Divergent changes in D-1 and D-2 dopamine binding sites in human brain during aging

Age-dependent decline in human brain dopamine D1 and D2 receptors

Dopamine D1 and D2 receptor binding was investigated in the caudate nucleus and putamen in 65 individuals ranging from 6 to 93 years of age. [3H]SCH 23390 and [3H]spiroperidol were used as ligands for D1 and D2 receptors, respectively. A full Scatchard analysis with several ligand concentrations was performed yielding Bmax and Kd values. The number of D1 and D2 receptors declined with age both in the caudate nucleus and putamen without any change in the mean dissociation constant. The ratio of D1:D2 receptors remained unchanged. The results show that the dopaminergic system degenerates in the aging striatum and may contribute to the frequent occurrence of extrapyramidal symptoms in the elderly.

Age-related changes in the human nigrostriatal dopaminergic system

A deficiency of dopamine in the striatum may contribute to the decline in motor function associated with aging. We examined the effect of aging on the densities of the D1 and D2 dopamine receptors, their high-agonist affinity sites, and the dopamine uptake sites in postmortem human putamen (n = 32; age range, 19-88 years). With aging a steady decrease was seen in dopamine uptake sites (p less than 0.001), confirming previous morphometric and biochemical data of an age-related loss of nigrostriatal axons. In contrast, the concentrations of the D1 and D2 dopamine receptors and their high-agonist affinity sites, which are believed to represent the functionally active receptors, were not affected by the aging process. These results may have implications for the pharmacological treatment of the age-related decline in motor function.

Striatal dopamine D2 receptors in tardive dyskinesia: PET study

The dopamine D2 receptors were investigated in vivo in eight neuroleptic-free patients with persistent tardive dyskinesia using positron emission tomography and 76Br-bromospiperone. The striatal receptor density, estimated by the striatum/cerebellum radioligand concentration ratio, was not elevated in patients as compared with age-matched controls but was positively correlated with the severity of orofacial dyskinesia assessed with the Abnormal Involuntary Movement Rating Scale. These results indicate that tardive dyskinesia is associated with normal levels of striatal D2 receptors but the severity of orofacial movements may depend on the relative density of striatal D2 receptors.

Effect of aging on concentrations of D2-receptor-containing neurons in the rat striatum

Striatal D2 dopamine receptor concentrations were shown to decrease 30-35% during the lifespan of Wistar rats as assessed both radiochemically and autoradiographically. Binding densities and degree of age-change varied within the striatum; the latter ranging from 17 to 44% in 4 different regions. Overall neuronal loss during aging was 19%, and also varied considerably within the different striatal regions. Thus, it appears that neuronal loss may account for up to roughly half of the striatal D2 receptor loss during aging.

Effects of ageing on the behavioural responses to dopamine agonists: decreased yawning and locomotion, but increased stereotypy

Sensorimotor function and the behavioural responses to a range of doses of subcutaneous apomorphine were assessed in mature (6-8 months) and old (23-26 months) Sprague-Dawley rats of comparable weight. In addition, the locomotor activity response of 12-month-old and 24-month-old rats to continuous infusions (14 days by osmotic minipump) of a selective dopamine D2 agonist. (+)-4-propyl-9-hydroxynaphthoxazine (PHNO, 10 micrograms/h) was investigated. Measures of spontaneous locomotor activity and motor coordination revealed impairments in the aged animals. Low doses of apomorphine (10-50 micrograms/kg), which preferentially activate dopamine autoreceptors, induced yawning, chewing mouth movements and penile grooming. The frequency of yawning and duration of penile grooming were significantly decreased in the old animals. In contrast, 200 micrograms/kg of apomorphine induced stereotyped sniffing and licking or gnawing, and these responses were significantly increased in the aged animals. There was a 25% decrease in striatal dopamine levels in the aged animals in this experiment. PHNO increased the amplitude of the circadian rhythms in locomotor activity exhibited by mature rats, and daytime tolerance to the stimulant effects of PHNO was reversed by stress in these animals. Both of these effects were attenuated in the aged rats. These findings suggest that (1) the dopamine receptors mediating yawning and stereotypy have different anatomical locations (2) ageing is associated with decreased responsiveness to stimulation of dopamine autoreceptors, consequent upon the loss of dopaminergic nerve terminals, and (3) while the functional response to selective stimulation of postsynaptic D2 receptors decreases with age, the postsynaptic response to a mixed D1/D2 agonist increases.

The influence of cholesterol on synaptic fluidity, dopamine D1 binding and dopamine-stimulated adenylate cyclase

The present study examined the influence of the synaptic cholesterol/phospholipid ratio on fluorescence polarization, the binding of SCH23390 to dopaminergic D1 binding sites and dopamine-stimulated adenylate cyclase activity. The cholesterol/phospholipid ratio of synaptic membranes from bovine caudate was modified by incubating the membranes with a lipid transfer protein and liposomes which were either loaded with or lacking cholesterol. The results of this study demonstrated that the number of binding sites (Bmax) for SCH23390 was insensitive to alterations in the synaptic cholesterol/phospholipid ratio and membrane order. However, when the cholesterol/phospholipid ratio was decreased by 30%, membrane order and binding affinity (Kd) were decreased. Despite the lack of change in the number of D1 binding sites, the activity of dopamine-stimulated adenylate cyclase was markedly inhibited by an elevated cholesterol/phospholipid ratio. The results of these studies are discussed in terms of their potential relevance to aging.

Parallel decrease in the density of dopamine D1 and D2 receptors in corpus striatum of rats from 3 to 25 months of age

The density (Bmax) and apparent dissociation constant (KD) of dopamine D1 and D2 receptors in striatum was estimated in rats of different ages (from 3.5 to 25 months) using 3H-SCH 23390 and 3H-spiperone as ligands. The density of D1 and D2 receptors decreases with age attaining 70 and 69% of the 3.5 months' value, respectively, whereas the KD's remain constant. The decreases in density of D1 and D2 receptors are parallel. Thus, throughout life the ratio between the density of D1 and D2 receptors remains constant.

Striatal dopamine receptor occupancy during and following withdrawal from neuroleptic treatment: correlative evaluation by positron emission tomography and plasma prolactin levels

The percentage occupation of striatal dopamine D2 receptors has been evaluated in 25 patients using 76Br-bromospiperone positron emission tomography (PET) and prolactin plasma levels (PRL) during oral neuroleptic treatment (11 studies), 1-90 days following discontinuation of such treatment (16 studies), and 1-120 days after last intramuscular administration of depot neuroleptics (nine studies). The PET-estimated occupation was highly significantly correlated in a sigmoid-like fashion to the logarithm of the chlorpromazine-equivalent dose of oral neuroleptics (suggesting a strict dose-occupation relationship during oral neuroleptic treatment and supporting the D2-mediated hypothesis of neuroleptic action), while PRL was weakly related to daily dosage. Following withdrawal, return to normal receptor availability, as estimated by PET, occurred within 5-15 days (suggesting that protracted effects of neuroleptics after withdrawal are not due to sustained D2 receptor occupation), but PRL values fell even more rapidly. Efficient treatment with depot neuroleptics resulted in marked PET-estimated D2 receptor occupation, stable over the whole 4-week drug-administration interval, suggesting that longer intervals could be appropriate; PRL values bore no relationship to PET-estimated occupation, indicating variable intersubject tolerance to neuro-endocrine dopamine blockade. Overall, PET was much more sensitive than PRL to estimate striatal D2 receptor occupation in vivo.

Behavioural effects of dopamine D-1 and D-2 receptor agonists in monkeys previously treated with haloperidol

The effects of dopamine D-1 and D-2 receptor agonists were evaluated in five Cebus apella monkeys. During a previous haloperidol treatment (2 years), three of the monkeys had developed oral tardive dyskinesia (tongue protrusion and/or chewing). The partial D-1 agonist, SKF 38393, induced/aggravated oral dyskinesia and slight sedation, but no non-oral repetitive movements. Conversely, the selective D-2 agonist, LY 171555, produced non-oral repetitive movements and increased reactivity (arousal), but no significant change in the oral movements. Apomorphine (a mixed D-1/D-2 agonist) induced non-oral repetitive movements, increased reactivity, and increased oral dyskinesia. Pretreatment with SKF 38393 inhibited the LY 171555-induced non-oral repetitive movements, while in four monkeys the SKF 38393-induced oral movements were inhibited by LY 171555. The results suggest that oral dyskinesia (tardive dyskinesia) is more related to D-1 receptor stimulation than to D-2 receptor supersensitivity.

Age changes in adrenergic and dopaminergic signal transduction mechanisms: parallels and contrasts

Age changes in signal transduction exhibit some interesting similarities in the adrenergic and dopaminergic systems of the brain. Although several differences exist with respect to current technical capabilities and characterization of complete signal-response pathways at the organismic level, these systems offer promising models for complimentary elucidation of the mechanisms of altered neurotransmitter action during aging.