Neuroendocrine and behavioral responses to challenge with the indirect serotonin agonist dl-fenfluramine in adults with obsessive-compulsive disorder

BIOL PSYCHIATRY 1992;31:19-34

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Neuroendocrine and Behavioral Responses to Challenge with the Indirect Serotonin Agonist d/-Fentturamine in Adults with Obsessive-Compulsive Disorder P. Anne McBride, Michael D. DeMeo, John A. Sweeney, James Halper, J. John Mann, and M. Katherine Shear

Neuroendocrine and behavioral responses to a single 60-mg oral dose of the indirect serotonin agonist dl-fenfluramine were assessed in unmedicated adults with obsessivecompulsive disorder (OCD) and neuroendocrine results contrasted with those in normal control subjects. Net fenfluramine-induced prolactin release did not differ significantly between OCD patients and normal controls. Prolactin responses in the OCD group were not significantly correlated with baseline Yale-Brown Obsessive Compulsive Scale scores for either obsessions or compulsions, but were positively correlated with the baseline Hamilton Depression Scale score and Hamilton Anxiety Scale score. No clear difference in the severity of patients' obsessions or compulsions was found following challenge with fenfluramine versus placebo. Although the present study does not demonstrate a serotonergic abnormality in OCD, this may be more a reflection of limitations of the tes, procedures than evidence that central nervous system (CNS) serotonergic function is normal in the disorder.

Introduction Recent efforts to elucidate the pathophysiology of obsessive-compulsive disorder (OCD) have focused largely on the function of the neurotransmitter serotonin. Pharmacological treatment studies have provided the most compelling evidence of a serotonergic role in OCD. Drugs that strongly inhibit serotonin reuptake by presynaptic serotonergic neurons (clomipramine, fluoxetine, fluvoxamine, and zimelidine) have repeatedly been shown to be effective in reducing obsessive-compulsive symptoms, whereas agents with less potent inhibitory effects on serotonin reuptake (for instance, amitryptyline, imipramine, and desipramine) appear to be relatively ineffective (Zohar and Insel 1987; Zak et al 1988). The likelihood that the serotonin reuptake inhibitors exert their effects in OCD by serotonergic mechanism has been strengthened by studies showing strong positive correlations between improvement in obsessive-compulsive symptoms during clomipramine

From the Laboratory of Psychopharmacology and the Anxiety Disorders Clinic of the Department of Psychiatry, Cornell University Medical College, New York, NY. Address reprint requests to Dr. P. Anne McBride, Department of Psychiatry, The New York Hospitai-Cornell Medical Center, 525 East 68th St, New York, NY 10021. Received February 15, 1991; revised July 20, 1991. © 1992 Society of Biological Psychiatry 0006.3223/92/$05.00

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treatment and drug-induced decreases in cerebrospinal fluid (CSF) levels of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) (Thoren et al 1980) and platelet serotonin concentration (Flament et al 1987), and by the fact that administration of the serotonin receptor antagonist metergoline appears to exacerbate obsessive-compulsive symptoms in OCD patients receiving clomipramine (Benkelfat et al 1989). Despite the therapeutic efficacy of the serotonin reuptake inhibitors in OCD, an underlying alteration in serotonergic function has not been convincingly demonstrated in the disorder. One small study reported elevated CSF levels of 5-HIAA in unmedicated OCD patients versus normal controls (Insel et al 1985). A second larger study failed to detect a significant difference in CSF 5-HIAA levels in patients and healthy volunteers, although the data suggested a possible bimodal distribution of values in the OCD group (Thoren et al 1980). Platelet serotonin uptake appears to be normal in OCD patients (Insel et al 1985; Weizman et al 1986). Reports have variably described normal (lnsel et al 1985) and reduced (Weizman et al 1986) numbers of platelet [3H]imipramine binding sites. Although decreased platelet serotonin content has been reported in adult OCD patients (Yaryura-Tobias et al 1977), adolescent patients appear to have a similar range of values compared to age-matched controls (Flament et al 1987). Recently, efforts to characterize central nervous system (CNS) serotonergic function in OCD have turned to the assessment of the behavioral and neuroendocrine responses of OCD subjects to pharmacological challenge with serotonin agonists. The nonselective direct serotonin receptor agonist m- chlorophenylpiperazine (m-CPP) has been most extensively employed. Two studies have shown a marked exacerbation of obsessive-compulsive symptoms following a single oral dose of m-CPP (Zohar et al 1987; Hollander et al 1988), although a third failed to detect a significant change in symptom severity following the intravenous administration of the drug (Charney et ai 19~8). The capacity of oral m-CPP to increase obsessive-compulsive symptoms appears to be abolished by chronic treatment with elomipramine (Zohar et al 1987) and fluoxetine (Hollander et al 1989). The foregoing findings suggest that postsynaptic serotonin receptor responsivity may be enhanced in OCD, and that serotonin reuptake inhibitors may exert their therapeutic effects in the disorder by downregulating receptor activity. However, two recent studies employing other direct serotonin receptor agonists call this hypothesis into question. Patients did not show an exacerbation of obsessive-compulsive symptoms following oral administration of either the selective serotonin~^ (5-HTm) receptor agonist ipsapirone (Lesch et al 1991) or MK-212 (Bastani et al 1990), a drug that appears to exert its effects primarily via 5-HT2 and 5-HTtc receptors. Neuroendocrine responses to direct serotonin receptor agonists have not provided evidence of postsynaptic serotonin receptor supersensitivity in OCD, but rather have suggested that receptor function is normal or reduced in the disorder. OCD subjects have been reported to have normal (Zohar et al 1987) or blunted (Hollander et al 1991) prolactin release following a single oral dose of m-CPP, as well as blunted cortisol release (Zohar et al 1987). Following intravenous m-CPP, female patients exhibited decreased prolactin secretion compared with controls, whereas male patients exhibited normal prolactin release; the cortisol response did not differ in OCD subject versus controls (Charney et al 1988). Neuroendocrine challenge studies with other direct serotonin receptor agonists have produced similar resuit~ to those employing m-CPP. The corticotropin and cortisol responses to oral ipsapirone were equivalent in patients and controls (Lesch et al 1991), whereas the prolactin and cortisol responses to oral MK-212 were blunted in OCD subjects (Bastini et al 1990).

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Although challenge tests employing direct serotonin receptor agonists have proven useful in evaluating postsynaptic serotonin receptor responsivity in psychiatric disorders, results do not provide information about the status of presynaptic serotonergic neurons. The use of challenge agents that augment presynaptic serotonergic function offer the advantage of permitting an assessment of the integrity of serotonin neuron circuits as a functional unit. Studies have suggested that the indirect serotonin agonists tryptophan and fenfluramine have no specific effect on the severity of obsessive-compulsive symptoms (Charney et al 1988; Hollander et al 1988). Relatively little has been published concerning neuroendocrine responses to indirect serotonin agonists in OCD. Charney et al (i988) have reported that prolactin release following intravenous tryptophan administration did not differ significantly between male and female patients and their respective gendermatched controls. Hollander et al (1991) did not find a significant alteration in the prolactin response to oral fenfluramine, whereas Hewlett et al (1989) have presented preliminary data suggesting that female patients may have blunted fenfluramine-induced prolactin release. In the present study, neuroendocrine and behavioral responses to fenfluramine challenge were assessed simultaneously in unmedicated OCD patients and neuroendocrine results contrasted with those in normal controls. Administered as a single dose, fentturamine enhances serotonergic transmission by stimulating serotonin release from the presynaptic neuron and by blocking serotonin reuptake (Borroni et al 1983). Prolactin release was used as the neuroendocrine outcome measure of the challenge test because data suggest that fenfluramine, in moderate doses, promotes prolactin secretion by its specific effects on serotonergic neurons (Quattrone et al 1978, 1983; Fuller et al 1982).

Methods

Subjects Twenty-one adults (13 men, 8 women) with OCD participated in the study. These subjects were recruited from applicants to the clinical and research programs of the Anxiety Disorders Clinic at an urban university medical center. The mean (_. SD) age of the OCD group was 34.3 ± 8.0 years with a range of 19-51 years; the mean ages of male and female subjects were 34.8 ± 8.4 and 33.6 ± 7.8 years, respectively. All were of normal weight and in excellent physical health. All subjects were drug-free a minimum of 1 month before testing and denied recent substance abuse. Female subjects were not taking oral contraceptives. Each subject initially received a clinical evaluation to confirm a history of OCD and to document the nature and severity of current symptoms. Diagnostic criteria were those specified by DSM-III-Revised (American Psychiatric Association 1987). Subjects were judged to be free of a comorbid Axis 1 disorder, and did not exhibit frank psychotic symptomatology. On the Yale-Brown Obsessive Compulsive Scale (YBOCS) (Goodman et al 1989), the mean score for obsessions was 12.9 ± 3.4 and for compulsions 12.7 __ 2.8; thus, most of the subjects l~ported obsessive-compulsive symptoms of moderate to extreme severity. The mean 21-item Hamilton Depression Scale (Hamilton 1960) score was 11.3 _ 7.2, and the mean Hamilton Anxiety Scale (Hamilton 1969) score 11.3 -5.8. Twenty-seven adults ,(16 men, 11 women) without personal or family histories of psychiatric or neurological disorder served as normal control subjects. All were of normal

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weight and in excellent physical health. The mean age of the control group was 31.1 -49.9 years with a range of 21-56 years; the mean ages of male and female subjects were 32.3 __ 10.6 and 29.4 _+ 8.9 years, respectively. All subjects were drug-free at testing and denied recent drug ingestion or histories of substance abuse. None of the female subjects had taken oral contraceptives in the preceding year. Most subjects were either hospital employees or medical students. Written informed consent was obtained from all subjects according to the requirements of the Committee on Human Rights in Research at the New York Hospital-Comell Medical Center.

Fenfluramine Challenge Test The fenfluramine challenge test was performed on an outpatient basis utilizing a placebocontrolled design. Each subject received a 60-mg oral dose of dl-fenfluramine hydrochloride (Pondimin) and placebo on different days. Food and fluids other than water were not permitted after 11 PM on nights preceding testing. Studies began at 8 AM with the insertion of an indwelling catheter into an antecubital vein. An intravenous infusion of 5% dextrose and 0.45% sodium chloride (200 ml/hr) was administered to prevent dehydration and hormonal changes due to the hypoglycemic effect of fenfluramine (McCrae 1975). Blood samples for measurement of plasma prolactin levels were drawn, via the catheter, both 15 rain before and immediately preceding the ingestion of fenfluramine or plac,~bo at 9 AM (hour 0), and then hourly for five successive hours. Plasma was also obtai',ned at hours O, 3, and 5 for assay of fenfluramine and norfenfluramine levels. A total '~of 100 ml of whole blood was required for all studies. Subjects rested quietly throughout the test, but did not sleep. In 18 of the 21 OCD subjects, the severity of obsessions and of compulsions was assessed immediately prior to the administration of fenfluramine or placebo (hour 0), and then hourly for five successive hours. At each assessment point, an investigator completed a YBOCS (minus items 2 and 7, which pertain to the level of interference with social and occupational activities) based upon the subject's report of symptoms during the preceding 30 min. The investigator then assigned a rating for the overall severity of obsessions and for the overall severity of compulsions (0 = none, 1 = mild, 2 = moderate, 3 = severe, 4 = extreme) that was representative of the subject's individual item scores on the obsessions and compulsions sections of the YBOCS. Placebo tablets identical to fenfluramine tablets were supplied by A. H. Robins Company. Although subjects were not told whether they had received fenfluramine or placebo on a given day, the investigator who rated the severity of obsessions and compulsions during challenge testing was aware of which treatment had been administered. Thus, these ratings are "single blind." Laboratory staff, however, lacked knowledge of both the treatment administered and the subject's group assignment. Clinical ratings obtained at study entry were completed prior to the initiation of challenge testing. The mean interval between the administration of fenfluramine and placebo was 8.9 - 7.5 days. In cases where fenfluramine was administered on the first test day, the placebo study was not performed for at least 1 week. The maximum interval between studies with active drug and placebo was 1 month. Although a number of OCD subjects and normal controls described impaired concentration, drowsiness, or mild dysphoria, anxiety, or elation following fenfluramine ingestion, none manifested more dramatic changes in mental status, such as agitation or ,i

Serotonergic Responsivity in OCD

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20 OCD

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Time ( h o u r s ) Figure 1. Plasmaprolactin levels (mean +_ SEM) in OCD subjects (n = 21) versus normal controls (n = 27) after a 60-mg oral dose of d/-fenfluramine (solid lines) and placebo (broken lines).

psychotic symptoms. Changes in mood and level of arousal were typically first noted 2 hr following fenfluramine ingestion. There were no clear differences in the effect of fenfluramine on mood, level of arousal, or anxiety level (rated hourly on a ten-item Likert scale) in OCD subjects versus controls. Somatic complaints included nausea, headache, fatigue, and light headedness, all minor in severity. Plasma prolactin levels were determined by an immunoradiometric assay using kits purchased from Hybritech Incorporated. The minimum detectable prolactin concentration is 0.3 ng/ml, and the interassay coefficient of variation is 4%. Plasma levels of fenfluramine and norfenfluramine were measured by a gas-liquid chromatographic method (Krebs et al 1984). The minimum detectable concentrations of fenfluramine and norfenfluramine are 2.0 and 5.0 ng/ml, respectively. Coefficients of variation are 3.4% for fenfluramine and 6.8% for norfenfiuramine.

Data Analysis The net change in plasma prolactin levels (APRL) following fenfluramine administration (hours 0-5) served as the neuroendocrine outcome measure of the challenge test. Because resting prolactin levels varied significantly over time during placebo studies (F = 13.07, df = 6,282, p < 0.001; Figure 1), fenfluramine-induced changes in hourly plasma prolactin levels were determined by subtracting an individual's plasma prolactin levels during the placebo trial from time-matched levels following administration of active drug. Baseline prolactin levels (average of levels of 8:45 and 9:00 AM) were not significantly different on the two study days (F = 0.04, df = 1,47, NS).

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A repeated-measures analysis of variance (ANOVA) was used to compare fenfluramineinduced changes in plasma prolactin levels between OCD and control subjects. The analysis was performed without and with the use of an index of plasma drug levels (the average of summed levels of fenfluramine and norfenfluramine at hours 3 and 5) as a covariate. As we have discussed elsewhere (McBride et al 1989), this index is a close approximation of both an individual's peak plasma drug level and the plasma drug level between hours 3 and 5, the period of maximal prolactin secretion during the fenfluramine challenge test. An additional repeated-measures ANOVA was performed to evaluate the effect of age on the prolactin response to fenfluramine challenge in OCD versus control subjects. This analysis was undertaken because we have previously observed an age-dependent decline in fenfluramine-induced prolactin release in healthy individuals that appears to be largely complete around the age of 30 years (McBride et al 1990). On the basis of this finding, age was treated as a dichotomous variable, with subjects grouped by age below 30 years and 30 years and above. Five of the 21 OCD subjects and 15 of the 27 normal controls were less than 30 years. In individual subjects, the net change in plasma prolactin levels following fenfluramine challenge was quantified by calculating the area under the curve (AUCaPRL) using the trapezoid rule. The correlation between values for AUCApRL and the peak change in prolactin levels was 0.95 (p < 0.001). Although values for AUCapRL and the index of plasma drug levels were significantly correlated with body weight in the total pool of subjects (r -- - 0 . 4 2 , df -- 40, p < 0.01, and r = - 0 . 4 2 , df = 40, p < 0.01, respectively), neither correlation remained significant when the analyses were performed by gender (women: r = - 0 . 2 5 , df = 16, NS, and r = - 0 . 0 8 , df = 16, NS; men: r = - 0 . 1 3 , df = 22, NS, and r = - 0 . 4 0 , df - 22, p < 0.10). The behavioral response to fenfluramine challenge was assessed by repeated-measures ANOVA comparing severity ratings for obsessions and for compulsions (hours 0-5) in OCD subjects following fenfluramine versus placebo. The two-tailed Student's t-test was used to contrast values for AUCaPRL and plasma drug levels between the experimental groups. The paired samples student's t-test was used to contrast severity ratings at specific time points following administration of fenfluramine and placebo. Correlation coefficients were calculated by simple linear regression analysis (Pearson's Product Moment), Standard deviations are reported as indices of group variability unless otherwise noted.

Results

Fenfluramine-lnduced Prolactin Release in OCD versus Normal Control Subjects Figure 1 illustrates mean plasma prolactin levels following the oral administration of dlfenfluramine hydrochloride (60 mg) and placebo to 21 subjects with OCD versus 27 normal controls. Fenfluramine resulted in a significant increase in plasma prolactin levels in both groups (treatment × time interaction: OCD subjectsuF = 15.24, df = 5,100, p < 0.001; normal control subjects--F = 21.79, df = 5,130, p < 0.001) that peaked 3-4 hr after drug ingestion. A comparison of fenfluramine-induced changes in plasma prolactin levels in OCD

Serotonergic Responsivity in OCD

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subjects versus normal controls (by ANOVA) failed to show a significant difference in net prolactin release between groups (treatment × diagnosis interaction: F = 0.06, df = 1,46, NS; treatment × diagnosis × time interaction: F = 0.19, df = 5,230, NS). Figure 2 depicts the magnitude of individual prolactin responses to fenfluramine challenge, quantified by the area under the curve (AUCApRL), in OCD versus control subjects subgrouped by gender. Mean values for AUCApRL did not differ between the diagnostic groups in male subjects (OCD: 16.9 4- 11.6 hr × ng/ml; controls: 24.4 4- 16.0 hr x ng/ml; t - 1.42, df - 27. NS), female subjects (OCD: 57.4 _ 52.4; controls: 49.1 _ 34.8; t = 0.42, df = 17, NS), orthe total pool (OCD: 32.3 _4_-38.1; controls: 34.4 4. 27.7; t = 0.23, df = 46, NS). The very modest difference in values for AUCApRL between OCD subjects and controls is reflected by a power analysis (using Cohen's d statistic) that yielded an effect size of only 0.06. Table 1 lists mean plasma levels of fenfluramine and its active metabolite, norfenfluramine, at hours 3 and 5 following drug ingestion in OCD subjects and normal controls. OCD subjects had significantly higher levels of both fenfluramine and norfenfluramine at each of the two sampling points. The average of summed levels of fenfluramine and norfenfluramine at the two sampling points, an index of subjects' plasma drug levels, was positively correlated with the magnitude of the prolactin response to fenfluramine challenge (AUCApRL) in the total pool of subjects (r = 0.29, df = 46, p < 0.05). Because the foregoing findings raised the possibility that OCD subjects might exhibit blunted prolactin responses relative to their plasma drug levels, the ANOVA contrasting net fenfluramine-induced prolactin release in patients and controls was repeated using the

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Table I. Plasma Levels of Fenfluramine and Norfenfluramine in OCD Subjects and Normal Controls" Compound (ng/ml)

OCD (n = 21)

Control (n = 27)

Hour 3 Fenfluramine Noffenfluramine Fenfluramine and norfenfluramine

65.6 +_ 21.6 14.9 +_ 5.3 80.5 +_ 26.3

50.3 +_ 12.3 b 11.7 -4- 4.0 c 62.0 _ 14.7 b

Hour 5 Fenfluramine N rfenfluramine Fenfluramine and norfenfluramine

68.9 _ 19.7 18.1 __ 5.0 87.0 _ 23.7

47.5 __ 12.8 d 13.3 +_ 4.6 d 60.8 +_ 15.8 d

°Values are the mea, ± SD. bp < 0.01, OCD versus control subjects. Cp < 0.05, OCD versus control subjects. ,tp < 0.001, OCD versus control subjects.

average of summed levels of fenfluramine and noffenfluramine at hours 3 and 5 as a covariate. Once again, prolactin responses did not differ significantly between groups (treatment x diagnosis interaction: F = 2.35, df = 1,45, NS; treatment x diagnosis x time interaction: F = 0.19, df = 5,230, NS).

Resting Plasma Prolactin Levels in OCD versus Normal Contr,,l Subjects Resting plasma prolactin levels during the entire course of the placebo trial were not significantly different in OCD subjects versus normal controls (Figure 1; resting prolactin x diagnosis interaetion--F = 0 . 0 0 , d f = 1,46, NS; resting prolactin x diagnosis x time interaction--F = 1.53, df = 6,276, NS).

Effect of Age on Fenfluramine-lnduced Prolactin Release in OCD versus Normal Control Subjects We have previously described an age-dependent decline in fenfluramine-induced prolactin release in healthy individuals that appears to be largely complete around the age of 30 years (McBride et al 1990). Furthermore, we have also found a differential effect of age on prolactin responses to fenfluramine challenge in patients with major depression versus normal controls (manuscript in preparation). In the present study (which included most of the subjects in the McBride et al 1990 study), net prolactin release (AUCAvaL)decreased with age in normal subjects (r = - 0 . 5 2 , df = 25, p < 0.01). Although prolactin responses were not significantly conelated with age in OCD subjects (r = - 0 . 2 7 , df = 19, NS), the linear regression equation was alm;3st identical to that in normal controls (Figure 3). Age and diagnosis did not have a significant interactive effect on the magnitude or timing of fenfluramine-induced prolactin secretion in an ANOVA where subjects were grouped by age below 30 years and 30 years and older (treatment x age x diagnosis interaction: F = 0.07, df = 1,44, NS; treatment x age × diagnosis × time interaction: F = 1.11, df = 6,264, NS). Thus, the lack of a statistically significant correlation between age and prolactin release in OCD subjects appears to be more a reflection of

Serotonergic Responsivity in OCD

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BIOL PSYCHIATRY ! 992;31:19-34

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Figure 3. The relationship between age and net fenfluramine-induced prolactin release, quantified by the area under the curve (AUC~pRL),in OCD subjects (solid symbols and regression line), and normal controls (open symbols and broken regression line). Male subjects are represented by squares, female subjects by circles.

scatter around the regression line (slope of line: OCD: - 1.30 +_ 1.05; controls: - 1.46 _ 0.48) than a true differential effect of age.

Correlations between Fenfluramine-lnduced Prolactin Release and Clinical Ratings at Study Entry in OCD Subjects At the time of induction into the study, OCD subjects' presenting symptomatology was quantified using conventional rating scales. Net fenfluramine-induced prolactin release (AUCAPRL) was not significantly correlated with the baseline Y':JOCS scores for either obsessions or compulsions (r = 0.31, df = 18, NS, and r . = 0.26, df = 18, NS, respectively). However, significant positive correlations were found between values for AUC~pRL and the 21-item Hamilton Depression Scale score and the Hamilton Anxiety Scale score (r = 0.58, df = 18, p < 0.01, and r = 0.60, df = 18, p < 0.005). The correlation between values for AUC ~PRL and the Hamilton Depression Scale score remained sig,ificant after the items pertaining to anxiety and obsessive-compulsive symptoms (items 10, 11, and 21) were eliminated (r = 0.54, p < 0.02).

Effects of Fenfluramine on the Severity of Obsessions and Compulsions in OCD Subjects As described in the methods section, the hour-by-hour severity of obsessions and of compulsions in OCD subjects was monitored throughout the fenfluramine and placebo challenge tests. Interestingly, most subjects reported only mild to moderate obsessive-

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Time (hours) Figure 4. Severity ratigs (mean - SEM) for obsessions in OCD subjects after a 60-mg oral dose of d/-fenfluramine (solid lines) and placebo (broken lines). Severity ratings were assigned by an investigator ba~ed on the patients' responses to the items pertaining to obsessions on the YaleBrown Obsessive Compulsive Scale. Anchor points are 0 -- none, I = mild, 2 = moderate, 3 = severe, 4 = :xtreme. Asterisk indicates p < 0.05; dagger p < 0.01; and double dagger p < 0.005, fenfluramine versus placebo.

compulsive symptoms imn,,~diately prior to the onset of challenge testing (hour 0) whereas, at the point of study entry, most had described their typical symptoms at moderate to extreme (the mean score on individual YBOCS items was 2.56 at study entry). Figure 4 illustrates slngle-bhaa ratings of the severity of obsessions following the administration of fenfluramine versus placebo. Although subjects reported at most mild to moderate obsessive symptoms on both s~:dy days, severity ratings differed significantly during challenge tests with active drug and placebo (treatment x time interaction: F = 8. l l, df = 5,85, p < 0.001). Fenfluramine administration was associated with a 65% decline in the severity of obsessions, maintained during hours 2-5 of the procedure, whereas placebo administration did not result in a consistent change in severity ratings. However, initial severity ratings were significantly higher on the day of fenfluramine administration than placebo administration (1.56 _ 0.92 versus 0.89 _ 0.90, t = 2.92, df = 17, p < 0.01), whereas severity ratings did not differ on the two study days at hours 3 and 4, tJae period during which plasma fe,fluramine and noffenfluramine levels typically peak (E.F. Coccaro, written communication, 1988). Subjects consictently repc,rted absent to mild compulsive symptoms throughout challenge studies with both fenfluramine and placebo. A repeated-measures ANOVA failed

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to reveal a significant difference in severity ratings on the two study days (treatment × time interaction: F - 1.31, df = 5,85, NS).

Discussion The present study failed to reveal a significant difference in the prolactin response to fenfluramine challenge in OCD subjects versus normal controls. This result is in agreement with mos*~studies that have assessed neuroendocrine responses of OCD patients to challenge with indh-ect serotonin agonists (Charney et al 1988; Hollander et al 1991). Furthermore, the magnitude of fenfluramine-induced prolactin release was not associated with the severity of patient's obsessions or compulsions at the time of induction into the study. In recent years, neuroendocrine challenge tests have been widely used to assess the responsivity of CNS serotonergic neuron circuits and postsynaptic serotonin receptors. Numerous studies in animals and humans have shown that plasma prolactin levels are increased after the administration of agents that augment CNS serotonergic transmission, including serotonin precursors, serotonin-releasing agents, and direct serotonin receptor agonists (Meltzer et al 1982, Mueller et al 1986, Siever et al 1984). In rats, the stimulation of prolactin release by the above agents is typically attenuated or blocked by electrolytically or pharmacologically induced lesions of the ascending serotonergic fibers of the raphi nuclei (Fessler et al 1984; Van de Kar and Bethea 1982). In humans, the prolactin response to m-CPP is abolished by pretreatment with the serotonin receptor antagonist metergoline (MueUer et al 1986). Indirect evidence suggests that serotonin facilitates prolactin release by promoting secretion of a hypothalamic prolactin releasing factor into the portal circulation (Ben-Jonathan et al 1989). Serotonin does not appear to interact directly with the pituitary lactotroph (prolactin-secreting cell) (Kato et al 1985). Although neuroendocrine challenge studies are a valuable tool in the assessment of CNS serotonergic responsivity in neuropsychiatric disorders, results must be interpreted with caution. In particular, it must be recognized that neuroendocrine responses refect the activity of only a circumscribed portion of the CNS serotonergic system. For instance, the prolactin response to m-CPP and other direct serotonin receptor agonists provides a measure of the responsivity of only those postsynaptic serotonin receptor complexes located in hypothalamic nuclei. Similarly, the prolactin response to fenfluramine or tryptophan reflects net serotonergic transmission at hypothalamic synapses. The functional activity of ascending serotonergic neurons that project to the hypothalamus may not necessarily reflect that of ascending serotonergic neurons that terminate in other areas. Thus, although the present study suggests that the responsivity of serotonin neuron circuits in the hypothalamus does not differ in subjects with OCD versus normal controls, one cannot conclude that this is also the case in other brain regions. Current models of the pathogenesis of OCD have postulated dysfunction of neuron circuits connecting the orbitofrontal cortex, basal ganglia, and thalamus (Modell et al 1989; Wise and Rapoport 1989). The fenfluramine challenge test does not assess serotonergic transmission between these structures. Although net fenfluramine-induced prolactin release was not significantly correlated with patients' YBOCS scores for obsessions or compulsions obtained at study entry, prolactin responses in the OCD group were positively correlated with both the baseline Hamilton Depression Scale score and the baseline Hamilton Anxiety Scale score. The presence of greater depressive symptomatology in OCD subjects with more robust pro-

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lactin responses is curious, as others have reported blunted (Siever et al 1984; Heninger et al 1984; Cowen and Charig 1987) or normal (Asnis et al 1988) prolactin responses to fenfluramine or tryptophan in patients suffering from major depression. Although the positive correlation between fenfluramine-induced prolactin release and the Hamilton Depression Scale score may be an artifact deriving from the relatively small sample size, it is also possible that different neurochemical processes are associated with depressive symptomatology in patients with a primary diagnosis of OCD versus major depression. The positive correlation between net fenfluramine-induced prolactin release and the Hamilton Anxiety Scale score is of interest because preclinical data have suggested that the serotonergic system may play a role in anxiety or fear (Chamey et al 1990). However, equivalent prolactin responses to both tryptophan and m-CPP have been reported in panic disorder patients versus normal controls (Charney and Heninger 1986; Charney et al 1987). Studies that assess symptom severity following challenge with a serotonin agonist have the advantage of an outcome measure that is more directly associated with pathogenic mechanisms in OCD than is a hormonal response. On the other hand, the severity of obsessive-compulsive symptoms during the course of a challenge test may be strongly influenced by a number of factors unrelated to the pharmacological effects of the drug, for example, anxiety or attention from an investigator. In some cases, symptom severity may be reduced in the testing situation because the subject is removed from the milieu in which symptoms typically occur. Furthermore, the quantification of changes in symptom severity is a relatively subjective process despite the development of standardized rating scales. In this study, statistical analyses indicated that fenfluramine administration resulted in a significant reduction in patients' obsessions compared with placebo administration, but had no effect on the severity of compulsions. However, several factors complicate the interpretation of these results. First of all, most of the OCD subjects stated that they were having little or no obsessive-compulsive symptomatology immediately prior to the administration of both t'enfluramine and placebo. Thus, there was little room to demonstrate a clear beneficial effect of fenfluramine on the symptoms. Secondly, the fact that initial severity ratings for obsessions were significantly higher on the day of fenfluramine administration than placebo administration may have accentuated the appearance of drug-induced symptomatic relief. The likelihood that the apparent improvement in obsessions following fenfluramine ingestion does not reflect a specific pharmacological effect of the drug is supported by the fact that severity ratings did not differ on the two study days at hours 3 and 4, the period during which plasma fenfluramine and noffenfluramine levels typically peak. Finally, the validity of the clinical ratings obtained during the challenge tests is somewhat diminished by the fact that they were elicited single-blind. Despite the foregoing limitations, it is reasonable to conclude that fenfluramine did not produce an exacerbatic~n of obsessions or compulsions, as has been reported following challenge with oral m-CPP. This result is in accord with other studies that have evaluated the acute effects of indirect serotonin agonists on symptom severity in OCD (Charney et al 1988; Hollander et al 1988). Discrepancies in behavioral responses to the various direct and indirect serotonin agonists may reflect differences in the affinity of each agonist for the different subtypes of postsynaptic serotonin receptors and for other monoamine receptors, m-CPP, the challenge agent that appears to have the greatest effect on obsessivecomp, lsive symptoms of those evaluated to date, binds with highest affinity at 5-HTIc receptors but also interacts with 5-HT~A, 5-HTtD, H-HT2, 5-HTa, and ot2-adrenergic

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receptors (Hamik and Peroutka 1989; Hoyer 1989). Because m-CPP may function as an antagonist at 5-HT2 receptors (Conn and Sanders-Bush 1987), it is possible that the drug potentiates obsessive-compulsive symptoms by inhibiting serotonergic transmission at this receptor subtype. The indirect serotonin agonists fenfluramine and tryptophan exert their effects on the serotonergic system by augmenting the activity of serotonin itself, which binds with high (nanomolar) affinity at the 5-HT~ family of receptors, moderate affinity at 5-HT3 receptors, and relatively low (micromolar) affinity at 5-HT2 receptors (Hoyer 1989). If altered responsivity of one or more postsynaptic serotonin receptor subtypes is ultimately shown to play a role in the induction of obsessive-compulsive symptoms, the lack of a behavioral response to fenfluramine or other indirect serotonin agonists might potentially reflect a reciprocal alteration in presynaptic neuron function. In summary, the present study revealed similar prolactin responses to fenfluramine challenge in adults suffering from OCD and normal controls. No clear difference in the severity of patients' obsessive-compulsive symptoms was found following fenfluramine versus placebo administration. However, these results cannot be taken as evidence that the CNS serotonergic system does not pl~.y a role in the pathogenesis of the disorder. Clearly, new techniques are needed to assess the functional integrity of serotonergic neuron circuits in those brain regions most implicated in OCD. Positron emission tomography (PET scan) may ultimately prove valuable in evaluating CNS serotonin receptor densities in vivo. Postmortem brain studies are needed to assess neuronal morphology, regional brain serotonin content, and binding indices of the various subtypes of serotonin receptors. Finally, greater consideration must be given to the interaction of the serotonergic system and other neurotransmitter systems in studies seeking to elucidate pathophysiological mechanisms in OCD. The serotonin reuptake inhibitors may exert their beneficial effects on obsessive-compulsive symptoms by altering the balance of neurotransmitter systems rather than by overriding a specific serotonergic defect. This project was supported in part by Grant:; MH37907 and MH40695 from the National Institute of Mental Health, Rockville, MD, and by a grant from the Ciba Geigy Corporation. Dr. McBride is the recipient of a Teacher-Scientist Award from the Andrew W. Mellon Foundation. Dis. DeMeo, Halper, and McBride were awarded Reader's Digest Research Fellowships.Dr. Mann is the recipientof an Irma T. HirschlCareerScientist Award. Technicalassistance was provided by Mrs. Anne Gallagher. The manuscript was typed by Mrs. Rosa Lacen-Baez and Ms. Rhonda R. Higgins. Measurementof plasma fenfluramineand norfenfluraminelevelswas kindly performed by George Wright, Ph.D., and Larry Cheng, Ph.D., at A.H. Robins Co., Richmond, VA. Fenfluramine hydrochloride(Pondimin) and placebo tablets were donated by A.H. Robins Co.

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