Pentobarbital
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Mechanism of action
Pentobarbital binds at a distinct binding site associated with a Cl- ionopore at the GABAA receptor, increasing the duration of time for which the Cl- ionopore is open. The post-synaptic inhibitory effect of GABA in the thalamus is, therefore, prolonged. All of these effects are associated with marked decreases in GABA-sensitive neuronal calcium conductance (gCa). The net result of barbiturate action is acute potentiation of inhibitory GABAergic tone. Barbiturates also act through potent (if less well characterized) and direct inhibition of excitatory AMPA-type glutamate receptors, resulting in a profound suppression of glutamatergic neurotransmission.
The exact mechanism(s) by which barbiturates exert their effect on the CNS, has not been fully elucidated. However, it is believed that such effects are related, at least partially, to the drugs' ability to enhance the activity of gamma-aminobutyric acid (GABA), the principal inhibitory neurotransmitter in the CNS, by altering inhibitory synaptic transmissions that are mediated by GABAA receptors. /Barbiturates General Statement/
Although the drugs act throughout the CNS, a site of particular sensitivity is the polysynaptic midbrain reticular formation which is concerned with the arousal mechanism. Barbiturates induce an imbalance in central inhibitory and facilitatory mechanisms influencing the cerebral cortex and the reticular formation. The significance of the effect of barbiturates on neurotransmitters is unclear. It appears that the drugs decrease the excitability of both presynaptic and postsynaptic membranes. It has not been determined which of the various actions of barbiturates at cellular and synaptic levels are responsible for their sedative and hypnotic effects. /Barbiturates General Statement/
Relatively low doses of the barbiturates depress the sensory cortex, decrease motor activity, and produce sedation and drowsiness. In some patients, however, drowsiness may be preceded by a period of transient elation, confusion, euphoria, or excitement, especially after subhypnotic doses of aprobarbital, pentobarbital, or secobarbital. /Barbiturates General Statement/
Larger doses distort judgment, cloud perception, suppress motor activity, and produce drowsiness and sleep. Still larger doses induce anesthesia. Barbiturate-induced sleep differs from physiologic sleep. Barbiturates reduce the rapid eye movement (REM) or dreaming stage of sleep. Stages III and IV sleep are also decreased. Although tolerance develops to the REM-suppressant effects during chronic administration, REM rebound occurs when the drugs are withdrawn, and the patient may experience markedly increased dreaming, nightmares, and/or insomnia. /Barbiturates General Statement/
For more Mechanism of Action (Complete) data for Pentobarbital (22 total), please visit the HSDB record page.
Pharmacodynamics
Pentobarbital, a barbiturate, is used for the treatment of short term insomnia. It belongs to a group of medicines called central nervous system (CNS) depressants that induce drowsiness and relieve tension or nervousness. Little analgesia is conferred by barbiturates; their use in the presence of pain may result in excitation.
Pharmacokinetics
Half-life
5 to 50 hours (dose dependent)
Plasma concentrations of pentobarbital decline in a biphasic manner with a half-life of about 4 hours for the first phase and 35-50 hours for the second phase.
The plasma half-life for pentobarbital in adults is 15 to 50 hours and appears to be dose dependent.
Following iv admin of pentobarbitone sodium 50 mg to 5 healthy subjects pentobarbitone was noted to have a distribution phase (alpha-phase) of about 4 hr, & elimination occurred with a harmonic mean beta-phase half-life of about 50 hr. This suggested that for pentobarbitone the body has a central plasma compartment & one or more extravascular compartments. ... /In another study/ findings of 7 healthy subjects /revealed/ an average beta-phase half-life of only 22.3 hr following iv admin of 100 mg pentobarbitone sodium. After oral admin the half-life was about the same. A more detailed knowledge of pentobarbitone pharmacokinetics was needed to explain the deviation ... /between these 2 different studies/. /Pentobarbitone sodium/
Absorption
Barbiturates are absorbed in varying degrees following oral, rectal, or parenteral administration.
Barbiturates are metabolized primarily by the hepatic microsomal enzyme system, and the metabolic products are excreted in the urine, and less commonly, in the feces. Approximately 25 to 50 percent of a dose of aprobarbital or phenobarbital is eliminated unchanged in the urine, whereas the amount of other barbiturates excreted unchanged in the urine is negligible.
Nearly all of an oral or rectal dose of pentobarbital is absorbed from the GI tract. Following oral administration of pentobarbital, peak plasma concentrations are usually reached in 30-60 minutes.
When pentobarbital is administered orally or rectally, the onset of action occurs within 15-60 minutes. The onset of action is within 1 minute following iv administration and within 10-25 minutes following im administration. Like secobarbital, pentobarbital probably has a duration of hypnotic effect of 1-4 hours following oral or rectal administration and about 15 minutes following iv administration.
Plasma pentobarbital concentrations of 1-5 ug/mL generally produce sedation, and plasma concentrations of 5-15 ug/mL produce sleep in most patients; however, plasma concentrations of greater than 10 ug/mL may produce deep coma, and those in excess of 30 mcg/mL are potentially lethal.
Approximately 35-45% of pentobarbital is bound to plasma proteins.
Metabolism
by hepatic microsomal enzyme system
Pentobarbital is metabolized by the liver chiefly by penultimate oxidation of the 1-methylbutyl substituent to a secondary alcohol, 5-ethyl-5-(3'-hydroxy-1'-methylbutyl) barbituric acid (hydroxypentobarbital) which is an inactive metabolite. Approximately 40-50% of an oral hypnotic dose of pentobarbital is excreted in urine as hydroxypentobarbital. The 1-methylbutyl substituent of pentobarbital can also be oxidized to form pentobarbital carboxylic acid, and small quantities of this metabolite have been found in urine.
The liver biotransforms most ... short acting barbiturates (90%-99%). Short-acting compounds /eg, pentobarbital & secobarbital/ are oxidized to more polar & inactive compounds (alcohols, ketones, phenol, or carboxylic acid). /Barbiturates/
Pentobarbital contains an asymmetric carbon atom in butyl side chain. Main route of metabolism is (omega-1)-oxidation to yield 5-ethyl-5-(3'-hydroxy-1'-methylbutyl)barbituric acid. This process creates a new center of asymmetry, thus giving rise to 4 possible diastereoisomeric metabolites.
Metabolized by liver by ... oxidation of 1-methylbutyl substituent to ... 5-ethyl-5-(3-hydroxy-1-methylbutyl)barbituric acid (hydroxypentobarbital) /and/ pentobarbital carboxylic acid ... Glucuronide conjugates of alcohols /and further unidentified oxidation products also found in urine of /humans/.
For more Metabolism/Metabolites (Complete) data for Pentobarbital (6 total), please visit the HSDB record page.
External links
Fact-sheets from PsychonautWiki. Harm-reduction reference only — not medical advice.