Acute Amphetamine Research Articles

Amphetamine treatment affects the extra‐hypothalamic vasopressinergic system in a sex‐ and nucleus‐dependent manner

The lateral septum (LS), a brain structure implicated in addictive behaviours, regulates the activation of dopaminergic neurones in the ventral tegmental area. Vasopressinergic projections from the extended amygdala to the LS, which are sexually dimorphic, could be responsible for the vulnerability to addiction in a sex-dependent manner. The present study aimed to investigate the modulatory effects of amphetamine (AMPH) on the expression of vasopressin (AVP) in the vasopressinergic extra-hypothalamic system in sensitised male and female rats. Adult male and female Sprague-Dawley rats underwent an AMPH-locomotor sensitisation protocol. Acute AMPH increased AVP mRNA expression in the medial amygdala (MeA), whereas AMPH-induced sensitisation increased AVP mRNA expression in the bed nucleus of the stria terminalis (BNST) only in females. Interestingly, the increase in AVP expression in BNST was higher in oestrus females compared to dioestrus females and acute AMPH resulted in a decrease in AVP levels in the LS, only in males. Thus, there are complex and region-specific interactions between AMPH and the extra-hypothalamic vasopressinergic system in the brain, underlying possible alterations in different behaviours caused by acute and chronic AMPH exposure.

Naltrexone modulates dopamine release following chronic, but not acute amphetamine administration: a translational study

The opioid antagonist naltrexone has been shown to attenuate the subjective effects of amphetamine. However, the mechanisms behind this modulatory effect are currently unknown. We hypothesized that naltrexone would diminish the striatal dopamine release induced by amphetamine, which is considered an important mechanism behind many of its stimulant properties. We used positron emission tomography and the dopamine D2-receptor radioligand [11C]raclopride in healthy subjects to study the dopaminergic effects of an amphetamine injection after pretreatment with naltrexone or placebo. In a rat model, we used microdialysis to study the modulatory effects of naltrexone on dopamine levels after acute and chronic amphetamine exposure. In healthy humans, naltrexone attenuated the subjective effects of amphetamine, confirming our previous results. Amphetamine produced a significant reduction in striatal radioligand binding, indicating increased levels of endogenous dopamine. However, there was no statistically significant effect of naltrexone on dopamine release. The same pattern was observed in rats, where an acute injection of amphetamine caused a significant rise in striatal dopamine levels, with no effect of naltrexone pretreatment. However, in a chronic model, naltrexone significantly attenuated the dopamine release caused by reinstatement of amphetamine. Collectively, these data suggest that the opioid system becomes engaged during the more chronic phase of drug use, evidenced by the modulatory effect of naltrexone on dopamine release following chronic amphetamine administration. The importance of opioid-dopamine interactions in the reinforcing and addictive effects of amphetamine is highlighted by the present findings and may help to facilitate medication development in the field of stimulant dependence.

Cardiovascular Complications of Acute Amphetamine Abuse: Cross-sectional study.

This study aimed to evaluate cardiovascular complications among patients who abuse amphetamines. This cross-sectional study took place between April 2014 and April 2015 among 3,870 patients referred to the Toxicology Emergency Department of Baharlou Hospital, Tehran University of Medical Sciences, Tehran, Iran. Those with clinical signs of drug abuse and positive urine screening tests were included in the study, while cases of chronic abuse were excluded. Cardiac complications were evaluated via electrocardiography (ECG) and transthoracic echocardiography. A total of 230 patients (5.9%) had a history of acute amphetamine abuse and positive urine tests. Of these, 32 patients (13.9%) were <20 years old and 196 (85.2%) were male. In total, 119 (51.7%) used amphetamine and methamphetamine compounds while 111 (48.3%) used amphetamines with morphine or benzodiazepines. The most common ECG finding was sinus tachycardia (43.0%), followed by sinus tachycardia plus a prolonged QT interval (34.3%). Mean creatine kinase-MB and troponin I levels were 35.9 ± 4.3 U/mL and 0.6 ± 0.2 ng/mL, respectively. A total of 60 patients (26.1%) were admitted to the Intensive Care Unit. The majority (83.3%) of these patients had normal echocardiography results. The mean aortic root diameter (ARD) was 27.2 ± 2.8 mm. Abnormalities related to the ARD were found in 10 patients (16.7%), three of whom subsequently died. According to these findings, cardiac complications were common among Iranian patients who abuse amphetamines, although the majority of patients had normal echocardiography and ECG findings.

Effects of amphetamine and methylphenidate on attentional performance and impulsivity in the mouse 5-Choice Serial Reaction Time Task.

Few studies have investigated the effects of conventional attention deficit-hyperactivity disorder (ADHD) medication in the mouse 5-choice serial reaction time task (5-CSRTT), and rat studies have yielded inconsistent results. We aimed to examine the effects of acute methylphenidate (MPH) and amphetamine (AMPH) treatment in the mouse 5-CSRTT. Trained male C57Bl/6J mice were tested in a variable stimulus duration schedule. Effects of AMPH (0.25, 0.5, and 1 mg/kg) and MPH (0.5, 1.0, and 2.0 mg/kg) on discriminative accuracy, omissions, and premature responses were assessed. Saline treatment data determined high- and low-attentive (LA), and high- and low-impulsive (LI) subgroups according to the upper and lower 30th percentiles, respectively. In the LA subgroup accuracy was improved by 0.5 mg/kg AMPH and 2 mg/kg MPH, while no effect was found in the high-attentive (HA) subgroup. Premature responses were increased by 1 mg/kg AMPH and 0.5 mg/kg MPH for all animals, and by 1 mg/kg AMPH for the LI subgroup. The use of variable stimulus duration, along with the division into high- and LA, and high-and LI subgroups, may improve the sensitivity of the 5-CSRTT when investigating drug effects on attention and impulsivity.

The expression of amphetamine sensitization is dissociable from anxiety and aversive memory: Effect of an acute injection of amphetamine

The repeated administration of amphetamine can lead to locomotor sensitization. Although the repeated administration of amphetamine has been associated with anxiety and impaired working memory, it is uncertain if expression of amphetamine sensitization is associated with modifications of emotional memories. To address this issue, rats were injected once daily with amphetamine for five consecutive days (1.5mg/kg). After four days of withdrawal, rats were delivered an acute amphetamine injection to assess the expression of sensitization. A single exposure to an elevated plus maze (EPM), 24h after the last injection of amphetamine, showed that amphetamine sensitization is not accompanied by anxiety. Next, aversive memory was assessed using an 11day inter-trial interval between the EPM Trial 1 and EPM Trial 2. Rats administered with saline showed a percentage of open arms time (% OAT) in Trial 2 that was comparable to Trial 1, demonstrating a reduction in the retrieval of aversive memory. However, rats sensitized after the EPM Trial 1 showed a significant decrease in the % OAT in Trial 2. Importantly, a decrease in the % OAT in Trial 2 compared to Trial 1 was also observed after a single injection of amphetamine 24h before Trial 2. These results show a facilitation in the retrieval of aversive memory, and suggest that a previous amphetamine injection is enough to produce a protracted activation of neural circuits necessary for the retrieval of aversive memory.

Alterations in ventral and dorsal striatal allosteric A2AR-D2R receptor-receptor interactions after amphetamine challenge: Relevance for schizophrenia

Striatal dopamine D2R homodimerization is increased in the dorsal striatum after acute amphetamine challenge and in the amphetamine-induced sensitized state, a well-known animal model of schizophrenia. Therefore, it was tested if the increase in D2R homoreceptor complexes found after acute amphetamine challenge in the saline or the amphetamine sensitized state leads to changes in the antagonistic adenosine A2AR-D2R interactions in the striatum. [3H]-raclopride binding was performed in membrane preparations from the ventral and dorsal striatum involving competition with the D2R like agonist quinpirole. In the ventral striatum CGS 21680 produced a significant increase of the KiH values (p<0.05) in the amphetamine sensitized group when expressed in percent versus the corresponding values in saline sensitized rats after amphetamine challenge. However, in the dorsal striatum a significant change did not develop in the KiH values when expressed in percent of the corresponding values in saline sensitized rats after amphetamine challenge. In fact, the non-significant change was in the opposite direction towards a reduction of the KiH values. Taken together, a reduced affinity of the high affinity D2 agonist binding site (KiH value) developed in the ventral but not in the dorsal striatum as a result of increased antagonistic allosteric A2AR-D2R interactions in the amphetamine-induced sensitized state versus the saline sensitized state after an acute amphetamine challenge. The selective reappearance of antagonistic A2AR-D2R receptor-receptor interactions in the ventral striatum after amphetamine challenge in the amphetamine sensitized rat may give one possible mechanism for the atypical antipsychotic-like actions of A2AR receptor agonists.

Opposite effects of acute and chronic amphetamine on Nurr1 and NF-κB p65 in the rat ventral tegmental area

Dopamine neurons are overstimulated by drugs of abuse and suffer molecular alterations that lead to addiction behavior. Nurr1 is a transcription factor crucial for dopamine neurons survival and dopamine production, activating the transcription of key genes like tyrosine hydroxylase (TH). Interestingly, nuclear factor-kappa B (NF-κB) has emerged as a new Nurr1 partner in response to inflammatory stimulus. In this study we evaluated the effects of single and repeated amphetamine administration in the expression of Nurr1 and the NF-κB p65 subunit in the rat ventral tegmental area (VTA). We found that acute amphetamine treatment increased Nurr1, p65 and TH protein levels in the VTA. On the other hand, chronic amphetamine treatment decreased Nurr1 and p65 protein levels, but TH was unchanged. Mammalian reporter assays in cell lines showed that p65 represses Nurr1 transcriptional activity in an artificial promoter driven by Nurr1 response elements and in the native rat TH promoter. These results indicate that Nurr1 and NF-κB p65 factors are involved in the adaptive response of dopamine neurons to psychostimulants and that both transcription factors could be regulating Nurr1-dependent transactivation in the VTA.

Improvement of attention with amphetamine in low- and high-performing rats.

Attentional deficits occur in a range of neuropsychiatric disorders, such as schizophrenia and attention deficit hyperactivity disorder. Psychostimulants are one of the main treatments for attentional deficits, yet there are limited reports of procognitive effects of amphetamine in preclinical studies. Therefore, task development may be needed to improve predictive validity when measuring attention in rodents. This study aimed to use a modified signal detection task (SDT) to determine if and at what doses amphetamine could improve attention in rats. Sprague-Dawley rats were trained on the SDT prior to amphetamine challenge (0.1, 0.25, 0.75 and 1.25mg/kg). This dose range was predicted to enhance and disrupt cognition with the effect differing between individuals depending on baseline performance. Acute low dose amphetamine (0.1 and 0.25mg/kg) improved accuracy, while the highest dose (1.25mg/kg) significantly disrupted performance. The effects differed for low- and high-performing groups across these doses. The effect of amphetamine on accuracy was found to significantly correlate with baseline performance in rats. This study demonstrates that improvement in attentional performance with systemic amphetamine is dependent on baseline accuracy in rats. Indicative of the inverted U-shaped relationship between dopamine and cognition, there was a baseline-dependent shift in performance with increasing doses of amphetamine. The SDT may be a useful tool for investigating individual differences in attention and response to psychostimulants in rodents.

High fat diet augments amphetamine sensitization in mice: Role of feeding pattern, obesity, and dopamine terminal changes

High fat (HF) diet-induced obesity has been shown to augment behavioral responses to psychostimulants that target the dopamine system. The purpose of this study was to characterize dopamine terminal changes induced by a HF diet that correspond with enhanced locomotor sensitization to amphetamine. C57BL/6J mice had limited (2hr 3 d/week) or extended (24 h 7 d/week) access to a HF diet or standard chow for six weeks. Mice were then repeatedly exposed to amphetamine (AMPH), and their locomotor responses to an amphetamine challenge were measured. Fast scan cyclic voltammetry was used to identify changes in dopamine terminal function after AMPH exposure. Exposure to a HF diet reduced dopamine uptake and increased locomotor responses to acute, high-dose AMPH administration compared to chow fed mice. Microdialysis showed elevated extracellular dopamine in the nucleus accumbens (NAc) coincided with enhanced locomotion after acute AMPH in HF-fed mice. All mice exhibited locomotor sensitization to amphetamine, but both extended and limited access to a HF diet augmented this response. Neither HF-fed group showed the robust amphetamine sensitization-induced increases in dopamine release, reuptake, and amphetamine potency observed in chow fed animals. However, the potency of amphetamine as an uptake inhibitor was significantly elevated after sensitization in mice with extended (but not limited) access to HF. Conversely, after amphetamine sensitization, mice with limited (but not extended) access to HF displayed reduced autoreceptor sensitivity to the D2/D3 agonist quinpirole. Additionally, we observed reduced membrane dopamine transporter (DAT) levels after HF, and a shift in DAT localization to the cytosol was detected with limited access to HF. This study showed that different patterns of HF exposure produced distinct dopamine terminal adaptations to repeated AMPH, which differed from chow fed mice, and enhanced sensitization to AMPH. Locomotor sensitization in chow fed mice coincided with elevated DAT function and increased AMPH potency; however, the enhanced behavioral response to AMPH after HF exposure was unique in that it coincided with reduced DAT function and diet pattern-specific adaptations.

Vitamin D3: A Role in Dopamine Circuit Regulation, Diet-Induced Obesity, and Drug Consumption.

The influence of micronutrients on dopamine systems is not well defined. Using mice, we show a potential role for reduced dietary vitamin D3 (cholecalciferol) in promoting diet-induced obesity (DIO), food intake, and drug consumption while on a high fat diet. To complement these deficiency studies, treatments with exogenous fully active vitamin D3 (calcitriol, 10 µg/kg, i.p.) were performed. Nondeficient mice that were made leptin resistant with a high fat diet displayed reduced food intake and body weight after an acute treatment with exogenous calcitriol. Dopamine neurons in the midbrain and their target neurons in the striatum were found to express vitamin D3 receptor protein. Acute calcitriol treatment led to transcriptional changes of dopamine-related genes in these regions in naive mice, enhanced amphetamine-induced dopamine release in both naive mice and rats, and increased locomotor activity after acute amphetamine treatment (2.5 mg/kg, i.p.). Alternatively, mice that were chronically fed either the reduced D3 high fat or chow diets displayed less activity after acute amphetamine treatment compared with their respective controls. Finally, high fat deficient mice that were trained to orally consume liquid amphetamine (90 mg/L) displayed increased consumption, while nondeficient mice treated with calcitriol showed reduced consumption. Our findings suggest that reduced dietary D3 may be a contributing environmental factor enhancing DIO as well as drug intake while eating a high fat diet. Moreover, these data demonstrate that dopamine circuits are modulated by D3 signaling, and may serve as direct or indirect targets for exogenous calcitriol.

Opposing effects of acute and chronic d-amphetamine on decision-making in rats

Amphetamine and other drugs of abuse have both short-term and long-lasting effects on brain function, and drug sensitization paradigms often result in chronic impairments in behavioral flexibility. Here we show that acute amphetamine administration temporarily renders rats less sensitive to reward omission, as revealed by a decrease in lose-shift responding during a binary choice task. Intracerebral infusions of amphetamine into the ventral striatum did not affect lose-shift responding but did increase impulsive behavior in which rats chose to check both reward feeders before beginning the next trial. In contrast to acute systemic and intracerebral infusions, sensitization through repeated exposure induced long-lasting increased sensitivity to reward omission. These treatments did not affect choices on trials following reward delivery (i.e. win-stay responding), and sensitization increased spine density in the sensorimotor striatum. The dichotomous effects of amphetamine on short-term and long-term loss sensitivity, and the null effect on win-stay responding, are consistent with a shift of behavioral control to the sensorimotor striatum after drug sensitization. These data provide a new demonstration of such a shift in a novel task unrelated to drug administration, and suggests that the dominance of sensorimotor control persists over many hundreds of trials after sensitization.

Brain Angiotensin II AT1 receptors are involved in the acute and long-term amphetamine-induced neurocognitive alterations.

Angiotensin II, by activation of its brain AT1-receptors, plays an active role as neuromodulator in dopaminergic transmission. These receptors participate in the development of amphetamine-induced behavioral and dopamine release sensitization. Dopamine is involved in cognitive processes and provides connectivity between brain areas related to these processes. Amphetamine by its mimetic activity over dopamine neurotransmission elicits differential responses after acute administration or after re-exposure following long-term withdrawal periods in different cognitive processes. The purpose of this study is to evaluate the AT1-receptor involvement in the acute and long-term amphetamine-induced alterations in long-term memory and in cellular-related events. Male Wistar rats (250-300g) were used in this study. Acute effects: Amphetamine (0.5/2.5mg/kgi.p.) was administered after post-training in the inhibitory avoidance (IA) response. The AT1-receptor blocker Losartan was administered i.c.v. before a single dose of amphetamine (0.5mg/kgi.p.). Long-term effects: The AT1-receptors blocker Candesartan (3mg/kg p.o.) was administered for 5days followed by 5 consecutive days of amphetamine (2.5mg/kg/day, i.p.). The neuroadaptive changes were evidenced after 1week of withdrawal by an amphetamine challenge (0.5mg/kgi.p.). The IA response, the neuronal activation pattern, and the hippocampal synaptic transmission were evaluated. The impairing effect in the IA response of post-training acute amphetamine was partially prevented by Losartan. The long-term changes induced by repeated amphetamine (resistance to acute amphetamine interference in the IA response, neurochemical altered response, and increased hippocampal synaptic transmission) were prevented by AT1-receptors blockade. AT1-receptors are involved in the acute alterations and in the neuroadaptations induced by repeated amphetamine associated with neurocognitive processes.

Amphetamine activates Rho GTPase signaling to mediate dopamine transporter internalization and acute behavioral effects of amphetamine

Acute amphetamine (AMPH) exposure elevates extracellular dopamine through a variety of mechanisms that include inhibition of dopamine reuptake, depletion of vesicular stores, and facilitation of dopamine efflux across the plasma membrane. Recent work has shown that the DAT substrate AMPH, unlike cocaine and other nontransported blockers, can also stimulate endocytosis of the plasma membrane dopamine transporter (DAT). Here, we show that when AMPH enters the cytoplasm it rapidly stimulates DAT internalization through a dynamin-dependent, clathrin-independent process. This effect, which can be observed in transfected cells, cultured dopamine neurons, and midbrain slices, is mediated by activation of the small GTPase RhoA. Inhibition of RhoA activity with C3 exotoxin or a dominant-negative RhoA blocks AMPH-induced DAT internalization. These actions depend on AMPH entry into the cell and are blocked by the DAT inhibitor cocaine. AMPH also stimulates cAMP accumulation and PKA-dependent inactivation of RhoA, thus providing a mechanism whereby PKA- and RhoA-dependent signaling pathways can interact to regulate the timing and robustness of AMPH's effects on DAT internalization. Consistent with this model, the activation of D1/D5 receptors that couple to PKA in dopamine neurons antagonizes RhoA activation, DAT internalization, and hyperlocomotion observed in mice after AMPH treatment. These observations support the existence of an unanticipated intracellular target that mediates the effects of AMPH on RhoA and cAMP signaling and suggest new pathways to target to disrupt AMPH action.

Emotional traits predict individual differences in amphetamine-induced positive mood in healthy volunteers.

Previous research on emotional correlates of individual differences in subjective responses to D-amphetamine has focused on relatively broad personality traits. Yet, emotional functioning is best characterized by several narrow subcomponents, each of which may contribute uniquely to amphetamine response. Here, we examine several specific subdomains of emotional functioning in relation to acute amphetamine response. At a baseline session, healthy stimulant-naive volunteers (N = 97) completed measures of several subdomains of baseline trait emotional functioning and then completed two counterbalanced experimental sessions during which they received a single oral dose of 20mg D-amphetamine or placebo. Acute subjective drug response measures were completed at repeated intervals before and after drug administration. Data from subjective measures that were significantly modulated by amphetamine were reduced using principal component analysis (amphetamine or placebo) into three higher-order factors of "positive mood," "arousal," and "drug high." Amphetamine did not significantly alter any "negative" subjective states. Separate multiple regression analyses were conducted regressing these three drug factors on baseline trait emotional functioning scales. The combined set of trait emotional functioning indicators accounted for approximately 22% of the variance in acute amphetamine-induced positive mood changes. Greater anticipatory pleasure and greater anxious distress each uniquely predicted greater amphetamine-induced positive mood. Trait emotional functioning did not significantly predict amphetamine-induced changes in arousal or drug high. Emotional traits appear to moderate drug-induced positive mood but not other dimensions of amphetamine effects. Different facets of emotional functioning may differentially modulate amphetamine's subjective effect profile.

Brain Region-Specific Trafficking of the Dopamine Transporter.

The dopamine transporter (DAT) is the key regulator of the dopamine neurotransmission in the CNS. In the present study, we developed a new approach for studying DAT localization and dynamics in intact neurons in acute sagittal brain slices from the knock-in mouse expressing epitope-tagged DAT. For the first time, the fluorescence imaging analysis of DAT was combined with the immunogold labeling of DAT and quantitative electron microscopy. In contrast to numerous studies of DAT trafficking in heterologous expression systems and dissociated cultured neurons, studies in intact neurons revealed a surprisingly low amount of endocytic trafficking of DAT at steady state and after acute amphetamine treatment and suggested that non-vesicular transport could be the main mechanism establishing DAT distribution within the dopaminergic neuron.

Withdrawal from Acute Amphetamine Induces an Amygdala-Driven Attenuation of Dopamine Neuron Activity: Reversal by Ketamine.

Drug addiction is a chronic disorder characterized by a cycle composed of drug seeking, intoxication with drug taking and withdrawal associated with negative affect. Numerous studies have examined withdrawal/negative affect after chronic use; however, very few have examined the effect of acute administration on the negative affective state after acute drug withdrawal. One dose of amphetamine was injected into Sprague-Dawley rats. Despair behavior using the modified forced swim test (FST) and dopamine (DA) activity in the ventral tegmental area using in vivo electrophysiological recordings were studied 18, 48 and 72 h after injection of amphetamine. The effects of inactivation of the basolateral amygdala (BLA) and ketamine administration on VTA DA neuron activity and passivity in the modified FST were examined. Eighteen hours following amphetamine withdrawal, there was a substantial decrease in the number of active DA neurons, as well as an increase in time spent immobile in the modified FST, which returned to baseline after 72 h. Inactivation of the BLA after acute amphetamine prevented the decrease in DA neuron tonic activity. Injection of ketamine also prevented the decrease in DA population activity but had no effect on immobility measured in the modified FST. The data support a model in which the negative affective state following acute amphetamine withdrawal is associated with a decrease in DA neuron population activity, driven by hyperactivity of the BLA. Although ketamine reversed the hypodopaminergic state following withdrawal, the failure to reduce immobility in the modified FST indicates that different processes underlying negative emotional state may exist between depression and drug withdrawal.

Physiological and behavioral effects of amphetamine in BACE1(-/-) mice.

β-Site APP-cleaving Enzyme 1 (BACE1) is a protease that has been linked to schizophrenia, a severe mental illness that is potentially characterized by enhanced dopamine (DA) release in the striatum. Here, we used acute amphetamine administration to stimulate neuronal activity and investigated the neurophysiological and locomotor-activity response in BACE1-deficient (BACE1(-/-) ) mice. We measured locomotor activity at baseline and after treatment with amphetamine (3.2 and 10 mg/kg). While baseline locomotor activity did not vary between groups, BACE1(-/-) mice exhibited reduced sensitivity to the locomotor-enhancing effects of amphetamine. Using high-performance liquid chromatography (HPLC) to measure DA and DA metabolites in the striatum, we found no significant differences in BACE1(-/-) compared with wild-type mice. To determine if DA neuron excitability is altered in BACE1(-/-) mice, we performed patch-clamp electrophysiology in putative DA neurons from brain slices that contained the substantia nigra. Pacemaker firing rate was slightly increased in slices from BACE1(-/-) mice. We next measured G protein-coupled potassium currents produced by activation of D2 autoreceptors, which strongly inhibit firing of these neurons. The maximal amplitude and decay times of D2 autoreceptor currents were not altered in BACE1(-/-) mice, indicating no change in D2 autoreceptor-sensitivity and DA transporter-mediated reuptake. However, amphetamine (30 µm)-induced potassium currents produced by efflux of DA were enhanced in BACE1(-/-) mice, perhaps indicating increased vesicular DA content in the midbrain. This suggests a plausible mechanism to explain the decreased sensitivity to amphetamine-induced locomotion, and provides evidence that decreased availability of BACE1 can produce persistent adaptations in the dopaminergic system.

Integration of neural networks activated by amphetamine in females with different estrogen levels: A functional imaging study in awake rats

Previous studies demonstrate that schizophrenia symptomatology in women is dependent upon estrogen levels. Estrogen has beneficial properties when administered in conjunction with antipsychotics, and estrogen also alters, in rats, dopamine neurotransmission, which is a common target of all antipsychotic medications, suggesting a possible interaction between the two. The aim of the current study was to investigate this possible interaction using functional magnetic resonance imaging in awake, female rats. Amphetamine-sensitized, ovariectomized rats receiving no, chronic low, or phasic high levels of estradiol replacement were used, and changes in blood-oxygen-level-dependent (BOLD) signal were recorded over time in response to an acute amphetamine injection. Increasing levels of estradiol enhanced BOLD activation in pathways previously known to be implicated in schizophrenia symptomatology, such as the mesocorticolimbic, habenular and olfactory pathways, as well as more widespread areas. We propose here the first comprehensive "amphetamine activation map" integrating brain regions where amphetamine-related BOLD activity is influenced by estrogen levels in sensitized female rats.

Dynamic increases in AMPA receptor phosphorylation in the rat hippocampus in response to amphetamine

Increasing evidence supports the critical role of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptors in psychostimulant action. These receptors are regulated via a phosphorylation-dependent mechanism in their trafficking, distribution, and function. The hippocampus is a brain structure important for learning and memory and is emerging as a critical site for processing psychostimulant effects. To determine whether the hippocampal pool of AMPA receptors is regulated by stimulants, we investigated and characterized the impact of amphetamine (AMPH) on phosphorylation of AMPA receptors in the adult rat hippocampus in vivo. We found that AMPH markedly increased phosphorylation of AMPA receptor GluA1 subunits at serine 845 (S845) in the hippocampus. The effect of AMPH was dose dependent. A single dose of AMPH induced a rapid and transient increase in S845 phosphorylation. Among different hippocampal subfields, AMPH primarily elevated S845 phosphorylation in the Cornu Ammonis area 1 and dentate gyrus. In contrast to S845, serine 831 phosphorylation of GluA1 and serine 880 phosphorylation of GluA2 were not altered by AMPH. In addition, surface expression of hippocampal GluA1 was up-regulated, while the amount of intracellular GluA1 fraction was concurrently reduced in response to AMPH. GluA2 protein levels in either the surface or intracellular pool were insensitive to AMPH. These data demonstrate that the AMPA receptor in the hippocampus is sensitive to dopamine stimulation. Acute AMPH administration induces dose-, time-, site-, and subunit-dependent phosphorylation of AMPA receptors and facilitates surface trafficking of GluA1 AMPA receptors in hippocampal neurons in vivo. Acute injection of amphetamine increased phosphorylation of α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor GluA1 subunits at a protein kinase A (PKA)-sensitive site (S845) in the rat hippocampus. This increase was dose- and time-dependent and correlated with an increase in surface GluA1 expression. Thus, amphetamine can upregulate GluA1 phosphorylation and surface trafficking of GluA1 in hippocampal neurons in vivo.

Quantification of [(11)C]yohimbine binding to α2 adrenoceptors in rat brain in vivo.

We quantified the binding potentials (BPND) of [11C]yohimbine binding in rat brain to alpha-2 adrenoceptors to evaluate [11C]yohimbine as an in vivo marker of noradrenergic neurotransmission and to examine its sensitivity to the level of noradrenaline. Dual [11C]yohimbine dynamic positron emission tomography (PET) recordings were applied to five Sprague Dawley rats at baseline, followed by acute amphetamine administration (2 mg/kg) to induce elevation of the endogenous level of noradrenaline. The volume of distribution (VT) of [11C]yohimbine was obtained using Logan plot with arterial plasma input. Because alpha-2 adrenoceptors are distributed throughout the brain, the estimation of the BPND is complicated by the absence of an anatomic region of no displaceable binding. We used the Inhibition plot to acquire the reference volume, VND, from which we calculated the BPND. Acute pharmacological challenge with amphetamine induced a significant decline of [11C]yohimbine BPND of ~38% in all volumes of interest. The BPND was greatest in the thalamus and striatum, followed in descending order by, frontal cortex, pons, and cerebellum. The experimental data demonstrate that [11C]yohimbine binding is sensitive to a challenge known to increase the extracellular level of noradrenaline, which can benefit future PET investigations of pathologic conditions related to disrupted noradrenergic neurotransmission.