NEUROMODULATORS ENHANCE TRANSMITTER RELEASE BY 2 SEPARATE MECHANISMS AT THE INHIBITOR OF CRAYFISH OPENER MUSCLE

A presynaptic voltage control method has been used to investigate the modulatory effects of serotonin (5-HT) and okadaic acid (OA) on the in hibitory junction of the crayfish opener muscle. Instead of using acti on potentials, we used 20 msec pulses depolarized to 0 mV to activate transmitter release. This approach allowed us to monitor two separate physiological parameters related to the release process. The first par ameter, transmitter release kinetics, is characterized as the delay wh en inhibitory postsynaptic conductance reaches its half-maximum (IPSG( 50)). The second parameter, the total area of IPSG (IPSG(area)), estim ates total transmitter output. We have reported previously that the F2 component of synaptic facilitation is associated with a decrease in I PSG(50) but without a change in IPSG(area). These results raised the p ossibility that IPSG(50) and IPSG(area) could be mediated by separate mechanisms that were modulated independently. To explore this possibil ity, we investigated the effects of 5-HT (100-200 nM) and OA (2.5 mu M ) on the two parameters. 5-HT and OA enhanced IPSG neither by changing the sensitivity of postsynaptic receptors, as tested by iontophoretic ally ejected GABA, nor by elevating resting and action potential-activ ated presynaptic free calcium, as monitored by fura-2 imaging. 5-HT an d OA decreased IPSG(50) by 3.0 +/- 1.4 and 3.6 +/- 1.1 msec, respectiv ely, and increased IPSG(area) by 50 +/- 21 and 37 +/- 6%, respectively . The ability of F2 facilitation to accelerate release kinetics was re duced in the presence of the modulators, suggesting that the mechanism underlying the accelerated release kinetics was shared by the two mod es of synaptic enhancement. This report demonstrates that the accelera tion in release kinetics and the increase in total release are two sep arate mechanisms for enhancing transmitter output and that these two m echanisms can be activated without changes in presynaptic calcium dyna mics.