Minor tranquilizers and brain defense systems.

In this article the participation of different brain neuron systems in the integration of fear responses, and in the anti-anxiety action of minor tranquilizers (MT) is discussed. As necessary background, the molecular mechanisms of MT action disclosed by recent neurochemical research are briefly described. The reviewed evidence suggests that MT decrease anxiety by impairing the functioning of brain neuron systems integrating defensive behavior. These systems are the septo-hippocampal system, together with its afferent mono-aminergic input from the rostral brain stem, which commands behavioral inhibition arising in situations involving threat of punishment or frustration. In addition, the brain aversive system comprising the dorsal periaqueductal gray matter, the periventricular gray matter and the amygdala integrates the active defense patterns of fight or flight as well as elaborates aversive motivation. The depressant action of MT upon both defense systems may be primarily due to facilitation of neurotransmission mediated by gamma-aminobutyric acid (GABA).