Abstract
Pavlovian fear conditioning has face validity for acquired anxiety disorders such as posttraumatic stress disorder (PTSD) and specific phobias. Memory formation mediated by synaptic plasticity, including long-term potentiation (LTP), underlies fear conditioning. The established roles of nitric oxide (NO) signaling in synaptic plasticity and LTP suggest that NO signaling may be critical for fear conditioning. Therefore, my research objectives were to investigate the roles of NO signaling in the acquisition and consolidation of fear memory following fear conditioning. I hypothesized that genetic and pharmacological inhibitions of NO signaling will impair fear conditioning, and that pharmacological facilitation of NO signaling will improve fear conditioning. I found that in the absence of the neuronal nitric oxide synthase gene (nNOS knockout mice) there were complete impairments in contextual and visually cued fear conditioning and a partial impairment in auditory cued fear conditioning compared to wild-type (WT) counterparts. A positive correlation was shown between the magnitudes of the physiological (stress hormone) and behavioral (freezing) responses to conditioned fearful stimuli in both genotypes. Investigations of basal expressions of memory related proteins that are downstream of NO signaling revealed significant dysregulations in naïve nNOS knockout mice compared to WT mice. Specifically, cyclic guanosine monophosphate (cGMP) was decreased while phosphorylated cyclic adenosine monophosphate binding protein (pCREB) was increased in the amygdala and hippocampus of nNOS knockout mice. Pharmacological inhibition of nNOS in WT mice resulted in impaired fear conditioning, and NO donor administration to nNOS knockout mice partially restored the fear conditioning deficits. Also, the nNOS inhibitor reduced expressions of cGMP and pCREB in WT mice, and NO donor administration partially restored the aberrant expressions of cGMP and pCREB in nNOS knockout mice. Thus, the behavioral and molecular results of the pharmacological experiments corroborated with the genetic experiments. It was also determined that multiple training sessions improved contextual and auditory, but not visually, cued fear conditioning in the nNOS knockout mice. Together, this line of investigations has revealed that the role of NO signaling in fear conditioning is dependent on a) the type of the conditioned stimulus, and b) the intensity of the training.