Thomas C. Jhou

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.

Cocaine Drives Aversive Conditioning via Delayed Activation of Dopamine-Responsive Habenular and Midbrain Pathways

Many strong rewards, including abused drugs, also produce aversive effects that are poorly understood. For example, cocaine can produce aversive conditioning after its rewarding effects have dissipated, consistent with opponent process theory, but the neural mechanisms involved are not well known. Using electrophysiological recordings in awake rats, we found that some neurons in the lateral habenula (LHb), where activation produces aversive conditioning, exhibited biphasic responses to single doses of intravenous cocaine, with an initial inhibition followed by delayed excitation paralleling cocaines shift from rewarding to aversive. Recordings in LHb slice preparations revealed similar cocaine-induced biphasic responses and further demonstrated that biphasic responses were mimicked by dopamine, that the inhibitory phase depended on dopamine D2-like receptors, and that the delayed excitation persisted after drug washout for prolonged durations consistent with findings in vivo. c-Fos experiments further showed that cocaine-activated LHb neurons preferentially projected to and activated neurons in the rostromedial tegmental nucleus (RMTg), a recently identified target of LHb axons that is activated by negative motivational stimuli and inhibits dopamine neurons. Finally, pharmacological excitation of the RMTg produced conditioned place aversion, whereas cocaine-induced avoidance behaviors in a runway operant paradigm were abolished by lesions of LHb efferents, lesions of the RMTg, or by optogenetic inactivation of the RMTg selectively during the period when LHb neurons are activated by cocaine. Together, these results indicate that LHb/RMTg pathways contribute critically to cocaine-induced avoidance behaviors, while also participating in reciprocally inhibitory interactions with dopamine neurons.

Efferent pathways of the mouse lateral habenula

The lateral habenula (LHb) is part of the habenula complex of the dorsal thalamus. Recent studies of the LHb have focused on its projections to the ventral tegmental area (VTA) and rostromedial tegmental nucleus (RMTg), which contain γ‐aminobutyric acid (GABA)ergic neurons that mediate reward prediction error via inhibition of dopaminergic activity. However, older studies in the rat have also identified LHb outputs to the lateral and posterior hypothalamus, median raphe, dorsal raphe, and dorsal tegmentum. Although these studies have shown that the medial and lateral divisions of the LHb have somewhat distinct projections, the topographic specificity of LHb efferents is not completely understood, and the relative extent of these projections to brainstem targets is unknown. Here we have used anterograde tracing with adeno‐associated virus–mediated expression of green fluorescent protein, combined with serial two‐photon tomography, to map the efferents of the LHb on a standard coordinate system for the entire mouse brain, and reconstruct the efferent pathways of the LHb in three dimensions. Using automated quantitation of fiber density, we show that in addition to the RMTg, the median raphe, caudal dorsal raphe, and pontine central gray are major recipients of LHb efferents. By using retrograde tract tracing with cholera toxin subunit B, we show that LHb neurons projecting to the hypothalamus, VTA, median raphe, caudal dorsal raphe, and pontine central gray reside in characteristic, but sometimes overlapping regions of the LHb. Together these results provide the anatomical basis for systematic studies of LHb function in neural circuits and behavior in mice. J. Comp. Neurol. 523:32–60, 2015.

Economic demand predicts addiction-like behavior and therapeutic efficacy of oxytocin in the rat

Significance Cocaine addiction is a major public health problem with no current pharmacotherapy approved by the US Food and Drug Administration. To accelerate discovery of treatments, we developed an animal model based on economics. Economics allows mathematical alignment of animal and human behavior, permitting more confident predictions of efficacy in addicts. Although economic models are strongly associated with addiction severity in humans, they have not yet been shown to be a marker of addiction in rats. In this report, we confirm that economic demand is strongly associated with addiction-like behavior in rats, and can predict the efficacy of a promising addiction therapy. Our findings indicate that this economic approach can be used to accelerate the development of novel addiction therapies. Development of new treatments for drug addiction will depend on high-throughput screening in animal models. However, an addiction biomarker fit for rapid testing, and useful in both humans and animals, is not currently available. Economic models are promising candidates. They offer a structured quantitative approach to modeling behavior that is mathematically identical across species, and accruing evidence indicates economic-based descriptors of human behavior may be particularly useful biomarkers of addiction severity. However, economic demand has not yet been established as a biomarker of addiction-like behavior in animals, an essential final step in linking animal and human studies of addiction through economic models. We recently developed a mathematical approach for rapidly modeling economic demand in rats trained to self-administer cocaine. We show here that economic demand, as both a spontaneous trait and induced state, predicts addiction-like behavior, including relapse propensity, drug seeking in abstinence, and compulsive (punished) drug taking. These findings confirm economic demand as a biomarker of addiction-like behavior in rats. They also support the view that excessive motivation plays an important role in addiction while extending the idea that drug dependence represents a shift from initially recreational to compulsive drug use. Finally, we found that economic demand for cocaine predicted the efficacy of a promising pharmacotherapy (oxytocin) in attenuating cocaine-seeking behaviors across individuals, demonstrating that economic measures may be used to rapidly identify the clinical utility of prospective addiction treatments.

Addiction-like Synaptic Impairments in Diet-Induced Obesity

BACKGROUND There is increasing evidence that the pathological overeating underlying some forms of obesity is compulsive in nature and therefore contains elements of an addictive disorder. However, direct physiological evidence linking obesity to synaptic plasticity akin to that occurring in addiction is lacking. We sought to establish whether the propensity to diet-induced obesity (DIO) is associated with addictive-like behavior, as well as synaptic impairments in the nucleus accumbens core considered hallmarks of addiction. METHODS Sprague Dawley rats were allowed free access to a palatable diet for 8 weeks then separated by weight gain into DIO-prone and DIO-resistant subgroups. Access to palatable food was then restricted to daily operant self-administration sessions using fixed ratio 1, 3, and 5 and progressive ratio schedules. Subsequently, nucleus accumbens brain slices were prepared, and we tested for changes in the ratio between α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-D-aspartate currents and the ability to exhibit long-term depression. RESULTS We found that propensity to develop DIO is linked to deficits in the ability to induce long-term depression in the nucleus accumbens, as well as increased potentiation at these synapses as measured by AMPA/N-methyl-D-aspartate currents. Consistent with these impairments, we observed addictive-like behavior in DIO-prone rats, including 1) heightened motivation for palatable food; 2) excessive intake; and 3) increased food seeking when food was unavailable. CONCLUSIONS Our results show overlap between the propensity for DIO and the synaptic changes associated with facets of addictive behavior, supporting partial coincident neurological underpinnings for compulsive overeating and drug addiction.

The Lateral Habenula Circuitry: Reward Processing and Cognitive Control.

There has been a growing interest in understanding the role of the lateral habenula (LHb) in reward processing, affect regulation, and goal-directed behaviors. The LHb gets major inputs from the habenula-projecting globus pallidus and the mPFC, sending its efferents to the dopaminergic VTA and SNc, serotonergic dorsal raphe nuclei, and the GABAergic rostromedial tegmental nucleus. Recent studies have made advances in our understanding of the LHb circuit organization, yet the precise mechanisms of its involvement in complex behaviors are largely unknown. To begin to address this unresolved question, we present here emerging cross-species perspectives with a goal to provide a more refined understanding of the role of the LHb circuits in reward and cognition. We begin by highlighting recent findings from rodent experiments using optogenetics, electrophysiology, molecular, pharmacology, and tracing techniques that reveal diverse neural phenotypes in the LHb circuits that may underlie previously undescribed behavioral functions. We then discuss results from electrophysiological studies in macaques that suggest that the LHb cooperates with the anterior cingulate cortex to monitor action outcomes and signal behavioral adjustment. Finally, we provide an integrated summary of cross-species findings and discuss how further research on the connectivity, neural signaling, and physiology of the LHb circuits can deepen our understanding of the role of the LHb in normal and maladaptive behaviors associated with mental illnesses and drug abuse.

Generality and opponency of rostromedial tegmental (RMTg) roles in valence processing

The rostromedial tegmental nucleus (RMTg), a GABAergic afferent to midbrain dopamine (DA) neurons, has been hypothesized to encode aversive stimuli. However, this encoding pattern has only been demonstrated for a limited number of stimuli, and its influence on the ventral tegmental (VTA) responses to aversive stimuli is untested. Here, we found that RMTg neurons show average inhibitions to rewarding stimuli and excitations to aversive stimuli of greatly varying sensory modalities and timescales. Notably, negative valence-encoding neurons are particularly enriched in subpopulations projecting to the VTA versus other targets. Additionally, RMTg neurons also dynamically encode “opponent” changes in motivational states induced by removal of sustained stimuli. Finally, excitotoxic RMTg lesions impair conditioned place aversion to multiple aversive stimuli, and greatly reduce aversive stimulus-induced inhibitions in VTA neurons, particularly in putative DA-like neurons. Together, our findings indicate a broad RMTg role in encoding aversion and potentially driving DA responses and behavior.

The entopeduncular nucleus drives lateral habenula responses to negative but not positive or neutral affective stimuli

Lateral habenula (LHb) neurons are activated by negative motivational stimuli and play key roles in the pathophysiology of depression. Early reports indicated the possibility that rostral entopeduncular nucleus (rEPN) neurons drive these LHb responses, but this influence remains untested. We find that both rEPN and LHb neurons in rats exhibit similar activation/inhibition patterns after negative/positive motivational stimuli, but that the rEPN influence on LHb firing is surprisingly selective. Temporary rEPN inactivation decreases LHb basal and burst firing, and eliminates LHb responses to footshock-predictive cues occurring 40-100ms but not 10-30ms post-stimulus, nor on responses to positive/neutral motivational stimuli. Additionally, rEPN inactivation partially but not fully reduces LHb responses to signaled footshocks, while excitotoxic rEPN lesions only partially diminish footshock-induced cFos in the LHb and its rostromedial tegmental nucleus targets. Together, our findings indicate an important but selective role of the rEPN in driving LHb responses to motivational stimuli.

Cocaine Self-Administration and Time-dependent Decreases in Prelimbic Activity

Cocaine self-administration causes dephosphorylation of proteins in prelimbic (PL) cortex that mediate excitation-transcription coupling, suggesting that cocaine causes decreased activity in PL neurons. Thus, we used in vivo single-unit extracellular recordings in awake, behaving rats to measure activity in PL neurons before, during, and after cocaine self-administration on the first and last session day (range 12-14 days). On the last day, cocaine suppressed 52% of recorded neurons in the PL cortex when compared to a 20 min baseline, significantly more than the 23% of neurons suppressed on the first day of cocaine. There was no change in the percentage of neurons that were excited on the first vs. the last day of self-administration (14% vs. 13%, respectively). To determine whether the tonic inhibitory shift was induced by the behavior or by the drug itself, rats received a final session in which cocaine availability was delayed for the first 30 min or in which cocaine was administered non-contingently in the absence of levers or cues in a subset of rats. These manipulations indicated that cocaine was both necessary and sufficient to induce a downshift in PL neuronal activity. However, this activity decrease was not observed in rats that received only non-contingent cocaine for 12-14 days, indicating that contingency during self-administration training contributes to the cocaine-induced tonic downshift in PL activity. These data suggest that the session-by-session decrease in PL neuronal activity induced by cocaine is a reflection of learned drug-seeking behavior and, hence, reducing this inhibition may lessen cocaine addiction.

Gene expression and neurochemical characterization of the rostromedial tegmental nucleus (RMTg) in rats and mice

The rostromedial tegmental nucleus (RMTg), also known as the tail of the ventral tegmental area (tVTA), is a GABAergic structure identified in 2009 that receives strong inputs from the lateral habenula and other sources, sends dense inhibitory projections to midbrain dopamine (DA) neurons, and plays increasingly recognized roles in aversive learning, addiction, and other motivated behaviors. In general, little is known about the genetic identity of these neurons. However, recent work has identified the transcription factor FoxP1 as enhanced in the mouse RMTg (Lahti et al. in Development 143(3):516–529, 2016). Hence, in the current study, we used RNA sequencing to identify genes significantly enhanced in the rat RMTg as compared to adjacent VTA, and then examined the detailed distribution of two genes in particular, prepronociceptin (Pnoc) and FoxP1. In rats and mice, both Pnoc and FoxP1 were expressed at high levels in the RMTg and colocalized strongly with previously established RMTg markers. FoxP1 was particularly selective for RMTg neurons, as it was absent in most adjacent brain regions. We used these gene expression patterns to refine the anatomic characterization of RMTg in rats, extend this characterization to mice, and show that optogenetic manipulation of RMTg in mice bidirectionally modulates real-time place preference. Hence, RMTg neurons in both rats and mice exhibit distinct genetic profiles that correlate with their distinct connectivity and function.