The Role of Sheath Cells in Glutamate Uptake by Insect Nerve Muscle Preparations

Octopamine inhibits synaptic transmission at the larval neuromuscular junction in Drosophila melanogaster

The effect of octopamine, a biogenic amine, on synaptic transmission at the neuromuscular junction (NMJ) in first instar larvae of Drosophila melanogaster was examined using the patch clamp technique. Muscle cells were voltage-clamped at -60 mV in the whole-cell configuration, and nerve-evoked excitatory junctional currents (EJCs) and miniature excitatory junctional currents (MEJCs) were recorded. Octopamine significantly decreased the mean amplitude of nerve-evoked EJCs in a dose-dependent manner and increased the failure rate. However, the mean amplitude and amplitude distribution of MEJCs were not affected by octopamine. These results suggest that octopamine is acting presynaptically. This effect was abolished by pretreatment with the octopamine receptor blocker, yohimbine. On the other hand, octopamine significantly decreased the decay time constant of MEJCs from 6.0+/-0.3 ms (mean+/-S.E., n=16) to 4.2+/-0.3 ms (n=14) (p<0.001), which might be the effect on the kinetic properties of junctional glutamate receptor channels. However, the mean open time of extrajunctional glutamate receptor channels was not changed by octopamine. Taken together, these results suggest that octopamine inhibits synaptic transmission by affecting both pre- and postsynaptic mechanisms.

Miniature endplate currents at the newly formed neuromuscular junction in Drosophila embryos and larvae

Miniature endplate currents (mepcs) were recorded in embryos and larvae of Drosophila melanogaster with the patch clamp technique in the whole cell configuration from abdominal ventral oblique muscles. Shortly after nerve-muscle contacts (16 h after egg laying) mepcs were small and infrequent. The mean amplitude of mepcs increased abruptly during late embryonic stages but did not change any further up to second instar larvae. The amplitude distribution of mepcs was skewed toward larger amplitudes and changed drastically to a more broader distribution at the time of hatching, but did not change any further. These muscle cells were found to be electrically coupled to those in the rostral neighbor segment. The synaptic current has a fast rising phase and decayed with a single exponential throughout the developmental period examined. The decay time constant increased with amplitude but the rise time did not. Neither the rise time nor the decay time constant changed with development. Many synaptic currents decayed in a stepwise manner revealing underlying single channel events. Two discrete steps were discerned, with amplitudes of about 7.6 and 16 pA at -60 mV. These two types of channels were observed throughout the stages examined.

Inhibition of GABA uptake potentiates the effects of exogenous GABA on locust skeletal muscle

Insect skeletal muscle is relatively insensitive to applied GABA, responses are elicited only when relatively high concentrations of GABA are used (greater than 10(-6) M). Pretreatment of the muscle with the GABA uptake inhibitors nipecotic acid, beta-aminobutyric acid or beta-alanine increases the sensitivity of the muscle to GABA by as much as 1000-fold. The evidence suggests the existence of a GABA uptake mechanism in the insect neuromuscular system which could reside in glial cells.

Inhibition of the glutamate uptake in the excitatory neuromuscular synapse of the locust by delta-philanthotoxin; a component of the venom of the solitary wasp Philanthus triangulum F. A high resolution autoradiographic study

Glutamate uptake in insect neuromuscular junctions is inhibited by delta-philanthotoxin. Two other components of the philanthus venom (beta- and gamma-philanthotoxin) do not affect this glutamate uptake. No synergism between the active delta-philanthotoxin and the two inactive components had to be assumed to explain the inhibition by the Philanthus venom.