Introduction

Regulatory peptides play key roles in the formation, development, and normal functioning of the nervous system. They are not understood fully despite the accumulating experimental data in recent years. The study of their mechanisms of action is of particular interest because regulatory peptides have potential in the creation of safe drugs on their basis with specific clinical properties and direct physiological effects. One representative of this class of drugs is the synthetic regulatory peptide Selank. It was designed and produced at the Institute of Molecular Genetics, Russian Academy of Sciences, in cooperation with the V.V. Zakusov Research Institute of Pharmacology, Russian Academy of Medical Sciences. Selank is a synthetic analog of the endogenous tuftsin molecule (the short Thr-Lys-Pro-Arg fragment of the human immunoglobulin G heavy chain), which was elongated at the C terminus via the addition of three natural L-amino acids (Pro-Gly-Pro) to improve its metabolic stability and yield a relatively longer duration (Ashmarin et al., 2005; Ashmarin, 2007).

Selank has pronounced anxiolytic activity and acts as a stable neuropsychotropic, antidepressant, and antistress drug that relieves aggression and fear reaction in different animal species (Kozlovskii and Danchev, 2002; Sollertinskaya et al., 2008; Semenova et al., 2010). Selank also has a nootropic action, which positively influences the formation of memory and learning processes (Kost et al., 2001; Sokolov et al., 2002; Semenova et al., 2007, 2009; Narkevich et al., 2008), and marked immunomodulatory activity (Uchakina et al., 2008; Ershov et al., 2009; Andreeva et al., 2010).

Clinical studies have shown that the effect of Selank is similar to that of tranquilizers at low doses, but is not accompanied by the unwanted side effects of benzodiazepine tranquilizers such as amnesia, withdrawal, and dependence (Seredenin et al., 1990, 1998). Benzodiazepines are allosteric modulators of the type-A γ-aminobutyric acid receptor (GABA_AR) and can increase the inhibitory action of GABA, the major inhibitory neurotransmitter in the CNS. The similarity of the spectrum of physiological effects of Selank and classical benzodiazepines (such as diazepam and phenazepam) suggests that there is a similar basis of their mechanism of action; that is, the allosteric modulation of GABA_A receptors.

Previously, it was shown that in the presence of Selank the amount of the specifically bound ligand, [³H] GABA, varied, and preliminary intranasal administration of peptide also induced changes in the number of specific binding sites of [³H] GABA but did not affect the affinity of the receptors (V’Yunova et al., 2014). Based on these data, the authors suggested that Selank can lead to a rapid change in the state of the GABAergic system by binding the peptide to GABA receptors and, thus, allosterically modulating the activity of GABA_A receptor.

In this study, we evaluated the contribution of the GABAergic system to the molecular mechanism responsible for the anxiolytic action of Selank. To test the hypothesis that Selank acts through GABA_A receptors, we investigated its effect on changes in the mRNA levels of the genes encoding the major subunits of the GABA receptors, transporters and ion channels involved in the transport of GABA, and those of other proteins involved in neurotransmission in rat brain 1 and 3 h after administration of the peptide. To identify the effects associated with the activation of GABA_A receptors, we analyzed the changes in the expression of the investigated genes in response to the action of the primary ligand, GABA.