Journal of Neurophysiology, Vol 65, Issue 5 1150-1157, Copyright © 1991 by APS
Delayed excitatory connections in the flight system of the locust
R. M. Robertson
Department of Biology, Queen's University, Kingston, Ontario, Canada.
1. Synaptic interactions between identified neurons in the flight system of
the locust were investigated by the use of standard intracellular recording
and staining techniques. The intent was to determine the distribution and
functional significance of delayed excitatory connections, which have been
previously described. 2. For one inhibitory connection it was demonstrated
that subthreshold depolarization of the presynaptic neuron was sufficient
to cause release of transmitter at the synapse. This established the
existence of graded interactions between spiking flight neurons. 3. Three
inhibitory interneurons were found to cause delayed excitatory responses in
several other neurons. Often these were coupled with direct inhibitory
connections between the same pre- and postsynaptic neurons, resulting in an
inhibitory/excitatory (I/E) postsynaptic potential (PSP). The two phases of
this PSP were variable. 4. Delayed excitatory connections appeared powerful
while the flight system was inactive. However, these connections were
disabled during flight rhythms at the phase when the presynaptic neuron was
depolarized and firing action potentials. This was likely to be due to the
nature of the disynaptic disinhibitory interaction being via (an)
intervening neuron(s) with oscillating membrane potentials and thresholds
for release of transmitter. 5. Thus connections demonstrated when flight
rhythms were not expressed changed their character during flight rhythms.
The delayed excitatory connections in this system probably reflect complex
circuits of inhibition mediated by graded interactions and have little
functional significance as phenomena in their own right.
