John A. Stanford

Glial Cell Line-Derived Neurotrophic Factor Increases Stimulus-Evoked Dopamine Release and Motor Speed in Aged Rhesus Monkeys

Changes in the functional dynamics of dopamine release and regulation in the basal ganglia have been posited to contribute to age-related slowing of motor functions. Here, we report the effects of glial cell line-derived neurotrophic factor (GDNF) on the stimulus-evoked release of dopamine and motor speed in aged monkeys (21–27 years of age; n = 10). Although no changes were observed in the vehicle controls (n = 5), chronic infusions of 7.5 μg of GDNF per day for 2 months into the right lateral ventricle initially increased hand movement speed up to 40% on an automated hand-reach task. These effects were maintained for at least 2 months after replacing GDNF with vehicle, and increased up to another 10% after the reinstatement of GDNF treatment for 1 month. In addition, upper-limb motor performance times of the aged GDNF-treated animals (n = 5) recorded at the end of the study were similar to those of five young adult monkeys (8–12 years of age). The stimulus-evoked release of dopamine was significantly increased, up to 130% in the right caudate nucleus and putamen and up to 116% in both the right and left substantia nigra of the aged GDNF recipients compared with vehicle controls. Also, basal extracellular levels of dopamine were bilaterally increased, up to 163% in the substantia nigra of the aged GDNF-treated animals. The data suggest that the effects of GDNF on the release of dopamine in the basal ganglia may be responsible for the improvements in motor functions and support the hypothesis that functional changes in dopamine release may contribute to motor dysfunctions characterizing senescence.

Striatal GDNF administration increases tyrosine hydroxylase phosphorylation in the rat striatum and substantia nigra.

Glial cell line‐derived neurotrophic factor (GDNF) improves motor dysfunction associated with aging in rats and non‐human primates, in animal models of Parkinsons disease, and may improve motoric function in patients with advanced Parkinsons disease. These improvements are associated with increased dopamine function in the nigrostriatal system, but the molecular events associated with this increase are unknown. In these studies, 100 µg of GDNF was injected into the striatum of normal aged (24‐month‐old) male Fischer 344 rats. The protein levels and phosphorylation of TH, ERK1/2, and related proteins were determined by blot‐immunolabeling of striatum and substantia nigra harvested 30 days after injection. In GDNF‐treated rats, TH phosphorylation at Ser31 increased ∼40% in striatum and ∼250% in the substantia nigra. In the substantia nigra, there was a significant increase in ERK1 phosphorylation. In striatum, there was a significant increase in ERK2 phosphorylation. Microdialysis studies in striatum showed that both amphetamine‐ and potassium‐evoked dopamine release in GDNF recipients were significantly increased. These data show that GDNF‐induced increases in dopamine function are associated with a sustained increase in TH phosphorylation at Ser31, which is greatest in the substantia nigra and maintained for at least one month following a single striatal administration of GDNF. These findings, taken from the nigrostriatal system of normal aged rats, may help explain the long lasting effects of GDNF on dopamine function and prior studies supporting that a major effect of GDNF involves its effects on dopamine storage and somatodendritic release of dopamine in the substantia nigra.

Effects of chronic intraputamenal infusion of glial cell line-derived neurotrophic factor (GDNF) in aged Rhesus monkeys

In this study, 17-23 year old Rhesus monkeys were used as an early model of Parkinsons disease (PD). Four animals received chronic infusions of GDNF and four received vehicle infusions into the right putamen via programmable pumps for 8 weeks. Weekly videotaping was performed to record general motor performance and a monkey movement analysis panel (mMAP) was used to quantify fine and coarse upper limb motor performance. The GDNF-treated animals showed significant improvements in their overall motor performance in the last 3 weeks of the study compared to controls. Fine motor time of the upper limbs improved significantly in both the GDNF-treated and control animals. After 8 weeks of drug administration, the animals were euthanized and tissue punches were taken from the basal ganglia for measures of dopamine (DA) and DA metabolite levels. In the right putamen, GDNF infusion produced a 217% increase in homovanillic acid (HVA) levels. In addition, DA levels increased by 50% in the right caudate nucleus and there were 122 and 76% increases in 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the right and left caudate nucleus, respectively. HVA levels were also seen to be increased by 212% in the right caudate nucleus. Finally, changes were seen in the right globus pallidus, with 390 and 171% increases in DA and HVA levels, respectively. These data support the hypothesis that GDNF may be beneficial for the treatment of damaged or degenerating DA neurons in aged monkeys and possibly in aged humans.

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, fluoxetines 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.

Aged Fischer 344 rats exhibit altered orolingual motor function: relationships with nigrostriatal neurochemical measures

The present study utilized a novel behavioral preparation to measure differences in orolingual motor function between young (6 months) and aged (24 months) Fischer 344 (F344) rats. Rats were trained to lick an isometric force-sensing operandum for water reinforcement so that the number of licks per session, licking rhythm and lick force could be compared between the two groups. The aged rats exhibited a greater number of licks per session, but a slowed licking rhythm, compared to the young rats. Lick force did not differ significantly between the groups. The dopamine (DA) uptake inhibitor nomifensine decreased all three measures in both groups. Analyses of whole brain tissue content of DA, 3,4 dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) in the substantia nigra and dorsal striatum revealed no significant differences between the two age groups. Differences were observed between the two groups with respect to relationships between behavioral and neurochemical tissue measures. Striatal DA content and the number of licks per session were positively correlated for the young rats but not for the aged rats. In the aged rats, but not the young rats, positive correlations were also observed between licking rhythm and the DOPAC+HVA/DA ratio in the substantia nigra. These findings suggest that age-related alterations in orolingual motor function may relate in part to functional changes in DA neuronal circuits.

Prenatal cocaine exposure alters potassium-evoked dopamine release dynamics in rat striatum

The emerging profile for the effects of prenatal cocaine exposure presents two prominent features in the exposed offspring: cognitive/attention deficits and an age-associated trend toward motor/tone abnormalities up to 2 years of age. One candidate mechanism underlying these clinical features is long-lasting alterations to dopamine (DA) neuron function. However, the impact of prenatal cocaine exposure on DA release in dopaminergic terminal fields in vivo in mature offspring is poorly understood. Long-Evans female rats were implanted with an i.v. access port, bred, and given saline or cocaine-HCl (3 mg/kg/ml) for gestational days (GD) 8-14 (1x/day), GD 15-21 (2x/day), or GD 8-21 (1x/day-GD 8-14, 2x/day-GD 15-21). Using in vivo high-speed chronoamperometric recordings, potassium-stimulated DA release was measured in striatum of anesthetized male offspring 90-150 days after birth. There was a trend toward increased potassium-evoked DA signal amplitudes in offspring exposed to cocaine at any time period examined. In offspring exposed to cocaine during GD 8-21 and GD 15-21, but not at GD 8-14, there were significant decreases in the clearance capacity of the potassium-evoked DA signal compared with control offspring. The time required to clear 80% of the evoked DA signal (T(80)) in striatum for DA was significantly prolonged (approximately 150% of control) and this effect was further increased in the mean-evoked DA concentration range for these two groups. We also measured total dopamine transporter (DAT) and tyrosine hydroxylase protein levels in these offspring by blot immunolabeling and found a small, but significant, decrease in DAT protein in striatum from offspring exposed at GD 8-21 and GD 15-21. Collectively, these data demonstrate that prenatal cocaine exposure during dopamine neuron neurogenesis has long-lasting effects on DA neuron function lasting into early adulthood which may be related in part to steady state DAT protein levels. These molecular events may be associated with established cognitive deficits and perhaps the trends seen in altered motor behavior.

Biochemical and Anatomical Changes in Basal Ganglia of Aging Animals

Deterioration of motor function is a hallmark of senescence in humans and other mammalian species. The similarity of age-related motor deterioration to the symptoms of Parkinsons disease has focused research efforts upon the basal ganglia and particularly the nigrotriatal dopaminergic system. In this chapter, age-related structural and functional characteristics of basal ganglia of several laboratory animal species were reviewed. Although some studies have demonstrated loss of substantia nigra pars compacta dopaminergic neurons with age, the degree of loss does not appear sufficient to account for the marked motor decrements that occur in senescence. Consequently, studies of age-related changes in nigrostriatal function have increased. Reported changes in the basal ganglia of aged animals include decreased numbers of dopamine receptors and transporters as well as decreased dopamine synthesis, release, and uptake. There is also evidence for changes in the interactions between dopamine and other neurotransmitters in the basal ganglia. These age-related changes in nigrostriatal dopaminergic function may account for the majority of the motor deficits that accompany normal aging.

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.

Aged F344 rats exhibit an increased proportion of dopamine agonist-excited striatal neurons

In order to study age-related differences in striatal electrophysiological activity in freely-moving animals, multi-wire electrode arrays were chronically implanted in the striatum of young (6-8 months) and aged (24-26 months) Fischer 344 rats. After recording baseline activity, d-amphetamine (D-AMPH; 1.0 mg/kg) and apomorphine (APO; 0.5 mg/kg) were administered to the two age groups. For both the D-AMPH and APO series, the percentage of striatal neurons that increased firing rates as a result of the DA agonists was 19% higher in the old animals than in the young animals. In addition, D-AMPH increased the firing rates of D-AMPH-excited neurons to a greater extent in the old animals than in the young animals. While the rate-increasing effects of APO did not differ significantly as a function of age, its effects were slightly greater in the old animals as well. These results suggest that age-related decreases in nigrostriatal DA function may result in alterations in the way in which the striatum integrates corticostriatal and nigrostriatal inputs to influence motor function.