Thomas K. Dalton

Possible "accelerated striatal aging" induced by 56Fe heavy-particle irradiation: implications for manned space flights.

The present experiments were carried out to determine the effects of energy deposition from energetic iron (56Fe particles, an important component of cosmic rays) on motor behavioral performance and to determine if the observed deficits were caused by alterations in the neostriatum (an important motor control area). Neostriatal function was assessed with two correlated parameters, i.e., motor behavioral performance (wire suspension task), and oxotremorine-enhanced K(+)-evoked release of dopamine from perifused striatal slices. Rats were exposed to one of several doses of 56Fe-particle irradiation (0.10-1.0 Gy) and tested on a wire suspension task at 3-180 days postirradiation. Results indicated that profound decrements occurred in both of these indices. The effects on K(+)-evoked release of dopamine were evident for as long as 180 days after irradiation, and a subsequent experiment indicated that these effects appeared as early as 12 h postirradiation. Since similar findings have been observed in aged rats, the results are discussed in terms of these particles producing a possible accelerated striatal aging effect.

Age-Related Decrements in the Muscarinic Enhancement of K(+)-Evoked Release of Endogenous Striatal Dopamine: An Indicator of Altered Cholinergic-Dopaminergic Reciprocal Inhibitory Control in Senescence

Previous experiments have indicated that the release of striatal dopamine (DA) is controlled by inhibitory DA autoreceptors which are mediated by inhibitory cholinergic heteroreceptors (HTRs). Activation of the HTRs by muscarinic or nicotine agonists potentiates the K+-evoked release of DA from the striatum. Present experiments were carried out to determine if this relationship is altered as a function of aging. Cross-cut striatal tissue slices obtained from 3 age-groups (6, 12-18 and 24 months) Wistar rats were superfused with a modified Krebs-Ringer basal release medium containing 2.5 mM KCl. After a 30-min equilibration period, a 5-min baseline fraction was collected from each chamber. The medium was then switched to one containing 30 mM KCl, and depending upon the experiment, 1 of 4 concentrations of a particular muscarinic (oxotremorine, pilocarpine, carbachol or bethanecol) or nicotinic (nicotine) agonist. In some experiments DA autoreceptor function was assessed directly with haloperidol. Six 5-min fractions were taken during depolarization. DA release was assessed using high performance liquid chromatography coupled to electrochemical detection. Results indicated that the efficacy of the muscarinic agonists was reduced in an age-dependent manner with the oldest age groups showing the smallest enhancement. The age at which the decline was seen was dependent on the muscarinic agonist that was applied. Deficits were seen as early as 12 months when full agonists (e.g. carbachol) were applied, but did not appear until 18 months when partial agonists (e.g. oxotremorine) were applied. These age-related alterations were not seen when haloperidol or nicotine were used to enhance the K+-evoked release of DA.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in muscarinic control of striatal dopamine autoreceptors in senescence: a deficit at the ligand-muscarinic receptor interface?

Research has indicated that the release of striatal dopamine (DA) is controlled by inhibitory DA autoreceptors which are in turn regulated by inhibitory muscarinic inhibitory cholinergic heteroreceptors (HTRs) located in close vicinity to the autoreceptors. Muscarinic activation enhances K+-evoked release of DA from striatal slices from mature but not senescent rats. Since it has been shown that age-dependent declines in Ca2+ mediated acetylcholine release can be restored by the ionophore A23187, it was of interest to determine if age-related decrements in Ca2+ mobilization might contribute to the alterations in muscarinic control of the striatal DA autoreceptors seen in senescence. Cross-cut striatal tissue slices obtained from two age-groups (6 and 24 months) of Wistar rats were superfused with a modified Krebs-Ringer medium containing 2.5 mM KCl. After a 30-min equilibration period, a 5-min baseline fraction was collected. The medium was then switched to one which contained 30 mM KCl and, depending upon the experiment, the muscarinic agonists carbachol, or oxotremorine or the Ca2+ mobilizing agents A23187 or inositoltrisphosphate (IP3) and enhancement of K+-evoked release of DA was examined. Six 5-min fractions were collected. DA release was determined by HPLC coupled to electrochemical detection. Results indicated that although deficits were seen in oxotremorine and carbachol enhancement of K+-evoked release of DA, these decrements were not observed when either A23187 or IP3 were utilized to enhance the K+-evoked release of DA.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional brain acetylcholine levels in rats acutely treated with ethanol or rendered ethanol-dependent.

Abstract : A study of brain acetylcholine (ACh) levels was undertaken in an attempt to reconcile discrepancies reported on the effects of ethanol treatment on the cholinergic system. ACh levels were measured in six areas of brains of male Sprague-Dawley rats after a single dose of ethanol or after the induction of ethanol dependence. At various times after treatment the rats were euthanatized by focused microwave irradiation. The excised brains were dissected into the following parts: cerebellum, brain stem, hypothalamus, hippocampus, caudate nucleus and cerebral cortex. After an acute dose of ethanol, ACh levels increased in most areas of the brain when blood ethanol concentrations were quite high. As blood ethanol declined, ACh levels decreased to below control values with similar results observed in ethanol-dependent rats still intoxicated. No significant changes in ACh levels were observed in ethanol-dependent rats undergoing a withdrawal syndrome. These data suggest that ethanol treatment exerts multiple effects on the cholinergic system, but they do not as yet support a role of ACh in the expression of the ethanol withdrawal syndrome. The results of this study provide further information on the development and exploration of models for chronic insults to the brain, such as long-term exposure to toxic chemicals and ionizing and nonionizing radiation. (Author)

Neurotransmitter-receptor binding in various brain regions in ethanol-dependent rats

Since chronic ethanol administration has been demonstrated to induce a number of alterations in neurotransmitter utilization, the possibility was investigated that the receptors, on which these transmitters act, are altered because of a modified synaptic input. Male Sprague-Dawley rats were rendered physically-dependent on ethanol by the oral administration of 9-13 g/kg of ethanol each day over a 4 day period. The binding of radioligands specific for alpha-adrenergic, beta-adrenergic, dopaminergic, serotonergic, muscarinic cholinergic, and GABAergic receptors was assessed at various intervals after withdrawal in several areas of the brain. No alteration in receptor binding was observed at any point under the conditions studied. The data suggest that the signs of an ethanol withdrawal syndrome are not mediated through changes in the ability of neurotransmitters to interact with their receptors.

Reductions of 56Fe Heavy-Particle Irradiation-Induced Deficits in Striatal Muscarinic Receptor Sensitivity by Selective Cross-Activation/Inhibition of Second-Messenger Systems

Recent experiments have revealed radiation-induced (600 MeV/u 56Fe energetic particles) losses of sensitivity of rodent neostriatal muscarinic receptors to stimulation by cholinergic agonists that appears as reductions in oxotremorine enhancement of K(+)-evoked dopamine release. These losses were postulated to be the result of radiation-induced alterations early in phosphoinositide-mediated signal transduction. Additional findings indicated that if the ligand-receptor-G protein interface was by passed no radiation deficits were seen. In the present study, radiation-induced deficits in K(+)-evoked dopamine release were examined in perifused striatal tissue obtained from rats exposed to 0, 0.1 or 1.0 Gy of 56Fe particles (600 MeV/u). Results showed that these deficits could be reduced by co-applying combinations of various pharmacological agents that were known to have differential effects on various second messengers such as 1,4,5-inositol-trisphosphate (IP3). Combinations included oxotremorine-carbachol, and either oxotremorine or carbachol with arginine vasopressin or arachidonic acid. These results are discussed in terms of putative radiation-induced changes in receptor-containing membranes which alter receptor-G protein coupling/uncoupling.

Transient Alterations in Neurotransmitter Activity in the Caudate Nucleus of Rat Brain after a High Dose of Ionizing Radiation

A single 10,000-rad dose of high-energy electrons induced an increase in dopaminergic and cholinergic activity in the caudate nucleus of the rat brain as assessed by K+-stimulated dopamine release in vitro and high-affinity choline uptake. These alterations occur during early transient incapacitation (ETI) and dissipate as the animal recovers behaviorally, in about 30 min after irradiation. Although the responses observed resemble those that result from blockade of dopamine receptors, no radiation-induced changes were found in dopamine-sensitive adenylate cyclase activity and [3H]haloperidol binding, two indices of dopaminergic receptor function. The data suggest that changes in dopaminergic and cholinergic activity are associated with the development of ETI and may play a role in the behavioral decrement observed under this condition.

An automated method for the determination of biogenic amines and their metabolites by high-performance liquid chromatography

An automated high-performance liquid chromatographic method has been developed that allows for the determination of a number of compounds related to catechol- and indoleamine metabolism. The compounds that can be measured include L-DOPA, dopamine, dihydroxyphenylacetic acid, homovanillic acid, 3-methoxytyramine, norepinephrine, 3-methoxy-4-hydroxyphenylglycol, dihydroxyphenylglycol, vanilmandelic acid, epinephrine, 5-hydroxytryptophan, serotonin, 5-hydroxyindoleacetic acid, and 5-hydroxytryptophol. Dihydroxybenzylamine is used as an internal standard. Although dihydroxyphenylglycol and vanilmandelic acid could be detected and quantified, they could not be separated from each other. The method is completely automated and is sensitive enough to detect amounts as low as 500 fmol. Up to 200 samples a week can be analyzed in the automated mode. Using this method, analyses of brain tissue can be accomplished with no need for a cleanup procedure. The value of this procedure lies in its ability to simultaneously determine various amines and metabolites from small tissue samples in the same animals and with automation to analyze a relatively large number of samples a day with little attention by a technician.

Relationship between Linear Energy Transfer and Behavioral Toxicity in Rats Following Exposure to Protons and Heavy Particles

Rats were exposed to protons (155 MeV) or to helium (165 MeV/amu), neon (522 MeV/amu) or argon (670 MeV/amu) particles to evaluate the behavioral toxicity of these types of radiations. Behavioral toxicity was assessed using the conditioned taste aversion paradigm. Exposure to all types of radiation produced dose-dependent increases in the intensity of the acquired taste aversion. However, the intensity of the aversions, measured as the dose that produced a 50% decrease in the intake of the sucrose-conditioned stimulus, did not show significant variation as a function of the linear energy transfer (LET) of the radiation. The results are discussed in terms of the relationship between LET and behavioral toxicity.

Iron-56 irradiation diminishes muscarinic but not {alpha}{sub 1}-adrenergic-stimulated low-K{sub m} GTPase in rat brain

Initial findings from our laboratory have indicated that muscarinic enhancement of K{sup +}-evoked release of dopamine from perifused striatal slices is reduced after exposure to {sup 56}Fe-particle irradiation. This finding suggested that there is a radiation-induced deficit in muscarinic receptor sensitivity. Subsequent findings have indicated that at least part of the loss in sensitivity may occur as a result of alterations in the initial steps of the signal transduction process and involve muscarinic receptor-G protein coupling/uncoupling. The present study was carried out to localize this deficit further by determining carbachol-stimulated low-K{sub m} guanosine triphosphatase (GTPase) activity in striatal and hippocampal tissue obtained from rats exposed to 0, 0.1 or 1.0 Gy of {sup 56}Fe-particle irradiation. In addition, to examine the specificity of the effect of {sup 56}Fe-particle irradiation, {alpha}{sub 1}-adrenergic-stimulated low-K{sub m} GTPase activity was also examined in these tissues. The results showed that there was a high degree of specificity in the effects of {sup 56}Fe particles. Decrements were observed in muscarinic-stimulated low-K{sub m} GTPase in striatum but not in hippocampus, and {sup 56}Fe-particle irradiation did not affect {alpha}{sub 1}-adrenergic low-K{sub m} GTPase activity in either brain tissue. 24 refs., 2 figs.