Effects of temperature on a central synapse between identified motor neurons in the locust

Changing the temperature from 10-40 degrees C modifies the transmission at an established monosynaptic connection between the fast extensor tibiae (FETi) and flexor tibiae motor neurons in the metathoracic ganglion of the locust Schistocerca gregaria (Forskål). Striking changes occur to the shape of the spikes, to membrane resistance, to the synaptic delay, and to the evoked synaptic potentials. In the presynaptic FETi motor neuron, raising the temperature reduces the amplitude of an antidromic spike recorded in the soma by a factor of 10 (40 mV to 4 mV), reduces the time taken to reach peak amplitude by 5 (3.5 to 0.7 ms) and decreases the duration at half maximum amplitude by 0.5. The conduction velocity of the spike in the axon is increased by 50% from 10 degrees C to 40 degrees C. Orthodromic spikes are affected by temperature in a similar way to the antidromic spikes. The membrane resistance of both pre- and postsynaptic motor neurons falls as the temperature is raised. The membrane resistance of FETi falls by a factor of 4 (about 4 M omega at 10 degrees C to 1 M omega at 40 degrees C). A contributory component to this fall could be the increase in the frequency of synaptic potentials generated as a result of inputs from other neurons. No temperature dependence could be demonstrated on the voltage threshold relative to resting potential for evoking orthodromic spikes, but because the resistance changes, the current needed to achieve this voltage must be increased at higher temperatures.(ABSTRACT TRUNCATED AT 250 WORDS)