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Age-related changes in adrenergic neuroeffector transmission
Autonomic Neuroscience: Basic and Clinical 96 (2002) 8 – 12 www.elsevier.com/locate/autneu
Review
Age-related changes in adrenergic neuroeffector transmission James R. Docherty * Department of Physiology, Royal College of Surgeons in Ireland, Dublin 2, Ireland
This work is dedicated to the memory of Dr. Meriel Borton, who died after an illness in June 2000
Abstract In this study we have looked at the effects of ageing on prejunctional control of noradrenergic neurotransmission in the cardiovascular system, in terms of a2-adrenoceptors, b2-adrenoceptors and the noradrenaline re-uptake process. These studies show diminished prejunctional a2- and prejunctional b-adrenoceptor-mediated responsiveness together with diminished noradrenaline re-uptake in rat tissues. The reduced prejunctional a2-inhibitory control and reduced re-uptake found in tissues from aged rats is more than likely to outweigh the effects of reduced b-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. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Cocaine; a-adrenoceptors; b-adrenoceptors; Noradrenaline; Neuronal re-uptake
1. Introduction Ageing is associated with a number of changes in noradrenergic neuro-effector transmission, resulting in altered response to the neurotransmitter. The release of noradrenaline from noradrenergic nerves is controlled by a number of prejunctional factors that modulate neurotransmission in ways appropriate to the situation. In this study, we have looked in detail at the effects of ageing on adrenergic neurotransmission, especially in the cardiovascular system. The cardiovascular system is controlled by the autonomic nervous system, which consists of the sympathetic and parasympathetic systems. The vasculature is innervated predominantly by the sympathetic division of the autonomic nervous system, with noradrenaline being the major neurotransmitter, whereas both the sympathetic and parasympathetic divisions of the autonomic nervous system control heart rate. Neurotransmission is likely to be affected by changes in nerve activity, density of innervation, prejunctional control of neurotransmission, and neurotransmitter disposition mechanisms, and in addition by alterations in target organ responsiveness, which may involve alterations in receptors, enzymes, second messengers or even alterations in tissue structure or alterations in external influences such as auta*
Tel.: +353-1-402-2269; fax: +353-1-402-2447. E-mail address: [email protected] (J.R. Docherty).
coids or reflex modulation. In this study we have looked at the effects of ageing on prejunctional control of noradrenergic neurotransmission.
2. Level of adrenergic nerve activity Increased muscle sympathetic nervous activity occurs in ageing, and this increased activity may be required, perhaps, because of diminished release per pulse or diminished postjunctional responsiveness. Several studies have reported a decreased release of neurotransmitter with ageing in response to electrical stimulation in isolated cardiovascular tissues, and this fits in with evidence for decreased noradrenergic innervation of tissues with age (see Docherty, 1990; Daly et al., 1988). However, other studies report an increased evoked release (Buchholz et al., 1998). The physiological effectiveness of noradrenergic nerve activity can be gauged by the effects of adrenoceptor antagonists, which act by blocking the ongoing actions of the endogenous neurotransmitter (thus indirectly demonstrating the ongoing level of nervous control of tissues). a-Adrenoceptor antagonists can be used to examine changes with age in noradrenergic control of blood pressure. If the noradrenergic component of peripheral vasoconstriction is altered (e.g. due to an increased role of other vasoconstrictors or a diminished role of vasodilators), or if the proportion of different subtypes of a-adrenoceptor involved in
1566-0702/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 1 5 6 6 - 0 7 0 2 ( 0 1 ) 0 0 3 7 5 - 7
J.R. Docherty / Autonomic Neuroscience: Basic and Clinical 96 (2002) 8–12
contractions is altered, the maximum effectiveness of an antagonist in terms of the hypotensive effect could be altered. In general, no change with age has been found in the hypotensive actions of a-adrenoceptor antagonists: the hypotensive actions of the a1-adrenoceptor antagonists prazosin and phentolamine (which act by blocking the vascular actions of noradrenaline) are reported to be unchanged by ageing in man (see Docherty, 1990). Likewise, b-adrenoceptor antagonists can be used to examine changes with age in noradrenergic control of heart rate and cardiac output. Studies have reported that b-adrenoceptor antagonists are effective at lowering blood pressure in the elderly (Klein et al., 1986), and that the b-adrenoceptor antagonist propranolol was as effective in older subjects at reducing cardiac output and heart rate during exercise (Conway et al., 1971), but produced larger falls in systolic blood pressure in the elderly. Maximum exercise heart rate declines with age (Jose et al., 1970) from about 200 beats/ min in young adults to about 150 –160 beats/min in the elderly. The intrinsic rate of the heart after blocking both vagal and sympathetic influences with atropine and propranolol (i.e. the intrinsic pacemaker rate of the heart in the absence of outside influences) is also reported to decrease with age (Jose et al., 1970), and this may be linked to a marked decrease in the number of pacemaker cells (Davies, 1976). Hence, a diminished intrinsic pacemaker rate together with attenuated responsiveness of pacemaker cells to b-adrenoceptor stimulation (see Docherty, 1990) are likely to explain the diminished maximal heart rate. Of course, noradrenergic nerves also release cotransmitters such as ATP: in one study of guinea-pig seminal vesicles, the noradrenergic but not the purinergic component of contractions declined with age (Pinna et al., 1997).
9
Hence, the relative importance of two cotransmitters may change with age.
3. Noradrenergic neuro-effector junction The noradrenergic (sympathetic) neuro-effector junction is the final level of control of neurotransmission. There are several possible sites at which ageing can modulate noradrenergic neuro-effector transmission (Fig. 1). We will now consider prejunctional a2-adrenoceptors, prejunctional badrenoceptors and neuronal re-uptake separately. 3.1. Prejunctional a2-adrenoceptors The best known prejunctional receptor on noradrenergic nerve terminals is the inhibitory a2-adrenoceptor and the effects of ageing on these receptors have been studied in most detail, and most studies report a decreased responsiveness in ageing (Docherty, 1990;Docherty, 1993; Daly et al., 1989). A decreased sensitivity of prejunctional a2-adrenoceptors should result in an increased release of neurotransmitter, and this coupled with a reported decline in the reuptake of noradrenaline (see below) with age should serve to maintain transmission even if postjunctional responsiveness declines with age. In our laboratory, a number of studies have looked at the function of prejunctional a2-adrenoceptors in ageing. Experiments were carried out employing rat vas deferens: prejunctional actions of the a2-adrenoceptor agonists xylazine or clonidine were gauged in terms of the ability to reduce the single pulse stimulation-evoked postjunctional isometric contraction. The potencies of xylazine and cloni-
Fig. 1. The noradrenergic neuroeffector junction, illustrating prejunctional a2-adrenergic receptor-mediated inhibition of release, prejunctional b-adrenergic receptor-mediated facilitation of release and neuronal re-uptake of noradrenaline.
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J.R. Docherty / Autonomic Neuroscience: Basic and Clinical 96 (2002) 8–12
dine were significantly reduced in tissues from 22-month rats. In fact, there is an age-related decline in both prejunctional a2- and 5-HT1-receptor responsiveness in rat vas deferens (Hyland and Docherty, 1985; Borton and Docherty, 1990), suggesting that the decline in response occurs at a postreceptor level, either at the level of the G protein or beyond (these two receptors may couple to the same Gprotein). Similarly, the a2-adrenoceptor agonist xylazine showed decreased potency with increasing age at inhibiting the cardioacceleration to nerve stimulation in the pithed rat heart (see Docherty, 1990). However, prejunctionally in the human saphenous vein, the inhibitory potency of xylazine at reducing transmitter release evoked by high frequency stimulation was not correlated with age (Borton and Docherty, 1990). Admittedly, these studies were of saphenous veins from patients in a limited age range, whereas animal studies were of a wide range of biological ages. Human saphenous vein studies of necessity employed high frequency rather than single-pulse stimulation: using highfrequency stimulation, the presence of the endogenous neurotransmitter at the prejunctional receptors is a complicating factor. 3.2. Prejunctional b-adrenoceptors Prejunctional b-adrenoceptors have been reported on some but not all noradrenergic nerve terminals, but in contrast to a2-adrenoceptors, they mediate a facilitation of neurotransmitter release. Since these are mainly b2-adrenoceptors (see Molderings et al., 1988), the most popular theory to explain the physiological function of these receptors is that they are targets for circulating adrenaline, which has much higher affinity for b2-adrenoceptors than noradrenaline, which has surprisingly low affinity (Starke et al., 1989) (see Fig. 1). The exact role of prejunctional adrenoceptors in the control of neurotransmission is not well understood, but these receptors may become activated when adrenaline from the adrenal medulla accumulates in noradrenergic nerves and is released together with noradrenaline as a cotransmitter. This development of b2-adrenoceptormediated facilitation has been proposed as a possible pathological change leading to hypertension (Majewski, 1983), but b-adrenoceptor antagonists may fail to inhibit the release of accumulated adrenaline from noradrenergic nerves in tissues such as the human saphenous vein (Molderings et al., 1988). However, the effects of ageing on the physiological function of b2-adrenoceptors can be studied by assessing the ability of b2-adrenoceptor agonists to increase nerve stimulation-evoked release of neurotransmitter in preparations, such as the rat isolated atrium, which have been loaded with radiolabelled noradrenaline. As can be seen from Fig. 2, the ability of the b2-adrenoceptor agonists procaterol and isoprenaline to increase stimulation-evoked release of noradrenaline was significantly attenuated in atria from aged animals. In fact, procaterol had only a very
Fig. 2. Prejunctional effects of the b-adrenergic receptor agonists procaterol and isoprenaline and the noradrenaline reuptake inhibitor cocaine in rat atria assessed as to the ability to increase 5-Hz nerve stimulation-evoked release of noradrenaline. Responses to procaterol, isoprenaline and cocaine were significantly greater in young animals (Student’s t-test: * P < 0.05). Adapted and expanded from (Hyland and Docherty, 1985).
limited effect on release of neurotransmitter in tissues from aged animals. Hence, results with both procaterol and isoprenaline suggest a diminished function of prejunctional b-adrenoceptors in atria from aged animals. A large number of studies of b-adrenoceptor function report age-related decreases in responsiveness in a wide range of tissues (Xiao and Lakatta, 1991), so that the decreased prejunctional responsiveness may be a further manifestation of this phenomenon. Whether these prejunctional b-adrenoceptors are important in noradrenergic neurotransmission, and whether age-related changes in these receptors are important, remains to be established. 3.3. Noradrenaline re-uptake The major route of deactivation of noradrenaline released from noradrenergic nerves is by re-uptake back into the nerve terminals (see Fig. 1). The effectiveness of the neuronal re-uptake system can be assessed using the neuronal uptake blocker cocaine: a decreased ability of cocaine to increase transmission should indicate a decline in the effectiveness of the re-uptake process. In rat atria, cocaine significantly increased stimulation-evoked release of noradrenaline in tissues from young animals but had little effect in tissues from aged animals (Fig. 2). The effects of age on the uptake process have been studied in rat heart with conflicting results, with reports of a reduced function (see Borton and Docherty, 1989), or even increased function of the uptake system (Daly et al., 1988) with age. Similarly, in blood vessels there are reports of both decreased (Hyland and Docherty, 1985) and increased function (Handa and
J.R. Docherty / Autonomic Neuroscience: Basic and Clinical 96 (2002) 8–12
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decreased noradrenaline transporter mRNA with age in rat locus coeruleus (Shores et al., 1999). 3.4. Effects of altered prejunctional control of neurotransmission These studies show diminished prejunctional a2- and prejunctional a-adrenoceptor-mediated responsiveness together with diminished noradrenaline re-uptake in rat tissues. The reduced prejunctional a2-inhibitory control and reduced re-uptake found in tissues from aged rats is more than likely to outweigh the effects of reduced badrenoceptor 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. 3.5. Plasma levels of noradrenaline Fig. 3. Effects of the neuronal noradrenaline re-uptake inhibitor cocaine in pithed rat assessed as to the ability of cocaine (1 mg kg 1) to increase pressor responses to noradrenaline. Cocaine significantly increased the pressor response in young animals (Student’s t-test: * P < 0.05). Responses are corrected for changes occurring in vehicle experiments. Adapted and expanded from (Hyland and Docherty, 1985).
Duckles, 1987) of the uptake system in ageing, or that loss of uptake does not explain increased transmitter release (Buchholz et al., 1998). Other studies of noncardiovascular autonomic nerves (Werneck De Avellar et al., 1990) and central neurones (Mcintosh and Westfall, 1987) report a decreased accumulation of noradrenaline with increasing age, with one study reporting an unchanged accumulation with age in rat trachea (Werneck De Avellar and Markus, 1990). The variable effects of ageing on the re-uptake process in blood vessels contrasts with the loss of re-uptake with age in the whole cardiovascular system in anaesthetised or pithed rats. In the pithed rat the pressor response to injected noradrenaline was significantly increased by cocaine in young but not aged rats (Fig. 3). Note that the pressor response to noradrenaline in the absence of cocaine is similar in young and old (Fig. 3). Hence, the loss of reuptake serves to preserve contractile function in the face of diminished responsiveness to noradrenaline. Such effects in man could explain the increased plasma levels of noradrenaline. In man, the difference between the age-related decline in the tachycardia to isoprenaline (not a substrate for reuptake) and the lack of an age-related change in the tachycardia to adrenaline was explained in terms of diminished re-uptake (White and Leenen, 1997). The re-uptake of other monoamines is also reported to decline with age (Moll et al., 2000). The transporters for noradrenaline, dopamine and serotonin are members of a family of neurotransmitter transporters. There is evidence that the number of transporters for noradrenaline in synaptosomes declines with age (Snyder et al., 1998), and there is
In man, one of the most consistently reported changes during ageing is an increased plasma level of noradrenaline, both at rest and in response to physiological stress (Zeigler et al., 1976). While some authors have sought to equate increased plasma levels of noradrenaline with increased adrenergic nerve activity, it has been suggested that plasma levels are a poor indicator of this (Bennet and Gardiner, 1988). Increased plasma levels of noradrenaline can be due to an increased rate of appearance in the plasma or to a decreased plasma clearance: an age-related increased rate of appearance of noradrenaline in the plasma has been reported (Macgilchrist et al., 1989). A decreased function of the disposition mechanisms for noradrenaline, particularly the re-uptake process, linked to decrease function of prejunctional receptors, is a possible explanation of the increased plasma levels.
4. Conclusion Ageing is associated with a decline in prejunctional modulation of neurotransmission, in terms of diminished noradrenaline re-uptake, diminished prejunctional a2-adre-
Table 1 Possible effects of ageing on noradrenergic neurotransmission Decreased nerve density Decreased w-conotoxin-sensitive Ca2 + channels Decreased depolarisation-induced Ca2 + entry Decreased a2-adrenergic receptor-mediated prejunctional inhibition of release Decreased b-adrenergic receptor-mediated prejunctional facilitation of release Decreased re-uptake of noradrenaline Decreased responsiveness of postjunctional receptors Increased plasma noradrenaline levels
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noceptor-mediated inhibition and diminished prejunctional b2-adrenoceptor-mediated facilitation. Table 1 summarizes the possible effects of ageing on noradrenergic neurotransmission. Overall, neurotransmission at rest may be changed little by ageing: prejunctional changes tending to increase neurotransmission may be cancelled out by diminished postjunctional responsiveness. However, the price to pay is a loss of fine control of neurotransmission and increased plasma levels of noradrenaline.
Acknowledgements Work carried out in the author’s laboratory was supported by the Royal College of Surgeons in Ireland, the Health Research Board (Ireland) and the Irish Heart Foundation.
References Bennett, T., Gardiner, S.M., 1988. Physiological aspects of the aging cardiovascular system. J. Cardiovasc. Pharmacol. 12 (Suppl. 8), S1 – S7. Borton, M., Docherty, J.R., 1989. The effects of ageing on neuronal uptake of noradrenaline in the rat. Naunyn-Schmiedeberg’s Arch. Pharmacol. 340, 139 – 143. Borton, M., Docherty, J.R., 1990. The effects of ageing on prejunctional 5hydroxytryptamine receptors in rat vas deferens. Naunyn-Schmiedeberg’s Arch. Pharmacol. 342, 130 – 135. Buchholz, J., Sexton, P., Hewitt, C.W., 1998. Impact of age om modulation of norepinephrine release from sympathetic nerves in the rat superior mesenteric artery. Life Sci. 62, 679 – 686. Conway, J., Wheeler, R., Sannerstedt, R., 1971. Sympathetic nervous activity during exercise in relation to age. Cardiovasc. Res. 5, 577 – 581. Daly, R.N., Goldberg, P.B., Roberts, R.J., 1988. Effects of age on neurotransmission at the cardiac sympathetic neuroeffector junction. J. Pharmacol. Exp. Ther. 245, 798 – 803. Daly, R.N., Goldberg, P.B., Roberts, J., 1989. The effect of age on presynaptic alpha2-adrenoceptor autoregulation of noradrenaline release. J. Gerontol. 44, B59 – B66. Davies, M.J., 1976. Pathology of the conducting system. In: Caird, F.L., Dall, J.L.C, Kennedy, R.D. (Eds.), Cardiology in Old Age. Plenum, New York, pp. 57 – 59. Docherty, J.R., 1990. Cardiovascular responses in ageing: a review. Pharmacol. Rev. 42, 103 – 125. Docherty, J.R., 1993. Effects of aging on the vasoconstrictor responses to noradrenaline and 5-hydroxytryptamine in the human saphenous vein. Cardiol. Elderly 1, 327 – 330. Handa, R.K., Duckles, S.P., 1987. Age-related changes in adrenergic vaso-
constrictor responses of the rat hindlimb. Am. J. Physiol. 253, H1566 – H1572. Hyland, L., Docherty, J.R., 1985. An investigation of age-related changes in pre- and postjunctional adrenergic receptors in human saphenous vein. Eur. J. Pharmacol. 110, 241 – 246. Jose, A.D., Stitt, F., Collison, D., 1970. The effects of exercise and changes in body temperature on the intrinsic heart rate in man. Am. Heart J. 79, 488 – 498. Klein, C., Gerber, J.G., Gal, J., Nies, A.S., 1986. Beta-adrenergic receptors in the elderly are not less sensitive to timolol. Clin. Pharmacol. Ther. 40, 161 – 164. Macgilchrist, A.J., Hawksby, C., Howes, L.G., Reid, J.L., 1989. Rise in plasma noradrenaline with age results from an increase in spillover rate. Gerontology 35, 7 – 13. Majewski, H., 1983. Modulation of noradrenaline release through activation of presynaptic b-adrenoreceptors. J. Auton. Pharmacol. 3, 47 – 60. Mcintosh, H.H., Westfall, T.C., 1987. Influence of aging on catecholamine levels, accumulation and release in F-344 rats. Neurobiol. Aging 8, 233 – 239. Molderings, G., Likungu, J., Zerkowski, H.R., Gothert, M., 1988. Presynaptic b2-adrenoceptors on the sympathetic nerve fibres of the human saphenous vein: no evidence for involvement in adrenaline-mediated positive feedback loop regulating noradrenergic transmission. NaunynSchmiedeberg’s Arch. Pharmacol. 337, 408 – 414. Moll, G.H., Mehnert, C., Wicker, M., Bock, N., Rothenberger, A., Ruther, E., Huether, G., 2000. Age-associated changes in the densities of presynaptic monoamine trasnporters in different regions of the rat brain from early juvenile life to late adulthood. Brain Res. 119, 251 – 257. Pinna, C., Rubino, A., Burnstock, G., 1997. Age-related changes in purinergic and adrenergic components of sympathetic neurotransmission in guinea-pig seminal vesicles. Br. J. Pharmacol. 122, 1411 – 1416. Shores, M.M., White, S.S., Veith, R.C., Szot, P., 1999. Tyrosine hydroxylase mRNA is increased in old age and norepinephrine uptake transporter mRNA is decreased in middle age in locus coeruleus of Brown – Norway rats. Brain Res. 826, 143 – 147. Snyder, D.L., Aloyo, V.J., Wang, W., Roberts, J., 1998. Influence of age and dietary restriciton on borepinephrine uptake into caqrdiac synaptosomes. J. Cardiovasc. Pharmacol. 32, 896 – 901. Starke, K., Gothert, M., Kilbinger, H., 1989. Modulation of neurotransmitter release by presynaptic autoreceptors. Physiol. Rev. 69, 864 – 989. Werneck De Avellar, M.C., Markus, R.P., 1990. Neuronal uptake of noradrenaline in the rat isolated trachea: effect of age. Comp. Biochem. Physiol., C: Comp. Pharmacol. Toxicol. 96, 287 – 290. Werneck De Avellar, M.C., Kobashi, Y.L., Markus, R.P., 1990. Age-related changes in neuronal uptake of noradrenaline. Naunyn-Schmiedeberg’s Arch. Pharmacol. 341, 295. White, M., Leenen, F.H., 1997. Effects of age on cardiovascular responses to adrenaline in man. Br. J. Clin. Pharmacol. 43, 407 – 414. Xiao, R.-P., Lakatta, E.G., 1991. Mechanisms of altered b-adrenergic modulation of the cardiovascular system with aging. Rev. Clin. Gerontol. 1, 309 – 322. Zeigler, M.G., Lake, C.R., Kopin, I.J., 1976. Plasma noradrenaline increases with age. Nature 261, 333 – 335.
Review
Age-related changes in adrenergic neuroeffector transmission James R. Docherty * Department of Physiology, Royal College of Surgeons in Ireland, Dublin 2, Ireland
This work is dedicated to the memory of Dr. Meriel Borton, who died after an illness in June 2000
Abstract In this study we have looked at the effects of ageing on prejunctional control of noradrenergic neurotransmission in the cardiovascular system, in terms of a2-adrenoceptors, b2-adrenoceptors and the noradrenaline re-uptake process. These studies show diminished prejunctional a2- and prejunctional b-adrenoceptor-mediated responsiveness together with diminished noradrenaline re-uptake in rat tissues. The reduced prejunctional a2-inhibitory control and reduced re-uptake found in tissues from aged rats is more than likely to outweigh the effects of reduced b-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. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Cocaine; a-adrenoceptors; b-adrenoceptors; Noradrenaline; Neuronal re-uptake
1. Introduction Ageing is associated with a number of changes in noradrenergic neuro-effector transmission, resulting in altered response to the neurotransmitter. The release of noradrenaline from noradrenergic nerves is controlled by a number of prejunctional factors that modulate neurotransmission in ways appropriate to the situation. In this study, we have looked in detail at the effects of ageing on adrenergic neurotransmission, especially in the cardiovascular system. The cardiovascular system is controlled by the autonomic nervous system, which consists of the sympathetic and parasympathetic systems. The vasculature is innervated predominantly by the sympathetic division of the autonomic nervous system, with noradrenaline being the major neurotransmitter, whereas both the sympathetic and parasympathetic divisions of the autonomic nervous system control heart rate. Neurotransmission is likely to be affected by changes in nerve activity, density of innervation, prejunctional control of neurotransmission, and neurotransmitter disposition mechanisms, and in addition by alterations in target organ responsiveness, which may involve alterations in receptors, enzymes, second messengers or even alterations in tissue structure or alterations in external influences such as auta*
Tel.: +353-1-402-2269; fax: +353-1-402-2447. E-mail address: [email protected] (J.R. Docherty).
coids or reflex modulation. In this study we have looked at the effects of ageing on prejunctional control of noradrenergic neurotransmission.
2. Level of adrenergic nerve activity Increased muscle sympathetic nervous activity occurs in ageing, and this increased activity may be required, perhaps, because of diminished release per pulse or diminished postjunctional responsiveness. Several studies have reported a decreased release of neurotransmitter with ageing in response to electrical stimulation in isolated cardiovascular tissues, and this fits in with evidence for decreased noradrenergic innervation of tissues with age (see Docherty, 1990; Daly et al., 1988). However, other studies report an increased evoked release (Buchholz et al., 1998). The physiological effectiveness of noradrenergic nerve activity can be gauged by the effects of adrenoceptor antagonists, which act by blocking the ongoing actions of the endogenous neurotransmitter (thus indirectly demonstrating the ongoing level of nervous control of tissues). a-Adrenoceptor antagonists can be used to examine changes with age in noradrenergic control of blood pressure. If the noradrenergic component of peripheral vasoconstriction is altered (e.g. due to an increased role of other vasoconstrictors or a diminished role of vasodilators), or if the proportion of different subtypes of a-adrenoceptor involved in
1566-0702/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 1 5 6 6 - 0 7 0 2 ( 0 1 ) 0 0 3 7 5 - 7
J.R. Docherty / Autonomic Neuroscience: Basic and Clinical 96 (2002) 8–12
contractions is altered, the maximum effectiveness of an antagonist in terms of the hypotensive effect could be altered. In general, no change with age has been found in the hypotensive actions of a-adrenoceptor antagonists: the hypotensive actions of the a1-adrenoceptor antagonists prazosin and phentolamine (which act by blocking the vascular actions of noradrenaline) are reported to be unchanged by ageing in man (see Docherty, 1990). Likewise, b-adrenoceptor antagonists can be used to examine changes with age in noradrenergic control of heart rate and cardiac output. Studies have reported that b-adrenoceptor antagonists are effective at lowering blood pressure in the elderly (Klein et al., 1986), and that the b-adrenoceptor antagonist propranolol was as effective in older subjects at reducing cardiac output and heart rate during exercise (Conway et al., 1971), but produced larger falls in systolic blood pressure in the elderly. Maximum exercise heart rate declines with age (Jose et al., 1970) from about 200 beats/ min in young adults to about 150 –160 beats/min in the elderly. The intrinsic rate of the heart after blocking both vagal and sympathetic influences with atropine and propranolol (i.e. the intrinsic pacemaker rate of the heart in the absence of outside influences) is also reported to decrease with age (Jose et al., 1970), and this may be linked to a marked decrease in the number of pacemaker cells (Davies, 1976). Hence, a diminished intrinsic pacemaker rate together with attenuated responsiveness of pacemaker cells to b-adrenoceptor stimulation (see Docherty, 1990) are likely to explain the diminished maximal heart rate. Of course, noradrenergic nerves also release cotransmitters such as ATP: in one study of guinea-pig seminal vesicles, the noradrenergic but not the purinergic component of contractions declined with age (Pinna et al., 1997).
9
Hence, the relative importance of two cotransmitters may change with age.
3. Noradrenergic neuro-effector junction The noradrenergic (sympathetic) neuro-effector junction is the final level of control of neurotransmission. There are several possible sites at which ageing can modulate noradrenergic neuro-effector transmission (Fig. 1). We will now consider prejunctional a2-adrenoceptors, prejunctional badrenoceptors and neuronal re-uptake separately. 3.1. Prejunctional a2-adrenoceptors The best known prejunctional receptor on noradrenergic nerve terminals is the inhibitory a2-adrenoceptor and the effects of ageing on these receptors have been studied in most detail, and most studies report a decreased responsiveness in ageing (Docherty, 1990;Docherty, 1993; Daly et al., 1989). A decreased sensitivity of prejunctional a2-adrenoceptors should result in an increased release of neurotransmitter, and this coupled with a reported decline in the reuptake of noradrenaline (see below) with age should serve to maintain transmission even if postjunctional responsiveness declines with age. In our laboratory, a number of studies have looked at the function of prejunctional a2-adrenoceptors in ageing. Experiments were carried out employing rat vas deferens: prejunctional actions of the a2-adrenoceptor agonists xylazine or clonidine were gauged in terms of the ability to reduce the single pulse stimulation-evoked postjunctional isometric contraction. The potencies of xylazine and cloni-
Fig. 1. The noradrenergic neuroeffector junction, illustrating prejunctional a2-adrenergic receptor-mediated inhibition of release, prejunctional b-adrenergic receptor-mediated facilitation of release and neuronal re-uptake of noradrenaline.
10
J.R. Docherty / Autonomic Neuroscience: Basic and Clinical 96 (2002) 8–12
dine were significantly reduced in tissues from 22-month rats. In fact, there is an age-related decline in both prejunctional a2- and 5-HT1-receptor responsiveness in rat vas deferens (Hyland and Docherty, 1985; Borton and Docherty, 1990), suggesting that the decline in response occurs at a postreceptor level, either at the level of the G protein or beyond (these two receptors may couple to the same Gprotein). Similarly, the a2-adrenoceptor agonist xylazine showed decreased potency with increasing age at inhibiting the cardioacceleration to nerve stimulation in the pithed rat heart (see Docherty, 1990). However, prejunctionally in the human saphenous vein, the inhibitory potency of xylazine at reducing transmitter release evoked by high frequency stimulation was not correlated with age (Borton and Docherty, 1990). Admittedly, these studies were of saphenous veins from patients in a limited age range, whereas animal studies were of a wide range of biological ages. Human saphenous vein studies of necessity employed high frequency rather than single-pulse stimulation: using highfrequency stimulation, the presence of the endogenous neurotransmitter at the prejunctional receptors is a complicating factor. 3.2. Prejunctional b-adrenoceptors Prejunctional b-adrenoceptors have been reported on some but not all noradrenergic nerve terminals, but in contrast to a2-adrenoceptors, they mediate a facilitation of neurotransmitter release. Since these are mainly b2-adrenoceptors (see Molderings et al., 1988), the most popular theory to explain the physiological function of these receptors is that they are targets for circulating adrenaline, which has much higher affinity for b2-adrenoceptors than noradrenaline, which has surprisingly low affinity (Starke et al., 1989) (see Fig. 1). The exact role of prejunctional adrenoceptors in the control of neurotransmission is not well understood, but these receptors may become activated when adrenaline from the adrenal medulla accumulates in noradrenergic nerves and is released together with noradrenaline as a cotransmitter. This development of b2-adrenoceptormediated facilitation has been proposed as a possible pathological change leading to hypertension (Majewski, 1983), but b-adrenoceptor antagonists may fail to inhibit the release of accumulated adrenaline from noradrenergic nerves in tissues such as the human saphenous vein (Molderings et al., 1988). However, the effects of ageing on the physiological function of b2-adrenoceptors can be studied by assessing the ability of b2-adrenoceptor agonists to increase nerve stimulation-evoked release of neurotransmitter in preparations, such as the rat isolated atrium, which have been loaded with radiolabelled noradrenaline. As can be seen from Fig. 2, the ability of the b2-adrenoceptor agonists procaterol and isoprenaline to increase stimulation-evoked release of noradrenaline was significantly attenuated in atria from aged animals. In fact, procaterol had only a very
Fig. 2. Prejunctional effects of the b-adrenergic receptor agonists procaterol and isoprenaline and the noradrenaline reuptake inhibitor cocaine in rat atria assessed as to the ability to increase 5-Hz nerve stimulation-evoked release of noradrenaline. Responses to procaterol, isoprenaline and cocaine were significantly greater in young animals (Student’s t-test: * P < 0.05). Adapted and expanded from (Hyland and Docherty, 1985).
limited effect on release of neurotransmitter in tissues from aged animals. Hence, results with both procaterol and isoprenaline suggest a diminished function of prejunctional b-adrenoceptors in atria from aged animals. A large number of studies of b-adrenoceptor function report age-related decreases in responsiveness in a wide range of tissues (Xiao and Lakatta, 1991), so that the decreased prejunctional responsiveness may be a further manifestation of this phenomenon. Whether these prejunctional b-adrenoceptors are important in noradrenergic neurotransmission, and whether age-related changes in these receptors are important, remains to be established. 3.3. Noradrenaline re-uptake The major route of deactivation of noradrenaline released from noradrenergic nerves is by re-uptake back into the nerve terminals (see Fig. 1). The effectiveness of the neuronal re-uptake system can be assessed using the neuronal uptake blocker cocaine: a decreased ability of cocaine to increase transmission should indicate a decline in the effectiveness of the re-uptake process. In rat atria, cocaine significantly increased stimulation-evoked release of noradrenaline in tissues from young animals but had little effect in tissues from aged animals (Fig. 2). The effects of age on the uptake process have been studied in rat heart with conflicting results, with reports of a reduced function (see Borton and Docherty, 1989), or even increased function of the uptake system (Daly et al., 1988) with age. Similarly, in blood vessels there are reports of both decreased (Hyland and Docherty, 1985) and increased function (Handa and
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decreased noradrenaline transporter mRNA with age in rat locus coeruleus (Shores et al., 1999). 3.4. Effects of altered prejunctional control of neurotransmission These studies show diminished prejunctional a2- and prejunctional a-adrenoceptor-mediated responsiveness together with diminished noradrenaline re-uptake in rat tissues. The reduced prejunctional a2-inhibitory control and reduced re-uptake found in tissues from aged rats is more than likely to outweigh the effects of reduced badrenoceptor 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. 3.5. Plasma levels of noradrenaline Fig. 3. Effects of the neuronal noradrenaline re-uptake inhibitor cocaine in pithed rat assessed as to the ability of cocaine (1 mg kg 1) to increase pressor responses to noradrenaline. Cocaine significantly increased the pressor response in young animals (Student’s t-test: * P < 0.05). Responses are corrected for changes occurring in vehicle experiments. Adapted and expanded from (Hyland and Docherty, 1985).
Duckles, 1987) of the uptake system in ageing, or that loss of uptake does not explain increased transmitter release (Buchholz et al., 1998). Other studies of noncardiovascular autonomic nerves (Werneck De Avellar et al., 1990) and central neurones (Mcintosh and Westfall, 1987) report a decreased accumulation of noradrenaline with increasing age, with one study reporting an unchanged accumulation with age in rat trachea (Werneck De Avellar and Markus, 1990). The variable effects of ageing on the re-uptake process in blood vessels contrasts with the loss of re-uptake with age in the whole cardiovascular system in anaesthetised or pithed rats. In the pithed rat the pressor response to injected noradrenaline was significantly increased by cocaine in young but not aged rats (Fig. 3). Note that the pressor response to noradrenaline in the absence of cocaine is similar in young and old (Fig. 3). Hence, the loss of reuptake serves to preserve contractile function in the face of diminished responsiveness to noradrenaline. Such effects in man could explain the increased plasma levels of noradrenaline. In man, the difference between the age-related decline in the tachycardia to isoprenaline (not a substrate for reuptake) and the lack of an age-related change in the tachycardia to adrenaline was explained in terms of diminished re-uptake (White and Leenen, 1997). The re-uptake of other monoamines is also reported to decline with age (Moll et al., 2000). The transporters for noradrenaline, dopamine and serotonin are members of a family of neurotransmitter transporters. There is evidence that the number of transporters for noradrenaline in synaptosomes declines with age (Snyder et al., 1998), and there is
In man, one of the most consistently reported changes during ageing is an increased plasma level of noradrenaline, both at rest and in response to physiological stress (Zeigler et al., 1976). While some authors have sought to equate increased plasma levels of noradrenaline with increased adrenergic nerve activity, it has been suggested that plasma levels are a poor indicator of this (Bennet and Gardiner, 1988). Increased plasma levels of noradrenaline can be due to an increased rate of appearance in the plasma or to a decreased plasma clearance: an age-related increased rate of appearance of noradrenaline in the plasma has been reported (Macgilchrist et al., 1989). A decreased function of the disposition mechanisms for noradrenaline, particularly the re-uptake process, linked to decrease function of prejunctional receptors, is a possible explanation of the increased plasma levels.
4. Conclusion Ageing is associated with a decline in prejunctional modulation of neurotransmission, in terms of diminished noradrenaline re-uptake, diminished prejunctional a2-adre-
Table 1 Possible effects of ageing on noradrenergic neurotransmission Decreased nerve density Decreased w-conotoxin-sensitive Ca2 + channels Decreased depolarisation-induced Ca2 + entry Decreased a2-adrenergic receptor-mediated prejunctional inhibition of release Decreased b-adrenergic receptor-mediated prejunctional facilitation of release Decreased re-uptake of noradrenaline Decreased responsiveness of postjunctional receptors Increased plasma noradrenaline levels
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noceptor-mediated inhibition and diminished prejunctional b2-adrenoceptor-mediated facilitation. Table 1 summarizes the possible effects of ageing on noradrenergic neurotransmission. Overall, neurotransmission at rest may be changed little by ageing: prejunctional changes tending to increase neurotransmission may be cancelled out by diminished postjunctional responsiveness. However, the price to pay is a loss of fine control of neurotransmission and increased plasma levels of noradrenaline.
Acknowledgements Work carried out in the author’s laboratory was supported by the Royal College of Surgeons in Ireland, the Health Research Board (Ireland) and the Irish Heart Foundation.
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