Use of toxins to disrupt neurotransmitter circuitry in the developing brain

Using cortical non-invasive neuromodulation as a potential preventive treatment in schizophrenia - A review

BACKGROUND

Evidence suggests that schizophrenia constitutes a neurodevelopmental disorder, characterized by a gradual emergence of behavioral and neurobiological abnormalities over time. Therefore, applying early interventions to prevent later manifestation of symptoms is appealing.

OBJECTIVE

This review focuses on the use of cortical neuromodulation in schizophrenia and its potential as a preventive treatment approach. We present clinical and preclinical findings investigating the use of neuromodulation in schizophrenia, including the current research focusing on cortical non-invasive stimulation and its possibility as a future preventive treatment.

METHODS

We performed a search in Medline (PubMed) in September 2020 using a combination of relevant medical subject headings (MeSH) and text words. The search included human and preclinical trials as well as existing systematic reviews and meta-analysis. There were no restrictions on language or the date of publication.

RESULTS

Neurodevelopmental animal models may be used to investigate how the disease progresses and thus which brain areas ideally should be targeted at a given time point. Here, abnormalities of the prefrontal cortex have been often identified as an early and persistent impairment in schizophrenia. Currently there is insufficient evidence to either support or refute the use of neuromodulation to the cortex in adult patients with already manifested symptoms. However, preclinical results show that early non-invasive neuromodulation to the prefrontal cortex of adolescent animals, sufficiently prevents later psychosis-relevant abnormalities in adulthood. This points to the promising potential of cortical non-invasive neuromodulation as a preventive treatment when applied early in the course of the disease.

CONCLUSION

Preclinical translational-oriented findings indicate, that neuromodulation to cortical areas offers the possibility of targeting early neuropathology and through this diminish the progression of a later schizophrenic profile. Further studies are needed to investigate whether such early cortical stimulation may serve as a future preventive treatment in schizophrenia.

Cognitive deficits caused by late gestational disruption of neurogenesis in rats: a preclinical model of schizophrenia

Late gestational disruption of neurogenesis in rats has been shown to induce behavioral abnormalities thought to mimic aspects of positive and negative symptoms of schizophrenia. Furthermore, it has been shown that the morphological changes produced by the perturbation are relevant to schizophrenia with reduced thickness of the hippocampus, thalamus, and cortical regions. In addition to the positive and negative symptoms, schizophrenia is associated with deficits in a wide variety of cognitive domains. In the present studies, we assessed whether the cognitive deficits are modeled by disruption of neurogenesis late during gestation (gestational day 17) in the rat. In the battery of tests utilized, we describe that rats in which neurogenesis was disrupted have deficits in a reversal-learning paradigm of the Morris water maze and in object recognition, and that they exhibit perseveration in the Porsolt forced swimming test. Additionally, we found deficient associative learning in an acquisition of an active avoidance paradigm and deficits in latent inhibition. No deficits were observed in the reference memory version of the Morris water maze and in a non-match-to position experiment, showing that the deficits are limited to certain aspects of cognition.

Disruption of neurogenesis on gestational day 17 in the rat causes behavioral changes relevant to positive and negative schizophrenia symptoms and alters amphetamine-induced dopamine release in nucleus accumbens

Gestational disruption of neurodevelopment has been proposed to lead to pathophysiological changes similar to those underlying schizophrenia. We induced such disruption by treating pregnant rat dams with methylazoxymethanol acetate (MAM) on gestational day 17 (GD17). Total brain size and that of the prefrontal cortex and hippocampus were reduced in adult rats exposed prenatally to MAM. When locomotor activity was assessed in an open field, MAM-exposed rats were hyper-responsive to a mild stress and to amphetamine (2 mg/kg, s.c.). They also engaged in less social interaction than controls. We studied, by microdialysis, the effect of amphetamine on extracellular dopamine in the nucleus accumbens and the medial prefrontal cortex of freely moving control and MAM-exposed rats. Amphetamine (2 mg/kg, s.c.) induced an increase in dopamine release that was larger in the nucleus accumbens of MAM-exposed rats than in controls, whereas no difference was seen in the medial prefrontal cortex. In controls, amphetamine infused into the medial prefrontal cortex (50 microM) led to a slight decrease in extracellular dopamine in the nucleus accumbens. This effect was absent in MAM-exposed rats, where a transient increase in nucleus accumbens dopamine levels was seen after amphetamine infusion. These results show that the late gestational disruption of neurogenesis in the rat leads to behavioral changes that mimic positive and negative schizophrenia symptoms, and also to a dysregulation of subcortical dopamine neurotransmission. This study contributes to the evaluation of the validity of the prenatal MAM GD17 treatment in rats as an animal model for schizophrenia.

Schizophrenia: from phenomenology to neurobiology

Schizophrenia is a common and debilitating illness, characterized by chronic psychotic symptoms and psychosocial impairment that exact considerable human and economic costs. The literature in electronic databases as well as citations and major articles are reviewed with respect to the phenomenology, pathology, treatment, genetics and neurobiology of schizophrenia. Although studied extensively from a clinical, psychological, biological and genetic perspective, our expanding knowledge of schizophrenia provides only an incomplete understanding of this complex disorder. Recent advances in neuroscience have allowed the confirmation or refutation of earlier findings in schizophrenia, and permit useful comparisons between the different levels of organization from which the illness has been studied. Schizophrenia is defined as a clinical syndrome that may include a collection of diseases that share a common presentation. Genetic factors are the most important in the etiology of the disease, with unknown environmental factors potentially modulating the expression of symptoms. Schizophrenia is a complex genetic disorder in which many genes may be implicated, with the possibility of gene-gene interactions and a diversity of genetic causes in different families or populations. A neurodevelopmental rather than degenerative process has received more empirical support as a general explanation of the pathophysiology, although simple dichotomies are not particularly helpful in such a complicated disease. Structural brain changes are present in vivo and post-mortem, with both histopathological and imaging studies in overall agreement that the temporal and frontal lobes of the cerebral cortex are the most affected. Functional imaging, neuropsychological testing and clinical observation are also generally consistent in demonstrating deficits in cognitive ability that correlate with abnormalities in the areas of the brain with structural abnormalities. The dopamine and other neurotransmitter systems are certainly involved in the treatment or modulation of psychotic symptoms. These broad findings represent the distillation of a large body of disparate data, but firm and specific findings are sparse, and much about schizophrenia remains unknown.

Schizophrenia and viral infection during neurodevelopment: a focus on mechanisms

The task of defining schizophrenia pathogenesis has fascinated and frustrated researchers for nearly a century. In recent years, unprecedented advances from diverse fields of study have given credence to both viral and developmental theories. This review considers possible mechanisms by which viral and developmental processes may interact to engender schizophrenia. Many of the current controversies in schizophrenia pathogenesis are reviewed in light of the viral hypothesis, including: epidemiological findings and the role of a genetic diathesis, phenotype heterogeneity, abnormalities in excitatory and inhibitory neurotransmitter systems, anomalous cerebral latereralization, and static vs progressive disease. The importance of animal models in elucidating the impact of viral infections on developing neurons is illustrated by recent studies in which neonatal rats are infected with lymphocytic choriomeningitis virus in order to examine alterations in hippocampal circuitry. Finally, consideration is given to a new hypothesis that some cases of schizophrenia could be instigated by a viral infection that disrupts developing inhibitory circuits, consequently unleashing glutamatergic neurotransmission leading to selective excitotoxicity, and a degenerative disease course.

The distribution of serotonergic nerves in microencephalic rats treated prenatally with methylazoxymethanol

Prenatal exposure of pregnant rats to methylazoxymethanol acetate (MAM) induces microencephaly in the offspring. In the present study of these microencephalic rats (MAM rats) we used quantitative autoradiography to investigate [3H] paroxetine binding sites, which are a selective marker of serotonin (5-HT) transporters (5-HTT). The binding in the accumbens, cortex, hippocampus, and dorsolateral thalamus was significantly increased in MAM rats, compared to the control rats, while there was a significant decrease in the dorsal raphe nucleus of the MAM rats. The levels of 5-HTT mRNA in the dorsal raphe nuclei were analyzed by in situ hybridization, which revealed a significant decrease in 5-HTT mRNA-positive neurons in the MAM rats compared to the control rats. The results imply serotonergic hyperinnervation in the cerebral hemispheres of MAM rats, while a target-dependent secondary degeneration of 5-HT neurons might be induced in the dorsal raphe nuclei of MAM rats.

Mechanisms underlying epileptogenesis in cortical malformations

The presence of developmental cortical malformations is associated with epileptogenesis and other neurological disorders. In recent years, animal models specific to certain malformations have been developed to study the underlying epileptogenic mechanisms. Teratogens (chemical, thermal or radiation) applied during cortical neuroblast division and migration result in lissencephaly and focal cortical dysplasia. Animals with these malformations have a lowered seizure threshold as well as histopathologies typical of those found in human dysgenic brains. Alterations that may promote epileptogenesis have been identified in lissencephalic brains, such as increased numbers of bursting types of neurons, and abnormal connections between hippocampus, subcortical heterotopia, and neocortex. A distinct set of pathological properties is present in animal models of 4-layered microgyria, induced with cortical lesions made during late stages of cortical neuroblast migration. Hyperexcitability has been demonstrated in cortex adjacent to the microgyrus (paramicrogyral zone) in in vitro slice preparations. A number of observations suggest that cellular differentiation is delayed in microgyric brains. Other studies show increases in postsynaptic glutamate receptors and decreases in GABA(A) receptors in microgyric cortex. These alterations could promote epileptogenesis, depending on which cell types have the altered receptors. The microgyrus lacks thalamic afferents from sensory relay nuclei, that instead appear to project to the paramicrogyral region, thereby increasing excitatory connectivity within this epileptogenic zone. These studies have provided a necessary first step in understanding molecular and cellular mechanisms of epileptogenesis associated with cortical malformations.

Effect of chronic amitriptyline administration on serotonergic receptors in rats with methylazoxymethanol-induced microencephaly

Methylazoxymethanol (MAM)-induced cortical hypoplasia resulted in a 20% decrease in the Bmax of 5-HT2A receptors in the frontal cortex with no change in the Bmax of 5-HT1A receptors. Chronic treatment with amitriptyline did not further decrease the Bmax of 5-HT2A receptors in the MAM-lesioned cortex, suggesting that the persistent down-regulation of cortical 5-HT2A receptors in MAM-lesioned rats was induced by serotonergic hyperinnervation.

Effects of prenatal methylazoxymethanol treatment on striatal dopaminergic systems in rat brain

To further examine the effects of prenatal methylazoxymethanol (MAM) treatment on striatal dopaminergic systems, the status of presynaptic dopamine transporters was examined by quantitative autoradiography of [3H]GBR 12935 binding. Significantly higher [3H]GBR 12935 binding was seen in MAM-lesioned striatum in comparison to the controls, indicating relative dopaminergic hyperinnervation in MAM-induced hypoplastic striatum. The effect of prenatal MAM treatment on extracellular levels of dopamine and its metabolites in the striatum was also examined using in vivo microdialysis. As measured in conscious freely-moving rats, prenatal MAM treatment significantly increased basal dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) release in the striatum in comparison with control rats. These data suggest that in accordance with morphological dopaminergic hyperinnervation, dopaminergic functions are significantly augmented in MAM-lesioned brains. Thus, it is suggested that MAM-induced microencephalic rats should serve as a good animal model for the study of augmented dopaminergic functions in the striatum.

Epidemiological and laboratory evidence of PCB-induced neurotoxicity

The purpose of this review is to provide a selective, but critical, assessment of important findings derived from both epidemiological and laboratory studies suggesting that: (1) exposure to polychlorinated biphenyls (PCBs) and related halogenated aromatic hydrocarbons induces significant neurological and behavioral dysfunctions in humans and laboratory animals, particularly following exposure during gestation and lactation; (2) the neurochemical actions of PCBs depend on their structure and the developmental status of the animal at the time of exposure; and (3) the mechanisms responsible for these changes may involve alterations in basic cellular signaling processes and endocrine function that influence the synthesis and activity of important central nervous system neurotransmitters, the organization of the developing brain, and the behavioral responses to these environmental contaminants.

Short exposure to methylazoxymethanol causes a long-term inhibition of axonal outgrowth from cultured embryonic rat hippocampal neurons

Methylazoxymethanol (MAM) is an alkylating agent that is used to induce microencephaly by killing mitotically active neuroblasts. We found that at later developmental times, MAM exposure can result in abnormal fiber growth in vivo. However, there have not been any previous studies on the effects of MAM on differentiating neurons. We examined the outcome of short exposure to MAM on postmitotic embryonic hippocampal cultures during the establishment of axonal polarity. At 0, 1, or 2 days in vitro (DIV), neurons were treated with 0.1 nM-1 microM MAM for 3 hr and then transferred to glial conditioned media. At 3 DIV, the cells were fixed and analyzed by immunofluorescent staining for neuron viability and differentiation. Control cells initiate several minor processes; one process elongates rapidly at about 1 DIV eventually becoming an axon, while extensive dendritic growth occurs after 3-4 DIV. Neurons treated with 1 microM MAM at 0 or 1 DIV showed a marked inhibition of neurite growth and withdrawal of axons without affecting cell viability. These cells continued to show minimal neurite outgrowth at 7 DIV, even when transferred to a glial coculture. In contrast, cells treated initially with MAM, after neuronal polarity is established at 2 DIV, showed no effect on axonal growth. To determine the effects of MAM on the neuronal cytoskeleton, we examined the in vitro assembly of brain microtubules in a one cycle assay. Exposure to MAM depleted the soluble pool of proteins, including microtubule-associated protein 1B (MAP1B) and MAP2, which are required for neurite outgrowth, through a nonspecific process. Under non-saturating conditions, there were no changes in the total amount of microtubules assembled or the coassembly of MAP1B and MAP2 in the presence of MAM. These results demonstrate that MAM can directly affect differentiating neurons, indicating that an early disruption of axonal outgrowth may have long-term effects.

Dihydropyridine ligand binding decreases earlier in adolescent than in infant swine after global cerebral ischemia

BACKGROUND AND PURPOSE

Voltage-dependent calcium channels (VDCCs) are thought to play a major role in the alteration of calcium homeostasis during ischemia. Tissue functional state as well as responsiveness to therapy with calcium channel blockers may be a function of regional changes in the density of VDCCs. This study determined whether VDCCs are altered by global ischemia in infant and adolescent swine.

METHODS

We employed the radioligand 3HPN200-110 to quantify the binding characteristics of VDCCs in cerebral cortex, caudate, and hippocampus by equilibrium binding analysis. Adolescent and infant pigs underwent 3, 5, 10, and 20 minutes of global cerebral ischemia without reperfusion by ligation of the brachiocephalic and left subclavian arteries combined with hypotension to a mean arterial blood pressure of 50 mm Hg. Brain cortex, hippocampus, and caudate samples were taken during ischemia and frozen immediately in liquid nitrogen, and crude synaptosomal membranes were isolated by differential centrifugation/filtration. 3HPN200-110 equilibrium binding assays were performed in the presence or absence of 1.0 mumol/L unlabeled nitrendipine to determine total and nonspecific binding.

RESULTS

Infant cortex maximal binding (Bmax) increased to 176% of control after 5 minutes of global cerebral ischemia and remained significantly elevated (172% of control) after 10 minutes before falling to near control levels by 20 minutes. Adolescent cortex Bmax increased to 157% of control levels after 5 minutes but did not remain elevated, falling to 131% of control by 10 minutes and near control by 20 minutes. Infant caudate and hippocampus binding were significantly elevated after 10 (124% and 149% of control, respectively) and 20 (115% and 120% of control, respectively) minutes of ischemia. Adolescent caudate and hippocampus binding was either not significantly different from control levels (hippocampus at 10 minutes) or less than control after 10 and 20 minutes of global cerebral ischemia. The decrease in binding following the initial upregulation, which appeared earlier in the adolescent than the infant pigs, may indicate decreased tolerance to ischemia in the adolescent.

CONCLUSIONS

The binding of 3HPN200-110 in brain is altered during 20 minutes of global cerebral ischemia, and these changes are region- and age-dependent.

Age-related differences in recovery of blood flow and metabolism after cerebral ischemia in swine

We tested two hypotheses: 1) that cerebral blood flow, oxygen consumption, and evoked potentials recover to preischemic values at 120 minutes of reperfusion more completely in 1-2-week-old piglets than in 6-10-month-old pigs after complete ischemia; and 2) that recovery of cerebral blood flow, oxygen consumption, and electrical function in piglets and pigs at 120 minutes of reperfusion is better after incomplete than after complete ischemia. During 30 minutes of ischemia produced by intracranial pressure elevation, cerebral blood flow determined by the microspheres technique was decreased to 0-1 ml/min/100 g with complete ischemia, to 1-10 ml/min/100 g with severe incomplete ischemia, or to 10-20 ml/min/100 g with moderate incomplete ischemia. During reperfusion after complete ischemia, both piglets and pigs demonstrated hyperemia but delayed hypoperfusion occurred in more brain regions in pigs, oxygen consumption returned to preischemic values in piglets but not in pigs (70 +/- 10% of preischemic values), and evoked potentials recovered better in piglets than in pigs (24 +/- 4% and 9 +/- 4% of preischemic values, respectively). Both piglets and pigs had fewer brain areas with hyperemia and hypoperfusion and improved oxygen consumption and electrical function during recovery from incomplete than from complete ischemia. We speculate that piglets tolerate complete ischemia better than pigs because of decreased reperfusion injury and that both groups recover better from incomplete than complete ischemia because of improved substrate supply during ischemia.

Differential target dependence in the developing brain: implications for mental retardation

During development of the brain, many neurons exhibit a dependence on other neuronal populations for their survival and differentiation (target dependence). Evidence suggests that some neural pathways are much more dependent on single target neuronal populations for their survival than are others (differential target dependence). This phenomenon has important implications both for animal models of congenital human brain damage and for ideas concerning the aetiology of behavioural abnormalities associated with human mental retardation. Predictions of the neuronal deficits likely to arise from exposure to cytotoxic agents (e.g. ionizing radiation, hyperthermia, viral infection) at a particular time must take differential target dependence into account. It is known that target dependence affects corticopetal pathways involved with the discriminative senses (e.g. vision), more than monoaminergic and cholinergic corticopetal pathways which are believed to be involved with arousal, selective sensory attention, sleep, memory and cortical vasomotor function. Following prenatal damage to superficial layers of the cerebral cortex, this effect of differential target dependence leads not only to a relative hyperinnervation of the cortex with monoaminergic and cholinergic projections, but a specific deficit in visual pathways. The implications of this combined deficit for the behaviour and rehabilitation of the mentally retarded are considered.

Patterns of transiently expressed acetylcholinesterase activity in cerebral cortex and dorsal thalamus of developing rats with cytotoxin-induced microencephaly

Previous studies have demonstrated that acetylcholinesterase (AChE) activity is expressed transiently by thalamocortical neurons of primary sensory systems in developing rat pups. In the present study, prenatal treatment with methylazoxymethanol acetate (MAM) on embryonic day 15, 16, or 17 resulted in rat pups with cerebral cortices markedly reduced in thickness and areal extent. Histochemical studies demonstrated that AChE staining occurs in fiber-like plexuses in primary visual, auditory, and somatosensory regions of developing cerebral cortex of MAM-treated animals, just as in normal developing rats, but that the transient patterns of AChE are found more superficially than normal and they occur in an abnormal patchy distribution. Neuronal somata in thalamic lateral geniculate, medial geniculate and ventral basal nuclei of MAM-treated animals show transient AChE staining indistinguishable from that seen in normal animals. These data indicate: (1) AChE is expressed transiently by thalamocortical neurons in MAM-treated animals, (2) intensity of the transiently expressed AChE is not affected by MAM-induced loss of cortical neurons, and (3) the abnormal AChE patterns in cortex likely reflect the abnormal distributions of thalamocortical terminal fields that are characteristic of MAM-treated animals.