[Ca2+]i transients. Under these experimental conditions, SCG neurones from both young and old animals were able to fully recover from high K+-evoked [Ca2+]i transients (Tsai et al., 1998). These data suggest that in peripheral adrenergic neurones plasma membrane calcium extrusion systems can by themselves control [Ca2+]i and their function is maintained with advancing age. In support of this study, we used vanadate, which at low concentrations (0.25 µ M) blocks PMCA function but does not significantly affect SERCA function. Under these conditions, the rate of recovery was diminished to a greater extent in SCG neu- rones from old as compared to young animals (Pottorf et al., 2000a). These data suggest that PMCA function may increase with advancing age as SERCA function declines. Although PMCA function has been reported to decline with age in synaptosomes derived from neurones in the CNS (Qin et al., 1998), our studies suggest that during normal aging PMCA function in peripheral adrenergic neurones is maintained with age. Model for age-related changes in [Ca2+]i regulation in peripheral adrenergic neurones One hypothesis suggested by numerous researchers is that advancing age leads to [Ca2+]i dysregulation and neuronal loss (Choi, 1992; Kirischuk & Verkhratsky, 1996; Ichas & Mazat, 1998; Thibault et al., 1998; Begley et al., 1999). However, this hypothesis may apply more to pathological conditions as opposed to normal aging. Thus, in light of our studies of peri- pheral adrenergic neurones we pose another hypothesis. That is advancing age in the absence of pathology results in subtle changes in the control of [Ca2+]i that may lead to altered neu- ronal function, and that other [Ca2+]i control mechanisms may allow neurones to adapt to an age-related decline in the control of [Ca2+]i. This model is summarized in Fig. 1(B) and emphasizes that loss of SERCA function may be compensated for by increased buffering by mitochondria and plasma membrane calcium extrusion so as to preserve some degree of cell viability in the face of advancing age. Consequences of age-related decline in SERCA Function in peripheral adrenergic neurones: CICR The function of neurones depends in part on the release of calcium from the SER in response to an elevation in [Ca2+]i mediated by voltage-gated Ca2+ channels (Usachev & Thayer, 1997, 1999a,b). This process has been termed CICR and is relevant in processes such as release of neurotransmitters and hormones. To sustain calcium release during neuronal activity requires refilling of the SER calcium through calcium influx and subsequent uptake into the SER via SERCA pumps (Kostyuk & Verkhratsky, 1994; Verkhratsky et al., 1994; Shmigol et al., 1996). Thus, buffering of [Ca2+]i transients and refilling [Ca2+]i stores by SERCA suggest that calcium release and [Ca2+]i buffering are intimately related processes. In CNS, SCG, and sensory neurones, SER Ca2+ stores can be rapidly refilled by activation of voltage gated calcium Age and Ca2+ homeostasis in peripheral neurones, J. N. Buchholz et al. 291 channels with high K+ or they can spontaneously refill within 3 –10 min following depletion with caffeine via activation of store operated calcium channels (Friel & Tsien, 1992; Shmigol et al., 1994, 1996; Usachev & Thayer, 1999b; Baba et al., 2003). In the discussion above, our work has consistently shown that SERCA mediated calcium uptake declines with age in isolated SCG neurones. Given that SERCA function declines with age in the SCG we studied how aging may alter the refilling and release of Ca2+ from the SER. We measured both rapid depolarization evoked refilling of SER calcium stores and spontaneous refilling following caffeine-evoked depletion of SER calcium stores in isolated Fura-2 loaded SCG neurones from rats aged 6, 12, 20, and 24 months. The data showed that both rapid and sponta- neous refilling of SER via SERCA declined with age. Overall, the data suggest that a functional consequence of reduced SERCA activity with advancing age is a compromise in the ability of SCG neurones to sustain release of [Ca2+]i during ongoing neuronal activity (Vanterpool et al., 2005). Altered expression of ryanodine receptors and selective modulators of CICR with advancing age Calcium-induced calcium release is mediated via the RyR channels and their function depends in part on density and regulation. The regulation of the function of RYRs is complex and overall this regulation serves to modulate the sensitivity of RYRs to cellular Ca2+ levels. These modulators include FK506 binding protein proteins, which serve to activate or inhibit channel state depending on its binding status, and activators such as phos- phorylation and intracellular molecules including cyclic adenosine diphosphate ribose (cADPr) (Hua et al., 1994; Ogawa et al., 2000; Marx & Marks, 2002). Specifically, cADPr levels are modulated by nitric oxide released from nNOS containing neurones. As RYRs are integral components in [Ca2+]i signaling in SCG neurones, we used molecular techniques of reverse transcription- polymerase chain reaction and enzyme-linked immuno- sorbent assay to test the hypothesis that the genetic and protein expression of the predominant RyR isoform(s) in adult rat SCG, along with selective modulators, are altered during late matu- ration and advancing age in F-344 rats aged 6, 12, and 24 months (Vanterpool et al., 2006). Surprisingly, we have found that RyR1 mRNA was undetect- able in the rat SCG, in contrast with other studies demonstrating RyR1 mRNA is expressed in excitable cells, including neurones (Fill & Copello, 2002). Thus, independent of late maturation and advancing age RyR1 does not appear to play a role in mediating the release of calcium in the SCG. However, RyR2 and RyR3 are the major receptor isoforms that regulate calcium release from RyR-sensitive stores in the SCG in all age groups. In addi- tion, during late maturation from 6 to 12 months RyR3 mRNA and protein levels increased and then decreased in senescent (24 month) animals while RyR2 mRNA and protein levels remained constant during late maturation and advancing age (Vanterpool et al., 2006). It is difficult to make a straightforward © 2007 Loma Linda University Journal compilation © Blackwell Publishing Ltd/Anatomical Society of Great Britain and Ireland 2007