Age-related loss of cholinergic-muscarinic coupling to PLC: comparison with changes in brain regional PLC subtypes mRNA distribution

Dissecting the age-related decline on spatial learning and memory tasks in rodent models: N-methyl-D-aspartate receptors and voltage-dependent Ca2+ channels in senescent synaptic plasticity

In humans, heterogeneity in the decline of hippocampal-dependent episodic memory is observed during aging. Rodents have been employed as models of age-related cognitive decline and the spatial water maze has been used to show variability in the emergence and extent of impaired hippocampal-dependent memory. Impairment in the consolidation of intermediate-term memory for rapidly acquired and flexible spatial information emerges early, in middle-age. As aging proceeds, deficits may broaden to include impaired incremental learning of a spatial reference memory. The extent and time course of impairment has been be linked to senescence of calcium (Ca²⁺) regulation and Ca²⁺-dependent synaptic plasticity mechanisms in region CA1. Specifically, aging is associated with altered function of N-methyl-D-aspartate receptors (NMDARs), voltage-dependent Ca²⁺ channels (VDCCs), and ryanodine receptors (RyRs) linked to intracellular Ca²⁺ stores (ICS). In young animals, NMDAR activation induces long-term potentiation of synaptic transmission (NMDAR-LTP), which is thought to mediate the rapid consolidation of intermediate-term memory. Oxidative stress, starting in middle-age, reduces NMDAR function. In addition, VDCCs and ICS can actively inhibit NMDAR-dependent LTP and oxidative stress enhances the role of VDCC and RyR-ICS in regulating synaptic plasticity. Blockade of L-type VDCCs promotes NMDAR-LTP and memory in older animals. Interestingly, pharmacological or genetic manipulations to reduce hippocampal NMDAR function readily impair memory consolidation or rapid learning, generally leaving incremental learning intact. Finally, evidence is mounting to indicate a role for VDCC-dependent synaptic plasticity in associative learning and the consolidation of remote memories. Thus, VDCC-dependent synaptic plasticity and extrahippocampal systems may contribute to incremental learning deficits observed with advanced aging.

In vitro sensitivity of cholinesterases and [3H]oxotremorine-M binding in heart and brain of adult and aging rats to organophosphorus anticholinesterases

Organophosphorus (OP) insecticides elicit toxicity via acetylcholinesterase inhibition, allowing acetylcholine accumulation and excessive stimulation of cholinergic receptors. Some OP insecticides bind to additional macromolecules including butyrylcholinesterase and cholinergic receptors. While neurotoxicity from OP anticholinesterases has been extensively studied, effects on cardiac function have received less attention. We compared the in vitro sensitivity of acetylcholinesterase, butyrylcholinesterase and [(3)H]oxotremorine-M binding to muscarinic receptors in the cortex and heart of adult (3 months) and aging (18 months) rats to chlorpyrifos, methyl parathion and their active metabolites chlorpyrifos oxon and methyl paraoxon. Using selective inhibitors, the great majority of cholinesterase in brain was defined as acetylcholinesterase, while butyrylcholinesterase was the major cholinesterase in heart, regardless of age. In the heart, butyrylcholinesterase was markedly more sensitive than acetylcholinesterase to inhibition by chlorpyrifos oxon, and butyrylcholinesterase in tissues from aging rats was more sensitive than enzyme from adults, possibly due to differences in A-esterase mediated detoxification. Relatively similar differences were noted in brain. In contrast, acetylcholinesterase was more sensitive than butyrylcholinesterase to methyl paraoxon in both heart and brain, but no age-related differences were noted. Both oxons displaced [(3)H]oxotremorine-M binding in heart and brain of both age groups in a concentration-dependent manner. Chlorpyrifos had no effect but methyl parathion was a potent displacer of binding in heart and brain of both age groups. Such OP and age-related differences in interactions with cholinergic macromolecules may be important because of potential for environmental exposures to insecticides as well as the use of anticholinesterases in age-related neurological disorders.

Age-associated changes in central nervous system glycerolipid composition and metabolism

In this review, changes in brain lipid composition and metabolism due to aging are outlined. The most striking changes in cerebral cortex and cerebellum lipid composition involve an increase in acidic phospholipid synthesis. The most important changes with respect to fatty acyl composition involve a decreased content in polyunsaturated fatty acids (20:4n-6, 22:4n-6, 22:6n-3) and an increased content in monounsaturated fatty acids (18:1n-9 and 20:1n-9), mainly in ethanolamine and serineglycerophospholipids. Changes in the activity of the enzymes modifying the phospholipid headgroup occur during aging. Serine incorporation into phosphatidylserine through base-exchange reactions and phosphatidylcholine synthesis through phosphatidylethanolamine methylation increases in the aged brain. Phosphatidate phosphohydrolase and phospholipase D activities are also altered in the aged brain thus producing changes in the lipid second messengers diacylglycerol and phosphatidic acid.

Differential modulation of phospholipase D and phosphatidate phosphohydrolase during aging in rat cerebral cortex synaptosomes

Phosphatidylcholine (PC) hydrolysis generates two important second messengers: phosphatidic acid (PA) and diacylglycerol (DAG). Phospholipase D (PLD) and phosphatidate phosphohydrolase (PAPase) are involved in their generation and therefore are key enzymes in signal transduction. Specific isoforms of these enzymes are activated by receptor occupancy in brain. Phosphatidylinositol 4,5-bisphosphate-dependent PLD (PIP2-PLD) and N-ethylmaleimide-insensitive PAPase (PAP2) have been suggested to act in series to generate the biologically active lipids PA and DAG. In the present study we examine age-induced changes mainly in PIP2-PLD and PAP2 activities in cerebrocortical synaptosomes from adult (4 months) and aged (28 months) Wistar rats. Aging increases the activity of both enzymes. Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) and cytosol (from cerebral cortex) stimulate PLD activity in adult and senescent synaptosomal membranes, the effect being greater in the latter. Under the same experimental conditions PAP2 activity was stimulated in aged membranes whereas in adult membranes GTPgammaS had no effect and cytosol showed a slight inhibitory effect. Diacylglycerol lipase (DGL) activity differs from that of PAP2 in aged rats and it was 21% inhibited with respect to synaptosomal membranes from adult rats. Increased sinaptosomal PLD activity in aged membranes appears to be independent of G protein regulation, whereas PAP2 activity is differentially regulated by GTPgammaS in aged membranes with respect to adult membranes. Our results suggest that under G-protein activation conditions, DAG production by the serial activation of PLD and PAP2 activities is increased in synaptosomal membranes from aged brain. The present paper demonstrates that PA generation (PLD activity) and degradation (PAPase activity) are differentially modulated during the aging process.

Heat acclimation and heat stress have different effects on cholinergic-induced calcium mobilization

There is evidence that the signal transduction array responsible for the secretion of water in evaporative cooling by the submaxillary gland of the rat is subject to heat acclimatory responses. The objectives of the present study were 1) to examine whether heat acclimation affects intracellular Ca2+ mobilization and, in turn, submaxillary glandular responsiveness; 2) to assess whether the acclimatory responses differ from those evoked on heat stress (HS). Experiments were conducted on submaxillary glands of rats acclimated at 34°C for 0, 2 [short-term heat acclimation (STHA)], and 30 [long-term heat acclimation (LTHA)] days. The resting cytosolic calcium concentration ([Ca2+]c) and the carbamylcholine-evoked calcium signal ([Ca2+]s) of dispersed glandular cells were measured using the fluorescent dye fura 2 AM. Inositol-1,4,5-trisphosphate (IP3)-sensitive endoplasmic reticulum Ca2+ stores were determined in permeabilized cells using fura 2 potassium salt. STHA resulted in a drop in both [Ca2+]s and IP3-sensitive Ca2+ stores. On LTHA, the [Ca2+]samplitude reverted to the preacclimation value, whereas the IP3-sensitive Ca2+ stores remained low. The drop in [Ca2+]s on STHA is in accord with the decreased glandular output (measured by 86Rb efflux) observed during this acclimation phase. However, after LTHA the enhanced glandular output despite reduced [Ca2+]s levels suggests an increased efficiency of cellular secretory mechanisms in that group. Collectively, the alterations in [Ca2+]ssupport our biphasic acclimation model (Horowitz M, Kaspler P, Marmari Y, and Oron Y. J Appl Physiol 80: 77–85, 1996.). In nonacclimated glands, HS caused an elevation in [Ca2+]s coincidentally with a decrease in the IP3 Ca2+ stores. In contrast, [Ca2+]s in both STHA and LTHA glands was not affected by HS, despite a marked increase in the IP3-sensitive Ca2+ stores in the LTHA glands. The opposing responses to HS and heat acclimation in calcium signaling and stores confirm the specificity of each process.

Effect of phospholipase C and protein kinase C following cholinergic denervation and hippocampal sympathetic ingrowth in rat hippocampus

Following cholinergic denervation of the hippocampus by medial septal lesions, an unusual neuronal reorganization occurs in which peripheral adrenergic fibers arising from the superior cervical ganglia grow into the hippocampus (hippocampal sympathetic ingrowth). We have reported previously that cholinergic denervation and hippocampal sympathetic ingrowth differentially affected cholinergically stimulated phosphoinositide hydrolysis, concentration and affinity of muscarinic receptors, Go-protein level and protein kinase C activity. To complete these studies, we determined whether cholinergic denervation and hippocampal sympathetic ingrowth influenced phospholipase C and protein kinase C expression in dorsal hippocampal membranes and cytosol. Using immunoblotting methods, the results showed that the 100,000 mol. wt subunit of phospholipase Cbeta was increased in the membrane fraction in the hippocampal sympathetic ingrowth group by 45% compared to controls and the 150,000 mol.wt subunit was increased by 75% and 59% compared to controls and cholinergic denervation, respectively. For protein kinase C detection, immunoblots were prepared using antibodies selective for "classical" protein kinase C members (alpha, beta, gamma) and for the "novel" protein kinase C subfamily members (delta, θ). Membrane protein kinase Cbeta was decreased in hippocampal sympathetic ingrowth by 35% compared to controls and by 41% compared to cytosolic hippocampal sympathetic ingrowth. Membrane protein kinase Cbeta was decreased in cholinergic denervation by 28% compared to controls. When compared to membranes from controls and the cholinergic denervation group, and to cytosolic fractions from the hippocampal sympathetic ingrowth groups, respectively, the following membrane protein kinase isoforms were found to be decreased by hippocampal sympathetic ingrowth: gamma by 55%, 40% and 57%; delta by 91.5%, 70% and 120%; theta; by 95%, 100% and 86%.In conclusion, our results may indicate the connection between the previously reported differential influence of hippocampal sympathetic ingrowth and cholinergic denervation on cholinergically stimulated phosphoinositol hydrolysis. The "normalization" of phosphoinositol hydrolysis found in hippocampal sympathetic ingrowth may be due to the increase in phospholipase Cbeta expression in hippocampal sympathetic ingrowth membrane fractions. Since the activation of protein kinase C is known to block phosphoinositol hydrolysis, hippocampal sympathetic ingrowth "normalization" of phosphoinositol hydrolysis may result from a reduction in protein kinase expression in hippocampal sympathetic ingrowth membranes.

Age-dependent increase in C7-1 gene expression in rat frontal cortex

We have previously reported that a fragment of mRNA, denoted as C7-1, which expression was significantly increased in the frontal cortex of aged rats. In the present study, we have cloned and sequenced the full length cDNA of the C7-1 gene. We have found that the open reading frame of this gene encoded a 463-amino-acid protein, which shared 84% identity in amino acid sequence with a subunit of vacuolar H(+)-ATPase (V-ATPase). Further Northern blot analysis revealed that there was an age-dependent increase in C7-1 gene expression in rat frontal cortex, but not in other brain areas. Moreover, application of C7-1 antisense oligonucleotide to cortical neuronal cultures markedly inhibited cell survival. These results together suggest that C7-1 is a marker for the aging process and that upregulation of C7-1 may be important in maintaining the normal function of V-ATPase during aging.

Metabotropic glutamate receptor-mediated hippocampal phosphoinositide turnover is blunted in spatial learning-impaired aged rats

Maximal phosphoinositide (PI) turnover was examined in the hippocampus of young and aged Long-Evans rats that were behaviorally characterized for spatial learning in the Morris water maze. The type 1 metabotropic glutamate receptor (mGluR) agonist 1S,3R ACPD was used to stimulate PI turnover and to determine the E(MAX) for each rat. Protein levels in hippocampus for type 1 mGluRs, Galphaq11, and phospholipase Cbeta-1 (PLCbeta-1) were also measured by quantitative Western blotting. The results show that PI turnover mediated by the mGluRs was blunted in the aged rats. The magnitude of the decrement in PI turnover was also significantly correlated with age-related spatial memory decline. The decrease in mGluR-mediated PI turnover occurred without changes in the protein level of either the mGluRs or the G-protein coupled to those receptors, Galphaq11. A significant decrease in the immunoreactivity of PLCbeta-1, however, was observed in the hippocampus of aged rats; PLCbeta-1 immunoreactivity was significantly correlated with spatial learning only when the young and aged rats were considered together. The decrement in mGluR-mediated signal transduction in the hippocampus that is related to cognitive impairment in aging may be attributable, at least in part, to a deficiency in the enzyme PLCbeta-1. That deficiency may also contribute to a blunted response in muscarinic stimulation of hippocampal PI turnover that we previously found in this same study population. An age-related alteration in this signal transduction system may provide a functional basis for cognitive decline independent of any loss of neurons in the hippocampus.

Cholinergic systems and long-term potentiation in memory-impaired apolipoprotein E-deficient mice

Impairments in cholinergic neurotransmitter systems of the basal forebrain are a hallmark of Alzheimer's disease pathophysiology. The presence of the epsilon4 allele of apolipoprotein E was recently implicated as a major risk factor in both familial and sporadic Alzheimer's disease. The present study examined the integrity of cholinergic and non-cholinergic systems in apolipoprotein E-deficient, memory-impaired mice. Choline acetyltransferase activity, hippocampal acetylcholine release, nicotinic and muscarinic (M1 and M2) receptor binding sites and acetylcholinesterase cell or terminal density showed no signs of alteration in either three-month or 9.5-month-old apolipoprotein E-deficient mice compared to controls. In contrast, long-term potentiation was found to be markedly reduced in these mice, but increases in the strength of stimulation induced the same level of long-term potentiation as that observed in controls. These alterations did not appear to be the consequence of modifications in the binding properties of glutamatergic receptors (N-methyl-D-aspartate and [RS]-alpha-amino-3-hydroxy-5-methylisoxazole propionic acid) but from defective regulation of the (RS)-alpha-amino-3-hydroxy-5-methylisoxazole propionic acid receptor by phospholipase A2 activity. These results support the notion that apolipoprotein E plays a fundamental role in neuronal plasticity, which could in turn affect cognitive performance through imbalances in extra- and intracellular lipid homeostasis.

Changes in the expression of protein kinase C (PKC), phospholipases C (PLC) and D (PLD) isoforms in spleen, brain and kidney of the aged rat: RT-PCR and Western blot analysis

The age-dependent changes of expression of protein kinase C (PKC), phospholipase C (PLC) and phospholipase D (PLD) isozymes were analyzed in spleen, brain and kidney of young-adult (12-16 week-old) and aged (82-88 week-old) rats. The activities of spleen cPKC and nPKC were significantly decreased by nearly 35 and 30% in aged rats compared to those of young adults, respectively (P < 0.05). The level of PKC beta1 was significantly decreased in aged rats as assessed by RT-PCR and Western blot analyses. In aged rat brain where the activity of cPKC was significantly decreased by nearly 25% (P < 0.05), PKC alpha and beta1 isozymes were significantly down-regulated. In kidney, the level of PKC beta2 was decreased. In spleen the both mRNA and protein levels of PLC beta2 and gamma2 were significantly down-regulated in aged rat (P < 0.05). PLC beta1 was also significantly lower in aged rat brain (P < 0.05) as assessed by RT-PCR and Western blotting. Moreover, PLC beta1 was significantly down-regulated in both mRNA and protein levels in aged rat kidney (P < 0.05). In contrast, the tissues examined, the expressions of PLD isozymes (PLD1a, 1b and 2) were rather stable in the course of aging. These results indicate that mRNAs of PLD isozymes were rather stable but that particular PKC and PLC isozymes were down-regulated in different tissues during aging, suggesting age-dependent decline of specific PKC and PLC isozymes in organs which may, at least in part, be implicated in tissue dysfunction with aging.

Galanin receptors in the hippocampus and entorhinal cortex of aged Fischer 344 male rats

Galanin (GAL) has been proposed to be an inhibitory modulator of cholinergic memory pathways because it acts within the hippocampus to inhibit the release and antagonize the postsynaptic actions of acetylcholine. Here we have used: 1) slice binding and quantitative autoradiography to assess the density and occupancy of GAL receptors; and 2) in situ hybridization histochemistry to assess expression of the GALR1 receptor subtype in the ventral hippocampus of 3-month-old and 21-month-old Fischer 344 male rats. We detected a small but significant (p < or = 0.0003) age-related reduction in 125I-GAL binding-site density in the ventral hippocampus and entorhinal cortex under standard binding conditions. Post-hoc analysis indicated that this reduction with age persisted in the CA1 radiatum and entorhinal cortex following GTP-induced desaturation to unmask pre-existent GAL receptors occupied by endogenous ligand. It was not associated with a significant change in peak GALR1 gene expression in the hippocampus. Because a portion of GAL receptors in this region have been postulated to function as presynaptic auto-receptors on cholinergic fiber terminals, the reduction in GAL binding sites with age may be a consequence of age-related alterations in GAL receptor expression by basal forebrain cholinergic neurons which project to the ventral hippocampus.

Effects of age on pilocarpine-induced c-fos expression in rat hippocampus and cortex

The effect of age on pilocarpine-induced expression of the immediate-early gene c-fos was examined in the hippocampus and cortex of Long-Evans rats. Rats were treated with either pilocarpine (25 mg/kg) or saline, and sacrificed 90 min. following injection. The level of c-fos mRNA and Fos-like protein expression was determined using in situ hybridization histochemistry, and immunocytochemistry, respectively. In saline-treated animals, comparable levels of c-fos mRNA and Fos-like protein were observed in the hippocampus and cortical regions of young (6 month) and aged (24-26 months) rats. The expression of Fos-like protein following pilocarpine treatment was increased, however, in frontal, retrosplenial, and cingulate cortex of aged compared to young rats. In frontal and retrosplenial cortex, the changes in Fos-like protein were accompanied by changes in c-fos mRNA expression. In contrast, no age difference was detected in the hippocampus or parietal cortex of pilocarpine-treated rats. These regionally-specific age differences in response to pilocarpine administration suggest that mechanisms localized to those areas play an important role in determining the response to cholinergic stimulation mediated through post-synaptic muscarinic receptors.

The role of the phosphoinositide signalling system in the pathogenesis of sporadic Alzheimer's disease: a hypothesis

Great advances have been made in recent years in our knowledge of the genetic mutations found in early onset familial Alzheimer's disease (AD) and their pathological consequences. The pathogenesis of sporadic AD, on the other hand, is less clear, although a central role of oxidative stress is indicated. In the AD brain, severe dysfunctions in the phosphoinositide signalling pathway have been reported. In view of the fact that (a) oxidative stress can adversely affect phosphoinositide breakdown and hence diacylglycerol-mediated activation of protein kinase C and (b) protein kinase C activation reduces the production of beta-amyloid peptide from amyloid precursor protein, it is possible that this represents a pathogenic pathway whereby oxidative stress can lead to amyloid deposition and the development of the disease.

Nicotinic and muscarinic cholinergic receptor binding in the human hippocampal formation during development and aging

High-affinity nicotine, alpha-bungarotoxin (alpha BT) and muscarinic receptor binding was measured in the human hippocampal formation in a series of 57 cases aged between 24 weeks gestation and 100 years. Changes in nicotine receptor binding during development and aging were more striking than differences in alpha BT and muscarinic binding. Nicotine binding was higher at the late foetal stage than at any other subsequent time in all areas investigated. In the hippocampus a fall in binding then occurred within the first six months of life, with little or no subsequent fall during aging, whereas in the entorhinal cortex and the presubiculum the major loss of nicotine binding occurred after the fourth decade. alpha BT binding was significantly elevated in the CA 1 region, but in no other region of the hippocampus, in the late foetus, and there was also a fall in alpha BT binding in the entorhinal cortex during aging from the second decade. The modest changes in total muscarinic binding, which appeared to reflect those in M1 and M3 + 4 rather than M2 binding, were a rise in the entorhinal cortex between the foetal stage and childhood and a tendency for receptors to fall with age in the hippocampus and subicular complex. These findings implicate mechanisms controlling the expression of nicotinic receptors to a greater extent than muscarinic receptors in postnatal development and aging in the human hippocampus.