Transient inactivation of the neonatal ventral hippocampus permanently disrupts the mesolimbic regulation of prefrontal cholinergic transmission: implications for schizophrenia

Imaging the Vesicular Acetylcholine Transporter in Schizophrenia: A Positron Emission Tomography Study Using [18F]-VAT

BACKGROUND

Despite longstanding interest in the central cholinergic system in schizophrenia (SCZ), cholinergic imaging studies with patients have been limited to receptors. Here, we conducted a proof-of-concept positron emission tomography study using [18F]-VAT, a new radiotracer that targets the vesicular acetylcholine transporter as a proxy measure of acetylcholine transmission capacity, in patients with SCZ and explored relationships of vesicular acetylcholine transporter with clinical symptoms and cognition.

METHODS

A total of 18 adult patients with SCZ or schizoaffective disorder (the SCZ group) and 14 healthy control participants underwent a positron emission tomography scan with [18F]-VAT. Distribution volume (VT) for [18F]-VAT was derived for each region of interest, and group differences in VT were assessed with 2-sample t tests. Functional significance was explored through correlations between VT and scores on the Positive and Negative Syndrome Scale and a computerized neurocognitive battery (PennCNB).

RESULTS

No group differences in [18F]-VAT VT were observed. However, within the SCZ group, psychosis symptom severity was positively associated with VT in multiple regions of interest, with the strongest effects in the hippocampus, thalamus, midbrain, cerebellum, and cortex. In addition, in the SCZ group, working memory performance was negatively associated with VT in the substantia innominata and several cortical regions of interest including the dorsolateral prefrontal cortex.

CONCLUSIONS

In this initial study, the severity of 2 important features of SCZ-psychosis and working memory deficit-was strongly associated with [18F]-VAT VT in several cortical and subcortical regions. These correlations provide preliminary evidence of cholinergic activity involvement in SCZ and, if replicated in larger samples, could lead to a more complete mechanistic understanding of psychosis and cognitive deficits in SCZ and the development of therapeutic targets.

Dorsal and Ventral Hippocampus Differentiate in Functional Pathways and Differentially Associate with Neurological Disease-Related Genes during Postnatal Development

The dorsal and ventral regions of the hippocampus are important in cognitive and emotional processing, respectively. Various approaches have revealed the differential molecular and structural characteristics, and functional roles of the hippocampus. Recent RNA sequencing (RNA-seq) technology has enriched our understanding of the hippocampus by elucidating more detailed information on gene expression patterns. However, no RNA-seq–based study on gene profiles in the developing hippocampus has been reported. Using RNA-seq–based bioinformatic analysis in conjunction with quantitative real-time polymerase chain reaction analysis and a comparison of in situ hybridization data obtained from the Allen Brain Atlas, we provide a thorough analysis of differentially expressed genes in the dorsal and ventral hippocampus at specific developmental ages representing the postnatally maturing hippocampus. Genes associated with particular functional pathways and marker genes for particular neurological diseases were found to be distinctively segregated within either the dorsal or ventral hippocampus at specific or at all developmental ages examined. We also report novel molecular markers enriched in the dorsal or ventral hippocampus. Taken together, this study provides insights into the molecular mechanisms underlying physiological functions linked to the dorsal or ventral hippocampus. The information provided in the study also contributes to a better understanding of brain functions and serves as a resource for future studies on the pathophysiology of dorsal and ventral hippocampal functions.

Performance in hippocampus- and PFC-dependent cognitive domains are not concomitantly impaired in rats exposed to 20cGy of 1GeV/n (56)Fe particles

NASA is currently conducting ground based experiments to determine whether the radiation environment that astronauts will encounter on deep space missions will have an impact on their long-term health and their ability to complete the various tasks during the mission. Emerging data suggest that exposure of rodents to mission-relevant HZE radiation doses does result in the impairment of various neurocognitive processes. An essential part of mission planning is a probabilistic risk assessment process that takes into account the likely incidence and severity of a problem. To date few studies have reported the impact of space radiation in a format that is amenable to PRA, and those that have only reported data for a single cognitive process. This study has established the ability of individual male Wistar rats to conduct a hippocampus-dependent (spatial memory) task and a cortex-dependent (attentional set shifting task) 90 days after exposure to 20cGy 1GeV/n (56)Fe particles. Radiation-induced impairment of performance in one cognitive domain was not consistently associated with impaired performance in the other domain. Thus sole reliance upon a single measure of cognitive performance may substantially under-estimate the risk of cognitive impairment, and ultimately it may be necessary to establish the likelihood that mission-relevant HZE doses will impair performance in the three or four cognitive domains that NASA considers to be most critical for mission success, and build a PRA using the composite data from such studies.

Neonatal Prefrontal Inactivation Results in Reversed Dopaminergic Responses in the Shell Subregion of the Nucleus Accumbens to NMDA Antagonists

Striatal dopaminergic dysregulation in schizophrenia could result from a prefronto-striatal dysconnectivity, of neurodevelopmental origin, involving N-methyl-d-aspartate (NMDA) receptors. The dorsomedian shell part of the nucleus accumbens is a striatal subregion of particular interest inasmuch as it has been described as the common target region for antipsychotics. Moreover, NMDA receptors located on the dopaminergic endings have been reported in the shell. The present study examines in adult rats the effects of early functional inactivation of the left prefrontal cortex on behavioral and dopaminergic responses in the dorsomedian shell part of the nucleus accumbens following administration of two noncompetitive NMDA receptor antagonists, ketamine, and dizocilpine (MK-801). The results showed that postnatal blockade of the prefrontal cortex led to increased locomotor activity as well as increased extracellular dopamine levels in the dorsomedian shell following administration of both noncompetitive NMDA receptor antagonists, and, more markedly, after treatment with the more specific one, MK-801, whereas decreased dopaminergic levels were observed in respective controls. These data suggest a link between NMDA receptor dysfunctioning and dopamine dysregulation at the level of the dorsomedian shell part of the nucleus accumbens. They may help to understand the pathophysiology of schizophrenia in a neurodevelopmental perspective.

Cholinergic systems are essential for late-stage maturation and refinement of motor cortical circuits

Previous studies reported that early postnatal cholinergic lesions severely perturb early cortical development, impairing neuronal cortical migration and the formation of cortical dendrites and synapses. These severe effects of early postnatal cholinergic lesions preclude our ability to understand the contribution of cholinergic systems to the later-stage maturation of topographic cortical representations. To study cholinergic mechanisms contributing to the later maturation of motor cortical circuits, we first characterized the temporal course of cortical motor map development and maturation in rats. In this study, we focused our attention on the maturation of cortical motor representations after postnatal day 25 (PND 25), a time after neuronal migration has been accomplished and cortical volume has reached adult size. We found significant maturation of cortical motor representations after this time, including both an expansion of forelimb representations in motor cortex and a shift from proximal to distal forelimb representations to an extent unexplainable by simple volume enlargement of the neocortex. Specific cholinergic lesions placed at PND 24 impaired enlargement of distal forelimb representations in particular and markedly reduced the ability to learn skilled motor tasks as adults. These results identify a novel and essential role for cholinergic systems in the late refinement and maturation of cortical circuits. Dysfunctions in this system may constitute a mechanism of late-onset neurodevelopmental disorders such as Rett syndrome and schizophrenia.

Identification of dorsal-ventral hippocampal differentiation in neonatal rats

The adult hippocampal formation (HF) is functionally, connectionally, and transcriptionally differentiated along the dorsal-ventral axis. At birth, the hippocampus appears shortened along its dorsal-ventral axis. We therefore questioned at what postnatal age the differentiated dorsal-ventral hippocampus is present. We first established that the ventral tissue in the short postnatal hippocampus remains ventral in the adult-like hippocampus. Second, using anatomical tracing techniques we report that, within the first postnatal week, the main input from the entorhinal cortex (EC) to HF is topographically organized. The terminal distribution of this input along the dorsal-ventral axis of HF was related to a dorsolateral-to-ventromedial axis of origin in EC, thus reflecting adult topography. Finally, we examined gene expression along the dorsal-ventral axis in the developing hippocampus. We found that several genes that were differentially enriched in the adult dorsal and ventral hippocampus were similarly enriched in the dorsal and ventral hippocampal poles at birth. The differentially expressed genes relate to different molecular pathways and biomarkers of disease. Taken together, these data lead us to conclude that the entire dorsal-ventral axis of HF is present at birth showing adult-like functional differentiation. Moreover, our findings indicate that the neonatal ventral hippocampus is enriched with biomarkers associated with mental illnesses. These include schizophrenia, affective and anxiety disorders, disorders previously deemed as ventral hippocampal associated disorders, as well as alcoholism. Our results thus suggest an early developmental susceptibility of the ventral HF to mental illness.

Transient inactivation of the ventral hippocampus in neonatal rats impairs the mesolimbic regulation of prefrontal glutamate release in adulthood

Cognitive deficits in schizophrenia (SZ) reflect maturational disruptions within a neural system that includes the ventral hippocampus (VH), nucleus accumbens (NAc), basal forebrain, and prefrontal cortex (PFC). A better understanding of these changes may reveal drug targets for more efficacious cognition enhancers. We have utilized an animal model in which the above distributed system is altered, during a sensitive period of development, by transiently inactivating the VH and its efferent projections. We determined the ability of NAc shell activation to evoke prefrontal glutamate release in adult male Wistar rats that had received saline (Sal) or tetrodotoxin (TTX) as neonates (PD7) or as adolescents (PD32). The nucleus accumbens shell (NAcSh) was activated by NMDA infusions (0.05-0.30 μg/0.5 μL). Basal and evoked glutamate levels were measured amperometrically using a glutamate-sensitive microelectrode. There were no differences in basal glutamate levels among the groups tested (overall 1.41 ± 0.26 uM). However, the dose-related stimulation of prefrontal glutamate levels seen in control rats treated with saline on PD7 (4.31 ± 0.22 μM after 0.15 μg) was markedly attenuated in rats treated with TTX on PD7 (0.45 ± 0.12 μM after 0.15 μg). This effect was age-dependent as infusions of TTX on PD32 did not alter the NMDA-induced increases in glutamate release (4.10 ± 0.37 μM after 0.15 μg). Collectively, these findings reveal that transient inactivation of VH transmission, during a sensitive period of development, leads to a functional mesolimbic-cortical disconnection that produces neurochemical and ultimately cognitive impairments resembling those seen in SZ.

Consequences at adulthood of transient inactivation of the parahippocampal and prefrontal regions during early development: new insights from a disconnection animal model for schizophrenia

The psychic disintegration characteristic of schizophrenia is thought to result from a defective connectivity, of neurodevelopmental origin, between several integrative brain regions. The parahippocampal region and the prefrontal cortex are described as the main regions affected in schizophrenia. Interestingly, latent inhibition (LI) has been found to be reduced in patients with schizophrenia, and the existence of a dopaminergic dysfunction is also generally well accepted in this disorder. In the present review, we have integrated behavioral and neurochemical data obtained in a LI protocol involving adult rats subjected to neonatal functional inactivation of the entorhinal cortex, the ventral subiculum or the prefrontal cortex. The data discussed suggest a subtle and transient functional blockade during early development of the aforementioned brain regions is sufficient to induce schizophrenia-related behavioral and dopaminergic abnormalities in adulthood. In summary, these results support the view that our conceptual and methodological approach, based on functional disconnections, is valid for modeling some aspects of the pathophysiology of schizophrenia from a neurodevelopmental perspective.

Cholinergic modulation of the medial prefrontal cortex: the role of nicotinic receptors in attention and regulation of neuronal activity

Acetylcholine (ACh) release in the medial prefrontal cortex (mPFC) is crucial for normal cognitive performance. Despite the fact that many have studied how ACh affects neuronal processing in the mPFC and thereby influences attention behavior, there is still a lot unknown about how this occurs. Here we will review the evidence that cholinergic modulation of the mPFC plays a role in attention and we will summarize the current knowledge about the role between ACh receptors (AChRs) and behavior and how ACh receptor activation changes processing in the cortical microcircuitry. Recent evidence implicates fast phasic release of ACh in cue detection and attention. This review will focus mainly on the fast ionotropic nicotinic receptors and less on the metabotropic muscarinic receptors. Finally, we will review limitations of the existing studies and address how innovative technologies might push the field forward in order to gain understanding into the relation between ACh, neuronal activity and behavior.

Memory reconsolidation in aversive and appetitive settings

Memory reconsolidation has been observed across species and in a number of behavioral paradigms. The majority of memory reconsolidation studies have been carried out in Pavlovian fear conditioning and other aversive memory settings, with potential implications for the treatment of post-traumatic stress disorder. However, there is a growing literature on memory reconsolidation in appetitive reward-related memory paradigms, including translational models of drug addiction. While there appears to be substantial similarity in the basic phenomenon and underlying mechanisms of memory reconsolidation across unconditioned stimulus valence, there are also notable discrepancies. These arise both when comparing aversive to appetitive paradigms and also across different paradigms within the same valence of memory. We review the demonstration of memory reconsolidation across different aversive and appetitive memory paradigms, the commonalities and differences in underlying mechanisms and the conditions under which each memory undergoes reconsolidation. We focus particularly on whether principles derived from the aversive literature are applicable to appetitive settings, and also whether the expanding literature in appetitive paradigms is informative for fear memory reconsolidation.

Early developmental elevations of brain kynurenic acid impair cognitive flexibility in adults: reversal with galantamine

Levels of kynurenic acid (KYNA), an endogenous α7 nicotinic acetylcholine receptor (α7nAChR) antagonist, are elevated in the brain of patients with schizophrenia (SZ) and might contribute to the pathophysiology and cognitive deficits seen in the disorder. As developmental vulnerabilities contribute to the etiology of SZ, we determined, in rats, the effects of perinatal increases in KYNA on brain chemistry and cognitive flexibility. KYNA's bioprecursor l-kynurenine (100mg/day) was fed to dams from gestational day 15 to postnatal day 21 (PD21). Offspring were then given regular chow until adulthood. Control rats received unadulterated mash. Brain tissue levels of KYNA were measured at PD2 and PD21, and extracellular levels of KYNA and glutamate were determined by microdialysis in the prefrontal cortex in adulthood (PD56-80). In other adult rats, the effects of perinatal l-kynurenine administration on cognitive flexibility were assessed using an attentional set-shifting task. l-Kynurenine treatment raised forebrain KYNA levels ∼3-fold at PD2 and ∼2.5-fold at PD21. At PD56-80, extracellular prefrontal KYNA levels were moderately but significantly elevated (+12%), whereas extracellular glutamate levels were not different from controls. Set-shifting was selectively impaired by perinatal exposure to l-kynurenine, as treated rats acquired the discrimination and intra-dimensional shift at the same rate as controls, yet exhibited marked deficits in the initial reversal and extra-dimensional shift. Acute administration of the α7nAChR-positive modulator galantamine (3.0mg/kg, i.p.) restored performance to control levels. These results validate early developmental exposure to l-kynurenine as a novel, naturalistic animal model for studying cognitive deficits in SZ.

Human electrophysiological correlates of learned irrelevance: effects of the muscarinic M1 antagonist biperiden

Learned irrelevance (LIrr) refers to a reduction in associative learning after pre-exposure of the conditioned and unconditioned stimulus in a non-contingent fashion. This paradigm might serve as a translational model for (pre)attentive information processing deficits in schizophrenia. This is the first study to investigate the event-related potentials (ERPs) of a within-subject LIrr paradigm in humans. Furthermore, the effects of the muscarinic M1 antagonist biperiden on LIrr were assessed. As expected, LIrr was found to be intact in young healthy volunteers after placebo. Furthermore, in the placebo condition P3b latency was decreased for target stimuli, which were pre-cued. This suggests that the predictability of the occurrence of these stimuli is mainly reflected by this ERP component. Biperiden had no effect on the behavioural LIrr measures, although prolonged reaction times were evident. Biperiden increased the N1 amplitude of the pre-exposed predictor letters, suggesting an effect of this drug on early perceptual processing. In conclusion, the within-subject paradigm used in the current study in combination with electroencephalography can reveal brain mechanisms involved in LIrr. M1 antagonism did not affect LIrr performance but seemed to influence early information processing.

Transient inactivation of the neonatal ventral hippocampus impairs attentional set-shifting behavior: reversal with an α7 nicotinic agonist

Cognitive deficits represent a core symptom cluster in schizophrenia that are thought to reflect developmental dysregulations within a neural system involving the ventral hippocampus (VH), nucleus accumbens (NAC), and prefrontal cortex (PFC). The present experiments determined the cognitive effects of transiently inactivating VH in rats during a sensitive period of development. Neonatal (postnatal day 7, PD7) and adolescent (PD32) male rats received a single bilateral infusion of saline or tetrodotoxin (TTX) within the VH to transiently inactivate local circuitry and efferent outflow. Rats were tested as adults on an attentional set-shifting task. Performance in this task depends upon the integrity of the PFC and NAC. TTX infusions did not affect the initial acquisition or ability to learn an intra-dimensional shift. However, TTX rats required a greater number of trials than did controls to acquire the first reversal and extra-dimensional shift (ED) stages. These impairments were age and region-specific as rats infused with TTX into the VH at PD32, or into the dorsal hippocampus at PD7, exhibited performance in the task similar to that of controls. Finally, acute systemic administration of the partial α7 nicotinic acetylcholine receptor (nAChR) agonist SSR 180711 (3.0 mg/kg) eliminated the TTX-induced performance deficits. Given that patients with schizophrenia exhibit hippocampal pathophysiology and deficits in the ED stages of set-shifting tasks, our results support the significance of transient hippocampal inactivation as an animal model for studying the cognitive impairments in schizophrenia as well as the pro-cognitive therapeutic potential of α7 nAChR agonists.

Early prefrontal functional blockade in rats results in schizophrenia-related anomalies in behavior and dopamine

Growing evidence suggests schizophrenia may arise from abnormalities in early brain development. The prefrontal cortex (PFC) stands out as one of the main regions affected in schizophrenia. Latent inhibition, an interesting cognitive marker for schizophrenia, has been found in some studies to be reduced in acute patients. It is generally widely accepted that there is a dopaminergic dysfunctioning in schizophrenia. Moreover, several authors have reported that the psychostimulant, D-amphetamine (D-AMP), exacerbates symptoms in patients with schizophrenia. We explored in rats the effects in adulthood of neonatal transient inactivation of the PFC on behavioral and neurochemical anomalies associated with schizophrenia. Following tetrodotoxin (TTX) inactivation of the left PFC at postnatal day 8, latent inhibition-related dopaminergic responses and dopaminergic reactivity to D-AMP were monitored using in vivo voltammetry in the left core part of the nucleus accumbens in adult freely moving rats. Dopaminergic responses and behavioral responses were followed in parallel. Prefrontal neonatal inactivation resulted in disrupted behavioral responses of latent inhibition and latent inhibition-related dopaminergic responses in the core subregion. After D-AMP challenge, the highest dose (1.5 mg/kg i.p.) induced a greater dopamine increase in the core in rats microinjected with TTX, and a parallel increase in locomotor activity, suggesting that following prefrontal neonatal TTX inactivation animals display a greater behavioral and dopaminergic reactivity to D-AMP. Transitory inactivation of the PFC early in the postnatal developmental period leads to behavioral and neurochemical changes in adulthood that are meaningful for schizophrenia modeling. The data obtained may help our understanding of the pathophysiology of this disabling disorder.