Final answer:
Low doses of CNS depressants lead to an initial excitatory response because inhibitory synapses are depressed before excitatory ones, enhancing GABA activity, which paradoxically induces a temporary phase of increased brain activity.
Step-by-step explanation:
Low doses of central nervous system (CNS) depressants produce an initial excitatory response primarily due to inhibitory synapses in the brain being depressed before excitatory synapses.
The action mechanism of CNS depressants, such as alcohol, barbiturates, and benzodiazepines, involves the enhancement of the neurotransmitter gamma-Aminobutyric acid (GABA). GABA's primary role is to suppress neuron activity, hence when GABAergic activity is increased at lower doses, the more selective inhibition of inhibitory neurons can lead to an excitatory effect.
Depressants exert their effects via numerous pharmacological mechanisms, including the facilitation of GABA or the inhibition of other neurotransmitters. These substances are capable of reducing arousal or stimulation in various brain areas. Yet, paradoxically, due to the initial suppression of neurons that usually inhibit brain functions, low doses of depressants might cause a short phase of increased neuronal activity, resulting in the perceived initial excitatory response.