Upregulation of cannabinoid type 1 receptors in dopamine D2 receptor knockout mice is reversed by chronic forced ethanol consumption

Fatty acid-binding protein 5 differentially impacts dopamine signaling independent of sex and environment

Epidermal/brain fatty acid-binding protein 5 (FABP5) plays an integral role in the intracellular trafficking of bioactive lipids/endocannabinoids and the subsequent initiation of cellular cascades affecting cannabinoid and dopamine (DA) systems. Social isolation (SI) and environmental enrichment (EE) during adolescence have been shown to impact DA signaling, and, specifically, DA transporter (DAT) and receptor levels of DA type 1 (D1) and 2 (D2); however, the relationship between FABP5, environment and DA signaling remains unclear. The present study quantified DAT and DA receptor levels in male/female FABP5-/- and FABP5+/+ mice raised in either SI or EE. Results showed that FABP5-/- mice had 6.09-8.81% greater D1 levels in striatal sub-regions of the caudal brain, independent of sex or environment. D1 levels were 8.03% greater only in the olfactory tubercle of enrichment-reared animals. In summary, these results supported that FABP5 plays an important function in regulating striatal DA signaling, and this may have important implications as a target with therapeutic potential for various psychiatric disorders.

Cannabinoid receptor availability modulates the magnitude of dopamine release in vivo in the human reward system: A preliminary multitracer positron emission tomography study

The established role of dopamine (DA) in the mediation of reward and positive reinforcement, reward processing is strongly influenced by the type 1 cannabinoid receptors (CB₁ Rs). Although considerable preclinical evidence has demonstrated several functional CB₁ R-DA interactions, the relation between human CB₁ R availability, DA release capacity and drug-reinforcing effects has been never investigated so far. Here, we perform a multitracer [¹⁸ F]MK-9470 and [¹⁸ F]fallypride positron emission tomography (PET) study in 10 healthy male subjects using a placebo-controlled and single-blinded amphetamine (AMPH) (30 mg) administration paradigm to (1) investigate possible functional interactions between CB₁ R expression levels and DA release capacity in a normo-DAergic state, relating in vivo AMPH-induced DA release to CB₁ R availability, and (2) to test the hypothesis that the influence of striatal DAergic signalling on the positive reinforcing effects of AMPH may be regulated by prefrontal CB₁ R levels. Compared with placebo, AMPH significantly reduced [¹⁸ F]fallypride binding potential (hence increase DA release; ΔBP_ND ranging from -6.1% to -9.6%) in both striatal (p < 0.005, corrected for multiple comparisons) and limbic extrastriatal regions (p ≤ 0.04, uncorrected). Subjects who reported a greater dopaminergic response in the putamen also showed higher CB₁ R availability in the medial and dorsolateral prefrontal cortex (r = 0.72; p = 0.02), which are regions involved in salience attribution, motivation and decision making. On the other hand, the magnitude of DA release was greater in those subjects with lower CB₁ R availability in the anterior cingulate cortex (ACC) (r = -0.66; p = 0.03). Also, the correlation between the DA release in the nucleus accumbens with the subjective AMPH effect liking was mediated through the CB₁ R availability in the ACC (c' = -0.76; p = 0.01). Our small preliminary study reports for the first time that the human prefrontal CB₁ R availability is a determinant of DA release within both the ventral and dorsal reward corticostriatal circuit, contributing to a number of studies supporting the existence of an interaction between CB₁ R and DA receptors at the molecular and behavioural level. These preliminary findings warrant further investigation in pathological conditions characterized by hypo/hyper excitability to DA release such as addiction and schizophrenia.

Abstinence from chronic methylphenidate exposure modifies cannabinoid receptor 1 levels in the brain in a dose-dependent manner

INTRODUCTION

Methylphenidate (MP) is a widely used psychostimulant prescribed for Attention Deficit Hyperactivity Disorder, and is also used illicitly by healthy individuals. Chronic exposure to MP has been shown to affect physiology, behavior, and neurochemistry.

METHODS

The present study examined its effect on the endocannabinoid system. Adolescent rats had daily oral access to either water (control), low dose MP (4/10 mg/kg), or high dose MP (30/60 mg/kg). After 13 weeks of exposure, half of the rats in each group were euthanized, however the remaining rats underwent a four-week long abstinence period. Cannabinoid receptor 1 binding (CB1) was measured with in vitro autoradiography using [3H] SR141716A.

RESULTS

Rats who underwent a 4-week abstinence period after exposure to chronic HD MP showed increased binding compared to rats with no abstinence period in several cortical and basal ganglia regions of the brain. In contrast to this, rats who underwent a 4-week abstinence period after exposure to chronic LD MP showed lower binding compared to rats with no abstinence period in mainly the basal ganglia regions and in the hindlimb region of the somatosensory cortex. Following 4 weeks of drug abstinence, rats who were previously given HD MP showed higher [ 3H] SR141716A binding than rats given LD MP in many of the cortical and basal ganglia regions examined. These results highlight biphasic effects of MP treatment on cannabinoid receptor levels. Abstinence from HD MP seemed to increase CB1 receptor levels while abstinence from LD MP seemed to decrease CB1 levels.

CONCLUSION

Given the prolific expression of cannabinoid receptors throughout the brain, many types of behaviors may be affected as a result of MP abstinence. Further research will be needed to help identify these behavioral changes.

A Single δ9-Tetrahydrocannabinol (THC) Dose During Brain Development Affects Markers of Neurotrophy, Oxidative Stress, and Apoptosis

δ⁹-tetrahydrocannabinol (THC) is one of the most used drugs during pregnancy and lactation and efficiently crosses the placental and blood–brain barriers. Despite the recent legalization initiatives worldwide, the adverse outcome pathway (AOP) of THC following exposure during brain development is incompletely understood. We have previously reported that a single injection of THC on postnatal day (PND) 10 altered adult spontaneous behavior and habituation rates in adult mice. Similar behavioral alterations have been reported following PND 10 exposure to the commonly used over-the-counter analgesic acetaminophen (AAP; also known as paracetamol); as both THC and AAP interact with the endocannabinoid system, we hypothesize that this system might be involved in the AOP of both these pharmaceuticals/drugs. Here, we report that a single THC dose on PND 10 decreased transcript levels of tropomyosin receptor kinase b (Trkb) 24 h after exposure in both the frontal and parietal cortex, and in the hippocampus in mice. An increase in the nuclear factor (erythroid-derived 2)-like 2 (Nrf2)/Kelch-like ECH-associated protein 1 (Keap1) ratio were also found in both the parietal cortex and hippocampus following neonatal exposure to THC. In addition, THC exposure increased transcript levels of cannabinoid receptor type 1 (Cb1r) in the parietal cortex and increased the apoptosis regulator BAX in the frontal cortex. This study is important for mainly 3 reasons: 1) we are starting to get information on the developmental neurotoxic AOP of PND 10 exposure to THC, where we suggest that transcriptional changes of the neurotrophic receptor Trkb are central, 2) our PND 10 exposure model provides information relevant to human exposure and 3) since PND 10 exposure to AAP also decreased Trkb transcript levels, we suggest THC and AAP may share key events in their respective AOP through endocannabinoid-mediated alterations of the brain-derived neurotrophic factor (BDNF)-TRKB signaling pathway.

The cannabinoid-1 receptor is abundantly expressed in striatal striosomes and striosome-dendron bouquets of the substantia nigra

Presynaptic cannabinoid-1 receptors (CB₁-R) bind endogenous and exogenous cannabinoids to modulate neurotransmitter release. CB₁-Rs are expressed throughout the basal ganglia, including striatum and substantia nigra, where they play a role in learning and control of motivated actions. However, the pattern of CB₁-R expression across different striatal compartments, microcircuits and efferent targets, and the contribution of different CB₁-R-expressing neurons to this pattern, are unclear. We use a combination of conventional techniques and novel genetic models to evaluate CB₁-R expression in striosome (patch) and matrix compartments of the striatum, and in nigral targets of striatal medium spiny projection neurons (MSNs). CB₁-R protein and mRNA follow a descending dorsolateral-to-ventromedial intensity gradient in the caudal striatum, with elevated expression in striosomes relative to the surrounding matrix. The lateral predominance of striosome CB₁-Rs contrasts with that of the classical striosomal marker, the mu opioid receptor (MOR), which is expressed most prominently in rostromedial striosomes. The dorsolateral-to-ventromedial CB₁-R gradient is similar to Drd2 dopamine receptor immunoreactivity and opposite to Substance P. This topology of CB₁-R expression is maintained downstream in the globus pallidus and substantia nigra. Dense CB₁-R-expressing striatonigral fibers extend dorsally within the substantia nigra pars reticulata, and colocalize with bundles of ventrally extending, striosome-targeted, dendrites of dopamine-containing neurons in the substantia nigra pars compacta (striosome-dendron bouquets). Within striatum, CB₁-Rs colocalize with fluorescently labeled MSN collaterals within the striosomes. Cre recombinase-mediated deletion of CB₁-Rs from cortical projection neurons or MSNs, and MSN-selective reintroduction of CB₁-Rs in knockout mice, demonstrate that the principal source of CB₁-Rs in dorsolateral striosomes is local MSN collaterals. These data suggest a role for CB₁-Rs in caudal dorsolateral striosome collaterals and striosome-dendron bouquet projections to lateral substantia nigra, where they are anatomically poised to mediate presynaptic disinhibition of both striosomal MSNs and midbrain dopamine neurons in response to endocannabinoids and cannabinomimetics.

Oral haloperidol or olanzapine intake produces distinct and region-specific increase in cannabinoid receptor levels that is prevented by high fat diet

Clinical studies show higher levels of cannabinoid CB1 receptors (CB1R) in the brain of schizophrenic patients while preclinical studies report a significant functional interaction between dopamine D2 receptors and CB1Rs as well as an upregulation of CB1Rs after antipsychotic treatment. These findings prompted us to study the effects of chronic oral intake of a first and a second generation antipsychotic, haloperidol and olanzapine, on the levels and distribution of CB1Rs in the rat brain. Rats consumed either regular chow or high-fat food and drank water, haloperidol drinking solution (1.5mg/kg), or olanzapine drinking solution (10mg/kg) for four weeks. Motor and cognitive functions were tested at the end of treatment week 3 and upon drug discontinuation. Two days after drug discontinuation, rats were euthanized and brains were processed for in vitro receptor autoradiography. In chow-fed animals, haloperidol and olanzapine increased CB1R levels in the basal ganglia and the hippocampus, in a similar, but not identical pattern. In addition, olanzapine had unique effects in CB1R upregulation in higher order cognitive areas, in the secondary somatosensory cortex, in the visual and auditory cortices and the geniculate nuclei, as well as in the hypothalamus. High fat food consumption prevented antipsychotic-induced increase in CB1R levels in all regions examined, with one exception, the globus pallidus, in which they were higher in haloperidol-treated rats. The results point towards the hypothesis that increased CB1R levels could be a confounding effect of antipsychotic medication in schizophrenia that is circumveneted by high fat feeding.

Contribution of heteromerization to G protein-coupled receptor function

G protein-coupled receptors (GPCRs) are a remarkably multifaceted family of transmembrane proteins that exert a variety of physiological effects. Although family A GPCRs are able to operate as monomers, there is increasing evidence that heteromerization represents a fundamental aspect of receptor function, trafficking and pharmacology. Most recently, it has been suggested that GPCR heteromers may play a crucial role as new molecular targets of heteromer-selective and bivalent ligands. The current review summarizes key recent developments in these topics.

Impaired Ethanol-Induced Sensitization and Decreased Cannabinoid Receptor-1 in a Model of Posttraumatic Stress Disorder

BACKGROUND AND PURPOSE

Impaired striatal neuroplasticity may underlie increased alcoholism documented in those with posttraumatic stress disorder (PTSD). Cannabinoid receptor-1 (CB1) is sensitive to the effects of ethanol (EtOH) and traumatic stress, and is a critical regulator of striatal plasticity. To investigate CB1 involvement in the PTSD-alcohol interaction, this study measured the effects of traumatic stress using a model of PTSD, mouse single-prolonged stress (mSPS), on EtOH-induced locomotor sensitization and striatal CB1 levels.

METHODS

Mice were exposed to mSPS, which includes: 2-h restraint, 10-min group forced swim, 15-min exposure to rat bedding odor, and diethyl ether exposure until unconsciousness or control conditions. Seven days following mSPS exposure, the locomotor sensitizing effects of EtOH were assessed. CB1, post-synaptic density-95 (PSD95), and dopamine-2 receptor (D2) protein levels were then quantified in the dorsal striatum using standard immunoblotting techniques.

RESULTS

Mice exposed to mSPS-EtOH demonstrated impaired EtOH-induced locomotor sensitization compared to Control-EtOH mice, which was accompanied by reduced striatal CB1 levels. EtOH increased striatal PSD95 in control and mSPS-exposed mice. Additionally, mSPS-Saline exposure increased striatal PSD95 and decreased D2 protein expression, with mSPS-EtOH exposure alleviating these changes.

CONCLUSIONS

These data indicate that the mSPS model of PTSD blunts the behavioral sensitizing effects of EtOH, a response that suggests impaired striatal neuroplasticity. Additionally, this study demonstrates that mice exposed to mSPS and repeated EtOH exposure decreases CB1 in the striatum, providing a mechanism of interest for understanding the effects of EtOH following severe, multimodal stress exposure.

Effects of Δ9-tetrahydrocannabinol in individuals with a familial vulnerability to alcoholism

BACKGROUND AND AIMS

A family history (FH) of alcoholism accounts for approximately 50% of the risk of developing alcohol problems. Several lines of preclinical evidence suggest that brain cannabinoid receptor (CB1R) function may mediate the effects of alcohol and risk for developing alcoholism including the observations that reduced CB1R function decreases alcohol-related behaviors and enhanced CB1R function increases them. In this first human study, we probed CB1R function in individuals vulnerable to alcoholism with the exogenous cannabinoid Δ(9)-tetrahydrocannabinol (Δ(9)-THC).

DESIGN, SETTING, AND PARTICIPANTS

Healthy volunteers (n = 30) participated in a three test day study during which they received 0.018 and 0.036 mg/kg of Δ(9)-THC, or placebo intravenously in a randomized, counterbalanced order under double-blind conditions.

MEASUREMENTS

Primary outcome measures were subjective "high," perceptual alterations, and memory impairment. Secondary outcome measures consisted of stimulatory and depressant subjective effects, attention, spatial memory, executive function, Δ(9)-THC and 11-hydroxy-THC blood levels, and other subjective effects. FH was calculated using the Family Pattern Density method and was used as a continuous variable.

FINDINGS

Greater FH was correlated with greater "high" and perceptual alterations induced by Δ(9)-THC. This enhanced sensitivity with increasing FH was specific to Δ(9)-THC's rewarding effects and persisted even when FH was calculated using an alternate method.

CONCLUSIONS

Enhanced sensitivity to the rewarding effects of Δ(9)-THC in high-FH volunteers suggests that alterations in CB1R function might contribute to alcohol misuse vulnerability.

Striatal CB1 and D2 receptors regulate expression of each other, CRIP1A and δ opioid systems

Although biochemical and physiological evidence suggests a strong interaction between striatal CB1 cannabinoid (CB1 R) and D2 dopamine (D2 R) receptors, the mechanisms are poorly understood. We targeted medium spiny neurons of the indirect pathway using shRNA to knockdown either CB1 R or D2 R. Chronic reduction in either receptor resulted in deficits in gene and protein expression for the alternative receptor and concomitantly increased expression of the cannabinoid receptor interacting protein 1a (CRIP1a), suggesting a novel role for CRIP1a in dopaminergic systems. Both CB1 R and D2 R knockdown reduced striatal dopaminergic-stimulated [(35) S]GTPγS binding, and D2 R knockdown reduced pallidal WIN55212-2-stimulated [(35) S]GTPγS binding. Decreased D2 R and CB1 R activity was associated with decreased striatal phosphoERK. A decrease in mRNA for opioid peptide precursors pDYN and pENK accompanied knockdown of CB1 Rs or D2 Rs, and over-expression of CRIP1a. Down-regulation in opioid peptide mRNAs was followed in time by increased DOR1 but not MOR1 expression, leading to increased [D-Pen2, D-Pen5]-enkephalin-stimulated [(35) S]GTPγS binding in the striatum. We conclude that mechanisms intrinsic to striatal medium spiny neurons or extrinsic via the indirect pathway adjust for changes in CB1 R or D2 R levels by modifying the expression and signaling capabilities of the alternative receptor as well as CRIP1a and the DELTA opioid system.

Positron emission tomography shows elevated cannabinoid CB1 receptor binding in men with alcohol dependence

BACKGROUND

Several lines of evidence link cannabinoid (CB) type 1 (CB (1) ) receptor-mediated endogenous CB (eCB) signaling to the etiology of alcohol dependence (AD). However, to date, only peripheral measures of eCB function have been collected in living humans with AD and no human in vivo data on the potentially critical role of the brain CB (1) receptor in AD have been published. This is an important gap in the literature, because recent therapeutic developments suggest that these receptors could be targeted for the treatment for AD.

METHODS

Medication-free participants were scanned during early abstinence 4 weeks after their last drink. Using positron emission tomography (PET) with a high-resolution research tomograph and the CB (1) receptor selective radiotracer [(11) C]OMAR, we determined [(11) C]OMAR volume of distribution ( V (T) ) values, a measure of CB (1) receptor density, in a priori selected brain regions in men with AD (n = 8, age 37.4 ± 7.9 years; 5 smokers) and healthy control (HC) men (n = 8, age 32.5 ± 6.9 years; all nonsmokers). PET images reconstructed using the MOLAR algorithm with hardware motion correction were rigidly aligned to the subject-specific magnetic resonance (MR) image, which in turn was warped to an MR template. Time-activity curves (TACs) were extracted from the dynamic PET data using a priori selected regions of interest delineated in the MR template space.

RESULTS

In AD relative to HC, [(11) C]OMAR V (T) values were elevated by approximately 20% (p = 0.023) in a circuit, including the amygdala, hippocampus, putamen, insula, anterior and posterior cingulate cortices, and orbitofrontal cortex. Age, body mass index, or smoking status did not influence the outcome.

CONCLUSIONS

These findings agree with preclinical evidence and provide the first, albeit still preliminary in vivo evidence suggesting a role for brain CB (1) receptors in AD. The current study design does not answer the important question of whether elevated CB (1) receptors are a preexisting vulnerability factor for AD or whether elevations develop as a consequence of AD.

Chronic Δ⁹-tetrahydrocannabinol exposure induces a sensitization of dopamine D₂/₃ receptors in the mesoaccumbens and nigrostriatal systems

Δ⁹-tetrahydrocannabinol (THC), through its action on cannabinoid type-1 receptor (CB₁R), is known to activate dopamine (DA) neurotransmission. Functional evidence of a direct antagonistic interaction between CB₁R and DA D₂-receptors (D₂R) suggests that D₂R may be an important target for the modulation of DA neurotransmission by THC. The current study evaluated, in rodents, the effects of chronic exposure to THC (1 mg/kg/day; 21 days) on D₂R and D₃R availabilities using the D₂R-prefering antagonist and the D₃R-preferring agonist radiotracers [¹⁸F]fallypride and [³H]-(+)-PHNO, respectively. At 24 h after the last THC dose, D₂R and D₃R densities were significantly increased in midbrain. In caudate/putamen (CPu), THC exposure was associated with increased densities of D₂R with no change in D₂R mRNA expression, whereas in nucleus accumbens (NAcc) both D₃R binding and mRNA levels were upregulated. These receptor changes, which were completely reversed in CPu but only partially reversed in NAcc and midbrain at 1 week after THC cessation, correlated with an increased functionality of D₂/₃R in vivo, based on findings of increased locomotor suppressive effect of a presynaptic dose and enhanced locomotor activation produced by a postsynaptic dose of quinpirole. Concomitantly, the observations of a decreased gene expression of tyrosine hydroxylase in midbrain together with a blunted psychomotor response to amphetamine concurred to indicate a diminished presynaptic DA function following THC. These findings indicate that the early period following THC treatment cessation is associated with altered presynaptic D₂/₃R controlling DA synthesis and release in midbrain, with the concurrent development of postsynaptic D₂/₃R supersensitivity in NAcc and CPu. Such D₂/₃R neuroadaptations may contribute to the reinforcing and habit-forming properties of THC.

Loss of dopamine D2 receptors induces atrophy in the temporal and parietal cortices and the caudal thalamus of ethanol-consuming mice

BACKGROUND

The need of an animal model of alcoholism becomes apparent when we consider the genetic diversity of the human populations, an example being dopamine D2 receptor (DRD2) expression levels. Research suggests that low DRD2 availability is associated with alcohol abuse, while higher DRD2 levels may be protective against alcoholism. This study aims to establish whether (i) the ethanol-consuming mouse is a suitable model of alcohol-induced brain atrophy and (ii) DRD2 protect the brain against alcohol toxicity.

METHODS

Adult Drd2+/+ and Drd2-/- mice drank either water or 20% ethanol solution for 6 months. At the end of the treatment period, the mice underwent magnetic resonance (MR) imaging under anesthesia. MR images were registered to a common space, and regions of interest were manually segmented.

RESULTS

We found that chronic ethanol intake induced a decrease in the volume of the temporal and parietal cortices as well as the caudal thalamus in Drd2-/- mice.

CONCLUSIONS

The result suggests that (i) normal DRD2 expression has a protective role against alcohol-induced brain atrophy and (ii) in the absence of Drd2 expression, prolonged ethanol intake reproduces a distinct feature of human brain pathology in alcoholism, the atrophy of the temporal and parietal cortices.