Biomedical subjects

D D Wheeler

Publications and source records attributed to D D Wheeler.

At least 19 recordsLinked to original sources

PubMed1994

Effects of cocaine on sodium dependent dopamine uptake in rat striatal synaptosomes.

Initial velocity of uptake of dopamine (DA) has been measured in the presence of 1 microM cocaine as a function of both [DA] and [Na]. Although DA uptake is overwhelmingly dependent on sodium, it appears that a small amount of DA uptake takes place in the absence of sodium. This contrasts with a previous study of the sodium dependence of uptake without cocaine (referred to below as control), in which uptake was found to be 100% sodium dependent. The data were fitted to several rapid equilibrium models and the minimal best fit model identified. The interaction of transporter (C), DA (S), and Na+ (Na) in this present model is identical to the reaction scheme found previously to fit control data (no cocaine). Whereas the control model required translocation only as CNa2S, in the presence of cocaine (I), two additional translocated species are required to fit the data (CS and CNaS). Another previous study of the interaction of carrier and cocaine at a constant [Na]0 predicted that cocaine interacts with a transporter site other than the DA binding site and that uptake takes place as CS and CSI. The present results are consistent with the assumption that the CS and CNaS forms of the present model are actually CSI and CNaSI, since they are required to fit a model of the sodium dependence in the presence of cocaine, but are not required in the absence of cocaine.

Animals↗

PubMed1993

A model of the sodium dependence of dopamine uptake in rat striatal synaptosomes.

Initial velocity of uptake of dopamine (DA) has been measured in rat striatal synaptosomes as a function of both [DA] and [Na]. Carrier mediated uptake is totally dependent on external sodium. The data were fitted to a rapid equilibrium model which has been found in previous studies to fit, with appropriate simplification, uptake data for glutamate, GABA, and choline in several brain regions under varying conditions. This model also gives a good fit to the dopamine data. The minimal best fit simplification of this model allows for DA uptake along with two sodium ions and predicts that apparent maximal velocity of uptake should increase with [Na], while the Michaelis-Menten constant should decrease. The minimal best fit model for DA, and a number of kinetic parameters which quantitate the model, are compared to those for the GABA, glutamate, and choline transporters. The results are consistent with a symmetrical, rapid equilibrium model, which has been presented previously for other neurotransmitters and precursors (18). This model offers a unifying basis for understanding the sodium and membrane potential dependence of neurotransmitter transport and the possible participation of transporters in depolarization induced release throughout the CNS.

Animals↗

PubMed1993

The effect of cocaine on membrane potential, on membrane depolarization by veratridine or elevated [K]o and on sodium/potassium permeability ratios in synaptosomes from the limbic cortex of the rat.

Effects of cocaine on the synaptosomal membrane potential (Em), on membrane depolarization induced by veratridine or elevated [K]o and on sodium/potassium permeability ratios (pNa/pK), have been measured in buffer containing either low or high [Na]. Fluorescence of the dye rhodamine 6G was used to measure the membrane potential. Cocaine began to reduce the Em (depolarized) at concentrations between 10(-6) and 10(-5) M in low [Na] buffer and between 10(-5) and 10(-4) M in high [Na] buffer. Maximum depolarization (with 10(-3) M cocaine) was 21 mV in low [Na] buffer and 11 mV in high [Na] buffer. Cocaine also reduced the depolarization caused by veratridine or elevated [K]o; the effective concentration of cocaine in reducing the response to veratridine was one-tenth that necessary to reduce the response to elevated [K]o. The antagonism by cocaine of the response to veratridine was similar to that found by other investigators; however, this action would tend to oppose depolarization and thus cannot explain the depolarizing effect of cocaine alone. The antagonism by cocaine of the depolarization caused by elevated [K] was consistent with a reduction in pK; such a change in pK could explain the observed reduction in Em caused by cocaine alone. The effect of cocaine (10(-3) M) on the Em was also measured as a function of [K]o at low and high [Na]o. Cocaine caused membrane depolarization at all [K]o's (3.9-19.2 mV), an effect that was somewhat greater in the low [Na] medium. These measurements of Em were fitted to the Goldman equation and the ratio of pNa/pK estimated. The presence of cocaine increased the estimate of pNa/pK by 45.7%, presumably by reducing pK.

Animals↗

PubMed1993

Dopamine uptake in five structures of the brain: comparison of rate, sodium dependence and sensitivity to cocaine.

The rate of uptake of dopamine (DA) per microgram protein and the sensitivity of uptake to sodium ([Na]o) and cocaine, have been measured in synaptosomes from five structures in the brain of the rat: striatum, nucleus accumbens, neocortex, limbic cortex and thalamus. Probably reflecting the number of DA terminals, there was a wide variation in the rate of uptake in the different structures: uptake was far greater in the striatum and nucleus accumbens (10-20-fold) than in the neocortex, limbic cortex or thalamus. Uptake in all structures was inhibited by cocaine. With high [Na]o, the IC50's varied from 1.10 microM for the thalamus to 3.32 microM for the nucleus accumbens. Maximum percentage inhibition varied from 76.0 for limbic cortex to 96.8 for nucleus accumbens and 97.7 for striatum; thus most uptake was carrier-mediated. With low [Na]o, IC50 for the nucleus accumbens was unchanged, while the IC50's for the striatum and limbic cortex were less. Maximum percentage inhibition was similar to that found for high [Na]o. Although the sensitivity to [Na] varied, synaptosomes from all areas showed Na-dependent uptake; lowering [Na] in the incubation medium from 133.2 to 9.5 mM reduced the uptake by a minimum of 36.9% in the neocortex to a maximum of 78.0% in striatum. Neither the rate of uptake nor the Na-dependence correlated precisely with the sensitivity to cocaine but the two structures which showed the greatest rate of uptake (nucleus accumbens and striatum) also showed the greatest sensitivity to [Na]o. Generally, the rate of uptake correlated with the density of DA terminals and number of cocaine binding sites as reported by others.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Effects of veratridine and cocaine on the kinetics of synaptosomal dopamine release.

Following preloading of striatal synaptosomes with 3H-dopamine (DA), the kinetics of release have been followed for a 60-min incubation period. DA appears to be totally releasable under both depolarizing (veratridine) and nondepolarizing conditions. Cocaine has no significant effect on release under either condition. Release is consistent with a model consisting of two parallel, linear compartments (when plotted as a log function). It is proposed that the slower compartment might represent the operation of the DA transporter, while the faster compartment might represent vesicular release.

Animals↗

PubMed1990

Aphasic subjects' comprehension of synthetic and natural speech.

This study investigated the ability of aphasic patients with mild auditory comprehension problems to respond to synthetic speech produced by an inexpensive speech synthesizer attached to a personal computer. Subjects were given four practice sessions with synthetic speech; testing of synthetic speech comprehension was performed during Sessions 1 and 4. During testing, aphasic subjects' comprehension of synthetic speech was compared with their comprehension of natural speech on four tasks: (a) picture identification, (b) following commands, (c) yes/no questions, and (d) paragraph comprehension with yes/no questions. Aphasic subjects comprehended natural speech better than synthetic speech in Session 1 but not in Session 4. Their synthetic speech scores improved between Sessions 1 and 4. There was also a significant difference among scores on the four tasks for both sessions. The means for picture identification were highest, followed by yes/no questions, commands, and finally paragraph comprehension for both sessions. Although performance by some subjects on some tasks was accurate enough to indicate that an inexpensive speech synthesizer could be a useful tool for working with mild aphasic patients, considerable caution in selecting both tasks and patients is warranted.

Adult↗

PubMed1989

Central actions of adenosine on pituitary secretion of prolactin, luteinizing hormone and thyrotropin.

Pituitary hormones were measured in plasma in unanesthetized male and female rats prepared with venous and ventricular cannulae following ventricular infusions of adenosine and two adenosine receptor agonists. Adenosine (10, 100, and 200 nmol) injected intraventricularly caused dose-related rises in plasma prolactin (PRL) in males; in females, only the 100- and 200-nmol doses increased PRL. Similar doses of adenosine had little effect on plasma levels of luteinizing hormone (LH) and thyrotropin (TSH) in the same animals. The adenosine receptor agonists L-N6-phenylisopropyladenosine (L-PIA) and 5'-N-ethyl-carboxamideadenosine (NECA) potently stimulated prolactin secretion at doses of 10 and 50 nmol when administered into the lateral ventricle, and at a dose of 2.5 nmol when administered into the third ventricle. The secretion of PRL was antagonized by the adenosine receptor antagonist theophylline (25 nmol), when theophylline was coadministered with NECA and L-PIA. TSH levels were reduced slightly but significantly following the 10- and 50-nmol infusions of L-PIA into the lateral ventricle. The less potent D-isomer of PIA (D-PIA) did not significantly stimulate PRL release. Coupled with studies indicating the presence of adenosine in the basal hypothalamus, our observations indicate a potential neuroendocrine role for this purine in prolactin secretion.

Adenosine↗

PubMed1988

Hypothalamic site of action for N-methyl-D-aspartate (NMDA) on LH secretion.

Excitatory amino acids have been shown to increase luteinizing hormone (LH) secretion following ventricular or systemic administration. In the present study we attempted to determine possible hypothalamic sites of action for the potent excitatory amino acid agonist, N-methyl-D-aspartate (NMDA). The ability of NMDA to enhance LH release was tested in male rats following infusion into the medial preoptic nucleus (MPO), anterior hypothalamic nucleus (AHY), ventromedial hypothalamic nucleus (VMH), and arcuate nucleus (ARC). In the MPO, infusion of 50 or 500 pmole NMDA increased pituitary LH secretion, resulting in a 2-7 fold increase in plasma LH. The 50 pmole dose was selected to test more caudal hypothalamic sites. Plasma LH levels were not affected following microinfusion of NMDA (50 pmole) into the AHY, VMH, and ARC. The present results indicate a regional specificity for NMDA in the enhancement of LH secretion. This regional specificity may reflect either a greater density of LHRH perikarya in the MPO or the presence of specific amino acid receptors on neuronal elements in the MPO, but not on neuronal elements in the other areas tested.

Animals↗

PubMed1987

Are there both low- and high-affinity glutamate transporters in rat cortical synaptosomes?

Kinetics of sodium dependent glutamic acid transport have been studied in rat cortical synaptosomes at sufficiently high glutamic acid concentrations ([G]) to delineate the "low affinity" transporter. Computer optimization techniques were used to fit the data to models which account for the sodium and substrate dependence of uptake. The data fit about equally well models consisting of two carriers (Model 1) or one carrier plus a linear component (Model 2). However, the results of further studies were inconsistent with Model 1, but totally consistent with Model 2. Thus the results are incompatible with the presence of both high- and low-affinity carriers. The carrier model found in previous studies of high affinity glutamate transport predicts the effects of high [G] and [Na] observed in the present study. The biphasic effect of [Na] on velocity of uptake is the logical consequence of the operation of this model. The rate equation for this model has been utilized to define and compute kinetic parameters which characterize the transporter. These kinetic functions are remarkably similar in shape and magnitude to previous estimates from the studies of the high affinity transport (low [G]). The results of other studies by the author which corroborate and expand the predictions of the kinetic model are discussed. These have been combined with the present results to formulate a rather comprehensive model of glutamate function. This model can be used to describe function in terms of mathematical equations and to make predictions from these equations. These equations relate velocity of uptake and the kinetic parameters to sodium and substrate concentration, velocity to membrane potential, distribution ratio to the electrochemical potential, and release to time, compartment sizes, and exchange constants. Such processes as concentration in the presynaptic terminal, depolarization induced release, re-uptake following stimulus induced release, and postsynaptic depolarization are all possible consequences of the operation of this model. The wide applicability of the model to the transport of other substrates in addition to glutamate is discussed.

Amino Acid Transport System X-AG↗

PubMed1986

Morphometric and autoradiographic analysis of crude synaptosomal preparations from rat cerebral cortex.

Morphometric and autoradiographic studies have been made of a crude synaptosomal preparation, which has been used extensively for membrane transport studies. When filters are used to separate membrane bound structures from incubation medium, the structures which survive filtration are those that are entrapped within the matrix of the filter structure. The population of membrane bound structures differs when one compares pellets of the preparation to sections of loaded 0.45 and 0.65 micron pore size filters. Both the relative numbers of synaptosomes, mitochondria, and other membrane bound structures (OMBS) and the mean size of each of the structures differ for pellet, 0.45 micron, and 0.65 micron filters. The percentage of total membrane bound volume attributable to synaptosomes increases from 28 in the crude preparation to 40 in 0.45 micron filters and 61 in 0.65 micron filters. The total volume of synaptosomes entrapped by differing pore size filters roughly correlates with the amount of substrate uptake. Neither mitochondrial volume nor the volume of other membrane bound structures was found to correlate with uptake. These results indicated that only the synaptosomes contribute measurably to this function. Autoradiographic studies confirm this conclusion. EM autoradiography following loading of the synaptosomal preparation with tritiated glutamate or GABA showed about 81% of the grains to be associated with synaptosomes. It is concluded that crude synaptosomal preparations may be used without further purification for membrane transport studies with unambiguous results.

Animals↗

PubMed1986

Endogenous GABA concentration in cortical synaptosomes from young and aged rats.

The objectives of this study are threefold: to compare the gamma-aminobutyric acid (GABA) concentration of a preparation of cortical synaptosomes from young male rats to that of aged rats; to compare the GABA concentration in synaptosomes retained by Millipore filtration for young and aged rats; and to compare the GABA concentration in the synaptosomal preparation to that in synaptosomes retained by Millipore filtration for young and aged rats. The GABA concentration is measured in cortical synaptosomes from Long-Evans rats at 2 months and 30 months of age. Concentration of the synaptosomal preparation declined significantly from 2.97 mM in 2-month animals to 2.68 mM in 30-month animals. In order to compare the results with previous studies of GABA transport, in which Millipore filters are used to separate synaptosomes from incubation medium, the GABA concentration is also measured following placement of synaptosomes on 0.65 mu pore size Millipore filters. For both young and aged animals, this population of synaptosomes is found to contain GABA at a concentration more than triple that in the overall population. This is the expected result if the filtration process reduces the non-synaptosomal component of the total preparation, since GABA uptake is known to be a function of the nerve-ending component. Comparison of 2-month and 30-month synaptosomes reveals that the GABA concentration of those synaptosomes retained by the filtration process declined to a greater extent in the aged group than did the content of the overall population (from 9.33 mM to 8.37 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

Age Factors↗

PubMed1986

Time course of the aging of the high affinity L-glutamate transporter in rat cortical synaptosomes.

Initial velocities of cortical synaptosomal glutamate uptake are measured as a function of both glutamate and sodium concentration in 10- and 18-month old animals and compared to previous results from 2- and 30-month old animals. As a percentage of 2-month values, mean velocity of uptake falls to 87% in the 10-month group and to 79% in the 18-month group, with no further change between 18 and 30 months. The data give minimal best fit to the same model for interaction of carrier with glutamate and sodium as found previously for 2- and 30-month old animals. Thus the basic mechanisms involved in transport do not change with age. However, the kinetic parameters which describe the transporter do change with age, the primary changes being in those parameters reflective of the transport capacity of the carrier. Va, apparent maximal velocity of uptake, declines most rapidly between 2 and 30 months, continues to decline rapidly between 10 and 18 months, and then declines slowly between 18 and 30 months. Jm, rate of uptake at infinite sodium concentration, declines most rapidly between 2 and 10 months, with a much slower decline between 10 and 30 months. Kt, apparent Michaelis-Menten constant, changes but little with age; at 30 months, Kt is only 6-11% less than at 2 months. Although the total change is only 9-10%, there appears to be a slow, steady decline in KNa, the sodium concentration giving an uptake equal to Jm/2, between 2 and 30 months of age.

Aging↗

PubMed1985

Aging of membrane transport mechanisms in the central nervous system--high affinity choline transport in rat cortical synaptosomes.

High affinity choline transport has been studied in cortical synaptosomes from rats ranging from 2 months to 30 months of age. Initial velocities were measured as a function of both choline and sodium concentration. These data give similar fits to a kinetic model which was previously found to give minimal best fit to data from 2-month-old animals. Thus, aging has apparently not altered the fundamental mechanism by which carrier, sodium, and choline interact in the process of uptake. Although some small differences in initial velocities were found between data from young and older animals, these differences showed no pattern and were not statistically significant. It is concluded that the choline transporter is unaffected by age, in contrast to the transporters for GABA and glutamic acid, whose transport capacities were found to decline with age.

Aging↗

PubMed1984

The effect of membrane potential on initial velocity of GABA uptake and steady state distribution ratio in rat cortical synaptosomes.

Initial velocity of synaptosomal GABA uptake and steady state distribution ratio (DR) have been measured as a function of medium potassium concentration [( K]o) and in the presence of veratridine. Previous studies of the effect of [K]o and veratridine on membrane potential have been utilized to relate GABA uptake to membrane potential. Both initial velocity of uptake and steady state DR are functions of membrane potential; initial velocity and in DR are linearly related to membrane potential. Final DR is determined by the electrochemical potential gradient for the transported ions. Previous studies of the relation of [Na]o to uptake have shown that either one or two sodium ions may be translocated with each GABA molecule. Present results are consistent with this assumption if it is also assumed that an anion is co-transported with each GABA molecule.

Animals↗

PubMed1984

Kinetics of D-aspartic acid release from rat cortical synaptosomes.

A kinetic study has been made of the release of D-aspartate from rat cortical synaptosomes following pre-loading with labelled D-aspartate, and the results compared to a previous study of the release of the acidic amino acids glutamate plus aspartate following pre-loading with labeled L-glutamate. Qualitatively, the results of the two studies are similar. The D-aspartate taken up during the pre-load period appears to be totally releasable. However, release is greatly increased by depolarizing media. The increased rate of release induced by increasing [K]o is independent of the [Ca]o, while veratrine-induced release is inhibited by [Ca]o. Release is from more than a single compartment, since plots of the log10 of the synaptosomal D-aspartate content (calculated from the label content) as a function of the incubation time are non-linear for all incubation solutions. In the previous study which utilized L-glutamate pre-loading, the results were consistent with either a model consisting of two passive compartments (that is, synaptosomal content T as a function of time is given by Ae-Kat + Be-Kbt, in which A and B are compartment sizes, Ka and Kb are exchange constants, and t is incubation time) or a model consisting of one passive compartment (Ae-Kat) and one saturated carrier compartment (T-Kbt, in which T = total content at zero time and Kb = maximal velocity). The present results with D-aspartate also give excellent fits to these models. However, there are some quantitative differences in the estimates of the compartment sizes and exchange constants, which are obtained by optimizing the fit of the data to the equation for each model. Although most of these quantitative differences appear to be minor, one difference between the two studies is of potential significance in interpretation of the results. In the glutamate study, all depolarizing media were found to reduce the exchange constant for the carrier mechanism, while in the present study, depolarizing media were found to increase the exchange constant, with the exception of veratrine-containing medium without calcium.

Animals↗

PubMed1984

Effect of hyperbaric oxygen on 2-deoxy-D-glucose and L-glutamate transport in rat cortical synaptosomes.

Initial velocities of uptake of L-glutamic acid and 2-deoxy-D-glucose (2-Dg) have been measured in cortical synaptosomes from rats which had been exposed to oxygen at high pressure (OHP) and compared to similar measurements in normobaric controls. Exposure to OHP had no significant effect on glutamate uptake at any combination of sodium and glutamate used. In contrast, OHP reduced 2-Dg uptake by an average of 17.5%. Although Kt was little affected, OHP exposure reduced apparent maximal transport capacity by 15%. Since hyperbaria with normal pO2 had no significant effect on uptake, the effect of OHP is an oxygen effect, rather than a pressure effect. The effects of OHP on uptake do not parallel the effects of age; glutamate transport capacity was reduced in aged animals, while 2-Dg transport was unaffected.

Aging↗

PubMed1983

A kinetic analysis of the release of acidic amino acids from rat cortical synaptosomes following pre-loading with [14C]glutamic acid.

The kinetics of the release of acidic amino acids have been studied in rat cortical synaptosomes. After pre-loading for 30 minutes in labeled glutamate, labeled glutamate and aspartate appear to be totally releasable. However, extra-synaptosomal Ca does not facilitate release. When elevated [K]0 was used to depolarize, release was unaffected by removal of Ca from the incubation medium. When veratridine was used as a depolarizing agent, presence of Ca in the incubation medium inhibited release. In all solutions, semi-log plots of synaptosomal label content as a function of time were non-linear, which is incompatible with release from a single compartment. Previous studies of the effect of membrane potential on transport led to the development of a carrier model which should participate in depolarization induced release (19). Under the conditions used in the present studies, this carrier should be saturated. When the data were fitted to a two compartment model, with release from compartment A linear with compartment size and release from compartment B via a saturated carrier, an excellent fit was obtained. Under control conditions, about 90% of the labeled amino acid is in compartment B and about 70% of the total release is from this compartment. Rate of release is greatly accelerated in depolarizing solutions. Under depolarizing conditions, there is a large shift of labeled amino acid from compartment B to compartment A and release from compartment A predominates. Analysis of the results under the several depolarizing conditions used shows that the present results are consistent with the predictions of the carrier model which has been developed from previous studies of the Na and membrane potential dependence of glutamate transport.

Amino Acids↗

PubMed1983

Effect of hyperbaric oxygen on GABA transport in rat brain synaptosomes.

Initial velocities of uptake of GABA have been measured in rat brain synaptosomes from animals which had been exposed to oxygen at high pressure (OHP) and compared to similar measurements in normobaric controls. For hypothalamus, no changes in GABA uptake occurred subsequent to exposure to OHP. For cortical synaptosomes, however, exposure to OHP resulted in a decreased velocity of GABA uptake at all combinations of [Na] and [GABA] used. The OHP data were found to fit the same transport model as found previously for control data. Thus, OHP exposure did not alter the basic mechanism by which sodium and GABA interact with the carrier in the process of transport. However, the constants which quantitate the model were changed by OHP exposure. As a consequence, the several kinetic parameters which are calculated from the model change in the OHP animals. These kinetic parameters are compared to similar calculations for both normobaric control animals and normobaric aged animals. Although the effects of OHP do not precisely parallel the effects of aging, the alterations in kinetic parameters are in several ways similar in the aged and OHP animals.

Aging↗