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Fentanyl is a powerful painkiller that elicits euphoria and positive reinforcement1. Fentanyl also leads to dependence, defined by the aversive withdrawal syndrome, which fuels negative reinforcement2,3(that is, individuals retake the drug to avoid withdrawal). Positive and negative reinforcement maintain opioid consumption, which leads to addiction in one-fourth of users, the largest fraction for all addictive drugs4. Among the opioid receptors, µ-opioid receptors have a key role5, yet the induction loci of circuit adaptations that eventually lead to addiction remain unknown. Here we injected mice with fentanyl to acutely inhibit γ-aminobutyric acid-expressing neurons in the ventral tegmental area (VTA), causing disinhibition of dopamine neurons, which eventually increased dopamine in the nucleus accumbens. Knockdown of µ-opioid receptors in VTA abolished dopamine transients and positive reinforcement, but withdrawal remained unchanged. We identified neurons expressing µ-opioid receptors in the central amygdala (CeA) whose activity was enhanced during withdrawal. Knockdown of µ-opioid receptors in CeA eliminated aversive symptoms, suggesting that they mediate negative reinforcement. Thus, optogenetic stimulation caused place aversion, and mice readily learned to press a lever to pause optogenetic stimulation of CeA neurons that express µ-opioid receptors. Our study parses the neuronal populations that trigger positive and negative reinforcement in VTA and CeA, respectively. We lay out the circuit organization to develop interventions for reducing fentanyl addiction and facilitating rehabilitation.

Psychedelic-assisted psychotherapy gained considerable interest as a novel treatment strategy for fear-related mental disorders but the underlying mechanism remains poorly understood. The serotonin 2A (5-HT2A) receptor is a key target underlying the effects of psychedelics on emotional arousal but its role in fear processing remains controversial. Using the psychedelic 5-HT2A/5-HT2C receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) and 5-HT2A receptor knockout (KO) mice we investigated the effect of 5-HT2A receptor activation on emotional processing. We show that DOI administration did not impair performance in a spontaneous alternation task but reduced anxiety-like avoidance behavior in the elevated plus maze and elevated zero maze tasks. Moreover, we found that DOI did not block memory recall but diminished fear expression in a passive avoidance task. Likewise, DOI administration reduced fear expression in an auditory fear conditioning paradigm, while it did not affect retention of fear extinction when administered prior to extinction learning. The effect of DOI on fear expression was abolished in 5-HT2A receptor KO mice. Administration of DOI induced a significant increase of c-Fos expression in specific amygdalar nuclei. Moreover, local infusion of the 5-HT2A receptor antagonist M100907 into the amygdala reversed the effect of systemic administration of DOI on fear expression while local administration of DOI into the amygdala was sufficient to suppress fear expression. Our data demonstrate that activation of 5-HT2A receptors in the amygdala suppresses fear expression but provide no evidence for an effect on retention of fear extinction.

Forebrain dopamine-sensitive (dopaminoceptive) neurons play a key role in movement, action selection, motivation, and working memory. Their activity is altered in Parkinson’s disease, addiction, schizophrenia, and other conditions, and drugs that stimulate or antagonize dopamine receptors have major therapeutic applications. Yet, similarities and differences between the various neuronal populations sensitive to dopamine have not been systematically explored. To characterize them, we compared translating mRNAs in the dorsal striatum and nucleus accumbens neurons expressing D1 or D2 dopamine receptor and prefrontal cortex neurons expressing D1 receptor. We identified genome-wide cortico-striatal, striatal D1/D2 and dorso/ventral differences in the translating mRNA and isoform landscapes, which characterize dopaminoceptive neuronal populations. Expression patterns and network analyses identified novel transcription factors with presumptive roles in these differences. Prostaglandin E2 (PGE2) was a candidate upstream regulator in the dorsal striatum. We pharmacologically explored this hypothesis and showed that misoprostol, a PGE2 receptor agonist, decreased the excitability of D2 striatal projection neurons in slices, and diminished their activity in vivo during novel environment exploration. We found that misoprostol also modulates mouse behavior including by facilitating reversal learning. Our study provides powerful resources for characterizing dopamine target neurons, new information about striatal gene expression patterns and regulation. It also reveals the unforeseen role of PGE2 in the striatum as a potential neuromodulator and an attractive therapeutic target.

The cerebellum, a primary brain structure involved in the control of sensorimotor tasks, also contributes to higher cognitive functions including reward, emotion and social interaction. Although the regulation of these behaviors has been largely ascribed to the monoaminergic system in limbic regions, the contribution of cerebellar dopamine signaling in the modulation of these functions remains largely unknown. By combining cell-type-specific transcriptomics, histological analyses, three-dimensional imaging and patch-clamp recordings, we demonstrate that cerebellar dopamine D2 receptors (D2Rs) in mice are preferentially expressed in Purkinje cells (PCs) and regulate synaptic efficacy onto PCs. Moreover, we found that changes in D2R levels in PCs of male mice during adulthood alter sociability and preference for social novelty without affecting motor functions. Altogether, these findings demonstrate novel roles for D2R in PC function and causally link cerebellar D2R levels of expression to social behaviors.

Adolescent cannabinoid exposure can have long-lasting effects on the brain, particularly in the prefrontal cortex, where the reelin protein plays a crucial role in neural organization. Chronic cannabinoid exposure leads to reelin deficiency and behavioral abnormalities, but the underlying mechanisms remain unclear. With the increasing use of synthetic cannabinoids (SCs) among young people, understanding these effects is crucial.

Parkinson's disease is caused by the progressive loss of dopamine innervation to the basal ganglia and is commonly treated with the dopamine precursor, L-DOPA. Prolonged administration of L-DOPA results in the development of severe motor complications, or dyskinesia, which seriously hamper its clinical use. Recent evidence indicates that L-DOPA-induced dyskinesia (LID) is associated with persistent activation of the mammalian target of rapamycin complex 1 (mTORC1) in the medium spiny neurons (MSNs) of the striatum, the main component of the basal ganglia. This phenomenon is secondary to the development of a strong sensitization at the level of dopamine D1 receptors, which are abundantly expressed in a subset of MSNs. Such sensitization confers to dopaminergic drugs (including L-DOPA) the ability to activate the extracellular signal-regulated protein kinases 1/2, which, in turn promote mTORC1 signaling. Using a mouse model of LID, we recently showed that administration of the allosteric mTORC1 inhibitor, rapamycin, reduces dyskinesia. This finding is discussed with respect to underlying mechanisms and potential significance for the development of future therapeutic interventions.

D-Amphetamine and methylphenidate are widely used in the treatment of attention-deficit/hyperactivity disorder. Both drugs increase extracellular norepinephrine and dopamine in the prefrontal cortex, where they are believed to exert their therapeutic effects. However, the molecular mechanisms underlying their action are poorly understood. To investigate the intracellular signaling pathways activated by D-amphetamine and methylphenidate in the prefrontal cortex in vivo in mice, we measured the cAMP-dependent Ser845 phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor GluR1 subunit and the active form of extracellular signal-regulated kinase (ERK). Administration of D-amphetamine (5–10 mg/kg) or methylphenidate (10–20 mg/kg) increased phosphorylation of GluR1. Basal and D-amphetamine–induced GluR1 phosphorylation was reduced by propranolol, a general β-adrenoceptor antagonist, and betaxolol, a β1-antagonist, but not by (±)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol (ICI-118,515), a β2-antagonist. The effect of methylphenidate was also blocked by propranolol and betaxolol. The D-amphetamine effect was slightly potentiated by prazosin, an α1-adrenoceptor antagonist, and mimicked by yohimbine, an α2 antagonist. Blockade of dopamine or N-methyl-D-aspartate (NMDA) receptors or serotonin depletion had no effect on D-amphetamine–induced GluR1 phosphorylation. D-amphetamine but not methylphenidate increased ERK phosphorylation. This effect required multiple signaling pathways because it was blocked by β1- and α1-adrenoceptor antagonists, by dizocilpine maleate (MK801), an NMDA antagonist, and by serotonin depletion. In contrast, blockade of dopamine receptors had no effect on D-amphetamine–induced ERK phosphorylation. Propranolol and betaxolol increased the hyperlocomotion produced by D-amphetamine and methylphenidate. Thus, both D-amphetamine and methylphenidate potently activate the cAMP pathway in the prefrontal cortex through β1-adrenergic receptors. This activation could have behavioral consequences and contribute to the treatment of attention-deficit/hyperactivity disorder.

Drug addiction is characterized by loss of control over drug consumption, with compulsive drug seeking and taking, despite obvious adverse consequences. It is a chronic brain disorder since the increased risk of relapse to active drug use remains high even after years of abstinence. Current research attempts to identify the cellular and molecular basis for the neuronal modifications underlying these complex behavioural alterations. Strong evidence indicates that the extracellular signal-regulated kinase (ERK) pathway is a conserved intracellular signalling module, critical for various types of learning and synaptic plasticity. Recent results show that drugs of abuse activate this pathway in the brain regions important for reward-controlled learning and addiction. Moreover, pharmacological and genetic approaches support a role of the ERK pathway in some of the long-term effects of addictive drugs. We suggest that drug-induced stimulation of the ERK pathway by dopamine in specific brain areas may be an exaggeration of its normal role in learning controlled by reward, and may contribute significantly to the acquisition of specific behaviours characteristic of drug addiction.

Tolerance and dependence induced by chronic Δ‐9‐tetrahydrocannabinol (THC) administration were investigated in mice. The effects on body weight, analgesia and hypothermia were measured during 6 days of treatment (10 or 20 mg kg−1THC twice daily). A rapid tolerance to the acute effects was observed from the second THC administration.The selective CB‐1 receptor antagonist SR 141716A (10 mg kg−1) was administered at the end of the treatment, and somatic and vegetative manifestations of abstinence were evaluated. SR 141716A administration precipitated several somatic signs that included wet dog shakes, frontpaw tremor, ataxia, hunched posture, tremor, ptosis, piloerection, decreased locomotor activity and mastication, which can be interpreted as being part of a withdrawal syndrome.Brains were removed immediately after the behavioural measures and assayed for adenylyl cyclase activity. An increase in basal, forskolin and calcium/calmodulin stimulated adenylyl cyclase activities was specifically observed in the cerebellum of these mice.The motivational effects of THC administration and withdrawal were evaluated by using the place conditioning paradigm. No conditioned change in preference to withdrawal associated environment was observed. In contrast, a conditioned place aversion was produced by the repeated pairing of THC (20 mg kg−1), without observing place preference at any of the doses used.This study constitutes a clear behavioural and biochemical model of physical THC withdrawal with no motivational aversive consequences. This model permits an easy quantification of THC abstinence in mice and can be useful for the elucidation of the molecular mechanisms involved in cannabinoid dependence.

Dyskinetic side effects of a Parkinson’s disease medication may involve dopamine D1 receptor–mediated activation of mTORC1.