Influence of aging on catecholamine levels, accumulation, and release in F-344 rats

Catecholamines Are the Key Trigger of COVID-19 mRNA Vaccine-Induced Myocarditis: A Compelling Hypothesis Supported by Epidemiological, Anatomopathological, Molecular, and Physiological Findings

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) mRNA vaccine-induced myocarditis is a rare but well-documented complication in young males. The increased incidence of sudden death among athletes following vaccination has been reported and requires further investigation. Whether the risk of myocarditis, a known major cause of sudden death in young male athletes, also increases after coronavirus disease 2019 (COVID-19) infection is unknown. The severity and implications of these critical adverse effects require a thorough analysis to elucidate their key triggering mechanisms. The present review aimed to evaluate whether there is a justification to hypothesize that catecholamines in a "hypercatecholaminergic" state are the key trigger of SARS-CoV-2 mRNA vaccine-induced myocarditis and related outcomes and whether similar risks are also present following COVID-19 infection. A thorough, structured scoping review of the literature was performed to build the hypothesis through three pillars: detection of myocarditis risk, potential alterations and abnormalities identified after SARS-CoV-2 mRNA vaccination or COVID-19 infection and consequent events, and physiological characteristics of the most affected population. The following terms were searched in indexed and non-indexed peer review articles and recent preprints (<12 months): agent, "SARS-CoV-2" or "COVID-19"; event, "myocarditis" or "sudden death(s)" or "myocarditis+sudden death(s)" or "cardiac event(s)"; underlying cause, "mRNA" or "spike protein" or "infection" or "vaccine"; proposed trigger, "catecholamine(s)" or "adrenaline" or "epinephrine" or "noradrenaline" or "norepinephrine" or "testosterone"; and affected population, "young male(s)" or "athlete(s)." The rationale and data that supported the hypothesis were as follows: SARS-CoV-2 mRNA vaccine-induced myocarditis primarily affected young males, while the risk was not observed following COVID-19 infection; independent autopsies or biopsies of patients who presented post-SARS-CoV-2 mRNA vaccine myocarditis in different geographical regions enabled the conclusion that a primary hypercatecholaminergic state was the key trigger of these events; SARS-CoV-2 mRNA was densely present, and SARS-CoV-2 spike protein was progressively produced in adrenal medulla chromaffin cells, which are responsible for catecholamine production; the dihydroxyphenylalanine decarboxylase enzyme that converts dopamine into noradrenaline was overexpressed in the presence of SARS-CoV-2 mRNA, leading to enhanced noradrenaline activity; catecholamine responses were physiologically higher in young adults and males than in other populations; catecholamine responses and resting catecholamine production were higher in male athletes than in non-athletes; catecholamine responses to stress and its sensitivity were enhanced in the presence of androgens; and catecholamine expressions in young male athletes were already high at baseline, were higher following vaccination, and were higher than those in non-vaccinated athletes. The epidemiological, autopsy, molecular, and physiological findings unanimously and strongly suggest that a hypercatecholaminergic state is the critical trigger of the rare cases of myocarditis due to components from SARS-CoV-2, potentially increasing sudden deaths among elite male athletes.

Decreased vesicular monoamine transporter 2 (VMAT2) and dopamine transporter (DAT) function in knockout mice affects aging of dopaminergic systems

Dopamine (DA) is accumulated and compartmentalized by the dopamine transporter (DAT; SLC3A6) and the vesicular monoamine transporter 2 (VMAT2; SLC18A2). These transporters work at the plasma and vesicular membranes of dopaminergic neurons, respectively, and thus regulate levels of DA in neuronal compartments that include the extravesicular cytoplasmic compartment. DA in this compartment has been hypothesized to contribute to oxidative damage that can reduce the function of dopaminergic neurons in aging brains and may contribute to reductions in dopaminergic neurochemical markers, locomotor behavior and responses to dopaminergic drugs that are found in aged animals. The studies reported here examined aged mice with heterozygous deletions of VMAT2 or of DAT, which each reduce transporter expression to about 50% of levels found in wild-type (WT) mice. Aged mice displayed reduced locomotor responses under a variety of circumstances, including in response to locomotor stimulants, as well as changes in monoamine levels and metabolites in a regionally dependent manner. Several effects of aging were more pronounced in heterozygous VMAT2 knockout (KO) mice, including aging induced reductions in locomotion and reduced locomotor responses to cocaine. By contrast, some effects of aging were reduced or not observed in heterozygous DAT KO mice. These findings support the idea that altered DAT and VMAT2 expression affect age-related changes in dopaminergic function. These effects are most likely mediated by alterations in DA compartmentalization, and might be hypothesized to be exacerbated by other factors that affect the metabolism of cytosolic DA. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

Age-dependent changes in dopaminergic projections from the substantia nigra pars compacta to the neostriatum

Age-dependent changes in dopaminergic (DA) innervation of the neostriatum (Str) were studied in male F344/N rats. Projections from the substantia nigra pars compacta (SNc) to the neostriatum were quantified using electrophysiological methods at age points from 6 to 24 months. The percentage of DA neurons activated antidromically by electrical stimulation (P-index) of Str increased between 18 and 24 months. Additionally, the percentage of DA neurons showing multiple antidromic latencies from striatal stimulation (M-index), which suggests axonal branching of individual DA neurons, increased significantly between 6 and 12 months and 6 and 24 months. These results suggest that DA neurons exhibit increased axonal branching in the aged brain.

Binding characteristics and sensitivity to endogenous dopamine of [11C]-(+)-PHNO, a new agonist radiotracer for imaging the high-affinity state of D2 receptors in vivo using positron emission tomography

[11C]-(+)-PHNO (4-propyl-9-hydroxynaphthoxazine) is a new agonist radioligand that provides a unique opportunity to measure the high-affinity states of the D2 receptors (D2-high) using positron emission tomography (PET). Here we report on the distribution, displaceablity, specificity and modeling of [11C]-(+)-PHNO and compare it with the well characterized antagonist D2 radioligand, [11C]raclopride, in cat. [11C]-(+)-PHNO displayed high uptake in striatum with a mean striatal binding potential (BP) of 3.95 +/- 0.85. Pre-treatment with specific D1 (SCH23390), D2 (raclopride, haloperidol) and D3 receptor (SB-277011) antagonists indicated that [11C]-(+)-PHNO binding in striatum is specific to D2 receptors. Within-subject comparisons showed that [11C]-(+)-PHNO BP in striatum was almost 2.5-fold higher than that measured with [11C]-(-)-NPA ([11C]-(-)-N-propyl-norapomorphine). Comparison of the dose-effect of amphetamine (0.1, 0.5 and 2 mg/kg; i.v.) showed that [11C]-(+)-PHNO was more sensitive to the dopamine releasing effect of amphetamine than [11C]raclopride. Amphetamine induced up to 83 +/- 4% inhibition of [11C]-(+)-PHNO BP and only up to 56 +/- 8% inhibition of [11C]raclopride BP. Scatchard analyses of [11C]-(+)-PHNO and [11C]raclopride bindings in two cats showed that the Bmax obtained with the agonist (29.6 and 32.9 pmol/mL) equalled that obtained with the antagonist (30.6 and 33.4 pmol/mL). The high penetration of [11C]-(+)-PHNO in brain, its high signal-to-noise ratio, its favorable in vivo kinetics and its high sensitivity to amphetamine shows that [11C]-(+)-PHNO has highly suitable characteristics for probing the D2-high with PET.

Reduced cortical noradrenergic neurotransmission is associated with increased neophobia and impaired spatial memory in aged rats

In the present study, young (5-month-old (mo)) and aged (24 mo) adult male Fischer-344 (F344) rats were assigned to experimental groups based upon their performance of a reference memory task in the Morris water maze and reactivity to a novel palatable taste in a gustatory neophobia task. Levels of norepinephrine (NE) and its metabolite 3-methoxy-4-hydroxy-phenylglycol (MHPG) were assayed via high performance liquid chromatography (HPLC) in brain regions associated with the locus coeruleus (LC)-hippocampus-cortex system and A1/A2-hypothalamic system. Binding of ligands specific for alpha-1, alpha-2, beta-1, and beta-2 receptors was assessed in hippocampus and cortex with receptor autoradiography. Impaired acquisition and retention of the water maze task and gustatory neophobia in aged rats was primarily associated with decreased NE activity in cingulate cortex (CC) as indicated by a significant reduction in the MHPG/NE ratio coupled with increased NE content. No significant changes in adrenergic receptor binding were detected in any region sampled. The results suggest that an aging-related reduction in cortical NE neurotransmission is associated with the expression of increased neophobia and deficits in spatial learning and memory performance occurring with advanced age in rats.

Presynaptic regulation of neurotransmitter release in the cortex of aged rats with differential memory impairments

Cluster analysis of water-maze reference-memory performances of 25-27-month-old (compared to 3-5-month-old) rats distinguished subpopulations of young adult rats (YOUNG), aged rats with no significant impairment (AU), aged rats with moderate impairment (AMI), and aged rats with severe impairment (ASI). In the frontoparietal cortex, we subsequently assessed the electrically evoked release of tritium in slices preloaded with [3H]choline, [3H]noradrenaline (NA), or [3H]serotonin (5-HT) and the effects of an agonist (oxotremorine, UK 14,304, and CP 93,129) of the respective autoreceptors. Cholinergic and monoaminergic markers were measured in homogenates. Overall, aged rats exhibited reduced accumulation of [3H]choline (-25%) and weaker evoked transmitter release (in % of accumulated tritium: -44%, -20%, and -34%, for [3H]acetylcholine, [3H]NA, and [3H]5-HT, respectively). In all rats, the inhibitory effects of the autoreceptor agonists on the evoked release of [3H] were comparable. Acetylcholinesterase (AChE), not choline acetyltransferase (ChAT), activity was reduced. The results suggest age-related modifications in the cholinergic, noradrenergic, and serotonergic innervation of the frontoparietal cortex, alterations of evoked transmitter release, but no interference with presynaptic autoinhibition of the release. Neither of these alterations seemed to account for the cognitive impairment assessed.

Neural Transplantation in Animal Models of Dementia

Neural transplantation provides a powerful novel technique for investigating the neurobiological basis and potential strategies for repair of a variety of neurodegenerative conditions. The present review considers applications of this technique to dementia. After a general introduction (section 1), attempts to replace damaged neural systems by transplantation are considered in the context of distinct animal models of dementia. These include grafting into aged animals (section 2), into animals with neurotransmitter-selective lesions of subcortical nuclei, in particular involving basal forebrain cholinergic systems (section 3), and into animals with non-specific lesions of neocortical and hippocampal systems (section 4). The next section considers the alternative use of grafts as a source of growth/trophic factors to inhibit degeneration and promote regeneration in the aged brain (section 5). Finally, a number of recent studies have employed transplanted tissues to model and study the neurodegenerative processes associated with ageing and Alzheimer's disease taking place within the transplant itself (section 6). It is concluded (section 7) that although neural transplantation does not offer any immediate prospect of therapeutic repair in clinical dementia, the technique does offer a powerful neurobiological tool for studying the neuropathological processes involved in both spontaneous degeneration and specific diseases of ageing. New understandings derived from neural transplantation may be expected to lead to rational development of novel strategies to inhibit the neurodegenerative process and to promote regeneration in the aged brain.

Age-related changes in adrenergic neuroeffector transmission

In this study we have looked at the effects of ageing on prejunctional control of noradrenergic neurotransmission in the cardiovascular system, in terms of alpha2-adrenoceptors, beta2-adrenoceptors and the noradrenaline re-uptake process. These studies show diminished prejunctional alpha2- and prejunctional beta-adrenoceptor-mediated responsiveness together with diminished noradrenaline re-uptake in rat tissues. The reduced prejunctional alpha2-inhibitory control and reduced re-uptake found in tissues from aged rats is more than likely to outweigh the effects of reduced beta-adrenoceptor facilitation, at least in normal conditions. Hence, assuming that such changes also occur in man, we might expect to find evidence of increased release of noradrenaline from noradrenergic nerves, and this could be reflected in plasma levels of noradrenaline.

Dopamine and serotonin uptake inhibitors on the release of dopamine and serotonin in the nucleus accumbens of young and aged rats

Nucleus accumbens (ACC) of young (4 months old) and aged (24 months old) Wistar rats were perfused with dopamine (DA) uptake blocker, cocaine, or the serotonin (5-HT) selective reuptake inhibitor, fluoxetine, through the microdialysis probe membrane, used to assess the dopamine transporter (DAT) or serotonin transporter (SERT) modulation. The basal extracellular DA release in the ACC was significantly lower in aged rats than young rats. Analysis of DA and 5-HT concentrations in the ACC with increased positive GFAP revealed that DA and DOPAC levels of aged rats were decreased to 55 and 60% of those in young rats, respectively. After co-perfusion with cocaine, both DA and 5-HT releases in the ACC were increased in the young and aged groups. However, the magnitude of the increased DA release was lower in aged rats than young rats. Co-perfusion with fluoxetine showed lower magnitude of the increased DA release in aged rats. It appears that the DAT and SERT system responds initially to ACC cell loss with age, and that especially ACC DAT in the aged rat is more degenerative compared with the young rats. These findings suggest that the serotonergic system with SERT in the remaining ACC neurons show an early adaptive response and resistance to the normal aging and maintain the multiple regulatory system in the ACC despite neural loss since the dopaminergic neurons in the aged animals are vulnerable to aging.

The effects of a specific alpha(2)-adrenoceptor antagonist, atipamezole, on cognitive performance and brain neurochemistry in aged Fisher 344 rats

The present experiments investigated the effects of a specific and potent alpha(2)-adrenoceptor antagonist, atipamezole, on cognitive performance and neurochemistry in aged rats. Aged control Fisher 344 rats, which had lower activities of choline acetyltransferase in the frontal cortex, were impaired in the acquisition of the linear arm maze task both in terms of repetition errors and their behavioural activity (the speed of arm visits), and they needed longer time to complete this task as compared to adult control rats. Atipamezole treatment (0.3 mg/kg) facilitated the acquisition of this task in the aged rats as they committed fewer errors and completed the task more quickly than saline-treated aged control rats. A separate experiment indicated that atipamezole enhanced the turnover of noradrenaline both in the adult and aged rats, but this effect was more pronounced in the aged rats. Furthermore, atipamezole enhanced significantly the turnover of serotonin and dopamine only in the aged rats when analysed in the whole brain samples. As alpha(2)-adrenoceptor antagonists are known to alleviate akinesia in the experimental models of Parkinson's disease, the present results could be especially relevant for the development of palliative treatment for demented Parkinsonian patients.

Tyrosine hydroxylase mRNA is increased in old age and norepinephrine uptake transporter mRNA is decreased in middle age in locus coeruleus of Brown-Norway rats

In normal aging, cell loss occurs in the locus coeruleus (LC), the major noradrenergic nucleus in the brain. This study examined changes in the LC of aged rats by measuring mRNA expression for tyrosine hydroxylase (TH) and the norepinephrine uptake transporter (NET). TH and NET mRNA expression were measured by in situ hybridization in young, middle-aged and aged rats. It appears that in middle age, the transporter system responds initially to LC cell loss by decreasing NET mRNA expression. Then, with further aging and cell loss, TH mRNA expression increases which may potentially increase NE synthesis in the remaining neurons. These findings suggest that multiple regulatory components are used to maintain stable noradrenergic synaptic levels despite neuronal loss. Published by Elsevier Science B.V.

Microdialysis studies of basal levels and stimulus-evoked overflow of dopamine and metabolites in the striatum of young and aged Fischer 344 rats

The technique of intracranial microdialysis was used to investigate the effects of aging on the striatal dopaminergic system of the anesthetized Fischer 344 rat. Microdialysis probes were implanted into the striatum of young (2-8 months) and aged (24-28 months) urethane anesthetized rats. Striatal dialysate levels were analyzed for dopamine (DA), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 5-hydroxyindoleacetic acid (5-HIAA), and serotonin (5-HT) by high performance liquid chromatography with electrochemical detection. As compared to the young animals, basal extracellular levels of DA and DOPAC were significantly decreased in two groups of aged animals. Stimulation with excess potassium added through the microdialysis probe produced a robust overflow of DA in the young and aged rat striatum, but the evoked overflow of DA was not diminished in the aged rat striatum as compared to young animals. In contrast, d-amphetamine-evoked overflow of DA was again robust in young and aged animals, but was greatly decreased in the aged rat striatum as compared to the signals recorded in the young rats. Taken together with previous reports, these data support the hypothesis that a major change in the regulation of DA release that occurs in aging involves changes in the function of the neuronal uptake of DA, which may be a compensatory property of DA neurons in senescence.

Age-related effects of rolipram on [3H]quinuclidinyl benzilate and [3H]phorbol 12,13-dibutyrate binding in the rat brain

Cholinergic neurotransmission and protein kinase C (PKC) in the brain play important roles in the processes of cognitive function. In this study, we examined the effect of chronic treatment with rolipram, a 3',5'-cyclic adenosine monophosphate (cyclic AMP)-selective phosphodiesterase inhibitor, on age-related changes in [3H]quinuclidinyl benzilate (QNB) and [3H]phorbol 12,13-dibutyrate (PDBu) binding, which labeled brain muscarinic cholinergic receptors and PKC, respectively. Rolipram was administered per os to young (15 weeks old) and old (80 weeks old) Wistar rats at dosage of 0.01 mg/kg and 0.1 mg/kg once a day over 4 weeks. Then, quantitative in vitro autoradiography was performed. Control old rats showed elevations in [3H]PDBu binding in the hippocampus and the cerebellum compared to young rats, but [3H]QNB binding was largely unchanged. Chronic treatment of the old rats with the higher dose of rolipram led to reductions in [3H]QNB and [3H]PDBu binding in many brain regions. However, the same treatment of the young rats induced no or minimal effect. Thus, the response of the brain to rolipram was different between young and old rats. These results suggest that the cyclic AMP-selective phosphodiesterase system in the brain is modified during aging, modulating subsequently cholinergic neurotransmission and PKC activity exclusively in old rat brains.

Low-level lead exposure selectively enhances dopamine overflow in nucleus accumbens: an in vivo electrochemistry time course assessment

Exposures to even very low levels of lead (Pb) alter behavioral and neurochemical functions. The current study was based on the hypothesis that excess synaptic dopamine (DA) availability may contribute to such disturbances and that the mesolimbic DA projection is more sensitive than the nigrostriatal system to Pb-induced DA-based alterations. In vivo electrochemical measurements of potassium chloride-evoked DA overflow and clearance were compared in dorsal striatum (STR) (nigrostriatal system) and nucleus accumbens (NAC)(mesolimbic system) of male rats after 11 weeks or 11 months of postweaning exposure to 0, 50, or 150 ppm Pb acetate drinking solutions. Pb increased evoked DA overflow selectively in NAC, with biphasic effects at 11 weeks, including increases greater than 400% at 50 ppm and concentration-related effects up to 265% of control at 11 months. Considered relative to 11-week control levels, continued exposure tended to attenuate the magnitude of Pb-related increases in DA overflow in NAC. Pb decreased clearance time in both brain regions, with these effects markedly augmented across time. These changes in DA function were observed at blood Pb values of only 15-16 micrograms/dl, underscoring their environmental relevance. The current findings support the hypothesis of excess DA availability as a mechanism of Pb-induced behavioral alterations and of a particular vulnerability of mesolimbic DA systems (NAC) to such effects. They also suggest that different mechanisms underlie Pb-related changes in amplitude and clearance and confirm previous reports of regional differences of DA systems in response to Pb exposure.

Age-related decline in striatal dopamine release and motoric function in brown Norway/Fischer 344 hybrid rats

The Brown Norway/Fischer 344 F1 hybrid rats (F344BNF1) is a newer rat model and is emerging as an important rodent model of aging. In the present study we used motoric performance tests, intracerebral microdialysis, and neurochemical measures of postmortem brain tissue to investigate the effects of aging in young (4-5 months), middle-aged (18-19), and old (24-25 months) F344BNF1 hybrid rats. We observed that old F344BNF1 rats exhibited decreased motoric performance, and lower levels of spontaneous and d-amphetamine-induced locomotor activity than those observed in young F344BNF1 rats. Microdialysis measures of extracellular basal levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and 4-hydroxy-3-methoxyphenylacetic acid (HVA) were significantly diminished in the striata of the middle-aged and old rats as compared to levels in young animals. In addition, d-amphetamine-evoked overflow of DA was significantly decreased in the middle-aged and aged rat striatum as compared to DA overflow in young F344BNF1 rats. Studies of postmortem brain tissue showed that the changes in overflow of DA correlated with significantly lower DA tissue content in ventral striatum and midbrain. Moreover, both dopamine turnover ratios (DOPAC/DA, HVA/DA) and the serotonin turnover ratio (5-HIAA/5-HT) were significantly elevated in the ventral striatum and nucleus accumbens. The results of this study demonstrate a correlation between reductions in striatal DA neurochemistry and diminished motor function in aged F344BNF1 rats.

Models of Alzheimer's disease: cellular and molecular aspects

Glucose metabolism in the brain is an important process that influences many normal cellular processes, from neurotransmitter synthesis to ATP production. While cortisol and insulin have opposing effects on glucose metabolism, desensitization of the neuronal insulin receptor results in metabolic abnormalities. In the normal aging brain, glucose/energy metabolism is decreased slightly. In the majority of cases. Alzheimer's disease is sporadic and has a late onset. Therefore, age-related variations in cellular metabolism following the principle of self-organized criticality may come into focus with respect to the etiopathogenesis of this neurodegenerative disorder. As a possible primary abnormal event in late-onset sporadic DAT, a prolonged desensitization of the neuronal insulin receptor is assumed to be responsible for cascade-like abnormalities in oxidative energy metabolism and related metabolism with impacts on amyloid formation.

Dopaminergic function in the neostriatum and nucleus accumbens of young and aged Fischer 344 rats

Age-dependent alterations in behavioral and neuronal functioning were assessed in young (2-3 month), middle-aged (12 month), and aged (24 month) Fischer 344 rats treated with the indirect dopamine agonist amphetamine (2.25 or 5 mg/kg), the D1 agonist SKF 38393 (7.5, 15, 30 mg/kg), or the D2 agonist quinpirole (0.3, 1.0, 3.0 mg/kg). Drug-induced changes in activity and stereotypy were measured during a 90-min testing session, with Fos immunohistochemistry being used to assess the neuronal response to dopamine agonist treatment. As expected, aged rats given amphetamine (5 mg/kg) had fewer activity counts and higher stereotypy scores than young rats. Middle-aged rats also had fewer activity counts but were similar in stereotypy scores to young rats. Amphetamine also induced different patterns of Fos immunoreactivity in the neostriatum and nucleus accumbens of young and aged rats, as Fos expression in aged rats exhibited a distinctive dorsal to ventral pattern of decline. In general, SKF 38393 had few age-related actions, although aged rats did show a slight relative increase in stereotypy. In contrast, the D2 agonist quinpirole substantially enhanced the motor activity and Fos expression of young rats, while only modestly affecting aged rats. Hence, these results suggest that the D2 receptor is more vulnerable to the effects of aging than the D1 receptor.

Delay in manifestations of aging by grafting NGF cultured chromaffin cells in adulthood

Dopamine agonists or grafts compensate impaired motor functions in aged rats. However, there is no evidence showing whether grafting in adulthood retard aging manifestations. Motor performance of 13-month-old rats was tested on 2 meter-long wooden beams which had a 15 degree inclination and whose thickness varied from 3, 6, 12, 18, to 24 mm. Rats at 14 months were randomly assigned to 3 groups: sham graft (Group 1); intrastriatal graft of chromaffin cells cultured with NGF (Group 2); intrastriatal graft of chromaffin cells (Group 3). Motor performance was tested at monthly intervals up until rats were 26 months old. Two more groups were included: 26-month-old naive rats (Group 4); and 3- to 5-month-old naive rats (Group 5) both evaluated only once. At 26 months, the basal activity of ventral mesencephalic dopaminergic neurons was recorded. Results showed in Group 2 delay of motor detriments seen in aged rats, maintenance of basal firing rates of nigral cells compared to those of younger rats, and greater survival of substantia nigra cells. It is suggested that NGF cultured chromaffin cells produce a delay of motor detriments in aged rats, as a result of inducing survival and firing rates of nigral cells comparable to those seen in young rats.

Decreased stress responsivity of central and peripheral catecholaminergic systems in aged 344/N Fischer rats

We investigated the effects of stress on central and peripheral sympatho-adrenal and sympatho-neural functions in healthy, intact young (3-4 mo) and aged (24 mo) male Fischer 344/N rats. Extracellular fluid (ECF) levels of the catecholamines norepinephrine (NE), dihydroxyphenylglycol (DHPG), methoxyhydroxyphenylglycol (MHPG), and dihydroxyphenylacetic acid (DOPAC) were obtained by microdialysis in the paraventricular nucleus (PVN) of the hypothalamus at baseline and during immobilization (IMMO). The baseline levels of these substances were similar in both age groups, and their concentrations increased significantly in response to IMMO. The IMMO-induced increases of NE and MHPG, however, were significantly smaller in old than in young rats. Plasma levels of the catecholamines NE, DHPG, MHPG, DOPAC, dihydroxyphenylalanine (DOPA), epinephrine (EPI), dopamine (DA), and HVA were also determined in young and old rats during IMMO. Basal levels of these substances were significantly higher in old than in young rats. The magnitude of the IMMO-induced increases in the majority of these compounds however, was significantly smaller in old than in young rats. We conclude that, at the basal state, aging in the Fischer rat is associated with normal PVN ECF, but high plasma catecholamine levels; at stress state, however, old rats have substantially lesser activation of their central and peripheral catecholaminergic systems than young rats.

Comparison of brain dopamine depletion induced by low-dose 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in young and aged rats

Effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on dopamine levels in the striatum (ST), substantia nigra (SN), olfactory tubercle (OT), ventral tegmental area (VTA) and nucleus accumbens (NA) were studied in young (1 month) and old (21 months) male Long-Evans rats. Following repeated treatment with MPTP at low dose (3 mg/kg i.p. daily for 8 days) both young and old rats showed a marked depletion of dopamine (DA) concentrations in the ST compared to those of age-matched saline-treated controls. However, a significant reduction in DA levels was observed in the OT and NA of old but not young rats treated with MPTP. In contrast, no changes in 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels in these three regions were seen in either young or old MPTP-treated rats. However, the ratio of DOPAC/DA and HVA/DA in the ST and NA were significantly higher in old MPTP-treated rats than those in old controls. The concentrations of DA, DOPAC and HVA remained unchanged in the SN and VTA of both young and old rats after MPTP treatment. These results indicate that multiple treatment of low-dose MPTP selectively causes more widespread damage to dopaminergic neurons in old rats than in young ones, and further support the view that effects of MPTP on the dopaminergic system in rats may depend on age.

Comparison of the subregional distributions of the monoamine vesicular transporter and dopamine uptake complex in the rat striatum and changes during aging

We have studied the heterogeneous distribution of the vesicular monoamine transporter, labelled with 3H dihydrotetrabenazine (3H TBZOH) and the dopamine uptake complex, labelled with 3H GBR12783 in the rat striatum. The ratio TBZOH/GBR12783 was higher in the anterior part of the striatum than in the caudal part. This discrepancy could not be explained by the contribution of serotoninergic innervation to 3H TBZOH binding, since the ratio TBZOH/citalopram was also higher in the anterior striatum than in the caudal striatum. The monoamine vesicular transporter and the dopamine uptake complex were more abundant in the lateral regions than in the regions situated near the midline. In the caudal striatum, the ventral part was richer in vesicular transporter than the dorsal part. In aged rats (30 months), a significant decrease in the density of both transporters was noticed in the middle part of the striatum. In the anterior part of the striatum, the ratio TBZOH/GBR12783 was elevated in aged rats compared to adult ones. This could participate in a functional adaptation of the partially diminished population of dopaminergic neurons during aging.

Selective changes of neurotransmitter receptors in middle-aged gerbil brain

Age-related alterations in major neurotransmitter receptors and voltage dependent calcium channels were analyzed by receptor autoradiography in the gerbil brain. [3H]Quinuclidinyl benzilate (QNB), [3H]cyclohexyladenosine (CHA), [3H]muscimol, [3H]MK-801, [3H]SCH 23390, [3H]naloxone, and [3H]PN200-110 were used to label muscarinic acetylcholine receptors, adenosine A1 receptors, gamma-aminobutyric acidA (GABAA) receptors, N-methyl-D-aspartate (NMDA) receptors, dopamine D1 receptors, opioid receptors, and voltage dependent calcium channels, respectively. In middle-aged gerbils (16 months old), the hippocampus exhibited a significant elevation in [3H]QNB, [3H]MK-801, [3H]SCH 23390, [3H]naloxone, and [3H]PN200-110 binding, whereas [3H]CHA and [3H]muscimol binding showed a significant reduction in this area, compared with that of young animals (1 month). On the other hand, the cerebellum showed a significant alteration in [3H]QNB, [3H]CHA, and [3H]naloxone binding and the striatum also exhibited a significant alteration in [3H]SCH 23390 and [3H]CHA binding in middle-aged gerbils. The neocortex showed a significant elevation only in [3H]CHA binding in middle-aged animals. The nucleus accumbens and thalamus also showed a significant alteration only in [3H]muscimol binding. However, the hypothalamus and substantia nigra exhibited no significant alteration in these bindings in middle-aged gerbils. These results demonstrate the age-related alterations of various neurotransmitter receptors and voltage dependent calcium channels in most brain regions. Furthermore, they suggest that the hippocampus is most susceptible to aging processes and is altered at an early stage of senescence.

Early age-dependent changes in noradrenaline efflux in the bed nucleus of stria terminalis: voltammetric data in rat brain slices

Fast cyclic voltammetry (FCV) was applied to the detection of stimulated noradrenaline (NA) efflux in the bed nucleus of stria terminalis (BSTV) of the superfused rat brain slice from rats of different ages. Three age groups were compared: young (5 weeks; 130 +/- 8 g), adult (12 weeks; 308 +/- 12 g), and mature rats (36 weeks; 575 +/- 21 g). The effect of train duration (20-99 pulses at 100 Hz) and frequency of stimulation (50 pulses at 10-500 Hz) on NA efflux were examined. The effect of the alpha 2 antagonist yohimbine (1 microM) was also investigated. NA efflux was significantly (p < 0.05) greater in young than adult or mature rats on all train durations and stimulus frequencies tested: Maximal NA efflux on the longest train (99 pulses, 100 Hz, 10 mA, 0.2 ms) was 423 +/- 48 nM (young), 135 +/- 24 nM (adult), and 155 +/- 26 nM (mature). There were no significant differences between adult and mature rats. Yohimbine (1 microM) elevated NA efflux to a greater extent at lower (10 & 20 Hz) than higher frequencies in all age groups. Yohimbine also potentiated NA efflux more (p < 0.05) in young and adult rats than in mature animals. There were no significant differences between young and adult rats. The results indicate early maturational differences in both NA efflux and its control by alpha 2 adrenoceptors. Interestingly, the decreases in NA efflux and in yohimbine response were not temporally linked.

Age influences abnormalities in striatal dopamine metabolism during and after transient forebrain ischemia

Age has been found to be a significant risk factor for brain ischemia and its mortality. After cerebral ischemia, the nigrostriatal dopaminergic system undergoes selective vulnerability with necrosis of striatal neurons. To study the effect of age and transient forebrain ischemia on striatal dopamine metabolism, investigations were performed in 1-year-old (adult) and 2-year-old (aged) male Wistar rats. A 15 min period of bilateral transient incomplete ischemia (ICI) was induced, and the concentrations of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), 3-methoxytyramine (3-MT), and homovanillic acid (HVA) were measured in the striatum by means of HPLC and electrochemical detection at the end of ischemia without reperfusion, and after 1 h, 24 h, 72 h, 144 h, and 288 h of postischemic cerebral reperfusion. In normal conditions, no 3-MT was detectable in either age group studied, and no other age-related changes could be found in DA or its metabolites. During ICI, an age-related difference became obvious in the 3-MT concentration, which was higher in aged animals. In this group, DOPAC dropped and DA turnover increased. After 1 h of postischemic reperfusion, the concentrations of DOPAC and HVA, as well as the turnover rate, had increased in both age groups, whereas an increase in the DA concentration became apparent in the adult animals only. The enhancement of the concentration of both DOPAC and HVA was more marked in adult animals than in aged ones. At 24 h of postischemic cerebral reperfusion, DA concentration was still elevated in both age groups, and HVA in the 1-year-old animals only. At 72 h of postischemic cerebral reperfusion, no differences were obvious between adult experimental animals and controls, whereas the elevated DA concentration persisted in aged animals, being higher than in the control group and in the 1-year-old rats. DA turnover was reduced. Longer periods of postischemic cerebral reperfusion were not found to be followed by any abnormalities compared with controls except for the DA concentration at 288 h (1-year-old group); nor were there any differences between the two age groups studied. The data obtained in this investigation clearly indicate age-related differences in the striatal dopaminergic neurotransmission after transient cerebral ischemia, in that in the aged brain reactions are markedly delayed after an injurious event such as ischemia.

Aging-induced decrease in dopaminergic-stimulated phosphoinositide metabolism in rat brain

Accumulation of [3H]inositol phosphate and [3H]inositol labeling of phosphoinositides were evaluated in brain slices of 3-, 6-, 12-, and 24-month-old Fischer-344 rats. The dopamine agonist, SKF38393, stimulated significantly lower accumulations of inositol trisphosphate, inositol bisphosphate, and inositol monophosphate in the striatum, hippocampus, and frontal cortex of 24-month-old rats compared with the 6-month-old animals. No differences, however, were observed between the 3, 6, and 12-month-old groups. Furthermore there were marked decrements of 41% to 58% in the labeling of phosphoinositides in the different brain regions of the aged animals. Dose-response studies in forebrain slices of the 6-month-old and 24-month-old animals showed aging-related decrements of 53% (p less than 0.001) and 48% (p less than 0.001) in the maximal SKF38393-stimulated labeling of phosphoinositides and accumulation of inositol phosphates, respectively. These data suggest that aging of the rat brain is associated with alterations in the basal turnover of the inositol cycle and in the sensitivity of the transduction pathway to dopamine receptor stimulation.

The effects of alpha-2 adrenoceptor antagonist, atipamezole, on spatial learning in scopolamine-treated and aged rats

In order to study whether noradrenergic drugs improve age-related cognitive dysfunctions the present experiments investigated whether atipamezole, a selective and specific alpha-2 antagonist, improves spatial learning impairment due to cholinergic blockade (scopolamine 0.8 mg/kg) or aging in rats. Previously, it has been shown that atipamezole dose-dependently (0.03-3.0 mg/kg) increases the turnover of noradrenaline in rat brain. According to the present results, atipamezole (0.1, 0.3, 0.6 mg/kg) did not affect spatial learning/memory when assessed in a free swim trial of the water maze task in control rats. Furthermore, atipamezole (0.1, 0.6 mg/kg) did not improve learning deficit in scopolamine treated young rats. Higher doses (greater than or equal to 1.0 mg/kg) of atipamezole could not be tested, because they induce floating behaviour in rats. In aged rats, which were screened to be impaired in the initial acquisition of the water maze task, 0.3 mg/kg atipamezole impaired further learning of this task. Because previous studies suggest that age-related learning impairment in the water maze may be, at least partly, due to a cholinergic deficit, the present results suggest that atipamezole which increases the release of noradrenaline in brain does not alleviate this learning deficit.

Preservation of the density of the dopamine uptake complex in aging Fischer 344 rat brain

This study was performed to determine if there are selective regional or global changes in the density or pharmacology of the dopamine uptake complex (DAUC) in aged rodent. Fifteen regions of the Fischer 344 rat (aged 4, 12, and 24 months) central nervous system were analyzed for the density of the DAUC employing [3H]GBR 12935 and in vitro quantitative autoradiography. Additionally, cocaine competitions were performed in the striatum of all of the animals. A 26-fold variation in the DAUC density was found in the regions sampled. However, no significant age-related changes were identified. Intrastriatal analysis of the DAUC density revealed binding heterogeneities; decreasing lateral to medial and decreasing dorsal to ventral gradients. No significant effect of aging on striatal gradients was observed. The proportion of high and low affinity sites for cocaine was unchanged in the three age groups. Taken together, these findings suggest a stability of this dopamine presynaptic marker in aging rat central nervous system.

Neurophysiological consequences of combined cholinergic and noradrenergic lesions

The present study investigates the effects of an anti-cholinesterase, tetrahydroaminoacridine (THA), on combined nucleus basalis (NB, quisqualic acid) and dorsal noradrenergic bundle (DNB, 60HDA) lesion-induced high-voltage spindle (HVS) activity. THA at 3 mg/kg, but not at 1 mg/kg, decreased HVS activity in NB- and DNB-lesioned rats. HVS activity in NB- and DNB-lesioned rats treated with THA at 6 mg/kg was lower than in saline-treated controls. In NB-lesioned rats subjected to an additional DNB lesioning, the HVS suppressing effect of THA at 3 mg/kg was decreased. The present results suggest that NB cholinergic and DNB noradrenergic systems interact in the regulation of HVS activity and that the efficacy of an anticholinesterase drug (THA) in reversing NB cholinergic lesion-induced thalamocortical activation deficit is decreased by combined DNB noradrenergic lesion.

Regional effects of aging on dopaminergic function in the Fischer-344 rat

To examine the consequences of aging on nigrostriatal (A9) and mesolimbic (A10) dopamine (DA) function, neurochemical and behavioral measurements were performed in male Fischer-344 rats 6, 12, 18, 24, and 30 months old. Regional analyses of dopaminergic overflow and uptake processes were examined using high-speed (5 Hz) in vivo electrochemical recordings and local drug application techniques. When potassium was used to elicit the presynaptic overflow of dopamine (DA) in striatal areas predominantly innervated by the substantia nigra, the amplitude of DA overflow was significantly lower in 24- and 30-month-old rats (p less than 0.05 and p less than 0.01 vs. 6 months, respectively). Furthermore, in ventral striatum (including the nucleus accumbens) which is innervated primarily by A10 DA cell bodies, potassium-evoked DA overflow release amplitudes were significantly lower in the 18, 24, and 30 month groups (p less than 0.01 vs. 6 months). In addition, age-related differences between the dorsal and ventral striatum were found in a preliminary investigation of DA diffusion and clearance. Local application of nomifensine, a DA uptake inhibitor, significantly increased (p less than 0.05 vs. control) the amplitude of the signal recorded after local application of 25-30 pmol DA in the ventral striatum of 6 month-old but not 24-month-old rats. High-performance liquid chromatography coupled with electrochemical detection (HPLC-EC) was used to analyze whole striatal DA levels, DA metabolite levels and turnover indices. However, no age-related differences in any of these variables were observed. Finally, a rod walking test was used to measure motor coordination and balance in animals prior to in vivo electrochemical recording.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic dietary pergolide preserves nigrostriatal neuronal integrity in aged-Fischer-344 rats

Pergolide, a potent D2 presynaptic agonist with postsynaptic D2 agonist activity and some D1 agonist activity was administered in the diet (0.5 mg/kg/day) of male Fischer 344 rats from age 3 to age 26 months. We hypothesized that the potent D2 presynaptic activity would reduce the baseline release of dopamine (DA) and thereby slow the formation of toxic oxidative metabolites that lead to age-related deterioration of nigrostriatal DA neurons. Pair-fed rats served as controls. We observed age-related losses of fluorescent DA cell bodies in the substantia nigra pars compacta and of fluorescent DA terminals in the striatum; chronic pergolide administration prevented these losses. Pergolide administration also prevented the age-related diminution of DA fluorescence intensity in substantia nigra cell bodies. A large decline in 3H-DA uptake with age was partially prevented by pergolide administration. We found no age-related alteration in the concentration of DA in the striatum and pergolide did not alter this concentration. Pergolide treatment resulted in only minor alterations in striatal 3H-spiperone binding and no change in dendritic arborizations of either DA substantia nigra neurons or medium spiny striatal neurons. Pergolide administration also prevented an age-related decline in circulating FSH levels. The uptake data and quantitative morphological findings suggest that pergolide administration in the diet for 2 years exerts a protective effect on age-related deterioration of DA nigrostriatal neurons. This finding was consistent with clinical reports of a subset of patients with Parkinson's disease in whom long-term efficacy of pergolide therapy is observed.

The relationships between aging, monoamine oxidase, striatal dopamine and the effects of MPTP in C57BL/6 mice: a critical reassessment

Although the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice have been reported to increase with age, they have not been characterized in the full spectrum of ages. Thus, in spite of a considerable body of scientific literature on the subject, previous reports leave unanswered the question of whether or not the increased susceptibility of fully mature mice is part of the aging process or simply a consequence of maturation. In the present study, the age-related effects of MPTP on striatal dopamine were studied in groups of C57BL/6 mice from young maturity to old age. The major increase in the effects of MPTP occurred between 2 and 10 months of age (equivalent to adolescence and young adulthood in humans). A slight additional increase was observed between 10 and 16 months (young adulthood and middle age) and the dopamine-depleting effects of MPTP significantly declined in truly aged animals (24 months). Of note also is the fact that normal concentrations of striatal dopamine did not decline in the later ages. Additional studies indicated that while neuronal sensitivity to the effects of 1-methyl-4-phenylpyridinium (MPP+; the putative toxic metabolite of MPTP) appears to remain constant, age-related changes in the activity of striatal monoamine oxidase type B (MAO B) paralleled the dopamine-depleting effects of MPTP in the 4 age groups. Indeed, MAO B activity increased between 2 and 16 months and declined slightly, but significantly, between 16 and 24 months. This pattern of age-related changes in MAO B, striatal dopamine and the sensitivity of the nigrostriatal system to toxic insult may provide insights into factors which have been implicated in age-related neurodegeneration and idiopathic Parkinson's disease.

DSP-4, a noradrenergic neurotoxin, produces more severe biochemical and functional deficits in aged than young rats

The present study examines the effects of noradrenergic lesions (either DSP-4 i.p. or 6-hydroxydopamine (6-OHDA) into the dorsal noradrenergic bundle on biochemical (noradrenaline (NA), dopamine (DA), serotonin (5-HT) and choline acetyltransferase (ChAT) activity) and cortical EEG (quantitative EEG (qEEG) and high-voltage spindle (HVS)) activity in young and aged rats. Near complete 6-OHDA NA lesions, but not partial DSP-4 NA lesions, increased HVS activity in young rats. DSP-4 and 6-OHDA lesions produced no significant changes in the 5-HT or DA levels or in the ChAT activity in young rats. In some of the aged rats, DSP-4 produced similar biochemical and HVS effects, as it induced in young rats. In the remainder of the aged rats, NA levels were greatly and 5-HT levels slightly decreased. DA levels and ChAT activity were unaltered in either set of aged rats. HVS activity was increased only in that group of aged rats with the greatly lowered NA content. These results suggest that: (1) some of the aged rats are more sensitive to DSP-4 treatment than young adult rats; and (2) NA depletions have to be complete to produce an increase in HVS activity in young and aged rats.

The effects of noradrenergic neurotoxin, DSP-4, on the performance of young and aged rats in spatial navigation task

The present study investigated whether an overactive noradrenergic system is related to the impairment in learning/memory in aged subjects. The effects of partial noradrenaline depletion (using the noradrenergic neurotoxin DSP-4) on the acquisition of a water maze task was investigated in young and aged rats, and hippocampal noradrenaline content was correlated with spatial learning performance in similar rats. DSP-4 treatment impaired markedly the acquisition of the water maze task in aged rats, but improved it slightly in young rats. DSP-4 treatment decreased swimming speed, and this effect tended to be more marked in young rats. In the group of control rats, hippocampal noradrenaline tended to correlate positively with spatial bias in aged rats (the rats with the highest noradrenaline content in the hippocampus tended to have the best spatial learning/memory), but negatively in young rats. These results do not support the hypothesis that spatial learning/memory impairment is due to an overactive noradrenergic system in aged rats. Further studies are needed to clarify the reasons of the marked age-related difference in the effects of DSP-4 on the performance of water maze task in rats.

The effects of atipamezole, an alpha-2 antagonist, on the performance of young and aged rats in the delayed nonmatching to position task

The present experiments were undertaken to study whether pharmacological activation of the noradrenergic system would improve age-related deficits in short-term memory. Thus, we investigated the effects the single dose administration (0.1, 0.3, 0.9 and 2.7 mg/kg, subcutaneously) or atipamezole, a specific alpha-2 adrenoceptor antagonist, had on the performance of young and aged rats in a delayed nonmatching to position task. After substantial training, aged rats made more errors at longer delays (4-30 seconds) than did young rats, although the percent correct responses at short delays (0-2 seconds) did not differ between young and aged rats. Atipamezole (0.1-0.9 mg/kg) did not improve the performance of young and aged rats in this task. Moreover, the highest dose (2.7 mg/kg) used increased the number of omissions and increased the latency to collect food pellets, indicating disruption of the performance of rats in this task. According to the present results, alpha-2 antagonist (administered peripherally at a single dose), which increases the release of noradrenaline, did not improve age-related deficit in short-term memory in rats.

Combination of atipamezole and tetrahydroaminoacridine/pilocarpine treatment suppresses high voltage spindle activity in aged rats

The present study evaluated the effects of combined alpha2-antagonist (atipamezole) and anticholinesterase (tetrahydroaminoacridine, THA) or muscarinic agonist (pilocarpine) treatments on the high voltage spindle (HVS) activity in aged rats. On their own, high doses of THA (3 mg/kg), pilocarpine (3 mg/kg) and atipamezole (3 mg/kg) suppressed HVS activity. Low doses of THA (1 mg/kg), pilocarpine (1 mg/kg) and atipamezole (1 mg/kg) did not suppress HVS activity. Combinations of low doses of atipamezole and THA or pilocarpine suppressed HVS activity. Our results suggest that 1) the administration of alpha2-antagonist blocked the age-related deficit of thalamocortical activation, 2) a combination of alpha2-antagonist and a cholinergic drug may more effectively stabilize age-related HVS activity than either of the treatments alone.

Effect of aging on dopamine metabolism in the rat cerebral cortex: a regional analysis

Age-related changes in the levels of dopamine (DA) and its metabolites were measured in seven cerebral cortical areas and in the striatum of 3, 10 and 27 month-old Sprague-Dawley rats. An age-related increase in DA levels was observed in the somatomotor (SM) cortex. In contrast, a decrease was observed in the temporal (T) cortex. Decreases in homovanillic acid (HVA) levels were observed in prelimbic (PL), pyriform (PY) and T cortex of aged rats, whereas significant increases in the levels of 3-methoxytyramine (3-MT) were observed in PL, prefrontal (PF), cingulate (C) as well as in T cortex. In the striatum, DA and HVA were decreased but the level of 3-MT remained unchanged. Norepinephrine (NE) levels increased in rats from 3 to 27 months in all the cortical areas. The increase in the levels of the DA extraneuronal metabolite, 3-MT, confirms our previous results showing that the release of DA might be increased with age in some cortical areas. The present results show that there is no general age-related decrease in the level of monoamines and of their metabolites in the rat cerebral cortex and that the changes display a complex, area-specific pattern.

Spatial memory deficits in aged rats: contributions of monoaminergic systems

Age-dependent changes in monoaminergic systems and their relationship to senescent memory decline were investigated in 4- and 25-26-month-old, female, Fischer 344 rats. Spatial memory performance was tested on an 8-arm radial maze, and levels of norepinephrine (NE), dopamine (DA) and metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid, serotonin (5-HT) and metabolite 5-hydroxyindoleacetic acid were measured in brain areas which contribute to memory function--basal forebrain cholinergic nuclei, subfields of the hippocampus, frontal and entorhinal cortex--and in monoaminergic cell body areas. The performance of aged subjects was significantly impaired as compared to young subjects, and alterations of 20-60% in monoamine and metabolite levels were measured in specific brain areas of aged rats. Decreased NE levels were found in basal forebrain nuclei and cortical areas but not in hippocampal subfields of aged rats. Changes in the 5-HT system were present in hippocampal, cortical and basal forebrain sites. Changes in the DA system were the most pervasive with aged rats showing decreased DA and/or metabolites in several basal forebrain nuclei, cortical areas, and the hippocampus. Aged rats showed 50% decreases of monoamines in locus coeruleus and substantia nigra and 30% decreases in the dorsal raphe nucleus. Some but not all of the changes correlated with memory performance. The present results in rats support evidence that age-dependent changes in monoaminergic function in discrete brain sites contribute to senescent memory decline and suggest that monoaminergic-cholinergic interactions within basal forebrain nuclei may be important in this decline.

Central monoamine metabolism in the male Brown-Norway rat in relation to aging and testosterone

Concentrations of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), noradrenaline (NA), free 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were determined in brain regions of 5-, 20-, and 32-month-old male Brown-Norway rats using high pressure liquid chromatography. In view of the activating effects of sex steroids on peptide and monoamine transmitter systems and the declining plasma testosterone levels with aging, the effects of testosterone supplementation on age-related changes in central monoamine metabolism were also studied. Age-related decreases in monoamine metabolism were observed in nigrostriatal, mesocortical and coeruleohippocampal systems. Marked reductions in DOPAC (35%) and HVA (50%) occurred in the ventral tegmental area between 20 and 32 months of age. 5-HT and 5-HIAA levels showed reductions and increases depending on the brain region. Testosterone administration resulted in elevations of HVA in the substantia nigra and MHPG in the locus coeruleus and hippocampus, which were most pronounced in young animals. It is concluded that there are marked differences in age-related changes between nigrostriatal, mesocortical and coeruleohippocampal systems and that testosterone exerts a stimulatory influence on some aspects of monoamine metabolism in young but not in aged animals.

Effect of age and monosodium-L-glutamate (MSG) treatment on neurotransmitter content in brain regions from male Fischer-344 rats

Peripheral administration of monosodium-L-glutamate (MSG) has been found to be neurotoxic in neonatal rats. When administered in an acute, subconvulsive dose (500 mg/kg i.p.), MSG altered neurotransmitter content in discrete brain regions of adult (6 month old) and aged (24 month old) male Fischer-344 rats. Norepinephrine (NE) content was reduced in both the hypothalamus (16%) and cerebellum (11%) of adult rats, but was increased in both the hypothalamus (7%) and cerebellum (14%) of aged rats after MSG treatment. MSG also altered the dopamine content in adult rats in both the posterior cortex and the striatum, causing a reduction (23%) and an increase (12%), respectively. Glycine content in the midbrain of aged rats increased (21%) after MSG injection. Of particular interest is the widespread monoamine and amino acid deficits found in the aged rats in many of the brain regions examined. NE content was decreased (11%) in the cerebellum of aged rats. Dopamine content was reduced in both the posterior cortex (35%) and striatum (10%) of aged rats compared to adult animals. Cortical serotonergic deficits were present in aged rats with reductions in both the frontal (13%) and posterior cortex (21%). Aged rats also displayed deficits in amino acids, particularly the excitatory amino acids. There were glutamate deficits (9-18% reductions) in the cortical regions (posterior and frontal) as well as midbrain and brain stem. Aspartate, the other excitatory amino acid transmitter, was reduced 10% in the brainstem of aged rats. These data indicate that an acute, subconvulsive, dose of MSG may elicit neurochemical changes in both adult and aged male Fisher-344 rats, and that there are inherent age-related deficits in particular neurotransmitters in aged male Fisher-344 rats as indicated by the reductions in both monoamines and amino acids.

Vesicular and ganglionic norepinephrine in the rat: progressive increase with age

This study analyzed dopamine (DA) and norepinephrine (NE) in the synaptic vesicles and cytoplasm of brains of rats of 2 months and 14 months. The data revealed a clear NE increase in the synaptic vesicles of the 14-month-old rats, contrasting with NE in the cytoplasmic fraction of the rat brain, which remained unchanged with age. Synaptic vesicles from different regions of rat brain, including those from the striatum, consistently exhibited higher NE than DA concentrations, suggesting that they are predominantly noradrenergic. In the brain, DA concentrations in vesicular and cytoplasmic fractions did not vary with age, whereas in the superior cervical ganglia DA and NE concentrations increased in the older rats. L-3,4-Dihydroxyphenylalanine administration significantly increased DA without affecting NE in the ganglia of rats of all ages. In the brain, such a treatment significantly raised DA only in the synaptic vesicles of the older rats, suggesting an increased facilitation of DA transport into the synaptic vesicles with age, which may account for the higher vesicular NE in the older rats.

Age-related changes in uptake and release on L-[3H]noradrenaline in brain slices of senescence accelerated mouse

High K+ and N-methyl-D-aspartate (NMDA) evoked L-[3H]noradrenaline (NA) release to a similar degree in the brain slices of 1-month-old senescence-accelerated resistant mice (SAM-R/1) and senescence-accelerated prone mice (SAM-P/8). However, 30 mM KCl-induced L-[3H]NA release significantly diminished in SAM-P/8 from 3 to 12 months without changing in SAM-R/1. In addition, NMDA-induced L-[3H]NA release was also reduced at 3 months and lowered to a level of spontaneous release at 12 months in SAM-P/8, but no age-related changes in SAM-R/1 were observed. It is suggested that NA release from NA nerve terminals responsive to depolarization is reduced in SAM-P/8 at an earlier stage than in SAM-R/1. Furthermore, NMDA receptors which could be localized in the soma and/or nerve terminals, seem to be involved in NA release and to be decreased with advancing age in SAM-P/8.

Brain amines and neocortical EEG in young and aged rats

1. Frontal and parieto-occipital electroencephalography (EEG) of young (4 months-old) and aged (17 and 22 months-old) Wistar rats were analyzed, both during movement and during waking immobility. 2. The levels of monoamines, serotonin and their metabolites were measured from the frontal cortex, parieto-occipital cortex, hippocampus, brainstem and midbrain. 3. In aged rats, as compared to young rats, the most apparent changes of the quantitative EEG spectrum were the decreased amplitude of alpha (5-10 Hz) and beta (10-20 Hz) frequency bands in the frontal and parieto-occipital cortices during both movement and waking immobility behavior (p less than 0.05). 4. The levels of dopamine (DA), homovanillinic acid (HVA), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) or the ratios of 5-HT/5-HIAA and DA/HVA did not differ between young and aged rats in any brain region studied, with the exceptions of brainstem DA and parieto-occipital 5-HIAA, which were elevated in aged rats (p less than 0.05). 5. In the frontal cortex, hippocampus and midbrain, noradrenaline (NA) levels of aged rats were slightly increased as compared to young rats (p less than 0.05). 6. NA levels of the parieto-occipital cortex and brainstem did not change during aging. 7. Furthermore, there were no clear correlations between the decreased amplitude of the quantitative EEG spectrum and monoamine or serotonin concentrations, or the ratios of 5-HT/5-HIAA and DA/HVA in the cerebral cortex of aging Wistar rat.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of age on brain cortical protein kinase C and its mediation of 5-hydroxytryptamine release

The effects of age on the activity and translocation of protein kinase C (PKC) and on the facilitation of 5-hydroxytryptamine (5-HT, serotonin) release induced by PKC activation with the phorbol ester phorbol 12-myristate 13-acetate were investigated. The activities of cortical PKC and its translocation in response to K+ depolarization and phorbol ester stimulation were reduced during aging in Fischer-344 rats. Parietal cortical brain slices from 6-, 12-, and 24-month-old animals were preloaded with [3H]5-HT and release was evoked by 65 mM K+ or the calcium ionophore A23187. 5-HT release induced by either K+ or A23187 was found to be reduced in 12- and 24-month-old as compared to 6-month-old animals. This decrease was not reversed by high extracellular Ca2+. Activation of PKC resulted in a facilitated transmitter release in tissue from 6- and 12-month-old animals but reduced [3H]5-HT release in slices from 24-month-old animals. These responses were prevented by the putative PKC inhibitor 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), but not by increasing extracellular or intracellular Ca2+. The results demonstrate an age-related change (1) in brain PKC activity and translocation and (2) in a physiological response to PKC stimulation. These results may have implications for other PKC-mediated functions that are altered during senescence.

Changes in EEG associated with sleep-awake behavior in young adult versus aged adult Fischer-344 rats

Age-related spontaneous cortical and hippocampal EEG changes associated with level of arousal in rats were investigated. The EEG of rats with chronically implanted electrodes were recorded using bipolar leads and simultaneously analyzed with a computer to obtain the power spectra. In the awake stage, EEGs in the aged rats consisted predominantly of slow waves when compared with those in the young rats. The cortical EEG activity of the aged rats exhibited specific irregular burst waves which consisted of two spectral peaks at 8-9 Hz and 15-16 Hz during the drowsy period. The relative powers of the cortical and hippocampal waves in the aged rats differed from those of the young rats during the slow-wave sleep stage. These findings suggest that the slowing of the EEG in the rats during the awake stage may be related to decreased brain activity associated with aging, and that the irregular burst waves in the cortical EEG in the aged rats appear to correlate with the changes observed in age-related human sleep patterns.