Studies on dopaminergic and GABAergic markers in striatum reveals a decrease in the dopamine transporter in schizophrenia

Impact of hypoxia-ischemia and dopamine treatment on dopamine receptor binding density in the preterm fetal sheep brain

Dopamine is often used to treat hypotension in preterm infants who are at risk of hypoxic-ischemic (HI) brain injury due to cerebral hypoperfusion and impaired autoregulation. There is evidence that systemically administered dopamine crosses the preterm blood-brain barrier. However, the effects of exogenous dopamine and cerebral HI on dopaminergic signaling in the immature brain are unknown. We determined the effect of HI and dopamine on D1 and D2 receptor binding and expressions of dopamine transporter (DAT) and tyrosine hydroxylase (TH) in the striatum of the preterm fetal sheep. Fetal sheep (99 days of gestation, term = 147days) were unoperated controls (n = 6) or exposed to severe HI using umbilical cord occlusion and saline infusion (UCO + saline, n = 8) or to HI with dopamine infusion (UCO + dopamine, 10 µg/kg/min, n = 7) for 74 h. D1 and D2 receptor densities were measured by autoradiography in vitro. DAT, TH, and cell death were measured using immunohistochemistry. HI resulted in cell death in the caudate nucleus and putamen, and dopamine infusion started before HI did not exacerbate or ameliorate these effects. HI led to reduced D1 and D2 receptor densities in the caudate nucleus and reduction in DAT protein expression in the caudate and putamen. Fetal brains exposed to dopamine in addition to HI were not different from those exposed to HI alone in these changes in dopaminergic parameters. We conclude that dopamine infusion does not alter the striatal cell death or the reductions in D1 and D2 receptor densities and DAT protein expression induced by HI in the preterm brain.NEW & NOTEWORTHY This is the first study on the effects of hypoxia-ischemia and dopamine treatment on the dopaminergic pathway in the preterm brain. In the striatum of fetal sheep (equivalent to ∼26-28 wk of human gestation), we demonstrate that hypoxia-ischemia leads to cell death, reduces D1 and D2 receptors, and reduces dopamine transporter. Intravenous dopamine infusion at clinical dosage used in preterm human infants does not alter the striatal cell death, D1 and D2 receptor density levels, and DAT protein expressions after hypoxia-ischemia in the preterm brain.

Altered levels of dopamine transporter in the frontal pole and dorsal striatum in schizophrenia

The dopamine hypothesis proposes that there is a hypodopaminergic state in the prefrontal cortex and a hyperdopaminergic state in the striatum of patients with schizophrenia. Evidence suggests the hyperdopaminergic state in the striatum is due to synaptic dopamine elevation, particularly in the dorsal striatum. However, the molecular mechanisms causing disrupted dopaminergic function in schizophrenia remains unclear. We postulated that the dopamine transporter (DAT), which regulates intra-synaptic dopamine concentrations by transporting dopamine from the synaptic cleft into the pre-synaptic neuron, could be involved in dopaminergic dysfunction in schizophrenia. Therefore, we measured levels of DAT in the cortex and striatum from patients with schizophrenia and controls using postmortem human brain tissue. Levels of desmethylimipramine-insensitive mazindol-sensitive [³H]mazindol binding to DAT were measured using in situ radioligand binding and autoradiography in gray matter from Brodmann’s area (BA) 10, BA 17, the dorsal striatum, and nucleus accumbens from 15 patients with schizophrenia and 15 controls. Levels of desmethylimipramine-insensitive mazindol-sensitive [³H]mazindol binding were significantly higher in BA 10 from patients with schizophrenia (p = 0.004) and significantly lower in the dorsal striatum (dorsal putamen p = 0.005; dorsal caudate p = 0.007) from those with the disorder. There were no differences in levels of desmethylimipramine-insensitive [³H]mazindol binding in BA 17 or nucleus accumbens. These data raise the possibility that high levels of DAT in BA 10 could be contributing to lower synaptic cortical dopamine, whereas lower levels of DAT could be contributing to a hyperdopaminergic state in the dorsal striatum.

Putative presynaptic dopamine dysregulation in schizophrenia is supported by molecular evidence from post-mortem human midbrain

The dopamine hypothesis of schizophrenia posits that increased subcortical dopamine underpins psychosis. In vivo imaging studies indicate an increased presynaptic dopamine synthesis capacity in striatal terminals and cell bodies in the midbrain in schizophrenia; however, measures of the dopamine-synthesising enzyme, tyrosine hydroxylase (TH), have not identified consistent changes. We hypothesise that dopamine dysregulation in schizophrenia could result from changes in expression of dopamine synthesis enzymes, receptors, transporters or catabolic enzymes. Gene expression of 12 dopamine-related molecules was examined in post-mortem midbrain (28 antipsychotic-treated schizophrenia cases/29 controls) using quantitative PCR. TH and the synaptic dopamine transporter (DAT) proteins were examined in post-mortem midbrain (26 antipsychotic-treated schizophrenia cases per 27 controls) using immunoblotting. TH and aromatic acid decarboxylase (AADC) mRNA and TH protein were unchanged in the midbrain in schizophrenia compared with controls. Dopamine receptor D2 short, vesicular monoamine transporter (VMAT2) and DAT mRNAs were significantly decreased in schizophrenia, with no change in DRD3 mRNA, DRD3nf mRNA and DAT protein between diagnostic groups. However, DAT protein was significantly increased in putatively treatment-resistant cases of schizophrenia compared to putatively treatment-responsive cases. Midbrain monoamine oxidase A (MAOA) mRNA was increased, whereas MAOB and catechol-O-methyl transferase mRNAs were unchanged in schizophrenia. We conclude that, whereas some mRNA changes are consistent with increased dopamine action (decreased DAT mRNA), others suggest reduced dopamine action (increased MAOA mRNA) in the midbrain in schizophrenia. Here, we identify a molecular signature of dopamine dysregulation in the midbrain in schizophrenia that mainly includes gene expression changes of molecules involved in dopamine synthesis and in regulating the time course of dopamine action.

Fine-mapping reveals novel alternative splicing of the dopamine transporter

The dopamine transporter gene (SLC6A3, DAT) has been implicated in the pathogenesis of numerous psychiatric and neurodevelopmental disorders, including schizophrenia (SZ). We previously detected association between SZ and intronic SLC6A3 variants that replicated in two independent Caucasian samples, but had no obvious function. In follow-up analyses, we sequenced the coding and intronic regions of SLC6A3 to identify complete linkage disequilibrium patterns of common variations. We genotyped 78 polymorphisms, narrowing the potentially causal region to two correlated clusters of associated SNPs localized predominantly to introns 3 and 4. Our computational analysis of these intronic regions predicted a novel cassette exon within intron 3, designated E3b, which is conserved among primates. We confirmed alternative splicing of E3b in post-mortem human substantia nigra (SN). As E3b introduces multiple in-frame stop codons, the SLC6A3 open reading frame is truncated and the spliced product may undergo nonsense mediated decay. Thus, factors that increase E3b splicing could reduce the amount of unspliced product available for translation. Observations consistent with this prediction were made using cellular assays and in post-mortem human SN. In mini-gene constructs, the extent of splicing is also influenced by at least two common haplotypes, so the alternative splicing was evaluated in relation to SZ risk. Meta-analyses across genome-wide association studies did not support the initial associations and further post-mortem studies did not suggest case-control differences in splicing. These studies do not provide a compelling link to schizophrenia. However, the impact of the alternative splicing on other neuropsychiatric disorders should be investigated. © 2010 Wiley-Liss, Inc.

Effects of in utero and lactational exposure to endosulfan in prefrontal cortex of male rats

The possible neurotoxic effects of the organochlorine pesticide endosulfan have been evaluated on male offspring rats exposed in utero and during lactation. Dams were treated with 0.61mg or 6.12mg endosulfan/(kgday) from the gestation beginning until the weaning. Male offspring rats were sacrificed at post-natal days (PND) 15, 30 and 60, and possible alterations in the content and metabolism of biogenic amines and amino acids were determined in prefrontal cortex using high-performance liquid chromatography (HPLC). Globally, endosulfan induced an increase in amino acid content in prefrontal cortex at PND 15 and PND 30. However, the levels of GABA at PND 15 and those of glutamine at PND 30 were not modified. At PND 60, a significant reduction in the content of GABA and taurine was observed, while the concentration of glutamate, aspartate and glutamine remained constant. Endosulfan did not modify norepinephrine and dopamine content, but serotonin concentration was increased at PND 30 and PND 60 and serotoninergic and dopaminergic metabolism was also modified. These results suggest that pre- and post-natal exposure to endosulfan affects biogenic amines and amino acids in prefrontal cortex, and those variations could be related to several alterations in the functions in which the prefrontal cortex is involved.

Metabotropic glutamate receptor protein expression in the prefrontal cortex and striatum in schizophrenia

We investigated the expression of metabotropic glutamate receptors (mGluR) in the prefrontal cortex (PFC) and striatum in schizophrenia. mGluRs modulate the release and reuptake of synaptic glutamate and mediate some molecular correlates of neuroplasticity, including long-term potentiation. The mGluRs are expressed widely in the PFC and striatum, regions often implicated in the pathophysiology of schizophrenia. Thus, we hypothesized that abnormal expression of mGluRs might contribute to glutamatergic dysfunction observed in the PFC and striatum in schizophrenia. Accordingly, we measured the expression of metabotropic glutamate receptors (mGluRs) in Brodmann areas 9, 11, 32, and 46 in the prefrontal cortex (PFC) and the caudate, putamen, and nucleus accumbens in schizophrenia (16 cases, 9 controls) by Western blot analysis. We found an increase in the expression of mGluR1a and mGluR2/3 immunoreactivity in the PFC in schizophrenia, while no changes in the expression of mGluR4a or mGluR5 were detected in this region. In the striatum we found no changes in the expression of any of the mGluRs studied. These results suggest that alterations of mGluR1a and mGluR2/3 expression in the PFC may contribute to the pathophysiology of schizophrenia, and support targeting these receptors for the generation of novel treatment modalities for this disabling illness.

The striatal dopamine transporter in first-episode, drug-naive schizophrenic patients: evaluation by the new SPECT-ligand[99mTc]TRODAT-1

Following the current hypothesis that acute schizophrenic psychotic illness is associated with a striatal 'hyperdopaminergic state', presynaptic integrity and dopamine transporter (DAT) density in first-episode, neuroleptic-naive schizophrenic patients was measured by single-photon-emission-tomography (SPECT) and compared with that in healthy control subjects. A new SPECT-ligand for assessment of the striatal DAT, the Technetium-99m-labelled tropane TRODAT-1 ([99mTc]TRODAT-1), was used. Ten inpatients suffering from a first acute schizophrenic episode and 10 age- and sex-matched healthy control subjects underwent SPECT with [99mTc]TRODAT-1. On the day of SPECT, psychopathological ratings were performed with the Brief Psychiatric Rating Scale (BPRS), the Positive and Negative Syndrome Scale (PANSS) and Schedule for Assessment of Negative Symptoms (SANS). Patients had not previously received any neuroleptic or antidepressant medication. Mean specific TRODAT-1 binding in the striatum did not differ significantly between the patient and the age- and sex-matched control group (1.25 vs. 1.28). Variance was significantly higher in the patient group. The data obtained with the new ligand in first-episode, drug-naive schizophrenic patients are in line with the PET results from the group of Laakso et al. in a comparable patient sample. [99mTc]TRODAT-1 seems to be a valuable new SPECT-ligand in the evaluation of the presynaptic site of the striatal dopaminergic synapse in schizophrenia.

An animal model of chronic placental insufficiency: relevance to neurodevelopmental disorders including schizophrenia

Evidence now suggests that compromised prenatal brain development may increase the risk for the manifestation of neurological disorders such as schizophrenia. We present a guinea-pig model which mimics a condition of human pregnancy, namely, chronic placental insufficiency. Previously we reported that at term there are changes in the brains of these offspring which are relevant to changes in patients with schizophrenia. The aim of this study was to examine whether deficits in brain structure persist to adolescence and young adulthood (8-12 weeks) and have implications for behavioral function. Reduced uteroplacental blood flow was induced via unilateral ligation of the uterine artery at mid-gestation. The brain was examined in control and prenatally compromised (PC) animals 8 weeks after birth using morphometric and immunohistochemical markers. In a separate cohort of animals, prepulse inhibition (PPI) of the acoustic startle response was assessed at 4, 8 and 12 weeks of age. Brain neurochemistry was examined by determining the concentrations of dopamine and its metabolite, dihydroxyphenylacetic acid (DOPAC), at 12 weeks using high performance liquid chromatography. In PC animals compared with controls there was a reduction in brain weight, persistent enlargement of the lateral ventricles, a reduction in the volume of the basal ganglia and septal region and no evidence of gliosis. No differences were observed in concentration of catecholamines in any brain region examined. At 12, but not 4 or 8, weeks of age, PPI was reduced in PC animals compared with controls. The findings of reduced brain weight, ventriculomegaly, reduced basal ganglia volume and absence of astrogliosis in the PC guinea-pig brain at adolescence parallel some of the changes observed in patients with schizophrenia. The impairment of PPI is comparable to sensorimotor gating deficits observed in patients with schizophrenia. These results indicate that adverse prenatal conditions lead to long-term alterations in brain structure and function which resemble alterations seen in patients with schizophrenia and therefore support the early neurodevelopmental hypothesis of schizophrenia.

Measurement of dopamine D2-like receptors in postmortem CNS and pituitary: differential regional changes in schizophrenia

In situ radioligand binding with autoradiography and anti-human dopamine D(2) receptor antibodies with Western blots have been used to measure the density of dopamine D(2)-like receptors in the caudate-putamen and pituitary from schizophrenic subjects who did or did not have residual antipsychotic drugs in their tissue at death. There was a significant decrease in the Ki for haloperidol displaceable [(125)I]iodosulpride binding in the pituitary (p < 0.01) and caudate-putamen (p < 0.05) from subjects with schizophrenia with residual drugs in their tissue. There was a significant decrease in the density of [(125)I]iodosulpride in the pituitary (p < 0.001) and a strong trend to a decrease in binding in the caudate-putamen (p = 0.055) from subjects with schizophrenia. By contrast, [(3)H]spiperone binding was decreased in the caudate-putamen (p < 0.05) with a trend to decreased binding in the pituitary (p = 0.07) from subjects with schizophrenia. There was no difference in the density of dopamine D(2) receptors in the caudate-putamen from subjects with schizophrenia (p = 0.31). All the findings on receptor densities were independent of drug status. [(125)I]iodosulpride binds to the dopamine D(2&3) receptors. We have shown that there is no change in the dopamine D(2) receptor in the caudate-putamen from subjects with schizophrenia and therefore, these data would be consistent with there being a decrease in the dopamine D(3) in the caudate-putamen from subjects with schizophrenia. Since dopamine D(3) receptors are absent or present at low concentrations in the pituitary, our data would suggest the dopamine D(2) receptor is decreased in that tissue from schizophrenic subjects.

The neonatal ventral hippocampal lesion model of schizophrenia: effects on dopamine and GABA mRNA markers in the rat midbrain

The neonatal ventral hippocampal lesion in the rat has been used as a model of schizophrenia, a human disorder associated with changes in markers of dopamine and gamma-aminobutyric acid (GABA) circuits in various regions of the brain. We investigated whether alterations in mRNA markers related to the activity of midbrain dopaminergic and GABAergic neurons are associated with this model. We used in situ hybridization histochemistry to assess expression of mRNAs for dopamine transporter (DAT), tyrosine hydroxylase (TH) and glutamate decarboxylase-67 (GAD67) in the midbrain of adult rats with neonatal and adult ibotenic acid lesions of the ventral hippocampus. Neonatally lesioned rats showed in adulthood significantly reduced expression of DAT mRNA in the substantia nigra and the ventral tegmental area but no changes in the expression of TH and GAD67 mRNAs in these midbrain regions. Adult lesioned rats showed no changes in the expression of any of these genes. As the neonatal ventral hippocampal lesion reproduces many aspects of schizophrenia and is used as an animal model of this disorder, these results suggest that the reduction in DAT mRNA could result from developmental neuropathology in the ventral hippocampus and may thus represent a molecular substrate of the disease process.

Potential role of dietary omega-3 essential fatty acids on some oxidant/antioxidant parameters in rats' corpus striatum

Omega-3 (omega-3) is an essential fatty acid (EFA) found in large amounts in fish oil. It contains eicosapentaenoic acid and docosahexaenoic acid (DHA). DHA is one of the building structures of membrane phospholipids of brain and necessary for continuity of neuronal functions. Evidences support the hypothesis that schizophrenia may be the result of increased reactive oxygen species mediated neuronal injury. Recent reports also suggest the protective effect of omega-3 EFA against neuropsychiatric disorders including schizophrenia. This study proposed to assess the changes in antioxidant enzyme and oxidant parameters in the corpus striatum (CS) of rats fed with omega-3 EFA diet (0.4g/kg/day) for 30 days. Eight control rats and nine rats fed with omega-3 were decapitated under ether anesthesia, and CS was removed immediately. Thiobarbituric acid-reactive substances (TBARS) and nitric oxide (NO) levels as well as total superoxide dismutase (t-SOD) and xanthine oxidase (XO) enzyme activities in the CS were measured. Rats treated with omega-3 EFA had significantly lower values of TBARS (P<0.001), NO (P<0.002) and XO (P<0.005) whereas higher values of t-SOD enzyme activity (P<0.002) than the control rats. These results indicate that omega-3 EFA rich fish oil diet reduces some oxidant parameters in CS. This may be revealed by means of reduced CS TBARS levels as an end product of lipid peroxidation of membranes in treated rats. Additionally, reduced XO activity and NO levels may support this notion. On the other hand, although the mechanism is not clear, omega-3 EFA may indirectly enhance the activity of antioxidant enzyme t-SOD. Taken together, this preliminary animal study provides strong support for a therapeutic effect of omega-3 EFA supplemented to classical neuroleptic regimen in the treatment of schizophrenic symptoms and tardive dyskinesia.

Presynaptic regulation of dopaminergic neurotransmission

The development of electrochemical recordings with small carbon-fiber electrodes has significantly advanced the understanding of the regulation of catecholamine transmission in various brain areas. Recordings in vivo or in slice preparations monitor diffusion of catecholamine following stimulated synaptic release into the surrounding tissue. This synaptic 'overflow' is defined by the amount of release, by the activity of reuptake, and by the diffusion parameters in brain tissue. Such studies have elucidated the complex regulation of catecholamine release and uptake, and how psychostimulants and anti-psychotic drugs interfere with it. Moreover, recordings with carbon-fiber electrodes from cultured neurons have provided analysis of catecholamine release and its plasticity at the quantal level.

Ketamine-induced changes in rat behaviour: A possible animal model of schizophrenia

It was investigated whether subchronic application of 30 mg/kg ketamine (Ket) induces reliable changes in behaviour and parameters of dopaminergic, glutamatergic, and serotonergic neurotransmissions, which might be the basis of an animal model in schizophrenia research. To test this, rats were injected with 30 mg/kg ip Ket daily for five consecutive days. In response to the first Ket injection, there was a decrease in activity time representing an acute Ket effect. Following the fifth injection, there were no differences between Ket- and saline (sal)-injected control rats in activity time, which might be a tolerance reaction. The following experiments were performed 2 or 4 weeks after Ket treatment. There were no effects on anxiety in either vehicle or Ket-treated rats using either low or high illumination levels in the elevated plus-maze. In the social interaction test, both groups of rats spent comparable times in social contact. The percentage of nonaggressive behaviour was decreased in Ket-treated rats. Two weeks after completion of the treatment, there was no effect on prepulse inhibition (PPI). Four weeks after the final Ket injection, latent inhibition (LI) was disrupted. There was no difference in the animals' activity in reaction to apomorphine (Apo) administration. Ket-treated rats injected with 0.1 mg/kg MK-801 showed an enhancement in locomotor activity. Ket treatment leads to an increase in D2 receptor binding in the hippocampus and a decrease in glutamate receptor binding in the frontal cortex. The authors did not find any changes in D1 receptor binding. The density of dopamine transporters was increased in the striatum. The density of 5-HT transporters was increased in the striatum, the hippocampus, and the frontal cortex. The results suggest that subchronic treatment with subanaesthetic doses of Ket induce schizophrenia-related alterations, which might be a useful animal model in the study of this disease.

Cannabis-sensitive dopaminergic markers in postmortem central nervous system: changes in schizophrenia

BACKGROUND

This study investigated if changes in pre-synaptic markers on dopaminergic neurons (dopamine transporter [DAT], tyrosine hydroxylase [TH]) were present in the caudate from subjects with schizophrenia who had Delta(9)(-)tetrahydrocannabinol (THC) in their blood at autopsy. These changes were posited because animal studies show that treatment with THC decreases dopamine uptake and TH in the striatum.

METHODS

Studies utilized caudate, obtained postmortem, from 14 schizophrenic and 14 control subjects. [(3)H]mazindol binding to caudate, measured using autoradiography, was taken as a measure of DAT; TH levels were estimated using an antihuman TH antibody and Western blotting.

RESULTS

There was decreased [(3)H]mazindol binding to DAT in the caudate from the schizophrenic subjects with no detectable blood THC levels (THC(-)) compared with THC(-) control subjects (mean +/- SEM: 240 +/- 19 vs. 296 +/- 14 fmol/mg estimated tissue equivalents, p =.01). There were no significant differences between levels of DAT in the caudate from schizophrenic and control subjects that had THC in their blood. Tyrosine hydroxylase was not different in any diagnostic cohort.

CONCLUSIONS

Our data suggests that DAT is decreased in the caudate from THC(-) subjects with schizophrenia, a change that may be reversed by ingesting THC from cannabis.