Neural events underlying learning in insects: changes in pacemaker

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Abstract
Computer-controlled operant-conditioning training procedures were used to raise (up-learning) or lower (down-learning) the mean frequency of discharge of the anterior adductor coxa motoneurone of the locust Schistocerca gregaria. Intracellular recordings were made from the soma of the motoneurone during training. The neurone appeared capable of spontaneous discharge in the absence of synaptic input since its mean pacemaker rate was measured after blocking synaptic inputs by infusing high Mg$^{2+}$/zero Ca$^{2+}$ saline into the neuropile associated with the neurone. Rates were determined before and after the training procedure was applied. It was found that a stable increase in the mean frequency of the pacemaker occurred during up-learning and a decrease during down-learning. The pacemaker shift accounted for a little over half the overall learning change. The remainder was attributed to changes in the activities of interneurones that directly, or indirectly, affect the motoneurone pacemaker. Conventional synaptic potentials that could have accounted for the remainder were not conspicuous in the soma recordings.