1. Low-frequency membrane potential oscillations recorded intracellularly from thalamocortical
(TC) cells of the rat and cat dorsal lateral geniculate nucleus (dLGN) and of the
rat ventrobasal nucleus (VB) maintained in vitro were investigated. On the basis of
their electrophysiological and pharmacological properties, four types of activity
were distinguished and named: the pacemaker oscillations, the spindle-like oscillations,
the 'very slow' oscillations and the 'N-methyl-D-aspartate' (NMDA) oscillations. 2.
The pacemaker oscillations (95 out of 173 cells) consisted of rhythmic, large-amplitude
(10-30 mV) depolarizations which occurred at a frequency of 1.8 +/- 0.3 Hz (range,
0.5-2.9 Hz) and could often give rise to single or a burst of action potentials. Pacemaker
oscillations were observed when the membrane potential was moved negative to -55 and
positive to -80 mV, but in a given cell the upper and lower limits of this voltage
range were separated by only 13.1 +/- 0.5 mV. Above -45 mV tonic firing consisting
of single action potentials was seen in the cells showing this or the other types
of low-frequency oscillations. 3. The spindle-like oscillations were observed in thirty-nine
(out of 173) TC cells and consisted of rhythmic (2.1 +/- 0.3 Hz), large-amplitude
depolarizations (and often associated burst firing) similar to the pacemaker oscillations
but occurring in discrete periods every 5-25 s and lasting for 1.5-28 s. The spindle-like
oscillations were observed when the membrane potential was moved negative to -55 and
positive to -80 mV and in two cells they were transformed into continuous pacemaker
oscillations by depolarization of the membrane potential to -60 mV. 4. Pacemaker and
spindle-like oscillations were unaffected by tetrodotoxin (TTX) or by selective blockade
of NMDA, non-NMDA, GABA(A), GABA(B), nicotinic, muscarinic, alpha- and beta-noradrenergic
receptors. 5. The 'very slow' oscillations consisted of a TTX-insensitive, slow hyperpolarization-depolarization
sequence (5-15 mV in amplitude) which lasted up to 90 s and was observed in nine dLGN
cells and in two VB cells. The pacemaker and the spindle-like oscillations were recorded
in one cell each which also showed the 'very slow' oscillations. 6. The 'NMDA' oscillations
were observed only in a 'Mg2+-free' medium (0 mM-Mg2+, 2-4 mM-Ca2+; 64 out of 72 cells)
and consisted of large-amplitude (10-25 mV) depolarizations that did not occur at
regular intervals and were intermixed with smaller depolarizations present on the
baseline and on the failing phase of the larger ones. The 'NMDA' oscillations were
voltage dependent (observed in the range from -60 to -85 mV), insensitive to TTX and
had a frequency of 1-4 Hz. Application of selective NMDA receptor antagonists reversibly
transformed the NMDA into the pacemaker or the spindle-like oscillations while blockade
of non-NMDA receptors as well as the other receptors mentioned above (paragraph 4)
had no effect on the 'NMDA' oscillations. 7. The pacemaker, the spindle-like and the
'very slow' oscillations were never observed in electrophysiologically identified
rat and cat dLGN interneurones (n = 12). Similarly, in a 'Mg2+-free' medium, interneurones
in the dLGN and cells in the ventral lateral geniculate nucleus did not show the 'NMDA'
oscillations but only slow, small-amplitude (< 4 mV) depolarizations that were reversibly
abolished by DL-2-amino-5-phosphono-valeric acid (25-mu-M). 8. These results indicate
that (i) single TC cells in different thalamic nuclei of different species are capable
of four types of low-frequency oscillatory activity that do not require the evoked
rhythmic recruitment of other neurones and (ii) activation of the NMDA receptors by
spontaneously released excitatory amino acid brings about the transformation of the
pacemaker and the spindle-like oscillations into the 'NMDA' oscillations.
