The Effects of Ghrelin on Spike Activity of the Suprachiasmatic Nucleus Neurones of the Rat

In one of the most important non-photic mechanisms of the circadian biological clock synchronization with environmental geophysical 24 h rhythm, information on feeding schedule, composition and calorie content of food is used. Hormone ghrelin, a product of the neuroendocrine oxyntic cells of the gastric mucosa to be a signal molecule within this mechanism. In experiments on sagittal hypothalamic slices of male Wistar rats, the effects of 25 nM ghrelin on spike activity and parameters of spike information coding were investigated. Application of ghrelin induced an increase in spike frequency and a decrease in entropy of interspike interval distribution in 32.1% of the neurones recorded. In 29.6% of the cells, opposite responses in the form of a reduction of activity and an increase in the entropy of interspike interval distribution were observed. Parameters of spike activity of the reminder 38.3% neurones of the suprachiasmatic nucleus did not change. The observed responses of the entropy of interspike interval distribution indicate the appropriate changes in a degree of irregularity of interspike interval induced by ghrelin. Application of selective high-affinity antagonist of GHS-R1a receptor, JMV 2959 (100 nM) did not induce responses of the investigated parameters of spike activity but completely prevented changes of both, spike frequency and entropy of interspike interval distribution observed in the presence of ghrelin. The obtained data show that hormone ghrelin by a direct influence on the suprachiasmatic nucleus in vitro modulates the activity level and spike code of relatively numerous population of neurones of the nucleus, wherein the effects of ghrelin are implemented via GHS-R1a receptors. The results of the current study provide additional evidence in favour of the hypothesis on the involvement of ghrelin in mechanisms of non-photic entrainment of the circadian biological clock in accordance with severity of food motivation and level of metabolism.