Age-dependent decrease in the affinity of muscarinic M1 receptors in neocortex of rhesus monkeys

Interaction of basal forebrain cholinergic neurons with the glucocorticoid system in stress regulation and cognitive impairment

A substantial number of studies on basal forebrain (BF) cholinergic neurons (BFCN) have provided compelling evidence for their role in the etiology of stress, cognitive aging, Alzheimer’s disease (AD), and other neurodegenerative diseases. BFCN project to a broad range of cortical sites and limbic structures, including the hippocampus, and are involved in stress and cognition. In particular, the hippocampus, the primary target tissue of the glucocorticoid stress hormones, is associated with cognitive function in tandem with hypothalamic-pituitary-adrenal (HPA) axis modulation. The present review summarizes glucocorticoid and HPA axis research to date in an effort to establish the manner in which stress affects the release of acetylcholine (ACh), glucocorticoids, and their receptor in the context of cognitive processes. We attempt to provide the molecular interactive link between the glucocorticoids and cholinergic system that contributes to BFCN degeneration in stress-induced acceleration of cognitive decline in aging and AD. We also discuss the importance of animal models in facilitating such studies for pharmacological use, to which could help decipher disease states and propose leads for pharmacological intervention.

Hippocampal M1 receptor function associated with spatial learning and memory in aged female rhesus macaques

Of the acetylcholine muscarinic receptors, the type 1 (M1) and type 2 (M2) receptors are expressed at the highest levels in the prefrontal cortex (PFC) and hippocampus, brain regions important for cognition. As equivocal findings of age-related changes of M1 and M2 in the nonhuman primate brain have been reported, we first assessed age-related changes in M1 and M2 in the PFC and hippocampus using saturation binding assays. Maximum M1 receptor binding, but not affinity of M1 receptor binding, decreased with age. In contrast, the affinity of M2 receptor binding, but not maximum M2 receptor binding, increased with age. To determine if in the elderly cognitive performance is associated with M1 or M2 function, we assessed muscarinic function in elderly female rhesus macaques in vivo using a scopolamine challenge pharmacological magnetic resonance imaging and in vitro using saturation binding assays. Based on their performance in a spatial maze, the animals were classified as good spatial performers (GSP) or poor spatial performers (PSP). In the hippocampus, but not PFC, the GSP group showed a greater change in T(2)*-weighted signal intensity after scopolamine challenge than the PSP group. The maximum M1 receptor binding and receptor binding affinity was greater in the GSP than the PSP group, but no group difference was found in M2 receptor binding. Parameters of circadian activity positively correlated with the difference in T(2)*-weighted signal intensity before and after the challenge, the maximum M1 receptor binding, and the M1 receptor binding affinity. Thus, while in rhesus macaques, there are age-related decreases in M1 and M2 receptor binding, in aged females, hippocampal M1, but not M2, receptor function is associated with spatial learning and memory and circadian activity.

Age-related increase of sI(AHP) in prefrontal pyramidal cells of monkeys: relationship to cognition

Reduced excitability, due to an increase in the slow afterhyperpolarization (and its underlying current sI(AHP)), occurs in CA1 pyramidal cells in aged cognitively-impaired, but not cognitively-unimpaired, rodents. We sought to determine whether similar age-related changes in the sI(AHP) occur in pyramidal cells in the rhesus monkey dorsolateral prefrontal cortex (dlPFC). Whole-cell patch-clamp recordings were obtained from layer 3 and layer 5 pyramidal cells in dlPFC slices prepared from young (9.6 ± 0.7 years old) and aged (22.3 ± 0.7 years old) behaviorally characterized subjects. The amplitude of the sI(AHP) was significantly greater in layer 3 (but not layer 5) cells from aged-impaired compared with both aged-unimpaired and young monkeys, which did not differ. Aged layer 3, but not layer 5, cells exhibited significantly increased action potential firing rates, but there was no relationship between sI(AHP) and firing rate. Thus, in monkey dlPFC layer 3 cells, an increase in sI(AHP) is associated with age-related cognitive decline; however, this increase is not associated with a reduction in excitability.

Kinetics of M1 muscarinic receptor and G protein signaling to phospholipase C in living cells

G protein-coupled receptors (GPCRs) mediate responses to external stimuli in various cell types. Early events, such as the binding of ligand and G proteins to the receptor, nucleotide exchange (NX), and GTPase activity at the Galpha subunit, are common for many different GPCRs. For G(q)-coupled M(1) muscarinic (acetylcholine) receptors (M(1)Rs), we recently measured time courses of intermediate steps in the signaling cascade using Förster resonance energy transfer (FRET). The expression of FRET probes changes the density of signaling molecules. To provide a full quantitative description of M(1)R signaling that includes a simulation of kinetics in native (tsA201) cells, we now determine the density of FRET probes and construct a kinetic model of M(1)R signaling through G(q) to activation of phospholipase C (PLC). Downstream effects on the trace membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP(2)) and PIP(2)-dependent KCNQ2/3 current are considered in our companion paper in this issue (Falkenburger et al. 2010. J. Gen. Physiol. doi:10.1085/jgp.200910345). By calibrating their fluorescence intensity, we found that we selected transfected cells for our experiments with approximately 3,000 fluorescently labeled receptors, G proteins, or PLC molecules per microm(2) of plasma membrane. Endogenous levels are much lower, 1-40 per microm(2). Our kinetic model reproduces the time courses and concentration-response relationships measured by FRET and explains observed delays. It predicts affinities and rate constants that align well with literature values. In native tsA201 cells, much of the delay between ligand binding and PLC activation reflects slow binding of G proteins to receptors. With M(1)R and Gbeta FRET probes overexpressed, 10% of receptors have G proteins bound at rest, rising to 73% in the presence of agonist. In agreement with previous work, the model suggests that binding of PLC to Galpha(q) greatly speeds up NX and GTPase activity, and that PLC is maintained in the active state by cycles of rapid GTP hydrolysis and NX on Galpha(q) subunits bound to PLC.

Effects of normal aging on prefrontal area 46 in the rhesus monkey

This review is concerned with the effects of normal aging on the structure and function of prefrontal area 46 in the rhesus monkey (Macaca mulatta). Area 46 has complex connections with somatosensory, visual, visuomotor, motor, and limbic systems and a key role in cognition, which frequently declines with age. An important question is what alterations might account for this decline. We are nowhere near having a complete answer, but as will be shown in this review, it is now evident that there is no single underlying cause. There is no significant loss of cortical neurons and although there are a few senile plaques in rhesus monkey cortex, their frequency does not correlate with cognitive decline. However, as discussed in this review, the following do correlate with cognitive decline. Loss of white matter has been proposed to result in some disconnections between parts of the central nervous system and changes in the structure of myelin sheaths reduce conduction velocity and the timing in neuronal circuits. In addition, there are reductions in the inputs to cortical neurons, as shown by regression of dendritic trees, loss of dendritic spines and synapses, and alterations in transmitters and receptors. These factors contribute to alterations in the intrinsic and network physiological properties of cortical neurons. As more details emerge, it is to be hoped that effective interventions to retard cognitive decline can be proposed.

Muscarinic receptor protein expression in the ocular tissues of the chick during normal and myopic eye development

Muscarinic receptor signalling has been implicated in both the embryonic and postnatal development of ocular structures as well as in myopic eye growth. A radioligand binding assay was used to determine whether changes in muscarinic receptor density and/or affinity occurred in the chick retina, choroid and sclera during early post-hatching development or with the induction of myopia. Specific receptor binding sites were saturable with increasing concentrations of the muscarinic receptor ligand [3H]N-methyl-scopolamine in the retina and choroid but not in the sclera. In normal eyes, binding density in the retina was not altered from age P5 to P10 (447+/-14 vs. 411+/-13 fmol/mg of protein, P=0.07). However, in the choroid, the number of receptor binding sites significantly increased between P5 and P10 (637+/-39 vs. 1125+/-121 fmol/mg of protein, P<0.01). Binding affinity (K(D)) was not altered with age in either the retina or choroid. Myopia was induced in chicks by deprivation of form vision, using translucent diffusers, from age P3. Despite the induction of significant degrees of ocular elongation and myopia at P5 (-8.7+/-0.3 D, P<0.01) and P10 (-22.5+/-1.3 D, P<0.01), neither muscarinic receptor density nor affinity were altered in the retina or choroid of myopic eyes. These findings indicate that regulation of muscarinic receptor numbers in the chick choroid is occurring in normal post-hatching development of this tissue. However, myopic eye enlargement was not associated with changes in muscarinic receptor protein expression in the chick retina and choroid.

Cholinergic activity of aged rhesus monkeys revealed by positron emission tomography

In the present study, the radiotracer [(18)F] (+)-4-fluorobenzyltrozamicol ((+)-[(18)F]FBT) and positron emission tomography (PET) were used to examine the vesicular acetylcholine transporter and determine if presynaptic cholinergic activity was altered with age in 23 rhesus monkeys that varied in age from 10 to 37 years. Binding of (+)-[(18)F]FBT in the basal ganglia was reduced significantly with increasing age of the monkeys. However, there were individual differences noted in that some middle-aged and aged monkeys demonstrated levels of (+)-[(18)F]FBT binding that were comparable to the binding measured in adult monkeys. These data indicate that presynaptic cholinergic function may decrease with age, but that there may be a differential susceptibility of the cholinergic system to the aging process in different individuals.

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.

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.

Selective muscarinic antagonists differentially affect in vivo acetylcholine release and memory performances of young and aged rats

Brain acetylcholine release and memory performance were investigated in young (three- to six-months) and old (20- to 24-months) rats. Acetylcholine release was measured in vivo in the cortex and hippocampus of freely-moving animals, under basal conditions and in the presence of the following muscarinic antagonists: scopolamine, (+/-)-5,11-dihydro-11-[[(2-[2-[(dipropylamino) methyl]-1-piperidinyl]ethyl) amino] carbonyl]-6H-pyrido(2,3-b)(1,4)-benzodiazepine-6-one (AFDX 384) and pirenzepine. The amount of acetylcholine released from the cortex and hippocampus of old rats was significantly reduced. In the presence of scopolamine and AFDX 384 but not of pirenzepine, the acetylcholine release was significantly higher in the old than the young rats, suggesting that changes in presynaptic M2/M4 muscarinic receptor function occur with ageing in the two brain regions. Cognitive capacities were evaluated using two different behavioural tasks: object recognition and passive avoidance response. Old rats were unable to discriminate between familiar and novel objects and had impaired performance in the passive avoidance test. AFDX 384 restored the performance in both tests. Furthermore, in young rats AFDX 384 reversed the impairment of both object recognition and passive avoidance response induced by scopolamine. The effect of AFDX 384 on acetylcholine release and behaviour in the old rats offers further support to a relationship between the age-related cholinergic hypofunction and cognitive impairment and indicates the blockade of presynaptic muscarinic receptors as a possible selective target for therapeutic strategies aimed at improving age-associated memory deficits.

Functional and neurobiological similarities of aging in monkeys and humans

Aging in humans may be accompanied by alterations in several functional abilities. However, there is a great deal of individual variability in the functions that may be altered with age within and across aged people. One potential source of age-related behavioral variation may lie in a differential vulnerability of neurobiological systems to the aging process in particular individuals. Aged monkeys demonstrate behavioral and brain alterations that have many parallels with those observed in aged humans and are valuable animal models in which to investigate the interrelationships between age, behavior and neurobiological measures. This review outlines the similarities of functional and neurobiological aging in monkeys and humans, notes the variability that exists in both behavioral and neural systems in aging, and identifies some of the areas of aging that are in need of further investigation.

Neurotrophins and the primate central nervous system: a minireview

The central nervous system (CNS) of primates is more complex than the CNS of other mammals. Details of the development and aging of the primate CNS have recently been revealed by various neurobiological techniques. It has become clear that the primate CNS has unique characteristics, for example, the capacity for the overproduction and elimination of fibers and synapses. Some differences have also been found in the distribution of and changes with development in levels of various neuroactive substances. Recent discoveries of a variety of neurotrophins in the mammalian CNS have led to research on the neurobiology of these molecules in the primate CNS. The distribution of and changes with development in levels of nerve growth factor (NGF) in the primate CNS are closely correlated with the cholinergic system of the basal forebrain. The administration of NGF into the monkey brain prevents the degeneration of the cholinergic neurons of the basal forebrain after axotomy, a result that suggests that neurotrophins might be very valuable agents for the future treatment of neurological diseases, such as Alzheimer's and Parkinson's diseases.

Adrenergic and serotonergic receptors in aged monkey neocortex

Autoradiography was employed to compare the distribution and density of adrenergic (alpha 1, alpha 2, and beta) and serotonergic (5-HT1 and 5-HT2) receptors in the neocortex of young adult (3 to 10 years of age) and aged (> 20 years of age) rhesus monkeys. The age-related changes in the density of adrenergic and serotonergic sites were area and layer specific. A decrease in the density of alpha 1 receptors occurred only in the superficial layers of the somatosensory cortex, whereas the density of alpha 2 receptors declined in layer I of the prefrontal cortex and in most layers of the motor and somatosensory regions. The increase in beta receptors was largely confined to the deep layers of the motor and somatosensory areas. The density of 5-HT1 sites decreased in most layers of the somatosensory cortex, while 5-HT2 receptors declined in the deep layers of the motor cortex and middle strata of the visual cortex. Overall, adrenergic and serotonergic receptors were least affected in the prefrontal cortex and most compromised in the motor and somatosensory cortex of aged primates.

Association of m1 and m2 muscarinic receptor proteins with asymmetric synapses in the primate cerebral cortex: morphological evidence for cholinergic modulation of excitatory neurotransmission

Muscarinic m1 receptors traditionally are considered to be postsynaptic to cholinergic fibers, while m2 receptors are largely presynaptic receptors associated with axons. We have examined the distribution of these receptor proteins in the monkey cerebral cortex and obtained results that are at odds with this expectation. Using immunohistochemistry with specific antibodies to recombinant m1 and m2 muscarinic receptor proteins, we have demonstrated that both m1 and m2 receptors are prominently associated with noncholinergic asymmetric synapses as well as with the symmetric synapses that characterize the cholinergic pathways in the neocortex. At asymmetric synapses, both m1 and m2 receptor immunoreactivity is observed postsynaptically within spines and dendrites; the m2 receptor is also found in presynaptic axon terminals which, in the visual cortex, resemble the parvicellular geniculocortical pathway. In addition, m2 labeling was also found in a subset of nonpyramidal neurons. These findings establish that the m2 receptor is located postsynaptically as well as presynaptically. The association of m1 and m2 receptors with asymmetric synapses in central pathways, which use excitatory amino acids as neurotransmitters, provides a morphological basis for cholinergic modulation of excitatory neurotransmission.