Toxicity of mild prenatal carbon monoxide exposure.

Rats prenatally exposed to a low concentration of carbon monoxide which results in carboxyhemoglobin levels equivalent to those maintained by human cigarette smokers, show reduced birth weight and decreased weight gain. Neuro-behavioral and biochemical testing of the offspring reveals lower behavioral activity levels through the preweaning period, altered central catecholamine activity, and reduction in total brain protein at birth.

Chronic lead administration in neonatal rats: electron microscopy of the retina.

The morphologic effects on the retina resulting from chronic lead exposure were assessed in neonatal rats. Newborn rats nursed from dams were given a low (0.115%) or a high (4.5%) concentration of lead in their diet. At day 21 the pups were weaned to the mother's diet. The retinas of the pups were studied by electron microscopy at various ages up to day 60. High and low lead concentrations produced necrosis of photoreceptor cells and cells of the inner nuclear layer. The high lead concentration, in addition, was associated with swelling of endothelial cells of the retinal vessels and narrowing of the lumen. Increased permeability of the retinal vessels and pigment epithelium to horseradish peroxidase was also observed under the high-dose condition. The authors conclude that lead can produce direct neuronal damage and, at high doses, produces retinal vascular lesions and alteration of the blood-retinal barrier.

Adenosine receptor interactions and anxiolytics.

[3H]-N6-cyclohexyladenosine and [3H]-1,3-diethyl-8-phenylxanthine label the A1 subtype of adenosine receptor in brain membranes. The affinities of methylxanthines in competing for A1 adenosine receptors parallel their potencies as locomotor stimulants. The adenosine agonist N6-(phenylisopropyl) adenosine is a potent locomotor depressant. Both diazepam and N6-(L-phenylisopropyl)adenosine cause locomotor stimulation in a narrow range of subdepressant doses. Combined stimulant doses of the two agents depress motor activity, as do larger doses of either one, given separately. Evidence supporting and against the hypothesis that some of the actions of benzodiazepines are mediated via the adenosine system is reviewed. A number of compounds interact with both systems, probably because of physico-chemical similarities between adenosine and diazepam. It is concluded that of the four classic actions of benzodiazepines, the sedative and muscle relaxant (but not anxiolytic or anticonvulsant) actions could possibly be mediated by adenosine.

Electrical self-stimulation of the brain: a model for the behavioral evaluation of toxic agents.

Rats implanted chronically with electrodes in the posterior lateral hypothalamus were trained to press levers in order to stimulate the brain electrically. Brief exposures to low oxygen concentrations reduced the lever pressing rate proportionately with the reduction in inspired oxygen. Similar reductions in self-stimulation rates could be observed in animals exposed to carbon monoxide or the organic solvent, trichloroethylene. Prolonged exposures of animals to hypoxia in chambers where self-stimulation rates as well as food and water intake via lever pressing were monitored, indicated that as oxygen concentration declined self-stimulation rates showed a marked increase for 12 hr followed by a decline. Food and water intake were depressed. This increase in self-stimulation was only observed at low (20 degrees C) ambient temperatures and was accompanied by central depletion of norepinephine. At high (30 degrees C) ambient temperatures, self-stimulation was depressed by hypoxia. The data show the importance of comparing acute with chronic exposure to toxic agents, as well as the influence of environmental temperature in influencing behavioral events. In addition, the data indicate that the self-stimulation technique offers unique advantages over behavior maintained by food or water reinforcers in evaluating toxic compounds.

Repeated exposure to diisopropylfluorophosphate (DFP) produces increased sensitivity to cholinergic antagonists in discrimination retention and reversal.

This experiment examined the effects of repeated exposure to diisopropylfluorophosphate (DFP), an organophosphate anticholinesterase, on the retention and reversal of a visual discrimination and on the number of muscarinic receptors in the brain. Rats were trained in a serial reversal procedure. After achieving stable performance, the rats were divided into two groups. One group received repeated injections of DFP, the other group received injections. To determine whether DFP-treated rats would be more sensitive than normal rats to stresses on the cholinergic system, each rat was injected with saline or one of three doses of scopolamine, a muscarinic receptor blocker, prior to testing on every 6th day. DFP alone caused no impairment in performance. Scopolamine produced a greater impairment in DFP-treated rats than in control rats. Similar results were obtained in a second behavioral task, match-to-sample in a water maze, using the same DFP treatment protocol and only one dose of scopolamine. The number of muscarinic receptors and acetylcholinesterase activity levels were reduced on the 2nd and 15th day after the end of DFP treatment. These results demonstrate that although repeated exposure to organophosphate anticholinesterases may not alter discrimination behavior directly, it may compromise the central nervous system so that it cannot react normally when challenged.

Xanthines alter behavior maintained by intracranial electrical stimulation and an operant schedule.

Caffeine and related alkylxanthines are widely used for recreation and therapeutic effects. In behavioral studies, both response rate-enhancing and attenuating effects have been described, depending upon the dose and behavioral measure used. Intracranial self-stimulation (ICSS) and differential reinforcement of low rates of responding (DRL) were assessed after rats were administered one of a range of doses of caffeine or aminophylline. These measures were chosen because of their demonstrated sensitivity to psychotropic drugs and the potential for comparing the alkylxanthine data to the extensive literature of amphetamine effects on ICSS and DRL behavior. Caffeine and aminophylline elicited dose- and drug-dependent changes in ICSS responding, and increased response rates and decreased reinforcements on the DRL schedule. These behavioral results are discussed with reference to alkylxanthine interactions with adenosine receptors.

Neonatal monosodium glutamate administration alters noradrenergic measures in the brainstem of the mouse.

Mice treated neonatally with MSG (4 mg/g) were compared to saline-injected controls on a number of neurochemical parameters of brainstem noradrenergic activity. MSG treatment resulted in an attenuation of brainstem norepinephrine (NE) decline after alpha-methyl-p-tyrosine administration. Neonatal MSG administration did not result in alterations in the steady state levels of brainstem NE or MOPEG. The synthesis of NE was slightly increased in the pons-medulla of MSG-treated mice as indexed by pargyline-induced NE accumulation. NE release, however, appeared diminished as reflected by a significant (p less than 0.05) decrease in the ratio of normetanephrine to NE found in the pons-medulla of MSG-treated mice given pargyline. The results suggest that MSG-induced damage to the arcuate nucleus produces selective alterations in brainstem NE systems. These alterations may reflect the toxic action of MSG on the opiomelanocortin neurons of the arcuate nucleus or other descending systems that are damaged by MSG. The loss of the descending opiomelanocortin input to the brainstem could result in these types of neurochemical consequences since the pharmacologic action of opiate drugs results in a selective enhancement of brainstem NE turnover in rodents.

Mechanisms of neurotoxicity and their relationship to behavioral changes.

In this review some of the evidence relating behavioral alterations induced by 2 neurotoxic chemicals, lead acetate and methyl mercury is presented with an attempt to relate these changes to the underlying neurobiological mechanisms. In the case of neonatal lead poisoning, the results of the early behavioral studies were confounded by excessive lead concentrations resulting in undernutrition of the pups. Subsequent studies in both rodents and monkeys have shown that blood-lead concentrations comparable to those seen in children can induce behavioral alterations that may be related to hippocampal damage. In the case of methyl mercury which is a potent cytotoxic agent, prenatal exposure results in widespread cortical, and cerebellar alterations characterized by reduced myelination, delayed migration and loss of neurons. These morphological alterations are accompanied by permanent alterations in learning and memory as well as altered pharmacological sensitivity in catecholaminergic systems. Recommendations are made for better formulated behavioral and neurobiological assays in neurotoxicology in order to lead to a better understanding of the toxicity of chemicals.

Regional brain and pituitary cyclic nucleotide response following central cholinergic stimulation in rats chronically exposed to lead.

Long Evans rats were exposed continuously to lead (1,000 ppm of lead acetate in the drinking water of dam or weanling) throughout gestation, lactation and until sacrifice at 60 days of age. A matched group of control animals was exposed to distilled water. On the day of sacrifice, control and lead treated animals were administered methylatropine 40 min. and oxotremorine (a muscarinic agonist) 10 min. prior to sacrifice by microwave irradiation. Locomotor activity was monitored during the 10 min. immediately preceding sacrifice. Levels of cyclic AMP and cyclic GMP were determined in 21 brain regions by radioimmunoassay. Locomotor activity following oxotremorine was significantly reduced in both lead treated and control animals. Levels of cyclic GMP were increased in septal region and frontal cortex and reduced in cerebellum and inferior colliculus of both groups of animals. The level of cyclic AMP in the pituitary increased 20-25 fold with a greater response noted in lead treated rats. Elevations of cyclic AMP were also noted in the interpeduncular nucleus, corpus striatum and substantia nigra in both groups. These data suggest that the cellular response to a muscarinic agonist was significantly affected by chronic exposure to lead in the pituitary but not in the remainder of the central nervous system.

Effects of chronic lead exposure on release of endogenous catecholamines: a preliminary communication.

The release of endogenous dopamine and norepinephrine from six brain regions (cortex, hippocampus, hypothalamus, striatum, s. nigra, n. accumbens-o. tubercle) of chronically lead-exposed rats was compared to release from controls. Prenatal and postnatal chronic lead intoxication was induced by 1000 ppm lead acetate in the drinking water of dams 70 days prior to mating and throughout gestation and lactation. Offspring were also maintained on 1000 ppm lead acetate until sacrificed at approximately 60 days of age. Release was measured in vitro from tissue minces prepared from both lead-exposed and control rats. There were no significant differences in spontaneous or potassium-stimulated norepinephrine release between lead-exposed and control rats. Potassium and amphetamine both stimulated striatal dopamine release; however, no marked differences between lead-exposed and control tissue were seen. Potassium-stimulated release of dopamine from the nucleus accumbens-olfactory tubercle area was increased in the lead-exposed animals. This is interesting in light of the postulated role of the n. accumbens in the central regulation of motor activity and the reported alterations in activity in lead-exposed rodents.

Flash evoked potentials from rat superior colliculus.

In view of reports that the superior colliculus evoked potential from rats is uniquely sensitive to toxic gases, the present study characterized normal flash evoked potentials from unanesthetized rats. The waveform was complex, with at least 5 positive and 5 negative peaks. The waveform originated in the SGS layer, and some components were stable over time if conditions of light intensity, stimulus frequency and dark adaptation were held constant. The greater complexity of the waveforms reported here compared to those described by others can be attributed to both an intense flash stimulus and unanesthetized preparation.

Carbon monoxide and flash evoked potentials from rat cortex and superior colliculus.

In view of the conflict between qualitative reports that flash evoked potentials from superior colliculus (SC) and visual cortex (VC) of rats are uniquely sensitive to low levels of carbon monoxide (CO), and a more quantitative report that the visual cortex evoked potential is not sensitive to low levels of CO, the present report documents the effects of different concentrations of CO upon flash evoked potentials from these areas. The amplitude of the P3-N4 component from the SC evoked potential demonstrated the best dose/effect relationship, increasing up to levels of 38% carboxyhemoglobin (COHb). Latencies were affected only when COHb levels reached 55%. The use of the visual system to determine effects of toxic agents upon the central nervous system in discussed.

Behavioral withdrawal following several psychoactive drugs.

The chronic administration of several psychoactive drugs has been suggested to produce behavioral withdrawal syndromes in the absence of physical withdrawal. The present study employed four representative psychoactive drugs, amphetamine, chlorpromazine, iproniazid, and desipramine, in a common behavioral paradigm using electrical stimulation of the brain to test for behavioral withdrawal. Behavior differing from both predrug and drug produced behavior occurred following the termination of amphetamine, iproniazid and chlorpromazine administration. The first two drugs produced an increase in self stimulation during administration, followed by a very significant decrease after the drugs were discontinued. Chlorpromazine administration on the other hand, produced a decrease in self stimulation rates, followed by a rebound increase after termination of treatment. No systematic effects were observed with desipramine. The relationship between the behavioral effects of these drugs during and following treatment and possible homeostatic mechanism influencing response tendencies is discussed.

Postnatal behavioral effects in mice after prenatal exposure to methylmercury.

CFW mice were injected with methylmercury hydroxide (1, 2, 3, 5 or 10 mg/kg as mercury) on Day 8 of gestation. Mice treated with 3, 5 or 10 mg/kg averaged 1/3 fewer pups than controls. Pups from these treated animals weighed less than controls and the weight differences persisted through weaning but were no longer significant at 56 days of age. Mice exposed to methylmercury in utero showed significant differences from controls in their behavior in a 2-way active avoidance shuttle box and in a punishment situation but not when tested in an open field, a water escape runaway or a conditioned suppression paradigm. Neither the mothers nor progeny of the mice exposed prenatally to methylmercury showed bahavioral deficits.

Evoked potential alterations following prenatal methyl mercury exposure.

Pregnant hooded rats were administered either 5 mg/kg CH3, Hg or 0 mg/kg CH3 Hg by gastric intubation on day seven of gestation. Female offspring were implanted with recording electrodes 60 days after birth and had their cortically recorded visual evoked potentials studied at four different flash intensities. Mercury exposed animals had higher P1-N1 and N1-P2 amplitudes and shorter P2 and N2 latencies than controls. The data provides evidence that a single ingestion of CH3 Hg by pregnant rats is sufficient to produce long term alterations in CNS activity.

Complex maze performance during carbon monoxide exposure in rats.

Most human victims of residential fires die of smoke inhalation. The cause of death of the victims is attributed to high levels of carboxyhemoglobin, but it is not clear why the victims are unable to escape even from locations remote from flaming combustion. In an attempt to provide a model of escape from toxic gases using animals, a complex maze was built for rats with 8 choice points. The animals were 24 hr water deprived and trained to remain in the start box for 15 min. Following this period, a rat was released in the maze and had to learn to avoid blind alleys and reach the goal box for water reinforcement within 15 min. Total time to traverse and total distance in the maze were recorded. Each animal was given one trial per day. After stable running times were established, different groups of six rats were exposed to 2000, 3000, 3500, and 4000 ppm of carbon monoxide (CO) when placed in the maze. Each animal was exposed to CO only once. On the day after CO exposure the rats were implanted with an arterial cannula and on the next day each animal was exposed to the same CO concentration it had previously experienced for 30 min. Blood samples were taken every 5 min. The effect of increasing CO concentrations was to increase maze running times as well as to decrease the number of animals reaching the goal. At 3500 ppm no animal reached the goal. At 2000 ppm, the animals that failed to reach the goal moved a greater distance than animals that reached the goal.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioral toxicology and risk assessment.

In order to establish safe exposure levels to toxic chemicals, risk assessment guidelines have been developed. These guidelines evaluate epidemiologic and animal research data on a particular chemical, as well as dose-response relationships, animal to human extrapolation and assessment of exposure levels of populations. Using the guidelines, risk characterization is established in order to determine a strategy for reducing undesirable risk to human populations. Using both human neonatal lead exposure data and results from rodent and primate studies, this review examines the possibility that behavioral measurements are sufficient to provide adequate risk assessment guidelines for lead intoxication of the developing organism. The overall trend in these data during the past 10 years has been to show that exposures to inorganic lead at levels previously considered safe have long-lasting significant alterations in behavioral measures, suggesting that central nervous system function has been altered irreversibly. The conclusion is drawn that behavioral toxicology can provide sensitive, quantitative and reliable data for risk assessment and that in the future these methodologies could be used to set exposure guidelines for other neurotoxic chemicals.

Evidence that exposure to methyl mercury during gestation induces behavioral and neurochemical changes in offspring of rats.

On day 15 of gestation, pregnant Sprague-Dawley rats were orally treated by gavage with 8 mg/kg of methyl mercury (MMC). At day 1 of postnatal life the levels of MMC in whole brain of exposed pups were found to be about 100 times higher than those of saline-exposed rats, while they were near to the control values at 21 days and practically normal at 60 days of age. Behavioral experiments showed that exposure to MMC in late gestation did not affect at any tested time (14, 21 and 60 days) locomotor activity or development of ultrasonic vocalization. An increased response to a challenge dose of amphetamine was, however, detected in MMC-exposed pups at day 14. This phenomenon was no longer evident at day 21 and 60 of age. In parallel, an increased density of dopamine receptors was found in the striatum at 14, but not at 21 and 60, days of age. From these data, we tentatively suggest that a high level of MMC induces a transient phenomenon of disuse-supersensitivity of the dopaminergic system. Moreover, further evidence that acute MMC exposure during prenatal life might induce permanent disturbances in learning and memory which could be partially related to a reduced functional activity of the glutamatergic system is provided.

An operant discrimination task allowing variability of reinforced response patterning.

Five pigeons were trained to perform a discrimination task allowing variability of reinforced response patterning. The task consisted of moving a stimulus light within an 4x4 matrix of lights from the top left position to the bottom right position by pecking on two keys in succession in order to obtain a reinforcement. A peck on one key moved the light one position to the right and a peck on the other key moved it one position down. After preliminary training on alternating fixed-ratio 3 schedules of reinforcement, the birds could peck on either key in any order, but more than three responses on a key resulted in a blackout followed by the return of the stimulus light to the start position. Results indicate that initially the birds used a wide variety of response patterns to obtain reinforcement, but with continued practice, response patterns became more stereotyped.