Burst responses in thalamic neurons have been typically associated with sleep states and not linked to sensory transmission. However, it has been demonstrated that bursts occur also during the wake state and could be implicated in high level processes such as attention. Bursts are due to a hyperpolarization of the membrane potential in relay cells linked to activation of specific ionic currents. We asked if this switch from tonic to burst mode is gradual or occurs abruptly upon reaching a threshold and how it is regulated by neurotransmitters acting at the thalamus.

Experiments were carried out in adult cats anaesthetized (with halothane, 0.1-5 % in nitrous oxide-oxygen 70 %-30 %), and neuromuscularly blocked (gallamine 10 mg kg^-1 h^-1). EEG, ECG, CO₂ and temperature were monitored (all procedures conformed to the Spanish Physiological Society, and the European Union (Statute No. 86/809). Extracellular recordings and iontophoretic ejection of drugs were made in A laminae of the lateral geniculate nucleus (LGN) using multibarrelled pipettes containing NaCl for recording and a selection of the following drugs: ACh, GABA, isoguvacin (GABA_A agonist), baclofen (GABA_B agonist), L-NitroArg (NOS inhibitor), atropine and scopolamine (ACh receptor antagonists). Spikes were collected during presentation of visual stimulus in control and during ejection of drugs. Spikes in tonic or burst mode were selected offline and the ratio in the different conditions compared. At the end of the experiment all animals were painlessly killed.

Our results show that (i) the percentage of spikes in bursts increases under conditions that supposedly hyperpolarize the cell, such as application of GABA agonists or ACh antagonist ejection; (ii) the increase in the percentage of bursts is not a regular process, bursts appearing suddenly at a particular level of drug ejection; and (iii) burst number is not directly related to cell responses: ejection of L-NitroArg, blocking NO synthesis, reduces thalamic activity presumably without modifying the membrane potential, since it did not affect the burst ratio. In a number of cells we could not increase the number of bursts; these cells form a group that in control had an extremely low percentage of burst (less than 2 %). These cells could confirm previous results showing a population of LGN cells which are unable to respond in burst mode.

These results fit with the existence of two different states in relay cells, associated with awake and sleep, the switch from one to the other under control of the modulatory systems to the thalamus. However, they also support the idea that an important number of bursts remain in the awake state, suggesting that bursts can play a role in visual processing.

This work was supported by MCYT and MEC (Acciones de Promoción y Movilidad), Spain.