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Jeremy D. Coplan, MD; Laszlo A. Papp, MD; Daniel Pine, MD; Jose Martinez, MA; Thomas Cooper, MA; Leonard A. Rosenblum, PhD; Donald F. Klein, MD; Jack M. Gorman, MD

Arch Gen Psychiatry. 1997;54(7):643-648.

Abstract
Background
Central noradrenergic (NA) dysregulation has provided a major theoretical framework for understanding the pathogenesis of panic disorder (PD). Using clonidine, an 2-adrenergic receptor agonist, as a probe of NA function, we investigated the hypothesis that the antipanic efficacy of the selective serotonin reuptake inhibitors may be associated with normalization of a putatively dysregulated NA system.

Methods
We report further analyses on data from 17 subjects with PD and 16 healthy volunteers who underwent measurement of the plasma NA metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG) immediately before and after oral clonidine administration. Thirteen patients with PD were rechallenged after 12 weeks during open fluoxetine hydrochloride treatment using the same clonidine paradigm; 13 healthy volunteers were rechallenged at 12 weeks, not having received treatment between challenges.

Results
Patients with PD, compared with healthy volunteers, have markedly elevated plasma MHPG volatility during the first clonidine challenge. Volatility describes the magnitude of within-subject plasma MHPG oscillatory activity as assessed by the root of the mean square successive difference. A greater degree of clinical global improvement was predicted by a greater magnitude of basal MHPG reduction with fluoxetine treatment. Antipanic response to fluoxetine was accompanied by a significant decrease of MHPG volatility to volunteer levels. Volunteer MHPG volatility remained unchanged from the first to second clonidine challenge.

Conclusions
Further evidence is provided for the hypothesis of NA dysregulation in PD as reflected by elevations of within-subject plasma MHPG volatility during clonidine challenge. Effective selective serotonin reuptake inhibitor—antipanic treatment in this clinical sample was paralleled by normalization of dysregulated NA function.

Author Affiliations
From the College of Physicians and Surgeons of Columbia University, New York State Psychiatric Institute (Drs Coplan, Papp, Pine, Klein, and Gorman and Messrs Martinez and Cooper) and the Phobia and Stress Clinic, Hillside Hospital, Long Island Jewish Medical Center (Drs Coplan, Papp, and Gorman) New York, NY; and the Department of Psychiatry and Primate Behavior Laboratory, SUNY Health Sciences Center at Brooklyn, Brooklyn, NY (Dr Rosenblum).

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