Biogenic Amines and the Sleep States

Due to a lack of adequate techniques, it is only very recently that a neuro pharmacological study of sleep has been successfully carried out. It was not until the technique of making continuous long-term electroencephalographic recordings had been perfected and suitable pharmacological techniques had been developed that this field was adequately explored. The fact that the monoamines themselves (serotonin and noradrenalin) do not cross the mammalian blood brain barrier presented a major technical difflculty. In birds, whose blood brain barrier is permeable, intravenous injection of serotonin or noradrenalin leads to slow-wave sleep, as indicated by hehavior and by electroencephalographic recording (25). Attempts to by pass this barrier in mammals through local application or through injection of serotonin into the brain have led, also, to an increase of cortical synchronization in short-term experiments (26). Injection into the ventricles has given questionable results (27). The use of precursors of these amines which readily cross the blood-brain barrier has been a step forward. The increase of cerebral serotonin (through intravenous or intraperitoneal injection of its precursor 5-hydroxytryptophan (5 HTP)) leads first to a state which resembles slow-wave sleep (28). However, this drug leads to a suppression of paradoxical sleep for 5 to 6 hours; the suppression is followed by a secondary rebound (29) The injection of dihydroxyphenylalanine (DOPA), which is a precursor of both dopamine (DA) and noradrenalin, produces an increase in waking (30, 31), whereas the injection of dihydroxyphenylserine, which is be lieved to be a direct precursor of noradrenalin (32), increases slow-wave sleep and paradoxical sleep (33). More recently it has been demonstrated that numerous drugs known to act on the concentrations of monoamines in the brain act in a rather predictable way on the sleep states. Producing a decrease in serotonin and noradrenalin, reserpine (0.5 milligram per kilogram in the cat), although inducing a "state of tranquility" (34), suppresses the appearance of slow-wave sleep for 12 hours and of paradoxical sleep for about 24 hours, whereas it triggers continuous PGO waves similar to those of paradoxical sleep. Secondary injection of 5-hydroxytryptophan, which restores a normal brain concentration of serotonin, results in the immediate reappearance of electroencephalographic patterns of slow wave sleep, whereas the injection of di hydroxyphenylalanine, which restores the catecholamine concentration following the administration of reserpine, leads to the reappearance of paradoxical sleep. Thus there was an implication that serotonin may be involved in slow wave sleep whereas the catecholamines rnay be involved in paradoxical sleep (30, 35). Monoamine oxidase (MAO) inhibitors (nialamide, iproniazid, phenylisopropylhydrazide), which act upon both brain monoamines by inhibiting their catalolism and by thus increasing their concentration in the brain, were shown to act dramatically upon sleep states. Most of the monoamine oxidase inhibitors utilized have a very specific suppressive effect upon paradoxical sleep and increase slow-wave sleep in the cat. This suppressive effect is so intense that it is even operative when the "need" for paradoxical sleep is greatly enhanced following paradoxical sleep deprivation (30, 36). This suggests that monoamine oxidase is necessary to the passage from slow-wave sleep to paradoxical sleep. These findings were of only limited significance in view of the enormous complexity of the biochemical mechanism of the brain. Most of these drugs were acting upon both indoleamines and catecholamines and therefore might very well be inter fering with the cyclic alteration of the brain's electrical activity.

More recently, however, some drugs that act selectively upon serotonin or catecholamines have been discovered. The most interesting ones for the neurophysiologist are those which inhibit the synthesis of monoamines (p-chlorophenylalanine for serotonin; alpha-methylparatyrosine for dopamine and noradrenalin, or disulfiram for noradrenalin). It is thus possible to alter selectively the metabolism of one monoamine in the study of the sleep states.

Other major steps came at this time, which permitted a more rigorous way of thinking by closing the gap between neuropharmacology, neuroanatomy, and neurophysiology. Indeed, histofluorescence techniques have made possible the precise topographical study of nerve cells containing monoamines (37). It was demonstrated that the serotonin containing neurons were located mostly in the raphe system (37, 38), the noradrenalin-containing neurons in the lateral part of the bulbopontine tegmentum (principally in the locus coeruleus), and the dopamine-containing neurons in the ventral part of the mesencephalon (37).

It was shown that the cell bodies of the monoamine-containing neurons send terminals to widespread regions of the spinal cord and brain, of which the ascending pathways, either mono- or polysynaptic, follow the medial forebrain bundle (37, 39). Moreover, it was shown that sectioning of the axons would suppress specific fluorescence of the corresponding terminals after 8 to 10 days (37). This finding made it possible to attack specific groups of monoamine containing neurons by classical neurophysiological techniques and to correlate such destruction with biochemical analysis, a critical period of time being allowed between destruction of the nerve cells and death of the animal (40).

Thus it was possible to alter the concentrations of monoamines in the brain either by inhibiting their synthesis or by destroying monoamine-containing nerve cells.

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