Most barbiturates have antiseizure properties. The discussion below is limited to the two barbiturates that exert maximal antiseizure action at doses below those required for hypnosis, a property that determines their clinical utility as antiseizure agents. The pharmacology of barbiturates is considered in Chapter 16.
Phenobarbital (luminal, others) was the first effective organic antiseizure agent. It has relatively low toxicity, is inexpensive, and is still an effective and widely used drug for this purpose.
Phenobarbital likely inhibits seizures by potentiation of synaptic inhibition through an action on the GABAa receptor. At therapeutic concentrations, phenobarbital enhances GABAa receptor—mediated current by increasing the duration of bursts of current without changing their frequency. At levels exceeding therapeutic concentrations, phenobarbital also limits sustained repetitive firing; this may underlie some of the antiseizure effects of higher concentrations of phenobarbital achieved during therapy of status epilepticus.
PHARMACOKINETIC PROPERTIES Oral absorption of phenobarbital is complete but slow; peak plasma concentrations occur several hours after a single dose. Phenobarbital is 40-60% bound to plasma and tissue proteins. Up to 25% of a dose is eliminated by renal excretion of the unchanged drug; the remainder is inactivated by hepatic CYPs. Phenobarbital induces UGTs and several CYPs, thereby stimulating degradation of drugs cleared by these mechanisms (see Table 19-2).
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Hypnosis has been defined as a state of heightened suggestibility in which the subject is able to uncritically accept ideas for self-improvement and act on them appropriately. When a hypnotist hypnotizes his subject, it is known as hetero-hypnosis. When an individual puts himself into a state of hypnosis, it is known as self-hypnosis.