Loss of ß-arrestin2 in D2 cells alters neuronal excitability in the nucleus accumbens and behavioral responses to psychostimulants and opioids.

Psychostimulants and opioids increase dopamine (DA) neurotransmission, activating D1 and D2 G protein-coupled receptors. ß-arrestin2 (ßarr2) desensitizes and internalizes these receptors and initiates G protein-independent signaling. Previous work revealed that mice with a global or cell-specific knockout of ßarr2 have altered responses to certain drugs; however, the effects of ßarr2 on the excitability of medium spiny neurons (MSNs), and its role in mediating the rewarding effects of drugs of abuse are unknown. D1-Cre and D2-Cre transgenic mice were crossed with floxed ßarr2 mice to eliminate ßarr2 specifically in cells containing either D1 (D1ßarr2-KO ) or D2 (D2ßarr2-KO ) receptors. We used slice electrophysiology to characterize the role of ßarr2 in modulating D1 and D2 nucleus accumbens MSN intrinsic excitability in response to DA and tested the locomotor-activating and rewarding effects of cocaine and morphine in these mice. Eliminating ßarr2 attenuated the ability of DA to inhibit D2-MSNs and altered the DA-induced maximum firing rate in D1-MSNs. While D1ßarr2-KO mice had mostly normal drug responses, D2ßarr2-KO mice showed dose-dependent reductions in acute locomotor responses to cocaine and morphine, attenuated locomotor sensitization to cocaine, and blunted cocaine reward measured with conditioned place preference. Both D2ßarr2-KO and D1ßarr2-KO mice displayed an enhanced conditioned place preference for the highest dose of morphine. These results indicate that D1- and D2-derived ßarr2 functionally contribute to DA-induced changes in MSN intrinsic excitability and behavioral responses to psychostimulants and opioids dose-dependently.

Cocaine Selectively Reorganizes Excitatory Inputs to Substantia Nigra Pars Compacta Dopamine Neurons.

Substantia nigra pars compacta (SNc) dopamine neurons and their targets are involved in addiction and cue-induced relapse. However, afferents onto SNc dopamine neurons themselves appear insensitive to drugs of abuse, such as cocaine, when afferents are collectively stimulated electrically. This contrasts with ventral tegmental area (VTA) dopamine neurons, whose glutamate afferents react robustly to cocaine. We used an optogenetic strategy to isolate identified SNc inputs and determine whether cocaine sensitivity in the mouse SNc circuit is conferred at the level of three glutamate afferents: dorsal raphé nucleus (DR), pedunculopontine nucleus (PPN), and subthalamic nucleus (STN). We found that excitatory afferents to SNc dopamine neurons are sensitive to cocaine in an afferent-specific manner. A single exposure to cocaine in vivo led to PPN-innervated synapses reducing the AMPA-to-NMDA receptor-mediated current ratio. In contrast to work in the VTA, this was due to increased NMDA receptor function with no change in AMPA receptor function. STN synapses showed a decrease in calcium-permeable AMPA receptors after cocaine, but no change in the AMPA-to-NMDA ratio. Cocaine also increased the release probability at DR-innervated and STN-innervated synapses, quantified by decreases in paired-pulse ratios. However, release probability at PPN-innervated synapses remained unaffected. By examining identified inputs, our results demonstrate a functional distribution among excitatory SNc afferent nuclei in response to cocaine, and suggest a compelling architecture for differentiation and separate parsing of inputs within the nigrostriatal system.SIGNIFICANCE STATEMENT Prior studies have established that substantia nigra pars compacta (SNc) dopamine neurons are a key node in the circuitry that drives addiction and relapse, yet cocaine apparently has no effect on electrically stimulated excitatory inputs. Our study is the first to demonstrate the functional impact of a drug of abuse on synaptic mechanisms of identified afferents to the SNc. Optogenetic dissection of inputs originating from dorsal raphé, pedunculopontine, and subthalamic nuclei were tested for synaptic modifications following in vivo cocaine exposure. Our results demonstrate that cocaine differentially induces modifications to SNc synapses depending on input origin. This presents implications for understanding dopamine processing of motivated behavior; most critically, it indicates that dopamine neurons selectively modulate signal reception processed by afferent nuclei.

Differential expression of long-term potentiation among identified inhibitory inputs to dopamine neurons.

The in vivo firing pattern of ventral tegmental area (VTA) dopamine neurons is controlled by GABA afferents originating primarily from the nucleus accumbens (NAc), rostromedial tegmental nucleus (RMTg), and local GABA neurons within the VTA. Although different forms of plasticity have been observed from GABA inputs to VTA dopamine neurons, one dependent on cyclic GMP synthesis and the other on adenylyl cyclase activation, it is unknown whether plasticity is differentially expressed in each. Using an optogenetic strategy, we show that identified inhibitory postsynaptic currents (IPSCs) from local VTA GABA neurons and NAc afferents exhibit a cyclic GMP-dependent long-term potentiation (LTP) that is capable of inhibiting the firing activity of dopamine neurons. However, this form of LTP was not induced from RMTg afferents. Only an adenylyl cyclase-mediated increase in IPSCs was exhibited by all three inputs. Thus discrete plasticity mechanisms recruit overlapping but different subsets of GABA inputs to VTA dopamine neurons.NEW & NOTEWORTHY We describe a mapping of plasticity expression, mediated by different mechanisms, among three distinct GABA afferents to ventral tegmental area (VTA) dopamine neurons: the rostromedial tegmental nucleus, the nucleus accumbens, and the local GABA neurons within the VTA known to synapse on VTA dopamine neurons. This work is the first demonstration that discrete plasticity mechanisms recruit overlapping but different subsets of GABA inputs to VTA dopamine neurons.

Chronic loss of noradrenergic tone produces ß-arrestin2-mediated cocaine hypersensitivity and alters cellular D2 responses in the nucleus accumbens.

Cocaine blocks plasma membrane monoamine transporters and increases extracellular levels of dopamine (DA), norepinephrine (NE) and serotonin (5-HT). The addictive properties of cocaine are mediated primarily by DA, while NE and 5-HT play modulatory roles. Chronic inhibition of dopamine ß-hydroxylase (DBH), which converts DA to NE, increases the aversive effects of cocaine and reduces cocaine use in humans, and produces behavioral hypersensitivity to cocaine and D2 agonism in rodents, but the underlying mechanism is unknown. We found a decrease in ß-arrestin2 (ßArr2) in the nucleus accumbens (NAc) following chronic genetic or pharmacological DBH inhibition, and overexpression of ßArr2 in the NAc normalized cocaine-induced locomotion in DBH knockout (Dbh -/-) mice. The D2/3 agonist quinpirole decreased excitability in NAc medium spiny neurons (MSNs) from control, but not Dbh -/- animals, where instead there was a trend for an excitatory effect. The Gαi inhibitor NF023 abolished the quinpirole-induced decrease in excitability in control MSNs, but had no effect in Dbh -/- MSNs, whereas the Gαs inhibitor NF449 restored the ability of quinpirole to decrease excitability in Dbh -/- MSNs, but had no effect in control MSNs. These results suggest that chronic loss of noradrenergic tone alters behavioral responses to cocaine via decreases in ßArr2 and cellular responses to D2/D3 activation, potentially via changes in D2-like receptor G-protein coupling in NAc MSNs.

Food restriction increases glutamate receptor-mediated burst firing of dopamine neurons.

Restriction of food intake increases the acquisition of drug abuse behavior and enhances the reinforcing efficacy of those drugs. However, the neurophysiological mechanisms responsible for the interactions between feeding state and drug use are largely unknown. Here we show that chronic mild food restriction increases the burst firing of dopamine neurons in the substantia nigra. Dopamine neurons from food-restricted mice exhibited increased burst firing in vivo, an effect that was enhanced by an injection of the psychomotor stimulant cocaine (10 mg/kg, i.p.). Food restriction also enhanced aspartic acid-induced burst firing of dopamine neurons in an ex vivo brain slice preparation, consistent with an adaptation occurring in the somatodendritic compartment and independent of a circuit mechanism. Enhanced burst firing persisted after 10 d of free feeding following chronic food restriction but was not observed following a single overnight fast. Whole-cell patch-clamp recordings indicated that food restriction also increased electrically evoked AMPAR/NMDAR ratios and increased D2 autoreceptor-mediated desensitization in dopamine neurons. These results identify dopamine neurons in the substantia nigra as a convergence point for the interactions between feeding state and drugs of abuse. Furthermore, increased glutamate transmission combined with decreased autoreceptor inhibition could work in concert to enhance drug efficacy in response to food restriction.

Disruption of NMDAR-dependent burst firing by dopamine neurons provides selective assessment of phasic dopamine-dependent behavior.

Midbrain dopamine (DA) neurons fire in 2 characteristic modes, tonic and phasic, which are thought to modulate distinct aspects of behavior. However, the inability to selectively disrupt these patterns of activity has hampered the precise definition of the function of these modes of signaling. Here, we addressed the role of phasic DA in learning and other DA-dependent behaviors by attenuating DA neuron burst firing and subsequent DA release, without altering tonic neural activity. Disruption of phasic DA was achieved by selective genetic inactivation of NMDA-type, ionotropic glutamate receptors in DA neurons. Disruption of phasic DA neuron activity impaired the acquisition of numerous conditioned behavioral responses, and dramatically attenuated learning about cues that predicted rewarding and aversive events while leaving many other DA-dependent behaviors unaffected.

High-frequency, short-latency disinhibition bursting of midbrain dopaminergic neurons.

During reinforcement and sequence learning, dopaminergic neurons fire bursts of action potentials. Dopaminergic neurons in vivo receive strong background excitatory and inhibitory inputs, suggesting that one mechanism by which bursts may be produced is disinhibition. Unfortunately, these inputs are lost during slice preparation and are not precisely controlled during in vivo experiments. In the present study we show that dopaminergic neurons can be shifted into a balanced state in which constant synaptic N-methyl-d-aspartate (NMDA) and GABA(A) conductances are mimicked either pharmacologically or using dynamic clamp. From this state, a disinhibition burst can be evoked by removing the background inhibitory conductance. We demonstrate three functional characteristics of network-based disinhibition that promote high-frequency, short-latency bursting in dopaminergic neurons. First, we found that increasing the total background NMDA and GABA(A) synaptic conductances increased the intraburst firing frequency and reduced its latency. Second, we found that the disinhibition burst is sensitive to the proportion of background inhibitory input that is removed. In particular, we found that high-frequency, short-latency bursts were enhanced by increasing the degree of disinhibition. Third, the time course over which inhibition is removed had a large effect on the burst, namely, that synchronous removal of weak inhibitory inputs produces bursts of high intraburst frequency and shorter latency. Our results suggest that fast, more precisely timed bursts can be evoked by complete and synchronous disinhibition of dopaminergic neurons in a high-conductance state.

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.

A dynamic role for GABA receptors on the firing pattern of midbrain dopaminergic neurons.

Dopaminergic neurons are subject to a significant background GABAergic input in vivo. The presence of this GABAergic background might be expected to inhibit dopaminergic neuron firing. However, dopaminergic neurons are not all silent but instead fire in single-spiking and burst firing modes. Here we present evidence that phasic changes in the tonic activity of GABAergic afferents are a potential extrinsic mechanism that triggers bursts and pauses in dopaminergic neurons. We find that spontaneous single-spiking is more sensitive to activation of GABA receptors than phasic N-methyl-D-aspartate (NMDA)-mediated burst firing in rat slices (P15-P31). Because tonic activation of GABA(A) receptors has previously been shown to suppress burst firing in vivo, our results suggest that the activity patterns seen in vivo are the result of a balance between excitatory and inhibitory conductances that interact with the intrinsic pacemaking currents observed in slices. Using the dynamic clamp technique, we applied balanced, constant NMDA and GABA(A) receptor conductances into dopaminergic neurons in slices. Bursts could be produced by disinhibition (phasic removal of the GABA(A) receptor conductance), and these bursts had a higher frequency than bursts produced by the same NMDA receptor conductance alone. Phasic increases in the GABA(A) receptor conductance evoked pauses in firing. In contrast to NMDA receptor, application of constant AMPA and GABA(A) receptor conductances caused the cell to go into depolarization block. These results support a bidirectional mechanism by which GABAergic inputs, in balance with NMDA receptor-mediated excitatory inputs, control the firing pattern of dopaminergic neurons.

Regulation of firing frequency in a computational model of a midbrain dopaminergic neuron.

Dopaminergic (DA) neurons of the mammalian midbrain exhibit unusually low firing frequencies in vitro. Furthermore, injection of depolarizing current induces depolarization block before high frequencies are achieved. The maximum steady and transient rates are about 10 and 20 Hz, respectively, despite the ability of these neurons to generate bursts at higher frequencies in vivo. We use a three-compartment model calibrated to reproduce DA neuron responses to several pharmacological manipulations to uncover mechanisms of frequency limitation. The model exhibits a slow oscillatory potential (SOP) dependent on the interplay between the L-type Ca(2+) current and the small conductance K(+) (SK) current that is unmasked by fast Na(+) current block. Contrary to previous theoretical work, the SOP does not pace the steady spiking frequency in our model. The main currents that determine the spontaneous firing frequency are the subthreshold L-type Ca(2+) and the A-type K(+) currents. The model identifies the channel densities for the fast Na(+) and the delayed rectifier K(+) currents as critical parameters limiting the maximal steady frequency evoked by a depolarizing pulse. We hypothesize that the low maximal steady frequencies result from a low safety factor for action potential generation. In the model, the rate of Ca(2+) accumulation in the distal dendrites controls the transient initial frequency in response to a depolarizing pulse. Similar results are obtained when the same model parameters are used in a multi-compartmental model with a realistic reconstructed morphology, indicating that the salient contributions of the dendritic architecture have been captured by the simpler model.

In vivo patch-clamp recordings reveal distinct subthreshold signatures and threshold dynamics of midbrain dopamine neurons.

The in vivo firing patterns of ventral midbrain dopamine neurons are controlled by afferent and intrinsic activity to generate sensory cue and prediction error signals that are essential for reward-based learning. Given the absence of in vivo intracellular recordings during the last three decades, the subthreshold membrane potential events that cause changes in dopamine neuron firing patterns remain unknown. To address this, we established in vivo whole-cell recordings and obtained over 100 spontaneously active, immunocytochemically-defined midbrain dopamine neurons in isoflurane-anaesthetized adult mice. We identified a repertoire of subthreshold membrane potential signatures associated with distinct in vivo firing patterns. Dopamine neuron activity in vivo deviated from single-spike pacemaking by phasic increases in firing rate via two qualitatively distinct biophysical mechanisms: 1) a prolonged hyperpolarization preceding rebound bursts, accompanied by a hyperpolarizing shift in action potential threshold; and 2) a transient depolarization leading to high-frequency plateau bursts, associated with a depolarizing shift in action potential threshold. Our findings define a mechanistic framework for the biophysical implementation of dopamine neuron firing patterns in the intact brain.

Selective D2 and D3 receptor antagonists oppositely modulate cocaine responses in mice via distinct postsynaptic mechanisms in nucleus accumbens.

The dopamine D3 receptor (D3R) has emerged as a promising pharmacotherapeutic target for the treatment of several diseases including schizophrenia, Parkinson's disease, and substance use disorders. However, studies investigating the D3R's precise role in dopamine neurotransmission or how it may be exploited to modulate responses to drugs of abuse have produced contrasting results, in part because most D3R-targeted compounds often also interact with D2 receptors (D2R). To resolve this issue, we set out to systematically characterize and compare the consequences of selective D2R or D3R antagonists on the behavioral-stimulant properties of cocaine in mice, and to identify putative neurobiological mechanisms underlying their behavior-modifying effects. Pretreatment with the selective D2R antagonist L-741,626 attenuated, while pretreatment with the selective D3R antagonist PG01037 enhanced, the locomotor-activating effects of both acute cocaine administration as well as sensitization following repeated cocaine dosing. While both antagonists potentiated cocaine-induced increases in presynaptic dopamine release, we report for the first time that D3R blockade uniquely facilitated dopamine-mediated excitation of D1-expressing medium spiny neurons in the nucleus accumbens. Collectively, our results demonstrate that selective D3R antagonism potentiates the behavioral-stimulant effects of cocaine in mice, an effect that is in direct opposition to that produced by selective D2R antagonism or nonselective D2-like receptor antagonists, and is likely mediated by facilitating D1-mediated excitation in the nucleus accumbens. These findings provide novel insights into the neuropharmacological actions of D3R antagonists on mesolimbic dopamine neurotransmission and their potential utility as pharmacotherapeutics.

Dorsal Raphe Dual Serotonin-Glutamate Neurons Drive Reward by Establishing Excitatory Synapses on VTA Mesoaccumbens Dopamine Neurons.

Dorsal raphe (DR) serotonin neurons provide a major input to the ventral tegmental area (VTA). Here, we show that DR serotonin transporter (SERT) neurons establish both asymmetric and symmetric synapses on VTA dopamine neurons, but most of these synapses are asymmetric. Moreover, the DR-SERT terminals making asymmetric synapses on VTA dopamine neurons coexpress vesicular glutamate transporter 3 (VGluT3; transporter for accumulation of glutamate for its synaptic release), suggesting the excitatory nature of these synapses. VTA photoactivation of DR-SERT fibers promotes conditioned place preference, elicits excitatory currents on mesoaccumbens dopamine neurons, increases their firing, and evokes dopamine release in nucleus accumbens. These effects are blocked by VTA inactivation of glutamate and serotonin receptors, supporting the idea of glutamate release in VTA from dual DR SERT-VGluT3 inputs. Our findings suggest a path-specific input from DR serotonergic neurons to VTA that promotes reward by the release of glutamate and activation of mesoaccumbens dopamine neurons.

Chemical synthesis of 7-oxygenated 12α-hydroxy steroid derivatives to enable the biochemical characterization of cytochrome P450 8B1, the oxysterol 12α-hydroxylase enzyme implicated in cardiovascular health and obesity.

Cholic acid is the endogenous 12α-hydroxylated bile acid, which possesses enhanced cholesterol absorption properties compared to its 12-desoxy counterpart, chenodeoxycholic acid. The oxysterol 12α-hydroxylase enzyme is cytochrome P450 8B1 (P450 8B1), which regioselectively and stereoselectively incorporates the 12α-hydroxy group in 7α-hydroxycholest-4-en-3-one, the biosynthetic precursor of cholic acid. Despite the vital role of P450 8B1 activity in cardiovascular health, research studies of other 12α-hydroxy steroid derivatives are rare. A synthetic route to incorporate a C12α-hydroxy group into the C12-methylene (-CH2-) in dehydroepiandrosterone derivatives is disclosed. The incorporation of the C12-oxygen was accomplished through a copper mediated Schönecker oxidation of an imino-pyridine intermediate, introducing the 12ß-hydroxy group. The resulting 12ß-hydroxy steroid derivative was oxidized to the C12-ketone, which was stereoselectively reduced with lithium tri-sec-butylborohydride to afford the 12α-hydroxy stereochemistry. The C7-position was oxidized to yield the various 7-keto, 7ß-hydroxy, and 7α-hydroxy derivatives. Furthermore, 7-ketodehydroepiandrosterone and 12 α-hydroxy-7-ketodehydroepiandrosterone both displayed NMDA receptor antagonistic activities at 10 µM concentrations. These C12α-hydroxy steroids will be used as tools to identify new biochemical properties of the enzymatic products of P450 8B1, the oxysterol 12α-hydroxylase.

A Central Move for CB2 Receptors.

The function of the CB2 cannabinoid receptor in the brain has long been a matter of debate. In this issue of Neuron, Stempel et al. (2016) describe a mechanism whereby endocannabinoid production leads to a cell-intrinsic hyperpolarization that controls self activity.

Differentiation and Characterization of Dopaminergic Neurons From Baboon Induced Pluripotent Stem Cells.

UNLABELLED: : The progressive death of dopamine producing neurons in the substantia nigra pars compacta is the principal cause of symptoms of Parkinson's disease (PD). Stem cells have potential therapeutic use in replacing these cells and restoring function. To facilitate development of this approach, we sought to establish a preclinical model based on a large nonhuman primate for testing the efficacy and safety of stem cell-based transplantation. To this end, we differentiated baboon fibroblast-derived induced pluripotent stem cells (biPSCs) into dopaminergic neurons with the application of specific morphogens and growth factors. We confirmed that biPSC-derived dopaminergic neurons resemble those found in the human midbrain based on cell type-specific expression of dopamine markers TH and GIRK2. Using the reverse transcriptase quantitative polymerase chain reaction, we also showed that biPSC-derived dopaminergic neurons express PAX6, FOXA2, LMX1A, NURR1, and TH genes characteristic of this cell type in vivo. We used perforated patch-clamp electrophysiology to demonstrate that biPSC-derived dopaminergic neurons fired spontaneous rhythmic action potentials and high-frequency action potentials with spike frequency adaption upon injection of depolarizing current. Finally, we showed that biPSC-derived neurons released catecholamines in response to electrical stimulation. These results demonstrate the utility of the baboon model for testing and optimizing the efficacy and safety of stem cell-based therapeutic approaches for the treatment of PD. SIGNIFICANCE: Functional dopamine neurons were produced from baboon induced pluripotent stem cells, and their properties were compared to baboon midbrain cells in vivo. The baboon has advantages as a clinically relevant model in which to optimize the efficacy and safety of stem cell-based therapies for neurodegenerative diseases, such as Parkinson's disease. Baboons possess crucial neuroanatomical and immunological similarities to humans, and baboon pluripotent stem cells can be differentiated into functional neurons that mimic those in the human brain, thus laying the foundation for the utility of the baboon model for evaluating stem cell therapies.

Noradrenergic inhibition of midbrain dopamine neurons.

Receptors that couple to phosphoinositide hydrolysis, which include metabotropic glutamate receptors (mGluRs) and muscarinic receptors, are known to either activate or inhibit the activity of dopamine cells depending on the pattern of receptor activation. Transient activation of alpha1 adrenoceptors with norepinephrine (NE) resulted in an outward current in midbrain dopamine neurons recorded in brain slices. The NE-mediated outward current was induced by activation of a potassium conductance through release of calcium from intracellular stores. Unlike the mGluR-mediated outward current, the outward current induced by alpha1 adrenoceptors often consisted of multiple peaks. Activation of alpha1 adrenoceptors also induced a wave of calcium release that spread through the soma and proximal dendrites without a decline in amplitude or rate of propagation and therefore differed qualitatively from that induced by mGluRs. Finally, the alpha1 adrenoceptor-activated outward current was more sensitive to the calcium store-depleting agents ryanodine and caffeine. Thus, although both alpha1 adrenoceptors and mGluRs mobilize calcium from intracellular stores, the mechanisms and pools of calcium differ. The results suggest that noradrenergic innervation of dopamine cells can directly inhibit the activity of dopamine cells. Psychostimulants, such as amphetamine, will therefore have a direct effect on the firing pattern of dopamine neurons through a combination of actions on dopamine and alpha1 adrenoceptor activation.

Cocaine self-administration selectively decreases noradrenergic regulation of metabotropic glutamate receptor-mediated inhibition in dopamine neurons.

Stimulant drugs of abuse have several effects on neural activity, including altering the excitability of dopamine neurons via the noradrenergic and glutamatergic systems. Thus, an interaction between noradrenergic and glutamatergic systems may play a role in drug-seeking behavior. Although many of the direct pharmacological effects of psychostimulants on dopamine neuron physiology are well established, the neurophysiological bases of drug-seeking behavior have yet to be fully elucidated. The present study measured short-term (3 d) and long-term (14 d) access to cocaine, by self-administration or passive exposure, and the regulation of metabotropic glutamate receptor (mGluR)-mediated inhibition of dopamine cells in rat midbrain slices. The results indicated that alpha-adrenoreceptor modulation of the mGluR-mediated inhibition is selectively reduced in animals that self-administered cocaine for 3 d. This effect was not observed in slices from either yoked cocaine animals, which were given cocaine in an amount and pattern equal to that used for the self-administering animals, or saline control animals. However, after 14 d of cocaine, alpha-adrenoreceptor regulation of the mGluR-mediated inhibition was equally reduced in both self-administering and yoked cocaine animals relative to saline controls. The results suggest that alpha-adrenoreceptor regulation of the mGluR-mediated inhibition is an adaptive cellular mechanism involved in early cocaine self-administration that is distinct from a direct pharmacological effect of cocaine on dopamine neurons. The noradrenergic system could therefore serve to alter the reward value of stimuli that have significant effects on dopamine neuron firing pattern through mGluRs.

Cocaine increases dopaminergic neuron and motor activity via midbrain α1 adrenergic signaling.

Cocaine reinforcement is mediated by increased extracellular dopamine levels in the forebrain. This neurochemical effect was thought to require inhibition of dopamine reuptake, but cocaine is still reinforcing even in the absence of the dopamine transporter. Here, we demonstrate that the rapid elevation in dopamine levels and motor activity elicited by cocaine involves α1 receptor activation within the ventral midbrain. Activation of α1 receptors increases dopaminergic neuron burst firing by decreasing the calcium-activated potassium channel current (SK), as well as elevates dopaminergic neuron pacemaker firing through modulation of both SK and the hyperpolarization-activated cation currents (Ih). Furthermore, we found that cocaine increases both the pacemaker and burst-firing frequency of rat ventral-midbrain dopaminergic neurons through an α1 adrenergic receptor-dependent mechanism within the ventral tegmental area and substantia nigra pars compacta. These results demonstrate the mechanism underlying the critical role of α1 adrenergic receptors in the regulation of dopamine neurotransmission and behavior by cocaine.