Fear Potentiation and Fear Inhibition in Posttraumatic Stress Disorder

Title of addendum: fear potentiation, conditional discrimination, and fear inhibition in
posttraumatic stress disorder.

Principal Investigator: Erica Duncan, M.D.

Co-Investigator: Seth Norrholm, M.D.

Background:

Fear potentiation of the acoustic startle response (ASR) can be demonstrated in both animals
and humans (Davis et al., 1993; Grillon et al., 1991). The neural correlates of fear
potentiated startle are well studied (Davis et al., 1993) and this paradigm has face validity
with cardinal symptoms of posttraumatic stress disorder (PTSD). Central to the clinical
problem of PTSD is the inability of these patients to inhibit their fear to stimuli
reminiscent of their traumatic experience, even during safe conditions. The elucidation of
mechanisms of fear inhibition that may be abnormal in these patients has the potential to
advance the development of effective treatments for these patients.

Two conditioning paradigms would permit the study of fear inhibition, but each has potential
confounds complicating the interpretation of results. Extinction paradigms have the problem
that the same stimulus causes both excitation of the response and later inhibition of
response, so that it is difficult to tease apart whether a given experimental manipulation
that enhances extinction does so by increasing inhibition or disrupting excitation.
Conditioned inhibition (using an A+/AX- paradigm) has the advantage of having two separate
stimuli for excitation and inhibition (A is excitatory; X is inhibitory), but has the
disadvantage of potential second-order conditioning (i.e., it is possible that A will
second-order condition X), which greatly complicates the separation of excitation and
inhibition.

The Davis lab has recently developed a discrimination procedure that avoids these problems
and allows for the independent evaluation of excitation and inhibition of fear conditioning.
The procedure, referred to as a conditional discrimination (abbreviated as AX+/BX-), is based
on a paradigm used in earlier learning theory experiments (Rescorla and Wagner, 1972; Bouton
and Swartzentruber, 1986). In this experiment, the response to X is conditional upon the
presence of either A or B. A becomes excitatory with training as the subject learns that A
and X presented together predicts the unconditioned stimulus (US). B becomes inhibitory in
that B presented with X predicts safety from the US. The presentation of AB results in a
reduced response compared to the response to A alone because B is inhibitory. Accordingly, a
novel stimulus, C, paired with A results in a greater response compared to AB. X becomes
excitatory to the degree that B is inhibitory. Testing of this paradigm with rats in the
Davis lab has produced data in good accord with predicted values (see Figure 1 in Appendix),
and does indeed allow for independent evaluation of excitatory and inhibitory processes
within the conditioning test session. To date, this paradigm has not been well studied in
human subjects. The translation of this paradigm to human subjects would present an important
advance in methodology for studying the neurobiology of fear inhibition in clinical
populations with anxiety disorders such as PTSD.

Specific Aims:

1. To establish methodology to assess baseline acoustic startle, fear potentiation of
acoustic startle in a classical conditioning paradigm, and conditional discrimination
(AX+/BX-) in 15 normal control subjects. Fear extinction will be assessed in one to
three separate repeated experimental sessions.

2. To compare 15 subjects with PTSD, 15 subjects with a history of combat exposure but no
PTSD, and 15 normal controls using this acoustic startle methodology. We hypothesize
that PTSD subjects will have normal baseline startle, intact fear potentiation, and
impaired inhibition of fear potentiation as measured by the AX+/BX- paradigm. We predict
that combat controls will have results intermediate between PTSD subjects and normals.

Methods

Subjects: 75 subjects with PTSD ages 18-80, 75 age and gender matched normal controls, and 75
age and gender matched subjects with combat history but no PTSD will be recruited. Subjects
will be screened and excluded for Axis I and II disorders (other than PTSD for the patient
group), for significant medical illness, and for auditory or visual impairment.

Assessment: PTSD subjects will be rated for current psychopathology with the Clinician
Administered PTSD Scale and the Mississippi Scale for Combat-Related PTSD. In addition, PTSD
subjects and combat controls will be rated for severity of combat exposure with the Combat
Exposure Scale. In all subjects, childhood trauma will be assessed with the Early Trauma
Inventory. Current depression and anxiety symptoms will be assessed using the Hamilton
Depression and Hamilton Anxiety scales, respectively. Attention and distractibility will be
assessed by means of the Continuous Performance Test, and motor function will be assessed
with the Finger Tapping Test. Cognitive function will be assessed with the Benton Visual
Retention Test, the California Verbal Learning Test, Wechsler Abbreviated Scale of
Intelligence, Wechsler Memory Scale, Selective Reminding Test, and the Trails Test. Results
of these tests will be used as covariates in the analysis of conditioning data in order to
control for potential group differences in attention or memory. Total time of assessment
should be about four to five hours.

Procedure: Acoustic stimuli will be delivered binaurally through headphones (Maico,TDH39P).
Acoustic startle stimuli will be 40 ms 116 dB white noise bursts with near instantaneous rise
time delivered by a computerized electromyographic (EMG) startle response monitoring system
(SR-LAB, San Diego Instruments). The eyeblink component of the acoustic startle response will
be measured via EMG of the right orbicularis oculi muscle. EMG activity will be amplified and
digitized using the SR-LAB (San Diego Instruments). According to methods established by Davis
and Grillon (Grillon et al., 1991; Grillon and Ameli, 1998), aversive stimuli (US) will be
40-250 ms airblasts with an intensity of 100 psi directed to the larynx, emitted by an
compressed air tank attached to a polyethylene tubing and controlled by a solenoid switch. A,
B, C, and X will be green, purple, orange, or blue lights (counterbalanced color assignment
across subjects) or pictures. Subjects will be asked to press a button to indicate that they
have learned the discrimination between the different colored lights or pictures. The test
session will begin with startle probes to reduce initial startle reactivity and rule out
nonstartlers. The training phase of the session will consist of trials in which stimuli A and
X are paired with the US (AX+), and trials in which stimuli B and X are not paired with the
US (BX-). The test phase will include startle probes alone or during presentation of the cues
AB and AC (the order will be counterbalanced across subjects). Between the presentation of AB
and AC, there will be reinforced trials of AX. Total time of test session will be
approximately 20 minutes.

Statistical Analysis: The basic approach will be a mixed model ANOVA, using Diagnostic Group
as a between-subjects factor (for Specific Aim 2), and within-subjects factors of trial type
(Cue + Startle vs. Startle Alone), and type of cue (AX, BX, AB, AC).

Future Directions: This work will be coordinated with a series of preclinical studies planned
in the Davis lab. The effects of pharmacologic probes on fear conditioning and fear
inhibition will be explored in normal controls as a potential method for identifying new
pharmacologic treatments for PTSD. Treatment effects on fear conditioning and inhibition will
be studied in PTSD subjects. Data from the current project will serve as pilot data for
future NIH R01 and VA Merit Review grant submissions.

References:

Bouton ME, Swartzentruber D (1986). Analysis of the associative and occasion-setting
properties of contents participating in a Pavlovian discrimination. J Exp Psych Anim Behav
Process 12:333-50.

Davis M, Falls WA, Campeau S, Kim M (1993). Fear-potentiated startle: a neural and
pharmacological analysis. Behav Brain Res 58:175-98.

Grillon C, Ameli R (1998). Effects of threat and safety signals on startle during
anticipation of aversive shocks, sounds, or airblasts. J Psychophysiol 12:329-337.

Grillon C, Ameli R, Woods SW, Merikangas K, Davis M (1991). Fear-potentiated startle in
humans: effects of anticipatory anxiety on the acoustic blink reflex. Psychophysiology
28:588-595.

Rescorla RA, Wagner AR (1972). A theory of Pavlovian conditioning: variations in the
effectiveness of reinforcement and nonreinforcement. New York: Appleton-Century-Crofts.

Inclusion Criteria:

- Age 18-80

- Diagnosis of PTSD

- Healthy controls (no history of PTSD)

- Combat controls (no history of PTSD)

Exclusion Criteria:

- No drug or alcohol abuse or dependence