Br. J. Pharmacol. (1991), 102, 429-433
C) Macmillan Press Ltd, 1991
Effects of acute paroxetine administration on tryptophan metabolism and disposition in the rat 'Abdulla A.-B. Badawy & Christopher J. Morgan South Glamorgan Health Authority, Biomedical Research Laboratory, Whitchurch Hospital, Cardiff CF4 7XB 1 The effects of acute oral administration of paroxetine on tryptophan metabolism and disposition were examined in the rat. 2 Basal liver tryptophan pyrrolase activity was inhibited by paroxetine in vitro and after oral administration. Maximum inhibition was caused by a 1 mg kg- dose. 3 Paroxetine administration also inhibited pyrrolase activity that had previously been enhanced by hormonal induction by cortisol or cofactor activation by haematin. The cortisol induction of the enzyme was, however, not inhibited by pretreatment of rats with paroxetine. 4 Paroxetine increased tryptophan availability to the brain, because of the above pyrrolase-inhibitory mechanism. Cerebral 5-hydroxytryptamine (5-HT) synthesis was accordingly enhanced, though this was apparent only with doses of the drug of up to 1 mg kg -. With larger doses, decreased 5-HT turnover, probably as a result of 5-HT uptake inhibition, was the more dominant feature. 5 Paroxetine lowered circulating corticosterone concentration, but did not influence those of albumin, non-esterified fatty acids or glucose. 6 It is concluded that, in addition to inhibiting brain 5-HT turnover, paroxetine also, in common with 20 other antidepressants, enhances 5-HT synthesis by increasing brain tryptophan concentration secondarily to inhibition of liver tryptophan pyrrolase activity.
Introduction Paroxetine [(-)-trans-4-(4'-fluorophenyl)3-(3',4'-methylenedioxyphenoxymethyl)piperidine] is an antidepressant (Lund Laursen et al., 1985; Feighner & Boyer, 1989; Rickels et al., 1989) drug with a profile of a potent and selective 5hydroxytryptamine (5-HT) uptake inhibitor (Thomas et al., 1987). Preliminary studies (A.M. Johnson, personal communication) indicate that paroxetine can, as other 5-HT uptake inhibitors (Fuller et al., 1974; Claassen et al., 1977; Ross et al., 1981), decrease 5-HT turnover in rat brain. However, it is not known whether it is capable of enhancing 5-HT synthesis by increasing tryptophan availability to the brain secondarily to inhibition of activity of the major tryptophan-degrading enzyme, liver tryptophan pyrrolase (tryptophan 2,3-dioxygenase, EC 1.13.11.11), as can 20 other antidepressants of different chemical structures and pharmacological classes (Badawy & Evans, 1981; 1982; Badawy, 1986). In the present work, we show that acute administration of paroxetine to rats both decreases turnover and enhances synthesis of brain 5-HT by the above mechanisms.
Methods Animals and treatments Locally bred male Wistar rats (150-170g) were maintained on cube diet 41B (Oxoid) and water under a natural light: dark cycle and at 22 + 10C and were killed between 12 h 00 min and 13 h 30 min by stunning and cervical dislocation (for the determination of liver tryptophan pyrrolase activity) or by decapitation (for all other determinations). Paroxetine hydrochloride was administered orally in doses of 0.1-25mgkg-1 body weight (dose calculated as salt) and appropriate control rats received an equal volume (2 ml kg- 1) of the vehicle (distilled water) by the same route. Cortisol acetate (20mgkg-1) or haematin HCl (5mgkg-1) was dissolved in dimethylformamide and administered intraperitoneally; Author for correspondence.
control rats received an equal volume (1 ml kg 1) of this latter vehicle by the same route.
Chemicals and drugs Paroxetine hydrochloride was a gift from Smith Kline Beecham Pharmaceuticals; all other chemicals were purchased from BDH Chemicals and Sigma (both of Poole, Dorset) and were of the purest commercially available grades.
Chemical, enzymic and other determinations Tryptophan pyrrolase activity was determined in fresh liver homogenates either in the absence (holoenzyme activity) or in the presence (total enzyme activity) of added (2pM) haematin (Badawy & Evans, 1975; see also the fuller description of Badawy, 1981 and additional comments by Badawy et al., 1983). The activity of the apoenzyme (the haem-free predominant form in rat and human liver) was calculated by difference. Free (ultrafiltrable) serum, total (acid-soluble) serum, liver and brain tryptophan concentrations and those of brain 5-HT and its major metabolite 5-hydroxyindol-3-ylacetic acid (5-HIAA) and serum albumin, corticosterone, glucose and non-esterified fatty acids were all determined by standard procedures (for references, see Badawy et al., 1984). Serum was isolated from clotted blood by centrifugation at 3000g for 1520 min, whereas brain was rapidly removed (within 10s of the death of the animals), frozen in liquid nitrogen and then stored at -20°C overnight before analysis. For liver tryptophan determination, a portion of the tissue was also frozen in liquid nitrogen and then stored along with brain as described above. Statistical analysis of results was performed by Student's t test.
Results Effects of paroxetine on the basal activity of rat liver tryptophan pyrrolase The time course of the effects of acute oral administration of paroxetine (1 mg kg- 1) on the basal pyrrolase activity is
A.A.-B. BADAWY & C.J. MORGAN
430
Table 1 Effects of paroxetine in vitro on the basal activity of rat liver tryptophan pyrrolase
4~m
Liver tryptophan pyrrolase activity
Paroxetine concentration
0 1 fIM
10pM .'>
0D:°
2
4
6
8 '12 Time (h)
16
20
24
Figure 1 Time course of effects of acute oral administration of paroxetine (1 mg kg -) on the basal activity of rat liver tryptophan pyrrolase. Paroxetine was administered orally at zero-time and pyrrolase activity was determined at the various times shown as described in the Methods section either in the absence (holoenzyme activity; *), or in the presence (total enzyme activity; 0) of added (2pM) haematin. The apoenzyme activity (A) was obtained by difference. Values are means (s.e.mean shown by vertical bars except where error lies within size of symbol) for each group of 4 rats.
shown in Figure 1. Whereas the activity of the holoenzyme (the haem-containing form) was not significantly altered at any of the time intervals examined, that of the total enzyme was significantly decreased by 42, 52, 54, 46 and 27% (P = 0.02-0.005) at 0.5, 1, 2, 3 and 4 h respectively. Activity of the apoenzyme (the haem-free form, obtained by difference) was therefore decreased by 68, 85, 94, 79 and 41% at the above time intervals respectively (P = 0.02-0.001) and also by 29% (P < 0.05) at 5 h. Pyrrolase activity then returned to the control (zero time) values at 6 h and remained unaltered during the subsequent 18 h. We also found (results not shown) that a 0.5mgkg-1 oral dose of paroxetine exerted similar time course effects, except that the extent of the pyrrolase inhibition was marginally smaller. The results in Figure 2 show that maximum inhibition of basal liver tryptophan pyrrolase activity at 2 h after oral paroxetine administration was observed with the lmgkg-1 dose (45% and 89% for the total enzyme and apoenzyme activities respectively; P < 0.001) and that significant inhibition of these two activities (of 18% and 42% respectively; P = 0.02-0.01) was evident with a dose of the drug as small as 0.1 mgkg-'. 4
0.1 mm
1mM
(Ermol of kynurenine formed h -1 g- ' wet wt.) Holoenzyme
Total enzyme
Apoenzyme
1.6 + 0.1 1.7 + 0.1 1.6 + 0.1 1.7 + 0.2 1.6 + 0.2
3.9 + 0.2 3.1 + 0.1** 2.7 + 0.2* 2.5 + 0.2** 2.0 + 0.1***
2.3 + 0.1 1.4 + 0.1 1.1 + 0.1*** 0.8 + 0.1 0.4 + 0.0***
Paroxetine was added in vitro to liver homogenates from normal fed rats. Pyrrolase activity was determined as described in the Methods section either in the absence (holoenzyme activity) or in the presence (total enzyme activity) of added (2pM) haematin. The apoenzyme activity was obtained by difference. Values are means + s.e.mean for each group of four separate experiments. The significance of the differences between paroxetine-supplemented and control homogenates is indicated as follows: * P < 0.01; ** P
C) Macmillan Press Ltd, 1991
Effects of acute paroxetine administration on tryptophan metabolism and disposition in the rat 'Abdulla A.-B. Badawy & Christopher J. Morgan South Glamorgan Health Authority, Biomedical Research Laboratory, Whitchurch Hospital, Cardiff CF4 7XB 1 The effects of acute oral administration of paroxetine on tryptophan metabolism and disposition were examined in the rat. 2 Basal liver tryptophan pyrrolase activity was inhibited by paroxetine in vitro and after oral administration. Maximum inhibition was caused by a 1 mg kg- dose. 3 Paroxetine administration also inhibited pyrrolase activity that had previously been enhanced by hormonal induction by cortisol or cofactor activation by haematin. The cortisol induction of the enzyme was, however, not inhibited by pretreatment of rats with paroxetine. 4 Paroxetine increased tryptophan availability to the brain, because of the above pyrrolase-inhibitory mechanism. Cerebral 5-hydroxytryptamine (5-HT) synthesis was accordingly enhanced, though this was apparent only with doses of the drug of up to 1 mg kg -. With larger doses, decreased 5-HT turnover, probably as a result of 5-HT uptake inhibition, was the more dominant feature. 5 Paroxetine lowered circulating corticosterone concentration, but did not influence those of albumin, non-esterified fatty acids or glucose. 6 It is concluded that, in addition to inhibiting brain 5-HT turnover, paroxetine also, in common with 20 other antidepressants, enhances 5-HT synthesis by increasing brain tryptophan concentration secondarily to inhibition of liver tryptophan pyrrolase activity.
Introduction Paroxetine [(-)-trans-4-(4'-fluorophenyl)3-(3',4'-methylenedioxyphenoxymethyl)piperidine] is an antidepressant (Lund Laursen et al., 1985; Feighner & Boyer, 1989; Rickels et al., 1989) drug with a profile of a potent and selective 5hydroxytryptamine (5-HT) uptake inhibitor (Thomas et al., 1987). Preliminary studies (A.M. Johnson, personal communication) indicate that paroxetine can, as other 5-HT uptake inhibitors (Fuller et al., 1974; Claassen et al., 1977; Ross et al., 1981), decrease 5-HT turnover in rat brain. However, it is not known whether it is capable of enhancing 5-HT synthesis by increasing tryptophan availability to the brain secondarily to inhibition of activity of the major tryptophan-degrading enzyme, liver tryptophan pyrrolase (tryptophan 2,3-dioxygenase, EC 1.13.11.11), as can 20 other antidepressants of different chemical structures and pharmacological classes (Badawy & Evans, 1981; 1982; Badawy, 1986). In the present work, we show that acute administration of paroxetine to rats both decreases turnover and enhances synthesis of brain 5-HT by the above mechanisms.
Methods Animals and treatments Locally bred male Wistar rats (150-170g) were maintained on cube diet 41B (Oxoid) and water under a natural light: dark cycle and at 22 + 10C and were killed between 12 h 00 min and 13 h 30 min by stunning and cervical dislocation (for the determination of liver tryptophan pyrrolase activity) or by decapitation (for all other determinations). Paroxetine hydrochloride was administered orally in doses of 0.1-25mgkg-1 body weight (dose calculated as salt) and appropriate control rats received an equal volume (2 ml kg- 1) of the vehicle (distilled water) by the same route. Cortisol acetate (20mgkg-1) or haematin HCl (5mgkg-1) was dissolved in dimethylformamide and administered intraperitoneally; Author for correspondence.
control rats received an equal volume (1 ml kg 1) of this latter vehicle by the same route.
Chemicals and drugs Paroxetine hydrochloride was a gift from Smith Kline Beecham Pharmaceuticals; all other chemicals were purchased from BDH Chemicals and Sigma (both of Poole, Dorset) and were of the purest commercially available grades.
Chemical, enzymic and other determinations Tryptophan pyrrolase activity was determined in fresh liver homogenates either in the absence (holoenzyme activity) or in the presence (total enzyme activity) of added (2pM) haematin (Badawy & Evans, 1975; see also the fuller description of Badawy, 1981 and additional comments by Badawy et al., 1983). The activity of the apoenzyme (the haem-free predominant form in rat and human liver) was calculated by difference. Free (ultrafiltrable) serum, total (acid-soluble) serum, liver and brain tryptophan concentrations and those of brain 5-HT and its major metabolite 5-hydroxyindol-3-ylacetic acid (5-HIAA) and serum albumin, corticosterone, glucose and non-esterified fatty acids were all determined by standard procedures (for references, see Badawy et al., 1984). Serum was isolated from clotted blood by centrifugation at 3000g for 1520 min, whereas brain was rapidly removed (within 10s of the death of the animals), frozen in liquid nitrogen and then stored at -20°C overnight before analysis. For liver tryptophan determination, a portion of the tissue was also frozen in liquid nitrogen and then stored along with brain as described above. Statistical analysis of results was performed by Student's t test.
Results Effects of paroxetine on the basal activity of rat liver tryptophan pyrrolase The time course of the effects of acute oral administration of paroxetine (1 mg kg- 1) on the basal pyrrolase activity is
A.A.-B. BADAWY & C.J. MORGAN
430
Table 1 Effects of paroxetine in vitro on the basal activity of rat liver tryptophan pyrrolase
4~m
Liver tryptophan pyrrolase activity
Paroxetine concentration
0 1 fIM
10pM .'>
0D:°
2
4
6
8 '12 Time (h)
16
20
24
Figure 1 Time course of effects of acute oral administration of paroxetine (1 mg kg -) on the basal activity of rat liver tryptophan pyrrolase. Paroxetine was administered orally at zero-time and pyrrolase activity was determined at the various times shown as described in the Methods section either in the absence (holoenzyme activity; *), or in the presence (total enzyme activity; 0) of added (2pM) haematin. The apoenzyme activity (A) was obtained by difference. Values are means (s.e.mean shown by vertical bars except where error lies within size of symbol) for each group of 4 rats.
shown in Figure 1. Whereas the activity of the holoenzyme (the haem-containing form) was not significantly altered at any of the time intervals examined, that of the total enzyme was significantly decreased by 42, 52, 54, 46 and 27% (P = 0.02-0.005) at 0.5, 1, 2, 3 and 4 h respectively. Activity of the apoenzyme (the haem-free form, obtained by difference) was therefore decreased by 68, 85, 94, 79 and 41% at the above time intervals respectively (P = 0.02-0.001) and also by 29% (P < 0.05) at 5 h. Pyrrolase activity then returned to the control (zero time) values at 6 h and remained unaltered during the subsequent 18 h. We also found (results not shown) that a 0.5mgkg-1 oral dose of paroxetine exerted similar time course effects, except that the extent of the pyrrolase inhibition was marginally smaller. The results in Figure 2 show that maximum inhibition of basal liver tryptophan pyrrolase activity at 2 h after oral paroxetine administration was observed with the lmgkg-1 dose (45% and 89% for the total enzyme and apoenzyme activities respectively; P < 0.001) and that significant inhibition of these two activities (of 18% and 42% respectively; P = 0.02-0.01) was evident with a dose of the drug as small as 0.1 mgkg-'. 4
0.1 mm
1mM
(Ermol of kynurenine formed h -1 g- ' wet wt.) Holoenzyme
Total enzyme
Apoenzyme
1.6 + 0.1 1.7 + 0.1 1.6 + 0.1 1.7 + 0.2 1.6 + 0.2
3.9 + 0.2 3.1 + 0.1** 2.7 + 0.2* 2.5 + 0.2** 2.0 + 0.1***
2.3 + 0.1 1.4 + 0.1 1.1 + 0.1*** 0.8 + 0.1 0.4 + 0.0***
Paroxetine was added in vitro to liver homogenates from normal fed rats. Pyrrolase activity was determined as described in the Methods section either in the absence (holoenzyme activity) or in the presence (total enzyme activity) of added (2pM) haematin. The apoenzyme activity was obtained by difference. Values are means + s.e.mean for each group of four separate experiments. The significance of the differences between paroxetine-supplemented and control homogenates is indicated as follows: * P < 0.01; ** P
