Long-term potentiation at peripheral synapses

The effect of ions and second messengers on long-term potentiation of chemical transmission in avian ciliary ganglia

1. The effects of tetanic stimulation of the oculomotor nerve on transmission through the avian ciliary ganglion have been determined by use of the amplitude of the compound action potential recorded in the ciliary nerve, in the presence of hexamethonium (300 microM), as a measure of synaptic efficacy. 2. Tetanic stimulation for 20 s at 30 Hz potentiated the chemical phase of the compound action potential by at least 100% of its control level. This potentiation, reflecting an increase in synaptic efficacy, decayed over two distinct time courses: firstly, a rapid decay with a time constant in the order of minutes, and secondly, a slower decay, representing a smaller potentiation, with a time constant in the order of an hour. The large increase in synaptic efficacy is attributed to post-tetanic potentiation (PTP) whereas the smaller but longer lasting increase is attributed to long-term potentiation (LTP). 3. Higher frequencies of tetanic stimulation gave increased PTP and LTP. 4. In order to test whether the influx of calcium ions into the nerve terminal during the tetanus is likely to be involved in potentiation, facilitation was measured during PTP and LTP. Facilitation was reduced to approximately zero during PTP but recovered to normal values about 15 min into LTP. A requirement for the induction of LTP was shown to be the presence of calcium in the bathing solution. However, blocking synaptic transmission with a high concentration of hexamethonium (3 mM) during the tetanic stimulation did not block the induction of LTP. 5. Application of the muscarinic inhibitor, atropine (2 microM), did not affect the magnitude of PTP or LTP. 5. Application of the muscarinic inhibitor, atropine (2 tM), did not affect the magnitude of PTP or LTP.6. The activator of protein kinase C, phorbol 12,13-dibutyrate (2 microM) potentiated synaptic transmission and reduced the potentiation due to PTP although it did not affect that due to LTP, but the inhibitor of this kinase, staurosporine (0.5 microM), partially blocked the appearance of LTP without affecting PTP after the tetanus.7. An inhibitor of calmodulin, W-7 (5 microM), reversibly blocked the appearance of LTP significantly after a tetanus although the size of PTP was not affected.8. The results presented here suggest that the initiation of LTP in the ciliary ganglion is due to an influx of calcium ions into the calyciform nerve terminal during the tetanus and that the mechanism for LTP involves a calcium-calmodulin-dependent process.

Transient increase of calcium in pre- and postsynaptic organelles of rat superior cervical ganglion after tetanizing stimulation

The stimulation-dependent translocation and redistribution of intracellular calcium was studied in synapses of the rat superior cervical ganglion. For the cytochemical demonstration of calcium at the ultrastructural level the oxalate-pyroantimonate technique was used. Calcium-containing deposits were observed as electron-dense particles in synaptic vesicles and occasionally in presynaptic mitochondria. On the postsynaptic side, vesicles and vacuoles in dendrites contained increased amounts of reaction product. Following a short train of impulses (20 s, 20 c.p.s.), the ganglionic response to a single stimulus was increased. This potentiation effect which lasted about 1-5 min was accompanied by a change in the distribution of calcium at fine structural level. In ganglia fixed 1 and 5 min after the train, but not later, the number of calcium-containing synaptic vesicles and postsynaptic vacuoles increased significantly.