The Effects of Insulin on Spike Activity of the Suprachiasmatic Nucleus Neurones and Functional State of Afferent Inputs from the Arcuate Nucleus in Rats

In experiments on the sagittal hypothalamic slices of maleWistar rats, the effects of 15 nM insulin on the level of spikeactivity, parameters of spike information coding by the suprachiasmatic nucleusneurones, and functional state of afferent inputs to the neuronesfrom the arcuate nucleus were studied. Application of insulin induceda decrease in the frequency of action potential generation and anincrease in the entropy of interspike interval distribution in 33.3%of neurones recorded; in 12% of cells, the responses of oppositedirection were found; in the remaining 54.7% neurones, spike activitydid not change. The responses of the entropy of interspike interval distributionsuggest the related changes in a degree of interspike interval irregularityinduced by insulin. To characterise afferent inputs to the suprachiasmaticnucleus neurones from the arcuate nucleus, electrophysiologicaltechnique of the construction and analysis of the peristimulus time histogram(PSTH) was used. Statistically significant responses to the stimulationof the arcuate nucleus were recorded in 24 of 38 neurones of thesuprachiasmatic nucleus. In 6 of the neurones, the responses werein the form of a short-latency (< 20 ms) excitation, in 1 neuronein the form of a long-latency excitation, in 6 neurones in the formof a short-latency inhibition; in 11 neurones complex two- or three-phaseresponses in the form of different compositions of excitation and inhibitionwere observed. Application of 15 nM insulin induced a qualitativetransformation of the responses (disappearing of the initial responsesor emergence of new responses) in 5 neurones initially respondedto stimulation, and in 1 neurone initially not responded to stimulationof the arcuate nucleus. Statistically significant changes in thelatency or duration of the responses in the presence of insulinwere not found. The results of the study suggest the ability ofinsulin to influence the activity level and the spike code of arespectively numerous population of neurones in the suprachiasmaticnucleus circadian oscillator as well as modulate the functionalstate of afferent inputs to the circadian oscillator from hypothalamicarcuate nucleus playing an important role in the control of appetiteand metabolism.