The Effects of a Novel Non-catechol Dopamine Partial Agonist on Working Memory in the Aged Rhesus Monkey.

Aged-related declines in cognition, especially working memory and executive function, begin in middle-age and these abilities are known to be mediated by the prefrontal cortex (PFC) and more specifically the dopamine (DA) system within the PFC. In both humans and monkeys, there is significant evidence that the PFC is the first cortical region to change with age and the PFC appears to be particularly vulnerable to age-related loss of dopamine (DA). Therefore, the DA system is a strong candidate for therapeutic intervention to slow or reverse age related declines in cognition. In the present study, we administered a novel selective, potent, non-catechol DA D1 R agonist PF-6294 (Pfizer, Inc.) to aged female rhesus monkeys and assessed their performance on two benchmark tasks of working memory - the Delayed Non-match to Sample Task (DNMS) and Delayed Recognition Span Task (DRST). The DNMS task was administered first with the standard 10 s delay and then with 5 min delays, with and without distractors. The DRST was administered each day with four trials with unique sequences and one trial of a repeated sequence to assess evidence learning and retention. Overall, there was no significant effect of drug on performance on any aspect of the DNMS task. In contrast, we demonstrated that a middle range dose of PF-6294 significantly increased memory span on the DRST on the first and last days of testing and by the last day of testing the increased memory span was driven by the performance on the repeated trials.

D1 Agonist Improved Movement of Parkinsonian Nonhuman Primates with Limited Dyskinesia Side Effects.

Parkinson's disease is a progressive neurodegenerative disease characterized by striatal dopaminergic loss. L-DOPA treatment replaces lost dopamine and enables motor function; however, eventually, fluctuating efficacy and side effects associated with its use become challenging for many patients. Here, we demonstrate, in a clinically translatable nonhuman primate model of parkinsonian motor symptoms, that treatment with the partial D1 receptor agonist CVL-751, formerly known as PF-06649751, is just as effective as L-DOPA in enabling movement and reducing disability. Importantly, CVL-751 efficacy is observed with less of the concomitant dyskinesia side effect associated with L-DOPA treatment. Data presented suggest that partial D1 agonists may be an effective and important treatment strategy for the management of Parkinson's patients.

In Vivo Modulation of Hippocampal Excitability by M4 Muscarinic Acetylcholine Receptor Activator: Implications for Treatment of Alzheimer's Disease and Schizophrenic Patients.

Abnormal hippocampal activity has been linked to impaired cognitive performance in Alzheimer's disease and schizophrenia, leading to a hypothesis that normalization of this activity may be therapeutically beneficial. Our work suggests that one approach for hippocampal normalization may be through activation of the M4 muscarinic acetylcholine receptor. We used a brain penetrant M4 muscarinic acetylcholine receptor selective activator, PT-3763, to show dose-dependent attenuation of field potentials in Schaffer collateral (CA3-CA1) and recurrent associational connections (CA3-CA3) ex vivo in hippocampal slices. In vivo, systemic administration of PT-3763 led to attenuation of glutamate release in CA3 as measured by amperometry and to a dose-dependent decrease in population CA1 pyramidal activity as measured by fiber photometry. This decrease in population activity was also evident with a localized administration of the compound to the recorded site. Finally, PT-3763 reversed scopolamine-induced deficit in Morris water maze. Our results suggest that M4 muscarinic acetylcholine receptor activation may be a suitable therapeutic treatment in diseases associated with hyperactive hippocampal activity.

α5 nAChR modulation of the prefrontal cortex makes attention resilient.

A loss-of-function polymorphism in the α5 nicotinic acetylcholine receptor (nAChR) subunit gene has been linked to both drug abuse and schizophrenia. The α5 nAChR subunit is strategically positioned in the prefrontal cortex (PFC), where a loss-of-function in this subunit may contribute to cognitive disruptions in both disorders. However, the specific contribution of α5 to PFC-dependent cognitive functions has yet to be illustrated. In the present studies, we used RNA interference to knockdown the α5 nAChR subunit in the PFC of adult rats. We provide evidence that through its contribution to cholinergic modulation of cholinergic modulation of neurons in the PFC, the α5 nAChR plays a specific role in the recovery of attention task performance following distraction. Our combined data reveal the potent ability of this subunit to modulate the PFC and cognitive functions controlled by this brain region that are impaired in disease.

Nicotinic receptor subtypes differentially modulate glutamate release in the dorsal medial striatum.

The dorsal medial striatum is a crucial part of the neural network that subserves dynamic, goal-directed behaviors. Functional output of this nucleus is shaped, in part, by the influence of glutamatergic inputs. Striatal cholinergic systems have the capacity to modulate these excitatory inputs through presynaptic nicotinic acetylcholine receptors (nAChRs); however, the individual contribution of the two major nicotinic receptor subtypes, α4ß2 and α7, to such modulation is not well characterized. In the present experiments, glutamate biosensors were used to monitor nAChR-dependent glutamate release with high temporal precision in the dorsal medial striatum of rats. Both α4ß2 and α7 nAChRs were found to potently modulate glutamate release; however the two receptor subtypes do so in strikingly different ways. α7 nAChRs appear to enhance release from glutamatergic terminals. In contrast, α4ß2 nAChRs act as a brake on glutamate release via an interaction with local dopaminergic inputs and D2 receptors. Combined, the present data reveal the capacity of local striatal cholinergic signaling to dynamically modulate excitatory inputs through nAChRs.

Cross-site strain comparison of pharmacological deficits in the touchscreen visual discrimination test.

The low rate of success for identifying effective treatments for cognitive dysfunction has prompted recent efforts to improve pharmaceutical discovery and development. In particular, investigators have emphasized improving translation from pre-clinical to clinical research. A specific area of focus has been touchscreen technology; this computer-automated behavioral testing method provides an objective assessment of performance that can be used across species. As part of a larger multi-site study with partners from the Innovative Medicines Initiative (IMI), two US sites, AbbVie and Pfizer, conducted a cross-site experiment with a common protocol for the visual discrimination (VD) task using identical testing equipment, stimuli, and rats of the same strains, sex, and age from the same supplier. As most touchscreen-based rodent experiments have used Lister-Hooded rats that are not readily available outside of Europe, a strain comparison with male Long-Evans rats was conducted as part of the study. Rats were trained for asymptotic performance, and test sessions were performed once per week in a full crossover design with cognition-impairing drugs. Drugs tested were phencyclidine and S-ketamine (N-methyl-D-aspartate (NMDA) antagonists), D-amphetamine (indirect dopamine agonist), and scopolamine (muscarinic antagonist). Satellite brain and plasma samples were taken to confirm appropriate exposures. Results indicate that both rat strains show similar patterns of impairment, although Lister-Hooded rats were more sensitive than Long-Evans rats to three out of four drugs tested. This suggests that researchers should fully explore dose-response relationships in their strain of choice and use care in the interpretation of reversal of cognitive impairment.

MAM (E17) rodent developmental model of neuropsychiatric disease: disruptions in learning and dysregulation of nucleus accumbens dopamine release, but spared executive function.

RATIONALE: Gestational day 17 methylazoxymethanol (MAM) treatment has been shown to reproduce, in rodents, some of the alterations in cortical and mesolimbic circuitries thought to contribute to schizophrenia. OBJECTIVE: We characterized the behavior of MAM animals in tasks dependent on these circuitries to see what behavioral aspects of schizophrenia the model captures. We then characterized the integrity of mesolimbic dopamine neurotransmission in a subset of animals used in the behavioral experiments. METHODS: MAM animals' capacity for working memory, attention, and resilience to distraction was tested with two different paradigms. Cue-reward learning and motivation were assayed with Pavlovian conditioned approach. Measurements of electrically stimulated phasic and tonic DA release in the nucleus accumbens with fast-scan cyclic voltammetry were obtained from the same animals used in the Pavlovian task. RESULTS: MAM animals' basic attentional capacities were intact. MAM animals took longer to acquire the working memory task, but once learned, performed at the same level as shams. MAM animals were also slower to develop a Pavlovian conditioned response, but otherwise no different from controls. These same animals showed alterations in terminal DA release that were unmasked by an amphetamine challenge. CONCLUSIONS: The predominant behavioral-cognitive feature of the MAM model is a learning impairment that is evident in acquisition of executive function tasks as well as basic Pavlovian associations. MAM animals also have dysregulated terminal DA release, and this may contribute to observed behavioral differences. The MAM model captures some functional impairments of schizophrenia, particularly those related to acquisition of goal-directed behavior.

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.

A systemically-available kynurenine aminotransferase II (KAT II) inhibitor restores nicotine-evoked glutamatergic activity in the cortex of rats.

Kynurenic acid (KYNA) is a tryptophan metabolite that acts in the brain as an endogenous antagonist at multiple receptors, including glutamate and α7 nicotinic acetylcholine receptors. Increased levels of KYNA have been demonstrated in the brain of patients with a range of neurocognitive disorders, including schizophrenia, and are hypothesized to contribute to cognitive symptoms. Reducing KYNA levels by administering inhibitors of enzymes of the kynurenine pathway, particularly kynurenine aminotransferase II (KAT II), has been proposed as a treatment for such cognitive impairments. Here we report that administration of a systemically available KAT II inhibitor, PF-04859989, restores glutamate release events ("transients") evoked by pressure ejections of nicotine into the prefrontal cortex of rats exhibiting elevated KYNA levels. Nicotine-evoked glutamatergic transients can be reliably evoked and recorded after repeated pressure ejections of nicotine over 4-5 h. Systemic administration of l-kynurenine (100 mg/kg; i.p.) significantly increased frontal cortical KYNA levels and greatly attenuated the amplitude of nicotine-evoked glutamatergic transients. Systemic administration of PF-04859989 30 min prior to administration of l-kynurenine, but not when administered 30 min after l-kynurenine, restored glutamatergic transients recorded up to 75 min after the administration of the KAT II inhibitor. Furthermore, the KAT II inhibitor significantly reversed l-kynurenine-induced elevations of brain KYNA levels. The KAT II inhibitor did not affect nicotine-evoked glutamatergic transients in rats not pre-treated with l-kynurenine. Because PF-04859989 restores evoked glutamate signaling it therefore is a promising therapeutic compound for benefiting the cognitive symptoms of schizophrenia and other disorders associated with elevated brain KYNA levels.

Modulators in concert for cognition: modulator interactions in the prefrontal cortex.

Research on the regulation and function of ascending noradrenergic, dopaminergic, serotonergic, and cholinergic systems has focused on the organization and function of individual systems. In contrast, evidence describing co-activation and interactions between multiple neuromodulatory systems has remained scarce. However, commonalities in the anatomical organization of these systems and overlapping evidence concerning the post-synaptic effects of neuromodulators strongly suggest that these systems are recruited in concert; they influence each other and simultaneously modulate their target circuits. Therefore, evidence on the regulatory and functional interactions between these systems is considered essential for revealing the role of neuromodulators. This postulate extends to contemporary neurobiological hypotheses of major neuropsychiatric disorders. These hypotheses have focused largely on aberrations in the integrity or regulation of individual ascending modulatory systems, with little regard for the likely possibility that dysregulation in multiple ascending neuromodulatory systems and their interactions contribute essentially to the symptoms of these disorders. This review will paradigmatically focus on neuromodulator interactions in the PFC and be further constrained by an additional focus on their role in cognitive functions. Recent evidence indicates that individual neuromodulators, in addition to their general state-setting or gating functions, encode specific cognitive operations, further substantiating the importance of research concerning the parallel recruitment of neuromodulator systems and interactions between these systems.

Toward a neuro-cognitive animal model of the cognitive symptoms of schizophrenia: disruption of cortical cholinergic neurotransmission following repeated amphetamine exposure in attentional task-performing, but not non-performing, rats.

Impairments in attentional functions and capacities represent core aspects of the cognitive symptoms of schizophrenia. Attentional performance has been demonstrated to depend on the integrity and activity of cortical cholinergic inputs. The neurobiological, behavioral, and cognitive effects of repeated exposure to psychostimulants model important aspects of schizophrenia. In the present experiment, prefrontal acetylcholine (ACh) release was measured in attentional task-performing and non-performing rats pretreated with an escalating dosing regimen of amphetamine (AMPH) and following challenges with AMPH. In non-performing rats, pretreatment with AMPH did not affect the increases in ACh release produced by AMPH-challenges. In contrast, attentional task performance-associated increases in ACh release were attenuated in AMPH-pretreated and AMPH-challenged rats. This effect of repeated AMPH exposure on ACh release was already present before task-onset, suggesting that the loss of cognitive control that characterized these animals' performance was a result of cholinergic dysregulation. The findings indicate that the demonstration of repeated AMPH-induced dysregulation of the prefrontal cholinergic input system depends on interactions between the effects of repeated AMPH exposure and cognitive performance-associated recruitment of this neuronal system. Repeated AMPH-induced disruption of prefrontal cholinergic activity and attentional performance represents a useful model to investigate the cholinergic mechanisms contributing to the cognitive impairments of schizophrenia.

Increased striatal dopamine synthesis is associated with decreased tissue levels of tyrosine.

Tyrosine levels do not generally affect indices of dopamine (DA) synthesis or efflux under basal conditions, but can do so when DA synthesis is increased. One possibility is that a high rate of DA synthesis depletes the normally adequate pool of endogenous tyrosine. To study this, we administered drugs known to preferentially increase striatal DA synthesis and examined DOPA levels in striatal microdialysate during perfusion with NSD-1015. In additional groups, we also measured DA, tyrosine and large neutral amino acids in striatal microdialysate, as well as in tissue from striatum and medial prefrontal cortex (MPFC). gamma-butyrolactone (GBL) (750 mg/kg i.p.) increased DOPA levels in striatal microdialysate, increased tissue DA levels in the MPFC and striatum, but lowered tissue tyrosine levels only in striatum. In striatal microdialysate, GBL markedly lowered DA levels; tyrosine levels were only marginally lower. Haloperidol (HAL) (1.0 mg/kg s.c.)+/-amfonelic acid (AFA) (5 mg/kg i.p.) increased striatal DOPA accumulation, increased striatal DA efflux, lowered striatal tissue tyrosine levels, but did not affect microdialysate tyrosine levels. There were no consistent changes in levels of other large neutral amino acids. We conclude that increased tyrosine hydroxylation can significantly deplete the endogenous pool of tyrosine. Under such conditions, near normal extracellular tyrosine levels are maintained despite lower tissue levels. The data are consistent with a net transfer of tyrosine from non-DAergic cells to DA terminals in support of DA synthesis.

Clozapine-induced dopamine release in the medial prefrontal cortex is augmented by a moderate concentration of locally administered tyrosine but attenuated by high tyrosine concentrations or by tyrosine depletion.

RATIONALE: Tyrosine availability can affect indices of dopamine (DA) release in activated central DA systems. There are, however, inconsistencies between studies. One possibility is that the relationship between tyrosine availability and DA release is non-linear. OBJECTIVES: This study aimed to determine how tyrosine depletion as well as a range of administered tyrosine concentrations affect antipsychotic drug-induced extracellular DA levels in the MPFC or striatum. METHODS: A guide cannula was implanted over the medial prefrontal cortex or striatum of adult male rats. After a 24-h recovery period, a microdialysis probe was inserted. Microdialysate collection began on the following day. Some rats received vehicle or a tyrosine- and phenylalanine-free neutral amino acid solution NAA(-) (IP) prior to clozapine (CLZ 10 mg/kg IP). Others received vehicle, CLZ (10 mg/kg IP) or haloperidol (HAL) (1 mg/kg IP) while the probe was perfused with artificial cerebrospinal fluid containing tyrosine 0-200 mug/ml. RESULTS: NAA(-) reduced tyrosine levels in MPFC dialysate by 35%. This reduction did not affect basal MPFC DA levels but attenuated the peak of CLZ-induced MPFC DA levels. The NAA(-) effect could be reversed by administration of tyrosine. Infused tyrosine 12.5-200 mug/ml did not affect basal DA levels either in MPFC or striatum. Within the MPFC, tyrosine 50.0 mug/ml significantly increased CLZ-induced DA levels. Within the striatum, tyrosine 25.0 mug/ml significantly increased while 150.0 mug/ml significantly decreased HAL-induced DA levels. CONCLUSIONS: Basal extracellular levels of DA in the MPFC and striatum are not affected by wide changes in tyrosine availability. However, modestly increased brain tyrosine levels can augment CLZ-induced MPFC and HAL-induced DA levels. Very high tyrosine concentrations attenuate HAL-induced striatal DA levels. These data may explain inconsistencies in the literature and suggest that tyrosine availability could be exploited to modulate psychotropic drug-induced DA levels in the brain.