Hippocampus 5-HT1A-receptors attenuate cocaine-induced hyperlocomotion and the increase in hippocampal but not nucleus accumbens 5-HT

Brain structural differences in cocaine use disorder: Insights from multivariate and neurotransmitter analyses

Cocaine use disorder (CUD) is a chronic and relapsing neuropsychiatric disorder characterized by structural and functional brain lesions, posing a significant public health challenge. While the disruptive effects of cocaine on neurotransmitter systems (receptors/transporters) have been well established, the patterns of brain structural abnormalities in CUD and its interaction with other factors remain an ongoing topic of investigation. We employed source-based morphometry (SBM), a multivariate approach on 50 CUD participants and 50 matched healthy controls from the public SUDMEX CONN dataset. This method allowed us to identify co-varying patterns of brain tissue volume differences, and further explore the effect of average cocaine dosage through moderation analysis. Spatial correlation analysis was also performed to examine micro-macro structural consistency between tissue volume variations and chemoarchitectural distribution of dopamine and serotonin. Our SBM analysis findings were consistent with reward-related neuroadaptations in the striato-thalamo-cortical and limbic pathways and also exhibited co-localization with the distribution of dopamine and serotonin systems. The moderation analysis suggested that the average dosage positively strengthens cocaine consumption years' effect on brain structures. By integrating our findings of gray and white matter volume differences and corresponding neurotransmitter profiles, this comprehensive view not only strengthens our understanding of the brain's structural abnormalities in CUD, but also reveals potential mechanisms underlying its development and progression.

Serotonin Signaling through Lipid Membranes

Serotonin (5-HT) is a vital modulatory neurotransmitter responsible for regulating most behaviors in the brain. An inefficient 5-HT synaptic function is often linked to various mental disorders. Primarily, membrane proteins controlling the expression and activity of 5-HT synthesis, storage, release, receptor activation, and inactivation are critical to 5-HT signaling in synaptic and extra-synaptic sites. Moreover, these signals represent information transmission across membranes. Although the lipid membrane environment is often viewed as fairly stable, emerging research suggests significant functional lipid-protein interactions with many synaptic 5-HT proteins. These protein-lipid interactions extend to almost all the primary lipid classes that form the plasma membrane. Collectively, these lipid classes and lipid-protein interactions affect 5-HT synaptic efficacy at the synapse. The highly dynamic lipid composition of synaptic membranes suggests that these lipids and their interactions with proteins may contribute to the plasticity of the 5-HT synapse. Therefore, this broader protein-lipid model of the 5-HT synapse necessitates a reconsideration of 5-HT's role in various associated mental disorders.

Drugs as instruments: a new framework for non-addictive psychoactive drug use

Most people who are regular consumers of psychoactive drugs are not drug addicts, nor will they ever become addicts. In neurobiological theories, non-addictive drug consumption is acknowledged only as a "necessary" prerequisite for addiction, but not as a stable and widespread behavior in its own right. This target article proposes a new neurobiological framework theory for non-addictive psychoactive drug consumption, introducing the concept of "drug instrumentalization." Psychoactive drugs are consumed for their effects on mental states. Humans are able to learn that mental states can be changed on purpose by drugs, in order to facilitate other, non-drug-related behaviors. We discuss specific "instrumentalization goals" and outline neurobiological mechanisms of how major classes of psychoactive drugs change mental states and serve non-drug-related behaviors. We argue that drug instrumentalization behavior may provide a functional adaptation to modern environments based on a historical selection for learning mechanisms that allow the dynamic modification of consummatory behavior. It is assumed that in order to effectively instrumentalize psychoactive drugs, the establishment of and retrieval from a drug memory is required. Here, we propose a new classification of different drug memory subtypes and discuss how they interact during drug instrumentalization learning and retrieval. Understanding the everyday utility and the learning mechanisms of non-addictive psychotropic drug use may help to prevent abuse and the transition to drug addiction in the future.

The role of serotonin in drug use and addiction

The use of psychoactive drugs is a wide spread behaviour in human societies. The systematic use of a drug requires the establishment of different drug use-associated behaviours which need to be learned and controlled. However, controlled drug use may develop into compulsive drug use and addiction, a major psychiatric disorder with severe consequences for the individual and society. Here we review the role of the serotonergic (5-HT) system in the establishment of drug use-associated behaviours on the one hand and the transition and maintenance of addiction on the other hand for the drugs: cocaine, amphetamine, methamphetamine, MDMA (ecstasy), morphine/heroin, cannabis, alcohol, and nicotine. Results show a crucial, but distinct involvement of the 5-HT system in both processes with considerable overlap between psychostimulant and opioidergic drugs and alcohol. A new functional model suggests specific adaptations in the 5-HT system, which coincide with the establishment of controlled drug use-associated behaviours. These serotonergic adaptations render the nervous system susceptible to the transition to compulsive drug use behaviours and often overlap with genetic risk factors for addiction. Altogether we suggest a new trajectory by which serotonergic neuroadaptations induced by first drug exposure pave the way for the establishment of addiction.

Episodic memories and their relevance for psychoactive drug use and addiction

The majority of adult people in western societies regularly consume psychoactive drugs. While this consumption is integrated in everyday life activities and controlled in most consumers, it may escalate and result in drug addiction. Non-addicted drug use requires the systematic establishment of highly organized behaviors, such as drug-seeking and -taking. While a significant role for classical and instrumental learning processes is well established in drug use and abuse, declarative drug memories have largely been neglected in research. Episodic memories are an important part of the declarative memories. Here a role of episodic drug memories in the establishment of non-addicted drug use and its transition to addiction is suggested. In relation to psychoactive drug consumption, episodic drug memories are formed when a person prepares for consumption, when the drug is consumed and, most important, when acute effects, withdrawal, craving, and relapse are experienced. Episodic drug memories are one-trial memories with emotional components that can be much stronger than "normal" episodic memories. Their establishment coincides with drug-induced neuronal activation and plasticity. These memories may be highly extinction resistant and influence psychoactive drug consumption, in particular during initial establishment and at the transition to "drug instrumentalization." In that, understanding how addictive drugs interact with episodic memory circuits in the brain may provide crucial information for how drug use and addiction are established.

Contributions of serotonin in addiction vulnerability

The serotonin (5-hydroxytryptamine; 5-HT) system has long been associated with mood and its dysregulation implicated in the pathophysiology of mood and anxiety disorders. While modulation of 5-HT neurotransmission by drugs of abuse is also recognized, its role in drug addiction and vulnerability to drug relapse is a more recent focus of investigation. First, we review preclinical data supporting the serotonergic raphe nuclei and their forebrain projections as targets of drugs of abuse, with emphasis on the effects of psychostimulants, opioids and ethanol. Next, we examine the role of 5-HT receptors in impulsivity, a core behavior that contributes to the vulnerability to addiction and relapse. Finally, we discuss evidence for serotonergic dysregulation in comorbid mood and addictive disorders and suggest novel serotonergic targets for the treatment of addiction and the prevention of drug relapse.

Dissociating effects of cocaine and d-amphetamine on dopamine and serotonin in the perirhinal, entorhinal, and prefrontal cortex of freely moving rats

RATIONALE

Neuroimaging studies with humans showed widespread activation of the cortex in response to psychostimulant drugs. However, the neurochemical nature of these brain activities is not characterized.

OBJECTIVE

The aim of the present study was to investigate the effects of cocaine and d-amphetamine on dopamine (DA) and serotonin (5-HT) in cortical areas of the hippocampal network in comparison to the prefrontal cortex (PFC).

MATERIALS AND METHODS

We conducted in vivo microdialysis experiments in behaving rats measuring DA and 5-HT in the perirhinal cortex (PRC), entorhinal cortex (EC), and PFC, after application of cocaine (0, 5, 10, 20 mg/kg; i.p.) or d-amphetamine (0, 0.5, 1.0, 2.5 mg/kg; i.p.).

RESULTS

Cocaine and d-amphetamine dose-dependently increased DA and 5-HT levels in the PRC, EC, and PFC. A predominant DA response to d-amphetamine was only found in the PFC, but not in the PRC and EC. Cocaine increased DA and 5-HT to an equal extent in the PFC and PRC but induced a predominant 5-HT response in the EC. When comparing the neurochemical responses between the drugs at an equal level of behavioral activation, cocaine was more potent than d-amphetamine in increasing 5-HT in the PFC, while no differences were found in the PRC or EC or in the DA responses in all three cortical areas.

CONCLUSIONS

We conclude that cocaine and d-amphetamine increase DA and 5-HT levels in PRC and EC largely to the same extent as in the PFC.

Serotonin and psychostimulant addiction: Focus on 5-HT1A-receptors

Serotonin(1A)-receptors (5-HT(1A)-Rs) are important components of the 5-HT system in the brain. As somatodendritic autoreceptors they control the activity of 5-HT neurons, and, as postsynaptic receptors, the activity in terminal areas. Cocaine (COC), amphetamine (AMPH), methamphetamine (METH) and 3,4-methylenedioxymethamphetamine ("Ecstasy", MDMA) are psychostimulant drugs that can lead to addiction-related behavior in humans and in animals. At the neurochemical level, these psychostimulant drugs interact with monoamine transporters and increase extracellular 5-HT, dopamine and noradrenalin activity in the brain. The increase in 5-HT, which, in addition to dopamine, is a core mechanism of action for drug addiction, hyperactivates 5-HT(1A)-Rs. Here, we first review the role of the various 5-HT(1A)-R populations in spontaneous behavior to provide a background to elucidate the contribution of the 5-HT(1A)-Rs to the organization of psychostimulant-induced addiction behavior. The progress achieved in this field shows the fundamental contribution of brain 5-HT(1A)-Rs to virtually all behaviors associated with psychostimulant addiction. Importantly, the contribution of pre- and postsynaptic 5-HT(1A)-Rs can be dissociated and frequently act in opposite directions. We conclude that 5-HT(1A)-autoreceptors mainly facilitate psychostimulant addiction-related behaviors by a limitation of the 5-HT response in terminal areas. Postsynaptic 5-HT(1A)-Rs, in contrast, predominantly inhibit the expression of various addiction-related behaviors directly. In addition, they may also influence the local 5-HT response by feedback mechanisms. The reviewed findings do not only show a crucial role of 5-HT(1A)-Rs in the control of brain 5-HT activity and spontaneous behavior, but also their complex role in the regulation of the psychostimulant-induced 5-HT response and subsequent addiction-related behaviors.

Double dissociating effects of sensory stimulation and cocaine on serotonin activity in the occipital and temporal cortices

Visual cues that become associated with the consumption of psychostimulant drugs energize craving and the intake of the drug by mechanisms of which little is known. In two experiments using in vivo microdialysis in freely moving rats we compared the effects of visual and auditory stimulation with that of cocaine (0, 5, 10, 20mg/kg; i.p.) on the extracellular serotonin (5-HT) activity in the occipital and temporal cortices in relation to behavior. Visual stimulation increased 5-HT in the occipital, but not temporal cortex, parallel to an increase in locomotion. Auditory stimulation decreased 5-HT in the auditory, but not occipital cortex, thus, showing a double dissociated 5-HT response. These data suggest that a locally restricted 5-HT response to sensory stimulation may gate behavioral activity sense-modality selectively. Cocaine affected 5-HT in the occipital cortex and behavioral activity in the same direction as visual stimulation, but in an amplified and prolonged way. In the temporal cortex cocaine also caused an increase in 5-HT. The findings demonstrate common effects of visual stimulation and cocaine on 5-HT activity in the occipital cortex in relation to locomotor activity. The results suggest that concepts of how neutral visual cues become powerful energizers of addiction-related behaviors should be expanded to incorporate not only an acute enhancement of reward processing mechanisms, but, in parallel, also an amplified processing of visual stimuli in the occipital cortex.

Determining the region-specific contributions of 5-HT receptors to the psychostimulant effects of cocaine

Cocaine is a drug of abuse that has complex neurochemical and behavioural profiles. When it became evident that models that involve only dopamine do not fully explain the complex effects of cocaine on behaviour, the focus of research expanded to include the 5-hydroxytryptamine (5-HT) system in the brain. The 5-HT system comprises several subtypes of 5-HT receptors, which contribute differentially to the various behavioural effects of cocaine. In this article, we describe which subtypes regulate behaviours that are related to cocaine addiction and how they might provide new therapeutic approaches. Numerous subpopulations of each 5-HT receptor can be distinguished according to their location in the brain. We also discuss how these subpopulations relate to the effects of 5-HT-receptor stimulation at the systemic level. These insights provide a new receptor-based approach for understanding the 5-HT mechanisms that subserve the actions of cocaine and possible pharmacotherapies against cocaine addiction.

Activation of dopaminergic neurotransmission in the medial prefrontal cortex by N-methyl-d-aspartate stimulation of the ventral hippocampus in rats

Many behavioral functions-including sensorimotor, attentional, memory, and emotional processes-have been associated with hippocampal processes and with dopamine transmission in the medial prefrontal cortex (mPFC). This suggests a functional interaction between hippocampus and prefrontal dopamine. The anatomical substrate for such an interaction is the intimate interconnection between the ventral hippocampus and the dopamine innervation of the mPFC. The present study yielded direct neurochemical evidence for an interaction between ventral hippocampus and prefrontal dopamine transmission in rats by demonstrating that subconvulsive stimulation of the ventral hippocampus with N-methyl-d-aspartate (NMDA; 0.5 mug/side) activates dopamine transmission in the mPFC. Postmortem measurements revealed that bilateral NMDA stimulation of the ventral hippocampus, resulting in locomotor hyperactivity, increased the homovanillic acid/dopamine ratio, an index of dopamine transmission, in the mPFC; indices of dopamine transmission in any of five additionally examined forebrain regions (amygdala, nucleus accumbens shell/core, lateral prefrontal cortex, caudate putamen) were unaltered. In vivo microdialysis measurements in freely moving rats corroborated the suggested activation of prefrontal dopamine transmission by demonstrating that unilateral NMDA stimulation of the ventral hippocampus increased extracellular dopamine in the ipsilateral mPFC. The suggested influence of the ventral hippocampus on prefrontal dopamine may be an important mechanism for hippocampo-prefrontal interactions in normal behavioral processes. Moreover, it indicates that aberrant hippocampal activity, as found in neuropsychiatric diseases, such as schizophrenia and mood disorders, may contribute to disruption of certain cognitive and emotional functions which are extremely sensitive to imbalanced prefrontal dopamine transmission.

The 5-HT1A receptor and behavioral stimulation in the rat: effects of 8-OHDPAT on spontaneous and cocaine-induced behavior

RATIONALE

The contribution of the 5-HT(1A) somato-dendritic autoreceptor populations to spontaneous and cocaine-induced locomotion is unclear.

OBJECTIVES

To use a low dose range of +/-8-hydroxy-2-(di- n-propylamino)tetralin (8-OHDPAT) to preferentially stimulate 5-HT(1A) autoreceptors and a medium 8-OHDPAT dose range to stimulate both 5-HT(1A) autoreceptors and postsynaptic receptors as pretreatments prior to either saline or cocaine.

METHODS

In experiment 1, either a medium dose of 8-OHDPAT (0.4 mg/kg) or a low dose (0.05 mg/kg) was given as pretreatments 20 min before five separate 20-min open-field tests. In experiment 2, the pretreatments were changed to a low dose range of 8-OHDPAT (0.01-0.05 mg/kg), with or without WAY 100635 (0.01-0.05 mg/kg). In experiment 3, the 8-OHDPAT pretreatments (0.01, 0.025 or 0.05 mg/kg) were administered 20 min prior to saline or cocaine (10 mg/kg) tests. In experiment 4, a medium dose range (0.2-0.3 mg/kg) was given 20 min prior to saline or cocaine (10 mg/kg) tests.

RESULTS

Experiment 1 showed that 8-OHDPAT (0.4 mg/kg) tended to increase locomotor activity but that pretreatment with 0.05 mg/kg severely suppressed locomotor activity. In experiment 2, 8-OHDPAT in the low dose range inhibited locomotor activity and this effect was reversed by co-administration of WAY 100635. Experiment 3 showed that the low-dose 8-OHDPAT pretreatment reduced locomotor activity in saline but not cocaine tests. In experiment 4, 8-OHDPAT in the medium dose range enhanced locomotor activity in cocaine tests.

CONCLUSIONS

It is suggested that the facilitatory effect of 8-OHDPAT on cocaine-induced locomotor stimulation is mediated by inhibition of 5-HT(1A) somato-dendritic autoreceptors.