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

Abstract

In experiments on the sagittal hypothalamic slices of male Wistar rats, the effects of 15 nM insulin on the level of spike activity, parameters of spike information coding by the suprachiasmatic nucleus neurones, and functional state of afferent inputs to the neurones from the arcuate nucleus were studied. Application of insulin induced a decrease in the frequency of action potential generation and an increase in the entropy of interspike interval distribution in 33.3% of neurones recorded; in 12% of cells, the responses of opposite direction were found; in the remaining 54.7% neurones, spike activity did not change. The responses of the entropy of interspike interval distribution suggest the related changes in a degree of interspike interval irregularity induced by insulin. To characterise afferent inputs to the suprachiasmatic nucleus neurones from the arcuate nucleus, electrophysiological technique of the construction and analysis of the peristimulus time histogram (PSTH) was used. Statistically significant responses to the stimulation of the arcuate nucleus were recorded in 24 of 38 neurones of the suprachiasmatic nucleus. In 6 of the neurones, the responses were in the form of a short-latency (20 ms) excitation, in 1 neurone in the form of a long-latency excitation, in 6 neurones in the form of a short-latency inhibition; in 11 neurones complex two- or three-phase responses in the form of different compositions of excitation and inhibition were observed. Application of 15 nM insulin induced a qualitative transformation of the responses (disappearing of the initial responses or emergence of new responses) in 5 neurones initially responded to stimulation, and in 1 neurone initially not responded to stimulation of the arcuate nucleus. Statistically significant changes in the latency or duration of the responses in the presence of insulin were not found. The results of the study suggest the ability of insulin to influence the activity level and the spike code of a respectively numerous population of neurones in the suprachiasmatic nucleus circadian oscillator as well as modulate the functional state of afferent inputs to the circadian oscillator from hypothalamic arcuate nucleus playing an important role in the control of appetite and metabolism.