Accumulation of Cd(II) in the CNS depending on the route of administration: intraperitoneal, intratracheal, or intranasal

Neuropathological profile of the African Giant Rat brain (Cricetomys gambianus) after natural exposure to heavy metal environmental pollution in the Nigerian Niger Delta

Pollution by heavy metals is a threat to public health because of the adverse effects on multiple organ systems including the brain. Here, we used the African giant rat (AGR) as a novel sentinel host to assess the effect of heavy metal accumulation and consequential neuropathology upon the brain. For this study, AGR were collected from distinct geographical regions of Nigeria: the rain forest region of south-west Nigeria (Ibadan), the central north of Nigeria (Abuja), and in oil-polluted areas of south Nigeria (Port-Harcourt). We found that zinc, copper, and iron were the major heavy metals that accumulated in the brain and serum of sentinel AGR, with the level of iron highest in animals from Port-Harcourt and least in animals from Abuja. Brain pathology, determined by immunohistochemistry markers of inflammation and oxidative stress, was most severe in animals from Port Harcourt followed by those from Abuja and those from Ibadan were the least affected. The brain pathologies were characterized by elevated brain advanced oxidation protein product (AOPP) levels, neuronal depletion in the prefrontal cortex, severe reactive astrogliosis in the hippocampus and cerebellar white matter, demyelination in the subcortical white matter and cerebellar white matter, and tauopathies. Selective vulnerabilities of different brain regions to heavy metal pollution in the AGR collected from the different regions of the country were evident. In conclusion, we propose that neuropathologies associated with redox dyshomeostasis because of environmental pollution may be localized and contextual, even in a heavily polluted environment. This novel study also highlights African giant rats as suitable epidemiological sentinels for use in ecotoxicological studies.

Transcriptomics reveals the mechanism of selenium-enriched Lactobacillus plantarum alleviating brain oxidative stress under cadmium stress in Luciobarbus capito

Cadmium (Cd) is one of toxic metal in environment and is thought to affect nervous system. There were an increasing number of studies on selenium (Se)-enriched probiotics which were believed to produce bioactive nanoselenium. The antagonism of Se on heavy metals can significantly affect biological toxicity of heavy metals. This study aimed to elucidate possible mechanism of brain injury in Luciobarbus capito after Cd exposure and the mitigation of Se-enriched probiotics through transcriptome analysis. The results revealed 465 differentially expressed genes in the Cd and the control brains (Cd vs C), including 320 genes with upregulated expression and 145 genes with downregulated expression. In addition, we found that there were 4117 differentially expressed genes in the Se-enriched L. plantarum plus Cd and the control brains (S1L1-Cd vs C), including 2552 genes with upregulated expression and 1565 genes with downregulated expression. There were 147 differentially expressed genes in the Se-enriched L. plantarum plus Cd and the control brains (S1L1-Cd vs Cd), including 40 genes with upregulated expression and 107 genes with downregulated expression. Moreover, GO enrichment analysis indicated that the differentially expressed genes were involved in biological processes cellular component, and molecular function. KEGG enrichment analysis indicated that MAPK signaling pathway, calcium signaling pathway, and PI3K-Akt signaling pathway were significantly enriched. Subsequently, qRT-PCR was performed, and we selected 15 related differentially expressed genes for verification. The qRT-PCR results revealed the same trend as the RNA-Seq results. In conclusion, this study elucidated relieving effect of Se-enriched probiotics on Cd exposure-induced brain oxidative stress. This study provided a theoretical basis for further research on genes related to Cd poisoning and the amelioration of Se-enriched probiotics on Cd poisoning.

Temperature Effects During a Sublethal Chronic Metal Mixture Exposure on Common Carp (Cyprinus carpio)

The aquatic environment is the final sink of various pollutants including metals, which can pose a threat for aquatic organisms. Waterborne metal mixture toxicity might be influenced by environmental parameters such as the temperature. In the present study, common carp were exposed for 27 days to a ternary metal mixture of Cu, Zn, and Cd at two different temperatures, 10 and 20°C. The exposure concentrations represent 10% of the 96 h-LC₅₀ (concentration lethal for the 50% of the population in 96 h) for each metal (nominal metal concentrations of Cu: 0.08 μM; Cd: 0.02 μM and Zn: 3 μM). Metal bioaccumulation and toxicity as well as changes in the gene expression of enzymes responsible for ionoregulation and induction of defensive responses were investigated. Furthermore the hepatosomatic index and condition factor were measured as crude indication of overall health and energy reserves. The obtained results showed a rapid Cu and Cd increase in the gills at both temperatures. Cadmium accumulation was higher at 20°C compared to 10°C, whereas Cu and Zn accumulation was not, suggesting that at 20°C, fish had more efficient depuration processes for Cu and Zn. Electrolyte (Ca, Mg, Na, and K) levels were analyzed in different tissues (gills, liver, brain, muscle) and in the remaining carcasses. However, no major electrolyte losses were observed. The toxic effect of the trace metal ion mixture on major ion uptake mechanisms may have been compensated by ion uptake from the food. Finally, the metal exposure triggered the upregulation of the metallothionein gene in the gills as defensive response for the organism. These results, show the ability of common carp to cope with these metal levels, at least under the condition used in this experiment.

Cadmium-induced toxicity is rescued by curcumin: A review

Cadmium (Cd) is one of the most common environmental and occupational heavy metals with extended distribution. Exposure to Cd may be associated with several deleterious consequences on the liver, bones, kidneys, lungs, testes, brain, immunological, and cardiovascular systems. Overproduction of reactive oxygen species (ROS) as the main mechanism behind its toxicity causes oxidative stress and subsequent damages to lipids, proteins, and DNA. Therefore, antioxidants along with chelating agents have shown promising outcomes against Cd-induced toxicity. Curcumin with various beneficial effects and medical efficacy has been evaluated for its inhibitory activities against biological impairments caused by Cd. Thus, this article is intended to address the effectiveness of curcumin against toxicity following Cd entry. Curcumin can afford to attenuate lipid peroxidation, glutathione depletion, alterations in antioxidant enzyme, and so forth through scavenging and chelating activities or Nrf2/Keap1/ARE pathway induction. © 2017 BioFactors, 43(5):645-661, 2017.

A potential non-invasive glioblastoma treatment: Nose-to-brain delivery of farnesylthiosalicylic acid incorporated hybrid nanoparticles

New drug delivery systems are highly needed in research and clinical area to effectively treat gliomas by reaching a high antineoplastic drug concentration at the target site without damaging healthy tissues. Intranasal (IN) administration, an alternative route for non-invasive drug delivery to the brain, bypasses the blood-brain-barrier (BBB) and eliminates systemic side effects. This study evaluated the antitumor efficacy of farnesylthiosalicylic acid (FTA) loaded (lipid-cationic) lipid-PEG-PLGA hybrid nanoparticles (HNPs) after IN application in rats. FTA loaded HNPs were prepared, characterized and evaluated for cytotoxicity. Rat glioma 2 (RG2) cells were implanted unilaterally into the right striatum of female Wistar rats. 10days later, glioma bearing rats received either no treatment, or 5 repeated doses of 500μM freshly prepared FTA loaded HNPs via IN or intravenous (IV) application. Pre-treatment and post-treatment tumor sizes were determined with MRI. After a treatment period of 5days, IN applied FTA loaded HNPs achieved a significant decrease of 55.7% in tumor area, equal to IV applied FTA loaded HNPs. Herewith, we showed the potential utility of IN application of FTA loaded HNPs as a non-invasive approach in glioblastoma treatment.

Nasal Administration of Cholera Toxin as a Mucosal Adjuvant Damages the Olfactory System in Mice

Cholera toxin (CT) induces severe diarrhea in humans but acts as an adjuvant to enhance immune responses to vaccines when administered orally. Nasally administered CT also acts as an adjuvant, but CT and CT derivatives, including the B subunit of CT (CTB), are taken up from the olfactory epithelium and transported to the olfactory bulbs and therefore may be toxic to the central nervous system. To assess the toxicity, we investigated whether nasally administered CT or CT derivatives impair the olfactory system. In mice, nasal administration of CT, but not CTB or a non-toxic CT derivative, reduced the expression of olfactory marker protein (OMP) in the olfactory epithelium and olfactory bulbs and impaired odor responses, as determined with behavioral tests and optical imaging. Thus, nasally administered CT, like orally administered CT, is toxic and damages the olfactory system in mice. However, CTB and a non-toxic CT derivative, do not damage the olfactory system. The optical imaging we used here will be useful for assessing the safety of nasal vaccines and adjuvants during their development for human use and CT can be used as a positive control in this test.

Cadmium induces N-cadherin cleavage via ERK-mediated γ-secretase activation in C6 astroglia cells

N-cadherin has known to be involved in tumor progression and metastasis. However, it is still obscure about the signaling pathway involving in the processing of N-cadherin. Thus, we examined which signaling pathway plays a major role in the processing of N-cadherin in C6 glioma cells following treatment of cadmium (Cd), a highly ubiquitous heavy metal. A cleavage product of N-cadherin, N-cad/CTF2 was observed by the treatment of Cd to C6 cells in a time and concentration-dependent manner. The production of N-cad/CTF2 was inhibited by pretreatment of γ-secretase inhibitors or siRNA transfection of nicastrin, indicating that γ-secretase is involved in the cleavage. Interestingly, Cd could activate both ERK and JNK signaling pathways in C6 cells; however, γ-secretase-mediated N-cad/CTF2 production by Cd was completely blocked by MEK1/2 inhibitors PD184352 and U0126, but not by a JNK inhibitor SP600125, demonstrating that the ERK signaling pathway plays a major role in the cleavage. In addition, pretreatment of an antioxidant or Ca²⁺ blocker blocked the production of N-cad/CTF2 by Cd together with the inhibition of ERK1/2 phosphorylation. Collectively, these results suggest that Cd increases intracellular Ca²⁺ or ROS, which induces γ-secretase-dependent N-cad/CTF2 production via the activation of the ERK signaling pathway in C6 glial cells.

Nasal-to-CNS drug delivery: where are we now and where are we heading? An industrial perspective

Delivery of drug therapeutics across the blood-brain barrier is a challenging task for pharmaceutical scientists. Nasal-to-CNS drug delivery has shown promising results in preclinical efficacy models and investigatory human clinical trials. The further development of this technology with respect to the establishment of valid, predictable preclinical species models, translatable pharmacokinetic-pharmacodynamic relationships and definition of toxicology impact will help attract additional pharmaceutical investment in this drug-delivery approach. Further discoveries in nasal nanotechnology, targeted delivery devices and diagnostic olfactory imaging will serve to fuel the advancements in this area of drug delivery.

Functional rehabilitation of cadmium-induced neurotoxicity despite persistent peripheral pathophysiology in the olfactory system

Intranasal exposure to the heavy metal cadmium has been linked to olfactory dysfunction and neurotoxicity. Here, we combine optical imaging of in vivo neurophysiology, genetically defined anatomical tract tracing, mass spectrometry, and behavioral psychophysical methods to evaluate the persistent harmful effects of acute intranasal exposure to cadmium in a mouse model and to investigate the functional consequences of sensory rehabilitation training. We find that an acute intranasal instillation of cadmium chloride leads to an accumulation of cadmium in the brain's olfactory bulb that persists for at least 4 weeks. This is accompanied by persistent severe pathophysiology of the olfactory nerve, a gradual reduction in axonal projections from the olfactory epithelium, and complete impairment on an olfactory detection task. Remarkably, 2 weeks of odorant-guided operant conditioning training proved sufficient to restore olfactory detection performance to control levels in cadmium-exposed mice. Optical imaging from rehabilitated mice showed that this training did not cause any detectable restoration of olfactory nerve function, suggesting that the recovery of function was mediated by central neuroplasticity in which the brain learned to interpret the degraded sensory input. These data demonstrate that sensory learning can mask even severe damage from neurotoxicants and suggest that explicit sensory training may be useful in rehabilitation of olfactory dysfunction.

In vivo visualization of olfactory pathophysiology induced by intranasal cadmium instillation in mice

Intranasal exposure to cadmium has been related to olfactory dysfunction in humans and to nasal epithelial damage and altered odorant-guided behavior in rodent models. The pathophysiology underlying these deficits has not been fully elucidated. Here we use optical imaging techniques to visualize odorant-evoked neurotransmitter release from the olfactory nerve into the brain's olfactory bulbs in vivo in mice. Intranasal cadmium chloride instillations reduced this sensory activity by up to 91% in a dose-dependent manner. In the olfactory bulbs, afferents from the olfactory epithelium could be quantified by their expression of a genetically encoded fluorescent marker for olfactory marker protein. At the highest dose tested, cadmium exposure reduced the density of these projections by 20%. In a behavioral psychophysical task, mice were trained to sample from an odor port and make a response when they detected an odorant against a background of room air. After intranasal cadmium exposure, mice were unable to detect the target odor. These experiments serve as proof of concept for a new approach to the study of the neural effects of inhaled toxicants. The use of in vivo functional imaging of the neuronal populations exposed to the toxicant permits the direct observation of primary pathophysiology. In this study optical imaging revealed significant reductions in odorant-evoked release from the olfactory nerve at a cadmium chloride dose two orders of magnitude less than that required to induce morphological changes in the nerve in the same animals, demonstrating that it is a more sensitive technique for assessing the consequences of intranasal neurotoxicant exposure. This approach is potentially useful in exploring the effects of any putative neurotoxicant that can be delivered intranasally.

In vivo monitoring of the transfer kinetics of trace elements in animal brains with hyphenated inductively coupled plasma mass spectrometry techniques

The roles of metal ions to sustain normal function and to cause dysfunction of neurological systems have been confirmed by various studies. However, because of the lack of adequate analytical method to monitor the transfer kinetics of metal ions in the brain of a living animal, research on the physiopathological roles of metal ions in the CNS remains in its early stages and more analytical efforts are still needed. To explicitly model the possible links between metal ions and physiopathological alterations, it is essential to develop in vivo monitoring techniques that can bridge the gap between metalloneurochemistry and neurophysiopathology. Although inductively coupled plasma mass spectrometry (ICP-MS) is a very powerful technique for multiple trace element analyses, when dealing with chemically complex microdialysis samples, the detection capability is largely limited by instrumental sensitivity, selectivity, and contamination that arise from the experimental procedure. As a result, in recent years several high efficient and clean on-line sample pretreatment systems have been developed and combined with microdialysis and ICP-MS for the continuous and in vivo determination of the concentration-time profiles of metal ions in the extracellular space of rat brain. This article reviews the research relevant to the development of analytical techniques for the in vivo determination of dynamic variation in the concentration levels of metal ions in a living animal.

Toxicity studies on depleted uranium in primary rat cortical neurons and in Caenorhabditis elegans: what have we learned?

Depleted uranium (DU) is the major by-product of the uranium enrichment process for its more radioactive isotopes, retaining approximately 60% of its natural radioactivity. Given its properties as a pyrophoric and dense metal, it has been extensively used in armor and ammunitions. Questions have been raised regarding the possible neurotoxic effects of DU in humans based on follow-up studies in Gulf War veterans, where a decrease in neurocognitive behavior in a small population was noted. Additional studies in rodents indicated that DU readily traverses the blood-brain barrier, accumulates in specific brain regions, and results in increased oxidative stress, altered electrophysiological profiles, and sensorimotor deficits. This review summarizes the toxic potential of DU with emphasis on studies on thiol metabolite levels, high-energy phosphate levels, and isoprostane levels in primary rat cortical neurons. Studies in Caenorhabditis elegans detail the role of metallothioneins, small thiol-rich proteins, in protecting against DU exposure. In addition, recent studies also demonstrate that only one of the two forms, metallothionein-1, is important in the accumulation of uranium in worms.

Role of the olfactory receptor neurons in the direct transport of inhaled uranium to the rat brain

Uranium presents numerous industrial and military uses and one of the most important risks of contamination is dust inhalation. In contrast to the other modes of contamination, the inhaled uranium has been proposed to enter the brain not only by the common route of all modes of exposure, the blood pathway, but also by a specific inhalation exposure route, the olfactory pathway. To test whether the inhaled uranium enter the brain directly from the nasal cavity, male Sprague-Dawley rats were exposed to both inhaled and intraperitoneally injected uranium using the (236)U and (233)U, respectively, as tracers. The results showed a specific frontal brain accumulation of the inhaled uranium which is not observed with the injected uranium. Furthermore, the inhaled uranium is higher than the injected uranium in the olfactory bulbs (OB) and tubercles, in the frontal cortex and in the hypothalamus. In contrast, the other cerebral areas (cortex, hippocampus, cerebellum and brain residue) did not show any preferential accumulation of inhaled or injected uranium. These results mean that inhaled uranium enters the brain via a direct transfer from the nasal turbinates to the OB in addition to the systemic pathway. The uranium transfer from the nasal turbinates to the OB is lower in animals showing a reduced level of olfactory receptor neurons (ORN) induced by an olfactory epithelium lesion prior to the uranium inhalation exposure. These results give prominence to a role of the ORN in the direct transfer of the uranium from the nasal cavity to the brain.

Non-systemic delivery of topical brimonidine to the brain: A neuro-ocular tissue distribution study

To date, the risks of central nervous system (CNS) side effects of topically administered ophthalmic therapeutic agents are thought to be the consequence of systemic absorption of these drugs. This paper envisions the possibility of drug delivery to the CNS following ocular application through non-systemic routes. After single instillation of 50 microl of 3H-radiolabeled Alphagan solution (0.2%) in the cul de sac of the right eye, three male albino rabbits (2-2.5 kg) were sacrificed at each time point (5, 15, 30 and 60 min). Both sides (eyes) specimens of aqueous humor, cornea, iris, lens, vitreous, conjunctiva, sclera, ciliary body, choroid, retina, optic nerve, optic tract and olfactory bulb were weighed, and blood samples were measured, before combustion in tissue oxidizer and radioactive liquid scintillation counting. Significant 3H-brimonidine levels were found in right and left optic nerves and tracts with extremely low corresponding drug levels in blood. Uveal tract (ciliary body, iris and choroid tissues) brimonidine levels were relatively high in the treated eye, and the highest among contralateral eye tissues. Our data provide the first case of good CNS availability after ocular application of conventional ophthalmic therapeutic agent, through non-systemic routes. Similar neuro-ocular pharmacokinetic studies should be adopted as a routine ocular therapeutics evaluation study.

Inhibitory regulation of glutamate aspartate transporter (GLAST) expression in astrocytes by cadmium-induced calcium influx

After injury to the CNS, the accumulation of extracellular glutamate induces neuronal excitotoxicity, leading to secondary tissue damage. Astrocytes can reduce excess extracellular glutamate primarily through the astrocytic glutamate transporter-1 and the Na(+)-dependent glutamate/aspartate transporter (GLAST). In this study, we used an in vitro model of cadmium-induced cellular stress and found that glutamate uptake activity of astrocytes was suppressed because of cadmium-induced inhibition of GLAST expression. The blockage of cadmium-triggered Ca(2+) influx by Ca(2+) chelators elevated GLAST transcription and glutamate uptake activity in astrocytes, suggesting that the suppression of GLAST expression in cadmium-treated astrocytes was Ca(2+)-dependent. This was supported by the findings showing the reduction of GLAST mRNA in astrocytes after treatment with Ca(2+)-ionophore A23187. Cadmium reduced human GLAST promoter activity; however, it increased the binding of Ca(2+)-sensitive activator protein-1 (AP-1) and cAMP response element binding protein (CREB) to their specific elements derived from the human GLAST promoter. These results demonstrate that AP-1 and CREB may be coupled with Ca(2+)-dependent pathway triggered by cadmium to mediate the inhibition of GLAST transcription. Our results suggest that Ca(2+) influx into astrocytes after CNS injury could cause the down-regulation of GLAST expression, thus reducing the astrocytic glutamate uptake function, which in turn may exacerbate secondary damage after CNS injury.

The nose revisited: a brief review of the comparative structure, function, and toxicologic pathology of the nasal epithelium

The nose is a very complex organ with multiple functions that include not only olfaction, but also the conditioning (e.g., humidifying, warming, and filtering) of inhaled air. The nose is also a "scrubbing tower" that removes inhaled chemicals that may be harmful to the more sensitive tissues in the lower tracheobronchial airways and pulmonary parenchyma. Because the nasal airway may also be a prime target for many inhaled toxicants, it is important to understand the comparative aspects of nasal structure and function among laboratory animals commonly used in inhalation toxicology studies, and how nasal tissues and cells in these mammalian species may respond to inhaled toxicants. The surface epithelium lining the nasal passages is often the first tissue in the nose to be directly injured by inhaled toxicants. Five morphologically and functionally distinct epithelia line the mammalian nasal passages--olfactory, respiratory, squamous, transitional, and lymphoepithelial--and each nasal epithelium may be injured by an inhaled toxicant. Toxicant-induced epithelial lesions in the nasal passages of laboratory animals (and humans) are often site-specific and dependent on the intranasal regional dose of the inhaled chemical and the sensitivity of the nasal epithelial tissue to the specific chemical. In this brief review, we present examples of nonneoplastic epithelial lesions (e.g., cell death, hyperplasia, metaplasia) caused by single or repeated exposure to various inhaled chemical toxicants. In addition, we provide examples of how nasal maps may be used to record the character, magnitude and distribution of toxicant-induced epithelial injury in the nasal airways of laboratory animals. Intranasal mapping of nasal histopathology (or molecular and biochemical alterations to the nasal mucosa) may be used along with innovative dosimetric models to determine dose/response relationships and to understand if site-specific lesions are driven primarily by airflow, by tissue sensitivity, or by another mechanism of toxicity. The present review provides a brief overview of comparative nasal structure, function and toxicologic pathology of the mammalian nasal epithelium and a brief discussion on how data from animal toxicology studies have been used to estimate the risk of inhaled chemicals to human health.

Inhibitory effect of Cd2+ on glycine-induced chloride current in rat hippocampal neurons

The effects of cadmium (Cd(2+)) on glycine-induced Cl(-) current (I(Gly)) were investigated in acutely dissociated rat hippocampal CA1 neurons using the conventional whole-cell patch-clamp technique in this study. We found that Cd(2+) reversibly and concentration-dependently, reduced the amplitudes of I(Gly), with an IC(50) of 1.27 mM and Hill coefficient of 0.45. The depression of I(Gly) by Cd(2+) was independent of membrane voltage between -60 and +40 mV and did not involve a shift in the reversal potential of the current. A non-competitive inhibition was suggested by a double reciprocal plot of the effects of Cd(2+) on the concentration-response curve of the I(Gly). Since intracellular dialysis with 3mM Cd(2+) failed to modify I(Gly), it was suggested that the site of action of Cd(2+) is extracellular. The suppression of I(Gly) by Zn(2+) was unaffected by 3mM Cd(2+), which indicated that Zn(2+) and Cd(2+) bind to independent sites on glycine receptor. The results show that Cd(2+) decreases I(Gly) in acutely dissociated rat hippocampal neurons and the depression of I(Gly) by Cd(2+) may contribute to worsen the neurotoxicological impairment.

Effects of intranasal insulin on cognition in memory-impaired older adults: Modulation by APOE genotype

Raising insulin acutely in the periphery and in brain improves verbal memory. Intranasal insulin administration, which raises insulin acutely in the CNS without raising plasma insulin levels, provides an opportunity to determine whether these effects are mediated by central insulin or peripheral processes. Based on prior research with intravenous insulin, we predicted that the treatment response would differ between subjects with (epsilon4+) and without (epsilon4-) the APOE-epsilon4 allele. On separate mornings, 26 memory-impaired subjects (13 with early Alzheimer's disease and 13 with amnestic mild cognitive impairment) and 35 normal controls each underwent three intranasal treatment conditions consisting of saline (placebo) or insulin (20 or 40 IU). Cognition was tested 15 min post-treatment, and blood was acquired at baseline and 45 min after treatment. Intranasal insulin treatment did not change plasma insulin or glucose levels. Insulin treatment facilitated recall on two measures of verbal memory in memory-impaired epsilon4- adults. These effects were stronger for memory-impaired epsilon4- subjects than for memory-impaired epsilon4+ subjects and normal adults. Unexpectedly, memory-impaired epsilon4+ subjects showed poorer recall following insulin administration on one test of memory. These findings suggest that intranasal insulin administration may have therapeutic benefit without the risk of peripheral hypoglycemia and provide further evidence for apolipoprotein E (APOE) related differences in insulin metabolism.

Nasal drug administration: potential for targeted central nervous system delivery

Nasal administration as a means of delivering therapeutic agents preferentially to the brain has gained significant recent interest. While some substrates appear to be delivered directly to the brain via this route, the mechanisms governing overall brain uptake and exposure remain unclear. Some substrates utilize the olfactory nerve tract and gain direct access to the brain, thus bypassing the blood-brain barrier (BBB). However, most agents of pharmacologic interest likely gain access to the brain via the olfactory epithelium, which represents a more direct route of uptake. While the traditional BBB is not present at the interface between nasal epithelium and brain, P-glycoprotein (and potentially other barrier transporters) is expressed at this interface. In addition, work in this laboratory has demonstrated that P-glycoprotein throughout the brain can be modulated with nasal administration of appropriate inhibitors. The potential for targeted central nervous system delivery via this route is discussed.

Axonal transport of rubidium and thallium in the olfactory nerve of mice

Following intranasal administration of radioactive (86)Rb(+) and (201)Tl(+) in mice, we observed this direct transport via the olfactory nerve pathway. The (86)RbCl and (201)TlCl solutions were administered to two groups of mice, the unilateral intranasal and intravenous administration groups. After sacrifice, their heads were divided into the right and left side, which were then subdivided into seven parts; the nasal mucosa and brain regions were separated. Following the unilateral intranasal administration, uptake after 6 h by the olfactory bulb was significantly higher on the ipsilateral side ((86)Rb, 0.7 %dose; (201)Tl, 0.5 %dose) than on the contralateral side ((86)Rb, 0.08 %dose; (201)Tl, 0.15 %dose). Moreover, the (86)Rb and (201)Tl that accumulated in the olfactory bulb were gradually transported to other brain regions of the olfactory tract, the telencephalon and the diencephalon on the side corresponding to the nostril used for administration. Significant differences were observed between the right and left side of the brain regions 6 and 12 h after administration. Further, (201)Tl autoradiography clearly showed striped patterns of dense accumulation, localized in the region around the glomerular layer and granule cell layer of the olfactory bulb and around the olfactory cortex. These results provide clear evidence of axonal transport via the olfactory nerve pathway, from nasal cavity to the olfactory bulb, as well as to the olfactory cortex through the synaptic junctions. The olfactory transport of the (86)Rb(+) and (201)Tl(+) is thought to represent the behavior of K(+) in the olfactory system.

Carrier mediated transport of chlorpheniramine and chlorcyclizine across bovine olfactory mucosa: Implications on nose‐to‐brain transport

Delivery to the CNS via the nasal cavity has been pursued as a means to circumvent the blood-brain barrier (BBB), yet the mechanism of drug transport across this novel route is not well understood. Hydroxyzine and triprolidine have been reported to readily reach the CNS following nasal administration, whereas no measurable amounts of chlorcyclizine or chlorpheniramine, structurally similar antihistamines, were observed in the CSF. The permeation of chlorpheniramine and chlorcyclizine in vitro across the bovine olfactory mucosa was studied to investigate the biological and physicochemical characteristics that contribute to the limited CNS disposition of these compounds following nasal administration. The submucosal to mucosal fluxes (J(s-m)) of chlorcyclizine and chlorpheniramine across the olfactory mucosa were significantly greater than the mucosal to submucosal fluxes (J(m-s)). Moreover, the submucosal-mucosal permeability of both compounds was temperature dependent and saturable. In the presence of metabolic inhibitors (ouabain and 2,4-dinitrophenol) and P-glycoprotein (P-gp)/multidrug resistance protein 1 (MRP1) inhibitors (quinidine and verapamil), the J(m-s) increased and J(s-m) decreased significantly. These results indicate that chlorpheniramine and chlorcyclizine are effluxed from the olfactory mucosa by efflux transporters such as P-gp and MRP1. Transport studies across inert polymeric membranes demonstrated that the permeability of chlorpheniramine and chlorcyclizine decreased at donor concentrations higher than 3 mM suggesting that physicochemical properties such as self-aggregation also play a role in the reduced olfactory mucosal permeability of these compounds at higher concentrations.

The effects of environmental pollutants on complex fish behaviour: integrating behavioural and physiological indicators of toxicity

Environmental pollutants such as metals, pesticides, and other organics pose serious risks to many aquatic organisms. Accordingly, a great deal of previous research has characterized physiological mechanisms of toxicity in animals exposed to contaminants. In contrast, effects of contaminants on fish behaviour are less frequently studied. Because behaviour links physiological function with ecological processes, behavioural indicators of toxicity appear ideal for assessing the effects of aquatic pollutants on fish populations. Here we consider the many toxicants that disrupt complex fish behaviours, such as predator avoidance, reproductive, and social behaviours. Toxicant exposure often completely eliminates the performance of behaviours that are essential to fitness and survival in natural ecosystems, frequently after exposures of lesser magnitude than those causing significant mortality. Unfortunately, the behavioural toxicity of many xenobiotics is still unknown, warranting their future study. Physiological effects of toxicants in the literature include disruption of sensory, hormonal, neurological, and metabolic systems, which are likely to have profound implications for many fish behaviours. However, little toxicological research has sought to integrate the behavioural effects of toxicants with physiological processes. Those studies that take this multidisciplinary approach add important insight into possible mechanisms of behavioural alteration. The most commonly observed links with behavioural disruption include cholinesterase (ChE) inhibition, altered brain neurotransmitter levels, sensory deprivation, and impaired gonadal or thyroid hormone levels. Even less frequently studied are the implications of interrelated changes in behaviour and physiology caused by aquatic pollutants for fish populations. We conclude that future integrative, multidisciplinary research is clearly needed to increase the significance and usefulness of behavioural indicators for aquatic toxicology, and aim to highlight specific areas for consideration.

P-Glycoprotein attenuates brain uptake of substrates after nasal instillation

PURPOSE

Previous literature has suggested the absence of an effective barrier between the nasal mucosa and the brain for compounds administered via the nasal route. These experiments were conducted to elucidate the role of the blood-brain barrier efflux transporter P-glycoprotein (P-gp) in attenuating delivery of P-gp substrates to the brain after nasal administration in mice.

METHODS

Brain uptake of several radiolabeled P-gp substrates, was measured in P-gp-deficient and P-gp-competent mice following nasal instillation. Additional experiments were performed to assess the potential for enhancing brain uptake by inhibiting P-gp with intranasal rifampin.

RESULTS

All substrates examined were measurable in brain tissue within 2 min. Substrate accumulation in P-gp-deficient mice was higher than in P-gp-competent animals; the degree to which P-gp attenuated brain uptake after nasal administration was similar to that during in situ brain perfusion. Co-administration of rifampin enhanced brain uptake of relevant substrates, and resulted in complete elimination of P-gp-mediated transport for 3H- verapamil.

CONCLUSIONS

P-gp attenuates brain accumulation of intranasally-administered P-gp substrates. Thus, biochemical components of the blood-brain barrier, such as efflux transporters may influence brain penetration after nasal administration. Co-administration of a P-gp inhibitor enhances the brain uptake of relevant substrates, suggesting that the transporter barrier functions may be reversible.

Cadmium disrupts behavioural and physiological responses to alarm substance in juvenile rainbow trout (Oncorhynchus mykiss)

Alarm substance is a chemical signal released from fish skin epithelial cells after a predator causes skin damage. When other prey fish detect alarm substance by olfaction, they perform stereotypical predator-avoidance behaviours to decrease predation risk. The objective of this study was to explore the effect of sublethal cadmium (Cd) exposure on the behavioural and physiological responses of juvenile rainbow trout (Oncorhynchus mykiss) to alarm substance. Waterborne exposure to 2 microg Cd l(-1) for 7 days eliminated normal antipredator behaviours exhibited in response to alarm substance, whereas exposures of shorter duration or lower concentration had no effect on normal behaviour. Furthermore, dietary exposure to 3 microg Cd g(-1) in the food for 7 days, which produced the same whole-body Cd accumulation as waterborne exposure to 2 microg l(-1), did not alter normal behaviour, indicating that an effect specific to waterborne exposure alone (i.e. Cd accumulation in the olfactory system) results in behavioural alteration. Whole-body phosphor screen autoradiography of fish exposed to (109)Cd demonstrated that Cd deposition in the olfactory system (rosette, nerve and bulb) during waterborne exposure was greater than in all other organs of accumulation except the gill. However, Cd could not be detected in the brain. A short-term elevation in plasma cortisol occurred in response to alarm substance under control conditions. Cd exposures of 2 microg l(-1) waterborne and 3 microg g(-1) dietary for 7 days both inhibited this plasma cortisol elevation but did not alter baseline cortisol levels. Our results suggest that exposure to waterborne Cd at environmentally realistic levels (2 microg l(-1)) can disrupt the normal behavioural and physiological responses of fish to alarm substance and can thereby alter predator-avoidance strategies, with potential impacts on aquatic fish communities.

Cadmium-metallothionein interactions in the olfactory pathways of rats and pikes

Deposition of cadmium onto the olfactory epithelium results in transport of the metal along the primary olfactory neurons to the olfactory bulbs of the brain. The present investigation was undertaken to determine the intracellular ligand binding of cadmium during this process. (109)Cd(2+) was applied on the olfactory epithelium of rats and pikes, and the subcellular distribution of the metal in the olfactory pathways was then examined. Two groups of rats were used: one pretreated with intranasal instillations of nonlabeled cadmium and the other given physiological saline (controls). Cellular fractionations showed that the (109)Cd(2+) was predominantly present in the cytosol of all samples, both in the rats and the pikes. Gel filtrations of the olfactory epithelium of control rats killed 2 h after the (109)Cd(2+) instillation showed that the metal was recovered in two peaks with elution volumes corresponding to metallothionein (MT) and glutathione (GSH)-the latter peak being the predominant one. However, in the epithelium of the cadmium-pretreated rats killed at 2 h, (109)Cd(2+) was recovered in one peak corresponding to MT. In the olfactory epithelium and bulbs of both groups of rats killed at 48 h, as well as in the olfactory epithelium, nerves, and bulbs of pikes killed at this interval, (109)Cd(2+) was recovered in one peak corresponding to MT. Immunohistochemistry of the olfactory system of rats given cadmium in the right nasal cavity showed induction of MT in the neuronal, sustentacular, and basal cells of the right olfactory epithelium, in the nerve fascicles in the lamina propria of the right olfactory mucosa, and in the olfactory nerve layer of the right olfactory bulb. On the left side, the immunoreactivity was low in these structures. MT immunoreactivity was observed in the glomeruli of both the right and the left olfactory bulbs. However, the staining was homogeneously distributed within the entire glomeruli of the right bulb, whereas it showed a mesh-like pattern corresponding to the localization of astrocytes in the glomeruli of the left bulb. We conclude that exposure of the olfactory epithelium to cadmium results in induction of MT in the primary olfactory neurons and a transport of the metal in these neurons as a cadmium-metallothionein (CdMT) complex. Our results further indicate that GSH is a ligand that can interact with cadmium before the metal binds to MT.

Direct nose-brain transport of benzoylecgonine following intranasal administration in rats

In our previous research, cocaine applied intranasally in rats diffused or was transported directly from the nasal cavity to the brain. However, the direct nose-brain cocaine transport only contributes to an initial increase in the relative cocaine brain exposure. In this study, we have determined the nose-brain transport of a polar metabolite of cocaine, benzoylecgonine, to help understand factors affecting drug transport via this novel pathway. The nasal cavity of male Sprague-Dawley rats was isolated to prevent drainage of nasally applied dosing solution to non-nasal regions. Benzoylecgonine was then administered, either by intranasal administration or by intravenous (iv) injection. At different times postdose, blood and tissues from different regions of the brain were collected from groups of rats (n = 4 for each collection time) and benzoylecgonine concentrations in these samples were analyzed by high-performance liquid chromatography. Benzoylecgonine concentrations in plasma were at maximal levels immediately after iv dosing and declined as a function of time. Following intranasal administration, benzoylecgonine concentrations in plasma reached maximal levels between 15 and 30 min after dosing and declined as a function of time. To allow comparison of brain benzoylecgonine content after iv and intranasal administration, brain benzoylecgonine contents were normalized by plasma benzoylecgonine concentrations. The ratios of the area under the benzoylecgonine concentration-time curve (AUC) between the olfactory bulb and plasma following intranasal administration were 10-100 times higher than those obtained after iv dosing. The olfactory tract-to-plasma benzoylecgonine AUC ratios after intranasal administration were significantly higher than those after iv dosing up to 120 min following dosing. The brain tissue-to-plasma AUC ratios in cerebellum, brain stem, and cerebral cortex after intranasal administration were significantly higher than the corresponding ratios after iv administration up to 30 min following dosing. We conclude than nasally administered benzoylecgonine was transported directly from the nasal cavity to the brain and that the significant increase in brain levels was sustained for a prolonged period of time. Factors contributing to the observed differences in the nose-brain transport of cocaine and benzoylecgonine are discussed.

Cadmium toxicity in synaptic neurotransmission in the brain

Chronic exposure to cadmium causes central nervous system disorders, e.g. olfactory dysfunction. To clarify cadmium toxicity in synaptic neurotransmission in the brain, the movement of cadmium in the synapses was examined using in vivo microdialysis. One and 24 h after injection of (109)CdCl(2) into the amygdala of rats, (109)Cd release into the extracellular space was facilitated by stimulation with high K(+), suggesting that cadmium taken up by amygdalar neurons is released into the synaptic clefts in a calcium- and impulse-dependent manner. Moreover, to examine the action of cadmium in the synapses, the amygdala was perfused with artificial cerebrospinal fluid containing 10-30 microM CdCl(2). The release of excitatory neurotransmitters, i.e. glutamate and aspartate, into the extracellular space was decreased during perfusion with cadmium, while the release of inhibitory neurotransmitters, i.e. glycine and gamma-amino butyric acid (GABA), into the extracellular space was increased during the period. These results suggest that cadmium released from the amygdalar neuron terminals affect the degree and balance of excitation-inhibition in synaptic neurotransmission.

Neurobehavioural effects of occupational exposure to cadmium: a cross sectional epidemiological study

BACKGROUND

A patient with unexplained minor behavioural changes associated with an axonal sensorimotor polyneuropathy had a history of chronic occupational exposure to cadmium (Cd). Although animal studies have shown that Cd is a potent neurotoxicant, little is known about its toxicity for the human central nervous system. The aim of this study was to investigate the toxic potential of chronic occupational exposure to Cd on neurobehavioural functions.

METHODS

A cross sectional epidemiological study was conducted ina group of Cd workers and an age matched control group. Eighty nine adult men (42 exposed to Cd and 47 control workers) were given a blinded standardised examination that consisted of computer assisted neurobehavioural tests (neurobehavioural examination system), a validated questionnaire to assess neurotoxic complaints (neurotoxicity symptom checklist--60, NSC-60), and a standardised self administered questionnaire to detect complaints consistent with peripheral neuropathy and dysfunction of the autonomic nervous system. Historical and current data on biomonitoring of exposure to Cd, either the highest value of Cd in urine (CdU in microgram Cd/g creatinine) of each Cd worker during work (CdUmax) or the current value (CdUcurrent) of each control, were available as well as data on microproteinuria.

RESULTS

Cd workers (CdUmax: mean (range), 12.6 (0.4-38.4)) performed worse than the controls (CdUcurrent: mean (range), 0.7 (0.1-2.0)) on visuomotor tasks, symbol digit substitution (p = 0.008), and simple reaction time to direction (p = 0.058) or location (p = 0.042) of a stimulus. In multiple linear regression analysis, symbol digit substitution, simple direction reaction time test, and simple location reaction time test were significantly related to CdUmax, (beta = 0.35 (p < 0.001), beta = 0.25 (p = 0.012), and beta = 0.23 (p = 0.021) respectively). More complaints consistent with peripheral neuropathy (p = 0.004), complaints about equilibrium (p = 0.015), and complaints about concentration ability (p = 0.053) were found in the group exposed to Cd than in the control group, and these variables correlated positively with CdUmax (peripheral neuropathy: beta = 0.38, p < 0.001; equilibrium: beta = 0.22, p = 0.057; concentration ability: beta = 0.27, p = 0.020).

CONCLUSION

Slowing of visuomotor functioning on neurobehavioural testing and increase in complaints consistent with peripheral neuropathy, complaints about equilibrium, and complaints about concentration ability were dose dependently associated with CdU. Age, exposure to other neurotoxicants, or status of renal function could not explain these findings. The present study also indicates that an excess of complaints may be detected in Cd workers before signs of microproteinuria induced by Cd occur.

Direct transport of cocaine from the nasal cavity to the brain following intranasal cocaine administration in rats

Individuals who consume cocaine illegally have long since adopted or explored the nasal route of administration. This study was designed to determine in an animal model whether nasally applied cocaine could be transported directly from the nasal cavity to the central nervous system. Male Sprague-Dawley rats were used in the study. The nasal cavity was isolated to prevent drainage of nasally applied dosing solution to nonnasal regions. Cocaine was then administered, either by intranasal (in) administration or by intravenous (iv) injection. At different times post dose, blood and tissues from different regions of the brain were collected. Cocaine concentrations in plasma and tissue samples were analyzed by HPLC. After iv administration, similar cocaine contents in different brain regions were observed. Following in administration, cocaine content in samples collected within 60 min post dose were found to differ considerably in different brain regions. The highest content was observed in the olfactory bulb, followed by the olfactory tract and then the remaining part of the brain. To allow comparison of brain cocaine content after iv and in administration, brain cocaine contents were normalized by plasma cocaine concentrations. The ratios of the area under the cocaine concentration-time curve (AUC) between the olfactory bulb and plasma at early times following in administration were significantly higher than those obtained after the iv dose (13.4 +/- 5.56 vs 6.16 +/- 0.94, p < 0.05, for AUC ratio up to 2 min post dose; 9.39 +/- 1.47 vs 7.34 +/- 0.59, p < 0.05, for AUC ratio up to 4 min post dose). At 1 min post dose, the olfactory bulb-to-plasma cocaine concentration ratios following in administration was three times those obtained after iv administration. After 1 min, the olfactory bulb-to-plasma concentration ratios following in administration were found to be similar to or smaller than those obtained after iv administration. The tissue-to-plasma concentration ratios in other brain regions following in administration were found to be smaller than those obtained following iv dosing. We conclude that nasally administered cocaine was transported directly from the nasal cavity to the brain but that only a very small fraction of the dose was transported via the direct pathway.

Nasal route for direct delivery of solutes to the central nervous system: fact or fiction?

During this century, several investigators reported that certain viruses, metals, drugs, and other solutes could bypass systemic circulation and enter the brain and/or cerebrospinal fluid directly following nasal administration. Although evidence clearly suggests that the olfactory epithelium and its olfactory cells play a major role, little is known about the mechanisms of direct transport of solutes into the brain. An overview of what is known about these mechanisms may aid in further research in this field, including studies of direct drug delivery to the central nervous system. This review, in addition to summarizing the literature to date, clearly describes the intricate association of the anatomical features involved in direct entry of solutes into the brain following nasal administration. To aid in the understanding of the possible routes a solute can take after nasal administration, the anatomy of the olfactory epithelium and surrounding tissues is described, and a detailed scheme delineating the emerging pathways is presented. Techniques used in delineating these pathways and studies supporting a particular pathway are discussed in greater detail. Finally, some factors influencing the direct transport of solutes to the cerebrospinal fluid and brain are summarized.

Uptake of nickel into the brain via olfactory neurons in rats

Intranasal instillation of nickel ([63]Ni2+) in rats resulted in an uptake of the metal in the olfactory epithelium and a migration along primary olfactory neurons to the glomeruli of the olfactory bulb. The metal was then seen to pass to the interior of the bulb and further to the olfactory peduncle, the olfactory tubercle and the rostral parts of the prepiriform, frontal and cingulate corticis. These results indicate that (63)Ni2+ slowly passes to secondary and tertiary olfactory neurons. Intraperitoneal injection of (63)Ni2+ resulted in a low uptake in the brain, without preferential labelling of the olfactory pathways. Inhalation of nickel compounds can impair the olfactory system. An uptake of nickel in the olfactory neurons may underly these lesions.

Uptake of manganese and cadmium from the nasal mucosa into the central nervous system via olfactory pathways in rats

In the olfactory epithelium the primary olfactory neurones are in contact with the environment and via the axonal projections they are also connected to the olfactory bulbs of the brain. Therefore, the primary olfactory neurones provide a pathway by which foreign materials may gain access to the brain. In the present study we used autoradiography and gamma spectrometry to show that intranasal instillation of manganese (54Mn2+) in rats results in initial uptake of the metal in the olfactory bulbs. The metal was then seen to migrate via secondary and tertiary olfactory pathways and via further connections into most parts of the brain and also to the spinal cord. Intranasal instillation of cadmium (109Cd2+) resulted in uptake of the metal in the anterior parts of the olfactory bulbs but not in other areas of the brain. This indicates that this metal is unable to pass the synapses between the primary and secondary olfactory neurones in the bulbs. Intraperitoneal administration of 54Mn2+ or 109Cd2+ showed low uptake of the metals in the olfactory bulbs, an uptake not different from the rest of the brain. Manganese is a neurotoxic metal which in man can induce an extrapyramidal motor system dysfunction associated with occupational inhalation of manganese-containing dusts or fumes. We propose that the neurotoxicity of inhaled manganese is related to an uptake of the metal into the brain via the olfactory pathways. In this way manganese can circumvent the blood-brain barrier and gain direct access to the central nervous system.

Effect of central acute administration of cadmium on drinking behavior

The effect of acute third ventricle cadmium administration on the drinking behavior of adult male rats under different situations was studied. Injections of cadmium chloride (0.07, 0.7,and 7.0 ng/rat) significantly attenuated water intake in dehydrated rats. Drinking behavior induced by acute intracerebroventricular injections of carbachol (2 micrograms/rat) or angiotensin II (5 ng/rat) was also inhibited by central cadmium injections. Cadmium-induced blockade in water intake in dehydrated animals was reverted by the previous administration of a 5-HT2 antagonist (RP62203) in different doses (5 and 10 micrograms/rat). The data clearly reveal that cadmium elicits very fast actions on the central nervous system. It is suggested that cadmium-induced attenuation of water intake may rely on at least three different mechanisms: impairment of cholinergic and angiotensinergic systems in the brain and stimulation of a central serotonergic drive acting on 5-HT2 receptors. The study of cadmium neurotoxicity by observation of drinking behavior, a behavioral parameter easy to be recorded and measured, is proposed.

Uptake of 203Hg2+ in the olfactory system in pike

Inorganic mercury (203Hg2+) was applied to the olfactory chambers or was given i.v. to pike (Esox lucius) and the uptake of the metal in the olfactory system and the brain was examined by autoradiography and gamma spectrometry. Application of 203Hg2+ in the olfactory chambers resulted in an accumulation of the metal in the olfactory nerves and the anterior parts of the olfactory bulbs of the brain. The levels of 203Hg2+ in other brain areas, such as the telencephalon, the optic tecti and the cerebellum, remained low. Application of 203Hg2+ in only one olfactory chamber resulted in an uptake of the metal only in the ipsilateral olfactory nerve and olfactory bulb. Intravenous injection of the 203Hg2+ resulted in a labelling of the olfactory system and the brain, which was much lower than of the blood. These results indicate that the 203Hg2+ is taken up in the olfactory neurones from the olfactory receptor cells in the olfactory rosettes and is transported to the terminal parts of the olfactory neurones in the olfactory bulbs. The uptake of mercury as well as some other metals in the olfactory system may result in noxious effects and this may be an important component in the toxicology of metals in fish.

Effects of inhalation of cadmium on the rat olfactory system: behavior and morphology

To investigate the effects of cadmium on olfaction, two separate studies were conducted in which male adult rats were exposed to CdO, via inhalation, for 5 h per day, 5 days a week for 20 weeks. Target exposure values of 250 and 500 micrograms/m3 were measured at 200 and 325 micrograms/m3 for the low concentration in two experiments, and 550 and 660 micrograms/m3 for the high concentration. Prior to exposure, olfactory thresholds were obtained using a conditioned suppression technique. After 20 weeks of cadmium exposure, there was no evidence of anosmia in any of the rats nor were there any significant changes observed in olfactory thresholds. Although olfaction was not impaired, cadmium levels in the olfactory bulbs of exposed rats were significantly elevated compared to controls. Cardiac and respiratory histopathology were observed at all exposure levels, but there was no evidence of nasal pathology related to exposure to cadmium. Failure of cadmium to produce olfactory dysfunction may be due to the protective effects of metallothionein and/or to the highly resilient nature of the rodent olfactory system.

Uptake and transport of manganese in primary and secondary olfactory neurones in pike

gamma-spectrometry and autoradiography were used to examine the axoplasmic flow of manganese in the olfactory nerves and to study the uptake of the metal in the brain after application of 54Mn2+ in the olfactory chambers of pikes. The results show that the 54Mn2+ is taken up in the olfactory receptor cells and is transported at a constant rate along the primary olfactory neurones into the brain. The maximal velocity for the transported 54Mn2+ was 2.90 +/- 0.21 mm/hr (mean +/- S.E.) at 10 degrees, which was the temperature used in the experiments. The 54Mn2+ accumulated in the entire olfactory bulbs, although most marked in central and caudal parts. The metal was also seen to migrate into large areas of the telencephalon, apparently mainly via the secondary olfactory axons present in the medial olfactory tract. A transfer along fibres of the medial olfactory tract probably also explains the labelling which was seen in the diencephalon down to the hypothalamus. The results also showed that there is a pathway connecting the two olfactory bulbs of the pike and that this can carry the metal. Our data further showed a marked accumulation of 54Mn2+ in the meningeal epithelium and in the contents of the meningeal sacs surrounding the olfactory bulbs. It appears from our study that manganese has the ability to pass the synaptic junctions between the primary and the secondary olfactory neurones in the olfactory bulbs and to migrate along secondary olfactory pathways into the telencephalon and the diencephalon.(ABSTRACT TRUNCATED AT 250 WORDS)

Cadmium chloride (CdCl2)-induced metallothionein (MT) expression in neonatal rat primary astrocyte cultures

Metallothionein (MT) protein and mRNA levels were studied following exposure of rat neonatal primary astrocyte cultures to cadmium chloride (CdCl2). MT mRNA was probed on Northern blots with a 32P labeled synthetic cDNA probe specific for rat MT mRNA. The probe hybridizes to a single mRNA with a size appropriate for MT, approximately 550 bases. Expression of MT-I mRNA in astrocyte monolayers exposed to 2 x 10(-6) M CdCl2 for 6 h was increased approximately 5-fold (9.7 fg/micrograms total RNA) over MT-I mRNA levels in controls (2 fg/micrograms total RNA). MT-I mRNA could also be detected in untreated cells, suggesting constitutive MT expression in these cells. Western-blot analysis revealed a marked increase in MT protein levels upon exposure to CdCl2 (1 x 10(-6) M; 96 h). Consistent with the constitutive expression of MTs both at the mRNA level and protein level, we have also demonstrated a time-dependent increase in MT-immunoreactivity in astrocytes exposed to CdCl2. The present study suggests that astrocytes constitutively express MTs, and that MT-induction by CdCl2 may be an example of a generalized increase in MTs in response to heavy metal exposure, thus protecting astrocytes, and perhaps also indirectly, juxtaposed neurons from the neurotoxic effects of heavy metals.

A review of axonal transport of metals

Neurons have efficient mechanisms for the transport of organelles and chemical substances in axons to the nerve terminals and back to the cell bodies. Enzymes involved in transmitter synthesis, peptide transmitters and their precursors are examples of macromolecules that are transported down the axon, anterogradely. For final degradation and possible reuse, many constituents are transported back to the cell body, retrogradely. Retrograde transport is also a pathway by which certain toxins may bypass the blood-brain barrier and accumulate in neurons. In recent years, it has been shown that certain metals may accumulate in neurons following retrograde transport. The metals for which retrograde transport has been demonstrated include lead, cadmium and mercury. In this article recent findings regarding axonal transport of metals are reviewed. The putative mechanisms involved in the uptake of metals into the nerve terminal and the fate of metals in the cell body are outlined. Axonal transport of metals as a possible etiological factor in diseases of the human nervous system is discussed.