Psychopharmacology (1992) 108:225-228

Psychopharmacology © Springer-Verlag 1992

Effect of m-chlorophenylpiperazine on plasma arginine-vasopressin concentrations in healthy subjects* Ren6 S. Kahn 1, Mitchel A. Kling 2, Scott Wetzler 3, Gregory M. Asnis 3, and Herman van Praag 3 1 Department of Psychiatry, Mount Sinai Hospital/School of Medicine and the Bronx Veterans Administration Hospital, 140 West Kingsbridge Road, Bronx, NY 10468, USA 2 Clinical Neuroendocrinology Branch, N.I.M.H., Building 10, 9000 Rockville Pike, Bethesda, MD 20892, USA 3 Department of Psychiatry, Albert Einstein College of Medicine/Montefiore Medical Center, 111 East 210th Street, Bronx, NY 10467, USA Received October 11, 1991 / Final version February 7, 1992

Abstract. It has been demonstrated convincingly that ACTH and prolactin release are under the stimulatory control of serotonin (5-hydroxytryptamine, 5HT). Recent animal studies suggest that stimulation of 5HT activity also induces the release of arginine-vasopressin (AVP). More specifically, m-chlorophenylpiperazine (MCPP), a 5HT agonist widely used to examine 5HT receptor responsivity in human subjects, has been found to induce AVP release in rodents. This study examined whether MCPP increased plasma AVP levels in healthy human subjects. MCPP was administered orally to 17 healthy subjects in a placebo-controlled design in doses of 0.25 and 0.5 mg/kg. AVP was measured twice hourly over a 210 min period after administration of capsules. MCPP did not significantly alter AVP levels as compared to placebo. However, female subjects had significantly lower plasma AVP levels than males. Since it has been suggested that MCPP-induced AVP release in animals is due to stimulation of 5HT1¢ receptors, the fact that MCPP did not induce the release of AVP in humans suggests that either MCPP is not a potent 5HT1¢ agonist or that AVP is not released by stimulation of 5HT~lo) receptors in human subjects. The observation of gender differences in plasma AVP levels suggests that this factor should be taken into account in future studies of AVP secretion in plasma. Key words: Serotonin - Arginine-vasopressin - m-Chlorophenylpiperazine - Human

The serotonin (5-hydroxytryptamine, 5HT) challenge paradigm is being used increasingly to assess in vivo 5HT receptor function in human subjects. In this paradigm, a 5HT agonist is administered, and effects controlled by * This work was conducted at the Department of Psychiatry, Montefiore Hospital/Albert Einstein College of Medicine, New York, NY, USA Offprint requests to: R.S. Kahn

5HT are then measured (Murphy et al. 1986; van Praag et al. 1987). Evidence that 5HT induces the release of ACTH and prolactin is well documented (see Tuomisto and M/innist6 1985), and therefore these hormones have commonly been measured in response to 5HT agonist challenge. m-Chlorphenylpiperazine (MCPP) is one of the most frequently used probes in the 5HT challenge paradigm (see Kahn and Wetzler 1991). MCPP is a direct 5HT receptor agonist that readily crosses the blood-brain barrier. It binds potently to all 5HT receptors, but most potently to 5HTlc receptors (see Kahn and Wetzler 1991). It has been found to induce the release of ACTH, cortisol and prolactin in a dose-dependent manner, when administered orally (Kahn et al. 1990), and after intravenous administration (Kalus et al. 1992). Release of arginine-vasopressin (AVP) also appears to be 5HT mediated. Intracerebroventricular administration of 5HT increases AVP in rat plasma (Montes and Johnson 1990), which is a centrally mediated effect (Steardo and Iovino 1986). The 5HT reuptake inhibitor, fluoxetine, induces the release of AVP in peripheral blood and in hypophyseal portal veins of rats (Gibbs and Vale 1983). The mixed 5HT1/5HT2 agonist MK212 induces AVP release as does MCPP (Bagdy et al. 1990). The former effect was blocked by the 5HTac/2 antagonist, LY53857 (Brownfield et al. 1988). However, neither the selective 5HTla agonists, 8-hydroxy-2-(di-N-propylamino)-tetralin (8OHDPAT) and ipsapirone, nor the 5HT~,/ 5HTlb agonist RU24969, nor the 5HT2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) induced the release of AVP (Brownfield et al. 1988; Bagdy et al. 1990). These findings suggest that 1) stimulation of 5HT activity induces the release of AVP in rats, and 2) 5HTinduced AVP release may be linked to a specific 5HT receptor subtype, i.e. the 5HT~ receptor. The latter would contrast with the apparent lack of such 5HT receptor subtype specificity for 5HT-induced ACTH and prolactin release [which are increased by stimulation of either 5HT~a (Koenig et al. 1987; Gilbert et al. 1988;

226 W i l l o u g h b y et al. 1988; Lesch et al. 1990) or 5HTI~/Z receptors (Koenig et al. 1987; D i R e n z o et al. 1989)]. Thus, A V P response to 5 H T agonist stimulation could be a potential m a r k e r o f 5HTlo receptor sensitivity. M o r e specifically, since M C P P potently binds to 5HTIc receptors, A V P response to M C P P could be a potential m a r ker for 5HTlo receptor sensitivity. The present study examined the effect o f M C P P o n plasma A V P concentrations in healthy subjects in a doseresponse study. It represents the first a t t e m p t to study A V P response to a n y 5 H T agonist in h u m a n subjects. W e have previously reported d a t a f r o m the same subjects on M C P P ' s effect on A C T H , cortisol and prolactin release ( K a h n et al. 1990).

Results

As Fig. 1 indicates, baseline A V P did n o t differ between tests, b u t was significantly lower in females (placebo d a y : 1.44- 1.3 pg/ml; 0.25 m g / k g M C P P d a y : 1.24-0.5 pg/ml; 0.5 m g / k g M C P P d a y : 1 . 2 + 0 . 6 pg/ml) t h a n in males (placebo d a y : 3 . 2 + 1.2 pg/ml; 0.25 m g / k g M C P P d a y : 2 . 9 + 1.2 pg/ml; 0.5 m g / k g M C P P d a y : 2 . 6 + 1.6 pg/ml) (F-- 11.9, df= 1,13, P < 0.005). M C P P did n o t significantly alter A V P concentrations in males (main effect test: F=0.81, df=2,12, P=0.47; TestxTime: F=l.04, df--14, 84, P = 0.42), n o r in females (main effect test: F=0.18, df=2,10, P=0.84; TestxTime: F=0.37, df= 14,70, P = 0.98) [Fig. 1]. The same results were obtained b y c o m b i n i n g d a t a f r o m males and females.

Material and methods

Healthy subjects (n = 17, mean age = 32.7 + SD 12.6) including nine males and eight females participated in this study. All subjects were administered the Schedule for Affective Disorders and Schizophrenia interview, met Research Diagnostic Criteria for "never mentally ill" and had no history of medical disorders. They were within 25 % of their ideal body weight, had normal laboratory and physical exams, were free of drug and alcohol use for at least 4 weeks and gave written informed consent prior to participation in the study. Subjects who had used hormone preparations within 6 months of the test, or who had an abortion or miscarriage during that period were excluded from the study. The effects of MCPP on ACTH, cortisol, prolactin, temperature and behavioral response in these subjects (as well as their MCPP blood levels) have been published earlier (Kahn et al. 1990). Due to a delay in the AVP assay, results on this aspect of the study are published separately. Subjects fasted (except for water intake) from 11 p.m. on the night before the day of the procedure. At 9 a.m. an in-dwelling intravenous catheter was inserted. Subjects were not allowed to drink, eat, smoke or sleep during the procedure. At 10 a.m. subjects received orally either 0.25 mg/kg or 0.5 mg/kg MCPP or identical placebo capsules in a randomized double-blind design. Time between procedures was usually 7 days, with a minimum of 2 days. AVP was assessed at 30 rain intervals between 9 a.m. and 1.30 p.m. MCPP was obtained from Aldrich Chemical Company (Milwaukee, WI). Plasma immunoreactive AVP concentrations were measured by the Clinical Neuroendocrinology Branch, NIMH/IRP, Bethesda, MD, by radioimmunoassay (RIA) following Sep-Pak C-18 TM (Waters, Inc., Milford, MA) extraction as previously described (Rittmaster et al. 1987) except for the use of a different anti-AVP antiserum (Arnell Products Corp., New York, NY). Detection limit was 0.3 pmol/1. The intra-assay and inter-assay coefficients of variation were 6.3 % and 13.9 %, respectively, at an AVP dose of 5 pg/ml, and 4.1% and 12.3 %, respectively, at a dose of 11 pg/ml. All samples were assayed concurrently, with all samples from a given subject analyzed in the same extraction run.

Data analysis. Baseline levels (time 0, when capsules were administered) across the three challenge tests were compared using repeated measures Analysis of Variance (ANOVA). Since an earlier study indicated that MCPP blood levels may vary depending on gender (Kahn et al. I991) and since baseline AVP concentrations were different for males and females, results were analyzed separately for each gender. To assess the effects of MCPP, a repeated measures analysis of variance (ANOVA) was conducted with two repeated measures (Test: placebo, 0.25 mg/kg MCPP and 0.5 mg/kg MCPP; and Time: 0, 30, 60, 90, 120, 150, 180, 210 min). Significance level was set at P < 0.05. All results are presented as mean+ standard deviation, except in the figures were they are presented as mean :t: standard error.

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Fig. 1. Mean and standard error of the mean arginine-vasopressin concentrations after administration (at 0 min) of placebo (trian~ gles) 0.25 mg/kg MCPP (circles) and 0.5 mg/kg MCPP (squares) in healthy males 3-

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Fig. 2. Mean and standard error of the mean arginine-vasopressin concentrations after administration (at 0 rain) of placebo (triangles), 0.25 mg/kg MCPP (circles) and 0.5 mg/kg MCPP (squares) in healthy females

227

Discussion Results of this study suggest that M C P P in oral doses of 0.5 and 0.25 mg/kg does not induce the release of AVP in healthy human subjects. This finding contrasts with previous studies finding that 0.5 mg/kg MCPP PO induces the release o f A C T H (Kahn et al. 1990) and prolactin (Mueller et al. 1985; M u r p h y et al. 1989; K a h n et al. 1990). Apparently MCPP, in a dose that is sufficient to increase A C T H and prolactin levels, does not induce AVP release in human subjects. There are several possible explanations for the negative results observed in the present study. Insensitivity of the AVP assay is an unlikely explanation, as nearly all samples showed detectable immunoreactive AVP. An insufficient number o f subjects studied could have precluded the finding of a significant drug effect, although the probability of finding a large (Test x Time) effect with this sample size is 96% for all subjects combined and about 60% when each gender is analyzed separately (Cohen 1988). The probability of finding moderate effects given the sample size of this study is 55% (Cohen 1988). Thus, although this study may have missed small to moderate effects o f M C P P on AVP release, it is unlikely that a large effect of M C P P on AVP release would have escaped detection. Variability in AVP levels could have obscured subtle increases in AVP secretion. In particular, variability may have been increased in the female subjects due to differences in the phase of the menstrual cycle. Finally, since the half-life of AVP may only be 5-10 min (King et al. 1989; Stark et al. 1989), the sampling interval of 30 min may have been too long to detect (brief) AVP peaks. However, as a consequence of oral administration time of peak M C P P levels varied considerably between subjects (Kahn et al. 1990). Thus, it appears unlikely that an increase in mean AVP concentrations would have remained undetected. In sum, the present data suggest that MCPP is not a potent AVP releaser in humans. This is the first study examining the effect of any 5HT agonist on AVP release in humans. Its results suggest that increasing 5HT function does not induce AVP release in human subjects. However, until studies are conducted using higher M C P P doses and other 5HT agonists such a conclusion is speculative. It has been suggested that MCPP-induced AVP release in animals is due to stimulation of 5HTI~ receptors, since AVP is not released by DOI (a 5HTz agonist) and 8 O H D P A T (a 5HTla agonist) (Bagdy et al. 1990). The fact that M C P P did not induce the release o f AVP in humans suggests that either M C P P is not a potent 5HTtc agonist or that AVP is not released by stimulation of this, or any other, 5HT receptor in human subjects. Finally, A C T H release may be under the stimulatory control of both corticotropin releasing hormone (CRH) and AVP (Gibbs and Vale 1983). Since M C P P did not induce the release o f AVP, it is unlikely that MCPPinduced A C T H release is AVP-mediated. By inference, MCPP-induced A C T H release may be the result of it stimulating C R H release. This would be consistent with the finding that MCPP-induced A C T H release is sup-

pressed by pretreatment with anti-CRH rabbit serum (Calogero et al. 1990). In summary, results o f this study suggest that M C P P does not induce AVP release in a dose that increases A C T H and prolactin levels in humans. Thus, AVP release may not be a useful indicator o f 5HT receptor sensitivity in human subjects, at least when M C P P is used as a probe. Further studies, using higher doses o f M C P P or intravenously administered M C P P are needed to expand and confirm these findings. Finally, since females were found to have lower basal AVP plasma levels than males, future studies examining plasma AVP secretion should take into account the possibility of such gender differences.

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