Analysis of synaptic transmission at single identified boutons on rat spinal neurons in culture.

The spatial organization of receptor channels has a major influence on the speed and possible plasticity of synaptic signal transmission. We have studies glutamatergic synapses on neurons in organotypic cultures of rat spinal cord. In order to avoid the problems related to the analysis of currents of unknown origin within a neuron, we chose to examine the functional properties of single identified synapses. Iontophoretic mapping of the cell surface revealed hot spots of high glutamate sensitivity coincident with presynaptic boutons stained with the dye FM 1-43. Local application of KCl to these sites caused bursts of synaptic release. Hot spots typically consisted of 330 receptors with an average single-channel conductance of 8.3 pS. Evoked synaptic currents involved only about 40-50 receptors and nevertheless showed characteristics of saturation. This suggests that glutamate receptor clusters at sites of presynaptic terminals are organized into well separated subclusters opposite release sites.