Serotonin control of prolactin release: an intriguing puzzle

11PS -April 1983

171

Serotonin control of prolactin release:

an intriguing puzzle P. Preziosi

Department o f Pharmacology, School o f Medicme, Catholic Umversity o f the Sacred Heart, Largo F. Vito 1, 00168 Rome, ltaly

Serotonergic control of prolactin (PRL) secretion is suprahypophysial, and highly intricate: inhibitory or stimulatory presynaptic modulation may be involved, including possible stimulation of a (post-junctional?) prolactin-releasing factor. Pharmacological tools and neurophysiological studies do not, and cannot, provide conclusive results, but this control has important therapeutic implications. Serotonergic modulation of PRL secretion seems to he evanescent in comparison to the strong tonic dopaminergic control. A stimulant role in the release of the hormone has, however, been documented in teleostei, avian species and mammals1. Distinct evidence is the simultaneous afternoon increase of serotonin (5-HT) concentrations in the brain and PRL in the blood of rats, and the role of the serotonergic brain system in the increase of serum PRL during sleep in humans, during lactation in mammals and during nesting in birds, as well as during stress (e.g. ether stress or blood withdrawal). Serotonergic control is suprahypophysial Most authors exclude any direct action of 5-HT or its precursors, in vitro andin vivo , on the lactotrophs after injection into the hypophysial portal vessels (these cells, however, may be stimulated in hypothalamus,--hypophysial co-cultures from avian species). Parenteral administration of 5-HT is equally ineffective (there are a few conflicting opinions on high doses and, in any case, there is no evidence of any increase in serum PRL in patients with carcinoid tumours, despite the increase of the circulating 5-HT). 5-11T and its recentlydiscovered metabolite, 5-hydroxykynurenine, injected by the intracerebroventricular (i.c.v.) route increase serum PRL; melatonin shares this property, but antagonizes the 5-HT increase, but not that of the metabolite. 5-HT precursors (tryptophan and, especially, 5-hydroxytryptophan, 5-HTP) administered by parenteral routes (including i.c.v.) increase the blood concentrations of PRL in avian species, rats and humans. In vivo these reduce the inhibitory effects on PRL of dopamine (DA) and dopaminergic agents and restore the blood concentrations of the hormone,

reduced from their former high levels in nesting turkeys by the 5-HT brain depletor p-chlorophenylalanine (pCPA). Their effect persists: (1) in humans, in the presence of a reduced hypophysial production of TRH, a well-known physiological stimulant of PRL secretion; (2) in rats with isolated medio-basal hypothalamus (MBH) (it is absent if this has been destroyed); (3) still in the rat, even after catecholaminergic depletion with or-methyl..p- tyrosine (o~-MpT), though there are conflicting views, or blocking of the dopaminergic receptors D2 with chlorpromazine, fluphenazine or flupentixol (all manipulations that in themselves lead to a marked increase in the PRL blood levels). The effect is increased by parenteral administration, simultaneously with the precursors, of specific inhibitors of the reuptake of 5-HT such as fluoxetine, becoming clearly superior to that after blocking the dopaminergic receptors. These inhibitors do not cause increases in serum PRL on their own, nor do they appear to be active in vitro on isolated hypophyses even with the precursors. Fluoxetine also potentiates the increase of serum PRL connected, in the rat, with stress (including ether stress or blood withdrawal) and, in humans, with administration of insulin. The stimulant action of 5-HT on the release of PRL therefore takes place above the hypophysial peduncle and probably in the MBH. Electrical stimulation of this (see below) leads to remarkable increases in serum PRL.

Serotonergic agonists and antagonists are only moderately useful tools, since they lack specificity Serotonergic agonists [quipazine, D-fenflurarnine and the metabolite of trazodone,(- ) m-chlorophenylpiperazine, m-CPP] by the parenteral route (including

i.c.v.) all markedly increase PRL secretion. Of these compounds, a remarkable presynaptic 5-HT-releasing activity is typical of fenfluramine (followed by long lasting depletion of the neurotransmitter). The releasing activity ofrn-CPP is less marked, but is still present as has been demonstrated in my laboratory. It is abolished by the membrane carrier inhibitor clomipramine and in the presence of this compound blocks the K ÷ induced release of [aH]5-HT from hypothalamic synaptosomes. This effect is strongly counteracted by the serotonergic antagonist methiotepin. Qulpazine has dopaminergic activities (among other things it inhibits DA uptake) and antagonizes the PRL-releasing action of opiate agonists and fenfluramine (the L-form and racemate) has antidopamirb ergic activity. Their action on blood PRL is enhanced in conditions of presumed supersensitivity (see below and Refs 2, 3). It has been shown in my laboratory that serotonergic agonists do not influence spontaneous or K÷-indueed release of DA from the olfactory tubercle perfused synaptosomes (Fig. 1). Stimulation of PRL secretion by mescalin, 2,5-dimethoxy-4-methylamphetamine and 3,4-dimethoxyphenylethylamine4, and by harmalin-related /3carbolines5, is caused by agonistic effects at the level of the central serotonergic recepo---o K C i 1 S m M e ~ e KCI 1 5 m M - m C p p

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tzation). (Preraosi, P, Cerrito, F. and Vacca, M.: paper presentedat the 21st Congressof the Italian Society

of Pharmacology.

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Fig 2. Hypotheticalinterplay between seromnergic, opmte and dopaminergic neurons in the control of prolactin secretion. An opiate-dependent step ~s envisaged in the seromnergic control of prolaetm release. The serotonergtc system (cellular bodies outside and inside the mediobasal hypothalamus) may modulate presynapttcally and be modulated by the opiate neurons. Through opiate neurons the serotonergic system may mhibit the release of dopamine from the tubero-infundibular neurons The unknown postsynaptically-released prolactin releasing factor may be a peptide ( VIP, CCK, etc.). For explanation, see text

tors. Repeated administration of 5-HT agonists enhances the effects observed immediately on PRL release: e.g. 5-metboxy-N-dimethyl tryptamine enhances its own effects and those of other serotonergic agonists, N,N-dimethyltryptamine and quipazine. The two presumed serotonergic antagonists that have been most extensively studied, methysergide and metergoline, do not inhibit the effects on serotonergic neurons (nuclei of the raphe, geniculate, ventrolateral, optical rectum and amygdala) of microiontophoretic applications of 5-HT, but do abolish the activating effects of 5-HT on neurons without serotonergic innervation. For this reason it has even been ruled out that they can interact on the central serotonergic receptors. With methysergide, inhibitory effects. have been observed on 5-HT at the level of the pyramidal cells of the hippocampus and on the cortical neurons. It also blocks the central behaviourai toxic effects of 5-HTP and the uptake of 5-HT in various areas of the brain including the hypothalamus. Metergoline is a 5-HT agonist-antagonist, although the antagonist activity prevails. These compounds do not (at least consistently) modify the basal serum PRL levels in rats and humans (they reduce them in fowl) but they antagonize the hormone increase that takes place: (1) during sleep in humans; (2) in the afternoon in rats; (3) during suckling (metergoline inhibits lactation in humans); (4) after the administration of precursors of serotonin (with or without uptake inhibitors), serotonergic agonists, oestrogens, TRH (conflicting opinions) and pimozide; (5) during electri-

TIPS - April 1983

cal stimulation of the MBH in ovariectomized monkeys. There is evidence of interference with the dopaminergic receptors by the pyrroloethylamine or pyrazoloethylamine moiety common to the molecules of the above mentioned compounds. Methysergide is considered to be a dopaminergic antagonist-agonist, its action being linked to the metabolite methergine. Metergoline increases the DA turnover of the striatum and the nucleus accumbens (a reduction of the serotonergic neurotransmission is, on the other hand, accompanied by an increase of the DA metabolism in the striatum), displaces the haloperidol from the binding sites and does not antagonize, but in high doses potentiates, the increase in serum PRL levels induced by the D2receptor antagonist sulpiride. Methysergide and metergoline, unlike the other dopaminergic agonists, do not stimulate release of growth hormone in humans. Lisuride, a semisynthetic derivative of the ergot alkaloids, demonstrates anti-PRL activity, probably predominantly related to a hypophysial dopaminergic mechanism rather than a serotonergic mechanism, and even shows agonistic affinity for various types of serotonergic receptors. Serious doubts about the specificity of the antagonistic activity of the above compounds at 5-HT receptor level as an essential component of the anti-PRL activity, arise from the observation that pizotyline (pizotifen), a potent central antiserotonergic compound that crosses the blood brain barrier, shows no antagonizing capacity, even against induced hyperprolactinaemias in humans". Methysergide and metergoline may account for an action on the lactotrophs due neither to the dopaminergic activity nor to the 5-HT receptor blocking properties. Cyproheptadine is an even more spurious antagonist than those already mentioned,

with its antihistamine, anticholinergic and antidopaminergic properties. Chemical or surgical depletion of brain 5-HT Administration of brain 5-HT depletors, such as p-CPA, is followed by varying effects on the basal blood levels of PRL and on induced hyperprolactinaemias. After destruction of the serotonergic terminals with specific neurotoxins, the basal PRL values are modified only slightly or not at all, while the hyperprolactinaemias induced by serotonergic agonists with considerable 5-HT-releasing properties (see Refs 1 and 2) are inhibited and those from the purer receptor serotonergic agonists like m-CPP, are potentiated ~. In the presence of a long-lasting depletion, the persistence of a small functionally-active neuronal pool of 5-HT permits the maintenance or recovery of PRL secretion. Indeed, recovery of other hypophysial secretions takes place after suppression of a corresponding 5-HT inhibitory control by p-CPA of TSH or by other monoaminergic depletors for other hypophyseal hormones (e.g. reserpine in the case of the noradrenaiine responsible for the tonic control of ACTH secretion). Electrical stimulation of selected brain areas The electrical stimulation of the serotonergic raphe nuclei causes an increase in the serum PRL levels, and of the 5-h~,droxyindoleacetic acid (5-HIAA) levels in the forebrain, indicating a specific increase in turnover of 5-HT in this area (both the PRL and the 5-HIAA increases are missing after stimulation of the occipital cortex). Electrical stimulation of the MBH causes a release of PRL and this has been attributed to activation of the serotonergic pathways.

TABLE I. Effect of various serotonerglc manipulations on PRL release

Manipulaaons

Basal values

PRL Stimulated condiUons Sleep

5-HT On pituitary lactotrophs i.c.v.

iv. Precursors (TRP, 5-HTP)by parenteralroutes Reuptakeinhibitors(e.g. fluoxetme) Reuptake + precursors(parenteral) Receptoragonists(e.g. mCPP) Braindepletors(pCPA) Nerveendingdestruction(5,6 DHT, 5,7 DHT+DMI) Cellbody lesion Cellbody stimulation Receptorantagonists

Suckling

Stress

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(a) 5-HT receptor agonists block the increase. (b) Supersensitivity to agonists. (c) Decrease blocked by receptor agonists

71PS -April 1983

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O CONTROL

synthesized hormone seems greater. Increased PRL release from activation of the serotonergic system may inhibit the m FLUOXETINE( F ) ~ 5-HYDROXYTRYPTOPHAN (5"HTP) release of hypothalarnic DA, develop indei F'I'5-NTP+NALOXONE ( N A L ) pendently of that system or he ascribed to release of a PRF (see Ref, 1), for which D mCPp there is some evidenceL *Pj .,=o 01 Activation of the serotonergic system I ___mCPp+NAL 'm..=.PC_, .,=0 01 causes PRL release through inhibition of 70 the TI-DA. The reduction by methysergide *** Pc_B,~0 000~ of increase in the serum PRL levels induced A by the dopaminergic antagonist pimozide 60 E suggests a serotonergic stimulation of the dopaminergic inhibitory system, and if this o} c 50 were eliminated there would be more DA available and a partial antagonism of the Z 40 effects of the neuroleptic. L~ Depletion of brain 5-HT byp-CPA does < _d not inhibit, or just negligibly reduces, the on,30 increase in blood PRL levels caused by (i. a-MpT or blocking of the dopaminergic < 20 receptors. I have already reported that or) m-CPP, a serotonergic agonist, does not < ..J achieve inhibition of spontaneous or K +a. 10 induced release of DA from the olfactory tubercle synaptosomes. Thus dopaminergic o inhibition through the serotonergic system A B C O E O may he modulated by another neuronal syso tem, probably the opioid system (Fig. 2), O} =. 50 rather than by a direct effect. The DA turnover inhibition recently held LU Z .d.. to be responsible for the increase in blood o 40 rr PRL that takes place during lactation8 may U.,, ::::, I-be due to serotonergic activation, but o0 30 through modulation of the opiate neurons o c2_ ! :i:!: (naloxone has recently been found to be I-rr 20 capable of antagonizing the increase of O PRL release from sucklingS). The relationO ship between the serotonergic system con, !i!ii lO [ trolling PRL secretion and the opiate CO < I ::::: neurons could be highly specific. Our _J o. 0 researches have shown how, under condiA C D E tions of strong activation of the brain serotonergic system due to intervention at F~g 3. 7"hediagram shows prolactin increase 45 min a~er an i.p. rejection o f 5-hydroxyt~. ptophan (30 mg the presynaptic level (administration of kg-9 preceded 60 min before mlection by fluoxetine (10 mg kg-9 in unanestheazed rats In some expenments naloxone (0.4 mg kg-9 was given t.p. 5 rain before 5 MTP. Naloxone does not modify the hyper- 5-HTP by itself or combined with the prolactmaemia following i.p. admmistratngn o f the serotonergic agomst mCPP Plasma corucosterone was uptake inhibitor fluoxetine), naloxone suealways unmodtfied. Data are expressed as mean +_SE M (Prezaos:et al. as for Fig 1) ceeds in inhibiting the secretion-induced increase of PRL, but does not modify that Supersensitivity and subsensitivity phenomenon is not found in animals with provoked by the 5-HT receptor agonists Supersensitivity phenomena from man- degeneration of the ascending seroton- m-CPP nor the 5-HTP-induced increase in ipulations that can induce PRL release ergic pathways produced by p-chloro- plasma corticosterone (Fig. 3) which is through interference with the serotonergic amphetamine and has been interpreted probably mediated by other serotonergic system can be observed by administering as a subsensitivity phenomenon at the pathways. Even if the modification of a serotonergic precursors or agonists after a presynaptic level of the serotonergic given pharmacological response by naloxdepletion of 5-HT caused by (1) a tryp- terminals). A synopsis of different manipu- one should not be the only criterion for tophan deficient diet; (2) the inhibitor of lations involving PRL release is displayed implying that there are endogenous peptide tryptophan-hydroxylase by p-CPA; (3) in Table I, opiates responsible for it, at least one effect chemical denervation by neurotoxin (see of 5-HTP, the amnesic effect, has been Refs 2 and 3). Repeated doses of Interplay between serotonergic, found to he mediated by these peptides 1°, as zimelidine, an inhibitor of 5-HT uptake, dopaminergic and opiate neurons in the shown by its potentiation by the peptidase after causing an acute increase in the serum control of prolactin release inhibitor bacitracine. On the other hand, PRL levels brings about a decrease in the At the hypophysial level, 5-HT does not it must be borne in mind that there is a blood concentration of the hormone by antagonize the inhibitory effects of DA on possibility of co-existence, in the same blocking the serotonergic transmission (this PRL release, even though the amount of neurons, of 5-HT and biologically-active

n iii

N'

lIPS -A15ril 1983

174 peptides (and thus possibly also of opiates). The opiate neurons could themselves control the serotonergic neurons: in the rat at least, the increase of PRL secretion induced by morphine or morphine-like drugs is ascribed to activation of the serotonergic system.

A perspective High-affinity, selective and long-lasting central serotonergic antagonists may usefully contribute to the clarification of points that are still obscure in the serotonergic modulation of PRL release and possibly allow useful therapeutic control of hyperprolactinaemic syndromes due to hypothalamic neurotransmission derangements.

Acknowledgement Recent research from my laboratory reported in this review was supported by Grants 81.00321.04 and 81.01443.96 (Progetto finalizzato Controllo Crescita Neoplastica) from the Italian Research Council (CNR) devolved to Pharmacological Unit c/o UCSC.

Reading list 1 Preziosl,

P.

(1980)

in

Progress

in

Psychoneuroendocrinology

(Brambdla, F., Racagni, G. and De Wied, D., eds), pp 75-85, North Holland, Amsterdam, London and New York

2 Quattrone, A., Schettini, G., Di Renzo, G. F., Tedeschi, G. and Prezlosi, P. (1979) Brain Res 174, 71-79 3 Quattn)ne, A , Schettim, G., Annunzaato, L. and Di Renzo, G. F. (1981)Eur. J. Pharmacol. 76, 9-13 4 Meltzer, H. Y., Fessler, R. G., Simonovic, M. and Fang, V. S. (1978)Ltfe Sci. 23, 1185-1192 5 Buckholtz, N. S. and Ondo, J. G. (1980)Endocr. Res. Comrnun. 7, 221-230 6 Clarenbach, P., Del Pozo, E., Brownen, J , Heredia, E., Spiegel, R. and Cramer, H. (1980) Brain Res. 202, 357-363 7 Malarkey, N. B , D'Orisio, Th.N., Kennedy, M. and Cataland, S. (1981 ) Ltfe Sci. 28, 2489-2495 8 Selmanoff, M. and Wise, P M. (1981) Brain Res. 212, 101-115 9 Ferland, L , Kledzik, G S., Cusan, L. and Labrie, F. (1978) Mol. Cell. Endocrinol. 12, 267-272 10 Garzon, J.,Rubio, J. and DelRio, J. (1981)Ltfe Scz. 29, 17-25

Aminoglycoside nephrotoxicity Constantin Cojocel and Jerry B. Hook Department of Pharmacology and Toxicology, Centerfor Enwronmental Toxicology, Michtgan State University, East Lansing, MI 48824, U.S.A.

Introduction In routine clinical practice, patients are frequently subjected to treatment with one or more potentially nephrotoxic drugs. For example, long-term use of aminoglycoside antibiotics has been associated with nephrotoxicity which constitutes a major complication in the management of gramnegative bacterial infections. Aminoglycosides damage both glomerulus and tubule reversibly or irreversibly depending on various factors such as dose, duration of exposure, age, sex, condition of the nephron at the time of exposure, etc. 1The nephrotoxic lesion is manifest clinically as proteinuria and enzymuria detected early after onset of treatment together with decreased urine osmolality, increase in urinary casts and elevated blood urea nitrogen and serum creatinine concentrations which are considered as late indicators of nephrotoxicity. Ultrastructural damage to the endothelium of the glomerular capillaries and to the tubular epithelium appears to be a characteristic feature of aminoglycoside nephrotoxicity. Formation of cytosegresomes containing myeloid bodies is an early morphological

sign of nephrotoxicity. Depending on dosage and time of exposure the morphological lesions can range from cellular swelling to proximal tubular necrosis. Numerous studies indicate recovery of function and structure of glomeruli and tubules after cessation of aminoglycoside treatment and even describe regeneration of some tubular cells despite continued drug administration z. Due to the increasing clinical concem with aminoglycoside nephrotoxicity, considerable interest and effort has focused on elucidation of the mechanisms of nephrotoxicity of these compounds. An excellent general review on the effects of xenobiotics on renal function has been published recently in this journal a. The purpose of this paper is to review recent progress made toward elucidation of the mechanism(s) of aminoglycoside nephrotoxicity.

Properties of aminoglycoside molecule The discovery of streptomycin by Schatz, Bugie and Waksman 4 began the development and production of aminoglycosides which eventually resulted in the introduction of a number of amino-

Ik~ 1983,ElsevierSciencePublishersB V , Amsterdam XXXX- XXXX/83/$0100

Paolo Preziosigraduated in Medicine in 1952 from the University of Naples. After 2 years spent m the laboratory of the Nobel Prize-winner Professor C. Heymans at Ghent, he became Professor of Pharmacology in 1965 at the University Medical School of Naples. In 1979 he was appointed chairman of the Department of Pharmacology at the Catholic University of the Sacred Heart Medical School, in Rome. His major research interest has for the last decade centered around the connections between neurotransmitters and hypophysial hormones. He was rectpient of the Purkinje medal in 1971. At present he is Prestdent of the Italian Society for Pharmaceutical Sciences and the Italian Society of Toxicology.

glycosides of clinical utility such as neomycin (1949), kanamycin (1957), gentamicin (1963), tobramycin (1967), amikacin (1972) and netilmicin (1975). Aminoglycosides consist of one or more amino sugars joined by a glycosidic linkage. They are highly polar cations; the average pKa is 8.0 or greater. The number of amino groups per molecule (Fig. 1) is responsible for the degree of their cationic character and for their ability to interact with anionic components in biological membranes. The number of amino groups per aminoglycoside molecule, and their cationic structure, appears to be correlated to the nephrotoxic potential. Aminoglycosides have a low lipid solubility and low capacity to penetrate membranes. As highly polar cations they are poorly absorbed from the intestinal tract but rapidly absorbed from intramuscular and subcutaneous sites of injection. Because of their polar nature the aminoglycosides are largely excluded from most cells and exhibit negligible binding to plasma proteins. Renal handling of aminoglyeosides Aminoglycosides are quickly excreted in the urine by humans and experimental animals, predominantly within the first 6 hours, with the total amount excreted reaching 80 to 90% of the dose in the first 24 hours. Only 1 to 2% of the administered dose is excreted in bile. Glomerular filtration has been established as the major route of aminoglycoside elimination from the