Influence of age and dietary restriction on norepinephrine uptake into cardiac synaptosomes

Alteration of the cardiac sympathetic innervation is modulated by duration of diabetes in female rats

To evaluate the sympathetic innervation of the female diabetic heart, resting heart rate and sympathetic tone were assessed in vivo, and effect of tyramine on spontaneous beating rate, norepinephrine atrial concentrations, uptake, and release were determined in vitro in streptozotocin- (STZ-) treated rats and respective controls aged 3 months to 2 years. Resting bradycardia, decreased sympathetic tone, deceleration of spontaneous beating rate, and slightly declining carrier-mediated, but preserved exocytotic norepinephrine release from the atria were found in younger diabetic rats while the reactivity of the right atria to tyramine was not affected with age and disease duration. Diabetic two-year-old animals displayed symptoms of partial spontaneous recovery including normoglycemia, increased plasma insulin concentrations, fully recovered sympathetic tone, but putative change, in releasable norepinephrine tissue stores. Our data suggested that female diabetic heart exposed to long-lasting diabetic conditions seems to be more resistant to alteration in sympathetic innervation than the male one.

Tetrahydrobiopterin does not affect end-organ responsiveness to norepinephrine-mediated vasoconstriction in aged skin

We have recently demonstrated that tetrahydrobiopterin (BH(4)) augments reflex vasoconstriction (VC) in aged skin. Although this appears to occur through its role in norepinephrine (NE) biosynthesis, the extent with which vascular mechanisms are affected are unknown. We hypothesized that localized BH(4) supplementation would not affect the VC response to exogenous NE when sympathetic nerves were blocked. Two microdialysis fibers were placed in bretylium tosylate pretreated (presynaptically blocks neurotransmitter release from sympathetic adrenergic nerve terminals; iontophoresis, 200 μA for 20 min) 3-cm(2) forearm skin of 10 young (Y) and 10 older (O) subjects for perfusion of 1) Ringer (control) and 2) 5 mM BH(4). While local skin temperature was clamped at 34°C, six concentrations of NE (10(-12), 10(-10), 10(-8), 10(-6), 10(-4), 10(-2) M) were infused at each drug-treated site. Cutaneous vascular conductance (CVC) was calculated (CVC = laser Doppler flux/mean arterial pressure) and normalized to baseline (%ΔCVC(base)). Despite prejunctional adrenergic blockade, NE-mediated VC was blunted in aged skin at each NE dose (10(-12): -12 ± 2 vs. -21 ± 2; 10(-10): -15 ± 2 vs. -27 ± 1; 10(-8): -22 ± 2 vs. -32 ± 2; 10(-6): -27 ± 2 vs. -38 ± 1; 10(-4): -52 ± 3 vs. -66 ± 5; 10(-2): -62 ± 3 vs. -75 ± 4%ΔCVC(base); P < 0.01), and this response was not affected by pretreatment with BH(4) (P > 0.05). Localized BH(4) did not affect end-organ responsiveness to exogenous NE, suggesting that the effects of BH(4) on cutaneous VC are primarily isolated to the NE biosynthetic pathway.

Localized tyrosine or tetrahydrobiopterin supplementation corrects the age-related decline in cutaneous vasoconstriction

The attenuated reflex vasoconstriction in aged skin may be partly mediated by oxidant-induced reduction in functional substrate and cofactor availability for noradrenaline biosynthesis. We hypothesized that localized supplementation of tyrosine and tetrahydrobiopterin (BH(4)) in aged human skin could augment reflex- (whole-body cooling) and pharmacologically (tyramine, which displaces noradrenaline from axon terminals) induced vasoconstriction. Four microdialysis fibres were placed in the forearm skin of 10 young and 10 older subjects for infusion of (1) Ringer solution (control), (2) 0.5 mm L-tyrosine, (3) 5 mm BH(4), and (4) BH(4) + L-tyrosine. Cutaneous vascular conductance (CVC) was calculated (laser Doppler flux/mean arterial pressure) and normalized to baseline (% Delta CVC(base)). Vasoconstriction was attenuated at the control site in the older subjects during both whole-body cooling (young: 39 +/- 3, older: 17 +/- 3% Delta CVC(base); P < 0.01) and tyramine infusion (young: 41 +/- 3, older: 21 +/- 4% Delta CVC(base); P < 0.01). BH(4) (cold, young: 37 +/- 3, older: 36 +/- 3; tyramine, young: 41 +/- 2, older: 36 +/- 3% Delta CVC(base)) and tyrosine (cold, young: 37 +/- 4, older: 34 +/- 4; tyramine, young: 40 +/- 4, older: 45 +/- 4% Delta CVC(base)) both resolved the age-related decrease in cutaneous vasoconstriction, but BH(4) + tyrosine did not further augment vasoconstriction (cold, young: 38 +/- 4, older: 31 +/- 3; tyramine, young: 36 +/- 3, older: 36 +/- 5 Delta %CVC(base)). These data are consistent with the concept that reduced bioavailability of BH(4) and/or tyrosine may impair noradrenaline synthesis and contribute to the attenuated vasoconstrictor response in aged skin.

PET imaging of the effects of age and cocaine on the norepinephrine transporter in the human brain using (S,S)-[(11)C]O-methylreboxetine and HRRT

OBJECTIVES

The role of the norepinephrine transporter (NET) in cocaine dependence has never been demonstrated via in vivo imaging due to the lack of suitable NET radioligands. Here we report our preliminary studies evaluting the NET in individuals with cocaine dependence (COC) in comparison to healthy controls (HC) using (S,S)-[(11)C]methylreboxetine ([(11)C]MRB), the most promising C-11 labeled positron-emission tomography (PET) radioligand for NET developed to date.

METHODS

Twenty two human volunteers (10 COC and 12 HC) underwent dynamic (11)C-MRB-PET acquisition using a High Resolution Research Tomograph (HRRT). Binding potential (BP(ND)) parametric images were computed using the simplified reference tissue model (SRTM2) with occipital cortex as reference region. BP(ND) values were compared between the two groups.

RESULTS

Locus coeruleus (LC), hypothalamus, and pulvinar showed a significant inverse correlation with age among HC (age range = 25-54 years; P = 0.04, 0.009, 0.03 respectively). The BP(ND) was significantly increased in thalamus (27%; P < 0.02) and dorsomedial thalamic nuclei (30%; P < 0.03) in COC as compared to HC. Upon age normalization, the upregulation of NET in COC also reached significance in LC (63%, P < 0.01) and pulvinar (55%, P < 0.02) regions.

CONCLUSION

Our results suggest that (a) brain NET concentration declines with age in HC, and (b) there is a significant upregulation of NET in thalamus and dorsomedial thalamic nucleus in COC as compared to HC. Our results also suggest that the use of [(11)C]MRB and HRRT provides an effective strategy for studying alterations of the NET system in humans.

Local tetrahydrobiopterin administration augments cutaneous vasoconstriction in aged humans

Reflex vasoconstriction (VC) is attenuated in aged skin resulting in greater skin blood flow and heat loss during cold exposure. We hypothesized that adrenergic function is compromised due to depletion of tetrahydrobiopterin (BH(4)), an essential cofactor required for catecholamine synthesis, and therefore local BH(4) supplementation would functionally augment reflex and pharmacologically induced VC elicited by gradual whole-body cooling (skin temperature (T(sk)) = 30.5 degrees C) and tyramine infusion, respectively. Four microdialysis (MD) fibres were placed in the forearm skin of 11 young (Y) and 11 older (O) human subjects for infusion of (1) Ringer solution (control), (2) 5 mM BH(4), (3) 5 mM BH(4) + 10 mM ascorbate, and (4) 5 mM BH(4) + adrenoreceptor blockade (5 mM yohimbine + 1 mM propranolol). Laser Doppler flux (LDF) was measured over each MD site and cutaneous vascular conductance was calculated as CVC = LDF/MAP and expressed as per cent change from baseline (% DeltaCVC(base)). The VC response was lower at the control site in O during cooling (Y: -34 +/- 2% DeltaCVC(base), O: -17 +/- 2% DeltaCVC(base); P < 0.001) and tyramine infusion (Y: - 33 +/- 4% DeltaCVC(base), O: -15 +/- 3% DeltaCVC(base); P < 0.001). BH(4) infusion normalized O to Y values during both cooling (Y: -34 +/- 4% DeltaCVC(base), O: -34 +/- 2% DeltaCVC(base); P < 0.001) and tyramine (Y: -38 +/- 4% DeltaCVC(base), O: -35 +/- 3% DeltaCVC(base); P < 0.001), however, adding adrenoreceptor blockade abolished VC in aged skin indicating that BH(4) acts through adrenergic, not cotransmitter, mechanisms. Local BH(4) supplementation augments reflex and tyramine-induced VC in aged skin, suggesting that reduced BH(4) bioavailability may contribute to attenuated VC during whole-body cooling.

Expressions of cardiac sympathetic norepinephrine transporter and beta1-adrenergic receptor decreased in aged rats

Evidence suggests that the deterioration of communication between the sympathetic nervous system and cardiovascular system always accompanies the aging of human and animals. Cardiac sympathetic norepinephrine (NE) transporter (NET) on presynaptic membrane is a predominant component to eliminate released NE in the synaptic cleft and maintains the sensitivity of the beta-adrenergic receptor (beta-AR). In the present study, we investigated NET and beta1-AR mRNA levels and sympathetic nerve density in cardiac sympathetic ganglion and left ventricular myocardium in 2- and 16-month-old rats with Northern blot analysis and immunohistochemistry. The expression levels of NET mRNA, NET protein and beta1-AR mRNA in the ganglia or myocardia of 16-month-old rats were markedly reduced by 67%, 26%, and 43%, respectively, in comparison with those in 2-month-old rats. Our results also show that aging induces a strong decrease of the catecholaminergic nerve fiber density.

Age-associated changes in mRNA levels of Phox2, norepinephrine transporter and dopamine beta-hydroxylase in the locus coeruleus and adrenal glands of rats

Age-related changes in the gene expression of the transcription factors, Phox2a and 2b, and two marker proteins, norepinephrine transporter (NET) and dopamine beta-hydroxylase (DBH), of noradrenergic neurons were characterized in the locus coeruleus (LC) and adrenal glands using in situ hybridization. Analysis of changes was performed in rats that were 1-23 months of age. Compared to 1-month-old rats, there was a 62% increase of Phox2a messenger RNA (mRNA) in the LC of 3-month-old rats, and a decline of 37% in 23-month-old rats. In contrast, levels of Phox2b mRNA in the LC remained unchanged in 3-month-old rats, but declined to a 30% reduction in 23-month-old rats. Interestingly, mRNA levels of NET in the LC decreased with increasing age to a reduction of 29%, 30% and 43% in 3-, 8- and 23-month-old rats, respectively. Similarly, DBH mRNA in the LC declined with increasing age to a 56% reduction in 23-month-old rats. mRNA levels of Phox2a, Phox2b, NET and DBH in the adrenal medulla of 23-month-old rats were significantly lower than those of 1-month-old rats. Semi-quantitative reverse transcription assays of the same genes yielded data similar to in situ hybridization experiments, with beta-actin mRNA levels being unchanged across the ages. Taken together, these data reveal that reduced Phox2 mRNAs in the LC and adrenal medulla of aging rats are accompanied by a coincidental decline in mRNA levels of NET and DBH and suggest a possible relationship between Phox2 genes and the marker genes in noradrenergic neurons after birth.

Meal size and frequency affect neuronal plasticity and vulnerability to disease: cellular and molecular mechanisms

Although all cells in the body require energy to survive and function properly, excessive calorie intake over long time periods can compromise cell function and promote disorders such as cardiovascular disease, type-2 diabetes and cancers. Accordingly, dietary restriction (DR; either caloric restriction or intermittent fasting, with maintained vitamin and mineral intake) can extend lifespan and can increase disease resistance. Recent studies have shown that DR can have profound effects on brain function and vulnerability to injury and disease. DR can protect neurons against degeneration in animal models of Alzheimer's, Parkinson's and Huntington's diseases and stroke. Moreover, DR can stimulate the production of new neurons from stem cells (neurogenesis) and can enhance synaptic plasticity, which may increase the ability of the brain to resist aging and restore function following injury. Interestingly, increasing the time interval between meals can have beneficial effects on the brain and overall health of mice that are independent of cumulative calorie intake. The beneficial effects of DR, particularly those of intermittent fasting, appear to be the result of a cellular stress response that stimulates the production of proteins that enhance neuronal plasticity and resistance to oxidative and metabolic insults; they include neurotrophic factors such as brain-derived neurotrophic factor (BDNF), protein chaperones such as heat-shock proteins, and mitochondrial uncoupling proteins. Some beneficial effects of DR can be achieved by administering hormones that suppress appetite (leptin and ciliary neurotrophic factor) or by supplementing the diet with 2-deoxy-d-glucose, which may act as a calorie restriction mimetic. The profound influences of the quantity and timing of food intake on neuronal function and vulnerability to disease have revealed novel molecular and cellular mechanisms whereby diet affects the nervous system, and are leading to novel preventative and therapeutic approaches for neurodegenerative disorders.

Age-related changes in adrenergic neuroeffector transmission

In this study we have looked at the effects of ageing on prejunctional control of noradrenergic neurotransmission in the cardiovascular system, in terms of alpha2-adrenoceptors, beta2-adrenoceptors and the noradrenaline re-uptake process. These studies show diminished prejunctional alpha2- and prejunctional beta-adrenoceptor-mediated responsiveness together with diminished noradrenaline re-uptake in rat tissues. The reduced prejunctional alpha2-inhibitory control and reduced re-uptake found in tissues from aged rats is more than likely to outweigh the effects of reduced beta-adrenoceptor facilitation, at least in normal conditions. Hence, assuming that such changes also occur in man, we might expect to find evidence of increased release of noradrenaline from noradrenergic nerves, and this could be reflected in plasma levels of noradrenaline.

The role of neuronal and extraneuronal plasma membrane transporters in the inactivation of peripheral catecholamines

Catecholamines are translocated across plasma membranes by transporters that belong to two large families with mainly neuronal or extraneuronal locations. In mammals, neuronal uptake of catecholamines involves the dopamine transporter (DAT) at dopaminergic neurons and the norepinephrine transporter (NET) at noradrenergic neurons. Extraneuronal uptake of catecholamines is mediated by organic cation transporters (OCTs), including the classic corticosterone-sensitive extraneuronal monoamine transporter. Catecholamine transporters function as part of uptake and metabolizing systems primarily responsible for inactivation of transmitter released by neurons. Additionally, the neuronal catecholamine transporters, recycle catecholamines for rerelease, thereby reducing requirements for transmitter synthesis. In a broader sense, catecholamine transporters function as part of integrated systems where catecholamine synthesis, release, uptake, and metabolism are regulated in a coordinated fashion in response to the demands placed on the system. Location is also important to function. Neuronal transporters are essential for rapid termination of the signal in neuronal-effector organ transmission, whereas non-neuronal transporters are more important for limiting the spread of the signal and for clearance of catecholamines from the bloodstream. Besides their presynaptic locations, NET and DAT are also present at several extraneuronal locations, including syncytiotrophoblasts of the placenta and endothelial cells of the lung (NET), stomach and pancreas (DAT). The extraneuronal monoamine transporter shows a broad tissue distribution, whereas the other two non-neuronal catecholamine transporters (OCT1 and OCT2) are mainly localized to the liver, kidney, and intestine. Altered function of peripheral catecholamine transporters may be involved in disturbances of the autonomic nervous system, such as occurs in congestive heart failure and hypernoradrenergic hypertension. Peripheral catecholamine transporters provide important targets for clinical imaging of sympathetic nerves and diagnostic localization and treatment of neuroendocrine tumors, such as neuroblastomas and pheochromocytomas.