Participation of the nucleus accumbens dopaminergic system in the antidepressant-like actions of a diet rich in omega-3 polyunsaturated fatty acids

The beneficial effects of omega (ω)-3 polyunsaturated fatty acid (PUFA) supplementation on major depressive disorder have been actively studied, but the underlying mechanism remains unknown. The present study examined the involvement of the nucleus accumbens (NAc) dopaminergic systems in behavioral changes in mice fed a diet high in ω-3 PUFAs. Mice fed a diet containing about double the amount of ω-3 PUFAs (krill oil (KO) diet) exerted shorter immobility times in the forced swim test (FST) than mice fed a control diet, containing only α-linolenic acid (ALA) as ω-3 PUFAs. The shorter immobility times were observed in both male and female mice. A dopamine metabolite, 3,4-dihydroxyphenylacetic acid, increased in the NAc in male mice fed the KO diet when compared with those fed the control diet. In addition, dopamine, 3-methoxytyramine, and homovanillic acid increased in the NAc in female mice fed the KO diet. Notably, the effects of the KO diet on the immobility time in the FST were abolished by microinjection of sulpiride, an antagonist of D₂-like receptors, into the NAc. A similar microinjection of an antagonist selective for D₁-like receptors, SKF83566, also abolished the reduction in immobility in the FST. Moreover, we found that tyrosine hydroxylase-positive cells increased in the ventral tegmental area (VTA) in mice fed the KO diet. These results suggest that modulation of the VTA-NAc dopaminergic pathway is one of the mechanisms by which a KO diet rich in ω-3 PUFAs reduces the immobility behavior in the mouse FST.

Importance of kynurenine 3-monooxygenase for spontaneous firing and pharmacological responses of midbrain dopamine neurons: Relevance for schizophrenia

Kynurenine 3-monooxygenase (KMO) is an essential enzyme of the kynurenine pathway, converting kynurenine into 3-hydroxykynurenine. Inhibition of KMO increases kynurenine, resulting in elevated levels of kynurenic acid (KYNA), an endogenous N-methyl-d-aspartate and α*7-nicotinic receptor antagonist. The concentration of KYNA is elevated in the brain of patients with schizophrenia, possibly as a result of a reduced KMO activity. In the present study, using in vivo single cell recording techniques, we investigated the electrophysiological characteristics of ventral tegmental area dopamine (VTA DA) neurons and their response to antipsychotic drugs in a KMO knock-out (K/O) mouse model. KMO K/O mice exhibited a marked increase in spontaneous VTA DA neuron activity as compared to wild-type (WT) mice. Furthermore, VTA DA neurons showed clear-cut, yet qualitatively opposite, responses to the antipsychotic drugs haloperidol and clozapine in the two genotypes. The anti-inflammatory drug parecoxib successfully lowered the firing activity of VTA DA neurons in KMO K/O, but not in WT mice. Minocycline, an antibiotic and anti-inflammatory drug, produced no effect in this regard. Taken together, the present data further support the usefulness of KMO K/O mice for studying distinct aspects of the pathophysiology and pharmacological treatment of psychiatric disorders such as schizophrenia.

Testosterone and Corticosterone in the Mesocorticolimbic System of Male Rats: Effects of Gonadectomy and Caloric Restriction

Steroid hormones can modulate motivated behaviors through the mesocorticolimbic system. Gonadectomy (GDX) is a common method to determine how steroids influence the mesocorticolimbic system, and caloric restriction (CR) is often used to invigorate motivated behaviors. A common assumption is that the effects of these manipulations on brain steroid levels reflects circulating steroid levels. We now know that the brain regulates local steroid levels in a region-specific manner; however, previous studies have low spatial resolution. Using ultrasensitive liquid chromatography tandem mass spectrometry, we examined steroids in microdissected regions of the mesocorticolimbic system (ventral tegmental area, nucleus accumbens, medial prefrontal cortex). We examined whether GDX or CR influences systemic and local steroids, particularly testosterone (T) and steroidogenic enzyme transcripts. Adult male rats underwent a GDX surgery and/or CR for either 2 or 6 weeks. Levels of T, the primary steroid of interest, were higher in all brain regions than in the blood, whereas corticosterone (CORT) was lower in the brain than in the blood. Importantly, GDX completely eliminated T in the blood and lowered T in the brain. Yet, T remained present in the brain, even 6 weeks after GDX. CR decreased both T and CORT in the blood and brain. Steroidogenic enzyme (Cyp17a1, 3β-hydroxysteroid dehydrogenase, aromatase) transcripts and androgen receptor transcripts were expressed in the mesocorticolimbic system and differentially affected by GDX and CR. Together, these results suggest that T is synthesized within the mesocorticolimbic system. These results provide a foundation for future studies examining how neurosteroids influence behaviors mediated by the mesocorticolimbic system.

Tyrosine hydroxylase in the ventral tegmental area of rams with high or low libido-A role for dopamine

Dopamine synthesis in the ventral tegmental area (VTA) is necessary for the reinforcement of sexual behavior. The objective of this study determined if sexual stimuli initiates reward, and whether reward is attenuated in sexually inactive rams. Sexually active rams were exposed to urine from estrous (n=4) or ovariectomized (n=3) ewes with inactive rams (n=3) exposed to urine from estrous ewes. Following exposure, rams were exsanguinated and brains perfused. Alternating sections of the VTA were stained for Fos related antigens (FRA), tyrosine hydroxylase, and dopamine beta-hydroxylase activity. Forebrain tissue, mid-sagittal ventral to the anterior corpus callosum, was stained for dopamine D2 receptors. Concentrations of cortisol was determined prior to and following exposure. Exposure to ovariectomized-ewe urine in sexually active rams did not influence (P=0.6) FRA expression, but fewer (P<0.05) neurons were positive for tyrosine hydroxylase in the VTA. Sexually inactive rams had fewer (P<0.05) FRA and tyrosine hydroxylase positive neurons in the VTA than sexually active rams following exposure to estrous ewe urine. VTA neurons staining positive for dopamine beta-hydroxylase did not differ by sexual activity (P=0.44) or urine exposure (P=0.07). Exposure to stimulus did not influence (P=0.46) numbers of forebrain neurons staining positive for dopamine D2 receptors in sexually active rams, but fewer (P=0.04) neurons stain positive in inactive rams. Serum concentrations of cortisol did not differ (P≥0.52) among rams prior to or following stimulus. In conclusion sexual inactivity is unlikely due to stress, but may be partially a result of decreased tyrosine hydroxylase and/or the response to dopamine.

Nuclear organization of the substantia nigra, ventral tegmental area and retrorubral field of the common marmoset (Callithrix jacchus): A cytoarchitectonic and TH-immunohistochemistry study

It is widely known that the catecholamine group is formed by dopamine, noradrenaline and adrenaline. Its synthesis is regulated by the enzyme called tyrosine hydroxylase. 3-hydroxytyramine/dopamine (DA) is a precursor of noradrenaline and adrenaline synthesis and acts as a neurotransmitter in the central nervous system. The three main nuclei, being the retrorubral field (A8 group), the substantia nigra pars compacta (A9 group) and the ventral tegmental area (A10 group), are arranged in the die-mesencephalic portion and are involved in three complex circuitries - the mesostriatal, mesolimbic and mesocortical pathways. These pathways are involved in behavioral manifestations, motricity, learning, reward and also in pathological conditions such as Parkinson's disease and schizophrenia. The aim of this study was to perform a morphological analysis of the A8, A9 and A10 groups in the common marmoset (Callithrix jacchus - a neotropical primate), whose morphological and functional characteristics support its suitability for use in biomedical research. Coronal sections of the marmoset brain were submitted to Nissl staining and TH-immunohistochemistry. The morphology of the neurons made it possible to subdivide the A10 group into seven distinct regions: interfascicular nucleus, raphe rostral linear nucleus and raphe caudal linear nucleus in the middle line; paranigral and parainterfascicular nucleus in the middle zone; the rostral portion of the ventral tegmental area nucleus and parabrachial pigmented nucleus located in the dorsolateral portion of the mesencephalic tegmentum. The A9 group was divided into four regions: substantia nigra compacta dorsal and ventral tiers; substantia nigra compacta lateral and medial clusters. No subdivisions were made for the A8 group. These results reveal that A8, A9 and A10 are phylogenetically stable across species. As such, further studies concerning such divisions are necessary in order to evaluate the occurrence of subdivisions that express DA in other primate species, with the aim of characterizing its functional relevance.

Assessment of cytochrome C oxidase dysfunction in the substantia nigra/ventral tegmental area in schizophrenia

Perturbations in metabolism are a well-documented but complex facet of schizophrenia pathology. Optimal cellular performance requires the proper functioning of the electron transport chain, which is constituted by four enzymes located within the inner membrane of mitochondria. These enzymes create a proton gradient that is used to power the enzyme ATP synthase, producing ATP, which is crucial for the maintenance of cellular functioning. Anomalies in a single enzyme of the electron transport chain are sufficient to cause disruption of cellular metabolism. The last of these complexes is the cytochrome c oxidase (COX) enzyme, which is composed of thirteen different subunits. COX is a major site for oxidative phosphorylation, and anomalies in this enzyme are one of the most frequent causes of mitochondrial pathology. The objective of the present report was to assess if metabolic anomalies linked to COX dysfunction may contribute to substantia nigra/ventral tegmental area (SN/VTA) pathology in schizophrenia. We tested COX activity in postmortem SN/VTA from schizophrenia and non-psychiatric controls. We also tested the protein expression of key subunits for the assembly and activity of the enzyme, and the effect of antipsychotic medication on subunit expression. COX activity was not significantly different between schizophrenia and non-psychiatric controls. However, we found significant decreases in the expression of subunits II and IV-I of COX in schizophrenia. Interestingly, these decreases were observed in samples containing the entire rostro-caudal extent of the SN/VTA, while no significant differences were observed for samples containing only mid-caudal regions of the SN/VTA. Finally, rats chronically treated with antipsychotic drugs did not show significant changes in COX subunit expression. These findings suggest that COX subunit expression may be compromised in specific sub-regions of the SN/VTA (i.e. rostral regions), which may lead to a faulty assembly of the enzyme and a greater vulnerability to metabolic insult.

Loss of tyrosine hydroxylase expression within the nigro-striato-cortical pathways in the cirrhotic rat: the possible restorative effect of the neurosteroid dehydroepiandrosterone sulfate

Hepatic encephalopathy (HE) is a neuropsychiatric disorder occurring as a consequence of both acute and chronic liver failure. Advanced HE is generally accompanied with extrapyramidal symptoms including rigidity and tremor, which may reflect alterations of the dopaminergic system. Recently we reported a beneficial effect of the neuroactive steroid dehydroepiandrosterone sulfate (DHEAS) in cirrhotic rats, however the mechanisms of such an effect by DHEAS were not addressed. In the present study, we describe the changes of the dopaminergic system occurring in the cirrhotic rats and concomitantly we investigated the effect of DHEAS on this system in Sprague-Dawley rats using the expression of tyrosine hydroxylase (TH) as a neuronal marker. Rats were submitted to bile duct ligation (BDL) surgery and TH immunohistochemistry was assessed in the Substantia nigra pars compacta (SNc), striatum, ventral tegmental area (VTA) and the cortex. TH immunoreactivity showed a significant diminution in both SNc and VTA concomitantly with the cortical and the striatal outputs in the BDL rats vs. controls. Three daily injections of 5mg/kg of DHEAS to BDL rats significantly normalized TH expression decrease in both SNc and VTA as well as dopaminergic projections to the striatum and the cortex of BDL rats. The present data support an involvement of the dopaminergic system in mild HE and a possible beneficial effect of the neurosteroid DHEAS as a potential pharmacological treatment of mild HE.

GDNF is a fast-acting potent inhibitor of alcohol consumption and relapse

Previously, we demonstrated that the action of the natural alkaloid, ibogaine, to reduce alcohol (ethanol) consumption is mediated by the glial cell line-derived neurotrophic factor (GDNF) in the ventral tegmental area (VTA). Here we set out to test the actions of GDNF in the VTA on ethanol-drinking behaviors. We found that GDNF infusion very rapidly and dose-dependently reduced rat ethanol, but not sucrose, operant self-administration. A GDNF-mediated decrease in ethanol consumption was also observed in rats with a history of high voluntary ethanol intake. We found that the action of GDNF on ethanol consumption was specific to the VTA as infusion of the growth factor into the neighboring substantia nigra did not affect operant responses for ethanol. We further show that intra-VTA GDNF administration rapidly activated the MAPK signaling pathway in the VTA and that inhibition of the MAPK pathway in the VTA blocked the reduction of ethanol self-administration by GDNF. Importantly, we demonstrate that GDNF infused into the VTA alters rats' responses in a model of relapse. Specifically, GDNF application blocked reacquisition of ethanol self-administration after extinction. Together, these results suggest that GDNF, via activation of the MAPK pathway, is a fast-acting selective agent to reduce the motivation to consume and seek alcohol.

Role of calcineurin in nicotine-mediated locomotor sensitization

Calcineurin is a serine/threonine phosphatase that contributes to the effects of nicotine on calcium signaling in cultured cortical neurons; however, the role of calcineurin in behavioral responses to nicotine in vivo has not been examined. We therefore determined whether calcineurin blockade could alter nicotine-mediated locomotor sensitization in Sprague Dawley rats using systemic or brain region-specific administration of the calcineurin inhibitors cyclosporine or FK506. Systemic cyclosporine administration decreased calcineurin activity in the brain, attenuated nicotine-mediated locomotor sensitization, and blocked the effects of nicotine on DARPP32 (dopamine- and cAMP-regulated phosphoprotein-32) activation in the striatum. Direct infusion of calcineurin inhibitors cyclosporine or FK506 into the ventral tegmental area (VTA) also attenuated nicotine-mediated locomotor sensitization, whereas infusion of rapamycin, which binds to FK-binding protein but does not inhibit calcineurin, did not affect sensitization. Together, the data suggest that activation of calcineurin, particularly in the VTA, is a novel signaling event important for nicotine-mediated behavior and intracellular signaling.

Nicotine and ethanol activate protein kinase A synergistically via G(i) betagamma subunits in nucleus accumbens/ventral tegmental cocultures: the role of dopamine D(1)/D(2) and adenosine A(2A) receptors

Tobacco and alcohol are the most commonly used drugs of abuse and show the most serious comorbidity. The mesolimbic dopamine system contributes significantly to nicotine and ethanol reinforcement, but the underlying cellular signaling mechanisms are poorly understood. Nicotinic acetylcholine (nACh) receptors are highly expressed on ventral tegmental area (VTA) dopamine neurons, with relatively low expression in nucleus accumbens (NAcb) neurons. Because dopamine receptors D(1) and D(2) are highly expressed on NAcb neurons, nicotine could influence NAcb neurons indirectly by activating VTA neurons to release dopamine in the NAcb. To investigate this possibility in vitro, we established primary cultures containing neurons from VTA or NAcb separately or in cocultures. Nicotine increased cAMP response element-mediated gene expression only in cocultures; this increase was blocked by nACh or dopamine D(1) or D(2) receptor antagonists. Furthermore, subthreshold concentrations of nicotine with ethanol increased gene expression in cocultures, and this increase was blocked by nACh, D(2) or adenosine A(2A) receptor antagonists, Gbetagamma or protein kinase A (PKA) inhibitors, and adenosine deaminase. These results suggest that nicotine activated VTA neurons, causing the release of dopamine, which in turn stimulated both D(1) and D(2) receptors on NAcb neurons. In addition, subthreshold concentrations of nicotine and ethanol in combination also activated NAcb neurons through synergy between D(2) and A(2A) receptors. These data provide a novel cellular mechanism, involving Gbetagamma subunits, A(2A) receptors, and PKA, whereby combined use of tobacco and alcohol could enhance the reinforcing effect in humans as well as facilitate long-term neuroadaptations, increasing the risk for developing coaddiction.

Increased accumbens Cdk5 expression in rats after short-access to self-administered cocaine, but not after long-access sessions

Upregulation of cyclin-dependent kinase 5 (Cdk5) after chronic cocaine administration has led to speculation that Cdk5 plays an important role in drug addiction. However, as Cdk5 involvement is implicated in a variety of neural events, including neuronal development, synaptic plasticity and learning, a specific role in drug abuse is yet to be determined. The present study utilized cocaine self-administration and food-reinforced operant procedures to assess possible relationships between cocaine intake, food-reinforced operant responding, behavioral activity, and Cdk5 levels in the nucleus accumbens (NAcc), ventral tegmental area (VTA), and prefrontal cortex (PFC) in rats. In Experiment 1, animals undergoing daily cocaine self-administration (1-h/30 days) or food-reinforced operant sessions (20-min/30 days) showed significant between-group differences in operant responding and behavioral activity, but no significant differences in NAcc, VTA or PFC Cdk5 levels compared to a Handled Control group. In Experiment 2, animals that had self-administered cocaine in 10 daily 1-h sessions (Short-Access Cocaine) showed significantly greater NAcc Cdk5 expression compared to an Unhandled Control group, and no evidence of cocaine-induced behavioral sensitization. Animals given 4-h daily access to cocaine over the same number of sessions (Long-Access Cocaine) showed significantly enhanced cocaine-reinforced responding and locomotor activation by the end of the sessions, but no significant differences in Cdk5 expression compared to Control animals. These findings suggest that overexpression of Cdk5 may be a transient adaptation to cocaine experience that subsides with increased cocaine exposure and does not correspond with measures of cocaine-induced behavioral sensitization.

Calcium influx through L-type channels generates protein kinase M to induce burst firing of dopamine cells in the rat ventral tegmental area

Enhanced activity of the dopaminergic system originating in the ventral tegmental area is implicated in addictive and psychiatric disorders. Burst firing increases dopamine levels at the synapse to signal novelty and salience. We have previously reported a calcium-dependent burst firing of dopamine cells mediated by L-type channels following cholinergic stimulation; this paper describes a cellular mechanism resulting in burst firing following L-type channel activation. Calcium influx through L-type channels following FPL 64176 or (S)-(-)-Bay K8644 induced burst firing independent of dopamine, glutamate, or calcium from the internal stores. Burst firing induced as such was completely blocked by the substrate site protein kinase C (PKC) inhibitor chelerythrine but not by the diacylglycerol site inhibitor calphostin C. Western blotting analysis showed that FPL 64176 and (S)-(-)-Bay K8644 increased the cleavage of PKC to generate protein kinase M (PKM) and the specific calpain inhibitor MDL28170 blocked this increase. Prevention of PKM production by inhibiting calpain or depleting PKC blocked burst firing induction whereas direct loading of purified PKM into cells induced burst firing. Activation of the N-methyl-D-aspartic acid type glutamate or cholinergic receptors known to induce burst firing increased PKM expression. These results indicate that calcium influx through L-type channels activates a calcium-dependent protease that cleaves PKC to generate constitutively active and labile PKM resulting in burst firing of dopamine cells, a pathway that is involved in glutamatergic or cholinergic modulation of the central dopamine system.

Knockout of ERK1 enhances cocaine-evoked immediate early gene expression and behavioral plasticity

The ability of cocaine to produce lasting neural adaptations in mesocorticolimbic brain regions is thought to promote drug seeking and facilitate addiction in humans. The Ras-controlled Raf-MEK-ERK protein kinase signaling cascade has been implicated in the behavioral and neurobiological actions of cocaine in animals. However, these pharmacological studies have not been able to determine the specific role of the two predominant isoforms of ERK (ERK1 and ERK2) in these processes. We report here that deletion of the ERK1 isoform, which leads to increased ERK2 stimulus-dependent signaling, facilitates the development of cocaine-induced psychomotor sensitization and the acquisition of a cocaine conditioned place preference. Conversely, pharmacological blockade of ERK signaling attenuates the development of psychomotor sensitization to cocaine. Finally, cocaine-evoked gene expression in mesocorticolimbic brain regions is potentiated in ERK1-deficient mice. Thus, alterations in ERK signaling influence both the neurobiological impact of cocaine and its ability to produce enduring forms of drug experience-dependent behavioral plasticity. Our results suggest that enhanced ERK2 signaling following repeated drug exposure may facilitate the development of forms of cocaine-induced plasticity that contribute to addiction.

Synthesis, protein levels, activity, and phosphorylation state of tyrosine hydroxylase in mesoaccumbens and nigrostriatal dopamine pathways of chronically food-restricted rats

Chronic food restriction (FR) enhances the rewarding and motor-activating effects of abused drugs, and is accompanied by changes in dopamine (DA) dynamics and increased D-1 DA receptor-mediated cell signaling and transcriptional responses in nucleus accumbens (NAc). However, little is known about effects of FR on DA synthetic activity in the mesoaccumbens and nigrostriatal pathways. In Experiment 1 of the present study, tyrosine hydroxylase (TH) gene expression was measured in ventral tegmental area and substantia nigra, using real-time RT-PCR and in situ hybridization; no differences were observed between FR and ad libitum fed (AL) rats. In Experiment 2, TH protein levels, determined by Western blot, were found to be elevated in NAc and caudate-putamen (CPu) of FR relative to AL rats. In the absence of increased transcription, this may reflect a slowing of TH degradation. In Experiments 3 and 4, DA synthetic activity was assessed by Western blot measurement of TH phosphorylation at Ser40, and HPLC measurement of in vivo tyrosine hydroxylation rate, as reflected by DOPA accumulation following administration of a decarboxylase inhibitor (NSD-1015; 100 mg/kg, i.p.). Basal phospho-(Ser40)-TH levels did not differ between groups but DOPA accumulation was decreased by FR. Decreased DOPA synthesis, despite increased levels of TH protein, may reflect the inhibitory effect of increased DA binding to TH protein or decreased concentrations of cofactor tetrahydrobiopterin. Finally, in response to D-amphetamine (0.5 and 5.0 mg/kg, i.p.), phospho-(Ser40)-TH was selectively decreased in NAc of FR rats. This suggests increased feedback inhibition of DA synthesis-a possible consequence of postsynaptic receptor hypersensitivity, or increased extracellular DA concentration. These results indicate that FR increases TH protein levels, but may decrease the capacity for DA synthesis by decreasing TH activity. According to this scheme, the previously observed upregulation of striatal cell signaling and transcriptional responses to DA receptor agonist administration may include compensatory neuroadaptations.

Calmodulin-Stimulated Adenylyl Cyclase Gene Deletion Affects Morphine Responses

To define the roles of the calmodulin-stimulated adenylyl cyclases (AC1 and AC8) in morphine-induced analgesia, tolerance, physical dependence, and conditioned place preference, we used mice having targeted disruptions of either the AC1 or AC8 genes or both genes [double knockout mice (DKO)]. Mice lacking either AC1 or AC8 genes or DKO did not differ from wild-type mice in short-term antinociceptive responses to morphine measured in the tail-flick analgesia assay. Morphine tolerance that developed immediately within 3 h of morphine administration (10 mg/kg s.c.) was significantly attenuated in DKO mice and AC8 single knockout mice. Tolerance induced continually by daily injections of morphine (10 mg/kg s.c.) was also reduced in DKO mice. In DKO mice continually treated with morphine, there was a significant reduction in withdrawal behaviors, including reduced wet-dog shakes and forepaw tremor after naloxone injection (10 mg/kg i.p.). Morphine produced hyperlocomotion and conditioned place preference in wild-type mice, whereas DKO mice displayed significantly less hyperlocomotion and conditioned place preference. Furthermore, the significant increase in phosphorylated cAMP-response element binding protein (CREB) staining in ventral tegmental area induced by long-term morphine treatment was not evident in DKO mice, suggesting that CREB activation by morphine requires cAMP generated by AC1 and AC8. These results support the hypothesis that calmodulin-stimulated adenylyl cyclases are important mediators of the neuronal responses to morphine.

Progestin facilitation of lordosis in rodents involves adenylyl cyclase activity in the ventral tegmental area

Increasing cAMP, or activating dopamine type 1 (D(1)) or GABA(A)/benzodiazepine receptor complexes (GBRs), in the ventral tegmental area (VTA) enhances lordosis of rodents. Whether D(1)- and/or GBR-mediated increases in progestin-facilitated lordosis involve the cAMP-synthesizing enzyme, adenylyl cyclase, in the VTA, was investigated. In Experiment 1, ovariectomized estradiol (E(2); 10 microg at h 0)+progesterone (P; 250 microg at h 45)-primed hamsters first received bilateral infusions of the adenylyl cyclase inhibitor, 2',5'-dideoxyadenosine (DDA; 12 microM/side), or vehicle, and then were infused with the D(1) agonist, SKF38393 (100 ng/side), the GBR agonist, muscimol (100 ng/side), or vehicle, to the VTA. Lordosis was evaluated before and 30 min after each infusion. In Experiment 2, ovariectomized, E(2)-primed (10 microg at h 0) rats received VTA infusions of DDA (12 microM/side) or vehicle; SKF38393 (100 ng/side), muscimol (100 ng/side), or vehicle; and the neurosteroid, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP; 100 or 200 ng/side), or beta-cyclodextrin vehicle. Lordosis was assessed before the series of infusions, immediately after drug infusions and 10 or 60 min after 3alpha,5alpha-THP infusions. Progestin- or progestin plus SKF38393-or muscimol-mediated increases in lordosis were blocked by DDA pretreatment. Thus, in the VTA, progestins' membrane action may involve adenylyl cyclase.

Involvement of serine/threonine protein phosphatases sensitive to okadaic acid in restraint stress-induced hyperlocomotion in cocaine-sensitized mice

We used okadaic acid (OA), a potent preferential inhibitor of PP2A and PP5 but not PP1 (PP subfamilies), to examine the involvement of serine/threonine protein phosphatase (PP) in behavioral sensitization stimulated by treatment with cocaine in mice. Repeated administration of cocaine (10 mg kg(-1)) once a day for five consecutive days produced a progressive increase in locomotor activity that was maintained after the cessation of cocaine treatment, as revealed by the fact that a challenge dose of cocaine given on day 7 of withdrawal reproduced an enhanced stimulant effect. On the seventh day of withdrawal, OA-sensitive PP activity and expression of PP2A and PP5, but not PP1gamma, were increased in whole-cell extract of the nucleus accumbens and the ventral tegmental area in cocaine-sensitized mice, compared to saline-treated mice. Restraint stress increased locomotor activity in cocaine-sensitized mice on day 7 after drug administration was ceased. The locomotor activity was more susceptible to restraint-elicited enhancement in cocaine-sensitized mice than in saline-treated mice. The restraint-induced hyperlocomotion was suppressed by a single intracerebroventricular injection of OA immediately before restraint in cocaine-sensitized mice, but this suppression did not occur in saline-treated mice. The membrane fraction of the whole brain in cocaine-sensitized mice showed that OA-sensitive activity levels rise after mice are subjected to restraint, and this is concomitant with an increase in expression levels of PP2A and PP5, but not PP1gamma. These results suggest that the upregulated OA-sensitive PPs are involved in stress-induced hyperlocomotion in cocaine-sensitized mice. There may be intracellular mechanisms mediating psychostimulant cross-sensitization to stress underlying the spontaneous recurrence of its psychosis.

Genotype differences in behavior and tyrosine hydroxylase expression between wild-type and progesterone receptor knockout mice

Progesterone receptor (PR) activation can modulate the expression of male sexual behavior, both acutely in adulthood as well as during development, through long lasting effects on neural differentiation. One mechanism by which PR activation may affect behavior, during either epoch of life, is through alterations of the dopaminergic system. We investigated the effects of PR deletion on the sensitivity of sexual behavior to dopamine antagonism in male wild-type (WT) and progesterone receptor knockout (PRKO) mice and found that WT mice were more behaviorally sensitive to the effects of dopamine D1 receptor blockade. There were also genotype differences in tyrosine hydroxylase-immunoreactivity (TH-ir) in the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) as well as genotype differences in how TH expression changed in response to social and sexual experience. In particular, in the VTA, sexually experienced PRKO mice had significantly more cells expressing TH than sexually experienced WT mice. In the SNc, experienced PRKO males had significantly more cells expressing TH than naive PRKO males. Thus, it appears that PR deletion affects the display of sexual behavior and its modulation by dopamine, as well as the differentiation of dopaminergic cells and the plasticity of those cells in response to social environment and behavioral experience.

Programming effects of antenatal dexamethasone in the developing mesolimbic pathways

Elevated glucocorticoids, during pregnancy, alter emotionality and increase propensity to drug abuse later in life, albeit through substrates and mechanisms are largely unknown. In this study, we examined whether antenatal glucocorticoid exposure induces enduring structural changes in the nucleus accumbens (NAcc), an important relay point in the reward limbic circuitry. To this end, rat dams were exposed to the synthetic glucocorticoid dexamethasone (DEX) on days 18 and 19 of gestation, and stereological tools were used to assess the total volume of, and neuronal numbers in, the NAcc, as well as the density of mesencephalic dopaminergic inputs from the ventral tegmental area (VTA) to the NAcc in their adult offspring. Further, we used measures of bromodeoxyuridine incorporation into NAcc cells to examine whether DEX-induced effects on cell proliferation represent another mechanism through which glucocorticoids alter the structure of mesolimbic pathways and might influence addictive behavior. Our studies show that exposure to DEX during late gestation results in significantly reduced volumes and cell numbers in the NAcc. The latter measure correlated strongly with a reduced rate of cell proliferation in DEX-exposed animals. Moreover, the treatment resulted in a decreased number of cells expressing tyrosine hydroxylase in the VTA and an impoverished dopaminergic innervation of the NAcc. These observations, which identify glucocorticoid-sensitive structures and neurochemical targets within the developing "reward pathway," pave way for future studies designed to understand how early life events can predispose individuals for developing drug dependence in adolescent and adult life.

A simple and fast densitometric method for the analysis of tyrosine hydroxylase immunoreactivity in the substantia nigra pars compacta and in the ventral tegmental area

Parkinson's disease is a progressive dyskinetic disorder caused by degeneration of mesencephalic dopaminergic neurons in the substantia nigra pars compacta (SNpc) and, to a lesser extent, in the ventral tegmental area (VTA). Tyrosine hydroxylase (TH) is a rate-limiting enzyme for dopamine synthesis, therefore immunohistochemistry for TH can be used as an important marker of dopaminergic cell loss in these regions. Traditionally, immunohistochemical experiments are analyzed qualitatively by optical microscopic observation or more rarely semi-quantitatively evaluated by densitometry. A common problem with such papers is the lack of a clear explanation of the algorithms and macros employed in the semi-quantitative approaches. In this paper, we describe, in detail, an easy, fast and precise protocol for the analysis of TH immunoreactivity in SNpc and VTA using one of the most popular image analysis software packages (Image Pro-Plus). We believe that this protocol will facilitate the evaluation of mesencephalic TH immunoreactivity in various available animal models of Parkinson's disease.

Altered mesencephalic dopaminergic populations in adulthood as a consequence of brief perinatal glucocorticoid exposure

Early exposure to stressors is strongly associated with enduring effects on central nervous system function, but the mechanisms and neural substrates involved in this biological 'programming' are unclear. This study tested the hypothesis that inappropriate exposure to glucocorticoid stress hormones (GCs) during critical periods of development permanently alters the mesencephalic dopaminergic populations in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). Using a rat model, the synthetic GC dexamethasone was added to the maternal drinking water during gestational days 16-19 or over the first week of postnatal life. In adulthood, the effects upon tyrosine hydroxylase immunopositive (TH+) cell numbers in the midbrain, and monoamine levels in the forebrain, of the adult offspring were assessed and compared with control offspring whose dams received normal drinking water. In the VTA, both prenatal and postnatal dexamethasone treatment increased TH+ cell numbers by approximately 50% in males and females. Although prenatal dexamethasone treatment also increased TH+ cell numbers in the SNc by 40-50% in males and females, postnatal treatment affected females only by increasing TH+ cell numbers by approximately 30%. In comparison, similar changes were not detected in the monoamine levels of the dorsolateral striatum, nucleus accumbens or infralimbic cortex of either males or females, which is a feature likely to reflect adaptive changes in these pathways. These studies demonstrate that the survival or phenotypic expression of VTA and SNc dopaminergic neurones is profoundly influenced by brief perinatal exposure to GCs at times when endogenous levels are normally low. These findings are the first to demonstrate permanent changes in the cytoarchitecture within midbrain dopamine nuclei after perinatal exposure to stress hormones and implicate altered functionality. Thus, they have significance for the increasing use of GCs in perinatal medicine and indicate potential mechanisms whereby perinatal distress may predispose to the development of a range of psychiatric conditions in later life.

Effects of naltrexone on the ethanol-induced changes in the rat central dopaminergic system

AIMS

The opioid antagonist naltrexone may reduce ethanol reward, but the underlying neurochemical mechanisms has yet to be clarified. The afferent projections to the nucleus accumbens from the ventral tegmental area (VTA) provide a potential substrate by which endogenous opioids may modulate the dopaminergic rewarding effects of ethanol. We assessed mRNA levels of tyrosine hydroxylase (TH), a major regulatory enzyme in the dopamine synthesis and levels of dopamine and its metabolites after chronic ethanol administration with and without concomitant naltrexone.

METHODS

Sprague-Dawley rats were exposed to chronic ethanol consumption (5%, 4 weeks) with and without concomitant naltrexone administration. Levels of TH mRNA in the VTA and substantia nigra (SN) and dopamine and its metabolites in the striatum of the rats were measured by in situ hybridization and by high performance liquid chromatography, respectively.

RESULTS

Chronic ethanol consumption increased TH mRNA levels in the VTA, but did not cause any significant change in the SN. With naltrexone treatment, ethanol-induced increase in the TH mRNA level was reduced in the VTA. Chronic ethanol consumption did not cause any change in the levels of dopamine and its metabolites in most brain regions. Only in the striatum, ethanol consumption with naltrexone treatment significantly increases the dopamine level.

CONCLUSION

This finding supports the presence of interactions of opioid and dopaminergic systems in the VTA in mediating ethanol reward, and thus naltrexone attenuates the rewarding properties of ethanol by interfering with the ethanol-induced stimulation of the mesolimbic dopaminergic pathway.

Role of extracellular signal-regulated kinase in the ventral tegmental area in the suppression of the morphine-induced rewarding effect in mice with sciatic nerve ligation

We recently reported that micro-opioid receptor agonist morphine failed to induce its rewarding effects in rodents with sciatic nerve injury. In the present study, we investigated whether a state of neuropathic pain induced by sciatic nerve ligation could change the activities of the extracellular signal-regulated kinase (ERK) and p38 in the mouse lower midbrain area including the ventral tegmental area (VTA), and these changes could directly affect the development of the morphine-induced rewarding effect in mice. The sciatic nerve ligation caused a long-lasting and profound thermal hyperalgesia. A dose-dependent place preference induced by s.c. administration of morphine was observed in sham-operated mice, but not in sciatic nerve-ligated mice. We found here for the first time that nerve injury produces a sustained and significant reduction in protein levels of phosphorylated-ERK and -p38 in cytosolic preparations of the mouse lower midbrain. The inhibition of ERK activity by i.c.v. pre-treatment with either PD98059 or U0126 impaired the morphine-induced place preference. In contrast, i.c.v. treatment with a specific inhibitor of p38, SB203580, did not interfere with the morphine-induced rewarding effect. Immunohistochemical study showed a drastic reduction in phosphorylated-ERK immunoreactivity within tyrosine hydroxylase-positive cells of the VTA. These results suggest that a sustained reduction in the ERK-dependent signalling pathway in dopamine cells of the VTA may be implicated in the suppression of the morphine-induced rewarding effect under neuropathic pain.

Suppressing calcium/calmodulin-dependent protein kinase II activity in the ventral tegmental area enhances the acute behavioural response to cocaine but attenuates the initiation of cocaine-induced behavioural sensitization in rats

In the present experiments we administered an alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor antagonist (CNQX), N-methyl-D-aspartate (NMDA) receptor antagonist (AP-5), or l-type calcium channel blocker (diltiazem) directly into the ventral tegmental area (VTA) before each of four daily systemic cocaine injections in order to assess their influence on the initiation phase of behavioural sensitization. Results indicated that pretreatment with CNQX or AP-5 impaired the initiation of cocaine-induced behavioural sensitization. Intra-VTA administration of diltiazem significantly increased the behavioural activation induced by an acute cocaine injection, but impaired the development of cocaine-induced behavioural sensitization. Because AMPA and NMDA receptors, as well as l-type calcium channels are calcium permeable, we also investigated the role of the calcium-activated second messenger calcium/calmodulin-dependent protein kinase II (CaM-KII). Similar to the results obtained with diltiazem, administration of the CaM-KII inhibitor KN-93 into the VTA enhanced the acute behavioural response to cocaine but prevented the augmentation of cocaine-induced behavioural hyperactivity following repeated injections. Consistent with this finding, the behavioural hyperactivity produced by cocaine was markedly enhanced among homozygous alpha-CaM-KII knockout mice but the initiation of behavioural sensitization to cocaine was attenuated relative to wild-type mice. Separate experiments performed in rats demonstrated an increase in total protein levels of CaM-KII in the VTA 24 h after the last of seven daily injections of cocaine. Taken together, these results indicate that blocking l-type calcium channels or impairing CaM-KII activity in the VTA augments the acute behavioural hyperactivity induced by cocaine. The present findings also suggest that increased calcium influx through AMPA receptors, NMDA receptors and l-type calcium channels on dopaminergic neurons in the VTA contributes significantly to the initiation of behavioural sensitization by amplifying calcium signalling through CaM-KII.

The relationships between neurons containing dopamine and nitric oxide synthase in the ventral tegmental area

Ventral tegmental area (VTA) is a heterogeneous group of dopaminergic cells which contains interfascicular (IF), parabrachial (PBP) and rostral linear (RLi) nuclei. Neurons of this area are involved in the regulation of motor and motivational aspects of behavior and reveal high neuronal plasticity. Among many various neurotransmitters and neuromodulators, nitric oxide (NO) is localized in this region. In the present study, we investigated morphology and distribution of nitric oxide synthase (NOS)-positive neurons in VTA and their colocalization with dopaminergic neurons. The study was performed on six adult Wistar rats. After perfusional fixation, the brains were cut, immunostained for tyrosine hydroxylase (TH) and NOS and studied by confocal laser microscopy. In each of the three studied nuclei of VTA we investigated three different neuronal populations. Numerous TH-immunoreactive (TH-ir) and NOS-immunoreactive (NOS-ir) neurons are present in the studied region. Among them, a considerable number showed coexistence of both neurotransmitters. The populations of TH-ir and NOS-ir neurons interact with each other as manifested by the presence of NOS-ir endings on TH-ir neurons and vice versa. Taking the above into account, it may be suspected that NO is involved in the modulation of dopaminergic transmission.

Phospholipase Cgamma in distinct regions of the ventral tegmental area differentially modulates mood-related behaviors

Neurotrophic factor signaling pathways modulate cellular and behavioral responses to drugs of abuse. In addition, chronic exposure to morphine increases expression of phospholipase Cgamma1 (PLCgamma1) (a protein involved in neurotrophic signaling) in the ventral tegmental area (VTA), a neural substrate for many drugs of abuse. Using viral-mediated gene transfer to locally alter the activity of PLCgamma1, we show that overexpression of PLCgamma1 in rostral portions of the VTA (R-VTA) results in increased morphine place preference, whereas PLCgamma1 overexpression in the caudal VTA (C-VTA) results in avoidance of morphine-paired compartments. In addition, overexpression of PLCgamma1 in R-VTA causes increased preference for sucrose and increased anxiety-like behavior but does not affect responses to stress or nociceptive stimuli. In contrast, overexpression of PLCgamma1 in C-VTA decreases preference for sucrose and increases sensitivity to stress and nociceptive stimuli, although there was a tendency for increased anxiety-like behavior as seen for the R-VTA. These results show that levels of PLCgamma1 in the VTA regulate responsiveness to drugs of abuse, natural rewards, and aversive stimuli and point to the possibility that distinct topographical regions within the VTA mediate generally positive versus negative responses to emotional stimuli. Moreover, these data also support a role for drug-induced elevations in PLCgamma1 expression in the VTA in mediating long-term adaptations to drugs of abuse and aversive stimuli.

ERK1/2 activation in rat ventral tegmental area by the mu-opioid agonist fentanyl: an in vitro study

Opioid receptors in the ventral tegmental area, predominantly the mu-opioid receptors, have been suggested to modulate reinforcement sensitivity for both opioid and non-opioid drugs of abuse. The present study was conducted to study signal transduction proteins, which may mediate the functioning of mu-opioid receptors in the neurons of the ventral tegmental area. Therefore, brain slices of the ventral tegmental area were exposed in vitro to the specific mu-opioid agonist fentanyl and immunohistochemically stained for four different activated proteins using phospho-specific antibodies. Fentanyl dose-dependently activated extracellular signal-regulated protein in brain slices of the ventral tegmental area. This activation was reversible with naloxone. Furthermore, naloxone itself also activated extracellular signal-regulated protein kinase. Under the present conditions fentanyl did not affect extracellular signal-regulated protein kinase 1 and 2, Stat and cyclic AMP-response element-binding protein activity. The direct activation of extracellular signal-regulated protein kinase in ventral tegmental area slices by the mu-opioid agonist fentanyl may suggest a role of extracellular signal-regulated protein kinase in reward processes.

Tyrosine hydroxylase mRNA and protein are down-regulated by chronic clozapine in both the mesocorticolimbic and the nigrostriatal systems

The dopaminergic system is one of the most important targets for pharmacological treatment of schizophrenia. Despite substantial work on mechanisms of action, it is not clear which dopaminergic pathways mediate the therapeutic effects of antipsychotic drugs. It has been shown that chronic clozapine, an atypical antipsychotic, decreases dopamine levels in the mesocorticolimbic system but not in the nigrostriatal system. Because tyrosine hydroxylase is the rate-limiting enzyme in the biosynthesis of dopamine, we studied the effect of chronic clozapine in both dopaminergic systems. We demonstrated a decrease of tyrosine hydroxylase mRNA not only in the ventral tegmental area but also in the substantia nigra, the cell body areas of the mesocorticolimbic and the nigrostriatal systems, respectively. The reduced tyrosine hydroxylase mRNA level in these areas is accompanied by an ample reduction in the tyrosine hydroxylase protein level in their corresponding axonal terminal fields, the nucleus accumbens and the striatum. There was thus discordance between the clozapine-induced decrease of tyrosine hydroxylase mRNA and protein and the absence of an effect on dopamine levels in the nigrostriatal system. It has been suggested that reduced levels of dopamine in the mesocorticolimbic system are required for the antipsychotic effect of the drug. Therefore, the modulation of tyrosine hydroxylase gene expression by clozapine in the mesocorticolimbic system might be necessary for its antipsychotic effect; this effect might be of relevance when considering new atypical agents.

Opposite metabolic changes in the habenula and ventral tegmental area of a genetic model of helpless behavior

Congenitally helpless rats have been selectively bred to display an immediate helpless response to stress in order to model hereditary brain differences that contribute to depression vulnerability. Differences in regional brain metabolism between congenitally helpless and non-helpless rats were investigated using quantitative cytochrome oxidase histochemistry. The results indicated that congenitally helpless rats had 64-71% elevated metabolism in the habenula and a 25% elevation in the related interpeduncular nucleus. In contrast, helpless rats had 28% reduced metabolism in the ventral tegmental area (VTA) and 14-16% reductions in the basal ganglia and basolateral and central amygdala. The opposite metabolic changes in the habenula and ventral tegmental area may be especially important for determining the congenitally helpless rat's global pattern of brain activity, which resembles the metabolic activity pattern produced by dopamine antagonism.

Age-related decline in the tyrosine hydroxylase-immunoreactive innervation of the amygdala and dentate gyrus in mice

Numbers of catecholaminergic neurons are known to decline with aging. Whether projections of these neurons to the forebrain are similarly affected is not known. High densities of tyrosine hydroxylase-immunoreactive (TH-ir) fibers are found in the hippocampal formation (CA1-3, dentate gyrus) and in the amygdala of normal adult mice. We report here that densities of TH-ir fibers in the amygdala and hippocampus in aged mice (21-26 months) decrease dramatically and in a subregion-specific fashion. There is a reduction of 35% in the dentate gyrus, while hippocampal regions CA1 through CA3 are almost entirely spared. In the amygdala the lateral, basolateral, basomedial, and central nucleus were affected, with fiber reduction ranging from 19% to 34%. These results indicate that the age-related decline of TH-ir catecholaminergic cell bodies in the substantia nigra and the ventral tegmental area induces substantial losses of TH-ir fibers in the amygdala and dentate gyrus, but not in other areas of the hippocampal formation. This suggests that region-specific factors may be implicated in the regulation of maintenance vs. degeneration of TH-ir fibers during aging.

Involvement of alpha 7 nicotinic acetylcholine receptors in gene expression of dopamine biosynthetic enzymes in rat brain

Brain dopaminergic systems are critical in mediating the physiological responses to nicotine. The effects of several concentrations of nicotine (0.08, 0.17, or 0.33 mg/kg body weight) and involvement of alpha7 nicotinic acetylcholine receptors (nAChRs) in gene expression of key enzymes in dopamine biosynthesis were evaluated in the ventral tegmental area (VTA) and substantia nigra (SN), cell bodies of the mesocorticolimbic and nigrostriatal pathways. Nicotine elicited a dose-dependent elevation of mRNA for tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine biosynthesis in VTA and SN. The VTA was more sensitive to lower concentrations of nicotine with maximal response observed with the lowest dose of nicotine. Nicotine also elevated mRNA levels of GTP cyclohydrolase I (GTPCH), rate limiting in biosynthesis of TH's essential cofactor tetrahydrobiopterin in both dopaminergic locations. The changes in TH and GTPCH mRNAs were correlated. Pretreatment with the alpha7 nAChR antagonist methyllycaconitine prevented the nicotine-induced rise in TH or GTPCH mRNA in VTA and SN. Administration of alpha7 nAChR agonist 3-[2,4-dimethoxybenzilidene]anabaseine at 1 to 10 mg/kg or (E,E-3-(cinnamylidene)anabaseine at 0.3 to 1 mg/kg increased TH mRNA in VTA and SN, but not in peripheral catecholaminergic cells. Thus, agonists of alpha7 nAChRs have therapeutic potential for increasing TH gene expression in dopaminergic regions without some of nicotine's disadvantages, such as its harmful effects on the cardiovascular system. The findings indicate that nicotine may regulate dopamine biosynthesis by alterations in gene expression of TH and its cofactor. The alpha7 nAChRs are involved in mediating these effects of nicotine.

Differential regulation of glutamic acid decarboxylase mRNA and tyrosine hydroxylase mRNA expression in the aged manganese-treated rats

Recent studies have implicated chronic elevated exposures to environmental agents, such as metals (e.g. manganese, Mn) and pesticides, as contributors to neurological disease. Eighteen-month-old rats received intraperitoneal injections of manganese chloride (6 mg Mn/kg/day) or equal volume of saline for 30 days in order to study the effect of manganese on the dopamine- and GABA-neurons. The structures studied were substantia nigra, striatum, ventral tegmental area, nucleus accumbens and globus pallidus. First, we studied the enzymatic activity of mitochondrial complex II succinate dehydrogenase (SDH). We found an overall decrease of SDH in the different brain areas analyzed. We then studied the mRNA levels for tyrosine hydroxylase (TH) and the dopamine transporter (DAT) by in situ hybridization. TH mRNA but not DAT mRNA was significantly induced in substantia nigra and ventral tegmental area following Mn treatment. Correspondingly, TH immunoreactivity was increased in substantia nigra and ventral tegmental area. Manganese treatment significantly decreased GAD mRNA levels in individual GABAergic neurons in globus pallidus but not in striatum. We also quantified the density of glial fibrillary acidic protein (GFAP)-labeled astrocytes and OX-42 positive cells. Reactive gliosis in response to Mn treatment occurred only in striatum and substantia nigra and the morphology of the astrocytes was different than in control animals. These results suggest that the nigrostriatal system could be specifically damaged by manganese toxicity. Thus, changes produced by manganese treatment on 18-month-old rats could play a role in the etiology of Parkinson's disease.

Lack of persistent changes in the dopaminergic system of rats withdrawn from methamphetamine self-administration

A continuing challenge for studies in the neurobiology of drug abuse is to identify and characterize long-lived neuroadaptations that can trigger craving and relapse. We previously reported that rats that had actively self-administered methamphetamine for 5 weeks and were then withdrawn from methamphetamine for 24 h showed marked decreases in somatodendritic dopamine D(2) autoreceptor levels in the ventral tegmental area and median and dorsal part of the substantia nigra zona compacta with a corresponding down-regulation of dopamine D(1) receptors in the shell of the nucleus accumbens. The purpose of the present study was to determine whether neuroadaptive changes in dopamine receptors or transporters in the brains of rats withdrawn for 24 h from chronic methamphetamine self-administration are persistent changes that can be demonstrated long after withdrawal. A "yoked" procedure was used in which rats were tested simultaneously in groups of three, with only one rat actively self-administering methamphetamine while the other two received yoked injections of either methamphetamine or saline. In vitro quantitative autoradiography was used to determine densities of dopamine uptake sites and dopamine D(1) and D(2) receptors in different brain regions following 7- and 30-day periods of withdrawal from chronic methamphetamine self-administration. No changes in dopamine transporter and dopamine receptor numbers were detected in any brain region examined in rats self-administering methamphetamine compared with littermates receiving yoked infusions of either methamphetamine or saline. Thus, neuroadaptive changes in densities of dopamine receptors or transporters in certain brain areas may contribute to the reinforcing effects of methamphetamine during the acquisition and maintenance phases of self-administration, but do not appear to contribute to the long-lasting neuroadaptive effects of chronic methamphetamine self-administration, which may trigger craving and relapse.

Evidence for the involvement of cyclooxygenase activity in the development of cocaine sensitization

Phospholipase A2 (PLA(2)) activation generates the release of arachidonic acid (AA) and platelet-activating factor (PAF), two compounds which may be involved in neuroplasticity. In previous studies, we found that PLA(2) activation is involved in the development of stimulant sensitization. In the present study, we have examined the roles of AA and PAF in the development of stimulant sensitization using agonists and antagonists selective for PAF receptors or the induction of various AA cascade-mediated eicosanoids. Sprague-Dawley rats were treated for 5 days with cocaine (30 mg/kg) or D-amphetamine (1 mg/kg) preceded 15 min earlier by various antagonists, and then tested following a 10-day withdrawal period for cocaine (15 mg/kg) or D-amphetamine (0.5 mg/kg)-induced locomotion. Consistent with our earlier work, pretreatment with the PLA(2) inhibitor quinacrine (25 mg/kg) blocked the development of cocaine and amphetamine sensitization. The lipoxygenase (LOX) inhibitors nordihydroguaiaretic acid (NDGA) (5-10 mg/kg) and MK-886 (1 mg/kg) had no effect on cocaine sensitization. The PAF receptor antagonist WEB 2086 (5-10 mg/kg) reduced the development of cocaine sensitization. The cyclooxygenase (COX) inhibitors indomethacin (1-2 mg/kg), piroxicam (0.5-1 mg/kg), 6-methoxy-2-napthylacetic acid (6-MNA; 0.5-1 mg/kg), and NS-398 (0.5-1 mg/kg) blocked the development of cocaine sensitization. The COX inhibitors indomethacin (2 mg/kg) and 6-MNA (1 mg/kg) also reduced the development of amphetamine sensitization. Rats were administered bilateral intraventral tegmental area (VTA) injections of D-amphetamine (5 microg/side) or saline coadministered with indomethacin (0.5 microg/side) or vehicle three times over 5 days and were then tested after a 10-day withdrawal for D-amphetamine (0.5 mg/kg ip)-induced locomotion. Intra-VTA amphetamine induced a robust form of amphetamine sensitization, which was blocked by coadministration of indomethacin. Unilateral intra-VTA injections of PAF (1 microg) did not significantly alter cocaine (15 mg/kg ip)-induced locomotion when tested after a 3-day withdrawal. These findings suggest that COX, and possibly PAF, activity is involved in the development of stimulant sensitization. Neuroanatomical studies demonstrate that this may occur at the level of the VTA.

Search for the presence of glucocorticoid receptors in dopaminergic neurons of rat ventral tegmental area and substantia nigra

Using non-fluorescent immunocytochemical double-labelling procedure and specific antibodies visualizing GR (glucocorticoid receptors) and TH (tyrosine hydroxylase) we have been looking for the co-localization of both antigens in neurons of the rat ventral tegmental area and adjacent substantia nigra. This experimental direction has been inspired by the available data showing that alterations in the level of circulating glucocorticosteroids have distinct effects on the intensity of dopaminergic neurotransmission. Thus, it was of interest to find the anatomical background for the above interaction. It has been found that the rat ventral tegmental area and substantia nigra possess a relatively moderate number of cells with active GR, i.e. receptors which are condensed in the nuclei. Further, we found that dopaminergic neurons (TH-positive) of the ventral tegmental area and substantia nigra were not immunopositive for GR. This observation was in the sharp contrast to the results from the locus coeruleus, where the co-localization of GR with TH was a general rule. Above anatomical data indicate that glucocorticoid receptors influence the dopaminergic neurotransmission by an indirect mechanism, which possibly involves intermittent neurotransmitter.

Dextromethorphan increases tyrosine hydroxylase mRNA in the mesencephalon of adolescent rats

Dextromethorphan (DM), an antitussive widely available in over-the-counter, has been abused mostly in teenage groups at high doses. To examine effects of DM on the reward pathway, we injected a high dose of DM (40 mg/kg; intraperitoneally) into the adolescent rat and measured tyrosine hydroxylase (TH) mRNA by in situ hybridization in the ventral tegmental area (VTA) and the substantia nigra (SN). Remarkable increases in the level of TH mRNA were observed in the VTA and SN 2 h after DM injection. Stereotyped behavior and ataxia increased, and rearing decreased by DM administration. These results suggest that DM-induced increase in TH mRNA expression in mesencephalon contribute to the reinforcing property and the behavioral effects of DM.

Substance P endopeptidase-like activity is altered in various regions of the rat central nervous system during morphine tolerance and withdrawal

In this study the level of a substance P endopeptidase (SPE)-like activity was measured in different regions of the rat central nervous system (CNS) after chronic administration of morphine. Male rats (200-220 g) were randomly divided into four groups. Two groups were injected (s.c.) with morphine (10 mg/kg) twice daily, whereas the other two received saline under identical conditions. After 8 days, when animals were completely tolerant to morphine, one of the morphine-treated groups and one group of saline-injected rats were given naloxone (s.c. 2 mg/kg). Withdrawal signs were observed and recorded. The enzyme activity was measured in extracts of the various CNS tissues by following the conversion of synthetic substance P (SP) to its N-terminal fragment SP(1-7) using a radioimmunoassay detecting this product. In discrete CNS areas including periaqueductal grey, spinal cord, substantia nigra and ventral tegmental area (VTA) a significant increase in enzyme activity was observed in the withdrawal group, while tolerant rats exhibited decreased SPE-like activity in the striatum (see Table 1). The enhanced enzyme activity during withdrawal is in agreement with our previous observation that the levels of SP(1-7) in rat brain are affected following naloxone precipitated withdrawal. In some tissues, including VTA, a correlation between the SPE-like activity and the intensity of the opioid abstinence was observed. Our result suggests that the elevated SPE-like activity is responsible for enhanced release of SP(1-7) in rats during morphine withdrawal, affirming a modulatory or regulative role of this enzyme in this state of opioid dependence.

Orexin-a activates phospholipase C- and protein kinase C-mediated Ca2+ signaling in dopamine neurons of the ventral tegmental area

The orexin-orexin receptor system has been implicated in the regulation of wakefulness/sleep states. Behavioral and psycho-stimulant effects of orexins have also been shown. Mesolimbic dopamine neurons in the ventral tegmental area (VTA) are implicated in the regulation of reward and wakefulness/sleep, In the present study, we examined the effect of orexin-A on cytosolic [Ca2+]i concentration ([Ca2+]) in the isolated rat VTA dopamine neurons. Orexin-A (10-12-10-8 M) concentration dependently increased [Ca2+]i in dopamine-containing neurons. The [Ca2+]i responses to orexin-A were inhibited under Ca2+-free conditions and by blockers of voltage-gated L- and N-type [Ca2+]i channels, nitrendipine and omega-conotoxin, respectively. The [Ca2+]i responses were also abolished by a phosphatidylcholine-specific phospholipase C inhibitor, D609, and a protein kinase C (PKC) inhibitor, calphostin C. A PKC activator, TPA, mimicked orexin-A in increasing [Ca2+]i. These results indicate that orexin-A increases [Ca2+]i in VTA dopamine neurons via phosphatidylcholine-specific PLC- and PKC-mediated activation of L- and N-type Ca2+ channels. This effect may serve as the mechanism by which orexin regulates wakefulness/sleep states and exerts its behavioral and psychostimulant effects.

Anti-sense oligonucleotides, for progestin receptors in the VMH and glutamic acid decarboxylase in the VTA, attenuate progesterone-induced lordosis in hamsters and rats

Immunocytochemical (ICC) staining for progesterone (P) receptors (PRs) and glutamic acid decarboxylase (GAD), the enzyme responsible for GABA production, reveal that there are many PRs in the ventral medial hypothalamus (VMH) and many GAD containing neurons in the ventral tegmental area (VTA). To investigate P's action on lordosis in the VMH and VTA, anti-sense oligos specific to PRs and GAD(65&67) were intracerebrally infused into the VMH and VTA of 24 ovariectomized hamsters and 40 ovariectomized rats. Estradiol benzoate (2 microg) primed hamsters and rats were infused to the VMH and the VTA with either PR (250 ng/1.0 microl infusion) or GAD (500 ng/1.0 microl infusion) anti-sense oligos, their scramble controls, or saline vehicle at hour 0 and again at hour 24. At hr 44, rodents were subcutaneously injected with P (500 microg) and were tested for sexual receptivity with a male 4 h later. There were significant reductions in lordosis of hamsters and rats following PR anti-sense infusions to the VMH compared to scrambled or vehicle control infusions. Effects of PR anti-sense to the VMH were not different from combined VMH and VTA PR anti-sense infusions; however, VMH infusions reduced lordosis compared to VTA-only anti-sense infusions. GAD anti-sense infusions reduced lordosis when infused into the VTA, compared to scrambled or saline vehicle infusions. Lordosis responsiveness following VTA GAD anti-sense infusions was not different from combined VMH and VTA infusions, but VTA infusions of GAD anti-sense reduced lordosis compared to VMH-only anti-sense infusions. These data suggest that in the VMH, PRs are important for P-facilitated lordosis, whereas in the VTA, GABAergic neurons may be an important substrate for mediating P's actions on lordosis of rodents.

Stoichiometry of tyrosine hydroxylase phosphorylation in the nigrostriatal and mesolimbic systems in vivo: effects of acute haloperidol and related compounds

Electrical stimulation of the medial forebrain bundle increases (32)P incorporation into striatal tyrosine hydroxylase (TH) at Ser (19), Ser(31), and Ser(40). In the present studies, the effects of acute haloperidol and related drugs on sitespecific TH phosphorylation stoichiometry (PS) in the nigrostriatal and mesolimbic systems were determined by quantitative blot immunolabeling using phosphorylation statespecific antibodies. The striatum (Str), substantia nigra (SN), nucleus accumbens (NAc), and ventral tegmental area (VTA) from Sprague-Dawley rats were harvested 30-40 min after a single injection of either vehicle, haloperidol (2 mg/kg), raclopride (2 mg/kg), clozapine (30 mg/kg), or SCH23390 (0.5 mg/kg). In vehicle-injected control rats, Ser(19) PS was 1.5- to 2. 5-fold lower in Str and NAc than in SN and VTA, Ser(31) PS was two-to fourfold higher in Str and NAc than in SN and VTA, and Ser(40) PS was similar between the terminal field and cell body regions. After haloperidol, Ser(40) PS increased twofold in Str and NAc, whereas a smaller increase in SN and VTA was observed. The effects of haloperidol on Ser(19) PS were similar to those on Ser(40) in each region; however, haloperidol treatment increased Ser(31) PS at least 1.6-fold in all regions. The effects of raclopride on TH PS were comparable to those of haloperidol, whereas clozapine treatment increased TH PS at all sites in all regions. By contrast, the effects of SCH23390 on TH PS were relatively small and restricted to the NAc. The stoichiometries of site-specific TH phosphorylation in vivo are presented for the first time. The nigrostriatal and mesolimbic systems have common features of TH PS, distinguished by differences in TH PS between the terminal field and cell body regions and by dissimilar increases in TH PS in the terminal field and cell body regions after acute haloperidol.

Up-regulation of tyrosine hydroxylase mRNA in a sub-population of A10 dopamine neurons in Parkinson's disease

Neuronal injury has been consistently found in A10 midbrain dopamine neurons in Parkinson's disease (PD). To assess changes in neurotransmitter-related gene transcription, in these neurons in PD, tyrosine hydroxylase (TH) mRNA expression was examined in the ventral tegmental area (VTA) of seven PD cases and seven control subjects, using in situ hybridization histochemistry (ISHH). In controls, TH mRNA expression was found in both melanised and non-melanised neurons in the VTA. Neither population expressed dopamine beta-hydroxylase (DBH). Of the melanised neurons, 99% were TH mRNA positive. The level of the TH mRNA signal (expressed as grain density per cell) was similar in the two populations (melanised: 0.129+/-0.004 (mean+/-S.E.M.), n=142 vs. non-melanised: 0.138+/-0.006, n=89, P>0.05, Student's t-Test). In PD cases there was no significant change in TH mRNA expression in melanised neurons (0.138+/-0.003, n=196), and the proportion of positively labeled melanised neurons was 98%. However, non-melanised neurons showed significantly higher TH mRNA (0.163+/-0.006, n=87) than non-melanised neurons in control subjects (P<0.005) and melanised neurons in the PD cases (P<0.0005). This up-regulation of TH mRNA expression in non-melanised neurons may suggest the existence of a compensatory mechanism at presynaptic level.

Caspase-3: A vulnerability factor and final effector in apoptotic death of dopaminergic neurons in Parkinson's disease

Caspase-3 is an effector of apoptosis in experimental models of Parkinson's disease (PD). However, its potential role in the human pathology remains to be demonstrated. Using caspase-3 immunohistochemistry on the postmortem human brain, we observed a positive correlation between the degree of neuronal loss in dopaminergic (DA) cell groups affected in the mesencephalon of PD patients and the percentage of caspase-3-positive neurons in these cell groups in control subjects and a significant decrease of caspase-3-positive pigmented neurons in the substantia nigra pars compacta of PD patients compared with controls that also could be observed in an animal model of PD. This suggests that neurons expressing caspase-3 are more sensitive to the pathological process than those that do not express the protein. In addition, using an antibody raised against activated caspase-3, the percentage of active caspase-3-positive neurons among DA neurons was significantly higher in PD patients than in controls. Finally, electron microscopy analysis in the human brain and in vitro data suggest that caspase-3 activation precedes and is not a consequence of apoptotic cell death in PD.

Inhibition of catechol-O-methyltransferase (COMT) in the brain does not affect the action of dopamine and levodopa: an in vitro electrophysiological evidence from rat mesencephalic dopamine neurons

In order to study whether the membrane hyperpolarization and firing inhibition caused by dopamine and levodopa on rat midbrain dopamine cells are affected by the inhibition of brain catechol-O-methyl-transferase (COMT), intracellular electrophysiological recordings were made from these neurons maintained in vitro. Here we report that a treatment of the cerebral tissue with tolcapone, a central and peripheral inhibitor of COMT, does not change the membrane responses of midbrain dopamine neurons to dopamine and levodopa. The lack of modification of the dopaminergic effects by tolcapone suggests that the pharmacological inhibition of intracerebral COMT does not have detectable action on dopamine neurotransmission. Therefore, the therapeutic action of tolcapone in Parkinson's disease, might be dependent on the reduction of COMT activity in the extracerebral tissue.

Human midbrain dopamine neurons express serotonin 2A receptor: an immunohistochemical demonstration

We demonstrated intense serotonin (5-HT) 2A receptor immunoreactivity in the human ventral tegmental area (VTA) using by a recently raised antibody against 5-HT2A receptor. The substantia nigra (SN) neurons also showed 5-HT2A receptor immunoreactivity. Double immunohistochemistry of 5-HT2A receptor and tyrosine hydroxylase (TH) revealed many neurons doubly labeled by 5-HT2A receptor and TH in the VTA and SN. It is suggested that activity of human midbrain dopaminergic neurons might be strongly regulated via 5-HT2A receptors at the level of their originating nuclei.

Necessary role for ventral tegmental area adenylate cyclase and protein kinase A in induction of behavioral sensitization to intraventral tegmental area amphetamine

In the present study, we investigated the effects of selective activation or inhibition of ventral tegmental area (VTA) adenylate cyclase (AC) and protein kinase A (PKA) on long-term sensitization induced by repeated intra-VTA or peripheral amphetamine (AMPH). Selective inhibition of AC by SQ 22,536 (9-(tetrahydro-2-furanyl)-9H-purin-6-amine; 100 nmol/side bilateral into VTA) had no effect on acute basal locomotion but attenuated the locomotor stimulation induced by acute i.p. AMPH (1.5 mg/kg). Coinjection of SQ 22,536 (100 nmol/side) fully blocked the sensitization induced by repeated intra-VTA AMPH (15 nmol/side) but had no detectable effect on the sensitization induced by repeated i. p. AMPH. Persistent activation of AC by intra-VTA cholera toxin (500 ng/side) modestly increased acute locomotion and induced a robust sensitization to i.p. AMPH challenge 10 days after the last of three repeated VTA microinjections. Selective inhibition of PKA by Rp-adenosine-3',5'-cyclic monophosphothioate triethylamine (Rp-cAMPS; 25 nmol/side) had no effect on acute basal or AMPH-stimulated locomotion. Coinjection of Rp-cAMPS (25 nmol/side) fully blocked the sensitization induced by repeated intra-VTA AMPH but had no effect on sensitization induced by repeated i.p. AMPH. Intra-VTA microinjection of the selective PKA activator Sp-adenosine-3',5'-cyclic monophosphothioate triethylamine (Sp-cAMPS; 25-100 nmol/side) dose-dependently stimulated acute locomotion and exerted synergistic effects on locomotor activity when coinfused into the VTA with AMPH but had no detectable effect on acute i.p. AMPH-induced locomotion. Repeated intra-VTA Sp-cAMPS did not induce sensitization to AMPH challenge but potentiated the sensitization induced by repeated i.p. AMPH. These results suggest that VTA cAMP signal transduction is necessary for the induction of persistent sensitization to intra-VTA amphetamine and that peripheral and intra-VTA AMPH may not induce behavioral sensitization by identical mechanisms.

Regulation of tyrosine hydroxylase promoter activity by chronic morphine in TH9.0-LacZ transgenic mice

Levels of tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis, are known to be upregulated in specific brain regions by chronic administration of drugs of abuse. Chronic morphine administration increases TH levels in the locus coeruleus and ventral tegmental area, whereas chronic cocaine administration increases TH levels in the ventral tegmental area only. While such upregulation of TH has been related to behavioral effects of the drugs, the mechanism underlying these adaptations has remained controversial. To study the possibility that upregulation of TH occurs at the transcriptional level, we investigated the effect of chronic morphine or cocaine treatment on the activity of the TH gene promoter (9.0 kb), coupled to the LacZ reporter gene, in transgenic mice. These TH9.0-LacZ mice have been shown to exhibit correct tissue-specific expression and regulation of the reporter gene. We show here that chronic (but not acute) exposure of the TH9.0-LacZ mice to morphine increases the expression of beta-galactosidase (which is encoded by the LacZ gene) in the locus coeruleus by twofold compared with sham-treated mice. In contrast, beta-galactosidase expression in the ventral tegmental area was decreased 20-25% by chronic morphine and unaffected by chronic cocaine administration. Similar results were obtained after analysis of TH mRNA levels in these brain regions by in situ hybridization. These results suggest that chronic morphine upregulates TH expression via transcriptional mechanisms in the locus coeruleus but by post-transcriptional mechanisms in the ventral tegmental area.

Self-stimulation of lateral hypothalamus and ventral tegmentum increases the levels of noradrenaline, dopamine, glutamate, and AChE activity, but not 5-hydroxytryptamine and GABA levels in hippocampus and motor cortex

Self-stimulation (SS) rewarding experience induced structural changes have been demonstrated in the hippocampal and motor cortical pyramidal neurons. In the present study, we have evaluated whether these changes are accompanied by neurochemical alterations in the hippocampus and motor cortex in SS experienced rats. Self-stimulation experience was provided one hour daily over a period of 10 days through stereotaxically implanted bipolar stainless steel electrodes, bilaterally in lateral hypothalamus and substantia nigra-ventral tegmental area. Self-stimulation experience resulted in a significant (P < 0.001) increase in the levels of noradrenaline, dopamine, glutamate and AChE activity but not 5-hydroxytryptamine and GABA levels in hippocampus and motor cortex. Such alterations in the levels of neurotransmitters may enhance the cognitive functions in the SS experienced rats.

[The projections of the amygdaloid body, ventral tegmental area and substantia nigra to different segments of the nucleus accumbens in the canine brain]

Peculiarities of labelled cells distribution in amygdala, tegmental ventral field and substantia nigra were studied in dog by method based on retrograde axonal transport of horseradish peroxidase with marker injected into either lateral or medial segments of nucleus accumbens. Medical segment receives projection fibers only from amygdala basal nucleus and tegmental ventral field, i. e. from structures related to limbic system, while lateral one--from all dopaminergic parts of substantia nigra, associated with regulation of locomotor behavior as well.

Localization of GTP cyclohydrolase in monoaminergic but not nitric oxide-producing cells

The first and rate-limiting enzyme in tetrahydrobiopterin (BH4) biosynthesis is GTP cyclohydrolase (GTPCH). BH4 serves as the essential cofactor for aromatic L-amino acid hydroxylases, such as tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH), as well as for nitric oxide synthase (NOS). We hypothesized that to provide access to the cofactor, a close association exists between BH4-synthesizing and BH4-dependent enzymes, and we determined the relationship among GTPCH, neuronal NOS (nNOS), and TH in rat brain and adrenal gland using immunohistochemistry and in situ hybridization. Analyses of adjacent sections revealed specific localization of GTPCH in TH-containing cells of the substantia nigra, ventral tegmental area, hypothalamus, locus ceruleus, and adrenal medulla, and also in TPH-containing cells of the dorsal raphe nucleus and pineal gland. Thus, BH4 can be synthesized in all monoaminergic cells and is readily available for the enzymes requiring it. In contrast, analysis of adjacent sections showed that nNOS was not colocalized with GTPCH. Scattered nNOS-positive cells were found in the cortex, striatum, cerebellum, and olfactory bulb, all areas that receive monoaminergic innervation. The absence of GTPCH in nNOS cells suggests that nitric oxide-producing cells may either obtain biopterin from monoamine-containing processes which terminate in close proximity, or take up biopterin released into the blood. Double labelling of the same section for TH and nNOS revealed the TH nerve terminals connecting with the nNOS-positive cell bodies, suggesting the possibility that the BH4-containing nerve terminals may directly donate this cofactor to the nNOS-containing cells.