Inhibition from locus coeruleus of caudate neurons activated by nigral stimulation.

Electrophysiological studies were performed to determine if the locus coeruleus (LC) plays a role in regulating activity of caudate nucleus (CN) neurons. Cats were anesthetized with alpha-chloralose and when conditioning stimuli were applied to the LC, preceding the test stimulus, there was a significant inhibition of spike generation in 33 out of 93 CN neurons transsynaptically activated by test stimulus to the pars compacta of the substantia nigra (SN). An inhibition of spike generation with LC conditioning stimulation and iontophoretic application of noradrenaline was obtained in 9 out of the same 23 CN neurons and 8 neurons remained unaffected with either treatment. In contrast to these neurons, LC conditioning stimulation did not affect the spontaneous firing and spike generation of CN neurons antidromically excited by SN stimulation. Spike firing of CN neurons transsynaptically activated by motor cortex stimulation remained unaffected with LC conditioning stimulation. These results suggest that noradrenaline derived from the LC produced an inhibition of spike generation in about one-third of CN neurons tested and transsynaptically activated by SN stimulation.