Noradrenergic activation amplifies bottom-up and top-down signal-to-noise ratios in sensory thalamus

Thalamocortical cells receive sensory signals via primary sensory afferents and cortical signals via corticothalamic afferents. These signals are influenced by a variety of neuromodulators that are released in the thalamus during specific behavioral states. Hence, different neuromodulators may set different thalamic modes of sensory information processing. We found that noradrenergic activation affects sensory and corticothalamic signals in the whisker thalamus differently than cholinergic activation. Whereas cholinergic activation increases the spontaneous firing (noise) and enlarges the receptive fields of ventroposterior medial thalamus (VPM) cells, noradrenergic activation decreases spontaneous firing and focuses receptive fields. Consequently, for sensory signals, noradrenergic activation sets bottom-up thalamic processing to a focused and noise-free excitatory receptive field, which contrasts with the broad and noisy excitatory receptive field characteristic of cholinergic activation. For corticothalamic signals, noradrenergic activation sets top-down processing to a noise-free high-frequency signal detection mode, whereas cholinergic activation produces a noisy broadband signal detection mode. The effects of noradrenergic activation on signal-to-noise ratios of VPM cells were found to be mediated by nucleus reticularis thalamic (nRt) cells. Hence, a major role of nRt cells is to regulate the noise level of thalamocortical cells during sensory processing. In conclusion, different modulators establish distinct modes of bottom-up and top-down information processing in the sensory thalamus.