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membrane potential of thalamocortical neurons to such a degree as to inac-
tivate the T- and H-currents and return these neurons to single-spike firing
which the activity of large numbers of cortical neurons may be synchronized
during sleep.
The primary neuromodulatory systems in the brain stem that are involved
with EEG synchronization include the dorsal raphe (serotonin), locus
coeruleus (LC; noradrenaline), ventral periaqueductal gray/ventral tegmental
area (dopamine), and pedunculopontine/laterodorsal tegmentum (acetylcho-
line). Each of these systems exhibits state-based activity. For example, the dor-
sal raphe in a cat fires progressively less frequently as the cat proceeds from
by differential release of serotonin during wake, NREMS, and REMS, in
in both the pathological state of depression and the effects of antidepressants
that act to increase serotonergic tone (e.g., selective serotonin reuptake inhib-
itors, serotonin/norepinephrine reuptake inhibitors, tricyclic antidepressants).
often preceding onset of an episode of depression in individuals with recurrent
depression. Much debate has centered on whether the depression causes the
disruption in sleep or if the disruption in sleep leads to the depression. The
neurobiology indicates that it may perhaps be that upstream disruption of
serotonin networks leads to both sleep disruption and depression, but that
sleep disruption is more easily observed at lower levels of serotonin disruption.
The change in serotonin signaling that is hypothesized to accompany the use
of many antidepressants is often associated with a disruption of sleep, specif-
NREMS and nearly absent during REMS, the pharmacologic increase of
serotonin tone, such as might follow the use of a serotonin transport inhibitor,
would be expected to shift sleep into lighter states, away from REMS and
stage 3 of NREMS. This is somewhat borne out by sleep studies conducted
ance, of course, is that given the highly associated nature, and possibly etiol-
ogy, of mood and sleep, normalization of mood despite disruption of sleep
may be a more clinically favorable outcome.
The LC provides noradrenergic input to both the thalamus and cortex.
LC-derived noradrenaline is involved not only in thalamic desynchroni-
zation of cortical EEG but also in general mechanisms of attention. Based
on recordings done in primates, it has been postulated that the relationship
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