The effect of the ghrelin receptors inhibitor [D-Lys3]-GHRP-6 on the levels and metabolism of monoamines in symmetric brain areas of rats treated chronically with alcohol

Abstract

Aim. In the course of the study, the impact of the ghrelin receptor GHS-R1a on the condition of symmetric monoaminergic systems of the rat brain was investigated. In particular, it was intended to find out whether the treatment with the ghrelin receptor antagonist [D-Lys3]-GHRP-6, recover the original content of monoamines and their metabolites in the brain of chronic alcoholic rats.
 Methods. The experiments were performed on 22 Wistar male rats. Experimental animals instead of drinking water received 10 % ethanol solution. Rats of the control groups continued to consume tap water. 6 months after the beginning of forced chronic alcohol treatement, 6 rats treated with alcohol, and 6 rats received water, in a month, once in three days, were instilled intranasally with the ghrelin antagonist [D-Lys3]-GHRP-6 (1 мкг/мкл, with 10 µl to each nostril). The other animals in the same manner were administered an equivalent volume of saline. 80 minutes after the last intranasal administration of drugs, rats were decapitated. With the HPLC-method, in the hypothalamus, olfactory tubercle, striatum and hippocampus of the left and right sides of the brain the contents of noradrenaline (NA), dopamine (DA), dioxyphenylacetic acid (DOPAC), homovanillic acid (HVA), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were measured. The results were processed by Student’s t-test using the statistical software package GraphPad Prism 6.0.
 Results. In the control rats (not exposed to either ethanol or drug) in the left striatum revealed a significant predominance of 5-HIAA compared to the same parameter of the other side of the brain. Under the condition of chronic ethanol intake, the initial left-sided asymmetry disappeared. Ethanol increased the content of 5-HT in the left hippocampus, 5-HIAA in the right olfactory tubercle and DA – in the right hypothalamus. [D-Lys3]-GHRP-6, when administered intranasally to the intact rats, significantly increased the 5-HIAA/5-HT ratio in the right olfactory tubercle, and the 5-HIAA, DOPAC and HVA levels – in the right striatum. In contrast, the left-sided effects in hippocampus were observed: the 5-HT levels increased and the 5-HIAA/5-HT ratio decreased. When instilled to intact rats, [D-Lys3]-GHRP-6 does not alter the monoaminergic systems of the hypothalamus. Between the monoaminergic systems of intact animals and alcoholic rats treated with [D-Lys3]-GHRP-6, the significant differences were shown. So, in the left hippocampus of alcoholic rats treated with [D-Lys3]-GHRP-6, the 5-HT level was higher, and the 5-HIAA/5-HT ratio was lower than in the control intact animals. Besides, in the right striatum of alcoholic rats treated with [D-Lys3]-GHRP-6, the DA metabolites levels were higher than those in the intact control animals. When comparing two groups of rats treated with [D-Lys3]-GHRP-6 (consumed water and alcoholic), the only difference was found: the alcoholic animals the content of DA in the left hypothalamus was lower than that of rats consumed water.
 Conclusion. Thus, by its influence on the monoaminergic system of the brain, [D-Lys3]-GHRP-6 is not an antagonist of the ethanol. Rather ethanol, when administered chronically, reduces the reactivity of the majority of monoaminergic systems to the ghrelin antagonist. Herewith, the forced chronic treatement with ethanol selectively increases the sensitivity to the [D-Lys3]-GHRP-6 in the hypothalamus DA-ergic system