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Increased platelet 5-HT in patients with bipolar depression. Reply to Franke et al. [J. Affect. Disord. 52 (1999) 101–110]
Journal of Affective Disorders 60 (2000) 143–145 www.elsevier.com / locate / jad
Reply to Letter to the Editor
Increased platelet 5-HT in patients with bipolar depression. Reply to Franke et al. [J. Affect. Disord. 52 (1999) 101–110] a, b c a I-Shin Shiah *, Huei-Chen Ko , Jia-Fu Lee , Ru-Band Lu a
Department of Psychiatry, Tri-Service General Hospital, National Defense Medical Center, No. 8, Ting-Chow Road, Section 3, Taipei, Taiwan b Graduate Institute of Behavioral Medicine, National Cheng-Kung University, Tainan, Taiwan c Department of Psychiatry, Military Pei-Taou Hospital, Taipei, Taiwan Received 10 August 1999
We would like to thank Franke et al. for their comments on our recent paper (Shiah et al., 1999), providing us an opportunity to discuss our work further. In our paper, we reported a trend towards a higher platelet 5-HT content in 12 bipolar I depressed patients and a significant increase in 12 bipolar II depressed patients. We also showed that when the bipolar I and bipolar II patients were pooled, there was a significant increase in platelet 5-HT levels in bipolar depressives (n 5 24) compared to normal controls (n 5 20). Based on the comparisons of platelet 5-HT levels between bipolar depressives and normal controls, we concluded that bipolar depression is associated with an increase in platelet 5-HT. Franke et al. speculated that some outliers had confounded our results. We disagree with their opinion for the following reasons. Firstly, the 95% confidence interval for the difference in platelet 5-HT level between our bipolar patients and normal controls was 0.17 to 1.34 nM 5-HT / 10 9 platelets. Since the confidence interval does not include zero, the difference in platelet 5-HT level between the two groups is statistically significant at *Corresponding author. Tel.: 1 886-2-2365-2463; fax: 1 8862-2367-1006. E-mail address: [email protected] (I-S. Shiah).
the 0.05 level. Secondly, although some of our patients had low values of platelet 5-HT, the effect size for the difference in platelet 5-HT content between patients and normal controls is 0.796, and the magnitude of the effect size also indicates that platelet 5-HT levels significantly differ between bipolar depressed patients and normal controls. Thirdly, after exclusion of the only patient with the platelet 5-HT level (i.e. 4.9 nM 5-HT / 10 9 platelets) outside the range of 2 S.D., the difference in platelet 5-HT between bipolar patients (n 5 23) and normal controls (n 5 20) remains significant (Mann–Whitney U 5 134.0, P , 0.03). The 95% confidence interval for the difference was 0.11 to 1.16 nM 5-HT / 10 9 platelets. Given the above, we are very confident of our finding that platelet 5-HT levels are significantly increased in bipolar depressed patients. By looking at Fig. 1 in our paper, Franke et al. speculated that the patient with 3.5 nM 5-HT / 10 9 platelets and the corresponding HAMD score of 34 had a ‘disproportional’ effect on our result of a trend for a significant positive correlation between 21-item Hamilton Depression Rating Scale (HAMD) scores and platelet 5-HT levels. They suggested that this patient should be excluded from the data analysis. However, we see no justification for doing this. Instead, we think that this patient should be included
0165-0327 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0165-0327( 99 )00172-X
144
I-S. Shiah et al. / Journal of Affective Disorders 60 (2000) 143 – 145
Fig. 1. Platelet 5-HT levels and 21-item Hamilton Depression Rating Scale (HAMD) scores in non-psychotic bipolar I (n 5 10) and bipolar II (n 5 11) depressed patients.
in the data analysis because this patient’s platelet 5-HT level is within the range of 2 S.D. Furthermore, it would make more sense to determine if those patients with extreme values of platelet 5-HT have confounding effects on our results. As mentioned earlier, there is only one patient with the platelet 5-HT level outside the range of 2 S.D. If this patient is excluded from analysis, the trend for a significant positive correlation between HAMD scores and platelet 5-HT in bipolar patients remains unchanged (Spearman’s r 5 0.360, P 5 0.09, n 5 23). Another reasonable approach would be to determine if those patients with psychotic features (as mentioned in our paper, two bipolar I and one bipolar II patients) have an influence on the results. Spearman’s correlation analysis not including these three patients (HAMD scores 33,34,38) showed a significant positive correlation between the HAMD scores and platelet 5-HT levels (Spearman’s r 5 0.538, P , 0.013, n 5 21) (please see Fig. 1). This significant positive correlation in such a homogenous group of bipolar nonpsychotic depressed patients supports our assumption that the alteration in platelet 5-HT function in bipolar depression is related to the severity of depression. We, therefore, disagree with their speculation that the correlation found in the study is due to chance. With regard to the conflicting results of the relationship between platelet 5-HT level and severity of depression in previous studies, this is not surprising be-
cause most of the patients involved in the studies had unipolar depression and our study involved a pure group of bipolar depressed patients. To date, at least three studies that examined platelet 5-HT uptake in bipolar patients reported increased platelet 5-HT uptake in bipolar patients when compared to controls (Oxenkrug, 1979; Zemishlany et al., 1982; Modai et al., 1984). However, Franke et al. did not cite them. Of the three studies, Oxenkrug (1979) simultaneously measured platelet 5-HT content and 5-HT uptake in 100 normal controls and 17 bipolar depressed patients before and after 4 weeks’ treatment with amitriptyline. The results of the study showed (1) bipolar depressed patients had a trend for higher platelet 5-HT content and a significantly higher platelet 5-HT uptake compared to normal controls; and (2) the content and uptake of 5-HT in platelets of depressed patients were significantly decreased after 4 weeks of successful amitriptyline treatment. Similarly, Modai et al. (1984) reported that bipolar depressed patients had a significant increase in platelet 5-HT uptake whereas unipolar depressed patients had a significant decrease in platelet 5-HT uptake, when compared to normal controls, respectively. Zemishlany et al. (1982) measured platelet 5-HT uptake in three different groups: 10 patients with history of bipolar disorder, their immediate families (total 31 people) and 10 healthy controls. They showed that patients with bipolar disorder had significantly increased platelet 5-HT uptake compared to healthy controls but the difference in platelet 5-HT uptake between patients and their families was not significant. Based on the assumption that blood platelets correspond to serotonergic neurons in the brain, the three studies that reported increased platelet 5-HT uptake in bipolar patients appear to support our hypothesis of an increase in presynaptic 5-HT uptake in bipolar depression. Interestingly, a recent in vitro study (Southam et al., 1998) showed that a new mood stabilizer, lamotrigine, led to a dose-dependent inhibition of 5-HT uptake in human platelets and rat brain synaptosomes. The authors of this study hypothesized that the inhibition of 5-HT uptake, which would in turn enhance 5-HT neurotransmission, is one of the mechanisms contributing to the efficacy of acutely administered lamotrigine in bipolar depression (Southam et al., 1998). If this is the case, one
I-S. Shiah et al. / Journal of Affective Disorders 60 (2000) 143 – 145
may expect that lamotrigine treatment normalizes the increased presynaptic 5-HT uptake in bipolar depressed patients, and thus restores the 5-HT deficit in this condition. Further studies are warranted to verify this. Finally, Franke et al. presented their data of platelet 5-HT levels on 52 healthy subjects and 67 patients with major depression. Their results showed an extremely high variability of platelet 5-HT content in both depressed patients and healthy controls and no significant difference in the mean values of platelet 5-HT between the two groups. The range of platelet 5-HT level for their healthy controls was 1.7 to 4.9 nM 5-HT / 10 9 platelets, whereas the range for our healthy controls was 0.24 to 2.58 nM 5-HT / 10 9 platelets. As mentioned in our paper, our healthy controls were on a low-monoamine diet and were engaging in normal exercise for 1 week before testing. Their blood samples were all taken at 08:30 h, and most of them (18 out of 20) were tested in Spring. These might have reduced the confounding effects of diet, exercise, diurnal and seasonal variation on platelet 5-HT content in our healthy controls. However, it is unclear what caused the high variability of platelet 5-HT level in their healthy controls. With regard to the variability of platelet 5-HT level for their patients, as discussed in our paper, unipolar depression is considered more heterogeneous than bipolar depression, and this may explain why their data showed such a high variability. Furthermore, the confounding effect of medications cannot be completely excluded because the duration of drug-free period for their patients was only 10 days. In addition, they also reported a high variability of platelet 5-HT in a small group of bipolar I non-psychotic depressed patients (n 5 8)
145
and no difference in the mean platelet 5-HT level between these patients and normal controls. It is very difficult for them to make a conclusion that biochemical heterogeneity in platelet 5-HT content exists in bipolar depression by using such a small sample of bipolar patients. Their negative finding of no difference in platelet 5-HT between bipolar depression and normal controls should also be interpreted with caution because of the possibility of type II error due to small sample size. In conclusion, our findings of increased platelet 5-HT in bipolar depressed patients and the positive correlation between the severity of depression and platelet 5-HT remain intact to support the involvement of 5-HT in the pathophysiology of bipolar depression.
References Modai, I., Zemishlany, Z., Jerushalmy, Z., 1984. 5-Hydroxytryptamine uptake by blood platelets of unipolar and bipolar depressed patients. Neuropsychobiology 12, 93–95. Oxenkrug, G.F., 1979. The content and uptake of 5-HT by blood platelets in depressive patients. J. Neural Transm. 45, 285–289. Shiah, I.S., Ko, H.C., Lee, J.F., Lu, R.B., 1999. Platelet 5-HT and plasma MHPG levels in patients with bipolar I and bipolar II depressions and normal controls. J. Affect. Disord. 52, 101– 110. Southam, E., Kirkby, D., Higgins, G.A., Hagan, R.M., 1998. Lamotrigine inhibits monoamine uptake in vitro and modulates 5-hydroxytryptamine uptake in rats. Eur. J. Pharmacol. 358, 19–24. Zemishlany, Z., Munitz, H., Rotman, A., Wijsenbeek, H., 1982. Increased uptake of serotonin by blood platelets from patients with bipolar primary affective disorder–bipolar type. Psychopharmacology 77, 175–178.
Reply to Letter to the Editor
Increased platelet 5-HT in patients with bipolar depression. Reply to Franke et al. [J. Affect. Disord. 52 (1999) 101–110] a, b c a I-Shin Shiah *, Huei-Chen Ko , Jia-Fu Lee , Ru-Band Lu a
Department of Psychiatry, Tri-Service General Hospital, National Defense Medical Center, No. 8, Ting-Chow Road, Section 3, Taipei, Taiwan b Graduate Institute of Behavioral Medicine, National Cheng-Kung University, Tainan, Taiwan c Department of Psychiatry, Military Pei-Taou Hospital, Taipei, Taiwan Received 10 August 1999
We would like to thank Franke et al. for their comments on our recent paper (Shiah et al., 1999), providing us an opportunity to discuss our work further. In our paper, we reported a trend towards a higher platelet 5-HT content in 12 bipolar I depressed patients and a significant increase in 12 bipolar II depressed patients. We also showed that when the bipolar I and bipolar II patients were pooled, there was a significant increase in platelet 5-HT levels in bipolar depressives (n 5 24) compared to normal controls (n 5 20). Based on the comparisons of platelet 5-HT levels between bipolar depressives and normal controls, we concluded that bipolar depression is associated with an increase in platelet 5-HT. Franke et al. speculated that some outliers had confounded our results. We disagree with their opinion for the following reasons. Firstly, the 95% confidence interval for the difference in platelet 5-HT level between our bipolar patients and normal controls was 0.17 to 1.34 nM 5-HT / 10 9 platelets. Since the confidence interval does not include zero, the difference in platelet 5-HT level between the two groups is statistically significant at *Corresponding author. Tel.: 1 886-2-2365-2463; fax: 1 8862-2367-1006. E-mail address: [email protected] (I-S. Shiah).
the 0.05 level. Secondly, although some of our patients had low values of platelet 5-HT, the effect size for the difference in platelet 5-HT content between patients and normal controls is 0.796, and the magnitude of the effect size also indicates that platelet 5-HT levels significantly differ between bipolar depressed patients and normal controls. Thirdly, after exclusion of the only patient with the platelet 5-HT level (i.e. 4.9 nM 5-HT / 10 9 platelets) outside the range of 2 S.D., the difference in platelet 5-HT between bipolar patients (n 5 23) and normal controls (n 5 20) remains significant (Mann–Whitney U 5 134.0, P , 0.03). The 95% confidence interval for the difference was 0.11 to 1.16 nM 5-HT / 10 9 platelets. Given the above, we are very confident of our finding that platelet 5-HT levels are significantly increased in bipolar depressed patients. By looking at Fig. 1 in our paper, Franke et al. speculated that the patient with 3.5 nM 5-HT / 10 9 platelets and the corresponding HAMD score of 34 had a ‘disproportional’ effect on our result of a trend for a significant positive correlation between 21-item Hamilton Depression Rating Scale (HAMD) scores and platelet 5-HT levels. They suggested that this patient should be excluded from the data analysis. However, we see no justification for doing this. Instead, we think that this patient should be included
0165-0327 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0165-0327( 99 )00172-X
144
I-S. Shiah et al. / Journal of Affective Disorders 60 (2000) 143 – 145
Fig. 1. Platelet 5-HT levels and 21-item Hamilton Depression Rating Scale (HAMD) scores in non-psychotic bipolar I (n 5 10) and bipolar II (n 5 11) depressed patients.
in the data analysis because this patient’s platelet 5-HT level is within the range of 2 S.D. Furthermore, it would make more sense to determine if those patients with extreme values of platelet 5-HT have confounding effects on our results. As mentioned earlier, there is only one patient with the platelet 5-HT level outside the range of 2 S.D. If this patient is excluded from analysis, the trend for a significant positive correlation between HAMD scores and platelet 5-HT in bipolar patients remains unchanged (Spearman’s r 5 0.360, P 5 0.09, n 5 23). Another reasonable approach would be to determine if those patients with psychotic features (as mentioned in our paper, two bipolar I and one bipolar II patients) have an influence on the results. Spearman’s correlation analysis not including these three patients (HAMD scores 33,34,38) showed a significant positive correlation between the HAMD scores and platelet 5-HT levels (Spearman’s r 5 0.538, P , 0.013, n 5 21) (please see Fig. 1). This significant positive correlation in such a homogenous group of bipolar nonpsychotic depressed patients supports our assumption that the alteration in platelet 5-HT function in bipolar depression is related to the severity of depression. We, therefore, disagree with their speculation that the correlation found in the study is due to chance. With regard to the conflicting results of the relationship between platelet 5-HT level and severity of depression in previous studies, this is not surprising be-
cause most of the patients involved in the studies had unipolar depression and our study involved a pure group of bipolar depressed patients. To date, at least three studies that examined platelet 5-HT uptake in bipolar patients reported increased platelet 5-HT uptake in bipolar patients when compared to controls (Oxenkrug, 1979; Zemishlany et al., 1982; Modai et al., 1984). However, Franke et al. did not cite them. Of the three studies, Oxenkrug (1979) simultaneously measured platelet 5-HT content and 5-HT uptake in 100 normal controls and 17 bipolar depressed patients before and after 4 weeks’ treatment with amitriptyline. The results of the study showed (1) bipolar depressed patients had a trend for higher platelet 5-HT content and a significantly higher platelet 5-HT uptake compared to normal controls; and (2) the content and uptake of 5-HT in platelets of depressed patients were significantly decreased after 4 weeks of successful amitriptyline treatment. Similarly, Modai et al. (1984) reported that bipolar depressed patients had a significant increase in platelet 5-HT uptake whereas unipolar depressed patients had a significant decrease in platelet 5-HT uptake, when compared to normal controls, respectively. Zemishlany et al. (1982) measured platelet 5-HT uptake in three different groups: 10 patients with history of bipolar disorder, their immediate families (total 31 people) and 10 healthy controls. They showed that patients with bipolar disorder had significantly increased platelet 5-HT uptake compared to healthy controls but the difference in platelet 5-HT uptake between patients and their families was not significant. Based on the assumption that blood platelets correspond to serotonergic neurons in the brain, the three studies that reported increased platelet 5-HT uptake in bipolar patients appear to support our hypothesis of an increase in presynaptic 5-HT uptake in bipolar depression. Interestingly, a recent in vitro study (Southam et al., 1998) showed that a new mood stabilizer, lamotrigine, led to a dose-dependent inhibition of 5-HT uptake in human platelets and rat brain synaptosomes. The authors of this study hypothesized that the inhibition of 5-HT uptake, which would in turn enhance 5-HT neurotransmission, is one of the mechanisms contributing to the efficacy of acutely administered lamotrigine in bipolar depression (Southam et al., 1998). If this is the case, one
I-S. Shiah et al. / Journal of Affective Disorders 60 (2000) 143 – 145
may expect that lamotrigine treatment normalizes the increased presynaptic 5-HT uptake in bipolar depressed patients, and thus restores the 5-HT deficit in this condition. Further studies are warranted to verify this. Finally, Franke et al. presented their data of platelet 5-HT levels on 52 healthy subjects and 67 patients with major depression. Their results showed an extremely high variability of platelet 5-HT content in both depressed patients and healthy controls and no significant difference in the mean values of platelet 5-HT between the two groups. The range of platelet 5-HT level for their healthy controls was 1.7 to 4.9 nM 5-HT / 10 9 platelets, whereas the range for our healthy controls was 0.24 to 2.58 nM 5-HT / 10 9 platelets. As mentioned in our paper, our healthy controls were on a low-monoamine diet and were engaging in normal exercise for 1 week before testing. Their blood samples were all taken at 08:30 h, and most of them (18 out of 20) were tested in Spring. These might have reduced the confounding effects of diet, exercise, diurnal and seasonal variation on platelet 5-HT content in our healthy controls. However, it is unclear what caused the high variability of platelet 5-HT level in their healthy controls. With regard to the variability of platelet 5-HT level for their patients, as discussed in our paper, unipolar depression is considered more heterogeneous than bipolar depression, and this may explain why their data showed such a high variability. Furthermore, the confounding effect of medications cannot be completely excluded because the duration of drug-free period for their patients was only 10 days. In addition, they also reported a high variability of platelet 5-HT in a small group of bipolar I non-psychotic depressed patients (n 5 8)
145
and no difference in the mean platelet 5-HT level between these patients and normal controls. It is very difficult for them to make a conclusion that biochemical heterogeneity in platelet 5-HT content exists in bipolar depression by using such a small sample of bipolar patients. Their negative finding of no difference in platelet 5-HT between bipolar depression and normal controls should also be interpreted with caution because of the possibility of type II error due to small sample size. In conclusion, our findings of increased platelet 5-HT in bipolar depressed patients and the positive correlation between the severity of depression and platelet 5-HT remain intact to support the involvement of 5-HT in the pathophysiology of bipolar depression.
References Modai, I., Zemishlany, Z., Jerushalmy, Z., 1984. 5-Hydroxytryptamine uptake by blood platelets of unipolar and bipolar depressed patients. Neuropsychobiology 12, 93–95. Oxenkrug, G.F., 1979. The content and uptake of 5-HT by blood platelets in depressive patients. J. Neural Transm. 45, 285–289. Shiah, I.S., Ko, H.C., Lee, J.F., Lu, R.B., 1999. Platelet 5-HT and plasma MHPG levels in patients with bipolar I and bipolar II depressions and normal controls. J. Affect. Disord. 52, 101– 110. Southam, E., Kirkby, D., Higgins, G.A., Hagan, R.M., 1998. Lamotrigine inhibits monoamine uptake in vitro and modulates 5-hydroxytryptamine uptake in rats. Eur. J. Pharmacol. 358, 19–24. Zemishlany, Z., Munitz, H., Rotman, A., Wijsenbeek, H., 1982. Increased uptake of serotonin by blood platelets from patients with bipolar primary affective disorder–bipolar type. Psychopharmacology 77, 175–178.