Noradrenaline and Serotonin Selectively Modulate Thalamic Burst Firing by Enhancing a Hyperpolarization-activated Cation Current

Neurons in many regions of the mammalian nervous system generate action potentials in two distinct modes: rhythmic oscillations in which spikes cluster together in a cyclical manner, and single spike firing in which action potentials occur relatively independently of one another. Which mode of action potential generation a neuron displays often varies with the behavioural state of the animal. For example, the shift from slow-wave sleep to waking and attentiveness is associated with a change in thalamic neurons from rhythmic burst firing to repetitive single spike activity, and a greatly increased responsiveness to excitatory synaptic inputs. This marked change in firing pattern and excitability is controlled in part by ascending noradrenergic and serotonergic inputs from the brainstem, although the cellular mechanisms of this effect have remained largely unknown. Here we report that noradrenaline and serotonin enhance a mixed Na+/K+ current which is activated by hyperpolarization (Ih) and that this enhancement may be mediated by increases in intracellular concentration of cyclic AMP. This novel action of noradrenaline and serotonin reduces the ability of thalamic neurons to generate rhythmic burst firing and promotes a state of excitability that is conducive to the thalamocortical synaptic processing associated with cognition.