Justine Renard

Hippocampal Cannabinoid Transmission Modulates Dopamine Neuron Activity: Impact on Rewarding Memory Formation and Social Interaction

Disturbances in cannabinoid type 1 receptor (CB1R) signaling have been linked to emotional and cognitive deficits characterizing neuropsychiatric disorders, including schizophrenia. Thus, there is growing interest in characterizing the relationship between cannabinoid transmission, emotional processing, and dopamine (DA)-dependent behavioral deficits. The CB1R is highly expressed in the mammalian nervous system, particularly in the hippocampus. Activation of the ventral hippocampal subregion (vHipp) is known to increase both the activity of DAergic neurons located in the ventral tegmental area (VTA) and DA levels in reward-related brain regions, particularly the nucleus accumbens (NAc). However, the possible functional relationship between hippocampal CB1R transmission and VTA DA neuronal activity is not currently understood. In this study, using in vivo neuronal recordings in rats, we demonstrate that activation of CB1R in the vHipp strongly increases VTA DA neuronal firing and bursting activity, while simultaneously decreasing the activity of VTA non-DA neurons. Furthermore, using a conditioned place preference procedure and a social interaction test, we report that intra-vHipp CB1R activation potentiates the reward salience of normally sub-threshold conditioning doses of opiates and induces deficits in natural sociability and social recognition behaviors. Finally, these behavioral effects were prevented by directly blocking NAc DAergic transmission. Collectively, these findings identify hippocampal CB1R transmission as a critical modulator of the mesolimbic DA pathway and in the processing of reward and social-related behavioral phenomena.

Adolescent Cannabinoid Exposure Induces a Persistent Sub-Cortical Hyper-Dopaminergic State and Associated Molecular Adaptations in the Prefrontal Cortex

Abstract Considerable evidence suggests that adolescent exposure to delta‐9‐tetrahydrocanabinol (THC), the psychoactive component in marijuana, increases the risk of developing schizophrenia‐related symptoms in early adulthood. In the present study, we used a combination of behavioral and molecular analyses with in vivo neuronal electrophysiology to compare the long‐term effects of adolescent versus adulthood THC exposure in rats. We report that adolescent, but not adult, THC exposure induces long‐term neuropsychiatric‐like phenotypes similar to those observed in clinical populations. Thus, adolescent THC exposure induced behavioral abnormalities resembling positive and negative schizophrenia‐related endophenotypes and a state of neuronal hyperactivity in the mesocorticolimbic dopamine (DA) pathway. Furthermore, we observed profound alterations in several prefrontal cortical molecular pathways consistent with sub‐cortical DAergic dysregulation. Our findings demonstrate a profound dissociation in relative risk profiles for adolescent versus adulthood exposure to THC in terms of neuronal, behavioral, and molecular markers resembling neuropsychiatric pathology.

What Can Rats Tell Us about Adolescent Cannabis Exposure? Insights from Preclinical Research:

Marijuana is the most widely used drug of abuse among adolescents. Adolescence is a vulnerable period for brain development, during which time various neurotransmitter systems such as the glutamatergic, GABAergic, dopaminergic, and endocannabinoid systems undergo extensive reorganization to support the maturation of the central nervous system (CNS). ▵-9-tetrahydrocannabinol (THC), the psychoactive component of marijuana, acts as a partial agonist of CB1 cannabinoid receptors (CB1Rs). CB1Rs are abundant in the CNS and are central components of the neurodevelopmental changes that occur during adolescence. Thus, overactivation of CB1Rs by cannabinoid exposure during adolescence has the ability to dramatically alter brain maturation, leading to persistent and enduring changes in adult cerebral function. Increasing preclinical evidence lends support to clinical evidence suggesting that chronic adolescent marijuana exposure may be associated with a higher risk for neuropsychiatric diseases, including schizophrenia. In this review, we present a broad overview of current neurobiological evidence regarding the long-term consequences of adolescent cannabinoid exposure on adult neuropsychiatric-like disorders.

Cannabidiol Modulates Fear Memory Formation Through Interactions with Serotonergic Transmission in the Mesolimbic System.

Emerging evidence suggests that the largest phytochemical component of cannabis, cannabidiol (CBD), may possess pharmacotherapeutic properties in the treatment of neuropsychiatric disorders. CBD has been reported to functionally interact with both the mesolimbic dopamine (DA) and serotonergic (5-HT) receptor systems. However, the underlying mechanisms by which CBD may modulate emotional processing are not currently understood. Using a combination of in vivo electrophysiological recording and fear conditioning in rats, the present study aimed to characterize the behavioral, neuroanatomical, and pharmacological effects of CBD within the mesolimbic pathway, and its possible functional interactions with 5-HT and DAergic transmission. Using targeted microinfusions of CBD into the shell region of the mesolimbic nucleus accumbens (NASh), we report that intra-NASh CBD potently blocks the formation of conditioned freezing behaviors. These effects were challenged with DAergic, cannabinoid CB1 receptor, and serotonergic (5-HT1A) transmission blockade, but only 5-HT1A blockade restored associative conditioned freezing behaviors. In vivo intra-ventral tegmental area (VTA) electrophysiological recordings revealed that behaviorally effective doses of intra-NASh CBD elicited a predominant decrease in spontaneous DAergic neuronal frequency and bursting activity. These neuronal effects were reversed by simultaneous blockade of 5-HT1A receptor transmission. Finally, using a functional contralateral disconnection procedure, we demonstrated that the ability of intra-NASh CBD to block the formation of conditioned freezing behaviors was dependent on intra-VTA GABAergic transmission substrates. Our findings demonstrate a novel NAcVTA circuit responsible for the behavioral and neuronal effects of CBD within the mesolimbic system via functional interactions with serotonergic 5-HT1A receptor signaling.

Cannabinoid Transmission in the Hippocampus Activates Nucleus Accumbens Neurons and Modulates Reward and Aversion-Related Emotional Salience

BACKGROUND Cannabinoid receptor transmission strongly influences emotional processing, and disturbances in cannabinoid signaling are associated with various neuropsychiatric disorders. The mammalian ventral hippocampus (vHipp) is a critical neural region controlling mesolimbic activity via glutamatergic projections to the nucleus accumbens. Furthermore, vHipp abnormalities are linked to schizophrenia-related psychopathology. Nevertheless, the mechanisms by which intra-vHipp cannabinoid signaling may modulate mesolimbic activity states and emotional processing are not currently understood. METHODS Using an integrative combination of in vivo electrophysiological recordings and behavioral pharmacologic assays in rats, we tested whether activation of cannabinoid type 1 receptors (CB1R) in the vHipp may modulate neuronal activity in the shell subregion of the nucleus accumbens (NASh). We next examined how vHipp CB1R signaling may control the salience of rewarding or aversive emotional memory formation and social interaction/recognition behaviors via intra-NASh glutamatergic transmission. RESULTS We demonstrate for the first time that vHipp CB1R transmission can potently modulate NASh neuronal activity and can differentially control the formation of context-dependent and context-independent forms of rewarding or aversion-related emotional associative memories. In addition, we found that activation of vHipp CB1R transmission strongly disrupts normal social behavior and cognition. Finally, we report that these behavioral effects are dependent upon intra-NASh alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/N-methyl-D-aspartate receptor transmission. CONCLUSIONS Together, these findings demonstrate a critical role for hippocampal cannabinoid signaling in the modulation of mesolimbic neuronal activity states and suggest that dysregulation of CB1R transmission in the vHipp→NASh circuit may underlie hippocampal-mediated affective and social behavioral disturbances present in neuropsychiatric disorders.

Cannabidiol Counteracts Amphetamine-Induced Neuronal and Behavioral Sensitization of the Mesolimbic Dopamine Pathway through a Novel mTOR/p70S6 Kinase Signaling Pathway

Schizophrenia-related psychosis is associated with disturbances in mesolimbic dopamine (DA) transmission, characterized by hyperdopaminergic activity in the mesolimbic pathway. Currently, the only clinically effective treatment for schizophrenia involves the use of antipsychotic medications that block DA receptor transmission. However, these medications produce serious side effects leading to poor compliance and treatment outcomes. Emerging evidence points to the involvement of a specific phytochemical component of marijuana called cannabidiol (CBD), which possesses promising therapeutic properties for the treatment of schizophrenia-related psychoses. However, the neuronal and molecular mechanisms through which CBD may exert these effects are entirely unknown. We used amphetamine (AMPH)-induced sensitization and sensorimotor gating in rats, two preclinical procedures relevant to schizophrenia-related psychopathology, combined with in vivo single-unit neuronal electrophysiology recordings in the ventral tegmental area, and molecular analyses to characterize the actions of CBD directly in the nucleus accumbens shell (NASh), a brain region that is the current target of most effective antipsychotics. We demonstrate that Intra-NASh CBD attenuates AMPH-induced sensitization, both in terms of DAergic neuronal activity measured in the ventral tegmental area and psychotomimetic behavioral analyses. We further report that CBD controls downstream phosphorylation of the mTOR/p70S6 kinase signaling pathways directly within the NASh. Our findings demonstrate a novel mechanism for the putative antipsychotic-like properties of CBD in the mesolimbic circuitry. We identify the molecular signaling pathways through which CBD may functionally reduce schizophrenia-like neuropsychopathology. SIGNIFICANCE STATEMENT The cannabis-derived phytochemical, cannabidiol (CBD), has been shown to have pharmacotherapeutic efficacy for the treatment of schizophrenia. However, the mechanisms by which CBD may produce antipsychotic effects are entirely unknown. Using preclinical behavioral procedures combined with molecular analyses and in vivo neuronal electrophysiology, our findings identify a functional role for the nucleus accumbens as a critical brain region whereby CBD can produce effects similar to antipsychotic medications by triggering molecular signaling pathways associated with the effects of classic antipsychotic medications. Specifically, we report that CBD can attenuate both behavioral and dopaminergic neuronal correlates of mesolimbic dopaminergic sensitization, via a direct interaction with mTOR/p70S6 kinase signaling within the mesolimbic pathway.

Neuronal and molecular effects of cannabidiol on the mesolimbic dopamine system: Implications for novel schizophrenia treatments

HIGHLIGHTSCannabinoid compounds found in marijuana have complex effects on neuropsychiatric disorders.Evidence demonstrates differential roles for delta‐9‐tetrahydrocannabinol (THC) vs. cannabidiol (CBD) in regulating the mesolimbic dopamine system.THC and CBD produce opposing molecular and neuronal effects on the mesolimbic dopamine system.The differential effects of THC and CBD may underlie their pro vs. anti‐psychotic clinical properties. ABSTRACT Growing clinical and pre‐clinical evidence points to a critical role for cannabidiol (CBD), the largest phytochemical component of cannabis, as a potential pharmacotherapy for various neuropsychiatric disorders. In contrast to delta‐9‐tetrahydrocannabinol (THC), which is associated with acute and neurodevelopmental pro‐psychotic side‐effects, CBD possesses no known psychoactive or dependence‐producing properties. However, evidence has demonstrated that CBD strongly modulates the mesolimbic dopamine (DA) system and may possess promising anti‐psychotic properties. Despite the psychotropic differences between CBD and THC, little is known regarding their molecular and neuronal effects on the mesolimbic DA system, nor how these differential effects may relate to their potential pro vs. anti‐psychotic properties. This review summarizes clinical and pre‐clinical evidence demonstrating CBDs modulatory effects on DA activity states within the mesolimbic pathway, functional interactions with the serotonin 5‐HT1A receptor system, and their downstream molecular signaling effects. Together with clinical evidence showing that CBD may normalize affective and cognitive deficits associated with schizophrenia, CBD may represent a promising treatment for schizophrenia, acting through novel molecular and neuronal mesolimbic substrates.

Opiate Exposure State Controls a D2-CaMKIIα-Dependent Memory Switch in the Amygdala-Prefrontal Cortical Circuit

The mammalian basolateral amygdala (BLA) and medial prefrontal cortex (mPFC) comprise a functionally interconnected circuit that is critical for processing opiate-related associative memories. In the opiate-naïve state, reward memory formation in the BLA involves a functional link between dopamine (DA) D1 receptor (D1R) and extracellular signal-related kinase 1/2 (ERK1/2) signaling substrates, but switches to a DA D2 (D2R)/Ca2+/calmodulin-dependent protein kinase IIα (CaMKIIα)-dependent memory substrate following chronic opiate exposure and spontaneous withdrawal. Using conditioned place preference (CPP) in rats paired with molecular analyses, we examined the role of intra-mPFC CaMKII, ERK and DAergic activity during the formation of opiate associative memories, and how opiate exposure state may regulate the functions of these molecular memory pathways. We report that the role of CaMKIIα signaling is functionally reversed within the BLA-mPFC pathway depending on opiate exposure state. Thus, in the opiate-naïve state, intra-mPFC but not intra-BLA blockade of CaMKII signaling prevents formation of opiate reward memory. However, following chronic opiate exposure and spontaneous withdrawal, the role of CaMKII signaling in the BLA-mPFC is functionally reversed. This behavioral memory switch corresponds to a selective increase in the expression of D2R and CaMKIIα, but not other calcium/calmodulin-related molecules, nor D1R expression levels within the mPFC.

Adolescent THC Exposure Causes Enduring Prefrontal Cortical Disruption of GABAergic Inhibition and Dysregulation of Sub-Cortical Dopamine Function

Chronic adolescent marijuana use has been linked to the later development of psychiatric diseases such as schizophrenia. GABAergic hypofunction in the prefrontal cortex (PFC) is a cardinal pathological feature of schizophrenia and may be a mechanism by which the PFC loses its ability to regulate sub-cortical dopamine (DA) resulting in schizophrenia-like neuropsychopathology. In the present study, we exposed adolescent rats to Δ-9-tetra-hydrocannabinol (THC), the psychoactive component in marijuana. At adulthood, we characterized the functionality of PFC GABAergic neurotransmission and its regulation of sub-cortical DA function using molecular, behavioral and in-vivo electrophysiological analyses. Our findings revealed a persistent attenuation of PFC GABAergic function combined with a hyperactive neuronal state in PFC neurons and associated disruptions in cortical gamma oscillatory activity. These PFC abnormalities were accompanied by hyperactive DAergic neuronal activity in the ventral tegmental area (VTA) and behavioral and cognitive abnormalities similar to those observed in psychiatric disorders. Remarkably, these neuronal and behavioral effects were reversed by pharmacological activation of GABAA receptors in the PFC. Together, these results identify a mechanistic link between dysregulated frontal cortical GABAergic inhibition and sub-cortical DAergic dysregulation, characteristic of well-established neuropsychiatric endophenotypes.

Fear Memory Recall Potentiates Opiate Reward Sensitivity through Dissociable Dopamine D1 versus D4 Receptor-Dependent Memory Mechanisms in the Prefrontal Cortex

Disturbances in prefrontal cortical (PFC) dopamine (DA) transmission are well established features of psychiatric disorders involving pathological memory processing, such as post-traumatic stress disorder and opioid addiction. Transmission through PFC DA D4 receptors (D4Rs) has been shown to potentiate the emotional salience of normally nonsalient emotional memories, whereas transmission through PFC DA D1 receptors (D1Rs) has been demonstrated to selectively block recall of reward- or aversion-related associative memories. In the present study, using a combination of fear conditioning and opiate reward conditioning in male rats, we examined the role of PFC D4/D1R signaling during the processing of fear-related memory acquisition and recall and subsequent sensitivity to opiate reward memory formation. We report that PFC D4R activation potentiates the salience of normally subthreshold fear conditioning memory cues and simultaneously potentiates the rewarding effects of systemic or intra-ventral tegmental area (VTA) morphine conditioning cues. In contrast, blocking the recall of salient fear memories with intra-PFC D1R activation, blocks the ability of fear memory recall to potentiate systemic or intra-VTA morphine place preference. These effects were dependent upon dissociable PFC phosphorylation states involving calcium-calmodulin-kinase II or extracellular signal-related kinase 1–2, following intra-PFC D4 or D1R activation, respectively. Together, these findings reveal new insights into how aberrant PFC DAergic transmission and associated downstream molecular signaling pathways may modulate fear-related emotional memory processing and concomitantly increase opioid addiction vulnerability. SIGNIFICANCE STATEMENT Post-traumatic stress disorder is highly comorbid with addiction. In this study, we use a translational model of fear memory conditioning to examine how transmission through dopamine D1 or D4 receptors, in the prefrontal cortex (PFC), may differentially control acquisition or recall of fear memories and how these mechanisms might regulate sensitivity to the rewarding effects of opioids. We demonstrate that PFC D4 activation not only controls the salience of fear memory acquisition, but potentiates the rewarding effects of opioids. In contrast, PFC D1 receptor activation blocks recall of fear memories and prevents potentiation of opioid reward effects. Together, these findings demonstrate novel PFC mechanisms that may account for how emotional memory disturbances might increase the addictive liability of opioid-class drugs.