2.4 Relating stress and anxiety biologically
Anxiety is linked to fear, and the amygdala plays a central role in attaching emotional significance to what we perceive and ‘deciding’ if fear, and hence ‘fight or flight’, is an appropriate reaction. It is important for animals to remember threatening situations and to avoid them, hence the amygdala also plays a crucial role in consolidating and storing memories of emotionally arousing, stressful experiences, including unconscious fear memories. As you saw above, it is the amygdala that initiates the stress response. But how is it itself affected by the consequences of that stress, and how might this be linked to anxiety?
A part of the amygdala, the basolateral amygdala (BLA), is well-supplied with glucocorticoid receptors, raising the possibility that the amygdala could be directly affected by a rise in glucocorticoid levels following stress. It is tempting to speculate that the kind of effect described is relevant to disorders such as post-traumatic stress disorder (PTSD), which can result from a single, traumatic event. Chronic stress is also known to ‘boost’ dendrite formation in the amygdala in a similar way to that shown for acute stress (Mitra and Sapolsky, 2008). Dendrites are the part of neurons (brain cells) that receive incoming signals or information from other brain cells. Therefore the more dendrites a neuron has, the more signals (or synaptic inputs) from other neurons it can receive. Clearly the amygdala is profoundly structurally changed by stress, and as you will see below, this leads to it exerting a more powerful influence on other parts of the brain.
The amygdala is also rich in receptors for the inhibitory neurotransmitter GABA. A neurotransmitter is a substance or chemical that neurons also use to talk to one another since these chemicals can cross the gaps that exist between neurons in the brain. Stress is known to lower levels of the neurotransmitter GABA and hence GABA inhibition on the amygdala (Roozendaal et al., 2009). GABA is the main inhibitory neurotransmitter in the brain meaning that when neurons receive a GABA signal from other neurons this signal depresses or inhibits their activity. Overall therefore you can see that reducing the amount of this inhibitory neurotransmitter will have a net effect of increasing activity in the amygdala. Some drugs (such as benzodiazepines) prescribed to reduce anxiety bind to GABA receptors and reinforce the effects of GABA.
A hyperactive amygdala may contribute to the well-established vividness of emotionally significant memories, as the amygdala sends powerful inputs to the hippocampus to give emotional flavour to conscious memories. A more active amygdala, via its triggering effects on the HPA axis, might also intensify the stress response further. It might also underlie the emotional symptoms seen in affective and anxiety disorders.
Next, the amygdala’s activity in humans with generalised anxiety disorder (GAD) is considered.