Central neurotransmitter substances and aging: a review

What matters in aging is signaling for responsiveness

Biological responsiveness refers to the capacity of living organisms to adapt to changes in both their internal and external environments through physiological and behavioral mechanisms. One of the prominent aspects of aging is the decline in this responsiveness, which can lead to a deterioration in the processes required for maintenance, survival, and growth. The vital link between physiological responsiveness and the essential life processes lies within the signaling systems. To devise effective strategies for controlling the aging process, a comprehensive reevaluation of this connecting loop is imperative. This review aims to explore the impact of aging on signaling systems responsible for responsiveness and introduce a novel perspective on intervening in the aging process by restoring the compromised responsiveness. These innovative mechanistic approaches for modulating altered responsiveness hold the potential to illuminate the development of action plans aimed at controlling the aging process and treating age-related disorders.

The Relationship of Physical Activity History to Pattern-Reversal Evoked-Potential Components in Young and Older Men and Women

Latencies and peak-to-peak amplitudes of pattern-reversal evoked-potential (PREP) components of active and inactive community-dwelling healthy 61- to 77-year-olds were compared with those of active and inactive 18- to 31-year-olds to determine whether long-term physical activity involvement was associated with attenuation of age-related changes in sensory processes. Binocular PREPs were derived for each of 2 check sizes (22 × 15 ft and 41 × 30 ft of visual angle) to provide increasing challenge of spatial resolution. Analyses of the latencies revealed significant effects for age, gender, and check size such that latencies were longer for older than for young participants, men than for women, and small than for larger check sizes. Amplitudes were larger in older adults for the P100-N150 peak-to-peak difference, but physical activity history was not associated with reduction of the observed age-related increases in component latencies and amplitude. As such, physical activity does not appear to attenuate age-related decline in visual sensory processing.

Degradation of stimulus selectivity of visual cortical cells in senescent rhesus monkeys

Human visual function declines with age. Much of this decline is probably mediated by changes in the central visual pathways. We compared the stimulus selectivity of cells in primary visual cortex (striate cortex or V1) in young adult and very old macaque monkeys using single-neuron in vivo electrophysiology. Our results provide evidence for a significant degradation of orientation and direction selectivity in senescent animals. The decreased selectivity of cells in old animals was accompanied by increased responsiveness to all orientations and directions as well as an increase in spontaneous activity. The decreased selectivities and increased excitability of cells in old animals are consistent with an age-related degeneration of intracortical inhibition. The neural changes described here could underlie declines in visual function during senescence.

Involvement of hippocampal synaptic plasticity in age-related memory decline

This article examines the functional significance of Ca(2+)-dependent synaptic plasticity in relation to compromised memory function during aging. Research characterizing an age-related decline in memory for tasks that require proper hippocampal function is summarized. It is concluded that aged animals possess the mechanisms necessary for memory formation, and memory deficits, including rapid forgetting, result from more subtle changes in memory processes for memory storage or maintenance. A review of experimental studies concerning changes in hippocampal neural plasticity over the course of aging indicates that, during aging, there is a shift in mechanisms that regulate the thresholds for synaptic modification, including Ca(2+) channel function and subsequent Ca(2+)-dependent processes. The results, combined with theoretical considerations concerning synaptic modification thresholds, provide the basis for a model of age-related changes in hippocampal synaptic function. The model is employed as a foundation for interpretation of studies examining therapeutic intervention in age-related memory decline. The possible role of altered synaptic plasticity thresholds in learning and memory deficits suggests that treatments that modify synaptic plasticity may prove fruitful for the development of early therapeutic interventions in age-related neurodegenerative diseases.

Evaluation of cycloserine in the treatment of Alzheimer's disease

This multicenter study evaluated the efficacy and safety of cycloserine and measured its effects on explicit and implicit memory tests in patients with Alzheimer's disease (AD). Four hundred ten patients with AD, aged 50 years or older, were enrolled in this parallel-group, double-blind, placebo-controlled, randomized trial of 5, 15, or 50 mg cycloserine or placebo twice daily, and 403 entered the double-blind treatment phase. Two hundred sixty-five patients completed the entire 26-week treatment phase. There were no baseline differences among the four treatment groups. Cognitive Drug Research (CDR) efficacy assessments showed no differences between active treatments and placebo from baseline to study weeks 2, 6, 14, or 26. Patients receiving 15 mg of cycloserine improved significantly on one section of an implicit memory test. No differences among treatments were observed for any other assessment scales evaluated. The incidence and severity of adverse events were similar across treatment groups. Cycloserine was well tolerated but did not demonstrate consistent evidence of efficacy during the course of therapy. Higher doses may be necessary to achieve efficacy in the AD population and do not appear to be precluded by the adverse event profile seen in this study.

Expression of tyrosine hydroxylase in the aging, rodent olfactory system

Tyrosine hydroxylase (TH) mRNA, immunoreactivity, and activity were examined as a reflection of dopamine expression in juxtaglomerular neurons intrinsic to the olfactory bulbs of young (6-month-old), middle aged (18-month-old), and aged (25- to 29-month-old) rats and mice. TH expression was maintained at levels observed in young animals in the olfactory bulbs of aged animals from two mouse strains, C57Bl/6JNia and C57Bl/6NNia, and one rat strain, an F1 hybrid between F344 and Brown Norway strains. The parental F344 rat strain exhibited reductions in TH expression of about 20% in 26- to 29-month-old animals as compared to 6- and 18 month-old rats. However, there was significant inter-animal variability. Some aged F344 rats had TH levels that were similar and others had activity levels that were 50% of those in young and middle aged animals. Neither the general condition of the animals nor the presence of adrenal tumors predicted the individuals with reduced TH expression. Olfactory bulb size, estimated from protein content, did not differ between rats and mice of different ages. In addition, expression of olfactory marker protein, a protein found primarily in mature olfactory receptor neurons, also was unchanged indicating the maintenance of afferent innervation. These data suggest that, in contrast to other brain dopamine systems, the expression of the dopamine phenotype is maintained in the aging olfactory bulb.

Evaluation of multiple doses of milacemide in the treatment of senile dementia of the Alzheimer's type

A multicenter, double-blind, placebo-controlled, parallel group study was conducted to assess the safety and efficacy of three doses of milacemide in the treatment of patients with senile dementia of the Alzheimer type of mild to moderate severity. Patients were randomly assigned to receive one of three dosages of milacemide (400, 800, or 1200 mg/day) or placebo for 4 weeks followed by a single-blind 4-week placebo period. One hundred forty-eight men and women older than 50 years of age were enrolled, and 129 patients completed the study. The differences among treatment groups were not statistically different with respect to total scores on the Alzheimer's Disease Assessment Scale or any items and subscales that were examined, nor were significant differences on the Clinical Global Impression Scale found. Clinically significant increases in liver function tests, specifically aspartate aminotransferase and alanine aminotransferase (AST and ALT), were reported for five of the patients receiving milacemide, requiring their withdrawal from the study.

Age-related changes in pattern visual evoked potentials: differential effects of luminance, contrast and check size

We recorded visual evoked potentials (VEPs) to checkerboard pattern-reversal stimulation in 109 normal subjects (51 males and 59 females; aged 19-84 years) in order to study the aging effect on the multiple channels of the visual system in humans. Transient VEPs to 3 check sizes (15', 30' and 50') were obtained by monocular stimulation. Two test conditions were employed: (1) a high luminance (180 cd/m2) and a low luminance (11 cd/m2) both with a fixed contrast (90%), and (2) a high contrast (85%) and a low contrast (10%) both at a fixed luminance (57 cd/m2). The major features of our results included: (1) the presence of a curvilinear relationship between P100 latency and age for all conditions, while the P100 amplitude did not show any such aging effect, (2) the age-latency function was similar between the two luminance conditions, while it was different between the two contrast conditions, and (3) the differential age effect on the P100 latency caused by changes in contrast depended on the check size. These results suggest that age-related changes in the human visual system are not uniform, but rather are different in the specific functional subdivisions. It is thus hypothesized that aging may differentially influence the separate channels of the human visual system.

Pattern visual evoked potential luminance and multiple sclerosis

Pattern visual evoked potentials (PVEPs) were recorded from 111 patients classified as having possible, probable or definite multiple sclerosis. Patients were stimulated with a checkerboard pattern using high and low luminances in order to test the hypothesis that an attenuated pattern luminance increases the detection rate of PVEP abnormalities. With increasing certainty of diagnosis, there was a concomitant increase in the incidence of PVEP abnormalities. However, there was no evidence that stimulating with a lower luminance pattern enhanced the sensitivity of the test. The same findings were also apparent when the patient data was analyzed according to the presence or absence of a history of optic neuritis or other visual symptoms. It is concluded that, within the luminance limits used in this study, the role of varied luminance in detecting demyelinating lesions in the optic nerves using the PVEP is minimal, although there was some limited evidence that a high level of luminance may be more appropriate than a low level.

The influence of age on neurotransmitter turnover in the rat's superior colliculus

Measurements of the turnover of dopamine, noradrenaline and serotonin and their metabolites have been performed in the superior colliculus of adult and aged rats. The turnover of dopamine, noradrenaline and their metabolites after pargyline treatment was significantly lower in aged rats than in adults. On the contrary, the synthesis rate of serotonin (measured by accumulation of 5-hydroxytryptophan after decarboxylase blockade) and the turnover rate of serotonin (after pargyline treatment) did not change during aging. These findings suggest that aging has a different effect on catecholamines and serotonin turnover in the superior colliculus of the aged rats.

Senile changes of crystalline lens: effects on the delayed latency of pattern visually evoked potentials in phakic and pseudophakic eyes

We recorded visually evoked cortical potentials to pattern stimuli (PVECPs) in 22 patients who received a posterior chamber intraocular lens implant. The mean latency of the P100 obtained from patients was not significantly different from that of age-matched normal phakic subjects. Furthermore, we found that both groups showed a similar increased delay of the P100 latency with age. The results of this study indicated that the senile opacity of the crystalline lens does not contribute to changes of PVECPs with age. These alterations may be due to senile changes of the neural pathway.

Simultaneous measurement of serotonin and 5-hydroxyindoleacetic acid in rat brain using a liquid chromatographic method with electrochemical detection

A simple and sensitive method for the simultaneous measurement of 5-hydroxytryptamine and its main metabolite 5-hydroxyindoleacetic acid in rat brain is described. Brain tissue samples were only deproteinated and, without further extraction, were injected directly onto a high-performance liquid chromatography column and detected electrochemically. The detection limit for 5-hydroxytryptamine and 5-hydroxyindoleacetic acid was 16 and 8 g per injection, respectively. Within 8 min, the total separation of 5-hydroxytryptamine and 5-hydroxyindoleacetic acid is achieved. With this method over 100 analyses can be performed in a single working day. Brain samples from young and old, male and female Brown Norway rats were analysed for indoles by this method. The ratio 5-hydroxytryptamine 5-hydroxyindoleacetic acid, an index for serotonin turnover, showed significant differences between age groups and genders in the cortex, and a significant difference between genders in the hypothalamus.

Age-related alterations in noradrenergic input to the hippocampal formation: structural and functional studies in intraocular transplants

Intrinsic versus extrinsic determinants of age-related alterations in hippocampal noradrenergic transmission were investigated using intraocular allografts in rats. Three groups of animals were examined: young hippocampal transplants in young hosts, old transplants in old hosts and young transplants in old hosts. Postsynaptic sensitivity to noradrenaline (NA) was measured by extracellular recordings of spontaneous activity and superfusion with known concentrations of catecholamines in the anterior chamber of the eye. Hill plots demonstrated that the dose-response relationships of NA-induced depressions were linear and parallel in the 3 groups. Aged hippocampal grafts displayed a highly significant subsensitivity to NA of one order of magnitude. The EC50 for this group was 203.1 microM as compared to 29.2 in young grafts. Young intraocular grafts in old hosts responded similarly to transplants in young hosts, with an EC50 of 32.4 microM for the depressant actions of NA. Collaterals of the host iris sympathetic ground plexus invaded the hippocampal grafts. The density of this noradrenergic innervation was estimated by immunohistochemistry for tyrosine hydroxylase. A slightly increased density and fluorescence intensity of the noradrenergic fibers were observed in the old transplants as compared to the young transplants in young and old hosts. This was correlated with a significantly (P less than 0.01) increased content of NA in old transplants, as measured with high performance liquid chromatography. The old transplants also contained a large number of autofluorescent lipofuchsin granules, which were absent in the young transplants, regardless of the recipient age. Taken together, these results suggest the existence of alterations in pre- as well as postsynaptic noradrenergic mechanisms in the aging hippocampus. These changes were dependent on transplant age rather than host age, thus suggesting an involvement of intrinsic rather than extrinsic determinants in this model system.

Age-related changes of catecholamines and their metabolites content in the visual system of the rat

The changes in the content of the catecholamines in each structure of the geniculate and extrageniculate visual system of the rat during the aging period (6-30 months) have been studied. Dopamine was found at lower levels than noradrenaline in all the structures. The dopamine and noradrenaline showed different developmental profiles. Dopamine and its metabolite levels decreased in the lateral geniculate and visual cortex and increased in superior colliculus and posterior thalamus. Noradrenaline and its metabolites increased in all structures during the aging period. However, 3-methoxy-4-hydroxyphenylglycol/noradrenaline and normetanephrine/noradrenaline ratios decreased in all structures except in superior colliculus. These results suggest age-related changes in the catecholamines in the visual system of the rat.

Effect of aging on monoamines and their metabolites in the rat brain

Concentrations of dopamine (DA), norepinephrine (NE), serotonin (5-HT) and their acid metabolites were assayed in specific brain areas of Wistar rats of various ages. DA and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were significantly lower in striatum and mesolimbic areas of old (24 mos) rats than young adult (3 mos), but not mature (12 mos) rats. The decrease of homovanillic acid (HVA) was significant in mesolimbic areas but not in striatum. Neither cortical NE nor its metabolite methoxyhydroxyphenylglycol sulphate (MHPG-SO4) were significantly changed by aging. 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in the brainstem showed a tendency to a decrease and increase respectively in aged animals compared with young adults, but the differences were not statistically significant. However, the ratio of 5-HIAA to 5-HT concentrations was significantly higher in aged animals. The conclusion can be drawn that, in these brain areas, DA is more vulnerable to aging than NE and 5-HT, the metabolism of the latter being even enhanced.

Effects of age and sex on pattern electroretinograms and visual evoked potentials

Pattern-electroretinograms (P-ERGs) and visual evoked potentials (VEPs) were simultaneously recorded in 112 normal individuals aged 20-75. Two-sized checks subtending 15' and 31' were used as stimuli. A weighted regression analysis was used to determine which of the variables, sex or age, was significant. The latency of the a and b wave of the P-ERGs showed a progressive increase with age but no difference between sexes. The effect was statistically significant for both 15' and 31' checks. There was no statistically significant aging effect for VEPs elicited by 31' checks. Aging, however, affected N70, P100, and the interpeak interval between b wave to N70 and b wave to P100 for responses to 15' checks. Shorter VEP latencies were noted in females for both 15' and 31' checks. The simultaneous recording of P-ERGs and VEPs has demonstrated that aging is a major variable at the retinal level. The effects on the a and b waves are mostly due to optic changes with aging and only partially to aging changes in the neuronal retinal circuitry. The effect of aging on VEPs is different for different size stimuli. The cause is a random neuronal cell loss in the visual pathways from the optic nerve to the visual cortex as the individual ages. The difference in VEP data between sexes may be related to anatomical size and hormonal influences.

Growth and development of intraocular fetal cortex cerebri grafts in rats of different ages

Cortex cerebri pieces from fetal donors were homologously and bilaterally grafted to the anterior chamber of the eye of 1.5-, 3.0- and 7.5-month-old rats. Repeated stereoscopic in vivo measurements revealed that the grafts in the young group grew to a size twice as large as those in the older two groups. The degree of gliosis was studied immunohistochemically using antibodies against glial fibrillary acidic protein. Both grafts to young and to older hosts were clearly gliotic as compared to normal cerebral cortex. However, the gliosis was significantly more pronounced in grafts to 3.0- and 7.5-month-old hosts as compared to grafts to 1.5-month-old hosts. The vascular component was evaluated using antibodies against laminin. We found laminin immunofluorescence to be an excellent marker of brain tissue vascularization, particularly at the capillary level, revealing the entire capillary tree and endothelial budding. The density of the vascular plexus and the average thickness of the capillaries of cortex cerebri grafted to the youngest recipients was remarkably similar to normal cerebral cortex. In marked contrast, grafts to the older hosts had a clearly pathological vascular network characterized by few, thick-walled blood vessels and very few normal-looking capillaries. We conclude that host age factors profoundly influence development and growth of intraocular brain tissue grafts.

Monoamine oxidases A and B are differentially regulated by glucocorticoids and "aging" in human skin fibroblasts

Two forms of monoamine oxidase (MAO A and MAO B) exist which, although similar in a number of properties, can be distinguished on the basis of their substrate specificity, inhibitor sensitivity, kinetic parameters, and protein structure. These properties were used to study the molecular mechanism(s) by which glucocorticoid hormones and "aging," known to alter MAO activity in vivo, regulated the expression of MAO A and MAO B in cultured human skin fibroblasts. The addition of dexamethasone or hydrocortisone to cultures resulted in a dose- and time-dependent increase in total MAO activity, whereas the removal of hormone from cultures resulted in a time-dependent decrease in activity toward control levels. The response to dexamethasone was affected by culture conditions such as serum concentration, feeding frequency, and cellular "age." Cellular aging, in the absence of hormone, also resulted in increased levels of total MAO activity. The effects of hormones and aging on total MAO activity appeared to be selective for MAO A. The 6- to 14-fold increases in total activity were paralleled by similar increases in the activity and amount of active MAO A but less than 2- to 3-fold increases in the activity and amount of MAO B. Altered synthesis or degradation of the active enzyme appeared to account for the effects of hormones, aging, and various culture conditions on MAO activity. Inhibitor sensitivity, substrate affinity, electrophoretic mobility, and molecular turnover number of either form of the enzyme were not altered during dexamethasone treatment or during cellular aging. However, rates of active MAO synthesis were affected by hormone treatment and feeding frequency, rates of active MAO degradation by serum concentration, and rates of active MAO synthesis or degradation by aging. In summary, we have shown that glucocorticoids and cellular aging selectively affect the amount of MAO A at the level of active enzyme synthesis or degradation. Further, our finding that the expression of the two forms of MAO in human fibroblasts can be independently regulated supports the growing evidence that MAO A and MAO B are separate molecular entities.

Brain dopamine autoreceptors in aging rats

The function of dopamine (DA) autoreceptors is evaluated in vivo in striatum and mesolimbic regions of young adult (4 months), mature (14 months) and old (26 months) male Wistar rats. gamma-Butyrolactone (GBL)-induced dihydroxyphenylalanine (DOPA) accumulation in rats also treated with an inhibitor of aromatic L-aminoacid decarboxylase was used to determine the presence of synthesis-modulating nerve terminal autoreceptors while its reversal with apomorphine served as an index of autoreceptor stimulation. GBL-induced DOPA accumulation in striatum is very high at all three ages (130-150% increase in comparison with controls) as is its reversal by apomorphine (65-80% decrease in comparison with GBL alone). In mesolimbic regions, GBL has much less effect than in striatum (31% rise at 4 and 26 months, 12% rise at 14 months), but apomorphine's effect is of the same order of magnitude (down 60-80%). The conclusion can be drawn that aging does not significantly affect DA autoreceptor function in striatum and mesolimbic areas.

Memory function and brain biochemistry in normal aging and in senile dementia

One might argue that the decrease in the number of brain cells as a function of age could be the source of the functional age deficits in memory performance. However, this possibility seems less likely since the actual loss of neurons up to advanced age is relatively small. There are no good estimates of the loss of synapses. Golgi staining of cortical neurons would indicate that there is a loss with higher age. So far, however, the most convincing data of marked loss with age appear at the biochemical level. Most human data fail to demonstrate a decrease in cholinergic and serotonergic activity as a function of normal aging, although there is a loss of corresponding receptors. In AD/SDAT, however, there is a marked damage to these systems. Conceivably, acetylcholine may be providing informational rather than tone setting or balancing influence on memory function. This may explain the failure of cholinomimetic drugs to improve memory in AD/SDAT due to their inability to supply the informational properties of normal neuronal transmission. The catecholamines, noradrenaline and dopamine are both lost in normal aging and to a much higher degree in AD/SDAT. Animal data show that noradrenaline deficiency results in scattered attention. Such a pattern might also exist in the intact aged and through guidance by means of instructions, contextual cues, and a richer TBR information, the elderly are being forced to attend. This may promote and supersede the normal functions of the noradrenaline system by directions from external rather than internal influences, conceivably by potentiating the remaining noradrenaline neurons. The cortical motor areas are relatively spared from neuro-degenerative changes in normal aging and in AD/SDAT and this might provide a neuroanatomical basis for the elderly's and mildly to moderately demented patients' success in memory performance when motor action is involved. The role of dopamine in motor function and its stability with age in hippocampus may also provide a neurochemical basis for the preservation of memory when the subjects are allowed to act physically during encoding.

Influence of diet on plasma tryptophan and brain serotonin levels in mice

Groups of mice were maintained for up to 78 weeks on tryptophan restricted, protein restricted and control diets. Plasma tryptophan levels were significantly reduced by both forms of dietary restriction. Brain serotonin levels were significantly reduced only in mice on the tryptophan restricted diet, but not for mice on the protein restricted diet. The protein-restricted diet contains less of the large neutral amino acids which compete with tryptophan to enter the brain. It is known that protein restriction and tryptophan restriction extend lifespan. The results presented here suggest that extension of lifespan and lowering of brain serotonin are not related.

Brain catecholamine content and turnover in aging rats

The content and turnover of catecholamines were evaluated in various brain regions of young adult (4-months) and aged (24-months) male rats. Turnover was assessed from the concentrations of acid metabolites and the decline of catecholamine content after synthesis blockade with alpha-methyl-p-tyrosine. Dopamine was reduced by aging in striatum, mesolimbic areas, and hypothalamus. Dopamine metabolites and turnover rate were significantly lower in striatum and mesolimbic areas of aged than of young animals. Hypothalamic norepinephrine content and turnover rate were unchanged in aged compared to young rats but its metabolite (MHPG-SO4) was increased in the cortex. These findings point to an extensive impairment of brain dopamine metabolism in aging rats, whereas norepinephrine seems to be less impaired.

Neurotransmitter receptors in the central nervous system and aging: pharmacological aspect (review)

The present review is concerned with the modern ideas about age-related changes of neurotransmitter receptors in the central nervous system and the possible ways of their pharmacological regulation. Based on the experimental data, attention has been paid to substantiation of the degree of maintenance of the receptor reactions in the CNS during aging in response to various pharmacological manipulations. Finally, the concept about an important role of the disturbances of neurotransmitter processes and their receptor link in the mechanisms of aging of the brain has been postulated.

Monoamine levels and monoamine oxidase activity in different regions of rat brain as a function of age

The levels of norepinephrine, dopamine, 5-hydroxytryptamine and the activity of monoamine oxidase were estimated in cerebral cortex, cerebellum, medulla oblongata, hypothalamus, striatum and midbrain of 21-day-old, 3-, 6-, 12- and 24-month-old male albino rats of Wistar strain. No significant change with age was found in the levels of all the three amines in cerebral cortex and cerebellum, while medulla oblongata showed a significant decrease of all the amines by 24 months of age. Hypothalamic norepinephrine, dopamine and striatal dopamine showed a highly significant decrease by 24 months of age, whereas 5-hydroxytryptamine in hypothalamus, norepinephrine and 5-hydroxytryptamine in striatum and dopamine in midbrain did not show any appreciable change with age. Monoamine oxidase activity in all the regions except cerebellum showed a significant increase by 24 months of age compared to 3- and 6-month-old rats.

The locus coeruleus and its possible role in ageing and degenerative disease of the human central nervous system

The central noradrenergic pathways with the mammalian brain are principally based on that group of nerve cells within the reticular substance of the upper pons known as the locus coeruleus. The physiological role of these nerve cells appears to be one of maintaining homeostasis within the central nervous system, whatever adverse conditions prevail in the rest of the body, through governing the flow of blood through, and degree of water permeability of, the capillary bed. The extensive ramifications of these noradrenergic terminals mean that the atrophy and loss of nerve cells from locus coeruleus that occurs in old age, and especially so in degenerative diseases of the central nervous system such as Alzheimer's disease and other conditions, will have widespread repercussions for brain function. The chain of physiological disturbances set up as a result of this cell loss may mean a progressive failure of homeostasis within the brain, which in the extreme may culminate in that pattern of mental breakdown which is usually termed dementia.

The effect of aging on vasopressin system in Fisher 344 rats

Vasopressin (VP) was measured by RIA in the plasma, neurohypophysis (NH) and hypothalamus (HT) of young (2 months), adult (12 months) and old (30 months) male Fisher 344 rats, ten of each age. Plasma VP concentration was significantly lower in old compared to adult and to young rats. VP content in the NH expressed per mg weight was similar in all three groups, while the hypothalamic VP content was decreased in the aged rats. This suggests that reduced synthesis and release and/or increased degradation of VP occurs in aged rats. These data are in an agreement with our previous results obtained with Sprague-Dawley rats indicating that these differences are not strain-related. In a pilot experiment, we further studied VP release from isolated perifused NH of young and old rats. Two glands of each age were simultaneously perifused in individual microchambers with medium TC 199 and subsequently stimulated by Locke's medium containing 56 mM K+. Both, the initial and the basal VP release from the NH of the old rats, as well as the response to high K+, were about a half that of the young rats. Thus, a decreased VP release may contribute to the findings of lower plasma VP concentration in aged rats.

Effects of constant bright illumination on reproductive processes in the female rat

Physiological and behavioral reproductive changes in the female rat which occur under constant bright illumination (LL) are examined. The development of LL-induced persistent estrus (PE) is discussed first in relation to other conditions in which PE is displayed. Next, mechanisms are reviewed which may account for the LL-induced changes. These include: (1) role of the retina, the retinohypothalamic tract and the suprachiasmatic nucleus; (2) influence of adrenal, pineal and Harderian glands; and (3) disruptions in either the 4-day endocrine rhythms or circadian neural component of the estrous cycle. Additional topics which are examined include the ontogeny of age-induced PE and the effects of LL on hormone receptor binding, puberty, sexual receptivity and mating.

Selective changes of receptor binding in brain regions of aged rats

Binding to several receptors was compared in brain regions of 3 and 21-23 month-old rats. In crude membrane preparations of aged rats the number of dopamine antagonist receptors in striatum was much reduced (-53%). beta-Noradrenergic receptors (cortex) and benzodiazepine receptors (hippocampus and cerebellum) were less but significantly reduced and serotonergic receptors, alpha 1 noradrenergic receptors (both in cortex) and dopamine agonist receptors (striatum) were unchanged. For each receptor binding the KD values were the same in young and old animals. GABA receptor binding (hippocampus and cerebellum) evaluated at only one 3H-GABA concentration (8 nM) was similar in both groups when expressed per protein content but significantly reduced in aged rats when expressed per tissue wet weight because of the partial purification of the synaptic membranes used for 3H-GABA binding. In our experimental conditions age-related changes of specific binding sites in the central nervous system were selective for some receptors studied and did not seem to be due to general non-specific modification of brain tissue composition.

Selective reduction of one class of dopamine receptor binding sites in the corpus striatum of aged rats

In aged rats (21-23 vs 3 month) the neuroleptic receptor number was reduced in the striatum (--53%), in the limbic area (--36%), and not changed in the cortex. The in vitro pharmacological profile of the remaining [3H]spiroperidol receptors in each area was not modified in aged animals. The binding of [3H]ADTN (2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronapthalene) in the striatum and of [3H]serotonin ([3H]-5 HT) in the cortex was also the same in both age groups.

Quantitative ultrastructural changes in rat cortical synapses during early-, mid- and late-adulthood

Quantitative ultrastructural analysis of rat parietal cortex was undertaken to determine the nature of the synaptic changes occurring in the molecular layer over a series of ages in early- (3 months), mid- (6 and 10 months) and late- (17 months) adulthood. The total number of synapses remained constant until 10 months of age, but decreased significantly by 17 months. Asymmetrical synapses on dendritic shafts were lost earlier (by 6 months) than asymmetrical synapses on dendritic spines (by 17 months). Symmetrical axodendritic synapses remained constant throughout adulthood. Analysis of synaptic terminal parameters revealed the following. Both individual and total presynaptic terminal areas decreased over the age range studied. Individual and total postsynaptic terminal areas, however, remained constant over the 3--17-month period. Positive correlations were obtained for the relationships between presynaptic terminal area and both age and synaptic vesicle number. The presynaptic terminal area was largest and contained the greatest number of vesicles at 3 months of age. This age was, in addition, characterized by the least numbers of mitochondria in the presynaptic terminal and spine apparatus in the postsynaptic terminal. The vacuolar and tubular cisternae of the presynaptic terminal were considerably reduced at 17 months. These data suggest that in the molecular layer of the cerebral cortex the period of adulthood is characterized by a diversity of synaptic changes. The 3-month age may reflect the end of the developmental phase and may be marked by changes in synaptic functional activity. The asymmetrical axodendritic synapses may constitute an intermediate form of synapse, capable of being transformed into axospinous synapses as dendritic spines continue to be formed in the adult.

Changes in noradrenergic neurotransmission in rat cerebellum during aging

This study compared the electrophysiological effects of locally applied cyclic adenosine-mono-phosphate (N6cAMP) between Purkinje neurons from young (4-month) and old rats (15-months and older). Purkinje neurons from young rats were significantly more sensitive to locally applied norepinephrine and N6cAMP than neurons from old rats. GABA sensitivity between the two groups was unaltered. Our results suggest that the locus of the adrenergic subsensitivity observed in older animals may reside to a large extent at or beyond the level of cAMP generation.

Age-related electrophysiological changes in rat cerebellum

The spontaneous discharge of cerebellar Purkinje neurons was studied in 3-, 12-, 15-, and 20-month-old rats. Fluphenazine and haloperidol administered intraperitoneally produced a dose-dependent increase in the spontaneous discharge in 3-month-old animals, but were ineffective in 12-, 15- and 20-month-old rats. Intraperitoneal administration of amphetamine resulted in a dose-dependent decrease in spontaneous Purkinje neuron discharge in 3-month-old rats without affecting neurons from the older animals. Discrution of norepinephrine afferents by 6-OHDA or reserpine increased spontaneous discharge rate in 3-month-old animals. This treatment did not affect Purkinje neurons in 12-, 15- and 20-month-old rats. Local application of drug with the same multibarreled electrode revealed that neurons from older rats were significantly less sensitive to inhibition by norepinephrine than were Purkinje neurons from 3-month-old animals. Similar application of GABA did not reveal a differential sensitivity. For locus coeruleus activation to produce 50% inhibition in Purkinje neuron discharge, significantly higher stimulation currents were required in 15-month-old rats than in 3-month-old animals. Our results suggest a decreased postsynaptic sensitivity to norepinephrine in 12-, 15- and 20-month-old rats as compared to 3-month-old animals.

Interactions of oestrogens and hours of sleep on cortisol, FSH, LH, and prolactin in hypogonadal women

The effects of conjugated oestrogens 0.625 mg and placebo on adrenal and pituitary hormones were compared in 10 postmenopausal women during their sleep by means of double blind prospective crossover study. On placebo, the women had nocturnal variations in LH, prolactin and cortisol concentrations. However, oestrogen administration selectively blunted LH and prolactin changes, but not the rise in cortisol.

An overview of neuropsychiatric disturbances in the elderly

In no other period of life than old age do neurophysiologic alterations seem to affect behavior and cognition so significantly. Consequently, the relationship between neurology and psychiatry remains substantial when attempts are made to understand and deal with the many problems of aging. This study concerns improvement in the organization of such an approach to the neurologic and psychiatric problems of the elderly and to various interdisciplinary factors. Special consideration is given to changes in the central nervous system with aging, and to the differential diagnosis of dementia and pseudodementia.

Dopamine (DA) and serotonin metabolic levels in the cerebrospinal fluid (CSF) in Alzheimer's presenile dementia under basic conditions and after stimulation with cerebral cortex phospholipids (BC-PL)

HVA and 5-HIAA levels in CSF were investigated in ten patients with presenile Alzheimer's dementia. As reported by others, CSF levels of dopamine and serotonin metabolism are lower in patients with presenile dementia than in normal controls of the same age. After lumbar puncture, the patients were treated with cerebral cortex phospholipids (200 mg i.v. daily). This produced a significant increase in CSF levels of HVA and 5-HIAA, as well as a reduction of serum PL. The data are interpreted as proving an active pharmacological action of cerebral cortex phospholipids on the various processes of the neurotransmitter substances in demential syndromes.

Ageing of the spinal ganglion neurons in the rat: a radioautographic study following injection of [3H]lysine

After administration of [3H]lysine to 3- and 24-month-old rats, radioautography demonstrates a significantly less important uptake in the A-type spinal ganglion neurons and in the old animals. This is in agreement with the existence of at least two functional categories of neurons in the spinal ganglion and suggests that protein synthesis is diminished in the old animals. The very fact that incorporation varies between animals of the same age sustains the hypothesis according to which amplitude of ageing is essentially an individual physiological-dependent process.

Age-dependent changes in the latency of the pattern visual evoked potential

Pattern visual evoked potentials were recorded from 71 normal subjects aged 16-72 years, using a range of 5 luminance levels. At lower levels the latency increased after the fourth decade, while there was little age-dependent increase in latency at higher levels.