The Behavioral Physiology of Locus Coeruleus Neurons

Factors controlling discharge of known norepinephrine-containing locus coeruleus (NE-LC) neurons were studied in unanesthetized behaving rats, and these neurons' efferent impulse conduction properties were examined in anesthetized rats. Single-unit (SU) and multiple-unit (MU) extracellular recordings in unanesthetized preparations demonstrated the following: (1) Tonic discharge co-varied with stages of the sleep-waking cycle (S-WC), being highest during waking (W), lower during slow-wave sleep (SWS), and virtually absent during paradoxical sleep (PS). (2) Altered discharge predictably anticipated S-WC stages as well as phasic cortical activity such as spindles during SWS. (3) Discharge was reduced within active waking during grooming and sweet water consumption. (4) Bursts of impulses accompanied spontaneous or sensory-evoked interruptions of sleep, grooming, consumption, or other such ongoing behaviors. (5) Discharge was not linked to movement per se. (6) Field potentials (FPs) occurred spontaneously in NE-LC recordings, temporally synchronized with bursts of unit activity from the same electrodes during Wand SWS, but at highest rates during PS, when discharge was virtually absent. (7) Short-latency (15-50 msec), transient, biphasic unit responses and synchronous FPs were predictably evoked by non-noxious auditory, visual and somatosensory stimuli; individual recordings typically exhibited similar response patterns for each sensory modality. (8) The magnitudes of sensory-evoked response varied as a function of vigilance, such that largest responses occurred for stimuli which awakened animals and least responsiveness was exhibited during uninterrupted sleep. (9) Sensory responsiveness also decreased during grooming and sweet water consumption. (10) Transiently reduced discharge occurred in response to gustatory stimulation accompanying voluntary consumption of sweet water. (11) SU and MU recordings throughout the nucleus yielded remarkably homogeneous results. (12) Robust phasic discharge was markedly synchronized among neurons in MU populations. SU recordings of spontaneous and antidromic NE-LC impulse activity in anesthetized rats indicated the following: (1) Impulse conduction velocity fluctuated as a function of basal conduction latency, impulse rate, and number of impulses in, a train of activity. (2) Impulse conduction velocity increased briefly, then exhibited a more pronounced, gradual decrease during the same train of activity. (3) Large increases in conduction latency occurred during low-frequency trains of impulse activity. (4) Calculations indicated that these axons may modulate their own impulse flow as a result of ion fluxes associated with spike propagation. These results are interpreted in light of previous data on the postsynaptic physiology of norepinephrine to indicate that robust activity in the NE-LC system may participate in terminating CNS and behavioral processes which have minimal value in coping with phasic external events, and simultaneously enhance activity within systems primarily concerned with such immediate responses. Conversely, low levels of spontaneous or sensory-evoked NE-LC discharge may enable tonic, endogenously generated vegetative behaviors to proceed. In this way, the NE-LC system may bias global behavioral orientation between the external and internal environments.