Christopher D. Verrico

MDMA (Ecstasy) and human dopamine, norepinephrine, and serotonin transporters: implications for MDMA-induced neurotoxicity and treatment

Rationale3,4-Methylenedioxymethamphetamine (MDMA, designated as “Ecstasy” if illicitly marketed in tablet form) induces significant decrements in neuronal serotonin (5-HT) markers in humans, nonhuman primates, and rats as a function of dosing and dosing regimen. In rats, MDMA-mediated effects are attributed, in part, to selective high-affinity transport of MDMA into 5-HT neurons by the 5-HT transporter (SERT), followed by extensive 5-HT release.ObjectivesTo clarify whether SERT-selective effects of MDMA at human monoamine transporters can account for the reported MDMA-induced selective toxicity of serotonin neurons in primate brain.MethodsWe investigated the interaction of [3H](±, RS)- (+, S)- and (−, R)-MDMA with the human SERT, dopamine (DA) transporter (DAT), and norepinephrine (NE) transporter (NET) in stably transfected human embryo kidney (HEK)-293 cells.ResultsThe human DAT, NET, and SERT actively transported [3H]RS(±)-MDMA saturably, stereoselectively, and in a temperature-, concentration-, and transporter-dependent manner. MDMA exhibited the highest affinity for the NET≫SERT≥DAT, the same rank order for MDMA inhibition of [3H]DA, [3H]NE, and [3H]5-HT transport and stimulated release of the [3H]monoamines, which differed from reports derived from rodent monoamine transporters. The extent of MDMA-induced release of 5-HT was higher compared with release of DA or NE.ConclusionsThe affinity of MDMA for the human SERT in transfected cells does not clarify the apparent selective toxicity of MDMA for serotonin neurons, although conceivably, its higher efficacy for stimulating 5-HT release may be a distinguishing factor. The findings highlight the need to investigate MDMA effects in DAT-, SERT-, and NET-expressing neurons in the primate brain and the therapeutic potential of NET or DAT inhibitors, in addition to SERT-selective inhibitors, for alleviating the pharmacological effects of MDMA.

Repeated exposure to Δ9-tetrahydrocannabinol reduces prefrontal cortical dopamine metabolism in the rat

Long-term abuse of marijuana by humans can induce profound behavioral deficits characterized by cognitive and memory impairments. In particular, deficits on tasks dependent on frontal lobe function have been reported in cannabis abusers. In the current study, we examined whether long-term exposure to delta9-tetrahydrocannabinol, the active ingredient in marijuana, altered the neurochemistry of the frontal cortex in rats. Two weeks administration of delta9-tetrahydrocannabinol reduced dopamine transmission in the medial prefrontal cortex, while dopamine metabolism in striatal regions was unaffected. These data are consistent with earlier findings of dopaminergic regulation of frontal cortical cognition. Thus, cognitive deficits in heavy abusers of cannabis may be subserved by drug-induced alterations in frontal cortical dopamine transmission.

Cannabinoid CB1 Receptor Immunoreactivity in the Prefrontal Cortex: Comparison of Schizophrenia and Major Depressive Disorder

We recently showed that measures of cannabinoid 1 receptor (CB1R) mRNA and protein were significantly reduced in dorsolateral prefrontal cortex (DLPFC) area 9 in schizophrenia subjects relative to matched normal comparison subjects. However, other studies have reported unaltered or higher measures of CB1R levels in schizophrenia. To determine whether these discrepancies reflect differences across brain regions or across subject groups (eg, presence of depression, cannabis exposure, etc), we used immunocytochemical techniques to determine whether lower levels of CB1R immunoreactivity are (1) present in another DLPFC region, area 46, in the same subjects with schizophrenia, (2) present in area 46 in a new cohort of schizophrenia subjects, (3) present in major depressive disorder (MDD) subjects, or (4) attributable to factors other than a diagnosis of schizophrenia, including prior cannabis use. CB1R immunoreactivity levels in area 46 were significantly 19% lower in schizophrenia subjects relative to matched normal comparison subjects, a deficit similar to that observed in area 9 in the same subjects. In a new cohort of subjects, CB1R immunoreactivity levels were significantly 20 and 23% lower in schizophrenia subjects relative to matched comparison and MDD subjects, respectively. The lower levels of CB1R immunoreactivity in schizophrenia subjects were not explained by other factors such as cannabis use, suicide, or pharmacological treatment. In addition, CB1R immunoreactivity levels were not altered in monkeys chronically exposed to haloperidol. Thus, the lower levels of CB1R immunoreactivity may be common in schizophrenia, conserved across DLPFC regions, not present in MDD, and not attributable to other factors, and thus a reflection of the underlying disease process.

Reduced prefrontal cortical dopamine, but not acetylcholine, release in vivo after repeated, intermittent phencyclidine administration to rats

Subchronic administration of phencyclidine to rats or monkeys produces prefrontal cortical cognitive dysfunction, as well as reduced frontal cortical dopamine utilization. In the current study, the effects of subchronic exposure to phencyclidine on dopamine and acetylcholine release in the prefrontal cortex were assessed, using in vivo microdialysis in conscious rats. Subchronic exposure to phencyclidine (5 mg/kg twice daily for 7 days) reduced both basal extracellular concentrations of dopamine as well as the increase in dopamine release produced by an acute phencyclidine challenge. The increase in dopamine release induced by a high potassium concentration in the perfusate tended to be reduced after subchronic phencyclidine treatment, while basal and evoked acetylcholine release was unaffected. These data demonstrate that altered dopamine turnover in subjects after subchronic exposure to phencyclidine is directly reflective of reduced release, and as such, represents a functionally relevant phenomenon.

Repeated, Intermittent Δ9-Tetrahydrocannabinol Administration to Rats Impairs Acquisition and Performance of a Test of Visuospatial Divided Attention

The residual neuropsychological effects of marijuana abuse in man indicate a dysfunction of the attentional/executive systems. Moreover, experimental investigations suggest that repeated, intermittent (subchronic) Δ9-tetrahydrocannabinol (THC), the main psychoactive ingredient of marijuana, alters neurotransmission in the frontal cortex of rats and humans, a key neural site mediating attention and executive functions. In the present studies, the acquisition and performance of a test of visuospatial attention (the lateralized reaction time task) after subchronic THC administration (10.0 mg/kg twice daily for 14 days) was examined. Rats previously administered THC showed impairments in this self-paced version of the classic multiple-choice serial reaction time task, which persisted 14 days after the final drug administration. Longer time points were not examined. These attentional impairments were transiently reversible with an acute amphetamine (0.5 mg/kg) challenge. These behavioral data demonstrate that chronic THC administration to rats induces an attentional deficit, similar to that observed in humans who abuse marijuana. Finally, amphetamines ability to reverse the attentional impairments provides indirect evidence that monoaminergic deficits may be linked to the cognitive dysfunction.

Effects of repeated nicotine pre-treatment on mesoprefrontal dopaminergic and behavioral responses to acute footshock stress

The effects of acute and repeated nicotine administration on the stress response of rat mesoprefrontal dopaminergic pathways were examined. Rats were given daily injections of nicotine (0.15 or 0.60 mg/kg, s.c., freebase) or saline for 4 days, then challenged with either nicotine or saline. A regimen of inescapable electrical footshocks or no footshocks was then administered. Thirty minutes after final injection, rats were sacrificed, brains removed and dopamine (DA) and its metabolite dihydroxy-O-phenylacetic acid (DOPAC) were extracted from medial prefrontal cortex (mPFC), nucleus accumbens septi (NAS) and dorsolateral striatum and quantified by high performance liquid chromatography with electrochemical detection. Acute administration of low dose nicotine (0.15 mg/kg) produced an increase in DA utilization (increased DOPAC/DA ratio) in mPFC and NAS, but not striatum. High dose nicotine (0.60 mg/kg) produced activation in NAS, but not mPFC or striatum. Repeated low dose nicotine pre-treatment produced tolerance to the effects of nicotine challenge in the mPFC, and reduced its effects in NAS. Footshock stress preferentially increased DA utilization in mPFC and associated footshock stress-induced immobility responses, and these were reduced by low, but not high, dose repeated nicotine pre-treatment. Further, a single dose of the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine (MCA) 30 min prior to nicotine challenge dose-dependently blocked the reduction of mesoprefrontal DA stress responsivity and immobility responses produced by repeated nicotine pre-treatment. These results indicate that: (1) there are dose-dependent differential effects of acute and repeated nicotine pre-exposure on regional DA utilization; (2) low, but not high, dose repeated nicotine reduces both the mesoprefrontal DA and behavioral effects of acute footshock stress; and (3) these effects of repeated nicotine may depend on mecamylamine-sensitive nAChR stimulation. These results may have relevance to acute stress and nicotine dependence, particularly in schizophrenic disorders, which have high prevalence rates of co-morbid nicotine dependence, stress-induced symptom exacerbation and prefrontal cortical dysfunction.

Effects of repeated nicotine administration and footshock stress on rat mesoprefrontal dopamine systems : Evidence for opioid mechanisms

We have examined the effects of nicotine pre-treatment on mesoprefrontal dopamine (DA) function in the presence and absence of acute stress, and the involvement of endogenous opiate peptide systems (EOPS). Acute electrical footshock stress preferentially increases DA utilization in medial prefrontal cortex (mPFC) compared to nucleus accumbens (NAS) and striatal terminal fields, and this is correlated with profound locomotor immobility. Our recent studies have demonstrated that repeated, but not acute, nicotine pre-treatment significantly reduced mPFC DA utilization and footshock stress-induced immobility responses. There is increasing evidence that the biochemical and behavioral effects of nicotine are mediated by EOPS, and we hypothesized that the stress-reducing effects of repeated nicotine administration in these studies were mediated by EOPS. Accordingly, rats pre-treated subcutaneously with repeated nicotine were given a single dose of the opiate receptor antagonist naloxone (0.1–10.0 mg/kg, i.p.) or saline as a co-treatment with nicotine or saline 10 min prior to acute footshock stress. Naloxone had no effects on non-stressed or acute footshock stress-induced mPFC DA utilization, but dose-dependently antagonized repeated nicotines attenuation of stress-induced mesoprefrontal DA utilization and immobility responses. Furthermore, naloxone dose-dependently blocked repeated nicotines augmentation of accumbal DA utilization. These results suggest that EOPS may be involved in mediating repeated nicotine administration effects on mesoprefrontal dopaminergic and immobility responses to acute footshock stress.

Repeated Δ9-tetrahydrocannabinol exposure in adolescent monkeys: persistent effects selective for spatial working memory.

OBJECTIVE Epidemiological findings suggest that, relative to adults, adolescents are more vulnerable to the adverse persistent effects of cannabis on working memory. However, the potential confounds inherent in human studies preclude direct determination of a cause-and-effect relationship between adolescent cannabis use and heightened susceptibility to persistent working memory impairments. Consequently, the authors examined the effects of repeated exposure to Δ9-tetrahydrocannabinol (THC) on performance of spatial and object working memory tasks in adolescent monkeys. METHOD Seven pairs of male adolescent rhesus monkeys, matched for baseline cognitive performance, received vehicle or THC intravenously 5 days/week for 6 months. Performance on spatial and object memory tasks was assessed 23 or 71 hours after drug administration throughout the study. In addition, acute effects on working memory were also assessed at the beginning and end of the 6-month period. RESULTS Relative to the vehicle-exposed control animals, those with repeated THC exposure had a blunted trajectory of accuracy improvements on the spatial working memory task in a delay-dependent manner. Accuracy improvements on the object working memory task did not differ between groups. Relative to the acute effects of THC on working memory at the beginning of the study, neither sensitivity nor tolerance was evident after 6 months of THC exposure. CONCLUSIONS Because maturation of performance is later for spatial than for object working memory, these findings suggest that persistent effects of THC on cognitive abilities are more evident when exposure coincides with the developmental stage during which the underlying neural circuits are actively maturing.

A comparison of impulsivity, depressive symptoms, lifetime stress and sensation seeking in healthy controls versus participants with cocaine or methamphetamine use disorders:

Previous research has focused on developing theories of addiction that may explain behavior in cocaine- and methamphetamine-dependent individuals. The primary goal of this report was to compare and contrast the prevalence of self-reported measures of impulsivity, depression, lifetime stress and sensation-seeking in healthy controls versus individuals with cocaine or methamphetamine use disorders. Twenty-nine individuals with cocaine use disorders and 31 individuals with methamphetamine use disorders were matched with 31 healthy control participants on several demographic variables. All participants were administered behavioral questionnaires including the Barrett Impulsiveness Scale (assessing impulsivity), Beck Depression Inventory II (assessing depression), Life Stressor Checklist–Revised (assessing lifetime stress) and the Impulsive Sensation Seeking Scale (assessing sensation-seeking). When compared to healthy controls, individuals with cocaine and methamphetamine use disorders had significantly higher levels of impulsivity and sensation-seeking. In addition, when compared to healthy controls, individuals with cocaine use disorders had significantly higher Beck Depression Inventory II scores, while individuals with methamphetamine use disorders had significantly higher Life Stressor Checklist–Revised scores. The results revealed that there were significantly higher levels of impulsivity, depression and sensation-seeking in cocaine users and significantly higher impulsivity, lifetime stress and sensation-seeking in methamphetamine users when compared to healthy controls.

Endocannabinoid metabolism in the prefrontal cortex in schizophrenia

Adolescent cannabis use is associated with greater relative risk, increased symptom severity, and earlier age of onset of schizophrenia. We investigated whether this interaction may be partly attributable to disease-related disturbances in metabolism of the major cortical endocannabinoid 2-arachidonoylglycerol (2-AG). Transcript levels for the recently discovered 2-AG metabolizing enzyme, α-β-hydrolase domain 6 (ABHD6), were assessed using quantitative PCR in the prefrontal cortex of schizophrenia and healthy subjects (n=84) and antipsychotic- or tetrahydrocannabinol-exposed monkeys. ABHD6 mRNA levels were elevated in schizophrenia subjects who were younger and had a shorter illness duration but not in antipsychotic- or tetrahydrocannabinol-exposed monkeys. Higher ABHD6 mRNA levels may increase 2-AG metabolism which may influence susceptibility to cannabis in the earlier stages of schizophrenia.