Melatonin and N-acetylserotonin inhibit leukocyte rolling and adhesion to rat microcirculation.

The hormone melatonin produced by the pineal gland during the daily dark phase regulates a variety of biological processes in mammals. The aim of this study was to determine the effect of melatonin and its precursor N-acetylserotonin on the microcirculation during acute inflammation. Arteriolar diameter, blood flow rate, leukocyte rolling and adhesion were measured in the rat microcirculation in situ by intravital microscopy. Melatonin alone or together with noradrenaline did not affect the arteriolar diameter or blood flow rate. Melatonin inhibited both leukocyte rolling and leukotriene B₄ induced adhesion while its precursor N-acetylserotonin inhibits only leukocyte adhesion. The rank order of potency of agonists and antagonist receptor selective ligands suggested that the activation of MT₂ and MT₃ melatonin binding sites receptors modulate leukocyte rolling and adhesion, respectively. The effect of melatonin and N-acetylserotonin herein described were observed with concentrations in the range of the nocturnal surge, providing the first evidence for a possible physiological role of these hormones in acute inflammation.     

Comparison of the structure-activity relationships of melatonin receptor agonists and antagonists: lengthening the N-acyl side-chain has differing effects on potency on Xenopus melanophores

The potency and affinity of two series of melatonin receptor ligands were examined using the pigment aggregation response in a clonal line of Xenopus laevis melanophores and radioligand binding assays on native receptors in chicken brain, recombinant human mt₁ and MT₂ and Xenopus laevis mel_1c receptor subtypes. One series was based on melatonin and had a methoxy group at the 5-position of the indole ring, while the other was based on luzindole and lacked this substituent but did have a 2-benzyl moiety; the N-acyl group of each series of analogues was varied from one to five carbon atoms. All analogues in the melatonin series were full agonists in melanophores (pEC₅₀ 7.76–10.24), while all compounds in the luzindole series were competitive melatonin antagonists (pA ₂ 5.47–6.60). With the agonist series, increasing the N-acyl side-chain from one to three carbon atoms was well tolerated in both the functional and binding assays, but further lengthening of the side-chain progressively and dramatically reduced potency and affinity. In contrast, for the antagonist series neither potency nor binding affinity changed substantially with the length of the N-acyl chain, except at the recombinant MT₂ subtype where two of the analogues had a lower affinity. In binding assays, three of the five antagonists were MT₂-selective; the most selective analogue (N-pentanoyl 2-benzyltryptamine, MT₂ pK _i 8.03) having 89- and 229-fold higher affinity than at mt₁ or mel_1c receptor subtypes. The different structure-activity relationships of these receptor agonists and antagonists is discussed with regard to the possible binding sites of agonists and antagonists within the receptor protein. Received: 26 June 1998 / Accepted: 6 September 1998     

Inhibitory effect of locally administered heparin on leukocyte rolling and chemoattractant-induced firm adhesion in rat mesenteric venules in vivo

1. Anti-inflammatory actions of heparin and related glycosaminoglycans have been described in the literature. Here, we used intravital microscopy of the rat mesentery microcirculation to examine effects of locally administered heparin on leukocyte rolling and chemoattractant-induced firm adhesion.
2. It was found that topical application of heparin reduced N-formyl-methionyl-leucyl-phenylalanine (fMLP)-induced leukocyte adhesion. Notably, the inhibitory action of heparin was not dose-dependent, but rather a biphasic dose-response was found, i.e. low (2 and 20 iu ml⁻¹) and high (1000 iu ml⁻¹) concentrations of heparin significantly reduced adhesion, whereas an intermediate dose (200 iu ml⁻¹) was less effective.
3. Heparin, 2 and 20 iu ml⁻¹, decreased rolling leukocyte flux, while having no effect on blood flow or total leukocyte flux. By contrast, heparin, 200 and 1000 iu ml⁻¹, increased total leukocyte flux in parallel with a rise in volume blood flow resulting in recovery of the rolling leukocyte flux at these doses. Thus, the biphasic inhibitory action of heparin on fMLP-induced firm adhesion could in part be attributed to changes in leukocyte delivery (i.e. blood flow) and rolling leukocyte flux induced by heparin.
4. When compensating for the influence of different rolling levels on fMLP-evoked adhesion, a dose-related inhibitory effect of heparin on the firm adhesive response per se was revealed. Similar results were obtained in a static adhesion assay in vitro where heparin 200 and 1000 iu ml⁻¹ (but not 2 and 20 iu ml⁻¹) significantly inhibited fMLP-induced leukocyte adhesion in the absence of any modulatory influence on changes in rolling.
5. Our data show that locally administered heparin inhibits leukocyte rolling as well as chemoattractant-induced firm adhesion in vivo which thus may contribute to the postulated anti-inflammatory activity of this compound. However, because of interference with several microvascular functions, strict dose-dependent responses to heparin treatment were not found, which illustrates the complex interplay between local blood flow, leukocyte rolling and chemoattractant-induced adhesion as determinants of leukocyte recruitment to tissues in inflammation.

     

Melatonin inhibits tachykinin NK₂ receptor-triggered 5-HT release from guinea pig isolated colonic mucosa

BACKGROUND AND PURPOSE

Melatonin is involved in the regulation of colonic motility, and sensation, but little is known about the influence of melatonin on 5-hydroxytryptamine (5-HT) release from colonic mucosa. A tachykinin NK₂ receptor-selective agonist, [β-Ala⁸]-neurokinin A_4-10[βAla-NKA-(4-10)] can induce 5-HT release from guinea pig colonic mucosa via NK₂ receptors on the mucosal layer. The present study was designed to determine the influence of melatonin on 5-HT release from guinea pig colonic mucosa, evoked by the NK₂ receptor agonist, βAla-NKA-(4-10).

EXPERIMENTAL APPROACH

The effect of melatonin was investigated on the outflow of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) from muscle layer-free mucosal preparations of guinea pig colon, using high-performance liquid chromatography with electrochemical detection.

KEY RESULTS

Melatonin caused a sustained decline in the βAla-NKA-(4-10)-evoked 5-HT outflow from the muscle layer-free mucosal preparations, but failed to affect its metabolite 5-HIAA outflow. The specific MT₃ receptor agonist, 5-methoxycarbonylamino-N-acetyltryptamine mimicked the inhibitory effect of melatonin on βAla-NKA-(4-10)-evoked 5-HT outflow. A MT₃ receptor antagonist prazosin shifted the concentration-response curve of melatonin to the right in a concentration-dependent manner and depressed the maximum effect, but neither a combined MT₁/MT₂ receptor antagonist luzindole, nor a MT₂ receptor antagonist N-pentanoyl-2-benzyltryptamine modified the concentration–response curve to melatonin.

CONCLUSIONS AND IMPLICATIONS

Melatonin inhibits NK₂ receptor-triggered 5-HT release from guinea pig colonic mucosa by acting at a MT₃ melatonin receptor located directly on the mucosal layer, without affecting 5-HT degradation processes. Possible contributions of MT₁/MT₂ melatonin receptors to the inhibitory effect of melatonin appear to be negligible. Melatonin may act as a modulator of excess 5-HT release from colonic mucosa.     

Docking studies for melatonin receptors

Introduction: Melatonin is a neurohormone that controls many relevant physiological processes beyond the control of circadian rhythms. Melatonin’s actions are carried out by two main types of melatonin receptors; MT₁ and MT₂. These receptors are important, and not just because of the biological actions of its natural agonist; but also, because melatonin analogues can improve or antagonize their biological effect.

Area covered: The following article describes the importance of melatonin as a biologically relevant molecule. It also defines the receptors for this substance, as well as the second messengers coupled to these receptors. Lastly, the article describes the amino acid residues involved in the docking process in both MT₁ and MT₂ melatonin receptors.

Expert opinion: The biological actions of melatonin and their interpretations are becoming more relevant and therefore require the development of new pharmacological tools. Understanding the second messenger mechanisms involved in melatonin actions, as well as the characteristics of the docking of this molecule to MT₁ and MT₂ melatonin receptors, will permit the development of more selective agonists and antagonists which will help us to better understand this molecule as well to develop new therapeutic compounds.     

Influence of the novel antidepressant and melatonin agonist/serotonin2C receptor antagonist,agomelatine, on the rat sleep–wake cycle architecture

Rationale  The novel antidepressant, agomelatine, behaves as an agonist at melatonin MT₁ and MT₂ receptors and as an antagonist at serotonin (5-HT)_2C receptors. In animal models and clinical trials, agomelatine displays antidepressant properties and re-synchronizes disrupted circadian rhythms. Objectives  The objectives of this study were to compare the influence of agomelatine upon sleep–wake states to the selective melatonin agonists, melatonin and ramelteon, and to the selective 5-HT_2C receptor antagonist, S32006. Methods  Rats were administered with vehicle, agomelatine, ramelteon, melatonin, or S32006, at the onset of either dark or light periods. Polygraphic recordings were performed and changes determined over 24 h, i.e., number and duration of sleep–wake episodes, latencies to rapid eye movement (REM) and slow-wave (SWS) sleep, power band spectra of the electroencephalogram (EEG), and circadian changes. Results  Administered at light phase onset, no changes were induced by agomelatine. In contrast, administered shortly before dark phase, agomelatine (10 and 40 mg/kg, per os) enhanced duration of REM and SWS sleep and decreased wake state for 3 h. Melatonin (10 mg/kg, per os) induced a transient enhancement in REM sleep followed by a reduction in REM and SWS sleep and an increase in waking. Ramelteon (10 mg/kg, per os) provoked a transient increase in REM sleep. Finally, S32006 (10 mg/kg, intraperitoneally), administered at dark phase onset, mimicked the increased SWS provoked by agomelatine, yet diminished REM sleep. Conclusions  Agomelatine possesses a distinctive EEG profile compared with melatonin, ramelteon, and S32006, possibly reflecting co-joint agonist and antagonist properties at MT₁/MT₂ and 5-HT_2C receptors, respectively. An erratum to this article can be found at     

Cardiovascular effects of melatonin receptor agonists

Introduction: Melatonin synchronizes circadian rhythms with light/dark period and it was demonstrated to correct chronodisruption. Several melatonin receptor agonists with improved pharmacokinetics or increased receptor affinity are being developed, three of them are already in clinical use. However, the actions of melatonin extend beyond chronobiology to cardiovascular and metabolic systems as well. Given the high prevalence of cardiovascular disease and their common occurrence with chronodisruption, it is of utmost importance to classify the cardiometabolic effects of the newly approved and putative melatoninergic drugs.

Areas covered: In the present review, the available (although very sparse) data on such effects, in particular by the approved (circadin, ramelteon, agomelatine) or clinically advanced (tasimelteon, piromelatine = Neu-P11, TIK-301) compounds are summarized. The authors have searched for an association with blood pressure, vascular reactivity, ischemia, myocardial and vascular remodeling and metabolic syndrome.

Expert opinion: The data suggest that cardiovascular effects of melatonin are at least partly mediated via MT₁/MT₂ receptors and associated with its chronobiotic action. Therefore, despite the sparse direct evidence, it is believed that these effects will be shared by melatonin analogs as well. With the expected approval of novel melatoninergic compounds, it is suggested that the investigation of their cardiovascular effects should no longer be neglected.     

Focus on agomelatine

Abstract

Background:

Agomelatine is a novel antidepressant with agonist activity at melatonin receptors (MT₁ and MT₂), and antagonistic effects at the 5HT_2c serotonin receptor.     

Melatonin: A pleiotropic molecule regulating inflammation

Melatonin is a neurohormone produced by the pineal gland that regulates sleep and circadian functions. Melatonin also regulates inflammatory and immune processes acting as both an activator and inhibitor of these responses. Melatonin demonstrates endocrine, but also paracrine and autocrine effects in the leukocyte compartment: on one side, leukocytes respond to melatonin in a circadian fashion; on the other side, leukocytes are able to synthesize melatonin by themselves. With its endocrine and paracrine effects, melatonin differentially modulates pro-inflammatory enzymes, controls production of inflammatory mediators such as cytokines and leukotrienes and regulates the lifespan of leukocytes by interfering with apoptotic processes. Moreover, its potent antioxidant ability allows scavenging of oxidative stress in the inflamed tissues. The interesting timing of pro- and anti-inflammatory effects, such as those affecting lipoxygenase activity, suggests that melatonin might promote early phases of inflammation on one hand and contribute to its attenuation on the other hand, in order to avoid complications of chronic inflammation. This review aims at giving a comprehensive overview of the various inflammatory pathways regulated by this pleiotropic hormone.     

Melatonin-induced augmentation of collagen deposition in cultures of fibroblasts and myofibroblasts is blocked by luzindole – a melatonin membrane receptors inhibitor

BackgroundMelatonin has been proven to have a regulatory influence on collagen accumulation in different types of wound. It was found to inhibit collagen accumulation in the superficial wound model but increase it in the myocardial infarction scar. The aim of the study is to determine the mechanism of melatonin action in the two wound types in rats.MethodsCells were isolated from both the superficial wound (subcutaneously inserted polypropylene net) and myocardial infarction scar (induced by ligation of the left coronary artery) and were identified by electron microscopy.ResultsLong-shaped cells forming whirl-like structures in culture (mainly identified as fibroblasts) were isolated from the superficial wound model, while myofibroblasts growing in a formless manner were acquired from the infarcted heart scar. Melatonin (10^–7 M) increased collagen accumulation in both fibroblast and myofibroblast cultures. Luzindole (10^–6 M), the blocker of both MT₁ and MT₂ melatonin membrane receptors, inhibited the effect of melatonin on the two types of cells.ConclusionRegardless of various healing potentials demonstrated by the tested cells (different cell composition, growth and organization), their response to melatonin was similar. Moreover, in the two investigated cultures, augmentation of the collagen content by melatonin was reversed by luzindole, which indicates the possibility of melatonin membrane receptor involvement in that process. The present results suggest that the increased melatonin-stimulated deposition of collagen observed in the infarcted heart of rats could be dependent on activation of the melatonin membrane receptors on scar myofibroblasts.     

Inhibitory actions of ropivacaine on tumor necrosis factor-alpha-induced leukocyte adhesion and tissue accumulation in vivo

We have examined the effect of ropivacaine, a local anesthetic, on leukocyte-endothelium interactions induced by tumor necrosis factor-alpha (TNF-alpha) in vivo by the use of intravital microscopy in the mouse cremaster microcirculation. It was found that ropivacaine markedly reduced venular leukocyte adhesion and tissue recruitment in response to TNF-alpha challenge, whereas leukocyte rolling was unchanged. Thus, treatment with ropivacaine may be a useful pharmacological tool to control acute inflammation.     

Role of MT1 melatonin receptors in methamphetamine-induced locomotor sensitization in C57BL/6 mice

Rationale

Melatonin modifies physiological and behavioral responses to psychostimulants, with the MT₁ and MT₂ melatonin receptors specifically implicated in facilitating methamphetamine (METH)-induced sensitization in melatonin-proficient mice.

Objective

The objective of the study is to assess differences in locomotor sensitization after a single dose of methamphetamine in low-melatonin-expressing C57BL/6 wild-type and MT₁ receptor knockout (MT₁KO) mice, comparing with melatonin-expressing C3H/HeN mice.

Methods

Mice received a vehicle or methamphetamine (1.2 mg/kg, i.p.) pretreatment (day 1) during the light (ZT5-9) or dark (ZT 19–21) periods in novel test arenas. Locomotor sensitization was assessed by methamphetamine challenge after an eight-day abstinence (day 9). TH protein expression was evaluated by immunofluorescence and Western blot analysis.

Results

Methamphetamine pretreatment induced statistically significant locomotor sensitization upon challenge after eight-day abstinence in C3H and C57 wild-type mice during the light period. The magnitude of sensitization in C57 mice was diminished in the dark period and completely abrogated in MT₁KO mice. No differences were observed in tyrosine hydroxylase immunoreactivity in the mesolimbic dopamine system. Additional exposures to the test arenas after methamphetamine pretreatment (nights 2–6) enhanced sensitization.

Conclusions

Deletion of the MT₁ melatonin receptor abolishes sensitization induced by a single METH pretreatment. The magnitude of sensitization is also altered by time of day and contextual cues. We conclude that the MT₁ melatonin receptor is emerging as a novel target of therapeutic intervention for drug abuse disorders.     

Evaluation of neurobiological and antioxidant effects of novel melatonin analogs in mice

Based on the pharmacophore model of melatonin (MT₁) receptor, we recently synthesized a series of indole derivatives that showed anticonvulsant activity with low neurotoxicity and hepatotoxicity in rodents. In the present study, the three most potent C3-modified derivatives with hydrazine structure 3c, 3e, and 3f, with 2-chlorophenyl, 2-furyl, and 2-thienyl fragments, respectively, were selected, and their neurobiological activity was explored in mice. In Experiment #1, the dose-dependent anxiolytic effect of a single i.p. administration of the novel compounds at doses of 10, 30, and 60 mg/kg were studied in the open field (OF) test. In Experiment#2, the analgesic effect of 3c, 3e, and 3f (30–100 mg/kg) was tested in the hot plate test and formalin test. Experiment#3 was designed to assess the antidepressant-like activity of 3c, 3e, and 3f (10–60 mg/kg). The forced swimming test (FST) and tail suspension test (TST)-induced effect on markers of oxidative stress in the frontal cortex (FC), and the hippocampus was evaluated. Melatonin was used in the same doses as melatonin analogs in all three experiments as a positive control. Desipramine (10 mg/kg) was also applied as a control in the FST. The three melatonin analogs bearing hydrazide/hydrazone substitution at 3C of the indol scaffold demonstrated improved antidepressant-like activity compared to the melatonin. The tested substances are devoided of anxiolytic effects. The antioxidant activity of the melatonin analogs and analgesic potential is comparable to that of melatonin. The 3C substitution with hydrazide/hydrazone moiety substantially contributes to the antidepressant and antioxidant activity of the melatonin analogs.     

Melatonin mitigates neomycin-induced hair cell injury in zebrafish

Context: Ototoxicity due to medications, such as aminoglycosides, is irreversible, and free radicals in the inner ear are assumed to play a major role. Because melatonin has an antioxidant property, we hypothesize that it might mitigate hair cell injury by aminoglycosides.

Objective: The objective of this study was to evaluate whether melatonin has an alleviative effect on neomycin-induced hair cell injury in zebrafish (Danio rerio).

Methods: Various concentrations of melatonin were administered to 5-day post-fertilization zebrafish treated with 125?μM neomycin for 1?h. Surviving hair cells within four neuromasts were compared with that of a control group. Apoptosis was assessed via terminal deoxynucleotidyl transferase dUTP nick-end labeling assay. The changes of ultrastructure were confirmed using a scanning electron microscope.

Results: Melatonin alleviated neomycin-induced hair cell injury in neuromasts (neomycin?+?melatonin 100?μM: 13.88?±?0.91 cells, neomycin only: 7.85?±?0.90 cells; n?=?10, p?Conclusion: Melatonin is effective in alleviating aminoglycoside-induced hair cell injury in zebrafish. The results of this study demonstrated that melatonin has the potential to reduce apoptosis induced by aminoglycosides in zebrafish.     

Melancholic-Like Behaviors and Circadian Neurobiological Abnormalities in Melatonin MT1 Receptor Knockout Mice

Background:

Melancholic depression, described also as endogenous depression, is a mood disorder with distinctive specific psychopathological features and biological homogeneity, including anhedonia, circadian variation of mood, psychomotor activation, weight loss, diurnal cortisol changes, and sleep disturbances. Although several hypotheses have been proposed, the etiology of this disorder is still unknown.

Methods:

Behavioral, electrophysiological and biochemical approaches were used to characterize the emotional phenotype, serotonergic and noradrenergic electrical activity, and corticosterone in melatonin MT1 receptor knockout mice and their wild type counterparts, during both light and dark phases.

Results:

Melatonin MT₁ receptor knockout mice have decreased mobility in the forced swim and tail suspension tests as well as decreased sucrose consumption, mostly during the dark/inactive phase. These mood variations are reversed by chronic treatment with the tricyclic antidepressant desipramine. In addition, MT₁ receptor knockout mice exhibit psychomotor disturbances, higher serum levels of corticosterone the dark phase, and a blunted circadian variation of corticosterone levels. In vivo electrophysiological recordings show a decreased burst-firing activity of locus coeruleus norepinephrine neurons during the dark phase. The circadian physiological variation in the spontaneous firing activity of high-firing neuronal subpopulations of both norepinephrine neurons and dorsal raphe serotonin neurons are abolished in MT₁ knockout mice.

Conclusions:

These data demonstrate that melatonin MT₁ receptor knockout mice recapitulate several behavioral and neurobiological circadian changes of human melancholic depression and, for the first time, suggest that the MT₁ receptor may be implicated in the pathogenesis of melancholic depression and is a potential pharmacological target for this mental condition.     

Melatonin releasing PLGA micro/nanoparticles and their effect on osteosarcoma cells

Melatonin loaded poly(d,l-lactide-co-glycolide) (PLGA) nanoparticles and microparticles in the diameter of ～200?nm and 3.5?μm, respectively, were prepared by emulsion–diffusion–evaporation method. Melatonin entrapment into the particles was significantly improved with the addition of 0.2% (w/v) melatonin into the aqueous phase and encapsulation efficiencies were found as 14 and 27% for nanoparticles and microparticles, respectively. At the end of 40 days, ～70% of melatonin was released from both of particles, with high burst release. Both blank and melatonin loaded PLGA nanoparticles caused toxic effect on the MG-63 cells due to their uptake by the cells. However, when 0.05?mg microparticle that is carrying ～1.7?μg melatonin was added to the cm² of culture, inhibitory effect of melatonin on the cells were obviously observed. The results would provide an expectation about the usage of melatonin as an adjunct to the routine chemotherapy of osteosarcoma by encapsulating it into a polymeric carrier system.     

Influence of beta-blockers on melatonin release

Objective: Melatonin is a mediator in the establishment of the circadian rhythm of biological processes. It is produced in the pineal gland mainly during the night by stimulation of adrenergic beta₁- and alpha₁-receptors. Sleep disturbances are common side-effects of beta-blockers. The influence of specific beta-blockade as well as that of combined alpha-and beta-blockade on melatonin production has not been investigated in humans before. Methods: We performed a randomized, double-blind, placebo-controlled, cross-over study in 15 healthy volunteers. Subjects received single oral doses of 40 mg (R)-propranolol, 40 mg (S)-propranolol, 50 mg (R)-atenolol, 50 mg (S)-atenolol, 25 mg (R,S)-carvedilol, 120 mg (R,S)-verapamil or placebo at 1800 hours. Urine was collected between 2200 hours and 0600 hours, and 6-sulfatoxy-melatonin (aMT6s), the main metabolite of melatonin which is almost completely eliminated in urine, was determined by radioimmunoassay (RIA). Results: Mean nocturnal excretion of aMT6s in urine after intake of the drugs was as follows (in μg): placebo 26; (R)-propranolol 24 (−7%, NS); (S)-propranolol 5 (−80%, P < 0.001); (R)-atenolol 27 (+7%, NS); (S)-atenolol 4 (−86%, P < 0.01); (R,S)-carvedilol 23 (−10%, NS); (R,S)-verapamil 29 (+14%, NS). These data show that only the specifically beta-blocking (S)-enantiomers of propranolol and atenolol decrease the nocturnal production of melatonin whereas the non-beta-blocking (R)-enantiomers have no effect. Unexpectedly, (R,S)-carvedilol which inhibits both alpha- and beta-adrenoceptors does not decrease melatonin production. Conclusion: These findings indicate that beta-blockers decrease melatonin release via specific inhibition of adrenergic beta₁-receptors. Since lower nocturnal melatonin levels might be the reason for sleep disturbances, further clinical studies should investigate whether or not oral administration of melatonin might avoid this well-known side-effect of beta-blockers. The reason why (R,S)-carvedilol does not influence melatonin production remains to be determined. Received: 10 August 1998 / Accepted in revised form: 25 November 1998     

Evidence for direct vasoconstrictor activity of melatonin in “pressurized” segments of isolated caudal artery from juvenile rats

Summary Responses of isolated, 60 mmHg pressurized segments of the distal caudal artery of adult and juvenile Wistar rats to melatonin and the selective ₂-adrenoceptor agonist 5-bromo-6-[2-imidazolin-2-ylamino]-quinoxaline bitartrate (UK-14304) were examined using the Halpern pressure myograph. Melatonin showed no direct vasoconstrictor activity in vessels from adult rats, whereas UK-14304 produced moderate vasoconstriction (pD₂- 7.43+-0.09). In the presence of phenylephrine-induced tone, melatonin produced a variable but small constrictor response (< 10µm reduction in diameter) in some vessels; the response to 1 gmol/l UK-14304 was less than in the absence of tone. In vessels isolated from juvenile rats, melatonin caused concentration-dependent vasoconstriction with a maximum response about 70% of the maximum response elicited by UK-14304. Vessels from juvenile rats were more sensitive to melatonin(pD₂- 9.40+-0.07) than they were to UK-14304 (pD₂ -8.12+-0.14). In the presence of phenylephrine-induced tone, the vasoconstrictor responses to both melatonin and IK-14304 were markedly less; the sensitivity to melatonin was not different from that seen in the absence of tone. These findings indicate that pressurized segments of the isolated distal caudal artery may provide a simple and convenient, functional model of melatonin receptors. The findings also appear to implicate melatonin in thermoregulatory processes in juvenile rats. Send offprint requests to V. G. Wilson at the above address