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

Changes in Locomotor Activity and Oxidative Stress-Related Factors after the Administration of an Amino Acid Mixture by Generation and Age

Amino acids, as nutrients, are expected to improve sleep disorders. This study aimed to evaluate the generation- and age-dependent sleep-improving effects of γ-aminobutyric acid (GABA) and 5-hydroxytryptophan (5-HTP) coadministration. The differentially expressed genes and generation-related behavior after the administration of a GABA/5-HTP mixture were measured in a Drosophila model, while age-related changes in gene expression and oxidative stress-related parameters were measured in a mouse model. The GABA/5-HTP-treated group showed significant behavioral changes compared to the other groups. Sequencing revealed that the GABA/5-HTP mixture influenced changes in nervous system-related genes, including those involved in the regulation of the expression of behavioral and synaptic genes. Additionally, total sleep time increased with age, and nighttime sleep time in the first- and third-generation flies was significantly different from that of the control groups. The GABA/5-HTP mixture induced significant changes in the expression of sleep-related receptors in both models. Furthermore, the GABA/5-HTP mixture reduced levels of ROS and ROS reaction products in an age-dependent manner. Therefore, the increase in behavioral changes caused by GABA/5-HTP mixture administration was effective in eliminating ROS activity across generations and ages.

Opioid system and Alzheimer's disease

The opioid system may be involved in the pathogenesis of AD, including cognitive impairment, hyperphosphorylated tau, Aβ production, and neuroinflammation. Opioid receptors influence the regulation of neurotransmitters such as acetylcholine, norepinephrine, GABA, glutamate, and serotonin which have been implicated in the pathogenesis of AD. Opioid system has a close relation with Aβ generation since dysfunction of opioid receptors retards the endocytosis and degradation of BACE1 and γ-secretase and upregulates BACE1 and γ-secretase, and subsequently, the production of Aβ. Conversely, activation of opioid receptors increases the endocytosis of BACE1 and γ-secretase and downregulates BACE1 and γ-secretase, limiting the production of Aβ. The dysfunction of opioid system (opioid receptors and opioid peptides) may contribute to hyperphosphorylation of tau and neuroinflammation, and accounts for the degeneration of cholinergic neurons and cognitive impairment. Thus, the opioid system is potentially related to AD pathology and may be a very attractive drug target for novel pharmacotherapies of AD.

Plasticity of GABA(a) system during ageing: focus on vestibular compensation and possible pharmacological intervention

The lesion of the vestibular end organ evokes static and dynamic symptoms, which spontaneously regress during a complex process known as 'vestibular compensation'. Vestibular compensation is age-dependent and involves several transmitter-identified pathways in the central nervous system. In this paper we studied the time course of vestibular compensation in adult (3 months) and old (24 months) rats and correlated behavioral recovery with modifications of glutamic acid decarboxylase (GAD) mRNA expression and benzodiazepine receptor density in different brain areas. Compensation in adult rats was complete 28 days after hemilabyrinthectomy, whereas old rats still showed significant behavioral impairment. A higher GABAergic tone was found in old rats, as indicated by higher benzodiazepine receptor density in lateral vestibular nucleus and higher mRNA level for glutamic acid decarboxylase in cerebral cortex and medial vestibular nucleus. In adult, compensated rats, benzodiazepine receptor density in the vestibular nuclei was normal 28 days after lesion, whereas GAD mRNA level was higher in anterior cingulate cortex, only. On the contrary, these parameters were still altered in anterior cingulate and somatosensory cortex, basal ganglia, vestibular nuclei and cerebellum in old rats 28 days after vestibular lesion. We also evaluated the effect of the ergoline derivative nicergoline on behavioral and neurochemical correlates of vestibular compensation in old rats. Nicergoline treatment attenuated the severity of oculomotor and postural symptoms after vestibular lesion and reversed most of these age- and lesion-induced alterations in GAD mRNA expression. Thus, lesion-related alterations of the GABAergic transmission and behavioral profile after vestibular lesion are age-dependent.

Caloric restriction retards the aging associated changes in gamma-aminobutyric acidA receptor gene expression in rat cerebellum

It is widely accepted that calorie restriction is an effective way of delaying the aging process. Also, there is an indication that the beneficial effects exerted by dietary manipulation may be due to a direct effect at the molecular level like gene expression. The studies were conducted to determine whether calorie restriction prevents any age-related changes in the structural and molecular aspects of the GABAA-BZ receptor. In aged (24-month old diet ad libitum) rats, the binding of [35S]t-butyl-bicyclophosphorothionate (TBPS) was significantly reduced in the cerebellum. In contrast, [35S]TBPS binding remained unchanged in the cerebellum of calorie restricted old rats. In order to evaluate the molecular basis of these changes, the alpha sub-unit mRNA levels were measured. The GABAA receptor alpha1 sub-unit mRNA level remained unchanged in both the old groups of rats. The alpha2 subunit mRNA level was significantly decreased in the cerebellum of aged rats (24-month old ad libitum), whereas it remained unchanged in the cerebellum of calorie restricted old animals. These findings indicate a selective age and diet related modulation in the stoichiometry of the GABAA receptor in aging.

Absence of modifications of the pharmacological properties of the GABAA receptor complex during aging, as assessed in 3- and 24-month-old rat cerebral cortex

We have investigated the possible modifications of the pharmacological properties of Type I and Type II benzodiazepine binding sites of the gamma-aminobutyric acidA (GABAA) receptor complex in cortical membranes from 3- and 24-month-old Wistar or Fischer rats. No major changes were found in the binding parameters of [3H]zolpidem (a Type I-specific ligand) or [3H]flunitrazepam (a non-selective benzodiazepine). Neither the Kd values nor the Bmax for either ligand were modified during aging in cortical membranes from Wistar or Fischer rats. Consequently, the proportion of Type I binding sites was also unmodified in aged cortical membranes. The absence of modifications of Type I and Type II binding sites was also confirmed by Cl 218,872 displacement of [3H]flunitrazepam binding in aged cortical membranes from Wistar rats. Furthermore, the [3H]muscimol binding and the allosteric interactions between Type I or total benzodiazepine binding sites and GABA binding sites also remained unaltered with aging in cortical membranes from Wistar rats. Moreover, the pattern and proportion of the [3H]flunitrazepam photoaffinity labeled peptides were also unmodified by aging. These results demonstrate the absence of modifications in Type I or total benzodiazepine binding sites of the GABAA receptor complex from adult and aged cortical membranes in Fischer or Wistar rats.

Adverse effects of antidepressants in the elderly

Depression is a common problem in old age and the use of antidepressant drugs is particularly prevalent among elderly patients. Limited data suggest that dose requirements may be lower in the elderly because of age-related changes in pharmacokinetics and perhaps also in sensitivity. The side effect profiles of the various antidepressants are reviewed with regard to their potential to cause specific problems in the older patients. Anticholinergic actions, orthostatic hypotension and sedative effects warrant particular care in the elderly.

Aging related alterations in GABAA receptor subunit mRNA levels in Fischer rats

The influence of aging on the binding of ligands to picrotoxin binding sites as well as steady state levels of mRNA for various alpha subunits of gamma-aminobutyric acid (GABA) receptor complex was investigated in male Fischer F-344 rats. In aged rats, the binding of [35S]t-butyl-bicyclophosphorothionate (TBPS) was significantly reduced. This decrease in TBPS binding derived from a reduced density of binding sites, rather than from affinity changes, in both cerebral cortex and cerebellum. In aged rats, alpha 1 mRNA level decreased approximately 70% between age 6 months and 24 months in the cerebral cortex (P less than 0.005). In contrast, alpha 1 mRNA remained unchanged in the cerebellum of old rats. The association of a decrease in picrotoxin binding sites in the cerebral cortex with a decline in alpha 1 mRNA level in the cerebral cortex and in alpha 2 mRNA level in the cerebellum is indicated. alpha 6 mRNA level increased with age in the cerebellum. These findings indicate a selective age related modulation in the stoichiometry of GABAA receptor in aging.

Brain monoamine oxidase and aging: A review

The present state of research on the age-related dynamics of brain monoamine oxidase (MAO) activity and its role in the development of age pathology is described. Special attention is given to the role of MAO in the pathogenesis of parkinsonism and to the mechanisms of its interaction with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a possible etiological factor of parkinsonism development in aging. The mechanisms of action of the selective MAO-B inhibitor, deprenyl, and their peculiarities in old age are analyzed. This study provides evidence pointing to a need for the use of deprenyl in geriatric practice as an effective drug for parkinsonism treatment and as a potent geroprotector.

Enkephalin-induced stimulation of humoral and cellular immune reactions in aged rats

Twenty-month-old Wistar rats received intraperitoneal injections of the opioid pentapeptide, methionine-enkephalin (Met-Enk) in periods before and after immunization with cellular and soluble antigens. Animals were treated with 0.2 mg of Met-Enk/kg b.w., a dose previously found to increase immune capacity in young adult rats. Saline-treated 20-month-old, and Met-Enk-treated rats and saline-treated 8-week-old controls were set up for each experimental group. Immune performance was evaluated by plaque-forming cell response, antibody production and various immunoinflammatory reactions. At autopsy, thymus and spleen were weighed and processed for histological examination. The results showed that 0.2 mg dose of Met-Enk produced significant enhancement of both humoral and cellular immune responses in senescent rats. Methionine-enkephalin treatment also induced a significant increase in thymus and spleen weights in these animals. Analysis of the cellular make up of these organs revealed the enlargement of cortical and medullary areas, and pronounced pyroninophilia in the subcortical zone of the thymus and thymus-dependent areas of the spleen. The results suggest that Met-Enk exerts an immunorestorative activity in aged animals, and that changes in the opioid system may play an important role in the maintenance of immune functions during senescence.

Effects of acute infusion of the muscarinic cholinergic agonist arecoline on verbal memory and visuo-spatial function in dementia of the Alzheimer type

1. Treatment of patients with dementia of the Alzheimer type (DAT) with arecoline, a muscarinic cholinergic receptor agonist, reportedly improves performance on a picture recognition memory task, but not on other memory measures. To examine further possible performance improvements following arecoline treatment, patients with DAT were treated with a 30 min intravenous infusion of arecoline (5 mg). 2. Psychometric testing was done at five time points (two before and three following the infusion). Patients were tested on a memory task (Buschke selective reminding) and a test of visuo-spatial performance (figure copying). 3. No net change from baseline was seen in mean scores following arecoline infusion. However, the changes in performance on the two tasks were correlated (p less than 0.02) over subjects at 10 min but not at 1.5 or 5.5 hr following the infusion. 4. This result suggests that although individual patients vary in their response to a given dose of arecoline, their responses are consistent across types of tasks. Thus the lack of a mean drug effect may be due to individual differences in response rather than to a lack of response.

Age-related loss of t-[35S]butylbicyclophosphorothionate binding to the gamma-aminobutyric acidA receptor-coupled chloride ionophore in rat cerebral cortex

Muscimol and t-butylbicyclophosphorothionate (TBPS) are known to label two distinct sites within the gamma-aminobutyric acidA (GABAA) receptor complex, i.e., the GABA recognition site and the chloride ionophore, respectively. Age-dependent changes in the specific binding of [3H]muscimol and [35S]TBPS were compared in membranes prepared from the cerebral cortex of rats, 2-800 days old. Perinatal (day 2) binding of muscimol and TBPS represented 8 and 20% of the respective values for adults (day 180). After the first week, muscimol binding increased more rapidly than TBPS binding. Levels near those of adults were reached at day 20 and remained practically unchanged in adulthood (day 180). In aged (780-day-old) rats, the binding of TBPS was significantly reduced, whereas muscimol binding did not change compared with adult values. This decrease of TBPS binding derived from a reduced density of binding sites, rather than from affinity changes. The allosteric responsiveness of TBPS binding to exogenous GABA was also reduced in aged animals. These findings indicate an age-related change in the molecular (structural) organization of the GABAA receptor-chloride ionophore complex in rat cerebral cortex.

Age-dependent vulnerability of brain choline acetyltransferase activity to transient cerebral ischemia in rats

Male Fischer-344 rats aged 6, 12, or 24 months were subjected to four-vessel occlusion cerebral ischemia to assess age-dependent ischemic vulnerability of cholinergic and GABAergic neurons based on choline acetyltransferase (EC 2.3.1.6) and glutamic acid decarboxylase (EC 4.1.1.15) activities. Activities of both enzymes were similar (p greater than 0.05) in 6- (n = 5) and 12- (n = 5) month-old rats. Mean +/- SEM choline acetyltransferase activities in the cortex, hippocampus, striatum, and cerebellum of 6-month-old controls were 75 +/- 5, 123 +/- 9, 415 +/- 9, and 50 +/- 4 nmol acetylcholine/hr/mg protein, respectively, and were 20-30% lower (p less than 0.05) in all brain regions except the cerebellum in 24-month-old controls. Choline acetyltransferase activity was unaffected by ischemia in 6- and 12-month-old rats but was reduced by 30-60% in 24-month-old rats. Mean +/- SEM glutamic acid decarboxylase activities in the cortex, hippocampus, striatum, and cerebellum of 6-month-old controls were 98 +/- 8, 86 +/- 7, 144 +/- 13, and 125 +/- 9 nmol gamma-aminobutyric acid/hr/mg protein, respectively, and 25-35% lower in all regions of 24-month-old controls. After 30 minutes of ischemia and 5 days of recovery, glutamic acid decarboxylase activities were reduced (p less than 0.05) in all brain regions and age groups. However, its activity was decreased (p less than 0.05 compared with age-matched controls) by 55% in the cortex and 79% in the hippocampus of 24-month-old rats compared with 30% and 45% in younger rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Microinfusion of clonidine and yohimbine into locus coeruleus alters EEG power spectrum: effects of aging and reversal by phosphatidylserine

1. The behavioural and electrocortical (ECoG) power spectrum effects of clonidine, and yohimbine, an agonist and an antagonist at alpha 2-adrenoceptors, after their unilateral microinfusion into the rat locus coeruleus (LC) in young (50-70 days old) and old (13-15 months old) rats were studied. 2. Clonidine (0.09, 0.19, 0.28 and 0.56 nmol) microinfused into the LC of young rats induced dose-dependent behavioural and ECoG slow wave sleep (SWS) with a significant increase in total voltage power and power in the lower frequency bands. In contrast, yohimbine (1.3 and 2.6 nmol) infused into the LC of young rats produced ECoG desynchronization and a significant decrease in total voltage power. 3. In contrast to young rats, clonidine (0.19 and 0.28 nmol) given into the LC did not affect behaviour and the ECoG power spectrum in old rats. However, after higher doses of clonidine (0.56 and 1.2 nmol) a small and short-lasting period of behavioural and ECoG SWS was still evident. Similarly, in old rats yohimbine, at a dose (1.3 nmol) which was stimulative in young animals, did not significantly affect behaviour and ECoG power spectrum. Higher doses of yohimbine (2.6 and 5.2 nmol) were required to induce behavioural and ECoG changes similar to those observed with lower doses of yohimbine in young rats. 4. Chronic treatment with phosphatidylserine (30 mg kg-1, orally, daily for 21 and 30 days), was able gradually to restore in old rats, in comparison with a vehicle-treated group, the responsiveness of alpha 2-adrenoceptors to clonidine and yohimbine given into the LC.

Mechanisms of altered hormone and neurotransmitter action during aging: the role of impaired calcium mobilization

Age-related changes in hormone and neurotransmitter regulation of physiological functions result from various mechanistic alterations. In many cases changes in the receptors for these agents appear to be closely linked to altered responsiveness. In other instances, receptors are unaffected by aging, and various post-receptor changes result in functional deterioration. Examples of the latter situation include stimulation of cyclic AMP production and high-affinity association of steroid receptor-hormone complexes with nuclear acceptor sites in various cell and tissue types. One of the most noteworthy post-receptor changes appears to be an impaired ability to stimulate calcium mobilization in many aged systems resulting in reductions in various biological responses. Although the processes which govern regulation of calcium fluxes vary with cell type, many such dysfunctions can be at least partially reversed if sufficient calcium can be transported to appropriate cellular sites. Thus, elucidation of the molecular mechanisms involved in impaired calcium mobilization may provide the basis for new therapeutic strategies.

Impaired coupling of naloxone sensitive opiate receptors to adenylate cyclase in PMNLs of aged male subjects

Met-enkephalin (Met-enk) in 10(-9)-10(-7) M concentrations enhanced the extracellular cytotoxicity of polymorphonuclear leukocytes (PMNL) of young adult humans partly by stimulation of the "respiratory burst" in these cells. Meanwhile adenylate cyclase was inhibited and guanylate cyclase was stimulated. All the observed effects were abolished by 10(-5) M naloxone. On the other hand, a positive receptor coupling to adenylate cyclase was found when Met-enk was added in higher (10(-6)-10(-5) M) concentrations to PMNLs. The elevated cAMP level resulted in decreased extracellular cytotoxicity of PMNLs by a naloxone insensitive way. In PMNLs obtained from healthy aged male subjects, Met-enk induced in all of the applied concentrations an increased cAMP level and no change in cGMP level, with subsequent decrease of cytotoxicity, i.e. an impaired negative coupling of naloxone sensitive opiate receptors was detected with aging.

The GABA/benzodiazepine receptor-chloride ionophore complex: nature and modulation

1. A high affinity, saturable, stereospecific binding site for Benzodiazepines has been found to be functionally and possibly structurally related to a GABA receptor-chloride ionophore complex. 2. There are both central (CNS) as well as "peripheral" binding sites, involving multiple organs. 3. Evidence strongly suggests that mutually exclusive Benzodiazepine agonists and antagonists bind to the same receptor, possibly in an agonist-antagonist-inverse agonist continuum. 4. The search for an endogenous ligand has been inconclusive and the question of such a substance remains open. 5. Although the relationship between this receptor and the Limbic System remains unclear, it seems certain that the Benzodiazepine receptor plays an important role in the modulation of Limbic System excitability.

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.