Degeneration and gliosis in rat retina and central nervous system following 3,3′-iminodipropionitrile exposure

3,3'-Iminodipropionitrile (IDPN) exposure causes a neurofilamentous axonopathy and olfactory, audiovestibular and visual toxicity. Many events relevant to these effects and the neurotoxic properties of nitriles as a class remain to be elucidated. We characterized the gliosis associated with the IDPN-induced retinal degeneration in comparison to other effects on the visual and central nervous systems. Gliosis was quantified using an ELISA for the intermediate filament protein, glial fibrillary acidic protein (GFAP). IDPN (0-400 mg kg-1 day-1x3 days, i.p.) caused corneal opacity and dose- and time-dependent increases in retinal GFAP, up to 26-28 fold of control values at 4 weeks post-exposure; a second peak occurred at 16 weeks. In contrast, GFAP peaked at 1 week in olfactory bulbs (OB), cingulate cortex and hippocampus. Cerebellum and striatum showed no gliosis. Retinal dopamine decreased within 2 weeks. Delayed GFAP increases occurred in superior and inferior colliculi. Retina and superior colliculi also showed increased [3H]PK-11195 binding. Histological analysis demonstrated progressive degeneration and gliosis in retina and colliculi. Taken together, the data indicate that primary and secondary degenerative events occur in the retina, and that this retinal degeneration induces GFAP increases in retina and superior colliculus. In addition, GFAP assays demonstrated that the retinal toxicity of IDPN is enhanced by CCl4 hepatotoxicity and blocked by methimazole inhibition of flavin-mono-oxygenases, similarly to its ototoxicity. GFAP assays also indicated that neither vestibulotoxic doses of crotononitrile nor olfatotoxic doses of dichlobenil damage the retina. The data support the use of GFAP assays for assessing the retinal toxicity of IDPN and other nitriles.     

Protection by 2-deoxy-D-glucose against beta,beta'-iminodipropionitrile-induced neurobehavioral toxicity in mice.

This investigation was undertaken to study the effect of 2-deoxy-D-glucose (2-DG) on beta, beta'-iminodipropionitrile (IDPN)-induced neurobehavioral toxicity in mice. Animals were divided into five groups of nine animals each. One of the groups served as control and received vehicle only, whereas the remaining four groups were treated with IDPN (250 mg/kg, i.p.) daily for 11 days. 2-DG was injected intraperitoneally in the doses of 0 (vehicle only), 100, 300, and 600 mg/kg daily 30 min before IDPN administration. The animals were observed for dyskinetic behavior including vertical (retrocollis) and horizontal (laterocollis) head movements and circling. Twenty-four hours after the last dose of IDPN, the animals were sacrificed by decapitation and striata were isolated from the brain for the analysis of serotonin (5-HT). Our results showed that 2-DG significantly and dose dependently attenuated the incidence and severity of IDPN-induced neurobehavioral toxicity. Administration of 2-DG also protected mice against IDPN-induced increase in striatal 5-HT levels. Further studies are warranted to investigate the neuroprotective mechanism of 2-DG against IDPN-induced neurotoxicity.     

D1 but not D4 Dopamine Receptors are Critical for MDMA-Induced Neurotoxicity in Mice

MDMA, an addictive psychostimulant-consumed worldwide, has the ability to induce neurotoxic effects and addiction in laboratory animals and in humans through its effects on monoaminergic systems. MDMA-induced neurotoxicity in mice occurs primarily in dopaminergic neurons and does not significantly affect the serotonergic system. As the neurotoxic effects of MDMA in mice involve excessive dopamine (DA) release, DA receptors are highly likely to play a role in MDMA neurotoxicity, but the specific dopamine receptor subtypes involved have not previously been determined definitively. In this study, dopamine D1 and D4 receptor knock-out mice (D1R^?/? and D4R^?/?) were used to determine whether these receptors are involved in MDMA neurotoxicity. D1R inactivation attenuated MDMA-induced hyperthermia, decreased the reduction of dopamine and dopamine metabolite levels, and protected against dopamine terminal loss and reactive astrogliosis as determined in the striatum, 7 days after MDMA treatment. In sharp contrast, inactivation of D4R did not prevent hyperthermia or the neurotoxic effects of MDMA. Altogether, these results indicate that D1R, but not D4R, plays a significant role in the dopaminergic striatal neurotoxicity observed after exposure to MDMA.     

Axonal degeneration and axonal caliber alterations following combined beta,beta'-iminodipropionitrile (IDPN) and acrylamide administration

A new model of neurofilamentous axonal abnormality is described which employs combined administration of beta,beta'-iminodipropionitrile (IDPN) and acrylamide (AC). The model was developed to test the hypothesis that IDPN-induced swelling increases the vulnerability of the distal axon to a second neurotoxic chemical insult. Rats were given a single intraperitoneal (IP) injection of IDPN (1.5 g/kg) one week before receiving a single injection of AC (75 mg/kg, IP). Axonal degeneration was observed at multiple levels along the sciatic nerve at two weeks (with reference to IDPN administration), and was not progressive up to five weeks. Quantitation of degenerating fibers demonstrated that the extent of degeneration increased distally along the sciatic nerve. Single administration of either IDPN or AC did not produce degeneration. Thus, IDPN-induced neurofilamentous swellings alter the susceptibility of the axon to AC neurotoxicity. Two variations of this model were also studied. First, rats given five daily injections of AC (30 mg/kg, IP) beginning one week following IDPN administration developed accumulations of fast axonally transported materials in IDPN-induced microtubule channels. Second, rats given chronic injections of AC (30 mg/kg, IP, five days/week, for four weeks), to reduce the delivery of neurofilaments to the proximal axon, developed less prominent axonal enlargements when challenged with IDPN. Thus, axonal atrophy can mask the development of neurofilamentous axonal swellings.     

Evidence of zinc protection against 2,5-hexanedione neurotoxicity: correlation of neurobehavioral testing with biomarkers of excretion

Risk prevention of human exposure against n-hexane neurotoxicity is relevant towards the protective measures to be proposed in occupational toxicology. Metabolic studies have identified 2,5-hexanedione (2,5-HD) as the main neurotoxic metabolite of n-hexane, which reacts with amino groups of lysine in axonal neurofilaments forming 2,5-dimethylpyrrole adducts, which are responsible for n-hexane neurotoxicity. In the present study, we have investigated the interaction of zinc with 2,5-HD, by correlating the decrease of pyrrole derivatives excretion with changes of neurobehavioral effects. Two subchronic experiments (11 and 8 weeks of exposure) were performed in Wistar rats exposed to different doses of 2,5-HD (200, 400 mg/kg per day) and to the mixture of 2,5-HD + zinc acetate (200 + 300 mg/kg per day) and (400 + 500 mg/kg per day). The results obtained show a significant increase in the excretion of pyrroles in the groups exposed to 2,5-HD alone as compared to controls, and a significant decrease in the excretion of pyrrole derivatives in the groups of rats co-exposed to 2,5-HD + zinc acetate when compared to the rats exposed to 2,5-HD alone. These biochemical changes were immediately evident after the first day of exposure. Simultaneously, neurobehavioral testing (rearing and ambulation in open field) was performed weekly in the same groups of rats. The results demonstrated a significant decrease in neurobehavioral dysfunction in rats co-exposed to 2,5-HD and zinc acetate. At the end of the exposure period, pyrroles levels returned to control values progressively, and the recovery of the neurotoxic effects was gradually established depending on the dose of exposure. The results suggest that zinc is a potential chemo-protector against 2,5-HD neurotoxicity which was identified by neurobehavioral testing. Moreover, pyrrole derivatives are good predictive biochemical biomarkers of 2,5-HD exposure and could be used as a complementary tool to characterize its neurotoxic effects.     

Pathology of the rat vestibular sensory epithelia during subchronic 3,3'-iminodipropionitrile exposure: hair cells may not be the primary target of toxicity

3,3'-Iminodipropionitrile (IDPN) is a neurotoxic compound that causes proximal neurofilamentous axonopathy and loss of the vestibular sensory hair cells. During subchronic exposure, the hair cells are eliminated by extrusion of the virtually intact cell from the sensory epithelia towards the luminal cavity. We describe the alterations of the vestibular epithelia before and during hair cell extrusion. Adult male Long-Evans rats were exposed to 0.2% IDPN in the drinking water for 1, 3, 5, 8 or 14 weeks, or to 0.1% IDPN for 14 weeks. Protrusion and subsequent extrusion of hair cells were observed in the cristae and utricular maculae of rats exposed to 0.2% IDPN for at least 5 weeks. At earlier time points and at lower doses, we observed the following pathological signs: blebbing of hair cells, swelling, retraction and fragmentation of the afferent nerve terminals, detachment of hair cells from the surrounding structures and loss of the pre- and post-synaptic membrane thickenings between hair cells and their afferent terminals. Widespread enlargement of the intercellular spaces also preceded and accompanied the extrusion process. The present data challenge the hypothesis that IDPN specifically affects hair cells.     

Potentiation of 3-hydroxyglutarate neurotoxicity following induction of astrocytic iNOS in neonatal rat hippocampal cultures

Neuronal damage in glutaryl-CoA dehydrogenase deficiency (GDD) has previously been addressed to N-methyl-D-aspartate (NMDA) receptor-mediated neurotoxicity of the accumulating neurotoxic metabolite 3-hydroxyglutarate. However, acute encephalopathic crises in GDD patients are typically precipitated by febrile illness or even routine vaccinations, suggesting a potentiating role of inflammatory cytokines. In the present study we investigated the effect of interleukin-1beta and interferon-gamma on 3-hydroxyglutarate toxicity in rat cortical astrocyte cultures and neonatal rat hippocampal cultures. A cotreatment of both culture systems with interleukin-1beta and interferon-gamma induced the protein expression of astrocytic inducible nitric oxide synthase (iNOS), resulting in increased nitric oxide (NO) production. Cytokine pretreatment alone had no effect on cell viability but potentiated 3-hydroxyglutarate neurotoxicity. NOS inhibition by aminoguanidine and L-NAME prevented an iNOS-mediated potentiation of 3-hydroxyglutarate neurotoxicity but failed to protect neurons against 3-hydroxyglutarate alone. In contrast, superoxide dismutase/catalase as well as MK-801 prevented toxicity of 3-hydroxyglutarate alone as well as its potentiation by iNOS, supporting a central role of NMDA receptor stimulation with subsequently increased superoxide anion production. It is concluded that the potentiation of 3-hydroxyglutarate neurotoxicity is most probably due to an induction of astrocytic iNOS and concomitantly increased NO production, enabling enhanced peroxynitrite formation. Thus, we provide evidence for a neuroimmunological approach to the precipitation of acute encephalopathic crises in GDD by inflammatory cytokines.     

Methamphetamine Induces Long-Term Alterations in Reactivity to Environmental Stimuli: Correlation with Dopaminergic and Serotonergic Toxicity

Methamphetamine (METH) abuse is known to induce persistent cognitive and behavioral abnormalities, in association with alterations in serotonin (5-HT) and dopamine (DA) systems, yet the neurobiological mechanisms underpinning this link are elusive. Thus, in the present study we analyzed the long-term impact of an acute toxic regimen of METH (4 mg/kg, subcutaneous × 4 injections, 2 h apart) on the reactivity of adult male rats to environmental stimuli, and correlated it to toxicity on 5-HT and DA innervations. Two separate groups of METH-injected rats were compared to their saline-treated controls on object exploration and startle paradigms, at either 1 or 3 weeks after METH administration, respectively. Twenty-four hours after behavioral testing, animals were sacrificed, and the neurotoxic effects of the METH schedule on DA and 5-HT terminals were measured through immunochemical quantification of their transporters (DAT and 5-HTT). At both 1 and 3 weeks after treatment, METH-injected rats exhibited a significant decline in the number of exploratory approaches to unfamiliar objects, which was significantly correlated with a parallel reduction in DAT immunoreactivity (IR) in the nucleus accumbens (NAc) core. Furthermore, METH-treated rats displayed a significant enhancement in startle magnitude after 3 (but not 1) weeks, which was inversely correlated with a decrement in 5-HTT IR in the Cg3 infralimbic area of prefrontal cortex. Our results suggest that METH induces long-term changes in object exploration and startle responsiveness, which may be respectively underpinned by reductions in DAergic and 5-HTergic brain terminals.     

Regulation of aberrant neurofilament phosphorylation in neuronal perikarya. III. Alterations following single and continuous beta, beta'-iminodipropionitrile administrations.

beta,beta'-Iminodipropionitrile (IDPN) administration produces giant neurofilament-filled axonal swellings in the first proximal internodes of large myelinated sensory and motor fibers without any accompanying axonal degeneration. In the present study, we asked whether proximal giant axonal swellings are sufficient to elicit aberrant neurofilament (NF) phosphorylation in neuronal perikarya. Rats were given a single intraperitoneal (i.p.) injection of IDPN (2 g/kg) followed by IDPN (0.1%) in the drinking water (continuous IDPN exposure) or tap water (single IDPN exposure) for two days to 7 weeks. Immunoreactivity to phosphorylated NF (pNF) epitopes (using monoclonal antibodies 6-17 and 7-05) was observed in L4 and L5 dorsal root ganglia (DRG) neurons beginning between one and 5 days, corresponding to the development of proximal giant axonal swellings. Quantitation of DRG neurons demonstrated maximal numbers of immunoreactive cell bodies to pNF epitopes (46-51%) by one week. The number of immunostained DRG cells was maintained in animals given continuous IDPN exposure, but declined significantly (P less than 0.001) in rats given a single injection of IDPN to 26 +/- 0.80% and 6 +/- 0.04% at 3 and 5 weeks, respectively. Ventral and dorsal root fibers, which undergo axonal atrophy distal to axonal swellings, showed intense immunoreactivity to pNF epitopes and a marked reduction or a complete lack of immunostaining to antibody 2-135 (directed against non-phosphorylated NF epitopes); pretreatment with alkaline phosphatase reversed this staining pattern. In a separate study, a similar alkaline phosphatase-sensitive lack of staining to antibody 2-135 was also observed in atrophic motor fibers in the DRG 4 weeks following nerve crush. It is suggested that aberrant NF phosphorylation in DRG neuronal cell bodies from IDPN-treated rats arises secondarily to an alteration in a retrogradely transported 'trophic' signal(s) to the neuron due to the presence of giant axonal swellings. Furthermore, pNFs in atrophic axons may correspond to stationary or slowly moving NFs in the axoplasm.     

Distal effects in a model of proximal axonopathy: 3,3'-iminodipropionitrile causes specific loss of neurofilaments in rat vestibular afferent endings

3,3'-Iminodipropionitrile (IDPN) is a neurotoxic compound that causes both a proximal neurofilamentous axonopathy and loss of the vestibular sensory hair cells. We used immunocytochemistry to examine changes in the expression of heavy, medium and light neurofilament (NF-H, NF-M, NF-L) proteins in the afferent terminals of vestibular sensory epithelia after IDPN exposure in rats. Acute, repeated and subchronic IDPN exposure induced a marked loss of NFs in the nerve terminals. The effect of subchronic IDPN was specific, as demonstrated by comparison with the synaptic membrane protein SNAP-25. In addition, Western blot analysis indicated specific loss of NFs in the vestibular receptors. Ultrastructural analysis revealed that afferent endings in the vestibular receptors were significantly preserved in animals exposed to subchronic IDPN, but that these endings showed NF segregation from microtubules followed by NF loss. These effects were closely paralleled by ultrastructural changes in the nerve terminals, particularly in the afferent contacts with the hair cells, and preceded hair cell loss. Thus, distal NF loss and nerve terminal pathology occur in the IDPN model of proximal neurofilamentous axonopathy. Similar distal pathology could also occur in human diseases characterized by proximal axonal swellings, particularly in amyotrophic lateral sclerosis.     

Behavioural disturbances and sensory pathology following allylnitrile exposure in rats

Animals exposed to allylnitrile develop permanent abnormalities in motor behaviour, similar to those caused by 3,3'-iminodipropionitrile (IDPN) and crotononitrile. IDPN and crotononitrile effects have been attributed to vestibular hair cell degeneration, but allylnitrile has been suggested to modify behaviour through neuronal degeneration in the CNS. Adult male Long-Evans rats were exposed to allylnitrile (0, 20, 40, 60 mg/kg per day, for 3 days) and the changes in rearing activity and rating scores in tests of vestibular function were assessed. Surface preparations of the vestibular sensory epithelia and the organ of Corti were observed for hair cell loss by scanning electron microscopy. Corneal transparency and concentrations in retina and olfactory bulbs of glial fibrillary acidic protein (GFAP), a marker for reactive gliosis, were also determined, as they are known targets of IDPN toxicity. In a dose-dependent manner, allylnitrile caused corneal opacity and gliosis in the retina and olfactory bulbs, decreased rearing activity and increased the rating scores in tests of vestibular dysfunction, and induced hair cell loss in both the vestibular sensory epithelia and the organ of Corti. The behavioural deficits correlated well with the loss of vestibular hair cells. We conclude that allylnitrile causes permanent modifications in behaviour by loss of vestibular function as IDPN and crotononitrile do and that all these chemicals share other toxic targets, such as the cornea, the retina, and the olfactory system. Data reported here and elsewhere indicate that a number of nitriles show similar neurotoxic properties.     

Exposure of mice to a predator odor increases acoustic startle but does not disrupt the rewarding properties of VTA intracranial self-stimulation

The present investigation assessed the propensity of an acute psychogenic stressor exposure to induce behavioral change in paradigms assessing fear/anxiety (acoustic startle) and motivation/anhedonia (intracranial self-stimulation) in CD-1 mice. In the acoustic startle paradigm, a 10-min exposure of 2-4 month old mice (young adult mice) to fox odor (2,5-dihydro-2,4,5-trimethylthiazoline; TMT) was associated with decreased acoustic startle relative to mice exposed to the control odor, butyric acid (BA), immediately and relative to both saline and BA exposure 24 h following odor exposure in the home cage. In contrast, a 2-min exposure of young adult mice to TMT was associated with an increase in startle relative to saline and BA during the immediate post-odor test session only. In young adult mice a 2-min and a 10-min exposure to BA resulted in a startle profile of mice reminiscent of saline-treated mice. In comparison to young adult mice, a 2-min exposure of mature adult mice (5-7 months old) to TMT enhanced startle for up to 48 h relative to both saline and BA, while a 10-min exposure of mature adult mice to TMT enhanced startle for 168 h post-odor exposure relative to saline-exposed mice only. However, the greatest increase in startle amplitude (i.e. 48 h) was acquired following the 2-min exposure of mature mice to TMT. Among mature adult mice, a 10-min exposure to BA in the home cage eventuated in enhanced startle relative to saline-exposed animals 168 h following odor exposure. In comparison, exposure of mice to 10 min of TMT depressed responding for VTA brain stimulation at the initial 80 Hz frequency, but was ineffective in elevating reward thresholds relative to mice merely exposed to saline. Mice assessed in the ICSS paradigm were approximately 2-4 months old at the time of surgery and 5-7 months old at the completion of testing. These data suggest that acute odor exposure may induce a fear gradient dependent upon the perceived stressor severity and that the resultant anxiety-like effects are dependent on the duration of odor exposure, age of the animals and the temporal interval between odor presentation and behavioral testing. Moreover, the anxiogenic properties of psychogenic stressors can be separated from their anhedonic effects. The implications of these data for clinical psychopathology are discussed.     

Altered prolactin response to M-chlorophenylpiperazine in monkeys previously treated with 3,4-methylenedioxymethamphetamine (MDMA) or fenfluramine

3,4-Methylenedioxymethamphetamine ("Ecstasy," MDMA) and fenfluramine, widely used by humans, are potent brain serotonin (5-HT) neurotoxins in animals. Thus, there is concern that humans previously exposed to these amphetamine derivatives may have incurred brain 5-HT neurotoxicity. However, assessing the status of brain 5-HT neurons in the living organism is challenging. To determine whether MDMA- and/or fenfluramine-induced 5-HT neurotoxicity can be detected during life using neuroendocrine methods, groups of monkeys previously treated with neurotoxic regimens of MDMA or fenfluramine, along with saline-treated controls, underwent neuroendocrine challenge with the direct 5-HT agonist and 5-HT-releasing drug, m-chlorophenylpiperazine (m-CPP). Animals treated 2 weeks previously with MDMA exhibited a nonsignificant reduction in the prolactin response to m-CPP. In contrast, monkeys treated 3 1/2 years previously with MDMA or 2 years previously with fenfluramine exhibited significantly increased prolactin responses to m-CPP. No significant differences in cortisol concentrations were noted between groups at any time point. These data indicate that neuroendocrine challenge with m-CPP is capable of detecting substituted amphetamine-induced 5-HT neurotoxicity in living primates, but that the recency of drug exposure is an important consideration. Changes in the neuroendocrine response to m-CPP over time in animals with substituted amphetamine-induced neurotoxicity may be related to aberrant 5-HT reinnervation of the basal forebrain that occurs over time in monkeys previously treated with neurotoxic doses of MDMA or fenfluramine.     

Persistent behavioral alterations in rats neonatally exposed to low doses of the organophosphate pesticide, parathion

Although developmental exposures of rats to low levels of the organophosphate pesticides (OPs), chlorpyrifos (CPF) or diazinon (DZN), both cause persistent neurobehavioral effects, there are important differences in their neurotoxicity. The current study extended investigation to parathion (PTN), an OP that has higher systemic toxicity than either CPF or DZN. We gave PTN on postnatal days (PND) 1-4 at doses spanning the threshold for systemic toxicity (0, 0.1 or 0.2 mg/kg/day, s.c.) and performed a battery of emotional and cognitive behavioral tests in adolescence through adulthood. The higher PTN dose increased time spent on the open arms and the number of center crossings in the plus maze, indicating greater risk-taking and overall activity. This group also showed a decrease in tactile startle response without altering prepulse inhibition, indicating a blunted acute sensorimotor reaction without alteration in sensorimotor plasticity. T-maze spontaneous alternation, novelty-suppressed feeding, preference for sweetened chocolate milk, and locomotor activity were not significantly affected by neonatal PTN exposure. During radial-arm maze acquisition, rats given the lower PTN dose committed fewer errors compared to controls and displayed lower sensitivity to the amnestic effects of the NMDA receptor blocker, dizocilpine. No PTN effects were observed with regard to the sensitivity to blockade of muscarinic and nicotinic cholinergic receptors, or serotonin 5HT(2) receptors. This study shows that neonatal PTN exposure evokes long-term changes in behavior, but the effects are less severe, and in some incidences opposite in nature, to those seen earlier for CPF or DZN, findings consistent with our neurochemical studies showing different patterns of effects and less neurotoxic damage with PTN. Our results reinforce the conclusion that low dose exposure to different OPs can have quite different neurotoxic effects, obviously unconnected to their shared property as cholinesterase inhibitors.     

Behavioral toxicity of trialkyltin compounds: a review

Triethyltin (TET) and trimethyltin (TMT) are neurotoxic organotin compounds which produce different patterns of toxicity in adult animals. Exposure to TET produces behavioral toxicity (decreased motor activity, grip strength, operant response rate and startle response amplitude) which reflects impaired neuromotor function. These deficits are consistent with the reported myelin vacuolation and cerebral edema produced by TET, and with its direct effects on muscle. Exposure to TMT produces both hyperactivity and impaired learning and performance. These impairments are consistent with reported neuronal cell death produced by TMT, particularly in limbic system structures. While the behavioral deficits produced by repeated exposure to TET are reversible when dosing is terminated, the behavioral impairments produced by a single exposure to TMT appears to be irreversible.     

Methamphetamine neurotoxicity increases brain expression and alters behavioral functions of CB1 cannabinoid receptors

Cannabis is the most common secondary illicit substance in methamphetamine (METH) users, yet the outcomes of the concurrent consumption of both substances remain elusive. Capitalizing on recent findings on the implication of CB₁ cannabinoid receptors in the behavioral effects of METH, we hypothesized that METH-induced neurotoxicity may alter the brain expression of CB₁, thereby affecting its role in behavioral functions. To test this possibility, we subjected rats to a well-characterized model of METH neurotoxicity (4 mg/kg, subcutaneous × 4 injections, 2 h apart), and analyzed their CB₁ receptor brain expression three weeks later. METH exposure resulted in significant enhancements of CB₁ receptor expression across several brain regions, including prefrontal cortex, caudate-putamen, basolateral amygdala, CA1 hippocampal region and perirhinal cortex. In parallel, a different group of METH-exposed rats was used to explore the responsiveness to the potent cannabinoid agonist WIN 55,212-2 (WIN) (0.5-1 mg/kg, intraperitoneal), within several paradigms for the assessment of emotional and cognitive functions, such as open field, object exploration and recognition, and startle reflex. WIN induced anxiolytic-like effects in METH-exposed rats and anxiogenic-like effects in saline-treated controls. Furthermore, METH-exposed animals exhibited a significantly lower impact of WIN on the attenuation of exploratory behaviors and short-term (90 min) recognition memory. Conversely, METH neurotoxicity did not significantly affect WIN-induced reductions in locomotor activity, exploration time and acoustic startle. These results suggest that METH neurotoxicity may alter the vulnerability to select behavioral effects of cannabis, by inducing distinct regional variations in the expression of CB₁ receptors.     

Depletion of serotonin using p-chlorophenylalanine (PCPA) and reserpine protects against the neurotoxic effects of p-chloroamphetamine (PCA) in the brain

The present study attempts to determine whether the neurotoxicity of p-chloroamphetamine (PCA) is dependent on a releasable pool of serotonin (5-HT). Rats treated with PCA alone or with reserpine and PCA exhibit a profound loss of 5-HT innervation in cerebral cortex after a 2-week survival period. However, depletion of 5-HT by combined treatment with p-chlorophenylalanine (PCPA) and reserpine provides substantial protection against the neurotoxic effects of PCA. These results indicate that release of 5-HT is a necessary step in the neurotoxicity of PCA and that a peripheral source of 5-HT is involved. We suggest that 5-HT release from platelets into the peripheral circulation may result in the formation of a neurotoxic 5-HT metabolite.     

Remodeling of cholinergic input to the hippocampus after noise exposure and tinnitus induction in Guinea pigs

Here, we investigate remodeling of hippocampal cholinergic inputs after noise exposure and determine the relevance of these changes to tinnitus. To assess the effects of noise exposure on the hippocampus, guinea pigs were exposed to unilateral noise for 2 hr and 2 weeks later, immunohistochemistry was performed on hippocampal sections to examine vesicular acetylcholine transporter (VAChT) expression. To evaluate whether the changes in VAChT were relevant to tinnitus, another group of animals was exposed to the same noise band twice to induce tinnitus, which was assessed using gap‐prepulse Inhibition of the acoustic startle (GPIAS) 12 weeks after the first noise exposure, followed by immunohistochemistry. Acoustic Brainstem Response (ABR) thresholds were elevated immediately after noise exposure for all experimental animals but returned to baseline levels several days after noise exposure. ABR wave I amplitude‐intensity functions did not show any changes after 2 or 12 weeks of recovery compared to baseline levels. In animals assessed 2‐weeks following noise‐exposure, hippocampal VAChT puncta density decreased on both sides of the brain by 20–60% in exposed animals. By 12 weeks following the initial noise exposure, changes in VAChT puncta density largely recovered to baseline levels in exposed animals that did not develop tinnitus, but remained diminished in animals that developed tinnitus. These tinnitus‐specific changes were particularly prominent in hippocampal synapse‐rich layers of the dentate gyrus and areas CA3 and CA1, and VAChT density in these regions negatively correlated with tinnitus severity. The robust changes in VAChT labeling in the hippocampus 2 weeks after noise exposure suggest involvement of this circuitry in auditory processing. After chronic tinnitus induction, tinnitus‐specific changes occurred in synapse‐rich layers of the hippocampus, suggesting that synaptic processing in the hippocampus may play an important role in the pathophysiology of tinnitus.     

Persistent behavioral alterations in rats neonatally exposed to low doses of the organophosphate pesticide,parathion

Although developmental exposures of rats to low levels of the organophosphate pesticides (OPs), chlorpyrifos (CPF) or diazinon (DZN), both cause persistent neurobehavioral effects, there are important differences in their neurotoxicity. The current study extended investigation to parathion (PTN), an OP that has higher systemic toxicity than either CPF or DZN. We gave PTN on postnatal days (PND) 1–4 at doses spanning the threshold for systemic toxicity (0, 0.1 or 0.2 mg/kg/day, s.c.) and performed a battery of emotional and cognitive behavioral tests in adolescence through adulthood. The higher PTN dose increased time spent on the open arms and the number of center crossings in the plus maze, indicating greater risk-taking and overall activity. This group also showed a decrease in tactile startle response without altering prepulse inhibition, indicating a blunted acute sensorimotor reaction without alteration in sensorimotor plasticity. T-maze spontaneous alternation, novelty-suppressed feeding, preference for sweetened chocolate milk, and locomotor activity were not significantly affected by neonatal PTN exposure. During radial-arm maze acquisition, rats given the lower PTN dose committed fewer errors compared to controls and displayed lower sensitivity to the amnestic effects of the NMDA receptor blocker, dizocilpine. No PTN effects were observed with regard to the sensitivity to blockade of muscarinic and nicotinic cholinergic receptors, or serotonin 5HT₂ receptors. This study shows that neonatal PTN exposure evokes long-term changes in behavior, but the effects are less severe, and in some incidences opposite in nature, to those seen earlier for CPF or DZN, findings consistent with our neurochemical studies showing different patterns of effects and less neurotoxic damage with PTN. Our results reinforce the conclusion that low dose exposure to different OPs can have quite different neurotoxic effects, obviously unconnected to their shared property as cholinesterase inhibitors.     

d-MDMA during vitamin E deficiency: effects on dopaminergic neurotoxicity and hepatotoxicity

The mechanism of 3,4-methylenedioxymethamphetamine (d-MDMA)-induced neurotoxicity may involve formation of toxic radical species. Endogenous defenses against toxic radical species include tissue stores of vitamin E, and thiols. We examined whether vitamin E deficiency could alter d-MDMA-induced neurotoxicity by administration of the drug to animals with diet induced vitamin E deficiency. Brain vitamin E levels in deficient mice were reduced 75% compared to sufficient animals. Animals received d-MDMA 5 or 10 mg/kg or saline (delivered every 2 hx4, s.c.). Diet slightly altered d-MDMA-induced temperature modulation. In brain, MDMA treatment reduced vitamin E, total antioxidant reserve and protein thiols 72 h after the first dose. In liver, MDMA treatment reduced glutathione and total antioxidant reserve at the same time point. The vitamin E-deficient group, treated with the low dose of d-MDMA, exhibited neurotoxic responses, including reduced striatal dopamine (47%) and elevated GFAP protein (3-fold): while the sufficient diet group was not altered. The higher d-MDMA dose caused neurotoxic responses in both diet groups. Liver toxicity was determined by histopathologic examination. d-MDMA caused hepatic necrosis that was more severe in vitamin E deficient than sufficient mice. These data indicate that (1) d-MDMA administration reduces antioxidant measures at a time coincident with d-MDMA-induced neuronal damage and (2) vitamin E deficiency increases susceptibility to d-MDMA-induced neurotoxicity and hepatic necrosis.