Effects of subacute toluene exposure on protein phosphorylation levels in rat frontoparietal and striatal membranes

Toluene at environmentally relevant low levels disrupts differentiation of astrocyte precursor cells

Recent findings that describe endocrine disruption caused by exposure to low levels of certain chemicals in the environment have led to a paradigm shift in the way toxicology studies are designed. Toluene at high levels damages the human central nervous system; however, the effects of toluene at low levels have not been studied. The authors used serum-free mouse embryo cells-a precursor of astrocytes-to predict the effect of chemicals on developing brain cells. When serum-free mouse embryo cells were exposed to low levels of toluene, induction of glial fibrillary acidic protein was inhibited. This study demonstrated that environmentally relevant low levels of toluene could disrupt normal prenatal brain development.

Tail pinch increases acetylcholine release in rat striatum even after toluene exposure

The effect of tail pinch on acetylcholine release in the striatum of freely moving rats was studied by microdialysis immediately after inhalation exposure to toluene (2000 ppm, 2 h) or exposure to air only. It has recently been found that toluene increases extracellular dopamine levels while decreasing acetylcholine release, and that dopamine uptake inhibition increases both extracellular dopamine levels and acetylcholine release, suggesting that toluene decreases acetylcholine release by a dopamine-independent mechanism. The present experiment was an attempt to study if a behaviourally induced increase of extracellular dopamine differs from that induced by toluene in affecting striatal acetylcholine release. Acetylcholine released increased during tailpinch in the unexposed as well as the toluene exposed group. No difference between the two groups in the acetylcholine release response to tailpinch was demonstrated. The result supports the conclusion that acute toluene exposure decreases acetylcholine release via a dopamine independent mechanism, and suggests that toluene exposure does not affect the striatal acetylcholine response to an acute stressful stimulus.

Effect of toluene inhalation on extracellular striatal acetylcholine release studied with microdialysis

Intracerebral microdialysis was performed on awake, freely moving rats in order to record effect of toluene exposure on acetylcholine release in striatum. Acetylcholine release decreased during (about 20%) and after (about 60%) toluene exposure (2 hr, 2000 p.p.m.) Striatal acetylcholine release is thought to be mediated by dopamine. In a previous work we found that extracellular dopamine levels increase during toluene exposure. A dopamine uptake inhibitor (LU 19-005, 2 mg/kg) was therefore injected subcutaneously and the effect of increased extracellular dopamine on acetylcholine release within the striatum was monitored in the absence of toluene exposure. LU 19-005 increased striatal dopamine levels six times and the acetylcholine levels increased to about 145% of basal value. The present study shows that toluene exposure decrease acetylcholine release while an injection of a dopamine uptake inhibitor fails to decrease acetylcholine release. Indicating that acute exposure of toluene decreases striatal acetylcholine release by a mechanism that is not mediated by increased extracellular dopamine levels. Our data suggest that toluene decrease acetylcholine release within the striatum and that this effect not is mediated by increased extracellular dopamine levels.

Extracellular dopamine levels within the striatum increase during inhalation exposure to toluene: a microdialysis study in awake, freely moving rats

An exposure chamber for microdialysis on awake, freely moving rats during exposure to volatile agents is described. Inhalation exposure to 1000 and 2000 ppm toluene for 2 h was accompanied by an increase in extracellular dopamine levels within the striatum, but did not affect the homovanillic acid level. Neither the dopamine nor the homovanillic acid level was affected by toluene 500 ppm or isoamylacetate. It is suggested that the action of inhaled toluene on the dopamine neuron differs from that of the anaesthetic halothane, possibly by interfering with dopamine reuptake. Microdialysis seems to be a useful tool for studying the effects of volatile agents on brain neurotransmission.

Effect of subacute toluene administration on the enkephalinergic neuromodulatory system in rats and protective action of ganglioside treatments

Gangliosides perform protective functions in the central nervous system. This paper describes a study of the effect of ganglioside administration on toluene neurotoxicity. Rat brain met-enkephalin immunostaining in the central amygdaloid nuclei showed changes in rats treated simultaneously with gangliosides and toluene with respect to rats treated with toluene alone. It is suggested that gangliosides prevent toluene neurotoxicity at this level, leading to hypothetical neurobehavioral changes.

Subacute exposure to low concentrations of toluene affects dopamine-mediated locomotor activity in the rat

The effects of low concentrations of toluene (40-80 ppm, 3 days, 6 h/day) were investigated on spontaneous and on apomorphine-induced locomotor activity in the rat, and were correlated to effects on S(-)[N-propyl-3H(N)]-propylnorapomorphine ([3H]NPA) binding in rat neostriatal membranes, on membrane fluidity, membrane leakage, and calcium levels in synaptosomes from the frontoparietal cortex, the neostriatum and the subcortical limbic area, and on serum hormone levels. Toluene exposure (80 ppm, post-exposure delay 18 h) alone did not affect locomotor activity, but attenuated apomorphine-induced (0.05 mg/kg, s.c.) suppression of rearing, and potentiated apomorphine-induced (1 mg/kg, s.c.) increases in locomotion and rearing. Toluene exposure increased the KD value of [3H]NPA binding without affecting the Bmax. All these effects were absent at 40 ppm of toluene or at a post-exposure delay of 42 h. Toluene exposure (80 ppm, post-exposure delay of 18 h) did not affect the serum levels of prolactin, TSH, corticosterone, or aldosterone, or synaptosomal membrane fluidity and calcium levels, whereas membrane leakage was increased in the neostriatum. The present study indicates that the reduction of D-2 receptor affinity by short-term, low-dose toluene exposure is accompanied by a reduced D-2 autoreceptor function and an enhanced postsynaptic D-2 receptor function.

Persistent effects of chronic exposure to styrene on the affinity of neostriatal dopamine D-2 receptors

We have investigated whether chronic exposure to styrene could inflict persistent effects on the binding characteristics of dopamine D-2 agonist binding sites in rat neostriatal membranes. Styrene exposure (1000 ppm, 6 months, 16 h/d overnight, and left without exposure for another 5 months) caused a marked increase (+160%) in the IC50 value of dopamine without significantly affecting the total amount of specifically bound [3H]raclopride. The specific [3H]raclopride binding in membranes from subcortical limbic areas was too low to yield acceptable displacement curves. These data indicate that chronic exposure to styrene can induce a persistent decrease in affinity of the neostriatal dopamine D-2 agonist binding sites, possibly mediated by membrane perturbations.

Effects of subacute toluene exposure on neuronal and glial marker proteins in rat brain

The behavior of marker proteins of neurons (gamma-enolase) and glial cells (alpha-enolase, beta-S100 protein and creatine kinase-B) was investigated quantitatively by using enzyme immunoassay systems in toluene-exposed rat brains. Three groups of animals were exposed to toluene vapor at 300 ppm, 1000 ppm, and 3000 ppm, respectively, 8 h/day, 6 days/week, for 2 weeks. After subacute repeated solvent exposure, both neuron-specific gamma-enolase and glial marker proteins displayed an overall concentration-dependent increase tendency in separate brain regions. In cerebrum, only the 3000 ppm group showed a significant increase in alpha-enolase by 27% and creatine kinase-B (CK-B) by 26%. alpha-Enolase and gamma-enolase exhibited a pronounced elevation in cerebellum relative to other brain regions, while beta-S100 protein appeared to be the most markedly altered marker in brainstem. The development of gliosis, which is a frequent phenomenon following CNS damage, is presumed to be responsible for the elevation of glial marker content. Energy metabolism disruption in brain tissues may also bring about the compensatory oversynthesis of glycolytic enzymes such as gamma-enolase, alpha-enolase and CK-B. The dose-dependent alteration patterns following toluene exposure suggest the feasibility of using these brain specific markers to evaluate solvent-induced CNS effects.

Ganglioside GM1 prevents and reverses toluene-induced increases in membrane fluidity and calcium levels in rat brain synaptosomes

The effects of exposure to ganglioside GM1 and to toluene in vitro upon synaptosomal integrity have been examined using fluorescence polarization of two probes: 1-[4(trimethylamino)phenyl]-1,3,5-hexatriene (TMA-DPH) and 1,6-diphenyl-1,3,5-hexatriene (DPH) to measure membrane anisotropy, and the fluorescent indicator fura-2 to assay levels of cytosolic calcium [( Ca2+]i). The anisotropy of both TMA-DPH and DPH was decreased by toluene, implying increased membrane fluidity. The decrease in TMA-DPH but not in DPH anisotropy was prevented by pretreatment with GM1 in concentrations as low as 10 microM. This is not an additive interaction since 10 microM of GM1 alone did not significantly modulate TMA-DPH anisotropy. When the GM1 treatment succeeded the addition of toluene the decrease in anisotropy of both probes was reversed. Toluene treatment increased [Ca2+]i in a dose- and time-dependent manner. This increase could partially be both prevented and reversed by treatment with 50 microM of GM1. These effects may reflect an additive interaction, since this concentration of GM1 alone reduced [Ca2+]i. The present results show that toluene increases membrane fluidity and intracellular calcium levels. These effects may be counteracted by the endogenous compound GM1.

Neurotensin modulates the binding characteristics of dopamine D2 receptors in rat striatal membranes also following treatment with toluene

The effects of neurotensin in vitro (1-100 nM) on the binding characteristics of [3H]N-propylnorapomorphine ([3H]NPA) were analysed in striatal membrane preparations of the adult male rat. Subsequently, it was investigated whether the modulatory effects of 10 nM neurotensin on [3H]NPA binding were altered by treatment with toluene in vivo (80 p.p.m., 3 days, 6 h day-1) and in vitro (19 mumol ml-1). Displacement of [3H]NPA binding by raclopride (IC50 about 15 nM) and SCH 23390 (without effect) indicated that [3H]NPA labelled only D2 dopamine receptors in the present study. Neurotensin was found to reduce the affinity of D2 receptors with a maximum response at 10 nM. At this concentration the KD value was increased by 30-40% without any consistent changes in the number of binding sites. The modulatory effect of neurotensin remained intact also following toluene treatment in vivo and in vitro, although at a higher KD range, since toluene alone increased the KD value of [3H]NPA binding by 40-50%. Thus, the mechanisms mediating the effects of neurotensin and toluene on the D2 receptor are likely to be different. When neurotensin and toluene treatments were combined, the KD values of [3H]NPA binding were about twice as high as in non-treated controls. These additive effects may lead to a severely decreased efficiency of dopamine D2-mediated neurotransmission in vivo.

Persistent effects of neonatal toluene exposure on regional brain catecholamine levels and turnover in the adult male rat

Effects of neonatal toluene exposure (80 ppm, day 1-7, 6 h/day) have been studied on regional brain catecholamine levels and utilization, and on serum levels of hypophyseal and adrenocortical hormones in the adult male rat. Catecholamine levels were measured by quantitative histofluorimetry in the forebrain and hypothalamus and by high pressure liquid chromatography with electrochemical detection in the substantia nigra. Catecholamine utilization was evaluated from the decrease in catecholamines seen after tyrosine hydroxylase inhibition using alpha-methyl-p-tyrosine methyl ester hydrochloride (alpha MT, 250 mg/kg, i.p., 2 h). Serum levels of thyroid stimulating hormone, corticosterone, aldosterone, prolactin and luteinizing hormone were measured by radioimmunoassays. Neonatal toluene exposure produced a reduction of dopamine levels and utilization selectively in the olfactory tubercle and substantia nigra of the adult rat. Furthermore, neonatal toluene exposure produced a significant reduction in the noradrenaline levels and utilization in the substantia nigra and an increase of noradrenaline utilization selectively in the subependymal layer of the median eminence and of the magnocellular part of the paraventricular hypothalamic nucleus. The serum hormone levels were not significantly influenced by neonatal toluene exposure as evaluated in adulthood. However, the alpha MT induced increase in serum prolactin levels was reduced following neonatal exposure to toluene. Neonatal toluene treatment was also found to alter the responses of the catecholamine neurons to subacute toluene exposure in adulthood. In some of the dopamine nerve terminal systems of the forebrain and in the dopamine cell body containing area of the substantia nigra neonatal toluene exposure appears to have made the dopamine neurons insensitive to adult subacute toluene exposure. In the hypothalamic noradrenaline nerve terminal systems, there were even reversed responses to subacute toluene exposure. The present results indicate that neonatal toluene exposure in doses at the threshold limit value produces persistent changes in dopamine and noradrenaline neurons of the forebrain, hypothalamus and substantia nigra in the presence of a relatively intact neuroendocrine system. In addition, neonatal toluene exposure appears to diminish or even counteract the responses to subacute toluene treatment in adulthood.

Effects of chronic toluene exposure on central monoamine and peptide receptors and their interactions in the adult male rat

The effects of chronic toluene exposure (CTE) (80 ppm, 6 h/day, 5 days/week, 3 months) were studied on neuropeptide and 5-hydroxytryptamine receptors, on protein phosphorylation levels and on catecholamine levels in various brain regions in the 15-month-old male rat. Behavioral parameters and serum levels of hypophyseal hormones and corticosterone were also analyzed. CTE selectively reduced [3H]neurotensin [( 3H]NT) binding in the basal layers of the orbital cortex. Instead, CTE increased the binding of [3H]etorphine in the nucleus accumbens and of [125I]vasoactive intestinal polypeptide [( 125I]VIP) in the area postrema and hypoglossal nucleus. Acute treatment with the irreversible monoamine receptor antagonist N-ethoxycarboxyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) increased the binding of [3H]NT in the orbital cortex in toluene exposed rats as compared with the reduced [3H]NT binding obtained in air exposed rats treated with EEDQ. Furthermore, the EEDQ induced increase in [125I]VIP binding in the area postrema and the hypoglossal nucleus was replaced by a reduced binding of [125I]VIP in EEDQ-treated CTE rats. CTE produced an overall increase in calcium-induced back phosphorylation and an overall decrease in cyclic adenosine monophosphate-induced back phosphorylation in the frontoparietal cortex. Noradrenaline stores tended to be reduced within various hypothalamic subnuclei and the serum prolactin levels were increased following CTE. However, no marked effects of CTE were seen on the behavioral parameters. In conclusion, the regional selectivity of CTE in disturbing [3H]NT and [125I]VIP binding may be due to the demonstrated vulnerability of monoamine-neuropeptide interactions to toluene.

Toluene induces changes in the morphology of astroglia and neurons in striatal primary cell cultures

Toluene (4.7-150) mumol per ml) was added for 30 or 60 min to astroglial and neuronal primary cell cultures from rat striatum and changes in cell morphology were analyzed by light microscopy. After 60 min incubation in 40 mumol toluene/ml, the cell bodies of the astrocytes appeared contracted, and their processes and nuclei were clearly visible. At higher doses of toluene the astrocytes seemed to be flattened and major cell damage was visualized by the uptake of vital dyes. The neurons, however, became affected and judged by morphological criteria only at the higher toluene doses. In conclusion, toluene induced morphological changes in primary astrocyte cultures and also in primary neuronal cultures at higher toluene concentrations.

Effects of toluene treatment in vivo and in vitro on the binding characteristics of [3H]neurotensin in rat striatal membranes

The effects of treatment with toluene in vivo (80 ppm, 3 days, 6 h/day) and in vitro (19 mumol/ml) were analyzed on the binding characteristics of [3H]neurotensin in rat striatal membranes. Exposure to toluene in vivo did not produce any significant effects on the binding characteristics of [3H]neurotensin. However, the addition of toluene in vitro caused a trend for a decreased Bmax value and produced a significantly reduced KD value of [3H]neurotensin binding. The absence of effects at 80 ppm indicates that the neurotensin receptor is relatively insensitive to toluene exposure, in contrast to, e.g. the dopamine agonist binding sites. Furthermore, the toluene response of the neurotensin receptor, as seen after treatment in vitro, is different from the responses seen in many monoamine receptors, which show decreased affinities following toluene exposure. It is possible that toluene is mediating its effects on the neurotensin receptor by changing the lipid micro-environment in which the receptor is situated. Another explanation would be that toluene selectively acts on the monoamine receptors, e.g. the more sensitive dopamine receptors, which through receptor-receptor interactions would cause the response seen in the neurotensin receptor. However, it cannot be excluded that the suggested receptor-receptor interaction itself is affected by toluene.

Effects of acute haloperidol treatment on regional catecholamine levels and utilization in rats exposed to toluene

The aim of the present investigation was to evaluate whether the responses of central catecholamine (CA) neurons to CA receptor blockade by haloperidol are altered upon toluene exposure. Male rats were exposed to air or toluene (80 ppm) for 5 and 4 days, 6 h day-1. CA levels and utilization were determined in discrete regions of the forebrain and hypothalamus as well as in the substantia nigra (SN) and anteromedial frontal cortex (AMFC). Serum levels of corticosterone, thyroid stimulating hormone, luteinizing hormone and prolactin were determined by radioimmunoassay procedures. Toluene exposure led to increased dopamine (DA) utilization in the AMFC and increased CA utilization in the paraventricular hypothalamic nuclei. In air-exposed rats haloperidol (1 mg kg-1, i.p., 2 h before killing) increased DA utilization in the marginal part of the nucleus caudatus putamen (CAUD). In toluene-exposed rats, haloperidol induced significant depletions of DA stores in the SN and in the medial and central parts of the CAUD. In the posterior nucleus accumbens (ACC) DA utilization was significantly increased. Combined haloperidol and toluene treatment selectively decreased DA levels in the ACC and SN, and significantly increased DA utilization in the CAUD, as compared with the air-exposed control group. Furthermore, after combined treatment, there was a specific increase in noradrenaline (NA) utilization in the SN and in CA utilization in the medial palisade zone of the median eminence. Serum prolactin levels were substantially raised in both the air and toluene groups after the haloperidol treatment. In conclusion, acute haloperidol treatment preferentially reduces DA levels and increases DA and NA utilization in the SN and in discrete tel- and diencephalic areas in rats exposed to toluene.