Marijuana - Psychopharmacology - How Canabis acts on the brain

**Petros Agapetos** · 11-15-2018, 03:25 AM

Abstract
Increases in mesolimbic dopamine transmission are observed when animals are treated with all known drugs of abuse, including cannabis, and to conditioned stimuli predicting their availability. In contrast, decreases in mesolimbic dopamine function are observed during drug withdrawal, including cannabis-withdrawal syndrome. Thus, despite general misconceptions that cannabis is unique from other drugs of abuse, cannabis exerts identical effects on the mesolimbic dopamine system. The recent discovery that endogenous cannabinoids modulate the mesolimbic dopamine system, however, might be exploited for the development of potential pharmacotherapies designed to treat disorders of motivation. Indeed, disrupting endocannabinoid signaling decreases drug-induced increases in dopamine release in addition to dopamine concentrations evoked by conditioned stimuli during reward seeking.

**Petros Agapetos** · 11-15-2018, 03:27 AM

All known drugs of abuse, including Δ9-tetrahydrocannabinol, the primary psychoactive component of Cannabis sativa, increase dopamine concentrations in terminal regions of the mesolimbic dopamine system (Di Chiara and Imperato 1988; Pierce and Kumaresan 2006). The mesolimbic dopamine system is a neural pathway that originates from A10 dopamine neurons in the ventral tegmental area of the midbrain and projects to limbic structures, most prominently the nucleus accumbens (Table 1) (Spanagel and Weiss 1999). Increases in nucleus accumbens dopamine are theorized to mediate the primary positive reinforcing and rewarding properties of all known drugs of abuse (Roberts et al. 1977; Wise and Bozarth 1985; Ritz et al. 1987). In addition, when animals are presented with conditioned stimuli that predict drug availability, transient dopamine events that are theorized to mediate the secondary reinforcing effects of drugs of abuse and initiate drug seeking are also observed in the nucleus accumbens (Phillips et al. 2003; Owesson-White et al. 2009). In contrast, the negative affective state that occurs during drug withdrawal is associated with a decrease in mesolimbic dopamine function, which might lead to compulsive drug seeking (Weiss et al. 2001; Koob 2009). This article reviews studies addressing the effects of cannabinoids and cannabinoid withdrawal on dopamine release, in addition to the effects of manipulating the endogenous cannabinoid system on drug- and cue-evoked dopamine release

**Petros Agapetos** · 11-15-2018, 03:31 AM

TONIC AND PHASIC DOPAMINE OVERVIEW
Before delving into the interaction between cannabinoids and the dopamine system, it is important first to develop a general understanding of the patterns of dopamine signaling and the common methods used to monitor dopamine transmission in vivo. Two distinct patterns of dopamine neural activity occur in the behaving animal. Midbrain dopamine neurons typically fire at low frequencies of 1–5 Hz, which is thought to produce a tone on high-affinity dopamine D2 receptors in terminal regions of the mesolimbic dopamine system, including the nucleus accumbens (Grace 1991; Dreyer et al. 2010). These tonic dopamine levels are detectable using techniques, such as in vivo microdialysis, that allow for neurochemical collection on a timescale of minutes. In contrast, when animals are presented with motivationally salient stimuli, such as conditioned cues that predict drug availability, midbrain dopamine neurons fire in high-frequency bursts (≥20 Hz), thereby producing transient increases in nucleus accumbens dopamine concentration that are sufficiently high to occupy low-affinity dopamine D1 receptors (Grace 1991; Phillips et al. 2003; Dreyer et al. 2010). These phasic dopamine events are detectable in vivo at the level of the dopamine neuron using single-unit electrophysiological recording techniques or at the neurochemical level within terminal fields of the mesolimbic dopamine system using fast-scan cyclic voltammetry, an electrochemical technique that allows for the detection of dopamine on the millisecond timescale.

CANNABINOIDS INCREASE TONIC DOPAMINE LEVELS
A long-held misconception was that cannabinoids, such as Δ9-tetrahydrocannabinol, fail to increase dopamine concentrations in the same manner as other drugs of abuse (Wickelgren 1997). This point was even disputed in the experimental literature; for example, a single edition of the journal Pharmacology, Biochemistry, and Behavior contained contrasting reports on whether cannabinoids increase dopamine levels in the brain (cf. Gardner and Lowinson 1991 vs. Castaneda et al. 1991). Currently, however, the existence of an overwhelming body of neurochemical evidence (Ng Cheong Ton et al. 1988; Chen et al. 1990, 1991, 1993; Tanda et al. 1997; Malone and Taylor 1999) unequivocally shows that cannabinoids do, indeed, increase dopamine concentrations in the nucleus accumbens. It should be noted, however, that genetic factors partially determine the magnitude of cannabinoid-induced increases in accumbal dopamine concentration (Gardner 2002, 2005), because Gardner and colleagues (Chen et al. 1991) reported that the dopamine-increasing potency of cannabinoids varies depending on the strain of rat assessed. Importantly, these cannabinoid-induced increases in nucleus accumbens dopamine are most prominently observed in the shell region of the nucleus accumbens and occur in a cannabinoid CB1 receptor-dependent manner. As illustrated in Figure 1A, the cannabinoid CB1 receptor agonist WIN55,212-2 increased dopamine concentrations within the nucleus accumbens shell, an effect that was blocked by the coadministration of the cannabinoid CB1 receptor antagonist rimonabant. Cannabinoid-induced increases in nucleus accumbens dopamine concentration are thought to arise from an increase in the mean firing rate of dopamine neurons within the ventral tegmental area. In accordance with this theory, using single-unit recording techniques, multiple reports indicate that cannabinoids increase the firing rate of ventral tegmental area dopamine neurons (French 1997; French et al. 1997; Gessa et al. 1998; Wu and French 2000). These parallel increases in cannabinoid-induced neural activity are shown in Figure 1B; specifically, WIN55,212-2 dose-dependently increased the frequency of dopamine neural firing (Gessa et al. 1998). Importantly, also, the cannabinoid-induced increases in dopamine neural activity were abolished following administration of rimonabant, which shows that cannabinoids increase dopamine neural activity through a CB1 receptor-dependent mechanism.

**Petros Agapetos** · 11-15-2018, 03:32 AM

CANNABINOIDS INCREASE PHASIC DOPAMINE EVENTS
The majority of the aforementioned neurochemical studies measured changes in tonic dopamine levels using in vivo microdialysis. To assess whether cannabinoids increase phasic dopamine release events, Cheer et al. (2004) measured nucleus accumbens dopamine concentrations in the behaving rat using fast-scan cyclic voltammetry. As illustrated in Figure 1C, WIN55,212-2 increased the frequency of phasic dopamine events detected in the nucleus accumbens shell (Cheer et al. 2004). Rather than resulting from the regular pacemaker firing that characterizes tonic dopamine signaling (e.g., Fig. 1B), these transient increases in accumbal dopamine release are thought to arise from high-frequency bursts of dopamine neural activity (Gonon 1988; Sombers et al. 2009). As would be predicted, therefore, Figure 1D shows that the cannabinoids Δ9-tetrahydrocannabinol and WIN55,212-2 both increased the frequency of bursts in addition to the number of impulses occurring during each burst of dopaminergic neural activity (Gessa et al. 1998).

**Petros Agapetos** · 11-15-2018, 03:33 AM

PHARMACOLOGICAL MECHANISMS OF CANNABINOID-INDUCED INCREASES IN DOPAMINE
After determining that cannabinoids increase both tonic and phasic dopamine neurotransmission, investigators began addressing the pharmacological mechanisms involved. Initially, in vitro synaptosomal studies suggested that cannabinoids might increase nucleus accumbens dopamine concentrations, in part, by binding to the dopamine transporter and thereby decreasing uptake into presynaptic terminals (Hershkowitz and Szechtman 1979; Poddar and Dewey 1980), which would be consistent with the pharmacological mechanism of action of other drugs of abuse, such as cocaine. If cannabinoids increase dopamine concentration by decreasing uptake, however, the width of electrically evoked dopamine release events should increase when assessed using in vivo FSCV. Electrically evoked dopamine events result in high concentrations of dopamine that saturate dopamine transporters, thus allowing changes in uptake to be discerned. As illustrated in Figure 1C, cannabinoids failed to alter the width of electrically evoked dopamine release events, thereby showing that cannabinoids do not increase dopamine by decreasing uptake (Cheer et al. 2004). Furthermore, dopamine uptake inhibitors typically decrease neural firing of dopamine neurons by activating inhibitory dopamine D2 autoreceptors (Einhorn et al. 1988). Thus, a cannabinoid-induced decrease in dopamine uptake would be inconsistent with a cannabinoid-induced increase in dopamine neural firing (Fig. 1B). Another possibility is that cannabinoids might directly stimulate dopamine neurons; however, this hypothesis is also unlikely, because of multiple reports that dopamine cell bodies lack cannabinoid CB1 receptors (Herkenham et al. 1991; Julian et al. 2003). An alternative model, proposed by Carl Lupica (Lupica and Riegel 2005), suggests that cannabinoids might increase dopamine release by indirectly disinhibiting dopamine neurons. In support of this model, application of cannabinoids to ventral tegmental area brain slices decreased GABAergic inhibitory postsynaptic currents in a GABAA receptor-dependent manner (Szabo et al. 2002) and failed to increase dopamine neural activity following pre-treatment of GABAA receptor antagonists (Cheer et al. 2000). Taken together, in support of Lupica’s model, these data suggest that cannabinoids increase dopamine neural firing by decreasing GABAergic inhibition of dopamine neural activity.

DOPAMINE LEVELS ARE DECREASED DURING CANNABIS WITHDRAWAL
A key feature of the addiction phenomenon—drug withdrawal—is theorized to produce a negative emotional state that drives persistent, relapsing drug seeking (Childress et al. 1988; Koob et al. 1998). It is now well accepted that withdrawal occurs in association with a decrease in mesolimbic dopamine function (Weiss et al. 2001). For example, when experimental animals are withdrawn from drugs of abuse (e.g., ethanol, morphine, cocaine, and amphetamine), decreased tonic dopamine concentrations are detected in the terminal regions of the mesolimbic dopamine system when assessed using in vivo microdialysis (Rossetti et al. 1992; Weiss et al. 1992, 1996). Although the use of cannabinoids, such as marijuana and hashish, was historically considered to be devoid of withdrawal symptoms (Todd 1946), we now know that cannabinoids do, indeed, produce clinically significant withdrawal symptoms. Over the course of several clinical studies (Jones et al. 1976; Budney et al. 1999; Haney et al. 1999), investigators documented a “cannabis-withdrawal syndrome,” which is composed of several core symptoms, including anxiety/nervousness, decreased appetite/weight loss, restlessness, sleep difficulties including strange dreams, chills, depressed mood, stomach pain/physical discomfort, shakiness, and sweating (Budney and Hughes 2006). It is likely, therefore, that these withdrawal symptoms contribute to cannabis dependence through negative reinforcement processes. The development of animal models of cannabis-withdrawal syndrome allowed for the investigation of the neural mechanisms involved. To date, two distinct models of cannabis withdrawal exist (Lichtman and Martin 2002). The first, involving abrupt forced abstinence from experimenter-administered cannabinoids (e.g., Δ9-tetrahydrocannabinol, WIN55,212-2), results in mild withdrawal symptoms that are relatively difficult to detect (Aceto et al. 1996, 2001). The second, involving precipitated withdrawal induced by a rimonabant challenge, results in immediately observable robust withdrawal symptoms (e.g., wet dog shakes) (Aceto et al. 1995; Tsou et al. 1995). The behavioral manifestations of precipitated cannabinoid withdrawal are accompanied by a decrease in ventral tegmental dopamine neural activity as assessed using single-unit recording techniques (Diana et al. 1999). The decreased dopamine neural activity occurs along the same time course as decreased tonic dopamine concentrations observed in the nucleus accumbens using in vivo microdialysis (Tanda et al. 1999). Precipitated withdrawal also induces a significant decrease in phasic dopamine neural activity (Diana et al. 1999). Taken together, these studies show that withdrawal from cannabinoids depresses mesolimbic dopamine function in the same manner as other drugs of abuse.

**Petros Agapetos** · 11-15-2018, 03:34 AM

A BRIEF INTRODUCTION TO THE ENDOCANNABINOID SYSTEM
The isolation of Δ9-tetrahydrocannabinol from C. sativa (Gaoni and Mechoulam 1964), the discovery that Δ9-tetrahydrocannabinol binds to a G-protein-coupled receptor in the brain (Devane et al. 1988), and the subsequent cloning of the cannabinoid CB1 receptor (Matsuda et al. 1990) led to the elucidation of a previously uncharacterized endogenous cannabinoid, or endocannabinoid system. Anandamide (Devane et al. 1992) and 2-arachydonoylglcycerol (Mechoulam et al. 1995) were the first, and remain the best, characterized endocannabinoids. Based on a growing recognition that cannabinoids modulate mesolimbic dopamine function, the endocannabinoid system is currently receiving a great deal of attention as investigators search for potential pharmacotherapies for addiction (Vries and Schoffelmeer 2005; Justinova et al. 2009).

ENDOCANNABINOIDS MIGHT INCREASE DOPAMINE RELEASE BY DISINHIBITING DOPAMINE NEURAL ACTIVITY
Lupica’s model (Lupica and Riegel 2005) concerning cannabinoid modulation of dopamine neural activity is also relevant in the context of endocannabinoid signaling. Endocannabinoids are unique from classical neurotransmitters in that they are formed and released on demand during periods of high neural activity (Freund et al. 2003). Thus, during phasic dopamine events, intracellular Ca2+ increases precipitously, which then activates enzymes (e.g., diaacylglycerol lipase, DGL) leading to the synthesis of endocannabinoids (Wilson and Nicoll 2002; Melis et al. 2004; Alger and Kim 2011). Once synthesized, endocannabinoids traverse the plasma membrane into the extra-synaptic space, where they retrogradely activate cannabinoid CB1 receptors on presynaptic terminals (Wilson and Nicoll 2001). Endocannabinoids binding to presynaptic cannabinoid CB1 receptors is known to result in the suppression of GABA-mediated inhibition (Wilson and Nicoll 2001), a form of synaptic plasticity known as depolarization-induced suppression of inhibition (Alger and Kim 2011). Within the ventral tegmental area, depolarization-induced suppression of inhibition should theoretically result in a net disinhibition of dopamine neural activity (cf. Fig. 2, A vs. B) (Lupica and Riegel 2005). A growing body of evidence suggests that 2-arachydonoylglycerol is the primary endocannabinoid involved in mediating such forms of synaptic plasticity (Melis et al. 2004; Tanimura et al. 2010).