Impaired recovery of brain muscarinic receptor sites following an adaptive down-regulation induced by repeated administration of diisopropyl fluorophosphate in aged rats

Effect of hormone replacement therapy in normalizing age related neuronal markers in different age groups of naturally menopausal rats

Aging of the normal brain is accompanied by changes in its structure, function, and metabolism. There are significant gender differences in aging brain. Most of these changes increase during menopausal condition in females when the level of estradiol and progesterone are decreased. The objective of this study was to determine the effect of estradiol and progesterone (separate as well as combined) hormones in neuronal tissues from naturally menopausal rats of different age groups. Results show decreased activity of Acetylcholine esterase (AChE) whereas the level of lipid peroxidation increased with age, and after the hormone treatments both AChE activity and level of lipid peroxidation returned to control values. The deposition of lipofuscin, a pigment that accumulated intraneuronally in brain and other tissues and is considered a marker of aging, was increased with aging and the hormone treatment decreased this deposition. The present study clearly shows reduction in risk factors associated with aging in the murine model system by hormone treatments, namely estrogen and progesterone by increasing the activity of acetylcholine esterase and decreasing the levels of lipid peroxidation and lipofuscin deposition in different parts of aging brain. This study suggests that hormone replacement therapy may either reduce or delay the onset of age related diseases like Alzheimer's, Parkinson's and other neurological disorders.

Neurochemical effects of chronic dietary and repeated high-level acute exposure to chlorpyrifos in rats

Very little is known about the effects of chronic exposure to relatively low levels of anticholinesterase insecticides or how the effects of chronic exposure compare to those of higher, intermittent exposure. To that end, adult male rats were fed an anticholinesterase insecticide, chlorpyrifos (CPF), for 1 year at three levels of dietary exposure: 0, 1, or 5 mg/kg/day (0+oil, 1+oil, and 5+oil). In addition, half of each of these groups also received a bolus dosage of CPF in corn oil ("spiked" animals; 60 mg/kg initially and 45 mg/kg thereafter) every 2 months (0+CPF, 1+CPF, 5+CPF). Animals were analyzed after 6 or 12 months of dosing, and again 3 months after cessation of dosing (i.e., "recovery" animals-six experimental groups with n = 4-6/group/time point). Cholinesterase (ChE) activity was measured in retina, whole blood, plasma, red blood cells, diaphragm, and brain [pons, striatum, and the rest of the brain (referred to simply as "brain")]. Muscarinic receptor density was assessed in retina, pons, and brain, whereas dopamine transporter density and the levels of dopamine and its metabolites were assessed in striatum. Cholinesterase activity at 6 and 12 months was not different in any of the tissues, indicating that a steady state had been reached prior to 6 months. The 1+oil group animals showed ChE inhibition only in the blood, whereas the 5+oil group exhibited > or = 50% ChE inhibition in all tissues tested. One day after the bolus dose, all three groups (0+CPF, 1+CPF, 5+CPF) showed > or = 70% ChE inhibition in all tissues. Muscarinic receptor density decreased only in the brain of the 5+oil and 5+CPF groups, whereas dopamine transporter density increased only at 6 months in all three spiked groups. Striatal dopamine or dopamine metabolite levels did not change at any time. Three months after CPF dosing ended, all end points had returned to control levels. These data indicate that, although chronic feeding with or without intermittent spiked dosages with CPF produces substantial biochemical changes in a dose- and tissue-related manner, there are no persistent biochemical changes.

Inhibitory effects of donepezil hydrochloride (E2020) on cholinesterase activity in brain and peripheral tissues of young and aged rats

Donepezil hydrochloride (donepezil: E2020: (+/-)-2-[(1-benzylpiperidin-4-yl)methyl]-5,6-dimethoxy-indan-1-one monohydrochloride)) is a centrally acting acetylcholinesterase inhibitor developed for the treatment of Alzheimer's disease. In the present study, its inhibitory effect on the activity of cholinesterase ex vivo was evaluated in the brain, plasma, erythrocytes, heart, small intestine, liver and pectoral muscle of young adult as well as aged rats, in comparison with that of tacrine (9-amino-1,2,3,4-tetrahydroacridine hydrochloride). In aged animals, cholinesterase activity in heart, small intestine and pectoral muscle was lower, whereas that in plasma and liver was higher than in young rats. Both groups showed the highest levels in the brain. Donepezil, at doses of 1.25, 2.5 and 5 mg/kg, p.o., inhibited brain, plasma, erythrocyte, liver and pectoral muscle cholinesterase activity in young rats in a dose-dependent manner but had less effect on cholinesterase activity in heart and small intestine. In aged animals, inhibition of cholinesterase activity in the brain, erythrocytes and pectoral muscle by donepezil was more potent than that in young animals. Tacrine, at doses of 5, 10 and 20 mg/kg, p.o., dose-dependently inhibited cholinesterase activity in all tissues of both young and aged animals, but most potently in heart, small intestine and liver. The inhibition of cholinesterase activity by tacrine in the brain, plasma, erythrocytes, heart and liver was more potent in aged rats than in tissues of young rats. Brain and plasma concentrations of unchanged donepezil and tacrine were measured in the same animals as used for the cholinesterase inhibition study. Brain and plasma concentrations of donepezil and tacrine were higher in aged than in young animals. It is concluded that the inhibitory effects of donepezil and tacrine on cholinesterase activity are greater in aged than in young rats, owing to differences in the tissue concentrations of these compounds between young and aged animals. It is also suggested that the effect of donepezil on cholinesterase activity is more tissue-selective than that of tacrine.

Chronic, low-level exposure to the cholinesterase inhibitor DFP. I. Time course of neurochemical changes in the rat pontomesencephalic tegmentum

Rats were repeatedly administered with a low dose of diisopropylfluorosphosphate (DFP; 0.2 mg/kg/day, SC, for 9 or 21 days), an irreversible cholinesterase (ChE) inhibitor. Control rats received a daily injection of oil vehicle. Neurochemical changes occurring in the pontomesencephalic tegmentum (PMT), a brain stem region critically involved in behavioral state control, were evaluated at various times of treatment and after DFP withdrawal. First, enzyme assay revealed a profile of ChE inhibition in the whole PMT which looked like that observed in the striatum; both the inhibition and recovery proceeded more slowly than they did in the plasma. Second, quantitative histochemistry indicated that ChE activity in the mesopontine cholinergic nuclei and the pontine reticular formation progressively decreased across the first days of DFP exposure, to reach an asymptotic level of inhibition after 6 days (74-82% inhibition). The inhibition was less pronounced in the locus coeruleus (49%). Third, [3H]QNB autoradiography showed that muscarinic receptor density was unchanged in any of the PMT areas selected. These results are discussed regarding the question of regional variation in susceptibility to anti-ChE agents. To what extent behavioral state alterations occur concomitantly with ChE activity changes is assessed in the companion article.

Transient global brain ischemia in young and aged rats: differences in severity and progression, but not localisation, of lesions evaluated by magnetic resonance imaging

A model of transient global brain ischemia consisting of bilateral occlusion of common carotid arteries for 10 min and mild hypoxia (15% O2-85% N2) for 20 min was studied by means of MRI in young and aged Fischer 344 rats (3-4 and 24-26 months, respectively). Ischemia was assessed by full suppression of spontaneous EEG activity, which reappeared and normalized similarly in the two age-groups. The survival of young with respect to aged rats was considerably higher both at 24 h (20/20, i.e. 100% vs 12/16, i.e. 75%) and at 48 h (16/20, i.e. 80% vs 6/16, i.e. 38%). The localisation of brain lesions, their severity and progression were evaluated by a diffusion-weighted MRI (DWI) sequence at 24 and 48 h post-ischemia. There were no DWI-detectable lesions in eight out of 20 young and two out of 12 aged rats. The localisation of DWI-detected lesions was rather similar in rats of the two age-groups. In fact, the cerebral cortex, mainly parietal, occipital and temporal lobes were damaged in 83% of young and 90% of aged rats. The respective percentages for the thalamus were 83 and 60%, for the striatum 58 and 50%, and for the hippocampus 25 and 30%. The lesions present in the cerebral cortex and the thalamus were considerably more severe in aged than in young rats. In conclusion, in spite of similar localisation of ischemic lesions in the two age-groups, their incidence was higher, appearance more rapid and severity more pronounced in aged with respect to young rats. This resulted in a considerably higher mortality of the former. The overall data indicate that the age issue is very important in experimental ischemia research.

Developmental and age-related changes in the D2 dopamine receptor mRNA subtypes in rat brain

The influence of ontogeny and aging on the D2 dopamine receptor mRNA in rat brain were examined using in situ hybridization histochemistry and Northern analysis utilizing oligonucleotide probes complementary to the different D2 mRNA subtypes. At birth, there was a high level of D2 dopamine receptor mRNA in corpus striatum relative to that found in the cerebral cortex and other brain areas. The hybridization signal of striatum (using a probe that hybridizes to both the D2A and D2B mRNA) increased during the first two postnatal weeks, reached a peak at day 16, then declined slightly. The D2A mRNA showed a similar distribution and developmental pattern. Intracisternal injection of 6-hydroxydopamine into neonates did not significantly alter the increase of the D2 dopamine receptor mRNAs, suggesting that neuronal input does not influence the ontogenetic development of this mRNA. In striatum, olfactory tubercule and inferior colliculus, the D2A mRNA declined between 3 and 24 months of age. By contrast, there was an age-related increase in the D2A mRNA in the anterior and intermediate lobes of the pituitary. The mRNA for the D2B dopamine receptor showed very low but nevertheless detectable levels in striatum, olfactory tubercule and pituitary. Like with the D2A mRNA, in 24-month-old rats the D2B mRNA declined in striatum and olfactory tubercule and increased in pituitary. These results show that there are differential tissue-related changes in the mRNAs for the D2 dopamine receptor during both development and aging.

Neurotransmitter receptor plasticity in aging

Neurotransmitter receptor plasticity is an important part of the compensatory processes by which the central nervous system adapts to pathological insult, long-term exposure to drugs or neuronal loss with advanced age. Receptor plasticity can be manifest as changes in the number of receptors (i.e., up- or down-regulation), changes in expression of mRNA for discrete receptor proteins, or alterations in receptor coupling to signal transduction systems. Evidence exists for impaired plasticity of neurons in the aged brain, which results in decreased ability to adjust to changes in their environment. However, such data are highly dependent on the neurotransmitter examined, the stimulus for receptor regulation and the animal model chosen for study. For example, senescent rats show an age-related impairment of muscarinic receptor up- or down-regulation after long-term exposure to cholinergic drugs. Thus, young rats exposed to chronic (three weeks) intracerebroventricular infusions of methylatropine or oxotremorine exhibit compensatory changes in the density of muscarinic receptors in frontal cortex and hypothalamus. In contrast, 3H-QNB binding is unaltered in the same brain regions of identically treated senescent rats. Similar observations of impaired muscarinic receptor plasticity in senescent animals have been confirmed by other investigators. Age-related differences in coupling of brain muscarinic receptors to G-proteins and in muscarinic receptor-stimulated phosphoinositide hydrolysis have also been reported. Interestingly, neuropeptides such as neurotensin, cholecystokinin and VIP can potentiate carbachol-stimulated phosphoinositide hydrolysis in frontal cortex of both young and aged rats. This adds another level at which cholinergic neurotransmission may be modulated in senescent animals. Potential age-related differences in the effects of chronic drug treatments or experimental brain lesions on muscarinic receptor coupling to second messenger systems or on expression of mRNA for particular muscarinic receptors are currently unknown. Hence, it is possible that senescent animals may show additional deficiencies in plasticity of muscarinic receptor mediated signal transduction or expression of muscarinic receptors subtypes.

Effects of excitatory amino acids on inositol phosphate accumulation in slices of the cerebral cortex of young and aged rats

The effects of glutamate, NMDA and quisqualate on carbachol- and norepinephrine-elicited formation of inositol phosphate (IP) were evaluated in slices prepared from the cerebral cortex of 3- and 24-month Sprague-Dawley rats. Glutamate, NMDA, and quisqualate antagonized the IP response to carbachol in a concentration-dependent fashion. This antagonism was more pronounced in aged than in young rats, both for glutamate (IC5O 0.114 and 0.210 mM) and NMDA (IC5O 0.0029 and 0.127 mM), but not for quisqualate. Glutamate (but not NMDA) also antagonized in a concentration-dependent fashion the IP response to norepinephrine, IC50s were 0.061 and 0.126 mM for aged and young rats, respectively; quisqualate had an inhibitory effect only at 1 mM concentration in the two age-groups, while in aged rats some stimulatory effect was present at 0.1 mM concentration. Glutamate, NMDA and quisqualate (1 mM) did not affect basal IP accumulation in either young or aged rats; quisqualate, however, at 0.1 mM concentration had some stimulatory effect, more pronounced in aged rats. This effect was probably responsible for the biphasic effect of quisqualate in this age-group. The most important finding consists of the demonstration of an age-related increase in the inhibitory effects of NMDA on carbachol-induced IP accumulation. This implies an altered modulation of cholinergic post-receptor mechanisms by glutamatergic mechanisms.

Effect of S-adenosyl-L-methionine on brain muscarinic receptors of aged rats

The number of muscarinic receptors in the striatum and hippocampus of aged rats is significantly lower than the number measured in young animals. The treatment of aged rats for 30 days with S-adenosyl-L-methionine (SAM) restored the number of muscarinic receptors to levels found in the striatum and hippocampus from young animals. We did not observe a clear-cut difference between the dissociation constants of untreated young and untreated or SAM-treated aged rats, whereas the binding capacity varied. Moreover, in vitro addition of SAM to hippocampal membranes from aged rats resulted in a significant increase in the number of binding sites. This in vitro effect was antagonized by S-adenosyl-L-homocysteine, a specific in vitro inhibitor of methyltransferase activity. The reduction in the muscarinic receptor density could be related to a decrease in neuronal membrane fluidity induced by aging, while its increase after SAM treatment might be ascribed to the ability of this methyl donor to increase the fluidity of cell membranes by stimulating phospholipid synthesis.

Age-related changes in acetylcholinesterase and its molecular forms in various brain areas of rats

A previous study conducted in this laboratory revealed a decrease in total cholinesterase (total ChE) in the cerebral cortex, hippocampus and striatum in aged rats (24 months) of various strains, as compared with young animals (3 months). The purpose of the present experiments was to extend the study to other brain areas (hypothalamus, medulla-pons and cerebellum) and to assess whether this decrease was dependent on the reduction of either specific acetylcholinesterase (AChE) or butyrylcholinesterase (BuChE) or both. By using ultracentrifugation on a sucrose gradient, the molecular forms of AChE were evaluated in all the brain areas of young and aged Sprague-Dawley rats. In young rats the regional distribution of total ChE and AChE varied considerably with respect to BuChE. The age-related loss of total ChE was seen in all areas. Although there was a reduction of AChE and, to somewhat lesser extent, of BuChE in the cerebral cortex, hippocampus, striatum, and hypothalamus (but not in the medulla-pons or the cerebellum), the ratio AChE/BuChE was not substantially modified by age. Two molecular forms of AChE, namely G4 (globular tetrameric) and G1 (monomeric), were detected in all the brain areas. Their distribution, expressed as G4/G1 ratio, varied in young rats from about 7.5 for the striatum to about 2.0 for the medulla-pons and cerebellum. The age-related changes consisted in a significant and selective loss of the enzymatic activity of G4 forms in the cerebral cortex, hippocampus, striatum, and hypothalamus, which resulted in a significant decrease of the G4/G1 ratio. No such changes were found in the medulla-pons or the cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

Size and charge isomers of acetylcholinesterase in the cerebral cortex of young and aged rats

Previous studies in this laboratory showed an age-related decline of acetylcholinesterase (AChE) activity in the cerebral cortex of rats. In the present study the age-related differences in enzymatic activity were evaluated in terms of individual molecular forms. Extracts containing total, soluble and membrane-bound AChE were analyzed both by ultracentrifugation in sucrose gradient and by non-denaturing gradient polyacrylamide gel electrophoresis. By ultracentrifugation two molecular forms, namely 10S and 4S (corresponding to tetrameric-G4 and monomeric-G1 forms, respectively) were separated in extracts of total and soluble AChE, while only 10S forms were present in extracts of membrane-bound AChE. Electrophoresis of soluble AChE extracts revealed slowly- and fast-migrating bands, grouped in two clusters of at least three bands each; membrane-bound AChE contained only a single slowly-migrating band. Electrophoresis of the single forms isolated by ultracentrifugation showed that slowly- and fast-migrating bands corresponded to G4 and G1 forms, respectively. Therefore, in soluble AChE no one-to-one relationship between charge- and size-isomers was observed; on the contrary, such relationship has been shown for membrane-bound AChE. This implies that soluble G4 forms and membrane-bound-G4 forms are electrophoretically different, being heterogeneous the former and homogeneous the latter. The age-related decline of total AChE, accompanied by a decrease of G4/G1 ratio, depended mainly on a decrease of membrane-bound AChE while soluble AChE and its G4/G1 ratio was unchanged. The qualitative pattern of charge isomers was not modified by aging.

Adaptive processes of the central and autonomic cholinergic neurotransmitter system: age-related differences

Potential age-related differences in the response of the ileum strip longitudinal and circular muscle to repeated treatment with diisopropyl fluorophosphate (DFP) were evaluated in Sprague-Dawley rats. The response was measured in terms of both biochemical parameters (acetylcholinesterase-AChE inhibition, muscarinic acetylcholine receptor binding sites-mAChRs, choline acetyltransferase-ChAT) and functional responsiveness (contractility of the isolated ileum stimulated by cholinergic agonists). The biochemical data were compared with those obtained for the cerebral cortex. Male 3- and 24-month old rats were s.c. injected with DFP on alternate days for 2 weeks (doses in mg/kg: first 1.1, two of 0.7 and four of 0.35). They were killed 48 hr and 7, 14, 21, 28 and 35 days after the last treatment. In the ileum strip of control rats there was a significant age-related decline of AChE, maximal density of 3H-QNB binding sites (Bmax) and ChAT. During the first week of DFP treatment the cholinergic syndrome was more pronounced in aged than in young rats, resulting in 35% and 10% mortality, respectively; subsequently the syndrome attenuated. At the end of DFP treatment ileal AChE were inhibited by about 30%; the down-regulation of mAChRs was about 50% in young and 35% in aged rats. No significant differences in the recovery rate of AChE were noted between young and aged rats (normalization within 7 days). On the contrary, mAChRs normalized within 5 weeks in young and 3 weeks in aged rats. This was probably due to more adaptive decline in the former group. There was a post-treatment increase of ChAT, transitory in young and persistent in aged rats. In spite of age-related marked loss of ileal mAChRs there were only little, although significant, changes in the contractile responsiveness of the isolated ileum to cholinergic agonists. Considerable DFP-induced down-regulation of mAChRs was not accompanied by changes in contractility stimulated by the agonists. The overall data indicate that age- and treatment-induced changes of AChE, mAChRs and ChAT in the ileum strip differ considerably from those observed in the cerebral cortex of the same rats.