THE FUNCTIONAL INFLUENCE OF BURST AND TONIC FIRING MODE ON SYNAPTIC-INTERACTIONS IN THE THALAMUS

Thalamocortical and perigeniculate (PGN) neurons can generate action p otentials either as Ca2+ spike-mediated high-frequency bursts or as to nic trains. Using dual intracellular recordings in vitro in monosynapt ically connected pairs of PGN and dorsal lateral geniculate nucleus (L GNd) neurons, we found that the functional effect of synaptic transmis sion between these cell types was strongly influenced by the membrane potential and hence the firing mode of both the pre- and postsynaptic neurons. Activation of single action potentials or low-frequency spike trains in PGN or thalamocortical neurons resulted in the generation o f PSPs that were 0.5-2.0 mV in amplitude. In contrast, the generation of Ca2+ spike-mediated bursts of action potentials in the presynaptic cell increased these PSPs to an average of 4.4 mV for the IPSP and 3.0 mV for the EPSP barrage, because of temporal summation and/or facilit ation. If the postsynaptic neuron was at a resting membrane potential (e.g., -65 mV), these PSP barrages could result in the activation of a low-threshold Ca2+ spike and burst of action potentials. These result s demonstrate that the burst firing mode of action potential generatio n is a particularly effective means by which perigeniculate and thalam ocortical neurons may influence one another. We propose that the activ ation of burst discharges in these cell types is essential for the gen eration of some forms of synchronized rhythmic oscillations of sleep a nd of epileptic seizures.