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

Effects of Concord grape juice on cognitive and motor deficits in aging

OBJECTIVE

Animals and humans show increased motor and cognitive declines with aging that are thought to be due to increased susceptibility to the long-term effects of oxidative stress and inflammation. Previous findings have suggested that reversals in these age-related declines might be accomplished by increasing the dietary intake of polyphenolics found in fruits and vegetables, especially those identified as being high in antioxidant and anti-inflammatory activities.

METHODS

We investigated the beneficial effects of two concentrations of Concord grape juice (10% and 50%) compared with a calorically matched placebo for their effectiveness in reversing age-related deficits in behavioral and neuronal functions in aged Fischer 344 rats.

RESULTS

Rats that drank the 10% grape juice from age 19 to 21 mo had improvements in oxotremorine enhancement of K+-evoked release of dopamine from striatal slices and in cognitive performance on the Morris water maze, and the 50% grape juice produced improvements in motor function.

CONCLUSIONS

These findings suggest that, in addition to their known beneficial effects on cancer and heart disease, polyphenolics in foods may be beneficial in reversing the course of neuronal and behavioral aging, possibly through a multiplicity of direct and indirect effects that can affect a variety of neuronal parameters.

Dietary supplementation with fruit polyphenolics ameliorates age-related deficits in behavior and neuronal markers of inflammation and oxidative stress

Dietary supplementation with fruit or vegetable extracts can ameliorate age-related declines in measures of learning, memory, motor performance, and neuronal signal transduction in a rat model. To date, blueberries have proved most effective at improving measures of motor performance, spatial learning and memory, and neuronal functioning in old rats. In an effort to further characterize the bioactive properties of fruits rich in color and correspondingly high in anthocyanins and other polyphenolics, 19-month-old male Fischer rats were fed a well-balanced control diet, or the diet supplemented with 2% extract from either blueberry, cranberry, blackcurrant, or Boysenberry fruit for eight weeks before testing began. The blackcurrant and cranberry diets enhanced neuronal signal transduction as measured by striatal dopamine release, while the blueberry and cranberry diets were effective in ameliorating deficits in motor performance and hippocampal HSP70 neuroprotection; these changes in HSP70 were positively correlated with performance on the inclined screen. It appears that the polyphenols in blueberries and cranberries have the ability to improve muscle tone, strength and balance in aging rats, whereas polyphenols in blueberries, cranberries and blackcurrants have the ability to enhance neuronal functioning and restore the brain's ability to generate a neuroprotective response to stress.

Muscarinic receptor subtype determines vulnerability to oxidative stress in COS-7 cells

Research has suggested that there may be increased brain-region selective vulnerability to oxidative stress in aging and that Vulnerability to oxidative stress may be important in determining regional differences in neuronal aging. We assessed whether one factor determining vulnerability to oxidative stress might involve qualitative/quantitative differences in receptor subtypes in various neuronal populations. COS-7 cells were transfected with one of five muscarinic receptor subtypes (M1-M5 AChR) to DA (1 mM for 4 h) and intracellular Ca2+ levels were examined via fluorescent imaging analysis prior to and following 750 microM oxotremorine (oxo). Results indicated that the ability of the cells to clear excess Ca2+ (i.e., Ca2+ Recovery) following oxo stimulation varied as a function of transfected mAChR subtype, with DA-treated M1, M2, or M4 cells showing greater decrements in Recovery than those transfected with M3 or M5 AChR. A similar pattern of results in M1- or M3-transfected DA-exposed cells was seen with respect to Viability. Viability of the untransfected cells was unaffected by DA. Pretreatment with Trolox (a Vitamin E analog) or PBN (a nitrone trapping agent) did not alter the DA effects on cell Recovery in the M1-transfected cells, but were effective in preventing the decrements in Viability. The calcium channel antagonists (L and N, respectively), Nifedipine and Conotoxin prevented both the DA-induced deficits in Recovery and Viability. Results are discussed in terms of receptor involvement in the regional differences in Vulnerability to oxidative stress with age, and that loss of neuronal function may not inevitably lead to cell death.

Galpha(i2) but not Galpha(i3) is required for muscarinic inhibition of contractility and calcium currents in adult cardiomyocytes

Parasympathetic stimulation of the heart acts through M(2)-muscarinic acetylcholine receptors to regulate ion channel activity and subsequent inotropic status. Although muscarinic signal transduction is mediated via pertussis toxin-sensitive G proteins Galpha(i/o), the specific signal transduction requirements of Galpha(i2) and Galpha(i3) in mediating muscarinic regulated L-type calcium currents (I(Ca, L)), intracellular calcium, and cell contractility remain to be determined. Adult ventricular myocytes were isolated from Galpha(i2)-null mice, Galpha(i3)-null mice, and their wild-type littermates. Cell shortening, intracellular calcium levels, and I(Ca, L) were all measured in response to isoproterenol, a beta-adrenergic receptor agonist, and carbachol, a cholinergic receptor agonist. With isoproterenol stimulation, myocytes from all groups demonstrated a marked increase in calcium currents, correlating with augmented intracellular calcium transient amplitude and cell shortening. Carbachol significantly attenuated the isoproterenol response in wild-type and Galpha(i3)-null cells but had no effect in Galpha(i2)-null cells. This study demonstrates that Galpha(i2), but not Galpha(i3), is required for muscarinic inhibition of the beta-adrenergic response in adult murine ventricular myocytes.

Effect of fruits, vegetables, or vitamin E--rich diet on vitamins E and C distribution in peripheral and brain tissues: implications for brain function

Age-related neurodegenerative conditions are the principal cause of declining cognitive and motor function during aging. Evidence support that fruits and vegetables containing generous amounts of antioxidant nutrients are important for neurological function. We investigated the effect of diets enriched with fruits or vegetables but low in vitamin E and a diet high in vitamin E on the distribution of vitamins C and E in the brain and dopamine release of Fischer 344 rat model, over an 8-month period. The low-vitamin E diet resulted in lowered alpha-tocopherol levels in brain and peripheral tissues, whereas the animals that received a diet enriched in vitamin E showed a significant increase, between 500-900%. Vitamin C concentration in plasma, heart, and liver was reduced in the vitamin E-supplemented group. It is concluded that supplementation or depletion of alpha-tocopherol for 8 months results in marked changes in vitamin E levels in brain tissue and peripheral tissues, and varied distribution of alpha-tocopherol throughout the different brain regions examined. In addition, compared to control group, rats supplemented with strawberry, spinach, or vitamin E showed a significant enhancement in striatal dopamine release. These findings suggest that other nutrients present in fruits and vegetables, in addition to the well-known antioxidants, may be important for brain function.

CNS-induced deficits of heavy particle irradiation in space: the aging connection

Our research over the last several years has suggested that young (3 mo) rats exposed to whole-body 56Fe irradiation show neuronal signal transduction alterations and accompanying motor behavioral changes that are similar to those seen in aged (22-24 mo) rats. Since it has been postulated that 1-2% of the composition of cosmic rays contain 56Fe particles of heavy particle irradiation, there may be significant CNS effects on astronauts on long-term space flights which could produce behavioral changes that could be expressed during the mission or at some time after the return. These, when combined with other effects such as weightlessness and exposure to proton irradiations may even supercede mutagenic effects. It is suggested that by determining mechanistic relationships that might exist between aging and irradiation it may be possible to determine the common factor(s) involved in both perturbations and develop procedures to offset their deleterious effects. For example, one method that has been effective is nutritional modification.

Reversals of age-related declines in neuronal signal transduction, cognitive, and motor behavioral deficits with blueberry, spinach, or strawberry dietary supplementation

Ample research indicates that age-related neuronal-behavioral decrements are the result of oxidative stress that may be ameliorated by antioxidants. Our previous study had shown that rats given dietary supplements of fruit and vegetable extracts with high antioxidant activity for 8 months beginning at 6 months of age retarded age-related declines in neuronal and cognitive function. The present study showed that such supplements (strawberry, spinach, or blueberry at 14.8, 9.1, or 18.6 gm of dried aqueous extract per kilogram of diet, respectively) fed for 8 weeks to 19-month-old Fischer 344 rats were also effective in reversing age-related deficits in several neuronal and behavioral parameters including: oxotremorine enhancement of K(+)-evoked release of dopamine from striatal slices, carbachol-stimulated GTPase activity, striatal Ca(45) buffering in striatal synaptosomes, motor behavioral performance on the rod walking and accelerod tasks, and Morris water maze performance. These findings suggest that, in addition to their known beneficial effects on cancer and heart disease, phytochemicals present in antioxidant-rich foods may be beneficial in reversing the course of neuronal and behavioral aging.

Membrane and receptor modifications of oxidative stress vulnerability in aging. Nutritional considerations

Evidence suggests that oxidative stress (OS) may contribute to the pathogenesis of age-related decrements in neuronal function and that OS vulnerability increases as a function of age. In addition to decreased endogenous protection, increases in OS vulnerability may result from changes in membrane lipids and distribution of receptor subtype. Using a PC-12 cell model system, we have shown that H2O2 or dopamine (DA) exposure induced deficits in the cell's ability to clear (extrude/sequester, E/S) Ca2+ that are similar to those seen in aging. When plasma membrane concentrations of sphingomyelin (SPM) were used, the SPM metabolite, sphingosine-1-phosphate was increased to the same levels as those seen in aging, and enhancement of OS-induced decreases in calcium E/S following KCL depolarization was observed. Differential decreases in CA2+ E/S were also seen following DA-induced OS in COS-7 cells transfected with one of five muscarinic receptor subtypes. Cells transfected with either M1, M2, or M4 receptors showed significantly greater vulnerability to OS (as expressed by greater decrements in calcium E/S and cell death) than those transfected with M3 or M5 receptors. The vitamin E analogue, Trolox, and the nitrone-trapping agent, PBN, were not effective in altering E/S decrements but were effective in preventing cell death 24 h after OS exposure. These findings suggest that putative regional (e.g., striatum and hippocampus) increases in OS vulnerability and loss of neuronal function in aging may be dependent upon membrane SPM concentration and receptor subtype. In related studies, attempts were made to determine whether increased OS protection via nutritional increases in antioxidant levels in rats [using diets supplemented with vitamin E (500IU/kg), strawberry extracts (9.4 g/kg dried aqueous extract, DAE), spinach (6.7 g/kg DAE), or blueberry extracts (10 g/kg DEA for six weeks)] would protect against exposure to 100% O2 (a model of accelerated neuronal aging). Results indicated that these diets were effective in preventing OS-induced decrements in several parameters (e.g., nerve growth factor decreases), suggesting that although there may be increases in OS vulnerability in aging, phytochemicals present in antioxidant-rich foods may be beneficial in reducing or retarding the functional central nervous system deficits seen in aging or oxidative insult.

Long-term dietary strawberry, spinach, or vitamin E supplementation retards the onset of age-related neuronal signal-transduction and cognitive behavioral deficits

Recent research has indicated that increased vulnerability to oxidative stress may be the major factor involved in CNS functional declines in aging and age-related neurodegenerative diseases, and that antioxidants, e.g., vitamin E, may ameliorate or prevent these declines. Present studies examined whether long-term feeding of Fischer 344 rats, beginning when the rats were 6 months of age and continuing for 8 months, with diets supplemented with a fruit or vegetable extract identified as being high in antioxidant activity, could prevent the age-related induction of receptor-mediated signal transduction deficits that might have a behavioral component. Thus, the following parameters were examined: (1) oxotremorine-enhanced striatal dopamine release (OX-K+-ERDA), (2) cerebellar beta receptor augmentation of GABA responding, (3) striatal synaptosomal 45Ca2+ clearance, (4) carbachol-stimulated GTPase activity, and (5) Morris water maze performance. The rats were given control diets or those supplemented with strawberry extracts (SE), 9.5 gm/kg dried aqueous extract (DAE), spinach (SPN 6.4 gm/kg DAE), or vitamin E (500 IU/kg). Results indicated that SPN-fed rats demonstrated the greatest retardation of age-effects on all parameters except GTPase activity, on which SE had the greatest effect, whereas SE and vitamin E showed significant but equal protection against these age-induced deficits on the other parameters. For example, OX-K+-ERDA enhancement was four times greater in the SPN group than in controls. Thus, phytochemicals present in antioxidant-rich foods such as spinach may be beneficial in retarding functional age-related CNS and cognitive behavioral deficits and, perhaps, may have some benefit in neurodegenerative disease.

Age-related neurodegeneration and oxidative stress: putative nutritional intervention

This review describes age-related changes that occur in neuronal function and cites evidence to show that these alterations may be the result of increased sensitivity to oxidative stress (OS). Evidence is presented to show that the abilities to mitigate the OS effects and to repair the damage from OS show decline as a function of age. Results from age- and OS-sensitive tests are given; these results indicate that one of the major sites of action of OS is the membranes, especially if compromised by high amounts of sphingomyelin, and one of the major effects of OS is to further alter the calcium disregulation in aging. It is suggested that attempts to increase antioxidant protection through diets comprised of fruits and vegetables identified as being high in total antioxidant activity might prevent or reverse the deleterious OS effects on neuronal aging.

Receptor- and age-selective effects of dopamine oxidation on receptor-G protein interactions in the striatum

The striatum contains a high concentration of oxidizable dopamine (DA), and the aged organism shows a decreased ability to respond to oxidative stress (OS), making this area extremely vulnerable to free radical insult. To determine the receptor specificity of this putative increase in OS sensitivity, striatal slices from 6- and 24-month-old animals were incubated (30 min, 37 degrees C) in a modified Krebs medium containing 0 to 500 microM DA with or without a preincubation (15 min) in a nitrone trapping agent, 1 or 5 mM alpha-phenyl-n-tert-butyl nitrone (PBN), and changes in low Km GTPase activity (an index of receptor-G protein coupling/uncoupling) assessed in muscarinic, 5-HT1A D1, and D2 receptors stimulated with carbachol, 8 OH-DPAT-HBr, SKF 38393, or quinelorane, respectively. DA exposure induced selective decreases in the stimulated activity in all of these receptor systems, and an overall increase in conjugated dienes (56%) of the young. In the case of carbachol and 8 OH-DPAT-HBr, the DA-induced deficits in GTPase stimulation were seen primarily in the young (61 and 32%, respectively), while DA-induced deficits in quinelorane (D2) stimulation were seen in both age groups. In the case of SKF 38393-stimulation (D1) the DA-induced deficits were higher in the striatal tissue from the old. The DA-induced decreases in carbachol stimulated GTPase activity in the tissue from the young could be prevented by pretreatment with PBN or the DA uptake inhibitor, nomifensin. No effect of nomifensin was seen in the old, because their DA uptake mechanisms were already compromised. These results suggest that although age-related declines in DA uptake may provide some protection against the OS effects in muscarinic or 5-HT1A receptors, other factors may increase the vulnerability of DA neurons to OS, even with reductions in DA uptake.

CNS effects of heavy particle irradiation in space: behavioral implications

Research from several sources indicates that young (3 mo) rats exposed to heavy particle irradiation (56Fe irradiation) produces changes in motor behavior as well as alterations in neuronal transmission similar to those seen in aged (22-24 mo) rats. These changes are specific to neuronal systems that are affected by aging. Since 56Fe particles make up approximately 1-2% of cosmic rays, these findings suggest that the neuronal effects of heavy particle irradiation on long-term space flights may be significant, and may even supercede subsequent mutagenic effects in their mission capabilities. It is suggested that among other methods, it may be possible to utilize nutritional modification procedures to offset the putative deleterious effects of these particles in space.

Cholesterol: a two-edged sword in brain aging

Previous research from several laboratories has indicated that cholesterol (CHO) accumulates in neuronal membranes and alters their structural and signal transduction (ST) properties during aging. The possible reasons for these increases in membrane CHO have not been specified. However, present findings suggest that such accumulation may actually serve to protect neuronal tissue from oxidative damage. Striatal slices (6, 24 month rats) were preincubated in 1 mM CHO (30 min) followed by incubation with H2O2 (10 microM, 30 min). The slices were then either superfused with 30 mM KCl in the presence or absence of 500 microM oxotremorine (Ox), and K(+)-evoked dopamine release (K(+)-ERDA) examined or assessed for carbachol-stimulated low K(m) GTPase activity. The results indicated that CHO incubation prior to H2O2 in either age group was effective in preventing H2O2 reductions in both non-Ox-enhanced K(+)-ERDA and Ox conditions, as well as sodium nitroprusside (SNP 150 microM)-induced decreases in K(+)-ERDA. In addition, H2O2-induced deficits in carbachol-stimulated low K(m) GTPase activity were reduced in the striatal tissue from the old animals pretreated with CHO. However, if the slices were incubated in H2O2 prior to CHO exposure, CHO enhanced the H2O2 effects in the tissue from the old animals. Thus, depending upon the order of exposure, CHO functioned to enhance or retard the effects of oxidative stress, in an age-dependent manner.

Oxidative stress and age-related neuronal deficits

Research from our laboratory has indicated that the loss of sensitivity that occurs in several receptor systems as a function of age may be an index of an increasing inability to respond to oxidative stress (OS). This loss occurs partially as a result of altered signal transduction (ST). Assessments have involved determining the nature of age-related reductions in oxotremorine enhancement of K(+)-evoked dopamine release (K(+)-ERDA) from superfused striatal slices. Using this model, we have found that 1. Reductions can be restored with in vivo administration of the free-radical trapping agent, N-tert-butyl-alpha-phenylnitrone (PBN); 2. Decrements in DA release induced by NO or H2O2 from striatal slices from both young and old animals could be restored with alpha-tocopherol or PBN; 3. ST decrements, such as those seen in aging, could be induced with radiation exposure; and 4. Pre-incubation of the striatal slices with cholesterol decreased subsequent deleterious effects of NO or OH. on DA release. Thus, cholesterol, which increases in neuronal membranes as a function of age, may function as a potent antioxidant and protectant against neuronal damage. These results suggest that therapeutic efforts to restore cognitive deficits in aging and age-related disease might begin with antioxidant reversal of ST decrements.

Age differences in sensitivity to H2O2- or NO-induced reductions in K(+)-evoked dopamine release from superfused striatal slices: reversals by PBN or Trolox

Previous research has indicated that many age-related functional alterations may be the result of a decreased ability of the organism to respond to oxidative stress (OS). However, this hypothesis is based on indirect indices of function (e.g., increased vulnerability of hepatocytes from senescent animals to H2O2-induced DNA damage, increases in lipofuscin accumulation). More direct tests of this hypothesis, especially as it relates to brain aging, have not been extensively undertaken. Present experiments were carried out to make such tests by examining age differences in the sensitivity to OS on reductions in striatal dopamine (DA) release. Thus, K(+)-evoked DA (K(+)-ERDA) release from superfused striatal slices from young (6-8 month) and old (24-25 month) animals was examined following either: (a) application of the NO-generator sodium nitroprusside or (b) preincubation with H2O2. In order to assess the specific effects of OS on muscarinic (mAChR) sensitivity, oxotremorine-enhancement of K(+) -ERDA was examined following incubation with H2O2. Results showed that the striatal tissue from the old animals showed greater sensitivity to both H2O2 and NO than young animals, and stimulated DA decreased at lower concentrations of these agents (e.g., NO--100 microM young, 30 microM old). In addition, H2O2 was also effective in reducing oxo-enhanced K(+)-ERDA and was more effective as a function of age. If the striatal tissue was incubated in either Trolox (alpha-tocopherol) or alpha-phenyl-n-tert-butyl nitrone (PBN) prior to OS, the negative effects of NO. and H2O2 were reversed in both age groups. Results are discussed in terms of age-related membrane and endogenous antioxidant alterations that could induce increases in sensitivity to OS and the specificity of antioxidants in reducing this sensitivity in key functional systems.

Changes in tissue responsiveness to hormones and neurotransmitters during aging

Responsiveness to many hormones and neurotransmitters by target tissues is altered during aging. Three model systems that are representative of these changes are reviewed. These are (1) striatal dopaminergic regulation of motor function, (2) IP3 and calcium-mediated electrolyte and neurosecretion by the alpha1-adrenergic and muscarinic cholinergic systems in parotid and corpus striatum, and (3) beta adrenergic and epidermal growth factor (EGF) stimulated DNA synthesis in hepatocytes.

Age-specific alterations in muscarinic stimulation of K(+)-evoked dopamine release from striatal slices by cholesterol and S-adenosyl-L-methionine

The present experiments were carried out in order to test the hypothesis that age-related signal transduction (ST) deficits may occur as a result of structural changes in the membrane that are reflected partially as increased membrane microviscosity. Oxotremorine (oxo) enhancement of K(+)-evoked release of dopamine (K(+)-ERDA) was examined in superfused striatal slices from mature (6 months) and old (24 months) Wistar rats incubated (1 or 4 h, 37 degrees C) with graded concentrations of S-adenosyl-L-methionine (SAM) or cholesterol hemisuccinate (CHO) in a modified Krebs medium. Tissue was then assessed for one of the following: (a) the degree of oxo-enhanced K(+)-ERDA, (b) carbachol stimulated low Km GTPase activity, or (c) alterations in membrane microviscosity. In other experiments the tissue was incubated in CHO followed by SAM (or the reverse), and oxo-enhanced K(+)-ERDA examined. Results indicated that SAM treatment increased all the parameters in the striatal tissue from old animals, while CHO had selective, opposite effects in the striatal tissue obtained from young animals. CHO-SAM, or the reverse, produced the same pattern of results. These results suggest that ST deficits may involve age-related structural alterations in membranes that interfere with receptor-G protein coupling/uncoupling.

In vivo or in vitro administration of the nitrone spin-trapping compound, n-tert-butyl-alpha-phenylnitrone, (PBN) reduces age-related deficits in striatal muscarinic receptor sensitivity

Previous research has indicated that age-related reductions in muscarinic (m) (e.g. oxotremorine, Oxo) agonist enhancement of striatal K(+)-evoked dopamine release (K(+)-ERDA) and decreased IP3 release upon m receptor (mAChR) agonist stimulation are partially the result of deficits in signal transduction (ST). The present experiments were carried out to test the hypothesis that these age-related ST deficits occur as a result of free radical-induced alterations in membranes containing receptor-G protein complexes. To test this hypothesis, the effects of in vivo and in vitro administration of the nitrone trapping agent, n-tert-butyl-alpha-phenylnitrone (PBN), on the Oxo-enhancement of K(+)-ERDA were examined. Results showed that: both in vivo (10 mg/kg/2 x day PBN i.p./14 days) in vitro (incubation of striatal slices 0-100 microM PBN/30 min) applications of PBN were effective in ameliorating age-related deficits in Oxo-enhanced K(+)-ERDA. The results of the in vivo administration of PBN indicate that the loss of mAChR sensitivity in aging may be the result of oxidative stress that can be restored by this nitrone trapping agent. These findings show that reductions of endogenous or exogenous free radicals may alter one important biomarker of aging, i.e. the loss of sensitivity in mAChR systems. However, these results, when considered along with those obtained with in vitro administration indicate that in addition, PBN may have acute effects (e.g. perhaps membrane structural alterations) which can also improve mAChR responsiveness.

Changes in G protein-mediated signal transduction in aging and Alzheimer's disease

Previous reports have shown that there are age-related reductions in muscarinic receptor (mAChR) sensitivity to agonist stimulation. Our research has elucidated the mechanisms involved in this loss. These studies have shown that this decline is the result of decreases in mAChR concentration, reductions in the number of neuronal cells, and altered phosphoinositide (PI)-mediated signal transduction (ST). The decrements in PI-mediated ST are observed as a reduced ability of muscarinic (m) agonists to enhance K(+)-evoked release of DA (K+ ERDA) from striatal slices from old rats. Additional experiments indicated that the locus of the ST deficits appears to be at the mAChR-G protein interface, since attempts to bypass this interface reduced m-enhanced K+ ERDA deficits in the striata from old rats. Moreover, it appears that the ability of mAChR to decouple from their respective G proteins is reduced as a function of age, since carbachol-stimulated low KM GTPase activity was found to be reduced in hippocampal and striatal tissue obtained from old rats. Similar findings were observed in this parameter in AD hippocampus and basal ganglia. Further reductions were seen in carbachol-stimulated low KM GTPase as a function of the duration of the disease. Results are discussed in terms of structural membrane alterations in aging and disease that may lead to reductions in the efficacy of receptor-G protein coupling/uncoupling.

The effects of aging on muscarinic receptor/G-protein coupling in the rat hippocampus and striatum

In the striatum and hippocampus, there is a loss of sensitivity to muscarinic agonists with age which has been traced to events early in the signal transduction pathway. Our laboratory has therefore focussed on investigations at this level. The current experiments investigate the effects of age on G-protein/receptor interactions by using competitive binding assays to measure the ability of GppNHp to decrease the proportion of receptors bound to G-proteins in the absence and the presence of added Mg2+. L-[3H]Quinuclidinyl benzilate was used as a nonselective ligand and [3H]pirenzepine as an M1 selective ligand. We find that: (1) muscarinic receptors and G-proteins in the striatum appear to become loosely coupled with age, with no change in Mg2+ sensitivity. (2) M1-receptor/G-protein complexes in the hippocampus display increased sensitivity to the presence of Mg2+ with age, with those from old but not young tissue requiring added Mg2+ in order to uncouple. This effect, however, may not be M1 specific.

Comparison of the level of mRNA encoding m1 and m2 muscarinic receptors in brains of young and aged rats

We compared the concentration of mRNA encoding the m1 and m2 muscarinic receptors in several brain regions obtained from young (5-8 months) and aged (24-28 months) male Fischer 344 rats. DNA-excess solution hybridization was employed as a quantitative measure of mRNA concentration. The results indicate the absence of changes in the m1 receptor message with aging in the cerebral cortex, hippocampus and striatum. While there was no statistically significant aging-associated alteration in the concentration of the message encoding the m2 receptor in the thalamus, midbrain, cerebellum and brainstem, there was a decrease in the message level in the hypothalamus.

The putative role of free radicals in the loss of neuronal functioning in senescence

One of the hallmarks of the aging process is a loss of sensitivity in central neuronal receptors to agonist stimulation. This appears to be especially true in central (hippocampal, striatal) muscarinic cholinergic systems and in the striatal dopamine systems. For these two systems, any decline in their sensitivity can be of extreme importance in determining the behavioral capabilities of the organism. Decrements in the striatal dopamine system may be reflected as motor behavioral deficits, while the central cholinergic systems play a major role in the processing of memory through the activation of muscarinic receptors (mAChR). Declines in the function of these receptors appear to be at least partially responsible for the marked deterioration of cognitive function in normal aging and, more notably, in Alzheimer's disease (AD). Previous work has indicated only minimal success in improving performance in tasks that assess memory in senescent animals or humans with pharmacological agents which enhance cholinergic functioning. The present review describes research that indicates that two of the factors involved in this decline in receptor sensitivity include: (a) decreased receptor concentrations and (b) age-related decrements in signal transduction pathways. Studies are reviewed that indicate that the oxidative neural damage that occurs via kainic acid or ionizing radiation parallel those seen in aging. It is suggested that the common mechanism that may exist among all of the age-, disease-, excitatory amino acid- or radiation-induced deficits in neuronal transmission may involve free-radical-mediated alterations in membrane integrity through lipid peroxidation.

Decrement of muscarinic receptor-stimulated low-KM GTPase in striatum and hippocampus from the aged rat

Previous studies have shown that there is an age-related loss of responsiveness in several different receptor systems (e.g. beta-adrenergic, dopaminergic and muscarinic). Our research, using perifused striatal slices and examining muscarinic agonist enhancement of K(+)-evoked dopamine release, has determined that at least part of the loss of sensitivity in muscarinic receptors (mAChR) may occur early in the post-receptor signal transduction process. The present study was carried out to further characterize and localize this deficit by examining carbachol- and oxotremorine-stimulated low-KM guanosine triphosphatase (GTPase) activity in striatal as well as hippocampal tissue obtained from adult (6 months) and old (24 months) Wistar rats. Receptor stimulated low-KM GTPase catalyzes the conversion of GTP to GDP to end the signal transduction cycle and is an indicator of receptor-G-protein coupling/uncoupling. The results showed that stimulated GTPase activity was significantly reduced in hippocampal and striatal tissue from the old animals. These findings suggest that there may be an age-related coupling/uncoupling deficit between muscarinic receptor and G-proteins, and that this deficit may contribute to the reduced mAChR responsiveness in senescence.

Age-related deficits of central muscarinic cholinergic receptor function in the mouse: partial restoration by chronic piracetam treatment

The effect of aging on muscarinic cholinergic receptor function in dissociated cell aggregates of the mouse brain was investigated using two biochemical models, i.e., carbachol-induced accumulation of inositol monophosphates and carbachol-induced desensitization of muscarinic cholinergic receptors as measured by the sequestration of specific 3H-N-methyl-scopolamine binding. While aging strongly reduced carbachol-induced inositol monophosphate accumulation, desensitization was not affected in the brains of aged animals. Chronic treatment of aged mice with the nootropic drug piracetam (500 mg/kg daily PO) significantly elevated the agonist-induced accumulation of inositol monophosphates possibly by increasing the available number of muscarinic cholinergic receptors not being in a desensitized state. The results support the hypothesis that nootropics like piracetam might act in part by restoring age-related deficits of central muscarinic cholinergic receptor function.

Chronic treatment with phosphatidylserine restores muscarinic cholinergic receptor deficits in the aged mouse brain

Chronic treatment (21 days) with phosphatidylserine (BC-PS) partially restored the reduced density of muscarinic cholinergic receptors in several regions of the aged (18 months) mouse brain. The effect was similar whether 3H-QNB or 3H-NMS was used as radioligand. The affinity of both radioligands was not altered by BC-PS treatment. Similar treatment of young (3 months) animals was without any effect on muscarinic cholinergic receptor density in all brain regions investigated. The effect was dose-dependent with elevations of receptor density between 15 and 28% for daily IP doses between 10 and 40 mg/kg, respectively. Similar treatment of aged mice with phosphatidylcholine (40 mg/kg) was without any effect. The data give further evidence that chronic treatment of aged animals with BC-PS reverses a variety of aged-related deficits of brain function.

Muscarinic receptor concentrations and dopamine release in aged rat striata

The extent to which age-related decreases in muscarinic enhancement of K(+)-evoked dopamine release (K(+)-ERDA) from perifused striatal slices is dependent upon the loss of striatal muscarinic receptors (mAChR) was determined. Both K(+)-ERDA and mAChR (M1, M2) concentrations were assessed from the same animals (3, 5-7 and 24-27 months). Results indicated associated decreases of 70% in oxotremorine-enhanced K(+)-ERDA and 36% in Bmax (3H-QNB) (3 and 24-27 months groups). Decrease of mAChR Bmax was not the result of membrane sequestration. Although both the concentrations of M1 and M2 muscarinic receptor subtypes decline with age, only the M2 receptor decline was correlated with the age-related decreases in muscarinic enhancement of K(+)-ERDA (r = .71, p less than 0.001). Results suggest that age-related decreases in mAChR concentrations as being partially responsible for deficits in muscarinic enhancement of K(+)-evoked release of DA.

Changes in dopamine release in vitro from the corpus striatum of young versus aged rats as a function of infusion modes of L-dopa, potassium, and amphetamine

In the present experiments we examined the effects of two different modes of infusion (two separate 10 min versus continuous infusions) of a depolarizing concentration of potassium (K+, 30 mM), amphetamine (AMPH, 10 microM), or L-DOPA (5 microM) upon dopamine (DA) release in vitro from superfused corpus striatal (CS) tissue fragments of young (2-4 months) and aged (18-24 months) male rats. The relative changes in DA release to two infusions of K+ and AMPH (R2/R1) were virtually identical for CS from both young and aged rats. In the case of L-DOPA, DA release from CS of young rats was markedly increased in response to the second compared to the first L-DOPA infusion (R2/R1 = 2.48 +/- 0.33, N = 7) and significantly greater than that of aged rats (R2/R1 = 1.24 +/- 0.17, N = 6). A continuous infusion of K+ and AMPH resulted in an overall greater amount of DA release from the CS of young versus aged rats. In contrast, DA release from CS of aged rats showed an overall more rapid and greater amount of DA release to continuous L-DOPA infusion than that from the CS of young rats. These results demonstrate age-dependent differences in DA release from the CS as a function of the infusion mode of specific secretagogues. Particularly interesting were the responses to L-DOPA where a pulsatile administration resulted in an overall greater amount of DA release from the CS of young rats while a continuous infusion produced a greater amount of DA release from the CS of aged rats.

Selective cross-activation/inhibition of second messenger systems and the reduction of age-related deficits in the muscarinic control of dopamine release from perifused rat striata

Possible alterations in muscarinic cholinergic (mACh) signal transduction in senescence were studied in rat neostriata. Acetylcholine (ACh) activation of striatal muscarinic heteroreceptors by carbachol or oxotremorine enhances K(+)-evoked release of dopamine from perifused striata of 6- but not 24-month-old rats. Present experiments determined the effects of simultaneous activation or activation/inhibition of more than one second messenger on K(+)-evoked release of DA from perifused striatal slices from these age groups. Combinations of carbachol (500 microns), which stimulates inositol-1,4,5-bisphosphate (IP3) production and inhibits cyclic AMP production, with oxotremorine (500 microns), which inhibits cyclic AMP production, in the presence of 30 mM KCl (in a modified Krebs-Ringer medium) reduced the age-related reduction in mAChR enhancement of DA release (analyzed by HPLC coupled to electrochemical detection; 5 min fractions were collected on ice in perchloric acid; flow rate 120 microliters/min). Combinations of these agonists with the putative second messenger arachidonic acid (10 microM), also enhanced K(+)-evoked release of DA in the striatal tissue from the 24-month group. IP3 activation was lower in the striatal tissue from old animals than those from young under all conditions, but cross-activation/inhibition actually may have lowered the IP3 threshold necessary for enhanced DA release to occur. In a subsequent experiment, pre-loading striatal tissue from young animals with either carbachol or oxotremorine under basal release conditions reduced the responding when the basal release medium was switched to one containing 30 mM KCl and combinations of the agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Pre- and post-synaptic cholinergic dysfunction in aged rodent brain regions: new findings and an interpretative review

Age-related impairment of dynamic aspects of central cholinergic neurotransmission has been indicated by many studies of aged rodents, but the regional distribution of cholinergic deficits and the relative contribution of presynaptic hypofunction and reduced acetylcholine release, loss of synaptic integrity or loss of muscarinic receptors remains unclear. This study therefore compared choline acetyltransferase activity (as a structural marker) and sodium-dependent high affinity choline uptake (which reflects both ongoing cholinergic neuronal activity and structural integrity) in the hippocampus, cortex and straitum of male C57BL mice at 3-4, 10-12 or 28-32 months of age. To evaluate the relationship of changes in muscarinic receptors to presynaptic alterations, binding of the antagonist 3H-quinuclidinyl benzilate was compared in membranes prepared from each of these brain regions. High affinity choline uptake was significantly reduced in all three brain regions by 28-32 months of age. This trend was already evident by 10-12 months of age, especially in hippocampus and cortex. By contrast, choline acetyltransferase activity was unchanged in striatum and actually increased in hippocampus and cortex of aged mice. Muscarinic binding was reduced significantly only in striatum and this effect was significant by 10-12 months of age. This decrease in antagonist binding was accompanied by a small but significant reduction in the relative proportion of high affinity agonist sites as defined by carbachol displacement. The impairment of high affinity choline uptake in the absence of a parallel reduction of choline acetyltransferase activity suggests a decline of ongoing cholinergic activity rather than loss of terminal integrity as the basis of presynaptic deficits in aging. This functional decline may be exacerbated by reduction of muscarinic receptors in striatum. Despite considerable literature support for the hypothesis that cholinergic mechanisms are impaired with age, several controversies leave important issues unresolved. Therefore, the present results are discussed in the context of a critical review with emphasis on dynamic properties of presynaptic function which require analysis in experimental animal models. The impact of normal aging on brain cholinergic systems is distinguished from the neurodegenerative changes in Alzheimer disease in that presynaptic function is compromised with a relative preservation of the integrity of innervation.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduction of motor behavioural deficits in senescence via chronic prolactin or estrogen administration: time course and putative mechanisms of action

The effects of chronic estrogen (E2), rat prolactin (rPRL), modified ovine prolactin (mPRL) administration on motor behavior (inclined screen performance) and striatal dopamine (DA) (D2subtype) receptor concentrations were examined in senescent (greater than 24 months of age) female rats, mPRL possesses no lactotrophic activity. Administration of either E2 or rPRL was effective in improving both inclined screen performance (increased time that the animal could remain on the screen by 95 and 413 s, respectively, compared to highest pre-injection performance) and striatal D2 receptor concentrations (14 and 20% respectively). These were indications, however, from separate analyses that improvements in inclined screen performance were seen prior to any increases in striatal D2 receptor concentrations. These early performance increases seemed instead to be the result of improved muscarinic receptor control over striatal DA autoreceptor function. Later improvements in inclined screen performance (at 6-7 days after the E2 injections were begun) were more dependent on increased striatal DA receptor concentrations. A second set of experiments which involved the injection of E2 into senescent male as well as female rats indicated that there were no sex differences in improvements in inclined screen performance, and that once the E2 injections were discontinued, performance returned to preadministration levels. The results are discussed in terms of two important processes that may be involved in mediating enhanced inclined screen performance following E2 administration: (1) enhancement of muscarinic receptor regulation of DA autoreceptor function; and (2) increases in striatal DA receptor density.