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Effects of bromocriptine on central dopaminergic receptors
Life Sciences Vol . 19, pp . 225-232, 1976 . Printed in the U.S .A .
Pergamon Press
EFFECTS OF BROMO(RIPTINE ON CENTRAL DOPAMINERGIC RECEPTORS M.Trabucchi, P.F .Spano, G.C .Tonon and L.Frattola Dept . of Pharmacology - University of Brescia Dept . of Neurology - University of Milan (Received in final form June 4, 1976) Bromocriptine injected to rats induces an increase of cAMP levels in the striatum in vivo . The time course of this increase is very similar to that of apomorphi ne . However bromocriptine does not stimulate striatal dopamine-sensitive adenylate cyclase but surprisingly antagonized the activation of this enzyme elicited by dopamine . Possible hypotheses on various sites of action of the drug are discussed .
Bromocriptine (2-bromo-.K-ergocriptine, CB 154) is an ergot polyps ptide derivative . It has been recently used by various authors as antiparkinsonian agent (1-5), as inhibitor of prolactin secretion in human galactorrhea and as inhibitor of GH secretion in acromegalic patients (7) . Among first studies on the mechanism of action of this drug are those performed by Corrodi et al . (8), showing that bromocriptine decreases dopamine turnover rate in rat striatum . Moreover Hökfelt and Fuxe (g) observed that bromocriptine decreases dopamine turno _ ver in median eminence . Both these studies have led to the hypothesis that in dopaminergic areas of the brain bromocriptine may directly stimulate dopamine receptors . To deeply investigate this hypothesis, Johnson et al . (10) have studied the activity of bromocriptine in several behavioral tests which involve activation of central dopaminergic mechanisms (i .e . controlateral rotation in animals with unilateral lesion of the substantia nigra, locomotor activity, stereotyped behaviour and antagonism of reserpine induced catalepsy) . These authors have o_b tained pharmacological evidence that bromocriptine stimulates the central dopaminergic system, through an activation of dopaminergic receptors . At the same time they have shown that an intact ax stem for dopamine synthesis or an intact granular dopamine store ge are necessary for the behavioral actions of the drug . METHODS Male Sprague-Dawley rats (Charles River, Milan) have been used in our experiments . The rats weighing between 110 and 130 grams were 225
226
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caged at constant temperature and humidity and exposed to a light cycle of twelve hours a day . The animals had free access to food and water . In the experiments where the formation of cAMP was measured in vi vo animals were killed by a beam of focussed microwave radiation (4 .5 kW, 2 .45 GAz), applied on the skull for 2-3 seconds (11) . Striatal cAMP was purified by column chromatography following the method of Mao and Guidotti (12) and determined with the activation of the cAMP dependent protein kinase as described by Kuo and Greengard (13) . Adenylate cyclase activity was measured as reported by Kebabian et al . (14) . The homogenate was incubated for 3 minutes and the reaction stopped by boiling the samples for 3 minutes . CAMP was _i solated and assayed following the method described above (13) . Protein was measured following the method of Lowry et al . (15) . Bromocriptine was kindly given by Sandoz (Basle) . The drug was so lubilized in acetic acid and the pH was corrected to the range of the buffer of the incubation medium . RESULTS The injection of bromocriptine to the rats 10 minutes before killi ng produces an increase of striatal cAMP concentration (table 1), which is already significant when the drug is given at a dose of 0.5 mg/Kg . This effect appears to be largely dose dependent with a maximal accumulation of striatal cAMP achieved at the dose of 2 mg/Kg i .p . After 2 mg/Kg of bromocriptine the level of cAMP in the striatum goes from 5 .32 _+ 0.29 to 10 .25 _+ 1 .34 with an incre_a se by about 90%. Moreover we studied the time course of cAMP accumulation in the striatum after an i .p . injection of 2 mg/Kg of bromocriptine into rats (fig . 1) . The maximum increase of striatal CAMP concentration is reached 10-15 minutes after the injection of the drug and two hours later the levels of cAMP have returned to control values . Apparently these data do not correlate with the time-course of the. behavioral changes elicited by bromocriptine, since from two to six hours after injection there is still a full behavioral effect of the drug . To explain the time dissociation between behavioral and in vivo biochemical effect of bromocriptine we have studied the action of the drug on the activity of dopamine stimulated ado nylate cyclase in striatal homogenates . As shown in table 2 bromocriptine when added to striatal homogena tea is completely inactive in stimulating dopamine sensitive adenylate cyclase . Up to concentrations of 10 -4 M bromocriptine fails to stimulate the enzyme activity, eventually showing a moderate inhibition . Bromocriptine effects have been compared in all the experiments to those of dopamine and apomorphine for a direct con trol of the method (data not shown) . On the other hand bromocriptine even at concentrations as low as 10-6 M significantly inhibits the stimulations of the enzyme act_i vity elicited by dopamine . Fig. 2 shows the stimulation of adenylate cyclase in rat striatal homogenates by dopamine in the presence and absence of bromocriptine (5 x 10-6 M) . From these data
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TABLE 1 cAMP çoncentration in rat striatum after injection of various doses of bromocriptine . Treatment Controls CB 154 0 .5 1 2 4
mg mg mg mg
pmoles/mg prot 5.32 7.95 9.51 10 .25 10 .09
± ± ± + ±
0 .29 1 .40 1 .02 1 .34 1 .20
(#) (#) (#) (#)
% increase 49 79 93 88
(*) P < 0.01 The drug was injected i .p . 10 1 minutes before killing the ani male . Each number is the mean _+ S.E .M . of at least 10 determi nations . '
minutes after the injection FIG . 1
Time curve of cAMP increase in rat striatum after i .p . bromocriptine injection (2 mg/Kg) . P <0 .01 as compared to controls . Each point represents the mean _+ S .E .M . of at least 12 determinations . bromocriptine appears to behave as a non-competitive inhibitor Of striatal dopamine stimulated adenylate cyclase . DISCUSSION Recently Johnson et al . (10) have reported that bromocriptine st_i mulates dopaminergic receptors of the brain . Moreover the experiments with the drug on the parkinsonian symptoms in man confirm the possibility that this drug may act through a dopaminergic mechanism .
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TABLE 2 Adenylate cyclase activity in rat striatum after addiction of DA and various bromocriptine (CB 154) concentrations . Treatment Control 6 DA 5x10 M -6 Bromocriptine 10 M lo- 5 M 1o-4 M
pmoles/mg prot/min 208 ± 10 373_+12 (*) 200 _+ 8 198 + 21 16A + 15
(#) P <0 .01 in respect to controls The numbers are the mean _+ S.E .M . of at least 15 determinations .
FIG. 2 Stimulation of adenylate cyclase in rat striatal homogenates by dopamine in the presence and absence of bromocriptine . The control values for cAMP formation are 195 + 15 pmoles/mg prot/ min . Each point represents the mean _+ S .E .M . of at least 10 determinations . Our data on the effect of bromocriptine upon the formation of cAMP in vivo in rat striatum may be regarded as a further demonstration of this fact . The time course of cAMP increase after bro
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229
criptine roughly parallels the one of apomorphine (16) . However there is no correlation with the behavioral activation .induced by the drug which starts after a lag-time of one hour and lasts for many hours (17) . To clarify this point experiments are in progress in our laborato ry to calculate the turnover rate of cAMP, which may be the only reliable biochemical correlate of the behavioral effect . Some particular problems in the interpretation of the effect of bromocriptine on the dopaminergic system are brought about by the experiments in vitro . Bromocriptine even at very high concentrations (10-4 M) is not active in stimulating the formation of cAMP in striatal homogenates . Normally, dopaminergic drugs such as apo morphine both in vitro and in vivo stimulate the formation of cAMP (14) . An explanation of this fenomenon could be linked to the possibility that in vivo bromocriptine is transformed into a metabolite which is active on the dopaminergic receptors . For example piribedil, a well known dopamine stimulating drug (18), is inactive by itself on the formation of CAMP in vitro, but its metabolite S 584 is capable to induce a stimulation of striatal adenylate cyclase (19) . However this interpretation is apparently in contrast with the rapid increase of striatal cAMP levels which have been observed in vivo after bromocriptine injection . A diffe rent line of interpretation is that bromocriptine may exerts its effect with a mechanism strictly linked to the release of dopamine . This fact would implicate a mode of action more similar to that of amphetamine then that of apomorphine (20) . Amphetamine in fact stimulates the formation of cAMP in vivo, but is not effecti ve in stimulating striatal dopamine-sensitive adenylate cyclase system in vitro (16) . A support to this hypothesis stems from the experiments by Johnson et al . (10) showing that sterotyped behaviour induced by bromocriptine is blocked by a previous treatment withC(-methylparatyrosine, an inhibitor of tyrosine hydroxylase (10) . Moreover bromocriptine, contrary to apomorphine, does not induce any behavioral activation in rats pretreated with reserpine (8) . On the other hand against the theory that bromocriptine may act through a mechanism similar to that of amphetamine is the fact that bromocriptine decreases dopamine turnover (8) at doses inducing an increase of spontaneus motor activity (2 .5 - 10 mg/Kg), whereas Costa has shown that amphetamine increase dopamine turnover in striatum (20) . In line with this view is also the observation that in rats unilaterally lesioned with 60HDA in the substr_a ntia nigra bromocriptine induces a turning controlateral to the lesioned side, whereas drugs releasing dopamine induce a turning homolateral to the lesion (10) . Another undemonstrated hypothesis which can be postulated is that bromocriptine may exerts its its activity on the presynaptic dopaminergic receptors (21) . The data on the decrease of the dopamine turnover, and on in vivo stimulation of the cAMP system may be interpreted in favor of this theory . On the other side it remains to be clarified the meaning of the
230
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blockade by bromocriptine of the striatal adenylate cyclase activation induced in vitro by dopamine . Even at very low concentration bromocriptine possesses a marked blocking activity which may be compared with that of some neuroleptics (22) although ., contrary to these drugs, the inhibition is non-competitive. Recent expe riments performed in parkinsonian patients (23) appear to indicate that bromocriptine may have in the molecule both a blocking and a stimulating activity on the dopaminergic system . Further work both in humans and in animals is necessary to get a better understanding on the intimate mode of action of this drug already used as a useful therapeutic agent . REFERENCES 1 . A.Galea Debono, I.Donaldson, C.D . Marsden, J.D .Parkes, Lancet 11, 987-988 (1975 2 . A .J .Lees, K.M .Shaw, G.M .Stern, Lancet ü, 709-710 (1975) 3 . D .B .Calne, P .F .Teychenne, L.E .Claveria, R .Eastman, J.K .Greenacre, A.Petrie, British Med . Journal 4 : 442-444 (1974) 4 . D .B .Calne, P .N .Leigh, P .F .Teychenne, A .N .Bamji, J.K .Greenacre, Lancet ii, 1355-1356 (1974) 5 . P .F .Teychenne, P .N .Leigh, J.L .Reid, J.K .Greenacre, A .Petrie and A.N .Bamji, Lancet ü, 473-476 (1975) 6 . E .Del Pozo, R.Brun Del Re, L .Vorga and H .Fiesen, J.C1in .Endocr . Metabolism 35, 768-771 (1972) 7 . Sachdev Y ., Tunbridge W.M .G ., Weightmann D .R ., .Gomez-Pan A., Duns A ., Hall R., S.K . Goolamali, Lancet ii, 1164-1168 (1975) 8 . H .Corrodi, K.Fuxe, T .H8kfelt, P.Lidbrink, U .Ungerstedt, J .Pharm . and Pharmac . 25, 409-412 (1973) 9 . T .H8kfelt, and K .Fuxe, In : Brain - endocrine interaction ad . Krigge, Scott and Weindl (1972) 10 . A .M .Johnson, D .M .Loew and J.M .Vigouret, British J. of Pharmacol . 76, 59-68 (1976) 11 . A .Guidotti, D .L .Cheney, M.Trabucchi, M .Doteuchi, C .Wong, and A .Hawkins, Neuropharmacology 13, 1115-1122 (1974) 12 . C.C .Mao, and A.Guidotti, Anal .Biochem, 59, 63-68 (1974) 13 . J.Kuo, and P .Greengard, In P .Greengard, R.Paoletti, G .A .Robi_n son (ads) Adv . in cyclic nucleotide Res . 2, 41-450, Raven Press, New York (1972) 14 . J.W .Kebabian, G.L .Petzold and P.Greengard, Proc .Nat .Acad .Sci . USA 69, 2145-2149 (1972) 15 . Lowry O.H ., N .J .Rosenbrough, A .L .Farr, R .J .Randall, J.Biol . Chum . 193, 265-275 (1951) 16 . A .Carenzi, A.Guidotti, A.Revuelta, E.Costa, J .Pharm .Exp .Ther . 194, 311-318 (1975) 17 . K .Fuxe, L .F .Agnati, H.Corrodi, B .J .Everitt, T .H8kfelt, A .L8fstr8m and U .Ungerstedt, Advances in Neurology vol . 9, 223-242, Raven Press, New York (1975) 18 . T .Y . Chase, A .C .Woods, G.A .Glaubiger, Arch,Nourol . 30, 383-386 (1974) 19 . R .J .Miller and L.L .Iversen, Naunyn-Schmiedeberg's Arch .Pharmacol . 282, 213-216 (1974) 20 . E .Costa, A .Groppetti, K .M .Naimzada, Brit . J .Pharmacol .44, 742-
Vol . 19, No . 2
Bromocriptine and Dopaminargic Receptors
231
751 (1972) 21 . L .O .Farnebo and B.Hamberger, In : Frontiers in catecholamine Re search ed . S .H .Snyder and S.Usdin New York, Pergamon Press pp 257-262 (1974) 22 . Y.C .Clement Cormier, J.W .Kebabian, F .I .Petzold, P .Greengard, Proc .Nat .Acad.Sci . USA 71, 1113-1118 (1974) 23 . L.Frattola, M.G .Albizzati, P .F .Spano, and M .Trabucchi, Pharmacol .Res .Comm . (in the press) .
Pergamon Press
EFFECTS OF BROMO(RIPTINE ON CENTRAL DOPAMINERGIC RECEPTORS M.Trabucchi, P.F .Spano, G.C .Tonon and L.Frattola Dept . of Pharmacology - University of Brescia Dept . of Neurology - University of Milan (Received in final form June 4, 1976) Bromocriptine injected to rats induces an increase of cAMP levels in the striatum in vivo . The time course of this increase is very similar to that of apomorphi ne . However bromocriptine does not stimulate striatal dopamine-sensitive adenylate cyclase but surprisingly antagonized the activation of this enzyme elicited by dopamine . Possible hypotheses on various sites of action of the drug are discussed .
Bromocriptine (2-bromo-.K-ergocriptine, CB 154) is an ergot polyps ptide derivative . It has been recently used by various authors as antiparkinsonian agent (1-5), as inhibitor of prolactin secretion in human galactorrhea and as inhibitor of GH secretion in acromegalic patients (7) . Among first studies on the mechanism of action of this drug are those performed by Corrodi et al . (8), showing that bromocriptine decreases dopamine turnover rate in rat striatum . Moreover Hökfelt and Fuxe (g) observed that bromocriptine decreases dopamine turno _ ver in median eminence . Both these studies have led to the hypothesis that in dopaminergic areas of the brain bromocriptine may directly stimulate dopamine receptors . To deeply investigate this hypothesis, Johnson et al . (10) have studied the activity of bromocriptine in several behavioral tests which involve activation of central dopaminergic mechanisms (i .e . controlateral rotation in animals with unilateral lesion of the substantia nigra, locomotor activity, stereotyped behaviour and antagonism of reserpine induced catalepsy) . These authors have o_b tained pharmacological evidence that bromocriptine stimulates the central dopaminergic system, through an activation of dopaminergic receptors . At the same time they have shown that an intact ax stem for dopamine synthesis or an intact granular dopamine store ge are necessary for the behavioral actions of the drug . METHODS Male Sprague-Dawley rats (Charles River, Milan) have been used in our experiments . The rats weighing between 110 and 130 grams were 225
226
Sromocriptine and Dopaminergic Receptors
Vol . 19, No . 2
caged at constant temperature and humidity and exposed to a light cycle of twelve hours a day . The animals had free access to food and water . In the experiments where the formation of cAMP was measured in vi vo animals were killed by a beam of focussed microwave radiation (4 .5 kW, 2 .45 GAz), applied on the skull for 2-3 seconds (11) . Striatal cAMP was purified by column chromatography following the method of Mao and Guidotti (12) and determined with the activation of the cAMP dependent protein kinase as described by Kuo and Greengard (13) . Adenylate cyclase activity was measured as reported by Kebabian et al . (14) . The homogenate was incubated for 3 minutes and the reaction stopped by boiling the samples for 3 minutes . CAMP was _i solated and assayed following the method described above (13) . Protein was measured following the method of Lowry et al . (15) . Bromocriptine was kindly given by Sandoz (Basle) . The drug was so lubilized in acetic acid and the pH was corrected to the range of the buffer of the incubation medium . RESULTS The injection of bromocriptine to the rats 10 minutes before killi ng produces an increase of striatal cAMP concentration (table 1), which is already significant when the drug is given at a dose of 0.5 mg/Kg . This effect appears to be largely dose dependent with a maximal accumulation of striatal cAMP achieved at the dose of 2 mg/Kg i .p . After 2 mg/Kg of bromocriptine the level of cAMP in the striatum goes from 5 .32 _+ 0.29 to 10 .25 _+ 1 .34 with an incre_a se by about 90%. Moreover we studied the time course of cAMP accumulation in the striatum after an i .p . injection of 2 mg/Kg of bromocriptine into rats (fig . 1) . The maximum increase of striatal CAMP concentration is reached 10-15 minutes after the injection of the drug and two hours later the levels of cAMP have returned to control values . Apparently these data do not correlate with the time-course of the. behavioral changes elicited by bromocriptine, since from two to six hours after injection there is still a full behavioral effect of the drug . To explain the time dissociation between behavioral and in vivo biochemical effect of bromocriptine we have studied the action of the drug on the activity of dopamine stimulated ado nylate cyclase in striatal homogenates . As shown in table 2 bromocriptine when added to striatal homogena tea is completely inactive in stimulating dopamine sensitive adenylate cyclase . Up to concentrations of 10 -4 M bromocriptine fails to stimulate the enzyme activity, eventually showing a moderate inhibition . Bromocriptine effects have been compared in all the experiments to those of dopamine and apomorphine for a direct con trol of the method (data not shown) . On the other hand bromocriptine even at concentrations as low as 10-6 M significantly inhibits the stimulations of the enzyme act_i vity elicited by dopamine . Fig. 2 shows the stimulation of adenylate cyclase in rat striatal homogenates by dopamine in the presence and absence of bromocriptine (5 x 10-6 M) . From these data
Vol . 19, No . 2
Bromocriptine and Dopaminergic Receptors
227
TABLE 1 cAMP çoncentration in rat striatum after injection of various doses of bromocriptine . Treatment Controls CB 154 0 .5 1 2 4
mg mg mg mg
pmoles/mg prot 5.32 7.95 9.51 10 .25 10 .09
± ± ± + ±
0 .29 1 .40 1 .02 1 .34 1 .20
(#) (#) (#) (#)
% increase 49 79 93 88
(*) P < 0.01 The drug was injected i .p . 10 1 minutes before killing the ani male . Each number is the mean _+ S.E .M . of at least 10 determi nations . '
minutes after the injection FIG . 1
Time curve of cAMP increase in rat striatum after i .p . bromocriptine injection (2 mg/Kg) . P <0 .01 as compared to controls . Each point represents the mean _+ S .E .M . of at least 12 determinations . bromocriptine appears to behave as a non-competitive inhibitor Of striatal dopamine stimulated adenylate cyclase . DISCUSSION Recently Johnson et al . (10) have reported that bromocriptine st_i mulates dopaminergic receptors of the brain . Moreover the experiments with the drug on the parkinsonian symptoms in man confirm the possibility that this drug may act through a dopaminergic mechanism .
228
Bro®ocriptine and Dopaminergic Receptors
vol . 19, No . 2
TABLE 2 Adenylate cyclase activity in rat striatum after addiction of DA and various bromocriptine (CB 154) concentrations . Treatment Control 6 DA 5x10 M -6 Bromocriptine 10 M lo- 5 M 1o-4 M
pmoles/mg prot/min 208 ± 10 373_+12 (*) 200 _+ 8 198 + 21 16A + 15
(#) P <0 .01 in respect to controls The numbers are the mean _+ S.E .M . of at least 15 determinations .
FIG. 2 Stimulation of adenylate cyclase in rat striatal homogenates by dopamine in the presence and absence of bromocriptine . The control values for cAMP formation are 195 + 15 pmoles/mg prot/ min . Each point represents the mean _+ S .E .M . of at least 10 determinations . Our data on the effect of bromocriptine upon the formation of cAMP in vivo in rat striatum may be regarded as a further demonstration of this fact . The time course of cAMP increase after bro
Vol . 19, No . 2
Browcriptine and Dopaminergic Receptors
229
criptine roughly parallels the one of apomorphine (16) . However there is no correlation with the behavioral activation .induced by the drug which starts after a lag-time of one hour and lasts for many hours (17) . To clarify this point experiments are in progress in our laborato ry to calculate the turnover rate of cAMP, which may be the only reliable biochemical correlate of the behavioral effect . Some particular problems in the interpretation of the effect of bromocriptine on the dopaminergic system are brought about by the experiments in vitro . Bromocriptine even at very high concentrations (10-4 M) is not active in stimulating the formation of cAMP in striatal homogenates . Normally, dopaminergic drugs such as apo morphine both in vitro and in vivo stimulate the formation of cAMP (14) . An explanation of this fenomenon could be linked to the possibility that in vivo bromocriptine is transformed into a metabolite which is active on the dopaminergic receptors . For example piribedil, a well known dopamine stimulating drug (18), is inactive by itself on the formation of CAMP in vitro, but its metabolite S 584 is capable to induce a stimulation of striatal adenylate cyclase (19) . However this interpretation is apparently in contrast with the rapid increase of striatal cAMP levels which have been observed in vivo after bromocriptine injection . A diffe rent line of interpretation is that bromocriptine may exerts its effect with a mechanism strictly linked to the release of dopamine . This fact would implicate a mode of action more similar to that of amphetamine then that of apomorphine (20) . Amphetamine in fact stimulates the formation of cAMP in vivo, but is not effecti ve in stimulating striatal dopamine-sensitive adenylate cyclase system in vitro (16) . A support to this hypothesis stems from the experiments by Johnson et al . (10) showing that sterotyped behaviour induced by bromocriptine is blocked by a previous treatment withC(-methylparatyrosine, an inhibitor of tyrosine hydroxylase (10) . Moreover bromocriptine, contrary to apomorphine, does not induce any behavioral activation in rats pretreated with reserpine (8) . On the other hand against the theory that bromocriptine may act through a mechanism similar to that of amphetamine is the fact that bromocriptine decreases dopamine turnover (8) at doses inducing an increase of spontaneus motor activity (2 .5 - 10 mg/Kg), whereas Costa has shown that amphetamine increase dopamine turnover in striatum (20) . In line with this view is also the observation that in rats unilaterally lesioned with 60HDA in the substr_a ntia nigra bromocriptine induces a turning controlateral to the lesioned side, whereas drugs releasing dopamine induce a turning homolateral to the lesion (10) . Another undemonstrated hypothesis which can be postulated is that bromocriptine may exerts its its activity on the presynaptic dopaminergic receptors (21) . The data on the decrease of the dopamine turnover, and on in vivo stimulation of the cAMP system may be interpreted in favor of this theory . On the other side it remains to be clarified the meaning of the
230
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Vol . 19, No . 2
blockade by bromocriptine of the striatal adenylate cyclase activation induced in vitro by dopamine . Even at very low concentration bromocriptine possesses a marked blocking activity which may be compared with that of some neuroleptics (22) although ., contrary to these drugs, the inhibition is non-competitive. Recent expe riments performed in parkinsonian patients (23) appear to indicate that bromocriptine may have in the molecule both a blocking and a stimulating activity on the dopaminergic system . Further work both in humans and in animals is necessary to get a better understanding on the intimate mode of action of this drug already used as a useful therapeutic agent . REFERENCES 1 . A.Galea Debono, I.Donaldson, C.D . Marsden, J.D .Parkes, Lancet 11, 987-988 (1975 2 . A .J .Lees, K.M .Shaw, G.M .Stern, Lancet ü, 709-710 (1975) 3 . D .B .Calne, P .F .Teychenne, L.E .Claveria, R .Eastman, J.K .Greenacre, A.Petrie, British Med . Journal 4 : 442-444 (1974) 4 . D .B .Calne, P .N .Leigh, P .F .Teychenne, A .N .Bamji, J.K .Greenacre, Lancet ii, 1355-1356 (1974) 5 . P .F .Teychenne, P .N .Leigh, J.L .Reid, J.K .Greenacre, A .Petrie and A.N .Bamji, Lancet ü, 473-476 (1975) 6 . E .Del Pozo, R.Brun Del Re, L .Vorga and H .Fiesen, J.C1in .Endocr . Metabolism 35, 768-771 (1972) 7 . Sachdev Y ., Tunbridge W.M .G ., Weightmann D .R ., .Gomez-Pan A., Duns A ., Hall R., S.K . Goolamali, Lancet ii, 1164-1168 (1975) 8 . H .Corrodi, K.Fuxe, T .H8kfelt, P.Lidbrink, U .Ungerstedt, J .Pharm . and Pharmac . 25, 409-412 (1973) 9 . T .H8kfelt, and K .Fuxe, In : Brain - endocrine interaction ad . Krigge, Scott and Weindl (1972) 10 . A .M .Johnson, D .M .Loew and J.M .Vigouret, British J. of Pharmacol . 76, 59-68 (1976) 11 . A .Guidotti, D .L .Cheney, M.Trabucchi, M .Doteuchi, C .Wong, and A .Hawkins, Neuropharmacology 13, 1115-1122 (1974) 12 . C.C .Mao, and A.Guidotti, Anal .Biochem, 59, 63-68 (1974) 13 . J.Kuo, and P .Greengard, In P .Greengard, R.Paoletti, G .A .Robi_n son (ads) Adv . in cyclic nucleotide Res . 2, 41-450, Raven Press, New York (1972) 14 . J.W .Kebabian, G.L .Petzold and P.Greengard, Proc .Nat .Acad .Sci . USA 69, 2145-2149 (1972) 15 . Lowry O.H ., N .J .Rosenbrough, A .L .Farr, R .J .Randall, J.Biol . Chum . 193, 265-275 (1951) 16 . A .Carenzi, A.Guidotti, A.Revuelta, E.Costa, J .Pharm .Exp .Ther . 194, 311-318 (1975) 17 . K .Fuxe, L .F .Agnati, H.Corrodi, B .J .Everitt, T .H8kfelt, A .L8fstr8m and U .Ungerstedt, Advances in Neurology vol . 9, 223-242, Raven Press, New York (1975) 18 . T .Y . Chase, A .C .Woods, G.A .Glaubiger, Arch,Nourol . 30, 383-386 (1974) 19 . R .J .Miller and L.L .Iversen, Naunyn-Schmiedeberg's Arch .Pharmacol . 282, 213-216 (1974) 20 . E .Costa, A .Groppetti, K .M .Naimzada, Brit . J .Pharmacol .44, 742-
Vol . 19, No . 2
Bromocriptine and Dopaminargic Receptors
231
751 (1972) 21 . L .O .Farnebo and B.Hamberger, In : Frontiers in catecholamine Re search ed . S .H .Snyder and S.Usdin New York, Pergamon Press pp 257-262 (1974) 22 . Y.C .Clement Cormier, J.W .Kebabian, F .I .Petzold, P .Greengard, Proc .Nat .Acad.Sci . USA 71, 1113-1118 (1974) 23 . L.Frattola, M.G .Albizzati, P .F .Spano, and M .Trabucchi, Pharmacol .Res .Comm . (in the press) .