Organization of ventral tegmental area projections to the ventral tegmental area-nigral complex in the rat

The rostromedial tegmental nucleus gates fat overconsumption through ventral tegmental area output in male rats

Excessive consumption of palatable foods that are rich in fats and sugars has contributed to the increasing prevalence of obesity worldwide. Similar to addictive drugs, such foods activate the brain's reward circuit, involving mesolimbic dopaminergic projections from the ventral tegmental area (VTA) to the nucleus accumbens (NAc) and the prefrontal cortex. Neuroadaptations occurring in this circuit are hypothesized to contribute to uncontrolled consumption of such foods, a common feature of most of eating disorders and obesity. The rostromedial tegmental nucleus (RMTg), also named tail of the VTA (tVTA), is an inhibitory structure projecting to the VTA and the lateral hypothalamus (LH), two key brain regions in food intake regulation. Prior research has demonstrated that the RMTg responds to addictive drugs and influences their impact on mesolimbic activity and reward-related behaviors. However, the role of the RMTg in food intake regulation remains largely unexplored. The present study aimed to investigate the role of the RMTg and its projections to the VTA and the LH in regulating food intake in rats. To do so, we examined eating patterns of rats with either bilateral excitotoxic lesions of the RMTg or specific lesions of RMTg-VTA and RMTg-LH pathways. Rats were exposed to a 6-week 'free choice high-fat and high-sugar' diet, followed by a 4-week palatable food forced abstinence and a 24 h re-access period. Our results indicate that an RMTg-VTA pathway lesion increases fat consumption following 6 weeks of diet and at time of re-access. The RMTg-LH pathway lesion produces a milder effect with a decrease in global calorie intake. These findings suggest that the RMTg influences palatable food consumption and relapse through its projections to the VTA.

Disruption of Electroencephalogram Coherence between Cortex/Striatum and Midbrain Dopaminergic Regions in the Knock-Out Mice with Combined Loss of Alpha, Beta, and Gamma Synucleins

The malfunctioning of the brain synucleins is associated with pathogenesis of Parkinson's disease. Synucleins' ability to modulate various pre-synaptic processes suggests their modifying effects on the electroencephalogram (EEG) recorded from different brain structures. Disturbances in interrelations between them are critical for the onset and evolution of neurodegenerative diseases. Recently, we have shown that, in mice lacking several synucleins, differences between the frequency spectra of EEG from different brain structures are correlated with specificity of synucleins' combinations. Given that EEG spectra are indirect characteristics of inter-structural relations, in this study, we analyzed a coherence of instantaneous values for EEGs recorded from different structures as a direct measure of "functional connectivity" between them.

METHODS

EEG data from seven groups of knock-out (KO) mice with combined deletions of alpha, beta, and gamma synucleins versus a group of wild-type (WT) mice were compared. EEG coherence was estimated between the cortex (MC), putamen (Pt), ventral tegmental area (VTA), and substantia nigra (SN) in all combinations.

RESULTS

EEG coherence suppression, predominantly in the beta frequency band, was observed in KO mice versus WT littermates. The suppression was minimal in MC-Pt and VTA-SN interrelations in all KO groups and in all inter-structural relations in mice lacking either all synucleins or only beta synuclein. In other combinations of deleted synucleins, significant EEG coherence suppression in KO mice was dominant in relations with VTA and SN.

CONCLUSION

Deletions of the synucleins produced significant attenuation of intra-cerebral EEG coherence depending on the imbalance of different types of synucleins.

Hypodopaminergic state of the nigrostriatal pathway drives compulsive alcohol use

The neurobiological mechanisms underlying compulsive alcohol use, a cardinal feature of alcohol use disorder, remain elusive. The key modulator of motivational processes, dopamine (DA), is suspected to play an important role in this pathology, but its exact role remains to be determined. Here, we found that rats expressing compulsive-like alcohol use, operationalized as punishment-resistant self-administration, showed a decrease in DA levels restricted to the dorsolateral territories of the striatum, the main output structure of the nigrostriatal DA pathway. We then causally demonstrated that chemogenetic-induced selective hypodopaminergia of this pathway resulted in compulsive-like alcohol self-administration in otherwise resilient rats, accompanied by the emergence of alcohol withdrawal-like motivational impairments (i.e., impaired motivation for a natural reinforcer). Finally, the use of the monoamine stabilizer OSU6162, previously reported to correct hypodopaminergic states, transiently decreased compulsive-like alcohol self-administration in vulnerable rats. These results suggest a potential critical role of tonic nigrostriatal hypodopaminergic states in alcohol addiction and provide new insights into our understanding of the neurobiological mechanisms underlying compulsive alcohol use.

Viral Tracing Confirms Paranigral Ventral Tegmental Area Dopaminergic Inputs to the Interpeduncular Nucleus Where Dopamine Release Encodes Motivated Exploration

Midbrain dopaminergic (DAergic) neurons of the ventral tegmental area (VTA) are engaged by rewarding stimuli and encode reward prediction error to update goal-directed learning. However, recent data indicate that VTA DAergic neurons are functionally heterogeneous with emerging roles in aversive signaling, salience, and novelty, based in part on anatomic location and projection, highlighting a need to functionally characterize the repertoire of VTA DAergic efferents in motivated behavior. Previous work identifying a mesointerpeduncular circuit consisting of VTA DAergic neurons projecting to the interpeduncular nucleus (IPN), a midbrain area implicated in aversion, anxiety-like behavior, and familiarity, has recently come into question. To verify the existence of this circuit, we combined presynaptic targeted and retrograde viral tracing in the dopamine transporter-Cre mouse line. Consistent with previous reports, synaptic tracing revealed that axon terminals from the VTA innervate the caudal IPN; whereas, retrograde tracing revealed DAergic VTA neurons, predominantly in the paranigral region, project to the nucleus accumbens shell, as well as the IPN. To test whether functional DAergic neurotransmission exists in the IPN, we expressed the genetically encoded DA sensor, dLight 1.2, in the IPN of C57BL/6J mice and measured IPN DA signals in vivo during social and anxiety-like behavior using fiber photometry. We observed an increase in IPN DA signal during social investigation of a novel but not familiar conspecific and during exploration of the anxiogenic open arms of the elevated plus maze. Together, these data confirm VTA DAergic neuron projections to the IPN and implicate this circuit in encoding motivated exploration.

From chronic stress and anxiety to neurodegeneration: Focus on neuromodulation of the axon initial segment

To adapt to the sustained demands of chronic stress, discrete brain circuits undergo structural and functional changes often resulting in anxiety disorders. In some individuals, anxiety disorders precede the development of motor symptoms of Parkinson's disease (PD) caused by degeneration of neurons in the substantia nigra (SN). Here, we present a circuit framework for probing a causal link between chronic stress, anxiety, and PD, which postulates a central role of abnormal neuromodulation of the SN's axon initial segment by brainstem inputs. It is grounded in findings demonstrating that the earliest PD pathologies occur in the stress-responsive, emotion regulation network of the brainstem, which provides the SN with dense aminergic and cholinergic innervation. SN's axon initial segment (AIS) has unique features that support the sustained and bidirectional propagation of activity in response to synaptic inputs. It is therefore, especially sensitive to circuit-mediated stress-induced imbalance of neuromodulation, and thus a plausible initiating site of neurodegeneration. This could explain why, although secondary to pathophysiologies in other brainstem nuclei, SN degeneration is the most extensive. Consequently, the cardinal symptom of PD, severe motor deficits, arise from degeneration of the nigrostriatal pathway rather than other brainstem nuclei. Understanding when and how circuit dysfunctions underlying anxiety can progress to neurodegeneration, raises the prospect of timed interventions for reversing, or at least impeding, the early pathophysiologies that lead to PD and possibly other neurodegenerative disorders.

A schizophrenia risk factor induces marked anatomical deficits at GABAergic-dopaminergic synapses in the rat ventral tegmental area: Essential evidence for new targeted therapies

To develop new therapies for schizophrenia, evidence accumulated over decades highlights the essential need to investigate the GABAergic synapses that presynaptically influence midbrain dopaminergic neurons. Since current technology restricts these studies to animals, and evidence accumulated in recent decades indicates a developmental origin of schizophrenia, we investigated synaptic changes in male rat offspring exposed to maternal immune activation (MIA), a schizophrenia risk factor. Using a novel combination of lentiviruses, peroxidase-immunogold double labeling, three-dimensional serial section transmission electron microscopy and stereology, we observed clear anatomical alterations in synaptic inputs on dopaminergic neurons in the midbrain posterior ventral tegmental area (pVTA). These changes relate directly to a characteristic feature of schizophrenia: increased dopamine release. In 3-month-old and 14-month-old MIA rats, we found a marked decrease in the volume of presynaptic GABAergic terminals from the rostromedial tegmental nucleus (RMTg) and in the length of the synapses they made, when innervating pVTA dopaminergic neurons. In MIA rats in the long-term, we also discovered a decrease in the volume of the postsynaptic density (PSD) and in the maximum thickness of the PSD at the same synapses. These marked deficits were evident in conventional GABA-dopamine synapses and in synaptic triads that we discovered involving asymmetric synapses that innervated RMTg GABAergic presynaptic terminals, which in turn innervated pVTA dopaminergic neurons. In triads, the PSD thickness of asymmetric synapses was significantly decreased in MIA rats in the long-term cohort. The extensive anatomical deficits provide a potential basis for new therapies targeted at synaptic inputs on midbrain pVTA dopaminergic neurons, in contrast to current striatum-targeted antipsychotic drugs.

MRI maps, segregation, and white matter connectivity of the human hypothalamus in health

The human hypothalamus is composed of several gray matter territories, forming 10 different structures mainly referred to as nuclei: the preoptic, suprachiasmatic, supraoptic, infundibular, paraventricular, dorsomedial, ventromedial, posterior (dorsal; dorsal hypothalamic area), and tuberomamillary nuclei, and the lateral hypothalamic area. The macroconnectivity, described since the middle of the 19th century, is currently probed using MRI methods, notably those relying on diffusion techniques. The structural connections can be grouped as follows: connections with the olfactory system; stria terminalis connections; stria medullaris connections; ansa lenticularis connections; subthalamus connections; optic tract connections; intrahypothalamic connections; hypothalamo-hypophysis connections; hypothalamic commissures; cortex connections.

Loss of Midbrain Dopamine Neurons and Altered Apomorphine EEG Effects in the 5xFAD Mouse Model of Alzheimer's Disease

Cognitive malfunction, synaptic dysfunction, and disconnections in neural networks are core deficits in Alzheimer's disease (AD). 5xFAD mice, a transgenic model of AD, are characterized by an enhanced level of amyloid-β and abnormal neurotransmission. The dopaminergic (DA) system has been shown to be involved in amyloid-β transformations and neuronal plasticity; however, its role in functional network changes in familial AD still remains unclear. In 5xFAD and non-transgenic freely moving mice, electroencephalograms (EEGs) were simultaneously recorded from the secondary motor cortex (MC), superficial layers of the hippocampal CA1 area (HPC), substantia nigra (SN), and ventral tegmental area (VTA). EEGs and their frequency spectra were analyzed before and after systemic injection of a DA receptor agonist, apomorphine (APO). In the baseline EEG from MC and HPC of 5xFAD mice, delta and alpha oscillations were enhanced and beta activity was attenuated, compared to control mice. In VTA and SN of 5xFAD mice, delta-theta activity was decreased and beta oscillations dominated. In control mice, APO suppressed delta activity in VTA to a higher extent than in MC, whereas in 5xFAD mice, this difference was eliminated due to attenuation of the delta suppression in VTA. APO increased beta activity in MC of mice from both groups while significant beta suppression was observed in VTA of 5xFAD mice. These mice were characterized by significant decrease of tyrosine hydroxylase immunopositive cells in both VTA and SN and of DA transporter in MC and hippocampal dentate gyrus. We suggest that the EEG modifications observed in 5xFAD mice are associated with alterations in dopaminergic transmission, resulting in adaptive changes in the cerebral networks in the course of familial AD development.

The rostromedial tegmental nucleus: a key modulator of pain and opioid analgesia

A recently defined structure, the rostromedial tegmental nucleus (RMTg; aka tail of the ventral tegmental area [VTA]), has been proposed as an inhibitory control center for dopaminergic activity of the VTA. This region is composed of GABAergic cells that send afferent projections to the ventral midbrain and synapse onto dopaminergic cells in the VTA and substantia nigra. These cells exhibit µ-opioid receptor immunoreactivity, and in vivo, ex vivo, and optogenetic/electrophysiological approaches demonstrate that morphine excites dopamine neurons by targeting receptors on GABAergic neurons localized in the RMTg. This suggests that the RMTg may be a key modulator of opioid effects and a major brake regulating VTA dopamine systems. However, no study has directly manipulated RMTg GABAergic neurons in vivo and assessed the effect on nociception or opioid analgesia. In this study, multiplexing of GABAergic neurons in the RMTg was achieved using stimulatory Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) and inhibitory kappa-opioid receptor DREADDs (KORD). Our data show that locally infused RMTg morphine or selective RMTg GABAergic neuron inhibition produces 87% of the maximal antinociceptive effect of systemic morphine, and RMTg GABAergic neurons modulate dopamine release in the nucleus accumbens. In addition, chemoactivation of VTA dopamine neurons significantly reduced pain behaviors both in resting and facilitated pain states and reduced by 75% the dose of systemic morphine required to produce maximal antinociception. These results provide compelling evidence that RMTg GABAergic neurons are involved in processing of nociceptive information and are important mediators of opioid analgesia.

GABAergic and glutamatergic effects on nigrostriatal and mesolimbic dopamine release in the rat

In this review, a series of experiments is presented, in which γ-amino butyric acid (GABA)ergic and glutamatergic effects on dopamine function in the rat nigrostriatal and mesolimbic system was systematically assessed after pharmacological challenge with GABAA receptor (R) and and N-methyl d-aspartate (NMDA)R agonists and antagonists. In these studies, [123I]iodobenzamide binding to the D2/3R was mesured in nucleus accumbens (NAC), caudateputamen (CP), substantia nigra/ventral tegmental area (SN/VTA), frontal (FC), motor (MC) and parietal cortex (PC) as well as anterior (aHIPP) and posterior hippocampus (pHIPP) with small animal SPECT in baseline and after injection of either the GABAAR agonist muscimol (1 mg/kg), the GABAAR antagonist bicuculline (1 mg/kg), the NMDAR agonist d-cycloserine (20 mg/kg) or the NMDAR antagonist amantadine (40 mg/kg). Muscimol reduced D2/3R binding in NAC, CP, SN/VTA, THAL and pHIPP, while, after amantadine, decreases were confined to NAC, CP and THAL. In contrast, d-cycloserine elevated D2/3R binding in NAC, SN/VTA, THAL, frontal cortex, motor cortex, PC, aHIPP and pHIPP, while, after bicuculline, increases were confined to CP and THAL. Taken together, similar actions on regional dopamine levels were exterted by the GABAAR agonist and the NMDAR antagonist on the one side and by the GABAAR antagonist and the NMDAR agonist on the other, with agonistic action, however, affecting more brain regions. Thereby, network analysis suggests different roles of GABAARs and NMDARs in the mediation of nigrostriatal, nigrothalamocortical and mesolimbocortical dopamine function.

Social deprivation substantially changes multi-structural neurotransmitter signature of social interaction: Glutamate concentration in amygdala and VTA as a key factor in social encounter-induced 50-kHz ultrasonic vocalization

Ultrasonic vocalizations are important for coordinating social behavior in rats. Examination of the neurochemical mechanisms that govern social behavior and ultrasonic vocalization emission is crucial for understanding the social impairments that occur in many neuropsychiatric disorders. To elucidate neurochemical changes in the brain structures related to social behavior and their mutual relationships, we conducted three-phase experiment. Neurochemicals were measured in the following behavioral situations: without social encounter, with short social encounter, with long social encounter in isolated and non-isolated rats. The aims of this study were to: (1) extract the most important neurotransmitters and their metabolites that are involved in social encounter-induced emission of 50 kHz calls; (2) to elucidate mutual relationships among the neurochemical changes in the selected, six brain structures, and analyze compound relationships by step analysis; (3) create a model of all-to-all neurotransmitter correlations; (4) find the neurochemical basis of 50-kHz USVs emission during social encounter. Our behavioral and neurochemical analysis indicated that social encounter was a triggering factor of the glutamatergic neurotransmission in the ventral tegmental area (VTA), hippocampus, and amygdala; serotonergic neurotransmission in the NAcc, CPu, and amygdala; the dopaminergic neurotransmission in the caudate putamen (CPu) and hippocampus; GABAergic neurotransmission in the hippocampus and VTA. Social encounter-induced 50-kHz USVs were bound up with changes in glutamate in amygdala and VTA, glycine in the amygdala, VTA, hippocampus, nucleus accumbens and CPu, and dopamine metabolites in VTA and CPu.

Pharmacological Manipulation of K v 7 Channels as a New Therapeutic Tool for Multiple Brain Disorders

K _v 7 ("M-type," KCNQ) K⁺ currents, play dominant roles in controlling neuronal excitability. They act as a "brake" against hyperexcitable states in the central and peripheral nervous systems. Pharmacological augmentation of M current has been developed for controlling epileptic seizures, although current pharmacological tools are uneven in practical usefulness. Lately, however, M-current "opener" compounds have been suggested to be efficacious in preventing brain damage after multiple types of insults/diseases, such as stroke, traumatic brain injury, drug addiction and mood disorders. In this review, we will discuss what is known to date on these efforts and identify gaps in our knowledge regarding the link between M current and therapeutic potential for these disorders. We will outline the preclinical experiments that are yet to be performed to demonstrate the likelihood of success of this approach in human trials. Finally, we also address multiple pharmacological tools available to manipulate different K _v 7 subunits and the relevant evidence for translational application in the clinical use for disorders of the central nervous system and multiple types of brain insults. We feel there to be great potential for manipulation of K _v 7 channels as a novel therapeutic mode of intervention in the clinic, and that the paucity of existing therapies obligates us to perform further research, so that patients can soon benefit from such therapeutic approaches.

The Rostromedial Tegmental Nucleus: Anatomical Studies and Roles in Sleep and Substance Addictions in Rats and Mice

The rostromedial tegmental nucleus (RMTg), a brake of the dopamine system, is specifically activated by aversive stimuli, such as foot shock. It is principally composed of gamma-aminobutyric acid neurons. However, there is no exact location of the RMTg on the brain stereotaxic atlas. The RMTg can be defined by c-Fos staining elicited by psychostimulants, the position of retrograde-labeled neurons stained by injections into the ventral tegmental area (VTA), the terminal field formed by axons from the lateral habenula, and some molecular markers identified as specifically expressed in the RMTg such as FoxP1. The RMTg receives a broad range of inputs and produces diverse outputs, which indicates that the RMTg has multiple functions. First, the RMTg plays an essential role for non-rapid eye movement sleep. Additionally, the RMTg serves a vital role in response to addiction. Opiates increase the firing rates of dopaminergic neurons in the VTA by acting on μ-opioid receptors on RMTg neurons and their terminals inside the VTA. In this review, we summarize the recent research advances on the anatomical location of the RMTg in rats and mice, its projections, and its regulation of sleep-wake behavior and addiction.

Habenular connections with the dopaminergic and serotonergic system and their role in stress-related psychiatric disorders

The habenula (Hb) is a phylogenetically old epithalamic structure differentiated into two nuclear complexes, the medial (MHb) and lateral habenula (LHb). After decades of search for a great unifying function, interest in the Hb resurged when it was demonstrated that LHb plays a major role in the encoding of aversive stimuli ranging from noxious stimuli to the loss of predicted rewards. Consistent with a role as an anti-reward center, aberrant LHb activity has now been identified as a key factor in the pathogenesis of major depressive disorder. Moreover, both MHb and LHb emerged as new players in the reward circuitry by primarily mediating the aversive properties of distinct drugs of abuse. Anatomically, the Hb serves as a bridge that links basal forebrain structures with monoaminergic nuclei in the mid- and hindbrain. So far, research on Hb has focused on the role of the LHb in regulating midbrain dopamine release. However, LHb/MHb are also interconnected with the dorsal (DR) and median (MnR) raphe nucleus. Hence, it is conceivable that some of the habenular functions are at least partly mediated by the complex network that links MHb/LHb with pontomesencephalic monoaminergic nuclei. Here, we summarize research about the topography and transmitter phenotype of the reciprocal connections between the LHb and ventral tegmental area-nigra complex, as well as those between the LHb and DR/MnR. Indirect MHb outputs via interpeduncular nucleus to state-setting neuromodulatory networks will also be commented. Finally, we discuss the role of specific LHb-VTA and LHb/MHb-raphe circuits in anxiety and depression.

Acute and chronic methylphenidate administration in intact and VTA-specific and nonspecific lesioned rats

Methylphenidate (MPD) is a psychostimulant used for the treatment of ADHD and works by increasing the bioavailability of dopamine (DA) in the brain. As a major source of DA, the ventral tegmental area (VTA) served as the principal target in this study as we aimed to understand its role in modulating the acute and chronic MPD effect. Forty-eight male Sprague-Dawley rats were divided into control, sham, electrical lesion, and 6-OHDA lesion groups. Given the VTA's implication in the locomotive circuit, three locomotor indices-horizontal activity, number of stereotypy, and total distance-were used to measure the animals' behavioral response to the drug. Baseline recording was obtained on experimental day 1 (ED 1) followed by surgery on ED 2. After recovery, the behavioral recordings were resumed on ED 8. All groups received daily intraperitoneal injections of 2.5 mg/kg MPD for six days after which the animals received no treatment for 3 days. On ED 18, 2.5 mg/kg MPD was re-administered to assess for the chronic effect of the psychostimulant. Except for one index, there was an increase in locomotive activity in all experimental groups after surgery (in comparison to baseline activity), acute MPD exposure, induction with six daily doses, and after MPD re-challenge. Furthermore, the increase was greatest in the electrical VTA lesion group and lowest in the 6-OHDA VTA lesion group. In conclusion, the results of this study suggest that the VTA may not be the primary nucleus of MPD action, and the VTA plays an inhibitory role in the locomotive circuit.

Heterogeneous dopamine signals support distinct features of motivated actions: implications for learning and addiction

Despite decades of research, investigations into effective neural and pharmacological therapies for many drugs of abuse, such as cocaine, have produced no FDA-approved approaches. This difficulty derives from the complexity of substance use disorders, which encompass a variety of behavioral, psychological, and neural circuit-based changes that occur as a result of repeated experience with the drug. Dopamine signaling has been demonstrated to play a key role in several aspects of drug abuse—from mediating its reinforcing properties and drug-seeking to triggering relapse—while also mediating a number of important aspects of normal (nondrug related) motivated behaviors and actions. Real-time recording methods such as in vivo voltammetry, electrophysiology, and calcium imaging demonstrate that the signaling properties of dopamine for motivationally relevant stimuli are highly dynamic and spatiotemporally circumscribed within afferent target regions. In this review, we identify the origins and functional consequences of heterogeneous dopamine release in the limbic system, and how these properties are persistently altered in the drug-experienced brain. We propose that these spatiotemporally parallel dopaminergic signals are simultaneously available to the animal, but that these circuits are impaired following prolonged drug experience by disrupting the location and content of dopamine signals in afferent target regions. These findings are discussed in the context of relapse and pathways to discovering new treatments for addiction disorders.

Differences in Nicotine Encoding Dopamine Release between the Striatum and Shell Portion of the Nucleus Accumbens

The aim of this work was to determine the effect of nicotine desensitization on dopamine (DA) release in the dorsal striatum and shell of the nucleus accumbens (NAc) from brain slices. In vitro fast-scan cyclic voltammetry analysis was used to evaluate dopamine release in the dorsal striatum and the NAc shell of Sprague–Dawley rats after infusion of nicotine, a nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine (Mec), and an α4β2 cholinergic receptor antagonist (DHβe). DA release related to nicotine desensitization in the striatum and NAc shell was compared. In both structures, tonic release was suppressed by inhibition of the nicotine receptor (via Mec) and the α4β2 receptor (via DHβe). Paired-pulse ratio (PPR) was facilitated in both structures after nicotine and Mec infusion, and this facilitation was suppressed by increasing the stimulation interval. After variable frequency stimulation (simulating phasic burst), nicotine infusion induced significant augmentation of DA release in the striatum that was not seen in the absence of nicotine. In contrast, nicotine reduced phasic DA release in NAc, although frequency augmentation was seen both with and without nicotine. Evaluation of DA release evoked by various trains (high-frequency stimulation (HFS) 100 Hz) of high-frequency stimulation revealed significant enhancement after a train of three or more pulses in the striatum and NAc. The concentration differences between tonic and phasic release related to nicotine desensitization were more pronounced in the NAc shell. Nicotine desensitization is associated with suppression of tonic release of DA in both the striatum and NAc shell that may occur via the α4β2 subtype of nAChR, whereas phasic frequency-dependent augmentation and HFS-related gating release is more pronounced in the striatum than in the NAc shell. Differences between phasic and tonic release associated with nicotine desensitization may underlie processing of reward signals in the NAc shell, and this may have major implications for addictive behavior.

Synaptic function and plasticity in identified inhibitory inputs onto VTA dopamine neurons

Ventral tegmental area (VTA) dopaminergic neurons are key components of the reward pathway, and their activity is powerfully controlled by a diverse array of inhibitory GABAergic inputs. Two major sources of GABAergic nerve terminals within the VTA are local VTA interneurons and neurons in the rostromedial tegmental nucleus (RMTg). Here, using optogenetics, we compared synaptic properties of GABAergic synapses on VTA dopamine neurons using selective activation of afferents that originate from these two cell populations. We found little evidence of co-release of glutamate from either input, but RMTg-originating synaptic currents were reduced by strychnine, suggesting co-release of glycine and GABA. VTA-originating synapses displayed a lower initial release probability, and at higher frequency stimulation, short-term depression was more marked in VTA- but not RMTg-originating synapses. We previously reported that nitric oxide (NO)-induced potentiation of GABAergic synapses on VTA dopaminergic cells is lost after exposure to drugs of abuse or acute stress; in these experiments, multiple GABAergic afferents were simultaneously activated by electrical stimulation. Here we found that optogenetically-activated VTA-originating synapses on presumptive dopamine neurons also exhibited NO-induced potentiation, whereas RMTg-originating synapses did not. Despite providing a robust inhibitory input to the VTA, RMTg GABAergic synapses are most likely not those previously shown by our work to be persistently altered by addictive drugs and stress. Our work emphasises the idea that dopamine neuron excitability is controlled by diverse inhibitory inputs expected to exert varying degrees of inhibition and to participate differently in a range of behaviours.

Controlling striatal function via anterior frontal cortex stimulation

Motivational, cognitive and action goals are processed by distinct, topographically organized, corticostriatal circuits. We aimed to test whether processing in the striatum is under causal control by cortical regions in the human brain by investigating the effects of offline transcranial magnetic stimulation (TMS) over distinct frontal regions associated with motivational, cognitive and action goal processing. Using a three-session counterbalanced within-subject crossover design, continuous theta burst stimulation was applied over the anterior prefrontal cortex (aPFC), dorsolateral prefrontal cortex, or premotor cortex, immediately after which participants (N = 27) performed a paradigm assessing reward anticipation (motivation), task (cognitive) switching, and response (action) switching. Using task-related functional magnetic resonance imaging (fMRI), we assessed the effects of stimulation on processing in distinct regions of the striatum. To account for non-specific effects, each session consisted of a baseline (no-TMS) and a stimulation (post-TMS) fMRI run. Stimulation of the aPFC tended to decrease reward-related processing in the caudate nucleus, while stimulation of the other sites was unsuccessful. A follow-up analysis revealed that aPFC stimulation also decreased processing in the putamen as a function of the interaction between all factors (reward, cognition and action), suggesting stimulation modulated the transfer of motivational information to cortico-striatal circuitry associated with action control.

The lateral habenula and the serotonergic system

The habenula (Hb) is an epithalamic structure differentiated into two nuclear complexes, medial (MHb) and lateral habenula (LHb). After decades of relative neglect, interest in the Hb resurged when it was demonstrated that LHb neurons play a key role in encoding disappointments and expectation of punishments. Consistent with such a role, the LHb has been implicated in a broad array of functions and pathologic conditions, notably in mechanisms of stress and pain, as well as in the pathophysiology of mood disorders. So far, the vast majority of research involving the LHb has focused on its role in regulating midbrain dopamine release. However, the LHb is also robustly interconnected in a reciprocal manner with a set of rostral serotonin (5-HT) nuclei. Thus, there is increasing evidence that the LHb is amply linked to the dorsal (DR) and median raphe nucleus (MnR) by a complex network of parallel topographically organized direct and indirect pathways. Here, we summarize research about the interconnections of the LHb with different subregions of the DR and MnR, as well as findings about 5-HT-dependent modulation of LHb neurons. Finally, we discuss the contribution of distinct LHb-raphe loops to stress and stress-related psychiatric disorders including anxiety and depression.

Alcohol preference and consumption are controlled by the caudal linear nucleus in alcohol-preferring rats

The neuroanatomical and neurochemical basis of alcohol drinking has been extensively studied, but the neural circuitry mediating alcohol reinforcement has not been fully delineated. In the present experiments, we used both neuroimaging and pharmacological tools to identify neural systems associated with alcohol preference and high voluntary alcohol drinking in alcohol-preferring AA (Alko Alcohol) rats. First, we compared the basal brain activity of AA rats with that of heterogeneous Wistar rats with manganese-enhanced magnetic resonance imaging (MEMRI). Briefly, alcohol-naïve rats were implanted with subcutaneous osmotic minipumps delivering 120 mg/kg MnCl2 over a 7-day period, and were then imaged using a three-dimensional rapid acquisition-relaxation enhanced pulse sequence. MEMRI analysis revealed that the most conspicuous subcortical activation difference was located in the caudal linear nucleus of raphe (CLi), with AA rats displaying significantly lower T1 signal in this region compared to Wistar rats. However, following long-term alcohol drinking, CLi activity was increased in AA rats. In the second experiment, the CLi was targeted with pharmacological tools. AA rats trained to drink 10% alcohol during 2-h sessions were implanted with guide cannulas aimed at the CLi and were given injections of the GABAA receptor agonist muscimol into the CLi before drinking sessions. Muscimol dose-dependently increased alcohol drinking, and co-administration of the gamma aminobutyric acid (GABA)A antagonist bicuculline blocked muscimol's effect. These findings suggest that the mediocaudal region of the ventral tegmental area, particularly the CLi, is important for the propensity for high alcohol drinking and controls alcohol reward via GABAergic transmission.

What can rodent models tell us about apathy and associated neuropsychiatric symptoms in Parkinson's disease?

In addition to classical motor symptoms, Parkinson's disease (PD) patients display incapacitating neuropsychiatric manifestations, such as apathy, anhedonia, depression and anxiety. These hitherto generally neglected non-motor symptoms, have gained increasing interest in medical and scientific communities over the last decade because of the extent of their negative impact on PD patients' quality of life. Although recent clinical and functional imaging studies have provided useful information, the pathophysiology of apathy and associated affective impairments remains elusive. Our aim in this review is to summarize and discuss recent advances in the development of rodent models of PD-related neuropsychiatric symptoms using neurotoxin lesion-based approaches. The data collected suggest that bilateral and partial lesions of the nigrostriatal system aimed at inducing reliable neuropsychiatric-like deficits while avoiding severe motor impairments that may interfere with behavioral evaluation, is a more selective and efficient strategy than medial forebrain bundle lesions. Moreover, of all the different classes of pharmacological agents, D2/D3 receptor agonists such as pramipexole appear to be the most efficient treatment for the wide range of behavioral deficits induced by dopaminergic lesions. Lesion-based rodent models, therefore, appear to be relevant tools for studying the pathophysiology of the non-motor symptoms of PD. Data accumulated so far confirm the causative role of dopaminergic depletion, especially in the nigrostriatal system, in the development of behavioral impairments related to apathy, depression and anxiety. They also put forward D2/D3 receptors as potential targets for the treatment of such neuropsychiatric symptoms in PD.

Marked differences in the number and type of synapses innervating the somata and primary dendrites of midbrain dopaminergic neurons, striatal cholinergic interneurons, and striatal spiny projection neurons in the rat

Elucidating the link between cellular activity and goal-directed behavior requires a fuller understanding of the mechanisms underlying burst firing in midbrain dopaminergic neurons and those that suppress activity during aversive or non-rewarding events. We have characterized the afferent synaptic connections onto these neurons in the rat substantia nigra pars compacta (SNpc) and ventral tegmental area (VTA), and compared these findings with cholinergic interneurons and spiny projection neurons in the striatum. We found that the average absolute number of synapses was three to three and one-half times greater onto the somata of dorsal striatal spiny projection neurons than onto the somata of dopaminergic neurons in the SNpc or dorsal striatal cholinergic interneurons. A similar comparison between populations of dopamine neurons revealed a two times greater number of somatic synapses on VTA dopaminergic neurons than SNpc dopaminergic neurons. The percentage of symmetrical, presumably inhibitory, synaptic inputs on somata was significantly higher on spiny projection neurons and cholinergic interneurons compared with SNpc dopaminergic neurons. Synaptic data on the primary dendrites yielded similar significant differences for the percentage of symmetrical synapses for VTA dopaminergic vs. striatal neurons. No differences in the absolute number or type of somatic synapses were evident for dopaminergic neurons in the SNpc of Wistar vs. Sprague-Dawley rat strains. These data from identified neurons are pivotal for interpreting their electrophysiological responses to afferent activity and for generating realistic computer models of neuronal networks of striatal and midbrain dopaminergic function.

Escalation in high fat intake in a binge eating model differentially engages dopamine neurons of the ventral tegmental area and requires ghrelin signaling

Binge eating is a behavior observed in a variety of human eating disorders. Ad libitum fed rodents daily and time-limited exposed to a high-fat diet (HFD) display robust binge eating events that gradually escalate over the initial accesses. Intake escalation is proposed to be part of the transition from a controlled to a compulsive or loss of control behavior. Here, we used a combination of behavioral and neuroanatomical studies in mice daily and time-limited exposed to HFD to determine the neuronal brain targets that are activated--as indicated by the marker of cellular activation c-Fos--under these circumstances. Also, we used pharmacologically or genetically manipulated mice to study the role of orexin or ghrelin signaling, respectively, in the modulation of this behavior. We found that four daily and time-limited accesses to HFD induce: (i) a robust hyperphagia with an escalating profile, (ii) an activation of different sub-populations of the ventral tegmental area dopamine neurons and accumbens neurons that is, in general, more pronounced than the activation observed after a single HFD consumption event, and (iii) an activation of the hypothalamic orexin neurons, although orexin signaling blockage fails to affect escalation of HFD intake. In addition, we found that ghrelin receptor-deficient mice fail to both escalate the HFD consumption over the successive days of exposure and fully induce activation of the mesolimbic pathway in response to HFD consumption. Current data suggest that the escalation in high fat intake during repeated accesses differentially engages dopamine neurons of the ventral tegmental area and requires ghrelin signaling.

The nucleus accumbens shell and the dorsolateral striatum mediate the reinforcing effects of cocaine through a serial connection

The reinforcing and addictive properties of cocaine are thought to rely on the dopaminergic innervation of the striatum. The ventromedial [i.e. nucleus accumbens shell (NAcc) shell] and dorsolateral [dorsolateral striatum (DLS)] regions of the striatum are serially connected, and it is thought that slowly developing neuroadaptations are responsible for the recruitment of the DLS in mediating habitual drug use after extended drug experience. Remarkably, we have recently shown that the DLS is also involved in cocaine self-administration after limited use, to modulate the reinforcing properties of the drug, a function usually ascribed to the NAcc shell. Here, we investigated whether the involvement of the DLS in cocaine reinforcement requires dopaminergic activity within the NAcc shell, by performing a pharmacological disconnection study. We infused the dopamine receptor antagonist α-flupenthixol unilaterally into the NAcc shell and infused this same antagonist into the contralateral DLS, thereby disrupting dopaminergic interconnectivity within the striatum. We show that this disconnection results in increased responding for cocaine under a fixed ratio-1 schedule of reinforcement in rats with limited cocaine experience. These data suggest that a functional dopaminergic interaction between the NAcc shell and the DLS mediates cocaine reinforcement during the early stages of drug use.

Dopamine neurons projecting to the posterior striatum form an anatomically distinct subclass

Combining rabies-virus tracing, optical clearing (CLARITY), and whole-brain light-sheet imaging, we mapped the monosynaptic inputs to midbrain dopamine neurons projecting to different targets (different parts of the striatum, cortex, amygdala, etc) in mice. We found that most populations of dopamine neurons receive a similar set of inputs rather than forming strong reciprocal connections with their target areas. A common feature among most populations of dopamine neurons was the existence of dense ‘clusters’ of inputs within the ventral striatum. However, we found that dopamine neurons projecting to the posterior striatum were outliers, receiving relatively few inputs from the ventral striatum and instead receiving more inputs from the globus pallidus, subthalamic nucleus, and zona incerta. These results lay a foundation for understanding the input/output structure of the midbrain dopamine circuit and demonstrate that dopamine neurons projecting to the posterior striatum constitute a unique class of dopamine neurons regulated by different inputs.

DOI:http://dx.doi.org/10.7554/eLife.10032.001

Most neurons send their messages to recipient neurons by releasing a substance called a ‘neurotransmitter’ that binds to receptors on the target cell. The sites of this type of signal transmission are called synapses. Some small populations of neurons modulate the activity of hundreds or thousands of these synapses all across the brain by releasing ‘neuromodulators’ that affect how they work. These neuromodulators are essential because they broadcast information that is likely to be useful to many brain regions, like a ‘news channel’ for the brain.

One important neuromodulator in the mammalian brain is dopamine, which contributes to motivation, learning, and the control of movement. Clusters of cells deep in the brain release dopamine, and people with Parkinson's disease gradually lose these cells. This makes it increasingly difficult for their brains to produce the correct amount of dopamine, and results in symptoms such as tremors and stiff muscles.

Individual dopamine neurons typically send information to a single part of the brain. This suggests that dopamine neurons with different targets might have different roles. To explore this possibility, Menegas et al. classified dopamine neurons in the mouse brain into eight types based on the areas to which they project, and then mapped which neurons send input signals to each type. These inputs are likely to shape the activity of each type (that is, their ‘message’ to the rest of the brain). The mapping revealed that most dopamine neurons do not receive substantial input from the area to which they project (i.e., they do not form ‘closed loops’). Instead, most of their input comes from a common set of brain regions, including a particularly large number of inputs from the ventral striatum.

However, Menegas et al. found one exception. Dopamine neurons that target part of the brain called the posterior striatum receive relatively little input from the ventral striatum. Their input comes instead from a set of other brain structures, and in particular from a region called the subthalamic nucleus. Electrical stimulation of the subthalamic nucleus can help to relieve the symptoms of Parkinson's disease. Therefore, the results presented by Menegas et al. suggest that this population of dopamine neurons might be particularly relevant to Parkinson's disease and that focusing future studies on them could ultimately be beneficial for patients.

DOI:http://dx.doi.org/10.7554/eLife.10032.002

Brainstem system of hippocampal theta induction: The role of the ventral tegmental area

This article summarizes the results of studies concerning the influence of the ventral tegmental area (VTA) on the hippocampal theta rhythm. Temporary VTA inactivation resulted in transient loss of the hippocampal theta. Permanent destruction of the VTA caused a long-lasting depression of the power of the theta and it also had some influence on the frequency of the rhythm. Activation of glutamate (GLU) receptors or decrease of GABAergic tonus in the VTA led to enhancement of dopamine release and increased hippocampal theta power. High time and frequency cross-correlation was detected for the theta band between the VTA and hippocampus during paradoxical sleep and active waking. Thus, the VTA may belong to the broad network involved in theta rhythm regulation. This article also presents a model of brainstem-VTA-hippocampal interactions in the induction of the hippocampal theta rhythm. The projections from the VTA which enhance theta rhythm are incorporated into the main theta generation pathway, in which the septum acts as the central node. The neuronal activity that may be responsible for the ability of the VTA to regulate theta probably derives from the structures associated with rapid eye movement (sleep) (REM) sleep or with sensorimotor activity (i.e., mainly from the pedunculopontine and laterodorsal tegmental nuclei and also from the raphe).

Long-range projection neurons of the mouse ventral tegmental area: a single-cell axon tracing analysis

Pathways arising from the ventral tegmental area (VTA) release dopamine and other neurotransmitters during the expectation and achievement of reward, and are regarded as central links of the brain networks that create drive, pleasure, and addiction. While the global pattern of VTA projections is well-known, the actual axonal wiring of individual VTA neurons had never been investigated. Here, we labeled and analyzed the axons of 30 VTA single neurons by means of single-cell transfection with the Sindbis-pal-eGFP vector in mice. These observations were complemented with those obtained by labeling the axons of small populations of VTA cells with iontophoretic microdeposits of biotinylated dextran amine. In the single-cell labeling experiments, each entire axonal tree was reconstructed from serial sections, the length of terminal axonal arbors was estimated by stereology, and the dopaminergic phenotype was tested by double-labeling for tyrosine hydroxylase immunofluorescence. We observed two main, markedly different VTA cell morphologies: neurons with a single main axon targeting only forebrain structures (FPN cells), and neurons with multibranched axons targeting both the forebrain and the brainstem (F + BSPN cells). Dopaminergic phenotype was observed in FPN cells. Moreover, four “subtypes” could be distinguished among the FPN cells based on their projection targets: (1) “Mesocorticolimbic” FPN projecting to both neocortex and basal forebrain; (2) “Mesocortical” FPN innervating the neocortex almost exclusively; (3) “Mesolimbic” FPN projecting to the basal forebrain, accumbens and caudateputamen; and (4) “Mesostriatal” FPN targeting only the caudateputamen. While the F + BSPN cells were scattered within VTA, the mesolimbic neurons were abundant in the paranigral nucleus. The observed diversity in wiring architectures is consistent with the notion that different VTA cell subpopulations modulate the activity of specific sets of prosencephalic and brainstem structures.

Conserved expression of the GPR151 receptor in habenular axonal projections of vertebrates

The habenula is a phylogenetically conserved brain structure in the epithalamus. It is a major node in the information flow between fronto-limbic brain regions and monoaminergic brainstem nuclei, and is thus anatomically and functionally ideally positioned to regulate emotional, motivational, and cognitive behaviors. Consequently, the habenula may be critically important in the pathophysiology of psychiatric disorders such as addiction and depression. Here we investigated the expression pattern of GPR151, a G protein-coupled receptor (GPCR), whose mRNA has been identified as highly and specifically enriched in habenular neurons by in situ hybridization and translating ribosome affinity purification (TRAP). In the present immunohistochemical study we demonstrate a pronounced and highly specific expression of the GPR151 protein in the medial and lateral habenula of rodent brain. Specific expression was also seen in efferent habenular fibers projecting to the interpeduncular nucleus, the rostromedial tegmental area, the rhabdoid nucleus, the mesencephalic raphe nuclei, and the dorsal tegmental nucleus. Using confocal microscopy and quantitative colocalization analysis, we found that GPR151-expressing axons and terminals overlap with cholinergic, substance P-ergic, and glutamatergic markers. Virtually identical expression patterns were observed in rat, mouse, and zebrafish brains. Our data demonstrate that GPR151 is highly conserved, specific for a subdivision of the habenular neurocircuitry, and constitutes a promising novel target for psychiatric drug development.

A subset of ventral tegmental area dopamine neurons responds to acute ethanol

The mechanisms by which alcohol drinking promotes addiction in humans and self-administration in rodents remain obscure, but it is well known that alcohol can enhance dopamine (DA) neurotransmission from neurons of the ventral tegmental area (VTA) and increase DA levels within the nucleus accumbens and prefrontal cortex. We recorded from identified DA neuronal cell bodies within ventral midbrain slices prepared from a transgenic mouse line (TH-GFP) using long-term stable extracellular recordings in a variety of locations and carefully mapped the responses to applied ethanol (EtOH). We identified a subset of DA neurons in the medial VTA located within the rostral linear and interfascicular nuclei that fired spontaneously and exhibited a concentration-dependent increase of firing frequency in response to EtOH, with some neurons responsive to as little as 20mM EtOH. Many of these medial VTA DA neurons were also insensitive to the D2 receptor agonist quinpirole. In contrast, DA neurons in the lateral VTA (located within the parabrachial pigmented and paranigral nuclei) were either unresponsive or responded only to 100mM EtOH. Typically, these lateral VTA DA cells had very slow firing rates, and all exhibited inhibition by quinpirole via D2 "autoreceptors". VTA non-DA cells did not show any significant response to low levels of EtOH. These findings are consistent with evidence for heterogeneity among midbrain DA neurons and provide an anatomical and pharmacological distinction between DA neuron sub-populations that will facilitate future mechanistic studies on the actions of EtOH in the VTA.

Lateral habenula and the rostromedial tegmental nucleus innervate neurochemically distinct subdivisions of the dorsal raphe nucleus in the rat

The lateral habenula (LHb) is an epithalamic structure differentiated in a medial (LHbM) and a lateral division (LHbL). Together with the rostromedial tegmental nucleus (RMTg), the LHb has been implicated in the processing of aversive stimuli and inhibitory control of monoamine nuclei. The inhibitory LHb influence on midbrain dopamine neurons has been shown to be mainly mediated by the RMTg, a mostly GABAergic nucleus that receives a dominant input from the LHbL. Interestingly, the RMTg also projects to the dorsal raphe nucleus (DR), which also receives direct LHb projections. To compare the organization and transmitter phenotype of LHb projections to the DR, direct and indirect via the RMTg, we first placed injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin into the LHb or the RMTg. We then confirmed our findings by retrograde tracing and investigated a possible GABAergic phenotype of DR-projecting RMTg neurons by combining retrograde tracing with in situ hybridization for GAD67. We found only moderate direct LHb projections to the DR, which mainly emerged from the LHbM and were predominantly directed to the serotonin-rich caudal DR. In contrast, RMTg projections to the DR were more robust, emerged from RMTg neurons enriched in GAD67 mRNA, and were focally directed to a distinctive DR subdivision immunohistochemically characterized as poor in serotonin and enriched in presumptive glutamatergic neurons. Thus, besides its well-acknowledged role as a GABAergic control center for the ventral tegmental area (VTA)-nigra complex, our findings indicate that the RMTg is also a major GABAergic relay between the LHb and the DR.

Control of the nigrostriatal dopamine neuron activity and motor function by the tail of the ventral tegmental area

Midbrain dopamine neurons are implicated in various psychiatric and neurological disorders. The GABAergic tail of the ventral tegmental area (tVTA), also named the rostromedial tegmental nucleus (RMTg), displays dense projections to the midbrain and exerts electrophysiological control over dopamine cells of the VTA. However, the influence of the tVTA on the nigrostriatal pathway, from the substantia nigra pars compacta (SNc) to the dorsal striatum, and on related functions remains to be addressed. The present study highlights the role played by the tVTA as a GABA brake for the nigrostriatal system, demonstrating a critical influence over motor functions. Using neuroanatomical approaches with tract tracing and electron microscopy, we reveal the presence of a tVTA-SNc-dorsal striatum pathway. Using in vivo electrophysiology, we prove that the tVTA is a major inhibitory control center for SNc dopamine cells. Using behavioral approaches, we demonstrate that the tVTA controls rotation behavior, motor coordination, and motor skill learning. The motor enhancements observed after ablation of the tVTA are in this regard comparable with the performance-enhancing properties of amphetamine, a drug used in doping. These findings demonstrate that the tVTA is a major GABA brake for nigral dopamine systems and nigrostriatal functions, and they raise important questions about how the tVTA is integrated within the basal ganglia circuitry. They also warrant further research on the tVTA's role in motor and dopamine-related pathological contexts such as Parkinson's disease.

The antero-posterior heterogeneity of the ventral tegmental area

The ventral tegmental area (VTA) is a brain region processing salient sensory and emotional information, controlling motivated behaviors, natural or drug-related reward, reward-related learning, mood, and participating in their associated psychopathologies. Mostly studied for its dopamine neurons, the VTA also includes functionally important GABA and glutamate cell populations. Behavioral evidence supports the presence of functional differences between the anterior VTA (aVTA) and the posterior VTA (pVTA), which is the topic of this review. This antero-posterior heterogeneity concerns locomotor activity, conditioned place preference and intracranial self-administration, and can be seen in response to ethanol, acetaldehyde, salsolinol, opioids including morphine, cholinergic agonists including nicotine, cocaine, cannabinoids and after local manipulation of GABA and serotonin receptors. It has also been observed after viral-mediated manipulation of GluR1, phospholipase Cγ (PLCγ) and cAMP response element binding protein (CREB) expression, with impact on reward and aversion-related responses, on anxiety and depression-related behaviors and on pain sensitivity. In this review, the substrates potentially underlying these aVTA/pVTA differences are discussed, including the VTA sub-nuclei and the heterogeneity in connectivity, cell types and molecular characteristics. We also review the role of the tail of the VTA (tVTA), or rostromedial tegmental nucleus (RMTg), which may also participate to the observed antero-posterior heterogeneity of the VTA. This region, partly located within the pVTA, is an inhibitory control center for dopamine activity. It controls VTA and substantia nigra dopamine cells, thus exerting a major influence on basal ganglia functions. This review highlights the need for a more comprehensive analysis of VTA heterogeneity.

Organization of monosynaptic inputs to the serotonin and dopamine neuromodulatory systems

Serotonin and dopamine are major neuromodulators. Here, we used a modified rabies virus to identify monosynaptic inputs to serotonin neurons in the dorsal and median raphe (DR and MR). We found that inputs to DR and MR serotonin neurons are spatially shifted in the forebrain, and MR serotonin neurons receive inputs from more medial structures. Then, we compared these data with inputs to dopamine neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). We found that DR serotonin neurons receive inputs from a remarkably similar set of areas as VTA dopamine neurons apart from the striatum, which preferentially targets dopamine neurons. Our results suggest three major input streams: a medial stream regulates MR serotonin neurons, an intermediate stream regulates DR serotonin and VTA dopamine neurons, and a lateral stream regulates SNc dopamine neurons. These results provide fundamental organizational principles of afferent control for serotonin and dopamine.

Role of lateral hypothalamus in two aspects of attention in associative learning

Orexin (hypocretin) and melanin-concentrating hormone (MCH) neurons are unique to the lateral hypothalamic (LH) region, but project throughout the brain. These cell groups have been implicated in a variety of functions, including reward learning, responses to stimulants, and the modulation of attention, arousal and the sleep/wakefulness cycle. Here, we examined roles for LH in two aspects of attention in associative learning shown previously to depend on intact function in major targets of orexin and MCH neurons. In experiments 1 and 2, unilateral orexin-saporin lesions of LH impaired the acquisition of conditioned orienting responses (ORs) and bilaterally suppressed FOS expression in the amygdala central nucleus (CeA) normally observed in response to food cues that provoke conditioned ORs. Those cues also induced greater FOS expression than control cues in LH orexin neurons, but not in MCH neurons. In experiment 3, unilateral orexin-saporin lesions of LH eliminated the cue associability enhancements normally produced by the surprising omission of an expected event. The magnitude of that impairment was positively correlated with the amount of LH damage and with the loss of orexin neurons in particular, but not with the loss of MCH neurons. We suggest that the effects of the LH orexin-saporin lesions were mediated by their effect on information processing in the CeA, known to be critical to both behavioral phenomena examined here. The results imply close relations between LH motivational amplification functions and attention, and may inform our understanding of disorders in which motivational and attentional impairments co-occur.

NMDA-glutamatergic activation of the ventral tegmental area induces hippocampal theta rhythm in anesthetized rats

Glutamate afferents reaching the ventral tegmental area (VTA) affect dopamine (DA) cells in this structure probably mainly via NMDA receptors. VTA appears to be one of the structures involved in regulation of hippocampal theta rhythm, and this work aimed at assessing the role of glutamatergic activation of the VTA in the theta regulation. Male Wistar rats (n=17) were divided into groups, each receiving intra-VTA microinjection (0.5 μl) of either solvent (water), glutamatergic NMDA agonist (0.2 μg) or antagonist (MK-801, 3.0 μg). Changes in local field potential were assessed on the basis of peak power (Pmax) and corresponding peak frequency (Fmax) for the delta (0.5-3 Hz) and theta (3-6 Hz) bands. NMDA microinjection evoked long-lasting hippocampal theta. The rhythm appeared with a latency of ca. 12 min post-injection and lasted for over 30 min; Pmax in this band was significantly increased for 50 min, while simultaneously Pmax in the delta band remained lower than in control conditions. Theta Fmax and delta Fmax were increased in almost entire post-injection period (by 0.3-0.5 Hz and 0.3-0.7 Hz, respectively). MK-801 depressed the sensory-evoked theta: tail pinch could not induce theta for 30 min after the injection; Pmax significantly decreased in the theta band and at the same time it increased in the delta band. Theta Fmax decreased 10 and 20 min post injection (by 0.4-0.5 Hz) and delta Fmax decreased in almost entire post injection period (by 0.3-0.7 Hz). NMDA injection generates theta rhythm probably through stimulation of dopaminergic activity within the VTA.

An update on the connections of the ventral mesencephalic dopaminergic complex

This review covers the intrinsic organization and afferent and efferent connections of the midbrain dopaminergic complex, comprising the substantia nigra, ventral tegmental area and retrorubral field, which house, respectively, the A9, A10 and A8 groups of nigrostriatal, mesolimbic and mesocortical dopaminergic neurons. In addition, A10dc (dorsal, caudal) and A10rv (rostroventral) extensions into, respectively, the ventrolateral periaqueductal gray and supramammillary nucleus are discussed. Associated intrinsic and extrinsic connections of the midbrain dopaminergic complex that utilize gamma-aminobutyric acid (GABA), glutamate and neuropeptides and various co-expressed combinations of these compounds are considered in conjunction with the dopamine-containing systems. A framework is provided for understanding the organization of massive afferent systems descending and ascending to the midbrain dopaminergic complex from the telencephalon and brainstem, respectively. Within the context of this framework, the basal ganglia direct and indirect output pathways are treated in some detail. Findings from rodent brain are briefly compared with those from primates, including humans. Recent literature is emphasized, including traditional experimental neuroanatomical and modern gene transfer and optogenetic studies. An attempt was made to provide sufficient background and cite a representative sampling of earlier primary papers and reviews so that people new to the field may find this to be a relatively comprehensive treatment of the subject.

Protracted maturation of forebrain afferent connections of the ventral tegmental area in the rat

The mesocorticolimbic dopamine system has long attracted the interest of researchers concerned with the unique gamut of behavioral and mental health vulnerabilities associated with adolescence. Accordingly, the development of the mesocorticolimbic system has been studied extensively, but almost exclusively with regard to dopaminergic output, particularly in the nucleus accumbens and medial prefrontal cortex. To the contrary, the ontogeny of inputs to the ventral tegmental area (VTA), the source of mesocorticolimbic dopamine, has been neglected. This is not a trivial oversight, as the activity of VTA neurons, which reflects their capacity to transmit information about salient events, is sensitively modulated by inputs. Here, we assessed the development of VTA afferent connections using the β subunit of cholera toxin (Ctβ) as a retrograde axonal tracer in adolescent (postnatal day 39) and early adult (8-9-week-old) rats. After intra-VTA injections of Ctβ, adolescent and early adult animals exhibited qualitatively similar distributions of retrogradely labeled neurons in the sense that VTA-projecting neurons were present at all of the same rostrocaudal levels in all of the same structures in both age groups. However, quantitation of retrogradely labeled neurons revealed that adolescent brains, compared with early adult brains, had significantly fewer VTA-projecting neurons preferentially within an interconnected network of cortical and striatopallidal forebrain structures. These findings provide a novel perspective on the development of the mesocorticolimbic dopamine system and may have important implications for age-dependent specificity in the function of this system, particularly with regard to adolescent impulsivity and mental health vulnerabilities.

Acute high fat diet consumption activates the mesolimbic circuit and requires orexin signaling in a mouse model

Overconsumption of palatable energy-dense foods has negative health implications and it is associated with obesity and several eating disorders. Currently, little is known about the neuronal circuitries activated by the acute ingestion of a rewarding stimulus. Here, we used a combination of immunohistochemistry, pharmacology and neuronal tracing analyses to examine the role of the mesolimbic system in general, and the orexin neurons in particular, in a simple experimental test in which naïve mice are allowed to spontaneously eat a pellet of a high fat diet (HFD) for 2 h. We found that acute HFD activates c-Fos expression in several reward-related brain areas, including the ventral tegmental area (VTA), nucleus accumbens, central amygdala and lateral hypothalamic area. We also found that: i- HFD-mediated orosensory stimulation was required for the mesolimbic pathway activation, ii- acute HFD differentially activates dopamine neurons of the paranigral, parabrachial pigmented and interfascicular sub-regions of the VTA, and iii- orexin neurons of the lateral hypothalamic area are responsive to acute HFD. Moreover, orexin signaling blockade, with the orexin 1 receptor antagonist SB-334867, reduces acute HFD consumption and c-Fos induction in the VTA but not in the other mesolimbic nuclei under study. Finally, we found that most orexin neurons responsive to acute HFD innervate the VTA. Our results show that acute HFD consumption recruits the mesolimbic system and that the full manifestation of this eating behavior requires the activation of orexin signaling.

Local changes in neurosteroid levels in the substantia nigra reticulata and the ventral tegmental area alter chronic ethanol withdrawal severity in male withdrawal seizure-prone mice

BACKGROUND

Allopregnanolone (ALLO) is a potent positive modulator of γ-aminobutyric acidA receptors (GABAA Rs) that affects ethanol (EtOH) withdrawal. Finasteride (FIN), a 5α-reductase inhibitor that blocks the formation of ALLO and other GABAergic neurosteroids, alters EtOH sensitivity. Recently, we found that Withdrawal Seizure-Prone mice from the first genetic replicate (WSP-1) exhibited behavioral tolerance to the anticonvulsant effect of intrahippocampal ALLO during EtOH withdrawal and that intrahippocampal FIN significantly increased EtOH withdrawal severity. The purpose of this study was to determine whether neurosteroid manipulations in the substantia nigra reticulata (SNR) and ventral tegmental area (VTA) produced effects during EtOH withdrawal comparable to those seen with intrahippocampal ALLO and FIN.

METHODS

Male WSP-1 mice were surgically implanted with bilateral guide cannulae aimed at the SNR or VTA at 2 weeks prior to EtOH vapor or air exposure for 72 hours. Initial studies examined the anticonvulsant effect of a single ALLO infusion (0, 100, or 400 ng/side) at a time corresponding to peak withdrawal in the air- and EtOH-exposed mice. Separate studies examined the effect of 4 FIN infusions (0 or 10 μg/side/d) during the development of physical dependence on the expression of EtOH withdrawal.

RESULTS

ALLO infusion exerted a potent anticonvulsant effect in EtOH-naïve mice, but a diminished anticonvulsant effect during EtOH withdrawal. Administration of FIN into the SNR exerted a delayed proconvulsant effect in EtOH-naïve mice, whereas infusion into the VTA increased EtOH withdrawal duration.

CONCLUSIONS

Activation of local GABAA Rs in the SNR and VTA via ALLO infusion is sufficient to exert an anticonvulsant effect in naïve mice and to produce behavioral tolerance to the anticonvulsant effect of ALLO infusion during EtOH withdrawal. Thus, EtOH withdrawal reduced sensitivity of GABAA Rs to GABAergic neurosteroids in 2 neuroanatomical substrates within the basal ganglia in WSP-1 male mice.

Braking dopamine systems: a new GABA master structure for mesolimbic and nigrostriatal functions

A new mesopontine structure exerting a strong influence on dopamine systems has recently been defined: the tail of the ventral tegmental area/rostromedial tegmental nucleus (tVTA/RMTg). This review presents a neuroanatomical, physiological, and behavioral overview of some of the recent and ongoing research on this brain region and its relationship with dopamine systems. The tVTA/RMTg sends dense GABA projections to VTA and substantia nigra neurons. The inhibitory influence of tVTA/RMTg on dopamine neurons is supported by both neuroanatomical and electrophysiology data. The latter studies also reveal the tVTA/RMTg as a substrate for morphine and cannabinoid action on dopamine cells. In primates, the tVTA/RMTg has been implicated in reward prediction error signals, through a basal ganglia-lateral habenula-tVTA/RMTg-dopamine-basal ganglia circuit. In rodents, the tVTA/RMTg has been shown to play a critical role in aversive behaviors, particularly those involving behavioral inhibition, such as freezing and avoidance. These findings highlight the functional importance of the tVTA/RMTg as a major GABA brake for dopamine systems.

Differential projections from the lateral habenula to the rostromedial tegmental nucleus and ventral tegmental area in the rat

The mesopontine rostromedial tegmental nucleus (RMTg) is a mostly γ-aminobutyric acid (GABA)ergic structure believed to be a node for signaling aversive events to dopamine (DA) neurons in the ventral tegmental area (VTA). The RMTg receives glutamatergic inputs from the lateral habenula (LHb) and sends substantial GABAergic projections to the VTA, which also receives direct projections from the LHb. To further specify the topography of LHb projections to the RMTg and VTA, small focal injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin were aimed at different subdivisions of the LHb. The subnuclear origin of LHb inputs to the VTA and RMTg was then confirmed by injections of the retrograde tracer cholera toxin subunit b into the VTA or RMTg. Furthermore, we compared the topographic position of retrogradely labeled neurons in the RMTg resulting from VTA injections with that of anterogradely labeled axons emerging from the LHb. As revealed by anterograde and retrograde tracing, LHb projections were organized in a strikingly topographic manner, with inputs to the RMTg mostly arising from the lateral division of the LHb (LHbL), whereas inputs to the VTA mainly emerged from the medial division of the LHb (LHbM). In the RMTg, profusely branched LHb axons were found in close register with VTA projecting neurons and were frequently apposed to the latter. Overall, our findings demonstrate that LHb inputs to the RMTg and VTA arise from different divisions of the LHb and provide direct evidence for a disynaptic pathway that links the LHbL to the VTA via the RMTg.

Connections of the lateral hypothalamic area juxtadorsomedial region in the male rat

The connections of the lateral hypothalamic area juxtadorsomedial region (LHAjd) were investigated in a series of pathway-tracing experiments involving iontophoretic co-injection of the tracers Phaseolus vulgaris-leucoagglutinin (PHA-L; for outputs) and cholera toxin B subunit (CTB; for inputs). Results revealed that the LHAjd has connections with some 318 distinct gray matter regions encompassing all four subsystems-motor, sensory, cognitive, and behavioral state-included in a basic structure-function network model of the nervous system. Integration of these subsystems is necessary for the coordination and control of emotion and behavior, and in that regard the connections of the LHAjd indicate that it may have a prominent role. Furthermore, the LHAjd connections, together with the connections of other LHA differentiations studied similarly to date, indicate a distinct topographic organization that suggests each LHA differentiation has specifically differing degrees of involvement in the control of multiple behaviors. For the LHAjd, its involvement to a high degree in the control of defensive behavior, and to a lesser degree in the control of other behaviors, including ingestive and reproductive, is suggested. Moreover, the connections of the LHAjd suggest that its possible role in the control of these behaviors may be very broad in scope because they involve the somatic, neuroendocrine, and autonomic divisions of the nervous system. In addition, we suggest that connections between LHA differentiations may provide, at the level of the hypothalamus, a neuronal substrate for the coordinated control of multiple themes in the behavioral repertoire.