Evidence suggests that serotonin1A (5-HT1A) receptor, serotonin transporter (5-HTT) and benzodiazepine (BZD) receptor function is abnormal in panic disorder (PD), postraumatic stress disorder (PTSD) and depression (MDD). The hypotheses for the role of 5-HT1A receptors have been obtained by assessing behavioral, neuroendocrine and temperature responses to the selective partial 5-HT1A agonist ipsapirone in anxiety disorders subjects and healthy controls, and examining effects on 5-HT1A receptor function in rats following antidepressant drug (AD) administration. 5-HT1A receptors knockout mice show behaviors that have been used as a model for anxiety disorders in humans. Moreover, 5-HT1A receptor agonists have been shown to be effective in the treatment of patients with anxiety disorders. The serotonin transporter is implicated in these disorders by virtue of the efficacy of selective serotonin reuptake inhibitors in relieving symptoms in these subjects. Evidence arguing for a role of BZD/GABA receptor dysfunction in anxiety disorders comes from studies showing anxiolytic and anxiogenic properties of BZD agonists and antagonists, respectively. BZD receptor sensitivity has been shown to be reduced in patients with anxiety disorders. Brain imaging studies using positron emission tomography (PET) and single photon emission computed tomography (SPECT) suggest decreased BZD receptor binding in PD and PTSD.

Animal studies link together serotonin and GABA suggesting a pathological pathway originating from 5-HT1A receptor deficit leading towards dysfunctions within GABAergic systems, resulting in increased levels of anxiety. Yet, association between disturbed interactions between 5-HT1A receptor binding and alterations in BZD receptor binding has not been explored in humans. The proposed study will advance knowledge regarding the neurobiology of PD and PTSD by employing PET and \[11C\]flumazenil, \[18F\]FC-WAY100635(\[18F\]FCWAY) and \[11C\]DASB to compare BZD receptor, 5-HT1A receptors and the serotonin transporter binding potential, respectively, between PD, PTSD and MDD patients and healthy controls. Because central 5-HT1A receptor density is down-regulated in rodents by corticosterone administration and by stress-mediated corticosterone secretion, assessments of HPA-axis activity will be assessed to determine whether down-regulation of 5-HT1A receptors correlates with cortisol hypersecretion in PD, PTSD and MDD.

The following hypotheses will be tested: 1) PD/PTSD/MDD patients have reduced GABA(A)-BZD receptor binding relative to healthy controls. 2) PD/PTSD/MDD patients have reduced 5-HT1A receptor binding potential relative to healthy controls. 3) 5-HT1A receptor binding will be more prominently reduced in PD and PTSD patients with comorbid MDD relative to anxiety disorders patients without comorbid MDD and healthy controls. 4) There will be a positive correlation between the reduction in 5-HT1A receptor binding and BZD binding in PD/PTSD/MDD patients. 5) There will be an inverse correlation between reduction in 5-HT1A receptor binding and BZD binding in PD/PTSD/MDD patients and cortisol secretion. 6) Serotonin transporter binding, measured using \[11C\]DASB, will be reduced in PD subjects relative to healthy controls.