Generation and propagation of subthreshold waves in a network of inferior olivary neurons

The cells of the inferior olivary (IO) nucleus generate a large repertoire of electrical signals, among them subthreshold oscillations of the membrane potential (STO). To date, subthreshold oscillations have been studied at the level of single-cell recordings, from which network properties were inferred. In this study we used whole cell patch recordings and optical imaging to address the following issues: 1) synchrony of STO in neighboring neurons; 2) stability of the oscillatory activity in the temporal and spatial domain; and 3) the size of the oscillating network. Recordings were made from 126 pairs of IO neurons in 13- to 30-day-old rats. An additional 262 neurons were recorded individually. The frequency of STO varied from 0.8 to 8.6 Hz. The frequency distribution revealed two subpopulations with peaks at about 3 and 6 Hz. The maximum amplitude among the cells varied from 2 to 25 mV. Oscillations in most neurons showed ongoing modulations in both frequency and amplitude. These modulations were largely abolished following bath application of 40 muM 6-cyano-7-nitroquinoxaline- 2,3-dione (CNQX), a competitive non-N-methyl-D-aspartate (non-NMDA) receptor antagonist, suggesting that they were caused by glutamatergic action. In 35 of 61 recorded pairs at least one neuron exhibited STO permitting us to compare frequency and phase relations. In 22 pairs there was coherent activity with zero phase difference between oscillations in the 2 cells. In these pairs, frequency and amplitude modulation occurred simultaneously in both neurons. Electrotonic coupling was tested in 13 pairs, that had coherent STO, and it was detected in 12. An additional seven pairs showed coherent oscillations but with a phase difference of 20-50 ms. Electrotonic coupling was observed in three of these pairs. Electrotonic coupling was also observed in two of five pairs in which only one neuron oscillated. No coupling was detected in one pair where both neurons oscillated but at different frequencies. Optical imaging using a voltage-sensitive dye (RH 414) was performed on 40 IO slices using an array of 128 photodiodes. Patches of oscillatory activity were observed in 10 slices. Among them six showed spontaneous oscillations, and four exhibited oscillations following extracellular stimulation. In agreement with cell pair recording, optical imaging demonstrated phase-shifted activity in the form of propagating waves of activity within an oscillating patch. We conclude that 1) STO exhibit ongoing modulations of frequency and amplitude that are probably caused by extrinsic inputs to the IO nucleus; 2) electrotonically coupled neurons show a high level of STO synchrony; and 3) the oscillatory activity can propagate within a network of coupled olivary neurons.