5-HT offsets homeostasis of synaptic transmission during short-term facilitation

Abstract

In this study, we approach the topic of vesicle recruitment and recycling by perturbing neurotransmission at the crayfish neuromuscular junction with altered electrical activity and the presence of the neuromodulator serotonin (5-HT). After induction of short-term facilitation (STF) with stimulus pulse trains (40 Hz, 20 pulses), the amount of synaptic transmission can be maintained at a relatively constant level, producing a plateau in the amplitude of the excitatory postsynaptic potentials (EPSPs) throughout the remaining stimuli within a train of a few hundred milliseconds. With an increase in the frequency of the stimuli within a train (60 Hz, 20 pulses), an altered plateau of larger EPSP amplitudes occurs. This suggests that differential rates of vesicle recruitment can be rapidly reached and maintained. Exposure of nerve terminals to 5-HT further enhances the EPSP amplitudes to yet a higher plateau level. The effect of 5-HT is more pronounced for 40-Hz pulse trains than for 60-Hz trains. This suggests that 5-HT can recruit vesicles into the readily releasable pool (RRP) and that the recruitment is limited at higher stimulation frequencies. The attainment of a larger amplitude in the plateaus of the EPSPs at 60 Hz compared with 40 Hz also suggests that the rapid induction of STF enhances the entry of vesicles into the RRP. By direct quantal counts, mean quantal content increases linearly during STF, and 5-HT offsets the linear release. We propose that 5-HT and electrically induced recruitment of vesicles from a reserve pool to the RRP may share similar recruitment mechanisms.

Physiological Recordings of High and Low Output NMJs on the Crayfish Leg Extensor Muscle

“…Since serotonin increases the number of vesicles that are released with evoked stimulation, and since it promotes recovery during depression, it is apparent that there is some modulation of the vesicle pools for enhancing the probability of fusion with serotonin present (Johnstone et al, 2008;Logsdon et al, 2006;Sparks et al, 2004). Models that would explain the dynamics of the vesicle pools within the presynaptic nerve terminal during low frequency stimulation, as opposed to a depressed state, also need to be considered among the various experimental protocols that are possible with this preparation.…”

Depressing Effect of Caffeine at Crayfish Neuromuscular Synapses II. Initial Search for Possible Sites of Action

“…In crayfish, serotonin's enhancing effects on EPSPs have a presynaptic loci (Strawn et al, 2000), do not involve changing intracellular Ca ++ concentrations (Delany et al, 1991) and is generated via IP 3 and cAMP second messenger cascades (Wang and Zucker, 1998). It appears that IP 3 triggered protein kinase activation is affecting some proteins in the exocytotic machinery making their role more efficient during exocytosis (Sparks and Cooper, 2004), probably by increasing the size of the releasable vesicle pool activated by the arrival of the action potential (Wang and Zucker, 1998).…”

Serotonin and Synaptic Transmission at Invertebrate Neuromuscular Junctions

“…Both electrophysiological and morphological studies imply that each of these two muscles is innervated by only 2 axons [66, 67]. Application of 5-HT to these NMJs appears to slightly depress synaptic strength [68,69]. We are not aware of any attempt to investigate actions of 5-HT on adult skeletal NMJs.…”

“…This notion of an internal release of Ca 2+ is also supported by experiments conducted by Glusman and Kravitz [91] in which they showed that a calcium-free bath, along with EGTA and high MgCl 2 , 5-HT could still cause spontaneous release of transmitter for lobster NMJs. The enhanced spontaneous and evoked fusion events relates to an increase in 'n' (number of sites) and 'p' (probability of release) to explain the enhanced 'm' (mean quantal content; m=np) after exposure to 5-HT [69, 93, 94]. An interesting observation, but not yet explained mechanistically, is that 5-HT produced an effect with low or zero extracellular calcium at a crayfish NMJ but 5-HT's effect depended on extracellular sodium concentration [89].…”

“…Vertebrate 5-HT 2 -like receptors were physiologically identified for Procambarus clarkii at NMJs [21-24, 69]. Since this subtype of receptor has been sequenced in a crab and crayfish [28] these may be the subtypes present at the NMJs; however the blockers for the vertebrate 5-HT 2 -like receptors could not block the entire 5-HT enhancement of synaptic enhancement [24].…”