optogenetic interrogation of neural circuits: technology for probing mammalian brain structures

Elucidation of the neural substrates underlying complex animal behaviors depends on precise activity control tools, as well as compatible readout methods. Recent developments in optogenetics have addressed this need, opening up new possibilities for systems neuroscience. Interrogation of even deep neural circuits can be conducted by directly probing the necessity and sufficiency of defined circuit elements with millisecond-scale, cell type-specific optical perturbations, coupled with suitable readouts such as electrophysiology, optical circuit dynamics measures and freely moving behavior in mammals. Here we collect in detail our strategies for delivering microbial opsin genes to deep mammalian brain structures in vivo, along with protocols for integrating the resulting optical control with compatible readouts (electrophysiological, optical and behavioral). The procedures described here, from initial virus preparation to systems-level functional readout, can be completed within 4-5 weeks. Together, these methods may help in providing circuit-level insight into the dynamics underlying complex mammalian behaviors in health and disease.

An intrinsic neuronal oscillator underlies dopaminergic neuron bursting

Dopaminergic neurons of the ventral midbrain fire high-frequency bursts when animals are presented with unexpected rewards, or stimuli that predict reward. To identify the afferents that can initiate bursting and establish therapeutic strategies for diseases affected by altered bursting, a mechanistic understanding of bursting is essential. Our results show that bursting is initiated by a specific interaction between the voltage sensitivity of NMDA receptors and voltage-gated ion channels that results in the activation of an intrinsic, action potential-independent, high-frequency membrane potential oscillation. We further show that the NMDA receptor is uniquely suited for this because of the rapid kinetics and voltage dependence imparted to it by Mg(2+) ion block and unblock. This mechanism explains the discrete nature of bursting in dopaminergic cells and demonstrates how synaptic signals may be reshaped by local intrinsic properties of a neuron before influencing action potential generation.

Targeted optogenetic stimulation and recording of neurons in vivo using cell-type-specific expression of Channelrhodopsin-2

A major long-term goal of systems neuroscience is to identify the different roles of neural subtypes in brain circuit function. The ability to causally manipulate selective cell types is critical to meeting this goal. This protocol describes techniques for optically stimulating specific populations of excitatory neurons and inhibitory interneurons in vivo in combination with electrophysiology. Cell type selectivity is obtained using Cre-dependent expression of the light-activated channel Channelrhodopsin-2. We also describe approaches for minimizing optical interference with simultaneous extracellular and intracellular recording. These optogenetic techniques provide a spatially and temporally precise means of studying neural activity in the intact brain and allow a detailed examination of the effect of evoked activity on the surrounding local neural network. Injection of viral vectors requires 30-45 min, and in vivo electrophysiology with optogenetic stimulation requires 1-4 h.

Optogenetic deconstruction of sleep-wake circuitry in the brain

How does the brain regulate the sleep–wake cycle? What are the temporal codes of sleep and wake-promoting neural circuits? How do these circuits interact with each other across the light/dark cycle? Over the past few decades, many studies from a variety of disciplines have made substantial progress in answering these fundamental questions. For example, neurobiologists have identified multiple, redundant wake-promoting circuits in the brainstem, hypothalamus, and basal forebrain. Sleep-promoting circuits have been found in the preoptic area and hypothalamus. One of the greatest challenges in recent years has been to selectively record and manipulate these sleep–wake centers in vivo with high spatial and temporal resolution. Recent developments in microbial opsin-based neuromodulation tools, collectively referred to as “optogenetics,” have provided a novel method to demonstrate causal links between neural activity and specific behaviors. Here, we propose to use optogenetics as a fundamental tool to probe the necessity, sufficiency, and connectivity of defined neural circuits in the regulation of sleep and wakefulness.

Multiple functions of dopamine neurons

Dopamine neurons carry phasic signals for a limited number of behavioural events. The events include, in descending order, reward, physically intense stimuli, risk and punishment. Recent neurophysiological studies have provided interesting details on these functions.

AGRP neurons are sufficient to orchestrate feeding behavior rapidly and without training

Two intermingled hypothalamic neuron populations specified by expression of agouti-related peptide (AGRP) or pro-opiomelanocortin (POMC) positively and negatively influence feeding behavior, respectively, possibly by reciprocally regulating downstream melanocortin receptors. However, the sufficiency of these neurons to control behavior and the relationship of their activity to the magnitude and dynamics of feeding are unknown. To measure this, we used channelrhodopsin-2 for cell type-specific photostimulation. Activation of only 800 AGRP neurons in mice evoked voracious feeding within minutes. The behavioral response increased with photoexcitable neuron number, photostimulation frequency and stimulus duration. Conversely, POMC neuron stimulation reduced food intake and body weight, which required melanocortin receptor signaling. However, AGRP neuron-mediated feeding was not dependent on suppressing this melanocortin pathway, indicating that AGRP neurons directly engage feeding circuits. Furthermore, feeding was evoked selectively over drinking without training or prior photostimulus exposure, which suggests that AGRP neurons serve a dedicated role coordinating this complex behavior.

Rewiring cells: synthetic biology as a tool to interrogate the organizational principles of living systems

The living cell is an incredibly complex entity, and the goal of predictively and quantitatively understanding its function is one of the next great challenges in biology. Much of what we know about the cell concerns its constituent parts, but to a great extent we have yet to decode how these parts are organized to yield complex physiological function. Classically, we have learned about the organization of cellular networks by disrupting them through genetic or chemical means. The emerging discipline of synthetic biology offers an additional, powerful approach to study systems. By rearranging the parts that comprise existing networks, we can gain valuable insight into the hierarchical logic of the networks and identify the modular building blocks that evolution uses to generate innovative function. In addition, by building minimal toy networks, one can systematically explore the relationship between network structure and function. Here, we outline recent work that uses synthetic biology approaches to investigate the organization and function of cellular networks, and describe a vision for a synthetic biology toolkit that could be used to interrogate the design principles of diverse systems.

Metabolic hormones, dopamine circuits, and feeding

Recent evidence has emerged demonstrating that metabolic hormones such as ghrelin and leptin can act on ventral tegmental area (VTA) midbrain dopamine neurons to influence feeding. The VTA is the origin of mesolimbic dopamine neurons that project to the nucleus accumbens (NAc) to influence behavior. While blockade of dopamine via systemic antagonists or targeted gene delete can impair food intake, local NAc dopamine manipulations have little effect on food intake. Notably, non-dopaminergic manipulations in the VTA and NAc produce more consistent effects on feeding and food choice. More recent genetic evidence supports a role for the substantia nigra-striatal dopamine pathways in food intake, while the VTA-NAc circuit is more likely involved in higher-order aspects of food acquisition, such as motivation and cue associations. This rich and complex literature should be considered in models of how peripheral hormones influence feeding behavior via action on the midbrain circuits.

Studying Cerebellar Circuits by Remote Control of Selected Neuronal Types with GABA(A) Receptors

Although GABA_A receptor-mediated inhibition of cerebellar Purkinje cells by molecular layer interneurons (MLIs) has been studied intensely at the cellular level, it has remained unclear how this inhibition regulates cerebellum-dependent behaviour. We have implemented two complementary approaches to investigate the function of the MLI-Purkinje cell synapse on the behavioural level. In the first approach we permanently disrupted inhibitory fast synaptic transmission at the synapse by genetically removing the postsynaptic GABA_A receptors from Purkinje cells (PC-Δγ2 mice). We found that chronic disruption of the MLI-Purkinje cell synapse strongly impaired cerebellar learning of the vestibular occular reflex (VOR), presumably by disrupting the temporal patterns of Purkinje cell activity. However, in PC-Δγ2 mice the baseline VOR reflex was only mildly affected; indeed PC-Δγ2 mice show no ataxia or gait abnormalities, suggesting that MLI control of Purkinje cell activity is either not involved in ongoing motor tasks or that the system compensates for its loss. To investigate the latter possibility we developed an alternative genetic technique; we made the MLI-Purkinje cell synapse selectively sensitive to rapid manipulation with the GABA_A receptor modulator zolpidem (PC-γ2-swap mice). Minutes after intraperitoneal zolpidem injection, these PC-γ2-swap mice developed severe motor abnormalities, revealing a substantial contribution of the MLI-Purkinje cell synapses to real time motor control. The cell-type selective permanent knockout of synaptic GABAergic input and the fast reversible modulation of GABAergic input at the same synapse illustrate how pursuing both strategies gives a fuller view.