PHASIC STIMULATION OF THE LOCUS-COERULEUS - EFFECTS ON ACTIVITY IN THE LATERAL GENICULATE-NUCLEUS

Neurons in the locus coeruleus (LC) encode information related to beha vioral state in a tonic pattern of firing and information related to t he occurrence of a sensory stimulus in a phasic pattern of firing. The effects of phasic stimulation of the LC (6 pulses at 30 Hz), designed to approximate its physiological activation by sensory stimuli, were studied in the lateral geniculate nucleus (LGN) of anesthetized rats. Phasic stimulation of the LC significantly increased neuronal firing i n the LGN with a mean latency 320 ms from onset of stimulation. Receiv er operating characteristic analyses on a trial-by-trial basis showed that phasic LC stimulation can result in a highly discriminable signal in the LGN. This increased neuronal firing rate in the LGN was specif ic for the site of stimulation and was reduced by the norepinephrine s ynthesis inhibitor alpha-methyl-p-tyrosine and by intravenous WB-4101 (alpha(1)-receptor antagonist). Neurons in the LGN have a single-spike firing mode when sensory information is faithfully relayed from retin a to cortex and a burst-firing mode when the transfer of this informat ion is degraded. Phasic LC stimulation reduced burst firing (2-5 ms in terspike intervals, ISIs) at low frequencies (less than or equal to 4 Hz) in the LGN, and for some neurons there was an absolute decrease in burst-like ISIs after LC stimulation, despite an increase in mean fir ing rate.