Mixed burst and tonic firing in the thalamus: a study in the feline lateral geniculate nucleus in vivo.

Compounds known to inhibit or disfacilitate cells in cat dorsal lateral geniculate nucleus (dLGN) were applied iontophoretically in vivo. Application of GABA, or agonists of GABA(A) and GABA(B) receptors, markedly decreased responses to low frequency periodic visual stimulation, but, while causing some increases in burst firing, cells continued to produce tonic spikes even when firing was reduced to near zero. Similar actions were seen with compounds manipulating the cholinergic system. Inhibition of local Nitric Oxide production reduced firing rates but did not affect burst firing. Significant levels of tonic firing were found mixed with burst firing throughout the recordings even under conditions most favourable for bursting. We suggest that the local synaptic input to an individual dLGN cell is sufficiently dynamic to prevent the prolonged periods of burst firing which can be evoked in brain slice preparations.