Cell-specific abnormalities of glutamate transporters in schizophrenia: sick astrocytes and compensating relay neurons?

Excitatory amino-acid transporters (EAATs) bind and transport glutamate, limiting spillover from synapses due to their dense perisynaptic expression primarily on astroglia. Converging evidence suggests that abnormalities in the astroglial glutamate transporter localization and function may underlie a disease mechanism with pathological glutamate spillover as well as alterations in the kinetics of perisynaptic glutamate buffering and uptake contributing to dysfunction of thalamo-cortical circuits in schizophrenia. We explored this hypothesis by performing cell- and region-level studies of EAAT1 and EAAT2 expression in the mediodorsal nucleus of the thalamus in an elderly cohort of subjects with schizophrenia. We found decreased protein expression for the typically astroglial-localized glutamate transporters in the mediodorsal and ventral tier nuclei. We next used laser-capture microdissection and quantitative polymerase chain reaction to assess cell-level expression of the transporters and their splice variants. In the mediodorsal nucleus, we found lower expression of transporter transcripts in a population of cells enriched for astrocytes, and higher expression of transporter transcripts in a population of cells enriched for relay neurons. We confirmed expression of transporter protein in neurons in schizophrenia using dual-label immunofluorescence. Finally, the pattern of transporter mRNA and protein expression in rodents treated for 9 months with antipsychotic medication suggests that our findings are not due to the effects of antipsychotic treatment. We found a compensatory increase in transporter expression in neurons that might be secondary to a loss of transporter expression in astrocytes. These changes suggest a profound abnormality in astrocyte functions that support, nourish and maintain neuronal fidelity and synaptic activity.

Vesicular glutamate transporter transcript expression in the thalamus in schizophrenia.

Previously, we have reported alterations in thalamic NMDA receptor subunit and excitatory amino acid transporter expression in schizophrenia, consistent with the hypothesis that thalamic glutamatergic dysfunction may contribute to the pathophysiology of this illness. We have generalized this hypothesis to include other molecules of the glutamate synapse. Using riboprobes specific for human brain-specific Na+-dependent inorganic phosphate transporter (BNPi) and differentiation-associated Na+/Pi co-transporter (DNPi), both vesicular glutamate transporters, in situ hybridization was performed in the thalami of persons with schizophrenia and comparison subjects. We detected increased expression of DNPi mRNA in the thalamus in schizophrenia, while BNPi mRNA was not expressed in the thalamus in any subjects. These findings support the hypothesis of glutamatergic dysfunction in the thalamus in schizophrenia.

Expression of excitatory amino acid transporter transcripts in the thalamus of subjects with schizophrenia.

OBJECTIVE: Recent investigations of schizophrenia have targeted glutamatergic neurotransmission, since phencyclidine, an N-methyl-D-aspartate (NMDA) receptor antagonist, can induce schizophreniform psychosis. The authors previously reported alterations in thalamic NMDA receptor subunit expression in schizophrenia, consistent with the hypothesis that thalamic glutamatergic hypofunction may contribute to the pathophysiology of this illness. In this study they generalized this hypothesis to include other molecules of the glutamate synapse, specifically excitatory amino acid transporters (EAATs), whose normal expression and regulation in the thalamus may also be disrupted in subjects with schizophrenia. METHOD: In situ hybridization with riboprobes specific for the human excitatory amino acid transporter transcripts EAAT1, EAAT2, and EAAT3 was performed in discrete thalamic nuclei in persons with schizophrenia and comparison subjects. RESULTS: Higher expressions of transcripts encoding EAAT1 and EAAT2, but not EAAT3, were detected in the thalamus of subjects with schizophrenia. CONCLUSIONS: These findings support the hypothesis of glutamatergic dysfunction in schizophrenia and suggest that molecules other than glutamate receptors are abnormally expressed in glutamatergic synapses in this illness.

Alterations in cerebral cortical galanin concentrations following neurotransmitter-specific subcortical lesions in the rat.

Galanin is associated with multiple projection neurons, and its immunoreactivity in the cerebral cortex may be derived from diverse sources. We investigated the effects of subcortical lesions on cerebral cortical galanin concentrations. Lesions of the anterior noradrenergic bundle (ANB) comparably reduced cerebral cortical galanin and norepinephrine (NE) concentrations. The effects of the ANB lesions on galanin were immediate and became most pronounced 1 week later. Extensive unilateral lesions of the nucleus basalis of Meynert (NBM) decreased galanin concentrations, although not as markedly as after ANB lesions. The NBM lesions had no additional effect in the presence of an ANB lesion. Decreases in cerebral cortical galanin concentrations depended upon the extent and the duration of the NBM lesion and were not as pronounced as the decreases in markers of cholinergic activity. Acute treatments with physostigmine, which inhibit cerebral cortical AChE, had no effect on galanin concentrations. The depletion of galanin following an NBM lesion was most pronounced within hours of the insult, while the depletion of ChAT following the same lesions required several days to develop. Cortical concentrations of galanin and 5-HT increased 1 hr after dorsal raphe nucleus (DRN) lesions and then decreased 7 d later. Six weeks later, galanin concentrations recovered in the cerebral cortex despite the continued depletion of 5-HT. These studies suggest that a substantial portion of cerebral cortical galanin may derive from noradrenergic neurons and may be modulated by cortically-projecting ACh and 5-HT neurons.

Frontal cortex as the site of action of physostigmine in nbM-lesioned rats.

The administration of a variety of cholinomimetic agents to nucleus basalis of Meynert-lesioned rats has been shown to alleviate their lesion-induced memory deficits. This experiment attempted to determine whether the frontal cortex was the site of the memory enhancing action of the cholinomimetic physostigmine. Different groups of rats received excitotoxic lesions of the basal forebrain, the frontal cortex or both. Immediately after one trial passive avoidance training, these rats were injected with either saline or a 0.06 mg/kg dose of physostigmine. Physostigmine enhanced the 72-hour retention test performance of sham-operated and basal forebrain-lesioned rats, but failed to affect the performance of rats with cortical lesions. These data were interpreted as consistent with the hypothesis that the memory-enhancing effects of physostigmine are at least partially mediated by the frontal cortex.

9-Amino-1,2,3,4-tetrahydroacridin-1-ols: synthesis and evaluation as potential Alzheimer's disease therapeutics.

The synthesis of a series of 9-amino-1,2,3,4-tetrahydroacridin-1-ols is reported. These compounds are related to 1,2,3,4-tetrahydro-9-acridinamine (THA, tacrine). They inhibit acetylcholinesterase in vitro and are active in a model that may be predictive of activity in Alzheimer's disease--the scopolamine-induced impairment of 24-h memory of a passive dark-avoidance paradigm in mice. Two compounds, (+/-)-9-amino-1,2,3,4-tetrahydroacridin-1-ol maleate (1a, HP-029) and (+/-)-9-(benzylamino)-1,2,3,4-tetrahydroacridin-1-ol maleate (1p, HP-128), were also active in reversing the deficit in 72-h retention of a one-trial dark-avoidance task in rats, induced by ibotenic acid lesions in the nucleus basalis magnocellularis. In addition, compound 1 p showed potent in vitro inhibition of the uptake of radiolabeled noradrenaline and dopamine (IC50 = 0.070 and 0.30 microM, respectively). Compounds 1a and 1p, which showed less acute toxicity in both rats and mice than THA, are in phase II and phase I clinical trials, respectively, for Alzheimer's disease.