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

HIV-Associated Apathy/Depression and Neurocognitive Impairments Reflect Persistent Dopamine Deficits

Individuals living with human immunodeficiency virus type 1 (HIV-1) are often plagued by debilitating neurocognitive impairments and affective alterations;the pathophysiology underlying these deficits likely includes dopaminergic system dysfunction. The present review utilized four interrelated aims to critically examine the evidence for dopaminergic alterations following HIV-1 viral protein exposure. First, basal dopamine (DA) values are dependent upon both brain region andexperimental approach (i.e., high-performance liquid chromatography, microdialysis or fast-scan cyclic voltammetry). Second, neurochemical measurements overwhelmingly support decreased DA concentrations following chronic HIV-1 viral protein exposure. Neurocognitive impairments, including alterations in pre-attentive processes and attention, as well as apathetic behaviors, provide an additional line of evidence for dopaminergic deficits in HIV-1. Third, to date, there is no compelling evidence that combination antiretroviral therapy (cART), the primary treatment regimen for HIV-1 seropositive individuals, has any direct pharmacological action on the dopaminergic system. Fourth, the infection of microglia by HIV-1 viral proteins may mechanistically underlie the dopamine deficit observed following chronic HIV-1 viral protein exposure. An inclusive and critical evaluation of the literature, therefore, supports the fundamental conclusion that long-term HIV-1 viral protein exposure leads to a decreased dopaminergic state, which continues to persist despite the advent of cART. Thus, effective treatment of HIV-1-associated apathy/depression and neurocognitive impairments must focus on strategies for rectifying decreases in dopamine function.

Microfabricated sampling probes for in vivo monitoring of neurotransmitters

Microfabricated fluidic systems have emerged as a powerful approach for chemical analysis. Relatively unexplored is the use of microfabrication to create sampling probes. We have developed a sampling probe microfabricated in Si by bulk micromachining and lithography. The probe is 70 μm wide by 85 μm thick by 11 mm long and incorporates two buried channels that are 20 μm in diameter. The tip of the probe has two 20 μm holes where fluid is ejected or collected for sampling. Utility of the probe was demonstrated by sampling from the brain of live rats. For sampling, artificial cerebral spinal fluid was infused in through one channel at 50 nL/min while sample was withdrawn at the same flow rate from the other channel. Analysis of resulting fractions collected every 20 min from the striatum of rats by liquid chromatography with mass spectrometry demonstrated reliable detection of 17 neurotransmitters and metabolites. The small probe dimensions suggest it is less perturbing to tissue and can be used to sample smaller brain nuclei than larger sampling devices, such as microdialysis probes. This sampling probe may have other applications such as sampling from cells in culture. The use of microfabrication may also enable incorporation of electrodes for electrochemical or electrophysiological recording and other channels that enable more complex sample preparation on the device.

The effect of aging on dopamine release and metabolism during sevoflurane anesthesia in rat striatum: an in vivo microdialysis study

We have previously reported that halothane anesthesia increases extracellular concentrations of dopamine (DA) metabolites in rat striatum using in vivo microdialysis techniques. Aging induces many changes in the brain, including neurotransmission. However, the relationship between aging and changes in neurotransmitter release during inhalational anesthesia has not been fully investigated. The aim of the present investigation was to evaluate the effect of sevoflurane on methamphetamine (MAPT)-induced DA release and metabolism in young and middle-aged rats. Male Sprague-Dawley rats were implanted with a microdialysis probe into the right striatum. The probe was perfused with a modified Ringer's solution and 40μl of dialysate was directly injected to an HPLC every 20min. Rats were administered saline, the same volume of 2mgkg(-1) MAPT intraperitoneally, or 5μM MAPT locally perfused. After treatments, the rats were anesthetized with 1% or 3% sevoflurane for 1h. Sevoflurane anesthesia significantly increased the extracellular concentration of DA only in middle-aged rats (52-weeks-old). In young rats (8-weeks-old), sevoflurane significantly enhanced MAPT-induced DA when administered both intraperitoneally and perfused locally, whereas no significant additive interaction was found in middle-aged rats. These results suggest that aging changes DA release and metabolism in rat brains primarily by decreasing the DA transporter.

Experimental manipulations blunt time-induced changes in brain monoamine levels and completely reverse stress, but not Pb+/-stress-related modifications to these trajectories

This study sought to further understand how environmental conditions influence the outcomes of early developmental insults. It compared changes in monoamine levels in frontal cortex, nucleus accumbens and striatum of male and female Long-Evans rat offspring subjected to maternal Pb exposure (0, 50 or 150ppm in drinking water from 2 months pre-breeding until pup weaning)+/-prenatal (PS) (restraint on GD16-17) or PS+offspring stress (OS; three variable stress challenges to young adults) determined at 2 months of age and at 6 months of age in littermates subsequently exposed either to experimental manipulations (EM: daily handling and performance on an operant fixed interval (FI) schedule of food reward), or to no experience (NEM; time alone). Time alone (NEM conditions), even in normal (control) animals, modified the trajectory of neurochemical changes between 2 and 6 months across brain regions and monoamines. EM significantly modified the NEM trajectories, and except NE and striatal DA, which increased, blunted the changes in monoamine levels that occurred over time alone. Pb+/-stress modified the trajectory of monoamine changes in both EM and NEM conditions, but these predominated under NEM conditions. Stress-associated modifications, occurring mainly with NEM OS groups, were fully reversed by EM procedures, while reversals of Pb+/-stress-associated modifications occurred primarily in nucleus accumbens, a region critical to mediation of FI response rates. These results extend the known environmental conditions that modify developmental Pb+/-stress insults, which is critical to ultimately understanding whether early insults lead to adaptive or maladaptive behavior and to devising behavioral therapeutic strategies. That time alone and a set of EM conditions typically used as outcome measures in intervention studies can themselves invoke neurochemical changes, moreover, has significant implications for experimental design of such studies.

Age-dependent alterations of corticostriatal activity in the YAC128 mouse model of Huntington disease

Huntington disease is a genetic neurodegenerative disorder that produces motor, neuropsychiatric, and cognitive deficits and is caused by an abnormal expansion of the CAG tract in the huntingtin (htt) gene. In humans, mutated htt induces a preferential loss of medium spiny neurons in the striatum and, to a lesser extent, a loss of cortical neurons as the disease progresses. The mechanisms causing these degenerative changes remain unclear, but they may involve synaptic dysregulation. We examined the activity of the corticostriatal pathway using a combination of electrophysiological and optical imaging approaches in brain slices and acutely dissociated neurons from the YAC128 mouse model of Huntington disease. The results demonstrated biphasic age-dependent changes in corticostriatal function. At 1 month, before the behavioral phenotype develops, synaptic currents and glutamate release were increased. At 7 and 12 months, after the development of the behavioral phenotype, evoked synaptic currents were reduced. Glutamate release was decreased by 7 months and was markedly reduced by 12 months. These age-dependent alterations in corticostriatal activity were paralleled by a decrease in dopamine D(2) receptor modulation of the presynaptic terminal. Together, these findings point to dynamic alterations at the corticostriatal pathway and emphasize that therapies directed toward preventing or alleviating symptoms need to be specifically designed depending on the stage of disease progression.

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.

Aging and sex differences in striatal dopaminergic function

In this report the potassium- (30 mM) and amphetamine- (10 microM) stimulated responses of dopamine (DA) and 3,4-dihydroxy phenylacetic acid (DOPAC) from superfused striatal tissue of female and male mice as sampled at 2, 6, 18 and 24 months of age were compared. When assessed relative to responses obtained from 2-month-old female mice, potassium-stimulated DA output of female mice was significantly decreased at 18 months of age and significantly increased at 24 months of age. In male mice, the only statistically significant change was an increase in potassium-stimulated DA in the 24 versus 2-month-old mice. In response to amphetamine-stimulation, DA responses from striatal tissue of 18-month-old females were significantly decreased and that of 24-month-old mice significantly increased relative to that of the 2-month-old females. In the case of male mice, amphetamine-stimulated DA responses of 6- and 18-month-old mice were significantly decreased compared with responses observed in the 2-month-old males. In addition, amphetamine-stimulated DA responses of the 24-month-old females were significantly greater than the 24-month-old males. In general, the response profiles for DOPAC to potassium and amphetamine stimulation were similar to that of DA for male, but not female, mice. These results demonstrate that sex differences in striatal dopaminergic function are differentially affected by age. Overall, striatal DA responsiveness of female mice shows more extreme age-related changes, particularly between the 2- and 6-month versus the 18- and 24-month-old mice and a discord between DA and DOPAC responses. Such extreme changes may be related to the presence (at 2 and 6 months) versus absence (at 18 and 24 months) of estrous cycles/gonadal steroid hormonal functions in female mice.

Spatial reference- (not working- or procedural-) memory performance of aged rats in the water maze predicts the magnitude of sulpiride-induced facilitation of acetylcholine release by striatal slices

Cluster analysis of water-maze reference-memory performance distinguished subpopulations of young adult (3-5 months), aged (25-27 months) unimpaired (AU) and aged impaired (AI) rats. Working-memory performances of AU and AI rats were close to normal (though young and aged rats differed in exploration strategies). All aged rats showed impaired procedural-memory. Electrically evoked release of tritium was assessed in striatal slices (preloaded with [(3)H]choline) in the presence of oxotremorine, physostigmine, atropine+physostigmine, quinpirole, nomifensine or sulpiride. Aged rats exhibited reduced accumulation of [(3)H]choline (-30%) and weaker transmitter release. Drug effects (highest concentration) were reductions of release by 44% (oxotremorine), 72% (physostigmine), 84% (quinpirole) and 65% (nomifensine) regardless of age. Sulpiride and atropine+physostigmine facilitated the release more efficiently in young rats versus aged rats. The sulpiride-induced facilitation was weaker in AI rats versus AU rats; it significantly correlated with reference-memory performance. The results confirm age-related alterations of cholinergic and dopaminergic striatal functions, and point to the possibility that alterations in the D(2)-mediated dopaminergic regulation of these functions contribute to age-related reference-memory deficits.

Altered basal and stimulated accumbens dopamine release in obese OLETF rats as a function of age and diabetic status

The Otsuka Long-Evans Tokushima Fatty (OLETF) rat lacking the CCK-1 receptor is hyperphagic, prefers palatable and high-calorie meals, and gradually develops obesity and type 2 diabetes. To determine dopamine levels in this strain, we used in vivo quantitative (no net flux) microdialysis at three different ages representing nondiabetic (8 wk), prediabetic (18 wk), and diabetic (56 wk) stages in OLETF and age-matched lean Long-Evans Tokushima Otsuka (LETO) controls. Results showed significantly elevated basal dopamine levels in the caudomedial nucleus accumbens of OLETF rats compared with LETO at younger ages (8 wk: 20.10 +/- 5.61 nM vs. 15.85 +/- 5.63 nM; 18 wk: 7.37 +/- 3.71 nM vs. 4.75 +/- 1.25 nM, means +/- SD). In contrast, at 56 wk of age, a profound decline in extracellular dopamine concentrations was seen in both strains with a tendency for a greater effect in OLETF rats (1.78 +/- 0.40 nM vs. 2.39 +/- 0.42 nM). Further, extracellular fraction, an index for reuptake, was higher in 56-wk-old OLETF compared with LETO (0.648 +/- 0.049 vs. 0.526 +/- 0.057). Potassium-stimulated dopamine efflux revealed an increased capacity of vesicular pool in OLETF rats compared with LETO across all age groups with an accentuated strain difference at 56 wk. These findings demonstrate altered striatal dopamine functions (i.e., increased stimulated release and uptake) in obese OLETF rat. This could be due to the lack of functional CCK-1 receptors, or metabolic and hormonal factors associated with the development of obesity and insulin resistance, or both.

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.

Oral methylphenidate fails to elicit significant changes in extracellular putaminal dopamine levels in Parkinson's disease patients: Positron emission tomographic studies

In this study, we assessed the changes of endogenous dopamine (DA) levels in response to methylphenidate in 5 patients with idiopathic Parkinson's disease (PD) and 6 healthy controls. Three-dimensional positron emission tomography was performed with the D2 receptor antagonist [11C]raclopride (RAC) at baseline and 1 hour following the administration of oral methylphenidate (0.8 mg/kg) to assess changes in dopamine levels indirectly. Oral methylphenidate produced no significant change in extracellular DA levels in the putamen, as estimated by comparing changes in RAC binding at baseline and 1 hour following its administration in PD subjects and healthy controls. However, there were small changes in RAC binding of opposite direction in caudate and ventral striatal regions compared between the two groups. Although there was no consistent improvement in motor function in the PD group, some patients did experience a subjective high in response to methylphenidate (MP). Failure of oral MP to alter extracellular DA levels in putamen could result from degeneration of presynaptic dopaminergic terminals, with consequent severe reductions in the levels of endogenous DA and dopamine transporter in PD subjects. Our data provide in vivo neurochemical support for the lack of clinical efficacy following MP in PD patients and are also in keeping with reduced DA release following amphetamine in PD subjects.

Dopamine and GABA increases produced by activation of glutamate receptors in the nucleus accumbens are decreased during aging

The aim of the present study was to investigate the effects of aging on the increases of dopamine and GABA induced by activation of ionotropic and metabotropic glutamate receptors in the nucleus accumbens of the freely moving rat. The effects of local perfusion of the agonists NMDA (10, 100 and 500 microM), AMPA (1, 20 and 100 microM) and ACPD (100, 500 and 1000 microM) on extracellular concentration of dopamine and GABA in the nucleus accumbens of young (2-4 months), middle-aged (10-14 months) and aged (24-32 months) male Wistar rats were studied using microdialysis. In young rats, perfusion of the agonists NMDA and AMPA, but not ACPD, produced an increase of dialysate concentrations of dopamine. Perfusion of the three glutamate agonists (NMDA, AMPA and ACPD) produced an increase of dialysate GABA. This increase was delayed in time compared with the increase of dopamine. In the nucleus accumbens of middle-aged and aged rats, the increases of dopamine induced by NMDA were significantly lower than those in young rats. Also the increases of dopamine induced by AMPA were lower in aged rats than those in young rats. The effects of AMPA, NMDA and ACPD on dialysate GABA were significantly lower in aged rats than in young rats. These findings suggest that aging changes the interaction between the neurotransmitters glutamate and dopamine and glutamate and GABA in the nucleus accumbens of the freely moving rat.

Differential effects of post-implantation time on potassium- versus D-amphetamine-evoked dopamine overflow in the striatum of F344 rats

Effects of post-implantation time on potassium (K+)- versus D-amphetamine (D-AMPH)-evoked striatal dopamine (DA) overflow were measured using microdialysis in freely moving young and aged Fischer 344 rats. In one group, samples were collected on the day of probe insertion (Day 1 group). In a second group, samples were collected 24 h after probe insertion (Day 2 group). While analyses revealed no significant differences between the two age groups, the 100 mM K+ stimulus evoked a significantly greater amount of DA overflow in the Day 1 group compared to the Day 2 group. The decrease in 3,4-dihydroxyphenylacetic acid (DOPAC) produced by K+ stimulation was not influenced by post-implantation time. The effect of the 250 microM D-AMPH stimulus on DA overflow did not differ between the Day 1 and Day 2 groups, nor did the decrease in DOPAC that accompanied D-AMPH stimulation. These results support the hypothesis that under some stimulus conditions, post-implantation time is an important variable in microdialysis studies.

Microdialysis studies of D-amphetamine-evoked striatal dopamine overflow in young versus aged F344 rats: effects of concentration and order of administration

In order to measure the effects of different concentrations of D-amphetamine (D-AMPH) infusions on striatal dopamine (DA) overflow in young versus aged rats, and to determine the influence of preceding infusions on subsequent stimuli, two microdialysis studies were conducted. In the first study, D-AMPH (100, 200, and 2000 microM) was infused in ascending order of concentration, while in the second study the order of administration was reversed. The order of administration significantly affected DA overflow and extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC). Generally, DA overflow was greater for a given concentration when it was administered first in a sequence than when it was administered later in a sequence. The two age groups did not differ on measures of DA overflow. The order of administration also significantly influenced the effect of D-AMPH on extracellular DOPAC, as the D-AMPH-related decreases in DOPAC were greater for a concentration when it was administered earlier versus later in the sequence. This effect was greater in the young rats than in the aged rats. D-AMPH also resulted in diminished levels of DOPAC in the aged rats compared to the young rats. These results suggest that between-groups studies may be more appropriate for determining the effects of different concentrations of D-AMPH on striatal DA overflow. They also demonstrate that while some measures of stimulus-evoked DA overflow may not differ between young and aged F344 rats, extracellular regulation of striatal DA (as measured by changes in DOPAC) may be altered. These alterations may contribute to age-related decreases in motor function.

High-frequency stimulation of the subthalamic nucleus enhances striatal dopamine release and metabolism in rats

High-frequency stimulation of the subthalamic nucleus is believed to exert its main effects via the basal ganglia output structures. Previously, we have shown a concomitant increase in striatal dopamine (DA) metabolites in normal and 6-hydroxydopamine-lesioned rats. The present study was designed to determine whether this increase in striatal DA metabolites reflects enhanced intraneuronal DA turnover or, alternatively, is due to increased DA release with subsequent rapid and efficient reuptake and/or metabolism. Thus, high-frequency stimulation of the subthalamic nucleus was performed in normal rats after inhibition of DA reuptake, metabolism or DA depletion. Extracellular levels of striatal DA and its metabolites were assessed using microdialysis. Our data suggest that subthalamic high-frequency stimulation increases striatal DA release and activates independent striatal DA metabolism. Since such changes could be triggered by modification of either the activity or the gene expression of the rate-limiting enzyme tyrosine hydroxylase, an activity assay and RT-PCR of striatal and nigral samples were performed. Subthalamic stimulation increased striatal tyrosine hydroxylase activity without affecting gene expression. We, therefore, conclude that the application of subthalamic high-frequency stimulation could partially compensate for the DA deficit by inducing increased striatal DA release and metabolism.

Microdialysis monitoring of methylphenidate in blood and brain correlated with changes in dopamine and rat activity

Methylphenidate (MPD), also called Ritalin, changes the extracellular levels of dopamine (DA) in the brain. This study coupled multiple-site microdialysis sampling with appropriate analytical methods to simultaneously profile the MPD concentration in blood and brain, while monitoring changes in the extracellular level of DA in the striatum of awake and freely moving rats. The animals' activity was also recorded. The maximum concentration of MPD in the blood and brain occurred during the first 20 min of sampling. The maximum DA concentration was reached in the first 20 min and gradually returned to the basal level after 3 h. The activity peak correlated well with the MPD and DA peaks and remained elevated for about 2.5 h. The ability to obtain and correlate data in this manner has the potential to reduce the number of animals required for a given study and to minimize interanimal variation.

Functional aspects of dopamine metabolism in the putative prefrontal cortex analogue and striatum of pigeons (Columba livia)

Dopamine (DA) in mammalian associative structures, such as the prefrontal cortex (PFC), plays a prominent role in learning and memory processes, and its homeostasis differs from that of DA in the striatum, a sensorimotor region. The neostriatum caudolaterale (NCL) of birds resembles the mammalian PFC according to connectional, electrophysiological, and behavioral data. In the present study, DA regulation in the associative NCL and the striatal lobus parolfactorius (LPO) of pigeons was compared to uncover possible differences corresponding to those between mammalian PFC and striatum. Extracellular levels of DA and its metabolites (homovanillic acid [HVA], dihydroxyphenylacetic acid [DOPAC]) and the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) were investigated by in vivo microdialysis of urethane-anesthetized pigeons under basal conditions and after systemic administration of D-amphetamine. DA was reliably determined only in LPO dialysates, and DA metabolite levels were significantly higher in LPO than in NCL. The HVA/DOPAC ratio, indicating extracellular lifetime of DA, was more than twice as high in NCL than in LPO dialysates. After amphetamine, DA increased in LPO while still being undetectable in NCL, and DA metabolites decreased in both regions. 5-HIAA slightly decreased in NCL dialysates. Amphetamine effects were delayed in NCL compared with the striatum. In conclusion, effects of amphetamine on the pigeon's ascending monoamine systems resemble those found in mammals, suggesting similar regulatory properties. The neurochemical differences between NCL and LPO parallel those between associative regions, such as PFC and dorsal striatum in mammals. They may reflect weaker regulation of extracellular DA, favoring DAergic volume transmission, in associative than striatal forebrain regions.

Relationship between blockade of dopamine transporters by oral methylphenidate and the increases in extracellular dopamine: therapeutic implications

Methylphenidate (Ritalin) is an effective drug in the treatment of attention deficit hyperactivity disorder. However, the doses required therapeutically vary significantly between subjects and it is not understood what determines these differences. Since methylphenidate's therapeutic effects are in part due to increases in extracellular DA secondary to blockade of dopamine transporters (DAT), the variability could reflect differences in levels of DAT blockade. Here we used PET to assess if for a given dose of methylphenidate the differences in DAT blockade account for the variability in methylphenidate-induced increases in extracellular DA. Ten healthy adult subjects were tested before and 60 min after oral methylphenidate (60 mg) with PET to estimate DAT occupancy (with [(11)C]cocaine as the radioligand) and levels of extracellular DA (with [(11)C]raclopride as the D2 receptor radioligand that competes with endogenous DA for binding to the receptor). Methylphenidate significantly blocked DAT (60 +/- 11%) and increased extracellular DA in brain (16 +/- 8% reduction in [(11)C]raclopride binding in striatum). However, the correlation between methylphenidate-induced DAT blockade and DA increases was not significant. These results indicate that for a given dose of methylphenidate, individual differences in DAT blockade are not the main source for the intersubject variability in MP-induced increases in DA. This finding suggests that individual differences in response to MP are due in part to individual differences in DA release, so that for an equivalent level of DAT blockade, MP would induce smaller DA changes in subjects with low than with high DA cell activity.

Acute locomotor effects of fluoxetine, sertraline, and nomifensine in young versus aged Fischer 344 rats

Spontaneous locomotor activity was measured in young (6-8 months) and aged (24-26 months) Fischer 344 (F344) rats. Following habituation to the activity monitors, aged rats demonstrated significantly diminished motor activity as quantified by total distance traveled and vertical activity. Movement speed did not differ significantly between the two groups. Following habituation, rats were administered acute doses of fluoxetine, sertraline, or nomifensine (1.0, 3.0, and 10.0 mg/kg). Fluoxetine diminished all three behavioral measures in the young rats, while in the old rats, fluoxetine's effects were limited to a robust attenuation of vertical activity. Sertraline decreased movement speed and vertical activity, but not total distance traveled, in the young rats. Unlike fluoxetine, sertraline produced no significant effects on any of the three behavioral variables in the old rats. Nomifensine increased behavioral scores for both age groups. The results are discussed in relation to acute motor side effects of selective serotonin reuptake inhibitors (SSRIs) in motor-impaired aged individuals, as these effects may influence their eventual use in the clinic.

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.

Cocaine and amphetamine-regulated transcript peptide (CART(62-76))-induced changes in regional monoamine levels in ratbrain

Cocaine- and amphetamine-regulated transcript (CART) is a novel neuropeptide with neurotransmitter-like effects. In the present study we examined the influence of CART peptide fragment, CART(62-76), on the levels of catecholamines (dopamine and norepinephrine), serotonin and their metabolites in five regions of the rat brain. CART(62-76) was administered at 0.5 or 5.0 microg dose intracerebroventricularly. A high-pressure liquid chromatograph coupled to electrochemical array detector was used to analyse the tissue homogenates. In the frontal cortex, CART(62-76) increased the levels of 5-hydroxyindoleacetic acid (5-HIAA), a metabolite of serotonin (5-HT). In the striatum, CART(62-76) decreased the levels of noradrenaline and 5-HT. In the hypothalamus, CART(62-76) increased the levels of 5-HIAA. CART(62-76) had no significant effect in the hippocampus and cerebellum. Our data suggest that CART(62-76) peptide has no major effect on dopaminergic pathways, but it modulates the activity of striatal noradrenergic and corticostriatal and hypothalamic serotoninergic system in the rat brain. These regionally selective neurochemical changes may explain the effects of CART peptides on appetitive, emotional and locomotor behaviour.

Nomifensine reveals age-related changes in K(+)-evoked striatal DA overflow in F344 rats

To investigate the influence of age-associated changes in DA uptake on measures of potassium-stimulated DA overflow in the striatum, microdialysis was conducted in anesthetized young (6-month-old) versus aged (24-month-old) F344 rats. Extracellular levels of DA, DOPAC, and HVA were measured under basal and potassium-stimulated (10, 25, 50, & 100 mM) conditions. Basal levels of DA and metabolites did not differ significantly between the two age groups. At the 50 and 100 mM concentrations, potassium stimuli significantly increased DA overflow and decreased DOPAC and HVA--effects that did not differ with age. The addition of the DA uptake inhibitor nomifensine (100 microM) to the perfusion solutions revealed differences between the two age groups. Nomifensine augmented potassium-evoked DA overflow at the 50 mM concentration in both groups, but only amplified the effect of the 100 mM concentration in the young animals. The results demonstrate that decreased DA transporter function in aged rats masks age-related differences in K(+)-evoked striatal DA release when microdialysis methods are used, resulting in net equalization of K(+)-evoked striatal DA overflow in young versus aged F344 rats.

Age-related changes in striatal function of freely-moving F344 rats

Multi-wire electrode arrays were used to record extracellular electrophysiological activity in striatal medium spiny-like neurons of freely-moving young (6-8 months) and aged (24-26 months) Fischer 344 rats. While overall basal firing rates did not differ between the two groups, d-amphetamine (5.0 mg/kg) increased firing rates more in the young rats. D-Amphetamine had heterogeneous effects on firing rates, however, exciting 63% of the neurons while inhibiting 37%. Neurons were classified according to their response to d-amphetamine (excited vs. inhibited) to examine age-related differences in firing rates and bursting activity. In the d-amphetamine-excited neurons, pre-drug intraburst firing rates were higher in the old rats. This effect was reversed by d-amphetamine. D-Amphetamine increased the percentage of spikes within bursts to a greater extent in the aged animals and decreased burst durations greater in the young group. In d-amphetamine-inhibited neurons, firing rates were diminished in the old rats more than they were in the young rats. These results demonstrate age-related alterations in striatal electrophysiological activity that may help explain motor deficits seen in senescence.

Age-related decline in the functional response of striatal group I mGlu receptors

In order to verify whether striatal group I metabotropic glutamate (mGlu) receptors undergo functional alteration in ageing, the effects induced by the selective agonist 3,5-dihydroxyphenylglycine (DHPG) in the striatum of young (3 months) and aged (24-25 months old) rats were compared. The ability of DHPG to stimulate phosphoinositide (PI) hydrolysis (striatal slices), to influence striatal dopamine release (in vivo microdialysis) and to potentiate the effects of NMDA on extracellular field potential amplitude (extracellular recordings on striatal slices) was reduced in the striatum of old vs young rats. These results show an age-dependent reduction in the functional response of striatal group I mGlu receptors, which may be one of the factors underlying the reduced ability aged striatum to integrate information.

Intracerebral infusion of H-dopamine and H-mannitol in the striatum of halothane-anaesthetized male rats. A dual-probe microdialysis study of long-distance diffusion

This report characterizes an in vivo intracerebral long-distance diffusion model using dual-probe microdialysis. Two probes 1 mm apart were implanted into the striatum of control and 6-hydroxydopamine (6-OHDA)-lesioned halothane-anaesthetized male rats. Either tritiated dopamine (500 nM 3H-DA) or mannitol (1.5 microM 3H-mannitol) was infused continuously for 5 h, while samples were collected from the other probe. Samples (10 microl) were counted by liquid scintillation. For the DA-infused rats, another 10 microL was separated with high-pressure liquid chromatography (HPLC)-electrochemical detection into individual fractions containing 3,4-dihydroxy phenylacetic acid (DOPAC) and homovanillinic acid (HVA), and counted for beta-decay. The total transfer of 3H-labelled compounds described the overall effect of cellular uptake, metabolism and clearance into the microcirculation, and was compared with that of an extracellular marker, 3H-mannitol. The migration reached steady-state levels, generating an equilibrium between delivery and removal from the extracellular space. The half-time of the steady-state values, t50%, was in all cases lower in 6-OHDA-treated rats compared with control. In addition, the t50% values of 3H-mannitol were lower than those following the 3H-dopamine infusion in both control or 6-OHDA-lesioned rats. However, it was not possible to detect any unmetabolized 3H-dopamine at the 1 mm distance. In conclusion, the dual-probe microdialysis approach proved to be a valid method to study in vivo diffusion and migration in the brain, and the intracerebral spread of compounds highly depends on the nature of the compound infused.

In vivo microdialysis studies of age-related alterations in potassium-evoked overflow of dopamine in the dorsal striatum of Fischer 344 rats

Intracerebral microdialysis was used to measure basal levels and potassium (K(+))-stimulated overflow of dopamine (DA), homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC), in the dorsal striatum of young (6 months) and aged (24 months) Fischer 344 (F344) rats. Basal levels of HVA were lower in aged rats whereas basal DA and DOPAC did not differ significantly between the two groups. The administration of three low to moderate doses of K(+) (10, 25, and 50 mM) through the microdialysis probe for one collection period revealed differences between the two age groups of F344 rats. DA overflow increased in a dose-dependent manner in the young but not aged rats. Extracellular levels of DOPAC and HVA decreased during the K(+) stimulation and there was a significant difference in the changes in HVA produced by K(+) stimulation in the young vs aged animals. These data support the hypothesis that low to moderate doses of K(+) may be necessary to demonstrate age-related differences in K(+)-evoked DA overflow, since previous microdialysis studies using higher doses have not reported age-related differences in DA overflow.