288 Age and Ca2+ homeostasis in peripheral neurones, J. N. Buchholz et al. superior mesenteric artery (Buchholz & Duckles, 1990; Buchholz et al., 1998). In addition, we have shown that the age-related increase in release is not dependent on any particular stimu- lation frequency as the increase occurs over a wide range of frequencies (Tsai et al., 1995). The change in function of vas- cular adrenergic neurones may be explained by multiple mech- anisms. For example, changes in density of adrenergic neurones, content, re-uptake, function of prejunctional inhibitory α2-autoreceptors, and calcium regulation (Pottorf et al., 2000b). Furthermore, additional complicating factors must also be addressed. Our studies and others have shown that the SCG (Dun et al., 1995; Wu et al., 1997) and the cerebral vasculature contain adrenergic and neuronal nitric oxide (nNOS) containing nerves; NO released from nNOS neurones augments stimulation- evoked release in both blood vessels and the CNS (Montague et al., 1994; Yamamoto et al., 1997; Zhang et al., 1998; Lee et al., 2000; Mbaku et al., 2000, 2003). Facilitation of the func- tion of adrenergic nerves via nNOS nerves may occur through enhancement of Ca2+ influx and/or Ca2+ release from internal stores. These mechanisms may also be altered with age and is the subject of early ongoing studies in our laboratory. content and adrenergic density in the periphery The content of in peripheral organs and blood vessels has been used as an index of adrenergic density. In the rat heart, content decreases with age (Martinez et al., 1981; Dawson & Meldrum, 1992) while in blood vessels the story is much less clear. For example, content in rat arteries (renal, femoral, and saphenous) increases with age, while in veins (renal, femoral, and saphenous) there is no change, and a decline in tail arteries (Handa & Duckles, 1987). Consistent with content in rat arteries, catecholamine histofluorescence, another measure of adrenergic nerve density, increases with age in rat superior mesenteric and renal arteries and portal vein (Mione et al., 1988). In contrast, spinal cord blood vessels show no age related change in adrenergic nerve density (Amenta et al., 1990). In another study in the internal carotid artery, sympathetic inner- vation declines with age. However, after intracerebral infusion of NGF, the number of sympathetic axons increases in aged animals (Isaacson & Crutcher, 1998). In a follow-up to this study using internal carotid and anterior cerebral arteries as models, content and numbers of tyrosine hydroxylase containing nerve fibres decline with age. However, after infusion of NGF, content and the number of tyrosine hydroxylase-containing neurones significantly increased in old animals (Dickason & Isaacson, 2002). These studies would suggest that there is an innate age-related ability to maintain a critical number of func- tioning sympathetic neurones. The NGF studies cited above offer some interesting possibilities with regards to therapeutic interventions in terms of maintaining cardiovascular homeostasis with advancing age. In light of these studies, there appears to be no clear relationship between age-related changes in adrenergic nerve density, increases in circulating plasma levels reported by others and increased stimulation-evoked release in our earlier studies. Thus, age-related changes in content may be species or vascular bed dependent. Transmitter uptake and function of inhibitory prejunctional α2-adrenoceptors Modulation of concentration in the synaptic cleft is medi- ated by a combination of re-uptake and activation of prejunc- tional α2-adrenceptors. The latter mechanism has been shown to attenuate release by decreasing stimulation-evoked calcium influx in adrenergic neurones (Schofield, 1990; Delcour & Tsien, 1993). Both mechanisms are critical to controlling the biophase concentrations of with ongoing moment-to-moment vascular adrenergic nerve activity (Illes, 1986; Langer & Rbilla, 1990; Buchholz et al., 1992; Insel, 1993; Esler et al., 1995). The effect of chemical agents such as cocaine and deoxycor- ticosterone that block the neuronal and extraneuronal uptake of NA, respectively, is reduced with age in the atria and vas def- erens in the pithed rat (Borton & Docherty, 1989; De Avellar et al., 1990). These studies suggest that uptake declines with age in peripheral adrenergic nerves. Contrasting studies using direct measurement of 3H-NA uptake support the idea that the function of uptake transporters does not change with age in peripheral adrenergic nerves (Duckles et al., 1985). In addition, in the rat heart (Limas, 1975) and tail artery (Buchholz & Duckles, 1990) the effect of cocaine and deoxycoricosterone on uptake increased with advancing age. These contrasting studies appear to be difficult to reconcile. However, in the tail artery model, when release of in the presence of uptake blockers was corrected for total release in the absence of uptake blockers, no change with age was observed (Buchholz & Duckles, 1990). These data suggest that the age-related change in the effectiveness of uptake blockers on may reflect the age-related differences in concentrations within the junctional cleft of the model under study. In light of the latter approach, overall the data would suggest that uptake with advancing age remains as a constant fraction of the amount of released. Taken together, direct measurement of uptake and effect of uptake blockers, an age-related increase in stimulation- evoked fractional release in peripheral adrenergic nerves is not explained by an age-related change in the function of transporters. Thus, age-related changes in release may possibly be explained by other mechanisms such as presynaptic inhibition. Studies measuring overflow have shown an age-related decline in the effect of prejunctional α2-adrenoceptor antago- nists in pithed rats, rat vas deferens and heart and tail artery (Hyland & Docherty, 1985; Docherty & Hyland, 1986; Daly et al., 1989; Buchholz & Duckles, 1990; Buchholz et al., 1992). These studies appear to support the idea of a general age-related decline in the function of feedback prejunctional α2-adrenoceptors. However, we have found that while the sensitivity of the prejunctional α2-adrenoceptors to the antagonist, idazoxan, declined with advancing age, there appears to be no age-related © 2007 Loma Linda University Journal compilation © Blackwell Publishing Ltd/Anatomical Society of Great Britain and Ireland 2007