7-Aminobutyric acid (GABA) is an inhibitory neurotransmitter in the CNS. Neurons using the GABA as neurotransmitter are among the most abundant in the CNS. GABAergic neurones occur mainly as local neurons or interneurons present in all area of the CNS involved in the local modulation of neuron activity and to a lesser extent, as projecting or principal neurones (cerebelar Purkinje cells, striatonig-ral, striathothalamic, and nigrothalamic pathways). There may be five or more types of GABA receptors, but GABA receptors GABAA and GABAB are the most studied. GABAa receptor blockers, such as bicuculline and picrotoxin, are both GABAa receptor-blocking agents that impede the GABAergic presynaptic inhibition of excitatory transmission of primary afferent neurones of the spinal cord, hence a general increase in neuronal activity, alertness, anxiety, spasms, seizures, and even death (Fig. 45).
Picrotoxinin is a sesquiterpene, which is found notably in the seeds Anamirta panicu-lata Coleb. (levant berries, Menispermaceae; Fig. 46). This substance is toxic, and as little as 20 mg induces epileptiform convulsions, myosis, and dyspnea with more or less
prolonged apnea. Picrotoxin (British Pharmacopoeia, 1963) has been used in the treatment of barbiturate poisoning (3-6 mg, intravenously) in Western medicine. Both compounds are elaborated in the Magnolidae (Ranunculales and Papaverales).
Bicuculline is an isoquinoline alkaloid elaborated from members of the family Fumariaceae (Papaverales), especially in Corydalis, Dicentra, and Fumaria species. Bicuculline, like picrotoxin, is a specific GABA receptor-blocking agent that impedes the
y - Aminobutyricacid Bicuculline
(GABA) (GABAergic antagonist)
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