Effects of methylmercury on neurotransmitter release from rat brain synaptosomes.

Although the effects of methylmercury (MeHg) at the neuromuscular junction have been well characterized, similar studies employing CNS preparations and transmitters have been limited. We found that MeHg (0.5-5.0 microM) produced a concentration-dependent increase in the spontaneous release of [3H]dopamine. gamma-[3H]aminobutyric acid, and [3H]acetylcholine from synaptosomes isolated from rat brain striatum, cortex, and hippocampus, respectively. At these same concentrations MeHg did not attenuate calcium-dependent depolarization-evoked 3H-transmitter release. MeHg did not appear to induce calcium influx into the nerve terminal since the increase in release persists in the absence of extrasynaptosomal calcium. The increase in spontaneous transmitter release induced by MeHg persisted in the presence of low extrasynaptosomal sodium, suggesting that MeHg's effects on release are not mediated by either Na+, K+-ATPase inhibition or selective increases in membrane sodium permeability. MeHg produced only a very small increase in 45Ca efflux from synaptosomes preloaded with 45Ca, whereas these same MeHg concentrations produced large increases in 45Ca efflux from preloaded isolated mitochondria. MeHg did increase the efflux of [3H]deoxyglucose phosphate from synaptosomes. An increase in the efflux of [3H]deoxyglucose phosphate is believed to reflect an increase in neuronal membrane permeability. The quantitative and temporal aspects of the MeHg-induced [3H]-deoxyglucose phosphate efflux were similar to those observed for MeHg-induced neurotransmitter release. These data suggest that the increase in spontaneous transmitter release induced by MeHg is mainly the result of transmitter leakage that occurs subsequent to MeHg-induced increases in synaptosomal membrane permeability. However, these results cannot exclude possible effects of MeHg on intrasynaptosomal calcium homeostasis.

Anodic stripping voltammetry procedure modified for improved accuracy of blood lead analysis.

In evaluating the accuracy and reliability of blood lead (PbB) measurements with the Environmental Science Associates Model 3010A Trace Metal Analyzer, intralaboratory comparison demonstrated that use of the operating conditions recommended by the manufacturer resulted in consistently underestimated PbB concentrations less than 400 micrograms/L and overestimated PbB values greater than 400 micrograms/L. At PbB concentrations less than 50 micrograms/L, measured concentrations were often registered as negative results. However, these negative values could be replicated to within +/- 10 micrograms/L, indicating good precision of the method, but obviously not good accuracy. In addition, lower-than-expected lead (Pb) values were measured in samples containing increased concentrations of copper (Cu), such as may occur in pregnant women. We modified the procedure to eliminate these inaccuracies by substituting manual peak-height measurements for reliance on the integrator and digital display of the instrument. We established the accuracy of the modified procedure by using calibration standards previously quantified by isotope dilution-mass spectroscopy. A quality-control program for monitoring PbB analysis is also described.

Effect of in vitro inorganic lead on dopamine release from superfused rat striatal synaptosomes.

The effect of inorganic lead in vitro in several aspects of [3H]dopamine release from superfused rat striatal synaptosomes was examined. Under conditions of spontaneous release, lead (1-30 microM) induced dopamine release in a concentration-dependent manner. The onset of the lead-induced release was delayed by approximately 15-30 sec. The magnitude of dopamine release induced by lead was increased when calcium was removed from the superfusing buffer. Lead-induced release was unaffected in the presence of putative calcium, sodium, and/or potassium channel blockers (nickel, tetrodotoxin, tetraethylammonium, respectively). Depolarization-evoked dopamine release, produced by a 1-sec exposure to 61 mM potassium, was diminished at calcium concentrations below 0.254 mM. The onset of depolarization-evoked release was essentially immediate following exposure of the synaptosomes to high potassium. The combination of lead (3 or 10 microM) with high potassium reduced the magnitude of depolarization-evoked dopamine release. This depression of depolarization-evoked release by lead was greater in the presence of 0.25 mM than 2.54 mM calcium in the superfusing buffer. These findings demonstrate multiple actions of lead on synaptosomal dopamine release. Lead can induce dopamine release by yet unidentified neuronal mechanisms independent of external calcium. Lead can also reduce depolarization-evoked dopamine release by apparent competition with calcium influx at the neuronal membrane calcium channel.

Altered central monoamine response to D-amphetamine in rats chronically exposed to inorganic lead.

Investigations of the mechanisms involved in the neurotoxicity resulting from chronic inorganic lead (Pb) exposure have centered on CNS biogenic amine function on the basis of behavioral and neurochemical findings. The following study examined the time course of the response of dopamine (DA) and 5-hydroxytryptamine (5-HT) neurons to d-amphetamine (AMPH) in rats chronically exposed to Pb from birth in order to further examine neurochemical mechanisms implicated by previous work. Offspring were exposed to 0.2% Pb acetate via the lactating dam and then weaned to the same drinking solution. At 120-140 days animals were injected with 1.0 mg/kg s.c. of the drug or with saline and sacrificed after various intervals. DA content in nucleus accumbens and corpus striatum in Pb-exposed animals was significantly higher than corresponding levels in controls at 20 minutes post-drug and remained significantly higher than baseline values at 80 minutes after the drug when DA concentrations in controls had returned to normal. These data suggest enhanced AMPH-induced DA synthesis in exposed rats. 5-Hydroxyindoleacetic acid (5-HIAA) content was significantly increased in three brain regions in exposed rats given AMPH compared to values in saline-injected exposed animals, indicating a compensation in these areas for the decreases in 5-HIAA values produced by Pb exposure alone. The results of this study reinforce the hypothesis that DA and 5-HT neurons are sensitive to relatively low levels of Pb exposure.

Influence of chronic inorganic lead exposure on regional dopamine and 5-hydroxytryptamine turnover in rat brain.

The results of previous behavioral studies utilizing chronic exposure to low amounts of inorganic lead (Pb) have suggested alterations in the function of biogenic amine neuronal systems. The following study was performed to provide evidence for the possible bases of these changes in pharmacological responsiveness in exposed animals. Dams were administered 0.2% Pb acetate in drinking water to expose their offspring to Pb via the maternal milk. Males were weaned to the same drinking solution. At 120-140 days a tracer dose of 1.0 mCi L-[3H]2,6-tyrosine (3H-TYR) and 0.5 mCi L-[3H(G)]tryptophan (3H-TRP) was injected through an indwelling jugular catheter, and norepinephrine (NE), dopamine (DA), 5-hydroxytryptamine (5-HT) and their respective precursors and metabolites were quantified by liquid chromatography with electrochemical detection with column eluate collected for liquid scintillation counting. At this level of exposure (blood lead (PbB) at day 90 in exposed animals = 43.1 +/- 1.7 micrograms/dl) no changes were observed in concentration of NE or DA or DA metabolites in any brain region. However, DA turnover was decreased in Pb-exposed animals in nucleus accumbens and frontal cortex. No changes in 5-HT content and turnover were observed in any brain region, but 5-hydroxyindoleacetic acid (5-HIAA) levels were decreased in 6 of the 9 brain regions examined. These findings are consistent with observations of an attenuated behavioral responsiveness to d-amphetamine (AMPH) in exposed animals, and suggest that the changes in DA and 5-HT neurons noted by other workers at higher levels of exposure persist when PbBs are in the range of 40 micrograms/dl.

Determination of gamma-aminobutyric and glutamic acids in rat brain by liquid chromatography with electrochemical detection.

The determination of glutamic and gamma-aminobutyric acids in rat brain regions by derivatization with o-phthaldialdehyde-thiol, isocratic separation by liquid chromatography, and quantification by electrochemical detection provides a simple and precise method for assessing changes in glutamatergic and GABAergic neuronal systems. Gamma-aminobutyric acid was eluted in 30-35 minutes followed by a washout step with 90% methanol to remove all amino acid derivatives with longer retention times. Homoserine was used as an internal standard. Significant increases in gamma-aminobutyric acid content in nucleus accumbens and substantia nigra could be detected 20 minutes after injection of 400 mg/kg valproic acid.

Simultaneous measurement of tyrosine, tryptophan and related monoamines for determination of neurotransmitter turnover in discrete rat brain regions by liquid chromatography with electrochemical detection.

Concomitant measurement of monoamine neurotransmitter turnover in discrete rat brain areas with the use of radiolabeled amino acid precursors permits simultaneous evaluation of interacting transmitter systems. [3H]Tyrosine and [3H]tryptophan were administered via indwelling catheters to unrestrained rats. Content and specific activity of norepinephrine, dopamine, 5-hydroxytryptamine, and the metabolites dihydroxyphenylacetic acid, homovanillic acid, and 5-hydroxyindoleacetic acid in addition to tyrosine and tryptophan were quantified by liquid chromatography with electrochemical detection and liquid scintillation counting. The method employs a simple extraction procedure without prior cleanup for chromatography. Neurotransmitter turnover rates that incorporated tyrosine- or tryptophan-specific activities were found to be two to four times greater than those that did not include them.

Effects of acrylamide on locomotion and central monoamine function in the rat.

Male rats receiving acrylamide (ACR) in their drinking water (100 ppm) for a six-week period displayed increased psychomotor stimulation to d-amphetamine (d-A; 1.0 mg/kg SC) under several conditions of handling and drug administration. Following behavioral tests a subset of the animals was sacrificed at 15, 50, 80 and 120 minutes following d-A and the brains removed and dissected for determinations of regional brain levels of several monoamine neurotransmitters and metabolites. ACR rats had elevated levels of 5-hydroxyindoleacetic acid (5-HIAA) in the striatum, septal area, and thalamus. The effect was most pronounced at 15 minutes post-drug with ACR rats not demonstrating a depression in 5-HIAA levels present in controls. Increases in accumben's dopamine and norepinephrine levels, evident after d-A, were of lesser magnitude in ACR-exposed rats. Decreases in dihydroxyphenylacetic acid and homovanillic acid, also evident after d-A, persisted for a longer duration in ACR-exposed rats. Light and electron microscopy of spinal cord, striatum, nucleus accumbens and thalamus did not reveal morphologic abnormalities. Sciatic nerves showed histopathological changes characteristic of multi-focal dying-back peripheral nerve degeneration. It was concluded that acrylamide's effect on the psychomotor stimulant properties of d-A may be related to changes in a serotonergic inhibitory system.

Responsiveness to d-amphetamine in lead-exposed rats as measured by steady state levels of catecholamines and locomotor activity.

Locomotor activity and brain catecholamine concentrations following d-amphetamine challenge were evaluated in lead-exposed rats. Sucklings were exposed to lead via its transfer from maternal drinking solutions (0.02% Pb (Ac)2, 109 ppm Pb) to the maternal milk supply. At 21 days of age half of each exposed litter was weaned to the maternal regimen, the remainder to distilled water. Spontaneous locomotor activity and lead in blood were qualified at 21, 30 and 90 days of age. At 90 days of age rats were also challenged with d-amphetamine sulfate (1.0 mg/kg, SC) and their drug-induced activity recorded for a 2 hours period. One week later the same animals received a similar dose of d-amphetamine and were sacrificed at 0, 0.5, 1, 3, 6, 12, 24 and 48 hours post injection for analysis of whole brain dopamine (DA) and norepinephrine (NE). Catecholamine concentrations did not differ between lead-exposed and control rats in the absence of a d-amphetamine challenge. However, over the 48 hr period following drug challenge, lead-exposed rats had lower whole brain steady state levels of NE than control animals. Similar changes were evident for rats exposed to lead continuously and for those exposed to lead only during the 21 day period from birth to weaning. The spontaneous locomotor activity of lead-exposed and control rats was comparable at all ages tested. However, drug-induced activity was greater in lead-exposed animals than controls. This facilitated drug response was statistically greater than that of controls for continuously lead-exposed rats. Those exposed only during the neonatal period showed similar trends.