A study is performed of the effect of the convulsants picrotoxin and bicucuUine, blockers of GABA-dependent C1--conductivity, on the rate of desensitization of muscimol-induced ~6C1-entry into synaptoneurosomes isolated from rat cortex. Both picrotoxin and bicuculline, despite the difference in the mechanisms of inhibition of the GABA receptor/C1-ionophore complex, markedly reduce the rate of desensitization. However, the initial moment of the action of both convulsants is characterized by inhibition of C1-transport alone, without a drop of the rate of desensitization.Key Words: desensitization; picrotoxin; bicuculline; synaptoneurosomes The GABA receptor/C1-ionophore complex is one of the most important molecular structures of preand postsynaptic membranes. Binding of GABA or its agonists (in particular, muscimol) with the GABA receptor opens up the coupled C1-channel and leads to a transient increase of the C1-permeability of neuronal membranes and, correspondingly, to spike inhibition. Long-term action of the transmitter results in desensitization of the receptor/ionophore complex, i.e., in a decrease of GABA-dependent C1-permeability [2,3].In our previous study we showed that pentylenetetrazole inhibition of muscimol-dependent 36C1 entry into synaptoneurosomes is accompanied by a considerable delay of desensitization. Here we report on experiments with the classic blockers of the GABA receptor/C1-ionophore complex bicuculline and picrotoxin, which act through different mechanisms. Bicuculline inhibits binding of GABA with the GABA receptor in a competitive manner MATERIALS AND METHODSIn our experiments we used the methods described earlier [1] with some modifications.White random-bred male rats weighing 180-200 g were decapitated, and the cerebral cortex was removed and homogenized manually (5 frictions) at 0-4~ in a glass homogenizer with a Teflon pestle in Krebs-Ringer medium containing (in mM): 145 NaC1, 5 KC1, 1 MgSO 4, 1 CaC 89 10 glucose, and 10 HEPES, pH 7.4 (20~ 15 ml/g tissue). The homogenate was sequentially •tered through 300-, 99-, 60-, and 27-~t capron sieves (Rakhmanov plant, Russia). The filtrate was centrifuged at 2700 g for 5 min, resuspended in the same volume of KrebsRinger medium, and recentrifuged under the same conditions. After the second centrifugation the pellet was suspended in Krebs-Ringer solution so that the final concentration of synaptoneurosomes was 4

Early in ontogeny, various parts of the nervous system, including the hippocampus of newborn rats, generate a special rhythmic activity, giant depolarizing potentials [1]. The latter are supposed to play an important role in the body growth and connection formation [2]. Giant potentials are accompanied by synchronous excitation of a large group of cells [3], which testifies to an increased neuronal activity in the hippocampus of newborn animals.When neuronal activity is high, adenosine triphosphate (ATP) is released into extracellular space to modulate synaptic transmission [4]. Histochemical analysis showed the existence of P2X2, P2X4, and P2X6 receptors in the newborn rat hippocampus [5]. Recently, ATP and adenosine were shown to modulate the frequency of giant depolarizing potentials and spontaneous postsynaptic potentials in the newborn rat hippocampus via P2X-subclass andenosine A1 and, presumably, ATP receptors [6,7]. However, the effects of many antagonists and agonists of purinergic receptors are nonspecific, which makes it difficult to evaluate the involvement of specific subtypes of purinergic receptors. The purpose of this study was to determine the role of P2X purinergic receptors in the modulation of giant depolarizing potentials.Sections of the hippocampus of two-to six-day-old rats ( n = 8) were made. The rats were preliminarily anesthetized with an intraperitoneal urethane injection (2 g/kg body weight). Sections (500 µ m thick) were obtained using a Leica VT1000S vibratome (Germany). Dissection was conducted with the use of a cooled artificial cerebrospinal fluid (ACSF) that contained (mM): NaCl, 130; KCl, 3.5; NaH 2 PO 4 , 1.2; NaHCO 3 , 25; MgCl 2 , 1.3; CaCl 2 , 2; glucose, 25. The fluid was oxygenized with carbogen (95% O 2 and 5% CO 2 ) (pH 7.3-7.4). After 1 h of incubation of the sections in ACSF at room temperature, the cellular and network activities were recorded using the patch-clamp method in the whole-cell configuration as described previously [7]. The patch-clamp electrodes were made from thin borosilicate glass; their resistance was 5 − 7 M Ω when filled with a solution of the following com-(a) (b) (c) 20 mV 10 s Fig. 1. Changes in the frequency of giant depolarizing neuronal potentials of the hippocampal NA3 field after the application of 50 µ M TNP-ATP and changes four minutes after washing of the latter. Records: (a) spontaneous activity in the control; (b) after TNP-ATP application; (c) after washing.