The rostromedial tegmental nucleus is essential for non-rapid eye movement sleep

The rostromedial tegmental nucleus (RMTg), also called the GABAergic tail of the ventral tegmental area, projects to the midbrain dopaminergic system, dorsal raphe nucleus, locus coeruleus, and other regions. Whether the RMTg is involved in sleep-wake regulation is unknown. In the present study, pharmacogenetic activation of rat RMTg neurons promoted non-rapid eye movement (NREM) sleep with increased slow-wave activity (SWA). Conversely, rats after neurotoxic lesions of 8 or 16 days showed decreased NREM sleep with reduced SWA at lights on. The reduced SWA persisted at least 25 days after lesions. Similarly, pharmacological and pharmacogenetic inactivation of rat RMTg neurons decreased NREM sleep. Electrophysiological experiments combined with optogenetics showed a direct inhibitory connection between the terminals of RMTg neurons and midbrain dopaminergic neurons. The bidirectional effects of the RMTg on the sleep-wake cycle were mimicked by the modulation of ventral tegmental area (VTA)/substantia nigra compacta (SNc) dopaminergic neuronal activity using a pharmacogenetic approach. Furthermore, during the 2-hour recovery period following 6-hour sleep deprivation, the amount of NREM sleep in both the lesion and control rats was significantly increased compared with baseline levels; however, only the control rats showed a significant increase in SWA compared with baseline levels. Collectively, our findings reveal an essential role of the RMTg in the promotion of NREM sleep and homeostatic regulation.

The anatomy of the human medial forebrain bundle: Ventral tegmental area connections to reward-associated subcortical and frontal lobe regions

Introduction

Despite their importance in reward, motivation, and learning there is only sparse anatomical knowledge about the human medial forebrain bundle (MFB) and the connectivity of the ventral tegmental area (VTA). A thorough anatomical and microstructural description of the reward related PFC/OFC regions and their connection to the VTA - the superolateral branch of the MFB (slMFB) - is however mandatory to enable an interpretation of distinct therapeutic effects from different interventional treatment modalities in neuropsychiatric disorders (DBS, TMS etc.). This work aims at a normative description of the human MFB (and more detailed the slMFB) anatomy with respect to distant prefrontal connections and microstructural features.

Methods and material

Healthy subjects (n = 55; mean age ± SD, 40 ± 10 years; 32 females) underwent high resolution anatomical magnetic resonance imaging including diffusion tensor imaging. Connectivity of the VTA and the resulting slMFB were investigated on the group level using a global tractography approach. The Desikan/Killiany parceling (8 segments) of the prefrontal cortex was used to describe sub-segments of the MFB. A qualitative overlap with Brodmann areas was additionally described. Additionally, a pure visual analysis was performed comparing local and global tracking approaches for their ability to fully visualize the slMFB.

Results

The MFB could be robustly described both in the present sample as well as in additional control analyses in data from the human connectome project. Most VTA- connections reached the superior frontal gyrus, the middel frontal gyrus and the lateral orbitofrontal region corresponding to Brodmann areas 10, 9, 8, 11, and 11m. The projections to these regions comprised 97% (right) and 98% (left) of the total relative fiber counts of the slMFB.

Discussion

The anatomical description of the human MFB shows far reaching connectivity of VTA to reward-related subcortical and cortical prefrontal regions - but not to emotion-related regions on the medial cortical surface - realized via the superolateral branch of the MFB. Local tractography approaches appear to be inferior in showing these far-reaching projections. Since these local approaches are typically used for surgical targeting of DBS procedures, the here established detailed map might - as a normative template - guide future efforts to target deep brain stimulation of the slMFB in depression and other disorders related to dysfunction of reward and reward-associated learning.

Nuclear organization of the substantia nigra, ventral tegmental area and retrorubral field of the common marmoset (Callithrix jacchus): A cytoarchitectonic and TH-immunohistochemistry study

It is widely known that the catecholamine group is formed by dopamine, noradrenaline and adrenaline. Its synthesis is regulated by the enzyme called tyrosine hydroxylase. 3-hydroxytyramine/dopamine (DA) is a precursor of noradrenaline and adrenaline synthesis and acts as a neurotransmitter in the central nervous system. The three main nuclei, being the retrorubral field (A8 group), the substantia nigra pars compacta (A9 group) and the ventral tegmental area (A10 group), are arranged in the die-mesencephalic portion and are involved in three complex circuitries - the mesostriatal, mesolimbic and mesocortical pathways. These pathways are involved in behavioral manifestations, motricity, learning, reward and also in pathological conditions such as Parkinson's disease and schizophrenia. The aim of this study was to perform a morphological analysis of the A8, A9 and A10 groups in the common marmoset (Callithrix jacchus - a neotropical primate), whose morphological and functional characteristics support its suitability for use in biomedical research. Coronal sections of the marmoset brain were submitted to Nissl staining and TH-immunohistochemistry. The morphology of the neurons made it possible to subdivide the A10 group into seven distinct regions: interfascicular nucleus, raphe rostral linear nucleus and raphe caudal linear nucleus in the middle line; paranigral and parainterfascicular nucleus in the middle zone; the rostral portion of the ventral tegmental area nucleus and parabrachial pigmented nucleus located in the dorsolateral portion of the mesencephalic tegmentum. The A9 group was divided into four regions: substantia nigra compacta dorsal and ventral tiers; substantia nigra compacta lateral and medial clusters. No subdivisions were made for the A8 group. These results reveal that A8, A9 and A10 are phylogenetically stable across species. As such, further studies concerning such divisions are necessary in order to evaluate the occurrence of subdivisions that express DA in other primate species, with the aim of characterizing its functional relevance.

[Analyzing A- and 0-rhythms of Conditioned Avoidance Reflex in the Rat Brain With Granger Causality]

It has been shown by the method of frequency decomposition of conditional Granger causality that under the execution of conditioned avoidance reflex θ-rhythm from the ventral hippocampus concurrently influences the ventral tegmental area and the series-connected basolateral amygdala and medial prefrontal cortex. Under the expectation of conditioned signal δ-rhythm from the prefrontal cortex influ- ences the ventral tegmental area and the amygdala.

Resting state networks distinguish human ventral tegmental area from substantia nigra

Dopaminergic networks modulate neural processing across a spectrum of function from perception to learning to action. Multiple organizational schemes based on anatomy and function have been proposed for dopaminergic nuclei in the midbrain. One schema originating in rodent models delineated ventral tegmental area (VTA), implicated in complex behaviors like addiction, from more lateral substantia nigra (SN), preferentially implicated in movement. However, because anatomy and function in rodent midbrain differs from the primate midbrain in important ways, the utility of this distinction for human neuroscience has been questioned. We asked whether functional definition of networks within the human dopaminergic midbrain would recapitulate this traditional anatomical topology. We first developed a method for reliably defining SN and VTA in humans at conventional MRI resolution. Hand-drawn VTA and SN regions-of-interest (ROIs) were constructed for 50 participants, using individually-localized anatomical landmarks and signal intensity. Individual segmentation was used in seed-based functional connectivity analysis of resting-state functional MRI data; results of this analysis recapitulated traditional anatomical targets of the VTA versus SN. Next, we constructed a probabilistic atlas of the VTA, SN, and the dopaminergic midbrain region (comprised of SN plus VTA) from individual hand-drawn ROIs. The combined probabilistic (SN plus VTA) ROI was then used for connectivity-based dual-regression analysis in two independent resting-state datasets (n = 69 and n = 79). Results of the connectivity-based, dual-regression functional segmentation recapitulated results of the anatomical segmentation, validating the utility of this probabilistic atlas for future research.

Cortico-Basal Ganglia reward network: microcircuitry

Many of the brain's reward systems converge on the nucleus accumbens, a region richly innervated by excitatory, inhibitory, and modulatory afferents representing the circuitry necessary for selecting adaptive motivated behaviors. The ventral subiculum of the hippocampus provides contextual and spatial information, the basolateral amygdala conveys affective influence, and the prefrontal cortex provides an integrative impact on goal-directed behavior. The balance of these afferents is under the modulatory influence of dopamine neurons in the ventral tegmental area. This midbrain region receives its own complex mix of excitatory and inhibitory inputs, some of which have only recently been identified. Such afferent regulation positions the dopamine system to bias goal-directed behavior based on internal drives and environmental contingencies. Conditions that result in reward promote phasic dopamine release, which serves to maintain ongoing behavior by selectively potentiating ventral subicular drive to the accumbens. Behaviors that fail to produce an expected reward decrease dopamine transmission, which favors prefrontal cortical-driven switching to new behavioral strategies. As such, the limbic reward system is designed to optimize action plans for maximizing reward outcomes. This system can be commandeered by drugs of abuse or psychiatric disorders, resulting in inappropriate behaviors that sustain failed reward strategies. A fuller appreciation of the circuitry interconnecting the nucleus accumbens and ventral tegmental area should serve to advance discovery of new treatment options for these conditions.

Circuit projection from suprachiasmatic nucleus to ventral tegmental area: a novel circadian output pathway

The suprachiasmatic nucleus (SCN) is a circadian pacemaker that synchronizes a number of vital processes. Although a great deal of research has focused on input pathways to SCN and on the central clock itself, relatively little is known about SCN output signaling pathways. The ventral tegmental area (VTA) has been extensively studied for its influence in motivated learning and, recently, for a potential role in arousal and sleep-wake regulation. Here we present data that SCN indirectly projects to VTA via the medial preoptic nucleus (MPON). Microinjection of the retrograde, transynaptic tracer pseudorabies virus (PRV) in rat VTA consistently labeled SCN neurons at time points indicative of an indirect circuit projection. To specify intermediate relay nuclei between SCN and VTA, putative relays were lesioned 1 week prior to PRV injections in VTA. Unilateral lesions of MPON reduced PRV labeling in SCN by 81.6% in the ipsilateral hemisphere and 75.8% in the contralateral hemisphere. Bilateral lesions of the caudal-dorsal lateral septum, another putative relay nucleus and dorsal injection control, did not significantly reduce PRV labeling in the SCN. Single-unit extracellular recordings under halothane anesthesia revealed a novel population of VTA neurons that selectively fired during the active circadian phase. These results show that SCN provides an indirect circuit pathway to VTA via MPON, and that VTA neurons exhibit a circadian rhythm in their impulse activity. This pathway may function in the circadian regulation of numerous behavioral processes including arousal and motivation.

Dopamine reward circuitry: two projection systems from the ventral midbrain to the nucleus accumbens-olfactory tubercle complex

Anatomical and functional refinements of the meso-limbic dopamine system of the rat are discussed. Present experiments suggest that dopaminergic neurons localized in the posteromedial ventral tegmental area (VTA) and central linear nucleus raphe selectively project to the ventromedial striatum (medial olfactory tubercle and medial nucleus accumbens shell), whereas the anteromedial VTA has few if any projections to the ventral striatum, and the lateral VTA largely projects to the ventrolateral striatum (accumbens core, lateral shell and lateral tubercle). These findings complement the recent behavioral findings that cocaine and amphetamine are more rewarding when administered into the ventromedial striatum than into the ventrolateral striatum. Drugs such as nicotine and opiates are more rewarding when administered into the posterior VTA or the central linear nucleus than into the anterior VTA. A review of the literature suggests that (1) the midbrain has corresponding zones for the accumbens core and medial shell; (2) the striatal portion of the olfactory tubercle is a ventral extension of the nucleus accumbens shell; and (3) a model of two dopamine projection systems from the ventral midbrain to the ventral striatum is useful for understanding reward function. The medial projection system is important in the regulation of arousal characterized by affect and drive and plays a different role in goal-directed learning than the lateral projection system, as described in the variation-selection hypothesis of striatal functional organization.

Ventral tegmental area neurons in learned appetitive behavior and positive reinforcement

Ventral tegmental area (VTA) neuron firing precedes behaviors elicited by reward-predictive sensory cues and scales with the magnitude and unpredictability of received rewards. These patterns are consistent with roles in the performance of learned appetitive behaviors and in positive reinforcement, respectively. The VTA includes subpopulations of neurons with different afferent connections, neurotransmitter content, and projection targets. Because the VTA and substantia nigra pars compacta are the sole sources of striatal and limbic forebrain dopamine, measurements of dopamine release and manipulations of dopamine function have provided critical evidence supporting a VTA contribution to these functions. However, the VTA also sends GABAergic and glutamatergic projections to the nucleus accumbens and prefrontal cortex. Furthermore, VTA-mediated but dopamine-independent positive reinforcement has been demonstrated. Consequently, identifying the neurotransmitter content and projection target of VTA neurons recorded in vivo will be critical for determining their contribution to learned appetitive behaviors.

Neuronal activation by stimuli that predict sexual reward in female rats

Conditioned stimuli (CSs) associated with paced copulation induce a conditioned partner preference for males bearing the CS. Here we examined the activation of Fos immunoreactivity (Fos-IR) following exposure to a CS previously paired with either paced or nonpaced copulation. Ovariectomized, hormone-primed rats received 10 sequential conditioning trials at 4-day intervals. In experiment 1, females in the odor-paired group learned to associate an almond odor on a male with paced copulation and an unscented male with nonpaced copulation. In the odor-unpaired group, females received the opposite association. In experiment 2, females associated two different strains of male, Long-Evans or Wistar, with paced or nonpaced copulation, respectively. A preference test indicated that females in both experiments developed a conditioned preference for the pacing-related males, as indicated by significantly more solicitations toward the male and a preference to copulate with the pacing-related male. Subsequently, females were exposed to the CS (odor or strain) alone for 1 h prior to kill and preparation of their brains for immunocytochemistry. In both experiments, the CS associated with paced copulation produced significantly more Fos-IR in the piriform cortex, medial preoptic area, and ventral tegmental area, relative to the same odor or strain cues associated with nonpaced copulation. These findings provide evidence that the state associated with paced copulation can be conditioned to environmental stimuli such as neutral odors or strain cues, which earn an incentive value via classical conditioning. The significance of the brain areas activated is discussed with regard to their role in sexual and other motivated behaviors.

Lateral habenula stimulation inhibits rat midbrain dopamine neurons through a GABA(A) receptor-mediated mechanism

Transient changes in the activity of midbrain dopamine neurons encode an error signal that contributes to associative learning. Although considerable attention has been devoted to the mechanisms contributing to phasic increases in dopamine activity, less is known about the origin of the transient cessation in firing accompanying the unexpected loss of a predicted reward. Recent studies suggesting that the lateral habenula (LHb) may contribute to this type of signaling in humans prompted us to evaluate the effects of LHb stimulation on the activity of dopamine and non-dopamine neurons of the anesthetized rat. Single-pulse stimulation of the LHb (0.5 mA, 100 micros) transiently suppressed the activity of 97% of the dopamine neurons recorded in the substantia nigra and ventral tegmental area. The duration of the cessation averaged approximately 85 ms and did not differ between the two regions. Identical stimuli transiently excited 52% of the non-dopamine neurons in the ventral midbrain. Electrolytic lesions of the fasciculus retroflexus blocked the effects of LHb stimulation on dopamine neurons. Local application of bicuculline but not the SK-channel blocker apamin attenuated the effects of LHb stimulation on dopamine cells, indicating that the response is mediated by GABA(A) receptors. These data suggest that LHb-induced suppression of dopamine cell activity is mediated indirectly by orthodromic activation of putative GABAergic neurons in the ventral midbrain. The habenulomesencephalic pathway, which is capable of transiently suppressing the activity of dopamine neurons at a population level, may represent an important component of the circuitry involved in encoding reward expectancy.

Rostral-caudal differences in the effects of intra-VTA muscimol on cocaine self-administration

We have found that dopamine (DA) in the ventral tegmental area (VTA) plays an important role in cocaine self-administration. DA in the VTA acts at D1-type receptors on the terminals of GABA afferents causing release of this neurotransmitter. Thus, the neurochemical pathways whereby VTA DA might be involved in cocaine self-administration may include GABA neurotransmission. In the present study, we investigated this possibility. Rats were prepared with intravenous catheters and bilateral guide cannulae positioned to allow microinjections directly into the VTA or a site 1 mm dorsal to it. The rats were then trained to self-administer cocaine (1.0 mg/kg/injection) under a fixed-ratio 1 schedule of reinforcement and tested with microinjections of muscimol (0, 0.05 and 0.1 microg/0.25 microl) or picrotoxin (0, 0.025 and 0.05 microg/0.25 microl) or trained under a progressive ratio (PR) schedule and tested with vehicle and 0.05 microg/0.25 microl muscimol. Muscimol in the VTA, but not immediately dorsal to it, significantly reduced cocaine intake under the FR1 schedule. Furthermore, when analyzed by rostral/caudal site of injection, it was found that rostral injections of muscimol significantly reduced cocaine self-administration whereas caudal injections produced non-significant decreases in self-administration. Inspection of individual records revealed no signs of non-specific behavioral effects of the muscimol treatments. Muscimol in the rostral VTA also significantly increased break points in responding under the PR schedule. Intra-VTA picrotoxin did not significantly affect cocaine self-administration. These data suggest that stimulation of GABA-A receptors in the VTA is involved in cocaine self-administration and reward and that this involvement is more pronounced in the rostral than in the caudal VTA.

Efferent connections of the rostral linear nucleus of the ventral tegmental area in the rat

The ventral tegmental area (VTA) is crucially involved in brain reward, motivated behaviors, and drug addiction. This district is functionally heterogeneous, and studying the connections of its different parts may contribute to clarify the structural basis of intra-VTA functional specializations. Here, the efferents of the rostral linear nucleus (RLi), a midline VTA component, were traced in rats with the Phaseolus vulgaris leucoagglutinin (PHA-L) technique. The results show that the RLi heavily innervates the olfactory tubercle (mainly the polymorph layer) and the ventrolateral part of the ventral pallidum, but largely avoids the accumbens. The RLi also sends substantial projections to the magnocellular preoptic nucleus, lateral hypothalamus, central division of the mediodorsal thalamic nucleus, lateral part of the lateral habenula and supraoculomotor region, and light projections to the prefrontal cortex, basolateral amygdala, and dorsal raphe nucleus. A similar set of projections was observed after injections in rostromedial VTA districts adjacent to RLi, but these districts also send major outputs to the lateral ventral striatum. Overall, the data suggest that the RLi is a distinct VTA component in that it projects primarily to pallidal regions of the olfactory tubercle and to their diencephalic targets, the central division of the mediodorsal thalamic nucleus and the lateral part of the lateral habenula. Because the rat RLi reportedly contains a lower density of dopaminergic neurons as compared with most of the VTA, its unusual projections may reflect a non-dopaminergic, putative GABAergic, phenotype, and this distinctive cell population seemingly extends beyond RLi boundaries into the laterally adjacent VTA. By being connected to the central division of the mediodorsal thalamic nucleus (directly and via ventral striatopallidal system) and to the magnocellular preoptic nucleus, the RLi and its surroundings may play a role in olfactory-guided behaviors, which are part of the approach responses associated with appetitive motivational states.

Regional cerebral blood flow changes associated with clitorally induced orgasm in healthy women

There is a severe lack of knowledge regarding the brain regions involved in human sexual performance in general, and female orgasm in particular. We used [15O]-H2O positron emission tomography to measure regional cerebral blood flow (rCBF) in 12 healthy women during a nonsexual resting state, clitorally induced orgasm, sexual clitoral stimulation (sexual arousal control) and imitation of orgasm (motor output control). Extracerebral markers of sexual performance and orgasm were rectal pressure variability (RPstd) and perceived level of sexual arousal (PSA). Sexual stimulation of the clitoris (compared to rest) significantly increased rCBF in the left secondary and right dorsal primary somatosensory cortex, providing the first account of neocortical processing of sexual clitoral information. In contrast, orgasm was mainly associated with profound rCBF decreases in the neocortex when compared with the control conditions (clitoral stimulation and imitation of orgasm), particularly in the left lateral orbitofrontal cortex, inferior temporal gyrus and anterior temporal pole. Significant positive correlations were found between RPstd and rCBF in the left deep cerebellar nuclei, and between PSA and rCBF in the ventral midbrain and right caudate nucleus. We propose that decreased blood flow in the left lateral orbitofrontal cortex signifies behavioural disinhibition during orgasm in women, and that deactivation of the temporal lobe is directly related to high sexual arousal. In addition, the deep cerebellar nuclei may be involved in orgasm-specific muscle contractions while the involvement of the ventral midbrain and right caudate nucleus suggests a role for dopamine in female sexual arousal and orgasm.

Neurotensin afferents of the ventral tegmental area in the rat: [1] re-examination of their origins and [2] responses to acute psychostimulant and antipsychotic drug administration

The ventral tegmental area (VTA) is involved in reward-related behaviours and the actions of psychostimulant drugs. It is influenced by afferents expressing a variety of neurotransmitters and neuromodulators; the innervation containing neurotensin is among the densest of these. Intra-VTA neurotensin activates dopaminergic neurons and plays an important role in the development of behavioural sensitization to psychostimulant drugs and possibly in schizophrenia. Using gold-coupled wheatgerm agglutinin as retrograde tracer in combination with nonisotopic in situ hybridization for neurotensin mRNA or neurotensin antibodies after colchicine treatment, the present study was undertaken to demonstrate the neurotensinergic neurons projecting to the VTA and determine whether (and in which subpopulations) neurotensin expression is regulated in VTA-projecting neurons after administrations of the psychostimulant drug methamphetamine or the antipsychotic haloperidol. This study reveals the lateral preoptico-rostral lateral hypothalamic continuum and the medial preoptic area as main sources for the neurotensin afferents of the VTA. Fewer neurotensinergic, VTA-projecting neurons are situated in the dorsal raphe, pedunculopontine and laterodorsal tegmental nuclei, lateral hypothalamic area, ventral endopiriform area, lateral septum, accumbens shell, parabrachial nucleus and different parts of the extended amygdala. The number of neurotensinergic VTA-projecting neurons increased significantly only after methamphetamine administration and exclusively in the accumbens shell. It is concluded that the widespread neurotensinergic VTA-projecting neurons, situated in areas involved in different reward-related behaviours, are well suited to convey distinct reward information to the VTA. The up-regulation of neurotensin expression selectively in VTA-projecting neurons in the accumbens shell following methamphetamine administration may be an important factor in the development of behavioural sensitization.

Reward-motivated learning: mesolimbic activation precedes memory formation

We examined anticipatory mechanisms of reward-motivated memory formation using event-related FMRI. In a monetary incentive encoding task, cues signaled high- or low-value reward for memorizing an upcoming scene. When tested 24 hr postscan, subjects were significantly more likely to remember scenes that followed cues for high-value rather than low-value reward. A monetary incentive delay task independently localized regions responsive to reward anticipation. In the encoding task, high-reward cues preceding remembered but not forgotten scenes activated the ventral tegmental area, nucleus accumbens, and hippocampus. Across subjects, greater activation in these regions predicted superior memory performance. Within subject, increased correlation between the hippocampus and ventral tegmental area was associated with enhanced long-term memory for the subsequent scene. These findings demonstrate that brain activation preceding stimulus encoding can predict declarative memory formation. The findings are consistent with the hypothesis that reward motivation promotes memory formation via dopamine release in the hippocampus prior to learning.

Reward-Motivated Learning: Mesolimbic Activation Precedes Memory Formation

We examined anticipatory mechanisms of reward-motivated memory formation using event-related FMRI. In a monetary incentive encoding task, cues signaled high- or low-value reward for memorizing an upcoming scene. When tested 24 hr postscan, subjects were significantly more likely to remember scenes that followed cues for high-value rather than low-value reward. A monetary incentive delay task independently localized regions responsive to reward anticipation. In the encoding task, high-reward cues preceding remembered but not forgotten scenes activated the ventral tegmental area, nucleus accumbens, and hippocampus. Across subjects, greater activation in these regions predicted superior memory performance. Within subject, increased correlation between the hippocampus and ventral tegmental area was associated with enhanced long-term memory for the subsequent scene. These findings demonstrate that brain activation preceding stimulus encoding can predict declarative memory formation. The findings are consistent with the hypothesis that reward motivation promotes memory formation via dopamine release in the hippocampus prior to learning.

Kappa opioids selectively control dopaminergic neurons projecting to the prefrontal cortex

Dopaminergic afferents arising from the ventral tegmental area (VTA) are crucial elements in the neural circuits that mediate arousal, motivation, and reinforcement. Two major targets of these afferents are the medial prefrontal cortex (mPFC) and the nucleus accumbens (NAc). Whereas dopamine (DA) in the mPFC has been implicated in working memory and attentional processes, DA in the NAc is required for responding to reward predictive cues. These distinct functions suggest a role for independent firing patterns of dopaminergic neurons projecting to these brain regions. In fact, DA release in mPFC and NAc can be differentially modulated. However, to date, electrophysiological studies have largely overlooked heterogeneity among VTA neurons. Here, we provide direct evidence for differential neurotransmitter control of DA neural activity and corresponding DA release based on projection target. Kappa opioid receptor agonists inhibit VTA DA neurons that project to the mPFC but not those that project to the NAc. Moreover, DA levels in the mPFC, but not the NAc, are reduced after local infusion of kappa opioid receptor agonists into the VTA. These findings demonstrate that DA release in specific brain regions can be independently regulated by opioid targeting of a subpopulation of VTA DA neurons. Selective control of VTA DA neurons projecting to the mPFC has important implications for understanding addiction, attention disorders, and schizophrenia, all of which are associated with DA dysfunction in the mPFC.

Regulation of drug reward by cAMP response element-binding protein: evidence for two functionally distinct subregions of the ventral tegmental area

The transcription factor cAMP response element binding protein (CREB) is implicated in the actions of drugs of abuse in several brain areas, but little information is available about a role for CREB in the ventral tegmental area (VTA), one of the key reward regions of the brain. Here, we demonstrate that chronic exposure to drugs of abuse induces CREB activity throughout the VTA. Using viral-mediated gene transfer, we expressed green fluorescent protein (GFP)-tagged CREB or mCREB (a dominant-negative form of CREB) in the VTA and, using a conditioned place-preference paradigm, found that CREB activation within the rostral versus caudal subregions of the VTA produces opposite effects on drug reward. We identified VTA subregion-specific differences in the proportion of dopaminergic and GABAergic neurons and in the dopaminergic projections to the nucleus accumbens, another brain region implicated in drug reward, and suggest that this may contribute to behavioral differences in this study. We also measured expression levels of tyrosine hydroxylase and the AMPA glutamate receptor subunit GluR1, both of which are known to contribute to drug reward in the VTA, and found that both of these genes are upregulated following the expression of CREB-GFP and downregulated following expression of mCREB-GFP, raising the possibility that CREB may exert its effects on drug reward, in part, via regulation of these genes. These results suggest a novel role for CREB in mediating drug-induced plasticity in the VTA and establish two functionally distinct subregions of the VTA in which CREB differentially regulates drug reward.

Prolonged increase in the sensitivity of the posterior ventral tegmental area to the reinforcing effects of ethanol following repeated exposure to cycles of ethanol access and deprivation

The posterior ventral tegmental area (VTA) is a neuroanatomical substrate mediating the reinforcing effects of ethanol in rats. Repeated alcohol deprivations produce robust ethanol intakes of alcohol-preferring (P) rats during relapse and increase the reinforcing effects of oral alcohol self-administration. The objective of this study was to test the hypothesis that alcohol drinking and repeated alcohol deprivations will increase the reinforcing effects of ethanol within the posterior VTA of P rats. Groups of female P rats were used (alcohol-naive, continuous access, and repeatedly deprived). Each rat was implanted with a guide cannula aimed at the posterior VTA. Depression of the active lever produced the infusion of 100 nl of artificial cerebrospinal fluid (CSF) or ethanol (25-300 mg%). Each rat was given only one ethanol concentration during the 4-h sessions conducted every other day. Compared with the infusions of artificial CSF, the alcohol-naive group reliably self-infused 75 and 150 mg% ethanol, but not the lower or higher concentrations. On the other hand, the continuous access group had significantly higher self-infusions of 50, 75, 150, and 300 mg% ethanol compared with artificial CSF infusions. The repeatedly deprived group also self-infused significantly more of 50, 75, 150, and 300 mg% ethanol than artificial CSF; moreover, the number of infusions for all four concentrations was higher in the repeatedly deprived versus the continuous access group. Chronic alcohol drinking by P rats increased the reinforcing effects of ethanol within the posterior VTA, and repeated alcohol deprivations produced a further increase in these reinforcing effects of ethanol.

Chronic ethanol drinking by alcohol-preferring rats increases the sensitivity of the posterior ventral tegmental area to the reinforcing effects of ethanol

BACKGROUND

The ventral tegmental area (VTA) is involved in regulating ethanol drinking, and the posterior VTA seems to be a neuroanatomical substrate that mediates the reinforcing effects of ethanol in ethanol-naive Wistar and ethanol-naive alcohol-preferring (P) rats. The objective of this study was to test the hypothesis that chronic ethanol drinking increases the sensitivity of the posterior VTA to the reinforcing effects of ethanol.

METHODS

Two groups of female P rats (one given water as its sole source of fluid and the other given 24-hr free-choice access to 15% ethanol and water for at least 8 weeks) were stereotaxically implanted with guide cannulae aimed at the posterior VTA. One week after surgery, rats were placed in standard two-lever (active and inactive) operant chambers and connected to the microinfusion system. Depression of the active lever produced the infusion of 100 nl of artificial cerebrospinal fluid (CSF) or ethanol. The ethanol-naive and chronic ethanol-drinking groups were assigned to subgroups to receive artificial CSF or 25, 50, 75, or 125 mg/dl of ethanol (n = 6-9/dose/group) to self-infuse (FR1 schedule) during the 4-hr sessions given every other day.

RESULTS

Compared with the infusions of artificial CSF, the control group reliably (p < 0.05) self-infused 75 and 125 mg/dl of ethanol but not the lower concentrations. The ethanol-drinking group had significantly (p < 0.05) higher self-infusions of 50, 75, and 125 mg/dl of ethanol than artificial CSF during the four acquisition sessions; the number of infusions of all three doses was higher in the ethanol-drinking group than in the ethanol-naive group. Both groups decreased responding on the active lever when artificial CSF was substituted for ethanol, and both groups demonstrated robust reinstatement of responding on the active lever when ethanol was restored.

CONCLUSIONS

Chronic ethanol drinking by P rats increased the sensitivity of the posterior VTA to the reinforcing effects of ethanol.

Remembrance of rewards past

Using event-related fMRI, Wittmann and colleagues report in this issue of Neuron that reward value enhances cue memory and that this process is associated with midbrain modulation of hippocampal consolidation. We propose that their findings introduce a novel mechanism by which positive arousal induced by reward anticipation may promote memory.

Glutamate-associated plasticity in the ventral tegmental area is necessary for conditioning environmental stimuli with morphine

We sought to determine if plasticity in the ventral tegmental area (VTA) of the midbrain is involved in learning to associate morphine exposure with a specific environment. For this, we tested whether activation of glutamate receptors and protein kinase A is needed for the acquisition and expression of a morphine-conditioned place preference (CPP). Rats received bilateral microinjections of either the NMDA antagonist AP5 (0.48 nmol/0.3 microl), the AMPA antagonist CNQX (0.21 nmol/0.3 microl), or vehicle into the VTA prior to each of three morphine-conditioning sessions. Both the AMPA and NMDA receptor antagonists blocked the development of morphine CPP when given into the VTA but not when given outside the VTA. In similar studies the protein kinase A (PKA) inhibitor, Rp-cAMPS (13 nmol/0.3 microl), blocked the acquisition of morphine CPP when given into the VTA immediately after morphine conditioning. In separate experiments, glutamate antagonists, or Rp-cAMPS, immediately prior to the preference test blocked the expression of morphine CPP when microinjected into the VTA. These data indicate that the VTA is an important site for synaptic modifications involved in the learning and memory of environmental cues predicting reward, and that glutamate input and PKA activation are crucial to this process.

Brief exposure to predator odor and resultant anxiety enhances mesocorticolimbic activity and enkephalin expression in CD-1 mice

The present study assessed alterations in mesolimbic enkephalin (ENK) mRNA levels after predator [2,5-dihydro-2,4,5-trimethylethiazoline (TMT)] and non-predator (butyric acid) odor encounter and/or light-dark (LD) testing in CD-1 mice immediately, 24, 48 and 168 h after the initial odor encounter and/or LD testing. The nucleus accumbens, ventral tegmental area, basolateral (BLA), central (CEA) and medial amygdaloid nuclei, prelimbic and infralimbic cortex were assessed for fos-related antigen (FRA) and/or ENK mRNA as well as neuronal activation of ENK neurons (FRA/ENK). Mice exposed to TMT displayed enhanced freezing and spent less time in the light of the immediate LD test relative to saline- or butyric acid-treated mice. Among mice exposed to TMT, LD anxiety-like behavior was associated with increased FRA in the prelimbic cortex and accumbal shell and decreased ENK-positive neurons in the accumbal core. Mice displaying high TMT-induced LD anxiety exhibited increased ENK-positive neurons in the BLA, CEA and medial amygdaloid nuclei relative to mice that displayed low anxiety-like behavior in the LD test after TMT exposure. In the BLA and CEA, 'high-anxiety' mice also displayed increased FRA/ENK after TMT exposure and LD testing. In contrast to neural cell counts, the level of ENK transcript was decreased in the BLA and CEA of 'high-anxiety' mice after TMT exposure and LD testing. These data suggest that increased FRA may regulate stressor-responsive genes and mediate long-term behavioral changes. Indeed, increased ENK availability in mesolimbic sites may promote behavioral responses that detract from the aversiveness of the stressor experience.

Reward-Related fMRI Activation of Dopaminergic Midbrain Is Associated with Enhanced Hippocampus- Dependent Long-Term Memory Formation

Long-term potentiation in the hippocampus can be enhanced and prolonged by dopaminergic inputs from midbrain structures such as the substantia nigra. This improved synaptic plasticity is hypothesized to be associated with better memory consolidation in the hippocampus. We used a condition that reliably elicits a dopaminergic response, reward anticipation, to study the relationship between activity of dopaminergic midbrain areas and hippocampal long-term memory in healthy adults. Pictures of object drawings that predicted monetary reward were associated with stronger fMRI activity in reward-related brain areas, including the substantia nigra, compared with non-reward-predicting pictures. Three weeks later, recollection and source memory were better for reward-predicting than for non-reward-predicting pictures. FMRI activity in the hippocampus and the midbrain was higher for reward-predicting pictures that were later recognized compared with later forgotten pictures. These data are consistent with the hypothesis that activation of dopaminergic midbrain regions enhances hippocampus-dependent memory formation, possibly by enhancing consolidation.

Injection of the 5-HT2C receptor agonist Ro60-0175 into the ventral tegmental area reduces cocaine-induced locomotor activity and cocaine self-administration

Previously, we have shown that systemic administration of the 5-HT(2C) receptor agonist Ro60-0175 reduces cocaine-induced locomotor activity and cocaine self-administration. Ro60-0175 also alters the activity of midbrain dopamine (DA) neurons of the ventral tegmental area (VTA), a region where 5-HT(2C) receptors are expressed. The present experiments investigated whether microinjections of Ro60-0175 into the VTA would alter the locomotor stimulant effect of cocaine and cocaine self-administration. In the tests for locomotor activity injection of 3 and 10, but not 1 microg, Ro60-0175 into the VTA reduced the locomotor stimulation resulting from injection of 10 mg/kg cocaine. In tests of cocaine self-administration, rats were trained to lever press for intravenous infusions of 0.25 mg cocaine delivered on either a fixed ratio 5 (FR5) or a progressive ratio schedule. Intra-VTA injection of Ro60-0175 at doses of 3 and 10 microg reduced responding for cocaine on both schedules without significantly altering the latency to initiate responding or the rate of responding. A subsequent experiment determined that the suppressant effect of intra-VTA Ro60-0175 (3 microg) on responding for cocaine was prevented by pretreatment with the selective 5-HT(2C) receptor antagonist SB242,084 (0.5 mg/kg). In a final experiment, intra-VTA injection of Ro60-0175 reduced responding for food reinforcement on the same progressive ratio schedule as used for cocaine self-administration. These results demonstrate that stimulation of 5-HT(2C) receptors in the VTA is sufficient to attenuate the stimulant and reinforcing effects of cocaine. These effects complement electrophysiological and neurochemical findings, and indicate that 5-HT(2C) receptors localized within the VTA modulate the activity of mesolimbic DA neurons.

Phospholipase Cgamma in distinct regions of the ventral tegmental area differentially modulates mood-related behaviors

Neurotrophic factor signaling pathways modulate cellular and behavioral responses to drugs of abuse. In addition, chronic exposure to morphine increases expression of phospholipase Cgamma1 (PLCgamma1) (a protein involved in neurotrophic signaling) in the ventral tegmental area (VTA), a neural substrate for many drugs of abuse. Using viral-mediated gene transfer to locally alter the activity of PLCgamma1, we show that overexpression of PLCgamma1 in rostral portions of the VTA (R-VTA) results in increased morphine place preference, whereas PLCgamma1 overexpression in the caudal VTA (C-VTA) results in avoidance of morphine-paired compartments. In addition, overexpression of PLCgamma1 in R-VTA causes increased preference for sucrose and increased anxiety-like behavior but does not affect responses to stress or nociceptive stimuli. In contrast, overexpression of PLCgamma1 in C-VTA decreases preference for sucrose and increases sensitivity to stress and nociceptive stimuli, although there was a tendency for increased anxiety-like behavior as seen for the R-VTA. These results show that levels of PLCgamma1 in the VTA regulate responsiveness to drugs of abuse, natural rewards, and aversive stimuli and point to the possibility that distinct topographical regions within the VTA mediate generally positive versus negative responses to emotional stimuli. Moreover, these data also support a role for drug-induced elevations in PLCgamma1 expression in the VTA in mediating long-term adaptations to drugs of abuse and aversive stimuli.

[Morphological analysis of information processing in basal ganglia of mammals]

The article reviews published and own data on special features of organization of afferent cortical and infracortical correctional systems of the total striopallidaric nuclear complex in various species of mammals. The article focuses on analysis of organization of these flows in a dog being a classical object of behavioural experiments and a rare object of neuromorphological studies. It was discovered that the general trend towards segregated and converged flows of functionally different information at the level of striopallidum is retained in all tested mammals, being an evidence of the fundamental nature of the discussed principles of organization of the projection systems of basal ganglia. The reviewed morphological data confirm reliability of the presented concept of basal ganglia functioning.

The motivational valence of nicotine in the rat ventral tegmental area is switched from rewarding to aversive following blockade of the alpha7-subunit-containing nicotinic acetylcholine receptor

RATIONALE

Within the mammalian ventral tegmental area (VTA), nicotine produces both aversive and rewarding motivational effects. However, the specific neuronal nicotinic acetylcholine receptor (nAChR) subtypes responsible for these effects are not clearly understood.

OBJECTIVES

In the present study, we challenged the motivational effects of nicotine directly in the VTA with nAChR subunit specific antagonists.

METHODS

Using an unbiased place-conditioning procedure as a behavioural assay, we performed bilateral microinfusions of nicotine over a wide range of concentrations (0.008, 8, 24 and 48 nmol/0.5 microl) and challenged the aversive and reinforcing behavioural effects of these nicotine doses with co-administration of di-hydro-beta-erythroidine (DHbetaE) (5 or 50 nmol/0.5 microl), a nAChR antagonist with higher relative affinity for the alpha4beta2 nAChR subunit, methyllycaconitine citrate (MLA) (0.4 or 4 nmol/0.5 microl), a nAChR antagonist that displays greater relative affinity for the alpha7 nAChR, and the NMDA receptor antagonist, d-2-amino-7-phosphoheptanoic acid (AP-7; 18 nmol/0.5 microl).

RESULTS

The alpha4beta2 antagonist DHbetaE blocked both the rewarding and aversive properties of intra-VTA nicotine. However, the alpha7 antagonist MLA blocked nicotine reward and switched the motivational valence of higher doses of nicotine (8-48 nmol/0.5 microl) from rewarding to aversive. The NMDA antagonist AP-7 blocked both the aversive and rewarding effects of intra-VTA nicotine.

CONCLUSIONS

These results suggest a functional dissociation between nAChR neural substrates within the VTA that mediate the bivalent motivational properties of nicotine and further suggest that nicotine may produce its motivational effects through a glutamatergic mechanism.

[Analysis of the morphological substrate of information processing in the dog brain pallidal complex based on organizational characteristics of its afferent projections]

Projections from functionally diverse cortical and subcortical structures (cortex, amygdaloid body, substantia nigra, ventral tegmental area, and thalamus) to the pallidum (globus pallidus, entopeduncular nucleus and ventral pallidum) were studied in dogs using the method of axonal transport of the retrograde markers. Anatomical aspects of both the functional heterogeneity of the pallidal structures and integrative processing of information which underlie the mechanisms of adaptive behavior, were analyzed on the basis of the results obtained.

Activation of ventral tegmental area cells by the bed nucleus of the stria terminalis: a novel excitatory amino acid input to midbrain dopamine neurons

We examined the role of excitatory amino acids (EAAs) in the activation of midbrain dopaminergic (DA) neurons evoked by stimulation of the ventromedial and ventrolateral (subcommissural) bed nucleus of the stria terminalis (vBNST). Using anesthetized rats and extracellular recording techniques, we found that 84.8% of ventral tegmental area (VTA) DA neurons were activated synaptically by single-pulse electrical stimulation of the vBNST. In contrast, similar stimulation did not affect the activity of presumed GABA neurons in the VTA. Three characteristic responses were observed in VTA DA neurons: short latency activation (<25 msec; 55.1% of cells), long latency activation (>65 msec; 56% of cells), and inhibition (61.8% of cells, usually followed by long latency excitation). Microinfusion of antagonists of EAA receptors (3 mm kynurenic acid, 100 microm AP-5, or 50 microm CNQX) from a micropipette adjacent to the recording electrode significantly reduced both short and long latency activations evoked in DA neurons by vBNST stimulation. Specific responses were attenuated similarly by AP-5 alone, CNQX alone, or a cocktail of AP-5+CNQX, indicating that joint activation of NMDA plus non-NMDA receptors was required. Stimulation of the vBNST by local microinfusion of glutamate increased the firing and bursting activity of VTA DA neurons. Similar microinfusion of GABA decreased bursting of VTA DA neurons without altering their firing rate. Retrograde and anterograde labeling and antidromic activation of vBNST neurons by VTA stimulation confirmed a direct projection from the vBNST to the VTA. These results reveal that inputs from the vBNST exert a strong excitatory influence on VTA DA neurons mediated by both NMDA and non-NMDA receptors.

Alterations in food intake elicited by GABA and opioid agonists and antagonists administered into the ventral tegmental area region of rats

Food intake is significantly increased following administration of mu-selective opioid agonists into the ventral tegmental area (VTA) region acting through multiple local opioid receptor subtypes. Since GABA receptor agonists in the VTA region are capable of eliciting feeding, the present study investigated whether feeding elicited by the mu-selective opioid agonist [D-Ala(2), NMe(4), Gly-ol(5)]-enkephalin (DAMGO) in the VTA region was altered by pretreatment into the same site with equimolar doses of either GABA(A) (bicuculline) or GABA(B) (saclofen) antagonists, and further, whether pretreatment with either general opioid or selective GABA receptor antagonists decreased feeding elicited by GABA(A) (muscimol) or GABA(B) (baclofen) agonists in the VTA region. DAMGO-induced feeding in the VTA region was dose-dependently decreased following pretreatment with either GABA(A) or GABA(B) antagonists in the absence of significant alterations in food intake by the antagonists per se. However, the presence of short-lived seizures following bicuculline in the VTA region suggests that this ingestive effect was caused by nonspecific actions. In contrast, GABA(B) receptors are involved in the full expression of mu-opioid agonist-induced feeding in this region since saclofen failed to elicit either seizure activity or a conditioned taste aversion. Pretreatment with naltrexone in the VTA region reduced intake elicited by baclofen, but not muscimol. Finally, baclofen-induced feeding was significantly reduced by saclofen, but not bicuculline, pretreatment in the VTA region. Therefore, possible coregulation between GABA(B) and opioid receptors in the VTA region, as suggested by immunocytochemical evidence, is supported by these behavioral effects upon ingestion.

The effects of ventral tegmental administration of GABA(A), GABA(B), NMDA and AMPA receptor agonists on ventral pallidum self-stimulation

The ventral pallidum (VP) is a basal forebrain structure that is interconnected with motor and limbic structures and may be considered as an interface between motivational and effector neural signals. Results from a considerable number of studies suggest that this structure is critically involved in reward-related behavior. The VP shares reciprocal connections with other reward-implicated regions, such as the ventral tegmental area (VTA). This anatomy predicts that drug-induced neuronal alterations in the VTA could profoundly alter the function of the VP. Here, using the curve-shift intracranial self-stimulation method, we studied the effects of muscimol (GABA(A) agonist), baclofen (GABA(B) agonist), NMDA and AMPA, microinjected bilaterally into the VTA on the rewarding efficacy of VP self-stimulation. Central injections of the highest dose of muscimol (0.128 microg) resulted in significant elevations in VP self-stimulation thresholds, indicating a reduction in the rewarding efficacy of the stimulation. Elevations in VP self-stimulation thresholds were also evident after intrategmental injections of higher doses of baclofen (0.12, 0.48 microg). By contrast, intrategmental activation of NMDA and AMPA receptors did not affect reward thresholds. These findings suggest that GABAergic and glutamatergic transmission in the VTA activate different circuits that may mediate different functions. Thus, the VTA--VP projection activated by GABA modulates VP stimulation reward, while the projection activated by glutamate may be involved in reward-unrelated effects, rather than in the processing of reward. The decreased rewarding efficacy of VP self-stimulation following intrategmental injections of muscimol and baclofen may be due to GABAergic modulation of ventral tegmental dopaminergic and nondopaminergic neurons projecting to the VP.

Factors that influence the persistence of stimulation-induced aversion

Brain stimulation reward in certain regions has been shown to produce analgesia to externally applied painful stimuli. In the present experiments, we studied how electrical self-stimulation of the dorsal raphe (DR) nucleus modifies the aversive effects of electrical stimulation of the nucleus reticularis gigantocellularis (Gi) or of the dorsal tegmentum (DTg). In the first study, the threshold for latency to escape aversive Gi stimulation was tracked before and after exposure to rewarding DR stimulation. Only a few sessions of DR self-stimulation were required to produce a complete and long-lasting inhibition of Gi aversion. In the second study, the aversion induced by DTg stimulation rapidly disappeared following a few test sessions at that site. Unlike our previous experience with Gi aversion that required either pairing with rewarding lateral hypothalamic (LH) or ventral tegmental area (VTA) pulses in order to increase the threshold for latency to escape Gi aversion, in this study, simply brief experience with rewarding DR stimulation in unpaired trials was sufficient to entirely suppress Gi-induced aversion. Even more surprising was the finding that unlike the Gi, aversion obtained from activation of the DTg does not persist, its threshold for escape quickly increases, and within a few sessions is no longer evident. One interpretation of these findings is that the aversion mechanisms associated with the Gi and DTg are differentially susceptible to analgesic processes.

GABA-containing neurons in the rat ventral tegmental area project to the prefrontal cortex

Dopamine-containing projections from the ventral tegmental area (VTA) to the prefrontal cortex (PFC) have been extensively characterized since their discovery over 25 years ago. However, the VTA projection to the PFC also contains a substantial nondopamine component, whose neurochemical phenotype is unknown. To examine if a portion of this nondopamine VTA projection contains GABA, anterograde and retrograde tract-tracing in the rat was combined with GABA immunocytochemistry and electron microscopy. Following injections of Fluoro-Gold (FG) into the PFC, many VTA neurons were retrogradely labeled, as visualized by immunoperoxidase staining for FG. A large portion of FG-labeled somata (58%) and dendrites (33%) within the VTA also contained immunogold-silver labeling for GABA. These dually labeled profiles exhibited a morphology similar to dopamine-containing cells within the VTA. To confirm and extend these findings, anterograde transport of biotinylated dextran amine (BDA) from the VTA was combined with immunogold-silver labeling for GABA within the PFC. Consistent with the results obtained from retrograde tracing, a portion of BDA-labeled terminals in the PFC also contained immunoreactivity for GABA. These dually labeled terminals formed symmetric synapses onto small caliber dendrites and dendritic spines. Some PFC dendrites contacted by GABA-containing VTA terminals were themselves GABA-labeled. The results of this investigation have identified a substantial population of GABA-containing neurons in the VTA that send axons to the PFC where they synapse on the distal processes of both pyramidal and local circuit neurons. This GABA-containing mesocortical pathway may provide substrates for both inhibitory and disinhibitory influences on PFC neuronal activity.

Intra-ventral tegmental area injection of rat cocaine and amphetamine-regulated transcript peptide 55-102 induces locomotor activity and promotes conditioned place preference

Cocaine- and amphetamine-regulated transcript (CART) is a novel mRNA that has been reported to be increased by acute psychostimulant administration, and that may be involved in the effects of psychostimulants. In this study, we examined the effect of centrally administered CART peptides on locomotor activity and conditioned place preference in the rat. CART peptide fragments were bilaterally injected into the ventral tegmental area. CART 55-102 (0.2-5.0 microg/side), an endogenously occurring peptide, dose dependently increased locomotor activity, whereas CART 1-26 (0.1-2.5 microg/side; not found endogenously) did not. The locomotor effects of CART 55-102 were dose dependently blocked by the dopamine D(2) receptor antagonist haloperidol (0.03-1.0 mg/kg i.p.). Four injections of 1.0 microg/side CART 55-102 induced a significant place preference, suggesting that CART 55-102 is reinforcing. Increases in locomotor activity after each of these CART 55-102 injections were similar and did not show tolerance or sensitization. This treatment regimen of CART 55-102 also did not produce sensitization to locomotor activity after a subsequent challenge with cocaine or amphetamine. When CART 55-102 (0.2-1.0 microg/side) was injected into the substantia nigra, no significant change in motor activity was observed. However, a higher dose of CART 55-102 (5.0 microg/side) induced a delayed increase in motor activity, suggesting a possible diffusion from the substantia nigra into the ventral tegmental area. Our findings suggest that CART 55-102 is behaviorally active and may be involved in the actions of psychostimulants. This is the first demonstration of the psychostimulant-like effects of CART peptides.

Limited collateralization of neurons in the rat prefrontal cortex that project to the nucleus accumbens

The specificity and selectiveness of a neuronal message depends in part on the number of recipient neurons that simultaneously receive this message. Hence, projections involved in higher order cognitive processes might be expected to exhibit a lower degree of collateralization than projections that mediate more basic brain functions. This study sought to determine the degree to which neurons projecting from the prefrontal cortex to the nucleus accumbens collateralize to major cortical and subcortical regions: the contralateral prefrontal cortex, the basolateral amygdala or the ventral tegmental area. Fluoro-Gold and cholera toxin-b were used to label prefrontal cortex neurons that project to these targets, and the proportion of neurons singly and dually labeled by immunofluorescence for these tracers was determined. The prefrontal cortex neurons projecting to these regions exhibited a partially complementary laminar distribution. Furthermore, of the neurons projecting to the nucleus accumbens, 13% sent a collateralized projection to the contralateral prefrontal cortex, 7% collateralized to the basolateral amygdala, and 3% sent a branched projection to the ventral tegmental area. No differences were observed in the degree of collateralization of neurons in superficial versus deep layers.Thus, the degree of collateralization of corticoaccumbens neurons was overall limited, but significantly greater to a cortical target than to subcortical regions. These branching patterns provide anatomical substrates for temporal and spatial coordination of activity in limbic circuits.

Medial prefrontal cortical output neurons to the ventral tegmental area (VTA) and their responses to burst-patterned stimulation of the VTA: neuroanatomical and in vivo electrophysiological analyses

During a delayed period in a delayed-response task, prefrontal cortical neurons show a change in neuronal firing rate that is dependent on a functional mesocortical dopamine input. This change in firing rate has been attributed to be part of the cellular processes underlying working memory. However, it is unclear what neural mechanisms activate mesocortical dopamine neurons to provide an optimal level of dopamine to modulate the firing of the medial prefrontal cortical (mPFC) neurons. This study examined the possibility of whether mPFC neurons that project to the ventral tegmental area (VTA) might activate the ascending mesocortical dopamine neurons. To determine the locations of the mPFC-->VTA neurons, cholera toxin subunit B was microinjected into the VTA. Retrogradely labeled mPFC neurons mainly reside in the deep lamina V and VI. In vivo single unit recording in urethane-anesthetized rats were also used to determine the responses of some of these neurons to burst-patterned stimulation of the VTA. Single-pulse stimulation (1 Hz) of the VTA antidromically activated burst firing mPFC-->VTA neurons. In response to burst-patterned stimulation of the VTA, which mimicked burst firing of VTA dopamine neurons (4-10 pulses at 10-15 Hz cycled at 0.5-3 Hz), the temporal structure of spontaneous burst firing patterns of these neurons but not their mean firing rate were changed. However, the mean firing rate of the non-VTA projecting neurons (i.e., no antidromic response to VTA stimulations) was either increased or decreased by similar burst-patterned stimulation of the VTA. These data suggest that burst-patterned stimulation of the ascending VTA-->mPFC or putative mesocortical dopamine neurons might have released dopamine and/or other neuromodulators to modulate the temporal code, rather than the rate code, of mPFC-->VTA neurons. Medial PFC neurons that project elsewhere (e.g., nucleus accumbens or mediodorsal thalamus) may mediate the sustained firing rate changes during, e.g., short-term working memory.

Enhanced amygdala kindling after electrical stimulation of the ventral tegmental area: implications for fear and anxiety

Electrical kindling refers to the seizure-generating properties of brain stimulation. In addition to producing epilepsy, the reorganization of forebrain neurocircuitry associated with kindling contributes to psychiatric disturbances involving fear and anxiety. The amygdala is a limbic structure that kindles readily and regulates the complex neurocircuitry underlying emotional responding. Dopamine-containing ventral tegmental area (VTA) neurons, known to be activated by threatening environmental stimuli, are an important component of the amygdala-based fear network. Using amygdala kindling as an indicator of sensitization development, we report here that repeated low-current, high-frequency stimulation of the VTA provoked afterdischarge in the central amygdala and enhanced kindling rate. By establishing a fundamental link between VTA activation and neural excitability in the central amygdala, the present results are consistent with the possibility of a common process underlying epileptogenisis and the fear motivational consequences of amygdala and VTA kindling. Considering the established role of the VTA and the amygdala in emotional responding, such a sensitization mechanism might mediate exaggerated fearfulness.

GABA(A), D1, and D5, but not progestin receptor, antagonist and anti-sense oligonucleotide infusions to the ventral tegmental area of cycling rats and hamsters attenuate lordosis

In hamsters, progesterone (P) in the hypothalamus and ventral tegmental area (VTA) is necessary for receptivity; in rats, hypothalamic P induces receptivity and midbrain P further enhances it. How P exerts its effects in the VTA on lordosis is of interest because few estrogen-induced P receptors (PRs) have been identified there. Sexual receptivity of rats and hamsters is enhanced when P's actions in the VTA are restricted to the membrane and when the gamma-aminobutyric acid (GABA)A agonist, muscimol, is infused into the VTA, but attenuated with infusions of the GABA(A) antagonist, bicuculline. The dopamine (DA) agonist. SKF38393, rapidly enhances receptivity when infused intravenously; this effect can be blocked by both DA receptor (DR) and PR antagonists. This study investigated the importance of PRs, glutamic acid decarboxylase (GAD), the enzyme responsible for GABA production, GABA(A) receptors (GBRs), and DRs in the VTA of cycling rats and hamsters for the expression of lordosis. Proestrous and diestrous animals implanted with bilateral VTA cannulae were pre-tested for receptivity, infused with either an antagonist (RU38486 (20 microg), bicuculline (100 ng), SCH23390 (100 ng)), anti-sense oligonucleotide (against PR (250 ng), GAD (500 ng), D1 (500 ng), D5 (250 ng)), or control infusions to each cannulae and re-tested. Vehicle and scrambled oligonucleotides were infused as controls and elicited similar effects. Antagonists of GBRs and DRs significantly reduced lordosis on post-tests compared to the PR antagonist and control conditions in rats and hamsters. Lordosis was significantly reduced, compared to controls, only by anti-sense oligonucleotides for GAD and D1- and D5-DR subtypes. These data suggest that in the VTA GABAergic and dopaminergic neurons may be more important in the mediation of sexual receptivity than neurons containing intracellular PRs.

Mesolimbic NMDA receptors are implicated in the expression of conditioned morphine reward

Systemic administration of a variety of N-methyl-D-aspartate (NMDA) receptor antagonists inhibits morphine's rewarding properties in the conditioned place preference test. In this study, we investigated the anatomical loci implicated in the inhibition of expression of morphine's reward by bilateral microinjections of a selective NMDA antagonist into the mesolimbic areas, including ventral tegmental area and nucleus accumbens. During conditioning, injections of 1 mg/kg morphine were associated with placing rats in one chamber of the place preference box; the exposures to the other chamber were associated with placebo administration. On the test day, drug-free control subjects demonstrated a marked preference for the morphine-associated chamber. Systemic administration of 5 mg/kg and 10 mg/kg of the competitive NMDA antagonist, NPC 17742 (2R,4R,5S-2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid), significantly reduced the expression of morphine-induced conditioned place preference; the dose of 10 mg/kg produced also an inhibition of locomotor activity. Similar attenuation of the expression of morphine-induced conditioned place preference was observed in rats receiving 15.6 and 62.5 ng/0.5 ml side of NPC 17742 injected bilaterally into the nucleus accumbens and ventral tegmental area. While the higher intra-accumbal dose of NPC 17742 produced behavioral stimulation, intra-tegmental injection did not affect locomotor activity. These findings suggest that activation of NMDA receptors in the nucleus accumbens and ventral tegmental area is necessary for the elicitation of approach by environments previously associated with morphine's rewarding action.

An electrophysiological characterization of ventral tegmental area dopaminergic neurons during differential pavlovian fear conditioning in the awake rabbit

Recent research has suggested that the mesencephalic dopaminergic (DA) system is activated by stress. For example, alterations in DA metabolites have been found in the ventral tegmental area (VTA) following footshock and immobilization in the rat [15,37]. Furthermore, this activation appears selective to DA neurons within the VTA since no changes were observed within the substantia nigra [15,16]. While this research suggests that DA neurons in the VTA are activated by aversive events, there has been a paucity of electrophysiological research designed to examine the sensory response characteristics of these DA neurons, and in particular their response to stimuli which predict aversive events. The present study was conducted to investigate the response characteristics of DA neurons within the VTA of the awake rabbit to acoustic stimuli which, via Pavlovian aversive conditioning procedures, came to predict the occurrence of a mild shock to the pinna. 45%, of the neurons meeting pre-established criteria for DA neurons demonstrated either significant excitation or inhibition to conditioned aversive stimuli. These neurons responded differentially to CS+ and CS- presentations. Some of these neurons (65%) demonstrated a greater increase in activity during the CS+ compared to the CS-, some (22%,) demonstrated a greater decrease in activity during the CS+ compared to the CS- and some (13%) demonstrated a greater increase in activity during the CS- compared to the CS+. Further, conditioned heart rate responses in the rabbits occurred during the recording of a majority of these neurons. These overall results suggest that conditioned aversive stimuli can affect the firing of VTA DA neurons and that these neurons comprise a heterogenous population with respect to their response profiles.

Effects of NMDA lesions of the medial basal forebrain on LH and VTA self-stimulation

Rewarding stimulation of the medial forebrain bundle (MFB) increases Fos-like immunoreactivity in many brain areas, including an ipsilateral, basal forebrain region extending from the medial preoptic area (MPO) to the lateral preoptic area, and substantia innominata. Excitotoxic lesions of the lateral portion of this region have been found to produce large sustained or transient increases in the number of pulses required to maintain half-maximal lever-pressing (required number of pulses) for MFB stimulation. In the present study, changes in self-stimulation of the lateral hypothalamus and ventral tegmental area were assessed following excitotoxic lesions of more medial structures, including the MPO and bed nucleus of the stria terminalis. Increases in the required number of pulses (up to 0.16 log10 units) were seen in only 2 of 10 subjects. In two other rats, the reward effectiveness of the stimulation was moderately increased after the lesion as manifested in decreases of up to 0.14 log10 units in the required number. No appreciable change from baseline was seen in the remaining six subjects. The simplest interpretation of these results is that neurons with cell bodies in the medial portion of the basal forebrain may make a smaller contribution to the rewarding effect of MFB stimulation than neurons in the lateral portion.

Anatomic evidence of a three-dimensional mosaic pattern of tonotopic organization in the ventral complex of the lateral lemniscus in cat

The ventral complex of the lateral lemniscus (VCLL, i.e., the ventral and intermediate nuclei) is composed of cells embedded in the fibers of the lateral lemniscus. These cells are involved in the processing of monaural information and receive input from the collaterals of the fibers ascending to the inferior colliculus. Whereas tonotopic organization is a feature of all other nuclei of the auditory system, this functional principle is debated in the VCLL. We have made focal injections of the tracer biotinylated dextran amine into different frequency band representations of the inferior colliculus in cat. Retrogradely labeled cells and terminal fibers (collaterals of efferent local axons and other ascending lemniscal fibers) were found in the ipsilateral VCLL. The spatial distribution of the labeling was analyzed using three-dimensional (3-D) reconstruction and computer graphical visualization techniques. A complex topographic organization was found. In all cases, labeled fibers and cells were distributed in multiple clusters throughout the dorsoventral extent of the VCLL. The shape, size, and location of the labeled clusters suggest an interdigitation of clusters assigned to different frequency-band representations. But an overall mediolateral distribution gradient was observed, with high frequencies represented medially and lower frequencies progressively more laterally. We conclude that the clusters may represent discontinuous frequency-band compartments as a counterpart to the continuous laminar compartments in the remaining auditory nuclei. The 3-D orderly mosaic pattern indicates that the VCLL preserves the spectral decomposition originated in the cochlea in a way that facilitates across-frequency integration.

Regional differences within the rat ventral tegmental area for muscimol self-infusions

The present study examined the effects of activating GABA(A) receptors in the anterior and posterior regions of the ventral tegmental area (VTA) on operant reinforcement behavior, using the technique of intracranial self-administration. Rats were given the opportunity to self-administer vehicle alone (artificial CSF) and vehicle containing 25, 50, and 100 microM muscimol, a GABA(A) agonist, into the anterior or posterior VTA during four sessions (3 h/session) in standard two-lever operant chambers. Rats received five times greater infusions of 50 and 100 microM muscimol than vehicle into the posterior VTA; both doses significantly increased responding above vehicle levels on the active and inactive (control) levers equally. When the response requirement for muscimol infusions was increased from a fixed-ratio 1 (FR1) to FR3 in a single-lever chamber, the total session responses increased approximately twofold. Muscimol was not self-infused when cannula placements were in the anterior VTA. The self-infusion of muscimol into the posterior VTA was attenuated by coadministration of picrotoxin. Overall, the results suggest that the activation of GABA(A) receptors in the posterior VTA produces goal-directed behavior.

Morphology and axonal projection patterns of auditory neurons in the midbrain of the painted frog, Discoglossus pictus

Acoustic signals are extensively used for guiding various behaviors in frogs such as vocalization and phonotaxis. While numerous studies have investigated the anatomy and physiology of the auditory system, our knowledge of intrinsic properties and connectivity of individual auditory neurons remains poor. Moreover, the neural basis of audiomotor integration still has to be elucidated. We determined basic response patterns, dendritic arborization and axonal projection patterns of auditory midbrain units with intracellular recording and staining techniques in an isolated brain preparation. The subnuclei of the torus semicircularis subserve different tasks. The principal nucleus, the main target of the ascending auditory input, has mostly intrinsic neurons, i.e., their dendrites and axons are restricted to the torus itself. In contrast, neurons of the magnocellular and the laminar nucleus project to various auditory and non-auditory processing centers. The projection targets include thalamus, tegmentum, periaqueductal gray, medulla oblongata, and in the case of laminar neurons--the spinal cord. Additionally, tegmental cells receive direct auditory input and project to various targets, including the spinal cord. Our data imply that both auditory and premotor functions are implemented in individual toral and tegmental neurons. Their axons constitute parallel descending pathways to several effector systems and might be part of the neural substrate for differential audiomotor integration.

GABAergic projections from the lateral lemniscus to the inferior colliculus of the rat

The objective of the present study was to provide direct evidence regarding GABAergic projections from the nuclei of the lateral lemniscus to the central nucleus of the inferior colliculus (ICC), and from the ICC to the opposite ICC. Projections of GABAergic neurons in the rat were investigated by a combination of fluorogold (FG) retrograde tracing and GABA immunocytochemistry. FG was first injected into a frequency-defined region (11-13 kHz) in the center of the ICC, and 1-2 weeks was allowed for retrograde transport. Vibratome sections were then cut through the brainstem and stained with GABA antibody. Double-labeling was taken as evidence of GABAergic neurons projecting to the ICC. The results from FG retrograde labeling alone showed that neurons in the dorsal nucleus of the lateral lemniscus (DNLL) bilaterally, in the intermediate and ventral nucleus of the lateral lemniscus (INLL and VNLL) ipsilaterally, and in the ICC contralaterally project to the ICC. GABA immunostaining alone showed substantial numbers of GABA positive neurons in the nuclei of the lateral lemniscus and the inferior colliculus. FG and GABA double-labeled neurons were present in all nuclei of the lateral lemniscus that project to the ICC. The greatest concentration of double-labeled neurons was found bilaterally in the DNLL, suggesting a prominent GABAergic projection from the DNLL to the ICC. The presence of many double-labeled neurons in the ipsilateral INLL and VNLL suggests that there are also GABAergic inputs from the INLL and VNLL to the ICC. No double-labeled neurons were found in the contralateral ICC, which suggests the possibility of a prominent non-GABAergic projection.

A dopamine-mu1 opioid link in the rat ventral tegmentum shared by palatable food (Fonzies) and non-psychostimulant drugs of abuse

The role of mu1 opioid receptors in the stimulation of dopamine transmission in the rat nucleus accumbens by an unusual palatable food (Fonzies) and non-psychostimulant drugs of abuse was investigated by the use of naloxonazine, a pseudo-irreversible antagonist of mu1 opioid receptors. Feeding of Fonzies stimulated dopamine release in the medial prefrontal cortex and in the shell, but not in the core of the nucleus accumbens. Pretreatment with naloxonazine given systemically (15 mg/kg i.p. 20 h before) completely prevented the stimulation of dopamine release in the shell of the nucleus accumbens by Fonzies without affecting that in the prefrontal cortex. Systemic pretreatment with naloxonazine reduced or, depending on the dose, abolished, the stimulation of dopamine release in the nucleus accumbens shell by morphine, nicotine and ethanol, but did not affect that by haloperidol. Naloxonazine also prevented the stimulatory effects of Fonzies, nicotine and morphine on nucleus accumbens dopamine transmission when infused bilaterally in the ventral tegmental area. The results indicate that mu1 opioid receptors in the ventral tegmentum play a major role in the stimulant effects of food and drugs of abuse on mesolimbic dopamine transmission.

Injection of SCH 23390 into the ventral tegmental area blocks the development of neurochemical but not behavioral sensitization to cocaine

Previous studies have indicated that dopamine D1 receptors in the ventral tegmental area (VTA) may play an important role in the development of sensitization to amphetamine. The present study was designed to determine if D1 receptors are also important in the development of cocaine-induced behavioral and neurochemical sensitization. Animals received intra-VTA injections of saline or the dopamine D1 receptor antagonist SCH 23390 (15 nmol/side) 5 min before receiving systemic injections of saline or cocaine (15 mg/kg) on 4 consecutive days. One week later animals were challenged with cocaine. Motor activity and extracellular dopamine concentrations in the nucleus accumbens were monitored on the day the animals received the first of their four daily treatments and/or on the day animals received their cocaine challenge injection. Intra-VTA SCH 23390 attenuated the acute response, but did not alter development of the sensitized motor-stimulant response to cocaine. In contrast to the behavioral data, intra-VTA SCH 23390 blocked both the acute cocaine-induced increase in extracellular dopamine in the nucleus accumbens and development of the sensitized response. These data provide partial support for the role of dopamine D1 receptors in the VTA in the development of cocaine-induced sensitization. The data also suggest, however, that additional mechanisms may play a role in the development of sensitization.