The role of calmodulin (CaM) in transmitter release was investigated using liposomes to deliver CaM and monoclonal antibodies against CaM (antiCaM) directly into the frog motor nerve terminal. Miniature endplate potentials (MEPPs) were recorded in a high K + solution, and effects on transmitter release were monitored using estimates of the quantal release parameters m (number of quanta released), n (number of functional transmitter release sites), p (mean probability of release), and var s p (spatial variance in p ). Administration of CaM, but not heat‐inactivated CaM, encapsulated in liposomes (1000 units ml −1 ) produced an increase in m (25%) that was due to an increase in n . MEPP amplitude was not altered by CaM. Administration of antiCaM, but not heat‐inactivated antiCaM, in liposomes (50 μl ml −1 ) produced a progressive decrease in m (40%) that was associated with decreases in n and p . MEPP amplitude was decreased (15%) after a 25 min lag time, suggesting a separation in time between the decreases in quantal release and quantal size. Bath application of the membrane‐permeable CaM antagonist W7 (28 μ M ) produced a gradual decrease in m (25%) that was associated with a decrease in n . W7 also produced a decrease in MEPP amplitude that paralleled the decrease in m . The decreases in MEPP size and m produced by W7 were both reversed by addition of CaM. Our results suggest that CaM increases transmitter release by mobilizing synaptic vesicles at the frog motor nerve terminal. British Journal of Pharmacology (2002) 137 , 719–727. doi: 10.1038/sj.bjp.0704923
Inositol 1,4,5-trisphosphate (IP(3)) and cyclic adenosine diphosphate-ribose (cADPR) are second messengers that enhance neurosecretion by inducing Ca(2+) release from smooth endoplasmic reticulum (SER). The putative intracellular messenger, nicotinic acid adenine dinucleotide phosphate (NAADP), releases Ca(2+) from stores that are distinct from SER. Evidence is presented here that NAADP causes a concentration-dependent increase in quantal output that is associated with an increase in probability of transmitter release at the frog neuromuscular junction. This effect is mimicked by A23187, a Ca ionophore that promotes Ca(2+) entry at the plasmalemma. The response to NAADP is potentiated by IP(3) but antagonized by cADPR. Thapsigargin completely blocks IP(3) and cADPR responses and decreases but does not prevent the response to NAADP. We conclude that NAADP, whose receptors are widely distributed in the brain, enhances neurosecretion by releasing Ca(2+) from an internal store near the plasmalemma, possibly from synaptic vesicles in the releasable pool. These data also support the hypothesis of a two-pool model for Ca(2+) oscillations at the presynaptic site.
Parkinson disease (PD) and dementia are neurodegenerative disorders that can be frequently seen in the elderly. Homocysteine (Hcy) is an intermediary metabolite from methylation, which is highly relevant to body physiologic activities including DNA metabolism. Elevated plasma level of homocysteine (eHcy) is seen in normal aging individuals and patients with neurologic disorders such as PD or dementia. Although clinical observations confirm the finding that eHcy is prevalent in PD patients, the former is not a recognized etiology causing PD but rather, an adverse outcome related to the therapy of dopaminergic supplementation. Notably, eHcy may exacerbate various medical and neurologic conditions such as cardiovascular diseases, stroke, mild cognitive impairment, all of which are potential risks for dementia. This chapter discusses the concerns of eHcy relative to dementia in PD.
NAADP (nicotinic acid-adenine dinucleotide phosphate) is an unusual second messenger thought to mobilize acidic Ca(2+) stores, such as lysosomes or lysosome-like organelles, that are functionally coupled to the ER (endoplasmic reticulum). Although NAADP-sensitive Ca(2+) stores have been described in neurons, the physiological cues that recruit them are not known. Here we show that in both hippocampal neurons and glia, extracellular application of glutamate, in the absence of external Ca(2+), evoked cytosolic Ca(2+) signals that were inhibited by preventing organelle acidification or following osmotic bursting of lysosomes. The sensitivity of both cell types to glutamate correlated well with lysosomal Ca(2+) content. However, interfering with acidic compartments was largely without effect on the Ca(2+) content of the ER or Ca(2+) signals in response to ATP. Glutamate but not ATP elevated cellular NAADP levels. Our results provide evidence for the agonist-specific recruitment of NAADP-sensitive Ca(2+) stores by glutamate. This links the actions of NAADP to a major neurotransmitter in the brain.
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