Relationship between glycine transporter 1 inhibition as measured with positron emission tomography and changes in cognitive performances in nonhuman primates.

Several lines of evidence suggest that schizophrenia is associated with deficits in glutamatergic transmission at the N-methyl-d-aspartate (NMDA) receptors. Glycine is a NMDA receptor co-agonist, and extracellular levels of glycine are regulated in the forebrain by the glycine type-1 transporters (GlyT-1). GlyT-1 inhibitors elevate extracellular glycine and thus potentiate NMDA transmission. This mechanism represents a promising new avenue for the treatment of schizophrenia. Here, the recently introduced positron emission tomography radiotracer [11C]GSK931145 was used to quantify the relationship between occupancy of GlyT-1 by a GlyT-1 inhibitor, Org 25935, and its impact on spatial working memory performances in rhesus monkeys. The effect of Org 25935 on working memory was assessed both in control conditions and during a state of relative NMDA hypofunction induced by ketamine administration, at a dose selected for each animal to reduce task performance by about 50%. Under control conditions, Org 25935 had no effect on working memory at GlyT-1 occupancies lower than 75% and significantly impaired working memory at occupancies higher than 75%. Under ketamine conditions, Org 25935 reversed the deficit in working memory induced by ketamine and did so optimally in the 40-70% GlyT-1 occupancy range. The results confirm the efficacy of this mechanism to correct working memory deficits associated with NMDA hypofunction. These data also suggest the existence of an inverted-U dose-response curve in the potential therapeutic effect of this class of compounds.

Under the curve: critical issues for elucidating D1 receptor function in working memory.

It has been postulated that spatial working memory operates optimally within a limited range of dopamine transmission and D1 dopamine receptor signaling in prefrontal cortex. Insufficiency in prefrontal dopamine, as in aging, and excessive transmission, as in acute stress, lead to impairments in working memory that can be ameliorated by D1 receptor agonist and antagonist treatment, respectively. Iontophoretic investigations of dopamine's influence on the cellular mechanisms of working memory have revealed that moderate D1 blockade can enhance memory fields in primate prefrontal pyramidal neurons while strong blockade abolishes them. The combined behavioral and physiological evidence indicates that there is a normal range of dopamine function in prefrontal cortex that can be described as an "inverted-U" relationship between dopamine transmission and the integrity of working memory. Both in vivo and in vitro studies have demonstrated a role for dopamine in promoting the excitability of prefrontal pyramidal cells and facilitating their N-methyl-d-aspartate inputs, while simultaneously restraining recurrent excitation and facilitating feedforward inhibition. This evidence indicates that there is a fine balance between the synergistic mechanisms of D1 modulation in working memory. Given the critical role of prefrontal function for cognition, it is not surprising that this balancing act is perturbed by both subtle genetic influences and environmental events. Further, there is evidence for an imbalance in these dopaminergic mechanisms in multiple neuropsychiatric disorders, particularly schizophrenia, and in related nonhuman primate models. Elucidating the orchestration of dopamine signaling in key nodes within prefrontal microcircuitry is therefore pivotal for understanding the influence of dopamine transmission on the dynamics of working memory. Here, we explore the hypothesis that the window of optimal dopamine signaling changes on a behavioral time-scale, dependent upon current cognitive demands and local neuronal activity as well as long-term alterations in signaling pathways and gene expression. If we look under the bell-shaped curve of prefrontal dopamine function, it is the relationship between neuromodulation and cognitive function that promises to bridge our knowledge between molecule and mind.

Antipsychotic treatment induces alterations in dendrite- and spine-associated proteins in dopamine-rich areas of the primate cerebral cortex.

BACKGROUND: Mounting evidence indicates that long-term treatment with antipsychotic medications can alter the morphology and connectivity of cellular processes in the cerebral cortex. The cytoskeleton plays an essential role in the maintenance of cellular morphology and is subject to regulation by intracellular pathways associated with neurotransmitter receptors targeted by antipsychotic drugs. METHODS: We have examined whether chronic treatment with the antipsychotic drug haloperidol interferes with phosphorylation state and tissue levels of a major dendritic cytoskeleton-stabilizing agent, microtubule-associated protein 2 (MAP2), as well as levels of the dendritic spine-associated protein spinophilin and the synaptic vesicle-associated protein synaptophysin in various regions of the cerebral cortex of rhesus monkeys. RESULTS: Among the cortical areas examined, the prefrontal, orbital, cingulate, motor, and entorhinal cortices displayed significant decreases in levels of spinophilin, and with the exception of the motor cortex, each of these regions also exhibited increases in the phosphorylation of MAP2. No changes were observed in either spinophilin levels or MAP2 phosphorylation in the primary visual cortex. Also, no statistically significant changes were found in tissue levels of MAP2 or synaptophysin in any of the cortical regions examined. CONCLUSIONS: Our findings demonstrate that long-term haloperidol exposure alters neuronal cytoskeleton- and spine-associated proteins, particularly in dopamine-rich regions of the primate cerebral cortex, many of which have been implicated in the psychopathology of schizophrenia. The ability of haloperidol to regulate cytoskeletal proteins should be considered in evaluating the mechanisms of both its palliative actions and its side effects.

Reversal of antipsychotic-induced working memory deficits by short-term dopamine D1 receptor stimulation.

Chronic blockade of dopamine D2 receptors, a common mechanism of action for antipsychotic drugs, down-regulates D1 receptors in the prefrontal cortex and, as shown here, produces severe impairments in working memory. These deficits were reversed in monkeys by short-term coadministration of a D1 agonist, ABT 431, and this improvement was sustained for more than a year after cessation of D1 treatment. These findings indicate that pharmacological modulation of the D1 signaling pathway can produce long-lasting changes in functional circuits underlying working memory. Resetting this pathway by brief exposure to the agonist may provide a valuable strategy for therapeutic intervention in schizophrenia and other dopamine dysfunctional states.

Behavioral changes and [123I]IBZM equilibrium SPECT measurement of amphetamine-induced dopamine release in rhesus monkeys exposed to subchronic amphetamine.

Previously we have shown that twelve weeks of repeated low-dose d-amphetamine (AMPH) exposure in rhesus monkeys induces a long-lasting enhancement of behavioral responses to acute low-dose challenge. The present study was designed to investigate the behavioral and neurochemical consequences of a six-week regimen of low-dose AMPH exposure (0.1-1.0 mg/kg, i.m., b.i.d.) in rhesus monkeys. SPECT imaging of AMPH's (0.4 mg/kg) ability to displace [123I]IBZM bound to D2 dopamine receptors in the striatum of saline control and AMPH-treated animals prior to and following chronic treatment was accomplished using a bolus/constant infusion paradigm. Following chronic AMPH treatment, all monkeys showed an enhanced behavioral response to acute AMPH challenge and a significant decrease in the percent of AMPH-induced displacement of [123I]IBZM in striatum compared to their pretreatment scans. These findings suggest that relatively small changes in presynaptic dopamine function may be reflected in significant alterations in the behavioral response to acute AMPH challenge.

Long-lasting psychotomimetic consequences of repeated low-dose amphetamine exposure in rhesus monkeys.

The dopamine hypothesis of schizophrenia posits that dopamine dysregulation plays a key role in the etiology of schizophrenia. In line with this hypothesis, repeated amphetamine (AMPH) exposure has been shown to alter dopamine systems and induce behaviors reminiscent of positive-like and negative-like symptoms in both human and nonhuman primates. The mechanisms by which AMPH produces disturbances in brain function and behavior are not fully understood. The present study has employed a novel AMPH regimen, 12 weeks of intermittent escalating low doses of AMPH, to produce a nonhuman primate model for the purpose of elucidating the behavioral and neural consequences of excessive dopamine exposure. Behavioral responses to acute AMPH challenge (0.4-0.46 mg/kg) were assessed prior to and following the chronic 12-week treatment regimen, and, at present monkeys have been followed out to 28 months post-treatment. After chronic treatment, enhanced behavioral responses to AMPH challenge were readily apparent at 5 days postwithdrawal, and, were still present at 28 months postwithdrawal. The enhanced behavioral responses to low-dose AMPH challenge that were observed in the present study resemble closely the behavioral profile that has been described for chronic high-dose AMPH treatment in monkeys; i.e., hallucinatory-like behaviors, static posturing, and fine-motor stereotypies were all exacerbated in response to AMPH injection. In some animals, acute challenges after chronic AMPH evoked aberrant behavioral responses that lasted for 4 days. AMPH-treated monkeys also exhibited a significant decrease in the incidence of motor stereotypies in the off-drug periods between challenges. The present results are the first to document persistent long-term behavioral effects of intermittent exposure to repeated low-dose AMPH treatment in nonhuman primates. These findings may lay the groundwork for the development of a primate mode of psychosis with possible positive-like and negative-like symptoms.

Sex differences in the effect of amphetamine on immediate early gene expression in the rat dorsal striatum.

Amphetamine (AMPH)-induced dopamine release in the striatum and AMPH-induced behavior in the rat have been demonstrated to be influenced by sex and hormonal status. The experiments reported here were conducted, therefore, to examine sex differences, hormonal influences and estrous cycle-dependent changes in AMPH-induced immediate early gene expression in the dorsal striatum. Cell counts were taken at three rostrocaudal levels from three to four regions of the dorsal striatum at each level (ventromedial, dorsomedial, dorsolateral, ventrolateral). The immunohistochemical localization of calbindin was used as a control. We report here that females on the afternoon of proestrus had a significantly greater percent of Fos-positive neurons after AMPH across the dorsolateral region of the middle and caudal striatum and in the ventrolateral region of the caudal striatum compared to females in diestrus, ovariectomized (OVX) females, castrated (CAST) males and intact males. There was no difference in AMPH-induced immediate early gene expression between OVX and diestrous rats. There were also no significant differences between CAST and intact males in AMPH-induced Fos expression, with the exception of the ventrolateral caudal striatum. In sum, the present findings indicate that AMPH-induced Fos expression is sexually dimorphic and modulated by gonadal hormones in lateral regions of the rat dorsal striatum.

Sex differences in striatal dopamine: in vivo microdialysis and behavioral studies.

Experiments were conducted to examine sex differences in striatal dopamine function using in vivo microdialysis in freely moving rats. We report here a sex difference in basal extracellular striatal dopamine determined by quantitative microdialysis (the no net flux method) when castrated and ovariectomized rats were compared. There was no sex difference in dopamine uptake into synaptosomes. This indicates that the sex difference in extracellular dopamine is most likely due to sex differences in dopamine release, synthesis, and/or metabolism. Within 30 min after a single injection (s.c.) of either estradiol benzoate (2.0 micrograms/100 g) or 17 beta-estradiol (1.5 micrograms/100 g) the amphetamine-stimulated release of dopamine was enhanced in the striatum of ovariectomized rats, but there was no effect in castrated male rats. The enhanced amphetamine-induced striatal dopamine release in ovariectomized rats was associated with an enhanced frequency of stereotyped head and limb movements and an increased peak in extra 1/4 turns. There were also sex differences in stereotyped behavior and extra 1/4 turns whether or not animals received estrogen treatment. Thus, there are sex differences in striatal extracellular dopamine and in the effect of estrogen on the striatal dopamine neurochemical and behavioral responses to amphetamine.