Aging of the rat mesostriatal system: Differences between the nigrostriatal and the mesolimbic compartments

The neurobiological effects of senescence on dopaminergic system: A comprehensive review

Over time, the body undergoes a natural, multifactorial, and ongoing process named senescence, which induces changes at the molecular, cellular, and micro-anatomical levels in many body systems. The brain, being a highly complex organ, is particularly affected by this process, potentially impairing its numerous functions. The brain relies on chemical messengers known as neurotransmitters to function properly, with dopamine being one of the most crucial. This catecholamine is responsible for a broad range of critical roles in the central nervous system, including movement, learning, cognition, motivation, emotion, reward, hormonal release, memory consolidation, visual performance, sexual drive, modulation of circadian rhythms, and brain development. In the present review, we thoroughly examine the impact of senescence on the dopaminergic system, with a primary focus on the classic delimitations of the dopaminergic nuclei from A8 to A17. We provide in-depth information about their anatomy and function, particularly addressing how senescence affects each of these nuclei.

Effect of senescence on the tyrosine hydroxylase and S100B immunoreactivity in the nigrostriatal pathway of the rat

Senescence is a natural and progressive physiological event that leads to a series of morphophysiological alterations in the organism. The brain is the most vulnerable organ to both structural and functional changes during this process. Dopamine is a key neurotransmitter for the proper functioning of the brain, directly involved in circuitries related with emotions, learning, motivation and reward. One of the main dopamine- producing nuclei is the substantia nigra pars compacta (SNpc), which establish connections with the striatum forming the so-called nigrostriatal pathway. S100B is a calcium binding protein mainly expressed by astrocytes, involved in both intracellular and extracellular processes, and whose expression is increased following injury in the nervous tissue, being a useful marker in altered status of central nervous system. The present study aimed to analyze the impact of senescence on the cells immunoreactive for tyrosine hydroxylase (TH) and S100B along the nigrostriatal pathway of the rat. Our results show an decreased expression of S100B⁺ cells in SNpc. In addition, there was a significant decrease in TH immunoreactivity in both projection fibers and TH⁺ cell bodies. In the striatum, a decrease in TH immunoreactivity was also observed, as well as an enlargement of the white matter bundles. Our findings point out that senescence is related to the anatomical and neurochemical changes observed throughout the nigrostriatal pathway.

Age-dependent alterations in key components of the nigrostriatal dopaminergic system and distinct motor phenotypes

The nigrostriatal dopaminergic (DA) system, which includes DA neurons in the ventral and dorsal tiers of the substantia nigra pars compacta (vSNc, dSNc) and DA terminals in the dorsal striatum, is critically implicated in motor control. Accumulating studies demonstrate that both the nigrostriatal DA system and motor function are impaired in aged subjects. However, it is unknown whether dSNc and vSNc DA neurons and striatal DA terminals age in similar patterns, and whether these changes parallel motor deficits. To address this, we performed ex vivo patch-clamp recordings in dSNc and vSNc DA neurons, measured striatal dopamine release, and analyzed motor behaviors in rodents. Spontaneous firing in dSNc and vSNc DA neurons and depolarization-evoked firing in dSNc DA neurons showed inverse V-shaped changes with age. But depolarization-evoked firing in vSNc DA neurons increased with age. In the dorsal striatum, dopamine release declined with age. In locomotor tests, 12-month-old rodents showed hyperactive exploration, relative to 6- and 24-month-old rodents. Additionally, aged rodents showed significant deficits in coordination. Elevating dopamine levels with a dopamine transporter inhibitor improved both locomotion and coordination. Therefore, key components in the nigrostriatal DA system exhibit distinct aging patterns and may contribute to age-related alterations in locomotion and coordination.

Neurobiological and Pharmacological Perspectives of D3 Receptors in Parkinson’s Disease

The discovery of the D3 receptor (D3R) subtypes of dopamine (DA) has generated an understandable increase in interest in the field of neurological diseases, especially Parkinson’s disease (PD). Indeed, although DA replacement therapy with l-DOPA has provided an effective treatment for patients with PD, it is responsible for invalidating abnormal involuntary movements, known as L-DOPA-induced dyskinesia, which constitutes a serious limitation of the use of this therapy. Of particular interest is the finding that chronic l-DOPA treatment can trigger the expression of D1R–D3R heteromeric interactions in the dorsal striatum. The D3R is expressed in various tissues of the central nervous system, including the striatum. Compelling research has focused on striatal D3Rs in the context of PD and motor side effects, including dyskinesia, occurring with DA replacement therapy. Therefore, this review will briefly describe the basal ganglia (BG) and the DA transmission within these brain regions, before going into more detail with regard to the role of D3Rs in PD and their participation in the current treatments. Numerous studies have also highlighted specific interactions between D1Rs and D3Rs that could promote dyskinesia. Finally, this review will also address the possibility that D3Rs located outside of the BG may mediate some of the effects of DA replacement therapy.

The Biology and Pathobiology of Glutamatergic, Cholinergic, and Dopaminergic Signaling in the Aging Brain

The elderly population is growing worldwide, with important health and socioeconomic implications. Clinical and experimental studies on aging have uncovered numerous changes in the brain, such as decreased neurogenesis, increased synaptic defects, greater metabolic stress, and enhanced inflammation. These changes are associated with cognitive decline and neurobehavioral deficits. Although aging is not a disease, it is a significant risk factor for functional worsening, affective impairment, disease exaggeration, dementia, and general disease susceptibility. Conversely, life events related to mental stress and trauma can also lead to accelerated age-associated disorders and dementia. Here, we review human studies and studies on mice and rats, such as those modeling human neurodegenerative diseases, that have helped elucidate (1) the dynamics and mechanisms underlying the biological and pathological aging of the main projecting systems in the brain (glutamatergic, cholinergic, and dopaminergic) and (2) the effect of defective glutamatergic, cholinergic, and dopaminergic projection on disabilities associated with aging and neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. Detailed knowledge of the mechanisms of age-related diseases can be an important element in the development of effective ways of treatment. In this context, we briefly analyze which adverse changes associated with neurodegenerative diseases in the cholinergic, glutaminergic and dopaminergic systems could be targeted by therapeutic strategies developed as a result of our better understanding of these damaging mechanisms.

Loss of Sensitivity to Rewards by Dopamine Neurons May Underlie Age-Related Increased Probability Discounting

Normative aging is known to affect how decisions are made in risky situations. Although important individual variability exists, on average, aging is accompanied by greater risk aversion. Here the behavioral and neural mechanisms of greater risk aversion were examined in young and old rats trained on an instrumental probability discounting task. Consistent with the literature, old rats showed greater discounting of reward value when the probability of obtaining rewards dropped below 100%. Behaviorally, reward magnitude discrimination was the same between young and old rats, and yet these same rats exhibited reduced sensitivity to positive, but not negative, choice outcomes. The latter behavioral result was congruent with additional findings that the aged ventral tegmental neurons (including dopamine cells) were less responsive to rewards when compared to the same cell types recorded from young animals. In sum, it appears that reduced responses of dopamine neurons to rewards contribute to aging-related changes in risky decisions.

Generation of a Novel Mouse Model of Parkinson's Disease via Targeted Knockdown of Glutamate Transporter GLT-1 in the Substantia Nigra

Parkinson's disease (PD) is a common neurodegenerative disease that is characterized by pathological dopaminergic (DA) neuronal death and α-synuclein aggregation. Glutamate excitotoxicity is a well-established pathogenesis of PD that involves dysfunctional expression of glutamate transporters. Glutamate transporter-1 (GLT-1) is mainly responsible for clearance of glutamate at synapses, including DA synapses. However, the role of GLT-1 in the aberrant synaptic transmission in PD remains elusive. In the present study, we generated small-interfering RNAs (siRNAs) to knockdown GLT-1 expression in primary astrocytes, and we report that siRNA knockdown of astrocytic GLT-1 decreased postsynaptic density-95 (PSD-95) expression in neuron-astrocyte cocultures in vitro. Using adeno-associated viruses (AAVs) targeting GLT-1 short-hairpin RNA (shRNA) sequences with a glial fibrillary acidic protein (GFAP) promoter, we abolished astrocytic GLT-1 expression in the substantia nigra pars compacta (SNpc) of mice. We found that GLT-1 deficiency in the SNpc induced parkinsonian phenotypes in terms of progressive motor deficits and nigral DA neuronal death in mice. We also found that there were reactive astrocytes and microglia in the SNpc upon GLT-1 knockdown. Furthermore, we used RNA sequencing to determine altered gene expression patterns upon GLT-1 knockdown in the SNpc, which revealed that disrupted calcium signaling pathways may be responsible for GLT-1 deficiency-mediated DA neuronal death in the SNpc. Taken together, our findings provide evidence for a novel role of GLT-1 in parkinsonian phenotypes in mice, which may contribute to further elucidation of the mechanisms of PD pathogenesis.

Age- and sex-related changes in the severity of physical and psychological dependence in morphine-dependent rats

Gender- and age-dependent effects on the severity of morphine dependence are still controversial. The aim of this study was to investigate the effects of age and sex on the severity of physical and psychological dependence in morphine-dependent rats. The adult/aged male and female Wistar rats were chronically treated with bi-daily doses (10 mg/kg, at 12 h intervals) of morphine for 14 days. Then, rats were tested for the severity of physical dependence on morphine (spontaneous withdrawal signs), anxiety-like (the elevated plus maze), depressive-like (sucrose preference test) and grooming behaviors after spontaneous morphine withdrawal. We found that the morphine withdrawal signs decreased after 3 and 7 days of withdrawal in female and male rats respectively, while there was no significant difference in overall dependence severity between the two sexes or ages. Also, we found that the withdrawal of morphine led to increased anxiety, depression and obsessive-compulsive behavior in the D (dependent)/Adult male and female rats. Also, the D/aged female and male rats exhibited a reduction in depressive-like behavior than the D/Adult rats. Moreover, the D/female rats exhibited a decreased obsessive-compulsive behavior in both age groups than male rats. We conclude that age has no effect on the duration of withdrawal from morphine and overall dependence severity. While, the duration of withdrawal from morphine was lower in female than male rats. Our results showed a sex difference on the duration of morphine withdrawal and an age difference in the expression of psychological dependence on morphine. Thus, therapeutic strategies may be different for opiate-dependent individuals in physical and psychological dimensions.

Tyrosine Hydroxylase Inhibition in Substantia Nigra Decreases Movement Frequency

Reduced movement frequency or physical activity (bradykinesia) occurs with high prevalence in the elderly. However, loss of striatal tyrosine hydroxylase (TH) in aging humans, non-human primates, or rodents does not reach the ~ 80% loss threshold associated with bradykinesia onset in Parkinson's disease. Moderate striatal dopamine (DA) loss, either following TH inhibition or decreased TH expression, may not affect movement frequency. In contrast, moderate DA or TH loss in the substantia nigra (SN), as occurs in aging, is of similar magnitude (~ 40%) to nigral TH loss at bradykinesia onset in Parkinson's disease. In aged rats, increased TH expression and DA in SN alone increases movement frequency, suggesting aging-related TH and DA loss in the SN contributes to aging-related bradykinesia or decreased physical activity. To test this hypothesis, the SN was targeted with bilateral guide cannula in young (6 months old) rats, in a within-subjects design, to evaluate the impact of nigral TH inhibition on movement frequency and speed. The TH inhibitor, α-methyl-p-tyrosine (AMPT) reduced nigral DA (~ 40%) 45-150 min following infusion, without affecting DA in striatum, nucleus accumbens, or adjacent ventral tegmental area. Locomotor activity in the open-field was recorded up to 3 h following nigral saline or AMPT infusion in each test subject. During the period of nigra-specific DA reduction, movement frequency, but not movement speed, was significantly decreased. These results indicate that DA or TH loss in the SN, as observed in aging, contributes as a central mechanism of reduced movement frequency.

Prolonged increase in ser31 tyrosine hydroxylase phosphorylation in substantia nigra following cessation of chronic methamphetamine

Methamphetamine (MA) exposure may increase the risk of motor or cognitive impairments similar to Parkinson's disease (PD) by middle age. Although damage to nigrostriatal or mesoaccumbens dopamine (DA) neurons may occur during or early after MA exposure, overt PD-like symptoms at a younger age may not manifest due to compensatory mechanisms to maintain DA neurotransmission. One possible compensatory mechanism is increased tyrosine hydroxylase (TH) phosphorylation. In the rodent PD 6-OHDA model, nigrostriatal lesion decreases TH protein in both striatum and substantia nigra (SN). However, DA loss in the SN is significantly less than that in the striatum. An increase in ser31 TH phosphorylation in the SN may increase TH activity in response to TH loss. To determine if similar compensatory mechanisms may be engaged in young mice after MA exposure, TH expression, phosphorylation, and DA tissue content were evaluated, along with dopamine transporter expression, 21 days after cessation of MA (24 mg/kg, daily, 14 days). DA tissue content was unaffected by the MA regimen in striatum, nucleus accumbens, SN, or ventral tegmental area (VTA), despite decreased TH protein in SN and VTA. In the SN, but not striatum, ser31 phosphorylation increased over 2-fold. This suggests that increased ser31 TH phosphorylation may be an inherent compensatory mechanism to attenuate DA loss against TH loss, similar to that in an established PD model. These results also indicate the somatodendritic compartments of DA neurons are more vulnerable to TH protein loss than terminal fields following MA exposure.

Dissociation of Striatal Dopamine and Tyrosine Hydroxylase Expression from Aging-Related Motor Decline: Evidence from Calorie Restriction Intervention

The escalating increase in retirees living beyond their eighth decade brings increased prevalence of aging-related impairments, including locomotor impairment (Parkinsonism) that may affect ~50% of those reaching age 80, but has no confirmed neurobiological mechanism. Lifestyle strategies that attenuate motor decline, and its allied mechanisms, must be identified. Aging studies report little to moderate loss of striatal dopamine (DA) or tyrosine hydroxylase (TH) in nigrostriatal terminals, in contrast to ~70%-80% loss associated with bradykinesia onset in Parkinson's disease. These studies evaluated the effect of ~6 months 30% calorie restriction (CR) on nigrostriatal DA regulation and aging-related locomotor decline initiated at 12 months of age in Brown-Norway Fischer F1 hybrid rats. The aging-related decline in locomotor activity was prevented by CR. However, striatal DA or TH expression was decreased in the CR group, but increased in substantia nigra versus the ad libitum group or 12-month-old cohort. In a 4- to 6-month-old cohort, pharmacological TH inhibition reduced striatal DA ~30%, comparable with decreases reported in aged rats and the CR group, without affecting locomotor activity. The dissociation of moderate striatal DA reduction from locomotor activity seen in both studies suggests that aging-related decreases in striatal DA are dissociated from locomotor decline.

Dopamine D2 gene expression interacts with environmental enrichment to impact lifespan and behavior

Aging produces cellular, molecular, and behavioral changes affecting many areas of the brain. The dopamine (DA) system is known to be vulnerable to the effects of aging, which regulate behavioral functions such as locomotor activity, body weight, and reward and cognition. In particular, age-related DA D₂ receptor (D2R) changes have been of particular interest given its relationship with addiction and other rewarding behavioral properties. Male and female wild-type (Drd₂ +/+), heterozygous (Drd₂ +/−) and knockout (Drd₂ −/−) mice were reared post-weaning in either an enriched environment (EE) or a deprived environment (DE). Over the course of their lifespan, body weight and locomotor activity was assessed. While an EE was generally found to be correlated with longer lifespan, these increases were only found in mice with normal or decreased expression of the D₂ gene. Drd₂ +/+ EE mice lived nearly 16% longer than their DE counterparts. Drd₂ +/+ and Drd₂ +/− EE mice lived 22% and 21% longer than Drd₂ −/− EE mice, respectively. Moreover, both body weight and locomotor activity were moderated by environmental factors. In addition, EE mice show greater behavioral variability between genotypes compared to DE mice with respect to body weight and locomotor activity.

Resveratrol and pinostilbene confer neuroprotection against aging-related deficits through an ERK1/2-dependent mechanism

Age-related declines in motor function may be due, in part, to an increase in oxidative stress in the aging brain leading to dopamine (DA) neuronal cell death. In this study, we examined the neuroprotective effects of natural antioxidants resveratrol and pinostilbene against age-related DAergic cell death and motor dysfunction using SH-SY5Y neuroblastoma cells and young, middle-aged, and old male C57BL/6 mice. Resveratrol and pinostilbene protected SH-SY5Y cells from a DA-induced decrease in cell viability. Dietary supplementation with resveratrol and pinostilbene inhibited the decline of motor function observed with age. While DA and its metabolites (DOPAC and HVA), dopamine transporter, and tyrosine hydroxylase levels remain unchanged during aging or treatment, resveratrol and pinostilbene increased ERK1/2 activation in vitro and in vivo in an age-dependent manner. Inhibition of ERK1/2 in SH-SY5Y cells decreased the protective effects of both compounds. These data suggest that resveratrol and pinostilbene alleviate age-related motor decline via the promotion of DA neuronal survival and activation of the ERK1/2 pathways.

Menopause and Parkinson's disease. Interaction between estrogens and brain renin-angiotensin system in dopaminergic degeneration

The neuroprotective effects of menopausal hormonal therapy in Parkinson's disease (PD) have not yet been clarified, and it is controversial whether there is a critical period for neuroprotection. Studies in animal models and clinical and epidemiological studies indicate that estrogens induce dopaminergic neuroprotection. Recent studies suggest that inhibition of the brain renin-angiotensin system (RAS) mediates the effects of estrogens in PD models. In the substantia nigra, ovariectomy induces a decrease in levels of estrogen receptor-α (ER-α) and increases angiotensin activity, NADPH-oxidase activity and expression of neuroinflammatory markers, which are regulated by estrogen replacement therapy. There is a critical period for the neuroprotective effect of estrogen replacement therapy, and local ER-α and RAS play a major role. Astrocytes play a major role in ER-α-induced regulation of local RAS, but neurons and microglia are also involved. Interestingly, treatment with angiotensin receptor antagonists after the critical period induced neuroprotection.

Exercise-Mediated Increase in Nigral Tyrosine Hydroxylase Is Accompanied by Increased Nigral GFR-α1 and EAAC1 Expression in Aging Rats

Exercise may alleviate locomotor impairment in Parkinson's disease (PD) or aging. Identifying molecular responses immediately engaged by exercise in the nigrostriatal pathway and allied tissue may reveal critical targets associated with its long-term benefits. In aging, there is loss of tyrosine hydroxylase (TH) and the glial cell line-derived neurotrophic factor (GDNF) receptor, GFR-α1, in the substantia nigra (SN). Exercise can increase GDNF expression, but its effect on GFR-α1 expression is unknown. Infusion of GDNF into striatum or GFR-α1 in SN, respectively, can increase locomotor activity and TH function in SN but not striatum in aged rats. GDNF may also increase glutamate transporter expression, which attenuates TH loss in PD models. We utilized a footshock-free treadmill exercise regimen to determine the immediate impact of short-term exercise on GFR-α1 expression, dopamine regulation, glutamate transporter expression, and glutamate uptake in 18 month old male Brown-Norway/Fischer 344 F1 hybrid rats. GFR-α1 and TH expression significantly increased in SN but not striatum. This exercise regimen did not affect glutamate uptake or glutamate transporter expression in striatum. However, EAAC1 expression increased in SN. These results indicate that nigral GFR-α1 and EAAC1 expression increased in conjunction with increased nigral TH expression following short-term exercise.

Initiation of calorie restriction in middle-aged male rats attenuates aging-related motoric decline and bradykinesia without increased striatal dopamine

Aging-related bradykinesia affects ∼ 15% of those reaching age 65 and 50% of those reaching their 80s. Given this high risk and lack of pharmacologic therapeutics, noninvasive lifestyle strategies should be identified to diminish its risk and identify the neurobiological targets to reduce aging-related bradykinesia. Early-life, long-term calorie restriction (CR) attenuates aging-related bradykinesia in rodents. Here, we addressed whether CR initiation at middle age could attenuate aging-related bradykinesia and motoric decline measured as rotarod performance. A 30% CR regimen was implemented for 6 months duration in 12-month-old male Brown-Norway Fischer 344 F1 hybrid rats after establishing individual baseline locomotor activities. Locomotor capacity was assessed every 6 weeks thereafter. The ad libitum group exhibited predictably decreased locomotor activity, except movement speed, out to 18 months of age. In contrast, in the CR group, movement number and horizontal activity did not decrease during the 6-month trial, and aging-related decline in rotarod performance was attenuated. The response to CR was influenced by baseline locomotor activity. The lower the locomotor activity level at baseline, the greater the response to CR. Rats in the lower 50th percentile surpassed their baseline level of activity, whereas rats in the top 50th percentile decreased at 6 weeks and then returned to baseline by 12 weeks of CR. We hypothesized that nigrostriatal dopamine tissue content would be greater in the CR group and observed a modest increase only in substantia nigra with no group differences in striatum, nucleus accumbens, or ventral tegmental area. These results indicate that initiation of CR at middle age may reduce aging-related bradykinesia, and, furthermore, subjects with below average locomotor activity may increase baseline activity. Sustaining nigral dopamine neurotransmission may be one component of preserving locomotor capabilities during aging.

α-Synuclein aggregation in the olfactory bulb of middle-aged common marmoset

The synaptic protein α-synuclein has been identified as a major component of Lewy bodies, a pathological hallmark of Parkinson's disease (PD). Prior to the formation of Lewy bodies, mislocalization and aggregation of the α-synuclein in brain tissue is frequently observed in various neurodegenerative diseases. Aberrant accumulation and localization of α-synuclein are also observed in the aging human brain, for which reason aging is regarded as a risk factor for neurodegenerative disease. To investigate changes in α-synuclein properties in the aging brain, we compared α-synuclein immunoreactivity in brain tissue of young (2-years-old) and middle-aged (6-years-old) common marmoset (Callithrix jacchus). Our analyses revealed marked changes in α-synuclein immunoreactivity in the olfactory bulb of common marmosets of these age cohorts. Perikaryal α-synuclein aggregations were formed in the olfactory bulb in middle-aged animals. We also observed signals of α-synuclein accumulation in hippocampus in this cohort; however, unlike in the olfactory bulb, hippocampal α-synuclein signals were localized in the synaptic terminals. We did not observe either of these features in younger marmosets, which suggest that aging may play a role in these phenomena. Our results using common marmoset brain corresponded with the observation that the α-synuclein aggregations were first occurred from olfactory bulb in human normal aged and PD brain. Therefore, common marmoset is expected as useful model for α-synuclein pathology.

Parkinson's disease as a result of aging

It is generally considered that Parkinson's disease is induced by specific agents that degenerate a clearly defined population of dopaminergic neurons. Data commented in this review suggest that this assumption is not as clear as is often thought and that aging may be critical for Parkinson's disease. Neurons degenerating in Parkinson's disease also degenerate in normal aging, and the different agents involved in the etiology of this illness are also involved in aging. Senescence is a wider phenomenon affecting cells all over the body, whereas Parkinson's disease seems to be restricted to certain brain centers and cell populations. However, reviewed data suggest that Parkinson's disease may be a local expression of aging on cell populations which, by their characteristics (high number of synaptic terminals and mitochondria, unmyelinated axons, etc.), are highly vulnerable to the agents promoting aging. The development of new knowledge about Parkinson's disease could be accelerated if the research on aging and Parkinson's disease were planned together, and the perspective provided by gerontology gains relevance in this field.

Enhanced performance of aged rats in contingency degradation and instrumental extinction tasks

Normal aging in rats affects behavioral performance on a variety of associative learning tasks under Pavlovian conditions. There is little information, however, on whether aging also impacts performance of instrumental tasks. Young (9-12 months) and aged (24-27 months) Fisher 344 rats were trained to press distinct levers associated with either maltodextrin or sucrose. The rats in both age groups increased their lever press frequency at a similar rate, suggesting that the initial acquisition of this instrumental task is not affected by aging. Using a contingency degradation procedure, we then addressed whether aged rats could adapt their behavior to changes in action-outcome contingencies. We found that young and aged rats do adapt, but that a different schedule of reinforcement is necessary to optimize performance in each age group. Finally, we also addressed whether aged rats can extinguish a lever press action as well as young rats, using 2 40-min extinction sessions on consecutive days. While extinction profiles were similar in young and aged rats on the first day of training, aged rats were faster to extinguish their lever presses on the second day, in spite of their performance levels being similar at the beginning of the session. Together these data support the finding that acquisition of instrumental lever press behaviors is preserved in aged rats and suggest that they have a different threshold for switching strategies in response to changes in action-outcome associations. This pattern of result implies that age-related changes in the brain are heterogeneous and widespread across structures.

Aging decreases L-type calcium channel currents and pacemaker firing fidelity in substantia nigra dopamine neurons

Substantia nigra dopamine neurons are involved in behavioral processes that include cognition, reward learning, and voluntary movement. Selective deterioration of these neurons is responsible for the motor deficits associated with Parkinson's disease (PD). Aging is the leading risk factor for PD, suggesting that adaptations occurring in dopamine neurons during normal aging may predispose individuals to the development of PD. Previous studies suggest that the unique set of ion conductances that drive spontaneous, rhythmic firing of action potentials could predispose substantia nigra dopamine neurons to selective neurodegeneration. Here we show, using patch-clamp electrophysiological recordings in brain slices, that substantia nigra dopamine neurons from mice 25-30 months of age (old) have comparable membrane capacitance and input resistance to neurons from mice 2-7 months of age (young). However, neurons from old mice exhibit slower firing rates, narrower spike widths, and more variable interspike intervals compared with neurons from young mice. Dopamine neurons from old mice also exhibit smaller L-type calcium channel currents, providing a plausible mechanism that likely contributes to the changes in impulse activity. Age-related decrements in the physiological function of dopamine neurons could contribute to the decrease in voluntary movement and other dopamine-mediated behaviors observed in aging populations. Furthermore, as pharmacological antagonism of L-type calcium channels has been proposed as a potential treatment for the early stages of PD, our results could point to a limited temporal window of opportunity for this therapeutic intervention.

Inhibition of cytoplasmic p53 differentially modulates Ca(2+) signaling and cellular viability in young and aged striata

The p53 protein, a transcription factor with many gene targets, can also trigger apoptosis in the cytoplasm. The disruption of cell homeostasis, such as Ca(2+) signaling and mitochondrial respiration, contributes to the loss of viability and ultimately leads to cell death. However, the link between Ca(2+) signaling and p53 signaling remains unclear. During aging, there are alterations in cell physiology that are commonly associated with a reduced adaptive stress response, thus increasing cell vulnerability. In this work, we examined the effects of a cytoplasmic p53 inhibitor (pifithrin μ) in the striatum of young and aged rats by evaluating Ca(2+) signaling, mitochondrial respiration, apoptotic protein expression, and tissue viability. Our results showed that pifithrin μ differentially modulated cytoplasmic and mitochondrial Ca(2+) in young and aged rats. Cytoplasmic p53 inhibition appeared to reduce the mitochondrial respiration rate in both groups. In addition, p53 phosphorylation and Bax protein levels were elevated upon cytoplasmic p53 inhibition and could contribute to the reduction of tissue viability. Following glutamate challenge, pifithrin μ improved cell viability in aged tissue, reduced reactive oxygen species (ROS) generation, and reduced mitochondrial membrane potential (ΔΨm). Taken together, these results indicate that cytoplasmic p53 may have a special role in cell viability by influencing cellular Ca(2+) homeostasis and respiration and may produce differential effects in the striatum of young and aged rats.

Brain renin-angiotensin system and dopaminergic cell vulnerability

Although the renin-angiotensin system (RAS) was classically considered as a circulating system that regulates blood pressure, many tissues are now known to have a local RAS. Angiotensin, via type 1 receptors, is a major activator of the NADPH-oxidase complex, which mediates several key events in oxidative stress (OS) and inflammatory processes involved in the pathogenesis of major aging-related diseases. Several studies have demonstrated the presence of RAS components in the basal ganglia, and particularly in the nigrostriatal system. In the nigrostriatal system, RAS hyperactivation, via NADPH-oxidase complex activation, exacerbates OS and the microglial inflammatory response and contributes to progression of dopaminergic degeneration, which is inhibited by angiotensin receptor blockers and angiotensin converting enzyme (ACE) inhibitors. Several factors may induce an increase in RAS activity in the dopaminergic system. A decrease in dopaminergic activity induces compensatory upregulation of local RAS function in both dopaminergic neurons and glia. In addition to its role as an essential neurotransmitter, dopamine may also modulate microglial inflammatory responses and neuronal OS via RAS. Important counterregulatory interactions between angiotensin and dopamine have also been observed in several peripheral tissues. Neurotoxins and proinflammatory factors may also act on astrocytes to induce an increase in RAS activity, either independently of or before the loss of dopamine. Consistent with a major role of RAS in dopaminergic vulnerability, increased RAS activity has been observed in the nigra of animal models of aging, menopause and chronic cerebral hypoperfusion, which also showed higher dopaminergic vulnerability. Manipulation of the brain RAS may constitute an effective neuroprotective strategy against dopaminergic vulnerability and progression of Parkinson's disease.

ser31 Tyrosine hydroxylase phosphorylation parallels differences in dopamine recovery in nigrostriatal pathway following 6-OHDA lesion

Compensatory mechanisms in dopamine (DA) signaling have long been proposed to delay onset of locomotor symptoms during Parkinson's disease progression until ~ 80% loss of striatal DA occurs. Increased striatal dopamine turnover has been proposed to be a part of this compensatory response, but may occur after locomotor symptoms. Increased tyrosine hydroxylase (TH) activity has also been proposed as a mechanism, but the impact of TH protein loss upon site-specific TH phosphorylation in conjunction with the impact on DA tissue content is not known. The tissue content of DA was determined against TH protein loss in the striatum and substantia nigra (SN) following 6-hydroxydopamine lesion in the medial forebrain bundle in young Sprague-Dawley male rats. Although DA predictably decreased in both regions following 6-hydroxydopamine, there was a significant difference in DA loss between the striatum (75%) and SN (40%), despite similar TH protein loss. Paradoxically, there was a significant decrease in DA against remaining TH protein in striatum, but a significant increase in DA against remaining TH in SN. In the SN, increased DA per remaining TH protein was matched by increased ser31, but not ser40, TH phosphorylation. In striatum, both ser31 and ser40 phosphorylation decreased, reflecting decreased DA per TH. However, in control nigral and striatal tissue, only ser31 phosphorylation correlated with DA per TH protein. Combined, these results suggest that the phosphorylation of ser31 in the SN may be a mechanism to increase DA biosynthesis against TH protein loss in an in vivo model of Parkinson's disease. Properties of dopamine biosynthesis were evaluated in the 6-OHDA model of Parkinson's disease by studying the impact of tyrosine hydroxylase (TH) protein loss on its own phosphorylation and dopamine (DA) tissue content in rat nigrostriatal pathway. A dichotomous response was observed between striatum and substantia nigra in that dopamine per remaining TH decreased in striatum, but increased in substantia nigra. Phosphorylation at ser31 reflected these differences, indicating that ser31 phosphorylation may be critical to maintain dopamine with progressive TH protein loss. Drawings are from slides purchased from Motifolio (http://motifolio.com/).

Memory deficits in aging and neurological diseases

Memory is central to our ability to perform daily life activities and correctly function in society. Improvements in public health and medical treatment for a variety of diseases have resulted in longer life spans; however, age-related memory impairments have been significant sources of morbidity. Loss in memory function is not only associated with aging population but is also a feature of neurodegenerative diseases such as Alzheimer's disease and other psychiatric and neurological disorders. Here, we focus on current understanding of the impact of normal aging on memory and what is known about its mechanisms, and further review pathological mechanisms behind the cause of dementia in Alzheimer's disease. Finally, we discuss schizophrenia and look into abnormalities in circuit function and neurotransmitter systems that contribute to memory impairment in this illness.

Molecular Pathogenesis and Pathophysiology of Parkinson’s Disease: New Targets for New Therapies

Parkinson’s disease (PD) is a complex chronic neurodegenerative disease of unknown etiology. A conceptual framework for all chronic diseases involves a series of channels or pathways (aging, genetic, environment, oxidative stress, mitochondrial damage, protein aggregation, etc.) and their interactions. Those channels with specificities may explain the ‘developmental’ program that through transcriptional reprogramming results in stressed dopamine neurons that eventually become dysfunctional or die, giving rise to the clinical manifestations of PD. In Chapter 2 we review the molecular mechanisms of those channels that may be implicated in the pathogenesis of PD and the pathophysiology of the disease based on the anatomo‐physiological complexity of the basal ganglia. This illustrates that understanding the molecular mechanisms of a disease may not be enough, or we have to reach an adequate system level to understand the disease process. Finally, we suggest that common therapies used for the treatment of other chronic diseases may be useful for the treatment (or help to advance the understanding) of PD, as well as new targets for new therapies that may be useful in the prevention of, or to stop the progression of, PD and other synucleinopathies.

The role of free radicals in the aging brain and Parkinson's Disease: convergence and parallelism

Free radical production and their targeted action on biomolecules have roles in aging and age-related disorders such as Parkinson's disease (PD). There is an age-associated increase in oxidative damage to the brain, and aging is considered a risk factor for PD. Dopaminergic neurons show linear fallout of 5-10% per decade with aging; however, the rate and intensity of neuronal loss in patients with PD is more marked than that of aging. Here, we enumerate the common link between aging and PD at the cellular level with special reference to oxidative damage caused by free radicals. Oxidative damage includes mitochondrial dysfunction, dopamine auto-oxidation, α-synuclein aggregation, glial cell activation, alterations in calcium signaling, and excess free iron. Moreover, neurons encounter more oxidative stress as a counteracting mechanism with advancing age does not function properly. Alterations in transcriptional activity of various pathways, including nuclear factor erythroid 2-related factor 2, glycogen synthase kinase 3β, mitogen activated protein kinase, nuclear factor kappa B, and reduced activity of superoxide dismutase, catalase and glutathione with aging might be correlated with the increased incidence of PD.

Cardiac workload and inhibitory control in younger and older adults

Our goal was to investigate age differences in the role played by cardiovascular regulation in response control. We questioned whether pre-test respiratory sinus arrhythmia (RSA; an index of phasic vagal cardiac control) and/or rate pressure product (RPP; a measure of cardiac workload) were associated with error rate and/or error-related electrocortical responses (ERPs) during a Go/NoGo inhibitory control task across three levels of working memory load. ERPs, RSA and RPP were indirectly associated with performance in young adults. Within the older group, higher resting RPP was directly associated with NoGo errors at all levels of load, an association not seen in the younger group. Thus, for older adults, excessive hemodynamic demands at rest were more relevant than on-task electrocortical responses in the prediction of inhibitory control errors. These data support the relevance of autonomic regulation in understanding age-related change in higher-order neurocognitive function.

Traumatic brain injury in adult rats causes progressive nigrostriatal dopaminergic cell loss and enhanced vulnerability to the pesticide paraquat

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of nigrostriatal dopaminergic neurons and the accumulation of alpha-synuclein. Both traumatic brain injury (TBI) and pesticides are risk factors for PD, but whether TBI causes nigrostriatal dopaminergic cell loss in experimental models and whether it acts synergistically with pesticides is unknown. We have examined the acute and long-term effects of TBI and exposure to low doses of the pesticide paraquat, separately and in combination, on nigrostriatal dopaminergic neurons in adult male rats. In an acute study, rats received moderate TBI by lateral fluid percussion (LFP) injury, were injected with saline or paraquat (10 mg/kg IP) 3 and 6 days after LFP, were sacrificed 5 days later, and their brains processed for immunohistochemistry. TBI alone increased microglial activation in the substantia nigra, and caused a 15% loss of dopaminergic neurons ipsilaterally. Paraquat increased the TBI effect, causing a 30% bilateral loss of dopaminergic neurons, reduced striatal tyrosine hydroxylase (TH) immunoreactivity more than TBI alone, and induced alpha-synuclein accumulation in the substantia nigra pars compacta. In a long-term study, rats received moderate LFP, were injected with saline or paraquat at 21 and 22 weeks post-injury, and were sacrificed 4 weeks later. At 26 weeks post injury, TBI alone induced a 30% bilateral loss of dopaminergic neurons that was not exacerbated by paraquat. These data suggest that TBI is sufficient to induce a progressive degeneration of nigrostriatal dopaminergic neurons. Furthermore, TBI and pesticide exposure, when occurring within a defined time frame, could combine to increase the PD risk.

The aging striatal dopamine function

Movement disorders are prevalent in the elderly and may have both central and peripheral origins. Age-related parkinsonism often results in movement disorders identical to some of the cardinal symptoms of typical Parkinson's disease (TPD). Nevertheless, there may be limited similarity in the underlying dysfunction of the sensory-motor circuitry since these two conditions exhibit different changes in the nigro-striatal pathway. In this short review, we highlight some of the key distinctions between aging and TPD regarding striatal dopaminergic activity and discuss them in the context of therapeutic strategies to alleviate motor decline in the elderly.

Age-related changes in dopamine signaling in Nurr1 deficient mice as a model of Parkinson's disease

The nuclear receptor related 1 (Nurr1) transcription factor contributes to the development and maintenance of dopamine (DA) neurons in the brain. We found that heterozygous Nurr1 knockout (Nurr1 +/-) influenced the age-dependent decline in the number of DA neurons and influenced DA signaling. We examined the DA marker, tyrosine hydroxylase, using immunohistochemistry, and we measured DA signaling using fast-scan cyclic voltammetry in 3 age groups of wild-type (Nurr1 +/+) and mutant (Nurr1 +/-) mice: 3-6, 9-12, and 15-23 mo old. Prior to significant loss of DA neurons and to the onset of parkinsonian symptoms, young Nurr1 +/- mice (3-6 mo) exhibited a decrease in peak evoked DA release that was partially countered by a decrease in the rate of DA reuptake. As peak evoked DA release declined with age for both the wild-type and Nurr1 +/- mice, both genotypes manifested decreased DA reuptake. As the DA release fell further with age, decreased DA reuptake eventually could not adequately compensate the Nurr1 +/- mice. The results indicated that Nurr1 deficiency led to impaired DA release even before significant DA neuron loss.

Age-related loss of orexin/hypocretin neurons

Aging is associated with many physiological alterations-such as changes in sleep patterns, metabolism and food intake-suggestive of hypothalamic dysfunction, but the effects of senescence on specific hypothalamic nuclei and neuronal groups that mediate these alterations is unclear. The lateral hypothalamus and contiguous perifornical area (LH/PFA) contains several populations of neurons, including those that express the neuropeptides orexin (hypocretin) or melanin-concentrating hormone (MCH). Collectively, orexin and MCH neurons influence many integrative homeostatic processes related to wakefulness and energy balance. Here, we determined the effect of aging on numbers of orexin and MCH neurons in young adult (3-4 months) and old (26-28 months) Fisher 344/Brown Norway F1 hybrid rats. Aged rats exhibited a loss of greater than 40% of orexin-immunoreactive neurons in both the medial and lateral (relative to the fornix) sectors of the LH/PFA. MCH-immunoreactive neurons were also lost in aged rats, primarily in the medial LH/PFA. Neuronal loss in this area was not global as no change in cells immunoreactive for the pan-neuronal marker, NeuN, was observed in aged rats. Combined with other reports of altered receptor expression or behavioral responses to exogenously-administered neuropeptide, these data suggest that compromised orexin (and, perhaps, MCH) function is an important mediator of age-related homeostatic disturbances of hypothalamic origin. The orexin system may represent a crucial substrate linking homeostatic and cognitive dysfunction in aging, as well as a novel therapeutic target for pharmacological or genetic restoration approaches to preventing or ameliorating these disturbances.

L-DOPA reverses motor deficits associated with normal aging in mice

We wished to determine whether L-DOPA, a common treatment for the motor deficits in Parkinson's disease, could also reverse the motor deficits that occur during aging. We assessed motor performance in young (2-3 months) and old (20-21 months) male C57BL/6 mice using the challenge beam and cylinder tests. Prior to testing, mice were treated with L-DOPA or vehicle. Following testing, striatal tissue was analyzed for phenotypic markers of dopamine neurons: dopamine, dopamine transporter, and tyrosine hydroxylase. Although the dopaminergic markers were unchanged with age or L-DOPA treatment, L-DOPA reversed the motor deficits in the old animals such that their motor coordination was that of a young mice. These findings suggest that some of the locomotor deficits that accompany normal aging are responsive to L-DOPA treatment and may be due to subtle alterations in dopaminergic signaling.

Vulnerability of mesostriatal dopaminergic neurons in Parkinson's disease

The term vulnerability was first associated with the midbrain dopaminergic neurons 85 years ago, before they were identified as monoaminergic neurons, when Foix and Nicolesco (1925) reported the loss of neuromelanin containing neurons in the midbrain of patients with post-encephalitic Parkinson's disease (PD). A few years later, Hassler (1938) showed that degeneration is more intense in the ventral tier of the substantia nigra compacta than in its dorsal tier and the ventral tegmental area (VTA), outlining the concept of differential vulnerability of midbrain dopaminergic (DA-) neurons. Nowadays, we know that other neuronal groups degenerate in PD, but the massive loss of nigral DA-cells is its pathological hallmark, having a pivotal position in the pathophysiology of the disease as it is responsible for the motor symptoms. Data from humans as well as cellular and animal models indicate that DA-cell degeneration is a complex process, probably precipitated by the convergence of different risk factors, mediated by oxidative stress, and involving pathogenic factors arising within the DA-neuron (intrinsic factors), and from its environment and distant interconnected brain regions (extrinsic factors). In light of current data, intrinsic factors seem to be preferentially involved in the first steps of the degenerative process, and extrinsic factors in its progression. A controversial issue is the relative weight of the impairment of common cell functions, such as energy metabolism and proteostasis, and specific dopaminergic functions, such as pacemaking activity and DA handling, in the pathogenesis of DA-cell degeneration. Here we will review the current knowledge about the relevance of these factors at the beginning and during the progression of PD, and in the differential vulnerability of midbrain DA-cells.

Entorhinal cortex structure and functional MRI response during an associative verbal memory task

Entorhinal cortex (ERC) volume in adults with mild cognitive impairment has been shown to predict prodromal Alzheimer's disease (AD). Likewise, neuronal loss in ERC has been associated with AD, but not with normal aging. Because ERC is part of a major pathway modulating input to the hippocampus, structural changes there may result in changes to cognitive performance and functional brain activity during memory tasks. In 32 cognitively intact older adults, we examined the relationship between left ERC thickness and functional magnetic resonance imaging (fMRI) activity during an associative verbal memory task. This task has been shown previously to activate regions that are sensitive to aging and AD risk. ERC was manually defined on native space, high resolution, oblique coronal MRI scans. Subjects having thicker left ERC showed greater activation in anterior cingulate and medial frontal regions during memory retrieval, but not encoding. This result was independent of hippocampal volume. Anterior cingulate cortex is directly connected to ERC, and is, along with medial frontal cortex, implicated in error detection, which is impaired in AD. Our results suggest that in healthy older adults, processes that engage frontal regions during memory retrieval are related to ERC structure.

Aging reveals a role for nigral tyrosine hydroxylase ser31 phosphorylation in locomotor activity generation

BACKGROUND

Tyrosine hydroxylase (TH) regulates dopamine (DA) bioavailability. Its product, L-DOPA, is an established treatment for Parkinson's disease (PD), suggesting that TH regulation influences locomotion. Site-specific phosphorylation of TH at ser31 and ser40 regulates activity. No direct evidence shows that ser40 phosphorylation is the dominating mechanism of regulating TH activity in vivo, and physiologically-relevant stimuli increase L-DOPA biosynthesis independent of ser40 phosphorylation. Significant loss of locomotor activity occurs in aging as in PD, despite less loss of striatal DA or TH in aging compared to the loss associated with symptomatic PD. However, in the substantia nigra (SN), there is equivalent loss of DA or TH in aging and at the onset of PD symptoms. Growth factors increase locomotor activity in both PD and aging models and increase DA bioavailability and ser31 TH phosphorylation in SN, suggesting that ser31 TH phosphorylation status in the SN, not striatum, regulates DA bioavailability necessary for locomotor activity.

METHODOLOGY AND PRINCIPAL FINDINGS

We longitudinally characterized locomotor activity in young and older Brown-Norway Fischer 344 F(1) hybrid rats (18 months apart in age) at two time periods, eight months apart. The aged group served as an intact and pharmacologically-naïve source of deficient locomotor activity. Following locomotor testing, we analyzed DA tissue content, TH protein, and TH phosphorylation in striatum, SN, nucleus accumbens, and VTA. Levels of TH protein combined with ser31 phosphorylation alone reflected inherent differences in DA levels among the four regions. Measures strictly pertaining to locomotor activity initiation significantly correlated to DA content only in the SN. Nigral TH protein and ser31 phosphorylation together significantly correlated to test subject's maximum movement number, horizontal activity, and duration.

CONCLUSIONS/SIGNIFICANCE

Together, these results show ser31 TH phosphorylation regulates DA bioavailability in intact neuropil, its status in the SN may regulate locomotor activity generation, and it may represent an accurate target for treating locomotor deficiency. They also show that neurotransmitter regulation in cell body regions can mediate behavioral outcomes and that ser31 TH phosphorylation plays a role in behaviors dependent upon catecholamines, such as dopamine.

Relationship of striatal dopamine synthesis capacity to age and cognition

Past research has demonstrated that performance on frontal lobe-dependent tasks is associated with dopamine system integrity and that various dopamine system deficits occur with aging. The positron emission tomography (PET) radiotracer 6-[(18)F]fluoro-l-m-tyrosine (FMT) is a substrate of the dopamine-synthesizing enzyme, aromatic amino acid decarboxylase (AADC). Studies using 6-[(18)F]fluorodopa (FDOPA) (another AADC substrate) to measure how striatal PET signal and age relate have had inconsistent outcomes. The varying results occur in part from tracer processing that renders FDOPA signal subject to aspects of postrelease metabolism, which may themselves change with aging. In contrast, FMT remains a purer measure of AADC function. We used partial volume-corrected FMT PET scans to measure age-related striatal dopamine synthesis capacity in 21 older (mean, 66.9) and 16 younger (mean, 22.8) healthy adults. We also investigated how striatal FMT signal related to a cognitive measure of frontal lobe function. Older adults showed significantly greater striatal FMT signal than younger adults. Within the older group, FMT signal in dorsal caudate (DCA) and dorsal putamen was greater with age, suggesting compensation for deficits elsewhere in the dopamine system. In younger adults, FMT signal in DCA was lower with age, likely related to ongoing developmental processes. Younger adults who performed worse on tests of frontal lobe function showed greater FMT signal in right DCA, independent of age effects. Our data suggest that higher striatal FMT signal represents nonoptimal dopamine processing. They further support a relationship between striatal dopamine processing and frontal lobe cognitive function.

Individual differences and developmental change in the ERN response: implications for models of ACC function

The anterior cingulate cortex (ACC) has been associated with conditions precipitating an increase in effortful processing or increased attention, including the presence of conflicting information and the detection of errors. The error-related negativity (ERN), an electrocortical response, has been used as a marker for these conditions. The ERN amplitude however is subject to developmental change across the lifespan as well as being sensitive to individual differences in personality, affect, and autonomic responsivity. In this review, we examine the implications of such influences for a standard ACC model of conflict processing, and outline the need of any model of ACC function to include mechanisms that allow for the integration of neurovisceral and cognitive domains.

Heroin abuse is characterized by discrete mesolimbic dopamine and opioid abnormalities and exaggerated nuclear receptor-related 1 transcriptional decline with age

Dysfunction of mesocorticolimbic dopaminergic neurons is considered a common feature of all drugs of abuse, yet few investigations have evaluated the dopamine (DA) system in nonstimulant human abusers. We examined mRNA expression levels of DA transporter (DAT), tyrosine hydroxylase (TH), dopamine D2 receptor, alpha-synuclein, and nuclear receptor-related 1 (Nurr1) in discrete mesocorticolimbic and nigrostriatal subpopulations of heroin users and control subjects. The chronic use of heroin was significantly associated with decreased DAT mRNA expression localized to the paranigral nucleus (PN) and the mesolimbic division of the ventral tegmental area (VTA) with no alterations in nigrostriatal populations. Consistently, the density of DAT immunoreactivity was significantly reduced in the nucleus accumbens but not in dorsal striatum, mesolimbic and nigrostriatal efferent targets, respectively. Significant alteration of the mRNA expression of Nurr1, a transcription factor that regulates DAT expression, was also confined to the PN. Moreover, the results revealed an exaggerated reduction of Nurr1 expression with age in heroin users (r = -0.8268, p < 0.001 vs controls, r = -0.6204, p = 0.0746). TH and alpha-synuclein mRNA levels were, in contrast, elevated in the VTA PN in heroin users with no change of the D2 receptor. Evaluating midbrain mu- and kappa-opioid receptors, relevant for the action of heroin and regulation of DA neurons, revealed dysregulation of G-protein coupling selective to the VTA PN. Altogether the current findings provide direct neurobiological evidence that midbrain reward circuits have the most prominent DA and opioid impairments in human heroin abusers and that abnormal Nurr1 transcription with opiate use may exacerbate limbic dysfunction with age.

Glutamate-dopamine-GABA interactions in the aging basal ganglia

The study of neurotransmitter interactions gives a better understanding of the physiology of specific circuits in the brain. In this review we focus mostly on our own results on the interaction of the neurotransmitters glutamate, dopamine and GABA in the basal ganglia during the normal process of aging. We review first the studies on the action of endogenous glutamate on the extracellular concentrations of dopamine and GABA in the neostriatum and nucleus accumbens during aging. It was found that there exists an age-related change in the interaction of glutamate, dopamine and GABA and that these effects of aging exhibit a dorsal-to-ventral pattern of effects with no changes in the dorsal parts (dorsal striatum) and changes in the most ventral parts (nucleus accumbens). Second we reviewed the data on the effects of different ionotropic and metabotropic glutamate receptor agonists on the extracellular concentrations of dopamine and GABA in the nucleus accumbens. The results obtained clearly show the different contribution of each glutamate receptor subtype in the age-related changes produced on the interaction of glutamate, dopamine and GABA in this area of the brain. Third the effects of an enriched environment on the action of AMPA and NMDA-receptor agonists in the nucleus accumbens of rats during aging are also evaluated. Finally, and since the nucleus accumbens has been suggested to play a role in emotion and motivation and also motor behaviour, we speculated on the possibility of a specific contribution for the different glutamatergic pathways terminating in the nucleus accumbens and their interaction with a decreased dopamine playing a relevant role in motor behaviour during aging.