Similarly, infreshly dissected hippocampal slices, electrical stimulation of the Schaffer collaterals, a glutama tergic fiber tract that connects neurons in the CA3 and CA1 fields, causes a Ca2+-dependent increase in 2-AG content . This stimulation has no effect on the levels of non-cannabinoid mono acylglycerols, such as 1-palmitoylglycerol, which indicates that 2-AG formation may not be attributed to a broad, non-specific increase in lipid turnover. Furthermore, electrical stimulation of the Scheffer col laterals does not modify hippocampal anandamide levels, suggesting that the biochemical pathways leading to the production of 2-AG and anandamide may be independently controlled . In further support of this idea, activationof D2 receptors, a potent stimulus for anandamide formation in the rat striatum, has no effect on striatal 2-AG levels .Neuronal and glial cells internalize 2-AG through a mechanism apparently similar to that implicated in anandamide transport. Thus, human astrocytoma and other tumor cells accumulate [3 H]anandamide and [ 3 H]2-AG with similar kinetic properties and this process is blocked by the anandamide transport inhibitor AM404 . In addition, anandamide and 2-AG prevent each other’s transport . Nevertheless, there also appear to be differences between anandamide and 2-AG accumulation. For example, [3 H]2-AG inter nalization in astrocytoma cells is reduced by exogenous arachidonic acid, whereas [3 H]anandamide internaliza tion is not. This discrepancy may be explained in two ways: arachidonic acid may directly interfere with a 2- AG carrier distinct from anandamide’s; or the fatty acid may indirectly prevent the facilitated diffusion of 2-AG by inhibiting its enzymatic conversion to arachi donic acid.
If the latter explanation is correct,vertical farming technology agents that interfere with the arachidonic acid esterification into phospholipids, such as triacsinC , should decrease [ 3 H]2-AG uptake. This was found indeed to be the case, at least inastrocytoma cells . Thus, while anandamide and 2-AG may be internalized through similar transport mechanisms, they appear to differ inhow their intracellular break down can affect the rate of transport into cells.After removal from the external medium, 2-AG is hydrolyzed to arachidonic acid and glycerol. In cell free preparations, FAAH cleaves anandamide and 2- AG at similar rates, which has led to suggest that this enzyme may contribute to the elimination of both com pounds. This appears to be unlikely, however, for three reasons. First, pig brain tissue contains two distinct 2- AG-hydrolase activities, both of which are chromato graphically different from FAAH . Second, inhibition of FAAH activity in intact neurons and astrocytoma cells prevents the hydrolysis of anandamide, but has no effect on 2-AG degradation . Finally, 2-AG hydrolysis is entirely preserved in FAAH-null mice . These findings suggest that, although 2-AG can be hydrolyzed by FAAH in vitro, different enzyme may be responsible for its degradation in vivo. A possible candidate for this role is MGL, a cytosolic serine hydrolase that cleaves 2- and 1-monoglycerides into fatty acid and glycerol .MGL is abundantly expressed in discrete areas of the rat brain—including the hippocampus, cortex, and cerebellum—where CB1 receptors are also found. Moreover, adenovirus-induced over expression of MGL enhances the hydrolysis of endogenously produced 2-AG in primary cultures of rat brain neurons . Finally, recent experiments indicate that silencing the MGL gene through RNA interference markedly impairs 2-AG degradation in intact HeLa cells . Although these results strongly support a role of MGL in2-AG hydrolysis, the development of additional experimental tools will be needed to demon strate such a role unambiguously.
Recreational stimulant use is a growing concern among young adults, with 4.4% and 5% to 35% of college students endorsing cocaine and recreational amphetamine use, respectively, and 16% of cocaine experimenters developing dependence within 10 years . To develop cost-effective prevention and intervention strategies, it is crucial to identify ultra–high risk recreational users. However, little is known about biobehavioral markers forecasting trajectory of occasional stimulant use to stimulant use disorder . Previous stimulant use research is predominantly cross-sectional, comparing individuals with chronic stimulant use with healthy individuals; although findings from these studies highlight brain disruptions related to drug use, they cannot disentangle whether disruptions preceded or were a result of chronic use. Young adulthood is a period of increased independence, often providing more opportunities for risky behavior such as drug experimentation. Risky behavior can be defined as actions that may be subjectively desirable but are potentially harmful and is typically quantified in young adults by their degree of substance use, unprotected sex, health habits, and crime engagement . Risk taking often occurs in clusters of maladaptive behaviors, suggesting underlying impairments in decision making . Decision making involves several brain processes, including learning, inhibition, and outcome assessment, specififically appraising positive or negative valence of choices . Functional magnetic resonance imaging research indicates that individuals with SUD show impaired decision making associated with altered brain activation in executive control and reward processing regions . Decision making is thought to involve a cooperative relationship between an impulsive system activated by immediate rewards and an inhibitory control system. Through learning, the control network allows individuals to resist immediate attraction to rewards in favor of longer-term advantageous outcomes .
In SUD, bio-behavioral indices of risk taking suggest an underlying imbalance between the control and impulsive systems. The control system integral to decision making comprises prefrontal cortex , theorized as responsible for learning the relationship between stimuli and outcome, working memory, and inhibiting behavior . SUD samples exhibit frontal lobe impairments associated with compromised decision making and increased risk behavior . For example, cocaine abusers exhibit dorsolateral PFC hypoactivation during response inhibition and prediction of uncertain outcomes ; in cocaine dependence, orbitofrontal cortex and DLPFC attenuation are linked to reduced ability to differentiate between variable monetary gains . Similarly, methamphetamine users inaccurately process success or failure of available options, a pattern associated with orbitofrontal cortex/DLPFC hypoactivation . Working in conjunction with frontal regions is striatum, an area associated with reward processing , selecting and initiating actions , and learning . During the Iowa Gambling Task , healthy individuals show stronger striatal activation to wins than to losses ,future vertical farming but amphetamine dependent individuals demonstrate hypersensitive striatal re sponses to rewards . Cocaine and methamphetamine users also exhibit striatal hyperactivation but frontal hypo activation during risky decision-making tasks such as the Iowa Gambling Task and the Balloon Analogue Risk Task that is linked to riskier behavioral performance .This suggests that such neural patterns during decision making pro mote favoring of risky incentives.Evidence from fMRI studies has led researchers to theorize that frontal lobe and striatum form a functional circuit with insular cortex and anterior cingulate cortex ; these regions coordinate to integrate emotional and autonomic information about rewards into goal-oriented behavior . ACC is proposed to be involved in emotion and behavior management based on its neural connections to both the emotion processing limbic system and the cognitive control center, PFC . Insula is proposed to play a role in interoceptive processing, wherein individuals integrate physiological cues to differentiate between risky and safe decisions and transform these cues into conscious feelings and behaviors . ACC and insula hypoactivation is evident in chronic stimulant users in response inhibition and error monitoring during decision making . Evidence for aberrant activity in key components of the PFC-limbic network has led researchers to suggest that weakened ability to accurately process information about options and control behaviors leads to favoring choices that offer immediate, rather than delayed, rewards . Cross-sectional studies of occasional stimulant users report decision-making impairments that parallel findings in stimulant-dependent individuals, including 1) weakened inhibitory control and reduced cognitive flexibility ; 2) neuropsychological impairments in executive functions ; and 3) frontal, striatal, and insular attenuation during a Risky Gains Task paired with reduced ability to differentiate between safe and risky decisions . Several research groups have recognized limitations of cross-sectional addiction research and have shifted toward a longitudinal approach to understand the transition to prob lematic substance use . Structural MRI studies show that decreased brain volume in frontocentral regions at age 14 years predicts binge drinking at age 16 and that frontos triatal regions are linked to heightened stimulant use in OSUs 1 to 2 years later .
However, fMRI has been less applied to predict the development of SUD. The current longitudinal study used follow-up clinical and drug use data from OSUs 3 years after an fMRI scan to determine whether baseline behavioral and blood oxygen level–dependent responses during the RGT 1) differentiated young adults who became problem stimulant users from those who desisted from stimulant use during the 3-year interim and 2) predicted cumulative baseline and follow-up stimulant and marijuana use across OSUs, regardless of clinical status , to address concerns regarding significant rates of marijuana and stimulant co-use . Analyses compared BOLD activity related to specific task requirements: decision contrasts compared BOLD activity during risk-taking choice trials versus safe choice trials; outcome contrasts compared BOLD activity on trials where each subject took a risk and subsequently earned a win or a loss. Categorical hypotheses were tested based on prior biobe havioral findings in stimulant- dependent individuals: 1) PSUs would exhibit riskier task performance than DSUs; 2) PSUs would show greater striatal BOLD signals than DSUs to out comes, particularly in response to risky wins; and 3) PSUs would exhibit lower PFC, insular, and cingulate BOLD signals during decision making. Because dimensional analyses were exploratory, no a priori hypotheses were tested.Three hypotheses were tested. First, consistent with the pre diction that PSUs would exhibit riskier task performance than DSUs, PSUs more frequently made a risky decision following a win compared with DSUs, while DSUs more frequently made a safe decision following a risky win. This pattern supports previous findings that PSUs are more reactive to rewards . Second, although it was predicted that PSUs would show greater activation in reward processing striatal regions to risky wins than to risky losses when compared with DSUs, our results demonstrated the opposite effect, with PSUs exhibiting lower striatal BOLD signals across outcomes than DSUs. However, this finding is consistent with a longitudinal study of sensation-seeking adolescents in which striatal hypoactivation predicted future problematic drug use; the authors theorized that lower striatal activity may lead to a compensatory mechanism in which one seeks out increased risk to gain greater stimulation, thereby balancing reward center hypo activation . PSUs exhibited greater temporo-occipital BOLD signals to wins than to losses, findings consistent with a recent meta-analysis reporting that 86% of addiction-related neu roimaging studies demonstrate significant visual cortex activity to drug cues . Although the RGT did not test drug-related responses, our results demonstrate an analo gous relationship to general reward cues, suggesting that PSUs may allocate greater visual attention to risky rewards than to risky losses. Middle temporal lobe is involved in memory of reward-based information critical for future oriented decision making, suggesting that PSUs may be less able to consolidate information about outcomes differently . Together, PSUs are characterized by visual attention and memory activation during risky rewards but blunted responsivity to loss outcomes. Our third prediction was supported in that PSUs exhibited lower PFC, insula, and cingulate BOLD signals than DSUs during risky feedback. These findings align with a recent study conducted by our research group demonstrating that during a task evaluating how individuals learn to make decisions, PSUs exhibited lower insula and ACC activation across all available outcomes than DSUs . Such pat terns are consistent with previous reports of PFC, insula, and ACC attenuations in chronic stimulant users that are linked with decreased ability to adapt behavior using prior experiences/ reduced inhibitory control, interoceptive awareness, and conflict monitoring, respectively . Thus, young adults pre disposed to SUD may have prior deficits in recruiting neural effort toward critical decision-making processes. Nonhypothesized group differences also emerged in thalamic, precuneus, and posterior cingulate regions that warrant discussion. PSUs showed relatively greater pre cuneus and posterior cingulate BOLD signals when making risky decisions than when making safe decisions when compared with DSUs. Such differences are consistent with previous findings in SUD samples that heightened activation of these areas during exteroceptive awareness may underlie the maintenance and exacerbation of substance use .