Neurophysiologic effects of repeated exposure to antidepressant medication: Are brain functional changes during antidepressant administration influenced by learning processes?

Abstract

Major Depressive Disorder (MDD) is a lifelong and recurrent illness, such that many individuals require multiple courses of antidepressant medication treatment. While some patients respond completely to each course of treatment, many do not, and with each unsuccessful antidepressant trial the likelihood that a patient will respond decreases. This raises the possibility that neurophysiologic response in subsequent antidepressant treatment may be influenced by learning processes including sensitization, habituation, and/or classical conditioning. Classical conditioning would entail the association of cues such as pill-taking (conditioned stimuli; CS) with the effects of active medication (unconditioned stimulus; US), such that later presentation of the CS alone would come to elicit a conditioned response (CR). Such effects could be revealed by blinded administration of placebo following a period of treatment with active medication. Habituation effects (tolerance), or sensitization effects (increased response), which require only repeated exposure to a stimulus, might be evidenced after repeated courses of antidepressant treatment. Knowledge of how learning processes impact neurophysiologic response to successive courses of antidepressant treatment would have relevance for clinical populations. Specific hypotheses, however, may be tested in healthy non-clinical samples to avoid potential confounding factors related to severity or chronicity of illness. Learning theories would suggest two hypotheses: (1) neurophysiologic response to placebo will differ between subjects who were previously treated with antidepressant treatment as compared to placebo (classical conditioning hypothesis); and (2) neurophysiologic response to an initial course of antidepressant treatment will differ from response to a repeated course of antidepressant treatment. Pilot data addressed these hypotheses in healthy never-depressed women who had previously received four weeks of venlafaxine IR, 150mg (antidepressant-experienced subjects; n=2) or matching placebo (antidepressant-naive subjects; n=4) under double-blind conditions. Six-and-a-half years later, we treated these six women with placebo for one week, followed by four weeks of double-blind treatment with venlafaxine IR, 150mg. Brain functional changes over the course of treatment were assessed using quantitative electroencephalography (qEEG) to compare prefrontal neurophysiologic responses between subjects who had, versus had not, previously been exposed to venlafaxine. Antidepressant-experienced versus antidepressant-naive subjects showed greater decreases in prefrontal cordance (PFC) during venlafaxine administration (sensitization hypothesis) but did not show significantly different PFC changes during treatment with placebo in this small pilot sample (classical conditioning hypothesis). Data suggest that brief treatment with antidepressant medication may have an enduring impact on neurophysiologic responses to a subsequent course of antidepressant treatment. Hypotheses should be tested in larger samples.