Basal ganglia dopamine receptor autoradiography and age-related movement disorders

Impact of Melatonin Deficit on Emotional Status and Oxidative Stress-Induced Changes in Sphingomyelin and Cholesterol Level in Young Adult, Mature, and Aged Rats

The pineal gland regulates the aging process via the hormone melatonin. The present report aims to evaluate the effect of pinealectomy (pin) on behavioral and oxidative stress-induced alterations in cholesterol and sphingomyelin (SM) levels in young adult, mature and aging rats. Sham and pin rats aged 3, 14 and 18 months were tested in behavioral tests for motor activity, anxiety, and depression. The ELISA test explored oxidative stress parameters and SM in the hippocampus, while total cholesterol was measured in serum via a commercial autoanalyzer. Mature and aged sham rats showed low motor activity and increased anxiety compared to the youngest rats. Pinealectomy affected emotional responses, induced depressive-like behavior, and elevated cholesterol levels in the youngest rats. However, removal of the pineal gland enhanced oxidative stress by diminishing antioxidant capacity and increasing the MDA level, and decreased SM level in the hippocampus of 14-month-old rats. Our findings suggest that young adult rats are vulnerable to emotional disturbance and changes in cholesterol levels resulting from melatonin deficiency. In contrast, mature rats with pinealectomy are exposed to an oxidative stress-induced decrease in SM levels in the hippocampus.

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.

Carotid body function in aged rats: responses to hypoxia, ischemia, dopamine, and adenosine

The carotid body (CB) is the main arterial chemoreceptor with a low threshold to hypoxia. CB activity is augmented by A(2)-adenosine receptors stimulation and attenuated by D(2)-dopamine receptors. The effect of aging on ventilatory responses mediated by the CB to hypoxia, ischemia, and to adenosine and dopamine administration is almost unknown. This study aims to investigate the ventilatory response to ischemia and to adenosine, dopamine, and their antagonists in old rats, as well as the effect of hypoxia on adenosine 3',5'-cyclic monophosphate (cAMP) accumulation in the aged CB. In vivo experiments were performed on young and aged rats anesthetized with pentobarbitone and breathing spontaneously. CB ischemia was induced by bilateral common carotid occlusions. cAMP content was measured in CB incubated with different oxygen concentrations. Hyperoxia caused a decrease in cAMP in the CB at all ages, but no differences were found between normoxia and hypoxia or between young and old animals. The endogenous dopaminergic inhibitory tonus is slightly reduced. However, both the ventilation decrease caused by exogenous dopamine and the increase mediated by A(2A)-adenosine receptors are not impaired in aged animals. The bradycardia induced by adenosine is attenuated in old rats. The CB's peripheral control of ventilation is preserved during aging. Concerns have also arisen regarding the clinical usage of adenosine to revert supraventricular tachycardia and the use of dopamine in critical care situations involving elderly people.

Striatal A2A adenosine receptor antagonism differentially modifies striatal glutamate outflow in vivo in young and aged rats

The effect of the adenosine A2A receptor antagonist SCH 58261 on glutamate release was investigated in the striatum of young and old rats by microdialysis experiments. SCH 58261 (50 nM) significantly decreased the spontaneous and K+-evoked glutamate outflow in young rats. In aged rats, spontaneous glutamate outflow was significantly reduced in comparison to young rats and SCH 58261 significantly increased spontaneous and K+-evoked glutamate outflow. It is suggested that the opposite effects of the A2A antagonist on glutamate outflow in young and aged rats can be respectively attributed to blockade of striatal A2A adenosine receptors located on glutamatergic terminals and on the striatal indirect output pathway.

Adenosine A1 and A2A receptor antagonists stimulate motor activity: evidence for an increased effectiveness in aged rats

The motor effects of selective adenosine A1 and A2A receptor antagonists were tested in young (2 months) and aged (24 months) Wistar rats. In young rats, both the selective A2A receptor antagonist 5-amino-7-(2-phenylethyl)-2-2(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazo++ + lo[1,5-c]pyrimidine (SCH 58261, minimal effective dose 2 mg/kg intraperitoneally (i.p.)) and the selective A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT, minimal effective dose 1.2 mg/kg i.p.) stimulated motor activity. In old rats, both compounds induced significant motor activation starting from doses lower than those required in young animals. Specifically, the minimal effective doses of SCH 58261 and CPT in aged rats were 1 and 0.6 mg/kg i.p, respectively. The results indicate that both adenosine A1 and A2A receptors play a functional role in the control of motor activity, and, therefore, the blockade of both receptor subtypes is involved in the motor stimulating properties of methylxanthines. Also the evidence indicates, for the first time, that in aged animals the motor inhibitory adenosinergic tone seems to be increased with respect to young animals.

Age-related alteration of the adenosine/dopamine balance in the rat striatum

An antagonistic interaction between adenosine A2A- and dopamine D2-receptors has been described. Radioligand binding experiments showed a predominant reduction in the number of D2 vs. A2A-receptors in the striatum of aged compared to young rats. The A2A-receptor-mediated antagonistic modulation of D2-receptor binding remained unchanged in aged animals. In striatal homogenates a significant increase in adenosine and no change in dopamine content was found in aged vs. young rats. These results reveal the existence of an age-dependent imbalance of adenosine vs. dopamine in favor of adenosine, which involves both presynaptic and postsynaptic mechanisms.

Mice deficient in G(olf) are anosmic

We have used gene targeting to examine the role of the G alpha subunit, G(olf), in olfactory signal transduction. Mice homozygous for a null mutation in G(olf) show a striking reduction in the electrophysiological response of primary olfactory sensory neurons to a wide variety of odors. Despite this profound diminution in response to odors, the topographic map of primary sensory projections to the olfactory bulb remains unaltered in G(olf) mutants. Greater than 75% of the G(olf) mutant mice are unable to nurse and die within 2 days after birth. Rare surviving homozygotes mate and are fertile, but mutant females exhibit inadequate maternal behaviors. Surviving homozygous mutant mice also exhibit hyperactive behaviors. These behavioral phenotypes, taken together with the patterns of G(olf) expression, suggest that G(olf) is required for olfactory signal transduction and may also function as an essential signaling molecule more centrally in the brain.

Effect of aging on dopaminergic receptors and uptake sites in the rat brain studied by receptor autoradiography

We studied the age-related alterations of dopaminergic receptors in the brain of Fisher 344 rats with various age (3 weeks and 6, 12, 18 and 24 months) using in vitro receptor autoradiography. [3H]SCH 23390, [3H]spiperone and [3H]nemonapride, and [3H]mazindol were used to label dopamine D1 receptors, dopamine D2 receptors and dopamine uptake sites, respectively. In immature rats (3 weeks old), [3H]SCH 23390 binding showed a significant increase in most brain regions compared to adult animals (6 months old), whereas [3H]spiperone and [3H]nemonapride bindings showed no significant alteration in any brain areas. In contrast, [3H]mazindol binding showed a significant decline in most brain regions. On the other hand, the age-related alterations in [3H]SCH 23390 binding were not observed in any brain regions. [3H]Spiperone and [3H]nemonapride bindings also showed no significant alteration in the brain during aging, except for a transient alteration in [3H]spiperone binding in the nucleus accumbens and hippocampus of 12 months old rats. However, [3H]mazindol binding showed a significant reduction in most brain areas of 12 months old rats. Thereafter, the age-related reduction in [3H]mazindol binding was observed in most brain regions of 18 and 24 months old rats. The results demonstrate that dopamine uptake sites are more susceptible to the aging process than both dopamine D1 and D2 receptors. Furthermore, our results suggest that dopaminergic receptors and dopamine uptake sites may develop with different patterns and speeds after birth. Our studies may provide valuable information concerning the effect of aging on dopaminergic systems.

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

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

Age-related loss of mRNA encoding adenosine A2 receptor in the rat striatum

The distributions of adenosine A2 receptor mRNA and adenosine A2 binding sites were studied by in situ hybridization histochemistry and receptor autoradiography in young (3 months) and old (24 months) rat striatum and compared to that of enkephalin mRNA. In the old striatum, the levels of adenosine A2 receptor mRNA and adenosine A2 binding sites were reduced by 32% and 20%, respectively. This decrease is similar to the 24% decrease of enkephalin mRNA and to the previously reported 20% neuronal loss. This indicated that, in contrast to the dopamine D2 receptor, the reduction of adenosine A2 receptors in the rat striatum during aging is mainly due to the neuronal loss.

Blunting of the neurotensin mRNA response to haloperidol in the striatum of aging rats: possible relationship to decline in dopamine D2 receptor expression

Neuroleptic drugs such as haloperidol (H) induce a rapid increase in neurotensin/neuromedin N (NT/N) gene expression in the dorsolateral striatum (DLSt) and nucleus accumbens (NA) in young adult rats. This effect may be mediated by post-receptor effectors that are activated by dopamine D2 receptor antagonism. The regional pattern of induction of neurotensin gene expression correlates with the side effect profile of particular neuroleptics. As motor side effects of H differ in aged animals, we hypothesized that the regional expression of the neurotensin gene may differ between young and old animals. We administered H or saline acutely to 3, 14, and 25 month-old Fischer 344 rats, followed by in situ hybridization and quantitative autoradiography for NT/N mRNA. There was a significant age effect on the H-induced NT/N mRNA response in the DLSt, but not the NA, of older animals. In addition to the blunted NT/N mRNA response, significant decreases in D2 receptor mRNA were observed in the lateral striatum of another group of young, middle-aged, and aged rats. Age-related blunting of the NT/N mRNA response to H in the DLSt may be due in part to a decrease in D2 receptors in this structure.

Decrease in human striatal dopamine D2 receptor density with age: a PET study with [11C]raclopride

The effect of age on human striatal dopamine D2 receptors was investigated with positron emission tomography (PET) using [11C]raclopride as a radioligand. Twenty-one healthy volunteers aged from 20 to 81 years were studied. An equilibrium method was applied and two separate PET scans with different specific activities of [11C]raclopride were performed. The maximal number of receptors (Bmax) and their dissociation constant (Kd) were calculated using Scatchard analysis. There was an age-dependent decline in the Bmax (r = -0.49; p = 0.02) of striatal D2 receptors while the Kd remained unchanged. The results show that there is an age-related loss of striatal D2 receptors, which, together with other changes in the brain nigrostriatal dopaminergic system, may contribute to extrapyramidal symptoms associated with aging.

Expression of the Proneurotensin Gene in the Rat Brain and Its Regulation by Antipsychotic Drugs

In this chapter, we have presented evidence for several potential levels of interaction of NT/N gene products with dopaminergic systems of the brain. We have focused on one manifestation of this interaction related to the effects of antipsychotic drugs on expression of the NT/N gene in two anatomically discrete populations of neurons. It appears that certain antipsychotic drugs can dramatically increase expression of this gene in the dorsolateral striatum by blocking dopamine D2 receptors, perhaps by increasing expression of the gene encoding the transcriptional regulator fos. In addition, a second group of NT cells in the shell region of the nucleus accumbens also respond to these drugs by increasing NT/N gene expression. Several other peptides have been suggested to respond to treatment with antipsychotic drugs. However, there are some important differences with respect to their effects on the NT cells we have studied. The most important of these is the differential responsiveness of the DLSt and nucleus accumbens NT neurons to typical and atypical antipsychotics. We showed that all antipsychotic drugs tested increased NT/N mRNA gene expression in the accumbens, a region thought to be involved in dopaminergic disturbances underlying psychosis. However, only the typical neuroleptics that have a high propensity to induce acute extrapyramidal motor side effects influenced NT/N gene expression in the dorsolateral striatum, a structure importantly involved in regulation of motor functions. We hypothesize, therefore, that NT/N-expressing neuronal systems in the nucleus accumbens may mediate some or all of the antipsychotic effects, whereas those in the dorsolateral striatum may be involved in motor effects of neuroleptic drugs. Thus, examination of the effects of these drugs on these neuronal populations will not only clarify their mechanism of action, but in addition may provide a useful "screening" assay for new drugs with enhanced antipsychotic activity, but reduced propensity to induce the debilitating extrapyramidal side effects that are a major cause of patient noncompliance. Future studies will focus on the effects of antipsychotic drugs on NT neurons in clinically relevant models of chronic administration, and on the molecular events involved in their effects on expression of the NT/N gene in the brain.

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.

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.