Plasticity of neuronal dynamics in the lateral habenula for cue-punishment associative learning.

The brain's ability to associate threats with external stimuli is vital to execute essential behaviours including avoidance. Disruption of this process contributes instead to the emergence of pathological traits which are common in addiction and depression. However, the mechanisms and neural dynamics at the single-cell resolution underlying the encoding of associative learning remain elusive. Here, employing a Pavlovian discrimination task in mice we investigate how neuronal populations in the lateral habenula (LHb), a subcortical nucleus whose excitation underlies negative affect, encode the association between conditioned stimuli and a punishment (unconditioned stimulus). Large population single-unit recordings in the LHb reveal both excitatory and inhibitory responses to aversive stimuli. Additionally, local optical inhibition prevents the formation of cue discrimination during associative learning, demonstrating a critical role of LHb activity in this process. Accordingly, longitudinal in vivo two-photon imaging tracking LHb calcium neuronal dynamics during conditioning reveals an upward or downward shift of individual neurons' CS-evoked responses. While recordings in acute slices indicate strengthening of synaptic excitation after conditioning, support vector machine algorithms suggest that postsynaptic dynamics to punishment-predictive cues represent behavioral cue discrimination. To examine the presynaptic signaling in LHb participating in learning we monitored neurotransmitter dynamics with genetically-encoded indicators in behaving mice. While glutamate, GABA, and serotonin release in LHb remain stable across associative learning, we observe enhanced acetylcholine signaling developing throughout conditioning. In summary, converging presynaptic and postsynaptic mechanisms in the LHb underlie the transformation of neutral cues in valued signals supporting cue discrimination during learning.

A neural substrate for negative affect dictates female parental behavior.

Parental behaviors secure the well-being of newborns and concomitantly limit negative affective states in adults, which emerge when coping with neonatal distress becomes challenging. Whether negative-affect-related neuronal circuits orchestrate parental actions is unknown. Here, we identify parental signatures in lateral habenula neurons receiving bed nucleus of stria terminalis innervation (BNSTLHb). We find that LHb neurons of virgin female mice increase their activity following pup distress vocalization and are necessary for pup-call-driven aversive behaviors. LHb activity rises during pup retrieval, a behavior worsened by LHb inactivation. Intersectional cell identification and transcriptional profiling associate BNSTLHb cells to parenting and outline a gene expression in female virgins similar to that in mothers but different from that in non-parental virgin male mice. Finally, tracking and manipulating BNSTLHb cell activity demonstrates their specificity for encoding negative affect and pup retrieval. Thus, a negative affect neural circuit processes newborn distress signals and may limit them by guiding female parenting.

Partial MCT1 invalidation protects against diet-induced non-alcoholic fatty liver disease and the associated brain dysfunction.

BACKGROUND & AIMS: Non-alcoholic fatty liver disease (NAFLD) has been associated with mild cerebral dysfunction and cognitive decline, although the exact pathophysiological mechanism remains ambiguous. Using a diet-induced model of NAFLD and monocarboxylate transporter-1 (Mct1+/-) haploinsufficient mice, which resist high-fat diet-induced hepatic steatosis, we investigated the hypothesis that NAFLD leads to an encephalopathy by altering cognition, behaviour, and cerebral physiology. We also proposed that global MCT1 downregulation offers cerebral protection. METHODS: Behavioural tests were performed in mice following 16 weeks of control diet (normal chow) or high-fat diet with high fructose/glucose in water. Tissue oxygenation, cerebrovascular reactivity, and cerebral blood volume were monitored under anaesthesia by multispectral optoacoustic tomography and optical fluorescence. Cortical mitochondrial oxygen consumption and respiratory capacities were measured using ex vivo high-resolution respirometry. Microglial and astrocytic changes were evaluated by immunofluorescence and 3D reconstructions. Body composition was assessed using EchoMRI, and liver steatosis was confirmed by histology. RESULTS: NAFLD concomitant with obesity is associated with anxiety- and depression-related behaviour. Low-grade brain tissue hypoxia was observed, likely attributed to the low-grade brain inflammation and decreased cerebral blood volume. It is also accompanied by microglial and astrocytic morphological and metabolic alterations (higher oxygen consumption), suggesting the early stages of an obesogenic diet-induced encephalopathy. Mct1 haploinsufficient mice, despite fat accumulation in adipose tissue, were protected from NAFLD and associated cerebral alterations. CONCLUSIONS: This study provides evidence of compromised brain health in obesity and NAFLD, emphasising the importance of the liver-brain axis. The protective effect of Mct1 haploinsufficiency points to this protein as a novel therapeutic target for preventing and/or treating NAFLD and the associated brain dysfunction. IMPACT AND IMPLICATIONS: This study is focused on unravelling the pathophysiological mechanism by which cerebral dysfunction and cognitive decline occurs during NAFLD and exploring the potential of monocarboxylate transporter-1 (MCT1) as a novel preventive or therapeutic target. Our findings point to NAFLD as a serious health risk and its adverse impact on the brain as a potential global health system and economic burden. These results highlight the utility of Mct1 transgenic mice as a model for NAFLD and associated brain dysfunction and call for systematic screening by physicians for early signs of psychological symptoms, and an awareness by individuals at risk of these potential neurological effects. This study is expected to bring attention to the need for early diagnosis and treatment of NAFLD, while having a direct impact on policies worldwide regarding the health risk associated with NAFLD, and its prevention and treatment.

Inhibition Within the Lateral Habenula-Implications for Affective Disorders.

The lateral habenula (LHb) is a key brain region implicated in the pathology of major depressive disorder (MDD). Specifically, excitatory LHb neurons are known to be hyperactive in MDD, thus resulting in a greater excitatory output mainly to downstream inhibitory neurons in the rostromedial tegmental nucleus. This likely results in suppression of downstream dopaminergic ventral tegmental area neurons, therefore, resulting in an overall reduction in reward signalling. In line with this, increasing evidence implicates aberrant inhibitory signalling onto LHb neurons as a co-causative factor in MDD, likely as a result of disinhibition of excitatory neurons. Consistently, growing evidence now suggests that normalising inhibitory signalling within the LHb may be a potential therapeutic strategy for MDD. Despite these recent advances, however, the exact pharmacological and neural circuit mechanisms which control inhibitory signalling within the LHb are still incompletely understood. Thus, in this review article, we aim to provide an up-to-date summary of the current state of knowledge of the mechanisms by which inhibitory signalling is processed within the LHb, with a view of exploring how this may be targeted as a future therapy for MDD.

Septal cholinergic input to CA2 hippocampal region controls social novelty discrimination via nicotinic receptor-mediated disinhibition.

Acetylcholine (ACh), released in the hippocampus from fibers originating in the medial septum/diagonal band of Broca (MSDB) complex, is crucial for learning and memory. The CA2 region of the hippocampus has received increasing attention in the context of social memory. However, the contribution of ACh to this process remains unclear. Here, we show that in mice, ACh controls social memory. Specifically, MSDB cholinergic neurons inhibition impairs social novelty discrimination, meaning the propensity of a mouse to interact with a novel rather than a familiar conspecific. This effect is mimicked by a selective antagonist of nicotinic AChRs delivered in CA2. Ex vivo recordings from hippocampal slices provide insight into the underlying mechanism, as activation of nAChRs by nicotine increases the excitatory drive to CA2 principal cells via disinhibition. In line with this observation, optogenetic activation of cholinergic neurons in MSDB increases the firing of CA2 principal cells in vivo. These results point to nAChRs as essential players in social novelty discrimination by controlling inhibition in the CA2 region.

Biophysical and synaptic properties of NMDA receptors in the lateral habenula.

Excitatory synaptic transmission in the lateral habenula (LHb), an evolutionarily ancient subcortical structure, encodes aversive stimuli and affective states. Habenular glutamatergic synapses contribute to these processes partly through the activation of AMPA receptors. Yet, N-methyl-d-aspartate receptors (NMDARs) are also expressed in the LHb and support the emergence of depressive symptoms. Indeed, local NMDAR blockade in the LHb rescues anhedonia and behavioral despair in rodent models of depression. However, the subunit composition and biophysical properties of habenular NMDARs remain unknown, thereby hindering their study in the context of mental health. Here, we performed electrophysiological recordings and optogenetic-assisted circuit mapping in mice, to study pharmacologically-isolated NMDAR currents in LHb neurons that receive innervation from different brain regions (entopeduncular nucleus, lateral hypothalamic area, bed nucleus of the stria terminalis, or ventral tegmental area). This systematic approach revealed that habenular NMDAR currents are sensitive to TCN and ifenprodil - drugs that specifically inhibit GluN2A- and GluN2B-containing NMDARs, respectively. Whilst these pharmacological effects were consistently observed across inputs, we detected region-specific differences in the current-voltage relationship and decay time of NMDAR currents. Finally, inspired by the firing of LHb neurons in vivo, we designed a burst protocol capable of eliciting calcium-dependent long-term potentiation of habenular NMDAR transmission ex vivo. Altogether, we define basic biophysical and synaptic properties of NMDARs in LHb neurons, opening new avenues for studying their plasticity processes in physiological as well as pathological contexts.

Stress undermines reward-guided cognitive performance through synaptic depression in the lateral habenula.

Weighing alternatives during reward pursuit is a vital cognitive computation that, when disrupted by stress, yields aspects of neuropsychiatric disorders. To examine the neural mechanisms underlying these phenomena, we employed a behavioral task in which mice were confronted by a reward and its omission (i.e., error). The experience of error outcomes engaged neuronal dynamics within the lateral habenula (LHb), a subcortical structure that supports appetitive behaviors and is susceptible to stress. A high incidence of errors predicted low strength of habenular excitatory synapses. Accordingly, stressful experiences increased error choices while decreasing glutamatergic neurotransmission onto LHb neurons. This synaptic adaptation required a reduction in postsynaptic AMPA receptors (AMPARs), irrespective of the anatomical source of glutamate. Bidirectional control of habenular AMPAR transmission recapitulated and averted stress-driven cognitive deficits. Thus, a subcortical synaptic mechanism vulnerable to stress underlies behavioral efficiency during cognitive performance.

Heterogeneous Habenular Neuronal Ensembles during Selection of Defensive Behaviors.

Optimal selection of threat-driven defensive behaviors is paramount to an animal's survival. The lateral habenula (LHb) is a key neuronal hub coordinating behavioral responses to aversive stimuli. Yet, how individual LHb neurons represent defensive behaviors in response to threats remains unknown. Here, we show that in mice, a visual threat promotes distinct defensive behaviors, namely runaway (escape) and action-locking (immobile-like). Fiber photometry of bulk LHb neuronal activity in behaving animals reveals an increase and a decrease in calcium signal time-locked with runaway and action-locking, respectively. Imaging single-cell calcium dynamics across distinct threat-driven behaviors identify independently active LHb neuronal clusters. These clusters participate during specific time epochs of defensive behaviors. Decoding analysis of this neuronal activity reveals that some LHb clusters either predict the upcoming selection of the defensive action or represent the selected action. Thus, heterogeneous neuronal clusters in LHb predict or reflect the selection of distinct threat-driven defensive behaviors.

Dysfunction of homeostatic control of dopamine by astrocytes in the developing prefrontal cortex leads to cognitive impairments.

Astrocytes orchestrate neural development by powerfully coordinating synapse formation and function and, as such, may be critically involved in the pathogenesis of neurodevelopmental abnormalities and cognitive deficits commonly observed in psychiatric disorders. Here, we report the identification of a subset of cortical astrocytes that are competent for regulating dopamine (DA) homeostasis during postnatal development of the prefrontal cortex (PFC), allowing for optimal DA-mediated maturation of excitatory circuits. Such control of DA homeostasis occurs through the coordinated activity of astroglial vesicular monoamine transporter 2 (VMAT2) together with organic cation transporter 3 and monoamine oxidase type B, two key proteins for DA uptake and metabolism. Conditional deletion of VMAT2 in astrocytes postnatally produces loss of PFC DA homeostasis, leading to defective synaptic transmission and plasticity as well as impaired executive functions. Our findings show a novel role for PFC astrocytes in the DA modulation of cognitive performances with relevance to psychiatric disorders.

Δ9-Tetrahydrocannabinol During Adolescence Attenuates Disruption of Dopamine Function Induced in Rats by Maternal Immune Activation.

The combination of prenatal, such as maternal infections, and postnatal environmental insults (e.g., adolescent drug abuse) increases risks for psychosis, as predicted by the two-hit hypothesis of schizophrenia. Cannabis abuse during adolescence is widespread and is associated with increased risk of psychoses later in life. Here, we hypothesized that adolescent Δ9-tetrahydrocannabinol (THC) worsens the impact of prenatal maternal immune activation (MIA) on ventral tegmental area (VTA) dopamine cells in rat offspring. Additionally, since substance abuse disorder is particularly prevalent among schizophrenia patients, we also tested how VTA dopamine neurons in MIA offspring respond to acute nicotine and cocaine administration. We used a model of neurodevelopmental disruption based on prenatal administration of the polyriboinosinic-polyribocytidilic acid [poly (I:C)] in rats, which activates the maternal immune system by mimicking a viral infection and induces behavioral abnormalities and disruption of dopamine transmission relevant to psychiatric disorders in the offspring. Male offspring were administered THC (or vehicle) during adolescence (PND 45-55). Once adult (PND 70-90), we recorded the spontaneous activity of dopamine neurons in the VTA and their responses to nicotine and cocaine. MIA male offspring displayed reduced number, firing rate and altered activity pattern of VTA dopamine cells. Adolescent THC attenuated several MIA-induced effects. Both prenatal [poly (I:C)] and postnatal (THC) treatments affected the response to nicotine but not to cocaine. Contrary to our expectations, adolescent THC did not worsen MIA-induced deficits. Results indicate that the impact of cannabinoids in psychosis models is complex.

Opposite responses to aversive stimuli in lateral habenula neurons.

Appropriate behavioural strategies to cope with unexpected salient stimuli require synergistic neuronal responses in diverse brain regions. Among them, the epithalamic lateral habenula (LHb) plays a pivotal role in processing salient stimuli of aversive valence. Integrated in the complex motivational circuit, LHb neurons are indeed excited by aversive stimuli, including footshock (Fs). However, whether such excitation is a common feature represented throughout the LHb remains unclear. Here, we combined single-unit extracellular recordings in anaesthetized mice with juxtacellular labelling to describe the nature, location and pharmacological properties of Fs-driven responses within the LHb. We find that, along with Fs-excited cells, about 10% of LHb neurons display Fs-mediated inhibitory responses. Such inhibited neuronal population, in contrast to Fs-excited neurons, display regular and high frequency activity at baseline and is clustered in the medial portion of the LHb. Juxtacellular labelling of Fs-excited and inhibited neurons unravels that both populations are of glutamatergic type, as they co-localized with the EAAC1 glutamatergic transporter but not with the GAD67 GABAergic marker. Moreover, while the excitatory responses to Fs require both AMPA and NMDA receptors, the inhibitory responses rely instead on GABAA channels. Taken together, our results indicate that two functionally and partly segregated LHb neuronal ensembles encode Fs in an opposite fashion. This highlights the neuronal complexity in the LHb for processing aversive external stimuli.

Punishment-Predictive Cues Guide Avoidance through Potentiation of Hypothalamus-to-Habenula Synapses.

Throughout life, individuals learn to predict a punishment via its association with sensory stimuli. This process ultimately prompts goal-directed actions to prevent the danger, a behavior defined as avoidance. Neurons in the lateral habenula (LHb) respond to aversive events as well as to environmental cues predicting them, supporting LHb contribution to cue-punishment association. However, whether synaptic adaptations at discrete habenular circuits underlie such associative learning to instruct avoidance remains elusive. Here, we find that, in mice, contingent association of an auditory cue (tone) with a punishment (foot shock) progressively causes cue-driven LHb neuronal excitation during avoidance learning. This process is concomitant with the strengthening of LHb AMPA receptor-mediated neurotransmission. Such a phenomenon occludes long-term potentiation and occurs specifically at hypothalamus-to-habenula synapses. Silencing hypothalamic-to-habenulainputs or optically inactivating postsynaptic AMPA receptors within the LHb disrupts avoidance learning. Altogether, synaptic strengthening at a discrete habenular circuit transforms neutral stimuli into salient punishment-predictive cues to guide avoidance.

Lateral Habenula Gone Awry in Depression: Bridging Cellular Adaptations With Therapeutics.

Depression is a highly heterogeneous disease characterized by symptoms spanning from anhedonia and behavioral despair to social withdrawal and learning deficit. Such diversity of behavioral phenotypes suggests that discrete neural circuits may underlie precise aspects of the disease, rendering its treatment an unmet challenge for modern neuroscience. Evidence from humans and animal models indicate that the lateral habenula (LHb), an epithalamic center devoted to processing aversive stimuli, is aberrantly affected during depression. This raises the hypothesis that rescuing maladaptations within this nucleus may be a potential way to, at least partially, treat aspects of mood disorders. In this review article, we will discuss pre-clinical and clinical evidence highlighting the role of LHb and its cellular adaptations in depression. We will then describe interventional approaches aiming to rescue LHb dysfunction and ultimately ameliorate depressive symptoms. Altogether, we aim to merge the mechanistic-, circuit-, and behavioral-level knowledge obtained about LHb maladaptations in depression to build a general framework that might prove valuable for potential therapeutic interventions.

Functional Principles of Posterior Septal Inputs to the Medial Habenula.

The medial habenula (MHb) is an epithalamic hub contributing to expression and extinction of aversive states by bridging forebrain areas and midbrain monoaminergic centers. Although contradictory information exists regarding their synaptic properties, the physiology of the excitatory inputs to the MHb from the posterior septum remains elusive. Here, combining optogenetics-based mapping with ex vivo and in vivo physiology, we examine the synaptic properties of posterior septal afferents to the MHb and how they influence behavior. We demonstrate that MHb cells receive sparse inputs producing purely glutamatergic responses via calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), heterotrimeric GluN2A-GluN2B-GluN1 N-methyl-D-aspartate (NMDA) receptors, and inhibitory group II metabotropic glutamate receptors. We describe the complex integration dynamics of these components by MHb cells. Finally, we combine ex vivo data with realistic afferent firing patterns recorded in vivo to demonstrate that efficient optogenetic septal stimulation in the MHb induces anxiolysis and promotes locomotion, contributing long-awaited evidence in favor of the importance of this septo-habenular pathway.

Limiting habenular hyperactivity ameliorates maternal separation-driven depressive-like symptoms.

Early-life stress, including maternal separation (MS), increases the vulnerability to develop mood disorders later in life, but the underlying mechanisms remain elusive. We report that MS promotes depressive-like symptoms in mice at a mature stage of life. Along with this behavioral phenotype, MS drives reduction of GABAB-GIRK signaling and the subsequent lateral habenula (LHb) hyperexcitability-an anatomical substrate devoted to aversive encoding. Attenuating LHb hyperactivity using chemogenetic tools and deep-brain stimulation ameliorates MS depressive-like symptoms. This provides insights on mechanisms and strategies to alleviate stress-dependent affective behaviors.

Aversive stimuli drive hypothalamus-to-habenula excitation to promote escape behavior.

A sudden aversive event produces escape behaviors, an innate response essential for survival in virtually all-animal species. Nuclei including the lateral habenula (LHb), the lateral hypothalamus (LH), and the midbrain are not only reciprocally connected, but also respond to negative events contributing to goal-directed behaviors. However, whether aversion encoding requires these neural circuits to ultimately prompt escape behaviors remains unclear. We observe that aversive stimuli, including foot-shocks, excite LHb neurons and promote escape behaviors in mice. The foot-shock-driven excitation within the LHb requires glutamatergic signaling from the LH, but not from the midbrain. This hypothalamic excitatory projection predominates over LHb neurons monosynaptically innervating aversion-encoding midbrain GABA cells. Finally, the selective chemogenetic silencing of the LH-to-LHb pathway impairs aversion-driven escape behaviors. These findings unveil a habenular neurocircuitry devoted to encode external threats and the consequent escape; a process that, if disrupted, may compromise the animal's survival.

Footshock-induced plasticity of GABAB signalling in the lateral habenula requires dopamine and glucocorticoid receptors.

The activity of lateral habenula (LHb) represents a substrate for the encoding of negative-valenced events. The exposure to aversive stimuli in naïve mice is sufficient to trigger a reduction in GABAB -mediated signaling in the LHb. This is ultimately instrumental for the hyperactivity of LHb neurons and for the establishment of depressive-like phenotypes. However, the mechanisms responsible for the induction of this aversion-driven plasticity are missing. Using ex-vivo patch-clamp recordings in slices, here we show that exposing mice to a series of inescapable footshocks (FsE) rapidly reduces baclofen-mediated GABAB currents in the LHb. This plasticity of GABAB signaling requires the activation of the dopamine and stress pathways. Indeed, the systemic administration of dopamine and glucocorticoids receptor antagonists prevents the FsE-induced reduction of GABAB currents in the LHb. To test whether the recruitment of these receptors occurs within the LHb, we exposed slices from control mice to either dopamine or corticosterone. Both manipulations failed to alter the amplitudes of baclofen-mediated GABAB currents. Altogether, these data suggest that dopamine and stress signaling are necessary for the induction of FsE-evoked GABAB plasticity in the LHb. However, the activation of these specific receptors may occur in structures different than the LHb, suggesting a circuit-based mechanism for this form of plasticity. These findings provide mechanistic insights on aversion-driven plasticity within the LHb.

Rescue of GABAB and GIRK function in the lateral habenula by protein phosphatase 2A inhibition ameliorates depression-like phenotypes in mice.

The lateral habenula (LHb) encodes aversive signals, and its aberrant activity contributes to depression-like symptoms. However, a limited understanding of the cellular mechanisms underlying LHb hyperactivity has precluded the development of pharmacological strategies to ameliorate depression-like phenotypes. Here we report that an aversive experience in mice, such as foot-shock exposure (FsE), induces LHb neuronal hyperactivity and depression-like symptoms. This occurs along with increased protein phosphatase 2A (PP2A) activity, a known regulator of GABAB receptor (GABABR) and G protein-gated inwardly rectifying potassium (GIRK) channel surface expression. Accordingly, FsE triggers GABAB1 and GIRK2 internalization, leading to rapid and persistent weakening of GABAB-activated GIRK-mediated (GABAB-GIRK) currents. Pharmacological inhibition of PP2A restores both GABAB-GIRK function and neuronal excitability. As a consequence, PP2A inhibition ameliorates depression-like symptoms after FsE and in a learned-helplessness model of depression. Thus, GABAB-GIRK plasticity in the LHb represents a cellular substrate for aversive experience. Furthermore, its reversal by PP2A inhibition may provide a novel therapeutic approach to alleviate symptoms of depression in disorders that are characterized by LHb hyperactivity.

Maternal Immune Activation Disrupts Dopamine System in the Offspring.

BACKGROUND: In utero exposure to maternal viral infections is associated with a higher incidence of psychiatric disorders with a supposed neurodevelopmental origin, including schizophrenia. Hence, immune response factors exert a negative impact on brain maturation that predisposes the offspring to the emergence of pathological phenotypes later in life. Although ventral tegmental area dopamine neurons and their target regions play essential roles in the pathophysiology of psychoses, it remains to be fully elucidated how dopamine activity and functionality are disrupted in maternal immune activation models of schizophrenia. METHODS: Here, we used an immune-mediated neurodevelopmental disruption model based on prenatal administration of the polyriboinosinic-polyribocytidilic acid in rats, which mimics a viral infection and recapitulates behavioral abnormalities relevant to psychiatric disorders in the offspring. Extracellular dopamine levels were measured by brain microdialysis in both the nucleus accumbens shell and the medial prefrontal cortex, whereas dopamine neurons in ventral tegmental area were studied by in vivo electrophysiology. RESULTS: Polyriboinosinic-polyribocytidilic acid-treated animals, at adulthood, displayed deficits in sensorimotor gating, memory, and social interaction and increased baseline extracellular dopamine levels in the nucleus accumbens, but not in the prefrontal cortex. In polyriboinosinic-polyribocytidilic acid rats, dopamine neurons showed reduced spontaneously firing rate and population activity. CONCLUSIONS: These results confirm that maternal immune activation severely impairs dopamine system and that the polyriboinosinic-polyribocytidilic acid model can be considered a proper animal model of a psychiatric condition that fulfills a multidimensional set of validity criteria predictive of a human pathology.

Adolescent Δ(9)-Tetrahydrocannabinol Exposure Alters WIN55,212-2 Self-Administration in Adult Rats.

Cannabis is the most commonly used illicit drug worldwide, and use is typically initiated during adolescence. The endocannabinoid system has an important role in formation of the nervous system, from very early development through adolescence. Cannabis exposure during this vulnerable period might lead to neurobiological changes that affect adult brain functions and increase the risk of cannabis use disorder. The aim of this study was to investigate whether exposure to Δ(9)-tetrahydrocannabinol (THC) in adolescent rats might enhance reinforcing effects of cannabinoids in adulthood. Male adolescent rats were treated with increasing doses of THC (or its vehicle) twice/day for 11 consecutive days (PND 45-55). When the animals reached adulthood, they were tested by allowing them to intravenously self-administer the cannabinoid CB1-receptor agonist WIN55,212-2. In a separate set of animals given the same THC (or vehicle) treatment regimen, electrophysiological and neurochemical experiments were performed to assess possible modifications of the mesolimbic dopaminergic system, which is critically involved in cannabinoid-induced reward. Behavioral data showed that acquisition of WIN55,212-2 self-administration was enhanced in THC-exposed rats relative to vehicle-exposed controls. Neurophysiological data showed that THC-exposed rats displayed a reduced capacity for WIN55,212-2 to stimulate firing of dopamine neurons in the ventral tegmental area and to increase dopamine levels in the nucleus accumbens shell. These findings-that early, passive exposure to THC can produce lasting alterations of the reward system of the brain and subsequently increase cannabinoid self-administration in adulthood-suggest a mechanism by which adolescent cannabis exposure could increase the risk of subsequent cannabis dependence in humans.