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Lithium decreases retinal melatonin levels in the frog
Neuroscience Letters, 96 (1989) 235 239
235
Elsevier ScientificPublishers Ireland Ltd. NSL 05830
Lithium decreases retinal melatonin levels in the frog Milena Kemali l, Palmiero Monteleone 2, Mario Maj 2, Nicola Milici 2 and Dargut Kemali 2 Zlstituto Cihernetica CNR, Arco Felice, Naples (Italy) and "-Department o['Medical Psychology and Psychiatry First Medical School, University of Naples, Naples (Italy)
(Received 26 July 1988; Revised version received 22 September 1988; Accepted 26 September 1988) Key wm'~Zs." Melatonin; Lithium: Haloperidol; Retina; Pigment screening; Illumination conditions The effect of the acute i.p. administration of lithium chloride (1 mg/kg) and of haloperidol ( I mg/kg) on retinal melatonin levels was studied in light-adapted and dark-adapted frogs of the species Rana escuh'nta. Two hours following drug administration, animals were killed by decapitation and a single retina from each frog was collected and homogenized in 1 ml of chilled 0.1 N HCI. After centrifugation, the pH of the supernatant was adjusted to 7.0. Melatonin was extracted by diethylether and assayed by double antibody radioimmunoassay (RIA). Lithium induced a significantdecrease of retinal melatonin levelsboth ill light-adapted (P < 0.006) and in dark-adapted (P < 0.01) animals, whereas no change was observed after haloperidol Ireatment. These results suggest thal the scleral aggregation of pigment granules induced by lithium in the frog retina is not mediated by a stimulation of melatonin synthesis.
Melatonin, the most widely investigated indole product of the pineal gland, has been long considered to be a compound unique to this gland, until recent studies [I I, 12] demonstrated its presence in the retina and other tissues. At present, it is definitively proved that melatonin is synthesized in the retina as well as in the pineal gland [14]. It has been suggested that, in lower vertebrates, e n d o g e n o u s m e l a t o n i n in the retina may play a role in the regulation of the d i u r n a l r h y t h m of eye p i g m e n t a t i o n u n d e r physiological c o n d i t i o n s [17]. This r h y t h m was first described by G a u p p et al. [3]: following exposure to light, m e l a n i n granules migrate a l o n g the finger-like processes of the retinal p i g m e n t epithelium (vitreal migration), whereas in the dark they aggregate within the cell bodies (scleral migration). This p h e n o m e n o n is k n o w n as p i g m e n t screening (PS), a n d its effect is to put the photoreceptors in the best conditions to receive i n c o m i n g light u n d e r bright or dim i l l u m i n a t i o n e n v i r o n m e n t s . Flight [2] suggested that m e l a t o n i n might regulate PS by f a v o u r i n g the aggregation of m e l a n i n granules in the dark. In fact, in two previous papers [5, 8], we showed that exogenously a d m i n i s t e r e d m e l a t o n i n produces a massive aggregation of m e l a n o -
Correspon&,nce. lstituto di Cibernetica del CNR, 1-80072 Arco Felice, Napoli, Italy.
0304-3940,'89,'$03.50 O 1989 Elsevier ScientificPublishers Ireland Ltd.
236
somes at the scleral surface of the retina in both light- and dark-adapted frogs. Moreover, it has been reported that peak retinal melatonin levels coincide with peak melanosome aggregation during the dark period [17]. It has been hypothesized [16] that lithium, commonly used in the prophylaxis of major affective disorders, may gain its therapeutic effects by modulating sensitivity to light and hence the capability for entrainment of circadian rhythms, through an influence on pigment movement in the retina. As a matter of fact, we previously showed that acutely administered lithium can induce a scleral aggregation of pigment granules in both light- and dark-adapted frogs [6]. It is possible that lithium produces this effect through a direct action on retinal melatonin synthesis. In order to verify this hypothesis, we studied the effect of the acute administration of lithium on melatonin levels in the retina of frogs kept in different conditions of illumination. Moreover, since dopamine (DA) has been thought to be directly or indirectly involved in the vitreal aggregation of melanosomes [7], we assessed also the effects of the DA antagonist haloperidol. Twenty-four frogs of the species Rana esculenta were utilized. Twelve were darkadapted and 12 were light-adapted. Light and dark adaptations were carried out by keeping the frogs in continuous light (a lamp of 60 W, 50 cm over the tank with the frogs) or darkness for a week. Four light-adapted and 4 dark-adapted frogs were injected intraperitoneally (i.p.) with a single dose of lithium chloride (1 mg/kg dissolved in 0.1 ml distilled water). Eight more frogs (4 for each light condition) were injected i.p. with haloperidot (1 mg/kg dissolved in 0.1 ml distilled water), The remaining 4 light-adapted and 4 dark-adapted frogs were injected i.p. with 0.1 ml saline. The injections were always performed at midday. Two h later, the animals were killed by decapitation. All the steps concerning dark-adapted frogs were carried out in dim red light. After decapitation, the eyes were enucleated and opened by means of ophthalmic scissors at the level of the ora serrata. The lenses were then extracted, and the whole retinas, with the attached pigment epithelium, were dissected out and immediately placed on solid C02 and stored frozen until extraction. At that time, a single retina for each frog was processed as follows. The retina was homogenized in 1 ml of chilled 0.1 N HC1 for 1 min. The homogenate was centrifuged for 10 min at 2000 g, at 4°C. The supernatant was collected in borosilicate glass tubes and its pH was adjusted to 7.0 with 0.5 M phosphate buffer pH 7.5 (approx. 1 ml) and 1 M NaOH (approx. 0.1 ml). The final volume was approx. 2 ml. After extraction by diethylether, melatonin was assayed by the double antibody radioimmunoassay (RIA) method using a commercial kit purchased from Eurodiagnostics (Apeldoorn, The Netherlands). The detection limits of the assay were 5-640 pg/ml. Melatonin antiserum was raised in rabbit (titre 1:3000) with the following cross-reactivity at 50% binding: 6-hydroxymelatonin 1%, 5-methoxy-tryptamine 0.05%, N-acetyl-serotonin 0.02%, tryptamine, serotonin and L-tryptophan<0.01%. Intra-assay coefficient of variation (CV) was 4.4 7.0% and inter-assay CV was 6.3 7.2%. Results were expressed as mean + S.E.M. and statistically analysed by one-way analysis of variance (ANOVA) and Student's t-test for unpaired data.
237
In each treatment condition, retinal melatonin levels were higher in dark- than in light-adapted frogs. This difference reached the statistical significance of P<0.005 (Student's t-test for unpaired data) in the lithium-treated group, P<0.007 in the haloperidol-treated animals and P < 0.002 in the control frogs. Moreover, one-way ANOVA revealed that retinal melatonin levels were significantly different among the 3 treatment groups in light-adapted frogs ( F = 12.387, P<0.002) as well as in darkadapted ones (F=5.545, P<0.02). In particular, lithium administration induced a significant decrease of retinal melatonin both in light- (P<0.006 vs light-adapted controls) and in dark-adapted (P <0.01) animals, whereas haloperidol treatment did not affect melatonin levels in either condition of illumination (Fig. 1). These results confirm that darkness stimulates melatonin synthesis in the frog retina. In line with this finding, a diurnal rhythm of melatonin, with higher levels during the dark period, has been reported in the retina of several species, including frogs [I 2, 13]. Moreover, our data indicate that acute lithium administration induces a clear-cut decrease of melatonin synthesis in the retina of both light- and darkadapted frogs, while haloperidol does not induce any change. As in the pineal gland, melatonin synthesis in the retina appears to be regulated by N-acetyltransferase (NAT). Iuvone and Besharse [4] demonstrated that NAT in the retina of Xenopus laevis undergoes a circadian rhythm of activity similar to that reported in the pineal, and that protein synthesis inhibitors block the dark-induced rise of retinal NAT activity, whereas cyclic AMP (cAMP) analogues increase it. These observations suggest that regulation of retinal NAT activity is similar to that
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Fig. 1. Melatonin levels (pg/retina) in light- and dark-adapted frogs following acute i.p. administration of saline (S), haloperidol 1 mg/kg (H) and lithium chloride 1 mg/kg (L). Bars represent mean values + S.E.M. averaged from 4 animals. Significant differences are the following: * P < 0 . 0 0 7 , **P<0.005, *** P < 0.002 (dark- vs light-adapted frogs); + P < 0.002 (light-adapted frogs treated with lithium vs saline ): P < 0 . 0 1 (dark-adapted frogs treated with lithium vs saline).
238 of pineal NAT: the increase of the enzyme activity during the dark period is modulated by a cAMP-dependent mechanism. It is possible, therefore, to hypothesize that acute lithium administration may decrease retinal melatonin levels by blocking the cAMP-dependent increase of N A T activity. In agreement with our results, Seggie [15] demonstrated that chronic lithium administration decreases retinal, serum and pineal melatonin levels in the rat. Moreover, Yocca and Friedman [18] showed that long-term treatment with lithium not only decreases the number of pineal fl-receptors, but also suppresses both isoproterenol-induced elevation of cAMP and pineal NAT activity in the rat. On the other hand, Zatz [19] reported that lithium added to pineal homogenates in vitro inhibits isoproterenol-stimulated adenylate cyclase, whereas it has no effect on fl-adrenergic receptor binding. These results suggest that chronic lithium administration decreases pineal melatonin levels by inhibiting noradrenaline-sensitive adenylate cyclase via a down-regulation of pineal fl-receptors. It is possible that, in our study, acute lithium administration achieved the same effect in the retina through the inhibition of calcium-mediated activation of this enzyme, it is useful to remind, in this connection, that inhibition of calcium-dependent enzyme systems (including adenylate cyclase, inositol-l-phosphatase, tryptophan hydroxylase and calcium ATPase) has been recently proposed as the basic molecular mechanism responsible for the therapeutic activity of lithium [10]. The failure of haloperidol to affect retinal melatonin levels in both light- and darkadapted frogs suggests that DA is not involved in the regulation ofmelatonin synthesis in the retina of the frog. Although some data in the literature indicate a stimulatory role of DA on pineal melatonin synthesis in the rat [1], species differences have been reported [9] and tissue differences may also be invoked. Since we previously showed that acute administration of lithium in frogs induces a PS similar to that produced by melatonin [6], it seemed possible to hypothesize that lithium could achieve this effect through the stimulation of retinal melatonin synthesis. In the light of the present results, this does not seem to be the case, since lithium even suppresses melatonin levels in the frog retina. Therefore, the effect of lithium on PS should not be considered a melatonin-mediated phenomenon. However, we cannot rule out that lithium, although decreasing retinal melatonin levels, might increase the sensitivity to retinal receptors to melatonin. At present, no data are available to support this hypothesis. The lack of effect of haloperidol on retinal melatonin is in line with the reported failure of this drug to affect PS in both light- and dark-adapted frogs [7]. In conclusion, the results of the present investigation show that, independently of environmental conditions of illumination, acute lithium administration decreases retinal melatonin levels in the frog, and support the view that the effect of lithium on PS in this species is not directly related to retinal melatonin synthesis. 1 Ebadi, M. and Govitrapong, P., Neural pathways and neurotransmittersaffectingmelatonin synthesis, J. Neural Transm., Suppl. 21 (1986) 125 155. 2 Flight, W.F.G., Morphological and functional comparison between the retina and the pineal organ of lower vertebrates, Prog. Brain Res., 52 (1979) 131 139.
239 3 Gaupp, E., Ecker, A. and Wiedersheim, R., Anatomie des Frosches, Abt. 3, Braunschweig, 1896. 4 luvone, P.M. and Besharse, J.C., Regulation of indolamine N-acetyltransferase activity in the retina: effects of light and dark, protein synthesis inhibitors and cyclic nucleotide analogs, Brain Res., 273 (1983) 1ll 119. 5 Kemali, M., Kemali, D., Lovero, N., Maj, M. and Milici, N., Lithium and melatonin: morphological moditications induced in frog retina pigment screening, Pharmacopsychiatry, 20 (1987) 224 226. 6 Kemali. M., Kemali, D., Maj, M., Lovero, N. and Milici, N., Frog retinal pigment screening and lithium, Comp. Biochem. Physiol., 86C (1987)421 ~1-23. 7 Kemali, M., Milici, N. and Kemali, D., Drugs and the frog retina. Effect of dopaminergic agents on the pigment screenings of light- and dark-adapted frogs, Neuropharmacology, 23 (1984) 381 385. 8 Kemali, M., Milici, N. and Kemali, D., Melatonin and LSD induce similar retinal changes in the frog, Biol. Psychiatry, 21 (1986) 981 985. 9 Lal, S., Isaac, 1., Pilapil, C., Nair, N.P.V., Hariharasubramian, N., Guyda, H. and Quirion, R., Effect of apomorphine on melatonin secretion in normal subjects, Prog. Neuro-Psychopharmacol. Biol. Psychiatry. II (1987) 229 233. 10 Meltzer, H.L., Lithium mechanisms in bipolar illness and altered intracellular calcium functions, Biol. Psychiatry, 21 (1986) 492 510. I 1 Pang, S.F., Brown, G.M., Grota, L.J., Chambers, J.W. and Rodman, R.L., Determination of N-acetylserotonin and melatonin activities in the pineal gland, Hardcrian gland, brain and serum of rats and chickens, Neuroendocrinology, 23 (1977) 1 13. 12 Pang, S.F., Yu, H.S., Suen, H.C. and Brown, G.M., Melatonin in the retina of rats: a diurnal rhythm, .1. Endocrinol., 87 (1980) 89 93. 13 Pang, S.F., Shiu, S.Y.W. and Tse, S.F., Effect of photic manipulation on the level of mclatonin in thc retinas of frogs, Gen. Comp. Endocrinol., 58 (1985) 464~-70. 14 Reiter, R.J., Richardson, B.A., Matthews, S.A., Lane, S.J. and Ferguson, B.N., Rhythms in immunoreactive melatonin in the retina and harderian gland of rats: persistence after pinealcctomy, Life Sci., 32(1983) 1229 1236. 15 Scggic, J., Lithium and the retina, Prog. Neuro-Psychopharmacol. Biol. Psychiatry, 12 (1988) 241 253. 16 Scggic, J., Melatonin, the rctinal-hypothalamic-pineal axis, and circadian rhythm regulation. In F.N. Johnson (Ed.), Lithium Therapy Monographs, Vol. 2, Karger, Basel, 1988, pp. 35 50. 17 Wiechmann, A.F., Melatonin: parallels in pineal gland and retina, Exp. Eye Res., 42 (1986) 507 527. 18 Yocca, F.D. and Friedman, E., Pineal rhythms. In F.N. Johnson (Ed.), Lithium Therapy Monographs. Vol. 2, Kargcr, Basel, 1988, pp. 211 219. 19 Zatz, M., Low concentrations of lithium inhibit the synthesis of cyclic AM P and cyclic G M P in the rat pineal gland, J. Neurochem., 32 (1979) 1315 1321.
235
Elsevier ScientificPublishers Ireland Ltd. NSL 05830
Lithium decreases retinal melatonin levels in the frog Milena Kemali l, Palmiero Monteleone 2, Mario Maj 2, Nicola Milici 2 and Dargut Kemali 2 Zlstituto Cihernetica CNR, Arco Felice, Naples (Italy) and "-Department o['Medical Psychology and Psychiatry First Medical School, University of Naples, Naples (Italy)
(Received 26 July 1988; Revised version received 22 September 1988; Accepted 26 September 1988) Key wm'~Zs." Melatonin; Lithium: Haloperidol; Retina; Pigment screening; Illumination conditions The effect of the acute i.p. administration of lithium chloride (1 mg/kg) and of haloperidol ( I mg/kg) on retinal melatonin levels was studied in light-adapted and dark-adapted frogs of the species Rana escuh'nta. Two hours following drug administration, animals were killed by decapitation and a single retina from each frog was collected and homogenized in 1 ml of chilled 0.1 N HCI. After centrifugation, the pH of the supernatant was adjusted to 7.0. Melatonin was extracted by diethylether and assayed by double antibody radioimmunoassay (RIA). Lithium induced a significantdecrease of retinal melatonin levelsboth ill light-adapted (P < 0.006) and in dark-adapted (P < 0.01) animals, whereas no change was observed after haloperidol Ireatment. These results suggest thal the scleral aggregation of pigment granules induced by lithium in the frog retina is not mediated by a stimulation of melatonin synthesis.
Melatonin, the most widely investigated indole product of the pineal gland, has been long considered to be a compound unique to this gland, until recent studies [I I, 12] demonstrated its presence in the retina and other tissues. At present, it is definitively proved that melatonin is synthesized in the retina as well as in the pineal gland [14]. It has been suggested that, in lower vertebrates, e n d o g e n o u s m e l a t o n i n in the retina may play a role in the regulation of the d i u r n a l r h y t h m of eye p i g m e n t a t i o n u n d e r physiological c o n d i t i o n s [17]. This r h y t h m was first described by G a u p p et al. [3]: following exposure to light, m e l a n i n granules migrate a l o n g the finger-like processes of the retinal p i g m e n t epithelium (vitreal migration), whereas in the dark they aggregate within the cell bodies (scleral migration). This p h e n o m e n o n is k n o w n as p i g m e n t screening (PS), a n d its effect is to put the photoreceptors in the best conditions to receive i n c o m i n g light u n d e r bright or dim i l l u m i n a t i o n e n v i r o n m e n t s . Flight [2] suggested that m e l a t o n i n might regulate PS by f a v o u r i n g the aggregation of m e l a n i n granules in the dark. In fact, in two previous papers [5, 8], we showed that exogenously a d m i n i s t e r e d m e l a t o n i n produces a massive aggregation of m e l a n o -
Correspon&,nce. lstituto di Cibernetica del CNR, 1-80072 Arco Felice, Napoli, Italy.
0304-3940,'89,'$03.50 O 1989 Elsevier ScientificPublishers Ireland Ltd.
236
somes at the scleral surface of the retina in both light- and dark-adapted frogs. Moreover, it has been reported that peak retinal melatonin levels coincide with peak melanosome aggregation during the dark period [17]. It has been hypothesized [16] that lithium, commonly used in the prophylaxis of major affective disorders, may gain its therapeutic effects by modulating sensitivity to light and hence the capability for entrainment of circadian rhythms, through an influence on pigment movement in the retina. As a matter of fact, we previously showed that acutely administered lithium can induce a scleral aggregation of pigment granules in both light- and dark-adapted frogs [6]. It is possible that lithium produces this effect through a direct action on retinal melatonin synthesis. In order to verify this hypothesis, we studied the effect of the acute administration of lithium on melatonin levels in the retina of frogs kept in different conditions of illumination. Moreover, since dopamine (DA) has been thought to be directly or indirectly involved in the vitreal aggregation of melanosomes [7], we assessed also the effects of the DA antagonist haloperidol. Twenty-four frogs of the species Rana esculenta were utilized. Twelve were darkadapted and 12 were light-adapted. Light and dark adaptations were carried out by keeping the frogs in continuous light (a lamp of 60 W, 50 cm over the tank with the frogs) or darkness for a week. Four light-adapted and 4 dark-adapted frogs were injected intraperitoneally (i.p.) with a single dose of lithium chloride (1 mg/kg dissolved in 0.1 ml distilled water). Eight more frogs (4 for each light condition) were injected i.p. with haloperidot (1 mg/kg dissolved in 0.1 ml distilled water), The remaining 4 light-adapted and 4 dark-adapted frogs were injected i.p. with 0.1 ml saline. The injections were always performed at midday. Two h later, the animals were killed by decapitation. All the steps concerning dark-adapted frogs were carried out in dim red light. After decapitation, the eyes were enucleated and opened by means of ophthalmic scissors at the level of the ora serrata. The lenses were then extracted, and the whole retinas, with the attached pigment epithelium, were dissected out and immediately placed on solid C02 and stored frozen until extraction. At that time, a single retina for each frog was processed as follows. The retina was homogenized in 1 ml of chilled 0.1 N HC1 for 1 min. The homogenate was centrifuged for 10 min at 2000 g, at 4°C. The supernatant was collected in borosilicate glass tubes and its pH was adjusted to 7.0 with 0.5 M phosphate buffer pH 7.5 (approx. 1 ml) and 1 M NaOH (approx. 0.1 ml). The final volume was approx. 2 ml. After extraction by diethylether, melatonin was assayed by the double antibody radioimmunoassay (RIA) method using a commercial kit purchased from Eurodiagnostics (Apeldoorn, The Netherlands). The detection limits of the assay were 5-640 pg/ml. Melatonin antiserum was raised in rabbit (titre 1:3000) with the following cross-reactivity at 50% binding: 6-hydroxymelatonin 1%, 5-methoxy-tryptamine 0.05%, N-acetyl-serotonin 0.02%, tryptamine, serotonin and L-tryptophan<0.01%. Intra-assay coefficient of variation (CV) was 4.4 7.0% and inter-assay CV was 6.3 7.2%. Results were expressed as mean + S.E.M. and statistically analysed by one-way analysis of variance (ANOVA) and Student's t-test for unpaired data.
237
In each treatment condition, retinal melatonin levels were higher in dark- than in light-adapted frogs. This difference reached the statistical significance of P<0.005 (Student's t-test for unpaired data) in the lithium-treated group, P<0.007 in the haloperidol-treated animals and P < 0.002 in the control frogs. Moreover, one-way ANOVA revealed that retinal melatonin levels were significantly different among the 3 treatment groups in light-adapted frogs ( F = 12.387, P<0.002) as well as in darkadapted ones (F=5.545, P<0.02). In particular, lithium administration induced a significant decrease of retinal melatonin both in light- (P<0.006 vs light-adapted controls) and in dark-adapted (P <0.01) animals, whereas haloperidol treatment did not affect melatonin levels in either condition of illumination (Fig. 1). These results confirm that darkness stimulates melatonin synthesis in the frog retina. In line with this finding, a diurnal rhythm of melatonin, with higher levels during the dark period, has been reported in the retina of several species, including frogs [I 2, 13]. Moreover, our data indicate that acute lithium administration induces a clear-cut decrease of melatonin synthesis in the retina of both light- and darkadapted frogs, while haloperidol does not induce any change. As in the pineal gland, melatonin synthesis in the retina appears to be regulated by N-acetyltransferase (NAT). Iuvone and Besharse [4] demonstrated that NAT in the retina of Xenopus laevis undergoes a circadian rhythm of activity similar to that reported in the pineal, and that protein synthesis inhibitors block the dark-induced rise of retinal NAT activity, whereas cyclic AMP (cAMP) analogues increase it. These observations suggest that regulation of retinal NAT activity is similar to that
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Fig. 1. Melatonin levels (pg/retina) in light- and dark-adapted frogs following acute i.p. administration of saline (S), haloperidol 1 mg/kg (H) and lithium chloride 1 mg/kg (L). Bars represent mean values + S.E.M. averaged from 4 animals. Significant differences are the following: * P < 0 . 0 0 7 , **P<0.005, *** P < 0.002 (dark- vs light-adapted frogs); + P < 0.002 (light-adapted frogs treated with lithium vs saline ): P < 0 . 0 1 (dark-adapted frogs treated with lithium vs saline).
238 of pineal NAT: the increase of the enzyme activity during the dark period is modulated by a cAMP-dependent mechanism. It is possible, therefore, to hypothesize that acute lithium administration may decrease retinal melatonin levels by blocking the cAMP-dependent increase of N A T activity. In agreement with our results, Seggie [15] demonstrated that chronic lithium administration decreases retinal, serum and pineal melatonin levels in the rat. Moreover, Yocca and Friedman [18] showed that long-term treatment with lithium not only decreases the number of pineal fl-receptors, but also suppresses both isoproterenol-induced elevation of cAMP and pineal NAT activity in the rat. On the other hand, Zatz [19] reported that lithium added to pineal homogenates in vitro inhibits isoproterenol-stimulated adenylate cyclase, whereas it has no effect on fl-adrenergic receptor binding. These results suggest that chronic lithium administration decreases pineal melatonin levels by inhibiting noradrenaline-sensitive adenylate cyclase via a down-regulation of pineal fl-receptors. It is possible that, in our study, acute lithium administration achieved the same effect in the retina through the inhibition of calcium-mediated activation of this enzyme, it is useful to remind, in this connection, that inhibition of calcium-dependent enzyme systems (including adenylate cyclase, inositol-l-phosphatase, tryptophan hydroxylase and calcium ATPase) has been recently proposed as the basic molecular mechanism responsible for the therapeutic activity of lithium [10]. The failure of haloperidol to affect retinal melatonin levels in both light- and darkadapted frogs suggests that DA is not involved in the regulation ofmelatonin synthesis in the retina of the frog. Although some data in the literature indicate a stimulatory role of DA on pineal melatonin synthesis in the rat [1], species differences have been reported [9] and tissue differences may also be invoked. Since we previously showed that acute administration of lithium in frogs induces a PS similar to that produced by melatonin [6], it seemed possible to hypothesize that lithium could achieve this effect through the stimulation of retinal melatonin synthesis. In the light of the present results, this does not seem to be the case, since lithium even suppresses melatonin levels in the frog retina. Therefore, the effect of lithium on PS should not be considered a melatonin-mediated phenomenon. However, we cannot rule out that lithium, although decreasing retinal melatonin levels, might increase the sensitivity to retinal receptors to melatonin. At present, no data are available to support this hypothesis. The lack of effect of haloperidol on retinal melatonin is in line with the reported failure of this drug to affect PS in both light- and dark-adapted frogs [7]. In conclusion, the results of the present investigation show that, independently of environmental conditions of illumination, acute lithium administration decreases retinal melatonin levels in the frog, and support the view that the effect of lithium on PS in this species is not directly related to retinal melatonin synthesis. 1 Ebadi, M. and Govitrapong, P., Neural pathways and neurotransmittersaffectingmelatonin synthesis, J. Neural Transm., Suppl. 21 (1986) 125 155. 2 Flight, W.F.G., Morphological and functional comparison between the retina and the pineal organ of lower vertebrates, Prog. Brain Res., 52 (1979) 131 139.
239 3 Gaupp, E., Ecker, A. and Wiedersheim, R., Anatomie des Frosches, Abt. 3, Braunschweig, 1896. 4 luvone, P.M. and Besharse, J.C., Regulation of indolamine N-acetyltransferase activity in the retina: effects of light and dark, protein synthesis inhibitors and cyclic nucleotide analogs, Brain Res., 273 (1983) 1ll 119. 5 Kemali, M., Kemali, D., Lovero, N., Maj, M. and Milici, N., Lithium and melatonin: morphological moditications induced in frog retina pigment screening, Pharmacopsychiatry, 20 (1987) 224 226. 6 Kemali. M., Kemali, D., Maj, M., Lovero, N. and Milici, N., Frog retinal pigment screening and lithium, Comp. Biochem. Physiol., 86C (1987)421 ~1-23. 7 Kemali, M., Milici, N. and Kemali, D., Drugs and the frog retina. Effect of dopaminergic agents on the pigment screenings of light- and dark-adapted frogs, Neuropharmacology, 23 (1984) 381 385. 8 Kemali, M., Milici, N. and Kemali, D., Melatonin and LSD induce similar retinal changes in the frog, Biol. Psychiatry, 21 (1986) 981 985. 9 Lal, S., Isaac, 1., Pilapil, C., Nair, N.P.V., Hariharasubramian, N., Guyda, H. and Quirion, R., Effect of apomorphine on melatonin secretion in normal subjects, Prog. Neuro-Psychopharmacol. Biol. Psychiatry. II (1987) 229 233. 10 Meltzer, H.L., Lithium mechanisms in bipolar illness and altered intracellular calcium functions, Biol. Psychiatry, 21 (1986) 492 510. I 1 Pang, S.F., Brown, G.M., Grota, L.J., Chambers, J.W. and Rodman, R.L., Determination of N-acetylserotonin and melatonin activities in the pineal gland, Hardcrian gland, brain and serum of rats and chickens, Neuroendocrinology, 23 (1977) 1 13. 12 Pang, S.F., Yu, H.S., Suen, H.C. and Brown, G.M., Melatonin in the retina of rats: a diurnal rhythm, .1. Endocrinol., 87 (1980) 89 93. 13 Pang, S.F., Shiu, S.Y.W. and Tse, S.F., Effect of photic manipulation on the level of mclatonin in thc retinas of frogs, Gen. Comp. Endocrinol., 58 (1985) 464~-70. 14 Reiter, R.J., Richardson, B.A., Matthews, S.A., Lane, S.J. and Ferguson, B.N., Rhythms in immunoreactive melatonin in the retina and harderian gland of rats: persistence after pinealcctomy, Life Sci., 32(1983) 1229 1236. 15 Scggic, J., Lithium and the retina, Prog. Neuro-Psychopharmacol. Biol. Psychiatry, 12 (1988) 241 253. 16 Scggic, J., Melatonin, the rctinal-hypothalamic-pineal axis, and circadian rhythm regulation. In F.N. Johnson (Ed.), Lithium Therapy Monographs, Vol. 2, Karger, Basel, 1988, pp. 35 50. 17 Wiechmann, A.F., Melatonin: parallels in pineal gland and retina, Exp. Eye Res., 42 (1986) 507 527. 18 Yocca, F.D. and Friedman, E., Pineal rhythms. In F.N. Johnson (Ed.), Lithium Therapy Monographs. Vol. 2, Kargcr, Basel, 1988, pp. 211 219. 19 Zatz, M., Low concentrations of lithium inhibit the synthesis of cyclic AM P and cyclic G M P in the rat pineal gland, J. Neurochem., 32 (1979) 1315 1321.