Altered dopamine receptor mediated signal transmission in the striatum of aged rats

Age-related alterations in the expression of genes and synaptic plasticity associated with nitric oxide signaling in the mouse dorsal striatum

Age-related alterations in the expression of genes and corticostriatal synaptic plasticity were studied in the dorsal striatum of mice of four age groups from young (2-3 months old) to old (18-24 months of age) animals. A significant decrease in transcripts encoding neuronal nitric oxide (NO) synthase and receptors involved in its activation (NR1 subunit of the glutamate NMDA receptor and D1 dopamine receptor) was found in the striatum of old mice using gene array and real-time RT-PCR analysis. The old striatum showed also a significantly higher number of GFAP-expressing astrocytes and an increased expression of astroglial, inflammatory, and oxidative stress markers. Field potential recordings from striatal slices revealed age-related alterations in the magnitude and dynamics of electrically induced long-term depression (LTD) and significant enhancement of electrically induced long-term potentiation in the middle-aged striatum (6-7 and 12-13 months of age). Corticostriatal NO-dependent LTD induced by pharmacological activation of group I metabotropic glutamate receptors underwent significant reduction with aging and could be restored by inhibition of cGMP hydrolysis indicating that its age-related deficit is caused by an altered NO-cGMP signaling cascade. It is suggested that age-related alterations in corticostriatal synaptic plasticity may result from functional alterations in receptor-activated signaling cascades associated with increasing neuroinflammation and a prooxidant state.

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.

Functional recovery of the striatal cholinergic system in aged rats by adenoviral vector-mediated gene transfer of dopamine D2 receptor

We previously reported that the striatal dopamine-acetylcholine (ACh) interaction was affected by the aging process, possibly via a decrease of the striatal D(2)R expression. In the current study, the ACh responses to the infusion of 0.1microM of the D(2)R agonist quinpirole were measured with microdialysis techniques after adenoviral vector-mediated gene transfer of D(2)R into the striatum of 25-month-old rats. The D(2)R gene-transferred rats showed significantly stronger responses of the striatal cholinergic neurons to the infusion of the D(2)R agonist than did control vector-transferred rats. The current study suggests that age-associated functional decline with decreased gene expression of the receptor may be restored by intervention.

Olfactory loss in aging

Olfactory loss is a common age-related complaint that may be caused by changes in the anatomy of the structures required for olfaction (for example, loss of olfactory receptor cells) or in the environment surrounding the receptor cell (for example, altered nasal mucus composition). However, aging, as well as age-related diseases and medications, may also alter the distribution, density, or function of specific receptor proteins, ion channels, or signaling molecules that affect the ability of neural elements throughout the olfactory pathway to signal and process odorant information. Although a great deal has been learned about the prevalence and nature of age-related olfactory loss, we are just beginning to explore avenues to prevent or alleviate this sensory deficit. Some studies suggest that, rather than being a necessary outcome of aging, age-associated factors such as chronic diseases, medications, and dental and sinus problems are the primary culprits in causing olfactory impairment. This idea suggests optimism in that, as we address these other age-related health issues, the prevalence of olfactory loss will lessen as well.

Increased sensitization to morphine-induced oral stereotypy in aged rats

Sensitization develops to the stereotypic biting behavior that appears with the repeated administration of high dose morphine to rats. Because there is evidence that this behavior is dopamine-mediated and that there are age-related changes in dopamine systems, we compared the development and expression of morphine-induced biting behavior in aged (24 months) and young rats (5 months). Animals were treated with four sensitizing 10 mg/kg doses of morphine or saline, followed by three weekly challenges with 4 mg/kg doses of morphine or saline. By the fourth sensitizing morphine dose and after the administration of each low dose challenge, the biting time was significantly greater for aged than for young morphine pre-treated rats. After the first weekly low dose challenge, the aged but not young animals expressed more biting than when they did after the last 10 mg/kg dose. These results indicate that sensitization to morphine-induced oral stereotypy is significantly greater in aged as compared to young rats. Age-related enhanced sensitivity to morphine-induced oral stereotypy might be related to age-induced increases in vulnerability to opioid-induced insults to the basal ganglia, and may be a model for certain diseases of this pathway.

Longitudinal follow-up study of adenoviral vector-mediated gene transfer of dopamine d2 receptors in the striatum in young, middle-aged, and aged rats: a positron emission tomography study

Overexpression of dopamine D(2) receptors by adenoviral vector-mediated gene transfer in the rat striatum was evaluated by positron emission tomography in vivo and by ex vivo autoradiography in 5-, 13-, and 24-month-old Fischer 344 rats. Each rat had hemilateral gene transfer of D(2) receptors mediated by adenoviral vectors (AdCMV.DopD(2)R) in the striatum with contralateral striatal injection of control vectors (AdCMV.LacZ). At day 2 or 3 after vector injection positron emission tomography or ex vivo autoradiography was performed. The binding potential of a radiolabeled D(2) receptors ligand, [11C]raclopride, was significantly higher in the D(2) receptors gene-transferred striatum than the control side in each age group at a similar degree. The binding potential in the AdCMV.DopD(2)R-injected striatum of 24-month-old rats was similar to that in the AdCMV.LacZ-injected striatum of 5-month-old rats (0.99+/-0.14 versus 0.91+/-0.08). A significant age-associated decrease of the binding potential of [11C]raclopride was found in the control vector-injected side, and a significant increase of the binding potential in the adenoviral vector-injected side in all three age groups, suggesting no aging effect on the overexpression of D(2) receptors. A group of rats underwent follow-up assessment by positron emission tomography. The overexpression of D(2) receptors decreased with time in all three groups; however, the decrease rate of the D(2) receptors expression was significantly smaller in the 24-month-old group than in the 5-month-old group. We confirmed that the adenoviral vector-mediated gene transfer of D(2) receptors compensated the decreased density of striatal D(2) receptors in the 24-month-old rats up to the level in the control striatum of 5-month-old rats, and the decrease rate of the overexpression was significantly smaller in aged rats.

The age-associated changes of dopamine-acetylcholine interaction in the striatum

The age-associated decline of dopamine D2 receptor (D2R) densities is one of the notable features of aging in mammalian brains. The concept of a functional dopamine (DA)-acetylcholine (ACh) interaction in the striatum has been strongly supported. However, the effects of this decline of D2R on the striatal DA-ACh interaction still remain to be elucidated. In the current study we examined the effects of different concentrations of a D2R agonist, quinpirole, on the striatal ACh release in three groups of rats at different ages (6-, 15-, 25-months-old) by in vivo microdialysis. The ACh release in the striatum was significantly decreased by quinpirole infusion in all three age groups. The old rats showed a significantly smaller decrease of the ACh release by quinpirole than that of the middle-aged rats at the dose of 0.1 microM and that of the young rats at the dose of 1 microM. The current study suggested that the striatal DA-ACh interaction was affected by the aging process.

Age-related alterations of nitric oxide production in the brains of seizure-susceptible EL mice

We evaluated age-related changes in nitric oxide (NO) production in the brains of EL mice, a strain highly susceptible to seizures. A group of EL(s) mice were tossed up weekly to induce convulsive seizures, while in a nonstimulated EL(ns) group induction of convulsive seizures was avoided. Brain levels of nitrite plus nitrate (NOx) in EL(ns) mice were significantly higher than in nonstimulated mice at 10 days, and also higher than levels at 15 and 50 weeks in either EL(s) or EL(ns) mice. A significantly higher number of NO-producing cells were demonstrated in the hippocampus and parietal cortex by staining for nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase in EL(s) mice at the ages of 15 and 50 weeks than in EL(ns) mice at the age of 6 weeks. In EL(ns) mice, significantly fewer neurons showed NADPH-diaphorase staining in the hippocampus, striatum and parietal cortex at the age of 50 weeks than at 6 weeks. The present results suggest that whole-brain NOx levels in EL(ns) and EL(s) mice and numbers of NADPH-diaphorase-positive neurons in EL(ns) mice decreased with aging, while increasing of numbers of such neurons in EL(s) mice were assumed to develop in compensation for reduction in whole-brain NOx levels.

Long-lasting effects of chronic mu-opioid intake on the signal transmission via dopamine D1 receptors in the limbic forebrain of drug deprived rats

Rats orally self-administered the potent and selective mu-opioid receptor agonist etonitazene for 8 weeks (free choice between three opioid solutions and water resulting in low drug intake, or forced intake of a single drug solution resulting in high opioid consumption). The signal transmission in membranes of the limbic forebrain (nucleus accumbens and olfactory tubercle) was studied during acute withdrawal (2 days of abstinence) and after 6 weeks of drug deprivation. Binding experiments with the dopamine (DA) D1 receptor antagonist [3H]SCH23390 revealed in the high consuming rats an increased binding density (Bmax) by 19% during withdrawal and a decreased Bmax by 17% after long-term abstinence compared with drug-naive controls (each P < 0.05). The addition of 500 nM DA reduced the [3H]SCH23390 binding affinity (Kd increased by 60-105%) and density (by 15-23%) in each of the five groups (P < 0.001). During acute withdrawal, the portion of Bmax inhibited by DA increased by 83% in the high consuming rats vs. the controls (P < 0.05). Full concentration-response curves of adenylyl cyclase (AC) stimulation by the DA D1 receptor agonist dihydrexidine and of inhibition of forskolin stimulated AC activity by the GTP analogue guanosine-5'-O-(3-thio)triphosphate (GTPgammaS) were performed: the former revealed a reduced maximum efficacy (Emax decreased by 23-37%), P < 0.001), the latter a reduced effective concentration (EC50 decreased by 60-103%, P < 0.05), in each etonitazene-experienced group vs. the controls.

Immunocytochemical study on the distribution of NOS-immunoreactive neurons in the cerebral cortex of aged rats

Nitric oxide (NO) involvement has been demonstrated in mechanisms of synaptic plasticity, particularly in hippocampal long-term potentiation, a mechanism that underlies certain forms of learning and memory. Several findings suggest that NO production may be decreased in the aged rats. Changes in the nNOS-containing neurons with ageing were demonstrated by immunocytochemistry. NOS-immunoreactive (IR) cells in aged rats were present in all cortical areas and the hippocampus, and the pattern of distribution was similar to that of the control group. The number of NOS-IR cells in the cerebral cortex was significantly decreased in the aged rats, but the extent of changes was variable in each area, and ranged from mild decrease (< 30%) to severe decrease (> 50%). Severely decreased areas were the cingulate cortex, parietal cortex area 1, temporal cortex area 1, 2, 3, medial part of occipital cortex area 2, monocular and binocular part of occipital cortex area 1, entorhinal cortex, hippocampus proper, dentate gyrus and subiculum. Morphologically, the number of dendritic branches seemed to be decreased in aged group and the length of dendrites of NOS-IR neurons showed a tendency to shorten. These results indicate the involvement of neuronal system containing NOS in the ageing brain, and provide the first morphological evidence for the loss of NOS neurons in the cerebral cortex of the aged rats by immunocytochemistry.

Neonatal ACTH administration elicits long-term changes in forebrain monoamine innervation. Subsequent disruptions in hypothalamic-pituitary-adrenal and gonadal function

The findings from this study demonstrated that the manipulation of the HPA system resulting from ACTH administration during neonatal development produces long-term, differential effects, not only on adrenocortical activity, but also on the activity and integrity of the forebrain monoamine systems. Increased concentrations of the monoamines within the forebrain regions studied at days 7 and 15, suggest a hastened maturation of these neural systems in animals neonatally treated with ACTH. The observed neurochemical alterations in these animals at one year are suggestive of an accelerated aging in the monoamine systems. A further consequence of these disturbances during development is an altered functioning of the HPG axis, as demonstrated by a delayed onset of puberty as previously reported, as well as significantly decreased proestrus plasma estradiol. Although deficits in sexual behavior also existed, it seems probable that these behavioral changes are a manifestation of altered neural systems regulating the ability to cope with a novel stimulus or situation, rather than a disruption of the "feminization" of the brain during sexual differentiation. This is in contrast to the male rat which exhibits permanent deficits in male typical sexual behavior following developmental ACTH treatment. The clinical relevance of these findings may be extensive. Perinatal exposure to events or agents that markedly increase ACTH and the corticosteroids may cause significant immediate and long-term changes in central monoamine functioning. These changes may constitute some of the most deleterious effects of stress exposure in infants and children. The alterations may be especially devastating in individuals with predispositions to stress-sensitive disorders such as anxiety, depression, and Tourette's syndrome. Finally, the use of ACTH in the treatment of infantile spasms may need to be reassessed in light of the possible long-term effects of ACTH on central monoamine functioning.

Role of nitric oxide in the effect of aging on spatial memory in rats

Since there is evidence that nitric oxide (NO) plays an important role as a diffusible messenger in learning and memory, we examined the role played by NO in the effect of aging on spatial memory in rats. The performance of aged rats (30 months old) in a radial-arm maze task was significantly impaired compared to that of adult rats (3 months old). The number of neurons containing NADPH-diaphorase (NADPH-d) reactivity in the cerebral cortex and striatum of aged rats was significantly less than that in the adult rats. The daily administration of NG-nitro-L-arginine methyl ester (L-NAME; 10-60 mg/kg, i.p.) resulted in a dose-dependent impairment of acquisition in the radial-arm maze task, while it failed to affect previously acquired performance, i.e., retention, in the adult rats. The content of 5-hydroxyindoleacetic acid in the hippocampus and of 3,4-dihydroxyphenylacetic acid in the striatum was significantly decreased and increased, respectively, in the L-NAME (60 mg/kg/day)-treated adult rats compared with that in controls. These findings demonstrate that NO production in the brain may be decreased in aged rats, suggesting that this alteration may be involved in memory processes, especially in the acquisition, but not in the retention, of spatial learning in rats, and further, that endogenous NO may be involved in the regulation of dopamine and 5-hydroxytryptamine metabolism.

DA1 and DA2 receptor regulation in the striatum of young and old rats after peripheral vestibular lesion

Anatomical, lesion and functional studies have indicated that the mesostriatal dopaminergic (DAergic) system may serve as supravestibular center in posture and locomotion control. Nevertheless, no data are available on the involvement of DAergic systems during vestibular compensation. This study was designed for the analysis of DA1 and DA2 receptors in the striatum by means of quantitative receptor autoradiography 28 days after unilateral or bilateral lesion of the labyrinth in 3-month-old rats. Considering the severe decline of DA content and receptors in striatum and the difference in behavioral recovery after vestibular lesions in old age, we also analyzed 24-month-old, lesioned and unlesioned rats. In young rats, hemilabyrinthectomy caused a bilateral increase (20-30%) of DA1 receptors and a two-fold increase of DA2 receptors. In old-rats, we observed a similar modification of DA2 receptors, and a 50% increase in DA1 receptors. Bilabyrinthectomy did not modify DA1 receptor density and decreased DA2 receptor density in young animals, whereas it produced an increase in both DA1 and DA2 in old rats. This study provides evidence for the involvement of the DAergic system during vestibular compensation. Our results also indicate great biochemical plasticity of the remaining DA receptors in the striatum of old rats.

Reduction in the number of NADPH-diaphorase-positive cells in the cerebral cortex and striatum in aged rats

Nitric oxide (NO) plays an important role as a diffusible messenger in learning and memory. To determine whether changes in NO production in the brain may be involved in aging-associated brain dysfunction, we measured the performance of aged rats in a radial arm maze task, and carried out histochemical examination of the changes in NADPH diaphorase (NADPH-d)-containing neurons in the brains of aged rats. The performance of aged rats (30 months old) in a radial arm maze task was significantly impaired compared to the performance of young rats (3 months old). The number of neurons containing NADPH-d reactivity in the cerebral cortex and striatum of aged rats was significantly reduced, by approximately 50 and 30 percent, respectively, compared to that in young rats. NO synthase activity in discrete brain regions of aged rats, i.e., in the cerebral cortex, striatum and hippocampus was not different from that in young rats, although the activity in the cerebellum of aged rats was significantly lower than that in young rats. These results suggest that the reduction in the number of NADPH-d-positive cells in the brains of aged rats may be involved in aging-associated learning impairment in rats.

Effects of chronic alcohol consumption and aging on dopamine D2 receptors in Fischer 344 rats

Aging and chronic alcohol consumption are each accompanied by significant changes in dopamine and dopamine receptors. This study extended previous work by investigating the combined effects of chronic alcoholism and aging on total dopamine D2 receptors in brain areas associated with the nigrostriatal and mesocorticolimbic systems. In addition, the effects of chronic alcohol consumption and aging on the high-affinity state of D2 receptors and their conversion to the low-affinity form is included. Quantitative autoradiography was used to assess [3H]spiperone-labeled D2 receptors in tissue sections from 5- to 14- and 24-month Fischer 344 rats that were pair-fed a control or 6.6% (v/v) ethanol-containing liquid diet for 6 weeks. In addition, D2 receptors were determined in rats given the control liquid diet ad libitum. The results of these experiments demonstrated age-related changes in the nigrostriatal system. There was an age-related loss of total dopamine D2 receptors in the rostral and caudal striatum (approximately 25% decrease in Bmax). This decline in D2 receptors may be associated with changes in motor function. Despite the age-related decline in D2 receptors, there were no significant differences in the proportion of striatal receptors in the high-affinity form or in their conversion to the low-affinity state. Both aging and chronic alcohol consumption produced significant changes in the concentration of D2 receptors in brain areas associated with the mesocorticolimbic system. That is, the specific binding of [3H]spiperone was decreased in the frontal cortex of aged rats. In addition, chronic alcoholism was associated with a significant increase (approximately 20%) in the Bmax for D2 receptors in the nucleus accumbens. Nonetheless, neither age nor chronic alcohol consumption altered the proportion of high-affinity D2 receptors in the nucleus accumbens or their conversion to the lower affinity state. The observed changes in D2 receptors in the frontal cortex and nucleus accumbens are of interest because of the involvement of the mesocorticolimbic dopamine areas in the rewarding properties of alcohol and other drugs of abuse. Although aging and chronic alcoholism both produced significant changes in dopamine D2 receptor concentrations, alcohol did not accentuate the age-related loss of D2 receptors. We cannot eliminate the possibility that a more prolonged exposure of higher ethanol dose may potentiate age-related changes in the dopaminergic system.

Tubulin stimulates adenylyl cyclase activity in rat striatal membranes via transfer of guanine nucleotide to Gs protein

Previous studies of rat cerebral cortex and rat C6 glioma cells have demonstrated that dimeric tubulin is capable of activating the G proteins Gs and Gil via transfer of guanine nucleotide from tubulin to Gs alpha and Gil alpha. To provide further information regarding cytoskeletal modulation of adenylyl cyclase, the present study examined effects of tubulin on the activation of the enzyme in rat striatal membranes. Tubulin, prepared from rat brain by polymerization with the hydrolysis-resistant GTP analog 5'-guanylylimidodiphosphate (GppNHp) caused significant activation of adenylyl cyclase by approximately 130%. Furthermore, tubulin-GppNHp activated SKF 38393-sensitive adenylyl cyclase and potentiated forskolin-stimulated activity of the enzyme. When tubulin, polymerized with the hydrolysis-resistant photoaffinity GTP analog [32p]p3 (4-azidoanilido)-p1-5'-GTP ([32P]AAGTP), was incubated with striatal membranes, AAGTP was transferred from tubulin to Gs alpha as well as Gi alpha with the extents of nucleotide transfers being 7.6 +/- 0.8% and 17.8 +/- 1.4% of AAGTP originally bound to tubulin, respectively. These results indicate that, in rat striatum, the tubulin dimer participates in the stimulatory regulation of adenylyl cyclase by transferring guanine nucleotide to Gs alpha, supporting the hypothesis that tubulin contributes to the regulation of neuronal signal transduction.

Effects of chronic alcohol consumption and aging on dopamine D1 receptors in Fischer 344 rats

The present study examined the hypothesis that chronic alcoholism augments the age-related loss of dopamine D1 receptors. This hypothesis was investigated because previous studies reported that both aging and chronic alcoholism produce significant changes in dopaminergic systems, and because chronic alcoholism potentiates some age-related CNS losses. In addition, this study investigated the effects of aging on D1 receptors in animals 1 and 7 days after withdrawal from chronic ethanol. Quantitative autoradiography was used to measure [3H]SCH 23390 binding to D1 receptors in brain areas associated with both the nigrostriatal and mesocorticolimbic dopamine systems. Receptors were assessed in 5-, 14-, and 24-month-old male Fischer 344 rats that were pair-fed a control or 6.6% (v/v) ethanol-containing liquid diet for 6 weeks. The results of these studies demonstrated that aging is associated with a significant decline in D1 receptors in the rostral and caudal striatum, and substantia nigra of both control and ethanol-fed rats. These receptor changes in the nigrostriatal system may be associated with motor abnormalities. In addition, there was an age-related decline in D1 receptors in two brain areas of the mesocorticolimbic system: the nucleus accumbens and frontal cortex. The latter findings may be important because of the involvement of this system with the rewarding properties of ethanol and other drugs of abuse. There were no age-related differences in the response of D1 receptors to ethanol withdrawal in the caudal and rostral striatum, substantia nigra, and nucleus accumbens.(ABSTRACT TRUNCATED AT 250 WORDS)

Restriction and functional changes of dopamine release in rat striatum from young adult and old rats

In order to investigate the age-related changes in dopaminergic activity in rats, we have utilized the K(+)- and veratridine-stimulated [14C]dopamine release from striatum in vitro as a functional index of responsiveness to these stimuli in aging. We found that the K(+)-stimulated dopamine release from old (12 months) rats decreased by more than 50% compared to that from young adult rats (3 months). Reserpine (5 mg/kg) led to a pronounced decrease of the K(+)-stimulated dopamine release of young adult as well as old rats. Whereas ouabain (10 mumol/l) decreased the K(+)-stimulated dopamine release from young adult rats, in old rats the K(+)-induced dopamine release was increased up to 250%. However, in old rats which were reserpine pretreated, ouabain was unable to stimulate the K(+)-induced dopamine release. In contrast, the veratridine-stimulated dopamine release of old rats was increased up to 200% compared to that of young adult rats and was highly sensitive to reserpine pretreatment but not to ouabain. However, reserpine did not alter this veratridine-stimulated dopamine release from young adult rats. The present data indicate that the age-related reduction of exocytosis-related, Ca(2+)-dependent release mechanisms (K+) are probably compensated via an increase in Ca(2+)-independent, uptake carrier-mediated release processes (veratridine).

LSD and structural analogs: pharmacological evaluation at D1 dopamine receptors

The hallucinogenic effects of lysergic acid diethylamide (LSD) have been attributed primarily to actions at serotonin receptors. A number of studies conducted in the 1970s indicated that LSD also has activity at dopamine (DA) receptors. These latter studies are difficult to interpret, however, because they were completed before the recognition of two pharmacologically distinct DA receptor subtypes, D1 and D2. The availability of subtype-selective ligands (e.g., the D1 antagonist SCH23390) and clonal cell lines expressing a homogeneous receptor population now permits an assessment of the contributions of DA receptor subtypes to the DA-mediated effects of LSD. The present study investigated the binding and functional properties of LSD and several lysergamide and analogs at dopamine D1 and D2 receptors. Several of these compounds have been reported previously to bind with high affinity to serotonin 5HT2 (i.e., 3H-ketanserin) sites in the rat frontal cortex (K0.5 5-30 nM). All tested compounds also competed for both D1-like (3H-SCH 23390) and D2-like (3H-spiperone plus unlabeled ketanserin) DA receptors in rat striatum, with profiles indicative of agonists (nH < 1.0). The affinity of LSD and analogs for D2 like receptors was similar to their affinity for 5HT2 sites. The affinity for D1 like receptors was slightly lower (2- to 3-fold), although LSD and several analogs bound to D1 receptors with affinity similar to the prototypical D1 partial agonist SKF38393 (K0.5 ca. 25 nM). A second series of experiments tested the binding and functional properties of LSD and selected analogs in C-6 glioma cells expressing the rhesus macaque D1A receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in muscarinic cholinergic, PCP, GABAA, D1, and 5-HT2A receptor binding, but not in benzodiazepine receptor binding in the brains of aged rats

We used in vitro quantitative autoradiography to investigate changes in neurotransmitter receptor binding, including muscarinic cholinergic, PCP, GABAA, benzodiazepine, D1 and 5-HT2A receptor, in the brains of aged rats, compared with such binding in young rats. Scatchard analysis revealed that the maximal number of binding sites for [3H]quinuclidinyl benzilate (QNB) in the caudate/putamen and accumbens was significantly decreased in aged rats compared with young rats, while its affinity remained unchanged. The specific binding of [3H]N-(1-[2-thienyl]cyclohexyl)3,4-piperidine (TCP) for the ion channels coupled with N-methyl-D-aspartate receptors in the caudate/putamen and hippocampus was significantly decreased in aged rats compared with young rats. The [3H]muscimol binding in aged rats was decreased in all brain regions examined compared with that in young rats, whereas [3H]flunitrazepam binding was not changed in any brain regions. The [3H]SCH23390 binding for dopamine D1 receptors was significantly increased in the parietal cortex, but decreased in the caudate/putamen and accumbens of aged rats compared with that in young rats. The [3H]ketanserin binding for 5-HT2A receptors in the cortex and accumbens was significantly decreased in aged rats compared with young rats. These results suggest that uneven changes in receptors for various neurotransmitters throughout the brain may be responsible for the decline of brain function in aged rats.

Alterations of inositol phosphate turnover in striatum of aged rats

Age-related inositol phosphate turnover in the rat central nervous system was investigated. Higher phospholipase-C activity and drastically higher (almost 2.5-fold) inositol 1,4,5-trisphosphate concentration in the corpus striatum (caudate-putamen) of extremely old (approximately 40 months) female Wistar rats in comparison to the young adult (approximately 3.5 months) rats were observed. Dopamine seems to slightly inhibit total inositol phosphate formation and this effect was antagonized by (-)-sulpiride.

Age-related alteration in signal transduction: involvement of the cAMP cascade

In the present study, we have investigated the involvement of the cAMP signal transduction pathways in young and aged rats. A significantly higher endogenous adenosine 3':5'-cyclic monophosphate (cAMP) level and a significant decline of the adenylate cyclase [AC, ATP pyrophosphate-lyase (cyclizing), EC.4.6.1.1.] activity were observed in striatal tissue from young rats (3 months) in comparison to aged rats (approximately 40 months). In the nucleus accumbens (NA), no age-dependent changes in the cAMP concentration and in the AC basal activity were found. To address the question, whether the interactions of guanine nucleotide-binding protein (G-protein) subunits (G alpha s and Gi) with AC have changed in the aging process, various pharmacological agents that modulate the AC activity (e.g., beta, tau-imidoguanine 5'-triphosphate (GppNHp), sodium fluoride (NaF), forskolin (FSK), and the combinations of GppNHp plus FSK, NaF plus FSK, and NaF plus ethanol (ETOH)) were applied. In addition, a [3H]FSK binding test was carried out. In striatal and NA tissue, the stimulation of the AC activity by FSK was inhibited by GppNHp (via Gi-protein) and was superadditive by the combination of FSK and NaF (via Gs-protein). The absolute AC activity upon stimulation by all agents used was significantly lower in the aged striatum compared to young striatum. In the NA, however, the AC activity showed an age-dependent reduction only upon FSK and upon FSK plus GppNHp stimulation. There was no difference in the specific [3H]FSK binding to the G alpha s protein-coupled catalytic subunit of the AC between young and aged animals both in the striatum and NA.(ABSTRACT TRUNCATED AT 250 WORDS)