Caffeine-dependent stimulus-triggered oscillations in the CA3 region of hippocampal slices from rats chronically exposed to lead

Neuron-derived factors negatively modulate ryanodine receptor-mediated calcium release in cultured mouse astrocytes

Changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i) produced by ryanodine receptor (RyR) agonist, caffeine (caf), and ionotropic agonists: N-methyl-d-aspartate (NMDA) receptor (NMDAR) agonist, NMDA and P2X7 receptor (P2X7R) agonist, 3'-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (BzATP) were measured in cultured mouse cortical astrocytes loaded with the fluorescent calcium indicator Fluo3-AM in a confocal laser scanning microscope. In mouse astrocytes cultured in standard medium (SM), treatment with caf increased [Ca²⁺]_i, with a peak response occurring about 10 min after stimulus application. Peak responses to NMDA or BzATP were observed about <1 min and 4.5 min post stimulus, respectively. Co-treatment with NMDA or BzATP did not alter the peak response to caf in astrocytes cultured in SM, the absence of the effects being most likely due to asynchrony between the response to caf, NMDA and BzATP. Incubation of astrocytes with neuron-condition medium (NCM) for 24 h totally abolished the caf-evoked [Ca²⁺]_i increase. In NCM-treated astrocytes, peak of [Ca²⁺]_i rise evoked by NMDA was delayed to about 3.5 min, and that induced by BzATP occurred about three minutes earlier than in SM. The results show that neurons secrete factors that negatively modulate RyR-mediated Ca²⁺-induced Ca²⁺ release (CICR) in astrocytes and alter the time course of Ca²⁺ responses to ionotropic stimuli.

Neurotoxicants, Micronutrients, and Social Environments

SUMMARY—Systematic research evaluating the separate and interacting impacts of neurotoxicants, micronutrients, and social environments on children's cognition and behavior has only recently been initiated. Years of extensive human epidemiologic and animal experimental research document the deleterious impact of lead and other metals on the nervous system. However, discrepancies among human studies and between animal and human studies underscore the importance of variations in child nutrition as well as social and behavioral aspects of children's environments that mitigate or exacerbate the effects of neurotoxicants.

In this monograph, we review existing research on the impact of neurotoxic metals, nutrients, and social environments and interactions across the three domains. We examine the literature on lead, mercury, manganese, and cadmium in terms of dispersal, epidemiology, experimental animal studies, effects of social environments, and effects of nutrition.

Research documenting the negative impact of lead on cognition and behavior influenced reductions by the Center for Disease Control in child lead-screening guidelines from 30 micrograms per deciliter (μg/dL) in 1975 to 25 μg/dL in 1985 and to 10 μg/dL in 1991. A further reduction is currently being considered. Experimental animal research documents lead's alteration of glutamate-neurotransmitter (particularly N-methyl-D-aspartate) activity vital to learning and memory. In addition, lead induces changes in cholinergic and dopaminergic activity. Elevated lead concentrations in the blood are more common among children living in poverty and there is some evidence that socioeconomic status influences associations between lead and child outcomes. Micronutrients that influence the effects of lead include iron and zinc.

Research documenting the negative impact of mercury on children (as well as adults) has resulted in a reference dose (RfD) of 0.1 microgram per kilogram of body weight per day (μg/kg/day). In animal studies, mercury interferes with glutamatergic, cholinergic, and dopaminergic activity. Although evidence for interactions of mercury with children's social contexts is minimal, researchers are examining interactions of mercury with several nutrients.

Research on the effects of cadmium and manganese on child cognition and behavior is just beginning. Experimental animal research links cadmium to learning deficits, manganese to behaviors characteristic of Parkinson's disease, and both to altered dopaminergic functioning.

We close our review with a discussion of policy implications, and we recommend interdisciplinary research that will enable us to bridge gaps within and across domains.

Interplay between non-NMDA and NMDA receptor activation during oscillatory wave propagation: Analyses of caffeine-induced oscillations in the visual cortex of rats

Generation and propagation of oscillatory activities in cortical networks are important features of the brain. However, many issues related to oscillatory phenomena are unclear. We previously reported neocortical oscillation following caffeine treatment of rat brain slices. Input to the primary visual cortex (Oc1) generates N-methyl-d-aspartate (NMDA) receptor-dependent oscillations, and we proposed that the oscillatory signals originate in the secondary visual cortex (Oc2). Because non-NMDA and NMDA receptors cooperate in synaptic transmission, non-NMDA receptors may also play an important role in oscillatory activities. Here we investigated how non-NMDA receptor activities contribute to NMDA receptor-dependent oscillations by using optical recording methods. After induction of stable oscillations with caffeine application, blockade of NMDA receptors abolished the late stable oscillatory phase, but elicited 'hidden' non-NMDA receptor-dependent oscillation during the early depolarizing phase. An interesting finding is that the origin of the non-NMDA receptor-dependent oscillation moved from the Oc1, during the early phase, toward the origin of the NMDA receptor-dependent oscillation that is fixed in the Oc2. In addition, the frequency of the non-NMDA receptor-dependent oscillation was higher than that of the NMDA receptor-dependent oscillation. Thus, in one course of spatiotemporal oscillatory activities, the relative balance in receptor activities between non-NMDA and NMDA receptors gradually changes, and this may be due to the different kinetics of the two receptor types. These results suggest that interplay between the two receptor types in the areas of Oc1 and Oc2 may play an important role in oscillatory signal communication.

Correlation between stimulation strength and onset time of signal traveling within the neocortical neural circuits under caffeine application

In general, strength of input to neocortical neural circuits affects the amplitude of postsynaptic potentials (PSPs), thereby modulating the way signals are transmitted within the circuits. Caffeine is one of the pharmacological agents able to modulate synaptic activities. The present study investigated how strength of input affects signal propagation in neocortical circuits under the application of caffeine. Spatio-temporal neural activities were observed from visual cortical slices of rats using optical recording methods with voltage-sensitive dye. Electrical stimulations were applied to white matter in the primary visual cortex with bath-application of caffeine. When the strength of stimulation was 0.3mA, signals propagated from the site of stimulation in the primary visual cortex toward the secondary visual cortex along vertical and horizontal pathways. When stimulation strength was reduced from 0.3mA to 0.07mA, start of signal propagation was delayed about 25ms without affecting field PSP amplitude or the manner of signal propagation. Conversely, co-application of caffeine and d-2-amino-5-phosphonovaleric acid (d-AP5) did not induce delays in signal start. These findings suggest that conversion of neural code from amplitude code to temporal code is inducible at the level of neocortical circuits in an N-methyl-d-aspartate (NMDA) receptor activity-dependent manner.

The potential of caffeine for functional modification from cortical synapses to neuron networks in the brain

Structure and function of the brain are use-dependent variables based on "synapse plasticity". Since synapses are driven by chemical transmitters, synaptic functions are liable to be modified by extrinsic chemicals displaying affinities for synaptic receptors or modulators. Caffeine is a widely used chemical substance that can invade synapses, and has several biochemical and metabolic actions on synaptic activities. This review focuses on the actions of caffeine on changes in structure and function in the region of the hippocampal formation and neocortex, which exhibit high synapse plasticity. At the synapse level, various synaptic receptors and channel activities are modulated by caffeine via mobilization of intracellular calcium, inhibition of phosphodiesterase, antagonism of adenosine receptors and GABA receptors. These actions of caffeine enable neurons to induce plastic changes in the properties of synaptic activities, such as synaptic transmission efficiency and morphology. At the network level, caffeine has the ability to activate cortical neural oscillators that deliver repetitive N-methyl-D-aspartate receptor-dependent signals to surrounding areas, causing strengthening of long-range inter-cortical communications. Caffeine might thus allow reorganization of cortical network functions via synaptic mobilizations.

Application of caffeine reveals input frequency-dependent determination of signal-traveling routes between primary and secondary visual cortices in rats

Upon entering the neocortex, neural signals are required to select which neocortical circuits to propagate through. The present study focused attention on use-dependent selection of signal-traveling routes. Rat brain slices including primary visual cortex (Oc1) and the medial part of the secondary visual cortex (Oc2M) were prepared. Electrical stimulation was delivered to white matter in Oc1 and spatiotemporal aspects of traveling signals were observed using optical recording methods under caffeine application. With an interstimulus interval (ISI) of 4-8s, signals traveled horizontally along deep layers from Oc1 to Oc2M, climbed within Oc2M, then returned along layer II/III from Oc2M to Oc1. Conversely, with an ISI of 40-64s, signals climbed within Oc1 and traveled horizontally along layer II/III from Oc1 to Oc2M in parallel with signals traveling along deep layers. Pharmacological experiments with antagonists for ionotropic glutamate receptors revealed that signal-traveling routes under higher-frequency stimulation were N-methyl-d-aspartate (NMDA) receptor activity-dependent, while those at the lower-frequency were non-NMDA receptor activity-dependent. These results suggest that neural circuits between Oc1 and Oc2M possess an input frequency-dependent gating system, in which signal-traveling routes might be affected by the relative balance of receptor activities between NMDA and non-NMDA receptors.

Common effects of lithium and valproate on mitochondrial functions: protection against methamphetamine-induced mitochondrial damage

Accumulating evidence suggests that mitochondrial dysfunction plays a critical role in the progression of a variety of neurodegenerative and psychiatric disorders. Thus, enhancing mitochondrial function could potentially help ameliorate the impairments of neural plasticity and cellular resilience associated with a variety of neuropsychiatric disorders. A series of studies was undertaken to investigate the effects of mood stabilizers on mitochondrial function, and against mitochondrially mediated neurotoxicity. We found that long-term treatment with lithium and valproate (VPA) enhanced cell respiration rate. Furthermore, chronic treatment with lithium or VPA enhanced mitochondrial function as determined by mitochondrial membrane potential, and mitochondrial oxidation in SH-SY5Y cells. In-vivo studies showed that long-term treatment with lithium or VPA protected against methamphetamine (Meth)-induced toxicity at the mitochondrial level. Furthermore, these agents prevented the Meth-induced reduction of mitochondrial cytochrome c, the mitochondrial anti-apoptotic Bcl-2/Bax ratio, and mitochondrial cytochrome oxidase (COX) activity. Oligoarray analysis demonstrated that the gene expression of several proteins related to the apoptotic pathway and mitochondrial functions were altered by Meth, and these changes were attenuated by treatment with lithium or VPA. One of the genes, Bcl-2, is a common target for lithium and VPA. Knock-down of Bcl-2 with specific Bcl-2 siRNA reduced the lithium- and VPA-induced increases in mitochondrial oxidation. These findings illustrate that lithium and VPA enhance mitochondrial function and protect against mitochondrially mediated toxicity. These agents may have potential clinical utility in the treatment of other diseases associated with impaired mitochondrial function, such as neurodegenerative diseases and schizophrenia.

NMDA receptor-dependent oscillatory signal outputs from the retrosplenial cortex triggered by a non-NMDA receptor-dependent signal input from the visual cortex

The retrosplenial cortex is located at a critical juncture between the visual cortex and hippocampal formation. Functions of the retrosplenial cortex at the local circuit level, however, remain unclear. Herein, we show how signals traveling from the visual cortex behave in local circuits of the retrosplenial cortex, using optical recording methods and application of caffeine to rat brain slices. Electrical signals evoked in the primary visual cortex penetrated into the deep layer of the retrosplenial granular a cortex (RSGa) and propagated further toward postsubiculum and upper layer. Non-N-methyl-D-aspartate (NMDA) receptor-dependent initial traveling signal from the visual cortex triggered NMDA receptor-dependent neural oscillation in the RSGa. Oscillatory signals originated from the local area in the deep layer of the RSGa, and the signal spread back and forth toward the visual cortex and postsubiculum, in addition to spreading toward the upper layer. From the perspective of the RSGa, extrinsic signal inputs from the visual cortex switched on neural oscillators in the RSGa that deliver NMDA receptor-dependent intrinsic signal outputs. Opening and strengthening of non-NMDA receptor-dependent input pathways from the visual cortex required NMDA receptor-dependent oscillatory neural activities. These input and output relationships indicate that the retrosplenial cortex may represent an important relay station between the visual cortex and hippocampal formation.