Abstracts

The anaesthetic agent isoflurane exerts its anticonvulsant effect on the central nervous system by suppressing excitation and enhancing inhibition. Most studies devoted to investigation of the mechanism of isoflurane action have been performed on culture and slice preparations from adult animals. However the mechanism of the anticonvulsant action of isoflurane on the immature brain is unclear. The influence of isoflurane on GABAergic system and neuronal network activity was studied in hippocampal slices from immature rat brain., Sprague-Dawley rat pups (postnatal days 8-12) were used throughout the study. Effects of isoflurane (2.5%) were recorded using extracellular and whole-cell patch technique. All intracellular recordings were performed at 0mV (reversal potential for excitatory synaptic transmission)., We studied the effect of isoflurane on multi-unit activity (MUA), giant depolarizing potentials (GDPs), spontaneous inhibitory postsynaptic currents (sIPSC) and tonic GABA currents recorded from the CA1 pyramidal cell layer while extracellular K⁺ ([K⁺][sub]o[/sub]) was changed in the range of 3.5-6mM. Application of isoflurane on hippocampal slices in normal [K⁺][sub]o[/sub] (3.5mM) solution significantly decreases amplitude and increase the inter-event interval of MUA. In whole-cell recordings isoflurane significantly increased the tonic GABA conductance; prolong decay and decrease the frequency of sIPSC. Increasing [K⁺][sub]o[/sub] to 6 mM led to increasing of frequency of GDPs. Adding of isoflurane to high [K⁺][sub]o[/sub] solution had a biphasic effect on GDPs. At first the frequency GDPs were two-fold increased followed by decreasing in amplitude (59.4 [plusmn] 1.1 %), increasing in duration (62.8 [plusmn] 3.3%) and frequency (43.2 [plusmn] 3.4%) of GDPs. There were not significant differences between the effect of isoflurane on MUA, sIPSCs and tonic GABA currents in two studied conditions (normal and high [K⁺][sub]o [/sub]solution). Gabazine added to extracellular solution in concentration, which selectively blocks synaptic GABA[sub]A [/sub]receptor (1[micro]M), increased MUA frequency, blocked sIPSCs, GDPs and had no apparent effect on tonic GABA conductance. In 6mM [K⁺][sub]o[/sub] solution application gabazine evoked interictal discharges. After application of isoflurane, the frequency of gabazine-induced interictal events decreased from 0.30 Hz to 0.25 Hz. Outward, bicuculline sensitive currents appeared in the presence of isoflurane simultaneously with interictal discharges. Anticonvulsive effect of isoflurane was completely abolished by adding 15 [mu]M bicuculline into extracellular solution., Our results demonstrate that isoflurane has multiple effects in the immature brain. Even though GABA is depolarizing in the immature brain, this volatile anaesthetic decreases the synchronization of neuronal activity, mainly through enhancing the GABAergic inhibition., (Supported by the Epilepsy Foundation.)