Barbiturates

Barbiturates are a class of drugs acting as **central nervous system (CNS) depressants**. They can invoke all levels of CNS mood alteration, ranging from paradoxical excitement to deep coma and death. Key systemic effects include: * **Respiratory**: Dose-dependent respiratory depression. At sedative doses, depression mimics normal sleep. As doses increase, the medullary respiratory center is progressively depressed. * **Cardiovascular**: Can cause tachycardia, decreased myocardial contractility, decreased stroke volume, and decreased mean arterial pressure. * **Gastrointestinal**: Generally reduces tone and motility of intestinal musculature. Thiobarbiturates may cause a transient increase in tone/motility after initial depression. * **Musculoskeletal**: Reduces sensitivity of the motor end-plate to acetylcholine, providing mild skeletal muscle relaxation. * **Hepatic**: Chronic administration (especially phenobarbital) leads to well-documented **hepatic microsomal enzyme induction**. > **Clinical Pearl**: Barbiturates possess **no true intrinsic analgesic activity**. They are primarily utilized in veterinary medicine for seizure management (e.g., phenobarbital), anesthesia induction (e.g., thiopental, methohexital), and humane euthanasia (e.g., pentobarbital).

Mecanismo: The exact mechanisms for the CNS effects caused by barbiturates are complex and multifactorial: * **Neurotransmitter Inhibition**: They inhibit the release of excitatory neurotransmitters including **acetylcholine**, **norepinephrine**, and **glutamate**. * **GABA Modulation**: Barbiturates enhance inhibitory neurotransmission by acting on **GABA** receptors. Pentobarbital is specifically noted to be **GABA-mimetic**, directly activating GABA receptors at higher doses. * **Calcium Channel Blockade**: At high anesthetic doses, barbiturates inhibit the uptake of calcium at nerve endings, further dampening synaptic transmission.

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