Possible role of mitochondria in posttetanic potentiation of GABAergic synaptic transmission in rat neocortical cell cultures

It has been previously demonstrated that mitochondria are of crucial importance for posttetanic potentiation (PTP) at neuromuscular junction. The aim of our study was to examine whether this may also be the case at a central synapse. To address this question, we studied possible mitochondrial involvement in PTP of GABAergic synaptic transmission in rat neocortical cultures, a preparation in which PTP has not been previously documented. Synaptic responses were evoked by local extracellular stimulation. Whole-cell patch-clamp technique was employed to record inhibitory postsynaptic currents (IPSCs) from postsynaptic neurons. Tetanic stimulation (30 Hz, 4 s) of the presynaptic neuron evoked an increase of IPSC amplitude, lasting for about 1 min. PTP was accompanied by a decrease of coefficient of variation of the IPSC and a decrease of paired-pulse (IPSC2/IPSC1) ratio, indicating involvement of presynaptic mechanism(s) in PTP. Possible role of mitochondria in PTP was addressed using drugs affecting Ca2+ uptake and subsequent Ca2+ efflux: carbonyl cyanide 3-chlorophenylhydrazone (CCCP) and tetraphenylphosphonium ions (TPP+). It was found that both CCCP (1-2 mu M) and TPP+ (10 mu M) either substantially decreased or eliminated PTP. These results further confirm presynaptic origin of PTP in neocortical neurons and suggest an important role of mitochondrial Ca2+ turnover in this form of synaptic plasticity at the central synapse.