I. O. Shymanskyy, G. V. Donchenko, A. P. Klimenko, T. M. Kuchmerovska
It has been previously shown that diabetes-associated central nervous system abnormalities are characterized by progressive alterations of neurotransmission. In particular, recent studies from our group have demonstrated that more early diabetes is accompanied by the increased spontaneous serotonin release from isolated synaptic endings; however the mechanism is still not clear. The current study was undertaken to estimate the relative importance of membrane potential and extracellular Ca2+ in the serotonin secretion process in diabetes. With the premise that increased phosphorylation of target proteins may be responsible for the increase in transmitter release we tested whether cAMP/PKA-mediated phosphorylations as well as mono-ADP-ribosylation of effector proteins were implicated in diabetes-associated brain failures. In addition, the effects of nicotinamide, a multiple-action compound, were examined. It was shown that diabetes caused a significant increase in spontaneous release of [2-14C]serotonin that was accompanied by synaptic membranes depolarization. Omission of Ca2+ from the incubation medium largely inhibited serotonin release only in untreated diabetes. Exposure of diabetic synaptosomes to cAMP-dependent protein kinase inhibitor H89, similar to Ca2+-free medium, downregulated serotonin release. The level of constitutively mono-ADP-ribosylated proteins of diabetic synaptosomes was elevated vs control. Protein mono-ADP-ribosylation induced by cholera toxin (CTX), activator of Gs-protein-coupled adenylyl cyclase, resulted in excessive 1.2-fold enhancement over basal level but to the less extent in diabetes as compared with that of control. Nevertheless, CTX as well as forskolin exerted more strong stimulating effect on serotonin release from diabetic synaptosomes as compared to control. H89 counteracted CTX-related action on this variable strongly suggesting that impaired serotonin release is, at least, dependent on Gs-protein-mediated phosphorylation. Nicotinamide treatment virtually normalized both protein mono-ADP-ribosylation and serotonin release as well as synaptosomal response to all stimuli used. The data suggest that alterations in protein mono-ADP-ribosylation may be involved as a possible mechanism responsible for the impaired neurotransmission in diabetes and nicotinamide may efficiently protect against ADP-ribosylation-mediated abnormalities in brain function.
Key words: nicotinamide, synaptosomes, serotonin release, membrane potential, mono-ADP-ribosylation, сАМР-dependent protein kinase.
The original article in Ukrainian is available for download in PDF format.
© The Ukrainian Biochemical Journal