Modulation of Presynaptic Transmitter Release by ATP Derived from Postsynaptic Sources. In: Rahamimoff R, Katz B, editors. Calcium, Neuronal Function and Transmitter Release. Boston: Springer US; 1986. pp. 357-74. [DOI: 10.1007/978-1-4613-2307-5_23]

Regulation of quantal transmitter secretion by ATP and protein kinases at developing neuromuscular synapses

The effects of endogenously released ATP on the maturation of developing neuromuscular synapses were investigated in Xenopus nerve-muscle co-cultures. The potentiating action of ATP (1 mM) on spontaneous acetylcholine release was inhibited by P2-purinoceptor antagonists suramin (0.3 mM) and reactive blue 2 (RB-2, 3 microM) in day 1 cultures. Bath application of suramin (10 microM) or RB-2 in day 1 cultures and prolonged treatment for 2 days dramatically decreased the amplitude of both spontaneous synaptic currents (SSCs) and evoked synaptic currents (ESCs) in the same cultures on day 3. Chronic treatment with 8-cyclopentyltheophylline (4 microM) or 6-cyano-2,3-dihydroxy-7-nitroquinoxaline (CNQX, 10 microM), P1-purinoceptor and glutamate receptor antagonists respectively, did not exert such an inhibitory effect. Chronic treatment with suramin or RB-2 for 2 days had no significant effect on the amplitude of either iontophoretic acetylcholine-induced whole-cell currents or single acetylcholine channel measurements in 3-day-old cultured myocytes. In addition, prolonged treatment for 2 days with various kinase inhibitors such as H-8 (10 microM), KN-62 (5 microM) and H-7 (10 microM) also decreased the amplitudes of both spontaneous and evoked synaptic currents in natural synapses, but not those of iontophoretic acetylcholine-induced currents. Furthermore, suramin and these protein kinase inhibitors also decreased the amplitude of spontaneous synaptic currents in manipulated synapses of 'vacated' nerve terminals. The results suggest that endogenously released ATP, acting in concert with various protein kinases, is involved in the maintenance and/or development of the quantum size of synaptic vesicles at embryonic neuromuscular synapses.

Retrograde signaling in the formation and maintenance of the neuromuscular junction

The neuromuscular junction is characterized by precise alignment between the nerve terminal and the postsynaptic apparatus formed by the muscle fiber. Organization of the neuromuscular junction during embryonic development, growth, and maintenance is coordinated by signals exchanged between motor neurons and their target muscle fibers. Identification of proteins such as agrin, likely to represent neuronal agents that direct the organization of the postsynaptic apparatus, has focused attention on characterization of proteins that mediate retrograde signals that regulate the organization and function of the nerve terminal. The results of these studies implicate a role for both adhesive and diffusible signals in coordinating the development, maturation, and maintenance of the motor nerve terminal. The diversity of molecules identified to date that appear to play a role in these processes implies a considerable level of redundancy in the transduction pathway. However, studies of early nerve-muscle interactions suggest that a common feature of many of these retrograde agents is activation of a protein kinase coupled with an increase in cytosolic Ca2+ concentration. While the molecular signals that regulate growth and maintenance of neuromuscular junctions are less well understood it seems likely that similar adhesive and diffusible factors will be involved.

Potentiation by endogenously released ATP of spontaneous transmitter secretion at developing neuromuscular synapses in Xenopus cell cultures

1. Previously we have shown that extracellular application of ATP, a substance co-stored and co-released with acetylcholine (ACh) in the peripheral nervous system, markedly potentiated the frequency of spontaneous synaptic currents (SSCs) produced by ACh. In the present study, we have further characterized the purinoceptor which mediates the potentiation effect of ATP and the role of endogenously released ATP. 2. Pretreatment with a P2-purinoceptor antagonist, suramin (0.3 mM), but not a P1-purinoceptor antagonist, 8-phenyltheophylline (0.1 mM), prevented the potentiating effect of ATP. 3. We studied the role of endogenously released ATP by examining the effect of a specific P2-purinoceptor antagonist on the frequency of spontaneous synaptic events at high-activity synapses (> or = 3 Hz) and found that suramin, but not 8-phenyltheophylline markedly reduced the frequency of SSCs at these high-activity synapses. In addition, desensitizing the P2-purinoceptor with alpha,beta-methylene ATP also produced similar effects to suramin. 4. Extracellular application of the L-type Ca2+ channel blockers, verapamil, nifedipine or diltiazem (10 microM each) reduced SSC frequency of high-activity synapses, while the N-type Ca2+ channel blocker, omega-conotoxin had no appreciable effect. The potentiating effect of ATP was further prevented by pretreatment with the L-type Ca2+ channel blockers. On the other hand, Bay K 8644, which is a depolarization-dependent L-type Ca2+ channel agonist, potentiated SSC frequency at these high-activity synapses. 5. These results suggest that endogenous release of ATP at developing neuromuscular synapses is responsible for the maintenance of high levels of spontaneous ACh release, which is known to play a crucial role in regulating postsynaptic differentiation.

Potentiation of miniature endplate potential frequency by ATP in Xenopus tadpoles

1. Extracellular application of ATP (1 mM), a substance co-stored and co-released with acetylcholine in peripheral nervous systems, potentiated the spontaneous secretion of acetylcholine (ACh) but had no effect on the amplitude and decay time constant of miniature endplate potentials (m.e.p.ps) at neuromuscular synapses in Xenopus tadpoles. 2. alpha,beta-Methylene ATP (0.3 mM) and GTP (1 mM) were also effective in increasing m.e.p.p. frequency. On the other hand, ADP, AMP and adenosine (all at 1 mM) decreased m.e.p.p. frequency. 3. Unlike the transient effect of ATP analogue and GTP on m.e.p.p. frequency, the phorbol ester TPA (2 microM) which is a protein kinase C activator, increased m.e.p.p. frequency consistently and the effects lasted as long as the presence of TPA. 4. Staurosporine (0.5 microM) and H-7 (10 microM), which are protein kinase C inhibitors, each decreased the basal level of m.e.p.p. frequency and markedly inhibited the effects of both ATP and TPA. 5. These results suggest that there is a basal activity of cytosolic protein kinases in the nerve terminals of Xenopus tadpoles and the effect of ATP is probably mediated by the binding of membrane surface purinoceptors which in turn activates cytosolic protein kinases and increases ACh release.