Selective 6OHDA-induced destruction of mesolimbic dopamine neurons: abolition of psychostimulant-induced locomotor activity in rats

The Effect of Chronic Treatment with the Inhibitor of Phosphodiesterase 5 (PDE5), Sildenafil, in Combination with L-DOPA on Asymmetric Behavior and Monoamine Catabolism in the Striatum and Substantia Nigra of Unilaterally 6-OHDA-Lesioned Rats

The use of phosphodiesterase inhibitors in the treatment of Parkinson's disease is currently widely discussed. The study aimed to investigate the impact of acute and chronic treatment with the phosphodiesterase 5 inhibitor, sildenafil, at low and moderate doses of 2 mg/kg and 6 mg/kg, and L-DOPA (12.5 mg/kg), alone or in combination, on asymmetric behavior and dopamine (DA) and serotonin metabolism in the striatum and substantia nigra of unilaterally 6-OHDA-lesioned rats. Acute administration of sildenafil at both tested doses jointly with L-DOPA significantly increased the number of contralateral rotations during a 2 h measurement compared to L-DOPA alone. The effect of a lower dose of sildenafil combined with L-DOPA was much greater in the second hour of measurement. However, the acute combined administration of a higher dose of sildenafil with L-DOPA resulted in an immediate and much stronger increase in the number of contralateral rotations compared to L-DOPA alone, already visible in the first hour of measurement. Interestingly, the chronic combined administration of 2 mg/kg of sildenafil and L-DOPA significantly reduced the number of contralateral rotations, especially during the first hour of measurement, compared to the long-term treatment with L-DOPA alone. Such an effect was not observed after the long-term combined treatment of a higher dose of sildenafil and L-DOPA compared to L-DOPA alone. The concentration of DA in the ipsilateral striatum and substantia nigra after the last combined chronic dose of sildenafil (2 or 6 mg/kg) and L-DOPA (12.5 mg/kg) was significantly higher than after L-DOPA alone. In spite of much stronger increases in the DA concentration in the ipsilateral striatum and substantia nigra, the number of contralateral rotations was reduced in the group of rats treated with the combination of 2 mg/kg sildenafil and L-DOPA compared to the group receiving L-DOPA alone. Moreover, the combined treatment with a low dose of sildenafil and L-DOPA had an opposite effect on DA catabolism, as assessed by DOPAC/DA and HVA/DA indexes, and these indexes were reduced in the ipsilateral striatum but increased in the contralateral striatum and substantia nigra compared to the treatment with L-DOPA alone. The results of the present study show that the addition of a low dose of a PDE5 inhibitor to the standard L-DOPA therapy differently modulates rotational behavior, the tissue DA concentration and its catabolism in the striatum and substantia nigra.

Preclinical evaluation of [18F]FP-CIT, the radiotracer targeting dopamine transporter for diagnosing Parkinson's disease: pharmacokinetic and efficacy analysis

BACKGROUND

N-(3-fluoropropyl)-2β-carboxymethoxy-3β-(4-iodophenyl) nortropane (FP-CIT), the representative cocaine derivative used in dopamine transporter imaging, is a promising biomarker, as it reflects the severity of Parkinson's disease (PD). 123I- and 18F-labeled FP-CIT has been used for PD diagnosis. However, preclinical studies evaluating [18F]FP-CIT as a potential diagnostic biomarker are scarce. Among translational research advancements from bench to bedside, translating preclinical findings into clinical practice is one-directional. The aim of this study is to employ a circular approach, beginning back from the preclinical stage, progressing to the supplementation of [18F]FP-CIT, and subsequently returning to clinical application. We investigated the pharmacokinetic properties of [18F]FP-CIT and its efficacy for PD diagnosis using murine models.

RESULTS

Biodistribution, metabolite and excretion analyses were performed in mice and PD models were induced in rats using 6-hydroxydopamine (6-OHDA). The targeting efficiency of [18F]FP-CIT for the dopamine receptor was assessed through animal PET/CT imaging. Subsequently, correlation analysis was conducted between animal PET/CT imaging results and immunohistochemistry (IHC) targeting tyrosine hydroxylase. Rapid circulation was confirmed after [18F]FP-CIT injection. [18F]FP-CIT reached the highest uptake of 23.50 ± 12.46%ID/g in the striatum 1 min after injection, and it was rapidly excreted within 60 min. The major metabolic organs of [18F]FP-CIT were confirmed to be the intestines, liver, and kidneys. Its uptake in the intestine was approximately 5% ID/g. The uptake in the liver gradually increased, with excretion beginning after reaching a maximum after 60 min. The kidneys exhibited rapid elimination after 10 min. In the excretion study, rapid elimination was verified, with 21.46 ± 9.53% of the compound excreted within a 6 h period. Additionally, the efficacy of [18F]FP-CIT PET was demonstrated in the PD model, with a high correlation with IHC for both the absolute value (R = 0.803, p = 0.0017) and the ratio value (R = 0.973, p = 0.0011).

CONCLUSIONS

This study fills the gap regarding insufficient preclinical studies on [18F]FP-CIT, including its ADME, metabolites, and efficiency. The pharmacological results, including accurate diagnosis, rapid circulation, and [18F]FP-CIT excretion, provide complementary evidence that [18F]FP-CIT can be used safely and efficiently to diagnose PD in clinics, although it is already used in clinics.

3-aminohydantoin derivate as a promising scaffold in dopaminergic neuroprotection and neurorescue in the in vivo and in vitro 6-hydroxydopamine models of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by a progressive loss of dopaminergic neurons in the substantia nigra, for which no disease-modifying treatments are available yet. Thus, developing new neuroprotective drugs with the potential to delay or stop the natural course of the disease is necessary. The aim of the present study was to evaluate the neuroprotective effects of a newly synthesized 3-aminohydantoin derivative named 3-amino-5-benzylimidazolidine-2,4-dione (PHAH). The possible neuroprotective and neurorescue effects of the synthesized compound were tested: (i) in N27 dopaminergic and BV-2 microglial cell lines treated with 6-hydroxydopamine (6-OHDA) and (ii) in the 6-OHDA rat model of PD. PHAH administration reduced proinflammatory markers, including nitric oxide synthase and interleukin-1β, in BV-2 cells activated by lipopolysaccharide. Although PHAH did not restore cell death induced by 6-OHDA, it was not cytotoxic for dopaminergic cells since cell viability, under the effect of the two concentrations, remained comparable to that of the control cells. Most interestingly, PHAH restored 6-OHDA-induced dopaminergic neurodegeneration in the substantia nigra and striatum and ameliorated 6-OHDA-induced oxidative stress in the rat brain. In summary, we have proven that in PD models, PHAH has neuroprotective effects in vivo and anti-inflammatory effects in vitro; however, these effects remain to be confirmed by carrying out certain specific behavioural tests as well as by exploring other neuroinflammatory markers. The present work also suggests that PHAH is a promising scaffold that can serve as the basis for the design and synthesis of other derivatives that can be potent antiparkinsonian agents.

The medial preoptic area and acute cocaine's stimulant effects in rats: Potential influences of estradiol and biological sex

The medial preoptic area (mPOA) in the hypothalamus is an important integrator of neuroendocrine signaling and a key regulator of both natural and drug-induced reward. Although the mPOA modulates sex differences in other behaviors, whether it also modulates sex differences in cocaine response remains unclear. To help us better understand the mPOA's role in sex differences associated with cocaine response, we examined cocaine-induced changes in locomotion and neural activity in the mPOA of male and female rats. In addition, neural activity in the striatum, a brain area known to be involved in cocaine response, was examined for comparison purposes. Fos, the protein product of the immediate early gene c-fos, was used as the marker of neural activity. Locomotion chambers were used to measure behavior, radioimmunoassays and vaginal lavages were used to determine hormonal status, and immunohistochemical assays were used to quantify Fos. To account for the effects of gonadal hormones, rats were left gonadally intact and categorized as either 'low-estradiol' or 'high-estradiol' based on their hormonal status on test day. Results indicate that high-estradiol females experienced greater cocaine-induced mPOA Fos-immunoreactivity (Fos-ir) and displayed greater cocaine-induced locomotion than low estradiol females. Conversely, high-estradiol males experienced less cocaine-induced mPOA Fos-ir and displayed less cocaine-induced locomotion than low-estradiol males. Cocaine-induced Fos-ir in the mPOA also correlated with cocaine-induced Fos-ir in areas of the striatum already associated with cocaine response. These findings further support the mPOA's role in the endocrine-mediated response to cocaine. It also identifies the mPOA as a contributor to sex differences in cocaine response and potential differences in vulnerability to developing cocaine use disorders.

Protective Role of Lactobacillus rhamnosus Probiotic in Reversing Cocaine-Induced Oxidative Stress, Glial Activation and Locomotion in Mice

Cocaine abuse is known to cause inflammation, oxidative injury and alterations in the gut microbiota. Although emerging studies have demonstrated the role of gut microbiota in modulating neurological complications and behavior, the mechanism(s) underlying these processes remain unclear. In the present study, we investigated the protective effect of Lactobacillus rhamnosus probiotic on cocaine-induced oxidative stress, glial activation, and locomotion in mice. In this study, groups of male C56BL6 mice were administered gut-resident commensal bacteria L. rhamnosus probiotic (oral gavage) concurrently with cocaine (20 mg/kg, i.p.) or saline for 28 days and assessed for oxidative stress and cellular activation in both the gut and brain as well as alterations in locomotion behavior. Cocaine-induced gut dysregulation was associated with increased formation of 4-hydroxynonenal (4-HNE) adducts, increased expression of pERK-1/2, pNF-kB-p65 and antioxidant mediators (SOD1, GPx1). In cocaine administered mice, there was increased activation of both microglia and astrocytes in the striatum and cortex of the brain as shown by enhanced expression of CD11b and GFAP, respectively. Cocaine administration also resulted in increased locomotor activity in the open field test in these mice. Administration of L. rhamnosus attenuated cocaine-induced gut oxidative stress and inflammation as well as glial activation and locomotion. These results suggest the potential of microbial-based interventions to attenuate cocaine-mediated behavioral responses and neuroinflammation, in addition to systemic inflammation and oxidative damage.

A high-affinity cocaine binding site associated with the brain acid soluble protein 1

Cocaine exerts its stimulant effect by inhibiting dopamine (DA) reuptake, leading to increased dopamine signaling. This action is thought to reflect the binding of cocaine to the dopamine transporter (DAT) to inhibit its function. However, cocaine is a relatively weak inhibitor of DAT, and many DAT inhibitors do not share cocaine’s behavioral actions. Further, recent reports show more potent actions of the drug, implying the existence of a high-affinity receptor for cocaine. We now report high-affinity binding of cocaine associated with the brain acid soluble protein 1 (BASP1) with a dissociation constant (Kd) of 7 nM. Knocking down BASP1 in the striatum inhibits [3H]cocaine binding to striatal synaptosomes. Depleting BASP1 in the nucleus accumbens but not the dorsal striatum diminishes locomotor stimulation in mice. Our findings imply that BASP1 is a pharmacologically relevant receptor for cocaine.

Sex Differences in Psychosis: Focus on Animal Models

Most psychiatric illnesses, such as schizophrenia, show profound sex differences in incidence, clinical presentation, course, and outcome. Fortunately, more recently the literature on sex differences and (to a lesser extent) effects of sex steroid hormones is expanding, and in this review we have focused on such studies in psychosis, both from a clinical/epidemiological and preclinical/animal model perspective. We begin by briefly describing the clinical evidence for sex differences in schizophrenia epidemiology, symptomatology, and pathophysiology. We then detail sex differences and sex hormone effects in behavioral animal models of psychosis, specifically psychotropic drug-induced locomotor hyperactivity and disruption of prepulse inhibition. We expand on the preclinical data to include developmental and genetic models of psychosis, such as the maternal immune activation model and neuregulin transgenic animals, respectively. Finally, we suggest several recommendations for future studies, in order to facilitate a better understanding of sex differences in the development of psychosis.

Dopaminergic denervation of the globus pallidus produces short-memory impairment in rats

Rats with low-level globus pallidus (GP) dopaminergic denervation can develop anxiety without any motor alterations. The aim of this study was to evaluate the effect of low-level 6-OHDA-induced unilateral and bilateral GP lesions in male Wistar rats (n = 8/group) on recognition memory, motor activity, and the number of TH+ neurons in the SNc. For unilateral- and bilateral-lesioned animals, there was a significant decrease in the number of TH+ neurons (27% and 42%, respectively) and in the object, location, and temporal order discrimination indexes of recognition memory tests. Motor activity was unaffected. Thus, GP dopamine denervation was detrimental to short-memory.

Effects of the selective dopamine D3 receptor antagonist PG01037 on morphine-induced hyperactivity and antinociception in mice

Recent preclinical studies have reported that pretreatment with the novel and highly-selective dopamine D₃ receptor (D₃R) antagonists R-VK4-40 or VK4-116 attenuates the abuse-related behavioral effects of oxycodone while enhancing its analgesic properties. However, whether these observed effects are generalizable to the broad class of D₃R antagonists and/or extend to opioids other than oxycodone has not been extensively explored. The present study sought to assess the impact of pretreatment with another selective D₃R antagonist, PG01037, on several behavioral effects of morphine in mice. C57Bl/6 J mice were pretreated with PG01037 (0-10 mg/kg) and tested for 1) hyperlocomotion induced by acute morphine (5.6-56 mg/kg), 2) locomotor sensitization following repeated morphine (56 mg/kg), 3) antinociception following acute morphine (18 mg/kg), and 4) catalepsy following administration of PG01037 alone or in combination with morphine (56 mg/kg). PG01037 dose-dependently attenuated morphine-induced hyperlocomotion and morphine-induced antinociception at doses that did not alter basal locomotion or nociception alone, but did not prevent the induction of locomotor sensitization following repeated morphine administration. Moreover, PG01037 did not induce catalepsy either alone or in combination with morphine. These results suggest that attenuation of acute opioid-induced hyperactivity may be a behavioral effect shared among D₃R-selective antagonists, thus supporting continued investigations into their use as potential treatments for opioid use disorder. However, PG01037 is unlike newer, highly-selective D₃R antagonists in its capacity to reduce opioid-induced antinociception, indicating that modulation of opioid analgesia may vary across different D₃R antagonists.

Dynamic dichotomy of accumbal population activity underlies cocaine sensitization

Locomotor sensitization (LS) is an early behavioral adaptation to addictive drugs, driven by the increase of dopamine in the Nucleus Accumbens (NAc). However, the effect on accumbal population activity remains elusive. Here, we used single-cell calcium imaging in mice to record the activity of dopamine-1-receptor (D1R) and dopamine-2-receptor (D2R) expressing spiny projection neurons (SPNs) during cocaine LS. Acute exposure to cocaine elevated D1R SPN activity and reduced D2R SPN activity, albeit with high variability between neurons. During LS, the number of D1R and D2R neurons responding in opposite directions increased. Moreover, preventing LS by inhibition of the ERK signaling pathway decreased the number of cocaine responsive D1R SPNs, but had little effect on D2R SPNs. These results indicate that accumbal population dichotomy is dynamic and contains a subgroup of D1R SPNs that eventually drives LS. Insights into the drug-related activity dynamics provides a foundation for understanding the circuit-level addiction pathogenesis.

FFAR1/GPR40 Contributes to the Regulation of Striatal Monoamine Releases and Facilitation of Cocaine-Induced Locomotor Activity in Mice

The free fatty acid receptor 1 (FFAR1) is suggested to function as a G protein-coupled receptor (GPR40) for medium-to-long-chain free fatty acids. Previous studies on the expression of FFAR1 revealed that the nigrostriatal region is one of the areas which express abundant FFAR1 mRNA/protein in the central nervous system (CNS). However, the role of FFAR1 in the CNS has been still largely unclarified. Here, we examined a possible functional role of FFAR1 in the control of extracellular concentrations of striatal monoamines and cocaine-induced locomotor activity. Microdialysis analysis revealed that the basal level of extracellular dopamine (DA) was significantly elevated, while the basal serotonin (5-HT) level tended to be reduced in the striatum of FFAR1 knockout (-/-) mice. Interestingly, local application of a FFAR1 agonist, GW9508, markedly augmented the striatal 5-HT release in FFAR1 wild-type (+/+) mice, whereas topical application of a FFAR1 antagonist, GW1100, significantly reduced the 5-HT release. However, the enhanced 5-HT release was completely lost in -/- mice. Although acute administration of cocaine enhanced the locomotor activity in both +/+ and -/- mice, the magnitude of the enhancement was significantly reduced in -/- mice. In addition, intraperitoneal injection of GW1100 significantly decreased the cocaine-induced locomotor enhancement. These results suggest that FFAR1 has a facilitatory role in striatal 5-HT release, and the evoked 5-HT release might contribute to enhance cocaine-induced locomotor activity.

Altered hedonic, novelty-, stress- and D-amphetamine-induced response due to social isolation in peripuberty

Reduction in direct social contact with peers during early adolescence is thought to be a risk factor for an increase in depressive symptoms, but there is still no clear evidence to suggest early behavioral manifestations and their association with the later outcome of social distancing during this period. To address this question, we used social isolation paradigm in peripubertal rats as the rodent model of adolescence. The litter was an experimental unit. On postnatal day 29, each litter gave group-housed and single-housed males, which were reared and tested one week and two weeks thereafter. Psychomotor/emotional response to novelty in exploration-based tasks, behavioral and neuronal responses to the drug reward (D-amphetamine), motivation/hedonic behavior, physiological and response to physiological stress were examined. Social isolation in peripubertal rats manifested through: hyper-reactivity/agitation and the state anxiety/risk-taking at an early stage; reduced behavioral response to D-amphetamine and altered neural processing of this stimulus, at a later stage; consummatory hypohedonia that deepened over time without changing the motivation to eat; unchanged body weight gain and resting blood corticosterone, cortisol and glucose levels over time; altered blood biochemistry (silenced corticosterone and increased glucose) due to overnight fasting only at an early stage. Our results highlight that the outcome of reduced direct social contact with peers during peripuberty is dynamic, with the cluster of atypical early symptoms that evolve into the syndrome that is delicate for assessment through routinely measurable behavior and biomarkers of stress, but with progressive consummatory hypohedonia and unaffected motivation to eat as stable marks.

Akathisia and Restless Legs Syndrome: Solving the Dopaminergic Paradox

Akathisia is an urgent need to move that is associated with treatment with dopamine receptor blocking agents (DRBAs) and with restless legs syndrome (RLS). The pathogenetic mechanism of akathisia has not been resolved. This article proposes that it involves an increased presynaptic dopaminergic transmission in the ventral striatum and concomitant strong activation of postsynaptic dopamine D₁ receptors, which form complexes (heteromers) with dopamine D₃ and adenosine A₁ receptors. It also proposes that in DRBA-induced akathisia, increased dopamine release depends on inactivation of autoreceptors, whereas in RLS it depends on a brain iron deficiency-induced down-regulation of striatal presynaptic A₁ receptors.

Neuroanatomy and Neurochemical Mechanisms of Time-Dependent Sensitization

Time-dependent sensitization (TDS) is a phenomenon described in rodents as an enhancement in the behavioral and neurochemical responses to intermittent exposure to psychostimulant drugs. Time-dependent sensitization also occurs after repeated encounters with environmental stress. Several features of TDS parallel those of multiple chemical sensitivity (MCS) in humans, and these similarities have led to the hypothesis that MCS may be explained in part by a similar sensitization process that occurs in rodents. In the studies presented here, we discuss some of the critical features of TDS following repeated exposure to cocaine and environmental stress, including the anatomical and neurochemical pathways utilized in expressing TDS. In addition, we discuss the possible neurochemical basis for individual differences in responsiveness to stimuli, including novelty and cocaine. The striking similarities between TDS and MCS suggest it may be possible to develop an animal model of MCS, using TDS in rodents as its basis.

Microglia depletion by PLX3397 has no effect on cocaine-induced behavioral sensitization in male mice

Cocaine and other addictive drugs are known to stimulate microglia, and microglia in turn have been shown to play roles in both the development and mitigation of drug dependence. For instance, cocaine can directly bind to surface receptors on microglia and trigger their release of interleukin-1β, which promotes addictive behaviors; however, cocaine also indirectly stimulates microglia by elevating dopamine, which causes microglia to impair long-lasting neuronal changes related to cocaine use. The seemingly opposing roles of microglia beg the question of what the net effect of microglial presence is on cocaine-induced behavioral changes. Here, we depleted microglia from the mouse brain by treating mice with PLX3397 and subjected the mice to cocaine-induced behavioral sensitization, a model for studying long-lasting neuronal changes associated with drugs of abuse. Although cocaine treatment had little effect on microglial abundance, PLX3397 treatment dramatically decreased the number of microglia in the nucleus accumbens and hippocampus in control mice and in mice subjected to cocaine sensitization. Importantly, loss of microglia did not appear to affect either the acute locomotor response to cocaine treatment or sensitization after repeated doses of cocaine. In conclusion, while our data do not contradict previous findings indicating that different microglial-derived factors can have seemingly opposite effects on behaviors associated with cocaine use, they suggest that microglia do not have a net effect on cocaine-induced long-lasting behavioral changes.

The non-receptor tyrosine kinase Pyk2 modulates acute locomotor effects of cocaine in D1 receptor-expressing neurons of the nucleus accumbens

The striatum is critical for cocaine-induced locomotor responses. Although the role of D1 receptor-expressing neurons is established, underlying molecular pathways are not fully understood. We studied the role of Pyk2, a non-receptor, calcium-dependent protein-tyrosine kinase. The locomotor coordination and basal activity of Pyk2 knock-out mice were not altered and major striatal protein markers were normal. Cocaine injection increased Pyk2 tyrosine phosphorylation in mouse striatum. Pyk2-deficient mice displayed decreased locomotor response to acute cocaine injection. In contrast, locomotor sensitization and conditioned place preference were normal. Cocaine-activated ERK phosphorylation, a signaling pathway essential for these late responses, was unaltered. Conditional deletion of Pyk2 in the nucleus accumbens or in D1 neurons reproduced decreased locomotor response to cocaine, whereas deletion of Pyk2 in the dorsal striatum or in A2A receptor-expressing neurons did not. In mice lacking Pyk2 in D1-neurons locomotor response to D1 agonist SKF-81297, but not to an anticholinergic drug, was blunted. Our results identify Pyk2 as a regulator of acute locomotor responses to psychostimulants. They highlight the role of tyrosine phosphorylation pathways in striatal neurons and suggest that changes in Pyk2 expression or activation may alter specific responses to drugs of abuse, or possibly other behavioral responses linked to dopamine action.

Critical View on the Usage of Ribavirin in Already Existing Psychostimulant-Use Disorder

Substance-use disorder represents a frequently hidden non-communicable chronic disease. Patients with intravenous drug addiction are at high risk of direct exposure to a variety of viral infections and are considered to be the largest subpopulation infected with the hepatitis C virus. Ribavirin is a synthetic nucleoside analog that has been used as an integral component of hepatitis C therapy. However, ribavirin medication is quite often associated with pronounced psychiatric adverse effects. It is not well understood to what extent ribavirin per se contributes to changes in drug-related neurobehavioral disturbances, especially in the case of psychostimulant drugs, such as amphetamine. It is now well-known that repeated amphetamine usage produces psychosis in humans and behavioral sensitization in animals. On the other hand, ribavirin has an affinity for adenosine A1 receptors that antagonistically modulate the activity of dopamine D1 receptors, which play a critical role in the development of behavioral sensitization. This review will focus on the current knowledge of neurochemical/ neurobiological changes that exist in the psychostimulant drug-addicted brain itself and the antipsychotic-like efficiency of adenosine agonists. Particular attention will be paid to the potential side effects of ribavirin therapy, and the opportunities and challenges related to its application in already existing psychostimulant-use disorder.

Novel prodrug PRX-P4-003, selectively activated by gut enzymes, may reduce the risk of iatrogenic addiction and abuse

OBJECTIVES

Prescription stimulants are vulnerable to oral and parenteral abuse. Intravenous forms of abuse may be most detrimental due to an enhanced risk of dependence, overdose, and infectious diseases. Our objective was to discover an orally active prodrug of a stimulant that would not be easily converted to its parent when injected, thus hindering intravenous abuse.

METHODS

Following an initial analysis of stimulant structures, the fencamfamine isomer [(-)-FCF; (N-ethyl-3-phenylbicyclo[2.2.1]heptan-2-amine)] was chosen as a parent drug due to its favorable biochemical properties. Subsequently, PRX-P4-003 {(-)-N-(Octadecanoyloxymethoxycarbonyl)-N-ethyl-3-phenylbicyclo[2.2.1]heptan-2-amine} qualified for further development. Experimental testing of PRX-P4-003 included radioligand binding assays, stability studies, and rodent pharmacokinetic and locomotor assays.

RESULTS

Prodrug PRX-P4-003 is a pharmacologically inactive, hydrophobic compound, whereas its parent (-)-FCF is a dopamine reuptake inhibitor with weaker effects on norepinephrine reuptake (K_i = 0.07 and 0.80 μM, respectively). PRX-P4-003 is metabolized to (-)-FCF in simulated intestinal fluid (with pancreatin) but not in simulated gastric fluid (with pepsin). Finally, PRX-P4-003 shows a significant oral but no intravenous increase in locomotion, correlating with its pharmacokinetics by these different routes of administration.

CONCLUSIONS

PRX-P4-003 is a novel prodrug stimulant enzymatically activated in the gut. Our data suggest a pancreatic, lipase-based mechanism of activation and as only 1% of this enzyme is found in the systemic circulation, PRX-P4-003 is unlikely to be bioactive if injected intravenously. Enzymatic release of (-)-FCF is needed prior to its systemic absorption, which may discourage oral abuse (e.g., by chewing). PRX-P4-003 is being developed for apathy in Alzheimer's disease and binge eating disorder.

Psychotomimetic-like behavioral effects of memantine in the mouse

A single administration of mice with memantine (1-amino-3,5-dimethyladamantane), a glutamatergic N-methyl-d-aspartate (NMDA) receptor antagonist, induced stereotyped behaviors in dose- and time-dependent manners. The predominant behavioral component of the stereotypy was a continuous, exaggerated sniffing which was accompanied by persistent locomotion. In contrast, a psychostimulant methamphetamine (METH) predominantly induced a stereotyped biting and other forms of intense stationary stereotypical behaviors. Memantine-induced stereotyped sniffing was attenuated by pretreatment with haloperidol, a dopamine D₂ receptor antagonist, in a dose-dependent manner. The memantine-induced stereotyped sniffing was also attenuated by pretreatment with betahistine (2-[2-(methylamino)ethyl]pyridine), an agent which increases histamine turnover and releases histamine in the brain. These observations suggest that memantine might induce stereotypies through neuronal mechanisms that are somewhat different from those of METH, but still overlap to a certain extent, since memantine-induced stereotypies can be attenuated by the mechanisms that also suppress METH-induced stereotypy. Importantly, these data suggests that the effects of memantine may be more limited to the ventral striatum including nucleus accumbens than those of METH, which is associated with dorsal striatal stimulation at high doses. In this respect memantine may also have pharmacological properties such as compartmentation (i.e. brain distribution) and neuronal mechanisms different from those of other NMDA receptor antagonists, such as ketamine, which may have important implications for therapeutic uses of these drugs.

Regulator of G protein signaling-12 modulates the dopamine transporter in ventral striatum and locomotor responses to psychostimulants

Regulators of G protein signaling are proteins that accelerate the termination of effector stimulation after G protein-coupled receptor activation. Many regulators of G protein signaling proteins are highly expressed in the brain and therefore considered potential drug discovery targets for central nervous system pathologies; for example, here we show that RGS12 is highly expressed in microdissected mouse ventral striatum. Given a role for the ventral striatum in psychostimulant-induced locomotor activity, we tested whether Rgs12 genetic ablation affected behavioral responses to amphetamine and cocaine. RGS12 loss significantly decreased hyperlocomotion to lower doses of both amphetamine and cocaine; however, other outcomes of administration (sensitization and conditioned place preference) were unaffected, suggesting that RGS12 does not function in support of the rewarding properties of these psychostimulants. To test whether observed response changes upon RGS12 loss were caused by changes to dopamine transporter expression and/or function, we prepared crude membranes from the brains of wild-type and RGS12-null mice and measured dopamine transporter-selective [³H]WIN 35428 binding, revealing an increase in dopamine transporter levels in the ventral-but not dorsal-striatum of RGS12-null mice. To address dopamine transporter function, we prepared striatal synaptosomes and measured [³H]dopamine uptake. Consistent with increased [³H]WIN 35428 binding, dopamine transporter-specific [³H]dopamine uptake in RGS12-null ventral striatal synaptosomes was found to be increased. Decreased amphetamine-induced locomotor activity and increased [³H]WIN 35428 binding were recapitulated with an independent RGS12-null mouse strain. Thus, we propose that RGS12 regulates dopamine transporter expression and function in the ventral striatum, affecting amphetamine- and cocaine-induced increases in dopamine levels that specifically elicit acute hyperlocomotor responses.

Traumatic brain injuries during development disrupt dopaminergic signaling

Traumatic brain injuries (TBI) sustained during peri-adolescent development produce lasting neuro-behavioral changes that render individuals at an increased risk for developing substance abuse disorders. Experimental and clinical evidence of a prolonged period of hypodopaminergia after TBI have been well documented, but the effect of juvenile TBI on dopaminergic dysfunction and its relationship with substance abuse have not been investigated. In order to determine the effect of juvenile brain injury on dopaminergic signaling, female mice were injured at 21days of age and then beginning seven weeks later were assessed for behavioral sensitization to amphetamine, a drug that increases synaptic dopamine availability. Together with a histological analysis of tyrosine hydroxylase, dopamine transporter, and dopamine D2 receptor expression, our data are indicative of a persistent state of hypodopaminergia well into adulthood after a juvenile TBI. Further, mice that sustained a juvenile TBI exhibited a significantly reduced activation of cFos in the urocortin-positive cells of the Edinger-Westphal nucleus in response to ethanol administration. Taken together, these data provide strong evidence for the vulnerability of juveniles to the development of lasting neuro-behavioral problems following TBI, and indicate a role of injury-induced hypodopaminergia as a risk factor for substance abuse later in life.

Depressive-like neurochemical and behavioral markers of Parkinson's disease after 6-OHDA administered unilaterally to the rat medial forebrain bundle

BACKGROUND

Although Parkinson's disease (PD) is characterized by progressive neurodegeneration of multiple neurotransmitter systems, 6-hydroxydopamine (6-OHDA) as a model substance is mainly used to selectively damage the nigrostriatal dopaminergic neurons and induce parkinsonian-like motor disturbances in rats. We hypothesized that high doses of this neurotoxin affecting other monoaminergic systems may also evoke the depressive-like behavior.

METHODS

The impact of 6-OHDA (8, 12, 16μg/4μl) administered unilaterally into the medial forebrain bundle on the sucrose solution intake (a measure of anhedonia) and on the tissue levels of noradrenaline (NA), dopamine (DA) and serotonin (5-HT) in the striatum (STR), substantia nigra (SN), prefrontal cortex (PFC) and hippocampus (HIP) was examined in rats pretreated or non-pretreated with desipramine.

RESULTS

The highest dose of 6-OHDA reduced the preference for 3% sucrose solution both in rats without and with desipramine pretreatment. All used doses of 6-OHDA dramatically decreased DA content in the studied brain structures on the ipsilateral side. NA levels were severely decreased in the ipsilateral STR, HIP and PFC of rats non-pretreated with desipramine and to a much lesser extent in those pretreated with desipramine. In the SN, moderate decreases in NA level were found both in rats pretreated and non-pretreated with desipramine. Higher doses of 6-OHDA reduced 5-HT content in the ipsilateral STR, HIP and PFC, but not in the SN, only in rats non-pretreated with desipramine.

CONCLUSIONS

Administration of the highest dose of 6-OHDA without desipramine pretreatment evoked neurochemical and behavioral changes resembling the advanced PD with coexisting depression.

The impact of Disrupted-in-Schizophrenia 1 (DISC1) on the dopaminergic system: a systematic review

Disrupted-in-Schizophrenia 1 (DISC1) is a gene known as a risk factor for mental illnesses possibly associated with dopamine impairments. DISC1 is a scaffold protein interacting with proteins involved in the dopamine system. Here we summarise the impact of DISC1 disruption on the dopamine system in animal models, considering its effects on presynaptic dopaminergic function (tyrosine hydroxylase levels, dopamine transporter levels, dopamine levels at baseline and after amphetamine administration) and postsynaptic dopaminergic function (dopamine D1 and D2 receptor levels, dopamine receptor-binding potential and locomotor activity after amphetamine administration). Our findings show that many but not all DISC1 models display (1) increased locomotion after amphetamine administration, (2) increased dopamine levels after amphetamine administration in the nucleus accumbens, and (3) inconsistent basal dopamine levels, dopamine receptor levels and binding potentials. There is also limited evidence for decreased tyrosine hydroxylase levels in the frontal cortex and increased dopamine transporter levels in the striatum but not nucleus accumbens, but these conclusions warrant further replication. The main dopaminergic findings are seen across different DISC1 models, providing convergent evidence that DISC1 has a role in regulating dopaminergic function. These results implicate dopaminergic dysregulation as a mechanism underlying the increased rate of schizophrenia seen in DISC1 variant carriers, and provide insights into how DISC1, and potentially DISC1-interacting proteins such as AKT and GSK-3, could be used as novel therapeutic targets for schizophrenia.

Sex differences in psychotomimetic-induced behaviours in rats

Animal model studies using equal numbers of males and females are sparse in psychiatry research. Given the marked sex differences observed in psychiatric disorders, such as schizophrenia, using both males and females in research studies is an important requirement. Thus the aim of this study was to examine sex differences in psychotomimetic-induced behavioural deficits relevant to psychosis. We therefore compared the acute effect of amphetamine or phencyclidine on locomotor activity and prepulse inhibition in adult male and female Sprague-Dawley rats. The results of this study were that: (1) amphetamine-induced distance travelled was greater in female rats than in male rats, (2) phencyclidine-induced locomotor hyperactivity was similar in male and female rats; (3) there were no sex differences in amphetamine- or phencyclidine-induced disruption of prepulse inhibition; (4) male rats had an increased startle response after amphetamine. These findings suggest that sensitivity to amphetamine, but not phencyclidine, differs between male and female rats, and that this sex difference is selective to locomotor hyperactivity and startle, but not prepulse inhibition. This study used two widely-used, validated preclinical assays relevant to psychosis; the results of this study have implications for psychiatry research, particularly for disorders where marked sex differences in onset and symptomology are observed.

Evaluation of 50-kHz ultrasonic vocalizations in animal models of mania: Ketamine and lisdexamfetamine-induced hyperlocomotion in rats

Drug-induced hyperlocomotion in rodents is frequently used as a behavioral model for mania. However, the use of locomotor activity as the single parameter in these animal models of mania may pose some limitations for developing new pharmacological treatments. Thus, alternative behavioral markers are required. Fifty-kHz ultrasonic vocalizations (USV), which are thought to represent positive affect, are increased by the administration of the psychostimulant d-amphetamine, an effect that can be prevented by lithium treatment, the gold standard antimanic drug for treating bipolar disorder. The aim of this study was to evaluate 50-kHz USV in two other pharmacological-induced animal models of mania: ketamine (KET)- and lisdexamfetamine (LDX)-induced hyperlocomotion. After systemic injection of LDX (10mg/kg, ip), racemic-ketamine (25mg/kg, ip) or S-ketamine (25mg/kg, ip), locomotor activity and 50-kHz USV emission were evaluated in rats. Furthermore, the effects of an antimanic treatment, namely lithium carbonate (100mg/kg, ip), on LDX-induced 50-kHz USV and hyperlocomotion were tested. Rats treated with racemic KET and S-KET showed increased locomotor activity, but these drug treatments did not significantly affect 50-kHz USV emission rates. On the other hand, LDX administration increased both locomotor activity and 50-kHz USV with both effects being reversed by lithium administration. The present findings suggest that 50-kHz USV can differentiate between drug-induced models of mania, which may represent different types of manic episodes. Thus, measuring 50-kHz USV might serve as an additional valuable behavioral variable to assess mania-like phenotypes in rat models.

Contribution of the nitric oxide donor molsidomine and the antiparkinsonian drug l-DOPA to the modulation of the blood pressure in unilaterally 6-OHDA-lesioned rats

BACKGROUND

Interaction between dopaminergic and nitrergic neurotransmission in the brain plays a crucial role in the control of motor function and in the regulation of blood pressure (BP). In Parkinson's disease (PD), dopaminergic denervation of the striatum leads to disturbances in the nitrergic system in the basal ganglia. Recently, it has been demonstrated that addition of a low dose of the nitric oxide donor molsidomine to l-DOPA therapy improves dopaminergic neurotransmission in the denervated nigrostriatal system and weakens dyskinesias in rodent models of the disease.

METHODS

The aim of the present study was to examine the impact of chronic administration of molsidomine (2mg/kg) and l-DOPA (25mg/kg), alone and in combination, on systolic (SBP) and diastolic (DBP) blood pressure in the anesthetized, unilaterally 6-OHDA-lesioned rats. The measurement of SBP and DBP was performed 24h after the penultimate and immediately after the last drug doses.

RESULTS

In 6-OHDA-lesioned rats receiving saline, spontaneous, small decreases in SBP and DBP were observed during the measurements lasting 60min. Administration of molsidomine alone or in combination with l-DOPA distinctly decreased the BP in 6-OHDA-lesioned rats already after 10min compared to those treated with saline or l-DOPA alone, respectively. In both groups, the molsidomine-mediated declines in BP persisted till the end of measurement but they disappeared after 24h.

CONCLUSIONS

Our results indicate that in this PD model molsidomine evokes a short-lasting decrease in BP in contrast to conventional antihypertensive drugs that maintain long-term effect and worsen orthostatic hypotension in parkinsonian patients.

Cannabidiol Prevents Motor and Cognitive Impairments Induced by Reserpine in Rats

Cannabidiol (CBD) is a non-psychotomimetic compound from Cannabis sativa that presents antipsychotic, anxiolytic, anti-inflammatory, and neuroprotective effects. In Parkinson's disease patients, CBD is able to attenuate the psychotic symptoms induced by L-DOPA and to improve quality of life. Repeated administration of reserpine in rodents induces motor impairments that are accompanied by cognitive deficits, and has been applied to model both tardive dyskinesia and Parkinson's disease. The present study investigated whether CBD administration would attenuate reserpine-induced motor and cognitive impairments in rats. Male Wistar rats received four injections of CBD (0.5 or 5 mg/kg) or vehicle (days 2-5). On days 3 and 5, animals received also one injection of 1 mg/kg reserpine or vehicle. Locomotor activity, vacuous chewing movements, and catalepsy were assessed from day 1 to day 7. On days 8 and 9, we evaluated animals' performance on the plus-maze discriminative avoidance task, for learning/memory assessment. CBD (0.5 and 5 mg/kg) attenuated the increase in catalepsy behavior and in oral movements - but not the decrease in locomotion - induced by reserpine. CBD (0.5 mg/kg) also ameliorated the reserpine-induced memory deficit in the discriminative avoidance task. Our data show that CBD is able to attenuate motor and cognitive impairments induced by reserpine, suggesting the use of this compound in the pharmacotherapy of Parkinson's disease and tardive dyskinesia.

Behavioral Responses to Amphetamine Challenge following 6-Ohda Hippocampal Lesions in Rats

The effects of 6-hydroxydopamine (6-OHDA, 8 μ g) retrograde selective lesions of bilateral dopaminergic pathways, projecting from the ventral tegmental area of Tsai (VTA-A10) to the hippocampal formation (HF), were studied through assessment of male Sprague-Dawley rats behavior, before and following amphetamine challenge (day 15 and day 30, respectively after 6-OHDA injections). Significant differences in locomotor and exploratory activities and motor coordination were evidenced between and within groups by one-way analysis of variance (ANOVA). These VTA-hippocampal dopaminergic pathway lesions may provide a morphological basis for the interpretation of the psychomotor deficits shown in this study.

Dopamine Regulation of Lateral Inhibition between Striatal Neurons Gates the Stimulant Actions of Cocaine

Striatal medium spiny neurons (MSNs) form inhibitory synapses on neighboring striatal neurons through axon collaterals. The functional relevance of this lateral inhibition and its regulation by dopamine remains elusive. We show that synchronized stimulation of collateral transmission from multiple indirect-pathway MSNs (iMSNs) potently inhibits action potentials in direct-pathway MSNs (dMSNs) in the nucleus accumbens. Dopamine D2 receptors (D2Rs) suppress lateral inhibition from iMSNs to disinhibit dMSNs, which are known to facilitate locomotion. Surprisingly, D2R inhibition of synaptic transmission was larger at axon collaterals from iMSNs than their projections to the ventral pallidum. Targeted deletion of D2Rs from iMSNs impaired cocaine's ability to suppress lateral inhibition and increase locomotion. These impairments were rescued by chemogenetic activation of Gi-signaling in iMSNs. These findings shed light on the functional significance of lateral inhibition between MSNs and offer a novel synaptic mechanism by which dopamine gates locomotion and cocaine exerts its canonical stimulant response. VIDEO ABSTRACT.

The medial preoptic area modulates cocaine-induced locomotion in male rats

Cocaine-induced locomotion is mediated by dopamine in the nucleus accumbens (NAc). Recent evidence indicates that the medial preoptic area (mPOA), a region in the rostral hypothalamus, modulates cocaine-induced dopamine in the NAc. Specifically, rats with lesions of the mPOA experienced a greater increase in dopamine following cocaine administration than rats with sham lesions. Whether the mPOA similarly influences cocaine-induced locomotion is not known. Here we examined whether radiofrequency or neurotoxic lesions of the mPOA in male rats influence changes in locomotion that follow cocaine administration. Locomotion was measured following cocaine administration in male rats with neurotoxic, radiofrequency, or sham lesions of the mPOA. Results indicate that bilateral lesions of the mPOA facilitated cocaine-induced locomotion. This facilitation was independent of lesion type, as increased locomotion was observed with either approach. These findings support a role for the mPOA as an integral region in the processing of cocaine-induced behavioral response, in this case locomotor activity.

Deficits in behavioral sensitization and dopaminergic responses to methamphetamine in adenylyl cyclase 1/8-deficient mice

The cAMP/protein kinase A pathway regulates methamphetamine (METH)-induced neuroplasticity underlying behavioral sensitization. We hypothesize that adenylyl cyclases (AC) 1/8 mediate these neuroplastic events and associated striatal dopamine regulation. Locomotor responses to METH (1 and 5 mg/kg) and striatal dopamine function were evaluated in mice lacking AC 1/8 (DKO) and wild-type (WT) mice. Only 5 mg/kg METH induced an acute locomotor response in DKO mice, which was significantly attenuated versus WT controls. DKO mice showed a marked attenuation in the development and expression of METH-induced behavioral sensitization across doses relative to WT controls. While basal and acute METH (5 mg/kg)-evoked accumbal dialysate dopamine levels were similar between genotypes, saline-treated DKO mice showed elevated tissue content of dopamine and homovanillic acid in the dorsal striatum (DS), reflecting dysregulated dopamine homeostasis and/or metabolism. Significant reductions in DS dopamine levels were observed in METH-sensitized DKO mice compared to saline-treated controls, an effect not observed in WT mice. Notably, saline-treated DKO mice had significantly increased phosphorylated Dopamine- and cAMP-regulated phosphoprotein levels, which were not further augmented following METH sensitization, as observed in WT mice. These data indicate that AC 1/8 are critical to mechanisms subserving drug-induced behavioral sensitization and mediate nigrostriatal pathway METH sensitivity. Calcium/calmodulin-stimulated adenylyl cyclase (AC) isoforms 1 and 8 were studied for their involvement in the adaptive neurobehavioral responses to methamphetamine. AC 1/8 double knockout (DKO) mice showed heightened basal locomotor activity and dorsal striatal dopamine responsivity. Conversely, methamphetamine-induced locomotor activity was attenuated in DKO mice, accompanied by reductions in dopamine and HVA content and impaired DARPP-32 activation. These findings indicate AC 1/8 signaling regulates the sensitivity of the nigrostriatal pathway subserving stimulant and neuroadaptive sensitizing effects of methamphetamine. 3-MT, 3-methoxytyramine; Ca(2+), calcium; CaM, calmodulin; cdk5; cyclin-dependent kinase 5; DA, dopamine; DARPP-32, dopamine- and cAMP-regulated phosphoprotein; D1R, dopamine D1 receptor; HVA, homovanillic acid; PKA, protein kinase A.

Pretreatment or Posttreatment with Aripiprazole Attenuates Methamphetamine-induced Stereotyped Behavior in Mice

Aripiprazole is a third-generation atypical antipsychotic and a dopamine D₂ receptor partial agonist. In the present study, we investigated whether a single administration of aripiprazole to mice, either as a pretreatment or as a posttreatment, would affect stereotypy induced by methamphetamine (METH). Pretreatment of male ICR mice with aripiprazole (1 or 10 mg/kg, i.p.) attenuated the incidence of METH-induced stereotypical behavior in a dose-dependent manner. Pretreatment of mice with 1 mg/kg aripiprazole produced an increase in the locomotor activity in mice treated with METH compared with mice treated with vehicle plus METH and with 10 mg/kg aripiprazole plus METH. This increase in locomotion is indicative of a rightward shift in the dose–response curve for METH, consistent with a shift in the type of stereotypical behavior observed from biting to sniffing. Aripiprazole posttreatment, after METH-induced stereotypical behavior, was fully expressed and also significantly attenuated overall stereotypy in an aripiprazole dose-dependent manner. These data suggest that the antagonism of METH effects by aripiprazole should be investigated as a potential treatment for acute METH overdose.

The pharmacology of neurokinin receptors in addiction: prospects for therapy

Addiction is a chronic disorder in which consumption of a substance or a habitual behavior becomes compulsive and often recurrent, despite adverse consequences. Substance p (SP) is an undecapeptide and was the first neuropeptide of the neurokinin family to be discovered. The subsequent decades of research after its discovery implicated SP and its neurokinin relatives as neurotransmitters involved in the modulation of the reward pathway. Here, we review the neurokinin literature, giving a brief historical perspective of neurokinin pharmacology, localization in various brain regions involved in addictive behaviors, and the functional aspects of neurokinin pharmacology in relation to reward in preclinical models of addiction that have shaped the rational drug design of neurokinin antagonists that could translate into human research. Finally, we will cover the clinical investigations using neurokinin antagonists and discuss their potential as a therapy for drug abuse.

The role of the GABA system in amphetamine-type stimulant use disorders

Abuse of amphetamine-type stimulants (ATS) has become a global public health problem. ATS causes severe neurotoxicity, which could lead to addiction and could induce psychotic disorders or cognitive dysfunctions. However, until now, there has been a lack of effective medicines for treating ATS-related problems. Findings from recent studies indicate that in addition to the traditional dopamine-ergic system, the GABA (gamma-aminobutyric acid)-ergic system plays an important role in ATS abuse. However, the exact mechanisms of the GABA-ergic system in amphetamine-type stimulant use disorders are not fully understood. This review discusses the role of the GABA-ergic system in ATS use disorders, including ATS induced psychotic disorders and cognitive dysfunctions. We conclude that the GABA-ergic system are importantly involved in the development of ATS use disorders through multiple pathways, and that therapies or medicines that target specific members of the GABA-ergic system may be novel effective interventions for the treatment of ATS use disorders.

Social defeat stress-induced sensitization and escalated cocaine self-administration: the role of ERK signaling in the rat ventral tegmental area

RATIONALE

Intermittent social defeat stress can induce neuroadaptations that promote compulsive drug taking. Within the mesocorticolimbic circuit, repeated cocaine administration activates extracellular signal-regulated kinase (ERK).

OBJECTIVE

The present experiments examine whether changes in ERK phosphorylation are necessary for the behavioral and neural adaptations that occur as a consequence of intermittent defeat stress.

MATERIALS AND METHODS

Rats were exposed to four brief intermittent defeats over the course of 10 days. Ten days after the last defeat, rats were challenged with cocaine (10 mg/kg, i.p.) or saline, and ERK activity was examined in mesocorticolimbic regions. To determine the role of ERK in defeat stress-induced behavioral sensitization, we bilaterally microinjected the MAPK/ERK kinase inhibitor U0126 (1 μg/side) or vehicle (20 % DMSO) into the ventral tegmental area (VTA) prior to each of four defeats. Ten days following the last defeat, locomotor activity was assessed for the expression of behavioral cross-sensitization to cocaine (10 mg/kg, i.p.). Thereafter, rats self-administered cocaine under fixed and progressive ratio schedules of reinforcement, including a 24-h continuous access "binge" (0.3 mg/kg/infusion).

RESULTS

We found that repeated defeat stress increased ERK phosphorylation in the VTA. Inhibition of VTA ERK prior to each social defeat attenuated the development of stress-induced sensitization and prevented stress-induced enhancement of cocaine self-administration during a continuous access binge.

CONCLUSIONS

These results suggest that enhanced activation of ERK in the VTA due to brief defeats is critical in the induction of sensitization and escalated cocaine taking.

Locomotor activating effects of cocaine and scopolamine combinations in rats: isobolographic analysis

Muscarinic cholinergic receptors are currently receiving renewed interest as viable targets for treating various psychiatric disorders. Dopaminergic and muscarinic systems interact in complex ways. The goal of this study was to quantify the interaction between a systemically administered psychomotor stimulant and muscarinic antagonist at the behavioral level. Through isobolographic analysis of locomotor activity data, we assessed the effects of three cocaine/scopolamine mixtures in terms of deviation from simple dose addition (additivity), at four effect levels. All three mixtures produced some more-than-additive (synergistic) effects, as lower doses were needed to produce the given effects relative to the calculated effect of additive doses. A mixture with comparable contributions from cocaine and scopolamine produced significantly more-than-additive effects at all but the lowest effect level examined. A mostly-cocaine mixture was more-than-additive only at low effect levels, whereas a mostly-scopolamine mixture produced effects more consistent with additivity, with only the highest effect level barely reaching significant synergism. Our study confirms and quantifies previous findings that suggested synergistic effects of stimulants and muscarinic antagonists. The synergism implies that cocaine and scopolamine stimulate locomotor activity through nonidentical pathways, and was most pronounced for a mixture containing cocaine and scopolamine in comparable proportions.

Serotonin/dopamine interactions in a hyperactive mouse: reduced serotonin receptor 1B activity reverses effects of dopamine transporter knockout

Knockout (KO) mice that lack the dopamine transporter (SL6A3; DAT) display increased locomotion that can be attenuated, under some circumstances, by administration of drugs that normally produce psychostimulant-like effects, such as amphetamine and methylphenidate. These results have led to suggestions that DAT KO mice may model features of attention deficit hyperactivity disorder (ADHD) and that these drugs may act upon serotonin (5-HT) systems to produce these unusual locomotor decreasing effects. Evidence from patterns of brain expression and initial pharmacologic studies led us to use genetic and pharmacologic approaches to examine the influence of altered 5-HT_1B receptor activity on hyperactivity in DAT KO mice. Heterozygous 5-HT_1B KO and pharmacologic 5-HT_1B antagonism both attenuated locomotor hyperactivity in DAT KO mice. Furthermore, DAT KO mice with reduced, but not eliminated, 5-HT_1B receptor expression regained cocaine-stimulated locomotion, which was absent in DAT KO mice with normal levels of 5-HT_1B receptor expression. Further experiments demonstrated that the degree of habituation to the testing apparatus determined whether cocaine had no effect on locomotion in DAT KO or reduced locomotion, helping to resolve differences among prior reports. These findings of complementation of the locomotor effects of DAT KO by reducing 5-HT_1B receptor activity underscore roles for interactions between specific 5-HT receptors and dopamine (DA) systems in basal and cocaine-stimulated locomotion and support evaluation of 5-HT_1B antagonists as potential, non-stimulant ADHD therapeutics.

Repeated exposure of adult rats to transient oxidative stress induces various long-lasting alterations in cognitive and behavioral functions

Exposure of neonates to oxidative stress may increase the risk of psychiatric disorders such as schizophrenia in adulthood. However, the effects of moderate oxidative stress on the adult brain are not completely understood. To address this issue, we systemically administrated 2-cyclohexen-1-one (CHX) to adult rats to transiently reduce glutathione levels. Repeated administration of CHX did not affect the acquisition or motivation of an appetitive instrumental behavior (lever pressing) rewarded by a food outcome under a progressive ratio schedule. In addition, response discrimination and reversal learning were not affected. However, acute CHX administration blunted the sensitivity of the instrumental performance to outcome devaluation, and this effect was prolonged in rats with a history of repeated CHX exposure, representing pro-depression-like phenotypes. On the other hand, repeated CHX administration reduced immobility in forced swimming tests and blunted acute cocaine-induced behaviors, implicating antidepressant-like effects. Multivariate analyses segregated a characteristic group of behavioral variables influenced by repeated CHX administration. Taken together, these findings suggest that repeated administration of CHX to adult rats did not cause a specific mental disorder, but it induced long-term alterations in behavioral and cognitive functions, possibly related to specific neural correlates.

Histone acetylation in the nucleus accumbens shell modulates ethanol-induced locomotor activity in DBA/2J mice

BACKGROUND

A growing body of literature suggests that epigenetic mechanisms, including histone acetylation, may play key roles in drug abuse and the development of addiction. Experiments in this study were designed to investigate the role of histone acetylation in ethanol (EtOH)-induced locomotor sensitization.

METHODS

Immunohistochemical, Western blotting, and site-directed pharmacological techniques were used to explore the roles of histone acetylation at histone H3 (acH3K9) in both the expression of and acquisition of EtOH-induced locomotor sensitization. A commonly used sensitization protocol, in which animals were exposed to repeated injections of a low dose of EtOH while in their home cage, was used to examine this behavioral phenomenon. Additionally, site-directed administration of the histone deacetylase inhibitor (HDACi) Trichostatin A (TSA), in the absence of repeated EtOH injections, was used to examine the role of hyperacetylation in the nucleus accumbens (NAC) shell in EtOH-induced locomotor sensitization.

RESULTS

Sensitized mice displayed elevated acH3K9 immunoreactivity (IR) localized to the shell of the NAC. This augmentation in acH3K9 IR was confirmed, in a separate experiment, using Western blot analyses. Next, repeated intra-accumbal infusions of TSA, in the absence of repeated EtOH injections, were sufficient to induce an augmented locomotor response to a later injection of a low dose (2.0 g/kg, intraperitoneally) of EtOH, indicative of cross-sensitization to this locomotor stimulation between TSA and EtOH. Finally, a local infusion of TSA into the shell of the accumbens was also associated with a significant increase in acH3K9 IR within this region.

CONCLUSIONS

Together, the present observations suggest that histone acetylation, particularly within the shell of the NAC, is important for the development and expression of EtOH-induced locomotor sensitization.

Persistent behavioral and neurochemical sensitization to an acute injection of methamphetamine following unpredictable stress

Prior research in humans and animals suggest that exposure to chronic stress alters the response to drugs of abuse, increasing vulnerability to drug addiction. Chronic unpredictable stress (CUS) has been shown to augment the increase of dopamine in the striatum when challenged with high doses of methamphetamine immediately following stress exposure, however it is not known whether this neurochemical stress-sensitization continues after the cessation of the stressors or if behavioral sensitization is also present. Therefore, the current study examined the immediate and delayed effects of CUS on methamphetamine-induced behaviors and striatal dopamine levels. Male rats were exposed to 10 days of CUS and then tested in either an open field box to assess locomotion or underwent in vivo microdialysis to measure striatal dopamine levels immediately following CUS or after a 1-2 week delay. All rats exposed to CUS showed a potentiated locomotor response immediately following an acute injection of 7.5mg/kg methamphetamine compared to non-stressed control rats. Both groups of CUS rats also showed augmented dopamine release and rectal temperatures following methamphetamine with prolonged increases in the CUS rats tested after a delay. These results suggest that CUS increases the sensitivity of a rat to a single injection of methamphetamine and that the increased sensitivity persists for up to 2 weeks following the last stressor.

Estrous cycle and sex affect cocaine-induced behavioural changes in CD1 mice

RATIONALE

Several findings on sex differences in cocaine response suggest a role for hormonal milieu in modulating the subjective effects of cocaine. Nitric oxide (NO) has been involved in the neurochemical, hormonal, and behavioral changes related to stress and anxiety. Within the brain, the anteroventral subdivision of the medial amygdala (MeAV) is an important area involved in processing emotional responses such as anxiety and a high density of NO-producing neurons is observed in this area.

OBJECTIVES

In this study, we hypothesize the possibility of sex/hormonal differences in response to cocaine and that these differences may reflect a change in the MeAV nitrergic system. We have examined cocaine's acute effects on nicotinamide adenine dinucleotide phosphate diaphorase (nadph-d) expression, as well as its effect on motor activity and anxiety in male and estrus and diestrus females.

RESULTS

Our results show that acute cocaine administration produces an increase in both anxiety behaviors and nadph-d expression in the MeAV. Male and diestrus female mice were more susceptible to these effects of cocaine than estrus female mice in which no differences were detected. In addition, we examined individual differences in male and female mice responding to intravenous cocaine reinforcement in a self-administration paradigm. Female mice acquired cocaine self-administration at a faster rate than males and showed a higher motivation to self-administer cocaine under a progressive ratio schedule of reinforcement.

CONCLUSIONS

Our data suggest a complex interaction between hormonal milieu and the behavioral and reinforcing effects of cocaine.

Clozapine pre-treatment has a protracted hypolocomotor effect on the induction and expression of amphetamine sensitization

Amphetamine locomotor sensitization is an animal model for the study of addiction and schizophrenia. The antipsychotic clozapine blocks the hyperlocomotion induced by an acute injection of amphetamine, but its effect on locomotor sensitization after repeated amphetamine administration remains unknown. In the present study we investigate the effect of repeated administration of clozapine on the induction and expression of amphetamine locomotor sensitization. We propose that repeated administration of clozapine blocks the induction and expression of amphetamine sensitization. Male Sprague-Dawley rats were classified according to their locomotor response to an acute saline injection in high responder saline (HRS) or low responder saline (LRS). Rats from both groups were injected once daily with amphetamine for 5 consecutive days. Horizontal locomotor activity was measured during 40 min. Four days after the last injection, an acute dose of amphetamine was administered to assess the expression of sensitization. Clozapine was injected once daily for 4 consecutive days before (pre-treatment) or after (treatment) induction of sensitization. Pre-treatment with clozapine significantly decreases both acute amphetamine-induced hyperlocomotion and the induction and expression of amphetamine sensitization only in LRS rats, showing a protracted hypolocomotor effect. On the other hand, clozapine treatment had no effect over locomotor response on the expression of amphetamine sensitization in either LRS or HRS rats. These data suggest that clozapine effect on amphetamine locomotor response depends on individual differences. Also, our results suggest that clozapine pre-treatment attenuates the neuroplasticity underlying amphetamine sensitization, but clozapine treatment is unable to reverse these changes once amphetamine sensitization has been induced.

Measuring dopaminergic function in the 6-OHDA-lesioned rat: a comparison of PET and microdialysis

BACKGROUND

[18 F]fluorodopa (FDOPA) positron emission tomography (PET) allows assessment of levodopa (LDOPA) metabolism and is widely used to study Parkinson's disease. We examined how [18 F]FDOPA PET-derived kinetic parameters relate the dopamine (DA) and DA metabolite content of extracellular fluid measured by microdialysis to aid in the interpretation of data from both techniques.

METHODS

[18 F]FDOPA PET imaging and microdialysis measurements were performed in unilaterally 6-hydroxydopamine-lesioned rats (n = 8) and normal control rats (n = 3). Microdialysis testing included baseline measurements and measurements following acute administration of LDOPA. PET imaging was also performed using [11C]dihydrotetrabenazine (DTBZ), which is a ligand for the vesicular monoamine transporter marker and allowed assessment of denervation severity.

RESULTS

The different methods provided highly correlated data. Lesioned rats had reduced DA metabolite concentrations ipsilateral to the lesion (p < 0.05 compared to controls), with the concentration being correlated with FDOPA's effective distribution volume ratio (EDVR; r = 0.86, p < 0.01) and DTBZ's binding potential (BPND; r = 0.89, p < 0.01). The DA metabolite concentration in the contralateral striatum of severely (>80%) lesioned rats was lower (p < 0.05) than that of less severely lesioned rats (<80%) and was correlated with the ipsilateral PET measures (r = 0.89, p < 0.01 for BPND) but not with the contralateral PET measures. EDVR and BPND in the contralateral striatum were not different from controls and were not correlated with the denervation severity.

CONCLUSIONS

The demonstrated strong correlations between the PET and microdialysis measures can aid in the interpretation of [18 F]FDOPA-derived kinetic parameters and help compare results from different studies. The contralateral striatum was affected by the lesioning and so cannot always serve as an unaffected control.

Molsidomine, a nitric oxide donor, modulates rotational behavior and monoamine metabolism in 6-OHDA lesioned rats treated chronically with L-DOPA

Some biochemical and histological studies of Parkinson's disease patients' brains and 6-OHDA-lesioned rats suggest that dopaminergic dennervation of the striatum leads to the nitrergic system hypofunction in this structure. Hence, recently the modulation of nitric oxide (NO)- soluble guanylyl cyclase-cyclic GMP signaling is considered to be a new target for the treatment of Parkinson's disease. The aim of our study was to examine the impact of chronic combined treatment with low doses of the NO donor molsidomine (2 and 4mg/kg) and L-DOPA (12.5 and 25mg/kg) on rotational behavior and monoamine metabolism in the striatum (STR) and substantia nigra (SN) of unilaterally 6-OHDA-lesioned rats. Chronic administration of molsidomine at a dose of 2mg/kg jointly with 25mg/kg of L-DOPA significantly decreased the number of contralateral rotations when compared to L-DOPA alone. Other combinations of the examined drug doses were less effective. The tissue DA levels in the ipsilateral STR and SN after the last chronic doses of molsidomine (2mg/kg) and L-DOPA (12.5 or 25mg/kg), were significantly higher than after L-DOPA alone. Chronic L-DOPA treatment alone or jointly with a lower dose of molsidomine decreased 5-HT levels and accelerated its catabolism in the examined structures. However, combination of a higher dose of molsidomine with L-DOPA (25mg/kg) did not reduce 5-HT content while its catabolism was less intensive. The obtained results show that low doses of molsidomine can modulate rotational behavior and tissue DA and 5-HT concentrations in the STR and SN of 6-OHDA-lesioned rats treated chronically with L-DOPA.

Drugs that prevent mouse sleep also block light-induced locomotor suppression, circadian rhythm phase shifts and the drop in core temperature

Exposure of mice to a brief light stimulus during their nocturnal active phase induces several simultaneous behavioral or physiological responses, including circadian rhythm phase shifts, a drop in core body temperature (Tc), suppression of locomotor activity and sleep. Each response is triggered by light, endures for a relatively fixed interval and does not require additional light for expression. The present studies address the ability of the psychostimulant drugs, methamphetamine (MA), modafinil (MOD) or caffeine (CAF), to modify the light-induced responses. Drug or vehicle (VEH) was injected at CT11 into constant dark-housed mice then exposed to 5-min 100μW/cm(2) light or no light at CT13. Controls (VEH/Light) showed approximately 60-min phase delays. In contrast, response was substantially attenuated by each drug (only 12-15min delays). Under a 12-h light:12-h dark (LD12:12) photoperiod, VEH/light-treated mice experienced a Tc drop of about 1.3°C coincident with locomotor suppression and both effects were abolished by drug pre-treatment. Each drug elevated activity during the post-injection interval, but there was also evidence for CAF-induced hypoactivity in the dark prior to the photic test stimulus. CAF acutely elevated Tc; MA acutely lowered it, but both drugs reduced Tc during the early dark (ZT12.5-ZT13). The ability of the psychostimulant drugs to block the several effects of light exposure is not the result of drug-induced hyperactivity. The results raise questions concerning the manner in which drugs, activity, sleep and Tc influence behavioral and physiological responses to light.