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G P Cooper

Publications and source records attributed to G P Cooper.

At least 19 recordsLinked to original sources
PubMed1990

Effects of inorganic mercury on [3H]dopamine release and calcium homeostasis in rat striatal synaptosomes.

Inorganic mercury (Hg2+) in vitro increases spontaneous transmitter release from nerve terminals. The mechanisms of action are not well understood but may involve alterations in intraterminal Ca2+ dynamics. In this study we describe actions of Hg2+ in vitro on isolated mammalian CNS striatal nerve terminals (synaptosomes). Cobalt (2 mM) completely blocked the effect of 2 microM Hg2+ on spontaneous [3H]dopamine release. Cadmium (100 microM) was equipotent to Co2+ in blocking depolarization-dependent [3H]dopamine release, but did not alter the 2 microM Hg2(+)-induced spontaneous [3H]dopamine release. Depolarization-dependent [3H]dopamine release was not altered by 5 microM Hg2+. It appears that the site of action of Hg2+ on spontaneous [3H]dopamine release is not the Ca2+ channel. The effects of Hg2+ on intraterminal ionized Ca2+ [( Ca2+]i) were evaluated using the Ca2(+)-specific fluorescent probe, fura-2. Hg2+ (1-8 microM) had no effect on [Ca2+]i in 1.2 mM Ca2(+)-containing buffers. In nominal Ca2+ media, 4 and 8 microM Hg2+ significantly decreased [Ca2+]i. Following exposure to 4 and 8 microM Hg2+ the quenching of extrasynaptosomal fura-2 by Mn2+ was increased, suggesting that Hg2+ facilitated the leakage of fura-2. This apparent leakage was probably due to a nonspecific increase in membrane permeability since 2 microM Hg2+ produced a Co2(+)-insensitive increase in [3H]deoxyglucose phosphate efflux. Hg2+ did not increase the leakage of either lactate dehydrogenase or soluble protein from synaptosomes. Hg2+ produced a concentration-dependent (1-8 microM) increase in 45Ca2+ efflux from superfused synaptosomes which was insensitive to blockade either by 2 mM Co2+ or by 100 microM Cd2+. These data suggest that the transmitter releasing action of Hg2+ involves interactions with sites that also interact with Co2+ but not with Cd2+. Furthermore, Hg2+ may have direct transmitter releasing actions (i.e., Ca2(+)-mimetic properties), as well as nonspecific actions on plasma membrane permeability which may not necessarily be linked to [3H]dopamine release.

Animals↗
PubMed1989

Effects of mercuric chloride on [3H]dopamine release from rat brain striatal synaptosomes.

Electrophysiological studies employing amphibian neuromuscular preparations have shown that mercuric chloride (HgCl2) in vitro increases both spontaneous and evoked neurotransmitter release. The present study examines the effect of HgCl2 on the release of [3H]dopamine from synaptosomes prepared from mammalian brain tissue. Mercuric chloride (3-10 microM) produces a concentration-dependent increase in spontaneous [3H]dopamine release from "purified" rat striatal synaptosomes, in both the presence and absence of extra-synaptosomal calcium. The effects of HgCl2 on transmitter release from amphibian neuromuscular junction preparations resemble those produced by the Na+, K+-ATPase inhibitor ouabain. Experiments were performed to determine whether the HgCl2 effects on mammalian synaptosomal dopamine release are a consequence of Na+, K+-ATPase inhibition. Na+, K+-ATPase activity in lysed synaptosomal membranes is inhibited by HgCl2 (IC50 = 160 nM). However, mercuric chloride in the presence of 1 mM ouabain still increased [3H]dopamine release. The specific inhibitor of Na+-dependent, high-affinity dopamine transport, RMI81,182 inhibited ouabain-induced [3H]dopamine release whereas it had no effect on HgCl2-induced [3H]dopamine release. These data suggest that augmentation of spontaneous [3H]dopamine release by HgCl2 probably is not mediated by an inhibition of Na+, K+-ATPase and HgCl2 does not act directly on the dopamine transporter.

Animals↗
PubMed1989

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.

Action Potentials↗
PubMed1988

Calcium efflux and neurotransmitter release from rat hippocampal synaptosomes exposed to lead.

The results of several studies, employing various tissue preparations, have demonstrated that in vitro Pb exposure has similar effects on the release of several different transmitter substances. Pb has been observed to attenuate depolarization-evoked release and increase spontaneous (depolarization-independent) release. The current study confirms that Pb in vitro increases the spontaneous release of [3H]acetylcholine (ACh) from superfused synaptosomes prepared from rat hippocampus. Additionally, hippocampal synaptosomes, preloaded with 45Ca, were superfused under conditions similar to those used in the [3H]ACh-release studies. Exposure to 1-30 microM Pb produced a concentration-dependent increase in the efflux of 45Ca that was quantitatively and temporally related to the Pb-induced release of [3H]ACh from the hippocampal synaptosomes. Depolarization-evoked [3H]ACh release with high potassium did not produce a corresponding increase in 45Ca efflux. It is concluded that the Pb-induced increase in spontaneous transmitter release is apparently due to either an increase in intraneuronal ionized calcium or the stimulation by Pb of Ca-activated molecules mediating transmitter release.

Acetylcholine↗
PubMed1987

Effects of divalent cations on acetylcholine release from digitonin-permeabilized rat cortical synaptosomes.

A preparation of rat brain synaptosomes, partially permeabilized by the cholesterol-specific detergent digitonin, was developed to study the effects of the heavy metals Pb+2 and Cd+2 and of other divalent cations on acetylcholine release from nerve terminals. Consistent with the cation specificities of the transmitter release process in intact nerve terminals, 100 microM free Ba+2, Sr+2 and Ca+2 each induced a release of acetylcholine (ACh) while Ni+2, Co+2 and Mn+2 did not. 100 microM Pb+2 and Cd+2 also induced a release of ACh from the permeabilized synaptosomes. This suggests that, similar to Ba+2 and Sr+2, both Pb+2 and Cd+2 may induce transmitter release through a direct action on the transmitter release apparatus.

Acetylcholine↗
PubMed1984

Effects of Pb2+ and Cd2+ on acetylcholine release and Ca2+ movements in synaptosomes and subcellular fractions from rat brain and Torpedo electric organ.

In this work we examined the effects of Pb2+ and Cd2+ on (a) [3H]ACh release and voltage-sensitive Ca2+ channels in rat brain synaptosomes, and (b) 45Ca2+ binding to isolated brain mitochondria and microsomes, and synaptic vesicles isolated from Torpedo electric organs. Pb2+ (Ki approximately 1.1 microM) and Cd2+ (Ki approximately 2.2) competitively block the K+-evoked influx of 45Ca2+ through the 'fast' calcium channels in synaptosomes. The Kis obtained with synaptosomes are in good agreement with the Ki values obtained from electrophysiological experiments at the frog neuromuscular junction (KPb:0.99 microM, KCd: 1.7 microM)7. The Ki for the inhibition of ACh release from synaptosomes by Cd2+ is 4.5 microM. Pb2+ is a less effective inhibitor of transmitter release (Ki approximately 16 microM) because it secondarily augments spontaneous transmitter efflux. Cd2+ has no effect on spontaneous release at concentrations less than or equal to 100 microM. The enhancing effect of Pb2+ on spontaneous release is (a) not abolished by omission of Ca2+ from the bathing medium, (b) is delayed by 1-2 min after the beginning of Pb2+ exposure, (c) is reversed upon the removal of Pb2+. In the presence of physiological concentrations of ATP (1 mM), Mg2+ (1 mM) and Pi (2 mM), 1-10 microM Pb2+ inhibits calcium uptake but Pb2+ greater than 10 microM causes a several-fold stimulation of passive binding of calcium to the organelles. This effect is associated with Pb2+-induced enhancement of Pi uptake. Cd2+ inhibits Ca2+ binding at all concentrations tested (1-50 microM) and reduces the Pb2+-induced Ca2+-binding to organelles. Neither Pb2+ nor Cd2+ have any discernible effects on spontaneous loss of calcium from mitochondria or microsomes preloaded with 45Ca. In summary, these data are consistent with the notion that Pb2+ and Cd2+ are potent blockers of presynaptic voltage-sensitive Ca2+ channels and the evoked release of transmitter which is contingent on Ca2+ influx through these channels. Our results are not consistent with the hypothesis that Pb2+ augments spontaneous release by interfering with intraterminal Ca2+-buffering by mitochondria, endoplasmic reticulum, or synaptic vesicles.

Acetylcholine↗
PubMed1984

Cadmium: effects on transmitter release at the frog neuromuscular junction.

Cd2+ competitively antagonizes Ca2+ in the evoked release of acetylcholine from nerve terminals in the frog sciatic-sartorius neuromuscular junction. The dissociation constant between Cd2+ and its receptor sites was calculated to be 1.7 microM. In agreement with previous work we found that brief exposures to 100-500 microM Cd2+ had little or no effect on spontaneous transmitter release. However, in contrast to other reports, prolonged exposures to 100-100 microM Cd2+ or tetanic nerve stimulation during brief exposures to high concentrations of Cd2+ produced substantial increases in spontaneous transmitter release. We conclude that Cd2+, when present at low concentrations, is a specific Ca2+ channel blocker. At higher concentrations, Cd2+ is either relatively impermeable to the presynaptic nerve membrane or, if it does enter the nerve terminal, it is less effective than such metals as Pb2+, La3+ or Hg2+ in increasing the rate of spontaneous transmitter release.

Acetylcholine↗
PubMed1984

Effects of lead on neuromuscular transmission in the frog.

The acute effects of Pb2+ on synaptic transmission at the frog neuromuscular junction were measured using conventional microelectrode techniques. Experiments were performed on preparations bathed in high magnesium/low calcium Ringer solution in order to record subthreshold endplate potentials (EPPs). The effects of Pb2+ on the muscle membrane and postsynaptic membrane were minimal since relatively high doses of Pb2+ caused no significant change in the input resistance of the muscle fiber and in the amplitude of the acetylcholine (ACh) iontophoteric potential when the ACh micropipette was highly localized. However, when the ACh micropipette was moved away from the receptors, the resulting ACh potential was reduced significantly by Pb2+. Pb2+ is a potent blocker of the EPP. Extracellular recordings from motor nerve terminals showed that endplate currents (EPCs) were reduced by Pb2+ while the nerve terminal potentials were unaffected. Therefore, Pb2+ blocks evoked transmitter release at a step following the depolarization of the nerve terminal. The blocking effect on the EPP was overcome when [Ca2+]o was raised. The log-log relationship between [Ca2+]o (abscissa) and EPP amplitude was shifted to the right in the presence of 1 microM Pb2+; the mean +/- S.E. slopes were 4.16 +/- 0.12 (control) and 4.05 +/- 0.13 (Pb2+). Reciprocal plots relating [Ca2+]o-1 to (EPP)-1/5 confirmed that Pb2+ competitively antagonized the action of Ca2+. The dissociation constant between Pb2+ and the Ca2+ receptor site was found to be 0.99 microM. Pb2+ is about 3 X 10(3) times more potent than is Mg2+, about 150 times more potent than is either Mn2+ or Co2+, and about 3 times more potent than Cd2+ is in blocking evoked release of ACh. After Pb2+ decreased the EPP, the MEPP frequency began to increase; this was probably the result of intracellular Pb2+ disrupting the Ca2+ sequestering activity of mitochondria and/or other intraterminal organelles. [Ca2+]i was thereby increased and an increase in MEPP frequency followed. Decreased MEPP amplitudes were observed when the MEPP frequency had been increased by Pb2+. Pb2+ may affect most chemical synapses in a manner which is similar to its effects on the neuromuscular junction and that this may be one of its important neurotoxic effects.

Acetylcholine↗
PubMed1984

Interactions of lead and cadmium on acetylcholine release at the frog neuromuscular junction.

The interactive effects of Cd2+ and Pb2+ on evoked and spontaneous transmitter release were studied in the sciatic nerve-sartorius muscle preparation of the frog (Rana pipiens). Either Pb2+ or Cd2+ competitively inhibited the actions of Ca2+ in bringing about evoked release, as measured by the endplate potential (EPP) amplitude. Combinations of Pb2+ and Cd2+ were additive in their effects on the EPP. The rate of spontaneous transmitter release was measured as the miniature endplate potential (MEPP) frequency. In contrast to their effects on the EPP, exposure of preparations to combinations of Pb2+ and Cd2+ actually increased the MEPP frequency less than exposures of the preparations to Pb2+ alone. The degree of reduction in MEPP frequency produced by Cd2+ depended upon the relative ratio of Pb2+ and Cd2+ ions. These results suggest that Pb2+ and Cd2+ ions competed for a common presynaptic receptor site during evoked release. They also suggest that Pb2+ ions may enter the nerve terminal, possibly through the Ca2+ channel, and that this entry is inhibited by Cd2+.

Acetylcholine↗
PubMed1984

Heavy metals: effects on synaptic transmission.

The acute effects of Pb++, Cd++ and Hg++ on synaptic transmission were studied on the in vitro sciatic nerve-sartorius muscle preparation of the frog, using electrophysiological techniques. Biochemical procedures were used to examine the effects of Pb++ and Cd++ on in vitro preparations of synaptosomes. In the electrophysiological studies Pb++ was shown to be a powerful competitive inhibitor of action potential-evoked release of acetylcholine (ACh) as judged by its depressant effects on the amplitude of endplate potentials (EPPs). The dissociation constant between Pb++ and the presynaptic Ca++ receptor is about 1 microM. Pb++ also increases spontaneous transmitter release as determined by the frequency of miniature endplate potentials (MEPPs). The increase in MEPP frequency is assumed to be due to an intracellular action of Pb++ to reduce the ability of nerve terminal organelles to buffer Ca++ and thereby, increases the intracellular concentration of Ca++. Cd++ also blocks evoked ACh release by a competitive inhibitory mechanism which appears similar to that for Pb++. The dissociation constant for Cd++ is about 2.8 microM. In contrast to Pb++, Cd++, does not increase resting MEPP frequency. Hg++ is unique in that it first causes an increase in evoked ACh release and then a sudden and complete blockade; the MEPP frequency follows a similar time course. The mechanism underlying these effects of Hg++ is uncertain. In rat brain synaptosomes, Pb++ and Cd++ competitively inhibit the K+-stimulated influx of 45Ca++. The dissociation constants for the interaction of Pb++ and Cd++ with Ca++ channels is 1.1 microM and 2.2 microM respectively. These data strongly support the idea that the electrophysiological effects of Pb++ and Cd++ on the EPP are due to a reduction of voltage-gated Ca++ entry into presynaptic nerve terminals.

Acetylcholine↗
PubMed1983

Influence of heavy metals on synaptic transmission: a review.

The acute effects of Pb++, Cd++ and Hg++ have been studied on the amphibian neuromuscular junction. These heavy metal ions primarily affect those presynaptic mechanisms which underlie neurotransmitter release; no significant postsynaptic effects were observed. All experiments were performed on the isolated sciatic nerve/sartorius muscle preparation. Conventional electrophysiological techniques using intracellular recordings were used to monitor acetylcholine (ACh) release. Ringer solutions usually contained high Mg++ and low Ca++ concentrations so that endplate potentials (EPPs) could be recorded under contraction-free conditions. Pb++, Cd++ and Hg++ were added to the Ringer solutions as chloride salts. Of the two forms of transmitter release, Pb++ blocked one (evoked release or EPP amplitude) and stimulated the other (the rate of spontaneous release or MEPP frequency). When a preparation was first exposed to a moderate dose of Pb++, the EPP amplitude decreased within about 1-2 min; however, at that time, the MEPP frequency was just beginning to increase. Low concentrations of Pb++ often reduced the EPP greatly without altering the MEPP frequency. Evidence is provided for a competitive interaction between Pb++ and Ca++ ions in evoked release which is believed to occur on the extracellular side of the nerve terminal. The dissociation constant between Pb++ and the presynaptic Ca++ receptor is about 1 microM. The increase in MEPP frequency is assumed to be due to an intracellular action of Pb++ which may reduce the ability of nerve terminal organelles to sequester Ca++ and thereby increase the intracellular concentration of ionized Ca++. Cd++ also blocks evoked ACh release by a competitive inhibitory mechanism similar to that for Pb++. Cd++ is slightly less potent than Pb++, the dissociation constant for Cd++ being around 2.8 microM. In contrast to Pb++, Cd++ does not increase resting MEPP frequency. Hg++ is unique in that it first causes an increase in evoked ACh release and then a sudden and complete blockade; the MEPP frequency follows a similar time course. The mechanism underlying these effects of Hg++ is uncertain.

Acetylcholine↗
PubMed1981

Acrylamide neurotoxicity: effects on far-field somatosensory evoked potentials in rats.

These experiments were designed to determine the sites of acrylamide induced damage in the specific ascending somatosensory system. Far-field somatosensory evoked potentials (SSEPs) were measured in adult rats given 400 ppm acrylamide in drinking water for 4, 8, or 16 days. Treated rats showed a statistically significant weight loss after 8 days of exposure and became visibly ataxic after about 12 days exposure. SSEPs were collected using modified techniques of Wiederholt and Iragui-Madoz [9] which enhance three far-field peaks arising from (I) the spinal posterior columns, (II) the posterior column nuclei and medial lemniscus, and (III) the thalamus and thalamo-cortical radiations. The three far-field peaks occur prior to the first cortical peak (P1). Tibial nerve stimulation voltage and frequency profiles revealed statistically significant alterations in all SSEP peaks after 16 days exposure. These findings indicate that damage may have occurred throughout the ascending somatosensory system including the spinal cord, dorsal column nuclei, medial lemniscus, thalamus, and sensory radiations.

Acrylamides↗
PubMed1979

Behavioral deficits in adult rats following neonatal lead exposure.

Rats exposed to lead via the maternal milk were tested at maturity on three different visual discrimination tasks. Starting at parturition the dams were given either tap water, 0.20% sodium acetate, 0.02% lead acetate, or 0.20% lead acetate in the drinking water. At weaning, the pups from all the groups were placed on normal chow and tap water. At 20 days of age, the concentration of lead in the blood and brain of the high lead-exposed offspring was approximately 6 times that of controls (11 microgram% vs 66 microgram%). A significant deficit was found in the ability of the high lead-exposed group to acquire a simultaneous visual discrimination task conducted in an operant chamber. No significant differences were observed in the ability of lead-exposed rats to acquire either a successive visual discrimination task or a cued go/no-go discrimination. Thee results suggest that early lead exposure can affect certain behavioral processes and that the effects may persist even after the rat has reached maturity.

Animals↗
PubMed1977

Behavioral effects of low level neonatal lead exposure.

Rats exposed to lead via maternal milk were tested at various stages of development on a number of behavioral tasks. Beginning at paturition, the dams were given either tap water, 0.02%, or 0.10% lead acetate in the drinking water. Pups from all three groups were weaned to normal chow and tap water at 21 days of age. The mean lead concentration of the dam's blood and of neonatal (20 days of age) brain and blood were all below 50 microgram/100 ml. No significant differences were found between the high lead-exposed group and controls in general as measured by wheel running over a 21 day period beginning at 30 days of age. However, there was a significant difference in wheel running behavior during the first three hr of testing. Both lead-exposed groups were found to display significantly less aggressive behavior as measured by the shock-elicited aggression test. Low level lead exposure had no discernable effect on the acquisition and subsequent reversal of a successive brightness discrimination task. Lead exposure under these conditions appears to affect some aspects of emotional behavior, while having little effect on general activity or cognitive function.

Aggression↗