Anxiety disorders have been linked to low activity of the ______ neurotransmitter.

The correct option is Gamma Aminobutyric Acid.

Gamma-Aminobutyric Acid (commonly abbreviated as GABA) is the primary inhibitory neurotransmitter in the mammalian central nervous system. Its principal role is to reduce neuronal excitability throughout the nervous system, essentially acting as a natural brake or calming agent for brain activity.

Under normal conditions, GABA binds to its receptors on postsynaptic neurons, leading to an influx of negatively charged chloride ions. This hyperpolarizes the neuron, making it less likely to fire an action potential. This physiological mechanism prevents over-excitation and maintains a balance in brain activity.

When there is low activity or a deficiency of GABA, the inhibitory control over the nervous system is weakened. Consequently, neurons can become overexcited and fire too frequently or intensely. This state of neuronal hyperexcitability is clinically and biologically linked to the heightened state of arousal, tension, and apprehension characteristic of anxiety disorders. Therefore, low activity of Gamma Aminobutyric Acid is directly associated with anxiety.