Figure 6.

LC-noradrenergic pathway modulation of sensory thalamic neuron responsiveness. Poststimulus time histograms (PSTH’s) histograms illustrate the effects of increasing tonic frequencies of locus coeruleus (LC) electrical stimulation on whisker-evoked responses of simultaneously recorded VPM thalamic neurons. Spike train activity for individual cells was recorded from an awake rat before (control) and during tonic activation (0.5, 1.0, or 5.0 Hz at 10 uA) of the ipsilateral LC nucleus. Each histogram represents the neuronal discharge pattern generated during n=30 whisker stimulus presentations (stimulus onset = 0). A. In some cells (top row) an increasing range of tonic activation of LC produced a monotonic suppression of whisker-evoked discharge. B. In other cells (middle row) whisker-evoked discharge was progressively enhanced across the range of stimulus frequencies tested. C. However, in many cells (bottom row) stimulus-evoked responses were enhanced at low LC stimulation frequencies (0.5–1.0 Hz) and suppressed at higher stimulus frequencies (5.0 Hz), yielding an inverted-U response function for LC-induced modulatory actions in these neurons. Thus, both facilitation and suppression of responsiveness to synaptic input can be observed simultaneously in neighboring thalamic neurons with increased tonic output from the LC. Inset numbers represent the summed probability that the neuron will discharge in response to the whisker pad stimulation.