Spontaneous excitatory postsynaptic currents in crayfish neuromuscular junctions in the absence and presence of serotonin and 3,4-diaminopyridine.

Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded under voltage clamp in short fibres (l less than or equal to 0.6 mm) from opener muscles and the contractor epimeralis muscle of small crayfish. From the amplitude distributions of sEPSCs which could be approximated by a Gaussian function, a mean amplitude ã = -1.16 nA +/- 0.28 (SE) was found for sEPSCs in 16 fibres of the claw opener voltage clamped to E = -60 mV (19-22 degrees C). In the opener of the first walking leg and in the contractor epimeralis muscle ã = -1.1 nA +/- 0.21 (SE; n = 6, -100 mV less than or equal to E less than or equal to -60 mV, 5-10 degrees C) and ã = -2.0 nA +/- 0.2 (SE; n = 4, E = -60 mV, 19-22 degrees C) were obtained. On average about 300-500 synaptic channels were estimated to open during a sEPSC. 'Giant' sEPSCs (gsEPSCs) were also observed. The amplitudes of gsEPSCs were up to 14 times larger than the amplitude of an average normal sEPSC. Moreover, the lifetime of gsEPSCs was up to about 3 times longer than that of sEPSCs. Like sEPSCs, gsEPSCs could not be abolished by 0.1 mumol/l tetrodotoxin. The rate at which sEPSCs and gsEPSCs occurred could be markedly enhanced by serotonin (1 mumol/l) and 3,4-diaminopyridine (1 mmol/l).