Ventral tegmental area astrocytes orchestrate avoidance and approach behavior.

The ventral tegmental area (VTA) is a heterogeneous midbrain structure, containing neurons and astrocytes, that coordinates behaviors by integrating activity from numerous afferents. Within neuron-astrocyte networks, astrocytes control signals from distinct afferents in a circuit-specific manner, but whether this capacity scales up to drive motivated behavior has been undetermined. Using genetic and optical dissection strategies we report that VTA astrocytes tune glutamatergic signaling selectively on local inhibitory neurons to drive a functional circuit for learned avoidance. In this circuit, astrocytes facilitate excitation of VTA GABA neurons to increase inhibition of dopamine neurons, eliciting real-time and learned avoidance behavior that is sufficient to impede expression of preference for reward. Loss of one glutamate transporter (GLT-1) from VTA astrocytes selectively blocks these avoidance behaviors and spares preference for reward. Thus, VTA astrocytes selectively regulate excitation of local GABA neurons to drive a distinct avoidance circuit that opposes approach behavior.

Generating bursts (and pauses) in the dopamine midbrain neurons.

Dopamine (DA) midbrain neurons project to several striatal and cortical target areas and are essentially involved in a puzzling variety of important brain functions such as action selection and motor performance, motivation and reward-based learning, but also working memory and cognition. These neurons act via the release of their (main) neurotransmitter, dopamine, which binds to metabotropic dopamine receptors of the D1 or D2 type on target neurons. Axonal but also dendritic dopamine release is essentially controlled by calcium-triggered exocytosis of dopamine-filled synaptic vesicles primarily driven by electrical activity of the dopamine neuron, which generates patterns of actions potentials in the somato-dendritic domain and distributes them along its axonal tree. Thus, recording the behaviorally relevant pattern of electrical activity in DA neurons and identifying the underlying biophysical mechanisms that integrate afferent synaptic inputs and intrinsic excitability constitute a crucial element for defining the physiological roles of the midbrain DA system. Electrical activity of midbrain DA neurons in vivo is characterized by tonic background activity in a narrow frequency range (ca. 1-8Hz) interrupted by either transient (i.e. phasic, <500ms) sequences of high-frequency firing (>15Hz), so called "bursts", or transient pauses of electrical activity, where DA neurons generate no action potentials. This review focuses on the properties of these phasic activity changes in midbrain DA neurons. It updates recent progress on the expanding behavioral contexts, associated with phasic electrical activity in DA neurons beyond the classical (canonical) reward prediction error model. The review also highlights recently defined contributions of synaptic inputs for burst and pause generation and the roles of distinct postsynaptic ion channels in midbrain DA neurons.

Detection of bursts and pauses in spike trains.

Midbrain dopaminergic neurons in vivo exhibit a wide range of firing patterns. They normally fire constantly at a low rate, and speed up, firing a phasic burst when reward exceeds prediction, or pause when an expected reward does not occur. Therefore, the detection of bursts and pauses from spike train data is a critical problem when studying the role of phasic dopamine (DA) in reward related learning, and other DA dependent behaviors. However, few statistical methods have been developed that can identify bursts and pauses simultaneously. We propose a new statistical method, the Robust Gaussian Surprise (RGS) method, which performs an exhaustive search of bursts and pauses in spike trains simultaneously. We found that the RGS method is adaptable to various patterns of spike trains recorded in vivo, and is not influenced by baseline firing rate, making it applicable to all in vivo spike trains where baseline firing rates vary over time. We compare the performance of the RGS method to other methods of detecting bursts, such as the Poisson Surprise (PS), Rank Surprise (RS), and Template methods. Analysis of data using the RGS method reveals potential mechanisms underlying how bursts and pauses are controlled in DA neurons.

Dynamic regulation of midbrain dopamine neuron activity: intrinsic, synaptic, and plasticity mechanisms.

Although the roles of dopaminergic signaling in learning and behavior are well established, it is not fully understood how the activity of dopaminergic neurons is dynamically regulated under different conditions in a constantly changing environment. Dopamine neurons must integrate sensory, motor, and cognitive information online to inform the organism to pursue outcomes with the highest reward probability. In this article, we provide an overview of recent advances on the intrinsic, extrinsic (i.e., synaptic), and plasticity mechanisms controlling dopamine neuron activity, mostly focusing on mechanistic studies conducted using ex vivo brain slice preparations. We also hope to highlight some unresolved questions regarding information processing that takes place at dopamine neurons, thereby stimulating further investigations at different levels of analysis.

Electrophysiological properties of catecholaminergic neurons in the norepinephrine-deficient mouse.

To determine how norepinephrine affects the basic physiological properties of catecholaminergic neurons, brain slices containing the substantia nigra pars compacta and locus coeruleus were studied with cell-attached and whole-cell recordings in control and dopamine beta-hydroxylase knockout (Dbh -/-) mice that lack norepinephrine. In the cell-attached configuration, the spontaneous firing rate and pattern of locus coeruleus neurons recorded from Dbh -/- mice were the same as the firing rate and pattern recorded from heterozygous littermates (Dbh +/-). During whole-cell recordings, synaptic stimulation produced an alpha-2 receptor-mediated outward current in the locus coeruleus of control mice that was absent in Dbh -/- mice. Normal alpha-2 mediated outward currents were restored in Dbh -/- slices after pre-incubation with norepinephrine. Locus coeruleus neurons also displayed similar changes in holding current in response to bath application of norepinephrine, UK 14304, and methionine-enkephalin. Dopamine neurons recorded in the substantia nigra pars compacta similarly showed no differences between slices harvested from Dbh -/- and control mice. These results indicate that endogenous norepinephrine is not necessary for the expression of catecholaminergic neuron firing properties or responses to direct agonists, but is necessary for auto-inhibition mediated by indirect alpha-2 receptor stimulation.

Amphetamine selectively blocks inhibitory glutamate transmission in dopamine neurons.

Amphetamine is a highly addictive psychostimulant that promotes the release of the catecholamines dopamine and norepinephrine. Amphetamine-induced release of dopamine in the midbrain inhibits the activity of dopamine neurons through activation of D2 dopamine autoreceptors. Here we show that amphetamine may also excite dopamine neurons through modulation of glutamate neurotransmission. Amphetamine potently inhibits metabotropic glutamate receptor (mGluR)-mediated IPSPs in dopamine neurons, but has no effect on ionotropic glutamate receptor-mediated EPSCs. Amphetamine desensitizes the mGluR-mediated hyperpolarization through release of dopamine, activation of postsynaptic alpha1 adrenergic receptors, and suppression of InsP3-induced calcium release from internal stores. By selectively suppressing the inhibitory component of glutamate-mediated transmission, amphetamine may promote burst firing of dopamine neurons. Through this mechanism, amphetamine may enhance phasic release of dopamine, which is important in the neural processing of reward.

GABAA receptor stimulation blocks NMDA-induced bursting of dopaminergic neurons in vitro by decreasing input resistance.

The effects of the GABAA agonist, isoguvacine, on NMDA-induced burst firing of substantia nigra dopaminergic neurons were studied with intracellular and whole cell recordings in vitro. NMDA application caused the neurons to fire in rhythmic bursts. Although the NMDA-induced bursty firing pattern was insensitive to hyperpolarization by current injection, it was reversibly abolished by the selective GABAA agonist, isoguvacine. The block of the rhythmic burst pattern by isoguvacine application occurred regardless of whether the chloride reversal potential was hyperpolarizing (ECl-=-70 mV) or depolarizing (ECl-=-40 mV). In either case, the input resistance of the dopaminergic neurons was dramatically decreased by application of isoguvacine. It is concluded that GABAA receptor activation by isoguvacine disrupts NMDA receptor-mediated burst firing by increasing the input conductance and thereby shunting the effects of NMDA acting at a distally located generator of rhythmic burst firing.

GABA(A) and GABA(B) antagonists differentially affect the firing pattern of substantia nigra dopaminergic neurons in vivo.

The effects of local pressure application of the selective GABA(A) antagonists, bicuculline, gabazine, and picrotoxin, and the selective GABA(B) antagonists, 2-OH-saclofen and CGP-55845A, on the spontaneous activity of electrophysiologically identified substantia nigra dopaminergic neurons were recorded in vivo in urethane anesthetized rats. Blockade of GABA(A) inputs by bicuculline powerfully and reversibly induced burst firing in dopaminergic neurons along with a modest (25%) increase in firing rate, but the increase in burst firing was not correlated with the increase in firing rate. Picrotoxin and gabazine also produced an increase in burst firing without an increase in firing rate. In contrast, local application of GABA(B) antagonists did not produce bursting but rather caused a modest shift to a more regular firing pattern in 50% of the cases. These data demonstrate that dopaminergic neurons in vivo are under tonic GABAergic inhibition mediated by GABA(A) receptors and suggest that GABAergic afferents to substantia nigra comprise a major pathway by which the firing pattern of dopaminergic neurons is controlled in vivo.

Striatal, pallidal, and pars reticulata evoked inhibition of nigrostriatal dopaminergic neurons is mediated by GABA(A) receptors in vivo.

Dopaminergic neurons express both GABA(A) and GABA(B) receptors and GABAergic inputs play a significant role in the afferent modulation of these neurons. Electrical stimulation of GABAergic pathways originating in neostriatum, globus pallidus or substantia nigra pars reticulata produces inhibition of dopaminergic neurons in vivo. Despite a number of prior studies, the identity of the GABAergic receptor subtype(s) mediating the inhibition evoked by electrical stimulation of neostriatum, globus pallidus, or the axon collaterals of the projection neurons from substantia nigra pars reticulata in vivo remain uncertain. Single-unit extracellular recordings were obtained from substantia nigra dopaminergic neurons in urethane anesthetized rats. The effects of local pressure application of the selective GABA(A) antagonists, bicuculline and picrotoxin, and the GABA(B) antagonists, saclofen and CGP-55845A, on the inhibition of dopaminergic neurons elicited by single-pulse electrical stimulation of striatum, globus pallidus, and the thalamic axon terminals of the substantia nigra pars reticulata projection neurons were recorded in vivo. Striatal, pallidal, and thalamic induced inhibition of dopaminergic neurons was always attenuated or completely abolished by local application of the GABA(A) antagonists. In contrast, the GABA(B) antagonists, saclofen or CGP-55845A, did not block or attenuate the stimulus-induced inhibition and at times even increased the magnitude and/or duration of the evoked inhibition. Train stimulation of globus pallidus and striatum also produced an inhibition of firing in dopaminergic neurons of longer duration. However this inhibition was largely insensitive to either GABA(A) or GABA(B) antagonists although the GABA(A) antagonists consistently blocked the early portion of the inhibitory period indicating the presence of a GABA(A) component. These data demonstrate that dopaminergic neurons of the substantia nigra pars compacta are inhibited by electrical stimulation of striatum, globus pallidus, and the projection neurons of substantia nigra pars reticulata in vivo. This inhibition appears to be mediated via the GABA(A) receptor subtype, and all three GABAergic afferents studied appear to possess inhibitory presynaptic GABA(B) autoreceptors that are active under physiological conditions in vivo.

GABAergic control of rat substantia nigra dopaminergic neurons: role of globus pallidus and substantia nigra pars reticulata.

Dopaminergic neurons in vivo fire spontaneously in three distinct patterns or modes. It has previously been shown that the firing pattern of substantia nigra dopaminergic neurons can be differentially modulated by local application of GABA(A) and GABA(B) receptor antagonists. The GABA(A) antagonists, bicuculline or picrotoxin, greatly increase burst firing in dopaminergic neurons whereas GABA(B) antagonists cause a modest shift away from burst firing towards pacemaker-like firing. The three principal GABAergic inputs to nigral dopaminergic neurons arise from striatum, globus pallidus and from the axon collaterals of nigral pars reticulata projection neurons, each of which appear to act in vivo primarily on GABA(A) receptors (see preceding paper). In this study we attempted to determine on which afferent pathway(s) GABA(A) antagonists were acting to cause burst firing. Substantia nigra dopaminergic neurons were studied by single unit extracellular recordings in urethane anesthetized rats during pharmacologically induced inhibition and excitation of globus pallidus. Muscimol-induced inhibition of pallidal neurons produced an increase in the regularity of firing of nigral dopaminergic neurons together with a slight decrease in firing rate. Bicuculline-induced excitation of globus pallidus neurons produced a marked increase in burst firing together with a modest increase in firing rate. These changes in firing rate were in the opposite direction to what would be expected for a monosynaptic GABAergic pallidonigral input. Examination of the response of pars reticulata GABAergic neurons to similar manipulations of globus pallidus revealed that the firing rates of these neurons were much more sensitive to changes in globus pallidus neuron firing rate than dopaminergic neurons and that they responded in the opposite direction. Pallidal inhibition produced a dramatic increase in the firing rate of pars reticulata GABAergic neurons while pallidal excitation suppressed the spontaneous activity of pars reticulata GABAergic neurons. These data suggest that globus pallidus exerts significant control over the firing rate and pattern of substantia nigra dopaminergic neurons through a disynaptic pathway involving nigral pars reticulata GABAergic neurons and that at least one important way in which local application of bicuculline induces burst firing of dopaminergic neurons is by disinhibition of this tonic inhibitory input.