Using high-performance liquid chromatography, gaschromatography and chromato-mass spectrometry methods a novel endogenous cyclic dipeptide cyclo-prolylglycine was identified in rat brain. Its content according to gas chromatography is 2.8 + 0.3 nmol/g wet brain. Synthetic cyclo-prolylglycine has demonstrated antiamnesic activity in the passive avoidance test in rats at a dose of 0.1 mg/kg i.p. Cyclic dipeptide cyclo-prolylglycine seems to be a memory facilitating substance and its presence in rat brain suggests the existence of a new mechanism of memory regulation.Key words: Cyclo-prolylglycine; Endogenous cyclic dipeptide; Memory facilitating substance displays activity due to interaction with one vasopressin receptor subtype. We have found out, however, that unlike piracetam, which facilitates the initial processing of memory engram as well as consolidation, the dipeptide pGlu-Asn-NH2 improved only the phase of information input and retrieval, but did not influence consolidation [10]. On the other hand, proline-containing dipeptide analogues of piracetam facilitated consolidation [11]. The cyclic dipeptide cyclo-(Pro-Gly) turned out to be the most active of them [12]. This cyclic dipeptide can be assumed to be one of the endogenous ligands of hypothetic 'nootropic receptors', which selectively regulates the processes of training and memory. This work is devoted to the identification of endogenous cyclo-(Pro-Gly) in rat brain.

The metabolism of a new piracetam analogue, the dipeptide cognitive enhancer N-phenylacetyl-L-prolylglycine ethyl ester (GVS-111) was studied in vivo. GVS-111 itself was not found in rat brain 1 h after 5 mg/kg i.p. administration up to limit of detection (LOD) under high performance liquid chromatography (HPLC) conditions. Three substances corresponding to the three possible GVS-111 metabolites, namely phenylacetic acid, prolylglycine and cyclo-prolylglycine, were found in experimental rat brain samples as well as in controls using HPLC, gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) methods. Only cyclo-prolylglycine concentration increased (2.5-fold) 1 h after GVS-111 administration. Cyclo-prolylglycine formation from GVS-111 in the presence of plasma and brain enzymes was shown in vitro. These data could be considered as evidence that GVS-111 is prodrug which converts in the body to cyclo-prolylglycine, and which is identical to the endogenous cyclopeptide that produces the nootropic activity.

Experiments on two mouse strains with opposite reactions to emotional stress showed selectivity of the anxiolytic effect of endogenous dipeptide cycloprolylglycine. In the open field test cycloprolylglycine (0.01-0.10 mg/kg intraperitoneally) dose-dependently (1.8-2.1-fold) increased motor activity of BALB/c mice with manifest fear reaction and had no effect on C57Bl/6 mice with active behavior. The content of endogenous cycloprolylglycine in mouse brain correlated with the type of emotional stress reaction: its content in the brain of C57Bl/6 mice 1.5 times surpassed that in BALB/c mice. It is concluded that cycloprolylglycine is involved in the endogenous regulation of fear reaction.

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Pharmacokinetics of GVS-111, a new acylprolyldipeptide with nootropic properties and its penetration across the blood-brain barrier were studied in rats using HPLC. It was found that the dipeptide is absorbed in the gastrointestinal tract, enters the circulation, and penetrates through the blood-brain barrier in an unmodified state.