Cerebral GABA and Fear Conditioning in PTSD

Posttraumatic stress disorder (PTSD) is a common and debilitating neuropsychiatric disorder in which an acute fear response to a traumatic event does not abate. This failure to recover from trauma is thought to be due at least in part to a deficit in learning not to fear situations and stimuli previously associated with the trauma (i.e., specifically due to a failure of extinction recall). Pavlovian fear conditioning can be simulated and measured experimentally in humans using a 2-day fear conditioning paradigm developed by our group, wherein conditioning and extinction learning phases are conducted on Day 1, and extinction recall is tested on Day 2.

Recent functional magnetic resonance imaging (fMRI) evidence indicates that PTSD is associated with hyper-responsivity of the insular cortex and hyporesponsivity of the ventromedial prefrontal cortex (VMPFC) during exposure to fear-inducing stimuli, consistent with altered excitability of brain regions mediating fear conditioning and extinction. As the brain's principal inhibitory neurotransmitter, GABA exerts a prominent role in modulating neuronal excitability. Interestingly, there are reports that adjunctive treatment with GABA-enhancing antiepileptics is efficacious in PTSD. There is also evidence, albeit inconsistent, that lower serum GABA levels predict a more chronic course of the illness. However, it is unclear whether serum levels accurately reflect brain GABA, which may contribute to inconsistency of serum findings. Moreover, it is possible that GABA alterations may vary in their presence, nature and significance across brain regions implicated in PTSD. The proposed study will examine the relationship of PTSD symptoms and behavioral fear conditioning deficits with regional brain gamma-aminobutyric acid (GABA) using proton magnetic resonance spectroscopy (1H-MRS).

We have the following aims and hypotheses:

1. To determine whether GABA alterations are associated with the categorical diagnosis of PTSD and not merely exposure to trauma. It is hypothesized that PTSD will be associated with higher GABA in VMPFC and lower GABA in the right insula.
2. To determine whether GABA levels are significantly associated with dimensional measures of PTSD symptom severity and individual symptom dimensions. It is predicted that higher GABA in the VMPFC and lower GABA in the right anterior insula will be associated with greater total symptom severity.
3. To determine whether GABA in VMPFC and right anterior insula are significantly associated with measures of extinction recall failure and anxiety sensitivity in PTSD. It is hypothesized that VMPFC GABA will be positively correlated with skin conductance response to a conditioned stimulus that had previously been extinguished and insula GABA will be negatively correlated with anxiety sensitivity.