The presynaptic effect dominated the postsynaptic one, resulting in increased EJP amplitudes in GABA concentrations of 1[0 sup.-5] mol [1.sup.-1] and above.
We also found that GABA increased the gastric mill 4 (gm4) muscle excitatory junctional potential (EJP) while decreasing muscle fiber input resistance, indicating that the effect on contraction is mediated at least partially by presynaptic actions at the neuromuscular junction.
Abbreviations: dgn, dorsal gastric nerve; dvn, dorsal ventricular nerve; EJP, excitatory junctional potential; GABA, [gamma]-aminobutyric acid; gm4, gastric mill 4 muscle; lvn, lateral ventricular nerve; LC/MS, reversed-phase liquid chromatography-mass spectrometry; MPN, modulatory proctolin neuron; POs, pericardial organs; STG.
Motoneuron-evoked EJPs at B19-Slrt synapses were first detected in the ipsilateral musculature on day 6 after crush of the buccal nerves.
This decline in spontaneous neuromuscular synaptic events coincided with a marked increase in the incidence of evoked EJPs recorded in these regenerating motor systems (Fig.
The only observed difference in these studies was that spontaneous mEJPs were still detected in greater than 50% of contralateral Slrt muscle fibers by 6-8 days post-crush, and less than 40% of muscle fibers exhibited evoked EJPs by days 9-11 post-crush.
For example, B19-evoked EJPs were detected in more than 80% of Slrt muscle fibers of the ipsilateral musculature.
This inhibitory connection forms at precisely the time (6-8 days post-crush) when synaptic boutons and evoked EJPs appear in the peripheral musculature, and it remains strong and sustained until day 14 post-crush.
In contrast, B19-evoked EJPs are not detected at this time (Fig.
Neuron B19 when stimulated in intact neuromuscular preparations induces high-amplitude EJPs, associated with strong contractions of the Slrt muscle, and lower amplitude EJPs and weaker contractions of the Aj muscle (Fig.
To determine whether these weak muscle contractions were associated with a reduction in synaptic efficacy, we performed quantitative electrophysiological analyses of EJP amplitudes.