PROPERTIES OF ATP RECEPTOR-MEDIATED SYNAPTIC TRANSMISSION IN THE RAT MEDIAL HABENULA

The properties of central ATP-mediated synaptic currents were studied using whole-cell patch-clamp recording in rat medial habenula slices. Release was shown to be calcium dependent with a Hill coefficient of s imilar to 2. The voltage dependence of synaptic current amplitudes was approximately Linear. Some reduction of the synaptic current amplitud es was observed at 10 mM extracellular calcium, suggesting calcium blo ck/permeability of the channels. This was confirmed by observation of current-voltage reversal potentials in different calcium concentration s. We estimate that the channels underlying half the synapses showed a negligible calcium permeability. In the other four out of eight synap ses the results suggest a very high calcium permeability with an estim ated P-Ca/P-Cs of >10. Thus, at -70 mV, in 1 mM calcium, more than 15% of the ATP-mediated synaptic current is estimated to be carried by ca lcium, but only at synapses with calcium-permeable channels. Net curre nt through these synaptic channels is also controlled by the voltage d ependence of synaptic current decay time constants (increasing e-fold for 158 mV depolarization) and by a strong dependence of transmitter r elease on the frequency of stimulation of the presynaptic neurone, wit h failure rates increasing 3-fold as stimulation rates were increased from 1 to 10 Hz. (C) 1997 Published by Elsevier Science Ltd.