Methylazoxymethanol treatment of fetal rats results in abnormally dense noradrenergic innervation of neocortex

D-serine reconstitutes synaptic and intrinsic inhibitory control of pyramidal neurons in a neurodevelopmental mouse model for schizophrenia

The hypothesis of N-methyl-D-aspartate receptor (NMDAR) dysfunction for cognitive impairment in schizophrenia constitutes the theoretical basis for the translational application of NMDAR co-agonist D-serine or its analogs. However, the cellular mechanism underlying the therapeutic effect of D-serine remains unclear. In this study, we utilize a mouse neurodevelopmental model for schizophrenia that mimics prenatal pathogenesis and exhibits hypoexcitability of parvalbumin-positive (PV) neurons, as well as PV-preferential NMDAR dysfunction. We find that D-serine restores excitation/inhibition balance by reconstituting both synaptic and intrinsic inhibitory control of cingulate pyramidal neurons through facilitating PV excitability and activating small-conductance Ca2+-activated K+ (SK) channels in pyramidal neurons, respectively. Either amplifying inhibitory drive via directly strengthening PV neuron activity or inhibiting pyramidal excitability via activating SK channels is sufficient to improve cognitive function in this model. These findings unveil a dual mechanism for how D-serine improves cognitive function in this model.

Etiology of Retinal and Cerebellar Pathology in Western Pacific Amyotrophic Lateral Sclerosis and Parkinsonism-Dementia Complex

Purpose

To reexamine the etiology of a unique retinal pathology (linear and vermiform sub-retinal tubular structures) described among subjects with and without neurodegenerative disease in former high-incidence foci of Western Pacific amyotrophic lateral sclerosis and parkinsonism-dementia complex (ALS/PDC) in Guam (USA) and the Kii peninsula of Honshu island (Japan).

Methods

Analysis of published and unpublished reports of 1) ALS/PDC and the retinal and cerebellar pathology associated therewith and 2) exogenous neurotoxic factors associated with ALS/PDC and the developing retina and cerebellum.

Results

ALS/PDC retinal and cerebellar pathology matches persistent retinal and cerebellar dysplasia found in laboratory animals given single in utero or postnatal systemic treatment with cycasin, the principal neurotoxic component in the seed of cycad plants traditionally used for food (Guam) or oral medicine (Kii-Japan), both of which have been linked to the human neurodegenerative disease.

Conclusion

ALS/PDC-associated retinal and cerebellar dysplasia could arise from in utero exposure to methylazoxymethanol, the genotoxic metabolite of cycasin that results from maternal ingestion of this azoxyglucoside. These results support the environmental toxic etiology of retinal and brain pathology in ALS/PDC.

My Life in Clinical Neuroscience: The Beginning

This chapter recounts the author's life from childhood until he opened his research laboratory as an Assistant Professor in the Department of Pharmacology and Experimental Therapeutics at Johns Hopkins School of Medicine in 1976. It emphasizes the importance of chance opportunities and generous mentoring in the initiation of his career in neuroscience and psychiatric research.

Developmental origins of adult diseases and neurotoxicity: epidemiological and experimental studies

To date, only a small number of commercial chemicals have been tested and documented as developmental neurotoxicants. Moreover, an increasing number of epidemiological, clinical and experimental studies suggest an association between toxicant or drug exposure during the perinatal period and the development of metabolic-related diseases and neurotoxicity later in life. The four speakers at this symposium presented their research results on different neurotoxic chemicals relating to the developmental origins of health and adult disease (DOHaD). Philippe Grandjean presented epidemiological data on children exposed to inorganic mercury and methylmercury, and discussed the behavioral outcome measures as they relate to age and stage of brain development. Donald A. Fox presented data that low-dose human equivalent gestational lead exposure produces late-onset obesity only in male mice that is associated with neurodegeneration. Didima de Groot presented results on prenatal exposure of rats to methylazoxymethanol and discussed the results in light of the etiology of western Pacific amyotrophic lateral sclerosis and Parkinson-dementia complex. Merle G. Paule addressed the long-term changes in learning, motivation and short-term memory in aged Rhesus monkeys following acute 24 h exposure to ketamine during early development. Overall, these presentations addressed fundamental issues in the emerging areas of lifetime neurotoxicity testing, differential vulnerable periods of exposure, nonmonotonic dose-response effects and neurotoxic risk assessment. The results indicate that developmental neurotoxicity results in permanent changes, thus emphasizing the need to prevent such toxicity.

Changes in postnatal norepinephrine alter alpha-2 adrenergic receptor development

Alpha-2 adrenergic receptors (A2AR) regulate multiple brain functions and are enriched in developing brain. Studies demonstrate norepinephrine (NE) plays a role in regulating brain maturation, suggesting it is important in A2AR development. To investigate this we employed models of NE absence and excess during brain development. For decreases in NE we used N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP4), a specific noradrenergic neurotoxin. Increased noradrenergic terminal density was produced by methylazoxymethanol acetate (MAM) treatment. A2AR density was assayed with [(3)H]RX821002 autoradiography. DSP4 lesions on postnatal day (PND) 3 produce A2AR decreases in many regions by PND 5. A2AR recover to control levels by PND 15 and 25 and there is no further change in total receptor density. We also assayed A2AR in brains lesioned with DSP4 on PND 13, 23, 33 and 43 and harvested 22 days post-lesion. A2AR levels remain similar to control at each of these time points. We examined A2AR functionality and high affinity state with epinephrine-stimulated [(35)S]GTPγS and [(125)I]p-iodoclonidine autoradiography, respectively. On PND 25, control animals and animals lesioned with DSP4 on PND 3 have similar levels of [(35)S]GTPγS incorporation and no change in high affinity state. This is in contrast to increases in A2AR high affinity state produced by DSP4 lesions of mature brain. We next investigated A2AR response to increases in norepinephrine levels produced by MAM. In contrast to DSP4 lesions, increasing NE results in a large increase in A2AR. Animals treated with MAM on gestational day 14 had cortical [(3)H]RX821002 binding 100-200% greater than controls on PND 25, 35, 45, 55 and 65. These data indicate that NE regulation of A2AR differs in developing and mature brain and support the idea that NE regulates A2AR development and this has long term effects on A2AR function.

Development of central neurotransmitter systems

Inter-neuronal communication is mediated primarily by chemical neurotransmitters, which are released from the nerve terminal, diffuse across the synaptic cleft and interact with specific receptors on adjacent neurons. The development of the biochemical machinery for neurotransmission is closely linked to the functional maturation of the brain's neuronal circuitry. Components essential for neurotransmission (e.g., synthetic enzymes, endogenous neurotransmitters, re-uptake processes and receptors) serve as specific biochemical markers for neuronal systems. The appearance of and developmental increases in these markers during fetal and postnatal life occur with the cessation of neuronal replication and initiation of neuropil elaboration. Discrete groups of neurotransmitter-specific neurons develop according to different timetables, resulting in a shifting pattern of their relative influence in the maturing brain. Human and animal studies demonstrate an early innervation of the neocortex by catecholaminergic axons while neurons using gamma-aminobutyric acid (GABA) mature somewhat later; and the ontogeny of the acetylcholine neurons lags behind both of these. Within each neuronal group the individual biochemical components for neurotransmission also follow differing time courses of maturation. Animal studies, in which cortical neurons were ablated by administering a toxin to the fetus, illustrate the interplay between intrinsic programmes and environmental influences in the assembly of neuronal circuits. The brain's preparation for independent life is characterized by a continual reorganization of neurotransmitter pathways.

Circuit-based framework for understanding neurotransmitter and risk gene interactions in schizophrenia

Many risk genes interact synergistically to produce schizophrenia and many neurotransmitter interactions have been implicated. We have developed a circuit-based framework for understanding gene and neurotransmitter interactions. NMDAR hypofunction has been implicated in schizophrenia because NMDAR antagonists reproduce symptoms of the disease. One action of antagonists is to reduce the excitation of fast-spiking interneurons, resulting in disinhibition of pyramidal cells. Overactive pyramidal cells, notably those in the hippocampus, can drive a hyperdopaminergic state that produces psychosis. Additional aspects of interneuron function can be understood in this framework, as follows. (i) In animal models, NMDAR antagonists reduce parvalbumin and GAD67, as found in schizophrenia. These changes produce further disinhibition and can be viewed as the aberrant response of a homeostatic system having a faulty activity sensor (the NMDAR). (ii) Disinhibition decreases the power of gamma oscillation and might thereby produce negative and cognitive symptoms. (iii) Nicotine enhances the output of interneurons, and might thereby contribute to its therapeutic effect in schizophrenia.

Circuit-based framework for understanding neurotransmitter and risk gene interactions in schizophrenia

Many risk genes interact synergistically to produce schizophrenia and many neurotransmitter interactions have been implicated. We have developed a circuit-based framework for understanding gene and neurotransmitter interactions. NMDAR hypofunction has been implicated in schizophrenia because NMDAR antagonists reproduce symptoms of the disease. One action of antagonists is to reduce the excitation of fast-spiking interneurons, resulting in disinhibition of pyramidal cells. Overactive pyramidal cells, notably those in the hippocampus, can drive a hyperdopaminergic state that produces psychosis. Additional aspects of interneuron function can be understood in this framework, as follows. (i) In animal models, NMDAR antagonists reduce parvalbumin and GAD67, as found in schizophrenia. These changes produce further disinhibition and can be viewed as the aberrant response of a homeostatic system having a faulty activity sensor (the NMDAR). (ii) Disinhibition decreases the power of gamma oscillation and might thereby produce negative and cognitive symptoms. (iii) Nicotine enhances the output of interneurons, and might thereby contribute to its therapeutic effect in schizophrenia.

An examination of calcium current function on heterotopic neurons in hippocampal slices from rats exposed to methylazoxymethanol

PURPOSE

To study voltage-dependent calcium currents (VDCCs) on hippocampal heterotopic neurons by using whole-cell patch-clamp techniques in brain slices prepared from methylaxozymethanol (MAM)-exposed rats.

METHODS

Whole-cell voltage-clamp recordings were obtained from visually identified neurons in acute brain slices by using an infrared differential interference contrast (IR-DIC) video microscopy system. Heterotopic neurons were compared with normotopic pyramidal cells in hippocampal slices from MAM-exposed rats or CA1 pyramidal neurons in slices from controls.

RESULTS

Heterotopic neurons expressed a prominent VDCC, which exhibited a peak current maximum around -30 mV (holding potential, -60 mV) and an inactivation time constant of 48.2 +/- 2.4 ms (n = 91). VDCC peak current and inactivation time constants were similar for normotopic (n = 92) and CA1 pyramidal cells (n = 40). Pharmacologic analysis of VDCC, on heterotopic, normotopic, and CA1 pyramidal cells, revealed an approximately 70% blockade of peak Ca2+ current with nifedipine and amiloride (L- and T-type channel blockers, respectively). Inhibition of VDCC, for all three cell types, also was similar when more specific Ca2+ channel antagonists were used [e.g., omega-conotoxin GVIA (N-type), omega-agatoxin KT (P/Q-type), and sFTX-3.3 (P-type)]. VDCC modulation by norepinephrine (NE) or adrenergic receptor-specific agonists [clonidine (alpha2), isoproterenol (beta), and phenylephrine (alpha1)] was similar for heterotopic and CA1 pyramidal cells.

CONCLUSIONS

Heterotopic neurons do not appear to exhibit Ca2+ channel abnormalities that could contribute to the reported hyperexcitability associated with MAM-exposed rats.

A radialization factor in normal cortical plate restores disorganized radial glia and disrupted migration in a model of cortical dysplasia

Treatment of pregnant ferrets on embryonic day 24 (E24) with the antimitotic methylazoxy methanol (MAM) leads to a specific constellation of effects in newborn kits, which include a very thin and poorly laminated neocortex, disruption of radial glial cell morphology with early differentiation into astrocytes, and abnormal positioning of Cajal-Retzius cells. We suggest that MAM treatment on E24 results in this model of cortical dysplasia by eliminating a population of cells that produce a factor capable of maintaining radial glia in their normal morphology. The abnormal radial glia, either alone or in combination with other abnormal features, are likely to prevent proper migration into the cortical plate. To test the possibility that normal cortex can provide the missing substance that influences radial glia, slices of E24 MAM-treated cortex were removed at postnatal day 0 (P0) and cultured adjacent to explants of P0 normal cortical plate. By labelling a small number of cells with injections of fluorescent dextrans into the cultured slices, we found that abnormal radial glia in MAM treated slices cocultured adjacent to normal cortical plate were restored toward normal, in comparison to E24 MAM treated slices cultured alone and in other control conditions. We also found that abnormally positioned Cajal-Retzius cells move into the marginal zone and that neurons are able to migrate into the cortical plate more effectively in the coculture condition. These data indicate that normal cortical plate of ferrets contains a factor causing radial glia to maintain their elongated morphology; the improved position of radial glia encourages repositioning of Cajal-Retzius cells and improved neuronal migration into the cortical plate.

Hippocampal heterotopia with molecular and electrophysiological properties of neocortical neurons

Cortical malformations resulting from aberrant brain development can be associated with mental retardation, dyslexia, and intractable forms of epilepsy. Despite emerging interest in the pathology and etiology of cortical malformations, little is known about the phenotype of cells within these lesions. In utero exposure to the DNA methylating agent methylazoxymethanol acetate (MAM) during a critical stage in neurodevelopment results in animals with distinct clusters of displaced neurons in hippocampus, i.e. nodular heterotopia. Here we examined the molecular and electrophysiological properties of cells within hippocampal heterotopia using rats exposed to MAM during gestation. Molecular analysis revealed that heterotopic cells do not express mRNA markers normally found in hippocampal pyramidal cells or dentate granule cells (SCIP, Math-2, Prox-1, neuropilin-2). In contrast, Id-2 mRNA, normally abundant in Layer II-III supragranular neocortical neurons but not in CA1 pyramidal neurons, was prominently expressed in hippocampal heterotopia. Current-clamp analysis of the firing properties of heterotopic neurons revealed a striking similarity with supragranular cortical neurons. In particular, both cells were characterized by small hyperpolarizing 'sag' potentials, high input resistance values, slow spike-train afterhyperpolarizations, and the absence of a depolarizing afterpotential. Normotopic CA1 pyramidal neurons (e.g. pyramidal cells with normal lamination adjacent to a heterotopia) in the MAM brain exhibited molecular and electrophysiological properties that were nearly identical to those of age-matched CA1 pyramidal neurons from control rats. We conclude that neuronal heterotopiae in the hippocampus of MAM-exposed rats are comprised of neurons with a Layer II-III supragranular cortex phenotype. The MAM model, therefore, may serve as a useful tool in examination of the factors influencing aberrant brain development and epilepsy.

Hippocampal heterotopia lack functional Kv4.2 potassium channels in the methylazoxymethanol model of cortical malformations and epilepsy

Human cortical malformations often result in severe forms of epilepsy. Although the morphological properties of cells within these malformations are well characterized, very little is known about the function of these cells. In rats, prenatal methylazoxymethanol (MAM) exposure produces distinct nodules of disorganized pyramidal-like neurons (e.g., nodular heterotopia) and loss of lamination in cortical and hippocampal structures. Hippocampal nodular heterotopias are prone to hyperexcitability and may contribute to the increased seizure susceptibility observed in these animals. Here we demonstrate that heterotopic pyramidal neurons in the hippocampus fail to express a potassium channel subunit corresponding to the fast, transient A-type current. In situ hybridization and immunohistochemical analysis revealed markedly reduced expression of Kv4.2 (A-type) channel subunits in heterotopic cell regions of the hippocampus of MAM-exposed rats. Patch-clamp recordings from visualized heterotopic neurons indicated a lack of fast, transient (I(A))-type potassium current and hyperexcitable firing. A-type currents were observed on normotopic pyramidal neurons in MAM-exposed rats and on interneurons, CA1 pyramidal neurons, and cortical layer V-VI pyramidal neurons in saline-treated control rats. Changes in A-current were not associated with an alteration in the function or expression of delayed, rectifier (Kv2.1) potassium channels on heterotopic cells. We conclude that heterotopic neurons lack functional A-type Kv4.2 potassium channels and that this abnormality could contribute to the increased excitability and decreased seizure thresholds associated with brain malformations in MAM-exposed rats.

Laminar specific alterations of thalamocortical projections in organotypic cultures following layer 4 disruption in ferret somatosensory cortex

The developing neocortex influences the growth of thalamocortical projections. Layer 4 in particular receives the majority of input from the thalamus and is important in instructing thalamic afferents to terminate. Previous in vivo experiments demonstrated that disruption of layer 4 during corticogenesis in ferret somatosensory cortex by application of methylazoxy methanol acetate (MAM) prevents proper termination of thalamic afferents in appropriate cortical regions. To further explore the role of layer 4 in thalamocortical development, we prepared organotypic cocultures consisting of normal gestational day 0 (P0) ferret thalamus paired with normal, embryonic day 33 (E33), or E38 MAM-treated cortex obtained from ferrets at either P0 or P7. Injection of MAM on E33 disrupts layer 4 formation, whereas similar injections on E38 interfere with layer 2 formation. The cocultures grew together for a number of days, then discrete injections of either fluorescent dextrans or 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) were made into the thalamic piece. The labeled thalamic afferents that grew into the cortical slice were analysed and the sites of their terminations quantified after 3, 5, or 7-10 days in culture (DIC). Our results varied somewhat with the amount of time in culture, but the preponderance of thalamic fibers in normal cortex terminated in layer 4, whereas their counterparts in E33 MAM-treated cortex grew beyond the cortical plate and many fibers terminated inappropriately within lower cortical layers or white matter. Terminal distribution of thalamic fibers in E38 MAM-treated cortex looked similar to normal. These results demonstrate that the cells of layer 4 provide thalamic afferents with important positional and termination cues.

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.

Prenatal malnutrition-induced functional alterations in callosal connections and in interhemispheric asymmetry in rats are prevented by reduction of noradrenaline synthesis during gestation

Prenatal malnutrition results in increased concentration and release of central noradrenaline, a neurotransmitter that is an important regulator of normal regressive events such as axonal pruning and synaptic elimination. This suggests that some of the functional disturbances in brain induced by prenatal malnutrition could be due at least in part to increased noradrenaline activity that may enhance regressive events during early stages of development. To test this hypothesis we studied whether chronic administration of alpha-methyl-p-tyrosine, an inhibitor of tyrosine hydroxylase, to rats during gestation might prevent long-term deleterious effects of prenatal malnutrition on functional properties of interhemispheric connections of the visual cortex, and on asymmetry of visual evoked responses. The experiments were conducted on normal and malnourished rats 45-50 d of age. Prenatal malnutrition was induced by restricting the food consumption of pregnant rats to 40%, from d 8 postconception to parturition. At birth, prenatally malnourished rats had significantly greater whole-brain noradrenaline concentration as well as significantly enhanced noradrenaline release in the visual cortex. At 45-50 d of age, the malnourished group had a significantly smaller cortical area, exhibiting transcallosal evoked responses; in addition, the amplitude of these responses was significantly smaller. Malnourished rats showed a significant reduction of the normal interhemispheric asymmetry of visual evoked responses. The addition of 0.3% alpha-methyl-p-tyrosine to the diet of malnourished pregnant rats during the last 2 wk of gestation prevented functional disorders induced in the offspring by prenatal malnutrition on interhemispheric connectivity of visual areas and on interhemispheric bioelectrical asymmetry, probably by reducing the elevated brain noradrenaline activity and thereby restoring the normal trophic role of this neurotransmitter.

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.

Experimentally induced disorders of neuronal migration produce an increased propensity for electrographic seizures in rats

Disorders of neuronal migration in humans are associated with intractable epilepsy and some evidence suggests a causal relationship. This study evaluated electroencephalograms (EEG) of rats with experimentally induced disorders of neuronal migration. Fetal Sprague-Dawley rats were exposed to 196 cGy external irradiation on days 16 and 17 of gestation. This produced adult offspring with diffuse cortical dysplasias, agenesis of the corpus callosum, periventricular heterotopias, and dispersion of the pyramidal cell layer of the hippocampus. Epidural electrodes were implanted in four experimental (irradiated on gestational day 17) and four control rats. EEGs were recorded without anesthesia and in the presence of the anesthetic agents ketamine, acepromazine, and xylazine. In the presence of acepromazine, xylazine, or a combination of the two drugs, two of the four experimental rats had prolonged ictal activity on EEG. In one of the rats the ictal activity progressed to electrographic status epilepticus. Ketamine alone did not produce ictal EEG activity. None of the control rats demonstrated ictal activity under any treatment condition. This study demonstrates that disorders of neuronal migration are associated with an increased propensity for seizures in the presence of certain sedating agents.

Clonidine treatment during gestation prevents functional deficits induced by prenatal malnutrition in the rat visual cortex

It has been shown that prenatal malnutrition results at birth in increased concentration of noradrenaline (NA) in the brain. Besides, it is known that NA is an important regulator of normal regressive processes occurring during synaptogenesis such as cell death, axonal pruning and synaptic elimination. The present study was designed to investigate (i) whether prenatal malnutrition enhances the NA release in the visual cortex and (ii) whether or not chronic administration of clonidine during gestation may prevent long-term deleterious effects of fetal malnutrition on functional properties of interhemispheric connections of the visual cortex and on the interhemispheric asymmetry of visual evoked responses. Prenatal malnutrition was induced by restricting food consumption to pregnant rats from Day 8 postconception to parturition. Results show that at birth, prenatally malnourished rats had higher NA release than normals. At 45-50 days of age, the malnourished group exhibited (a) reduced peak-to-peak amplitude and diminished extent of the projecting field of transcallosal evoked responses, and (b) abolished interhemispheric asymmetry of visual evoked responses. Clonidine administration to malnourished mothers from Day 14 postconception to parturition (10 g/kg/day s.c.), prevented in the offspring disorders induced by prenatal malnutrition on cortical NA release, on interhemispheric connectivity of visual areas and on interhemispheric bioelectrical asymmetry, probably by restoring the normal trophic role of NA during synaptogenesis. Results are discussed in relationship to normal regressive events occurring during early brain development.

Effects of foetal treatment with methylazoxymethanol on noradrenergic synapses in rat cerebral cortex

Methylazoxymethanol (MAM)-induced cerebral hypoplasia resulted in a significant increase in densities of norepinephrine uptake sites in cerebral cortex, suggesting that norepinephrinergic axon terminals were compressed in the smaller brain volumes. The density of beta-adrenergic receptors in MAM-lesioned cerebral cortex was decreased probably due to down-regulation, while there were no changes in the proportions and affinities of agonist high-affinity sites and agonist low-affinity sites in the desensitized beta-receptors.

Development of neurons and synapses in ochratoxin A-induced microcephalic mice: a quantitative assessment of somatosensory cortex

Ochratoxin A (a mycotoxin) is known to cause cell death in the developing brain of embryos 1-2 days after treatment. Microcephaly was observed with high frequency in mice by prenatal treatment with ochratoxin A. Using a stereological method, the numerical densities of neurons and synapses were investigated in the somatosensory cortex of 6-week-old microcephalic mice. The numerical density of neurons in ochratoxin A-treated mice represented a 39% increase compared to the control mice, but there was no difference in the numerical density of synapses. The somatosensory cortices of control mice had about 13,000 synapses per neuron, whereas ochratoxin A-treated mice had about 9,400 synapses per neuron. The deficits in synapse-to-neuron ratios seen in ochratoxin A-induced microcephalic brain seemed to result from a reduced dendritic growth.

Loss of striatal somatostatin neurons following prenatal methylazoxymethanol

Prenatal administration of methylazoxymethanol acetate (MAM), which kills neuroblasts undergoing mitosis, was used to lesion striatal somatostatin neurons. Previous [3H]thymidine autoradiographic studies had indicated that striatal somatostatin neurons undergo their final mitotic division at Gestational Days (G) 15 and 16. Therefore, pregnant Sprague-Dawley rats received an intraperitoneal injection of MAM (25 mg/kg) on G15. Neurochemical and histological examination of the mature offspring indicated the loss of half the striatal aspiny interneurons in which somatostatin, neuropeptide Y, and NADPH diaphorase coexist, with relative sparing of the cholinergic interneurons and medium spiny projection cells. This prenatal MAM treatment was without apparent effect on the patch-matrix organization of the striatum.

Morphometrical and microdensitometrical studies on peptide- and tyrosine hydroxylase-like immunoreactivities in the forebrain of rats prenatally exposed to methylazoxymethanol acetate

Methylazoxymethanol acetate (MAM Ac) injected into pregnant rats at a dose of 25 mg/kg at gestational day 15 causes microcephaly due to an atrophy of various telencephalic areas, mainly neocortex, hippocampus and basal ganglia. Previous studies demonstrated alterations in various neurochemical markers of classical transmitter systems in these regions. The present paper deals with changes in peptide and tyrosine hydroxylase (TH)-containing neurons in MAM Ac-induced microcephaly using immunocytochemistry coupled with computer-assisted morphometry and microdensitometry. No change in the number of vasoactive intestinal polypeptide (VIP)-immunoreactive neurons in the neocortex and neuropeptide Y (NPY)-immunoreactive neurons in the nucleus caudatus-putamen was found whereas cholecystokinin (CCK)-and NPY-immunoreactive neurons in the neocortex and CCK- and VIP-immunoreactive neurons in the hippocampus were decreased. The reduction of the latter peptide containing neuronal populations led to a maintained density of cells in MAM Ac-exposed rats, due to the parallel reduction of the overall mass of these regions. TH immunoreactivity was found to be unchanged in the basal ganglia, and increased in the cerebral cortex in agreement with previous reports on noradrenaline cortical system after MAM Ac exposure. The present results show a heterogenous vulnerability of different peptide immunoreactive neuronal populations to MAM Ac exposure. The sparing of VIP- and NPY-immunoreactive neurons may be due to their late development in the neocortex and striatum, respectively. The hypothesis is introduced that cortical VIP interneurons can develop independent of marked alterations in the intrinsic circuitry of the cortical region.

Relationships between brain CT scan findings and cortisol in psychotic and nonpsychotic depressed patients

In this report, data are presented on pre- and postdexamethasone cortisol levels, neuropsychological testing, and computed tomography (CT) scan findings in 30 depressed patients (15 psychotic and 15 nonpsychotic). Particularly significant findings were observed when data from the unipolar subgroup (n = 22) were analyzed separately. Unipolar psychotic depressed patients had significantly larger (p less than 0.05) anterior pole and cella media ventricle-to-brain ratios (VBRs) and significantly greater (p less than 0.05) left and right inferior parietal brain "atrophy" than nonpsychotic depressed patients. Higher rates of Dexamethasone Suppression Test (DST) nonsuppression were observed in psychotic depressed patients and in patients with larger cella VBRs. Inferior parietal brain atrophy and large VBRs were also associated with greater cognitive impairment on psychometric testing. Implications of these findings are discussed.

Neurochemical effects of prenatal treatment with ochratoxin A on fetal and adult mouse brain

Ochratoxin A (OA) is a mycotoxin produced by several storage fungi, such as Aspergillus ochraceus and several Penicillium species. OA (3 mg/kg) was given intraperitoneally to pregnant mice on day 11 of gestation (day 1 = day of insemination), and neurochemical changes in brains of their offspring were examined at fetal and adult stages. OA treatment produced retardation of intrauterine growth as well as microencephaly and reductions in total weight and DNA content of fetal brains. Specific activities of lysosomal enzymes in fetal brains began to increase by the 2nd day after treatment and to reach peak activities by the 3rd or 4th day after injection, indicative of cell death in the developing brains. Examination of brain regions of offspring three months after birth revealed that both tissue weight and DNA content were reduced to 80% of control in cerebral hemispheres (CHs; cerebral cortex and subjacent white matter, hippocampus, and amygdala) and to 90% of control in remainder of the brain (BGDM; basal ganglia, diencephalon, and mesencephalon). Total content of noradrenaline (NA), dopamine (DA) 5-hydroxytryptamine (5-HT) in treated CH showed about 15% reduction, although, expressed on a tissue weight basis, concentrations of these monoamines were increased by about 15%. Total DA content in BGDM was also reduced to 85% of controls, but total content of NA and 5-HT in BGDM and pons-medulla oblongata did not change. These results suggest that synaptogenesis of monoamine neurons in the cerebrum is impaired by prenatal treatment with OA, and that dopaminergic neurons show a slight selective vulnerability to the toxin.

Hyperactivity and instrumental learning deficits in methylazoxymethanol-treated rat offspring

Several changes of spontaneous motor and learned behaviours were obtained in the male offspring of pregnant rats that were treated on gestation day 15 with the antimitotic agent methylazoxymethanol (MAM, 25 mg/kg). MAM-treated offspring, when tested at adult ages, showed notable increases in motor activity parameters as measured by direct observation or in automated photocell test cages. This hyperactive state was accompanied by clear impairments by MAM offspring in the acquisition of instrumental learning in a radial arm maze and in a circular swim maze. In Skinner boxes, MAM offspring made fewer responses during the Fixed Ratio (FR) 1 schedule but did not differ from the saline offspring in the acquisition of the difficult differential-reinforcement-of-low-rates (DRL) 72 sec task. Neurochemical assays indicated that the MAM rats had elevated noradrenaline and dopamine levels in several brain regions. These findings are discussed with regard to possible alterations of habituation processes in MAM rats.

Brain changes in rats induced by prenatal injection of methylazoxymethanol

Various doses (0, 1, 5, 10, 15, 20, or 25 mg/kg) of methylazoxymethanol acetate (MAM), a potent alkylating agent, were injected singly into pregnant rats intraperitoneally on day 15 of gestation. Relationships between brain weights and neurochemical changes in the cerebral hemispheres (CHs; cerebral cortex and subjacent white matter, hippocampus, amygdala) and remainder of the brain (BGDM; basal ganglia, diencephalon, and mesencephalon) were examined at 60 days of age in offspring; varying degrees of microencephaly were observed. Dose-dependent reductions in the weights of CH and BGDM were observed. Reductions in total DNA content positively correlated with decreases in brain weights also observed. Dose-dependent elevations of noradrenaline (NA) and dopamine (DA) were observed in CH at MAM levels 10 mg/kg and above; dose-dependent elevations of 5-hydroxytryptamine (5-HT) were observed at 15 mg/kg and above; and in BGDM at 20 mg/kg and above dose-dependent elevations for NA and 5-HT were observed; dose-dependent elevations at 15 mg/kg and above were observed for DA. Monoamine concentrations were negatively correlated with brain weights or total DNA contents. NA and DA concentrations increased to the extent of approximately 1.3 times of control at a time when an 18% loss of CH weight was noted in animals treated with 10 mg/kg MAM. It is suggested that the above variables might be appropriately sensitive neurochemical markers for detecting minor developmental anomalies in the brain.

Alteration of mouse cerebellar circuits following methylazoxymethanol treatment during development: immunohistochemistry of GABAergic elements and electron microscopic study

Methylazoxymethanol (MAM) injected postnatally affects cerebellar development in mice. A single injection at the fifth postnatal day produces hypogranular cerebella whereas a single injection at birth produces, in addition, a disorderly cytoarchitecture of the folium and alteration of Purkinje cell positioning (Bejar et al.: Exp. Brain Res. 57:279-285, '85). In the present study we have used immunohistochemistry with anti-GABA immune serum and electron microscopy to further characterize these alterations. In addition to the already-described nonoccupied dendritic spines of Purkinje cells both in mice injected the day of birth and or at the fifth postnatal day, we have observed, in animals injected at birth, the absence of pericellular baskets around Purkinje cells and the presence of heterologous synapses between mossy fibres and Purkinje cell dendrites. These heterologous synapses apparently disappear after postnatal day 20. By using an appropriate timing of MAM injection, different types of hypogranular cerebella, phenocopies of different mutants, can be obtained in large enough number to carry out extensive biochemical studies at each developmental age.

Astroglial development in microencephalic rat brain after fetal methylazoxymethanol treatment

Treatment of pregnant rats on gestation day 15 with methylazoxymethanol (MAM) leads to a marked microencephaly in the offspring with a considerable atrophy in cerebral cortex, hippocampus and striatum. The development of the astrocytic populations in these atrophic regions was studied by means of immunohistochemistry using an antiserum against glial fibrillary acidic protein (GFA). The distribution and density of GFA-positive structures were not notably altered in the parietal cortex, hippocampal formation and striatum after prenatal MAM-treatment as compared to control. Also the individual astrocytes were morphologically similar in experimental and control animals in all regions analyzed. We suggest that an adjustment of the astrocytic development has occurred in response to the changed neuronal environment. Alternatively, MAM-treatment may affect neuronal and glial precursor cells leading to a seemingly normal astrocytic cell density.

The angiogenesis of micrencephalic rat brains caused by methylazoxymethanol acetate. I. Superficial venous system. A quantitative analysis

The angiogenesis of the rat cerebrum was studied under pathologic conditions caused by the administration of the neurotoxin methylazoxymethanol acetate (MAMAc) in the time (E14) of neuroblast migration. The sinovenous junction of the main superficial cerebral veins and the morphological changes of the veins were examined by a quantitative analytic method. The hypoplastic areas of the brains showed extremely malformed venous systems with pathologic changes of the sinovenous junctions depending on the degree of disturbance of the neuroblast migration. These findings suggest the primary role of the neuronal maturation in the angioarchitectonic development and the direct dependency of the vascular differentiation on the neuroblast migration of the drained territory.

Monoamine neurotransmitter metabolism in microencephalic rat brain after prenatal methylazoxymethanol treatment

Administration of methylazoxymethanol (MAM) in the fetal stage leads to forebrain microencephaly with a severe atrophy in cerebral cortex, striatum, and hippocampus. The concentration of endogenous monoamines was markedly increased in the atrophic regions while total amount was largely unchanged. Striatal dopamine and cortical noradrenaline nerve terminals from MAM treated animals showed unaltered sedimentation properties in a sucrose density gradient and were estimated to have normal transmitter levels. gamma-Butyrolactone induced increase in dopamine levels and its counteraction by apomorphine was essentially unaltered after MAM. These data give further support for the view that the monoamine nerve terminal fields develop to their normal size in the atrophic regions leading to a hyperinnervation. Analysis of monoamine metabolite levels, increase of monoamines after monoamine oxidase inhibition, and disappearance of catecholamines after tyrosine hydroxylase inhibition were conducted to obtain information on monoamine turnover. The results indicated an essentially unaltered, or a small reduction of, monoamine turnover in the atrophic regions when calculated per monoamine nerve terminal, while increased when calculated per unit weight of the tissue.

Development of central neurotransmitter-specified neuronal systems: implications for pediatric neuro-psychiatric disorders

An increasing focus on the mechanism of synaptic neurochemistry in pediatric neurology, may lead to a better understanding of the pathophysiology of many disorders and result in a more rational approach to their pharmacotherapy. With the burgeoning list of putative neurotransmitters in brain, and the growing evidence of co-localization of many of these neurotransmitters, chemical neurotransmission likely involves a higher degree of complexity than appreciated heretofore. The potential role of neurotransmitter dysfunction in the pathophysiology of neurologic and behavior disorders of children, should not be considered as restricted to those disorders that involve selective neuronal loss, but may encompass a much wider spectrum of syndromes due to metabolic abnormalities, as well as disturbances of the finer features of chemical neurotransmission.

Effects of Purkinje cell degeneration on the noradrenergic projection to mouse cerebellar cortex

We have examined the effects of a genetically programmed target cell death on the noradrenergic afferent projection to mouse cerebellar cortex. We have observed that the noradrenergic axon terminals originating in in locus coeruleus are maintained in the cerebellar cortex of the Purkinje cell degeneration (pcd) mutant mouse in spite of the absence of Purkinje cells, the targets for the noradrenergic projection. The number of noradrenergic terminals in the atrophic mutant cerebellar cortex is approximately normal as assessed by counts of fibers exhibiting catecholamine fluorescence and by measurement of high affinity uptake of tritium-labeled norepinephrine (NE) by synaptosomes prepared from cerebellar cortex. An increased density of NE fibers is observed which appears to be a consequence of reduced cerebellar mass in the mutant. Although the number of noradrenergic terminals is unaffected, morphological and biochemical alterations are observed in this system. The fibers are more brightly fluorescent in mutant than in normal mice and their pattern is less orderly. The content of the endogenous transmitter, NE, is increased from 150 to 170% whereas the activity of the rate-limiting enzyme tyrosine hydroxylase (TH) is reduced to about 60% of normal values. These changes appear to be permanent as they are still present in 6 month-old mutant animals, the oldest studied. No alterations in either NE content or TH activity are found in pcd/pcd hippocampus, another target for the locus coeruleus axons.

Effects of cytotoxic deletions of somatic sensory cortex in fetal rats

Pregnant rats were injected on the 14th day of gestation with the cytotoxic drug methylazoxymethanol acetate. This compound causes the death of neural precursor cells that were synthesizing DNA at the time of injection. After birth, the progeny of treated mothers grew to maturity with a neocortex that was greatly reduced in area by the death of all cells, particularly at the frontal and occipital poles but at medial and lateral margins of neocortex as well. In the remaining cortex layers II through IV failed to develop. The experiment deprived growing thalamocortical axons, which innervate the somatic sensory cortex late in development, of part of their normal target area and of a substantial number of their definitive target cells. It also deprived them of any cues they might have received from these target cells migrating through them as the axons accumulate beneath the cortical plate. Anatomical experiments indicated that, despite these defects, thalamocortical axons could still colonize the sensorimotor areas and form synapses in their typically bilaminar pattern, though the outer, denser lamina of terminations occurred abnormally at the level of the apices of layer V pyramidal cell bodies. Receptive field mapping of single and multiunit responses in the somatic sensory region showed brisk responses and receptive fields of normal size. It also indicated the formation of a body map that was topographically intact except for deletions at its periphery; that is, a total map was not compressed into a smaller area. This suggests that somatic sensory thalamocortical fibers recognize only remaining cortical target cells in appropriate fields. Moreover, successful ones among them seem to recognize neighborhood relations and conserve synaptic space at the expense of those that would have innervated the deleted peripheral parts of the area. Pyramidal neurons in the remaining cortical layers and in ectopic islands of cells that had incompletely migrated from the neuroepithelium, probably on account of destruction of radial glial cell precursors, were shown by retrograde labeling to send their axons only to appropriate subcortical targets. Pyramidal neurons, though recognized as such, also adopted a variety of abnormal orientations, such as inversion, apparently in an attempt to grow apical dendrites toward major zones of synaptic terminations.

Neurotransmitter specific alterations in dementing disorders: insights from animal models

Recent years have witnessed considerable change in the conceptualization of the pathophysiology of the cognitive impairments in dementing disorders, as a result of synaptic neurochemical analyses. Profound reductions in the forebrain cholinergic projections occur in Alzheimer's disease. In GM1 gangliosidosis, variable alterations in neurotransmitter related processes that are located in synaptic membranes have been described. Exploitation of animal models of human disorders resulting in dementia may further clarify the dynamic alterations in the biochemical processes required for effective neurotransmission in cortex.

Infrapyramidal mossy fibers in the hippocampus of methylazoxymethanol acetate-induced microcephalic rats

Treatment of pregnant rats with methylazoxymethanol acetate results in the invasion of mossy fibers into the infrapyramidal region of the hippocampus in the offspring. Since such an invasion of mossy fibers has also been reported in neonatal hyperthyroidism, prenatal ethanol exposure and neonatal lesion of CA3, a common etiology for this phenomenon is proposed.

Development of catecholaminergic phenotypic characters in the mouse locus coeruleus in vivo and in culture

While abundant studies have begun to elucidate ontogeny of the peripheral nervous system, molecular mechanisms underlying brain development remain obscure. To approach this problem, we initiated parallel in vivo and in vitro studies of the mouse locus coeruleus (l.c.), a brainstem noradrenergic nucleus. The catecholaminergic enzymes tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) were used to monitor phenotype expression and development. TH catalytic activity and immunocytochemical reactivity were initially detectable on gestational Day 13 (E-13) in vivo, and adult levels of activity were approximately by the third postnatal week. Immunotitration studies indicated that the developmental increase was due to accumulation of enzyme molecules and not enzyme activation. The in vivo developmental profile of DBH approximated that of TH. To begin defining regulatory mechanisms, explants of embryonic brainstem were placed in culture. Explantation on E-12, prior to expression of TH or DBH, resulted in the de novo appearance of these phenotypic characters after 4 days. Explantation on E-18, after the enzymes are already expressed, was followed by a striking sixfold rise in TH activity. Immunotitration studies revealed that the increase in TH activity in E-18 cultures was attributable to increased molecule number, reproducing the in vivo results. Moreover, the E-18 explants, cultured for 3 weeks, attained higher plateau levels of TH activity than E-12 cultures, and this differences was due to increased molecule number. Morphometric analysis indicated that 3-week E-12 cultures actually had more l.c. cells than E-18 cultures, indicating that differences in TH were not due to increased cells in the E-18 l.c. Finally, systemic study revealed that the development of TH activity in culture increased progressively from E-11 to E-12 to E-13, suggesting that critical regulatory events occur at this time. Our studies suggest that the l.c. is an excellent model for the study of brain development in vivo and in vitro. Initial phenotypic expression and dramatic development occur in culture in the absence of normal targets, normal afferent innervation and, presumably, normal humoural milieu.

Alzheimer's disease: a disorder of cortical cholinergic innervation

Great emphasis is being placed on identification of neurotransmitter systems involved in the symptomatic manifestations of neurological and psychiatric disorders. In the case of Alzheimer's disease, which now seems to be one of the most common causes of mental deterioration in the elderly, compelling evidence has been developed that acetylcholine-releasing neurons, whose cell bodies lie in the basal forebrain, selectively degenerate. These cholinergic neurons provide widespread innervation of the cerebral cortex and related structures and appear to play an important role in cognitive functions, especially memory. These advances reflect a close interaction between experimental and clinical neuroscientists in which information derived from basic neurobiology is rapidly utilized to analyze disorders of the human brain.

Postnatal development of aminergic projections to frontal cortex: effects of cortical lesions

The effects of lesions induced by fetal treatment with methylazoxymethanol acetate (MAM) on the developmental synaptic chemistry of the frontal cortex of rats were examined. Treatment at 14 days' gestation (DG) resulted in a 50% decrease in frontal cortical mass whereas treatment at 15 DG caused a 40% decrement. No difference in concentration between MAM and control rats was observed in the postnatal development of the GABA-ergic markers, glutamate decarboxylase, and GABA. In contrast, the concentrations of the presynaptic markers for the dopaminergic, noradrenergic, and serotonergic terminals were generally elevated in the MAM lesioned frontal cortex throughout postnatal development. The effects of MAM lesion on cholinergic markers were mixed, with minimal alterations in concentration during the first 2 postnatal weeks followed by an increasing disparity thereafter. The results of this study provide additional evidence in support of the differential impact of the cortical hypoplasia on intrinsic neurons as contrasted to aminergic afferents.

Fetally-induced noradrenergic hyperinnervation of cerebral cortex results in persistent down-regulation of beta-receptors

Methylazoxymethanol acetate (MAM)-induced cortical hypoplasia resulted in a 2-fold increase in the concentration of norepinephrine and its metabolite, 3-methoxyl-4-hydroxyphenylglycol sulfate, and a 26% decrease in the Bmax for the beta-receptor ligand, [3H]dihydroalprenolol (DHA) in the rat cortex. Chronic treatment with desmethylimipramine did not further decrease DHA-labeled sites although prior lesion of the noradrenergic terminals with 6-hydroxydopamine markedly increased the number in the MAM-lesioned cortex.

Neurotransmitter chemistry of lissencephalic cortex induced in ferrets by fetal treatment with methylazoxymethanol acetate

Treatment of pregnant ferrets with 15 mg/kg of methylazoxymethanol acetate (MAM) at 33 days of fetal gestation results in offspring with cortical hypoplasia and lissencephally. Neurochemical analysis of 5 areas of cortex from 8-week-old offspring of MAM- or vehicle-treated jills indicated an overall enrichment in markers for catecholaminergic (tyrosine hydroxylase, norepinephrine) and cholinergic (choline acetyltransferase, acetylcholine) terminals but minimal change in the concentration of GABAergic markers (glutamate decarboxylase, gamma-aminobutyric acid); however, there did not appear to be a direct, inverse relationship between the concentration of catecholaminergic and cholinergic markers and the degree of hypoplasia in cortical subareas unlike what has been found previously in the rat.