Neuronal Apoptosis In Rats Research Articles

Evaluation of pathways involved in pentachlorophenol-induced apoptosis in rat neurons

Pentachlorophenol (PCP) (C 6HCl 5O) is a synthetic toxic organochloride fungicide for humans which exhibit neurotoxic properties. In the present research, we describe the potential pathways implicated in PCP-induced apoptosis in an acute model of toxicity in rat cerebellar granule neurons (CGNs). In our experiments, acute exposure of CGNs to micromolar concentrations of PCP induced the transcriptional activity of genes related to the classical apoptosis pathway (caspase 3, caspase 8, Bad), oxidative stress and glutathione metabolism (glutathione peroxidase-1, catalase, glutathione-S-transferase-3 and superoxide dismutase-1), and mitogenic response (cyclin D1, cdk2, cdk4, cdkn2b). Results from Western blot also shown significative increases in the expression of cyclins D1, E and A and cdk4. The mitogenic response was also related to a significative increase in the phosphorylation of retinoblastoma protein (Rb). PCP would cause apoptosis up-regulating the transcriptional activity of p53 gene and also increasing their activation by phosphorylation, concomitant with a decrease in the sirtuin 1 content. In conclusion, acute exposure of CGNs to PCP induces the classical p53 apoptotic pathway, promotes the up-regulation of several genes related to oxidative stress and the over-expression of molecules involved in the cell cycle control.

Administration of Human Protein-C concentrate prevents apoptotic brain cell death after experimental sepsis

Activated Protein C renders anti-apoptotic properties in neurons and endothelial cells. The aim of the present study was to evaluate the in vivo cytoprotective role of Protein C zymogen (PC) administration in septic rat brain. Male Wistar rats ( n = 60) were subjected to sepsis via Cecal Ligation and Puncture (CLP). Animals were randomly divided either to receive 100 IU/kg human PC concentrate at 1, 7 and 13 h post CLP (CLP + PC group) or placebo treatment (CLP group). At pre-specified time points (6, 12, 24, 36, 48 and 60 h post CLP) five animals from either group were euthanized and the brain tissue was removed. Apoptosis in both neurons (Neu-N+) and astroglia (GFAP+) was assessed by flow cytometry using 7-aminoactinomycin D (7AAD). Immunohistochemical detection of cleaved caspase 3, bax, bcl-2, cytochrome c and caspase 8 was also performed. PC treated animals had significantly reduced apoptosis in neurons at 6 and 24 h post CLP ( p = 0.04 and p = 0.016 respectively) and necrosis at 6, 12 and 60 h post CLP ( p = 0.008, p = 0.012 and p = 0.032 respectively). Astrocyte necrosis was also decreased in septic rats receiving PC (6, 12 and 60 h post CLP p = 0.008, p = 0.016 and p = 0.008 respectively). In addition, active caspase 3, bax, cytochrome c and caspase 8 expression was significantly decreased during early sepsis (6–36 h) while bcl-2 expression was increased (24 h p = 0.001 and 60 h p = 0.001) in the PC treated animals compared to placebo. PC concentrate administration in experimental sepsis produced a time dependent inhibition of apoptosis in rat neurons and astrocytes. The inhibition of sepsis related apoptosis concerned both the mitochondrial and caspase 8 dependent pathways.

Simulated weightlessness aggravates hypergravity-induced impairment of learning and memory and neuronal apoptosis in rats

It has been demonstrated that altered gravity may lead to impairments in cognitive functions. However, the effect of a combination of hypergravity and weightlessness on cognitive functions is not well understood. In the present study, we report the effects of high sustained hypergravity after 7 days’ simulated weightlessness on learning and memory abilities and neuronal apoptosis in rats. In the Y-maze tests, hypergravity (HG) or simulated weightlessness (SW) significantly decreases accuracy, and increases reaction time of rats compared to that of controls. On the contrary, in the passive avoidance test, HG or SW treatment significantly shortens latency and prolongs total time compared to those of controls. In addition, TUNEL staining shows a few apoptotic cells in cortex and hippocampus in the HG, SW and HG + SW groups, and the number of TUNEL positive cells was found to be the most in the HG + SW group. Furthermore, rats with combined HG and SW treatment reveal a synergistic effect in both the Y-maze and the passive avoidance tests, as well as increased neuronal cell death. These findings suggest that simulated weightlessness may exacerbate hypergravity-induced impairment of learning and memory, likely caused by neuronal cell death in rats.

M- and T-tropic HIVs promote apoptosis in rat neurons.

Neuronal loss, reactive astrocytes, and other abnormalities are seen in the brain of individuals with acquired immune deficiency syndrome-associated Dementia Complex (ADC). Human immunodeficiency virus-1 (HIV-1) is believed to be the main agent causing ADC. However, little is known about the molecular and cellular mechanisms of HIV-1 neurotoxicity considering that HIV-1 does not infect post-mitotic neurons and that viral load does not necessarily correlate with ADC. Various viral proteins, such as the envelope protein gp120 and the transcription activator Tat, have been shown to induce neuronal apoptosis through direct and indirect mechanisms both in vitro and in vivo. Progeny HIV-1 virions can also cause neuronal death. However, it has not been fully established yet whether HIV-1 promotes neuronal apoptosis by a direct mechanism. To explore the neurotoxic effect of HIV-1, we exposed rat cerebellar granule cells and cortical neurons in culture to two different strains of HIV-1, IIIB and BaL, T- and M-tropic strains that utilize CXCR4 and CCR5 coreceptors, respectively, to infect cells. We observed that both viruses elicit a time-dependent apoptotic cell death in these cultures without inducing a productive infection as determined by the absence of the core protein of HIV-1, p24, in cell lysates. Instead, neurons were gp120 positive, suggesting that the envelope protein is shed by the virus and then subsequently internalized by neurons. The CXCR4 receptor antagonist AMD3100 or the CCR5 receptor inhibitor D-Ala-peptide T-amide blocked HIV IIIB and HIV Bal neurotoxicity, respectively. In contrast, the N-methyl-D-aspartate receptor blocker MK801 failed to protect neurons from HIV-mediated apoptosis, suggesting that HIV-1 neurotoxicity can be initiated by the viral protein gp120 binding to neuronal chemokine receptors.

Nitric oxide-mediated neuronal apoptosis in rats with recurrent febrile seizures through endoplasmic reticulum stress pathway

Nitric oxide (NO), as a neurotransmitter, exerts various physiological and pathological effects on the brain. Excess NO is toxic to neurons and may cause neuronal apoptosis. However, the cascade of NO-mediated apoptosis is not fully understood. We utilized a recurrent febrile seizures (FS) rat model and found that plasma NO was increased, neuronal apoptosis was evident, the expression of glucose-regulated protein78 (GRP78, a well-established marker of ER stress) was elevated, and caspase-12 (an ER stress-specific proapoptosis molecule) was activated in the hippocampus in a time-dependent manner after recurrent FS. Administration of sodium nitroprusside (SNP, an NO donor) enhanced neuronal apoptosis, down-regulated the expression of GRP78, and increased that of caspase-12 in FS+SNP groups compared with FS groups. In contrast, treatment with N(G)-nitrol-l-arginine methyl ester (l-NAME, a competitive NO synthase inhibitor) inhibited neuronal apoptosis, up-regulated the expression of GRP78, and decreased that of caspase-12 in FS+l-NAME groups compared with FS groups. These results suggest that NO mediates neuronal apoptosis caused by recurrent FS, and that the ER stress pathway is involved in NO-mediated neuronal apoptosis.

Inhibitory effect of acupuncture on neuronal apoptosis in rats after cerebral ischemia

Background Delayed neuronal death after total cerebral ischemia may accompany with apoptosis, but acupuncture may play a certain role in protecting nerve through inhibiting ischemic neuronal apoptosis. Objective To observe the effect of acupuncture on neuronal apoptosis in rats after cerebral ischemia and analyze its cerebral protective mechanism. Design Contrast observation among groups. Setting Heilongjiang University of Traditional Chinese Medicine. Materials A total of 30 male healthy Wistar rats of general grade and weighing (250±20) g were randomly divided into three groups, including sham operation group, cerebral ischemia group and acupuncture group with 10 rats in each group. Apoptosis in situ kit was provided by Baolingman Company, Germany. Methods The experiment was carried out in the Laboratory Center, Heilongjiang University of Traditional Chinese Medicine from May to November 2004. ▪ Rats in the cerebral ischemia group and the acupuncture group were used to establish total cerebral ischemic models with four vessels occlusion; in addition, models in the sham operation group were established with the same method as mentioned above. However, four vessels of rats in the sham operation were exposured and cerebral ischemia did not occur. Rats in the acupuncture group were given acupuncture treatment after operation. Needle of 40 mm in length was used to acupuncture bilateral Zusanli (St 36) and Quchi (LI 11) with the depth of 3 mm, and then bilateral acupoints were connected with KWD-808II omnipotenc impulse electro-therapeutic apparatus (frequency: 1 Hz; thin waves; voltage: 2 V) once a day for totally 30 minutes. Meanwhile, needle of 25 mm in length was used to acupuncture Baihui (Du 20) with the depth of 3 mm, and then the needle was twirled once every 5 minutes for 30 minutes in total. The course was 7 days. ▪ Neuronal injuries in hippocampal CA1 area after cerebral ischemia were observed with Nissl body staining method at 7 days after treatment; neuronal apoptosis was observed with TUNEL staining; manifestations of neuronal apoptosis in cerebral cortex and hippocampal CA1 area were observed with electron microscope. Main outcome measures Neuronal injuries in hippocampal CA1 area after cerebral ischemia; neuronal apoptosis in cerebral cortex and hippocampal CA1 area after cerebral ischemia; morphological changes under electron microscope. Results Among 30 Wistar rats, 24 rats were involved in the final analysis. ▪ Expression of positive neurons in cerebral cortex and hippocampal CA1 area with Nissl body staining: Neuronal defect was obvious in cerebral cortex and hippocampal CA1 area in the cerebral ischemia group as compared with that in the sham operation group ( P < 0.05), and neuronal defect was decreased in hippocampal CA1 area in the cerebral ischemia group as compared with that in the acupuncture group ( P < 0.05). ▪ Expression of positive neurons in cerebral cortex and hippocampal CA1 area with TUNEL staining: Positive neurons with TUNEL staining were not observed in the sham operation group, but positive neurons were increased in the cerebral ischemia group as compared with those in the acupuncture group ( P < 0.05). ▪ Observational results of electron microscope: Neuronal apoptosis was not found in the sham operation group; neuronal apoptosis was rarely found in the acupuncture group; neuronal apoptosis was typical in the cerebral ischemia group. Conclusion Delayed neuronal death after total cerebral ischemia may accompany with apoptosis, but acupuncture may play a certain role in protecting nerve through inhibiting ischemic neuronal apoptosis.

Effects of Shenfu injection on hypoxic-ischemic brain damage: experiment with neonatal rats

To investigate the effect of Shenfu (ginseng and aconite root) injection on hypoxic-ischemic brain damage (HIBD). Sixty 7-day-old Sprague-Dawley rats undergoing ligation of left common carotid artery and then put into a container with 8% O2 and 92% N(2) for 2 h so as to establish HIBD models, were randomly divided into 3 groups: Shenfu injection pretreatment group (since 4 days before the experiment Shenfu injection 10 ml/kg was injected intraperitoneally once a day for 4 days), Shenfu injection treatment group [Shenfu injection 10 ml/kg was injected intraperitoneally immediately after hypoxic-ischemia (HI) insult once a day for 7 days], and control group (normal saline 10 ml/mg was injected intraperitoneally immediately after HI insult once a day for 7 days). Twenty neonatal rats underwent sham operation as control group. The 4 groups were further divided into subgroups of 6 rats according to the time points: 2 hours before and 2 hours, 12 hours, 24 hours, 3 days, 7 days, 14 days, and 28 days after HI insult. 3, 7, 14, and 28 days after the HI insult the body weight was observed and the survival rate was observed 28 d after the HI insult. At different time points the rats of different subgroups were killed and their brains were taken out. Flow cytometry was used to calculate the neuron apoptosis rate in the hippocampal CA1 region. The body weight increase levels 3, 7, 14 and 28 days after HI insult of the control group were all significantly less than those of the sham operation group (e.g 7 days: 8.8 g +/- 2.1 g vs 14.0 g +/- 2.9 g, all P < 0.01) and the body weight increase levels 3, 7, 14, and 28 days after HI insult of the control group were all significantly less than those of the Shenfu injection pretreatment group (e.g 7 d: 11.7 g +/- 3.3 g) and Shenfu injection treatment group (e.g 7 d: 10.9 g +/- 2.7 g, P < 0.01 or P < 0.05). The survival rate 28 d after HI insult of the control group was 60%, significantly less than those of other groups (all P < 0.05), and there was no significant difference in the survival rate among the other groups (all P > 0.05). Compared with the sham operation group the neuron apoptosis rates of the hippocampal CA1 region of the Shenfu injection treatment group and Shenfu injection pretreatment group began to increase 2 hours after HI insult, peaked 24 hours after, then gradually decreased, and recovered to normal 14 days after. The neuron apoptosis rates 2, 12, 24, 72 hours, and 7 days after HI insult of the Shenfu injection pretreatment group were all significantly lower than those of the control group (e.g 24 hours: 16.0% +/- 4.2% vs 11.9% +/- 2.3% vs 18.1%, P < 0.01 or P < 0.05), and the neuron apoptosis rates 72 hours and 7 days after HI insult of the Shenfu injection treatment group were significantly lower than those of the control group. Shenfu injection can enhance the physical development and elevate the survival rate of neonatal rats with HI insult, and significantly prevents apoptosis of the hippocampus neurons from HI insult.

CORRELATIVE FACTORS OF NEURONAL APOPTOSIS IN HIPPOCAMPUS AFTER STATUS CONVULSION

Objectives: To explore the influences of age and duration of status convulsion (SC) on hippocampal neuron apoptosis by observing the dynamic change of neuron apoptosis in rats with different age after SC termination.

Antiapoptotic effect both in vivo and in vitro of A20 gene when transfected into rat hippocampal neurons

To evaluate the antiapoptotic effect of the A20 gene in primary hippocampal neurons both in vivo and in vitro. Primary hippocampal neurons in embryonic day 18 (E18) rats were transfected with the A20 gene by using the new Nucleofector electroporation transfection method. We then examined, whether A20-neurons possessed anti-apoptotic abilities after TNF-alpha stimulation in vitro. A20-neurons and pcDNA3-neurons were transplanted into the penumbra of the brains of rats that had been subjected to 90-min of ischemia induced by left middle cerebral artery occlusion (MCAO). A20-neurons resisted TNF-alpha induced apoptosis in vitro. The apoptosis rate of neurons overexpressing A20 (28.46%+/-3.87%) was lower than that in neurons transfected with pcDNA3 (53.06%+/-5.36%). More A20-neurons survived in the penumbra both 3-d and 7-d after transplantation than did sham pcDNA3 neurons. The novel function of A20 may make it a potential targets for the gene therapy for neurological diseases.

Lack of alpha-synuclein does not alter apoptosis of neonatal catecholaminergic neurons.

alpha-Synuclein is an abundant neuronal protein of uncertain function linked to Parkinson's disease. Numerous studies have proposed an antiapoptotic function for alpha-synuclein, based on overexpression experiments in cell lines. To explore whether alpha-synuclein has such a physiological function, we assessed the response of wild type or alpha-synuclein null neonatal mouse sympathetic neurons to nerve growth factor deprivation, a well-characterized stimulus of neuronal apoptosis. There was no difference in the rate of neuronal loss, neuronal apoptosis, or c-jun phosphorylation. Furthermore, the absence of alpha-synuclein did not alter the magnitude of naturally occurring cell death in vivo in substantia nigra pars compacta. Therefore, alpha-synuclein is unlikely to play a significant role in apoptotic signalling in catecholaminergic neurons of the neonatal nervous system.

Developmental neurotoxicity of ketamine: morphometric confirmation, exposure parameters, and multiple fluorescent labeling of apoptotic neurons.

Ketamine is a widely used pediatric anesthetic recently reported (C. Ikonomidou et al., 1999, Science 283, 70-74) to enhance neuronal death in neonatal rats. To confirm and extend these results, we treated four groups of PND 7 rats with seven sc doses, one every 90 min, of either saline, 10 mg/kg ketamine, 20 mg/kg ketamine, or a single dose of 20 mg/kg ketamine. The repeated doses of 20 mg/kg ketamine increased the number of silver-positive (degenerating) neurons in the dorsolateral thalamus to a degree comparable to previous results (Ikonomidou et al., 1999, Science 283, 70-74), i.e., 28-fold vs. 31-fold respectively. However, blood levels of ketamine immediately after the repeated 20 mg/kg doses were about 14 micrograms/ml, about seven-fold greater than anesthetic blood levels in humans (J. M. Malinovsky et al., 1996, Br. J. Anaesth. 77, 203-207; R. A. Mueller and R. Hunt, 1998, Pharmacol. Biochem. Behav. 60, 15-22). Levels of ketamine in blood following exposure to the multiple 10 mg/kg doses of ketamine or to a single 20 mg/kg dose ranged around 2-5 micrograms/ml; although these blood levels are close to an anesthetic level in humans, they failed to produce neurodegeneration. To investigate the mode of ketamine-induced neuronal death, coronal sections were stained with both Fluoro-Jade B (a green fluorescent stain selective for neurodegeneration) and DAPI (a blue DNA stain), as well as for caspase-3 (using an antisera labeled red with rhodamine). These histochemical results confirmed the developmental neurotoxicity of ketamine, demonstrated that Fluoro-Jade B (FJ-B), like silver methods, successfully stained degenerating neurons in neonatal rats, and indicated that ketamine acts by increasing the rate of neuronal apoptosis.

Apoptosis of neurons in rats with experimental allergic encephalomyelitis

Experimental allergic encephalomyelitis (EAE) is a rat model of multiple sclerosis (MS). In MS and EAE, calpain expression is increased in cells of the central nervous system including astrocytes, oligodendrocytes and microglia; as well as infiltrating macrophages and T cells. However, calpain expression in neurons has not previously been reported in the EAE model. Since degeneration of neurons has been demonstrated in progressive cases of MS, and calpain has been implicated in cell death in EAE, it is hypothesized that neurons expressing increased levels of calpain may undergo apoptosis. To test the hypothesis, spinal cords were collected from control and EAE rats from 4 to 12 days after challenge with guinea‐pig myelin and complete Freund's adjuvant. Apoptosis in neurons was determined by combined TUNEL and immunoflourescent staining using a neuron‐specific marker (NeuN). Calpain expression in neurons was determined using double‐immunoflourescent staining with antibodies specific for calpain and NeuN. At 8 and 9 days postchallenge, neither apoptosis nor increased calpain expression were detected in neurons in control or 9 day EAE spinal cord sections. In contrast, at 11 days postinduction neurons in acute EAE were TUNEL‐positive, indicating that cells underwent apoptosis. Further studies are in progress to determine if and when oligodendrocytes in EAE rats are apoptotic. By correlating cell‐specific increases in calpain expression with apoptosis in EAE, a better understanding of the role of calpain in EAE and MS might be achieved. This increased knowledge could suggest a potential benefit of calpain inhibitors in the treatment of EAE and MS.Acknowledgements: Supported in part by grants from NIH‐NINDS.

Room E, 10/16/2000 2: 00 PM - 4: 00 PM (PS) Cortical Spreading Depression Induced Preconditioning Decreases Neuronal Apoptosis in Rats Subjected to Focal Ischemia

<b>Room E, 10/16/2000 2: 00 PM - 4: 00 PM (PS) Cortical Spreading Depression Induced Preconditioning Decreases Neuronal Apoptosis in Rats Subjected to Focal Ischemia</b>

Room F, 10/16/2000 9: 00 AM - 11: 00 AM (PS) Effect of Isoflurane on Neuronal Apoptosis in Rats Subjected to Focal Ischemia

Room F, 10/16/2000 9: 00 AM - 11: 00 AM (PS) Effect of Isoflurane on Neuronal Apoptosis in Rats Subjected to Focal Ischemia

Endothelin B receptor deficiency is associated with an increased rate of neuronal apoptosis in the dentate gyrus

The dentate gyrus retains neuronal proliferative potential throughout life. Using immature endothelin B receptor-deficient (sl/sl) rats, a rabbit model of pneumococcal meningitis and autopsy brains from humans who died from pneumococcal meningitis, we explored the role of endothelin B receptors in physiological and pathological neuronal apoptosis in the dentate gyrus. At postnatal days 3–4, the rate of apoptosis in the dentate gyrus was high in all rats, declining to low levels in wild-type rats (+/+) on days 14 and 22, but remaining high in both homozygous (sl/sl) and heterozygous (sl/+) endothelin B receptor-deficient rats. Increased apoptosis was not significantly compensated for by neuronal proliferation. Hippocampal neuronal cultures also exhibited genotype-dependent apoptosis with the highest rate in neurons from homozygous endothelin B receptor-deficient (sl/sl) rats. In rabbit and human pneumococcal meningitis, increased apoptosis in the dentate gyrus was associated with loss of neuronal endothelin B receptor immunoreactivity. In conclusion, endothelin B receptors appear to act as neuronal survival factors in the dentate gyrus in rodents and man, both during postnatal development and under pathological conditions.

Activity of voltage-operated calcium channels in rat cerebellar granule neurons and neuronal survival

Neuronal activity and Ca 2+ channel activation play important roles in neuronal survival and development. In cerebellar granule neurons, the culture conditions can induce differential expression of various membrane receptor proteins. To test the hypothesis that culture conditions might affect the activity of voltage-operated Ca 2+ channels, the present study analysed the differences in Ca 2+ signalling between granule neurons grown in the presence of normal (5 mM) or high (25 mM) KCl. The Ca 2+ transients evoked by 50 mM KCl developed similarly in both cultures, as a function of age. In contrast, when compared with neurons grown in 25 mM KCl, a proportion of the neurons grown in normal KCl showed, between days in vitro 4 and 6, a higher Ca 2+ transient in response to 12.5 mM KCl. These neurons were less sensitive to the effect of 10 μM nifedipine and, conversely, more sensitive to the effects of 10 μM ω-conotoxin MVIIC when stimulated with 50 mM KCl, indicating that they express preferentially, at this stage, the N- and/or Q-type Ca 2+ channels. This period of maximal activity of the N/Q-type Ca 2+ channels was associated with a significant increase in the rate of neuronal apoptosis. The present study also shows, by comparing the rates of neuronal apoptosis, that the long-term maintenance in 25 mM KCl appears to “synchronize” and sensitize the neuronal population to the apoptotic process. These results illustrate the differential effect the culture conditions can have on the expression and activity of Ca 2+ channels, which, in turn, can modulate neuronal survival.

Neuronal apoptosis in rats is accompanied by rapid impairment of cellular respiration and is prevented by scavengers of reactive oxygen species

Apoptosis of cerebellar granule cells induced by potassium withdrawal is accompanied by a very rapid decrease in both cell and mitochondrial respiration supported by glucose and succinate, respectively. The respiratory control ratio, which is an index of oxidative phosphorylation and therefore reflects the ability of mitochondria to produce ATP, is reduced by 50% within the first 2 h after the beginning of apoptosis. insulin-like growth factor I (IGF-I), actinomicin D or cycloheximide, previously reported to inhibit apoptosis, fully prevent the impairment of cellular respiration while scavengers of reactive oxygen species partially inhibit apoptosis and restore cellular respiration.

Olfactory neurons are protected from apoptosis in adult transgenic mice over-expressing the bcl-2 gene.

The olfactory system provides a useful in vivo model for studying neuronal apoptosis. The synaptic target deafferentation (olfactory bulb ablation) of the sensory epithelium induces a massive and synchronous wave of retrograde apoptosis in the large population of olfactory sensory neurons. The proto-oncogene bcl-2 is involved in the regulation of cell death and is able to block apoptosis in motoneurones. We demonstrate here that olfactory neurons over-expressing the human Bcl-2 protein in transgenic mice are long-term protected from apoptotic death following ipsilateral olfactory bulbectomy. We kinetically assessed neuronal death 32 h, 50 h and 5 days following unilateral olfactory bulbectomy, in adult C57BL6 (wild-type) and transgenic mice with olfactory neurons over-expressing the Human bcl-2 gene. Using the TUNEL method and morphometric analysis of olfactory epithelium, we confirmed the occurrence of a wave of neuronal death in wild-type mice but failed to detect a significant rate of neuronal apoptosis in the olfactory epithelium of transgenic animals. Apoptotic death of olfactory neurons probably shares common pathways with apoptotic processes occuring in other neuronal types, including motoneurons.

Susceptibility to lead toxicity.

Adverse effects of lead on human health have been recognized for centuries. Nevertheless, current public health measures require a greater understanding of the pathobiology of chronic low level exposure (1-3). Such considerations are related to the question of what level of lead intake is harmful and why specific clinical manifestations of lead poisoning are encountered under particular circumstances. The recognition of factors, both synergistic and antagonistic, which influence the toxicity of lead is essential for adequate understanding of the effects of environmental lead on human health. The immense body of literature already written about lead contains many clues to and impressions of such factors, both adverse and beneficial, modifying the toxicity of lead. This brief review will discuss a number of such factors. Few of these have as yet been subjected to rigorous experimental confirmation. A consideration of antagonisms and synergisms is based on certain assumptions with regard to the metabolism of lead; that is, the daily intake and excretion of lead as well as the movement of lead between various tissues and effects on cells and subcellular organelles.