Stacy A. Castner

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.

Enhancement of working memory in aged monkeys by a sensitizing regimen of dopamine D1 receptor stimulation

A natural consequence of aging is a loss of dopamine function and associated deficits in working memory in both human and nonhuman primates. Specifically, deficiency of D1 receptor signaling has been implicated in age-related cognitive decline. Here, we report that an intermittent, sensitizing regimen of the D1 dopamine agonist ABT-431 dramatically enhances working memory performance in aged rhesus monkeys, while either producing impairment or having little effect on performance in young adult monkeys. Importantly, cognitive enhancement in the aged monkeys was still evident for >1 year after cessation of D1 treatment. Because intermittent exposure to low doses of amphetamine and other stimulants has been shown to enhance responsiveness to subsequent stimulant exposure, our findings suggest that sensitization of D1 signaling may provide a novel neurobiological mechanism for improving a core cognitive process in conditions in which dopamine function has deteriorated, such as in normal aging and Parkinsons disease.

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.

Immediate and Sustained Improvements in Working Memory After Selective Stimulation of α7 Nicotinic Acetylcholine Receptors

BACKGROUND Nicotine improves cognition in humans and animal models of neuropsychiatric disorders. Here, we sought to establish whether selective stimulation of the neuronal nicotinic α7 receptor could improve spatial working memory in nonhuman primates. METHODS Beginning with an estimated dose range from rodent studies, the dose of the α7 agonist AZD0328 was titrated for a significant impact on working memory in rhesus macaques after acute administration. After training to stability on the spatial delayed response task, subjects were administered AZD0328 (1.6 ng/kg-.48 mg/kg; intramuscular) or vehicle 30 min before cognitive testing. AZD0328 (1 ng/kg-1.0 μg/kg; intramuscular) was then administered in a repeated, intermittent ascending dose regimen where each dose was given in two bouts for 4 days with a 1-week washout in between bouts, followed by 2-week washout. RESULTS Acute AZD0328 improved cognitive performance when the dose was titrated down to .0016 and .00048 mg/kg from a cognitively impairing dose of .48 mg/kg. In a subgroup, sustained enhancement of working memory was evident for 1 month or more after acute treatment. Immediate and sustained cognitive enhancement was also found during and after repeated administration of AZD0328 at .001 mg/kg. CONCLUSIONS These findings demonstrate that extremely low doses of a nicotinic α7 agonist can have profound acute and long-lasting beneficial consequences for cognition, dependent upon the integrity of dorsolateral prefrontal cortex. Thus, the α7 receptor might have a fundamental role in the neural circuitry of working memory and in the synaptic plasticity upon which it might depend.

Amphetamine sensitization impairs cognition and reduces dopamine turnover in primate prefrontal cortex

BACKGROUND Amphetamine (AMPH) sensitization in monkeys produces long-lasting behavioral changes that model positive (hallucinatory-like behaviors) and negative (psychomotor depression) symptoms of schizophrenia. The extent to which this model produces the core deficit in schizophrenia--working memory impairment--is unknown. METHODS Two groups of rhesus monkeys were sensitized to AMPH over 6 weeks. In one group, acquisition of cognitive tasks (delayed response, visual discrimination, delayed nonmatch-to-sample) was examined beginning 6+ months postsensitization. The second group was pretrained to stability on delayed response before sensitization. Regional postmortem concentrations of dopamine and its metabolites were examined in tissue from age-matched AMPH-naive and AMPH-sensitized monkeys using high-performance liquid chromatography with electrochemical detection (HPLC-ECD). RESULTS The AMPH-sensitized monkeys were profoundly impaired in their ability to acquire cognitive tasks compared with AMPH-naïve monkeys. Pretrained monkeys showed impaired delayed response performance for several months following sensitization. Analysis by HPLC revealed that AMPH sensitization significantly reduced dopamine turnover in prefrontal cortex and striatum. CONCLUSIONS Impairments in the acquisition and performance of spatial delayed response in association with reduced dopamine turnover in prefrontal cortex following AMPH sensitization provide further support for the relevance of this model to both the etiology and the treatment of cognitive dysfunction in schizophrenia.

Reduction of Brain Kynurenic Acid Improves Cognitive Function

The elevation of kynurenic acid (KYNA) observed in schizophrenic patients may contribute to core symptoms arising from glutamate hypofunction, including cognitive impairments. Although increased KYNA levels reduce excitatory neurotransmission, KYNA has been proposed to act as an endogenous antagonist at the glycine site of the glutamate NMDA receptor (NMDAR) and as a negative allosteric modulator at the α7 nicotinic acetylcholine receptor. Levels of KYNA are elevated in CSF and the postmortem brain of schizophrenia patients, and these elevated levels of KYNA could contribute to NMDAR hypofunction and the cognitive deficits and negative symptoms associated with this disease. However, the impact of endogenously produced KYNA on brain function and behavior is less well understood due to a paucity of pharmacological tools. To address this issue, we identified PF-04859989, a brain-penetrable inhibitor of kynurenine aminotransferase II (KAT II), the enzyme responsible for most brain KYNA synthesis. In rats, systemic administration of PF-04859989 dose-dependently reduced brain KYNA to as little as 28% of basal levels, and prevented amphetamine- and ketamine-induced disruption of auditory gating and improved performance in a sustained attention task. It also prevented ketamine-induced disruption of performance in a working memory task and a spatial memory task in rodents and nonhuman primates, respectively. Together, these findings support the hypotheses that endogenous KYNA impacts cognitive function and that inhibition of KAT II, and consequent lowering of endogenous brain KYNA levels, improves cognitive performance under conditions considered relevant for schizophrenia.

Neuroplasticity as a target for the pharmacotherapy of anxiety disorders, mood disorders, and schizophrenia

Current treatments for psychiatric disorders were developed with the aim of providing symptomatic relief rather than reversing underlying abnormalities in neuroplasticity or neurodevelopment that might contribute to psychiatric disorders. This review considers the possibility that psychiatric treatments might be developed that target neuroplasticity deficits or that manipulate neuroplasticity in novel ways. These treatments might not provide direct symptomatic relief. However, they might complement or enhance current pharmacotherapies and psychotherapies aimed at the prevention and treatment of psychiatric disorders. In considering neuroplasticity as a target for the treatment of psychiatric disorders, we build on exciting new findings in the areas of anxiety disorders, mood disorders, and schizophrenia.

Amphetamine sensitization alters dendritic morphology in prefrontal cortical pyramidal neurons in the non-human primate.

Amphetamine (AMPH) sensitization in the nonhuman primate induces persistent aberrant behaviors reminiscent of the hallmark symptoms of schizophrenia, including hallucinatory-like behaviors, psychomotor depression, and profound cognitive impairment. The present study examined whether AMPH sensitization induces similarly long-lasting morphologic alterations in prefrontal cortical pyramidal neurons. Three to 3½ years postsensitization, sensitized, and AMPH-naïve control monkeys were killed. Blocks of prefrontal cortex were Golgi-impregnated for elucidation of pyramidal dendritic morphology in layers II/superficial III (II/IIIs), deep III, and V/VI. In AMPH-sensitized animals as compared to AMPH-naïve controls, pyramidal dendrites in layer II/IIIs exhibited reduced overall dendritic branching and reduced peak spine density (22%) on the apical trunk. Across all layers, the distance from soma to peak spine density along the apical trunk was decreased (126.38±7.65 μm in AMPH-sensitized compared to 162.98±7.26 μm in AMPH-naïve controls), and basilar dendritic length was reduced (32%). These findings indicate that chronic dopamine dysregulation, consequent to AMPH sensitization, results in enduring, atrophic changes in prefrontal pyramidal dendrites that resemble the pathologic alterations described in patients with schizophrenia and may contribute to the persistence of schizophrenia-like behavioral changes and cognitive dysfunction associated with sensitization. These findings may also provide key insights into the etiologic origin of the pronounced behavioral disturbances and cognitive dysfunction associated with schizophrenia.

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 AMPHs (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.

Glycine transporter inhibition reverses ketamine-induced working memory deficits

Glycine transporter inhibitors have recently been reported to improve symptoms in patients with schizophrenia. Here we used acute ketamine in the nonhuman primate to test the effectiveness of the novel glycine transporter inhibitor, PF-3463275, in a model of cognitive dysfunction relevant to schizophrenia. PF-3463275 (0.01–0.17 mg/kg; subcutaneously) or a vehicle was given before the administration of ketamine (median dose of 1.0 mg/kg intramuscularly) or placebo (saline). Ketamine induced hallucinatory-like behaviors that were not reversed by PF-3463275. In contrast, all doses of PF-3463275 alleviated the deficit in spatial working memory induced by ketamine. Theses findings build upon those in patients by providing translational support for targeting glycine transporter in adjunctive treatment for cognitive dysfunction in schizophrenia.